Multi-Responsive Thermally Activated Delayed Fluorescence Materials: Optical ZnCl2 Sensors and Efficient Green to Deep-red OLEDs

Thermally activated delayed fluorescence (TADF) is an emission mechanism whereby both singlet and triplet excitons can be harvested to produce light. Significant attention has been devoted to developing TADF materials for organic light-emitting diodes (OLEDs), while their use in other organic electronics applications such as sensors, has lagged. We have developed and systematically studied a family of TADF emitters, TPAPyAP, TPAPyBP, and TPAPyBPN containing a triphenylamine (TPA) donor and differing nitrogen-containing heterocyclic pyrazine-based acceptors. Depending on the acceptor strength, these three compounds emit with photoluminescence maxima (λPL), of 516 nm, 550 nm, and 575 nm in toluene. Notably, all three compounds showed a strong and selective spectral response to the presence of ZnCl2, making them the first optical TADF sensors for this analyte. We also demonstrate that these three emitters can be used in vacuum-deposited OLEDs, which showed moderate efficiencies. Of note, the device with TPAPyBPN in 2,8-bis(diphenyl-phoshporyl)-dibenzo[b,d]thiophene (PPT) host emitted at 657 nm and showed an EQEmax 12.5%. This electroluminescence was significantly red-shifted yet showing comparable efficiency compared to a device fabricated in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host (λEL= 596 nm, EQEmax = 13.6%).