Solar Energy Harvesting with Carbon Nitrides and N‐Heterocyclic Frameworks: Do We Understand the Mechanism?

Abstract

The photocatalytic splitting of water into molecular hydrogen and molecular oxygen with sunlight is the dream reaction for solar energy conversion. For decades, transition metal oxide semiconductors and supramolecular organometallic structures have been extensively explored as photocatalysts for solar water splitting. More recently, polymeric materials consisting of triazine or heptazine building blocks have attracted considerable attention as hydrogen‐evolution photocatalysts. The mechanism of hydrogen evolution with these materials is discussed throughout the current literature in terms of the familiar concepts developed for photoelectrochemical water splitting with semiconductors since the 1970s. We discuss in this Minireview an alternative mechanistic paradigm for photoinduced water splitting with carbon nitrides, which focusses on the specific features of the photochemistry of aromatic N‐heterocycles in aqueous environments. Using ab initio electronic‐structure calculations, it is shown that a water molecule which is hydrogen‐bonded to an N‐heterocycle can be decomposed into hydrogen and hydroxyl radicals by two simple sequential photochemical reactions. This concept is illustrated by the discussion of excited‐state reaction paths and their energy profiles for hydrogen‐bonded complexes of pyridine, triazine and heptazine with a water molecule. It is shown that the excited‐state hydrogen‐transfer and hydrogen‐detachment reactions are essentially barrierless, in sharp contrast to water oxidation in the electronic ground state, where high barriers prevail. We also discuss in some detail the products of possible reactions of the highly reactive photogenerated hydroxyl radicals with the chromophores. We hypothesize that the challenge of efficient solar hydrogen generation with carbon nitride materials is less the decomposition of water as such, but rather the controlled recombination of the photogenerated radicals to the closed‐shell products H2 and H2O2.

Triangular boron carbon nitrides: an unexplored family of chromophores with unique properties for photocatalysis and optoelectronics

“…The hydrogen bonding is essential for the excited-state PCET reaction which leads to the abstraction of an H-atom from the substrate molecule. 37,41,71 In contrast to optoelectronics, the energy and the properties of the lowest bright 1 ππ* state play a decisive role, in addition to the properties of the S1(ππ*) state. The photon is absorbed by the bright 1 ππ* state which then rapidly (on sub-picosecond time scales) decays to the S1 state by internal conversion.…”

Photoinduced water oxidation in pyrimidine–water clusters: a combined experimental and theoretical study

“…Despite extensive research efforts in the past ten years, the photophysical and photochemical processes underlying the photocatalytic hydrogen evolution reaction with graphitic carbon nitride materials remain a mystery. 7,8 To gain insight into the fundamental photophysics of heptazine-based materials, a heptazine-derived molecular photocatalyst, tri-anisole-heptazine (TAHz), was recently synthesized by Schlenker and coworkers. 9 It was shown that photoexcited TAHz can oxidize water via an H-atom transfer reaction, resulting in the quenching of the excited state of TAHz and the liberation of OH radicals that were detected using terephthalic acid.…”

Synthesis, electrochemical, photophysical, and photovoltaic properties of new fluorescent compounds: 3 H ‐benzofuro[2,3‐ b ]pyrazolo[4,3‐ f ]quinoline

“…In the present study, which is conducted as continuation of our investigations on synthesizing new heterocyclic dyes as organic photosensitizers in DSSCs, 35,36 we have aimed at a synthesis of 3 H ‐benzofuro[2,3‐ b ]pyrazolo[4,3‐ f ]quinolines as novel fluorophores as high‐performance organic photosensitizers. The photovoltaic performances of the title compounds are remarkable and comparable to recently heterocyclic dyes as organic photosensitizers 37‐39 . In the process, their physical spectral and analytical data proved to corroborate the structure of the pentacylic heterocyclic compounds.…”