(PHOTO)ISOMERIZATION DYNAMICS OF MEROCYANINE DYES IN SOLUTION

The dynamics for isomerization of the first excited singlet state and of an unstable ground state conformer have been measured for a merocyanine dye in a variety of solvents. It is shown that the rates of both isomerizations depend on the temperature, viscosity and polarity of the surrounding medium. These effects can be explained in terms of the large-scale torsional motion and change in dipole moment that accompany isomerization. As the barrier to isomerization of the first excited singlet state increases, fluorescence intensifies but there is no accompanying increase in triplet state population. The rate of decay of the triplet state depends on polarity of the solvent, due to solvent-induced changes in the energy gap, but not on viscosity. These various findings are discussed in terms of a potential energy diagram that involves intermediate population of a "twisted" excited singlet state.