Whereas commonly studied organic molecules for nonlinear optics contain an electron-donor group D and an electron-acceptor group A linked by a pi-conjugated system, we now report the behaviour of some molecules in which a rigid, saturated hydrocarbon bridge separates D and A groups by at least three sigma-bonds. The electronic spectra of these systems and their hyperpolarizability values as determined by second-harmonic-generation (SHG) measurements reveal that the chromophores cannot be regarded as electronically isolated, but that a nonnegligible electronic interaction occurs, probably mediated via the interconnecting sigma-bonds. The relative weakness of this through-bond interaction results in a smaller value for the oscillator strength of the intramolecular charge-transfer transition in such nonconjugated D/A systems as compared to fully conjugated ones. The lowering effect of the smaller transition dipole moment on the magnitude of the molecular hyperpolarizability, beta, appears to be partly compensated, however, by the large difference between ground- and excited-state dipole moment achievable in these systems.
