The first hyperpolarizabilities of the mesoionic compounds (MICs) 2-(4-chlorophenyl)-3-methyl-4-phenyl-1,3-thiazolium-5-thiolate (MIC-1), 2-(4-chlorophenyl)-3-methyl-4-(4-methylphenyl)-1,3-thiazolium-5-thiolate (MIC-2), and (2-(4-chlorophenyl)-3-methyl-4-(4-methoxyphenyl)-1,3-thiazolium-5-thiolate) (MIC-3), dissolved in dimethyl sulfoxide (DMSO), are reported here for the first time to our knowledge. Hyper-Rayleigh scattering experiments were performed with an excitation source operating at λ = 1180 nm (ω = 8475 cm–1). The measured hyperpolarizability, β­(2ω), was used to calculate the static hyperpolarizability, β(0), by applying the classical two-level model. The values obtained for β(0) were 10.1 × 10–30 esu (MIC-1), 8.7 × 10–30 esu (MIC-2), and 10.4 × 10–30 esu (MIC-3) which are smaller than previous theoretical predictions that did not consider features related to the liquid phase.

We synthesized the mesoionic compound 2-(4chlorophenyl)-3-methyl-4-(4-methylphenyl)-1,3-thiazole-5-thiolate and measured its refractive and absorptive nonlinear optical response in different temporal and spectral regimes. The experiments were performed by using the Z-scan technique with two pulsed light sources: the second harmonic (at 532 nm) of a modelocked and Q-switched Nd−YAG laser (100 ps, 10 Hz) and a Ti: Sapphire laser system (100 fs, 1 kHz) operating at 800 nm. The observation and characterization of nonlinear refraction, two-and three-photon absorption, and excited state absorption of the mesoionic compound dissolved in dimethyl sulfoxide, in different concentrations, are presented and discussed with basis on the population redistribution in a three-energy-level model that allows the determination of the parameters which characterize the nonlinear response.