Boron-based donor-spiro-acceptor compounds exhibiting thermally activated delayed fluorescence (TADF)

Stanoppi, Marco; Lorbach, Andreas

doi:10.1039/c8dt01255e

Cited by 47 publications

(27 citation statements)

References 54 publications

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“…In contrast, p ‐NMe 2 ‐ F Xyl F Bf exhibits two different radiative decay processes, a prompt ( τ =9.2 ns) and a delayed ( τ =1.6 μs) one. This can be caused by different processes namely TTA (triplet–triplet annihilation) or TADF (thermally activated delayed fluorescence) . Due to the low concentrations (≤10 −5 m ) of the compound employed, the lack of dependence on the concentration and the temperature dependence of the lifetime, we can attribute the observed behavior to TADF.…”

Section: Resultsmentioning

confidence: 99%

Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9‐Borafluorenes

Rauch

Fuchs

Friedrich

et al. 2020

Chemistry A European J

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Three differentp erfluoroalkylated borafluorenes (F Bf)w ere prepared and their electronic and photophysical properties were investigated. The systemsh ave four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo-aryl moieties. They differ with regard to the para substituents on their exo-aryl moieties, being ap roton(F Xyl F Bf, F Xyl:2 ,6-bis(trifluoromethyl)phenyl), at rifluoromethyl group (F Mes F Bf, F Mes:2 ,4,6-tris(trifluoromethyl)phenyl) or ad imethylamino group (p-NMe 2-F Xyl F Bf, p-NMe 2-F Xyl:4-(dimethylamino)-2,6-bis(trifluoromethyl)phenyl), respectively.A ll derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron-deficient derivative F Mes F Bf wasa lso chemically reduced and its radicala nion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 ms; however, the underlying mechanismsresponsible for this differ.T he donor-substitutedd erivative p-NMe 2-F Xyl F Bf exhibitst hermally activated delayedf luorescence (TADF) from ac harge-transfer (CT) state, whereas the F Mes F Bf and F Xyl F Bf borafluorenese xhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition-dipole moments.

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Section: Resultsmentioning

confidence: 99%

Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9‐Borafluorenes

Rauch

Fuchs

Friedrich

et al. 2020

Chemistry A European J

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“…In terms of chemical and thermal stabilities, it is anticipated that four‐coordinate boron compounds are more suitable for OLED materials. Although there are limited examples of four‐coordinate boron‐containing TADF emitters reported, the operational lifetimes of these OLEDs are still considerably poor, which may be ascribed to ligand cleavage that leads to the formation of facial isomers…”

Section: Methodsmentioning

confidence: 99%

Four‐Coordinate Boron Emitters with Tridentate Chelating Ligand for Efficient and Stable Thermally Activated Delayed Fluorescence Organic Light‐Emitting Devices

Chan

Lai

et al. 2019

Angewandte Chemie

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An ew class of four-coordinate donor-acceptor fluoroboron-containing thermally activated delayed fluorescence (TADF) compounds bearing atridentate 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligand has been successfully designed and synthesized. Upon varying the donor moieties from carbazole to 10H-spiro[acridine-9,9'-fluorene] to 9,9dimethyl-9,10-dihydroacridine,these boron derivatives exhibit aw ide range of emission colors spanning from blue to yellow with alarge spectral shift of 2746 cm À1 ,with high PLQYs of up to 96 %i nt he doped thin film. Notably,v acuum-deposited organic light-emitting devices (OLEDs) made with these boron compounds demonstrate high performances with the best current efficiencies of 55.7 cd A À1 ,p ower efficiencies of 58.4 lm W À1 and external quantum efficiencies of 18.0 %. More importantly,l ong operational stabilities of the green-emitting OLEDs based on 2 with half-lifetimes of up to 12 733 hours at an initial luminance of 100 cd m À2 have been realized. This work represents for the first time the design and synthesis of tridentate dppy-chelating four-coordinate boron TADF compounds for long operational stabilities,s uggesting great promises for the development of stable boron-containing TADF emitters.The leap-forward development of thermally activated delayed fluorescence (TADF) materials that are capable to realize 100 %o fa ni nternal quantum efficiency (IQE) via reverse intersystem crossing (RISC) from the lowest triplet excited state (T 1 )t ot he lowest singlet excited state (S 1 )h as attracted al ot of research interests [1] since the first demonstration of highly efficient organic light-emitting devices (OLEDs) based on purely organic TADF materials by Adachi and co-workers in 2012. [2] From the molecular design viewpoint, as mall S 1 -T 1 energy gap (DE ST )a nd high photoluminescence quantum yield (PLQY) are the most important and closely associated physical parameters for efficient TADF emitters. [1] Generally,asmall DE ST can be achieved by spatially separating the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) through as imple donor-acceptor structure with atwisted conformation or anon-conjugated bridge. [1] A high PLQY can be accomplished by enhancing light absorption through increasing the oscillator strength, and the introduction of phenyl linkers,r igid structures,d ual emitting units and multiple donors. [1b] On the other hand, the nature of excited states and the heteroatom effect are found to play important roles in the RISC process,enabling efficient TADF to occur even with DE ST larger than 0.3 eV. [1d, 3] Forp ractical applications,o nly TADF emitters with chemical stability can provide the potential for realizing long lifetime of OLEDs. [1b] Considering all these factors,T ADF emitters based on the bipolar triazine-carbazole system have been attracting particular interests due to the ease of functionalization and good chemical stabilities of the triazine and carbazole moieties. [4] Highly efficient blue-, gr...

