Diazulenylmethyl Cations with a Silicon Bridge: A π‑Extended Cationic Motif to Form J‑Aggregates with Near-Infrared Absorption and Emission

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 pK_R⁺ 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 J-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 J-aggregates.