【作者】 路伟；

【导师】 江黎明；

【作者基本信息】 浙江大学 ， 高分子化学与物理， 2014， 博士

【摘要】 手性是自然界中普遍存在的一种现象,是生命过程的基本特征之一。最为人们熟知的是蛋白质的α-螺旋结构和DNA的双螺旋结构,它们的基本组成单元分别是左旋(L-型)氨基酸和右旋(D-型)核糖,但都具有右手螺旋的二级结构。设计、合成手性功能分子和手性功能聚合物吸引了化学、材料科学、生命科学等诸多学科的广泛关注。其中,手性分子开关因其在化学传感、对映体识别、分子器件及信息存储等领域的广阔应用前景而成为一个热点研究方向。手性光学开关是指手性体系在特定外界因素(光、电、热、pH等)刺激下其光学性质表现出可逆变化的过程。通常,以位阻烯烃、偶氮苯、联萘、螺吡喃为基本骨架的小分子光学开关机制基于π-键或σ-键的断裂与形成；超分子聚集体、螺旋聚合物、有机折叠体以及配体呈螺旋排列的金属配合物则通过非共价弱相互作用(氢键、π-π堆积、静电吸引等)的破坏和重组实现手光切换功能。尽管手性开关体系已有大量文献报道,但是,构筑结构简单、响应快速、高度可逆、长使用寿命的手性分子开关仍然极具挑战。本论文从模型分子入手,详细研究了手性水杨醛Schiff碱和水杨酰胺衍生物的光谱性质以及在酸-碱作用下的手光信号反转现象。研究发现,在两类手性化合物溶液中加入碱(TBA+OH-),导致CD光谱显著变化,其重要特征是Cotton效应红移、符号反转；若酸化则使该过程完全逆转,循环五次未见信号强度衰减,由此实现了可酸碱调控的手性光学开关。DFT分子构象分析和量子化学理论计算表明,在此脱质子／质子化过程中,伴随着分子内氢键的断裂与形成。基于TDDFT的ECD理论计算与实测CD曲线相当吻合；而且,由激子手性规则可准确预测脱质子前后的Cotton效应符号。此类手光反转源于C-C单键旋转所引起的两种相对稳定构象之间的转变,具有切换快速、高度可逆和无损输出等优点。上述发现为合理设计功能单体、手性聚合物基分子开关和化学传感器奠定了基础。按照这个思路,我们开展了以下研究工作：利用ATRP聚合技术和后修饰策略将手性邻羟亚苄胺基(SA)发色团引入聚合物侧链,成功合成了一类新型的手性聚甲基丙烯酸甲酯衍生物P(s-BAPM)s。结果表明,在酸碱诱导下,该聚合物在DMSO溶液中表现出与上述小分子手性开关相同的手光信号反转行为,获得了酸碱可控的聚合物基手性光学开关分子。作为CD/荧光双检测通道化学传感器,该聚合物可在Cl-、Br-、CH3CO2-和C6H5CO2-等多种阴离子共存时高选择性识别氟离子,而且对扁桃酸显示出良好的对映选择性识别效果。我们进一步设计了含手性亚胺结构单元的苯乙烯型功能单体(VNP),分别利用RAFT聚合技术和传统的自由基聚合方法制备了均聚物PVNP和共聚物Poly(HEMA-co-VNP)。研究发现,在常见的阴离子(F-、H2PO4-、AcO-、Cl-、Br-和I-)中,只有HSO4-能诱导聚合物的CD光谱发生独特的变化,在～382nm处出现新的(-)-Cotton效应,此外,还伴随荧光发射大大增强。因此这两种聚合物可作为双检测通道传感器高选择性识别硫酸氢根离子。由硫酸氢根诱导的聚合物光谱变化可被碱完全复原,当交替加入HSO4-和OH-时,可观察到CD信号以"ON"-"OFF"方式可逆转变,循环5次未见强度明显衰减。共聚物Poly(HEMA-co-VNP)具有优良的亲水性和成膜性,采用简单的旋涂法可在石英片上形成高质量的薄膜。将薄膜浸入NaHSO4水溶液1分钟后取出晾干,在荧光显微镜下观察到明亮的荧光发射。该薄膜探针对HSO4-的检测下限为50μM,比相应的溶液传感器更加灵敏。借助于普通的紫外灯光照可裸眼观察HSO4-试样形成的荧光斑点,经重复检测—纯水冲洗数次其检测效果保持稳定。这是迄今为止报道的第一例可用于检测水性介质中HSO4-离子的聚合物薄膜荧光传感器。更多还原

