Systematic Study via Controlling the Molecular Design for Reducing the Triplet Delayed Lifetime in Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs

A strategy for developing high-performance thermally activated delayed fluorescence (TADF) emitters with shorter delayed lifetimes (τ_D) is highly sought after. To address this, we have systematically constructed three emitters based on open and fused molecular structures by opting for the same 3,6-di-<i>tert</i>-butyl-9<i>H</i>-carbazole (tCz) as the donor unit and varying the acceptor core. NP-tCz (fused), BPh-tCz (open), and PPy-tCz (N-substituted open) showed gradually decreased excited singlet-triplet energy gap (Δ<i>E</i>_ST) values of 0.48 eV, 0.18 eV, and 0.11 eV, respectively, in a 10 wt % emitter: 3,3'-di(9<i>H</i>-carbazol-9-yl)-1,1'-biphenyl (<i>m</i>CBP) blend. NP-tCz displayed photoluminescence (PL) maxima at 463 nm, exhibiting both room temperature phosphorescence (RTP) and TADF signatures, owing to its large Δ<i>E</i>_ST and prolonged τ_D of 40 ms. In contrast, BPh-tCz showed a PL peak at 478 nm with typical TADF behavior and a τ_D of 456 μs, while PPy-tCz exhibited PL maxima at 498 nm, also characteristic of TADF, with the shortest τ_D of 116 μs. The emissive layer in the organic light-emitting diodes (OLEDs) comprised 10 wt % emitter blended in <i>m</i>CBP host. The OLEDs based on NP-tCz, BPh-tCz, and PPy-tCz exhibited electroluminescence peaks at 478, 484, and 499 nm, respectively, with maximum external quantum efficiency of 7.2%, 11.9%, and 26.9%. PPy-tCz (open) containing N-substitutional doping is the most efficient among the three emitters, offering the shortest τ_D and high efficiency. Deeper molecular-level insights into the excited-state deactivation processes of these emitters were obtained from reliable quantum-chemical calculations, which complemented and supported the experimental observations. These findings offer a pathway to control the delayed lifetime and improve efficiency by tailoring molecular structures through open and fused configurations.