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.