Large Dynamic Ranges in Photorefractive NLO Polymers and NLO-Polymer-Dispersed Liquid Crystals Using a Bifunctional Chromophore as a Charge Transporter

Abstract

We report on the synthesis and characterization of two photorefractive side-chain polymethacrylates, functionalized with amino-donor-containing chromophores. Carbazole units were neither copolymerized nor added to the polymers as separate components, and charge transport was provided by the chromophores. The polymer glass transition temperature was lowered to room temperature with appropriate plasticizers. After the addition of a sensitizer, (2,4,7-trinitro-9-fluorenylidene)malononitrile (TNFM) or C 60 , a charge-transfer (CT) complex is formed between the chromophore and sensitizer, and photoconductivity can be observed upon excitation of the CT complex at 680 and 780 nm. The excellent photorefractive properties at both wavelengths were probed using the techniques of four-wave mixing and two-beam coupling. We show that the grating buildup speed is dominated by the charge generation. Because of the absence of carbazole units, the chromophore number densities in the samples are very high (up to 15.3 x 10 20 cm -3 ), leading to refractive index modulation amplitudes exceeding 5 x 10 -3 at only 50 V/μm and 780 nm, which is among the best results obtained so far in polymeric photorefractive media. Meanwhile, the fully functionalized polymers offer the major advantage of a very good phase stability. As a proof of principle, we have also prepared polymer-dispersed liquid crystals (PDLC) using the bifunctional NLO polymers as the polymer matrix. PDLCs were prepared by a thermally induced phase separation after addition of 20 wt % of a liquid-crystal molecule that shows a nematic phase at room temperature. In the PDLC, a refractive index modulation amplitude of 1.8 x 10 -2 was observed at 780 nm and 50 V/μm. To the best of our knowledge, this exceeds the refractive index modulation amplitudes observed so far in fully functionalized polymers at 780 nm.