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.