Efficient Thermally Activated Delayed Fluorescence Emitters Research Articles

De(sulfon)amidative Cyclization: The Synthesis of Dibenzolactams and Dibenzosultams for Organic Light Emitting Diode Materials

AbstractThis study addresses a challenge in organic synthetic chemistry: the direct cleavage of amide bonds, which is typically hampered by the thermodynamic stability of the C(Ar)−C(acyl) bond. Previous methods often rely on “CO” extrusion‐jointing transition metal‐catalyzed process and require activated tertiary amides, limiting their applicability due to incompatibility with reactive functional groups such as halogens. Herein, we report a transition metal‐free approach for the deamidative cyclization of biaryl diamides via a radical process, yielding dibenzolactam derivatives. Along this line, we have developed the desulfonamidative cyclization of biaryl disulfonamides to produce dibenzosultams through direct nucleophilic aromatic substitution, demonstrating high selectivity for unsymmetrical structures. Additionally, unsymmetrical sulfamoyl biaryl amides, containing both amide and sulfonamide functionalities, can selectively undergo desulfonamidative coupling with the amide to form dibenzolactams, which offers a complementary synthetic pathway to unsymmetric dibenzolactams. These protocols exhibit excellent compatibility with reactive functional groups, including halogens, providing an innovative synthetic toolbox for the development of thermally activated delayed fluorescence (TADF) materials used in organic light emitting diodes (OLEDs). DMAC‐PDO, incorporating a dibenzolactam as the acceptor unit, serves as an efficient blue TADF emitter with a maximum external quantum efficiency (EQEmax) of 23.4 %.

De(sulfon)amidative Cyclization: The Synthesis of Dibenzolactams and Dibenzosultams for Organic Light Emitting Diode Materials.

This study addresses a challenge in organic synthetic chemistry: the direct cleavage of amide bonds, which is typically hampered by the thermodynamic stability of the C(Ar)-C(acyl) bond. Previous methods often rely on "CO" extrusion-jointing transition metal-catalyzed process and require activated tertiary amides, limiting their applicability due to incompatibility with reactive functional groups such as halogens. Herein, we report a transition metal-free approach for the deamidative cyclization of biaryl diamides via a radical process, yielding dibenzolactam derivatives. Along this line, we have developed the desulfonamidative cyclization of biaryl disulfonamides to produce dibenzosultams through direct nucleophilic aromatic substitution, demonstrating high selectivity for unsymmetrical structures. Additionally, unsymmetrical sulfamoyl biaryl amides, containing both amide and sulfonamide functionalities, can selectively undergo desulfonamidative coupling with the amide to form dibenzolactams, which offers a complementary synthetic pathway to unsymmetric dibenzolactams. These protocols exhibit excellent compatibility with reactive functional groups, including halogens, providing an innovative synthetic toolbox for the development of thermally activated delayed fluorescence (TADF) materials used in organic light emitting diodes (OLEDs). DMAC-PDO, incorporating a dibenzolactam as the acceptor unit, serves as an efficient blue TADF emitter with a maximum external quantum efficiency (EQEmax) of 23.4 %.

Fluorene vs. Spirobifluorene: Effect of the π‐System on TADF Properties

AbstractThere are many options to design a molecular structure that could result in thermally activated delayed fluorescence (TADF). One promising strategy is to use the donor‐π‐acceptor motive where an electron‐donating unit is linked to an electron‐acceptor via an aryl moiety like phenyl. While this approach is widely used and well understood, the performance of the chromophores can be limited by different energy loss pathways, e. g. internal conversion, or by π‐stacking. To circumvent these problems rigid structures with sterically demanding substituents are applied. In this work, we designed two TADF emitters based on phenothiazine and nitrile linked via spiro‐9,9’‐bi[fluorene] or 9,9‐dimethylfluorene and compared the effect of the linker on the physical properties of the dyes. This work emphasizes the importance of careful design of conjugated spacer for efficient TADF emitters.

