Designing Non-Centrosymmetric Molecular Crystals: Optimal Packing May Be Just One Carbon Away

Molecular nonlinear optical (NLO) crystals feature important advantages compared to inorganic counterparts, such as low dielectric constants, ultrafast response times, and large electro-optic coefficients. Conjugated push-pull chromophores connecting electron-donating with accepting groups are often employed in the design of these crystals. However, associated large molecular dipole moments induce antiparallel or centrosymmetric conformations in the solid-state, which leads to NLO inactivity. The cation-anion hydrogen bond interactions of a hydroxy-piperidino electron donor group are combined with increased van der Waals volume effects induced by an ethyl modification of the electron-accepting moiety. This produces non-centrosymmetric packing in the organic salt EHPSI-4NBS ((E)-1-ethyl-2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl)-3,3-dimethyl-3H-indol-1-ium 4-nitrobenzenesulfonate). Converting a methyl group into ethyl changes the packing symmetry in the molecular crystal to switch on NLO activity. This behavior is attributed to the increased size of the ethyl group, which pushes apart the van der Waals contacts of the cation that lead to centrosymmetric packing in the methyl derivative. To test the NLO properties of EHPSI-4NBS, THz generation experiments are performed at 1200 nm pump wavelength. Spectral amplitude similar to DAST ((E)-4-(4-(dimethylamino)styryl)-1-methylpyridin-1-ium tosylate) crystal is observed with generation profile from 0 to 3.8 THz.