NLO properties of a triphenlyguanidine salt: The importance of pseudo-symmetry

Using a combined experimental and computational approach, we were able to trace the quasi-vanishing of the second-order nonlinear optical (NLO) properties of a newly synthesized organic salt, bis(triphenylguanidinium) l-malate, to the pseudo-centrosymmetry of its structure. The employed experimental techniques were single crystal X-ray diffraction to determine the structure, “open-aperture” Z-scan technique to measure the nonlinear absorption, Kurtz and Perry powder method and Maker fringes techniques for second- and third-harmonic generation. The molecular hyperpolarizability tensors (β and γ) were calculated within Density Functional Theory (DFT) and the macroscopic second- and third-order susceptibility tensors were estimated by combining the supermolecule approach with the oriented gas model. The calculations performed with global hybrid GGA and range-separated functionals reproduced the correct order of magnitude of the observed second- and third-order susceptibilities.