EXCITED-STATE RELAXATION IN CYANINE DYES - A REMARKABLY EFFICIENT REVERSE INTERSYSTEM CROSSING FROM UPPER TRIPLET LEVELS

The upper triplet states of Merocyanine 540 and related cyanine dyes h ave been generated by a two-color, two-pulse method. The decay of this state partitions between internal conversion to T-1 and remarkably ef ficient reverse intersystem crossing to the singlet manifold. Thus, UV or visible laser generation of the T-1 state (either directly or by a sensitized route) followed by excitation with a 640 nm pulse ultimate ly results in fluorescence and isomerization from the lowest excited s inglet state (S-1), 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 ob served for direct excitation into the singlet manifold, we conclude th at both the singlet and triplet states are of the same geometry and th at isomerization does not occur directly from either T-1 or T-n.