A recording of this talk is available until 4/23.

Revisiting Scenarios for Photocatalytic Water Oxidation

Prof. Wolfgang Domcke -- Department of Chemistry, Technology University Munich

Host: Cody Schlenker
 

Abstract: During the past decade, polymeric carbon nitride materials received vast attention as metal-free and chemically highly stable photocatalysts for hydrogen evolution from water. The extensive literature on water splitting with these carbon nitrides addresses materials properties, such as morphology, band gap or charge carrier mobilities. However, the photophysical properties of the molecular chromophores (heptazine) and the photochemistry of the water oxidation reaction have remained obscure. In this talk, an alternative perspective of photocatalytic water splitting with organic materials is presented which focusses on the photochemical reactivity of molecular N-heterocycles with protic substrate molecules. The proposed scenario has evolved from ab initio computational studies performed at the Technical University of Munich and spectroscopic and kinetic studies performed at the University of Washington. It has been discovered that a functionalized derivative of heptazine, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This discovery allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules. The unique properties of the excited electronic states of the Hz molecule, such as inverted S₁ and T₁ states, are highlighted. The potential-energy landscape relevant for the PCET reaction has been characterized by computational studies. Guided by these data, rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy could be demonstrated, providing insight into the branching mechanisms among nonreactive locally excited states of the chromophore and reactive intermolecular charge-transfer states. Extrapolating from these results, a general scenario is proposed which unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions. A novel family of heterocyclic chromophores is proposed which exhibit unprecedented tunability of their optical and photocatalytic properties.