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Figure S7-a) A comparison is shown between aerated (red) and degassed (black), steadystate fluorescence spectra of DPTZ-DBTO2 in MCH solution at RT. Upon degassing the solution, the fluorescence integral increases by a factor of ca. 13. b) The emissions obtained in aerated and degassed solutions very closely match each other.
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Here we report a comprehensive photophysical investigation of a new emitter molecule DPTZ-DBTO2, showing thermally activated delayed fluorescence (TADF), with near-orthogonal electron donor (D) and acceptor (A) units. We show that DPTZ-DBTO2 has minimal singlet-triplet energy splitting due to its near-rigid molecular geometry. However, the electron...
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The understanding of the external enhancement effects from host matrixes on thermally activated delayed fluorescence (TADF) emitters is crucial but incomprehensive at present. Herein, a series of phosphine oxide (PO) acceptors mDBSOSPO (m = 2, 3, and 4, corresponding to PO substitution position) and 4,4'‐bis(9‐carbazolyl)‐2,2'‐dimethylbiphenyl (CDB...
Citations
... [4][5][6][7][8] The dihedral angle between the D/A units affecting the HOMO-LUMO wavefunction overlap plays the dominating role in reducing ∆EST. [9][10][11][12][13][14] Considering Fermi's golden rule, the reverse intersystem crossing rate krISC exhibits the following proportionality (Equation 1) to the transition matrix element 〈S1|HSOC|T1〉 of SOC between S1 and T1 and to the energy gap ∆EST: [8,15] ∝ 〈 1 |̂| 1 〉 • − ∆ (1) Here kB is the Boltzmann constant and T the temperature. Beside the intrinsic rate dynamics controlled by the molecular motif, TADF dynamics is susceptible to the local environment and therefore is heavily influenced by the local dielectric and steric properties of the complementary host material as highlighted in Figure 1 a. [16][17][18][19][20][21] More specifically the energetic arrangement of 1 CT, 3 CT and 3 LE states is directly targeted by the host polarity, while the host rigidity imposes constrains to the conformational freedom along the dihedral angle thereby indirectly affecting the energy landscape and the SOC. ...
Donor-Acceptor (D-A) type thermally activated delayed fluorescence (TADF) emitters which constitute the key functional units in proposed Gen3 Organic Light Emitting Diodes (OLEDs), are sensitive to the rigidity and polarity of their local environment. In particular, the torsional freedom of the D-A dihedral angle and the excited state dipole moments of the occurring charge transfer states, condition a distribution of TADF dynamics over the emitter ensemble, concealed in standard optical spectroscopy. Here we apply spectroscopy on the single molecule level to directly access individual emitter properties, and, thus, bypass the downside of ensemble averaging. By photon correlation data and locally resolved spectral information on single D-A type TADF molecules embedded in technological relevant host materials of different polarity and rigidity, we derive host-dependent characteristics and distributions in the TADF dynamics. Those can be related to the conformational freedom and the dielectric environment imposed by the specific local host rigidity and polarity, thereby pointing out new selection criteria for host-emitter combinations in OLEDs.
... [1][2][3] In this regard, triplet harvesting via thermally activated delayed fluorescence (TADF) has emerged as a challenger technology to the incumbent roomtemperature phosphorescence in OLEDs. 1,[4][5][6] The efficiency of TADF relies in part on minimizing the energy gap between the lowest excited singlet and triplet states. This is usually achieved using donor-acceptor charge-transfer chromophores, where orthogonally oriented D and A units, i.e. twisted D-A molecules, 1,6-9 minimises the electron exchange energy and therefore the splitting between the lowest excited singlet and triplet states, provided they exhibit the same spatial symmetry. ...
Intramolecular charge transfer (ICT) is a fundamental chemical process whereby excitation moves charge from an electron donor to an electron acceptor within the same molecule. Thermally activated delayed fluorescence (TADF) exploits the ICT property to harvest triplet excited states, leading to extensive optoelectronic applications, including OLEDs. However, the highly twisted conformation of TADF molecules results in limited device lifetimes. Rigid molecules offer increased stability, yet their typical planarity and π-conjugated structures impede ICT. Herein, we introduce a new paradigm for achieving dispersion-free triplet harvesting in ICT molecules. Using fused indolocarbazole-phthalimide molecules, we demonstrate remarkably stable co-planar ICT states, yielding blue/green-TADF with good photoluminescence quantum yield and a small singlet-triplet energy gap (∆EST) <50 meV. The formation of ICT is dictated by the bonding connectivity between the donor and acceptor fragments, leading to excited-state conjugation breaking, stabilising the planar ICT excited state, revealing a new criteria for designing efficient TADF materials.
