Efficient Thermally Activated Delayed Fluorescence Research Articles

Highly Efficient Sky-Blue π-Stacked Thermally Activated Delayed Fluorescence Emitter with Multi-Stimulus Response Properties.

Organic materials with multi-stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π-stacked thermally activated delayed fluorescence (TADF) material with multi-stimulus response (MSR) properties, named SDMAC, was designed and synthesized using distorted 9,9-dimethyl-10-phenyl-9,10-dihydroacridine as a donor. SDMAC possesses a rigid π-stacked configuration with intramolecular through-space interactions and exhibits aggregation-induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties of SDMAC can be used in displays and lighting. Using SDMAC as an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light-emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP-TADF OLEDs based on the through-space charge transfer strategy. The CP organic light-emitting diodes (CP-OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.

Highly Efficient Sky‐Blue π‐Stacked Thermally Activated Delayed Fluorescence Emitter with Multi‐Stimulus Response Properties

AbstractOrganic materials with multi‐stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π‐stacked thermally activated delayed fluorescence (TADF) material with multi‐stimulus response (MSR) properties, namedSDMAC, was designed and synthesized using distorted 9,9‐dimethyl‐10‐phenyl‐9,10‐dihydroacridine as a donor.SDMACpossesses a rigid π‐stacked configuration with intramolecular through‐space interactions and exhibits aggregation‐induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties ofSDMACcan be used in displays and lighting. UsingSDMACas an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light‐emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP‐TADF OLEDs based on the through‐space charge transfer strategy. The CP organic light‐emitting diodes (CP‐OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.

(Invited) Structural Analysis of Organic Semiconducting Materials By Solid State NMR

Recently, our group have successfully developed several blue and green thermally activated delayed fluorescence (TADF) emitters showing high external quantum efficiencies in OLEDs through high-throughput screening based on quantum chemical calculations. Although a number of highly efficient TADF emitters have been reported, the study about their conformation and aggregated structure in an amorphous states, which is expected to be the origin of high device performance, is still limited. Thus we have carried out multiscale simulations and solid-state NMR (ssNMR) analysis of organic amorphous thin films. The ssNMR is one of the powerful technique to acquire the structural information of amorphous aggregated materials, which has been difficult by diffraction techniques due to the lack of long range order for amorphous state. In this presentation, we demonstrate ssNMR analysis of amorphous organic semiconducting materials.Reference(1) “Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy”, Suzuki, K.; Kubo, S.; Aussenac, F.; Engelke, F.; Fukushima, T.; Kaji, H. Angew. Chem. Int. Ed. 2017, 56, 14842.(2) “Efficient blue thermally activated delayed fluorescence emitters showing very fast reverse intersystem crossing”, Ren, Y., Wada, Y., Suzuki, K., Kusakabe, Y., Geldsetzer, J. and Kaji, H. Appl. Phys. Express, 2021, 14, 071003.(3) “Solution-processable thermally activated delayed fluorescence emitters for application in organic light emitting diodes”, Suzuki, K.; Adachi, C.; Kaji, H. J. Soc. Inf. Disp. 2017, 25, 480.(4) “Combined Inter- and Intramolecular Charge-Transfer Processes for Highly Efficient Fluorescent Organic Light-Emitting Diodes with Reduced Triplet Exciton Quenching”, Moon, C.-K.; Suzuki, K.; Shizu, K.; Adachi, C.; Kaji, H.; Kim, J.-J. Adv. Mater. 2017, 131, 6614.(5) “Triarylboron-based Fluorescent Organic Light-emitting Diodes with External Quantum Efficiencies Exceeding 20%”, Suzuki, K.; Kubo, S.; Shizu, K.; Fukushima, T.; Wakamiya, A.; Murata, Y.; Adachi, C.; Kaji, H.; Angew. Chem. Int. Ed. 2015, 54, 15231.(5) “Purely organic electroluminescent material realizing 100% conversion from electricity to light”, Kaji, H.; Suzuki, H.; Fukushima, T.; Shizu, K.; Suzuki, K.; Kubo, S.; Komino, T.; Oiwa, H.; Suzuki, F.; Wakamiya, A.; Murata, Y.; Adachi, C. Nat. Commun. 2015, 6, 8476.

