Three-Dimensional Nonlinear Optical Chromophores Based on Through-Space Delocalization

Six permutations of 4-fold donor and/or acceptor substitution of paracyclophane at the 4, 7, 12, and 15 positions were synthesized to probe the phenomenon of three-dimensional delocalization on the nonlinear optical properties of organic materials. The interplay between through-bond intramolecular charge transfer (ICT) as well as three-dimensional, or through-space, ICT processes gives rise to large quadratic hyperpolarizability values. The determination of dipolar (beta(J)(=1)) and octupolar (beta(J)(=3)) irreducible tensor contributions to the overall beta tensor value is made possible by the polarized harmonic light scattering technique at 1.32 microm. The electric field-induced second-harmonic generation technique was also used at 1.91 microm for comparison. Significant experimental beta values for members of the series made of two centrosymmetric benzene-like units are a clear signature of a purely through-space ICT between two aryl subunits. The two configurational isomers that pair two dipolar donor-acceptor chromophores also exhibit octupolar character. Analysis of these two with an additive model for beta(J)(=1) and beta(J)(=3) reveals a strong three-dimensional inter-ring charge transfer.