The mechanism of excited-state proton transfer in 1-naphthol–piperidine clusters

Abstract

The geometries of 1-naphthol–(piperidine)_n (1-NpOH–(Pip)_n) (n = 0–3) clusters have been calculated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods to investigate excited-state proton transfer (ESPT) in the low-lying singlet excited states, L_a and L_b. For the n = 1 cluster, no PT structure was found in L_b and L_a as well as the ground state, S₀. For n = 2, optically accessible L_b from S₀ shows the PT structure. We therefore concluded that the threshold size of ESPT is n = 2, which is consistent with previous experimental results. ESPT in 1-NpOH–(Pip)_n is simply triggered by optical excitation to L_b. It is essentially different from the 1-NpOH–(NH₃)_n cluster in which an internal conversion process is required to promote ESPT. From the calculated structures, the importance of the solvation of the π-ring is strongly suggested rather than the proton affinity in ESPT.