Electronic spectra of 7-azaindole/ammonia clusters and their photochemical reactivity

The S1-S0 electronic spectra of 7-azaindole-(NH3)n clusters (n=1-3) were measured by mass-selected two-color resonance-enhanced multiphoton ionization spectroscopy. The laser-induced fluorescence spectrum obtained by monitoring the UV fluorescence shows well-structured vibrational bands for the monomer and 7-azaindole-(NH3)1,2 clusters, while no signals appear for the 7-azaindole-(NH3)3 cluster. The action spectrum obtained by monitoring visible emission shows no signal for all species, which suggests little reactivity for excited-state proton/hydrogen transfer. From the observed and calculated IR spectra, the geometry of 7-azaindole-(NH3)1,2 was concluded to be a hydrogen-bonded bridge form, which is similar to the photochemically reactive 7-hydroxyquinoline-(NH3)3 cluster. The difference in the photochemical reactivity is discussed on the basis of excited-state quantum chemical calculations.