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![Figure 5. Molecular structures of CDCBs. Me, methyl; Ph, phenyl [6].](profile/Thomas-Penfold/publication/314384358/figure/fig4/AS:469969948418051@1489060975817/Molecular-structures-of-CDCBs-Me-methyl-Ph-phenyl-6_Q320.jpg)
Thermally activated delayed fluorescence (TADF) has recently emerged as one of the most attractive methods for harvesting triplet states in metal-free organic materials for application in organic light emitting diodes (OLEDs). A large number of TADF molecules have been reported in the literature with the purpose of enhancing the efficiency of OLEDs...
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... situation is similar to the existing requirements when selecting hosts for OLEDs based on heavy metal complexes. Some of the most fre- quently used hosts are mCP, CBP, DPEPO, mCBP, TPBi, TCTA, 1, 5-DCN, and TAPC, see figure 10 for molecular structures [10,[46][47][48][49][50][51][52][53][54]. ...
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... The latter phenomenon is termed thermally activated delayed fluorescence (TADF) and has been intensively studied. 11,[14][15][16][17][18][19][20] A key property of TADF emitters is a small energy gap ∆E ST between S 1 and T 1 states 17,18 ...
Efficient OLEDs must quickly convert singlet and triplet excitons into photons. Molecules with an inverted singlet-triplet energy gap (INVEST) are promising candidates for this task. However, typical INVEST molecules have drawbacks like too low oscillator strengths and excitation energies. High-throughput screening could identify suitable INVEST molecules, but existing methods are problematic: The workhorse method TD-DFT cannot reproduce gap inversion, while wavefunction-based methods are too slow. This study proposes a state-specific method based on unrestricted Kohn-Sham DFT with common hybrid functionals. Tuned on the new INVEST15 benchmark set, this method achieves an error of less than 1 kcal/mol, which is traced back to error cancellation between spin contamination and dynamic correlation. Applied to the larger and structurally diverse NAH159 set in a black-box fashion, the method maintains a small error (1.2 kcal/mol) and accurately predicts gap signs in 83% of cases, confirming its robustness and suitability for screening workflows.
... The proportion of generated excitons of different spins under electrical excitation, i.e., singlets and triplets, is a ratio of 1:3 in OLEDs, intrinsically determined by the spin of charge carriers after bimolecular recombination [6,7]. Therefore, the kinetic behavior of singlets and triplets plays different roles in device performance [8,9]. ...
... In this derivation, each kinetic process involved is assumed with a constant rate. In the early stages, such a treatment was successfully applied to describe the photophysical properties of TADF systems [9]. Recently, there have been reports about the impact of different geometrical configurations on the triplet-state energy in donor-acceptor type TADF emitters [27]. ...
The efficiency of electroluminescent devices based on thermally activated delayed fluorescence (TADF) is a complex interplay of spin-allowed radiative and nonradiative singlet and triplet recombination as well as spin-flip (reverse) intersystem crossing processes. An analytical description of exciton dynamics based on the various singlet and triplet transition rates would strongly facilitate the evaluation of the effects of different processes on device performance. We present unified and analytical expressions for the exciton densities, photoluminescence quantum yield (PLQY), and charge-to-photon (CTP) conversion efficiency in TADF-based electroluminescent devices. The derivation is based on a three-level model that can also be applied to conventional fluorescence- or phosphorescence-based devices. The model allows us to analytically calculate the fundamental kinetic rates in TADF systems as well as the CTP efficiency in devices with only PLQY and transient photoluminescence decay as experimental inputs. Evaluation of the individual kinetic processes reveals that TADF emitters with PLQY as high as around 90% that exhibit pronounced delayed fluorescence, intuitively treated as potential candidates for high-performance electroluminescent devices, can still result in a CTP efficiency of only 50%–60% due to the direct competition between triplet recombination and reverse intersystem crossing.
