Studies of excited-state absorption and photoisomerization of cyanine dyes by using laser-induced anharmonic thermal gratings

The laser-induced anharmonic thermal grating technique has been used to study the excited-state absorption and photoisomerization of two cyanine dyes, 3,3′-diethyloxatricarbocyanine iodide (DOTCI) and 1,3,3,1′,3′,3′-hexamethylindotricarbocyanine iodide (HITCI). An anomalous excitation intensity dependence of the diffraction efficiency is observed for both dye molecules; the anomaly is caused by diffraction from multiple thermal gratings which are 180° out of phase. The previously developed theory for anharmonic thermal gratings based on a quasi-plane-wave approximation (QPWA) cannot quantitatively fit the measured results. A more recently developed, general numerical method for calculating diffraction from anharmonic thermal gratings accounts for the observed anomalous behavior in terms of an excited-state and trap-state formation model. For DOTCI, the nonlinear absorption in ethanol is caused by the saturation of the ground-state absorption and strong excited-state absorption, while for HITCI, in addition to these two absorption processes, photoisomer absorption may also be an important process contributing to the nonlinear absorption at the excitation wavelength (532 nm).