Solvation-Dependent Latency of Photoacid Dissociation and Transient IR Signatures of Protonation Dynamics

We elucidate the characteristic proton pathways and the transient infrared signatures of intermediate complexes during the first picoseconds of photoinduced protonation dynamics of a photoacid (<i>N</i>-methyl-6-hydroxyquinolinium) in aqueous solution from first-principles molecular dynamics simulations. Our results indicate that the typical latency time between photoexcitation and proton dissociation ranges from 1 ps to longer time time scales (∼100 ps). The rate-limiting step for the actual dissociation of the proton into the solvent is the solvation structure of the first accepting water molecule. The nature of the proton pathway in water (stepwise or concerted) is not unique but determined by the coordination number of the accepting water molecules along the hydrogen bond chain. We find a characteristic uncommon infrared mode at ∼1300 cm^–1 of the transient photobase-Eigen cation complex immediately after photodissociation that we predict to be observable experimentally in time-resolved IR spectroscopy. A broad continuous absorption band from 1500 to 2000 cm^–1 arises from the acidic proton imminently before dissociation