Exploiting new feasibility of a phenylquinoline unit for establishing efficient green thermally activated delayed fluorescent emitter with short delayed fluorescent lifetime

Realizing the confluence of high photoluminescence quantum yields (PLQYs) and short delayed fluorescent lifetimes remains a challenge for thermally activated delayed fluorescent (TADF) emitters. A green TADF emitter, 10-(2,4-diphenylquinolin-6-yl)-10 H -phenoxazine (PhQLPXZ), is exploited by directly linking an 2,4-diphenylquinoline unit with an electron-donating (D) 10 H -phenoxazine moiety. For the first time, the phenylquinoline (PhQL) unit is revealed as an electron-accepting unit for fabricating efficient TADF emitters. The dihedral angle between the donor and the acceptor is nearly 90°, which induces a totally separation of frontier orbitals. As a result, the emitter reveals a small singlet-triplet splitting energy of 0.09 eV and a short delayed fluorescence lifetime of 1.3 μs. The green organic light-emitting diodes (OLEDs) employing PhQLPXZ as the emitter exhibit a maximum external quantum efficiency of 15.7%. This study demonstrates for the first time that phenylquinoline derivatives can be employed for constructing TADF emitters for high-efficiency fluorescent OLEDs. • Phenylquinoline derivatives were employed to construct a green TADF emitter with a short delayed lifetime of 1.3 μs. • The molecule endows a vertical dihedral angle (97°) between the acceptor and the donor and high photoluminescence quantum yield over 70% in film. • The device with a doping concentration of 6% exhibited a high EQE of 15.7%, which is one of the best among the devices based on quinolone cored emitters.

Multiplying the efficiency of red thermally activated delayed fluorescence emitter by introducing intramolecular hydrogen bond

As one of the three primary colors, red organic light-emitting diodes (OLEDs) are indispensable in practical applications. However, red emitters are generally subject to severe non-radiative exciton loss due to their narrow energy gap. In this work, three new thermally activated delayed fluorescence (TADF) emitters were developed, namely 4-(acenaphtho[1,2-b]quinoxalin-9-yl)-N,N-diphenylaniline (TPA-AP), 4-(acenaphtho[1,2-b]pyrido[2,3-e]pyrazin-10-yl)-N,N-diphenylaniline (TPA-APy), and 4-(acenaphtho[1,2-b]pyrazino[2,3-e]pyrazin-9-yl)-N,N-diphenylaniline (TPA-APm), employing a series of finely modified acenaphtho[1,2-b]quinoxaline (AP) derivatives as acceptor units. Among three TADF emitters, an intramolecular hydrogen bond is formed between the donor (D) and acceptor (A) units in TPA-APm. Consequently, the overlap of the frontier molecular orbitals (FMOs) of TPA-APm can increase appropriately, and the fluorescence radiative rate (kF) of TPA-APm is nearly twofold than that of TPA-AP and TPA-APy. Furthermore, the non-radiative decay rate (knr) of TPA-APm is less than that of TPA-AP and TPA-APy by an order of magnitude, which is attributed to the improved molecular rigidity caused by intramolecular hydrogen bond. As a result, TPA-APm-based OLEDs achieved a multiplied external quantum efficiency (EQE) of 21.1% with the electroluminescence peak at 590 nm, comparing to only 7.0% and 11.5% for the TPA-AP-based and TPA-APy-based devices, respectively. These results demonstrate appropriate intramolecular hydrogen bond can suppress the influence of non-radiative decay by simultaneously enhancing molecular rigidity and facilitating the fluorescence process, and have great potential in the design of efficient red TADF emitters.

Intramolecular-locked triazatruxene-based thermally activated delayed fluorescence emitter for efficient solution-processed deep-blue organic light emitting diodes

Developing efficient deep blue thermally activated delayed fluorescence (TADF) emitters for solution-processed organic light emitting diodes (OLEDs) remains a formidable challenge. Herein, three new TADF emitters, namely FTAT-MBO, FTAT-HBO and FTAT-FBO, are designed and synthesized by incorporating intramolecular-locked triazatruxene with deep highest occupied molecular orbital and high triplet level as the electron donor and the boron-fused units with different substituents of methyl, hydrogen, and fluorine as the electron acceptors. Three compounds show blue emission (445–473 nm) with photoluminescence quantum yields up to 90% and bandwidths down to 54 nm. They also exhibit distinct TADF effect with small singlet − triplet energy splitting (ΔEST) below 0.2 eV and short delay fluorescence lifetimes (down to 1.8 μs). Solution-processed TADF OLEDs based on FTAT-MBO and FTAT-HBO as emitters displayed deep blue emission with the maximum external quantum efficiency (EQE) and Commission Internationale de l’éclairage (CIE) coordinates of 10.2% and (0.15, 0.08); 11.4% and (0.15, 0.12); respectively, while the solution-processed sky-blue OLEDs based on FTAT-FBO achieved the maximum EQE of 17.5% and CIE coordinates of (0.15, 0.25). This study demonstrates the potential of the intramolecular-locked triazatruxene donor unit to construct efficient TADF materials for the fabrication of solution-processed deep-blue to sky-blue OLEDs.

