Imaging the Photochemistry of the Hydrogen-Bonded Heptazine-Water Complex with Femtosecond Time-Resolved Spectroscopy: A Computational Study

Abstract: Graphitic carbon nitride ($g$-CN) has attracted vast interest as a promising inexpensive metal-free photocatalyst for water splitting with solar photons. The heptazine (Hz) molecule is the building block of graphitic carbon nitride. The photochemistry of the Hz molecule and derivatives thereof in protic environments has been the subject of several recent experimental and computational studies. In the present work, the hydrogen-bonded Hz$\cdots$H$_2$O complex was adopted as a model system for the exploration of photoinduced electron and proton transfer processes in this complex with quasi-classical nonadiabatic trajectory simulations, using the $ab$ $initio$ ADC(2) electronic-structure method and a computationally efficient surface-hopping algorithm. The population of the optically excited bright $^1\pi\pi^*$ state of the Hz chromophore relaxes through three $^1n\pi^*$ states and a low-lying charge-transfer state, which drives proton transfer from H$_2$O to Hz, to the long-lived optically dark S$_1$($\pi\pi^*$) state of Hz. The imaging of this ultrafast and complex dynamics with femtosecond time-resolved transient absorption (TA) pump-probe (PP) spectroscopy and two-dimensional (2D) electronic spectroscopy (ES) was computationally explored in the framework of the quasi-classical doorway-window approximation. By comparison of the spectra of the Hz$\cdots$H$_2$O complex with those of the free Hz molecule, the effects of the hydrogen bond on the ultrafast internal conversion dynamics can be identified in the spectroscopic signals. Albeit the TA PP and 2D ES spectroscopies are primarily sensitive to electronic excited-state dynamics and less so to proton transfer dynamics, they nevertheless can provide mechanistic insights which can contribute to the acceleration of the optimization of photocatalysts for water splitting.