Cu(i) vs. Ru(ii) photosensitizers: elucidation of electron transfer processes within a series of structurally related complexes containing an extended π-system.

Heteroleptic Cu(i) complexes are a promising alternative towards traditional Ru(ii) photosensitizers. In particular, Cu(i) complexes of the type [Cu(P^P)(N^N)]+, where N^N represents a diimine and P^P a bulky diphosphine ligand, are already successfully applied for photocatalysis, organic light-emitting diodes or dye-sensitized solar cells. Therefore, this study aims for 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)]2+ (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, i.e. upon visible light excitation they undergo a metal-to-ligand charge transfer to the diimine ligand(s). In contrast, the reduced Cu(i)- and Ru(ii)-dppz complexes show considerably different electronic transitions. The singly reduced Cu(i)-dppz complexes are able to accumulate an additional electron at the phenanthroline moiety upon blue-light excitation, which is beneficial for multi-electron-transfer reactions. Upon low-energy light irradiation electronic transitions from the dppz- anion to the xant ligand are excited, which could shorten the lifetime of the photosensitizer intermediates in an unwanted way.