Spectroscopy and photophysics of flavin-related compounds: Isoalloxazines

Electronic structure and S0-Si, T1-Ti, and S0-Ti transition energies and oscillator strengths were calculated using the TD-DFT method for a series of tetramethyl-substituted isoalloxazines. The order of excited states remains generally the same in these and other isoalloxazines, apart from the cases where the steric interaction of the substituents renders the molecule slightly non-planar. The two lowest close-lying singlet excited states, determining the photophysical properties, have n,π* and π,π* character, with their exact ordering depending on the substitution pattern. The relatively strong dependence of the photophysical properties of these compounds of solvent properties is interpreted in terms of the proximity effect of these two lowest excited states. The calculated results are compared to the experimental spectral and photophysical properties.