... Thermally activated delayed fluorescence (TADF) materials have attracted increasing attention as efficient emitters for fabricating OLEDs. The main advantage of TADF-based OLEDs is that almost 100% internal quantum efficiency can be achieved without employing precious metals [2][3][4][5][6][7][8] . In basic terms, the best way to achieve high TADF efficiency involves the spatial separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) by connecting the donor and acceptor groups [3][4][5][6][7] . ...
... The main advantage of TADF-based OLEDs is that almost 100% internal quantum efficiency can be achieved without employing precious metals [2][3][4][5][6][7][8] . In basic terms, the best way to achieve high TADF efficiency involves the spatial separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) by connecting the donor and acceptor groups [3][4][5][6][7] . However, such donor-acceptor-type designs have the significant drawback of a broad emission band caused by large structural relaxation in the excited state. ...
An ultrapure deep-blue multi-resonance-induced thermally activated delayed fluorescence material (DOB2-DABNA-A) is designed and synthesized. Benefiting from a fully resonating extended helical π-conjugated system, this compound has a small ΔE ST value of 3.6 meV and sufficient spin–orbit coupling to exhibit a high-rate constant for reverse intersystem crossing ( k RISC = 1.1 × 10 6 s –1 ). Furthermore, an organic light-emitting diode employing DOB2-DABNA-A as an emitter is fabricated; it exhibits ultrapure deep-blue emission at 452 nm with a small full width at half maximum of 24 nm, corresponding to Commission Internationale de l’Éclairage (CIE) coordinates of (0.145, 0.049). The high k RISC value reduces the efficiency roll-off, resulting in a high external quantum efficiency (EQE) of 21.6% at 1000 cd m –2 .
... In principle, the realization of TADF in organic emitters requires minimization of the energy gap between the lowest excited triplet and singlet excited states (ΔE ST ). 2 In practice, this requirement can be met in donor−acceptor (D−A) systems with large D−A dihedral angles, resulting in chargetransfer (CT) character of excited states. 3 However, most of the developed OLED devices based on organic TADF-active emitters still suffer from a strong efficiency roll-off and low stability as a consequence of the accumulation of long-lived triplet excitons. In fact, a spin-flip transition between the states of the same CT nature is a forbidden process, and its spin− orbit coupling (SOC) is too low to enable rISC in a submicrosecond domain to meet the application requirements. ...
... This, together with their broad and structureless shape, indicates strong CT character of such a T 1 −S 0 transition, similar to that of S 1 −S 0 observed in the PL spectra ( Figure S1). Hence, in more polar CBP and DPEPO hosts, the lowest triplet excited state of both emitters is attributed to 3 CT. ...
... Due to the presence of three donor units in both emitters of C 3 symmetry), the 1 CT and 3 CT states, as well as 3 LE D , are three times degenerated. However, to simplify calculations, only one of each three existing 1 CT, 3 CT, and 3 LE D states was considered, which is justified further. ...
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.
... A gradual spectral shift into the low-energy area was observed, so, with the radiation shift from λ = 493 nm at the beginning of the area, a fast emission region (t = 0-10 ns) to λ = 535 nm, and in the long-wave area a slowed-down emitting band (t = 10−70 μs) was observed. The spectral red shift with the delayed emission is a feature of the exciplex caused by the effect of polarization under the influence of the host medium [42,43]. a b To increase further the efficiency of using the exciplex approach in the technology of organic light-emitting devices, it is advisable to develop molecules with a high PLQY level for the use of D and A, while in order to minimize the quenching of triplet states in the TADF process, the donor and acceptor molecules should contain high-energy triplet energy levels [44,45]. ...
