Novel nonlinearity–transparency–thermal stability trade-off of thiazolylazopyrimidine chromophores for nonlinear optical application
Introduction
Chromophore-containing polymeric electro-optic materials have been intensively studied during the past two decades due to their promising applications in information processing and fiber communication. This interest is motivated not only by the large NLO response but also by the versatility, ease of processing, and possibility of tailoring the physicochemical properties by the molecular engineering approach. For the application of NLO polymers, it is important to optimize the comprehensive properties, especially the transparency. The transparency in the near-infrared region (1.0–1.7 μm), such as 1.3 μm and 1.5 μm, is required, rather than that in the visible range. Usually, NLO chromophores, the active components in the NLO polymers, must possess a good nonlinearity–transparency–thermal stability trade-off for nonlinear optical application. In spite of different types of chromophores used for NLO application, azo chromophores have attracted much attention because they can be readily prepared with a wide range of donor and acceptor groups and also because the planarity of the azo bridge versus non-planarity of other systems such as stilbenes should contribute to larger π electron transfer properties of the system and result in larger NLO responses. Although there has been tremendous progress in enhancing the first hyperpolarizability of the chromophore, the harmony and balance of the aforementioned three issues for NLO chromophore remain unsolved, which limits its practical applications in photonics.
Recently, some authors reported the synthesis and characterization of new chromophores in which heteroaromatic rings, such as thiazole [1], [2], [3], [4], benzothiazole [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] and thiophene [16], [17], [18], [19], [20] are employed as conjugating units, which exhibit good nonlinear optical properties. However, the color chemistry studies have evidenced that the replacement of a benzene ring by a less aromatic heterocycle will result in a significant bathochromic shift of the visible absorption band due to the deficiency of electron density on the heterocycle C-atom [21]. Hence the hetarylazo chromophores, which exhibit larger molecular hyperpolarizabilities than that of their benzenoid analogues, are generally accompanied by a large bathochromic shift of the visible absorption band. In the past years we have synthesized some hetarylazo chromophores derived from amino-substituted thiazole [22] and benzothiazole [23], [24], [25] as diazo components and aniline derivatives as coupling components. The hetarylazo chromophores possess large β values but produce a pronounced bathochromic shift when compared to the corresponding benzenoid compounds.
To overcome bathochromic shifts of the visible absorption band of conventional azo chromophores, it has been shown in the literature [26] that nitrogen heterocycles were introduced into NLO chromophores to improve optical transparency and thermal stability. However, some chromophores with high transparency and good thermal stability are generally accompanied by low β responses. In continuation of our work, in order to improve optical transparency, thermal stability and maintain good nonlinearity, the pyrimidine ring was introduced into the chromophores and some novel thiazolylazopyrimidine chromophores were prepared through an azo coupling reaction. These new NLO chromophores are distinguished by good transparency, thermal stability and large optical nonlinearity for nonlinear optical application. Furthermore, the thiazolylazopyrimidine chromophores containing amino groups and hydroxyl groups can be easily incorporated into polymers through covalent bonds to improve relaxation behavior, which gives these chromophores broad applications.
Section snippets
Materials
C.I. Disperse Red 1 (DR1, purity >95%), 2-amino-5-nitrothiazole, 4-nitroaniline, 2-chloropyrimidine, 2-aminopyrimidine and N-ethyl-N-(2-hydroxyethyl) aniline were obtained from Aldrich Chemical Co. All the regents and solvents were analytically pure and used without further purification.
Synthesis of 2-2′-hydroxyethylaminopyrimidine
2-Chloropyrimidine (26 g, 1 mol) and ethanolamine (34.5 g, 2.5 mol) in ethanol (150 mL) were refluxed for 1 h, the solvent evaporated, the residue poured into cold water (100 mL), saturated with sodium chloride, the pH
Preparation of the azo chromophores
The synthetic procedures of the azo chromophores (1a,b,c and 2a) are shown in Scheme 1, Scheme 2. The chromophores were obtained by coupling pyrimidine derivatives and aniline derivatives, respectively, with relevant diazotised amines.
Two types of nitrosating reagent, nitrosyl chloride from NaNO2/HCl solution and nitrosylsulphuric acid from NaNO2/concentrated H2SO4, were used for diazotisation of the variously substituted amines. Most of the mono-substituted carbo-aromatic anilines were
Conclusion
The new NLO thiazolylazopyrimidine chromophores have been synthesized by diazo coupling reaction for potential NLO applications. The thiazolylazopyrimidine chromophores 1a and b exhibit large molecular hyperpolarizabilities with the values of 510 × 10−30 esu and 500 × 10−30 esu, respectively, which are larger than that of chromophore 1c derived from 4-nitroaniline. In comparison with analogous aniline based chromophore 2a, the thiazolylazopyrimidine chromophores 1a and b display 40–50 nm blue shift of
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (50377005), “the New Century Excellent Talents Program” of ministry of education of China (NCET-04-0482), and “the Six Top Talents Foundation” of Jiangsu Province of China (06-A-033).
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