Excited-state proton transfer in 1-naphthol·(NH3)n complexes: the threshold size is n=4

Resonant two-photon ionization, fluorescence and fluorescence-detected excitation spectra of 1-naphthol·(NH3)n clusters are reported and interpreted with emphasis on determining the smallest clusters for which excited-state proton transfer (ESPT) can occur. Consistent with the earliest studies but in contrast to more recent work, the ESPT threshold is found at n=4, not n=3. The confusion is shown to have arisen from weak, broad fluorescence of n=1–3 clusters extending to ∼400 nm. Excitation spectra of genuine ESPT emission at longer wavelengths gives no indication of contributions from n<4.