Stable Borepinium and Borafluorenium Heterocycles: A Reversible Thermochromic “Switch” Based on Boron–Oxygen Interactions

The first examples of N‐heterocyclic carbene (NHC) and cyclic(alkyl)(amino) carbene (CAAC) stabilized borepinium and borafluorenium heterocycles are reported herein. The optical properties of the heterocyclic borenium cations were tuned by varying the Lewis base and by changing the number of atoms in the ring. More importantly, functionalizing the cationic boron ring system in the NHC‐borafluorenium cation affords a temperature‐sensitive molecule with reversible colorimetric “turn off/turn on” properties in solution. Notably, this is the first report of thermochromism in these cationic species. This property, which is mediated by an intermolecular boron–oxygen bond equilibrium, was examined in detail by X‐ray crystallography, variable temperature‐UV/Vis absorption spectroscopy (VT‐UV/Vis), and density functional theory (DFT). Optical tuning: The first examples of thermochromic N‐heterocyclic carbene (NHC)‐ and cyclic(alkyl)(amino) carbene (CAAC)‐stabilized borepinium and borafluorenium heterocycles are reported. The optical properties of the heterocyclic borenium cations were tuned by varying the Lewis base and by changing the number of atoms in the ring (see graphic).