We have studied unsymmetrical acetophenone azines X-Ph-C(Me)=N-N=C(Me)-Ph-Y because of the NLO properties of their ferroelectric crystal structures. The crystal structures feature polar stacking of layers of perfectly polar aligned azines. While our primary interest always has been with unsymmetrical azines, we have also studied the symmetrical azines to learn about the layer architectures. Here we report a detailed analysis of the crystal architecture of symmetrical p-chloroacetophenone azine 1 (LIKHUI). Ab initio studies established that 1 is C2-symmetric in the gas phase. The small difference between the phenyl twists in crystals of 1 was intriguing and rendered each molecule asymmetric (C1) rather than dissymmetric (C2), and we were looking for a possible cause of a potential pseudo-Jahn−Teller effect (PJTE). However, our analysis showed qualitatively different coordination environments of the two chloroarene moieties and that 1 presents a case of crystal environment induced symmetry reduction (CEISR). Morphologies of idioteloamphiphile monolayers (IAMs) are classified, and the crystal structure of 1 is of the type “AABB Kick/Flat”. Careful analysis of the crystal structure showed arene-arene contacts of the types edge-to-face (e|f), face-to-edge (f|e), and face-to-face (f|f), a new arene-azine interaction (e|Az), Cl---π and Cl---Cl interactions. Arene interaction inventory analyses show in a compelling fashion that the arenes Ai and As in each azine engage in qualitatively different intermolecular interactions. Since the discovery of CEISR in the crystal structure of 1, we have been able to demonstrate the same phenomenon for several symmetrical acetophenone azines. Our approach to dissimilarity analysis does not rely on the presence of arenes and can be generalized to any molecular system with two moieties of the same constitution. Many known cases of symmetry reduction might have been thought to be caused by PJTE and need to be reexamined as potential cases of CEISR.

Symmetrical para-chloroacetophenone azine: Asymmetry instead of dissymmetry due to crystal environment induced symmetry reduction (CEISR)

ACS Fall 2021

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