Photoinduced Water–Heptazine Electron-Driven Proton Transfer: Perspective for Water Splitting with g-C₃N₄

Abstract

Heptazine-assembled polymeric carbon nitride (CN) materials have fascinated the research community as a photocatalyst for hydrogen evolution while less attention has been devoted to the mechanistic features of the host materials. Using excited-state nonadiabatic dynamics simulations, the molecular-level picture of the decomposition of heptazine hydrogen bonded to water molecule(s) (heptazine−water complex) into heptazinyl and hydroxyl biradical products is revealed. Dynamics simulations show that hydrogen detachment from the water molecule to the heptazine occurs within tens of femtoseconds and suggest that excited-state deactivation via N− H••••••O−H electron-driven proton transfer (EDPT) is the dominant and most relevant excited-state deactivation process in heptazine−water complexes leading to conical intersection. The observation of photorelaxation-induced water splitting by heptazine is proof of the water-splitting reaction principle, which presents further challenges for computational and experimental investigations of the deactivation of heptazinyl and OH biradical products for efficient hydrogen evolution.

Polymer photocatalysts for solar-to-chemical energy conversion

“…Photoluminescence (PL) spectroscopy has been extensively used to study the photophysics of melon and early results pointed to the generation of singlet Frenkel excitons highly confined to the heptazine units upon photoexcitation owing to the lack of significant conjugation within the 1D strand. [93][94][95] This gives rise to high ExBE (~1 eV ) 34,96 making exciton dissociation difficult. The material was thus suggested to be an optical quasi-monomer.…”

A straightforward synthesis of a new family of molecules: 2,5,8-trialkoxyheptazines. Application to photoredox catalyzed transformations

“…Especially, their fluorescence emission 3 has been exploited in liquid crystalline materials 4 and OLEDs devices. 5 Finally, even more attracting, heptazine polymers have demonstrated outstanding photocatalytic properties, 6,7 and the heptazine core might well be one of the rare molecular platforms which could give birth to molecular catalysts able to perform water splitting [8][9][10] , the holy Grail of all photocatalytic processes. Two recent reviews have made a point on this highly interesting family of heterocyclic compounds 11,12 .…”

Barrierless Heptazine-Driven Excited State Proton-Coupled Electron Transfer: Implications for Controlling Photochemistry of Carbon Nitrides and Aza-Arenes

“…We have elected to examine all-organic materials with long excited state lifetimes and good photostability. Recently, the nitrogen-rich heptazine (Hz) unit has garnered increased attention for applications in photocatalytic hydrogen production. ,− The mechanism of molecular Hz-driven ES-PCET has been studied computationally, − and our group reported the first experimental evidence for this process resulting in neutral radical species . This Hz-based chromophore exhibits a singlet lifetime of 296.9 ± 0.4 ns, the same order of magnitude as [Ru­(bpy) 3 ] 2+ , a compound which is ubiquitous in the study of photochemical transformations. , Interestingly, this roughly 300 ns lifetime is significantly longer than first row transition-metal complexes as well as traditional strongly absorbing organic chromophores, which typically have lifetimes of no more than tens of nanoseconds .…”