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 nonlin ear optical properties, we have synthesized and investigated several series of soluble push-pull diphenylpolyenes of increasing length and having vari ous donor (D) and acceptor (A) end groups. Their linear and NLO properties have been studied by performing electro-optical absorption measurements (EO AM) and third-harmonic generation (THG) experiments in solution. Each push- pull molecule exhibits an intense ICT absorption band in the visible charac terized by an increase in dipole moment upon excitation (Delta mu). By leng thening the polyenic chain, linear increases in excited-state dipoles are a chieved while the ground-state dipole is maintained constant. The polyenic chain thus allows for long-distance intramolecular charge transfer. In addi tion, tuning of the magnitude of the ICT phenomenon and of the nonlinear re sponses is achieved by varying the donor and acceptor end groups: increasin g the D/A strength leads to concomitant enhancements of Delta mu, quadratic (beta) and cubic (gamma) hyperpolarizabilities values. Giant Delta mu value s (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, sta bility and transparency. (C) 1999 Elsevier Science B.V. All rights reserved .