Isomerization dynamics of 1,1′-diethyl-4,4′-cyanine (1144C) studied by different third-order nonlinear spectroscopic measurements

The isomerization of 1, 1-diethyl-4,4 ' -cyanine (1144C) in two solvents, ethanol and hexanol, was studied by three different third-order four-wave mixing techniques: three-pulse photon echo peak shift (3PEPS), transient grating (TG), and pump-probe (PP) to provide complimentary information on different aspects of the reaction dynamics. A double-sided Feynman diagram analysis was used to assist in the formulation of the kinetic behavior in the TG and PP signals. The ground-state recovery process can be described as a single exponential process which is dominated by the bond-twisting motion on the excited-state surface. The rate is strongly viscosity and temperature dependent. A larger yield of the isomer is obtained in less viscous solvents or with greater excess excitation energy. Apparently, contradictory wavelength-dependent decay behavior was found in the TG and PP signals. This can be understood by considering the different real and imaginary contributions to the signals from hot ground-state absorption and its subsequent recovery to the equilibrated state and from the product state.