Photochemistry of Flavins. Ii. Photophysical Properties of Alloxazines and Isoalloxazines

Pulsed laser photolysis at 347nm has been used to study the transient spectroscopy of alloxazine, lumichrome, lumiflavin, and riboflavin in acidic (pH 2.2) aqueous solution and in ethanol. Intersystem crossing quantum yields (φ ISC ) were determined by a modification of the comparative laser excitation method which utilizes the variation of the triplet yield with intensity in conjunction with a kinetic model for the various photophysical and photochemical processes occurring during the pulse. Fluorescence quantum yields and lifetimes are also reported. Correction for quenching of the excited singlet state by H + ions shows that, in neutral aqueous solution, intersystem crossing for flavins is an efficient process (φ ISC ˜ 0.7) which, in conjunction with fluorescence, accounts for the fate of all absorbed photons. For alloxazine (φ ISC ˜ 0.45) and lumichrome (φ ISC ˜ 0.7) the results are more difficult to interpret owing to interconversion between alloxazine and isoalloxazine structures in the singlet excited state. For all four compounds, the quantum yield of products derived from the singlet excited state is estimated as ˜0.04. There is evidence of biphotonic product formation at high laser energies. In ethanol, where φ ISC for lumichrome is about twice that of lumiflavin, internal conversion between the excited singlet and ground states appears to be a significant process. Complete triplet‐triplet absorption spectra in the region 260–750nm are reported. For lumichrome at pH 2.2 there is spectral evidence for isomeric triplet states which appear to be in equilibrium.