Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry

TL;DR

This paper combines spectroscopy and computational chemistry to understand how heptazine-based photocatalysts can photooxidize water, revealing mechanisms of proton-coupled electron transfer and demonstrating laser control of these reactions.

Contribution

It provides new insights into the photochemistry of heptazine complexes, including the first demonstration of laser-controlled PCET reactions in water oxidation catalysts.

Findings

Heptazine derivatives can photooxidize water and phenol in homogeneous reactions.

Spectroscopic and computational methods elucidate the PCET mechanism.

Laser control can influence the branching pathways of the reactions.

Abstract

We present a conspectus of recent joint spectroscopic and computational studies which provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This 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 and derivatives thereof are highlighted. The potential energy landscape relevant for the PCET reaction has been characterized by judicious computational studies. These data provided the basis for the demonstration of rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy, which sheds light on the branching mechanisms occurring by the interaction of nonreactive locally excited states of the chromophore with reactive intermolecular charge-transfer states. Extrapolating from these results, we propose a general scenario 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.