Cu(I) vs. Ru(II) complexes — photophysical and electrochemical behavior of a series of structurally related photosensitizers

in: Physical Chemistry Chemical Physics (2018)
Zhang, Ying; Traber, Philipp; Zedler, Linda; Kupfer, Stephan; Gräfe, Stefanie; Schulz, Martin; Karnahl, Michael; Dietzek, Benjamin; Frey, Wolfgang
Heteroleptic Cu(I) complexes are a promising alternative towards traditional Ru(II) photosensitizers. In particular, Cu(I) complexes of the type [Cu(N^N)(P^P)]+, where N^N represents a diimine and P^P a bulky diphosphine ligand, are already successfully applied for photo(redox)catalysis, organic light-emitting diodes (OLEDs) or dye-sensitized solar cells (DSSCs). Therefore, this study aims at the systematic comparison of three novel heteroleptic Cu(I) compounds, composed of xantphos (xant) as P^P ligand and different diimine ligands with an extended π-system in the backbone, with their structurally related Ru(II) analogues. In these Ru(II) photosensitizers [Ru(bpy)2(N^N)]+ (bpy = 2,2’-bipyridine) the same N^N ligands were used, namely, dipyrido[3,2-f:2’,3’-h]quinoxaline (dpq) and dipyrido[3,2-a:2’,3’-c]phenazine (dppz). To gain an in-depth understanding of the photoinduced charge transfer processes, the photophysical features of these complexes and their electrochemically oxidized/reduced species were studied by a combination of UV-Vis absorption, resonance Raman and spectroelectrochemistry. (TD)DFT calculations were applied to qualitatively analyze these measurements. As a result, the heteroleptic Cu(I) complexes exhibit comparable charge transfer properties to their Ru(II) analogues: Upon visible light excitation they undergo a metal-to-ligand charge transfer (i.e. to the diimine ligands). However, the reduced Ru/Cu-dppz complexes show considerably different electronic transitions. These differences might impact the function of such Ru/Cu-dppz-based photosensitizers when incorporated into multi-electron transfer cascades, e.g. in the photocatalytic reduction of protons to hydrogen.

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