Elongated push-pull diphenylpolyenes for nonlinear optics:: molecular engineering of quadratic and cubic optical nonlinearities via tuning of intramolecular charge transfer

Push-pull polyenes are of particular interest for nonlinear optics (NLO) as well as model compounds for long-distance intramolecular charge transfer (ICT). In order to tune the ICT phenomenon and control the linear and nonlinear optical properties, we have synthesized and investigated several series of soluble push-pull diphenylpolyenes of increasing length and having various donor (D) and acceptor (A) end groups. Their linear and NLO properties have been studied by performing electro-optical absorption measurements (EOAM) and third-harmonic generation (THG) experiments in solution. Each push-pull molecule exhibits an intense ICT absorption band in the visible characterized by an increase in dipole moment upon excitation (Delta mu). By lengthening the polyenic chain, linear increases in excited-state dipoles are achieved while the ground-state dipole is maintained constant. The polyenic chain thus allows for long-distance intramolecular charge transfer. In addition, tuning of the magnitude of the ICT phenomenon and of the nonlinear responses is achieved by varying the donor and acceptor end groups: increasing the D/A strength leads to concomitant enhancements of Delta mu, quadratic(beta) and cubic (gamma) hyperpolarizabilities values. Giant Delta mu values (up to more than 30 D) and enhanced non-resonant quadratic and cubic NLO molecular responses (i.e. beta(0) = 500 x 10(-30) esu and gamma(0) = 8000 x 10(-36) esu) have been attained while maintaining suitable solubility, stability and transparency. (C) 1999 Elsevier Science B.V. All rights reserved.