Novel Deep Red Thermally Activated Delayed Fluorescence Molecule with Aggregation-Induced Emission Enhancement: Theoretical Design and Experimental Validation.

Thermally activated delayed fluorescence (TADF) molecules with deep red luminescence have shown great applications in organic light-emitting diodes (OLEDs). However, the development of high efficient deep red TADF emitters is full of resistance, and new design strategies are highly desired. This work theoretically predicts the luminescence properties and photophysical mechanism of a spiro-acridine based molecule DBPz-2spAc in toluene and aggregation states. Experiments further show that the solid state can effectively suppress nonradiative energy loss and thus improve luminescence efficiency. OLEDs based on DBPz-2spAc show high luminescence efficiency. In addition, studies based on spiro-acridine derivatives indicate that bending the degree of acridine in excited state will directly affect the nonradiative energy loss. This study improves the understanding of the luminous behavior of spiro-acridine derivative based TADF emitters in solution and in aggregation state, which should pave the way for the design of efficient deep red TADF materials.

Substituent effect on TADF properties of 2-modified 4,6-bis(3,6-di-tert-butyl-9-carbazolyl)-5-methylpyrimidines.

The interest in organic materials exhibiting thermally activated delayed fluorescence (TADF) significantly increased in recent years owing to their potential application as emitters in highly efficient organic light emitting diodes (OLEDs). Simple modification of the molecular structure of TADF compounds through the selection of different electron-donating or accepting fragments opens great possibilities to tune the emission properties and rates. Here we present the synthesis of a series of novel pyrimidine–carbazole emitters and their photophysical characterization in view of effects of substituents in the pyrimidine ring on their TADF properties. We demonstrate that electron-withdrawing substituents directly connected to the pyrimidine unit have greater impact on the lowering of the energy gap between singlet and triplet states (ΔEST) for efficient TADF as compared to those attached through a phenylene bridge. A modification of the pyrimidine unit with CN, SCH3, and SO2CH3 functional groups at position 2 is shown to enhance the emission yield up to 0.5 with pronounced TADF activity.

A facile strategy for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

By enhancing electron-withdrawing abilities of the secondary acceptor attachments into the main acceptor core, their charge transfer triplet ( 3 CT) energy levels are gradually reduced, while the locally excited triplet ( 3 LE A ) levels are well maintained. The well-aligned excited states, consequently, results in the enhancement of the rates of reverse intersystem crossing (RISC), and boosting the device efficiencies in red OLEDs. • A facile strategy to modulate CT states while maintaining LE state is proposed. • CT energy levels are gradually reduced, while the 3 LE A ones are well maintained. • Fast k RISC , small ΔE ST and high PLQY can be realized simultaneously. • mDPBPZ-DPXZ-based red OLED exhibits much higher EQE compared to its counterparts. Realizing highly efficient reverse intersystem crossing (RISC) process for red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge. Herein we demonstrate that charge transfer (CT) energy levels are gradually reduced with enhancing electron-withdrawing abilities of the secondary acceptor (A) attachments. Meanwhile, their local excited triplet ( 3 LE A ) state levels are well maintained due to the conjugations between the main framework and the attachments are all similar for these A segments. The optimized molecular energy level alignments are obtained in the case of mDPBPZ-DPXZ, in which 3 CT and the 3 LE A states are almost degenerate, and gives rise to a small singlet–triplet energy difference ( ΔE ST ) of 0.02 eV, a fast RISC rate of 1.54 × 10 6 s −1 and a high photoluminescence quantum yield of 93%, simultaneously. And the corresponding device achieves an external quantum efficiency of 21.6%, significantly higher than the other counterparts. This work demonstrates that the energy alignment between the CT and 3 LE A state can be easily managed by introducing secondary A attachments on original A framework, which shifts the 1 CT and 3 CT state to lower energies without affecting the 3 LE A energy. It provides a facile design strategy to promote the RISC process, which is particular useful in designing highly efficient red TADF emitters.

