Non-linear optical (NLO) materials are essential in optical computing, optical signal processing, and data storage. Non-centrosymmetric conjugated organic molecules are candidates for NLO materials. We have synthesized and crystallized series of unsymmetrical acetophenone azines X-Ph-C(R)=N-N=C(R)-Ph-Y and of butadiene analogs X-Ph-C(R)=CH-CH=C(R)-Ph-Y. Usually X represents a halogen atom or NO2 and Y represents a π donating alkoxide (RO; R = Me, Et, Pr, Dec). These molecules were designed to have modest intra-molecular dipoles and to ensure enough inter-molecular interactions to stabilize the desired parallel-aligned dipole lattices. In fact, several azines crystallize with perfect polar order. Arene-arene double-T contacts are the key to the crystal structures because their attractive power helps to overcome the intrinsic electrostatic repulsion of parallel-aligned side-by-side dipoles and thereby make such polar NLO materials possible. The NLO properties of some of the crystals were confirmed with second harmonic generation (SHG) measurements. Ab initio calculations were performed to explore the conformational space of the monomers and to study packing effects on conformations. Reaction energies ΔG were computed for the asymmetrization reactions, that is, the formation of two asymmetric azines from the respective symmetric azines. The small magnitudes of the ΔG values explain the difficulty encountered in the separation symmetric and asymmetric azines in the product. This study provides for a better understanding of the morphology of these organic NLO crystals and guidance for improved synthesis and purification of NLO materials.

Non-linear optical properties of crystalline unsymmetrical 4-halo-4’-n-Alkoxyacetophenone azines and of butadiene analogs. Structural and computational analysis of the crystal architectures

ACS Fall 2020 Virtual Meeting

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