【Abstract】 Chirality can be observed on various levels in nature, which is one of basic characteristics of living process. The well-known α-helix protein and double-helix DNA possess the right-handed second structure although the former and the latter consist of levorotatory amino acids and dextrorotatory ribose, respectively. Research related to chiral molecules and chiral polymers has been received increasing attention from different scientific fields including chemistry, materials and life science. Especially, chiroptical switches has become one of the most topics due to the potential applications in the chemosensing, enantioselective recognition, molecular device, data storage and so on.A chiral molecular switch is a molecule that can be reversibly shifted between two or more stable states with distinct chiroptical properties. The molecules may be shifted between the states in response to environmental stimuli, such as changes in pH, light, temperature, an electrical current, or in the presence of a ligand. Usually, chiral molecular switches based on the sterically overcrowded olefins, diarylethenes, and helicenes, the reversible and stereochemically controlled transformation of one optically pure diastereomer to another is realized through the cleavage and formation of a π-bond even of σ-bond. For chiral supramolecular aggregates, helical polymers, foldamers and metal complexes, the chiroptical switch is driven by the the cleavage and reorganization hydrogen-bonding, π-π stacking, electrostatic interaction or metal coordination. Although a lot of chiral switch systems have been reported, the development of a chiral molecular switch with structural simplicity, rapid responsiveness, high reversibility, and long lifetime is still challenging.Herein we describe a new approach to construct chiral molecular switches with the features of stimuli-controlled and reversible chiroptical inversion. Its fundamental concept involves an interchange of two conformations presenting diverse chiroptical properties by acid/base-mediated C-C single bond rotation. As a proof of principle, two simple chiral molecular switches have been established using N-salicylidene Schiff base and salicylamide as the peculiar skeleton motifs. Upon addition of base in solution, the circular dichroism (CD) spectra of these chiral compounds displayed unique changes featuring an inversion of the Cotton effect’s signs, and the original CD profiles can be recoverd thoroughly by acidification. Various spectroscopic studies as well as the conformational analysis combining with time-dependent density functional theory (TDDFT) calculations allowed clear elucidation of the chiroptical inversion mechanism. Furthermore, such a chiroptical inversion may be interpreted in terms of the exciton chirality rule. Compared to the existing molecule-based chiral inversion systems, this promising new type of chiroptical switching molecule is relatively unique as it does not involve any covalent bond formation/breakage and thus possesses distinct advantages, such as fast switching rate, high reversibility and fatigue resistance, and the nondestructive readout. Such dynamic chiroptical inversion systems are expected to find potential applications in molecular recognition, chemosensors, or the construction of molecular-scale devices. More importantly, these findings suggest that the use of the conformational transition about a single bond may serve as the basis for designing chiroptical inversion systems.Thus, by using ATRP technique in combination with post-modification the stimuli-responsive chiral salicylidenimino (SA) motif was covalently attached to the pendant group of polymethacrylates to construct a new kind of acid/base-controlled polymeric chiroptical switches. As a chemosensor, the resulting polymer showed high selectivity for F-relative to other anions including Cl-, Br-, CH3CO2, and C6H5CO2in DMSO solution as judged from fluorescence and circular dichroism spectrophotometric titrations. Moreover, this dynamic chiroptical inversion system can be employed as an enantioselective chemosensor for a-hydroxy acids such as mandelic acid.In addition, we designed a new functional monomer containing chiral imine unit, N-{[2-(4-vinylbenzyloxy)-1-naphthyl]-methylene}-(,S)-2-phenylglycinol (VNP). The homopolymer PVNP and copolymer poly(HEMA-co-VNP) were synthesized through RAFT and conventional free radical polymerization, respectively. The two polymers showed a highly selective red-shifted emission and a unique chiroptical response upon HSO4-binding in organic solution. Interestingly, the HSO4--induced CD or fluorescence signal can be totally reversed with addition of base and eventually recovered the initial state, leading to a reproducible molecular switch with two distinguished "on" and "off’ states. As expected, the copolymer has excellent hydrophilicity, flexibility and good film-forming properties. Thus, high-quality film probe could be easily fabricated on quartz plates through spin-casting techniques. The resultant polymeric films can recognize HSO4-ion among a series of common anions in aqueous solution with high selectivity and sensitivity; the detection limit determined based on the S/B criteria is ca.50μM. The promising new film probe for HSO4-has distinct characteristics such as rapid response, enough stability in aqueous media, and practicality. To the best of our knowledge, until now no similar reports on polymer-based fluorescent sensors, particularly polymeric film probe, have been made for the selective detection of bisulfate ions in aqueous media.更多还原