The presence of the effect of thermally activated delayed fluorescence (TADF) in organic light-emitting materials (emitters), manifested in the "collecting" of triplet excitons in organic semiconductor complexes that do not contain noble metals, creates excellent prerequisites for the application of TADF materials in the technology of manufacturing organic light-emitting diodes (OLED). The significant progress in solving theoretical and technical problems, achieved in the process of development of highly efficient TADF materials, paves the way for the formation of the future of organic electronics. This review presents the analyses of the nature of the long-term fluorescence generation mechanism at the molecular level and the up-to-date strategies for designing TADF donor-acceptor materials, as well as exciplex intermolecular complexes. Special attention is focused on the analysis of TADF emitter ambipolar materials with a highly twisted, rigid molecular structure, which reveal a tendency towards the multi-channel emission mechanisms and their implementation in a variety of OLED structure architectures.
... 24,25 For TADF emitters, the emission has taken from the singlet state with a certain time delay than that of normal fluorescence by the reverse intersystem crossing (rISC) from the triplet state. 26 On the other hand, RTP occurred from the populated triplet states with a long time interval (Scheme 1B). ...
... After detailed analysis of the intriguing PL of a Au 38 (PET) 26 (PET = 2-phenylethanethiolate) NC, Lianshun et al. 80 established the coexistence of fluorescence, phosphorescence, and TADF emissions in Au 38 (PET) 26 . The NCs showed an NIR emission at 865 nm with a QY of 1.8% at room temperature. ...
... The PL spectrum showed a small decrease in intensity in pure oxygenated atmosphere than that in N 2 atmosphere ( Figure 9B). This confirmed the existence of triplet state in Au 38 (PET) 26 . Under N 2 atmosphere, the lifetime of the Au 38 (PET) 26 comprises three components: 35.3 ns (12.7%, τ 1 ), 448.9 ns (33.3%, τ 2 ), and 2.3 μs (54.0%, τ 3 ) ( Figure 9C). ...
The emerging research area of ultrasmall, luminescent coinage metal nanoclusters exhibiting delayed emission through thermally-activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) has garnered considerable interest in diverse applications such as imaging, therapy, and optoelectronics. To utilize the metal nanoclusters for advanced applications, it is crucial to understand the excited state dynamics governing delayed emission. Here, we comprehensively review the recent advancement in the design strategies of metal nanoclusters aimed at achieving control over their long-lived luminescence. The design strategies include solvent-induced aggregation, crystallization-induced self-assembly, complexation methods, ligand engineering, metal core doping, and composition regulation. Through these strategies, long-lived emission can be achieved which can provide TADF and RTP luminescence mechanism. Our primary goal is to harness the versatile capabilities of long-lived metal nanoclusters to advance their optoelectronic properties and broaden their range of practical applications.
... Dias et al. demonstrated that coupling between various triplet states facilitates the TADF mechanism, with an intermediate triplet state playing a crucial role. 64 The TADF mechanism via the mediated triplet state is attributed to vibrational frequency resonance in MR-TADFs, elucidated in recent work by Kim et al. 65 They expound on the role of an intermediate state in the ISC and RISC mechanism and showed that in MR-TADF compounds the S 1 ,T 1 ,T 3 states play an analogous role to CT 1 /CT 3 /LE 3 in the D−A type TADF system. Drummond et al. illustrated that high-lying triplet excitons, which were initially nonaccessible, can be made accessible and involved in the mechanism under specific energy conditions, spin−orbit coupling, and the nature of interacting states. ...
... The kinetics of emission of the toluene solutions of 2PO-CF3Ph is studied in detail, as both deoxygenated and air equilibrated toluene solutions exhibit the highest quantum yield of all samples of the compound at room temperature. [46]. Detailed information on the study of the kinetics (Table 2, Fig. 9) is presented in Supplemental information. ...
... [30][31][32] And, in the context of TADF emitters, it is important that they have a nonvanishing electrical PDM which is required for enabling the reverse intersystem crossing process. 33,34 However, their SOP has not been studied systematically yet. Importantly, one material from this class (BCPO, as mentioned above) has been found to exhibit a record-high GSP of more than 150 mV nm −1 , which is by a factor of 2 to 4 higher than other prototypical SOP materials, like Alq 3 or TPBi, having similar or even higher PDM, as shown in Fig. 2. ...
