Raman spectroscopy and theoretic study of hyperpolarizability effect in diiodobutenyl-bis -thioquinolinium triiodide at low temperature

Various structural features, such as proton disorder or noncovalent interactions, determine the existence of switchable nonlinear optical properties under varying external conditions. Thus, for the single crystal of diiodobutenyl‐bis‐thioquinolinium triiodide with the bridge hydrogen atom, previously characterized under ambient conditions by C2/c symmetry, we have measured Raman spectra in the temperature range from 298 K down to 113 K. Variations in low‐wavenumber region of Raman spectra at temperatures below 153 K have been attributed to the change of the bridge hydrogen atom position in the [NHN]⁺ fragment, thus lowering the crystal symmetry from C2/c to Cc. Quantum chemical calculations in the solid state for noncentrosymmetric Cc structure predict high hyperpolarizability and second‐order electric susceptibilities, comparable to those of modern nonlinear optical materials. This indicates the emergence of nonlinear optical properties in the low‐temperature phase of the studied crystal. Copyright © 2017 John Wiley & Sons, Ltd.