Proton transfer from 1-naphthol to water: Small clusters to the bulk

Abstract

Excited-state proton transfer from 1-naphthol to water was studied as a function of solvent system size, from supersonically cooled neutral clusters, 1-naphthol⋅(H2O)n, n=1–50, to bulk ice and water. Occurrence or nonoccurrence of proton transfer was detected and studied using cluster-size-specific laser-spectroscopic techniques: resonant two-photon ionization (R2PI) and laser-induced fluorescence emission. Depending on cluster size or solution phase, three qualitatively different types of excited-state behavior were observed: (1) For small clusters, n≤7, both the R2PI and fluorescence spectra of the clusters were similar in nature to the spectra of bare 1-naphthol; (2) the medium-size clusters (n=8–20) show incremental spectral shifts which indicate successive stages of molecular solvation, and the spectra approach that of 1-naphthol in bulk ice at n≊20; (3) the fluorescence spectra for large clusters, n≥20, show increasing emission intensity below 25 000 cm−1, characteristic of the emission of the excited-state 1-naphtholate anion. Excited-state proton transfer appears to occur in the largest observed clusters (n≥30), yet the fluorescence spectra do not converge fully to that of 1-naphtholate anion in bulk water. These three behaviors are discussed in terms of a model based on three distinct excited states connected by two electronic and geometric rearrangement processes. The model accounts in a unified way for the complete range of aqueous solvation behavior observed here as well as in many other solvent systems studied previously. The extent of proton transfer reaction is largely solvent controlled, the major determinants being the proton affinity of the solvent or solvent cluster, and its ability to resolvate the nascent ion pair on a subnanosecond time scale. In bulk ice, the slow solvent relaxation results in complete absence of excited-state proton transfer from both 1- and 2-naphthol.