Electronic spectra and photophysics of δ-carboline (5H-pyrido[3,2-b]indole) Original Research Article

The absorption, excitation, fluorescence and phosphorescence spectra of δ-carboline, 5H-pyrido[3,2-b]indole, have been investigated in non-aqueous solvents of various polarity at room temperature and 77 K. The δ-carboline fluorescence shows time-resolved red shifts, solvatochromic effects, non-exponential decays and anomalous temperature dependence. These results suggest that the fluorescence emission of δ-carboline originates from two close low lying excited states which do not equilibrate on the time scale of fluorescence emission: a non-polar locally excited state (LE) and a polar intramolecular charge transfer state (CT). The contributions of the LE and CT states to the total fluorescence spectrum of δ-carboline progressively change as the solvent polarity increases. The possibility of a vibronically induced coupling of these excited states could also reasonably explain the reduction of the fluorescence quantum yield and the increase of the phosphorescence quantum yield of δ-carboline as compared with carbazole.