# 2. Catalight Young Scientist Symposium: Artificial Photosynthesis

Europe/Berlin
Online Conference

#### Online Conference

CYSS2021 will be hosted online using Zoom. The poster session will follow a dual-format approach using a chat-platform and videoconference rooms.
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Description

The CataLight Young Scientist Symposium (CYSS) is an annual conference organized by PhD students and junior PostDocs of SFB/TRR 234 CataLight aimed at researchers in an early career stage in the field of light-driven catalysis and related fields. By employing an online format, we want to allow for the participation of young scientists, in particular PhD students, from all over the globe and offer them an opportunity to share their work, which have been especially sparse over the last year due to the cancellation of on-site conferences. Therefore the event will emphasize on talks from young scientists accompanied by selected invited talks.

In line with the research area of CataLight the symposium aims to cover all facets of the highly interdisciplinary field of artifical photosynthesis and water-splitting. We therefore welcome contributions from a diverse range of fields, includign design and synthesis of catalytic systems and support materials, (spectroscopic) characterization and theoretical research. For an in-depth explanation of our aims & scope, see our Scientific Programme.

CYSS2021 will take place on Tuesday and Thursdays between 21st and 30st September 2021.

To facilitate in-depth discussion discussion or even collaboration between participants of CYSS, we're accompanying the symposium with an instant messaging service that can be used to contact the speakers or any other participants with more in-depth questions or exchange contact information for further discussion.

CYSS is supported by the SFB/TRR 234 CataLight. We gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (German Research Foundation - DFG).

Contact the Organizers
• Tuesday, 21 September
• 09:15 09:30
Conference Opening 15m
• 09:30 10:00
Efficient one and two photon mechanisms: From heavy atom and spin effects in photocatalysis to blue-light driven upconversion 30m
Speaker: Prof. Christoph Kerzig (University of Mainz)
• 10:05 10:55
Molecules & Materials I
• 10:05
Organic Conjugated Porous Polymer-Pd Composites: An Enabling Photocatalyst 20m

Organic conjugated porous polymers (CPPs) are a class of semiconducting materials with significant photochemical prospects. Choice over the monomers endow us the ability to manually fine tune the properties of the final CPPs. Herein, we report an account of our ongoing research on semiconducting CPP-Pd composites as a state-of-the-art heterogeneous photocatalysts.

Using azobenzene as a building unit, CPPs amenable to substantial photo-activity and Pd-chelation ability have been synthesized. Loading of Pd(0) into these CPP nanoparticles resulted in Mott-Schottky heterojunction, which was exploited for visible-light-induced Suzuki coupling under both batch and flow-reactor setup.

Successively, Pd(II) were loaded on another azobenzene-based CPP. Instead of a Schottky junction, a heterogenized azobenezene-Pd(II) complex was formed. Upon visible light illumination, this material manifested a polymer-to-metal energy transfer. This high-energy Pd(II)-system was then utilized for photocatalytic Sonogashira reaction using both aromatic and aliphatic substrates.

Finally, another azobenzene-containing CPP was formed and loaded with various amounts of Pd(0)/Pd(II) mixed system. A combination of both these aforementioned processes led to a cooperative performance between Pd(0) and Pd(II) , endowing excellent activity in photocatalytic selective hydrogenation.

Speaker: Dr Ipsita Nath (COMOC, Department of Chemistry, Ghent University, Ghent, Belgium)
• 10:30
Achieving Inter-Laboratory Comparability of Photochemical Experiments with a Modular, Documented and Characterized Experimental Setup 20m

In recent years, many new photon-mediated reactions were developed and a strong interest to improve the reaction rates grew. Standardized experimental protocols and characterized reactors are one possibility to identify suited reaction conditions that lead to the desired enhancements. Le et al. describe a standardized photoreactor used for an acceleration of numerous photocatalytic transformations.[1] While the proposed photoreactor led to an increase in the reaction rates, characterization falls short. For example, no photonic characterization in terms of received photon fluxes was reported. Without a fully covered photonic evaluation of the proposed setups, a quantitative comparison of e.g. homogenous and heterogeneous photocatalysts cannot be achieved. For this reason, the development of knowledge driven reactor designs and experimental protocols is needed to enable a better understanding of the influence of different process parameters.
Recently, we established accurate methods for the measurement of photon fluxes, using actinometry and radiometry.[2][3] In addition, rapid prototyping of photoreactors, using 3D printing, facilitates a fast comparison of geometric parameters of the proposed photoreactors.[4] The evaluation of a reactor setup using a holistic approach can lead to inter-laboratory comparability and control over photoinduced processes. This is one step to the realization of the overall goal, the utilization of sunlight for photoinduced transformations. Our contribution will show an experimental platform that was developed to meet the above requirements.
Realizing sunlight driven transformations raises another challenge to reaction control since the light intensity changes during the course of a day and year. Therefore, operation under unsteady irradiation is relevant for technical application. The easiest way to examine unsteady irradiation is a simple pulsed irradiation that can be realized either by switching the light source on and off, using shutters or in continuous reactors.
Taking the photosensitized water oxidation catalysis as an example, pulsed irradiation was realized by continuous pumping of the reaction solution through a partially irradiated capillary. We found significantly higher turnover numbers and frequencies for pulsed operation at low light intensity as shown in Figure 1 (left). In further experiments, reaction and operation conditions were varied systematically. Additionally, a modular photoreactor was developed that enables control over the used radiation source, reaction temperature and provides a defined irradiation geometry as shown in Figure 1 (right). The results will be shown in this contribution.

References

[1] C. Le, M. K. Wismer, Z. Shi, R. Zhang, D. V. Conway, G. Li, P. Vachal, I. W. Daves, D. W.C. MacMillan, ASC Central Science, 2017, 3, 647-653,http://dx.doi.org/10.1021/acscentsci.7b00159
[2] B. Wriedt, D. Ziegenbalg, Flow Chemistry, 2019, 301-305, https://doi.org/10.1007/s41981-019-00072-7
[3] B. Wriedt, D. Kowalczyk, D. Ziegenbalg, ChemPhotoChem, 2018, 2, 913–921, https://doi.org/10.1002/cptc.201800106
[4] F. Guba, Ü. Tastan, K. Gugeler, M. Buntrock, T. Rommel, D. Ziegenbalg, Chemie Ingenieur Technik, 2019, 91, No. 1–2, 17–29, https://doi.org/10.1002/cite.201800035

Speaker: Daniel Kowalczyk (Institute of chemical engineering)
• 10:55 11:10
Coffee Break 15m
• 11:10 12:00
Molecules & Materials I
• 11:10
All-Perovskite Photoelectrochemical Cell for Artificial Photosynthesis of Solar Hydrogen 20m

Perovskite photoelectrodes are promising for efficient photoelectrochemical (PEC) overall water splitting (OWS) due to its photocurrent, photovoltage and early onset potential [1, 2]. However, activating their photostability in electrolyte is tough, yet this has proven challenging because degradation performance of perovskite photoanode and photocathode in different electrolyte [3, 4]. Here, we report the first use of pure FAPbI3 thin-film with above requirements by flipping n-i-p structure for development of integrated photocathode, which displays photoelectrochemically efficient performance with photocurrent density of -19 mA cm-2 at 0 V vs. RHE and alkaline stable for > 15 hours with enough O2-H2 gas productivity. Integrated FAPbI3/HEC photocathode show positive onset potential of 1.1 V vs. RHE, which allows to combine with water oxidating and alkaline stable FAPbI3/OEC photoanode (21 mA cm-2 at 1.23 V vs. RHE) for the development of all-perovskite dual configurations in transparent archetype conjoined PEC cell. This unassisted photonic OWS system using parallelly “wired” configuration shows a maximum solar-to-hydrogen (STH) conversion efficiency of >5% with photostability of > 10 hours, while its arising “wireless” standalone artificial leaf shows the STH conversion efficiency of 4.1% with 3 hours of photostability in fully-alkaline media.

Speaker: Dharmesh Hansora (Chemical Engineering Department, Ulsan National Institute of Science and Technology (UNIST))
• 11:35
Synthesis of novel crystalline phase of ZnO synthesized using anti-diabetic drug metformin as template 20m

Zinc oxide is a well-explored material semiconductor material that has been reported and studied for around a century. It has been utilized for sensing, optoelectronic studies, antimicrobial properties, etc. Like other semi-conductors, the properties of ZnO can also be tuned by changing the morphology, porosity, structure defect crystallization phase, etc. Various methods have been employed to enhance the porosity and fabrication of novel crystal phases. However, to date, there has been no other crystalline phase reported other than hexagonal wurtzite and cubic zinc blende phase. Here, we have employed an anti-adiabatic drug, metformin, as a template for the synthesis of ZnO in the hydrothermal pathway. As a result, we have obtained two different types of ZnO crystal structures with triclinic phases. Two different structures of ZnO was obtained while using two different pathway wherein one path it has been salt to oxide synthesis whereas in the other it is a transformation of one crystalline phase to other. These materials NZO-1 and NZO-2 have been thoroughly characterized using PXRD, TEM, and XPS. The PXRD patterns are indexed to identify the crystalline plane. In both cases, the materials are found to be nanorods composed of self-assembled spherical ZnO nanoparticles. NZO-1 in presence of a photo-sensitive covalent organic framework has shown enhanced semiconducting properties for photoelectrochemical water oxidation compared to ZnO hexagonal wurtzite phase. NZO-2, on the other hand, was surface phosphorylated which has shown good proton-conducting properties under hydrous conditions. Both of them have shown unique optoelectronic properties under different conditions. NZO-2 in particular has shown red emission under laser irradiation.

Speaker: Sauvik Chatterjee (Indian Association for the Cultivation of Science)
• 14:15 15:15
"Scientific Speed Dating" 1h

Get to know the other participants.
Present your research and your interests in 2 minutes. To multiple groups of 5 fellow young researchers.

• 15:30 16:00
Nanocarbon in Photochemistry and Electrochemistry 30m

Artificial photosynthesis, a process in which the light energy is converted into the chemical bond, represents an important area of research aimed toward generation of Sun-derived fuels and value-added chemicals. To successfully utilize photons from Sun, such solar reactors need to contain light-absorbing chromophores that efficiently and rapidly channel the absorbed energy toward desired catalytic sites where useful chemistry can takes place with high selectivity and low kinetic barriers. The Glusac group investigates molecular chromophores and catalysts for artificial photosynthesis. We utilize advanced time-resolved laser spectroscopy techniques to investigate mechanisms of energy and charge migration in molecular excited states and evaluate the parameters that control undesired energy losses through fast charge recombination. We also explore molecular electrocatalysts that are able to receive electrons and holes from excited chromophores and covert then into desired products.

Three projects are currently under investigation in our labs: (i) Bio-inspired CO2 reduction using metal-free NAD+/NADH analogs, where we look for strong hydride donors that can selectively reduce CO2 to methanol and that can be recycled photochemically; (ii) Light-harvesting by graphene quantum dot assemblies, where we explore excited-state energy and charge redistributions in chromophore-catalyst assemblies using advanced time-resolved laser spectroscopy methods; (iii) Carbon-based platforms for electrocatalysis. In this project, we investigate methods to decorate carbon electrodes with molecular catalytic motifs that can perform useful chemistry.