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Theoretical study for the TADF nature of through-space conjugated [2.2]paracyclophane derivatives and the design for red-light emission

The thermally activated delayed fluorescence (TADF) nature of a series of [2.2]paracyclophane derivatives with spatially conjugated structures were studied by DFT/TD-DFT method. Through-space charge transfer, rather than the usual through-bond charge transfer exist between the frontier molecular orbitals of these molecules. The singlet-triplet energy gaps which affect the reverse intersystem crossing (RISC) process were compared among the molecules reported in the experimental study to explain why 2 and 4 are more likely to show TADF emission than 1 and 3. Especially, when discussing the intersystem crossing (ISC) process according to the excited states properties and the spin-orbit coupling matrix elements, the influence of the spatially conjugated structure was also analyzed detailedly. Transition dipole moments of the absorption states were greatly enhanced in the conjugated structure and the ISC between some high-energy states was promoted in consequence, indirectly leading to the promotion of the RISC process which generates S1 excitons. In addition, we designed 5–8 by replacing the benzophenone group with the thianthrene-9,9′,10,10′-tetraoxide group, in order to obtain red-light emission through decreasing the LUMO energy level. The calculation results show that the new molecules have larger radiation rate and smaller internal conversion rate, which indicates that the designed molecules are potential efficient red-light TADF materials.

Controlling the conjugation extension inside acceptors for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

Increasing the reverse intersystem crossing (RISC) rates remains a challenge for red thermally activated delayed fluorescence (TADF) emitters, and energy level alignments play an important role. In this work, we demonstrate that locally excited triplet energy levels from acceptor segments (3LEA) can be well modulated by changing the conjugation extension inside acceptors. Our newly designed compounds pDPAPQ-PXZ and pDPBPZ-PXZ show nearly identical charge-transfer (CT) properties because of similar donor and acceptor strengths. On the other hand, the 3LEA state of pDPBPZ-PXZ was significantly lower than that of pDPAPQ-PXZ only because of its greater π-delocalization. In the case of pDPBPZ-PXZ, ideal molecular energy level alignments are obtained with near-degenerate CT featuring singlet and triplet states (1CT, 3CT) and 3LEA. As a result, pDPBPZ-PXZ simultaneously achieves an approximately 10-fold improved kRISC and a dramatically improved photoluminescence quantum yield of 85.6% compared with pDPAPQ-PXZ. The corresponding OLED successfully doubles the external quantum efficiency to 20.3%. This work demonstrates that the conjugation extensions of acceptors play a fatal role in determining 3LEA levels and that they can be isolated from controlling the acceptor strengths. This provides a promising approach toward enhancing RISC efficiency and developing highly efficient red TADF emitters.

Polycyclic phenazine-derived rigid donors construct thermally activated delayed fluorescence emitters for highly efficient orange OLEDs with extremely low roll-off

Two new thermally activated delayed fluorescence (TADF) molecules, tBuInPz-BO and InPz-BO, are constructed via connecting novel rigid polycyclic phenazine-derived donors with oxygen-bridged boron acceptor. Both emitters exhibit fast reverse intersystem crossing rates and high photoluminescence quantum yields, owing to the high molecular structural rigidity. As a result, the orange organic light-emitting diode (OLED) fabricated with tBuInPz-BO shows a maximum external quantum efficiency (EQEmax) of 22.0% with the electroluminescence peak at 585 nm. Moreover, the device displays extremely low efficiency roll-off. At high luminance of 1000 and 10000 cd/m2, the EQE values still maintain a high level of 20.2% and 14.8%, respectively, achieving state-of-the-art electroluminescence performance among TADF-based orange OLEDs. This work reveals the great potential of the new phenazine-derived donors for constructing highly efficient TADF emitters.

Rational design of CN substituted dibenzo[a,c]phenazine acceptor for color tuning of thermally activated delayed fluorescent emitters

