Spiral Donor Design Strategy for Blue Thermally Activated Delayed Fluorescence Emitters.

Thermally activated delayed fluorescence (TADF) emitters with a spiral donor show tremendous potential toward high-level efficient blue organic light-emitting diodes (OLEDs). However, the underlying design strategy of the spiral donor used for blue TADF emitters remains unclear. As a consequence, researchers often do "try and error" work in the development of new functional spiral donor fragments, making it slow and inefficient. Herein, we demonstrate that the energy level relationships between the spiral donor and the luminophore lead to a significant effect on the photoluminescent quantum yields (PLQYs) of the target materials. In addition, a method involving quantum chemistry simulations that can accurately predict the aforementioned energy level relationships by simulating the spin density distributions of the triplet excited states of the spiral donor and corresponding TADF emitters and the triplet excited natural transition orbitals of the TADF emitters is established. Moreover, it also revealed that the steric hindrance in this series of molecules can form a nearly unchanged singlet (S1) state geometry, leading to a reduced nonradiative decay and high PLQY, while a moderated donor-acceptor (D-A) torsion in the triplet (T1) state can induce a strong vibronic coupling between the charge-transfer triplet (3CT) state and the local triplet (3LE) state, achieving an effective reverse intersystem crossing (RISC) process. Furthermore, an electric-magnetic coupling is formed between the high-lying 3LE state and the charge-transfer singlet (1CT) state, which may open another RISC channel. Remarkably, in company with the optimized molecular structure and energy alignment, the pivotal TADF emitter DspiroS-TRZ achieved 99.9% PLQY, an external quantum efficiency (EQE) of 38.4%, which is the highest among all blue TADF emitters reported to date.

J-Aggregation Enhances the Electroluminescence Performance of a Sky-Blue Thermally Activated Delayed-Fluorescence Emitter in Nondoped Organic Light-Emitting Diodes.

A pivotal thermally activated delayed-fluorescence (TADF) emitter, DspiroAc-TRZ, was developed, and it exhibits greatly enhanced electroluminescence performance in nondoped organic light-emitting diodes (OLEDs) owing to the concurrent manipulation of aggregation behavior and monomolecular structure. The delicate non-planar packing pattern in the DspiroAc-TRZ crystal can not only lead to highly efficient solid-state luminescence but also form a loose intermolecular packing pattern, greatly decreasing the HOMO or LUMO overlaps in dimers and shortening the triplet exciton diffusion length. In addition, the rigid donor and acceptor moieties in DspiroAc-TRZ can rigidify the molecular backbone, resulting in a tiny geometric vibrational relaxation in the excited state. Impressively, high photoluminescent quantum yields of 78.5 and 83.7% were achieved for the DspiroAc-TRZ single crystal and nondoped film. A high external quantum efficiency (EQE) of 25.7% was achieved in a nondoped sky-blue TADF OLED, which is higher than any reported EQE value of nondoped sky-blue TADF OLEDs so far.

Nondoped Red Fluorophores with Hybridized Local and Charge-Transfer State for High-Performance Fluorescent White Organic Light-Emitting Diodes.

Two red fluorophores (TPABTPA and TPABCHO) with hybridized local and charge-transfer properties were systematically studied. TPABTPA and TPABCHO enabled nondoped organic light-emitting diodes (OLEDs) with excellent external quantum efficiency (EQE) of 11.1% and 5.0%, respectively, attributed to high exciton utilization efficiency of 82% and 46%, respectively. Furthermore, TPABTPA and TPABCHO were utilized as complementary emitters for a sky-blue thermally activated delayed fluorescence material to fabricate two-color fluorescent white OLEDs (WOLEDs) in a fully nondoped emissive-layer configuration. Furthermore, device performance was optimized through a simple device engineering strategy by sandwiching a suitable interlayer between the emitting layers. As a result, the optimized TPABTPA- and TPABCHO-based WOLEDs successfully achieved high EQEs of 23.0% and 8.6%, respectively, along with a low efficiency roll-off and good spectral stability, due to high exciton utilization efficiency of the emitters and importantly efficient suppression of a nonradiative energy-transfer process.

Adamantane-Substituted Acridine Donor for Blue Dual Fluorescence and Efficient Organic Light-Emitting Diodes.

To date, blue dual fluorescence emission (DFE) has not been realized because of the limited choice of chemical moieties and severe geometric deformation of the DFE emitters leading to strong intramolecular charge transfer (ICT) with a large Stokes shift in excited states. Herein, an emitter (1'r,5'R,7'S)-10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-spiro [acridine-9,2'-adamantane] (a-DMAc-TRZ) containing a novel adamantane-substituted acridine donor is reported, which exhibits unusual blue DFE. The introduction of the rigid and bulky adamantane moiety not only suppressed the geometry relaxation in excited state, but also induced the formation of quasi-axial conformer (QAC) and quasi-equatorial conformer (QEC) geometries, leading to deep-blue conventional fluorescence and sky-blue thermally activated delayed fluorescence (TADF). The resulting organic light-emitting diodes (OLEDs) achieved a maximum external quantum efficiency (EQE) of about 29 %, which is the highest reported for OLEDs based on dual-conformation emitters.

White-light emission from a single heavy atom-free molecule with room temperature phosphorescence, mechanochromism and thermochromism.

