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(HO) 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-(HO). Excited state calculations of DAH(HO) reproduce these spectral features. The conformer-selected IR spectrum of DAH(HO) 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(HO). It is concluded that the excited state proton transfer in DAH is inhibited by water-insertion.