Speaker: Prof. Ksenija Glusac (University of Illinois at Chicago)
• 16:05 16:30
Spectroscopy I
• 16:05
Mechanistic Insights through In Situ Reaction Monitoring in Photocatalytic Synthesis 20m

Mechanistic Insights through In Situ Reaction Monitoring in Photocatalytic Synthesis

Amiera Madani ab, Jamal A. Malik a, Cristian Cavedona ab, Bartholomäus Pieber a
a) Department of Biomolecular Systems, Max-Planck-Institute of Colloids and Interfaces,
Am Mühlenberg 1, 14476 Potsdam, Germany;
b) Department of Chemistry and Biochemistry, Freie Universität Berlin,
Arnimallee 22, 14195 Berlin, Germany

In situ reaction monitoring is a powerful tool to track reactions in real time under synthetically relevant conditions.1 Such investigations unveil intermediates and allow to determine rate dependencies to understand reaction kinetics, which is essential to elucidate the underlying mechanism.
In this talk, I will present how in situ reaction monitoring was used to provide mechanistic insights in two photocatalytic reactions. First, a comprehensive kinetic examination of a dual nickel/photocatalytic C–O arylation using a homogeneous photocatalyst or a heterogeneous photocatalyst will be discussed. 2-3 We used in situ infrared spectroscopy for in-depth kinetic studies of both catalytic systems have been carried out by applying variable time normalization analysis (VTNA). The studies revealed arguments against the current mechanistic hypothesis, which states that the photocatalyst is only involved to trigger reductive elimination.
Second, I will present our efforts to track a newly developed photo oxidative cleavage of benzyl ether protecting groups using a LED-NMR setup.4 These investigations support the underlying mechanistic hypothesis and supports the notion that the reaction ceases upon light removal.emphasized text

1. Nielsen, C. D. T.; Burés, J., Chem. Sci 2019, 10, 348-353. https://doi.org/10.1039/C8SC04698K
2. Welin, E. R.; Le, C.; Arias-Rotondo, D. M.; McCusker, J. K.; MacMillan, D. W. C., Sci 2017, 355, 380-385. DOI: 10.1126/Science.Aal2490
3. Malik, J. A.; Madani, A.; Pieber, B.; Seeberger, P. H., J. Am. Chem. Soc. 2020, 142, 11042-11049.https://dx.doi.org/10.1021/jacs.0c02848
4. Cavedon, C.; Sletten, E. T.; Madani, A.; Niemeyer, O.; Seeberger, P. H.; Pieber, B., Org Lett 2021, 23, 514-518.https://dx.doi.org/10.1021/acs.orglett.0c04026
Speaker: Amiera Madani (Max-Planck-Institute of Colloids and Interfaces)
• 16:30 16:40
Coffee Break 10m
• 16:40 17:30
Spectroscopy I
• 16:40
Operando redox state kinetics in photo- and electro-catalytic schemes for water splitting 20m

To design water-splitting catalysts for energy conversion and storage with better metal atom utilisation and activity[1-2], it is necessary to understand how the photo- and electro-catalytic performance of catalyst materials relates to their chemical configuration at the atomic level. Herein, we use operando time-resolved spectroelectrochemistry to investigate the kinetics of multi-reduced and multi-oxidized localised intermediates. Mono-metallic and bi-metallic molecular catalysts were investigated to rationalize more complex multi-metallic systems. First, earth-abundant molecular cobalt catalysts for H2- and CO2-reduction were studied on a photoelectrode based on mesoporous TiO2 [3]. The accumulation of charge in the catalyst, necessary for catalysis, was observed in hundreds of microseconds and was critically quenched by valence band holes under microseconds. It was then shown that regulating the applied potential, the excitation light intensity and the photoelectrode surface coverage was an effective approach to prevent recombination while simultaneously favouring charge accumulation. Second, the electrochemical activity of molecular and inorganic iridium-based water-oxidation catalysts were compared per iridium state [4]. Three multi-electron oxidation states involving Ir3+, Ir4+ and Ir4.x+ were identified in both catalysts. The generation of Ir4.x+-based states was found to be the potential determining step for catalytic water-oxidation, whilst H2O2 oxidation was observed to be driven by the generation of Ir4+ states. The TOFs over potential indicate a first order reaction mechanism for H2O2 oxidation by Ir4+ states in IrOx and for water oxidation by the molecular catalyst, and a potential-dependent mechanism for water oxidation in IrOx involving the co-operative interaction of multiple Ir4.x+ states. This insight into the intrinsic reaction kinetics based on localised states from our spectroelectrochemical data offers a promising alternative to more widely applied Tafel and Butler-Volmer analyses for disordered metal oxide electrocatalysts such as IrOx, which is of interest to enhance the performance and cost effectiveness of electrolysers and to develop earth-abundant catalysts.

[1] Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5⁰C Climate Goal, International Renewable Energy Agency, 2020
[2] J. K. Nørskov, A. Latimer and C. F. Dickens, Research needs towards sustainable production of fuels and chemicals, 2019
[3] Bozal-Ginesta, C., Mesa, C.A., Eisenschmidt, A., Francàs, L., Shankar, R., Antón-García, D., Warnan, J., Willkomm, J., Reynal, A., Petit, C., Reisner, E., Durrant, J.R., Chemical Science, 2021, 12, 946-959, DOI: 10.1039/d0sc04344c
[4] Bozal-Ginesta, C., Rao, R.R., Mesa, C.A., Liu, X., Hillman, S., Stephens, I., Durrant, J.R., Operando spectroelectrochemical analysis of active state kinetics in water-oxidation IrOx electrocatalysts, submitted

Speaker: Carlota Bozal Ginesta (Imperial College London)
• 17:05
Operando x-ray spectroscopy studies of mesoporous FeNiOx catalysts for alkaline water splitting 20m

A carrier of renewable energy that will play a key role in the replacement of fossil fuels is molecular hydrogen, which can be generated by electrocatalytic water splitting. The anodic reaction of the water electrolysis process, the oxygen evolution reaction (OER) limits the overall efficiency of water electrolysis due to high overpotentials.[1] Thus to enable the large-scale production of hydrogen, suitable electrocatalysts are required.
In search for cost-effective, abundant and stable electro-catalytic materials for OER, transition metal oxides have been found to be promising candidates in alkaline solutions.[2]
An in-depth understanding of the behavior of the involved electrocatalytic materials under reaction conditions is crucial for the development of efficient and stable electrocatalysts for water splitting. Operando x-ray spectroscopy enables the observation of the electronic structure of the catalyst under electrocatalytic reaction conditions and provides insight into processes occurring at the electrode/electrolyte interface.
We propose mesoporous FeNiOx films as model systems for the spectroscopic investigation of the electrode/electrolyte interface under alkaline OER conditions. Their exceptionally high porosity and their film thickness of about 100 nm make these nanostructured systems ideal to track their electronic structure during catalytic cycles via synchrotron-radiation based soft x-ray spectroscopy methods, such as x-ray absorption spectroscopy (XAS) in total electron yield (TEY) and total fluorescence yield (TFY) mode. To perform operando XAS, we use a flow-cell design pioneered by the Schlögl group at the Fritz Haber Institute Berlin (Tesch, Bonke) [3]) which allows us to study the electrode/electrolyte interface at a defined potential.

References
[1] N. T. Suen, S. F. Hung, Q. Quan, N. Zhang, Y. J. Xu, and H. M. Chen, Chem. Soc. Rev., vol. 46, no. 2, pp. 337–365, 2017, doi: 10.1039/c6cs00328a.
[2] B. M. Hunter, H. B. Gray, and A. M. Müller, Chem. Rev., vol. 116, no. 22, pp. 14120–14136, 2016, doi: 10.1021/acs.chemrev.6b00398.
[3] M. F. Tesch et al., Angew. Chemie - Int. Ed., vol. 58, no. 11, pp. 3426–3432, 2019, doi: 10.1002/anie.201810825.

Speaker: Hanna Trzesniowski (Helmholtz-Zentrum Berlin für Materialien und Energie)
• Thursday, 23 September
• 09:30 10:00
A search for new photophysics and photochemistry with metal complexes 30m

This talk will focus on three different topics. In the first part, I will present recent results obtained by Mirjam Schreier, Xingwei Guo and Björn Pfund on photo-triggered hydrogen atom transfer (photo-HAT) between an iridium hydride complex and olefins.1 Whilst photoinduced electron transfer (PET) is very common for transition metal complexes, photo-HAT is rare.
In the second part, I will talk about unusually photostable ruthenium(II) complexes investigated by Lucius Schmid and Christoph Kerzig, and how these complexes (with their high triplet energies) are useful for photoredox and energy transfer catalysis.2
In the final part, I will tell about our latest adventures in the development diisocyanide complexes as Earth-abundant analogs of well-known ruthenium(II) polypyridines. This will include Jakob Bilger’s deep red molybdenum(0) emitter that he used for triplet-triplet annihilation upconversion,3 and some new first-row transition metal luminophores.

(1) Schreier, M. R.; Pfund, B.; Guo, X.; Wenger, O. S. Chem. Sci. 2020, 11, 8582.
(2) Schmid, L.; Kerzig, C.; Prescimone, A.; Wenger, O. S. JACS Au 2021, doi: 10.1021/jacsau.1c00137.
(3) Bilger, J. B.; Kerzig, C.; Larsen, C. B.; Wenger, O. S. J. Am. Chem. Soc. 2021, 143, 1651.

Speaker: Prof. Oliver Wenger (University of Basel)
• 10:05 10:30
Molecules & Materials II
• 10:05
Acridine based covalent organic frameworks for photocatalytic C-N cross coupling reactions 20m

The field of Covalent Organic Frameworks (COFs) – crystalline and porous polymers that are solely consisting of organic building blocks reticulated via covalent bonds – have gained increasing attention in the last decade. [1,2] COFs have since emerged as a powerful tool towards a plethora of different applications including gas storage & separation, energy storage and catalysis.[2] Keys to the success are the porosity, structural tunability and the possibility to integrate functional linkers in the COF backbone. However, the usage of COFs as heterogenous photosensitizer have mostly been studied in the activation of H2O and CO2 while only few examples of COFs have been used to catalyze organic transformations. [3,4]
Herein, we demonstrate the synthesis of a library of 2,6-diaminoacridine based conjugated covalent organic frameworks. This class of acridine COFs is then used as fully organic, heterogeneous photocatalysts in Ni(II)-mediated metallaphotocatalytic C-N cross coupling. Through its broad absorption in the visible light range, acridine COFs can also harvest lower energetic green light to drive the reaction. [5]

[1] K. Geng, T. He, R. Liu, S. Dalapati, K. T. Tan, Z. Li, S. Tao, Y. Gong, Q. Jiang, D. Jiang, Chem. Rev. 2020, 120, 16, 8814–8933, 10.1021/acs.chemrev.9b00550.
[2] X. Zhao, P. Pachfule, A. Thomas, Chem. Soc. Rev. 2021, 10.1039/d0cs01569e, DOI 10.1039/d0cs01569e.
[3] T. Banerjee, K. Gottschling, G. Savasci, C. Ochsenfeld, B. V. Lotsch, ACS Energy Lett. 2018, 3, 2, 400–409, 10.1021/acsenergylett.7b01123.
[4] L. Wang, R. Wang, X. Zhang, J. Mu, Z. Zhou, Z. Su, ChemSusChem 2020, 13, 11, 2973–2980, 10.1002/cssc.202000103.
[5] S. Gisbertz, S. Reischauer, B. Pieber, Nat. Catal. 2020, 3, 8, 611–620, 10.1038/s41929-020-0473-6.

Speaker: Mr Michael Traxler (TU Berlin)
• 10:30 10:40
Coffee Break 10m
• 10:40 11:30
Molecules & Materials II
• 10:40
The Wavelength Matters 20m

The combination of photo- and nickel catalysis (metallaphotocatalysis) has emerged as a powerful strategy for carbon–carbon and carbon–heteroatom cross couplings [1]. Key to the success are redox or photosensitization events between a nickel- and a photocatalyst (PC). These protocols rely on a few photocatalysts that can only convert a small portion of visible light (<500 nm) into chemical energy. The high-energy photons that excite the photocatalyst can result in unwanted side reactions. Dyes that absorb a much broader spectrum of light are not applicable due to their short-lived excited states. We demonstrate a self-assembling catalyst system that overcomes this limitation. Immobilization of a nickel catalyst on dye-sensitized titanium dioxide results in a material that catalyzes carbon-heteroatom and carbon-carbon bond formations. The modular approach of dye-sensitized metallaphotocatalysts (DSMPs) accesses the entire visible light spectrum and allows tackling selectivity issues resulting from low-wavelengths strategically. The concept overcomes current limitations of metallaphotocatalysis by unlocking the potential of dyes that were previously unsuitable [2].

References:
[1] J. Twilton, C. Le, P. Zhang, M. H. Shaw, R. W. Evans, D. W. C. MacMillan, Nat.Rev. Chem. 2017, 1, 0052.
[2] S. Reischauer, V. Strauss, B. Pieber, ACS Catal. 2020, 10, 13269−13274.

