Gotfredsen Henrik, Hergenhahn Janko, Duarte Fernanda, Claridge Timothy D W, Anderson Harry L
Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Oxford, OX1 3TA, U.K.
J Am Chem Soc. 2024 Sep 11;146(36):25232-25244. doi: 10.1021/jacs.4c09267. Epub 2024 Aug 26.
Extended π-systems often form supramolecular aggregates, drastically changing their optical and electronic properties. However, aggregation processes can be difficult to characterize or predict. Here, we show that butadiyne-linked 8- and 12-porphyrin nanorings form stable and well-defined bimolecular aggregates with remarkably sharp NMR spectra, despite their dynamic structures and high molecular weights (12.7 to 26.0 kDa). Pyridine breaks up the aggregates into their constituent rings, which are in slow exchange with the aggregates on the NMR time scale. All the aggregates have the same general two-layer sandwich structure, as deduced from NMR spectroscopy experiments, including H DOSY, H-H COSY, TOCSY, NOESY, and H-C HSQC. This structure was confirmed by analysis of residual dipolar couplings from C-coupled H-C HSQC experiments on one of the 12-ring aggregates. Variable-temperature NMR spectroscopy revealed an internal ring-on-ring rotation process by which two π-π stacked conformers interconvert via a staggered conformation. A slower dynamic process, involving rotation of individual porphyrin units, was also detected by exchange spectroscopy in the 8-ring aggregates, implying partial disaggregation and reassociation. Molecular dynamics simulations indicate that the 8-ring aggregates are bowl-shaped and highly fluxional, compared to the 12-ring aggregates, which are cylindrical. This work demonstrates that large π-systems can form surprisingly well-defined aggregates and may inspire the design of other noncovalent assemblies.
扩展的π体系常常形成超分子聚集体，从而极大地改变其光学和电子性质。然而，聚集过程可能难以表征或预测。在此，我们表明，尽管丁二炔连接的8卟啉和12卟啉纳米环具有动态结构和高分子量（12.7至26.0 kDa），但它们能形成稳定且结构明确的双分子聚集体，其核磁共振谱（NMR）非常尖锐。吡啶将聚集体分解为其组成的环，这些环在NMR时间尺度上与聚集体进行缓慢交换。通过包括氢扩散排序谱（H DOSY）、氢氢化学位移相关谱（H-H COSY）、全相关谱（TOCSY）、核Overhauser效应谱（NOESY）和氢碳异核单量子相干谱（H-C HSQC）在内的NMR光谱实验推断，所有聚集体都具有相同的一般双层夹心结构。通过对其中一个12环聚集体进行碳耦合氢碳异核单量子相干实验的剩余偶极耦合分析，证实了该结构。变温核磁共振光谱揭示了一种内环旋转过程，通过该过程，两个π-π堆积构象异构体通过交错构象相互转化。在8环聚集体中，通过交换光谱还检测到了一个较慢的动态过程，该过程涉及单个卟啉单元的旋转，这意味着部分解聚和重新缔合。分子动力学模拟表明，与呈圆柱形的12环聚集体相比，8环聚集体呈碗状且具有高度的流动性。这项工作表明，大π体系可以形成令人惊讶的结构明确的聚集体，并可能启发其他非共价组装体的设计。