Steric effects and electronic manipulation of multiple donors on S0/S1 transition of Dn-A emitters

Multiple donor-acceptor (D-A) combinations represent a promising category of thermally activated delayed fluorescence (TADF) materials, offering potential for superior efficiency and stability. However, current systems are predominantly composed of limited donor groups, primarily carbazole-based derivatives. In this work, we developed a series of D-A type materials incorporating helical π-expanded carbazole (CzNaph) and 7H-dinaphtho[1,8-bc:1′,8′-ef]azepine (AzNaph), alongside traditional carbazole, ranging from mono- to tetra-substituted configurations (Dn-A). Through systematic investigation of geometric and electronic structures, the number and positioning of multiple donors are confirmed with significant manipulations on charge transfer characteristics and the S1 state via steric effects. Density functional theory (DFT) calculations reveal that varying the number of π-extended donors within the acceptor framework produces emission colors from ultraviolet to red, providing a diverse range of emitters. Furthermore, the reduced reorganization energy of S1 observed in tetra-substituted Cz and CzNaph, as well as MonoAzN, indicates lower structural relaxation, highlighting these materials' potential as stable luminescent candidates. This study underscores the importance of diverse composing units in achieving efficient and stable TADF emitters with multiple and hetero-donor configurations.

Donor-only substituted benzene achieves thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a promising mechanism for harvesting triplet excitons in organic light-emitting diodes (OLEDs). The donor–acceptor (D–A) design is the most conventional strategy for developing efficient TADF emitters. A subsequently emerged approach, known as the multiple resonance (MR) effect, also employs electron-donating and electron-withdrawing functional groups. Thus, developing TADF materials has traditionally relied on ingenuity in selecting and combining two functional units. Here, we have realized a TADF molecule by utilizing only a carbazole donor moiety. This molecule is an unusual example in the family of TADF materials and offers better insight into the electronic structures in the excited states for luminescent materials.

Excited-State Engineering Enables Efficient Deep-Blue Light-Emitting Diodes Exhibiting BT.2020 Color Gamut.

Organic luminescent materials that exhibit thermally activated delayed fluorescence (TADF) can convert non-emissive triplet excitons into emissive singlet states through a reverse intersystem crossing (RISC) process. Therefore, they have tremendous potential for applications in organic light-emitting diodes (OLEDs). However, with the development of ultra-high definition 4K/8K display technologies, designing efficient deep-blue TADF materials to achieve the Commission Internationale de l'Éclairage (CIE) coordinates fulfilling BT.2020 remains a significant challenge. Here, an effective approach is proposed to design deep-blue TADF molecules based on hybrid long- and short-range charge-transfer by incorporation of multiple donor moieties into organoboron multiple resonance acceptors. The resulting TADF molecule exhibits deep-blue emission at 414nm with a full width at half maximum (FWHM) of 29nm, together with a thousand-fold increase in RISC rate. OLEDs based on the champion material achieve a record maximum external quantum efficiency (EQE) of 22.8% with CIE coordinates of (0.163, 0.046), approaching the coordinates of the BT.2020 blue standard. Moreover, TADF-assisted fluorescence devices employing the designed material as a sensitizer exhibit an exceptional EQE of 33.1%. This work thus provides a blueprint for future development of efficient deep-blue TADF emitters, representing an important milestone towards meeting the blue color gamut standard of BT.2020.

Efficient Blue Thermally Activated Delayed Fluorescence Molecules with Sandwich‐Structured Through‐Space Charge Transfer for Fabricating High‐Performance OLEDs

