Thermally activated delayed fluorescence (TADF) materials, combining high fluorescence quantum efficiency and short delayed emission lifetime, are highly desirable for application in organic light-emitting diodes (OLEDs) with negligible external quantum efficiency (EQE) roll-off. Here, we present the pathway for shortening the TADF lifetime of highly emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives. Tiny manipulation of the molecular structure with methyl groups was applied to tune the singlet–triplet energy-level scheme and the corresponding coupling strengths, enabling the boost of the reverse intersystem crossing (rISC) rate (from 0.7 to 6.5) × 10 6 s –1 and shorten the TADF lifetime down to only 800 ns in toluene solutions. An almost identical TADF lifetime of roughly 860 ns was attained also in solid films for the compound with the most rapid TADF decay in toluene despite the presence of inevitable conformational disorder. Concomitantly, the boost of fluorescence quantum efficiency to near unity was achieved in solid films due to the weakened nonradiative decay. Exceptional EQE peak values of 26.3–29.1% together with adjustable emission wavelength in the range of 502–536 nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated by lowering the TADF lifetime.

An effective way to suppress non-radiative triplet quenching and enhance rISC up to 100% in benzophenone-derived blue TADF emitters is proposed.

Construction of rigid TADF compounds allows us to minimize the conformational disorder and obtain single-exponential DF in solid hosts with an exceptional RISC rate of nearly 6 × 106 s−1 and high emission yield.

In this work, we report on the synthesis and photophysical investigation of a new star-shaped triazine-carbazole derivative 2,4,6-tris­(3-(3,6-di-tert-butyl-9H-carbazol-9-yl)­phenyl)-1,3,5-triazine. Comparative study of the photophysical properties of the newly synthesized emitter along with its para-substituted isomer 2,4,6-tris­(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)­phenyl)-1,3,5-triazine was performed. While para-linkage caused higher oscillator strength of the lowest energy absorption band and high fluorescence quantum yield, the meta-linkage resulted in stronger charge transfer character as well as higher triplet energy. Delayed emission of meta-isomer was found to be 3 orders of magnitude more intense than that of para-isomer. Temperature dependent measurements of meta-isomer confirmed the thermally activated delayed fluorescence origin of its delayed fluorescence with the activation energy of 0.07 eV. Organic light emitting diode containing this emitter dispersed in bis­[2-(diphenylphosphino)­phenyl] ether oxide with emission spectrum peak at 475 nm was fabricated. Commission Internationale de l’Éclairage color coordinates corresponded to a sky-blue emission color (0.16, 0.23). The turn-on voltage of the electroluminescent device was found to be in the range of 5–6 V with a maximum external quantum efficiency of 9.5%. These results confirm the importance of the linking pattern between donor and acceptor moieties in the molecular design of thermally activated delayed fluorescence emitters.

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Efficient deep-blue naphthyridine-based TADF emitters featuring narrow-band-emission with low roll-off were designed by using the H-bonding and sterically controlled charge-transfer interactions.

The carbazole substitution pattern strongly modifies the rates of radiative and nonradiative emission, opening the possibility to boost the emission yield.