Speaker: Susanne Reischauer
• 11:05
Reactivity of Nitric Oxide towards Nonheme Diiron(II) Complexes Containing Bridging Thiolate and Terminal Hydrosulfide 20m

As compared to Carboxylate-bridged nonheme diiron(II) complexes, the diiron(II)-hydrosulfide and thiolate complexes are very rare in Literature. In this context, complexes [Fe2(N-Et-HPTB)(R1S)](BF4)2 (R1 = Me, Et, tBu, Ph), N-Et-HPTB is the anion of N, N,N′, N′-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane) could be synthesized directly from the addition of sodium Salt of thiolate into a mixture of Fe(BF4)2·6H2O, HN-Et-HPTB, and Et3N. The reaction of (Cp2Fe)(BF4) with 1b yielded [FeII2(N-Et-HPTB)(DMF)3](BF4)3·DMF (4) (when crystallized from DMF/diethyl ether), which might indicate the formation of a transient ethane thiolate bridged {FeIIFeIII} species, followed by a rapid internal redox reaction to generate diethyl disulfide and the solvent coordinated diiron(II) complex, 5. This possibility was supported by a comparative cyclic voltammetric study of 1a−1c and 5. Reaction of [Fe2(N-Et- HPTB)(R1S)](BF4)2 with Ph3CSNO produces [Fe2(N-Et-HPTB)(NO)2(DMF)2](BF4)3. While attempted synthesis of diiron(II)-hydrosulfide complexes using HS− produced insoluble precipitate, Reaction of Fe(BF4)2·6H2O, Et3N and HN-Et-HPTB with RSH (R = PhCH2,tBu) /NaStBu in DMF at RT yielded the desired complex, [Fe2(N-EtHPTB)(SH)(H2O)](BF4)2·DMF. Treatment of 2a with 1 eq. of (Cp2Fe)(BF4) resulted into the isolation of an unprecedented, mixed-valence diiron(II, III)- hydrosulfide complex, [Fe2(N-Et- HPTB)(SH)(H2O)(DMF)2](BF4)3. The reaction of 2a with 6 equ. Ph3CSNO produces [Fe2(N-Et-HPTB)(SH)(NO)2(DMF)](BF4)2.

Speaker: Dr Nabhendu Pal (Indian Association for The Cultivation of Science )
• 14:15 15:15
Poster Session
• 14:15
Influence of reaction conditions and “innocent” cations on the activity of molecular molybdenum sulfides towards light-driven HER catalysis 1h

Over the past years, molybdenum sulfides have obtained significant interest as earth abundant hydrogen evolution reaction (HER) catalysts.[1] However, the highest activities are observed on amorphous structures, thus structural information on active sites or the catalytic mechanism on a molecular level are difficult to determine.[2] We have therefore developed molecular analogues, so called thiomolybdate clusters. First light-driven homogeneous HER studies showed that a prototype cluster, [Mo3S13]2- shows high catalytic activity. It was found that selective ligand exchange is the basis for the loss of activity.[3] For a better understanding of the mechanism and deactivation with the related ligand exchange, the reaction was studied under various reaction conditions. Besides, the dependency on “classic” reaction conditions, like pH or solvent, a strong influence of the catalytic activity upon presence of ammonium cations was discovered. Catalytic studies with various catalysts, photosensitiser and electron donors showed, all chosen reactions were influenced by the presence of ammonium ions. The exact role of the cation within the catalysis is not completely understood yet and might differ between the different reactions investigated.

[1] C. G. Morales-Guio, X. Hu, Acc. Chem. Res. 2014, 47, 2671.
[2] B. Seo, S. H. Joo, Nano Converg. 2017, 4, 19.
[3] M. Dave, A. Rajagopal, M. Damm-Ruttensperger, B. Schwarz, F. Nägele, L. Daccache, D. Fantauzzi, T. Jacob, C. Streb, Sustainable Energy Fuels 2018, 2, 1020.

Speaker: Magdalena Heiland (Uni Ulm, AC I)
• 14:15
Interaction of dot-in-rod CdSe@CdS nanostructures with redoxactive dopamine and polydopamine 1h

Semiconductor dot-in-rod nanostructures have been majorly investigated due to their tunable optical and electronic properties. In particular, CdSe@CdS nanorods (NR) are ideal candidates for light harvesting materials used in photovoltaics or photocatalysis.
Investigations of the electron and/or hole transfer between NRs and redoxactive materials, e.g., molecular catalysts, are of special interest. Here, we investigate the interaction of photoexcited NR with the redoxactive molecule dopamine by using steady-state and time-resolved photoluminescence (PL) spectroscopy. Since the molecular structure and the redox properties of dopamine depend on pH and the presence of oxygen, we study the pH dependence of the PL quenching in acidic and basic media in absence and presence of oxygen. To disperse the NR in a wide pH range, a ligand exchange with an uncharged, hydrophilic surface ligand has to be performed first. Additionally, the effect of coating the NR with the redoxactive polymer polydopamine is presented as well.

Speaker: Ms Xhesilda Fataj (Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany)
• 14:15
Metal-organic frameworks for photocatalytic carbon dioxide reduction and hydrogen production 1h

As we venture into the post-Covid-19 era, moderating the release of greenhouse gases into the atmosphere, like carbon dioxide, which cause global warming and decreasing our dependence on traditional fossil fuels for energy are pressing environmental challenges facing our planet. As an alternative to fossil-fuel-derived energy, solar energy can be used. To harvest this energy, materials need to be designed for large-scale energy utilization at an affordable price. One such class of materials that can be used for this application is metal-organic frameworks (MOFs). MOFs can be used as light-harvesting antennae and catalytic centers for solar energy conversion. This is in part due to the molecular, building-block approach used to synthesize these materials that provide them with chemical tunability and a hierarchical organization. This results in catalytic properties that can also be used in photocatalytic processes to synthesize fine chemicals. In this presentation, energy transfer principles in processes photocatalyzed by MOFs are discussed. Specific applications related to photocatalytic CO2 reduction and hydrogen production are emphasized. A way forward on how to design these materials for these applications is provided.

Speaker: Eyas Mahmoud
• 14:15
Photocatalysis on Thin-Films: Integration of Molecular Clusters onto Oxide Supports 1h

With the increasing energy demand, the replacement of unsustainable fossil fuels with renewable sources of energy is desperately needed. The ample supply of water and sunlight on earth makes solar fuels, such as hydrogen generated via the photocatalytic water splitting reaction, one of the most promising solutions.
Heterogeneous photocatalysis, in which a solid-state semiconducting catalyst is suspended in a solvent, can generate hydrogen from water upon irradiation with suitable light [1,2]. However, despite the simplicity and scalability of this original approach, such particulate systems suffer from several drawbacks: (a) The photocatalyst particles tend to agglomerate over the course of the photocatalytic reaction, which leads to a decrease of the active surface area and increases the charge carrier diffusion length – both resulting in a diminishing performance. (b) Such suspended, particulate systems are hard to characterize down to the nanoscale and atomic resolution, especially when considering the dynamic nature of the active sites that often require in situ and operando studies.
Implementation of the thin-film form of a photocatalyst can overcome these challenges. Not only does it provide us with advantages of enhanced stability (individual particles are held in place) and straightforward catalyst recovery and reuse, it also opens up the possibility to better understand the photocatalytic system by means of advanced electrochemical and (in situ) spectroscopic tools.
In this work, we apply this concept to investigate a set of promising hybrid photocatalysts comprising solid-state light-absorbing supports (exampling TiO2 and BiVO4) and a molecular co-catalyst (exampling MoSx clusters). We design a protocol for the assembly of thin-film photocatalysts based on spin-coating and atomic layer deposition (to fabricate the support films) and drop-casting/dip-coating (to immobilize the clusters). We show the feasibility of such photocatalysts towards photocatalytic hydrogen evolution reaction (HER) and provide insights into the catalyst stability (e.g. leaching through TXRF) and active state of the [Mo3S13] cluster (e.g. its transformation through XPS). Furthermore, by applying several photo- and electro-chemical tools, we investigate the reaction mechanism to understand the synergistic effects between the catalyst and the support.

1. T. Kawai, T. Sakata, Chemical Physics Letters, 1980, 87-89
2. K. Domen, S. Naito, M. Soma, T. Onishi, and K. Tamaru, J. Chem. Soc., Chem. Commun., 1980, 543-544
Speaker: Stephen Nagaraju Myakala (TU WIEN)
• 14:15
Photophysical Properties of Heteroleptic Cu(I) Photosensitizers bearing 5,6- and 4,7-disubstituted Phenanthrolines 1h

Abstract attached below

Speaker: Florian Döttinger (Department of Energyconversion, Institute of Physical and Theoretical Chemistry, Technische Universität Braunschweig)
• 14:15
Re Polypyridyl Complexes for Photoinduced PCET Reactivity utilizing the Disulfide/Dithiol Switch 1h

Since the sun is the primary source of energy on Earth, the use of solar energy for the production of sustainable fuels is of pivotal importance for our society. Recently, light-driven proton-coupled electron-transfer (PCET) processes received great attention because they may provide approaches for new solar-to-fuel transformations.
This study targets a rational design of systems that enable excited-state proton-coupled multi-electron transfer reactions (ES-PCET). Herein we present a series of Rhenium photosensitizers decorated with a sulfurated bipyridine ligand which exhibits promising features as the disulfide/dithiol unit can be considered as a 2H/2e– switch.
Electrochemical measurements, as well as (transient-) absorption and emission spectroscopy in combination with DFT calculations for assignment of optical transitions provide detailed understanding of the ground state and photophysical properties of the complex, which in turn serve to rationally design a system capable of achieving a multi-PCET driven by light.

Speaker: Manuel Oelschlegel
• 14:15
Spectroscopic characterization of the interaction between CdSe quantum dots and FeFe-H2ase mimics 1h

## Spectroscopic characterization of the interaction between CdSe quantum dots and FeFe-H2ase mimics

Alexander Schleusener1,2, Mathias Micheel1, Stefan Benndorfer2, Wolfgang Weigand2, Maria Wächtler1,2

1 Leibniz Institute of Photonic Technology, Jena
2 Friedrich-Schiller University, Jena

FeFe-hydrogenase (FeFe-H2ase) enzymes have been thoroughly investigated in combination with semi-conducting nanostructures for the photocatalytic reduction of protons to hydrogen. Despite the promis-ing activity of these semiconductor-enzyme hybrids, the electron transfer process to the enzyme pro-ceeds on the same time scale as the intrinsic recombination processes of the semiconductor material. The origin of the slow electron transfer process can be attributed to the complex protein structure of the enzyme and the accompanying large distance to the surface of the semiconductor [1, 2]. To overcome this efficiency-limiting step, small and compact mimics of the active site of the enzymes, which enable the proximity to the semiconducting surface, were investigated in combination with CdSe quantum dots (QD). The interaction was evaluated based on emission quenching experiments as well as time-resolved techniques like time-resolved emission and fs-transient-absorption spectroscopy. The results for a FeFe-H2ase mimic without a coupling group, show that the contact is sufficiently close that the electron trans-fer process from the photoexcited QD can proceed on a sub-10 ps timescale as well as hot-electron transfer on a sub-200 fs timescale [3]. Furthermore, by introducing potential coupling groups like –COOH or –NH2 the affinity to the QD surface could be improved which results in more stable semiconductor-molecular hybrids. Thin films of CdSe QD with the initial ligand system (TOPO and phosphonic acids) were functionalized with the mimics and showed in the first catalytic test an up to six-fold increased amount of Hydrogen compared to the non-functionalized films. The results pave the way for a general design principle to stable semiconductor-molecular hybrids.

