Studies of the photophysics and spectroscopy of alloxazine and related compounds in solution and in the solid state

The absorption and emission spectra and the fluorescence lifetimes and quan tum yields of alloxazine and several of its N- and C- methyl-substituted de rivatives, including two isoalloxazines, have been measured in a nonpolar s olvent (1,2-dichloroethane), a polar aprotic solvent (acetonitrile), and a polar protic solvent (ethanol). The excited state decays are all single exp onential, suggesting that only one emitting species is present in all cases . The spectroscopic data show that the emitting species is the pi,pi* elect ronic excited state corresponding to the ground state of the absorbing mole cule; there is no evidence of excited state proton transfer for any of thes e solutes in any of the three solvents. The longer lifetimes and larger flu orescence quantum yields of the isoalloxazines compared with the alloxazine s can be attributed exclusively to significantly slower rates of radiationl ess relaxation in the excited isoalloxazines. The remission and emission sp ectra of these same compounds adsorbed on cellulose are similar to the corr esponding absorption and emission spectra in homogeneous solution, suggesti ng that the emitting species are the same in the adsorbed state as they are in homogeneous solution. However, the adsorbed excited species exhibit non exponential temporal decay, attributed to inhomogeneities in the binding of the chromophore to cellulose. The emission spectra of polycrystalline samp les of alloxazine and lumichrome suggest the possible occurrence of double intermolecular excited state proton transfer, but this interpretation is no t supported by the further photophysical data obtained here.