Fluorescence Quenching of Benzaldehyde in Water by Hydrogen Atom Abstraction

We computed the mechanism of fluorescence quenching of benzaldehyde in water through relaxed potential energy surface scans. Time‐dependent density functional theory calculations along the protonation coordinate from water to benzaldehyde reveal that photoexcitation to the bright ππ* (S₃) state is immediately followed by ultrafast decay to the nπ* (S₁) state. Evolving along this state, benzaldehyde (BA) abstracts a hydrogen atom, resulting in a BAH^. and OH^. radical pair. Benzaldehyde does not act as photobase in water, but abstracts a hydrogen atom from a nearby solvent molecule. The system finally decays back to the ground state by non‐radiative decay and an electron transfers back to the OH^. radical. Proton transfer from BAH⁺ to OH^? restores the initial situation, BA in water.