| The main contribution of this thesis is exploring the new organic two-photon materials with strong two-photon absorption (TPA) and two-photon excited fluorescence (TPEF). We designed and synthesize multi-branched pyrazine derivatives with two, three or four branches: 2,6-bistyrylpyrazine(A), 2,5-bistyrylpyrazine(B), 2,3,5-tristyrylpyrazine(C), 2,3,5,6-tetrastyrylpyrazine(D). The results are summarized into the following:1. Syntheses and electron structures of two-, three- and four-branched pyrazine derivativesThe compounds were synthesized by the Aldol condensation reaction. The charge density distributions were calculated by PM3 method. The results show that, in each molecule, the electrons transfer to the pyrazine acceptor from donors at the end of branches.2. The preparation and characterization of organic nano-particles of pyrazine derivativesNano- or micron-particles mostly composed of pyrazine derivatives molecules were prepared by reprecipitation and solvent-exchange method successfully. ESEM, and TEM were used to characterize the structure of the particles.3. Linear absorption and single-photon excited fluorescence (SPEF)The linear absorption spectra, the single-photon excited fluorescence (SPEF) spectra, and the quantum yields of pyrazine derivatives were measured. From the results, we get the following conclusion:⑴With the solvent polarity increasing, the SPEF peak positions (λm fax) of all the compounds blue-shift, and showed negative solvatokinetic effect.(2) With the increase of branch number, theλamaxand theλm faxshowed red-shift, and both theεand the I maxincreased with the branch numbers increasing.(3) It is noteworthy that, although both A and B are two-branched compounds, the SPA spectra showed obvious red-shifts from A to B (88 nm). This was a clear evidence that meta-branching produces much weaker electronic coupling effect between the branches than para-branching.(4) The optical properties of organic nano-particles were obtained via testing the solid fluorescence of the particles in different aggregation state. It revealed that the maximum fluorescence emission peak shifted to the shorter wavelength when the small particles were improved their dispersion and order.4. The two-photon absorption and emission properties(1) The relationship between the output fluorescence intensity and the input power of Compounds C and D were determined. The TPEF intensity showed a quadratic dependence on the input power, which suggested the two-photon excitation mechanism.(2) TPEF was similar to SPEF, their peak positions showed the same changing tendencies as the branch number changed (λB<λC<λD); the TPEF intensities of different compounds changed in the same order as that of SPEF intensities. It indicated that both the emissions for a given compound were from the same excited state, though their initial Frank-Condon states may be different.(3) For compounds B, C and D, the ratio ofσbetween them (1: 2.8: 3.7) is near to that of the square of branch number (1:2.25:4)In summary, we have studied the nonlinearities of a series of pyrazine derivatives containing two, three and four branches, respectively. They exhibit strong TPEF in the blue-violet to yellow-green region. Considering their comprehensively better photophysical and physical properties, the four-branched compounds are potential second- and third-order NLO materials. Better second- and third-order properties will be obtained by designing molecules with various branches. |
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