• Rational color tuning of thermally activated delayed fluorescence emitters by molecular engineering. • CN substituent management of dibenzophenazine acceptor to manage the light-emitting properties. • High external quantum efficiency from yellow to deep red organic light-emitting diodes. Thermally activated delayed fluorescent (TADF) emitters with emission wavelength beyond 600 nm and high external quantum efficiency (EQE) above 20 % are rarely reported because of the inherent limitation of the energy gap law. Rigid and planar dibenzo[ a,c ]phenazine (BP) core was proven to be a useful acceptor to develop highly efficient red TADF emitters by minimizing non-radiative loss mechanism. In this work, by varying the number and substitution position of donor/acceptor unit attached to the BP acceptor, four long wavelength TADF emitters (3-AcBPmCN, 3-AcBPdCN, 3-AcBPtCN and 11-ACBPdCN) were developed to control the emission wavelength from yellow (552 nm) to deep-red (637 nm) region. A directly attached 9,9-dimethyl-9,10-dihydroacridine (Ac) donor and newly developed 3-CNBP, 11,12-CNBP, 3,11,12-CNBP and 3,6-CNBP acceptor units were utilized in the synthesis of TADF emitters of 3-AcBPmCN, 3-AcBPdCN, 3-AcBPtCN and 11-AcBPdCN respectively. With the increasing number of cyano units on BP acceptor, decreased HOMO-LUMO energy gap and singlet–triplet energy gap (Δ E ST ) were reported for three TADF emitters of 3-AcBPmCN, 3-AcBPdCN and 3-AcBPtCN. Further, it was demonstrated that in between the two donor-substitution positions of BP acceptor (3 and 11), 11-position substituted 11-AcBPdCN TADF emitter exhibited higher EQE (23.1 %) than the 3-position substituted 3-AcBPdCN TADF emitter (17.3 %) with the similar color coordinate (0.53, 0.46).

Phenylpyridine and carbazole based host materials for highly efficient blue TADF OLEDs

In this work, two host materials for blue TADF (thermally activated delayed fluorescence) OLEDs (organic light-emitting diodes) were designed and synthesized for improving the lifetime by incorporating pyridine core carbazole connected at different liking positions. These materials exhibit good thermal stability, high bond dissociation energy (BDE), and longer lifetime than the reference host. In particular, a blue TADF OLEDs system consisting of 4N-oCBP (9-(3-(2-(9 H -carbazol-9-yl)phenyl)pyridin-4-yl)-9 H -carbazole host and TCz-Trz (2,4,6-tris(2-(9 H -carbazol-9-yl)phenyl)-1,3,5-triazine) dopant exhibited the best device performances with a maximum external quantum efficiency (EQE) of 16.2% and an improvement of 88% in operational lifetime (initial luminance of 200 cd/m 2 ). This improved lifetime could be explained by the C–N BDE in the host molecular structure. The carbazole-based host molecule with different linking positions to the pyridine core developed in this study has considerable potential for improving the efficiency and lifetime of the TADF OLEDs device. • Two host materials for blue TADF OLEDs were designed and synthesized for improving the lifetime by incorporating pyridine core carbazole connected at different liking positions. • This improved lifetime could be explained by the C–N BDE in the host molecular structure.

Modulating Non‐Radiative Deactivation via Acceptor Reconstruction to Expand High‐Efficient Red Thermally Activated Delayed Fluorescent Emitters

AbstractThe development of high‐efficient red thermally activated delayed fluorescent (TADF) materials is crucial to expanding their applications. Here, the acceptor reconstruction strategies of acceptor bonding and acceptor fusing in donor–acceptor‐type materials to modulate their nonradiative deactivation process and emission color for expanding high‐efficient red TADF emitters are presented. They are applied to design novel red TADF emitters TXO‐b‐TPA and TXO‐f‐TPA based on non‐red thioxanthone oxide (TXO) acceptor. Compared to TXO‐based emitter TXO‐TPA, these acceptor reconstruction strategies enable red‐shifted (48–88 nm) and efficient red TADF emission (600–640 nm). Their radiative and nonradiative transition processes can be modulated by the different molecular orbital and excited state distributions based on their acceptor structural differences, which results in a dramatic enhancement of their organic light‐emitting diode (OLED) device efficiencies from 1.21/3.63% (TXO‐f‐TPA) to 20.9% (TXO‐b‐TPA). These results confirm that the acceptor reconstruction strategies provide a viable solution for overcoming the limitations of previous red TADF emitters. Moreover, the retrieval of reported TADF materials can further demonstrate that these acceptor reconstruction strategies can be transferrable to more non‐red TADF materials to expand high‐efficient red‐shift and/or red TADF materials.

A Calix[3]acridan‐Based Host–Guest Cocrystal Exhibiting Efficient Thermally Activated Delayed Fluorescence

AbstractA supramolecular strategy to construct thermally activated delayed fluorescence (TADF) materials through host–guest charge transfer interactions was proposed. Consequently, a new class of macrocycle namely calix[3]acridan was conveniently synthesized in 90 % yield. The host–guest cocrystal formed by calix[3]acridan and 1,2‐dicyanobenzene exhibited efficient TADF properties due to intense intermolecular charge transfer interactions. Moreover, the spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital resulted in a very small singlet–triplet energy gap of 0.014 eV and hence guaranteed an efficient reverse intersystem crossing for TADF. Especially, a high photoluminescence quantum yield of 70 % was achieved, and it represents the highest value among the reported intermolecular donor–acceptor TADF materials.

