Proton transfer in 7-hydroxyquinoline·(NH[sub 3])[sub n] solvent clusters.

Mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline·(NH[sub 3])[sub n] clusters with n=2-16. For n=2 and 3, the absorption and emission spectra exhibit discrete and narrow bands, characteristic of nonreactive hydrogen bonded ammonia-chain clusters. For n>=4, the S[sub 1]←S[sub 0] R2PI spectra are completely broadened, with an onset at approx. 355 nm. For n=4-7, a weak fluorescence emission is observed, Stokes-shifted by approx. 185 nm, with a maximum at approx. 540 nm, which shows discrete structure on a broad background. From comparison to fluorescence emission observed in bulk solution, we conclude that S[sub 1] state enol→keto tautomerization occurs. For the n>=7 or 8 clusters, the fluorescence emission spectra become completely unstructured and shift to the blue, peaking at 435-450 nm. This emission indicates the occurrence of either S[sub 1]-state proton transfer to the ammonia solvent cluster and formation of the 7-HQ anion, or of the ground-state intracluster enol→keto tautomerization reaction. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]