Radiationless Transitions in Cyanine Dyes

The quantum yields of fluorescence and the maximum quantum yield of triplet produced have been measured for three cyanine dyes in thin films of polyvinyl alcohol. The fraction of excited dye molecules which neither fluoresce nor undergo intersystem crossing to the triplet state and which, therefore, must undergo a nonradiative transition from the excited singlet state to the ground state is equal to or greater than 0.86±0.05 for 1,1′-diethyl−2,2′-cyanine iodide, 0.82±0.04 for 1,1′-diethyl−6-bromo−2,2′-cyanine iodide, and 0.64±0.06 for 1,1′-diethyl−6-iodo−2,2′-cyanine iodide.