[1] K. A. Brown, M. B. Wilker, M. Boehm, G. Dukovic, P. W. King, J. Am. Chem. Soc. 2012, 134, 12, 5627–5636, DOI 10.1021/ja2116348
[2] M. B. Wilker, J. K. Utterback, S. Greene, K. A. Brown, D. W. Mulder, P. W. King, G. Dukovic, J. Phys. Chem. C 2018, 122, 1, 741–750, DOI 10.1021/acs.jpcc.7b07229
[3] A. Schleusener, M. Micheel, S. Benndorf, M. Rettenmayr, W. Weigand, M. Wächtler, J.Phys.Chem.Lett.2021, 12, 4385−4391, DOI 10.1021/acs.jpclett.1c01028

Speaker: Alexander Schleusener (Istituto Italiano di Tecnologia)
• 14:15
Time-Resolved Spectroscopy of new Fe-Based Photosensitizers 1h

During the past 10 years, progress has been made in exploring first-row transition metals such as iron as replacements for scarce metals in many solar cell and photocatalysis applications.[1] Prior to our efforts, it was found by means of ultrafast spectroscopy that the relevant excited states of early iron-based light-harvesting complexes typically deactivate in less than a ps, a timescale not suitable for most electron-transfer reactions.[2] In the work described here, we use a class of complexes called iron-carbenes where careful ligand design has allowed access to a much wider range of timescales where excited state lifetimes are now reaching up to 2 ns.[3,4] Fe-carbenes was for the first time successfully used in photocatalysis in 2017[5], and since then a bi-metallic dyad connecting an iron photosensitizer with a cobalt catalytic centre has been synthesized.[6] Also, the record efficiency for iron-based dye-sensitized solar cells is held by an Fe(II)-carbene complex.[7]

The latest advance within this field, a set of iron-carbene push-pull photosensitizers, will be presented here.[8] The new complexes employ a linear design strategy, where an electron withdrawing anchor group and an electron donating group are placed on one each ligand. The first step to utilize the energy harvested by the photosensitizer is for the excited electron to be transferred to a second unit, may it be a catalytic centre or towards an electrode for power extraction. The push-pull design introduced has resulted in a directional charge transfer excitation, where the excited electron is guided towards the anchor site of the complex. The electron donating group introduced has also broadened the absorption spectra of the complexes, in order to harvest more of the sun light.

To investigate the fundamental steps of electron transfer, photosensitizers bound to titania nanoparticles have been characterized by ultrafast transient absorption spectroscopy. The electron injection from the excited complex to the nanoparticles has been proven to be ultrafast, competing with ISC on the sub-ps timescale. Also, a recombination process with a lifetime of ~100 fs has been identified, eventually resulting in the return of ~90% of the initially excited molecules to ground state. Such ultrafast recombination has not before been resolved for these kinds of systems, and thus the study presented has shed light over what is the major bottleneck limiting the performance. The new results open up a viable route for improvement in order to use these systems for applications such as solar cells and heterogeneous catalysis.

[1] O.S. Wenger, Photoactive Complexes with Earth-Abundant Metals, J. Am. Chem. Soc. 140 (2018) 13522–13533. https://doi.org/10.1021/jacs.8b08822.
[2] J. E. Monat, J. K. McCusker, Femtosecond Excited-State Dynamics of an Iron(II) Polypyridyl Solar Cell Sensitizer Model, J. Am. Chem. Soc. 122 (2000) 4092–4097. https://doi.org/10.1021/ja992436o
[3] K.S. Kjær et al., Luminescence and Reactivity of a Charge-Transfer Excited Iron Complex with Nanosecond Lifetime, Science 363 (2019) 249–253. https://doi.org/10.1126/science.aau7160.
[4] L. Lindh et al., Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis, Catalysts 10 (2020) 315. https://doi.org/10.3390/catal10030315
[5] P. Zimmer et al., N-Heterocyclic Carbene Complexes of Iron as Photosensitizers for Light-Induced Water Reduction, European Journal of Inorganic Chemistry 11 (2017) 1504-1509 https://doi.org/10.1002/ejic.201700064
[6] P. Zimmer et al., Towards Noble-Metal-Free Dyads: Ground and Excited State Tuning by a Cobalt Dimethylglyoxime Motif Connected to an Iron N-Heterocyclic Carbene Photosensitizer, European Journal of Inorganic Chemistry 48 (2018) 5203-5214 https://doi.org/10.1002/ejic.201800946
[7] A. Reddy-Marri et al., Record Power Conversion Efficiencies for Iron (II)-NHC-Sensitized DSSCs from Rational Molecular Engineering and Electrolyte Optimization, Journal of Materials Chemistry A 9 (2021) 3540–3554. https://doi.org/10.1039/D0TA10841C.
[8] L. Lindh et al., Dye-Sensitized Solar Cells based on Fe N-heterocyclic Carbene Photosensitizers with Improved Rod-like Push-Pull Functionality, submitted (2021)

Speaker: Linnea Lindh (Lund University)
• 15:30 16:00
Energy upconversion using semiconductor nanocrystal-organic hybrid materials 30m
Speaker: Prof. Troy van Voorhis (Massachusetts Institute of Technology)
• 16:05 16:30
Theory I
• 16:05
Unprecedented Water-Assisted Chemical Route Towards the Oxygen Evolution Reaction at the Hydrated (110) Ruthenium Oxide Surface: heterogeneous catalysis via DFT-MD & metadynamics simulations 20m

The biggest (or one of the biggest) issue that the world human population faces in this century is the urgent need for clean and renewable technologies. Herein, spin-polarized Density Functional Theory Molecular Dynamics (DFT-MD) simulations, coupled with advanced enhanced sampling methods in the well-tempered metadynamics framework, are applied to gain a global understanding of RuO2 aqueous interface in catalyzing the Oxygen Evolution Reaction (OER), and hence possibly help in the design of novel catalysts in the context of photochemical water oxidation. Notwithstanding RuO2 is one of the most active catalysts toward OER, a plethora of fundamental details on its catalytic properties are still elusive, severely limiting its large-scale deployment.
An atomistic understanding of structural, electronic and mechanical properties of bulk rutile RuO2 and
explicit solvent effects on (110)-RuO2 facet are provided in the context of (photo)electrochemical conditions.
We focus on the comprehension of the mechanistic interplay between surface wettability, interfacial water dynamics and surface chemical activity. Moreover, we provide a dependence of physical and chemical properties, such as surface electric field and work function, from different degrees of surface wettability of the (110)-RuO2 facet.
Furthermore, the present study quantitatively assesses the kinetics, thermodynamics and the involved energies behind the OER at the (110)-RuO2 catalyst surface revealing plausible pathways composing the reaction network of the O2 evolution in both gas-phase and explicit solvent model. Albeit a unique efficient pathway has been identified in the gas-phase OER, a lowest-energy-
requiring reaction route is possible when (110)-RuO2 is in contact with liquid water.
By estimating the free-energy surfaces associated to these processes, we reveal an unprecedented water-assisted OER
mechanism which involves a crucial proton-transfer-step assisted by the local water environment making the OER a spontaneous process, i.e. no overpotential is required.
These findings pave the way toward the systematic usage of those techniques for the fine assessment of the activity of catalysts, fully including the entropic contributions due to finite-temperature and explicit-solvent effects.
The proposed study will be embedded into the “Solar light to chemical energy conversion” priority research program at the University of Zurich, LightChECconsortium (www.lightchec.uzh.ch).

Speaker: Fabrizio Creazzo (University of Zurich)
• 16:30 16:40
Coffee Break 10m
• 16:40 17:30
Theory I
• 16:40
Charge Separation in Oxygenic Photosynthesis: The Role of Protein Electrostatics 20m

Photosystem II is a multi-subunit pigment-protein complex that utilizes sunlight to trigger charge-separation and catalyse water oxidation. The charge separation cascade is initiated in the reaction centre (RC), which is composed of six pigments (four Chlorophyll a and two Pheophytin a) arranged symmetrically along the D1 and D2 core poly-peptides. Biological evolution favoured productive electron transfer only along the D1 side with the precise nature of the initial excitation event(s) remaining under debate. In this work, (1-2) we employ multiscale quantum mechanics/molecular mechanics (QM/MM) coupled with high-level computations (full time-dependent density functional theory with range-separated functionals benchmarked against coupled-cluster theory) to investigate the excited state profile of the RC. Our results describe for the first time at a fundamental electronic structure level precisely how differential protein electrostatics create the observed excitation asymmetry within the RC. By simultaneous quantum chemical treatment of multimeric pigment assemblies, we identify the critical pairs of RC pigments associated with low-lying charge-transfer states and we eventually propose a novel model to describe excitation of Photosystem II reaction centre based on two parallel charge-separation pathways. Among others, our new model explains the triggering of charge separation by direct absorption of far-red photons (700-800 nm), i.e. beyond the known “red-limit” (680 nm) of oxygenic photosynthesis.

References
1) A. Sirohiwal, F. Neese, D. A. Pantazis, J. Am. Chem. Soc., 2020, 142, 42, 18174-18190.
2) A. Sirohiwal, F. Neese, D. A. Pantazis, J. Chem. Theory Comput., 2021, 17, 3, 1858-1873.

Speaker: Abhishek Sirohiwal (Max-Planck-Institut für Kohlenforschung)
• 17:05
Förster resonance energy transfer between chromophores embedded into lipid membranes 20m

Through photosynthesis, organisms transform light energy into chemical energy to be used as fuel in metabolic processes. These processes are comprised of highly directional energy and electron transfer mechanisms, which are achieved in complex biological environments such as phospholipid membranes.[1]
To explore the possibilities of controlling energy transfer processes of chromophores embedded into membranes, we investigated experimentally and through computational simulations the Förster Resonance Energy Transfer (FRET) in an artificial bio-inspired system. Such system comprises an N-substituted perylene diimide (C4-PDI-C4 in Figure 1) as donor and an alkylated ruthenium tris-bipyridyl derivative (Ru(bpy)2bpyC9 in Figure 1) as acceptor embedded into lipid bilayer membranes. Time dependent evolution of the chromophores' assembly was explored by means of classical molecular dynamic simulations in combination with umbrella sampling methods. Furthermore, using density functional theory calculations, we computed optical properties such as absorption and emission spectra as well as the magnitude and orientation of the transition dipole moments with special attention to the different conformers to include conformational effects.
The membrane confines both position and relative arrangement of the chromophores. Since the FRET is highly dependent on distance and directionality of the transition dipole moments,[2] the embedding in the membrane not only controls the rate of energy transfer, but also the participating electronic states. Distinct arrangements of the perylene diimide donor lead to a population of specific excited states of the ruthenium-based acceptor due to the alignment of the transition dipole moments. Thus, our study shows how the control of the insertion modes into the membrane opens the possibility for direct tuning of the FRET efficiency.
[1] Pannwitz, A. et al.: “Roadmap towards solar fuel synthesis at the water interface of liposome membranes”, Chem. Soc. Rev. 2021, 50 (8), 4833-4855
[2] Anzola, M. et al.: “Understanding Förster Energy Transfer through the Lens of Molecular Dynamics”, J. Chem. Theory Comput. 2020, 16, 7281-7288

Speaker: Mr Richard Jacobi (Institute of Theoretical Chemistry, University of Vienna)
• 17:30 18:30
"Scientific Speed Dating" 1h

Get to know the other participants.
Present your research and your interests in 2 minutes. To multiple groups of 5 fellow young researchers.

• Tuesday, 28 September
• 09:30 10:00
Light-driven nitrogen fixation 30m
Speaker: Prof. Vera Krewald (TU Darmstadt)
• 10:05 10:30
Theory II
• 10:05
Bending Air out of a Cube: Flexibility Enhances Reactivity in a Mn4O4 Water Oxidation Catalyst 20m

In nature, photosystem II catalyzes water splitting as part of photosynthesis to fuel cell growth and produce the oxygen all aerobic life depends on. Within photosystem II, the water oxidation half-reaction is catalyzed by the oxygen-evolving complex (OEC), a highly efficient Mn4CaOx cluster that has inspired attempts to develop synthetic catalysts of similar structure.1 We propose a catalytic cycle for the highly active synthetic water oxidation catalyst (WOC) [Mn4V4O17(OAc)3]3- (1),2 a model system for the OEC. Comparison among multiple pathways shows that the pre-catalyst 1 is activated by oxidation and ligand exchange;3 water oxidation then proceeds through a series of proton-coupled electron transfer steps, with a predicted thermodynamic overpotential of 0.71 V.4 In-depth investigations of the ligand exchange, O-O bond formation, and O2 evolution steps reveals the highly dynamic interplay between redox isomerism and Jahn-Teller effects in the catalytic cycle: both serve to enhance catalytic reactivity by redistributing electrons between mixed-valence metal centers and weakening key bonds through Jahn-Teller distortions, introducing flexibility to the otherwise-rigid cubane core of 1. These results are of general importance both for understanding water oxidation on molecular catalysts as well as advancing the design of Mn-containing WOCs.

