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Conventional organic molecules for applications in second-order non-linear optics are donor–acceptor substituted π systems that show only one intense charge-transfer (CT) transition. Thus, only a single element of the second-order polarizability tensor, β, is significant in these one-dimensional systems. The advantages and optimization strategies for two new classes of molecules with multiple CT transitions and two-dimensional second-order polarizability are reviewed. These are donor–acceptor substituted π systems that lack a dipole and have a molecular symmetry of C3 or higher, and dipolar molecules of symmetry C2v. A basic introduction to the field is also given.

The title compounds are formed by the nucleophilic aromatic substitution reaction between sym-trichlorotrinitrobenzene and amines. The yields and relative rates of formation depend critically on the degree of alkylation of the nucleophile. Contrary to the usual behavior, the introduction of a second and third donor is facilitated in the case of ammonia and monoalkylamines, while this is not the case for secondary amines. This behavior is rationalized on the basis of intramolecular hydrogen bonding and substantiated by an X-ray analysis of 3,5-dichloro-2,4,6-trinitroaniline.

Is it possible to identify molecular architectures that allow prediction of crystal structures? Can the factors that determine these structures be identified? To answer these questions, analogues of 1, which forms layer structures, were synthesized in which some or all of the nitro groups were replaced with sulfonyl acceptors. Surprisingly, the expected drastic change in the conformational and/or crystallographic behavior was not observed.