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...
moreThe 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. ■ INTRODUCTION Recent developments of nonlinear (NL) optical materials are concerned with the design of new systems with enhanced second-and third-order optical properties aiming their uses in optoelectronic devices, 1,2 all-optical limiters, 3 and THz imaging and spectroscopy, 4 among other applications. Important works have been reported with focus on the exploitation of new organic molecules because they present large NL optical coefficients, fast response times, straightforward synthesis, and processability. In particular, molecules with electronic push−pull systems, which give rise to π-electrons delocalization, present large optical hyperpolarizabilities and fast response for nonresonant excitation. 5−12 Organic molecules formed by push−pull systems have three components; an electro-acceptor group (R A), an electron-donor group (R D) and a π-conjugated spacer to link the R A and R D. 9,10 The goal is to engineer the components in order to maximize the optical hyperpolarizabilities. Among the push−pull organic systems known, the mesoionic compounds (MICs) deserve special attention because of their remarkable physical and chemical properties that enable applications as antioxidant, 13 anti-inflammatory, 14 antibacterial, 15 and antitumor sumstances, 16 as well as the large NL optical susceptibility as already proposed 17−19 and experimentally demonstrated. 20−22 Indeed, the MICs' properties point to NL biophotonics as a possible avenue for future applications. MICs are planar five-membered heterocyclic betaines, with at least one side-chain whose atom is also in the ring plane where the positive and negative charges are separated and delocalized within a π-electrons system that originate permanent electric dipole moments of ≈5 D. The region, which includes the side chain atom, is associated with highest occupied molecular orbital (HOMO) and a negative π-charge, while the other is associated with lowest unoccupied molecular orbital (LUMO) and a positive π-charge. 18 The molecules are neutral, they present π-electron delocalization, and they are composed of resonant (mesomeric) structures. In previous theoretical studies the potential of MICs for NL optics was considered on the basis of their high hyper-polarizability values, their stability and the possibility of improving their optical response by manipulation of their chemical structure. 17,19 The studies revealed that certain MICs present relatively high third-order NL optical response in the visible and near-infrared regions that were verified in experiments with pulsed lasers operating in various temporal regimes. For example, two-photon absorption cross sections in