Photorelaxation via Water-Mediated Electron Transfer in Fully Solvated Heptazine

Theoretical photochemistry simulations often treat the solvent as static, with only the solute evolving out of equilibrium. However, in polar solvents like water, dynamic solvent effects critically influence photorelaxation. We perform nonadiabatic excited-state molecular dynamics simulations of heptazine (C₆N₇H₃) fully solvated in water and identify a previously unrecognized solvent-mediated photorelaxation pathway. It involves direct hybridization between the 1b₁ lone pair p-orbitals of water and lone pair orbitals on heptazine nitrogen atoms, bypassing hydrogen bonding. This drives water-to-heptazine electron transfer (ET), forming transient (H₂O)_n⁺ clusters (2 ≤ n ≤ 4) stabilized in hemibonded configurations. The electron-hole pair evolves within a droplet of ∼125 water molecules and recombine on a subpicosecond time scale via back-ET from a pyramidalized aromatic carbon─acting as an intermediate carbanion─to the hemibonded (H₂O)₂⁺ cluster. Our findings highlight the critical role of explicit solvent fluctuations and finite-size effects in excited-state dynamics, providing new insights into photorelaxation and radical formation relevant to the photocatalytic cycle of heptazine.