Photoconductive crystals for nonlinear optics: molecular design and crystal structure

Acceptor-substituted carbazole derivatives were newly designed as novel nonlinear optical materials with photoconductive properties. Various 3-, 6- and 9-substituted carbazole derivatives were synthesized systematically and their optical second-harmonic generation (SHG) activity and crystallographic structures were examined. Enhancement of molecular hyperpolarizability ( beta ) was achieved and an intracharge-transfer band was observed by introducing electron-withdrawing groups in 3 and/or 6 positions. The hydrogen bonding in the 9-hydroxyethyl substituent plays an important role in generating non-centrosymmetric packing of carbazole molecules. Single crystals of 9-hydroxyethylcarbazole (HEK), 3-nitro-9-hydroxyethylcarbazole (NHEK) and 3,6-dinitro-9-hydroxyethylcarbazole (DNHEX) can be obtained by the solution growth technique. The HEK and NHEK crystals were determined to have non-centrosymmetric crystallographic structures with tetragonal and monoclinic unit cells. These crystals have unique space groups I4 1 and P2 1 , and hydrogen bonding was formed in adjacent hydroxy groups. In these crystals each carbazole ring is aligned along the same direction. On the other hand, DNHEK has a centrosymmetric unit cell with space group P1. In this case the hydrogen bond involves a hydrogen on a hydroxy oxygen interacting with the nitro oxygen in an adjacent molecule. The highly aligned carbazole system is attractive in terms of structure related properties such as the carrier transport process and second-order nonlinear optical responses.