References:
(1) Lubitz, W.; Reijerse, E. J.; Messinger, J. Solar Water-Splitting into H2 and O2: Design Principles of Photosystem II and Hydrogenases. Energy Environ. Sci. 2008, 1 (1), 15–31. https://doi.org/10.1039/B808792J.
(2) Schwarz, B.; Forster, J.; Goetz, M. K.; Yücel, D.; Berger, C.; Jacob, T.; Streb, C. Visible-Light-Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster. Angew. Chem. Int. Ed. 2016, 55 (21), 6329–6333. https://doi.org/10.1002/anie.201601799.
(3) Cardenas, G.; Trentin, I.; Schwiedrzik, L.; Hernández-Castillo, D.; Lowe, G. A.; Kund, J.; Kranz, C.; Klingler, S.; Stach, R.; Mizaikoff, B.; Marquetand, P.; Nogueira, J. J.; Streb, C.; González, L. Activation by Oxidation and Ligand Exchange in a Molecular Manganese Vanadium Oxide Water Oxidation Catalyst. 2021. https://doi.org/10.33774/chemrxiv-2021-7w62s.
(4) Schwiedrzik, L.; Brieskorn, V.; González, L. Flexibility Enhances Reactivity in a Bioinspired Mn4O4 Cubane Water Oxidation Catalyst: Interplay between Redox Isomerism and Jahn-Teller Effects. submitted 2021.

Speaker: Mr Ludwig Schwiedrzik (Institute of Theoretical Chemistry, University of Vienna, Vienna, Austria)
• 10:30 10:40
Coffee Break 10m
• 10:40 11:30
Theory II
• 10:40
Redox Isomerism in the S3 State of the Oxygen-Evolving Complex Resolved by Coupled Cluster Theory 20m

Abstract
The active site of the oxygen-evolving complex of photosystem II is an oxo-bridged Mn4CaO5 cluster that stores four oxidizing equivalents required to oxidize water into dioxygen by cycling through five oxidation states known as the S0-S4 states.[1] Recent structural and spectroscopic investigations for the final metastable S3 state support contradictory redox formulations ranging from the widely accepted Mn(IV)4 oxo-hydroxo model to a Mn(III)2Mn(IV)2 peroxo model.[2–4] Density functional theory energetics of suggested S3 redox isomers are inconclusive because the different redox isomers have different numbers of total unpaired electrons. Retrieval of the full correlation energy is critical for computing accurate relative energies.
The domain-based local pair natural orbital approach to coupled cluster theory, DLPNO-CCSD(T),[5] offers a highly efficient way to extend the applicability of the “gold standard” coupled cluster theory to large systems. In this work, we leverage the ability of DLPNO-CCSD(T) to provide reference values for spin state energetics in order to achieve a reliable estimation of the energy difference between S3 Mn(IV)4 oxo-hydroxo and the Mn(III)2Mn(IV)2 peroxo structures.[6] The results enable us to provide a realistic and reliable energy profile of these redox isomers and to assess their relevance for the interpretation of experimental observations and for the mechanism of water oxidation.

References
[1] D. A. Pantazis, in Solar‐to‐Chemical Conversion (Ed.: H. Sun), Wiley, 2021, pp. 41–76.
[2] M. Suga, F. Akita, M. Sugahara, M. Kubo, Y. Nakajima, T. Nakane, K. Yamashita, Y. Umena, M. Nakabayashi, T. Yamane, T. Nakano, M. Suzuki, T. Masuda, S. Inoue, T. Kimura, T. Nomura, S. Yonekura, L.-J. Yu, T. Sakamoto, T. Motomura, J.-H. Chen, Y. Kato, T. Noguchi, K. Tono, Y. Joti, T. Kameshima, T. Hatsui, E. Nango, R. Tanaka, H. Naitow, Y. Matsuura, A. Yamashita, M. Yamamoto, O. Nureki, M. Yabashi, T. Ishikawa, S. Iwata, J.-R. Shen, Nature 2017, 543, 131–135.
[3] H. Isobe, M. Shoji, T. Suzuki, J.-R. Shen, K. Yamaguchi, Journal of Photochemistry and Photobiology A: Chemistry 2021, 405, 112905.
[4] D. A. Marchiori, R. J. Debus, R. D. Britt, Biochemistry 2020, 59, 4864–4872.
[5] C. Riplinger, F. Neese, J. Chem. Phys. 2013, 138, 034106.
[6] M. Drosou, D. A. Pantazis, Chem. Eur. J. 2021, 27, 1-12.

Speaker: Maria Drosou (National and Kapodistrian University of Athens)
• 11:05
Temperature dependent emission lifetime of Ru(II) complexes with bi-pyridine ligands 20m

A fundamental challenge for the development of sustainable energy technologies is the efficient harvesting of solar energy to drive chemical processes. The design of light-driven artificial catalytic systems usually implies the use of a photosensitizer and a photo-redox catalyst. In this sense, an efficient photosensitizer should possess long-lived emission lifetimes but this feature is severely hindered when working close to room temperature due to the fast non-radiative decay to the ground state. In this work, we elucidate from first principle simulations the origins of the temperature-dependent emission lifetimes of three Ru-based photosensitizers (Figure 1), where one of them was shown experimentally to possess a temperature-independent emission (R= tBu).
Inspired by recently published protocols,[1,2] for the three ruthenium photosensitizers we have computed radiative, temperature-independent non-radiative and temperature-dependent non-radiative rate constants. Our methodology provides a quantitative agreement with experimental emission lifetimes over a wide range of temperatures, opening a tractable route to the future computational design of novel ruthenium photosensitizers. In addition, we found that a balance in the reaction barriers of the $^3MLCT$→$^3MC$→$S_0$ non-radiative pathway would account for the temperature-independent emission lifetime of the R = tBu derivative. Despite that electronic energies have been often used to study these decay pathways on iridium and ruthenium complexes,[1,2,3] we found that Gibbs free energy reaction barriers are essential for a qualitative and quantitative agreement with the experiments. Our results also advise caution when interpreting kinetic parameters obtained by fits of the experimental data.

Acknowledgments: This work is funded by the Deutsche Forschungsgemeinschaft and the Austria Science Fund through the Catalight Transregio (project I 3987).

References:
[1] D. Escudero, Chem. Sci. 2016, 7, 1262–1267.
[2] X. Zhang, D. Jacquemin, Q. Peng, Z. Shuai, D. Escudero, J. Phys. Chem. C 2018, 122, 6340–6347.
[3] A. Soupart, I. M. Dixon, F. Alary, J. L. Heully, Theor. Chem. Acc. 2018, 137, 1–11.

Speaker: Mr David Hernández Castillo (Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna)
• 16:00 16:30
Towards Solar Factories 30m

The solar-driven photocatalytic splitting of water into hydrogen and oxygen is a potential source of clean and renewable fuel. However, four decades of global research have proven this multi-step reaction to be highly challenging. Here, I will present our strategies, and most recent results, in taking photocatalyst production to new and unexplored frontiers, while exploring solar to chemical conversion that goes beyond water splitting. I will focus on unique design of innovative nano scale particles, which harness nano phenomena for improved activity, and methodologies for the construction of sophisticated heterostructures. I will share our design rules and accumulated insights, which enabled us to demonstrate efficient, and stable, full-cycle endothermic redox transformations, realizing a genuine solar-to-fuel energy conversion, with state of the art efficiencies of up to 4.2%.

Speaker: Prof. Lilac Amirav (Schulich Faculty of Chemisty, Technion - Israel Institut of Technology)
• 16:35 17:25
Molecules & Materials III
• 16:35
A 'Defective' Conjugated Porous Poly-Azo as Multifunctional Photocatalyst 20m

Organic conjugated porous polymers (CPPs) present themselves as a class of highly enabling and sustainable photocatalysts. They are routinely used as metal-free heterogeneous amorphous semiconductors for various organic transformations. To date, most research have focused on the synthesis of various CPP chemical structures to tune their bandgap and valance and conduction band (VB-CB) potentials. However, a more fundamental physicochemical assessment of the materials to establish concise structure-property relations is still lacking.
Herein we report the synthesis and characterization of an azobenzene-based organic π–conjugated porous polymer (AzoCPP) as heterogeneous dual photocatalyst manifesting net-oxidative bromination of arenes and dehydroxylation of boronic acids to corresponding phenols. Hierarchical porosity and high surface area of the nano-sized AzoCPP allowed superior catalyst-substrate contact during catalyses, whereas the inherent structural defect present in the CPP backbone resulted in low-energy sinks functioning as de facto catalytic sites. A combination of these two structure-property aspects of AzoCPP, in addition to the dielectric constant manipulation of the system, led to an excellent catalytic performance. The protocols remained valid for a wide substrate scope and the catalyst was recycled multiple times without substantial loss in catalytic activity. With the aid of subsequent control experiments and analytical characterizations, mechanisms for each catalysis are proposed and duly corroborated.

Speaker: Dr Jeet Chakraborty (COMOC, Department of Chemistry, Ghent University, Ghent, Belgium)
• 17:00
Surface-anchoring and modification of molecular [Mo3S13]2- cluster co-catalysts for photocatalytic water splitting 20m

The ever-increasing energy consumption by human societies leads to the unprecedented need for green and renewable fuels. Hydrogen is a suitable alternative, however still today the most cost-effective source remains from fossil fuels. Photocatalysis is a promising strategy to generate hydrogen from renewable sources, such as water and sunlight. However, the efficiencies of contemporary photocatalytic systems are not yet enough to contribute substantially to the world energy demand. One important issue that requires urgent attention is the design of earth-abundant, tunable and selective co-catalyst.
Among various candidates, transition-metal-based metal sulfides – such as those from the $MoS_x$-family – have shown excellent co-catalytic properties due to the presence of suitable active sites for electrochemical $H_2$ production [1]. More recently, thiomolybdates – such as $[Mo_3S_{13}]^{2-}$ molecular clusters deposited on carbonaceous supports – have demonstrated superior electrochemical hydrogen generation efficiency associated with the presence of abundant and exposed sulfur ligands [2]. Compared to other co-catalysts reported elsewhere, such clusters feature well-defined molecular structures, compositions, and geometries, which may allow for in-depth studies and understanding of the active sites, reaction mechanisms, and dynamic nature of the catalytic processes [3].
Motivated by these factors, here we construct and investigate a set of promising earth-abundant photocatalysts comprised of various photoactive oxide supports (anatase and rutile $TiO_2$, $BiVO_4$, $V_2O_5$, $WO_3$) and $[Mo_3S_{13}]^{2-}$ as a model HER co-catalyst. We show that the clusters undergo strong and irreversible binding to the oxide surface upon impregnation and that this surface-anchoring follows a monolayer adsorption model. We further suggest certain structural rearrangement of the original cluster upon binding. Next, we demonstrate a volcano-like photocatalytic performance towards hydrogen evolution reaction (HER) as a function of the co-catalyst loading. We discuss the underlying contributions to this activity trend. Finally, we investigate the impact of the cluster structure and integrity on photocatalytic performance by subjecting it to heat treatments in air and $N_2$. Detailed assessment using XPS and other spectroscopic techniques suggests the role of the S ligands on the HER performance. These investigations provide a better understanding of catalyst-support interactions and the effect of thermal treatment on the active sites of the promising thiomolybdate co-catalysts.
References
1. Xie, J., Zhang, H., Li, S., Wang, R., Sun, X., Zhou, M., Zhou, J., Lou, X.W.(. and Xie, Y. Adv. Mater., (2013), 25, 5807-5813. https://doi.org/10.1002/adma.201302685
2. Kibsgaard, J., Jaramillo, T. & Besenbacher, F. Nature Chem., (2014), 6, 248–253. https://doi.org/10.1038/nchem.1853
3. M. Dave, A. Rajagopal, M. Damm-Ruttensperger, B. Schwarz, F. Nagele, L. Daccache, D. Fantauzzi, T. Jacob and C. Streb. Sustainable Energy Fuels, (2018),2, 1020-1026. https://doi.org/10.1039/C7SE00599G

