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Thermally activated delayed fluorescence

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Thermally activated delayed fluorescence (TADF) is a process through which surrounding thermal energy changes population of excited states of molecular compounds and thus, alters light emission. The TADF process usually involves an excited molecular species in a triplet state, which commonly has a forbidden transition to the singlet ground state, termed phosphorescence. By absorbing nearby thermal energy, the triplet state can undergo reverse intersystem crossing (RISC) converting the triplet state population to an excited singlet state, which then emits light to the singlet ground state in a delayed process termed delayed fluorescence. Accordingly, in many cases, the TADF molecules show two types of emission, a delayed fluorescence and a prompt fluorescence. This is found for specific organic molecules, but also for selected organo-transition metal compounds, such as Cu(I) complexes. Along with traditional fluorescent molecules and phosphorescent molecules, TADF compounds belong to the three main light-emitting material groups used in organic light-emitting diodes (OLEDs).

History

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The first evidence of thermally activated delayed fluorescence in a fully organic molecule was discovered in 1961 using the compound eosin.[1] The emission detected was termed "E-type" delayed fluorescence, but the mechanism was not completely understood. In 1986, this mechanism was further investigated and described in detail using aromatic thiones,[2] but a practical use was only identified much later.

Application of the TADF mechanism for efficient light generation in OLEDs was proposed in 2008 and subsequently studied by Yersin and coworkers [3] (originally designated as "singlet harvesting mechanism"). Since 2009, the mechanism was extensively investigated by Chihaya Adachi and coworkers as well as by other research groups. A series of papers were published, reporting effective TADF molecular design strategies focusing on different TADF compounds.[4][5][6][7] Extensive studies of green, orange, and blue emitting OLEDs based on organic TADF materials spiked interest in the TADF field. This mechanism was soon considered as possible high efficiency alternative to traditional fluorescent and also phosphorescent compounds used in lighting displays so far. TADF materials are being considered the third generation of OLEDs following fluorescent and phosphorescent based devices.[7][8]

Mechanism

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Photoluminescence pathways and associated energy levels

The steps of the TADF mechanism are displayed in the figure at right (where it is assumed that the ground state is a singlet state, which is usually but not always the case). In the electroluminescent process, which is observed in OLEDs, an electrical excitation leads to population of singlet and triplet states of the TADF molecules. From the singlet state an allowed transition can occur to the electronic singlet ground state on a time scale of 10 to 100 nanoseconds for organic TADF molecules. This emission represents the prompt fluorescence. On the other hand, from the excited triplet state, the electron can undergo a forbidden de-excitation to the ground state as radiative transition, called phosphorescence, or as non-radiative process. However, this occurs on a much slower time scale, being on the order of microseconds to seconds. Thus, usually, thermal activation from the triplet to the excited singlet state, the reverse intersystem crossing, populates the singlet state in a fast process and quenches the triplet state population. As a consequence, delayed fluorescence is observed. Accordingly, when a TADF material becomes electronically excited, it exhibits prompt fluorescence and delayed fluorescence, usually occurring at (almost) the same wavelength. Selected organo-transition metal compounds can show both TADF and relatively fast phosphorescence.[9]

In an OLED based on traditional fluorescent materials only harvesting of the singlet state population is possible. Thus, due to spin statistics only 25% of the excitation can be exploited. On the other hand, both phosphorescent and TADF materials have the ability to harvest the excitation from both singlet and triplet states, theoretically allowing these materials to convert close to 100% of the electrically generated excitons, giving them a large advantage over traditional fluorescent-based materials. However, due to light out-coupling losses in OLED devices, the external quantum efficiency (EQE) is, without employing specific out-coupling enhancement strategies, substantially lower, lying roughly around 5% and 22%, respectively.[2]

Spin statistics

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Electronic states of materials used in light emitting devices typically contain some type of spin coupling. In phosphorescent materials, for example, heavy transition metals are used to take advantage of spin-orbit coupling. In most TADF materials, the excited and ground state electrons couple to have not only a combined total spin quantum number S, but also a combined z-component of the spin Sz. When this spin coupling phenomenon is considered, in a random situation, three possible electron combinations of total spin S=1 and one combination of total spin S=0 are occurring. This corresponds to the observed 75% triplet states and 25% singlet states generated under electrical excitation.

