Institute for Complex Molecular Systems, Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
J Am Chem Soc. 2011 Aug 10;133(31):12238-46. doi: 10.1021/ja204543f. Epub 2011 Jul 15.
We present our results on the mixing of different porphyrin molecules in supramolecular assemblies. Herein, chiral amplification experiments reveal the subtle role of the structural (mis)match between these monomers. We show that according to the "sergeant-and-soldiers" principle, a chiral porphyrin "sergeant" efficiently mixes with achiral "soldiers" in the same helical aggregate and strongly biases its handedness. However, when we mix two porphyrin enantiomers in a majority-rules experiment, no chiral amplification is observed at all, which is due to their narcissistic self-sorting into conglomerate-like aggregates. The mixing between two enantiomers in the same stack only occurs in a diluted-majority-rules experiment, in which enantiomeric mixtures of sergeants are diluted with achiral soldiers. The different outcomes of these chiral amplification phenomena are verified by modeling studies that reveal high mismatch penalties, which are ascribed to the high stereocenter loading of 12 methyl groups onto the monomers. Mixed-metal chiral amplification experiments between copper- and zinc-porphyrins show the same distinction in their mixing behavior, which is further supported by fluorescence measurements. The selective removal of chiral Zn-porphyrins from these mixed-metal systems is performed with the Lewis base quinuclidine that depolymerizes the Zn-porphyrins upon axial ligation. This extraction process proceeds at different time scales, depending on the mixed state: slow extraction kinetics for the mixed sergeant-and-soldiers and diluted-majority-rules systems and an instant extraction for the phase-separated majority-rules system. By simultaneously monitoring the supramolecular chirality during extraction, a chiral memory effect is observed for both mixed systems that show slow extraction kinetics. For the sergeant-and-soldiers system, the remaining supramolecular backbone contains achiral monomers only, which give rise to a long lasting chiral memory with slow, entropy-driven atropisomerization. Yet in case of the diluted-majority-rules system, the remaining backbone contains a mixture of achiral and chiral monomers in its unpreferred helicity; giving rise to a short chiral memory, in which the fast atropisomerization is enthalpy-driven due to the high mismatch penalty.
我们展示了在超分子组装中混合不同卟啉分子的结果。在这里，手性放大实验揭示了这些单体之间结构（不）匹配的微妙作用。我们表明，根据“长官与士兵”原理，手性卟啉“长官”可以有效地与同一螺旋聚集体中的非手性“士兵”混合，并强烈偏向其手性。然而，当我们在多数规则实验中混合两种卟啉对映体时，根本没有观察到手性放大，这是由于它们自恋地自我分类成聚集体样聚集体。在同一堆积中混合两种对映体仅发生在稀释多数规则实验中，其中“长官”的对映体混合物与非手性“士兵”稀释。这些手性放大现象的不同结果通过建模研究得到验证，这些研究揭示了高错配罚分，这归因于单体上 12 个甲基的高立体中心负载。铜-和锌-卟啉之间的混合金属手性放大实验显示出它们混合行为的相同区别，这进一步得到荧光测量的支持。用路易斯碱喹啉啶选择性地从这些混合金属体系中去除手性 Zn-卟啉，该路易斯碱通过轴向配位使 Zn-卟啉解聚。这个提取过程在不同的时间尺度上进行，这取决于混合状态：混合的“长官和士兵”和稀释多数规则体系的缓慢提取动力学以及相分离的多数规则体系的瞬间提取。通过在提取过程中同时监测超分子手性，观察到两个混合体系都具有手性记忆效应，其提取动力学较慢。对于“长官和士兵”体系，剩余的超分子主链仅包含非手性单体，这导致具有缓慢、熵驱动的前手性异构化的持久手性记忆。然而，在稀释多数规则体系的情况下，剩余的主链在其不优先的螺旋中包含非手性和手性单体的混合物；由于高错配罚分，导致快速前手性异构化是焓驱动的，因此具有短暂的手性记忆。
