Electronic spectra and photophysics of the α-carboline (1-azacarbazole) monomer.

The UV-vis electronic absorption and emission spectra of α-carboline or 1-azacarbazole, 9H-pyrido[2,3-b]indole, AC, have been investigated in aprotic solvents. Radiative, k(r), non-radiative, k(nr), rate constants and natural lifetimes, τ(N), of the AC monomer in hexane and acetonitrile, obtained from the experimentally determined fluorescence quantum yields and fluorescence lifetimes, have been compared with those theoretically estimated. The closeness between these experimental and theoretical data, the small Stokes shifts, the mirror image relationship between the absorption and fluorescence spectra and the close correspondence between the absorption and fluorescence excitation spectra, provide good evidences that the emission of AC monomer occurs directly from its lowest singlet excited state. The mono- and multi-parametric analyses of the AC solvatochromism indicate that the polarity-polarizability, the hydrogen bond donor and the hydrogen bond acceptor properties of the solvent preferentially stabilize the singlet excited over the ground state. These analyses also reveal that photoexcitation reinforces the hydrogen bond donor and acceptor properties of the AC, becoming the pyridinic nitrogen atom more basic and the pyrrolic group more acid.