Ultrafast Investigation of Vibrational Relaxation in the S1 Electronic State of HITC

The condensed phase relaxation dynamics of electronically excited 1,1‘,3,3,3‘,3‘-hexamethylindotricarbocyanine iodide HITC (a cyanine dye) has been examined by transient bleach/stimulated emission experiments. These measurements were performed using tunable pump and probe laser pulses with ～200 fs time resolution. The dynamics observed was assigned to vibrational relaxation in the S₁ state of HITC. Solvation effects make a negligible contribution to these experiments because the dipole moment of HITC only changes by a small amount when the S₁ ← S₀ transition is excited. Experiments performed with variable wavelength pump and probe pulses show that vibrational relaxation is faster at high energies in the S₁ state. At low energies in the S₁ state the vibrational relaxation times depend on the solvent. The measured relaxation times at low energies are1.7 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 increases with solvent dipole moment. To explain these observations, we propose that the torsional motion associated with isomerization in the S₁ state of HITC is responsible for vibrational deactivation at low energies. Rotation about the central C?C bond in the HITC polyene chain produces an internal charge transfer state, creating a large dipole moment along the long axis of the molecule. Thus, the torsional vibration generates an oscillating dipole which can couple to the dipole moments of the solvent molecules, providing a mechanism for energy exchange between HITC and the solvent. The rate of energy exchange will increase with the solvent dipole moment and decrease with the solvent viscosity because highly viscous solvents hinder the torsional vibration and, so, reduce the magnitude of the induced dipole moment in HITC.