Influence of H-bonds on the photoionization of aromatic chromophores in water: The aniline molecule.

This article focuses on the photophysical and photoionization mechanisms of aniline in aqueous solution following UV excitation at 267 nm. Time-resolved pump–probe and pump–repump–probe experiments reveal the simultaneous formation of two excited states: the bright ππ* state responsible for typical photophysical relaxation with a ∼1 ns lifetime, and a charge transfer πσ* state along the N–H bond that leads to autoionization and the generation of solvated electrons and radical cations with a ∼500 ps lifetime. The dual relaxation pathways are explained by different hydrogen-bonding interactions between water molecules and the aniline amino group, with theoretical calculations supporting the role of specific aniline–water cluster geometries in facilitating charge transfer. These findings suggest a general mechanism for aromatic molecules containing N–H or O–H bonds in water to form stable separated charges at excitation energies near their electronic absorption thresholds.