ULTRAFAST INVESTIGATION OF VIBRATIONAL-RELAXATION IN THE S-1 ELECTRONIC-STATE OF HITC

The condensed phase relaxation dynamics of electronically excited 1,1' ,3,3,3',3'-hexamethylindotricarbocyanine iodide HITC (a cyanine dye) h as been examined by transient bleach/stimulated emission experiments. These measurements were performed using tunable pump and probe laser p ulses with similar to 200 fs time resolution. The dynamics observed wa s assigned to vibrational relaxation in the SI state of HITC. Solvatio n effects make a negligible contribution to these experiments because the dipole moment of HITC only changes by a small amount when the S-1 <-- S-0 transition is excited. Experiments performed with variable wav elength pump and probe pulses show that vibrational relaxation is fast er at high energies in the S-1 state. At low energies in the S-1 state the vibrational relaxation times depend on the solvent. The measured relaxation times at low energies are 1.7 ps in acetonitrile, 3.2 ps in dimethyl sulfoxide, 2.4 ps in methanol, 3.5 ps in ethanol, 7.1 ps in 1-butanol, and 6.4 ps in ethylene glycol. These results show that the vibrational relaxation rate decreases with solvent viscosity and incre ases with solvent dipole moment. To explain these observations, we pro pose that the torsional motion associated with isomerization in the S- 1 state of HITC is responsible for vibrational deactivation at low ene rgies. Rotation about the central C-C bond in the HITC polyene chain p roduces an internal charge transfer state, creating a large dipole mom ent along the long axis of the molecule. Thus, the torsional vibration generates an oscillating dipole which can couple to the dipole moment s of the solvent molecules, providing a mechanism for energy exchange between HITC and the solvent. The rate of energy exchange will increas e with the solvent dipole moment and decrease with the solvent viscosi ty because highly viscous solvents hinder the torsional vibration and, so, reduce the magnitude of the induced dipole moment in HITC.