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Comprehensive Summary: Conjugated fused‐ring structures have attracted extensive attention due to their high molecular rigidity to restrain excited‐state relaxation and non‐radiative decay, and further to enhance the luminance efficiency for emissive materials. Herein, we develop a series of donor‐acceptor type thermally activated delayed fluorescence (TADF) emitters by introducing fused‐ring 5H‐benzofuro[3,2‐c]carbazole (32BFCz) as electron donating unit. Through optimizing the numbers and structure of donor and acceptor moieties, three compounds named 32BFCzA, mCF3BFCzOXD and dCF3BFCzOXD are designed, which are composed by mono‐32BFCz/trifluoromethylpicolinonitrile, penta‐BFCz/3‐(trifluoromethyl)phenyl)‐1,3,4‐oxadiazol‐2‐yl)benzene and penta‐32BFCz/3,5‐bis(trifluoromethyl)phenyl)‐1,3,4‐oxadiazol‐2‐yl)benzene as donor/acceptor groups, respectively. They all exhibited green emission with peak value ranging from 532 to 540 nm. Through rationally tuning the donor/acceptor properties to reduce the distance between positive and negative charges, the ground state dipole moments decline from 8.93, 6.18 to 5.30 Debye, which further induced sequentially reduced singlet‐triplet energy splits (ΔESTs), increased photoluminescence quantum yields (PLQYs), enhanced reverse intersystem crossing rate (kRISCs) and diminished delayed fluorescence lifetime from 32BFCzA, mCF3BFCzOXD to dCF3BFCzOXD. Therefore, gradually increased external quantum efficiency (EQE) is accordingly achieved when employing them as emitters for TADF OLEDs. dCF3BFCzOXD with the mostly optimized donor/acceptor structures demonstrates the highest efficiency with maximum EQE to 21.4%. Our work provides guidelines on the design of high‐efficiency D‐A type TADF materials. [ABSTRACT FROM AUTHOR]

A blue thermally activated delayed fluorescence (TADF) emitter 4'-(trifluoromethyl)-[1,1'-biphenyl]-2,3,4,5,6-pentayl-pentakis(9 H -carbazole) 5CzDPhCF 3 is designed by involving the non-conjugated CF 3 moiety as electron-withdrawing unit, which dominates the acceptor properties by the electron inductive effect instead of other widely reported conjugated effects in TADF materials. Inserting a simple phenyl bridge into the light blue TADF emitter of (6-(trifluoromethyl)benzene-1,2,3,4,5-pentayl)pentakis(9 H -carbazole) (5CzPhCF 3 ) to dilute the electron-withdrawing capability and reduce D-A interactions, significantly hypsochromicly shifted emission by 48 and 64 nm is observed in 5CzDPhCF 3 with deep blue emission peak at 435 and 431 nm in CH 2 Cl 2 and film, respectively. 5CzDPhCF 3 exhibits transient lifetime of 6.9 ns and 0.14 μs, comparative HOMO of ∼ -5.55 eV, whereas higher LUMO of −2.25 eV than −2.56 eV of the control 5CzPhCF 3 . The triplet energy and singlet-triplet energy gap (Δ E ST ) for 5CzDPhCF 3 is 2.95 and 0.23 eV respectively, higher than 2.80 and 0.02 eV of 5CzPhCF 3 . And the photoluminescence quantum yield measured for 5CzDPhCF 3 and 5CzPhCF 3 is 22/27% and 24/43% in toluene at ambient/N 2 atmosphere, respectively. Due to the larger Δ E ST for less efficient reverse intersystem crossing in 5CzDPhCF 3 for TADF emission, a trade-off between the color purity and electroluminescent (EL) efficiency can be found. A maximum external quantum efficiency of 2.0 and 9.5% is achieved in the vacuum-deposited TADF devices based on 5CzDPhCF 3 and 5CzPhCF 3 emitter, respectively. And the phenyl-bridged 5CzDPhCF 3 containing device displays pure deep blue EL emission with satisfactory CIE of (0.157, 0.066), while 5CzPhCF 3 only shows light blue emission with CIE of (0.175, 0.327). To the best of our knowledge, the CIE of 5CzDPhCF 3 is among the only few examples on deep blue TADF emission with CIE(y) values < 0.07. [ABSTRACT FROM AUTHOR]

