Highly birefringent liquid-crystalline polymers for photonic applications: synthesis of liquid-crystalline polymers with side-chain azo-tolane mesogens and their holographic properties

Abstract

We have synthesized highly birefringent liquid-crystalline polymers (LCPs) containing an azobenzene group directly connected to a tolane moiety (P3ATm), in which the alkyl spacer length (CH₂)_m was varied as m = 6, 9 and 12. All LCPs showed a nematic phase in a broad temperature range (>170 °C). The homogeneously aligned LCP films showed a high value of birefringence (about 0.4) and a large change in the birefringence could be induced by photoinduced alignment change. Grating formation of LCPs has been explored in a glassy state. The diffraction efficiency (η) showed a maximum when an order–disorder change was completed in the bright areas of the interference patterns. Studies on the effects of light intensity on the grating formation suggested that the diffraction efficiency was high (η = 20%) even though the alignment change of the azo-tolane moieties was small. Furthermore, the behavior of the grating formation was found to be dependent on the alkyl spacer length of P3ATm.