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 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 Tn.