AbstractExploring robust blue luminescent materials is of high significance but challenging for the commercialization application of organic light‐emitting diodes (OLEDs). In this work, it is wished to report two blue thermally activated delayed fluorescence (TADF) molecules with a through‐space charge transfer feature. They have sandwich‐structured molecular backbones comprised of one triphenyltriazine acceptor and two 9‐phenylcarbazole or 3,6‐di(tert‐butyl)‐9‐phenylcarbazole donors that are linked at the 1,8,9‐positions of a 3,6‐di(tert‐butyl)carbazole holder, and exhibit high thermal stability and strong blue delayed fluorescence with high photoluminescence quantum yields and fast reverse intersystem crossing. The blue OLEDs use 2TBCTC as an emitter and provide outstanding maximum external quantum efficiencies (EQEmaxs) of up to 32.76% with Commission Internationale de I'Eclairage coordinates of (0.160, 0.206). To the best of knowledge, 2TBCTC is the most efficient blue TADF emitter based on through‐space charge transfer in the literature. Moreover, the interlayer‐sensitizing OLED employs 2TBCTC as a sensitizer for blue multi‐resonance TADF (MR‐TADF) molecule (v‐DABNA) as guest emitter attains an excellent EQEmax of 32.69% with a narrow full width at half maximum of 18 nm, validating the excellent potential of 2TBCTC as sensitizer for MR‐TADF emitters and the feasibility and effectiveness of the interlayer sensitization strategy for blue hyperfluorescence OLEDs.

Modulation of Delayed Fluorescence Guided by Conformational Effect-Mediated Thermally Enhanced Phosphorescence in Phenothiazines-Quinoline-Cl Conjugates.

Triplet energy harvesting via thermally activated delayed fluorescence (TADF) from pure organic systems has attracted great attention in organic light-emitting diodes, sensing, and photocatalysis. However, the realization of thermally enhanced phosphorescence (TEP)-guided efficient TADF with a high rate of reverse intersystem crossing (kRISC) still needs to be discovered. Herein, we report two phenothiazine-quinoline conjugates (P2QC, P2QMC) comprising two phenothiazine donors covalently attached to the chlorine-substituted quinolinyl acceptor. Spectroscopic analysis in conjunction with quantum chemistry calculations reveals that TEP in P2QC originated due to slow internal conversion from higher-lying triplet to lowest triplet (T2' → T1') of the quasi-axial (QA) conformer and TADF (kRISC = 1.44 × 108 s-1) originated from the quasi-equatorial (QE) conformer caused by a low singlet-triplet gap (ΔES1-T1 = 0.11 eV) and triplet energy transfer from QA to QE owing to the degenerate ground state of the conformers. In contrast, TADF (kRISC = 0.74 × 108 s-1) and dual phosphorescence under ambient conditions are observed in P2QMC. This study provides a sustainable guideline for developing efficient TADF emitters via conformation effects and energy transfer mechanisms.

Modulation of nonradiative emission decay rate by spacer unit in donor-acceptor TADF compounds

Highly emissive thermally activated delayed fluorescence (TADF) compounds typically are constructed from twisted donor and acceptor units, usually with the help of an additional spacer unit. Such molecular structure ensures low singlet-triplet energy gap, enabling the utilization of triplet states and boosting the emission yield. Here we show that emission yield also depends on the molecular structure of spacer unit. Selecting structurally flexible biphenyl spacer unit instead of rigid fluorene was shown to selectively enhance the nonradiative decay rate of triplet states (kNRT), leading to the lowered yield of delayed fluorescence (from 0.55 to 0.43). However, more rapid kNRT led in weaker OLED external quantum efficiency (EQE) roll-off due to lowered triplet lifetime, though at the cost of lower peak efficiency. The presented work highlights the need of careful design of spacer fragments for efficient TADF emitters.

