Triplet Excited States Modulated by Push-Pull Substituents in Monocyclometalated Iridium(III) Photosensitizers.

A series of novel monocyclometalated [Ir(tpy)(btp)Cl] complexes (-) were synthesized using 2,2':6',2″-terpyridine (tpy) and 2-(2-pyridyl)benzo[]thiophene (btp) ligands, as well as their derivatives bearing electron-donating -butyl (-Bu) and electron-withdrawing trifluoromethyl (CF) groups. - exhibited visible-light absorption stronger than that of the known complex [Ir(tpy)(ppy)Cl] (; ppy = 2-phenylpyridine). Spectroscopic and computational studies revealed that two triplet states were involved in the excited-state dynamics. One is a weakly emissive and short-lived ligand to ligand charge-transfer (LLCT) state originating from the charge transfer from the btp to the tpy ligand. The other is a highly emissive and long-lived ligand-centered (LC) state localized on the btp ligand. Interestingly, the excited state dominant with LLCT was completely changed to the LC state upon the introduction of substituents on both the tpy and btp ligands. For instance, the excited state of the parent complex was weakly emissive (Φ = 2%) and short-lived (τ = 110 ns) in CHCl; conversely, , fully furnished with -Bu and CF groups, displayed intense phosphorescence with a prolonged lifetime (τ = 14 μs). This difference became increasingly prominent when the solvent was changed to aqueous CHCN, most probably due to the LLCT stabilization. The predominant excited-state nature was switchable between the LLCT and LC states depending on the substituents employed; this was demonstrated through investigations of and , bearing either the -Bu or the CF group, where the complexes exhibited properties intermediate between those of and . All of the Ir(III) complexes were tested as photosensitizers in photocatalytic H evolution over a Co molecular catalyst, and outperformed the others, including , due to improvement in the following key properties: visible-light-absorption ability, excited-state lifetime, and reductive power of the one-electron-reduced species against the catalyst.