... Historically, one of the first three-state TADF models and its more advanced versions were developed to explain the photophysics of very strong D−A-and D−A−D-type TADF emitters, for example, PTZ-DBTO2 44 and diPTZ-DBTO2. 45 We can thus answer the above-mentioned question in the following manner: In contrast to weaker DA-type TADF emitters, in strong ones, the ΔE 1 CT− 3 CT itself as well as its f luctuations is to low be a critical rISC parameter. It is the negligible SOC value that is the main factor limiting the ef f iciency of the 3 The results presented here support previously developed TADF models and complete them with the information on the case of (1) strongly stabilized and degenerate 1,3 CT states with reduced ΔE 1 CT− 3 CT and θ DA rotation, (2) strong 3 LE-state impact on rISC, and (3) efficient rISC acceleration by bromine HAs via the 3 LE− 1 CT SOC enhancement. ...
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.
... Dilute toluene solutions of PP1 and PP2 exhibit an increase in PL intensity by factors of 1.16 and 1.36, respectively, upon deoxygenation ( Figure S11). This is a manifestation of triplet exciton utilization in emission, which is quenched in the presence of oxygen due to the electronic excitation being externally converted through collisional interactions with molecular oxygen [42]. For this reason, the PLQY of the films of the compounds can be increased if the oxygen-free environment is ensured. ...
Two derivatives of phenyl pyrimidine as acceptor unit and triphenylamino or 4,4′-dimethoxytriphenylamino donor groups were designed and synthesized as emitters for organic light-emitting diodes (OLEDs) aiming to utilize triplet excitons in the electroluminescence. Thermogravimetric analysis revealed high thermal stability of the compounds with 5% weight loss temperatures of 397 and 438 °C. The theoretical estimations and photophysical data show the contributions of local excited and charge transfer states into emission. The addition of the methoxy groups led to the significant improvement of hole-transporting properties and the bathochromic shift of the emission from blue to green-blue spectral diapason. It is shown that mixing of the compounds with the organic host results in facilitation of the delayed emission. The singlet–triplet energy splitting was found to be too big for the thermally activated delayed fluorescence. No thermal activation of the long-lived emission was detected. No experimental evidence for triplet–triplet annihilation and room temperature phosphorescence were detected making the hot exciton mechanism the most probable one. The OLEDs based on the compounds reached the maximum external quantum efficiency of up to 10.6%.
... Therefore, it has been proposed that the presence of the locally excited triplet state ( 3 LE) serves as an intermediate triplet state with a considerable change in MO. It contributes to rapid vibrational coupling known as spin-vibronic coupling (SVC), which results in the triplet equilibrium with a CT triplet state ( 3 CT) and thus mediates the direct spin-orbit coupling (SOC) between CT states [17][18][19][20] . Among many notable efforts to generalize the spin interconversion process [21][22][23] , the use of electron spin resonance has shown that a strongly mixed CT/LE character driven by an available rotational or torsional freedom leads to a reversible spin-flip route 16,24 . ...
Despite significant progress made over the past decade in thermally activated delayed fluorescence (TADF) molecules as a material paradigm for enhancing the performance of organic light-emitting diodes, the underlying spin-flip mechanism in these charge-transfer (CT)-type molecular systems remains an enigma, even since its initial report in 2012. While the initial and final electronic states involved in spin-flip between the lowest singlet and lowest triplet excited states are well understood, the exact dynamic processes and the role of intermediate high-lying triplet (T) states are still not fully comprehended. In this context, we propose a comprehensive model to describe the spin-flip processes applicable for a typical CT-type molecule, revealing the origin of the high-lying T state in a partial molecular framework in CT-type molecules. This work provides experimental and theoretical insights into the understanding of intersystem crossing for CT-type molecules, facilitating more precise control over spin-flip rates and thus advancing toward developing the next-generation platform for purely organic luminescent candidates.
... Small EST is a prerequisite for efficient reverse intersystem crossing (RISC) along with a large spin-orbit coupling (SOC) between singlet and triplet states. [90][91][92] By taking the leverage of these features, twisted D-A systems are a promising scaffold for achieving both TADF as well as luminescence in NIR region without using excessively large -conjugated systems. Furthermore, the twisted D-A geometry impacts packing structure in the solid state where close ··· contacts are inhibited to suppress aggregationcaused quenching (ACQ) while J-aggregate formation is induced by electrostatic interactions, leading to a red shifted emission in the condensed phase. ...
