FIRST OPTICALLY ACTIVE DIARYLPOLYSILANES : FACILE HELICAL SCREW SENSE CONTROL WITH ONLY (S)-ENANTIOPURE SIDE CHAINS
FIRST OPTICALLY ACTIVE DIARYLPOLYSILANES : FACILE HELICAL SCREW SENSE CONTROL WITH ONLY (S)-ENANTIOPURE SIDE CHAINS
复制标题
第一个光学活性二芳基聚硅烷：仅具有 (S)-对映体纯侧链的简便螺旋螺杆感应控制
DOI：
10.1021/ja991680c
复制
发表时间：
1999
影响因子：
15
通讯作者：
H. Nakashima
中科院分区：
文献类型：
--
作者：
J. Koe;and Michiya Fujiki;H. Nakashima
与无机半导体 LED（例如 III-V 和 II-VI 宽带隙材料）相比，基于有机聚合物的发光二极管 (LED) 具有吸引力，原因有几个，包括低成本、易于制造大面积薄膜、可加工性以及通过选择取代基实现发射波长的化学可调性。 1 另一个优点是，发射光的圆偏振 (CP) 可以通过对聚合物高阶结构进行适当的化学修饰来获得，而不是使用圆偏振滤光片，对于相同的发光输出强度，圆偏振滤光片可能需要更强烈地驱动器件，从而增加功耗并缩短寿命。 CP 电致发光的第一个例子是针对含有对映体纯手性侧链的可溶性聚（对亚苯基亚乙烯基）共聚物进行报道的。 2 在聚硅烷中，3-5 因其独特的光学和电子特性而备受关注，6 据报道，最大的电致发光 (EL) 效率是二芳基聚硅烷、双（对正丁基苯基）聚硅烷，7, 8 在这些实验室中观察到其在 407 nm 处的发射。我们现在正在研究含有对映纯手性侧链的二芳基聚硅烷，并在此报告了带有 (S)-2-甲基丁基的光学活性螺旋二芳基聚硅烷均聚物的首次合成，以及对圆二色性 (CD) 光谱中与优先 P（正）或 M（负）聚合物螺旋螺旋方向相关的 Cotton 效应的符号和强度的影响和轻松控制，仅通过每个 Si 重复单元的数量和芳基环取代位置(S)-对映体手性取代基。对于螺旋聚合物主链构象，手性侧链的存在以及它们之间产生的立体关系可能会引起聚合物主链的优先螺旋方向。对于此处感兴趣的聚硅烷，这将导致圆二色性 (CD) 光谱中出现棉花效应，与电子离域硅主链的 σ-σ* 跃迁导致的 UV 吸收一致。最低能量的聚硅烷 σ-σ* 跃迁对电子和空间取代基效应都很敏感：6, 9 前者通过硅-硅和硅侧取代基轨道重叠，后者通过它们对主链全局和局部构象的影响。尽管仍然是一个备受争议的话题（特别是在固态），但溶液状态下的聚硅烷主链构象已经根据介观片段内的微观螺旋或全反式域进行了讨论。 10 对于双芳基取代的聚硅烷，虽然理论上指出了取代基对芳基环主链轨道混合产生的异常长 UV λmax 值的贡献，11a 也有人建议它们可能采用全反式构象。 11b 聚硅烷通常不具有光学活性，这是由于采用了全反式构象，或者在螺旋聚硅烷的情况下，由于具有等量的 P（正）和 M（负）螺旋方向螺旋。然而，当改变 P 和 M 旋向的相对能量时，正如在许多二烷基 12 和烷基芳基聚硅烷 13 体系中所证明的那样，通过引入对映体纯手性侧链或端基，14 表现出旋光性。二芳基聚硅烷 H-(SiPh2) 30-H 模型的力场计算 15 在扭转角 165 和 195 处产生局部最小值，对应于 157 螺旋构象并表明非全反式结构。在这些计算的基础上，为了获得具有普遍螺旋感的二芳基聚硅烷，我们设计了四种 [(Ar')(Ar'') Si] n …
Organic polymer-based light-emitting diodes (LEDs) are attractive for several reasons, including low cost, ease of fabrication of large area thin films, processability, and chemical tunability of emission wavelength through choice of substituents, compared with inorganic semiconducting LEDs such as III-V and II-VI wide band gap materials. 1 An additional advantage is that circular polarization (CP) of emitted light may be obtained by suitable chemical modification of the polymer higher order structure, rather than by use of circularly polarizing filters, which for the same luminescence output intensity may require the device to be driven harder, increasing power consumption and decreasing the lifetime. The first example of CP electroluminescence was reported for a soluble poly (p-phenylenevinylene) copolymer containing enantiopure chiral side chains. 2 Among polysilanes, 3-5 of interest for their unique optical and electronic properties, 6 the greatest reported electroluminescence (EL) efficiency has been that of a diarylpolysilane, bis (p-n-butylphenyl) polysilane, 7, 8 for which emission at 407 nm was observed in these labs. We are now investigating diarylpolysilanes containing enantiopure chiral side chains and report here the first synthesis of optically active helical diarylpolysilane homopolymers bearing (S)-2-methylbutyl groups and the effects on, and facile control of, the sign and intensity of the Cotton effect associated with a preferential P (plus) or M (minus) polymer helical screw sense in the circular dichroism (CD) spectra by the number per Si repeat unit and aryl ring substitution position of only (S)-enantiomer chiral substituents. For a helical polymer backbone conformation, the presence of chiral side chains, and their resulting stereorelationship with each other, may induce a preferential screw sense of the polymer main chain. For the polysilanes of interest here, this would result in the appearance of a Cotton effect in the circular dichroism (CD) spectrum, coincident with the UV absorption due to the σ-σ* transition of the electronically delocalized silicon main chain. The lowest energy polysilane σ-σ* transition is sensitive to both electronic and steric substituent effects: 6, 9 the former through silicon-silicon and silicon-side substituent orbital overlaps and the latter through their effects on the main chain global and local conformations. Though still a subject of much debate (especially in the solid state) polysilane backbone conformations in the solution state have been discussed in terms of microscopic helical or all-trans domains within mesoscopic segments. 10 For bisarylsubstituted polysilanes, while the substituent contribution to the unusually long UV λmax values from aryl ring-backbone orbital mixing is indicated theoretically, 11a it has also been suggested that they may adopt an all-trans conformation. 11b Polysilanes are generally optically inactive, due either to the adoption of an all-trans conformation or, in the case of a helical polysilane, to equivalent amounts of both P (plus) and M (minus) screw sense helices. When the relative energies of P and M screw senses are altered, however, as has been demonstrated in a number of dialkyl-12 and alkyl-arylpolysilane13 systems, through the incorporation of enantiopure chiral side chains or end groups, 14 optical activity is exhibited. Force field calculations15 for the model diarylpolysilane H-(SiPh2) 30-H produce local minima at torsion angles of 165 and 195, corresponding to a 157 helical conformation and suggesting a non-all-trans structure. On the basis of these calculations and to obtain diarylpolysilanes with a prevailing screw sense, we designed four diarylpolysilanes of the type [(Ar′)(Ar′′) Si] n …