Novel second-order nonlinear optical chromophores containing multi-heteroatoms in donor moiety: Design, synthesis, DFT studies and electro-optic activities
Graphical abstract
Introduction
Organic and polymeric electro-optic (EO) materials have drawn much attention because of their attractive potential applications in optical data transmission and optical information processing [1]. Compared with traditional inorganic and semiconductor materials, the organic EO materials have many advantages such as larger nonlinear optical coefficients, simpler preparation and lower cost [2], [3], [4], [5], [6]. Generally, the nonlinear optical (NLO) chromophore molecules possessing a dipolar D-π-A type structure is the core component in such materials, which turned the design and preparation of NLO chromophores into a hot research spot in the area of organic EO materials [7], [8]. Much great attentions have been paid to the rational design of chromophores with not only high first-order hyperpolarizability but also good thermal and photochemical stabilities, and in addition, have good solubility and compatibility with polymer matrix.
It is a well-established fact that the conjugation length and donor/acceptor strength of these D-π-A type push–pull chromophore molecules can cause dramatic influences to their second-order nonlinear responses [3], [5], [6], [7]. In order to search for highly efficient NLO chromophores, finding an optimal combination of their donor/acceptor still represents one of the critical challenges [9], [10], [11]. In the past decade, the researches on NLO chromophores have mainly focused on the design of electron bridges and electron acceptors [12], [13], [14], [15]. However, the electron donors, which are important components of NLO chromophores, have received much less attention as the general class of traditional alkyl and aryl amines were considered to be the idea electron donors and were constantly-used. These amines have been proven to possess suitable energy levels of the nitrogen nonbonding lone pair, which is well matched to donate electrons into the π* of the attached aromatic ring [7], [16].
In recent years several studies have demonstrated that introducing additional heteroatoms into the donor moiety of chromophores could dramatically modulate the conjugation of the π-electron networks and the electronic character of the charge transfer kinetics, which could further influence the chromophores' first-order hyperpolarizability [5], [17], [18], [19]. Meanwhile, additional heteroatoms can also provide abundant opportunities for further modifications [5], [20]. However, such kind of electron donors are usually confronted with problems like their complicated synthetic steps and first-order hyperpolarizability, which are still needed to improved. In order to investigate the influence of additional heteroatoms in traditional donor moiety of the NLO chromophores and to achieve better first-order hyperpolarizability and macroscopic EO activities, we designed a series of chromophores in this article with one or two heteroatoms in ortho-position towards the π-bridge where they can more effectively conjugate with the π-system to donate lone pair electrons. Oxygen atom and nitrogen atom were selected as candidates due to their potential suitable electron-donating abilities. For reasonable comparison, these NLO chromophores all choose 2-dicyanomethylene-3-cyano-4,5,5-dimethyl-2,5-dihydrofuran (TCF) as electron acceptors. 1H NMR, MS and crystal structure analysis were carried out to demonstrated the preparation of these chromophores. Thermal stability, photophysical properties, density functional theory (DFT) calculations and EO activities of these chromophores were systematically studied and compared to illustrate the influences of the suitable additional heteroatoms on rational NLO chromophore designs.
Section snippets
Materials and instrument
All chemicals are commercially available and are used without further purification unless otherwise stated. N, N-dimethyl formamide (DMF) was distilled over calcium hydride and stored over molecular sieves (pore size 3Å). Acetone was dried with anhydrous MgSO4, then distilled and stored over molecular sieves (pore size 3Å). The 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran(TCF) acceptor was prepared according to the literature [21]. TLC analyses were carried out on 0.25 mm thick
Synthesis and characterization
Scheme 1 showed the synthetic approach for the chromophores A–D. Compounds 9 and TCF were prepared according to the literature [21], [22]. Starting from the aromatic amine or substituted aromatic amine, chromophores A–D were synthesized in good overall yields through simple 2–3 step reactions: SN2 substitution reaction with bromoethane, Vilsmeier reaction, and the final Knoevenagel condensation with TCF acceptor. This route has been established for the high yield synthesis of this kind of NLO
Conclusion
A series of organic NLO chromophores with multi-heteroatoms donors have been synthesized and systematically characterized by NMR, MS, UV–Vis absorption spectra, and single-crystal X-ray diffraction methods. The new compounds obtained good yields which are ensured by the simple work-up procedures. Thermogravimetric analysis showed good thermal and thermoxidative stabilities of the four chromophores. The effects of bathochromic and solvatochromic behavior on the UV–Vis absorption were also
Acknowledgments
We are grateful to the Directional Program of the Chinese Academy of Sciences (KJCX2.YW.H02), Innovation Fund of Chinese Academy of Sciences (CXJJ-11-M035) and National Natural Science Foundation of China (No. 11104284 and No. 61101054) for the financial support.
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