Excited-state proton transfer in gas-phase clusters: 2-Naphthol⋅(NH3)n

Abstract

Excited-state proton transfer was studied in supersonically cooled neutral acid-base clusters of 2-naphthol⋅(NH3)n with n=1-10, using size-selective two-color resonant two-photon ionization (R2PI) and fluorescence emission techniques. The smaller clusters (n≤3) show vibrationally resolved spectra and do not exhibit excited-state proton transfer. Two rotamers (cis and trans) were observed for each cluster size; these exhibit very different electronic-vibrational couplings to the intermolecular modes, reflecting the orientation of the (NH3)n cluster relative to the electronic transition dipole moment of the aromatic chromophore. Excited-state proton transfer occurs for n≥4, as evidenced by (a) a large spectral red shift (≊8000 cm−1) of the fluorescence emission band; (b) large increase in the emission bandwidth; (c) similarity of the fluorescence band position and width to that of 2-naphtholate anion in neutral or basic aqueous solution. In absorption, the S1←S0 bands are substantially broadened for clusters with n≥4, but are not shifted to the red, as is the case for the analogous 1-naphthol⋅(NH3)n clusters. The cluster-size dependence of the proton transfer reaction is mainly controlled by the cluster proton affinity.