Excimer states in microhydrated adenine clusters

We present femtosecond pump–probe mass and photoelectron spectra for adenine (A) and microhydrated A_m(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).