Achieving Submicrosecond Thermally Activated Delayed Fluorescence Lifetime and Highly Efficient Electroluminescence by Fine-Tuning of the Phenoxazine–Pyrimidine Structure

Abstract

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.

TD-DFT and Experimental Methods for Unraveling the Energy Distribution of Charge-Transfer Triplet/Singlet States of a TADF Molecule in a Frozen Matrix

“…Especially, the SOC of 1 CT and 3 CT states of TADF molecules with orthogonal D-A structure such as acridine donor-based ones are too weak for typical RISC rate (10 4 ~10 6 s -1 ) indicating the presence of another RISC pathway. 12,[25][26][27][28][29][30][31][32] Recent researches emphasize that locally excited triplet ( 3 LE) or hybrid local and charge-transfer state ( 3 HLCT) that has sufficient SOC with the 1 CT state must exist for efficient RISC process. 12,[30][31][32][33][34] In addition, the energy difference between 1 CT, 3 CT, 3 HLCT, and 3 LE should also be minimized for a faster RISC rate.…”

“…For example, Adachi et al enhanced RISC rate by reducing 3 HLCT-3 LE gap and Monkman et al enhanced the RISC rate by reducing 3 CT- 3 LE gap. 12,34 These reports were successful in enhancing RISC, but the excited states involved in the investigation were limited to discrete states at relaxed geometries, 12,[30][31][32][33][34] not considering the conformational distribution of TADF molecules in solid organic films inevitably generated 18,28,35,36 during thermal deposition due to the rotatable D-A linkage.…”

“…36 In addition, it has been pointed out by several reports that multiexponential decay and spectral shift over time are the evidence of the conformational distribution of TADF molecules in solids. 28,[37][38][39][40][41][42][43] However, the relation of conformational distribution, configurations of excited state transition orbitals, and excited state energies have not been clearly explained. Moreover, while 1 CT states are widely investigated through time-resolved spectroscopy of photoluminescence (PL) in the full time range of fluorescence, triplet states are mostly investigated by PL measurement at a fixed delay time due to difficulties in phosphorescence measurement at room temperature and long-lasting phosphorescence emission up to several tens of seconds.…”

TD-DFT and Experimental Methods for Unraveling the Energy Distribution of Charge-Transfer Triplet/Singlet States of a TADF Molecule in a Frozen Matrix

“…Especially, the SOC of 1 CT and 3 CT states of TADF molecules with orthogonal D-A structure such as acridine donor-based ones are too weak for typical RISC rate (10 4 ~10 6 s -1 ) indicating the presence of another RISC pathway. 12,[25][26][27][28][29][30][31][32] Recent researches emphasize that locally excited triplet ( 3 LE) or hybrid local and charge-transfer state ( 3 HLCT) that has sufficient SOC with the 1 CT state must exist for efficient RISC process. 12,[30][31][32][33][34] In addition, the energy difference between 1 CT, 3 CT, 3 HLCT, and 3 LE should also be minimized for a faster RISC rate.…”

“…For example, Adachi et al enhanced RISC rate by reducing 3 HLCT-3 LE gap and Monkman et al enhanced the RISC rate by reducing 3 CT- 3 LE gap. 12,34 These reports were successful in enhancing RISC, but the excited states involved in the investigation were limited to discrete states at relaxed geometries, 12,[30][31][32][33][34] not considering the conformational distribution of TADF molecules in solid organic films inevitably generated 18,28,35,36 during thermal deposition due to the rotatable D-A linkage.…”