Concomitance, Reversibility, and Switching Ability of Centrosymmetric and Non-Centrosymmetric Crystal Forms: Polymorphism in an Organic Nonlinear Optical Material

Reversibility between two concomitant polymorphs and their switching ability is demonstrated for the novel organic nonlinear optical (NLO) material Z-3-(3-methoxyphynyl)-2-(4-nitrophenyl)acrylonitrile. The appearances of their concomitant as well as exclusive polymorphic forms were discovered upon systematic crystallization experiments using various solvents. Determination of X-ray crystal structures confirmed that the polymorphs crystallize in centrosymmetric (P2(1)/n) and non-centrosymmetric (Fdd2) space groups. A search using the Cambridge Structural Database revealed that there exist only 10 such polymorphic pairs. Reversible phase transitions and the switching ability between the polymorphs can be achieved via both heat and solvents as stimuli. Structural analysis confirmed that the polymorphs have two-dimensional similarity. Further, quantitative and qualitative analysis of interaction energies based on the UNI force field and energy frameworks indicate that they possess very similar energies, although the centrosymmetric form is slightly lower in energy. A comparably high value of second harmonic generation activity and the NLO properties as estimated based on gas-phase and in-crystal calculations suggest that the non-centrosymmetric form could be a potential NLO material. To the best of our knowledge, this study is the rarest of its kind to show the reversibility and switching ability between centrosymmetric and non-centrosymmetric polymorphic phases in an organic material, induced via both thermal and solvent stimuli.