Influence of photoisomerization on saturated absorption of 3,3′-diethyloxadicarbocyanine iodide (DODCI) studied by diffraction from laser-induced, anharmonic thermal gratings

Saturated optical absorption of 3,3′-diethyloxadicarbocyanine iodide (DODCI) in alcohol solvents has been studied using the laser-induced anharmonic thermal grating technique. The probe angle dependence of diffraction intensity is resolved into two Gaussian profiles, centered at first and second Bragg incidence, respectively. The diffraction transients observed at these angles have time dependences described by single exponentials with a 1: 1 4 ratio of time constants, in agreement with thermal relaxation of an anharmonic Bragg grating. The excitation intensity dependence of the diffraction signals is interpreted using Rullière's kinetic model with an assumption that the non-radiative transition of the excited photoisomer proceeds mainly via internal conversion. A set of kinetic equations for Rullière's model has been solved numerically to describe the time evolution of the populations of all levels under a step pulse excitation. The weaker absorption by the photoisomer and reverse-photoisomerization have made the effective saturation intensity of DODCI very large in comparison with the predictions of a simple three-level model. The absorption cross section of DODCI photoisomer in ethanol is estimated from this work to be (4.7 ± 0.2) × 10^-18 cm² (at 532 nm), the back-photoisomerization yield is 0.017 ± 0.005 in methanol, and an upper limit to the photoisomer excited state lifetime is 1.3 ± 0.3 ns in ethylene glycol.