Non-classical donor-acceptor chromophores for second order nonlinear optics

The development of highly efficient organic nonlinear optical materials for opto-electronic applications, which are promising for high-speed information processing and telecommunications, has drawn considerable attention over the last decade."] Although various prototype optoelectronic devices have been successfully demonstrated, the improvement of practical devices still requires further materials and product device developments.[*] Opto-electronic components based on second-order nonlinear optical effects such as linear electro-optic, nonlinear optical, and photorefractive effects require materials with large second-order optical nonlinearities. The optimization of both the molecular hyperpolarizability /3 and the non-centrosymmetric molecular order in the bulk are essential prerequisites to the design of useful molecular or polymeric materials with large secondorder optical nonlinearitie~.[~] It has been theoretically and experimentally shown that the origin of large optical nonlinearities of a onedimensional, rod-shaped, dipolar molecule such as a donoracceptor disubstituted benzenes, biphenyls, polyenes, stilbenes, and tolanes arises from the highly polarizable charge asymmetry of the n-conjugated system."*31 Although over the last few yearsr4] there has been tremendous progress in the understanding and optimization of the first-order molecular hyperpolarizability of these classical dipolar chromophores, these highly hyperpolarizable chromophores, especially donor-acceptor disubstituted polyenes, are usually limited in practical applications because of their chemical instability. Furthermore, the formation of efficient acentric crystals, and the good orientational order and temporal stability in electrically poled polymers of classical dipolar chromophores, are in general challenging materials