Speaker: Samar Batool (TU Wien, Austria)
• 17:25 17:40
Coffee Break 15m
• 17:40 18:30
Molecules & Materials III
• 17:40
Sensitization-initiated electron transfer via upconversion: mechanism and photocatalytic applications 20m

The field of photoredox catalysis has established new methodologies for synthetic applications within the last decade. While mono-photonic mechanisms are very well established, the intrinsic limitation by the energy of one (one) photon for challenging photocatalytic applications become more and more apparent. A strategy to tackle this problem is the use of more than one photon per catalytic turnover.[1] Especially for systems with several (photo-)active catalysts, mechanistic insights are challenging to achieve and a good (mechanistic) understanding is often difficult to obtain.
As an example, sensitization-initiated electron transfer (SenI-ET) has recently been established as strategy for photoredox catalysis.[2-3] For a system consisting of [Ru(bpy)₃] (Cl)₂ and pyrene with DiPEA as sacrificial reagent the original mechanistic proposal lead to a controversial debate about the actual pathway for the formation of a pyrenyl radical anion as catalytically active species for substrate activation.[4-5] Spectroscopic investigations indicate a two-photonic mechanism, but a direct spectroscopic proof for the actual formation of the pyrenyl radical anion has not been found with this system.[6]
We demonstrate in our study that fac-[Ir(ppy)₃] and 2,7-di-tert-butylpyrene with N,N-dimethylaniline as sacrificial electron donor can catalyse photochemical reactions through a sensitization-initiated electron transfer pathway via triplet-triplet annihilation upconversion followed by reductive quenching of the formed singlet state.[7] This combination of sensitizer, annihilator and sacrificial reagent allows the direct spectroscopic observation of the pyrenyl radical anion responsible for substrate activation as well as all other catalytically active species of the proposed catalytic cycle. The detailed spectroscopic analysis is complemented with selected photocatalytic applications.

[1] F. Glaser, C. Kerzig, O. S. Wenger, Angew. Chem., Int. Ed. 2020, 59, 10266–10284, doi: 10.1002/anie.201915762.
[2] C. Kerzig, M. Goez, Chem. Sci. 2016, 7, 3862–3868, 10.1039/c5sc04800a.
[3] I. Ghosh, R. S. Shaikh, B. König, Angew. Chem., Int. Ed. 2017, 56, 8544–8549, 10.1002/anie.201703004.
[4] M. Marchini, G. Bergamini, P. G. Cozzi, P. Ceroni, V. Balzani, Angew. Chem., Int. Ed. 2017, 56, 12820–12821, 10.1002/anie.201706217.
[5] I. Ghosh, J. I. Bardagi, B. König, Angew. Chem., Int. Ed. 2017, 56, 12822–12824, 10.1002/anie.201707594.
[6] M. S. Coles, G. Quach, J. E. Beves, E. G. Moore, Angew. Chem., Int. Ed. 2020, 59, 9522–9526, 10.1002/anie.201916359.
[7] F. Glaser, C. Kerzig, O. S. Wenger, Chem. Sci. 2021, 10.1039/D1SC02085D.

Speaker: Felix Glaser (University of Basel)
• 18:05
How a fused naphthalimide unit enables advanced Cu(I) photosensitizers 20m

The design of novel and efficient photosensitizers that are based on earth-abundant metals are at the heart of our current research.[1-3] In this respect, we are especially focused on heteroleptic Cu(I) photosensitizers (CuPS) of the type [(P^P)Cu(N^N)]+ bearing one diphosphine (P^P) and one diimine ligand (N^N). It is important that these photosensitizers fulfil some basic requirements, i.e. a strong absorption, reversible redox processes, a high (photo)stability and a long-lived excited state.[1-3]
For this purpose, we developed a new kind of phenanthroline-based ligand with an extended π-system in the backbone, where a naphthalimide unit is directly fused to the phenanthroline core (biipo and dmbiipo, see figure). The resulting Cu(I) and Ru(II) complexes were extensively studied regarding their photophysical, electrochemical and photocatalytic properties.[4,5]
For the Cu(I) complex it was found that the MLCT excitation relaxes into a ligand-centered dark state, with a remarkably long lifetime of several microseconds in an acetonitrile solution and several hundred microseconds in the solid state.[5]
For an MLCT charge-separated excited state, the Cu(I) center would be formally oxidized to Cu(II) which prefers a different coordination environment. The so-called exciplex quenching mechanism is a major deactivation pathway for such heteroleptic Cu(I) complexes.[1] However, computational studies and time-resolved spectroscopic measurements (i.e. step scan FTIR) showed that the flattening in the excited state does not occur for these Cubiipo complexes. This lack of flattening is a strong indication for a purely ligand centered excited triplet state with a copper center in a d10 configuration, which may provide an enhanced catalytic activity due to the suppression of the exciplex quenching mechanism.[5]
Therefore, we tested this Cu(I) complex in the photocatalytic isomerization of E-stilbene. It was discovered that Cudmbiipo is much more active than the related Cu(I) complex without such a naphthalimide unit or typical reference complexes like [Ru(bpy)3]2+. This illustrates, that it is not necessary for a complex to have an emissive excited state to be able to efficiently convert a substrate molecule. Instead, the excited state lifetime and the S0-T1 energy difference are crucial factors for the photocatalytic activity.[5]
[1]: Y. Zhang, M. Schulz, M. Wächtler, M. Karnahl, B. Dietzek, Coord. Chem. Rev., 2018, 356, 127-146, DOI: 10.1016/j.ccr.2017.10.016.
[2]: M. Heberle, S. Tschierlei, N. Rockstroh, M. Ringenberg, W. Frey, H. Junge, M. Beller, S. Lochbrunner, M. Karnahl, Chem. Eur. J., 2017, 23, 312-319, DOI: 10.1002/chem.201604005.
[3]: M. Rentschler, M. A. Schmid, W. Frey, S. Tschierlei, M. Karnahl, Inorg. Chem., 2020, 59, 14762−14771, DOI: 10.1021/acs.inorgchem.9b03687.
[4]: Y. Yang, J. Brückmann, W. Frey, S. Rau, M. Karnahl, S. Tschierlei, Chem. Eur. J., 2020, 26, 17027-17034, DOI: 10.1002/chem.202001564.
[5]: M. A. Argüello Cordero, P. Boden, M. Rentschler, P. Di Martino Fumo, W. Frey, Y. Yang, M. Gerhards, M. Karnahl, S. Lochbrunner and S. Tschierlei, 2021, submitted.

Speaker: Mr Martin Rentschler (Department of Energy Conversion, Institute of Physical and Theoretical Chemistry, Technische Universität Braunschweig)
• 18:45 19:45
Poster Session
• 18:45
[FeFe] Hydrogenase based prototype Photosensitizer-Catalyst dyad for hydrogen generation under visible light 1h

Inspired by the active center of the natural [FeFe] hydrogenase,[1] we have designed a compact and noble metal-free photosensitizer (PS) catalyst (CAT) dyad for photocatalytic hydrogen evolution under visible light. It represents a prototype dyad for utilizing π-conjugated oligothiophenes as effective and inexpensive light absorbers and overcomes excitability in the UV range, reported for a similar complex.[2] The dyad and its interaction with the sacrificial donor 1,3-dimethyl-2-phenylbenzimidazoline (BIH) were studied by steady-state and time-resolved spectroscopy coupled with electrochemical techniques and visible light-promoted photocatalytic investigations. For the first time, EPR spectroscopy during photocatalysis revealed the formation of an active [FeIFe0] species, which presumably drives photocatalysis effectively (TON ≈ 300) on a long-term basis.

Speaker: Mr Philipp Buday (Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena)
• 18:45
A Base Metal Iron(II)-Cobalt(III) Dyad for Photocatalytic Hydrogen Evolution 1h

Photocatalytic hydrogen production provides a sustainable method to generate hydrogen as future energy carrier. Currently, noble metal-based compounds are widely used in two-component water reduction systems where the electron transfer between a photosensitive and a catalytically active complex is a diffusion driven process (Fig.a).
Combining photosensitizer and water reduction catalyst in one single molecule leads to bimetallic dyads with higher catalytic activity and/or robustness.[1,2] Typically used metals in dyads are ruthenium, iridium and platinum. To enhance the sustainability of these systems first-row transition metals such as iron and cobalt are suited alternatives regarding their higher availability and lower toxicity.[3]
The photocatalytic process in dyads involves several steps. First, the photosensitive site is excited by a light source. Second, the excited electron is transferred via a bridging ligand to the catalytically active site, where protons are reduced to molecular hydrogen (Fig.b). Additionally, intermolecular transfer of excited electrons can occur. Third, the photosensitizer is regenerated by a sacrificial reductant (not shown). In this work an iron (II) N-heterocyclic carbene photosensitizer was combined with a cobalt (III) dimethylglyoxime fragment. We investigated the system’s stability with NMR and performed ultrafast pump-probe X-ray experiments in combination with transient optical absorption spectroscopy to characterize ultrafast charge transfer dynamics. Finally, the iron-cobalt dyad (Fig.c) was successfully tested in hydrogen evolution reaction.[4]
[1] Tschierlei S. et al., Chem. Eur. J. 2009, 15, 7678-7688, DOI:10.1002/chem.200900457
[2] Jacques A. et al., Eur. J. Inorg. Chem. 2016, 1779-1783, DOI:10.1002/ejic.201501467
[3] Zimmer P. et al., Eur. J. Inorg. Chem. 2018, 5203-5214. DOI: 10.1002/ejic.201800946
[4] Huber-Gedert M. et al., Chem. Eur. J. 2021, 27, 9905-9918., DOI: 10.1002/chem.202100766

Speaker: Mrs Marina Huber-Gedert (Universität Paderborn)
• 18:45
Design and characterization of a jet loop reactor to improve the mass transfer effect on photocatalytic water splitting process 1h

To meet the high energy demand and reduce the greenhouse gas emission from fossil fuels, photocatalytic water splitting for hydrogen production using solar energy has been increasingly drawing attention to researchers. Although photocatalytic water splitting research has been around for a while since 1972, [1] most of the focus has been put on developing efficient photocatalysts for water splitting. However, reaction engineering concerns must be studied in order to put photocatalytic water splitting process in practical use.
Mass transfer effects on photocatalytic systems can be the most overlooked problem in the photocatalytic field. [2] When the mass transfer rates of the products from the photocatalyst active sites are slower than the water reduction/oxidation rates, the overall efficiency of the whole reaction will drop. We have found similar effects recently in our research on the photocatalytic reduction of nitrobenzene. [3] Therefore, it is necessary to study mass transfer effects on photocatalytic water splitting process to efficiently use this process for hydrogen production.
Jet loop reactor offers excellent mixing performance, making it especially interesting for the application in mass transfer limited reaction systems. [4] In this work, a jet loop reactor is designed to study the mass transfer effect on photocatalytic water splitting. Radiometry and actinometry are used to characterize the irradiation field in the jet loop reactor. An overhead stirrer with 3D printed propellers is used to improve the mixing in the jet loop reactor. Photocatalytic water splitting reaction will be performed in the jet loop photoreactor using polymeric carbon nitride (CNx)-based molecular photocatalyst. Parameters such as flow rate and light irradiation rate will be studied for investigating mass transfer effects on photocatalytic water splitting process.

References
[1] Z. Xing, X. Zong, J. Pan and L. Wang, Chem. Eng. Sci., 2013, 104, 125–146, DOI: 10.1016/j.ces.2013.08.039.
[2] B. Ipek and D. Uner, Water Chem., 2020, DOI:10.5772/intechopen.89235.
[3] F. Guba, F. Gaulhofer and D. Ziegenbalg, J. Flow Chem., 2021, DOI:10.1007/s41981-021-00174-1.
[4] N. Fajrina and M. Tahir, Int. J. Hydrogen Energy, 2019, 44, 540–577, DOI: 10.1016/j.ijhydene.2018.10.200.

