Anisotropy of third-order nonlinear optical responses in metallophthalocyanine aggregates

Abstract

Anisotropy of transient absorption change and electroabsorption response in metallophthalocyanine (MPc) aggregates were investigated in terms of the influence of intermolecular interaction to the 2D character of the molecule. In an MPc aggregate intermolecular interaction with less symmetry might result in appearance of 1D character, depending on the nature of interaction and arrangement of molecules. In experiment, polarization- dependent electroabsorption response in various MPcs doped in PMMA presented a 1D character at the red-shifted absorption band due to coplanar intermolecular interaction and a 2D one at the blue-shifted band due to cofacial interaction. Polarization-dependent subpicosecond transient absorption change in various aggregate vanadylphthalocyanine, evaporated and doped-polymer films in phase-I and phase-II aggregation, presented a variety of anisotropy: in phase-I aggregate doped in polystyrene, a wavelength-dependent anisotropy similar to that of electroabsorption response was obtained; in phase-I aggregates doped in PMMA or evaporated, a 2D character was observed in almost whole wavelength region; in phase-II aggregates both 1D and 2D characters were in the red-shifted absorption band. These result are partly elucidated by the exciton coupling model in 2D molecules. It is also suggested that pump wavelength dependence of anisotropy and existence of two or more aggregate species might play important rolls.