Singlet oxygen production and photoisomerization: two competitive processes for merocyanine 540 irradiated with visible light.

The quantum yields of singlet oxygen production by merocyanine 540 have been measured during visible light irradiation performed in methanol and ethanol. These appear to be one hundred times smaller than the quantum yield for rose bengal measured under the same conditions. Flash photolysis experiments demonstrate the ability of merocyanine 540 molecules to isomerize under visible light irradiation: the isomerization quantum yields are about 0.65 in both ethanol and methanol. This information combined with the fluorescence quantum yield data account for the low values for singlet oxygen production.