The π-extended ligand of a ruthenium complex stores two photo-generated electrons, mimicking a key step in photosynthesis.

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.

The π-extended dipyrido[3,2-a:2′,3′-c]phenazine (dppz) ligand of the Ru(II) complex [Ru(bpy) 2 (oxo-dppqp)]- 3″-4,5,6]quinolino[2,3-h]phenazin-15-one, bpy = 2,2′-bipyridine) enables the mononuclear complex for visible-light-driven accumulation of two electrons on a single ligand structure. Although this has been shown before, the excited-state physics underlying this promising feature are exploited in this work. The photophysics of the complex was investigated by excitation-wavelengthdependent resonance Raman and transient absorption spectroscopy in combination with time-dependent density functional theory. The results show that excitation with visible light leads to the population of the two excited-state branches: (i) the population of a short-lived 3 MLCT state in which the excess electronic density is localized on the pyridoquinolinone moiety of the extended ligand (τ = 105 ps) and (ii) the population of a more long-lived 3 ππ* state (τ = 9 ns). Notably, the long-lived 3 ππ* state rather than a 3 MLCT state is prone to reductive quenching by the sacrificial electron donor and, hence, presents the critical excited-state intermediate in the photochemical charge accumulation experiments.