Murai Masahito, Abe Mikiya, Ogi Soichiro, Yamaguchi Shigehiro
Department of Chemistry, Graduate School of Science and Integrated Research Consortium on Chemical Science (IRCCS), Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan.
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan.
J Am Chem Soc. 2022 Nov 9;144(44):20385-20393. doi: 10.1021/jacs.2c08372. Epub 2022 Oct 28.
We disclose a series of silicon-bridged diazulenylmethyl cations as stable and one-dimensionally π-extended carbocations. Connecting nonbenzenoid azulenes to a carbocation center at an appropriate position while reinforcing a planar arrangement effectively delocalizes a positive charge over the π-conjugated skeleton. This structural constraint endows the carbocations with not only high chemical stability with the p value of around 9.5, despite the absence of any electron-donating substituents, but also an intense absorption in the red region due to the effective enhancement of the transition dipole moment. X-ray crystallographic analysis revealed an offset π-stacked arrangement with the outer seven-membered ring overlapping in a face-to-face manner, in which both the steric bulk of the vertically oriented substituents on the silicon atom and the location of the counter anion should play a crucial role. Reflecting this molecular arrangement, the π-extended cations exhibited a red-shifted absorption in the near-infrared (NIR) region in both the solid state and aggregated state in solution, indicative of the formation of -aggregates. More pronounced redshifts in the absorption band upon the formation of the aggregates were observed by proper choice of the substituents on the silicon bridge and the counter anions, and the aggregates exhibited sharp fluorescence bands with the maximum emission wavelength exceeding 800 nm. These results demonstrate the impact of the nonbenzenoid aromatic stabilization of a carbocation and the efficacy of the present design strategy for the construction of a promising π-extended cationic motif that can form NIR-absorbing and emissive -aggregates.
我们公开了一系列硅桥连二氮杂萘基甲基阳离子，它们是稳定的一维π-扩展碳正离子。在适当位置将非苯型薁连接到碳正离子中心，同时加强平面排列，可有效地使正电荷在π共轭骨架上离域。这种结构限制不仅赋予碳正离子高化学稳定性（p值约为9.5），尽管没有任何供电子取代基，而且由于跃迁偶极矩的有效增强，使其在红色区域有强烈吸收。X射线晶体学分析揭示了一种错位π堆积排列，其中外部七元环以面对面的方式重叠，硅原子上垂直取向取代基的空间位阻和抗衡阴离子的位置都应起关键作用。反映这种分子排列，π-扩展阳离子在固态和溶液聚集态下在近红外（NIR）区域均表现出红移吸收，表明形成了π-聚集体。通过适当选择硅桥上的取代基和抗衡阴离子，在聚集体形成时吸收带出现更明显的红移，并且聚集体表现出尖锐的荧光带，最大发射波长超过800 nm。这些结果证明了碳正离子的非苯型芳香稳定化的影响以及本设计策略对于构建有前景的π-扩展阳离子基序（可形成近红外吸收和发射的π-聚集体）的有效性。