Near infrared (NIR) light (700–1400 nm) can be used in numerous biological/medical as well as technological applications. In this work we review the most recent examples of highly-efficient NIR organic...
... 6,7 It is well-known that to harness triplet energy from the current state-of-the-art TADF emitter, triplet excitons are converted into a singlet via an endothermic RISC. [1][2][3]8 Although organic TADF emitters offer the advantages of high stability, low cost, and easy fabrication over traditional metal complexes containing heavy noble metals that exhibit 100% internal quantum efficiency (IQE) 9,10 and large SOC for ISC, the time constant of the delayed fluorescence (DF) component falls in the microsecond or even millisecond range, resulting in a slow rate in RISC (k RISC ) of ∼10 −5 −10 6 s −1 , which can lead to large buildups of triplets at high excitation densities and subsequent losses due to triplet−triplet and triplet−charge quenching. 11,12 Recently, it has been shown that (a) an increase in the spatial separation of the highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO) leads to strong TADF, 8 (b) vibronic (nonadiabatic) coupling between the locally excited triplet ( 3 LE) triplet and lowest excited charge transfer ( 3 CT) triplet states of twisted D−A geometries increases k RISC by 4 orders of magnitude, 13 and (c) the introduction of multiple donors in TADF molecules results in the population of a higher-lying intermediate excited triplet state, which opens up additional RISC channels via a secondorder spin−vibronic mechanism. ...
... [1][2][3]8 Although organic TADF emitters offer the advantages of high stability, low cost, and easy fabrication over traditional metal complexes containing heavy noble metals that exhibit 100% internal quantum efficiency (IQE) 9,10 and large SOC for ISC, the time constant of the delayed fluorescence (DF) component falls in the microsecond or even millisecond range, resulting in a slow rate in RISC (k RISC ) of ∼10 −5 −10 6 s −1 , which can lead to large buildups of triplets at high excitation densities and subsequent losses due to triplet−triplet and triplet−charge quenching. 11,12 Recently, it has been shown that (a) an increase in the spatial separation of the highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO) leads to strong TADF, 8 (b) vibronic (nonadiabatic) coupling between the locally excited triplet ( 3 LE) triplet and lowest excited charge transfer ( 3 CT) triplet states of twisted D−A geometries increases k RISC by 4 orders of magnitude, 13 and (c) the introduction of multiple donors in TADF molecules results in the population of a higher-lying intermediate excited triplet state, which opens up additional RISC channels via a secondorder spin−vibronic mechanism. 14−16 Thus, the research community has focused on minimizing ΔE ST to accelerate thermal RISC. ...
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.
... Hence, the energy difference between singlet and triplet (ΔEST) is usually large (c.a 0.5 -1.0 eV) in most organic light emitting molecules as shown in Figure 1. TADF process [26] attempts to overcome this issue, based on designing special organic molecules, where the ΔEST is small as much as possible [27]. EQE (external quantum efficiency), represents the percentage of the emitted photons outside the device to the number of charges injected into it, these can be as low as 5 % for organic fluorophores from 25 % singlet excitons, after a light-out coupling efficiency ~20 %. ...
... Their work has proven that when 1-(and 9) position(s) of the phenothiazine undergo a substitution reaction with a bulky group, a strong phosphorescence will take place and no TADF will be appear at room temperature. The work of Dias et al. [27], showed that the rigid near perpendicular D-A-D molecular geometry is critical to minimize ΔEST. ...
