Molecular Design of Heptazine-based Photocatalysts: Effect of Substituents on Photocatalytic Efficiency and Photostability.

Polymeric carbon nitride materials consisting of heptazine (Hz) building blocks are of high current interest as promising heterogeneous photocatalysts for the water oxidation reaction. However, the preparation of these materials via high-temperature pyrolysis leads to poorly constrained chemical composition and molecular structure, which hampers the reproducibility of reported data and represents a major obstacle for unravelling the underlying reaction mechanisms. Recently, a derivative of the Hz molecule, trianisole-heptazine (TAHz), was synthesized and was shown to catalyze the oxidation of water to hydroxyl radicals under 365 nm LED light in a homogenous reaction. The possibility of water photo-oxidation with a precisely defined molecular catalyst in neat solvents opens new perspectives for clarifying the reaction mechanisms to rationally design improved photocatalysts. In the present work, the effects of chemical substituents on the three CH positions of Hz on the photocatalytic reactivity were explored with wave-function-based ab initio electronic-structure calculations for hydrogen-bonded complexes of Hz and three selected Hz derivatives (TAHz, tri-chloro-Hz and tri-cyano-Hz) with a water molecule. While anisole is an electron donating substituent, Cl is a weakly electron withdrawing substituent and CN is a strongly electron withdrawing substituent. It is shown that the barrier for the photoinduced abstraction of an H-atom from the water molecule is raised (lowered) by electron donating (electron withdrawing) substituents. The energy of the 2pisigma* state, which drives the photodetachment reaction of the H-atom from the reduced chromophore, is lowered (raised) by electron donating (electron withdrawing) substituents. The highly mobile and reactive hydroxyl radicals generated by water oxidation can recombine with the reduced chromophore radicals to yield photohydrates. Among the four chromophores studied, TAHz stands out on account of the metastability of its photohydrate, which suggests self-healing of the photocatalyst after oxidation of TAHzH radicals by OH radicals. All four chromophores exhibit inverted S1/T1 gaps. This feature eliminates long-lived triplet states and therefore avoids the activation of molecular oxygen. Robustness with respect to atmospheric oxygen under irradiation and the capability of self-healing, identified herein for the TAHz molecule, are important features which future technologically relevant photocatalytic materials should possess.