Hydration-controlled excited-state relaxation in protonated dopamine studied by cryogenic ion spectroscopy.

Ultraviolet (UV) and infrared (IR) spectra of protonated dopamine (DAH + ) and its hydrated clusters DAH + (H 2 O) 1-3 are measured by cryogenic ion spectroscopy. DAH + monomer and hydrated clusters with up to two water molecules show a broad UV spectrum, while it turns to a sharp, well-resolved one for DAH + -(H 2 O) 3 . Excited state calculations of DAH + (H 2 O) 3 reproduce these spectral features. The conformer-selected IR spectrum of DAH + (H 2 O) 3 is measured by IR dip spectroscopy, and its structure is assigned with the help of quantum chemical calculations. The excited state lifetime of DAH + is much shorter than 20 ps, the cross correlation of the ps lasers, revealing a fast relaxation dynamics. The minimal energy path along the NH → π proton transfer coordinate exhibits a low energy barrier in the monomer, while this path is blocked by the high energy barrier in DAH + (H 2 O) 3 . It is concluded that the excited state proton transfer in DAH + is inhibited by water-insertion.