Thermally activated delayed fluorescence

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).

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]

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]

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 S_z. 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.

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 k_RISC, and of reverse intersystem crossing, given by k_RISC, both determined by spin-orbit coupling, should be as fast as possible. In particular, k_RISC 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 k_RISC time. Thus, singlet state population is faster.

Another key property is the energy difference between the singlet and triplet state energy levels, called ΔE_ST. In particular, as k_RISC 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 ΔE_ST.

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 ΔE_DQ 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]

The chemical structure of many commonly used TADF materials reflects the requirement to minimize the ΔE_ST by displaying a twisted structure where one part of the molecule is oriented on a plane parallel to the other. 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. Because the pairs of carbazole groups are antiplanar, the differences between the HOMO and LUMO energy levels is minimized and the compound can more easily transfer between the triplet and singlet states.

Besides having an overall twisted conformation, high efficiency TADF materials contain both electron donating and electron accepting moieties and incorporate the same kind of planar twist between them. The interactions between these electron accepting and electron donating groups reduces the overlap of the HOMO and LUMO energy levels even further. Thus, many highly efficient TADF materials contain multiple carbazole groups as electron donors and can 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 low ΔE_ST.

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 their improved contrast, response time, wider viewing angle, and the possibility of making flexible displays. The first generation of OLEDs were based on fluorescent materials, which includes most OLED or AMOLED displays that are currently commercially available. The second generation of OLED materials employ phosphorescence light emission, which has the advantage of higher theoretical efficiency, but may still be lacking in other areas like the poor lifetimes seen in phosphorescent blue emitters.

Many consider the third generation of OLEDs to be TADF materials because of their already impressive quantum efficiencies and performance in small test devices. In practice, these newer TADF materials still have difficulties with solution processability in larger practical devices and many blue TADF molecules show poor performance and lifetimes. If these challenges can be addressed, TADF-based OLEDs show promise in replacing current LCD-based displays and OLED-based displays especially in curved televisions and flexible phone screen designs. In 2019, Taiwan-based Wisechip launched the world's first hyperfluorescent OLED display^[12] that uses TADF emitters (developed by Kyulux) in a Hyperfuorescence structure.

TADF-based materials have a unique advantage in some imaging techniques because of their longer lifetime over promptly fluorescing materials. For instance, the TADF exhibiting molecule ACRFLCN exhibits a strong sensitivity towards triplet oxygen making it an effective molecular oxygen sensor.^[13] 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.^[14]

TADF compounds can also be synthesized to exhibit a tunable color change based on the macroscopic particle size in powder form. In this way, these compounds can shift the color of their light emission through mechanical grinding in 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 mechanisms. The compound named SCP shows dual emission peaks in its photoluminescence spectrum and changes from a green color to blue through mechanical grinding.^[15]

Research into TADF materials has provided impressive results and devices made with these compounds have already achieved comparable device performance and comparable 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 molecule with a reasonable lifetime. Creating a long lifetime blue OLED has been a challenge not only for TADF, but for fluorescent and phosphorescent materials as well due to the higher energy light degradation pathways. Another difficulty in producing efficient TADF materials is the lack of a reliable molecular design strategy. The combination of donating and accepting groups and the twisted molecular structure provide good fundamental starting concepts for new synthesis, but the difficulty in predicting HOMO and LUMO energy levels and the control of excitons through the material make it challenging to pinpoint which moieties will prove the most effective.

• Fluorescence
• Phosphorescence
• OLED
• Light-emitting diode
• Singlet state
• Triplet state
• Intersystem crossing

• TADF OLED emitters, introduction and market status
• What is thermally activated delayed fluorescence?