Fluolab【Angew.Chem.】开启反芳香性新篇章：在界面上将18-π卟啉精准扩张至20-π体系
文章标题： π‐Enlargement in Porphyrin Macrocycles at Interfaces
通讯作者： Ana Barragán, Aurelio Gallardo, Nazario Martín, David Écija
文章概要
引言
卟啉作为一种基础的杂环大环化合物，不仅在生物系统的氧气运输和光合作用中扮演核心角色，在催化、传感及光电材料等尖端技术领域也展现出巨大的应用潜力。长期以来，科学家们一直致力于通过溶液化学手段改良卟啉的宏观结构以调控其性能，而诱导和研究大环体系中的反芳香性已成为近年来的研究热点。通过将电子计数增加到超越传统的18-π框架，可以构建具有独特氧化还原和光学行为的4n π电子回路。然而，由于溶解度挑战和构象不稳定等限制，在溶液中合成高度平面的反芳香性扩张卟啉极具挑战。本研究另辟蹊径，利用表面共价合成技术，在超高真空环境下成功实现了卟啉大环的骨架重排，开创性地在界面上完成了从芳香性体系向反芳香性体系的跨越。
Chemical scheme of the on-surface transformation of precursor 1 into the 20-π expanded porphyrin 2, and reaction products 3, 3_1, 3_2 and 4.
主要实验及结论
研究团队精心设计并合成了一种ABAB型卟啉前驱体，其特征是在对角（trans）位点的内源位置修饰了两个三氟甲基（-CF3） 基团。实验过程中，将该前驱体沉积在保持于200至300摄氏度高温的Ag(111)表面上。通过结合扫描隧道显微镜（STM） 和非接触原子共力显微镜（nc-AFM） 观察发现，前驱体发生了深度脱卤反应，原本属于三氟甲基的碳原子被成功插入到卟啉大环骨架中，形成了两个亚乙烯基桥联，从而将原本的18-π电子系统扩张为20-π乙烯基卟啉体系。与此同时，外围的蒽基取代基通过氧化环化反应进一步增强了整个分子的π共轭程度。
On-surface synthesis of the 20-π expanded porphyrin (2), alongside concomitant reaction products by depositing precursor 1 on a Ag(111) substrate held at 200 °C. (a) Overview STM image after deposition of precursor 1 on an Ag(111) substrate held at 200 °C. The different transformed expanded porphyrin species identified are marked by the blue squares. = 50 mV, = 50 pA, scale bar = 4.0 nm. (b, c) Top and side views of the adsorption geometry simulated by DFT of the transformed species 2, respectively. (d, e) Constant-current (with chemical sketch superimposed. = 500 mV, = 50 pA, scale bar = 5 Å) and constant-height ( = 5 mV, scale bar = 5 Å) STM images, respectively, of the transformed 20-π expanded porphyrin 2. (f, g) Laplace-filtered constant-height frequency-shift nc-AFM image (Z-offset: 150 pm above STM set point: 5 mV, 50 pA. Scale bar = 5 Å), acquired with a CO-functionalized tip, and corresponding calculated frequency-shift image, of species 2. (h–j) Constant-current STM ( = -100 mV, = 50 pA, scale bar = 5 Å), constant-height STM ( = 5 mV, scale bar = 5 Å nm), and Laplace-filtered constant-height frequency-shift nc-AFM (Z-offset: 100 pm above STM set point: 5 mV, 50 pA) images of species 3, respectively. (k-m) Constant-current STM ( = 500 mV , = 50 pA, scale bar = 5 Å), constant-height STM ( = 5 mV, scale bar = 5 Å), and Laplace-filtered constant-height frequency-shift nc-AFM (Z-offset: 140 pm above STM set point: 5 mV, 50 pA) images of a section of oligomer 4.
First three steps of the reaction mechanism for the incorporation of a carbon atom into the porphyrin backbone upon deposition of 1 on a Ag(111) substrate held at 200 °C and subsequent dehalogenation of -CF3 moieties. Free energy profile of the first three steps of the reaction mechanism proposed for the transformation of 1 into 2, calculated within the QM/MM formalism.
量子化学计算进一步揭示了这一转化背后的奥秘。密度泛函理论（DFT） 计算表明，扩张后的20-π卟啉在界面上保持了极其优异的平面构象，这为增强反芳香性提供了结构基础。通过NICS(0)计算得出的数值高达19.0 ppm，有力地证明了其强烈的反芳香特征。此外，扫描隧道谱（STS）测量显示该分子的带隙极窄，仅为0.2 eV左右，这种低带隙特性正是反芳香化合物的典型电子特征，预示着其在低能电子跃迁器件中的应用前景。有趣的是，这种特定的扩张反应在Au(111)表面上并未发生，充分证明了银基底在诱导界面化学反应中的关键模板作用。研究团队最后还探索了该扩张体系的配位能力，成功实现了钴原子的金属化，形成了一种独特的二配位氮-钴中心，这与传统卟啉的四配位模式形成了鲜明对比。
Antiaromaticity calculations and electronic characterization of 2 on Ag(111). (a) NICS(0) calculation of 2. NICS(0) positive (negative) values indicate antiaromatic (aromatic) character. To facilitate the interpretation of the NICS(0) values we plotted the positive (negative) in blue (red) and added clock-(counter clock-)wise arrows, based on their relation to the induced magnetic current density. (b) Long-range d_I_/d_V_ spectra (right panel) acquired on the positions indicated by colours in the topographic STM image (left panel).
Metallation of 20-π (2.1.2.1) porphyrin on Ag(111) by cobalt. Scanning probe microscopy of distinct products obtained after deposition of precursor 1 on Ag(111) substrate held at 300°C and dosing of Co atoms for 4 min. (a, b) Constant-current ( = 0.5 V, = 30 pA, scale bar = 5 Å) and constant-height ( = 5 mV, scale bar = 5 Å) STM images of the Co-metallated transformed 20-π expanded porphyrin, 2-Co. (c) Corresponding experimental Laplace-filtered constant-height frequency-shift nc-AFM image, acquired with a CO functionalized tip (Z-offset: 145 pm above STM set point: 5 mV, 50 pA. Scale bar = 5 Å). d) Top and side view of the DFT simulation of the relaxed geometry of 2-Co on Ag(111). (e–g) Constant-current STM ( = 0.5 V, = 50 pA, scale bar = 5 Å), constant-height STM ( = 5 mV, scale bar = 5 Å), and Laplace-filtered constant-height frequency-shift nc-AFM (Z-offset: 160 pm above STM set point: 5 mV, 50 pA) images of the species 3 metallated with Co, 3-Co, respectively. (h–j) Constant-current STM ( = 0.5 V, = 50 pA, scale bar = 6 Å), constant-height STM ( = 5 mV, scale bar = 6 Å), and Laplace-filtered constant-height frequency-shift nc-AFM (Z-offset: 200 pm above STM set point: 5 mV, 50 pA) images of a metallated oligomer 4-Co.
总结及展望
这项研究成功展示了一种在界面上精确操纵大环分子骨架的新策略，通过碳原子插入技术实现了卟啉从芳香性到反芳香性的受控转变。表面环境不仅稳定了在溶液中难以存在的扩张体系，更通过界面相互作用强制分子保持平面化，锁定了其独特的物理化学性质。这一成果不仅丰富了表面科学在原子级精准合成领域的手段，也为设计新型反芳香性功能材料、开发高效光电转换器件以及构建特殊配位环境的单分子催化剂开辟了全新的路径。未来，这种基于界面的骨架扩张策略有望应用于更多复杂的大环体系，助力探索分子电子学中更深层次的物理规律。