Polar Stacking of Dipole Parallel-Aligned Monolayers of Unsymmetrical 1,4-Diphenyl-1,3-butadienes Creates Nonlinear Optical Materials: Insights from Experiments Guide Structure Assignments

A series of 1-(4′-halophenyl)-4-(4″-methoxyphenyl)buta-1,3-dienes, (H, MeO, Y)-1,4-diphenylbutadienes with halogens Y = F ( 1 ), Cl ( 2 ), Br ( 3 ), and I ( 4 ) are described. Crystal structure analysis establishes that 1 – 4 present a new class of highly dipole parallel-aligned polar organic molecular materials for nonlinear optics (NLO). Building on previous studies of polar crystals of unsymmetrical acetophenone azines, (R, X, Y)-azines, X–Ph–CR═N–N═CR–Ph–Y (R = Me; X = R’O, PhO, R’OPh; Y = F, Cl, Br, I), we suspected that the all-carbon analogs (R, R’O, Y)-1,4-diphenylbutadienes, R’O–Ph–CR═CH–CH═CR–Ph–Y, could crystallize with polar lattices and feature stronger NLO effects because of their improved conjugation and higher dipole moments. The key concept for the construction of polar crystals involves dipole parallel-aligned beloamphiphile monolayers (PBAMs) held together by double T-contacts. Butadienes engage in new types of (fe|fe) double T-contacts while PBAMs of azines employ (ef|fe) double T-contacts. Polar PBAM stacking results in perfect ( 1 ) or near-perfect ( 2 – 4 ) three-dimensional (3D) dipole parallel-alignment depending on the nature of the interlayer noncovalent interactions. The crystal architectures are discussed for 1 – 4, and packing effects on chromophore conformations are assessed by comparison to potential energy surface analyses of the free molecules. The lower excitation energies of butadienes 1 – 4 are the primary origin of their higher hyperpolarizabilities relative to their azine analogs. Hyperpolarizabilities of 1 – 4 were computed for the free molecules in the gas phase and in solution phase (continuum models), measured in solution phase using the Lippert–Mataga equation (LME), and measured in the solid-state by powder second-harmonic generation (SHG). Overall, the results demonstrate that the series of (MeO, Y)-1,4-diphenylbutadienes presents a new class of polar crystalline materials with better performance parameters compared to the analogous (MeO, Y)-acetophenone azines. The results highlight that the optimization of a crystal property, such as the nonlinear optical (NLO) performance, must consider not only the desired molecular property but the design of the chromophore must be holistic with a view to the resulting supramolecular structure and account for the crucial role of interlayer interactions to control solid-state macroscopic properties.