Spectral and photophysical properties of the δ-carboline anhydrobase (N1-methyl-5H-pyrido[3,2-b]indole)

The spectral and photophysical properties of the delta-carboline anhydrobase or N-1-methyl-5H-pyrido[3,2-b]indole, Ndelta-C, have been studied by using absorption and fluorescence spectroscopies. In non-aqueous solvents, the longer wavelength absorption bands of Ndelta-C shift to the blue as the solvent polarity and solvent hydrogen-bonding donor properties increase and its fluorescence spectra show dual emission and non-exponential decays. To account for these experimental results a singlet state diagram involving, coupling of npi*-pipi* states, charge transfer and geometrical relaxation is proposed. In aqueous acidic media, the changes in the absorption and fluorescence spectra with acidity can be rationalised on the basis of two fluorescent ground state conformational isomers of cationic nature. The normal one, where the positive charge is on the pyridinic nitrogen, and its conformational isomer, with the positive charge localised on the pyrrolic nitrogen. (C) 2004 Elsevier B.V. All rights reserved.