Speaker: Mr Pengcheng Li (Institute of Chemical Engineering, Ulm University)
• 18:45
Efficient one-photon driven catalysis by coupled energy and electron transfer 1h

Efficient one-photon driven catalysis by coupled energy and electron transfer

M.-S. Bertrams, O. S. Wenger, C. Kerzig

Maria-Sophie Bertrams, Johannes Gutenberg University of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany

Electron-transfer activation steps of substrates (S) for chemical reactions often require high redox potentials. The available redox energy for light-initiated substrate activations is not only limited by the photon itself, but also by the typically rather inefficient conversion of its energy into reduction or oxidation power.[1] The combination of electron and energy transfer mechanisms paved the way for accessing highly reactive aryl radical anions using the pooled energy of two visible photons.[2-4] In this work we present a novel one-photon driven type of the sensitization-initiated electron transfer mechanism with a water-soluble p-terphenyl (TDS) radical anion as reactive species. The radical anion of p-terphenyl has been used as photoredox catalyst in energy-demanding CO$_2$ reduction reactions, but its direct excitation requires hazardous UV-light (< 320 nm).[5] Our photoredox cycle is initiated with one visible photon absorbed by an iridium complex. The corresponding triplet-excited complex transfers its energy (TTET) to TDS and the resulting $^3$TDS is reduced by a sacrificial donor to the desired catalytically active radical (ET).
Spectroscopic investigations reveal detailed mechanistic insights and confirm the one-photon nature of our catalytic cycle, which converts almost 80 % of the initial photon energy in redox energy available for substrate activation. Finally, we successfully performed blue light driven (pinacol coupling, dehalogenation and hydrogenation) reactions in water, using a conventional 440 nm LED as energy input and the ascorbate monoanion (HAsc$^-$) as sacrificial electron donor.

Speaker: Maria-Sophie Bertrams
• 18:45
Magnetic properties of Chromium and Molybdenum molecular qubits candidates 1h

We explore pseudo tetrahedral complexes containing different metals and aryl ligands as potential molecular qubits. The energy difference between the triplet ground state and the first singlet excited state, together with the zero-field splitting (ZFS) constants D and E, are computed using multireference methods and compared with experimental values. This study allowed us to validate our computational procedure and predict the ZFS parameters of yet unsynthesized qubits. In the long term, we plan to develop novel theoretical and computational tools to explore molecular qubits.

Speaker: Arturo Sauza de la Vega (University of Chicago)
• 18:45
NiFe oxyhydroxide modified mesoporous titania thin films for artificial photosynthesis 1h

The serious effects of global warming make the transition towards renewable energy sources crucial for our society. Artificial photosynthesis arises as an interesting approach, by mimicking the way nature transforms CO2 and water into complex organic fuels, using solar energy.[1,2] One of the main limitations of the water splitting reaction in this process is the sluggish oxygen evolution reaction (OER). Ni-Fe oxyhydroxides (NiFeOx) are state-of-the-art OER electrocatalysts,[3] while semiconductors, such as TiO2, can be used to absorb solar light. Combining these two materials at the nanoscale is an interesting starting point towards the design of integrated systems for solar fuel production.
In this work, we present the synthesis of ordered thin mesoporous TiO2 films prepared via a sol-gel approach, electrochemically modified by NiFeOx. This soft chemistry synthesis allows fine tuning of the composition and crystallinity of the TiO2 skeleton,[4] which determines the photocatalytic performance. Furthermore, the deposition of NiFeOx is useful in improving the reaction kinetics. The materials were characterized by means of photoelectrochemical techniques, SEM, EDX and AFM. The amount of catalyst and its Ni/Fe ratio was optimized to reach the best OER activity.[5] Our results show that at certain potentials, there is a synergy between the photoactive species and the OER catalyst, making these systems a promising composite photoanode for artificial photosynthesis.

[1] Dogutan, D. K.; Nocera, D. G. Artificial Photosynthesis at Efficiencies Greatly Exceeding That of Natural Photosynthesis. Acc. Chem. Res. 2019, 52 (11), 3143–3148. https://doi.org/10.1021/acs.accounts.9b00380.
[2] Gust, D.; Moore, T. A.; Moore, A. L. Solar Fuels via Artificial Photosynthesis. Acc. Chem. Res. 2009, 42 (12), 1890–1898. https://doi.org/10.1021/ar900209b.
[3] Dionigi, F.; Strasser, P. NiFe-Based (Oxy)Hydroxide Catalysts for Oxygen Evolution Reaction in Non-Acidic Electrolytes. Adv. Energy Mater. 2016, 6 (23), 1600621. https://doi.org/10.1002/aenm.201600621.
[4] Violi, I. L.; Perez, M. D.; Fuertes, M. C.; Soler-Illia, G. J. A. A. Highly Ordered, Accessible and Nanocrystalline Mesoporous TiO 2 Thin Films on Transparent Conductive Substrates. ACS Appl. Mater. Interfaces 2012, 4 (8), 4320–4330. https://doi.org/10.1021/am300990p.
[5] Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel–Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136 (18), 6744–6753. https://doi.org/10.1021/ja502379c.

Speaker: Priscila Vensaus (Nanosystems Institute, University of San Martín)
• 18:45
Oxygen-deficient Zirconia (ZrO2-x) as Core in ZrO2-x@TiO2 Core-Shell composite for Cocatalyst-free Photocatalytic H2 Reforming 1h

Heterojunction formation by coupling two semiconductors with different band properties is a well-known method to fabricate a highly active photocatalysts. In addition, forming oxygen defects on the surface of metal oxide semiconductors, i.e. TiO2 and ZrO2, was reported to expand the light absorption to the visible range, which in turn increases the photocatalytic activity using sunlight [1,2]. The focus of the present study was the investigation of defect introduction in ZrO2 via different reducing agents and its effect on the photocatalytic hydrogen evolution from methanol reforming. Ultimately, the influence of reduced ZrO2 (ZrO2-x) as core in ZrO2-x@TiO2 on the photocatalytic activity shall be explored.

For the defect introduction, two reducing agents (Mg and NaBH4) were used. The reduction process was performed at 973 K for 4h in reducing atmosphere (10 vol.-% H2 in N2, 50 mL.min-1), followed by etching with 2M HCl for 24h. TiO2 shell will be grown on ZrO2-x surface by sol-gel method according to the procedure of Ferreira-Neto et al. [3]. Photocatalytic H2 reforming was performed according to Sinhamahapatra et al. [1] without cocatalyst.

Preliminary studies show that NaBH4-reduced ZrO2-x (ZrO2-x-NaBH4) exhibits photocatalytic activity for H2 reforming while pristine ZrO2 and Mg-reduced ZrO2-x (ZrO2-x-Mg) are inactive (Fig.1). X-ray diffraction patterns reveal that the reduction by Mg results in the formation of tetragonal phase, while the monoclinic phase is maintained during the reduction with NaBH4 (Fig.2). Literature suggests that the presence of the tetragonal phase strongly diminishes photocatalytic activity [2]. Thus, preserving the monoclinic phase during the reduction of ZrO2 is possibly crucial for the synthesis of an efficient ZrO2-x@TiO2 core-shell catalyst.

References
[1] A. Sinhamahapatra, J.-P. Jeon, J. Kang, B. Han, J.-S. Yu, Sci. Rep., 6, 27218, DOI: 10.1038/srep27218.
[2] S.R. Teeparthi, E.W. Awin, R. Kumar, Sci. Rep., 8, 5541, DOI: 10.1038/s41598-018-23648-0
[3] E.P.F. -Neto, S. Ullah, M.B. Simoes, A.P. Perissinotto, F.S. de Vicente, P.-L.M. Noeske, S.J.L. Ribeiro, U.P.R. -Filho, Colloids Surf. A, 570, 293-305, DOI: 10.1016/j.colsurfa.2019.03.036.

Speaker: Rian Kurniawan (Institute of Chemical Technology, Universität Leipzig)
• 18:45
Xanthine-Based α-Diimines for the Light-Driven Nickel-Catalyzed O-Arylation of Carboxylic Acid 1h

The development of light-driven reactions has gained a lot of interest in recent years.[ ]Metallaphotoredox catalysis is an example of a successful development in this area: light-induced excitation of metal complexes can facilitate redox events or energy-transfer processes and the active catalyst can be regenerated with rather simple chemicals like amines.[ ] With the exception of some copper(I) and chromium(III) catalysts, photoredox catalysis based on 3d elements is relatively underdeveloped. Nevertheless, recent publications have shown that certain nickel α-diimine complexes have been used in light-driven cross-coupling reactions in combination with some photocatalysts. This approach is also referred as Ni/photo(redox) dual catalysis.[ ] Particularly, the Ni-catalyzed arylation of carboxylates has been intensively discussed in the literature.[ ] Several mechanistic scenarios have been proposed, which involve the participation of different Nin+ species (n = 0, 1, 2, 3), with the role of the photocatalyst/photosensitizer still being debated. In this regard, we have considered the possibility to study the Ni-catalyzed arylation of carboxylates by choosing a ligand system not only intended to stabilize the nickel species, but also photoactive per se. Arylated xanthines have been known as fluorescent molecules for more than a decade,[ ] but their photophysical properties as well as its potential as photocatalysts are relatively unexplored. Herein, a family of α-diimine ligands that combines a xanthine and a pyridine scaffold has been prepared, resulting in fluorescent cores. The coordination chemistry of some of the members of the ligand family towards nickel was studied. Both, the fluorescent ligands and metal complexes were fully characterized by a combination of different physical methods, including electronic spectroscopy. The in situ combination of the xanthine-based chelates with an appropriate Ni source supported the light-driven arylation of aromatic carboxylates. Unlike any other methodology reported so far, external photocatalysts are not required for such cross-coupling to proceed.

Speaker: Rafael Emilio Rodriguez-Lugo (Universität Regensburg)
• Thursday, 30 September
• 09:30 10:00
A molecular view at the interface between water and a photocatalyst 30m
Speaker: Prof. Ellen Backus (University of Vienna)
• 10:05 10:30
Spectroscopy II
• 10:05
Electron hopping across a dye-sensitized NiO surface 20m

The lateral self-exchange hole transfer between dye molecules immobilized on a mesoporous semiconductor has been studied rather extensively for mesoporous titanium dioxide films in the context of dye-sensitised photoanodes (and n-DSCs) for solar harvesting and gives insight to charge recombination, one of the key factors that determines the efficiency of such systems.

In the present work, the lateral electron transfer dynamics between dyes have been studied for the first time across a sensitised NiO surface for photocathodes. The lateral electron hopping between TIP dye molecules on the mesoporous NiO surface have been investigated with spectroelectrochemistry where an applied potential step initiates lateral electron hopping between the dye molecules on the NiO surface. From the kinetic trace, the magnitude of electron hopping for the TIP could be extracted and gives insight into the lateral self-exchange dynamics for p-type dye-sensitised systems.

Speaker: Sina Wrede (Uppsala University)
• 10:30 10:40
Coffee Break 10m
• 10:40 11:30
Spectroscopy II
• 10:40
Transient Absorption 2D Correlation Spectroscopy – A Kinetic Model Free Approach to the Analysis of Ultrafast Spectroscopy Data 20m

Time-resolved femtosecond transient absorption (fs-TA) spectroscopy is a powerful method to investigate the photoinduced processes in molecular systems, that is, their relaxation and reaction pathways upon photoexcitation on time scales between a few femtoseconds and hundreds of nanoseconds. Analysis of the resulting set of spectra generally requires advanced techniques which try to fit a kinetic model to the data, for example via global lifetime analysis or multivariate curve resolution. While these methods are widely used, they require a priori information on the sampled system, as they require a kinetic model, i.e., the number of processes contributing to the data. Choosing the right number of processes is often not an easy task and heavily influences the results.1

2D correlation spectroscopy (2DCOS), a method popular especially in the field of vibrational spectroscopy, offers a way to analyze systematic changes in datasets recorded under a changing external variable, by recovering the cross-correlation function of the spectral variables.2 2DCOS is ideally suited to TA spectroscopy, as the external variable (time) is inherent to the method. Furthermore, it does not require a specific kinetic model or other a priori information. TA-2DCOS allows the extraction of the number of kinetic processes contributing to the data set alongside qualitative spectral signatures. We demonstrate that TA-2DCOS can reproduce the results obtained by common, model dependent, methods for well understood systems. 2DCOS can therefore serve as an alternative analysis method for fs-TA spectra and offer an ideal starting point for quantitative methods, if a priori information is not available.3

Acknowledgement: This work was supported by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) in the framework of SFB/TRR 234 CataLight (subprojects C2 and A1, project number 364549901) and project BO 4700/4-1.

