Control of crystal packing in SHG active 7,7-bis(n-pentylamino)-8,8-dicyanoquinodimethane by intermolecular H-bonds and alkyl chain interactions

Abstract

Our recent studies have shown that simple alkyl chains can be used as crystal design elements to drive the formation of noncentrosymmetric lattices of interest in quadratic nonlinear optical applications. In a series of alkyl substituted diaminodicyanoquinodimethanes second harmonic generation was observed only in the systems with intermediate chain length. The molecular and crystal structures of 7,7-bis(n-pentylamino)-8,8-dicyanoquinodimethane, one of the SHG active crystals in this series, are presented in this paper. Dipolar, H-bonding and alkyl chain interactions are observed between the molecules in the crystal. Semiempirical quantum chemical computations on the molecule and detailed analysis of the experimental molecular and crystal structures reveal the critical role played by alkyl chain interactions in the formation of the noncentric lattice and the resultant SHG capability of this material.