Selective Formation and SHG Intensity of Noncentrosymmetric and Centrosymmetric 1,1,2,2-Tetramethyl-1-(4-(N,N-dimethylamino)phenyl)-2-(2′-cyanophenyl)disilane Crystals under External Stimuli

In this paper, selective formation of noncentrosymmetric (Cc; alpha-crystal) and centrosymmetric (P2(1)/c; beta-crystal) 1,1,2,2-tetramethyl-1-(4-(N,N-dimethylamino)phenyl)-2-(2'-cyanophenyl)disilane (1) was reported and both crystals were characterized by single-crystal X-ray diffraction (XRD) analysis, indicating multiple CH-pi interactions created the crystal packing. The second harmonic generation (SHG) activity was attributed to the crystal packing and the orientation of dipole moment. Noncentrosymmetric crystal packing in a parallel manner exhibited SHG activity (intensity: 2.1 times vs urea). On the contrary, centrosymmetric structure in an antiparallel form showed no SHG activity (intensity: 0.0 times vs urea). As the calculated crystal energies of two polymorphs were very close to each other, the regulation of alpha- and beta-crystals is succeeded through supercooled liquid-to-crystal and crystal-to-crystal phase transitions, controlling the SHG activity. Powder XRD and solid-state C-13 NMR spectroscopy were applied for the qualitative analysis of alpha- or beta-polymorphs.