Second- and third-order nonlinear optical properties of chiral phthalocyanine films

Vanadyl phthalocyanine derivatives having optically active side chains and the corresponding racemic isomers were synthesized and examined as nonlinear optical materials. These dyes were soluble in organic solvents and gave uniform thin films using spin coating. The thin films (neat or polymer doped) of each phthalocyanines showed the second- and third-order nonlinear optical responses under appropriate experimental conditions. The nonlinear optical susceptibilities of the optically active derivatives are larger than those of the corresponding racemic isomers. To clarify this enhancement phenomenon, we measured the electronic absorption- and circular dichloic spectra, and X-ray diffraction of the thin films. These results suggested that the optically active dyes forms one-dimensional columnar aggregates with one-handed helical sense and the columns further aligned into honeycomb-like chiral superstructures. It was surmised from the experimental results that the chiral superstructures enhance the nonlinear optical responses relative to the racemic analogues.