Molecular Beam Studies of the “Super” Photoacid 5-Cyano-2-naphthol in Solvent Clusters

Clusters of the strong excited-state acid 5-cyano-2-naphthol with water, ammonia, methanol, and Me 2 SO were generated in a molecular beam and investigated by resonant two-photon ionization (R2PI) and fluorescence spectroscopies. In the free molecule and the smallest complexes these show that the S 1 state is highly analogous to that of the parent 2-naphthol. Fluorescence attributable to excited-state proton transfer (ESPT) was observed for ammonia and water clusters, but not methanol or Me 2 SO. The ESPT size thresholds are, for ammonia 3 or 4; for water 8−10. The cluster ESPT characteristics of 5-cyano-2-naphthol show many similarities with those of the somewhat weaker excited-state acid 1-naphthol.