Through simple peripheral tert -butyl or methoxy substitution at 3,6-position of carbazole to increase electron donating property, emission colors of donor-acceptor (D-A) type thermally activated delayed fluorescence (TADF) emitters are rationally tuned. Due to enhanced intramolecular D-A interactions, the intramolecular charge transfer (ICT) transition absorption is bathochromically shifted from 451 to 474 and 490 nm in DCM solution and 458 to 476 and 497 nm in film for 4CzCNPy, 4tCzCNPy and 4oCzCNPy, respectively. And the singlet, triplet as well as HOMO energy levels were also gradually down-shifted from bare 4CzCNPy to 4tCzCNPy and 4oCzCNPy. 4CzCNPy shows green emission in DCM, while 4tCzCNPy and 4oCzCNPy demonstrates yellow and deep red, with peak values from 536 to 561 and 634 nm, respectively. TADF OLEDs based on yellow emitter of 4tCzCNPy exhibit a maximum external quantum efficiency (EQE) of 22.2%, while maximum EQE of 17% and 2.9% is achieved for the green 4CzCNPy and orange-red 4oCzCNPy. [Display omitted] • A simple color tunable approach through peripheral modification is reported. • Emission colors of D-A type TADF emitters are tuned via enhancing donor properties. • PL peaks are red-shifted by up to 100 nm in DCM solution and >60 nm in doped films. • Maximum efficiency of 22.2% is achieved for 4tCzCNPy-based TADF OLED. [ABSTRACT FROM AUTHOR]

A newly synthesized organic compound, 4,5,6,7-tetrakis-carbazol-9-yl-2-methyl-isoindole-1,3-dione (4CzPTANMe) exhibits polymorphism, mechanochromic luminescence, and thermally activated delayed fluorescence (TADF), simultaneously. 4CzPTANMe exists six solvent-induced aggregation states including five crystalline forms with fluorescence emission from green to red. Interestingly, 4CzPTANMe shows typical TADF characteristics in crystalline states while no TADF property in disordered amorphous state. Crystal structure analysis together with photophysical measurements reveal that the polymorph-dependent TADF of 4CzPTANMe can be facilely tuned by the solvents in the crystals. In addition, a doped light-emitting device based on 4CzPTANMe as an emitter exhibits greenish-yellow emission with a peak at 538 nm, a maximum current efficiency of 15.9 cd/A and a decent external quantum efficiency of 5.96%. 4CzPTANMe exhibits weak intermolecular interactions triggered polymorphism, mechanochromic luminescence, and thermally activated delayed fluorescence, simultaneously. [Display omitted] • Polymorphism, MCL, and TADF are combined in one compound. • CzPTANMe exhibits TADF characteristics in crystalline states, but no TADF property in disordered amorphous state. • A doped OLED shows a decent external quantum efficiency of 5.96%. [ABSTRACT FROM AUTHOR]

Three 3,3′-bicarbazole derivatives, 4,4′-(9 H ,9′ H -[3,3′-bicarbazole]-9,9′-diyl)bis(2-methylbenzonitrile) ( p CNBCz m Me, 1 ), 4,4′-(9 H ,9′ H -[3,3′-bicarbazole]-9,9′-diyl)bis(3-(trifluoromethyl)benzonitrile) ( p CNBC zo CF 3 , 2 ) and 4,4′-(9 H ,9′ H -[3,3′-bicarbazole]-9,9′-diyl)bis(2-(trifluoromethyl)benzonitrile) ( p CNBCz m CF 3 , 3 ) were designed and synthesized through a simple one-step catalyst-free C N coupling reaction, by using 9 H ,9′ H -3,3′-bicarbazole and alkyl substituted fluorocyanobenzene as starting materials. Compounds 1 – 3 exhibit high thermal stabilities with T d above 400 °C and T g from 134 to 165 °C. They show thermally activated delayed fluorescence (TADF) characteristics with microsecond scale long lifetimes, similar triplet energy of ∼2.65 eV whereas different absorption and photoluminescence behavior according to the substitution of CH 3 or CF 3 alkyl units at different meta - or ortho - positions. Low singlet-triplet band gaps ( △E ST ) of 0.30, 0.19 and 0.14 eV are observed for 1 – 3 , respectively. By partially blocking the electroactive sites at the 3,3′-position of carbazole to prevent electropolymerization, these 3,3′-bicarbazole derivatives perform favorable electro-oxidative stability. From both experimental and theoretical results, the introduction of electron-withdrawing CF 3 in 2 and 3 lowers both of the HOMO and LUMO levels compared to the weak electron-donating CH 3 -substituted 1 . The hole and electron transport properties can also be tuned through different alkyl on different ortho - or meta -positions. It is interesting that the electron-donating CH 3 meta -structured 1 exhibit one order higher electron mobility than the strong electron-withdrawing CF 3 ortho -positioned 2 and meta -structured 3 , while ortho -CF 3 linked compound 2 with more twisted geometry showed the poorest hole-transport properties. By using the three TADF materials as hosts to conduct double host/dopant TADF systems for solution processed green TADF devices, maximum power efficiencies are achieved at 29.9, 22.7 and 30.9 lm/W for 1, 2 and 3 based devices respectively, in simple structure of ITO/PEDOT:PSS (40 nm)/ 1 – 3 :4CzCNPy (40 nm)/TmPyPB (60 nm)/LiF (0.8 nm)/Al (100 nm). [ABSTRACT FROM AUTHOR]