The upper triplet states of Merocyanine 540 and related cyanine dyes have been generated by a two-color, two-pulse method. The decay of this state partitions between internal conversion to T1 and remarkably efficient reverse intersystem crossing to the singlet manifold. Thus, UV or visible laser generation of the T1 state (either directly or by a sensitized route) followed by excitation with a 640 nm pulse ultimately results in fluorescence and isomerization from the lowest excited singlet state (S1), as evidenced by the production of the ground state photoisomer absorption and emission coincident with the second laser pulse. Since identical behavior (isomerization and fluorescence) is observed for direct excitation into the singlet manifold, we conclude that both the singlet and triplet states are of the same geometry and that isomerization does not occur directly from either T1 or T n .

The photophysical properties of 3,3'-dialkylthiacarbocyanine iodides and chlorides were measured in various solvents. It was found that photoisomerization and fluorescence are the major contributors to the deactivation of the excited singlet state; intersystem crossing occurs with only a very low efficiency. In ethanol, a triplet yield of 0.004 and a singlet oxygen quantum yield of 0.002 were determined. The photophysical parameters of these dyes are not substantially influenced by the length of the alkyl chain or the size of the halide counterion. The substitution of an ethyl with an octadecyl-chain only slightly hinders photoisomerization, and the replacement of the chloride with an iodide reduces only marginally the fluorescence lifetimes and fluorescence quantum yields in chloroform. A significant external heavy-atom effect is observed using dibromoethane as a solvent: triplet and singlet oxygen yields increase 7-10-fold, and the triplet lifetime decreases from 55 microseconds to 15 microseconds.