Photorefractivity in Nematic Liquid Crystals Using a Donor−Acceptor Dyad with a Low-Lying Excited Singlet State for Charge Generation

The photorefractive effect was observed in room-temperature nematic liquid crystal composites consisting of a eutectic mixture of 65% 4‘-( n -pentyl)-4-cyanobiphenyl and 35% 4‘-( n -octyloxy)-4-cyanobiphenyl doped with a donor−acceptor dyad in which 9-( N -pyrrolidinyl)perylene-3,4-dicarboximide is directly linked to 1,8:4,5-naphthalenetetracarboxydiimide. This dyad has a broad, intense absorption band that extends from 500 to 750 nm. Measurements of two-beam asymmetric energy exchange (beam-coupling) and the diffraction efficiency of the refractive index grating produced in these liquid crystal thin films were carried out. A comparison is made between the covalent donor−acceptor dyad and the corresponding noncovalent donor−acceptor pair. In each case the covalent dyad outperforms the noncovalent donor−acceptor pair. The results show that aminoaryleneimides are promising new charge donating chromophores that are capable of extending photorefractive performance of nematic liquid crystal composites into the near-infrared region of the spectrum.