Linear and second-order nonlinear optical properties of ionic organic crystals.

The linear and second-order nonlinear optical susceptibilities of three ionic organic crystals, 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST), 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS), and 4-N,N-dimethylamino-4'-N'-phenyl-stilbazolium hexafluorophosphate (DAPSH), have been calculated by adopting a two-step multi-scale procedure, which consists in calculating: (i) the ion properties using ab initio or density functional theory methods and then (ii) in accounting for the crystal environment effects using classical electrostatic models. Provided that the ionic properties are evaluated at the second-order Møller-Plesset level and that the dressing field effects using point charges are accounted for, the agreement with experiment is excellent and enables to explain the origin of the larger χ((2)) response of DAPSH with respect to DAST and DSTMS. The study has also demonstrated that including the dressing field leads to a decrease of the χ((2)) response of ionic crystals whereas its effect is opposite for molecular crystals. Moreover, the results have also demonstrated that this multi-scale approach can be used to interpret the impact of the nature and position of the counterion on the linear and nonlinear optical susceptibilities of ionic crystals. Finally, it has been shown that the use of a conventional exchange-correlation functional like B3LYP leads to severe overestimations of χ((1)) but large underestimations of χ((2)) whereas the use of homogeneous dipole field is not recommended because it usually leads to overestimations of the linear and nonlinear optical susceptibilities.