Factors affecting TADF

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Several key kinetic properties of TADF materials determine their ability to efficiently generate light through delayed fluorescence, while minimizing thermal loss pathways. The rates of reverse intersystem crossing, referred to as kRISC, and of reverse intersystem crossing, given by kRISC, both determined by spin-orbit coupling, should be as fast as possible. In particular, kRISC should be faster than the rates of non-radiative triplet relaxation pathways. Most non-radiative triplet pathways like triplet-triplet annihilation, triplet quenching, or thermal decay occur on the order of microseconds or longer, which usually is long compared to the kRISC time. Thus, singlet state population is faster.

Another key property is the energy difference between the singlet and triplet state energy levels, called ΔEST. In particular, as kRISC depends exponentially on this energy gap, it should be small, that is smaller than a few times the available thermal energy (≈25.6 meV at room temperature) to effectively allow for fast reverse intersystem crossing.[9] Minimization of this energy gap is thus, considered to be one of the most important strategies in synthesizing potential TADF materials. The most effective strategies employed so far to synthesize molecules with donor and acceptor moieties spaced apart or twisted with respect to each other. This effectively reduces the energy gap ΔEST.

Moreover, the TADF decay time, representing another key parameter, should be as short as possible in order to reduce unwanted chemical reactions during excited state population.[10] This represents a further challenge and requires specific molecular design strategies.[9]

When the ground state is not a singlet state, different strategies of improving TADF performance may exist that have no counterpart in the singlet ground state case. For example, in the doublet copper(II) porphyrin molecule, the emissive state is a doublet state formed by antiferromagnetic coupling between a triplet excited porphyrin ligand and a ground state Cu(II) ion, and a quartet state formed by their ferromagnetic coupling lies slightly below the emissive state. In this case, the doublet-quartet gap ΔEDQ is mainly determined by the distance between the ligand and the metal, rather than the distance between the donor and the acceptor (both the porphyrin ligand in this case).[11]

Chemical structure

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Chemical structure of TADF material 4CzIPN

The chemical structures of many commonly used TADF materials reflect the requirement to minimize the ΔEST by displaying a twisted structure. One of the most commonly used and successful TADF materials 2,4,5,6-Tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), contains this type of structure as the bottom and top carbazole groups can be viewed as flat and coplanar while the bottom left and bottom right carbazole groups can be thought of as coming into and out of the page. This type of molecule contains electron donating and electron accepting moieties showing small orbital overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). As a consequence, small singlet-triplet splitting, small ΔEST, is resulting.

Many highly efficient TADF materials contain multiple carbazole groups as electron donors and, for example, incorporate electron acceptors, like triazines, sulfoxides, benzophenones, and spiro-based groups. The table below shows several examples of these compounds that have been reported to yield high efficiencies and relatively small ΔEST.

High Efficiency TADF Compounds [8]
Chemical Name Photoluminescence wavelength (nm) Electroluminescence wavelength (nm) ΔEST (eV) Device Efficiency, EQE (%)
34TCzPN 448 475 0.16 21.8
DMAC-TRZ 495 495 0.046 26.5
Ac-MPM 489 489 0.19 24.5
DMAC-DPS 465 476 0.08 19.5
DTCBPy 518 514 0.04 27.2
ACRSA 485 490 0.04 16.5
POB-PZX 482 503 0.028 22.1
PXZ-Mes3B 507 502 0.071 22.8

In a recent design strategy, electron donating and accepting moieties are separated by two bridges, leading to the DSH molecule. In this situation, very small orbital overlap between HOMO and LUMO is resulting. Thus, ultra-small energy gap ΔEST between the lowest excited singlet and triplet states of only about 1 meV is obtained. For this specific molecule, an ultra-short emission decay time, lying in the sub-microsecond time range, is attained. An OLED device fabricated, shows EQE of ≈ 19 %.[12][13]

Chemical structure of the DSH molecule
Chemical structure of the DSH molecule showing ultra-small energy gap ΔEST and ultra-fast emission decay time for OLED application.

