Fabrication of Flexible High‐Temperature Film Thermometers and Heat‐Resistant OLEDs Using Novel Hot Exciton Organic Fluorophores

The development of organic thermosensitive fluorophores for use in heat-resistant organic light emitting diodes (OLEDs) and large-area and flexible high-temperature sensing remains challenging due to the susceptibility of such materials to thermally facilitated nonradiative decay. A series of "hot exciton" materials ("C1" and "C2") based on pyrrole-substituted triarylphosphine oxides that exhibit high heat resistance have been developed. At a temperature of 260 degrees C, the films retain 42% (C1) and 29% (C2) of their room temperature fluorescence. This is thanks to thermally facilitated reverse intersystem crossing (RISC) from a high-lying triplet to a singlet state. By combining the novel fluorophores with a yellow emitter with an extremely large Stokes shift, flexible and large-area ratiometric film thermometers are fabricated that demonstrate naked-eye high-temperature sensing. The relative sensitivity, S-r, of the film thermometer is higher than 1% K-1 in the high-temperature region (393 to 470 K), with the maximum S-r reaching 1.26% K-1 at 430 K. Using these blue emitters, heat-resistant cyan and white OLEDs are also fabricated. With thermally populated singlets and nearly 100% exciton harvesting via fast RISC, the C1-based cyan OLED exhibits a nearly 12-fold enhancement in electroluminescence on heating from room temperature to 530 K, while the corresponding white OLED displays a 5.7-fold electroluminescence enhancement.