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“…Other than the applications mentioned above, a few more four-coordinated boron compounds were reported in studies of TADF behavior. [120,121] The four-coordinate boron acceptors demonstrated potential for design of TADF emitters by realizing a high EQE over 20%, although not many emitters have been documented in the literature. The material stability of four-coordinate boron emitters was a critical issue, but the updated materials may presumably circumvent the lifetime issues through acceptor design and a combinatorial approach for both donor and acceptor.…”

Section: Four-coordinate Boron-based Tadf Emittersmentioning

confidence: 99%

“…Other than the applications mentioned above, a few more four‐coordinated boron compounds were reported in studies of TADF behavior. [ 120,121 ]…”

Section: Four‐coordinate Boron‐based Tadf Emittersmentioning

confidence: 99%

Three‐ and Four‐Coordinate, Boron‐Based, Thermally Activated Delayed Fluorescent Emitters

Kothavale

Lee

2020

Advanced Optical Materials

137

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After the first report of organometallic complex-based phosphorescent emitting materials, [5] it was possible to realize 100% internal quantum efficiency in phosphorescent OLEDs (PhOLEDs). [6] Using strong spin-orbit coupling (SOC) of heavy metals such as platinum or iridium, it was possible to surpass the limit of spin statistics and utilize all triplet excitons as well as singlet excitons in PhOLEDs. [7-10] After a huge improvement in the external quantum efficiencies (EQEs) in PhOLEDs, it was possible to commercialize green and red PhOLEDs as replacements for green and red fluorescent OLEDs. However, blue fluorescent OLEDs could not be replaced with PhOLEDs due to lifetime issues. [11,12] Although the currently commercialized OLEDs are mainly based on PhOLEDs, serious issues such as high cost, need for rare materials, and environmental sustainability of the phosphorescent emitters used have forced researchers to identify an alternative to PhOLEDs. [13,14] Recently, TADF OLEDs have emerged as a viable alternative to PhOLEDs due to their capability to exhibit 100% internal quantum efficiency (like PhOLEDs) by harvesting both singlet and triplet excitons through a reverse intersystem crossing (RISC) mechanism. [15-19] As the emitters used in TADF OLEDs are purely organic materials, they are inexpensive, readily available, and offer a large number of design opportunities. They can be easily tuned for different colors and can also serve as efficient host materials in the emitting layers. After the first report of a high EQE TADF OLED in 2012, [20] a number of highly efficient blue, green, and red TADF OLEDs have been reported. [21-36] Although short device lifetime and poor color purity are the main obstacles in commercial application of TADF OLEDs, they are promising because they have obtained high EQEs equal to or higher than that of the PhOLEDs. Commercially applicable, highly efficient, stable, and pure color TADF emitters must have four important characteristics of small singlet-triplet energy gap (ΔE ST), high photoluminescence quantum yield (PLQY), small full width at half-maximum (FWHM), and stability of the emitting materials. However, it is difficult to achieve all four factors in a single molecular design. [17] For example, a small ΔE ST can be obtained through separation of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) using highly twisted molecular structures. However, a high PLQY will be realized by maximizing the overlap between the HOMO and LUMO. The two factors are contradictory, but they are necessary Recent developments in purely organic-material-based thermally activated delayed fluorescence (TADF) emitters are helping to make them suitable for commercial applications in the near future. In spite of their high external quantum efficiencies, the broad emission spectra and short device lifetimes are the main barriers to using TADF emitters. Among the classes of materials, boron-embedded polycyclic aromatic compounds have shown potential as TADF emitte...

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Boron-based donor-spiro-acceptor compounds exhibiting thermally activated delayed fluorescence (TADF)

Cited by 47 publications

References 54 publications

Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9‐Borafluorenes

Highly Stable, Readily Reducible, Fluorescent, Trifluoromethylated 9‐Borafluorenes

Four‐Coordinate Boron Emitters with Tridentate Chelating Ligand for Efficient and Stable Thermally Activated Delayed Fluorescence Organic Light‐Emitting Devices

Three‐ and Four‐Coordinate, Boron‐Based, Thermally Activated Delayed Fluorescent Emitters

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