Spectroscopy and photophysics of trimethyl-substituted derivatives of 5-deazaalloxazine. Experimental and theoretical approaches

Photophysical and structural properties of 5-deazaalloxazine derivatives were studied, with methyl substituent at the position 8 or 9 of the benzene ring and at the N(1) and N(3) positions. Crystal structures of the compounds were determined by X-ray diffraction method. onic structure and S0 → Si, S0 → Ti and T1 → Ti transition energies and oscillator strengths were calculated for 1,3,8- and 1,3,9-trimethyl-5-deazaalloxazine, using the TD-DFT methods. The calculations suggest that for 5-deazaalloxazine derivatives the lowest-lying singlet excited states have pure π,π∗ nature. This is in contrast to the alloxazine derivatives, where two lowest-lying transitions are of π,π∗ and n,π∗ character, being isoenergetic in most cases. The theoretical data are compared to the experimental results, involving steady-state and time-resolved spectra. 1,3,8-Trimethyl-5-deazaalloxazine and 1,3,9-trimethyl-5-deazaalloxazine have longer fluorescence lifetimes and higher values of fluorescence quantum yield compared to the alloxazine derivatives in the same solvents, whereas the absorption maxima and emission maxima of studied compounds are blue-shifted as compared to trimethyl derivatives of alloxazine.