Accumulation of mono-reduced [Ir(piq)2(LL)] photosensitizers relevant for solar fuels production.

A series of nine [Ir(piq)2(LL)]+.PF6- photosensitizers, where piqH = 1-phenylisoquinoline, was developed and investigated for excited-state electron transfer with sacrificial electron donors that included triethanolamine (TEOA), triethylamine (TEA) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) in acetonitrile. The photosensitizers were obtained in 57-82% yield starting from the common [Ir(piq)2µ-Cl]2 precursor and were all characterized by UV-Vis absorption as well as by steady-state, time-resolved spectroscopies and electrochemistry. The excited-state lifetimes ranged from 250 to 3350 ns and excited-state electron transfer quenching rate constants in the 109 M-1 s-1 range were obtained when BIH was used as electron donor. These quenching rate constants were three orders of magnitude higher than when TEA or TEOA was used. Steady-state photolysis in the presence of BIH showed that the stable and reversible accumulation of mono-reduced photosensitizers was possible, highlighting the potential use of these Ir-based photosensitizers in photocatalytic reactions relevant for solar fuels production.