Two heavy atom-free luminophores (SHB2t and SDB2t) with simple molecular structures have been synthesized via Suzuki coupling reactions in which both display white-light emission with prompt fluorescence and room temperature phosphorescence (RTP) in the solid state. The impressive RTP of the luminophores is produced by a synergistic effect of the strong intermolecular hydrogen bonding in addition to the spin-orbit coupling of the sulfonyl oxygen atoms and the moderate singlet-triplet energy gaps (ΔEST). These factors facilitate the intersystem crossing (ISC) process to generate triplet excitons in which the molecular conformations become immobilized to effectively suppress radiationless decay. Under the stimuli of mechanical force and solvent vapor, the RTP of SHB2t and SDB2t can be simply turned off and on by breaking and reforming the robust hydrogen bonding, which leads to remarkable and reversible mechanochromism between white and deep-blue emission. Moreover, two different thermochromic processes have been observed for the pristine and ground samples of SDB2t, in which a tricolor switching system between white, deep-blue and blue emission has been successfully achieved through the sequential control of grinding, heating and fuming. From detailed studies we have determined that the mechanism for the thermochromism of SDB2t is correlated with the rearrangement of the white-light emitting molecules to a new packing mode without RTP emission.

Structure-Performance Investigation of Thioxanthone Derivatives for Developing Color Tunable Highly Efficient Thermally Activated Delayed Fluorescence Emitters.

Thioxanthone derivatives consisting of undecorated carbazole as an electron donor and thioxanthone (TXO) or 9H-thioxanthen-9-one-S,S-dioxide (SOXO) as an electron acceptor in a donor-acceptor (D-A) or donor-acceptor-donor (D-A-D) structure were developed as thermally activated delayed fluorescence emitters to fabricate highly efficient fluorescent organic light emitting diodes. Their emission color was successfully tuned from blue to yellow by changing the sulfur atom valence state of the thioxanthone unit to tune intramolecular charge transfer effect. Their thermal, electrochemical, photophysical, and electroluminescent properties, and theoretical calculations were systematically investigated to illustrate the molecular structure and property relationships. Maximum external quantum efficiency (EQE) of 13.6% with Commission Internationale de L'Eclairage coordinates of (0.37, 0.57) was achieved for green light emission CzSOXO consisting of SOXO and carbazole in a D-A structure. Blue light emission CzTXO and DCzTXO consisting of TXO and carbazole in a D-A and D-A-D structure could also give EQE values exceeding 11%. Their efficiency roll-off with increasing current density was simulated by adopting triplet-triplet annihilation model, indicating that the TXO derivatives suffer more severe efficiency roll-off because of their relatively long delayed fluorescence lifetime (τ(D)).

Intermolecular Electronic Coupling of Organic Units for Efficient Persistent Room-Temperature Phosphorescence.

Although persistent room-temperature phosphorescence (RTP) emission has been observed for a few pure crystalline organic molecules, there is no consistent mechanism and no universal design strategy for organic persistent RTP (pRTP) materials. A new mechanism for pRTP is presented, based on combining the advantages of different excited-state configurations in coupled intermolecular units, which may be applicable to a wide range of organic molecules. By following this mechanism, we have developed a successful design strategy to obtain bright pRTP by utilizing a heavy halogen atom to further increase the intersystem crossing rate of the coupled units. RTP with a remarkably long lifetime of 0.28 s and a very high quantum efficiency of 5 % was thus obtained under ambient conditions. This strategy represents an important step in the understanding of organic pRTP emission.

Achieving very bright mechanoluminescence from purely organic luminophores with aggregation-induced emission by crystal design.

Although bright organic mechanoluminescence (ML) has been observed for a few luminophores with aggregation-induced emission (AIE), details of the positive effect of AIE on ML performance remain unclear and a feasible design principle for AIE-ML compounds has not yet been presented. Herein, an effective strategy for the molecular design of efficient AIE-ML materials is demonstrated, based on tetraphenylethene (TPE) building blocks with formyl substituents, which yield non-centrosymmetric crystal structures with prominent piezoelectric properties for molecular excitation combined with AIE features for intense emission. Following this approach, three AIE-active compounds have been developed and are found to show unique ML characteristics. Furthermore, the results of single crystal X-ray analysis and density functional theory (DFT) calculations suggest that the ML performance would probably be further enhanced by creating molecules with larger dipolar moments and enhanced AIE properties.

Achieving remarkable mechanochromism and white-light emission with thermally activated delayed fluorescence through the molecular heredity principle.

Achieving high contrast mechanochromism (Δλem,max > 100 nm) and white-light emission under mild conditions from a single compound with a simple structure is a great challenge. Herein, we report a novel dual-emissive compound, namely SCP, with an asymmetric molecular structure that fully inherits the photophysical properties of the parent molecules SC2 and SP2. SCP shows high contrast, linearly tunable mechanochromism and bright white-light emission arising from a combination of traditional fluorescence and thermally activated delayed fluorescence (TADF). The origin of the dual-emission for SCP was demonstrated based on the analysis of the white-emitting single crystals. In addition, a mechanism of luminochromism for SCP driven by the application of mechanical force is proposed. These observations present a rational design strategy for the development of high performance multi-functional materials for white-light emission.