Thermochromic Afterglow from Benzene‐1,4‐Diboronic Acid‐Doped Co‐crystals

The accurate thermosensing requires a minimum impact of autofluorescence and light scattering from the samples. In this study, we discovered that commercially available benzene‐1,4‐diboronic acid (BDBA) doped co‐crystals with trimethylolpropane (TMP) exhibit excellent thermochromic dual phosphorescence properties over a wide temperature range from −132 to 40 °C, despite its simple structure that does not have any donor‐acceptor linkage. The dual phosphorescence was consisted of monomeric benzene‐1,4‐diboronate (BDBA ester) and aggregation‐stabilized species. With an increase in temperature, the emission intensity from the monomeric state significantly decreased, whereas that originating from the aggregated state remained almost constant owing to the difference in their thermal stabilities. Further investigation revealed that molecular distortions in singlet excited states enable efficient intersystem crossing, causing efficient phosphorescence from the monomeric state of BDBA ester. The afterglow color of benzene‐1,4‐diboronic acid (BDBA)‐doped co‐crystals with trimethylolpropane (TMP) varied from bluish to greenish color towards an increase of temperature from −132 to 40 °C. Based on the photophysical properties of co‐crystals and TD‐DFT calculations, the molecular distortions at the singlet excited states enhanced the spin‐orbit coupling to improve the intersystem crossing rate of BDBA ester.