ChemInform Abstract: Synthesis and Second-Order Optical Nonlinearity of Carbazolyl-Substituted Furan Chromophores with High Thermal Stability and Good Transparency (original) (raw)

Journal of Chemical Research

Organic and polymeric second-order nonlinear optical (NLO) materials have attracted much attention for their potential application involving optical modulation, molecular switching, optical memory and information storage. 1 One of the major challenges in this area of research is to design and synthesise second-order NLO chromophores simultaneously achieving large first molecular hyperpolarisability (β), good chemical and thermal stability, and high optical transparency. 2 Polymers containing a carbazole moiety can be used in many functional materials, such as photoconductors 3 and nonlinear optical 4 and photorefractive 5 materials. The thermally stable carbazole molecule has a structure isoelectronic with diphenylamine, and thus the introduction of electron acceptor(s) in the 3 and /or 6 position(s) causes intramolecular charge transfer and a mesomeric dipole moment, resulting in a strong second-order nonlinear optical response. 6 We have designed and synthesised a series of new carbazole chromophores in which the furan unit is used as one of the conjugation moieties (see Scheme 1, compounds 3a-e). Furan is used because it has a lower resonance stabilisation energy than benzene; thus chromophores with the furan moiety have proved to display molecular hyperpolarisabilities elevated compared with their benzene analogues. 7 To our knowledge, only three experimental works have been reported for the chromophores containing the furan moiety, in which the electron-donating groups are always alkylamino groups. 8-10 We hope that the new chromophores here may represent new opportunities for the optimisation of all main properties such as nonlinearity, transparency and thermal stability. The general procedure for the synthesis of 3a-e is shown in Scheme 1. Compound 1 was synthesised in 83% yield by the Wittig reaction between (2-furylmethyl)triphenylphosphonium bromide and 3-formyl-N-hexylcarbazole. Formylation of 1 with POCl 3 and DMF afforded 2 in 87% yield. Various acceptor groups were introduced by Knoevenagel condensation with the formyl group. Thus 3a, 3d and 3e were obtained from the condensation of 2 with malononitrile, 3-dicyanomethylene-1,5,5-trimethylcyclohexene and 3-phenyl-4-(3,5,5-trimethyl-2-418 J. CHEM. RESEARCH (S), 2001