Quinolinium-Derived Acentric Crystals for Second-Order NLO Applications with Transparency in the Blue

Derivatives of quinolinium iodide have been found to exhibit second-order nonlinear optical (NLO) response in their crystalline form. The quaternary amine functionality is introduced as an electron-withdrawing group in NLO-active chromophores. While their electron-accepting capabilities are somewhat weaker than those of the nitro group, these organic salts show a much more favorable transparency-nonlinearity tradeoff for blue second harmonic generation (SHG) NLO applications. Here we present crystal growth and characterization via X-ray diffraction (XRD), NMR, FTIR, and optical spectroscopy measurements. Experimental linear optical features are fully consistent with INDO/SCI-SOS theoretical calculations. These latter provide a rationale for the NLO response of these materials. Calculations predict a sizable molecular nonlinearity, which parallels the wavelength of the lowest charge-transfer transition. In addition, a direct correlation between SHG powder response to the β crystallographic angle is observed.