Crystalline Zwitterionic Stilbazolium Derivatives with Large Quadratic Optical Nonlinearities

The synthesis and quadratic nonlinear optical properties of a series of zwitterionic 4-substituted 4'(n-sulfonatoalkyl)stilbazolium derivatives are described. The most active compounds in second harmonic generation (SHG) are obtained with the 3-sulfonatopropyl group. 4-Methylthio-4'(3-sulfonatopropyl)stilbazolium, 4-n-butyl-4'-(3-sulfonatopropyl)stilbazolium and (3-sulfonatopropy1)stilbazolium (further called MTSPS, BSPS, and SPS, respectively) were selected by the SHG powder test for further investigations. They present several crystal modifications depending on recrystallization conditions. MTSPS presents two highly SHG-active monohydrated crystalline phases, respectively, 2.4 and 1.3 times more than NPP in the Kurtz and Perry powder test a t 1.06 pm. The single-crystal X-ray structure of the most active phase is presented. MTSPSnH20 phase I crystallizes in the monoclinic space group P21, with two molecules per unit cell, with dimensions of a = 10.380 A, b = 11.124 A, c = 6.968 A, p = 99.93'. The conformation of the zwitterionic part of MTSPS molecule agrees well with molecular modeling calculations. The water molecule is involved in a hydrogen bond network linking the ribbonlike molecules along the c axis. In addition, within a sheet the hyperpolarizable chromophores are arranged so that an intermolecular charge transfer is possible. The electric-field-induced second-harmonic generation (EFISH) and ground-state dipole moment measurements of more soluble MTSPS and SPS analogs have been performed, leading for the first time to the hyperpolarizability p values of such zwitterionic species. Finally the quadratic nonlinear susceptibilities of MTSPS materials have been estimated by using the oriented gas model, giving values in reasonable agreement with the experimental ones deduced from semiquantitative SHG powder tests.