Spiro Boron‐Nitrogen Molecules Based Thermally Activated Delayed Fluorescence Emitter for Highly Efficient Solution‐Processed Organic Light‐Emitting Diodes

AbstractSpiro compounds with unique structural and electronic properties are beneficial for thermally activated delayed fluorescence (TADF) emitters. Herein, a series of spiro boron‐nitrogen (B‐N) compounds are reported with donor–acceptor (D‐A) interaction through homoconjugation. Three phenyl acridine‐containing spiro B‐N compounds exhibit highly efficient photoluminescence (PL) in degassed toluene and solid‐state with obvious TADF character. The separation of HOMO and LUMO for these compounds is confirmed by theoretical calculations, as is the small ΔEST of phenyl acridine‐containing compounds. These three compounds are used as emitting layers in solution‐processed organic light‐emitting diodes (OLEDs), resulting in deep‐blue to green emission colors with different acceptor moieties. Impressively, an excellent external quantum efficiency (EQE) of 22.1% is recorded for the TPA‐s‐FMesBF‐based OLED device, which is comparable with the highest value from solution‐processed OLEDs based on spiro D‐A emitting materials. This work presents a facile synthetic route to novel B, N‐substituted spiro compounds, as well as the structure‐property relationship of spiro D‐A compounds for highly efficient TADF emitters.

Towards red thermally activated delayed fluorescence emitter: Molecular design and property prediction of phenothiazine-triphenyltriazine derivative

A series of thermally activated delayed fluorescence (TADF) molecules with the linkage of the electron-donating group (phenothiazine, PTZ) and electron-withdrawing groups (2,4,6-triphenyl-1,3,5-triazine type, TRZ) have been designed and investigated for efficient red TADF emitters by means of quantum chemical calculations. The geometrical and electronic structures, absorption and emission properties, reverse intersystem crossing (RISC), radiative and nonradiative decay processes of the PTZ-TRZ-based molecules were analyzed. The calculated results indicate that the designed molecules possess the relatively large RISC rates (>105 s−1) and exhibit the red emission, indicating that they have the potential to be an efficient red emitting TADF material. Furthermore, the position of N atom in the TRZ acceptor group have significant effects on RISC process and emission color. This work provides in-depth understanding of the structure–property relationship of PTZ-TRZ-based TADF molecules, and offers the guideline for the screening of the efficient pure red TADF emitters.

Torsion Angle Analysis of a Thermally Activated Delayed Fluorescence Emitter in an Amorphous State Using Dynamic Nuclear Polarization Enhanced Solid-State NMR.

The torsion angle between donor and acceptor segments of a thermally activated delayed fluorescence (TADF) molecule is one of the most critical factors in determining the performance of TADF-based organic light-emitting diodes (OLEDs) because the torsion angle affects not only the energy gap between the singlet and triplet but also the oscillator strength and spin-orbit coupling. However, the torsion angle is difficult to analyze, because organic molecules are in an amorphous state in OLEDs. Here, we determined the torsion angle of a highly efficient TADF emitter, DACT-II, in an amorphous state by dynamic nuclear polarization enhanced solid-state NMR measurements. From the experimentally obtained chemical shift principal values of 15N on carbazole, we determined the average torsion angle to be 52°. Such quantification of the torsion angles in TADF molecules in amorphous solids will provide deep insight into the TADF mechanism in amorphous OLEDs.

Engineering the Macrocyclic Donor Structures towards Deep-Blue Thermally Activated Delayed Fluorescence Emitters.

Deep-blue thermally activated delayed fluorescence (TADF) molecules present promising potential in organic light-emitting diodes (OLEDs), especially for display applications. Here, an efficient molecular engineering approach to modifying the donor or acceptor features of the D-π-A-configured TADF molecules for deep-blue emission is reported. By introducing oxygen and sulfone as a bridge unit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized, respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescence quantum yields (PLQYs) and retains TADF characters, while the new emitter c-NN-MePym with the most blue-shifted emission only exhibits a pure fluorescent nature because of the electron-accepting feature of pyrimidine that is insufficient for inducing the TADF property. In the presence of macrocyclic donors, these new emitters show high horizontal dipole ratios (Θ// = 85-89%), which are beneficial for improving the light out-coupling efficiency. Deep-blue TADF OLEDs incorporating c-ON-MeTRZ as an emitter doped in the mCPCN host achieves a high maximum external quantum efficiency (EQEmax) of 30.2% together with 1931 Commission Internationale de I'Eclairage (CIE) coordinates of (0.14, 0.13), while the counter device employing c-NS-MeTRZ as a dopant gives EQEmax of 15.4% and CIE coordinates of (0.14, 0.09). The EQEmax of c-ON-MeTRZ- and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively, with a polar host DPEPO, which stabilizes the charge transfer (CT) S1 state to give lower ΔEST for improving the reverse intersystem crossing process. The efficient TADF character, high PLQYs, and high anisotropic emission dipole ratios work together to render the superior electroluminescence (EL) efficiencies. Based on the detailed characterizations of physical properties, theoretical analyses, and comprehensive study on the corresponding devices, a clear structure-property-performance relationship has been successfully established to verify the effective molecular design strategy of modulating the macrocyclic donor characters for efficient deep-blue TADF emitters.

