Molecular structure, symmetry, and shape as design elements in the fabrication of molecular crystals for second harmonic generation and the role of molecules-in-materials

Molecular materials nave emerged as a major theme in contemporary science and technology, thanks to the synergetic confluence of the power of synthetic chemistry, the predictive and analytical capability of condensed matter physics, and the versatility of materials science and engineering. The stringent demands in terms of molecular design and supramolecular assembly to be met in the fabrication of nonlinear optical (NLO) materials for applications such as optical second harmonic generation (SHG) make it a unique arena covering fascinating explorations in chemical synthesis, molecular and materials characterization, theoretical modeling, NLO studies, and photonics technology. Our search for conceptually novel and practically simple but effective approaches to the design of molecular building blocks for crystals exhibiting efficient SHG has led us to the avenues described in this Account. The focus has been on the incorporation of structural, symmetry, and shape features in dipolar NLO-phores to realize noncentrosymmetric or polar molecular assemblies suitable to elicit appreciable SHG responses. Attachment of n-alkyl chains of optimal length and remote functional groups were shown to be effective methods to steer the assembly of achiral NLO-phore units into SHG-active noncentrosymmetric lattices. Computational modeling of molecules, molecular dusters, and molecular assemblies in crystals provides valuable insight into the observed structure-function correlations. A systematic exploration of the impact of the placement of stereogenic centers in strongly zwitterionic NLO-phores on the molecular organization led us to the effective exploitation of C,C-2-symmetric units to form helical superstructures capable of efficient SHG. New materials developed are based on organic and metal-organic molecules as well as coordination polymers. The potential utility of molecular shape could be demonstrated through the realization of perfectly polar organization in a family of screw-shaped dipolar molecules. Combination of optical transparency, thermal stability, and feasibility of fabricating thin films with orientationally ordered crystallites are important aspects of these materials. Several examples presented in this Account highlight the significance of molecules-in-materials by illustrating that not only the individual molecules and their organization in the crystal lattice but also the intermolecular interactions exert critical impact on the nonlinear optical response of the materials. The role of cooperative interactions in some of the cases is pointed out. This Account projects a range of design strategies for molecular SHG crystals and avenues for expanding further on the present observations; the need to address and exploit the contribution of intermolecular interactions is specially noted. The different examples presented illustrate not only the fabrication of new families of materials based on interesting models proposed earlier but also the emergence of new models from the novel materials developed.