SINGLET OXYGEN PRODUCTION AND FLUORESCENCE YIELDS OF MEROCYANINE-540 - A COMPARATIVE-STUDY IN SOLUTION AND MODEL MEMBRANE SYSTEMS

Singlet oxygen and fluorescence quantum yields of merocyanine 540 were measured in solution (methanol, ethanol, n-heptanol) and in model membrane systems (cationic micelles, unilamellar dimyristoyl- and dipalmitoylphosphatidylcholine vesicles). Both singlet oxygen quantum yields and fluorescence quantum yields increase with increasing viscosity/rigidity of the surrounding medium: the yield of singlet oxygen production (24-degrees-C) goes from 0.002 in methanol to 0.04 in dipalmitoylphosphatidylcholine vesicles, and fluorescence yields (25-degrees-C) change from 0.14 to 0.61 in the same media. The data are consistent with previous findings that photoisomerization is in direct competition with intersystem crossing and radiative relaxation. Therefore, a singlet oxygen yield close to the maximum value of 0.11 can only be achieved after both photoisomerization and internal conversion are prevented by a highly viscous environment.