The achievement of perfect polar dipole alignment in ferroelectric lattices of organic non-linear optical (NLO) materials demonstrates the potential for crystal engineering by rational design to craft materials with advanced optical performance. The methoxyphenyl series of acetophenone azines feature polar stacking of parallel beloamphiphile monolayers (PBAMs). These third-generation chromophores feature superior triple T-contacts and improved crystal growth, and they excel because of their enhanced NLO properties.

Abstract

An improved design is described for ferroelectric crystals and implemented with the “methoxyphenyl series” of acetophenone azines, (MeO−Ph, Y)-azines with Y=F (1), Cl (2), Br (3), or I (4). The crystal structures of these azines exhibit polar stacking of parallel beloamphiphile monolayers (PBAMs). Azines 1, 3, and 4 form true racemates whereas chloroazine 2 crystallizes as a kryptoracemate. Azines 1–4 are helical because of the N−N bond conformation. In true racemates the molecules of opposite helicity (M and P) are enantiomers A(M) and A*(P) while in kryptoracemates they are diastereomers A(M) and B*(P). The stacking mode of PBAMs is influenced by halogen bonding, with 2–4 showcasing a kink due to directional interlayer halogen bonding, whereas fluoroazine 1 demonstrates ideal polar stacking by avoiding it. Notably, (MeO−Ph, Y)-azines display a stronger bias for dipole parallel alignment, attributed to the linearity of the biphenyl moiety as compared to the phenoxy series of (PhO, Y)-azines with their non-linear Ph−O−Ph moiety. The crystals of 1–4 all feature planar biphenyls and this synthon facilitates their crystallization through potent triple T-contacts and enhances their nonlinear optical (NLO) performance by increasing conjugation length and affecting favorable chromophore conformations in the solids.