Abstract
New push-pull second harmonic generation (SHG) chromophores 2 were synthesized in order to study their linear and nonlinear optical properties. The donor-acceptor π-conjugated systems 2 were prepared in good to excellent yields by a simple aldol-type condensation of the precursor aldehydes 1 with 6-nitro-2-methyl-1,3-benzothiazole in the presence of NaOH. Hyper-Rayleigh scattering in dioxane solutions was used to evaluate their second-order nonlinear optical properties. The experimental results showed that the optical (linear and nonlinear) properties are influenced by the electron-donor strength of the groups of the π-conjugated bridge. The vinyl-benzothiazole derivative 2b, with methoxyl as the donor group, exhibited the largest first hyperpolarizability β (1660 × 10−30 esu, 40 times greater than the standard reference pNA molecule) and the highest decomposition temperature (Td = 360 °C). The good balance between nonlinearity and thermal stability makes this compound a promising candidate as second harmonic generator (SHG) for nonlinear optics.
Highlights
Organic molecules comprising strong electron donor and acceptor groups connected by a πconjugated system are of fundamental importance in materials chemistry due to their numerous applications in modern technology, such as nonlinear optical (NLO) devices, poled polymers, photovoltaic cells, organic light-emitting diodes (OLEDs), semiconductor materials, and optical data storage devices [1,2,3,4].Several push-pull molecules containing benzothiazole as an electron-withdrawing group have already been reported, and typically exhibit favorable fluorescence, electrochemical, solvatochromic, photochromic, and NLO properties [5,6,7]
Benzothiazoles 2 with arylthienyl π-bridges were obtained in good yields (70–80%) by the condensation of 6-nitro-2-methyl-1,3-benzothiazole with formyl derivatives 1a-b in ethanol, in the presence of aqueous sodium hydroxide solution, at room temperature for 3–5 h (Scheme 1, Table 1)
It was prepared in 84% yield by the nitration of 2-methyl-1,3-benzothiazole using a mixture of nitric acid/sulfuric acid (3:1) as nitration agents [13] in sulfuric acid at 2–3 °C for 30 min
Summary
Organic molecules comprising strong electron donor and acceptor groups connected by a πconjugated system (often designated as donor-π-acceptor or “push-pull” chromophores) are of fundamental importance in materials chemistry due to their numerous applications in modern technology, such as nonlinear optical (NLO) devices, poled polymers, photovoltaic cells, organic light-emitting diodes (OLEDs), semiconductor materials, and optical data storage devices [1,2,3,4].Several push-pull molecules containing benzothiazole as an (auxiliary) electron-withdrawing group have already been reported, and typically exhibit favorable fluorescence, electrochemical, solvatochromic, photochromic, and NLO properties [5,6,7]. We have reported the synthesis and characterization of the UV-vis, solvatochromic, thermal, and second-order NLO properties of benzothiazole derivatives containing bithienyl [8,9], arylthienyl [10], and thienylpyrrolyl [11,12]. As an extension of our earlier work and in order to improve the thermal stability and NLO properties of heterocyclic chromophores of the benzothiazole type, we decided to synthesize arylthienyl-benzothiazoles bearing a vinyl spacer between the donor πexcessive heterocycle (thiophene) and the acceptor π-deficient benzothiazole moiety. Proceedings 2019, 9, 4 we report the synthesis and characterization of the optical and thermal properties of two vinylbenzothiazole derivatives as second harmonic generators (SHG) for NLO applications
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