Hyperpolarizabilities of substituted conjugated molecules. III. Study of a family of donor–acceptor disubstituted phenyl-polyenes

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Abstract
In Parts I and II of this series, we have made use of a finite field perturbated INDO scheme to compute molecular hyperpolarizabilities, to discriminate through simple symmetry considerations between respective σ and π electronic contributions, and to test these ideas on smaller size benzene monosubstitutes. Following the same approach, we report here investigation of a family of disubstituted donor–acceptor benzenes of increasing molecular size [NH2–C6H4– (CH=CH) n −NO2, with n=0,1 and 2] and of great experimental interest in nonlinear optics(second harmonic generation). Supposedly responsible for the higher value of the second order hyperpolarizability (tensor β i j k ) of this type of molecules, the charge transfer process is explored here and its importance confirmed. Results are compared to experimental data with the help of a simple ’’two level’’ charge transfer model to account for frequency dispersion and found in satisfactory agreement with previously noted tendencies. Also confirmed here are the chain length influence on β i j k and the prevailing part played by π electrons in second order nonlinear processes of conjugated molecules. This study is seen to bring a theoretical credit to the ever growing experimental interest in large charge transfer conjugated molecules in the field of nonlinear opticsmaterial research.