A number of organo-transition metal complexes, for example, based on Cu(I), Ag(I), Au(I), Au(III) metal centers, exhibit also distinct TADF behavior. In particular, Cu(I) compounds synthesized with different ligands display a wide range of ΔEST values, extending from around 33 to 160 meV.[12] The depicted Cu(I) compound shows an example.[14]

Copper(I) complex featuring TADF
Chemical structure of a Cu(I) complex displaying distinct TADF behavior, shown as an example.

Systematic photophysical including theoretical studies of a large number of Cu(I) compounds result in a detailed understanding of TADF properties. In particular, it is shown that the TADF decay time is not only given by the energy gap ΔEST, but also by the singlet excited state to the singlet ground state transition rate. Moreover, variation of spin-orbit coupling as realized by chemical change enables to modify ISC and reverse ISC rates as well as tuning in of phosphorescence in addition to TADF emission.[12]

Furthermore, it is referred to recent investigations with two-coordinate carbene-M(I)-amide complexes with M(I) = Cu(I), Ag(I), and Au(I). These compounds exhibit short-lived TADF at high emission quantum yield. Even robust materials for OLEDs showing long operational device live time (LT90 > 1000 hours at 1000 cd m-2) were reported.[15][16][17]

Applications

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Organic LEDs

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LG 4K Curved OLED TV

The vast majority of research on TADF-based materials is focused on improving the efficiency and lifetime of TADF-based OLEDs. Organic light-emitting diodes or OLEDs have provided an alternative to traditional liquid-crystal display (LCD) displays due to the improved contrast, response time, wider viewing angle, and the possibility of fabricating flexible displays. Most OLEDs that are currently commercially available employ phosphorescent organo-transition metal emitters, belonging to the second OLED generation emitters. They have the advantage of high operational lifetime for red and green emission color, however, poor lifetimes are still found for blue emitter materials. Thus, for generation of blue light, traditional organic molecules are applied.

Frequently, it is considered that TADF-based OLEDs may represent the third generation of OLEDs. However, the vast majority of research on TADF-based materials is still focusing on improving efficiency, operational device lifetime, and color purity, though first OLED displays[18] that use TADF emitters are already on the market. These devices are based on TADF emitters combined with color-pure fluorescent organic emitters. The TADF materials harvest efficiently all electrically excited excitons. They represent donors for efficient radiationless energy transfer to fluorescent acceptors, which finally emit light. The corresponding mechanism was named hyperfluorescence.[17]

Fluorescence imaging

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TADF-based materials have a unique advantage in some imaging techniques because of their longer emission lifetimes than promptly materials that show prompt fluorescence. For instance, the TADF exhibiting molecule ACRFLCN shows a strong sensitivity towards triplet oxygen making it an effective molecular oxygen sensor.[19] The fluorescein derivative DCF-MPYM has shown success in the field of bioimaging as its long lifetime allows time-resolved fluorescence imaging in living cells. These tailored organic compounds are especially promising in bioimaging applications because of their low cytotoxicity compared to traditional compounds like lanthanide complexes.[20]

Mechanoluminescence

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TADF compounds can also be synthesized to exhibit a tunable color change based on the macroscopic particle size in powder form. In these compounds, color shift of light emission through mechanical grinding can occur, a phenomenon termed mechanoluminescence. Specifically, asymmetric compounds with diphenyl sulfoxide and phenothiazine moieties have been synthesized displaying linearly tunable mechanochromism due to a combination of fluorescence and TADF. The compound named SCP shows dual emission peaks in its photoluminescence spectrum and changes from a green to blue color through mechanical grinding.[21]

Challenges

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Research of TADF materials has provided impressive results and devices made with these compounds have already achieved good device performance with high quantum efficiencies. However, the synthesis and application of TADF materials still has multiple challenges to overcome before they become commercially viable. Likely the biggest hurdle is the difficulty in producing a blue light emitting TADF molecules with a reasonable operational lifetime. Fabrication ofa long operational lifetime pf blue light emitting OLEDs is been a challenge not only for TADF, but also for phosphorescent materials. This is due degradation pathways at the high energy of blue light. Another difficulty in producing efficient TADF materials is the lack of sufficient knowledge concerning detailed structure-property relations for rational molecular design. Though, the combination of donating and accepting groups and the twisted or bridged molecular structure tpye already provide good fundamental starting concepts for new material concepts.

