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 l-naphthol(NH,),, 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 similar to 400 nm. Excitation spectra of genuine ESPT emission at longer wavelengths gives no indication of contributions from n < 4. (C) 1998 Elsevier Science B.V. All rights reserved.