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+(H2O)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+–(H2O)3. Excited state calculations of DAH+(H2O)3 reproduce these spectral features. The conformer-selected IR spectrum of DAH+(H2O)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+(H2O)3. It is concluded that the excited state proton transfer in DAH+ is inhibited by water-insertion.