On illumination flavin reacts from its triplet state with dihydroaromatic systems at room temperature yielding 1,5-dihydroflavin. Substrates which are substituted with methyl groups to hinder aromatisation (3,3-dimethyl-1-phenyl-1,4-cyclohexadiene, ergosterol, N-methylacridan, 1,3,10-trimethyl-1,5-dihydro-5-deazaisoalloxazine) yield at -40 °C 4a-substituted 4a,5-dihydroflavins (adducts), which on warming split homolytically, yielding a 1:1:1-mixture of 1,5-dihydroflavin/starting flavin/dimerized substrate after disproportionation and dimerisation, resp. In the case of unblocked substrates these adducts are not UV-detectable even at -80 °C but split heterolytically, yielding 1,5-dihydroflavin and oxidized substrate in a 1:1-ratio.