Excimer states in microhydrated adenine clusters

Abstract

We present femtosecond pump-probe mass and photoelectron spectra for adenine (A) and microhydrated Am(H2O)n clusters. Three distinct relaxation processes of photoexcited electronic states were distinguished: in unhydrated A, relaxation of the optically bright ππ* state occurred via the dark nπ* state with respective lifetimes of <0.1 and 1.3 ps. In microhydrated clusters A(H2O)n, relaxation via the nπ* state is quenched by a faster relaxation process, probably involving πσ* states. For the predominantly hydrogen-bonded adenine dimer (A2), excited state relaxation is dominated by monomer-like processes. When the adenine dimer is clustered with several water molecules, we observe a nanosecond lifetime from excimer states in π-stacked clusters. From the electron spectra we estimate adiabatic ionization potentials of 8.32 eV (A), 8.27 eV (A(H2O)1), 8.19 eV (A(H2O)2), 8.10 eV (A(H2O)3), 8.18 eV (A2), and 8.0 eV (A2(H2O)3-5). © the Owner Societies.