Light-driven chemical reactions have several advantages over thermal reactions, e.g. the potential use of sunlight as an energy source or the access to certain reaction pathways that are thermally forbidden. Efficient photosensitizers that are not based on rare noble metals like ruthenium or iridium are a major subject of our current research.[1-5] In this respect, we are especially focused on heteroleptic Cu(I) photosensitizers of the type [(P^P)Cu(N^N)]+ consisting of one diphosphine (P^P) and one diimine ligand (N^N). These photosensitizers have to fulfil some basic requirements, i.e. reversible redox processes, a high (photo)stability and long-lived excited states.[1-5]
An important de-excitation pathway for such Cu(I) photosensitizers is the so-called exciplex quenching, where a solvent molecules coordinates to the metal center in the excited complex (exciplex). Preventing this pathway by proper ligand design is a promising approach towards enhancing the excited state lifetimes of the resulting photosensitizers.[6,7] Therefore, we developed multidentate diimine ligands with different additional donor atoms in proximity of the metal center to understand and possibly overcome this quenching mechanism.[6,7]
A multi-method approach was used to study the impact of these additional donor atoms on the coordination behavior in the ground and the excited state as well as on the complexes' photophysical and electrochemical properties.[6,7] The related complexes with no additional donor atoms were investigated for comparison.[6,7]

[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] R. Giereth, I. Reim, W. Frey, H. Junge, S. Tschierlei, M. Karnahl, Sustainable Energy Fuels, 2019, 3, 692-700, DOI: 10.1039/c8se00521d.
[4] M.-A. Schmid, M. Rentschler, W. Frey, S. Tschierlei, M. Karnahl, Inorganics, 2018, 6, 134, DOI: 10.3390/inorganics6040134.
[5] R. Giereth, A. K. Mengele, W. Frey, M. Kloß, A. Steffen, M. Karnahl, S. Tschierlei, Chem. Eur. J., 2020, 26, 2675-2684, DOI: 10.1002/chem.201904379.
[6] M. Rentschler, S. Iglesias, M.-A. Schmid, C. Liu, S. Tschierlei, W. Frey, X. Zhang, M. Karnahl, D. Moonshiram, Chem. Eur. J., 2020, 26, 9527-9536, DOI: 10.1002/chem.201905601.
[7] M. Rentschler, M.-A. Schmid, W. Frey, S. Tschierlei, M. Karnahl, Inorg. Chem., 2020, DOI: 10.1021/acs.inorgchem.9b03687.