A Calix[3]acridan-Based Host-Guest Cocrystal Exhibiting Efficient Thermally Activated Delayed Fluorescence.

A supramolecular strategy to construct thermally activated delayed fluorescence (TADF) materials through host-guest charge transfer interactions was proposed. Consequently, a new class of macrocycle namely calix[3]acridan was conveniently synthesized in 90 % yield. The host-guest cocrystal formed by calix[3]acridan and 1,2-dicyanobenzene exhibited efficient TADF properties due to intense intermolecular charge transfer interactions. Moreover, the spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital resulted in a very small singlet-triplet energy gap of 0.014 eV and hence guaranteed an efficient reverse intersystem crossing for TADF. Especially, a high photoluminescence quantum yield of 70 % was achieved, and it represents the highest value among the reported intermolecular donor-acceptor TADF materials.

Theoretical Studies on the Photophysical Properties of the Ag(I) Complex for Thermally Activated Delayed Fluorescence Based on TD-DFT and Path Integral Dynamic Approaches.

Theoretical calculation not only is a powerful tool to deeply explore photophysical processes of the emitters but also provides a theoretical basis for material renewal and design strategy in the future. In this work, the interconversion and decay rates of the thermally activated delayed fluorescence (TADF) process of the rigid Ag(dbp)(P2-nCB) complex are quantitatively calculated by employing the optimally tuned range-separated hybrid functional (ω*B97X-D3) method combined with the path integral approach to dynamics considering the Herzberg–Teller and the Duschinsky rotation effects within a multimode harmonic oscillator model. The calculated results show that the small energy splitting ΔE(S1–T1) = 742 cm–1 (experimental value of 650 cm–1) of the lowest singlet S1 and triplet T1 state and proper vibrational spin–orbit coupling interactions facilitate the reverse intersystem crossing (RISC) processes from the T1 to S1 states. The kRISC rate is estimated to be 1.72 × 108 s–1 that is far more than the intersystem crossing rate kISC of 7.28 × 107 s–1, which will greatly accelerate the RISC process. In addition, the multiple coupling routes of zero-field splitting (ZFS) interaction can provide energetically nearby lying states, to speed up the RISC pathway, and restrict the phosphorescence decay rate. A smaller ZFS D-tensor of 0.143 cm–1, E/D ≈ 0.094 ≪ 1/3, and Δg > 0 are obtained, indicating that the excited singlet states are hardly mixed into the T1 state; thus, a lower phosphorescence decay rate (kp = 9.29 × 101 s–1) is expected to occur, and the T1 state has a long lifetime, which is helpful for the occurrence of the RISC process. These works are in excellent agreement with the experimental observation and are useful for improving and designing efficient TADF materials.

Creation of efficient solution-processed OLEDs via a strategy of the host-guest system constructing with two small cross-linkable TADF molecules

Based on the strategy of using small cross-linkable thermally activated delayed fluorescence (TADF) molecules act as the host or the guest, a highly efficient TADF polymer is successfully constructed to develop efficient solution-processed organic light-emitting diodes (OLEDs). In this paper, two small cross-linkable TADF monomers, namely DV-2CzBN and DV-2CzTBN, are designed and synthesized. Simultaneously, small molecular TADF monomers are linked together using two vinylbenzyl linkers to form polymers. Systematic studies demonstrate that the polymers based on the host-guest TADF system can maintain the intrinsic TADF nature unchanged, and also effectively inhibit the triplet excitons quenching. Encouragingly a solution-processed OLED based on the host-guest system TADF polymer showed an excellent device performance with the maximum current efficiency (CE) of 25.1 cd A −1 , the power efficiency (PE) of 19.6 lm W −1 and external quantum efficiency (EQE) of 11.4%, respectively, which are two times higher in device efficiency than the corresponding device with the polymers based on the single-molecule system. Here, a new series of non-conjugated polymers were designed and synthesized based on the strategy of the host-guest system constructing with two small cross-linkable TADF molecules. • A new series of TADF polymers have been synthesized based on two small cross-linkable TADF molecules. • The TADF characteristics of small cross-linkable molecule are perfectly inherited by these VBN-based polymers. • The TADF behaviors can be improved by managing the content of DV-2CzTBN unit in the polymers. • The VBN10-based device can afford a high TADF OLED by solution process with EQE of 11.4%.