Highly rigid polycyclic acceptors by incorporating quinoxaline and naphthalimideacridine towards red TADF emitters

The development of red thermally activated delayed fluorescence (TADF) emitters with the emission peak above 600 nm remains far behind blue and green emitters. Herein, the highly rigid polycyclic acceptors with strong electron-withdrawing ability, integrated by quinoxaline and 1,8-naphthalimideacridine units, were utilized to construct two new red emitters with emission peak at 640 nm. Both target molecules display the twisted configuration to decrease singlet-triplet splitting energy (ΔEST) for the TADF property. Red organic light-emitting diodes (OLEDs) employing the emitters achieve the maximum external quantum efficiencies of 5.2% with electroluminescence peaks over 600 nm. This work provides a feasible strategy of the new polycyclic acceptors for developing efficient red TADF emitters.

Observation of Long-Lived Charge-Separated States in Anthraquinone-Phenothiazine Electron Donor-Acceptor Dyads: Transient Optical and Electron Paramagnetic Resonance Spectroscopic Studies.

We prepared a series of phenothiazine (PTZ)-anthraquinone (AQ) electron donor-acceptor dyads to study the relationship between molecular structures and the possibility of charge transfer (CT) and intersystem crossing (ISC). As compared to the previously reported PTZ-AQ dyad with a direct connection of two units via a C-N single bond, the PTZ and AQ units are connected via a p-phenylene or p-biphenylene linker. Conformation restriction is imposed by attaching ortho-methyl groups on the phenylene linker. UV-vis absorption spectra indicate electronic coupling between the PTZ and AQ units in the dyads without conformation restriction. Different from the previously reported PTZ-AQ, thermally activated delayed fluorescence (TADF) is observed for the dyads containing one phenylene linker (PTZ-Ph-AQ and PTZ-PhMe-AQ). The prompt fluorescence lifetime in cyclohexane is exceptionally long (τPF = 62.0 ns, population ratio: 99.2%) and 245.0 ns (93.5%) for PTZ-Ph-AQ and PTZ-PhMe-AQ, respectively (normally τPF <20 ns); the delayed fluorescence lifetimes for these two dyads were determined as τDF = 2.4 μs (6.5%) and 7.6 μs (0.8%), respectively. For the dyad containing a biphenylene linker (PTZ-Ph2Me-AQ), no TADF was observed. Charge-separated (CS) states were observed for PTZ-Ph-AQ and PTZ-PhMe-AQ, and the lifetimes were determined as 7.0 and 1.3 μs, respectively, indicating the triplet spin multiplicity of the CS state. The 3CS state lifetimes are shortened to 100 ns and 440 ns for the two dyads, respectively, in the polar solvent acetonitrile. For dyads with a longer linker, i.e., PTZ-Ph2Me-AQ, the CS state lifetime is not sensitive to solvent polarity (τCS = 1.8 and 1.3 μs in cyclohexane and acetonitrile, respectively). In reference dyads, where the PTZ unit is oxidized to sulfoxide, no CT absorption band and TADF were observed, which is attributed to the increased CS state energy (>3 eV) becoming higher than that of the AQ triplet (3AQ*) state (ca. 2.7 eV). These experimental evidence show that the presence of 1CS, 3CS, and 3LE (LE: locally excited) states sharing similar energy is essential for the occurrence of TADF. Population of the long-lived 3CS state (with a lifetime of a few μs) does not produce by itself TADF, because the ISC process of 1CS→3CS is nonsufficient. Femtosecond transient absorption spectra show that charge separation (CS) occurs readily (<5 ps) for most dyads, even in nonpolar solvents. Nanosecond pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that either a spin correlated radical pair (SCRP) is formed, with the electron exchange energy 2J = +2.14 mT, or radical pairs with stronger interaction, |2J| > 6.57 mT. These studies are useful for in-depth understanding of the CS and ISC in compact electron donor-acceptor dyads and for design of efficient TADF emitters.

A Donor‐π‐Acceptor Type Red Thermally Activated Delayed Fluorescence Emitter Featuring Spiro as the Bridge Exhibiting Aggregation‐Induced Emission Activity and Outstanding Efficiencies in Organic Light‐Emitting Diodes

AbstractConstructing donor‐π‐acceptor (D‐π‐A)‐type molecular structures by employing a phenyl as the π‐bridge to link donor (D) and acceptor (A) units has been recognized as an effective way to develop highly efficient red thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs). However, flexible and relatively planar structures would open potential energy loss channels, such as nonradiative inactivation and aggregation‐induced triplet quenching processes. Here, a bulky spiro‐9,9′‐bifluorene unit is first implemented to serve as a bridging group to construct a D‐π‐A molecule, enabling it to have higher overall rigidity, more sufficient steric hindrance, prolonged molecule length, and obvious aggregation‐induced emission characteristics compared with a common phenyl bridge. As a result, energy dissipation routes are effectively relieved at the unimolecular level, together with mitigated interchromophore quenching, rendering a 100% photoluminescence quantum yield and a larger horizontal dipole ratio of 89%. The OLED based on 3‐(2‐(diphenylamino)‐9,9′‐spirobi[fluoren]‐7‐yl)dibenzo[a,c]phenazine‐11,12‐dicarbonitrile exhibits an excellent external quantum efficiency of nearly 37% at 612 nm, which is over 1.38‐fold enhancement compared with the phenyl bridge‐based control molecule. This work provides an instructive solution to design highly efficient red TADF emitters exploiting D‐π‐A‐type molecular structures.

Solution-Processed Pure Red TADF Organic Light-Emitting Diodes With High External Quantum Efficiency and Saturated Red Emission Color.

In spite of recent research progress in red thermally activated delayed fluorescence (TADF) emitters, highly efficient solution-processable pure red TADF emitters are rarely reported. Most of the red TADF emitters reported to date are designed using a rigid acceptor unit which renders them insoluble and unsuitable for solution-processed organic light-emitting diodes (OLEDs). To resolve this issue, a novel TADF emitter, 6,7-bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-5,8-dicarbonitrile (tBuTPA-CNQx) is designed and synthesized. The highly twisted donor-acceptor architecture and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital distribution lead to a very small singlet-triplet energy gap of 0.07eV, high photoluminescence quantum yield of 92%, and short delayed fluorescence lifetime of 52.4µs. The peripheral t-butyl phenyl decorated quinoxaline acceptor unit and t-butyl protected triphenylamine donor unit are proven to be useful building blocks to improve solubility and minimize the intermolecular interaction. The solution-processed OLED based on tBuTPA-CNQx achieves a high external quantum efficiency (EQE) of 16.7% with a pure red emission peak at 662 nm, which is one of the highest EQE values reported till date in the solution-processed pure red TADF OLEDs. Additionally, vacuum-processable OLED based on tBuTPA-CNQx exhibits a high EQE of 22.2% and negligible efficiency roll-off.

Enhancing reverse intersystem crossing rate of red thermally activated delayed fluorescence emitters by simultaneously involving locally excited triplet states from donor and acceptor segments

Accelerating reverse intersystem crossing (RISC) rates of red thermally activated delayed fluorescence (TADF) emitters is crucial yet remains ongoing challenges. Herein, we disclose an ingenious molecular design to dramatically boost such an up-conversion process by leveraging effective participation of localized excited triplet states from both donor and acceptor segments (3LED and 3LEA). Inspiringly, energy level alignment with 3LEA-3LED close to the charge-transfer singlet and triplet (1CT and 3CT) states are obtained in the case of DBBPZ-IDPZ, by simultaneously employing polycyclic large-conjugation donor and acceptor units. Due to the joint contribution of 3LED and 3LEA, DBBPZ-IDPZ shows 15.7 folds accelerated RISC rate and delivers a higher photoluminescence quantum yield of 98.1 % relative to the control molecule (36.7 %). Eventually, the red OLED based on DBBPZ-IDPZ presents a significantly improved external quantum efficiency of 26.0 % with an emission maximum at 628 nm, which is approximately triple that of the control device (8.9 %). This is the first example of a delicate management of 3LED and 3LEA to simultaneously participate in RISC process for developing highly efficient red TADF emitters. Our work provides new insights into the molecular design tactic of high-performance red TADF OLEDs.

Thermally activated delayed fluorescence in a mechanically soft charge-transfer complex: role of the locally excited state.

Molecular design for thermally activated delayed fluorescence (TADF) necessitates precise molecular geometric requirements along with definite electronic states to ensure high intersystem crossing (ISC) rate and photoluminescence quantum yield (PLQY). Achieving all these requirements synchronously while maintaining ease of synthesis and scalability is still challenging. To circumvent this, our strategy of combining a crystal engineering approach with basic molecular quantum mechanical principles appears promising. A holistic, non-covalent approach for achieving efficient TADF in crystalline materials with distinct mechanical properties is highlighted here. Charge transfer (CT) co-crystals of two carbazole-derived donors (ETC and DTBC) with an acceptor (TFDCNB) molecule are elaborated as a proof-of-concept. Using temperature-dependent steady-state and time-resolved photoluminescence techniques, we prove the need for a donor-centric triplet state (3LE) to ensure efficient TADF. Such intermediate states guarantee a naturally forbidden, energetically uphill reverse intersystem crossing (RISC) process, which is paramount for effective TADF. A unique single-crystal packing feature with isolated D-A-D trimeric units ensured minimal non-radiative exciton loss, leading to a high PLQY and displaying interesting mechanical plastic bending behaviour. Thus, a comprehensive approach involving a non-covalent strategy to circumvent the conflicting requirements of a small effective singlet-triplet energy offset and a high oscillator strength for efficient TADF emitters is achieved here.

Tailoring Donor‐Acceptor Emitters to Minimise Localisation Induced Quenching of Thermally Activated Delayed Fluorescence

AbstractBy inverting the common structural motif of thermally activated delayed fluorescence (TADF) materials to a rigid donor core and multiple peripheral acceptors, it has been shown possible to achieve both a reverse intersystem crossing (rISC) rate 1×107 s−1 and a unity photoluminescence quantum yield [Dos Santos et al. Adv. Sci. 5 (2018) 1700989]. However, the rISC rate in this motif is quenched by localisation of the excited state electronic structure which causes in‐equivalence between the peripheral acceptors. In this paper, we explore a series of related molecular targets which seek to reduce the effect of localisation on the rISC rates. This includes structures that contain donors exhibiting three‐fold or four‐fold symmetry and different degrees of steric hindrance around the donor‐acceptor bond with the objective of using steric hindrance to exert finer conformational control of the excited state dynamics to enhance functional properties. We demonstrate that a triazatruxene central donor is most effective for TADF owing to the energetic position of the locally excited state, while cyano‐benzonitriles are the most effective acceptors for reducing the effect of localisation on the electronic structure. These also push the emission into the blue region of the spectrum, opening the possibilities for this to be a pathway to develop efficient blue TADF emitters.