Photophysical properties of structurally and electronically modified flavin derivatives determined by spectroscopy and theoretical calculations

Four different riboflavin (RF) derivatives, two electronically modified compounds (1- and 5-deazariboflavin, 1DRF and 5DRF) and two sterically modified compounds (7,8-didemethyl- and 8-isopropylriboflavin, DMRF and iprBF), were subjected to a combination of time-resolved measurements (absorption and fluorescence) and high-level quantum chemical investigations. Both alkyl-modified flavins showed similar fluorescence properties as the parent compound, yet 5DRF had a larger quantum yield of fluorescence (Φ_F = 0.52) than RF (Φ_F = 0.27). Interestingly, 1DRF did not show fluorescence at all under these steady state conditions. The triplet quantum yield was different for the modified flavins such that no triplet formation was found for 1DRF, whereas the other compounds all formed triplet states (Φ_TR for 5DRF of 0.64 and 0.50 and 0.23 for iprRF and DMRF, respectively). The triplet states of the two alkyl-modified flavins decayed with similar time constants as the parent compound, whereas a shorter lifetime was measured for 5DRF (τ_TR = 15 μs, compared to τ_TR = 29 μs for RF). In the calculations, the flavin derivatives were modeled as lumiflavins, that is, without the ribityl chain. We conclude that for aqueous solutions of DMRF, iprRF, and 5DRF intersystem crossing (ISC) takes place from the S^{1 1}(ππ^*) to the T₂ ³(ππ^*) state by a vibronic spin-orbit coupling mechanism, a process common to most flavins, whereas ISC is slow in excited 1DRF due to the absence of a close-by triplet state.