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 is devoted to developing TADF materials for organic light-emitting diodes (OLEDs), while their use in other organic electronics applications such as sensors, has lagged. A family of TADF emitters, TPAPyAP, TPAPyBP, and TPAPyBPN containing a triphenylamine (TPA) donor and differing nitrogen-containing heterocyclic pyrazine-based acceptors is developed and systematically studied. Depending on the acceptor strength, these three compounds emit with photoluminescence maxima (λ_PL), of 516, 550, and 575 nm in toluene. Notably, all three compounds show a strong and selective spectral response to the presence of ZnCl₂, making them the first optical TADF sensors for this analyte. It is demonstrated that these three emitters can be used in vacuum-deposited OLEDs, which show moderate efficiencies. Of note, the device with TPAPyBPN in 2,8-bis(diphenyl-phoshporyl)-dibenzo[b,d]thiophene (PPT) host emits at 657 nm and shows a maximum external quantum efficiency (EQE_max) of 12.5%. This electroluminescence is significantly red-shifted yet shows comparable efficiency compared to a device fabricated in 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host (λ_EL = 596 nm, EQE_max = 13.6%).