Speaker: Mr Julian Hniopek (Leibniz-Insitute of Photonic Technology)
• 11:05
Metal-Ligand Covalency as a New Design Principle for Fe(II) Photosensitizers: Insights from Resonant Inelastic X-Ray Scattering 20m

There have been significant efforts in recent years to destabilize ligand-field with respect to charge-transfer excited states in Fe(II) complexes, towards the goal of accessing long-lived luminescent charge-transfer excited states and developing viable, sustainable and affordable alternatives to Ru(II) and other precious metal photosensitizers,[1] which have found wide application in solar fuel, photovoltaic, photocatalytic and biological applications. The most effective strategy to date has been to increase 10Dq using strong-field ligands,[2] but this appears to have hit a ceiling at sub-optimal charge-transfer excited state lifetimes – necessitating conceptually new molecular design strategies.

The Herbert group recently reported a new strategy, employing weak-field amido ligands that enhance metal-ligand covalency.[3] We have therefore investigated the electronic structure and excited-state ordering of this new class of Fe(II) photosensitizer using resonant inelastic x-ray scattering (RIXS). RIXS probes Raman-allowed valence excited states with atomic specificity, and is therefore a powerful tool for investigating the electronic structure of transition-metal complexes from both metal and ligand perspectives.

This presentation will focus on determining the electronic structure of Fe(II) complexes bearing amido ligands using N 1s2p (K-edge) and Fe 2p3d (L-edge) RIXS, and on a previously unreported competition between ligand-field strength and metal-ligand covalency with respect to destabilizing ligand-field relative to charge-transfer excited states, derived from Tanabe-Sugano analysis of the Fe 2p3d (L-edge) RIXS data.

[1] Oliver S. Wenger, Chem. Eur. J., 2019, 25, 6043-6052, https://doi.org/10.1002/chem.201806148
[2] Pavel Chabera et al., J. Phys. Chem. Lett., 2018, 9, 459-463, https://doi.org/10.1021/acs.jpclett.7b02962
[3] Jason D. Braun et al., Nat. Chem., 2019, 11, 1144-1150, https://doi.org/10.1038/s41557-019-0357-z

Speaker: Dr Christopher Larsen (University of Geneva)
• 15:30 16:00
When molecules meet materials: Heterogenised molecular systems for CO2 reduction and H2 evolution reaction 30m

Conversion of CO2 and water into fuels and value-added chemicals using renewable electrical or solar energy offers a promising route to mitigate anthropogenic carbon footprint and reduce our reliance on fossil fuels and petroleum industry. However, the challenge lies to develop suitable catalysts that can lower the kinetic barriers for water splitting and CO2 activation, and drive the fuel synthesis selectively toward the desired product(s). Molecular catalysts fascinate synthetic chemists the most due to their tuneability, which allows us to tailor the structure to fine-tune their intrinsic properties. However, these molecular systems are somewhat disadvantaged by practical consideration because they often function in homogeneous solution and displays limited long-term stability. Having an effective scaffold to mount the catalyst on, representing 'heterogenisation' of the molecule, is a key part of building a practical system that brings together the benefits of homogeneous and heterogeneous catalysis. In this talk, I will explore two different approaches towards fabricating hybrid electro- and photo-catalytic materials, (1) utilisation of molecular catalysts as building blocks for synthesis of modular porous materials,[1] and (2) direct immobilisation of molecular complexes onto semiconducting materials for solar-driven transformations.[2,3] Final part of the talk will focus on demonstrating how the fuel-forming cathodic half-reaction (CO2 reduction) can be applied in a coupled electrolyser to produce value-added chemicals at the anode via organic electrooxidation.[4]

[1] S. Roy†, Z. Huang†, A. Bhunia, A. Castner, A. Kumar, X. Zou, S. Ott; J. Am. Chem. Soc., 2019, 141, 15942–15950
[2] S. Roy, E. Reisner; Angew. Chem. Int. Ed., 2019, 58, 12180–12184.
[3] S. Roy, M. Miller, J. Warnan, J.J. Leung, C.D. Sahm, E. Reisner; ACS Catal., 2021, 11, 1868–1876
[4] M. A. Bajada†, S. Roy†, J. Warnan†, K. Abdiaziz, A. Wagner, M. M. Roessler, E. Reisner; Angew. Chem. Int. Ed., 2020, 59, 15633–15641

Speaker: Dr Souvik Roy (University of Lincoln)
• 16:05 16:55
Molecules & Materials IV
• 16:05
Ultrathin Oxide Membranes for Photo- and Electrocatalytic Applications 20m

The recent emergence of ultrathin oxide layers has opened new pathways for the hierarchical integration of electrodes, catalysts, and membranes. Similar to thylakoids, new charge conducting molecule embedded, gas impermeable, and proton conducting oxide nanomembranes for coupling catalysts can be readily made. In my newly appointed position as group leader at UTwente I will study structure, performance, and electron & proton transfer dynamics of such molecule embedded ultrathin oxide membranes to close the photo- and electrosynthetic redox cycle on the nano-scale under separation of the incompatible catalysis environments and products.

Herein, I will present our most recent results in controlling and optimizing photoinduced charge transfer across ultrathin silica separation membrane with embedded molecular wires. Accurate measurement of wire and light absorber densities is accomplished by the polarized FT-IRRAS method. Guided by density functional theory (DFT) calculations, four wire derivatives featuring electron-donating (methoxy) and -withdrawing groups (sulfonate, perfluorophenyl) with highest occupied molecular orbital (HOMO) potentials ranging from 1.48 to 0.64 V vs NHE were synthesized and photocurrents evaluated. Combined with the established high proton conductivity and small-molecule blocking property, the charge transfer measurements demonstrate that oxidation and reduction catalysis can be efficiently integrated on the nanoscale under separation by an ultrathin silica membrane.

Speaker: Georgios Katsoukis (University of Twente)
• 16:30
Disclose the complexity involved in water oxidation catalysis based on Ru-bpn complexes 20m

By far ruthenium (Ru) complexes are the best water oxidation catalysts (WOCs) to produce proton (H+) as a source for H2 gas production from H2O. However, most of research works published in this area fail to address the real active species. This led to erroneous and vague conclusions and is only restricted the reported water oxidation catalyst, without offering the practical conclusion to design new family of complexes. Here a new family of Ru complexes with the formula [Ru(bpn)(pic)2]Cl2 (where bpn is 2,2′-bi(1,10-phenanthroline) and pic stands for 4-picoline) (1Cl2) is synthesized to investigate the true nature of active species involved in the electrochemical and chemical water oxidation of the Ru complexes by six coordinated nitrogen atoms. Extensive electrochemical (by using cyclic voltammetry, differential pulse voltammetry, and controlled potential electrolysis), structural (X-ray diffraction analysis), spectroscopic (UV-vis, NMR, and resonance Raman), and kinetic studies are performed. 12+ experiences a substitution reaction when it is chemically or electrochemically oxidized to RuIII, in which picoline is replaced by an hydroxido ligand to produce [Ru(bpn)(pic)(OH)]2+ (22+). The former complex is in equilibrium with an oxo-bridged species {[Ru(bpn)(pic)]2(μ-O)}4+ (34+) which is the major form of the complex in the RuIII oxidation state. The dimer formation is the rate determining step of the overall oxidation process (kdimer = 1.35 M-1 s-1), which is in line with the electrochemical data at pH = 7 (kdimer= 1.4 M-1 s-1). 34+ can be reduced to [Ru(bpn)(pic)(OH2)]2+ (42+). All species generated in situ at pH 7 have been thoroughly characterized by NMR, mass spectrometry, UV-Vis and electrochemical techniques. 12+ and 42+ are also characterized by single crystal X-ray diffraction analysis. Chemical oxidation of 12+ triggered by Ce2+ shows its capability to oxidize water to dioxygen.1

(1) Ghaderian, A.; Franke, A.; Gil-Sepulcre, M.; Benet-Buchholz, J.; Llobet, A.; Ivanović-Burmazović, I.; Gimbert-Suriñach, C. Dalton Transactions 2020, 49, 17375.

Speaker: Dr Abolfazl Ghaderian (The Institute of Chemical Research of Catalonia (ICIQ))
• 16:55 17:10
Coffee Break 15m
• 17:10 18:00
Molecules & Materials IV
• 17:10
Ligand-based redox reactions supporting the small molecule activation by low-coordinate iron complexes 20m

Redox non-innocent supporting ligands can be employed to facilitate multi-electron redox reactions and stabilize open-shell intermediates in 3d transition metal-mediated conversions.1 The tridentate meridional N3-ligand 2,6-di-(3,5-di-tert-butylpyrrol-2-yl)-pyridine (tbpyrr2py) is shown to support a diverse range of small molecule activation reactions of its low-coordinate iron complexes, specifically, sulfur atom transfer2, reversible N-N bond formation3, and dinitrogen activation under ambient conditions.4 In these examples, the tbpyrr2py ligand is found to support various redox processes, ranging from the oxidation of a high-valent iron(IV) to the two-fold reduction of a low-valent iron(II) species.
1. P. J. Chirik, K. Wieghardt, Science, 2010, 327, 5967, 794-795.
2. D. Sorsche, M.E. Miehlich, E. M. Zolnhofer, P.J. Carroll, K. Meyer, D.J. Mindiola, Inorg. Chem., 2018, 57, 18, 11552-11559.
3. J.R. Aguilar-Calderón, D. Fehn, D. Sorsche, M.E. Miehlich, P.J. Carroll, E. Zars, K. Meyer, D.J. Minidola, Inorg. Chem., 2021, published online (asap).
4. D. Sorsche, M.E. Miehlich, K. Searles, G. Gouget, E.M. Zolnhofer, S. Fortier, C.-H. Chen, M. Gau, P.J. Carroll, C.B. Murray, K.G. Caulton, M.M. Khusniyarov, K. Meyer, D.J. Mindiola, J. Am. Chem. Soc., 2020, 142, 18, 8147-8159.

Speaker: Dieter Sorsche (Universität Ulm)
• 17:35
Tuning the Electron Transport and Stability of the Photoanode in Water-Splitting Dye Cells 20m

Water-splitting dye-sensitized photoelectrochemical cells have been attracting more attentions recently because they provide a promising solution to solar energy conversion from sunlight to solar fuels. However, issues such as the photo-desorption of sensitizers and the low quantum efficiency of the photoanode limit their applications in real life. Also, in an aqueous environment near neutral pH, sensitizers desorb from the electrode surface due to the hydrolysis of anchoring groups. The charge injection efficiency and stability of sensitizers on the electrode have shown to be dependent on the anchoring groups.
To study the stability of photosensitizers in an aqueous environment, we have synthesized three types of dimeric ruthenium(II) polypyridyl complexes with different anchoring groups on the bipyridine ligands, namely carboxylate, phosphonate, and a combination of both.
In addition, the electron transport in the nanostructured semiconductor competes with the charge recombination. From a kinetic model for water-splitting dye-sensitized TiO2 nanoparticle electrodes in a previous study, it is suggested that the overall efficiency can be improved by accelerating electron diffusion in the TiO2 nanostructure.
We have demonstrated that the electron transport kinetics and the stability of sensitizers can be tuned by modifying the morphology of the electrode architecture and the anchoring groups of the sensitizers, respectively. Crystalline TiO2 nanowire arrays facilitate the electron diffusion and increase electron lifetime for recombination. Therefore, the nanowire array electrodes are promising to increase the efficiency of water-splitting dye cells.
From the photostability experiment results, the bonding strength of the sensitizers to the electrodes can be tuned by modifying the molecule with different anchoring groups. Carboxylate anchoring groups bind weakly to the electrode, while the phosphonate group shows stronger binding. The complex with mixed anchoring groups shows medium stability. In the next stage of study, we will look into the charge injection efficiency of these complexes in aqueous electrolyte to investigate if the anchoring groups have an impact on electron injection efficiency.

Speaker: Langqiu Xiao (University of Pennsylvania)
• 18:00 18:15
Conference Closing 15m