全偶氮苯官能化树枝状分子的合成及其非线性光学性能和自组装行为研究

【作者】 张文泉；

【导师】 施文芳；

【作者基本信息】 中国科学技术大学 ， 高分子化学与物理， 2007， 博士

【摘要】 偶氮苯基团的顺式和反式两种构象在光照或者加热条件下很容易实现相互转变，伴随着偶氮苯分子偶极矩、自由体积以及光谱学特征的变化。偶氮苯基团的光敏感性使其在非线性光学材料，智能高分子，可控药物释放，分子自组装以及光信息存储方面均具有广阔的应用前景。从1993年第一个含有偶氮苯基团的树枝状分子合成至今，已有大量的相关研究报道，树枝状分子独特的分子结构使得这类分子在众多领域表现出了与线形聚合物不同的性能和突出的优势，但是迄今为止，大多数的研究都集中在偶氮苯基团位于外围或者核的树枝状分子，关于以偶氮苯基团为结构单元的树枝状分子的研究还较少，而且其应用研究也处在探索阶段。本论文合成了以偶氮苯为结构单元的1到4代dendron和第1、2代的dendrimer，采用多种手段对其结构进行了表征，并探索了其在非线性光学材料、光致自组装方面的应用的可能性。一、以偶氮苯为结构单元的树枝状分子的合成及表征以4-羧基-4’-（1,2-丙二醇醚）-偶氮苯（CPA）为AB₂单体，含正丁基的偶氮苯分子4-羧基-4’-正丁基醚偶氮苯（CBA）为外围单体，采用“收敛法”合成了全偶氮苯官能团化的1到4代dendron和1、2代dendrimer。反应过程中利用甲氧基乙氧基甲基（MEM）保护基团有效地抑制了副反应的发生，合成了最多含31个偶氮苯基团的树枝状大分子。NMR，FTIR，GPC和MALDI-TOF-MS分析结果证明了所合成的树枝状大分子具有结构规整性和单分散性。紫外可见吸收光谱的研究证明，所有树枝状分子的偶氮苯基团在紫外光和可见光的照射下均可实现完全的光致顺反异构化，而且偶氮苯分子在此过程中稳定而不降解。利用核磁共振氢谱发现位于分子内部的偶氮苯比外围的偶氮苯更难发生顺反异构化。偶氮苯树枝状分子的优异的光敏性能为其作为光学材料的应用研究提供了可能性。二、以偶氮苯为结构单元的树枝状分子的主客体二阶非线性光学性能初步探索了以偶氮苯基团为结构单元的树枝状分子作为非线性光学材料的应用可能性。树枝状分子的高度支化的堆积结构使其与聚碳酸酯（PC）的相容性远远优于相同结构的单体小分子，可以实现较高的掺杂比例。同时还发现，树枝状分子的支化结构能够阻碍偶氮苯基团的聚集，从而有效地提高了生色团在主体中的浓度。利用紫外可见吸收光谱确定了树枝状分子在主体分子PC中的最佳掺杂比例，研究了不同极化条件对于样品非线性光学性能的影响。实验结果表明，dendron的信号二次谐波产生（SHG）信号大于dendrimer，随着dendron代数的增加，SHG信号减弱，表明分子的极性和偶氮苯生色团在PC中取向的难易程度决定样品的非线性信号的强度。较低代数dendron的各个偶氮苯基团在极化电场作用下较易取向一致，从而提高了整个分子的一阶超极化率并可得到较高的SHG强度(d₃₃)。通过样品的松弛过程研究了不同树枝状分子对于掺杂体系非线性信号保持能力的影响，结果证明，全生色团官能化的树枝状分子可以有效地提高样品非线性光学性能，当生色团可以在极化电场方向上有效取向时，树枝状分子可以形成锥形的分子结构并且材料的SHG信号强度大大提高，这为设计合成新的非线性材料提供了科学依据。三、以偶氮苯为结构单元的含磷树枝状分子的合成及其聚集和光响应行为制备了以偶氮苯为结构单元的磷酸二酯树枝状两亲性分子，采用¹H NMR、¹³C NMR、³¹P NMR和MALDI-TOF-MS对分子结构进行了表征，结果证实所合成的含磷树枝状分子结构具有规整性和单分散性。利用稀释法和超声法制备树枝状分子在水溶液中的聚集体，并对其在水中的自组装行为进行了研究，结果发现不同树枝状分子由于分子结构、亲水性和制样方法的不同可以导致偶氮苯基团的聚集形态特征的较大差别，从而获得不同的聚集体吸收光谱和形貌。研究了两亲性树枝状分子在水中形成的聚集体的光响应行为，发现偶氮苯基团的构象差异会引起聚集体结构的巨大变化。利用钙黄绿素作为荧光指示剂，对用超声法形成的G2囊泡对客体分子的包裹和可控释放进行了研究，发现G2囊泡可以稳定地包裹钙黄绿素，并且在紫外光照射下可控释放。更多还原

