Highly Strained, Radially π‑Conjugated Porphyrinylene Nanohoops

Small π-conjugated nanohoops are difficult to prepare, but offer an excellent platform for studying the interplay between strain and optoelectronic properties, and, increasingly, these shape-persistent macrocycles find uses in host–guest chemistry and self-assembly. We report the synthesis of a new family of radially π-conjugated porphyrinylene/phenylene nanohoops. The strain energy in the smallest nanohoop [2]­CPT is approximately 54 kcal mol–1, which results in a narrowed HOMO–LUMO gap and a red shift in the visible part of the absorption spectrum. Because of its high degree of preorganization and a diameter of ca. 13 Å, [2]­CPT was found to accommodate C60 with a binding affinity exceeding 108 M–1 despite the fullerene not fully entering the cavity of the host (X-ray crystallography). Moreover, the π-extended nanohoops [2]­CPTN, [3]­CPTN, and [3]­CPTA (N for 1,4-naphthyl; A for 9,10-anthracenyl) have been prepared using the same strategy, and [2]­CPTN has been shown to bind C70 5 times more strongly than [2]­CPT. Our failed synthesis of [2]­CPTA highlights a limitation of the experimental approach most commonly used to prepare strained nanohoops, because in this particular case the sum of aromatization energies no longer outweighs the buildup of ring strain in the final reaction step (DFT calculations). These results indicate that forcing ring strain onto organic semiconductors is a viable strategy to fundamentally influence both optoelectronic and supramolecular properties.小型π共轭纳米环（π-conjugated nanohoops）的合成极具挑战性，但其为探究张力与光电性质间的相互作用提供了优质研究平台；与此同时，这类形状持久的大环化合物（shape-persistent macrocycles）在主客体化学（host–guest chemistry）与自组装（self-assembly）领域的应用正不断拓展。本研究报道了一类全新的径向π共轭卟啉撑/亚苯基纳米环（radially π-conjugated porphyrinylene/phenylene nanohoops）的合成路线。其中尺寸最小的纳米环[2]CPT的张力能（strain energy）约为54 kcal·mol⁻¹，该特性使其最高占据分子轨道-最低未占据分子轨道（HOMO-LUMO）能隙收窄，同时吸收光谱的可见光波段出现红移。得益于高度的预组织性与约13 Å的直径，X射线晶体学（X-ray crystallography）表征显示，尽管富勒烯（fullerene）并未完全嵌入主体空腔，[2]CPT对C₆₀的结合亲和力（binding affinity）仍超过10⁸ M⁻¹。此外，本研究采用相同策略合成了π拓展型纳米环（π-extended nanohoops）[2]CPTN、[3]CPTN与[3]CPTA（其中N代表1,4-亚萘基（1,4-naphthyl），A代表9,10-亚蒽基（9,10-anthracenyl））；实验结果表明，[2]CPTN对C₇₀的结合强度是[2]CPT的5倍。我们在合成[2]CPTA时未能成功，这一结果凸显了当前制备张力型纳米环最常用实验方法的局限性：密度泛函理论（DFT, Density Functional Theory）计算显示，在该反应的最终步骤中，芳香化能（aromatization energies）的总和已不足以抵消环张力（ring strain）的累积。上述研究结果表明，向有机半导体（organic semiconductors）引入环张力，是从根本上调控其光电与超分子性质（supramolecular properties）的可行策略。