Interrogating the photogenerated Ir(iv) state of a water oxidation catalyst using ultrafast optical and X-ray absorption spectroscopy

Abstract

Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(IV) state of the Ir(III)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor–chromophore–Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI–¹*PMI–Ir(III) → NDI–PMI˙⁻–Ir(IV) and NDI–¹*PMI–Ir(III) → NDI˙⁻–PMI˙⁺–Ir(III) that each proceed with τ < 5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI˙⁻–PMI–Ir(IV). The fully charge-separated ion pair has a lifetime of 17.2 ± 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the L_III-edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(III) to Ir(IV), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donor–acceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.