Evolution of homochiral helical dye assemblies:: Involvement of autocatalysis in the "Majority-Rules" effect
Evolution of homochiral helical dye assemblies:: Involvement of autocatalysis in the "Majority-Rules" effect
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DOI：
10.1002/anie.200704550
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Wuerthner, Frank
中科院分区：
文献类型：
--
作者：
Lohr, Andreas;Wuerthner, Frank
尽管这一有趣的自然现象吸引了一代又一代不同学科的研究人员，但自然界中手性的演变仍然不清楚。[1，2]同手性发生的主要要求之一是在生物相关结构中从较小的初始对映体偏差中产生大量的对映体过剩。[2]在自催化不对称反应中观察到了这种“手性放大”，在螺旋大分子、[4，5]和超分子组件中观察到了这种“手性放大”。[6-8]这些系统中的一些手性放大被用“多数规则”效应解释，这意味着手性单体的轻微对映体过量决定了整体的螺旋意义。[8]过去，在超分子体系中只探索了“多数规则”效应的热力学。本文报道了手性双(花菁)染料自组装过程中“多数规则”效应的动力学。我们的研究揭示了从不同对映体过量的单体到明确定义的纳米棒的双(花菁)染料的复杂自组装序列，并为自催化参与“多数规则”效应提供了证据。我们目前的发现有助于从机制上理解同种异体发生和螺旋纳米结构的形成过程。我们最近报道了非手性双(花菁)染料1通过超分子聚合和分级自组装自组装成高度定义的纳米棒。[9]分子模拟研究表明，这些纳米棒是由六个螺旋缠绕的超分子单链聚合物创建的，发色团以卡片包装的方式组织，并螺旋缠绕在纳米棒的长轴上。通过应用在酰亚胺位置上带有两个(R)-2-辛基侧链的手性双(R，R)-2，我们通过原子力显微镜(AFM)和圆二色谱(CD)研究为这些超分子结构的螺旋性提供了直接证据。[10]更有趣的是，这种对手性染料(R，R)-2的研究揭示了一个罕见的超分子立体位移的例子，在从最初的运动学自组装纳米棒(记为H1)到热力学平衡自组装纳米棒(记为H2)的复杂转变过程中。这两种不同类型的纳米棒显示了不同的CD光谱，并且它们的形态螺旋间距也有很大的不同，AFM观察到。[10]这些纳米棒的螺旋意义由手性2-辛基侧链的绝对构型决定。因此，我们提出了一个问题，即在这些超分子组装中是否可以观察到手性的放大，特别是由“多数规则”效应引起的手性放大，如果是的话，这种放大的机制是什么？为了探讨这些问题，我们研究了不同对映体过量(Ee)的(R，R)-2和(S，S)-2单体共组装的聚集体的立体化学行为。[11]这些研究为手性放大在对映体染料2的自组装中提供了明确的证据，更有趣的是，揭示了自催化参与了“多数规则”导向的手性放大过程。动力学研究揭示了染料2的复杂自组装序列，如图1所示。由具有不同ee值的2的对映体混合物形成的H1和H2聚集体的各向异性因子g(Δε/ε)由CD和UV/Vis光谱在437 nm处确定(图2a和辅助信息)。由于最初的H1纳米棒的形成速度非常快，…
The evolution of homochirality in nature is still not clearly understood despite the fact that this intriguing natural phenomenon has attracted generations of researchers of various disciplines.[1, 2] One of the major requirements for homochirogenesis is the creation of a large enantiomeric excess in biologically relevant structures from a small initial enantiomeric bias.[2] Such “amplification of chirality” has been observed in autocatalytic asymmetric reactions,[3] in helical macromolecules,[4, 5] and supramolecular assemblies.[6–8] The chiral amplification in some of these systems has been explained by the “majority-rules” effect, which implies that a slight enantiomeric excess of chiral monomers dictates the overall helical sense.[8] In the past, only the thermodynamics of the “majority-rules” effect were explored in supramolecular systems. Here we report on the kinetics of the “majority-rules” effect in the self-assembly of chiral bis (merocyanine) dyes. Our studies reveal a complex selfassembly sequence of bis (merocyanine) dyes towards welldefined nanorods from monomers of different enantiomeric excess and provide evidence for the involvement of autocatalysis in the “majority-rules” effect. Our present findings contribute to the mechanistic understanding of homochirogenesis and the formation processes of helical nanostructures. We recently reported that achiral bis (merocyanine) dye 1 self-assembles into highly defined nanorods through supramolecular polymerization and hierarchical self-assembly.[9] Molecular modeling studies suggested that these nanorods are created from six helically intertwined supramolecular single-stranded polymers, with the chromophores organized in a card-pack fashion and helically wound around the long axis of the nanorods. By applying chiral bis (merocyanine) derivative (R, R)-2, which bears two (R)-2-octyl side chains at the imide positions, we have provided direct evidence for the helicity of these supramolecular structures through atomic force microscopy (AFM) and circular dichroism (CD) studies.[10] More interestingly, this investigation with chiral dye (R, R)-2 disclosed a rare example of a supramolecular stereomutation in the course of a complex transition process from initial kinetically self-assembled nanorods (denoted as H1) into thermodynamically equilibrated self-assembled nanorods (denoted as H2). These two different types of nanorods showed distinct CD spectra and their morphological helical pitch differs significantly, as observed by AFM.[10] The helical sense of these nanorods is governed by the absolute configuration of the chiral 2-octyl side chains. Thus, we raised the question as to whether amplification of chirality, in particular that arising from the “majority-rules” effect, can be observed in these supramolecular assemblies and, if so, what are the mechanistic pathways for such an amplification. To approach these questions, we have studied the stereochemical behavior of aggregates that are coassembled from enantiomeric (R, R)-2 and (S, S)-2 monomers of various enantiomeric excess (ee).[11] These studies provide clear evidence for chiral amplification in the self-assembly of enantiomeric dyes 2 and, more intriguingly, disclose the involvement of autocatalysis in the “majority-rules”-directed chiral amplification process. The complex self-assembly sequence for dyes 2 revealed by kinetic investigations is depicted in Figure 1.The anisotropy factors g (Δε/ε) for H1 and H2 aggregates formed from enantiomeric mixtures of 2 with various ee values were determined from CD and UV/Vis spectra at 437 nm (Figure 2a and Supporting Information). Since the formation of the initial H1 nanorods is very fast …