7-AZAINDOLE - THE LOW-TEMPERATURE NEAR-UV/VIS SPECTRA AND ELECTRONIC-STRUCTURE

7-Azaindole is isolated in an argon matrix and its absorption, fluorescence and phosphorescence are analyzed by comparison with indole spectra and INDO/Sl calculations. The absorption origin is a distinct band at 289.9 nm, but we could not identify the second {pi*,pi} transition in the spectrum. We predict it to be weak and near-degenerate with a pyridine-type {pi*,n} transition. The origin of the well-structured phosphorescence emission, at 414.9 nm, is strongly blue-shifted in comparison to the previously reported phosphorescence of 7-azaindole in hydrocarbon glasses. A long phosphorescence lifetime, tau = 3.78 s, indicates the S-0{pi} <-- T-1{pi*} radiative transition. The Phi(P)/Phi(F) ratio in 7-azaindole is smaller by an order of magnitude in the argon matrix and two orders of magnitude in glycerol/water glass, in comparison to that of indole. The sensitivity to protic media indicates that an S-0{n} <-- T-k {pi*} transition is responsible for the emission loss. No dimerizaton of 7-azaindole and tautomerization to 7H-pyrrolo[2,3-b]pyridine was observed in the argon matrix or glycerol/water glass; however, association with indole is suggested by phosphorescence band shifts and emission loss. It is expected that these features will further the use of 7-azaindole as a spectroscopic marker in a variety of environments.