Purely Organic Dyes with Thermally Activated Delayed FluorescenceA Versatile Class of Indicators for Optical Temperature Sensing

Carbazole-substituted dicyanobenzenes and anthraquinone based intramolecular charge-transfer dyes, two classes of organic thermally activated delayed fluorescence (TADF) emitters developed for organic light-emitting diode (OLED) technology, are prepared and investigated for their suitability for optical oxygen and temperature sensing. The new materials do not offer any improvement compared to the state-of-the-art probes in respect to oxygen sensing. In contrast, due to very high temperature dependency of the TADF decay times, the new dyes are particularly advantageous for optical thermometry. Contrary to many state-of-the-art optical thermometers, the dyes show only moderate decrease of TADF intensity at increased temperatures. The dyes immobilized into a nearly oxygen impermeable poly(vinylidene chloride-co-acrylonitrile) matrix feature excellent temperature sensitivity in the investigated temperature range (278-323 K) with 1.4-3.7% K-1 change of TADF lifetime at 298 K. Indicator-loaded cell penetrating cationic nanoparticles are prepared for biological applications and feature sensitivities from 2.2 to 2.8% K-1 change of TADF lifetime at 298 K. Moreover, fiber-optic minisensors enable fast monitoring of temperature fluctuations in small sample volumes. The outcome of our work demonstrates the high potential of organic TADF emitters for optical temperature sensing and imaging.