Hyperpolarizabilities of substituted conjugated molecules. III. Study of a family of donorndash;acceptor disubstituted phenylhyphen;polyenes

摘要

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 sgr; and pgr; 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 donorndash;acceptor benzenes of increasing molecular size lsqb;NH2ndash;C6H4ndash; (CH=CH)nminus;NO2, withn=0,1 and 2rsqb; and of great experimental interest in nonlinear optics (second harmonic generation). Supposedly responsible for the higher value of the second order hyperpolarizability (tensor bgr;ijk) 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 rsquo;rsquo;two levelrsquo;rsquo; 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 bgr;ijkand the prevailing part played by pgr; 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 optics material research.