Modulating the photophysical properties of isomeric thermally activated delayed fluorescence emitters through precise control of intra/intermolecular hydrogen bonding for nondoped OLEDs

The complex trade-offs in the design of thermally activated delayed fluorescence (TADF) molecules make it a significant challenge to simultaneously satisfy high photoluminescence quantum yield (Phi(PL)), rapid reverse intersystem crossing (RISC), small singlet-triplet energy gap (Delta E-ST), and high oscillator strength (f) through rational molecular design strategies. To overcome this challenge, an advanced approach that strategically regulates intra/intermolecular interactions of TADF molecules through isomer engineering achieving controllable aggregation behavior is proposed. We present four efficient isomeric TADF molecules which are constructed based on the integration of highly rigid and planar electronic donors with N-heterocyclic electronic acceptors. The intrinsic rigidity and planarity of the donors and acceptors, along with the resulting steric effects and weak interactions, were validated through reduced density gradient (RDG) analysis and X-ray crystallography. The resulting ordered molecular packing, facilitated by both intra/intermolecular interactions, helps reduce non-radiative decay and provides multiple pathways for reverse intersystem crossing in TADF molecules. Consequently, these four molecules exhibit exceptional properties, including high Phi(PL) (77-88 %), small Delta E-ST (0.05-0.28 eV), rapid radiative decay rate (k(r)(S) >10(-6)) and RISC rate (k(RISC) >10(4) s(-1)). Thanks to these advantages, the nondoped cyan-blue OLEDs demonstrate remarkable performance, with a maximum current efficiency (CEmax) of 32.4 cd/A and a maximum external quantum efficiency (EQE(max)) of 16.6 %, while also exhibiting minimal efficiency roll-off.