Boron-based heterocycles have been previously shown to exhibit properties useful for organic light emitting diodes (OLEDs), photovoltaics (PV), and small molecule sensors. Borafluorenium ions have been largely ignored in terms of optical properties but have been noted for their bright coloration in the solid state. Previously, our group studied the thermochromism of <i>N</i>-heterocyclic carbene (NHC) stabilized 3-methoxy-9-borafluorenium ions, which become colorless at low temperatures, yet are bright red at room temperature. As an extension to this work, we have synthesized a new class of cyclic(alkyl)(amino)carbene (CAAC) and NHC-stabilized 3,6-dimethoxy-9-borafluorenium ions which exhibit thermochromic, solvatochromic, and thermoluminescent properties. These compounds have been characterized by X-ray crystallography, UV-Vis spectroscopy, and NMR. Notably, the NHC-stabilized cations show noticeable changes from -20 °C to 20 °C, and the CAAC-stabilized cation exhibits a large increase in fluorescence as temperature decreases. The hitherto undiscovered thermoluminescence of borafluorenium ions changes how these compounds have the potential to be viewed as thermal sensors in the future. In this presentation, our recent developments on the thermal response of borafluorene-based materials will be discussed, and their optical properties highlighted.

Thermally responsive borafluorenium cations

ACS Spring 2022

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