Electronic and second-order nonlinear optical properties of conformationally locked benzylideneanilines and biphenyls. The effect of conformation on the first hyperpolarizability

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Abstract
The first hyperpolarizability β and electronic properties of N,N-dimethylamine and/or nitro-substituted benzylideneanilines and biphenyls are compared with those of conformationally locked model compounds, viz. identically substituted 2-phenyl-3,3-dimethyl-3H-indoles and fluorenes, respectively. Although PM3 semi-empirical calculations indicate that a small to moderate increase in β can be achieved by locking the π-systems in a planar conformation, experimental results show that the actual gain is either small or negligible. This mainly finds its origin in relatively narrow electronic absorption bands of the conformationally locked compounds, which are related to their rigidity. Moreover, single crystal X-ray structures of the donor–acceptor substituted 3H-indoles demonstrate that the presence of a saturated bridge does not necessarily lead to a more planar structure. Furthermore, it is shown that the difference in β of the two N,N-dimethylamino-nitro-substituted benzylideneaniline isomers is of electronic rather than of conformational origin.

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