Nonlinear Optical Properties and Liquid-Crystalline Behavior of New Polyesters with Dipole Moments Aligned Transverse to the Backbone

Abstract

Side-chain azobenzene-based second-order nonlinear optical (NLO) polyesters (PNPAR, PNPAPAR, PPAR-TC, PPAPAR-TC) containing nitro or tricyanovinyl groups as acceptors with their dipole moments aligned transverse to the polymer backbone, were synthesized and characterized, and their thermal properties, liquid-crystalline (LC) phase behavior, and NLO properties studied. Polymers containing nitro groups as acceptors were found to exhibit LC properties, while those containing tricyanovinyl groups as acceptors did not. The LC polymers were of smectic nature. The polymer PNPAR with a nitro group, when poled at the LC phase-transition temperature range, was found to exhibit higher NLO activity than that with a tricyanovinyl group (PPAR-TC). When polymer PNPAPAR containing a nitro group, also exhibiting an LC phase, was poled at a temperature different from that of its transition temperature, the NLO activity observed was comparable to, and/or lower than, that of the similar polymer (PPAPAR-TC) having a tricyanobvinyl group in the place of the nitro group. All the polymers were found to possess moderate temporal stability of second harmonic generation activity and were found to have stability over the whole process.