Role of Triplet States in the Photodynamics of Aniline
The dynamics of excited heteroaromatic molecules is a key to understanding the photoprotective properties of many biologically relevant chromophores that dissipate their excitation energy nonreactively and thereby prevent the detrimental effects of ultraviolet radiation. Despite their structural variability, most heteroaromatic compounds share a common feature of a repulsive <sup>1</sup>πσ* potential energy surface. This surface can lead to photoproducts, and it can also facilitate the population transfer back to the ground electronic state by means of a <sup>1</sup>πσ*/S<sub>0</sub> conical intersection. Here, we explore a hidden relaxation route involving the triplet electronic state of aniline, which has recently been discovered by means of time-selected photofragment translational spectroscopy [J. Chem. Phys. 2019, 151, 141101]. By using the recently available analytical gradients for multiconfiguration pair-density functional theory, it is now possible to locate the minimum energy crossing points between states of different spin and therefore compute the intersystem crossing rates with a multireference method, rather than with the less reliable single-reference methods. Using such calculations, we demonstrate that the population loss of aniline in the T<sub>1</sub>(<sup>3</sup>ππ<sup>*</sup>) state is dominated by C<sub>6</sub>H<sub>5</sub>NH<sub>2</sub>→C<sub>6</sub>H<sub>5</sub>NH⸱ + H⸱ dissociation, and we explain the long nonradiative lifetimes of the T<sub>1</sub>(<sup>3</sup>ππ<sup>*</sup>) state at the excitation wavelengths of 294‑264 nm.