Upon film growth by physical vapor deposition, the preferential orientation of polar organic molecules can result in a non-zero permanent dipole moment (PDM) alignment, causing a macroscopic film polarization. This effect, known as spontaneous orientation polarization (SOP), was studied in the case of different phosphine oxides. We investigate the control of SOP by molecular design and film-growth conditions. Our results show that using less polar phosphine oxides with just one phosphor-oxygen bond yields an exceptionally high degree of SOP with the so-called giant surface potential (slope) reaching more than 150mV/nm in a neat BCPO film grown at room temperature. Additionally, by altering the evaporation rate and the substrate temperature, we are able to control the SOP magnitude over a broad range from 0 to almost 300mV/nm. Diluting BCPO in a non-polar host enhances the PDM alignment only marginally, but combining temperature control together with dipolar doping can result in almost perfectly aligned molecules with more than 80% of their PDMs standing upright on the substrate on average.
... Thermally activated delayed fluorescence (TADF) compounds are promising emitters to be used in organic light-emitting diodes (OLED) [1][2][3][4][5][6][7][8][9][10][11][12]. These emitters have the advantage of low cost and high harvesting efficiency of both the singlet and triplet excitons and thus a high quantum efficiency for the electroluminescence [13]. The TADF emission process involves the reverse intersystem crossing (rISC) from the triplet (T 1 ) state to the emissive singlet (S 1 ) state. ...
... Later, a three-state model was proposed, i.e., the 1 CS, 3 CS, and a locally excited ( 3 LE) state should be involved in the TADF process. The rISC is facilitated by the 3 LE state, which shares a similar energy with the 3 CS state, e.g., the so called spin-vibronic coupling effect, and as a result, the rISC is fast and efficient, which can result in significant TADF [13,[27][28][29][30][31]. It was also proposed that vibration facilitates the ISC in the TADF emitters [32,33]. ...
... General experimental methods, 1 H NMR, 13 ...
A series of 1,8-naphthalimide (NI)-phenothiazine (PTZ) electron donor–acceptor dyads were prepared to study the thermally activated delayed fluorescence (TADF) properties of the dyads, from a point of view of detection of the various transient species. The photophysical properties of the dyads were tuned by changing the electron-donating and the electron-withdrawing capability of the PTZ and NI moieties, respectively, by oxidation of the PTZ unit, or by using different aryl substituents attached to the NI unit. This tuning effect was manifested in the UV–vis absorption and fluorescence emission spectra, e.g., in the change of the charge transfer absorption bands. TADF was observed for the dyads containing the native PTZ unit, and the prompt and delayed fluorescence lifetimes changed with different aryl substituents on the imide part. In polar solvents, no TADF was observed. For the dyads with the PTZ unit oxidized, no TADF was observed as well. Femtosecond transient absorption spectra showed that the charge separation takes ca. 0.6 ps, and admixtures of locally excited ( ³ LE) state and charge separated ( ¹ CS/ ³ CS) states formed (in n -hexane). The subsequent charge recombination from the ¹ CS state takes ca. 7.92 ns. Upon oxidation of the PTZ unit, the beginning of charge separation is at 178 fs and formation of ³ LE state takes 4.53 ns. Nanosecond transient absorption (ns-TA) spectra showed that both ³ CS and ³ LE states were observed for the dyads showing TADF, whereas only ³ LE or ³ CS states were observed for the systems lacking TADF. This is a rare but unambiguous experimental evidence that the spin–vibronic coupling of ³ CS/ ³ LE states is crucial for TADF. Without the mediating effect of the ³ LE state, no TADF is resulted, even if the long-lived ³ CS state is populated (lifetime τ CS ≈ 140 ns). This experimental result confirms the ³ CS → ¹ CS reverse intersystem crossing (rISC) is slow, without coupling with an approximate ³ LE state. These studies are useful for an in-depth understanding of the photophysical mechanisms of the TADF emitters, as well as for molecular structure design of new electron donor–acceptor TADF emitters.
