Spectroscopy and photophysics of cyanoalloxazines. Theoretical study

Electronic structure and the S0-Sn and T1-Tn absorption spectra were calculated using the Time-dependent density functional theory (TD-DFT) approach for a series of cyanoalloxazines. The results correspond quite well to the experimental spectroscopic parameters, available for 7- and 8-cyanoalloxazines. The lowest singlet excited states of the 6-, 7-, 8-, and 9-cyanoalloxazines are predicted to have the n,π* character, with the second excited state of the π,π* character, located about 1500 cm−1 higher, and with a much higher oscillator strength. The two lowest singlet excited states are believed to be strongly coupled due to the proximity effect, determining the high non-radiative decay rates from the S1 state and an enhanced intensity of the experimental S0-S1 band as compared to calculated oscillator strengths. The present theoretical results, obtained in the UB3LYP/6-31G(d) unrestricted formalism, were compared to those previously obtained for other alloxazine derivatives.