ODD-EVEN OSCILLATIONS OF SHG EFFICIENCIES IN TWIN NLO CHROMOPHORES

Twin NLO chromophores having two azobenzene units linked by a flexible polymethylene spacer of varying lengths are shown to exhibit odd-even oscillations in their second harmonic generation (SHG) efficiencies, when measured in the powder form. These twin systems were designed to also exhibit liquid cystallinity, and indeed most of them do exhibit a nematic mesophase. The anticipated odd-even oscillations, in both the ir isotropization transition temperatures (Ti) and isotropization entr opies (Delta Si), were also observed. The odd-even oscillation of the SHG efficiencies has been ascribed to a more effective cancellation of mesogenic dipoles in the even twins as compared to their odd counterp arts, due to a preferred centrosymmetric packing in the former case. B ased on the behaviour of these twin chromophoric molecules, it may be anticipated that such odd-even oscillations will also be observed in t he analogous main chain NLO polymers.