See also

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References

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  1. ^ C. A. Parker; C. G. Hatchard (1961). "Triplet-Singlet Emission in Fluid Solutions". Transactions of the Faraday Society. 57: 1894–1904. doi:10.1039/TF9615701894.
  2. ^ a b Andrzej Maciejewski; Marian Szymanski (1986). "Thermally Activated Delayed Fluorescence of Aromatic Thiones". J. Phys. Chem. 90 (23): 6314–6318. doi:10.1021/j100281a051.
  3. ^ DE patent 102008033563, Hartmut Yersin & Uwe Monkowius, "Komplexe mit kleinen Singulett-Triplett-Energie-Abständen zur Verwendung in opto-elektronischen Bauteilen (Singulett-Harvesting-Effekt)", published 2010-01-21, issued 2008-07-17, assigned to Merck Patent GmbH 
  4. ^ A. Endo; M. Ogasawara; A. Takahashi; D. Yokoyama; Y. Kato; C. Adachi (2009). "Thermally Activated Delayed Fluorescence from Sn(4+)-porphyrin complexes and their application to organic lightemitting diodes--a novel mechanism for electroluminescence". Adv. Mater. 21 (47): 4802–4806. Bibcode:2009AdM....21.4802E. doi:10.1002/adma.200900983. PMID 21049498. S2CID 29015731.
  5. ^ Ayakata Endo; Keigo Sato; Kazuaki Yoshimura; Takahiro Kai; Atsushi Kawada; Hiroshi Miyazaki; Chihaya Adachi (2011). "Efficient Up-conversion of Triplet Excitons into a Singlet State and its Application for Organic Light Emitting Diodes". Applied Physics Letters. 98 (8): 083302. Bibcode:2011ApPhL..98h3302E. doi:10.1063/1.3558906. hdl:2324/19151. S2CID 94698653.
  6. ^ H. Uoyama; K. Goushi; K. Shizu; H. Nomura; Chihaya Adachi (2012). "Highly Efficient Organic Light-emitting Diodes From Delayed Fluorescence". Nature. 492 (7428): 234–238. Bibcode:2012Natur.492..234U. doi:10.1038/nature11687. hdl:2324/25887. PMID 23235877. S2CID 4376505.
  7. ^ a b Yersin, Hartmut (2019). Highly efficient OLEDs: materials based on thermally activated delayed fluorescence. Weinheim, Germany: Wiley-VCH. p. 588. ISBN 9783527339006.
  8. ^ a b Zhiyong Yang; Zho Mao; Zongliang Xie; Yi Zhang; Siwei Liu; Juan Zhao; Jiarui Xu; Zhenguo Chi; Matthew P. Aldred (2017). "Recent Advances in Organic Thermally Activated Delayed Fluorescence Materials". Chem Soc Rev. 46 (3): 915–1016. doi:10.1039/c6cs00368k. PMID 28117864.
  9. ^ a b c Yersin, Hartmut; Monkowius, Uwe (2 September 2024). "Thermally Activated Delayed Fluorescence and Beyond. Photophysics and Material Design Strategies". Advanced Photonics Research: 2400111. doi:10.1002/adpr.202400111.
  10. ^ Noda, Hiroki; Nakanotani, Hajime; Adachi, Chihaya (June 2018). "Excited state engineering for efficient reverse intersystem crossing". Science Advances. 4 (6): eaao6910. doi:10.1126/sciadv.aao6910. PMC 6014720.
  11. ^ Wang, Xingwen; Wu, Chenyu; Wang, Zikuan; Liu, Wenjian (2023). "When do tripdoublet states fluoresce? A theoretical study of copper(II) porphyrin". Frontiers in Chemistry. 11. arXiv:2307.07886. Bibcode:2023FrCh...1159016W. doi:10.3389/fchem.2023.1259016. PMC 10667454. PMID 38025061.
  12. ^ a b c Yersin, Hartmut; Monkowius, Uwe (2024). "Thermally Activated Delayed Fluorescence and Beyond. Photophysics and Material Design Strategies". Advanced Photonics Research. doi:10.1002/adpr.202400111.