Utilizing purely organic materials with relatively highly emissive characteristics for designing organic light-emitting diodes (OLEDs) is fascinating since employing rare metals is unessential, hence, enormous research activities have been conducted in this field. Recently, additional efforts were devoted to designing special emitting materials demonstrating thermally activated delayed fluorescence (TADF). Molecular structures were ideally designed in a specific way using an electron-deficient molecule as an acceptor (A) and an electron-rich molecule as a donor (D) in order to minimize the overlapping between the highest occupied molecular orbital (HOMO) of the donor (D) and the lowest unoccupied molecular orbital (LUMO) of the acceptor (A). TADF requires a small band gap between the singlet excited and triplet excited states (ΔEST), usually less than (c.a 0.1 eV) of the luminescent material. This makes it possible to thermally harvest the excitons from the triplet excited state (T1) to the singlet excited state (S1) by a process known as reverse intersystem crossing (RISC) and then to harvest all the excitons for the fluorescence process. Thus, the design of the TADF molecular structure needs both an electron-rich segment as a donor and an electro-deficient segment as an acceptor. Up to the present, electron-deficient moieties (EDM) have been studied less than electron rich moieties (ERM) during the last few years. This review is highlighting the recent advances regarding the specific molecular designs of TADF emission materials for OLED applications, particularly, focusing on (dibenzothiophene-S,S-dioxide) as electron deficient molecule, which exhibits a strong electron-deficient unit and high ability of photo- and electroluminescence features. HIGHLIGHTS Purely organic materials with relatively highly emissive characteristics were used for designing organic light-emitting diodes (OLEDs) since employing rare metals is unessential, hence, enormous research activities have been conducted in this field. Recently, additional efforts were devoted to designing special emitting materials demonstrating thermally activated delayed fluorescence (TADF). GRAPHICAL ABSTRACT
... Recently, it has been demonstrated that when a streak camera or an optical parametric oscillation (OPO) laser is integrated into an intensified charge coupled device (ICCD), with the aid of the unequal-time-channel working manner of an ICCD, the resultant device can be used to accurately quantify the S PF and S DF in one time range, and hence, the φ PF and φ DF values of a TADF fluorophore can be precisely demerged. 26 Note that the span of the time range can even be extended to 10 orders of magnitude (10 0 −10 10 ns). 27 However, since neither streak camera (∼$300,000-$500,000) nor OPO laser (∼$100,000) is inexpensive, to date only few research groups can afford to use this ICCD-based transient PL characterization method. ...
Accurate quantification on the quantum yields (φ) of both the prompt fluorescence (PF) and the delayed fluorescence (DF) species is quite essential for the clarification of molecular design rationales for thermally activated delayed fluorescence (TADF) luminogens. Currently, most φPF and φDF data of TADF fluorophores were acquired through time-correlated single-photon counting (TCSPC) lifetime measurement systems. However, because of their equal-time-channel working manner, so far all the commercially available TCSPC systems cannot render accurate measurement on φPF of TADF materials due to the lack of enough valid data points in the faster decay region of the corresponding photoluminescence (PL) decay curves. Although an intensified charge coupled device (ICCD) system equipped with a streak camera or an optical parametric oscillation laser has been proven to be a powerful tool for accurate determination of φPF and φDF of TADF fluorophores, the ultrahigh cost of these ICCD systems makes them inaccessible to most users. Herein, by replacing the timing module of a commercial TCSPC system with a low-cost and versatile time-to-digital converter (TDC) module, we developed a modified TCSPC system that can work in an unequal-time-channel manner. The resultant TDC-TCSPC system can not only concurrently determine the accurate lifetime of PF and DF species whose lifetime span even exceeds 5 orders of magnitude in just one time window but also render accurate measurements on φPF and φDF of TADF fluorophores. The reliability of the TDC-TCSPC method was verified through TCSPC- and ICCD-based comparative experiments on ACMPS, a known TADF fluorophore. Our results not only can provide a low-cost and convenient test method for accurate determination of key experimental data of TADF materials but also will facilitate deeper understanding of the molecular design principles for high-performance TADF materials.
... In the multiple donors emitters, it was found that the resonance of multiple intermediate states corresponding to a part of molecular structure contributes to the efficient RISC process [17][18][19][20] . Moreover, because the CT state generally possess negligible H SOC , it was proposed that a three-state model involving a locally excited (LE) state is the key to RISC [21][22][23] . Adopting the abovementioned strategies, the excited state lifetime of the TADF emitters can be minimized to a few microseconds or sub-microsecond timerange, corresponding to the rate constant of RISC (k RISC ) >10 6 s − 1 11,16,19,24-33 . ...
Fast spin-flipping is the key to exploit the triplet excitons in thermally activated delayed fluorescence based organic light-emitting diodes toward high efficiency, low efficiency roll-off and long operating lifetime. In common donor-acceptor type thermally activated delayed fluorescence molecules, the distribution of dihedral angles in the film state would have significant influence on the photo-physical properties, which are usually neglected by researches. Herein, we find that the excited state lifetimes of thermally activated delayed fluorescence emitters are subjected to conformation distributions in the host-guest system. Acridine-type flexible donors have a broad conformation distribution or bimodal distribution, in which some conformers feature large singlet-triplet energy gap, leading to long excited state lifetime. Utilization of rigid donors with steric hindrance can restrict the conformation distributions in the film to achieve degenerate singlet and triplet states, which is beneficial to efficient reverse intersystem crossing. Based on this principle, three prototype thermally activated delayed fluorescence emitters with confined conformation distributions are developed, achieving high reverse intersystem crossing rate constants greater than 10⁶ s⁻¹, which enable highly efficient solution-processed organic light-emitting diodes with suppressed efficiency roll-off.
