The fluorescence emission spectra and fluorescence quantum yields of hemicyanine dyes LDS 698, LDS 722, and LDS 730 were measured in different media. No transient species was detected in the laser flash-photolysis experiments performed with Ar-saturated solutions of the dyes in methanol. However, in the presence of 0.08 M potassium iodide, the absorption of the triplet states was clearly observed. Oxygen consumption measurements in the absence and presence of a chemical trap (furfuryl alcohol) in MeOH : H2O (ϕr = 1 : 1) solutions of the dyes containing KI confirmed the generation of singlet molecular oxygen.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas