Thermally Activated Delayed Fluorescence Mechanism Research Articles

Application of the Heavy-Atom Effect for (Sub)microsecond Thermally Activated Delayed Fluorescence and an All-Organic Light-Emitting Device with Low-Efficiency Roll-off.

The feature of abundant and environmentally friendly heavy atoms (HAs) like bromine to accelerate spin-forbidden transitions in organic molecules has been known for years. In combination with the easiness of incorporation, bromine derivatives of organic emitters showing thermally activated delayed fluorescence (TADF) emerge as a cheap and efficient solution for the slow reverse intersystem crossing (rISC) problem in such emitters and strong efficiency roll-off of all-organic light-emitting diodes (OLEDs). Here, we present a comprehensive photophysical study of a tri-PXZ-TRZ emitter reported previously and its hexabromo derivative showing a remarkable enhancement of rISC of up to 9 times and a short lifetime of delayed fluorescence of 2 μs. Analysis of the key molecular vibrations and TADF mechanism indicates almost compete blockage of the spin-flip transition between the charge-transfer states of different multiplicity 3CT → 1CT. In such a case, rISC as well as its enhancement by the HA is realized via the 3LE → 1CT transition, where 3LE is the triplet state localized on the same brominated phenoxazine donor involved in the formation of the 1CT state. Interestingly, the spin-orbit coupling (SOC) with two other 3LE states is negligible because they are localized on different donors and not involved in 1CT. We consider this as an example of an additional "localization" criterion that completes the well-known El Sayed rule on the different nature of states for nonzero SOC. The applicative potential of such a hexabromo emitter is tested in a "hyperfluorescent" system containing a red fluorescent dopant (tetraphenyldibenzoperiflanthene, DBP) as an acceptor of Förster resonance energy transfer, affording a narrow-band red-emitting system, with most of the emission in the submicrosecond domain. In fact, the fabricated red OLED devices show remarkable improvement of efficiency roll-off from 2-4 times depending on the luminance, mostly because of the increase of the rISC constant rate and the decrease of the overall delayed fluorescence lifetime thanks to the HA effect.

Thermally Activated Delayed Fluorescence (TADF)-active Coinage-metal Sulfide Clusters for High-resolution X-ray Imaging.

The study of facile-synthesis and low-cost X-ray scintillators with high light yield, low detection limit and high X-ray imaging resolution plays a vital role in medical and industrial imaging fields. However, the optimal balance between X-ray absorption, decay lifetime and excitonic utilization efficiency of scintillators to achieve high-resolution imaging is extremely difficult due to the inherent contradiction. Here two thermally activated delayed fluorescence (TADF)-actived coinage-metal clusters M6 S6 L6 (M=Ag or Cu) were synthesized by simple solvothermal reaction, where the cooperation of heavy atom-rich character and TADF mechanism supports strong X-ray absorption and rapid luminescent collection of excitons. Excitingly, Ag6 S6 L6 (SC-Ag) displays a high photoluminescence quantum yield of 91.6 % and scintillating light yield of 17420 photons MeV-1 , as well as a low detection limit of 208.65 nGy s-1 that is 26 times lower than the medical standard (5.5 μGy s-1 ). More importantly, a high X-ray imaging resolution of 16 lp/mm based on SC-Ag screen is demonstrated. Besides, rigid core skeleton reinforced by metallophilicity endows clusters M6 S6 L6 strong resistance to humidity and radiation. This work provides a new view for the design of efficient scintillators and opens the research door for silver clusters in scintillation application.

Thermally Activated Delayed Fluorescence (TADF)‐active Coinage‐metal Sulfide Clusters for High‐resolution X‐ray Imaging

AbstractThe study of facile‐synthesis and low‐cost X‐ray scintillators with high light yield, low detection limit and high X‐ray imaging resolution plays a vital role in medical and industrial imaging fields. However, the optimal balance between X‐ray absorption, decay lifetime and excitonic utilization efficiency of scintillators to achieve high‐resolution imaging is extremely difficult due to the inherent contradiction. Here two thermally activated delayed fluorescence (TADF)‐actived coinage‐metal clusters M6S6L6 (M=Ag or Cu) were synthesized by simple solvothermal reaction, where the cooperation of heavy atom‐rich character and TADF mechanism supports strong X‐ray absorption and rapid luminescent collection of excitons. Excitingly, Ag6S6L6 (SC‐Ag) displays a high photoluminescence quantum yield of 91.6 % and scintillating light yield of 17420 photons MeV−1, as well as a low detection limit of 208.65 nGy s−1 that is 26 times lower than the medical standard (5.5 μGy s−1). More importantly, a high X‐ray imaging resolution of 16 lp/mm based on SC‐Ag screen is demonstrated. Besides, rigid core skeleton reinforced by metallophilicity endows clusters M6S6L6 strong resistance to humidity and radiation. This work provides a new view for the design of efficient scintillators and opens the research door for silver clusters in scintillation application.

Solid-state effect on the luminescence mechanism of thermally activated delayed fluorescence with aggregation induced emission: A theoretical perspective

An investigation of high-performance organic green thermally activated delayed fluorescence (TADF) material, PyB-DPAC, with aggregation-induced luminescence (AIE) characteristics was theoretically conducted by the first principle calculations method in this work. The polarizable continuum mode (PCM) and the combination of quantum mechanics and molecular mechanics (QM/MM) method were employed to consider the liquid-phase and solid-phase environments, respectively. Results show that TADF mechanism of the molecule is mainly due to the occurrence of efficient intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes between S1 and T2, which benefits from the small energy gap, large SOC and the limited changes of molecular geometry structures in solid-phase environment. The increase of radiative transition rate (Kr) and significant decrease of non-radiative transition rate (Knr) is responsible for the AIE property. Our calculations reasonably explained the experimental results and showed that PyB-DPAC is an excellent TADF material with AIE characteristics, this may help to better understand the influence of solid-state effect and the luminescence mechanism of AIE-TADF molecules.

Charge transporting and thermally activated delayed fluorescence materials for OLED applications.

The design and synthesis of effective charge transporting (CT) and thermally activated delayed fluorescence (TADF) materials are in high demand to obtain high-performing OLED devices. Recently, the significant development in the field of OLEDs has led to the creation of numerous charge transporting and TADF materials with diverse structures. To further improve the device performance, a better understanding of the structural characteristics and structure-property relationships of these materials is essential. Moreover, to enhance the efficiency of OLEDs, all the electrogenerated excitons should be constrained in EMLs. The TADF mechanism can theoretically register 100% IQE through a potent up-conversion method from non-radiative triplet excitons to radiative singlet excitons. In this review, the structural importance, classification, physical properties, and electroluminescence data of some recent charge transporting and TADF materials are summarized and discussed. Moreover, their molecular structural dependence on functional groups and linkers is classified, which can enhance their charge transporting or emitting ability. To offer a potential roadmap for the further development of charge transporting and TADF materials, it is hoped that this study will encourage researchers to acknowledge their important role in OLEDs.

What is TADF (thermally activated delayed fluorescence) compared to the mechanisms of FL (fluorescence), PH (phosphorescence), and TTA (triplet–triplet annihilation) based on a novel naphthalimide sulfonylphenyl derivative as a host?

One of the biggest challenges in the field of optoelectronics is to easily identify the mechanisms involved in light-emitting materials. There are several different mechanisms of emission, and the best known are fluorescence (FL), phosphorescence (PH), thermally activated delayed fluorescence (TADF), and triplet–triplet annihilation (TTA). In this study, simple ways to diagnose these mechanisms are presented with an example. N,N-di(benzenesulfonylphenyl)-N-(N-2-aminomethylpyridine-1,8-naphthalimide)amine is synthesized as a host compound due to its high HOMO-LUMO and ionization potential (IP) values and three emitters consisting of 50 wt% Bis (1phenylisoquinoline))(acetylacetonate)iridium(III) (Ir(piq)2(acac)), 10-(4-(4,6-Diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (DMAC-TRZ) and 1,1′-(2,5-Dimethyl-1,4-phenylene)dipyrene (DMPPP) were considered. Overall, the host compound, doped Ir(piq)2(acac) and doped DMPPP follow the mechanisms of FL, the combination of FL and PH, and the combination of FL and TTA, as the photoluminescence (PL) decay life time and the PL intensity increased with decreasing temperature. However, DMAC-TRZ doped with the host compound showed the TADF mechanism, as the PL decay lifetime and the PL intensity increase simultaneously with increasing temperature. In addition, the difference between the singlet excited state (S1) and the triplet excited state T1 (ΔEST) from the PL and PH spectra at 77 K for the spin-coated DMAC-TRZ-doped host show the lowest value in between the samples at 0.02 eV, and the reverse intersystem crossing (rISC) effect is activated. Moreover, the linear slopes of the power dependence analysis of the DMPPP-doped host were calculated to be 2.04 and 0.91, indicating that there are two components (photons) for this sample which is strong evidence for the TTA mechanism. Taking into account the synthesized host compound, (Ir(piq)2(acac):host), (DMAC-TRZ:host) and (DMPPP:host) as emitter layers, the organic light emitting diodes (OLEDs) were fabricated via solution processing, and the TADF OLED exhibited the highest efficiency among the other OLEDs.

Chemical Tailoring Assisted non-TADF to TADF Switching in Carbazole-Benzophenone Emitter - An In-silico Investigation.

Organic light-emitting diodes (OLEDs) have become one of the most popular lighting technologies since they offer several advantages over conventional devices. In carbazole-benzophenone (CzBP) OLED devices, the polymeric form of the compound is previously reported to be Thermally Activated Delayed Fluorescence (TADF)-active (ΔEST ≈0.12 eV), while the monomer (CzBP) (ΔEST ≈0.39 eV) does not. The present study examines the effect of chemical tailoring on the optical and photophysical properties of CzBP using DFT and TDDFT methods. The introduction of a single -NO2 group or di-substitution (-NO2 , -COOH or -CN) in the selected LUMO region of the reference CzBP monomer significantly reduces ΔEST ≈0.01 eV, projecting these systems as potential TADF-active emitters. Furthermore, the chemical modification of CzBP-LUMO alters the two-step TADF mechanism (T1 →T2 →S1 ) in CzBP (ES₁ >ET2 >ET₁ ) to the Direct Singlet Harvesting (T1 →S1 ) mechanism (ET2 >ES₁ >ET₁ ), which has recently been identified in the fourth-generation OLED materials.

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.

Afterglow OLEDs incorporating bright closely stacked molecular dimers with ultra-long thermally activated delayed fluorescence

•Molecular aggregates exhibiting robust luminescence properties •Long-lived thermally activated delayed fluorescence from dimers •Bright afterglow emission in an electroluminescence device Purely organic luminescent materials exhibiting afterglow emission after photoexcitation have been developed recently and hold great promise for future applications, whereas the afterglow from electroexcitation remains rare and inefficient. Here, afterglow electroluminescence is achieved based on an organic dimer system with high photoluminescence quantum yield and unprecedented long-lived thermally activated delayed fluorescence (TADF) characteristics. The strong π-π interactions and aggregation structures can suppress nonradiative transition and promote fluorescence emission. In addition, the molecular stacking also renders a moderate singlet-triplet energy gap, enabling a long-lived TADF afterglow. In combination with host-guest interaction optimization, efficient organic light-emitting diodes (OLEDs) based on the dimer system achieve high external quantum efficiency of 14.7% along with bright afterglow emission with lifetime of 157 ms. This work not only extends the scope of purely organic aggregates for novel optoelectronic properties and applications but also opens up new pathways for achieving efficient afterglow OLEDs with TADF mechanism. Purely organic luminescent materials exhibiting afterglow emission after photoexcitation have been developed recently and hold great promise for future applications, whereas the afterglow from electroexcitation remains rare and inefficient. Here, afterglow electroluminescence is achieved based on an organic dimer system with high photoluminescence quantum yield and unprecedented long-lived thermally activated delayed fluorescence (TADF) characteristics. The strong π-π interactions and aggregation structures can suppress nonradiative transition and promote fluorescence emission. In addition, the molecular stacking also renders a moderate singlet-triplet energy gap, enabling a long-lived TADF afterglow. In combination with host-guest interaction optimization, efficient organic light-emitting diodes (OLEDs) based on the dimer system achieve high external quantum efficiency of 14.7% along with bright afterglow emission with lifetime of 157 ms. This work not only extends the scope of purely organic aggregates for novel optoelectronic properties and applications but also opens up new pathways for achieving efficient afterglow OLEDs with TADF mechanism.

Merging thermally activated delayed fluorescence and two-photon ionization mechanisms for highly efficient and ultralong-lived organic afterglow

It is challenging to simultaneously achieve high afterglow efficiency and long afterglow duration in organic systems. Here we report dopant-matrix materials with afterglow efficiency of 92.4 ± 10.7 % and afterglow duration up to 1 h under ambient conditions. The materials exhibit thermally activated delayed fluorescence (TADF) mechanism at room temperature to harvest triplet energies and possess afterglow efficiency close to unity. At high excitation dose where two-photon ionization mechanism is switched on, the materials follow a power law decay of excited states due to retarded charge recombination and display hour-long afterglow duration. The TADF mechanism and two-photon ionization are respectively responsible for the high efficiency and hour-long duration. This two-mechanism strategy provides a new pathway to address efficiency-to-duration conflict in afterglow systems. Moreover, the present afterglow materials can serve as efficient donors of Förster resonance energy transfer, exhibit excellent processability, and function as background-free bioimaging agents.

Theoretical Insights into the Optical and Excited State Properties of Donor–Phenyl Bridge–Acceptor Containing Through-Space Charge Transfer Molecules

A comparative new strategy to enhance thermally activated delayed fluorescence (TADF) of through-space charge transfer (CT) molecules in organic light-emitting diodes (OLEDs) is investigated. Generally, TADF molecules adopt a twisted donor and acceptor structure to get a sufficiently small ΔEST and a higher value of the spin-orbit coupling matrix element (SOCME). However, molecules containing donor-phenyl bridge-acceptor (D-p-A) units and featuring π-stacked architectures have intramolecular CT contribution through space and exhibit high TADF efficiency. We have explored the insights into the TADF mechanism in D-p-A molecules using the density functional theory (DFT) and time-dependent DFT methods. The calculated optical absorption and ΔEST values are found to be in good agreement with available experimental data. Interestingly, we found the origin of the SOCME to be the twisted orientation of the donor and bridge moieties. Also, we predicted similar molecules with enhanced OLED efficiency with different substitutions.

Theoretical Studies on Thermally Activated Delayed Fluorescence Mechanism of Au(III) Complexes in Films: Insights from Quantum Mechanics/Molecular Mechanics Simulations

Organometallic complexes usually exhibit thermally activated delayed fluorescence (TADF) emission in films. However, the surrounding effects on TADF have not been explored and the underlying photophysical mechanism remains elusive. In this study, we have investigated structures, spectroscopic characteristics, radiative and nonradiative rates, and luminescence mechanisms of three Au(III) complexes in films using DFT and time-dependent DFT approaches combined with molecular dynamics and quantum mechanics/molecular mechanics simulations. The results show that the S1 and T1 states are of ligand-to-ligand charge transfer character owing to the efficient separation of the HOMO and the LUMO. Proper spin–orbit couplings and small energy gaps between the S1 and T1 states benefit the reverse intersystem crossing process enabling TADF. Moreover, the film environments play key roles in regulating geometric structures, electronic properties, and thereto luminescence. These results provide insights into understanding the TADF mechanism of organometallic complexes in films.

Fine‐Tuning the Photophysics of Donor‐Acceptor (D‐A3) Thermally Activated Delayed Fluorescence Emitters Using Isomerisation

AbstractHere two D–A3 regioisomers, comprising three dibenzothiophene‐S,S‐dioxide acceptor units attached to a central triazatruxene core, are studied. Both molecules show thermally activated delayed fluorescence (TADF), however, the efficiency of the TADF mechanism is strongly affected by the D–A substitution position. The meta‐ substituted emitter (1 b) shows a slightly higher‐lying singlet charge transfer state and a lower‐lying triplet state than that observed in the para‐ substituted emitter (1 a), resulting in a larger singlet–triplet splitting (ΔEST) of 0.28 eV compared to only 0.01 eV found in 1 a. As expected, this ΔEST difference strongly impacts the reverse intersystem crossing (rISC) rates and the para‐ isomer 1 a exhibits a much faster delayed fluorescence emission. Calculations show that the triplet energy difference between the two isomers is due to steric hindrance variances along the donor–acceptor rotation axis in these molecules: as 1 b is less restricted, rotation of its acceptor unit leads to a lower T1 energy, further away from the region of high density of states (the region where larger vibronic coupling is found, favouring rISC). Therefore, our results show how the substitution pattern has a marked effect on triplet state energies and character, verifying the key structural designs for highly efficient TADF materials.

The Interplay between ESIPT and TADF for the 2,2'-Bipyridine-3,3'-diol: A Theoretical Reconsideration.

Organic molecules with excited-state intramolecular proton transfer (ESIPT) and thermally activated delayed fluorescence (TADF) properties have great potential for realizing efficient organic light-emitting diodes (OLEDs). Furthermore, 2,2'-bipyridine-3,3'-diol (BP(OH)2) is a typical molecule with ESIPT and TADF properties. Previously, the double ESIPT state was proved to be a luminescent state, and the T2 state plays a dominant role in TADF for the molecule. Nevertheless, whether BP(OH)2 undergoes a double or single ESIPT process is controversial. Since different ESIPT channels will bring different TADF mechanisms, the previously proposed TADF mechanism based on the double ESIPT structure for BP(OH)2 needs to be reconsidered. Herein, reduced density gradient, potential energy surface, IR spectra and exited-state hydrogen-bond dynamics computations confirm that BP(OH)2 undergoes the barrierless single ESIPT process rather than the double ESIPT process with a barrier. Moreover, based on the single ESIPT structure, we calculated spin-orbit coupling matrix elements, nonradiative rates and electron-hole distributions. These results disclose that the T3 state plays a predominant role in TADF. Our investigation provides a better understanding on the TADF mechanism in hydrogen-bonded molecular systems and the interaction between ESIPT and TADF, which further provides a reference for developing efficient OLEDs.

Femtosecond transient absorption spectroscopy on the thermally activated delayed fluorescence of bis[4-(9-H-carbazole)phenyl] sulfone

Tracking the time-evolution process of singlet-triplet conversion is essential to achieve effective thermally activated delayed fluorescence (TADF). The femtosecond transient absorption (fs-TA) spectroscopy of Bis[4-(9-H-carbazole)phenyl] Sulfone (CZ-DPS) in dichloromethane (DCM) shows that the excited state absorption (ESA) signal (375 nm) and the stimulated emission (SE) signal (430 nm) reach their maximum values within 4.1 ps. Subsequently, along with the decay of the ESA and SE peaks, a noticeable triplet-triplet absorption (TTA) signal appears at 428 nm and the intensity gradually increases. Furthermore, an isosbestic point at 400 nm between ESA and TTA signals indicates the occurrence of the intersystem crossing (ISC) process from singlet to triplet states, and its time constant is 3.3 ns. Moreover, the observed long-lived lifetime of 12.1 μs in DCM solution is assigned to the delayed fluorescence. The large spin-orbit coupling (SOC) constant (1.039 cm−1) facilitates the ISC process of the CZ-DPS from the first excited state (S1) to the second triplet-state (T2). Furthermore, the small energy difference (ΔES1T2 = 0.03 eV) induces the effective reversed ISC process from the T2 to S1 states to achieve delay fluorescence emission. This work presents the time-resolved spectra changes and TADF mechanism of CZ-DPS molecule.

Diabatic Decomposition Perspective on the Role of Charge Transfer and Local Excitations in Thermally Activated Delayed Fluorescence.

Thermally activated delayed fluorescence (TADF) is a phenomenon that relies on the upconversion of triplet excitons to singlet excitons by means of reverse intersystem crossing (rISC). It has been shown both experimentally and theoretically that the TADF mechanism depends on the interplay between charge transfer and local excitations. However, the difference between the diabatic and adiabatic character of the involved excited states is rarely discussed in the literature. Here we develop a diabatization procedure to implement a four-state model Hamiltonian to a set of TADF molecules. We provide physical interpretations of the Hamiltonian elements and show their dependence on the electronic state of the equilibrium geometry. We also demonstrate how vibrations affect the TADF efficiency by modifying the diabatic decomposition of the molecule. Finally, we provide a simple model that connects the diabatic Hamiltonian to the electronic properties relevant to TADF and show how this relationship translates into different optimization strategies for rISC, fluorescence, and overall TADF performance.

More than 25,000 h device lifetime in blue phosphorescent organic light-emitting diodes via fast triplet up-conversion of n-type hosts with sub μs triplet exciton lifetime

Understanding and retardation of materials degradation phenomena in blue phosphorescent organic-light emitting diodes (PhOLEDs) are crucial to replace a fluorescent device with an efficient phosphorescent device in practical applications. Among the organic functional materials in the blue PhOLEDs, the host in the emitting layer is the most populated and fragile region in view of its density of states since singlet, triplet excitons, and polarons are continuously accumulated and consumed during operation. In this work, we presented novel n-type hosts which have thermally activated delayed fluorescence (TADF) features without loss of triplet energy even in the p-n mixed host structure. Both high triplet energy of mixed host and triplet up-conversion of a new n-type host within sub μs range allowed to demonstrate highly efficient and operationally stable blue PhOLEDs. Introducing 3,3-di(9H-carbazol-9-yl)biphenyl and 3-[bis(9H-carbazol-9-yl)-1,3,5-triazin-2-yl]-5-(triphenylsilyl)benzonitrile as a mixed host, blue PhOLEDs exhibited maximum external quantum efficiency of 20.9%, color coordinate of (0.16, 0.29), and device lifetime of 13,573 h at 100 cd m−2, which was 4.2 times longer than that of state of the art blue device. Furthermore, the optimized blue PhOLEDs demonstrated device lifetime longer than 25,000 h, which is one of the longest device lifetimes of blue PhOLEDs without any sensitization or outcoupling enhancement. This result indicated that triplet exciton recycling via TADF mechanism in the n-type host is one of the effective approaches to enhance the operational stability of blue PhOLEDs.

Thermally Activated Delayed Fluorescence of a Pyromellitic Diimide Derivative in the Film Environment Investigated by Combined QM/MM and MS-CASPT2 Methods.

Arylene diimide compounds exhibit thermally activated delayed fluorescence (TADF), but its mechanism remains elusive. Herein we studied the TADF mechanism of a carbazole-substituted pyromellitic diimide derivative (CzPhPmDI) in poly(methyl methacrylate) (PMMA) film by using DFT, TD-DFT, and MS-CASPT2 methods within the QM/MM framework. We found that the TADF mechanism involves three electronic states (i.e., S0, S1, and T1), but the T2 state is not involved because its energy is higher than the S1 state by 6.9 kcal/mol. By contrast, the T1 state is only 3.2 kcal/mol lower than the S1 state and such small energy difference benefits the reverse intersystem crossing (rISC) process from T1 to S1 thereto TADF. This point is seconded by relevant radiative and nonradiative rates calculated. At room temperature, the ISC rate from S1 to T1 is calculated to be 6.1 × 106 s-1, which is larger than the fluorescence emission rate, 2.2 × 105 s-1; thus, the dominant S1 population converts to the T1 state. However, in the T1 state, the rISC process (1.8 × 104 s-1) becomes the most important channel because of the negligible phosphorescence emission rate (3.5 × 10-2 s-1). So, the T1 population is still converted back to the S1 state to fluoresce enabling TADF. Unfortunately, the rISC process is blocked in low temperature. Besides, we found that relevant Huang-Rhys factors have dominant contribution from low-frequency vibrational motion related to the torsional motion of functional groups. These gained insights could provide useful information for the design of organic TADF materials with excellent luminescence efficiency.

Long-Lived Triplet Charge Separated State and Thermally Activated Delayed Fluorescence in a Compact Orthogonal Anthraquinone-Phenothiazine Electron Donor-Acceptor Dyad.

A long-lived triplet charge separated state (3CS state lifetime: 0.56 μs) was observed in a compact electron donor-acceptor dyad with electron donor phenothiazine (PTZ) and acceptor anthraquinone (AQ) directly connected by a single C-N bond (AQ-PTZ). The 1CS state energy (2.0 eV in cyclohexane) is lower than those of the 3AQ (2.7 eV) or the 3PTZ state (2.6 eV). By oxidation of the PTZ unit, thus increasing of the 1CS state energy (2.7 eV in cyclohexane), thermally activated delayed fluorescence (TADF) was observed [τ = 17.7 ns (99.9%)/1.5 μs (0.1%)]. Time-resolved electron paramagnetic resonance (TREPR) spectra confirm the electron spin multiplicity of the 3CS state, and the zero-field-splitting (ZFS) parameters |D| and |E| are 48.2 mT and 11.2 mT, respectively. These results are useful for design of compact electron donor-acceptor dyads to access the long-lived 3CS state and study the TADF mechanism.

Thermally Activated Delayed Fluorescence Mechanism of a Bicyclic "Carbene-Metal-Amide" Copper Compound: DFT/MRCI Studies and Roles of Excited-State Structure Relaxation.

Herein we investigated the luminescence mechanism of one "carbene-metal-amide" copper compound with thermally activated delayed fluorescence (TADF) using density functional theory (DFT)/multireference configuration interaction, DFT, and time-dependent DFT methods with the polarizable continuum model. The experimentally observed low-energy absorption and emission peaks are assigned to the S1 state, which exhibits clear interligand and partial ligand-to-metal charge-transfer character. Moreover, it was found that a three-state (S0, S1, and T1) model is sufficient to describe the TADF mechanism, and the T2 state should play a negligible role. The calculated S1-T1 energy gap of 0.10 eV and proper spin-orbit couplings facilitate the reverse intersystem crossing (rISC) from T1 to S1. At 298 K, the rISC rate of T1 → S1 (∼106 s-1) is more than 3 orders of magnitude larger than the T1 phosphorescence rate (∼103 s-1), thereby enabling TADF. However, it disappears at 77 K because of a very slow rISC rate (∼101 s-1). The calculated TADF rate, lifetime, and quantum yield agree very well with the experimental data. Methodologically, the present work shows that only considering excited-state information at the Franck-Condon point is insufficient for certain emitting systems and including excited-state structure relaxation is important.