Third-order nonlinear optical properties of fractal- and nonfractal-structured oligomers modeled after dendron parts in Cayley-tree-type dendrimers

Abstract

As an extension of our previous study on polarizabilities [J. Chem. Phys. 115, 1052 (2001)], we investigate the molecular third-order nonlinear optical properties, i.e., the second hyperpolarizabilities (γ), of three types of model oligomers (made of phenylene vinylenes), i.e., para-oligomers and meta-oligomers with fractal and nonfractal structures, which are considered to be models of dendron parts involved in Cayley-tree-type dendrimers. Similarly to the polarizability case, the chain-length dependence of γ of para-oligomers is found to be much larger than that of nonfractal meta-oligomers, while fractal meta-oligomers exhibit an intermediate chain-length dependency of γ between them. The spatial contributions of π-electrons to γ of these systems are elucidated using hyperpolarizability density analysis. It is found that the spatial contributions of π-electrons to γ of nonfractal meta-oligomers are much smaller than those of para-oligomers and are well decoupled at the meta-substituted benzene rings, while the fractal meta-oligomers show significantly large spatial contributions of π-electrons to γ in their linear-leg (para-substituted phenylene vinylene) regions similarly to the case of para-oligomers although the distributions are also well decoupled at the meta-substituted benzene rings. This feature supports the distinct features of chain-length dependency of γ for these oligomers.