  13. ^ Yersin, Hartmut; Czerwieniec, Rafał; Mataranga‐Popa, Larisa; Mewes, Jan‐Michael; Cheng, Gang; Che, Chi‐Ming; Saigo, Masaki; Kimura, Shuji; Miyata, Kiyoshi; Onda, Ken (August 2022). "Eliminating the Reverse ISC Bottleneck of TADF Through Excited State Engineering and Environment‐Tuning Toward State Resonance Leading to Mono‐Exponential Sub‐µs Decay. High OLED External Quantum Efficiency Confirms Efficient Exciton Harvesting". Advanced Functional Materials. 32 (34). doi:10.1002/adfm.202201772.
  14. ^ Czerwieniec, Rafał; Yersin, Hartmut (2015). "Diversity of Copper(I) Complexes Showing Thermally Activated Delayed Fluorescence: Basic Photophysical Analysis". Inorganic Chemistry. 54 (9): 4322–4327. doi:10.1021/ic503072u.
  15. ^ Feng, Xingyu; Yang, Jian‐Gong; Miao, Jingsheng; Zhong, Cheng; Yin, Xiaojun; Li, Nengquan; Wu, Chao; Zhang, Qizheng; Chen, Yong; Li, Kai; Yang, Chuluo (2022). "Au⋅⋅⋅H−C Interactions Support a Robust Thermally Activated Delayed Fluorescence (TADF) Gold(I) Complex for OLEDs with Little Efficiency Roll‐Off and Good Stability". Angewandte Chemie International Edition. 61 (40). doi:10.1002/anie.202209451.
  16. ^ Tang, Rui; Xu, Shuo; Lam, Tsz‐Lung; Cheng, Gang; Du, Lili; Wan, Qingyun; Yang, Jun; Hung, Faan‐Fung; Low, Kam‐Hung; Phillips, David Lee; Che, Chi‐Ming (2022). "Highly Robust Cu I ‐TADF Emitters for Vacuum‐Deposited OLEDs with Luminance up to 222 200 cd m −2 and Device Lifetimes (LT 90 ) up to 1300 hours at an Initial Luminance of 1000 cd m −2". Angewandte Chemie International Edition. 61 (33). doi:10.1002/anie.202203982.
  17. ^ a b Li, Tian-Yi; Zheng, Shu-Jia; Djurovich, Peter I.; Thompson, Mark E. (2024). "Two-Coordinate Thermally Activated Delayed Fluorescence Coinage Metal Complexes: Molecular Design, Photophysical Characters, and Device Application". Chemical Reviews. 124 (7): 4332–4392. doi:10.1021/acs.chemrev.3c00761.
  18. ^ "Wisechip launches the world's first Hyperfluorescence™ OLED display- a 2.7-inch yellow PMOLED – Kyulux". 17 October 2019. Retrieved 4 March 2020.
  19. ^ Kenjiro Hanaoka; Kazuya Kikuchi; Shigeru Kobayashi; Tetsuo Nagano (2007). "Time-resolved long-lived luminescence imaging method employing luminescent lanthanide probes with a new microscopy system". Journal of the American Chemical Society. 129 (44): 13502–13509. doi:10.1021/ja073392j. PMID 17927176.
  20. ^ Xiaoqing Xiong; Fengling Song; Jingyun Wang; Yukang Zhang; Yingying Xue; Liangliang Sun; Na Jiang; Pan Gao; Lu Tian; Xiajun Peng (2014). "Thermally activated delayed fluorescence of fluorescein derivative for time-resolved and confocal fluorescence imaging". Journal of the American Chemical Society. 136 (27): 9590–9597. doi:10.1021/ja502292p. PMID 24936960. S2CID 14036669.
  21. ^ Bingjia Xu; Wenlang Li; Jiajun He; Sikai Wu; Qiangzhong Zhu; Zhiyong Yang; Yuan-Chun Wu; Yi Zhang; Chongjun Jin; Po-Yen Lu; Zhenguo Chi; Siwei Liu; Jiarui Xu; Martin R. Bryce (2016). "Achieving very bright mechanoluminescence from purely organic luminophores with aggregation-induced emission by crystal design". Chemical Science. 7 (8): 5307–5312. doi:10.1039/c6sc01325b. PMC 6020548. PMID 30155182.
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