【Abstract】 Azobenzene moieties can readily undergo reversible trans-cis photoisomerization, leading to the large structural change as reflected in the dipole moment, spatial requirement, and absorption spectra, which make azobenzene moieties good candidates as a light induced "molecular switch", and have been attracting increasing interest for many applications. Incorporating of azobenzene moieties into the skeletons of dendrimers has been demonstrated to provide interesting properties and possible applications, as well as the contribution to theoretical study on the conformational information of dendrimers using the drastic changes in photochemical and photophysical properties between two isomers of azobenzene. Therefore, the design and synthesis of new dendrimers constructed with azobenzene moieties are pursued to fully understand the structure-property correlation and to develop new-style nonlinear optical materials.1. Synthesis and characterization of dendrons and dendrimers skeleton-constructed with azobenzene moietyA series of dendrons and dendrimers skeleton-constructed with azobenzene moiety based upon 4-carboxy-4’-（1,2-propanediolether）-azobenzene as an AB₂ monomer, via a convergent approach, proceeding in a repeated stepwise growth manner starting from 4-carboxy-4’-（n-butylether）-azobenzene as a peripheral monomer, were synthesized, and characterized by NMR, FTIR, and MALDI-TOF-MS analysis. Their regular molecular architecture and thus monodispersed molecular weights were confirmed by GPC. The UV-vis absorbance and ¹H NMR spectrum study indicated that the azobenzene moieties in CHCl₃ solution took fully trans-cis isomerization under UV irradiation, and reversely isomerization back to the trans by visible light irradiation or by heat.2. Second-harmonic properties of dendritic polymers skeleton-constructed with azobenzene moiety used for nonlinear optical materialsThe synthesized dendrons and dendrimers were doped in a high T_g polycarbonate （PC） as a host for nonlinear optical （NLO） materials. The optimal loading density and poling conditions were investigated using UV-vis spectral and second-harmonic generation （SHG） measurements. The results showed that the dendrons and dendrimers have good solubility in PC host, which increased with the generation increased. Moreover, the SHG measurements indicated that the dendrons possess higher nonlinearity than the dendrimers. The chromophores of lower generation dendrons were easier to orient along the poling electric field and gave a cone shape with the azobenzene branching units, which coherently contributed to the molecular hyperpolarizability and resulted in the higher SHG intensity. The temporal stability of the dendrons with 15% loading density was also investigated, and showed that the decaying in nonlinearity was slower for higher generation dendrons.3. Aggregation behaviors and photoresponsive properties of azobenzene constructed phosphate dendrimersThe first to third generation of amphiphilic azobenzene constructed phosphate dendrimers （Gl to G3） were synthesized via a convergent strategy and characterized by NMR and MALDI-TOF-MS measurements. The aggregation behaviors and photoresponsive properties of their aggregates in water were investigated. The TEM observation and UV-vis absorption spectrum study showed that the morphologies of the aggregates are greatly affected by the preparation method. The multiple aggregation states, including H-aggregation and J-aggregation, were formed for azobenzene moieties in G2 and G3 aggregates obtained via sonication, resulting from their different distance from the hydrophilic phosphate group. The controlled release property of G2 vesicle was investigated using calcein as a fluorescence indicator, showing the potential use as a photoregulatable carrier.更多还原