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Computational study of molecules for quadratic nonlinear optical applications: orthogonal orientation of dipole vector and hyperpolarisability pseudo vector

In push–pull organic molecules of interest in quadratic nonlinear optical (NLO) applications, the major hyperpolarisability component usually lies along the dipole axis. Most of these molecules crystallise with 
 the dipole moment vectors of near neighbors aligned antiparallel, leading to near-cancellation of their 
 hyperpolarisability tensor components and poor bulk NLO response. Appropriate design of these molecules can 
 often effect an orthogonal orientation of their molecular planes in the crystalline state even when the major dipole axes are aligned antiparallel. In this paper we show that if the hyperpolarisability pseudo vector (defined using the projections of the tensor components) in these molecules is oriented orthogonal to their 
 dipole vector, appreciable bulk NLO response can be generated. We present results of a semiempirical computational investigation carried out in a combinatorial chemical sense to delineate the structural features that encourage such orientation of the hyperpolarisability and dipole vectors within a molecule.