Thermally activated delayed fluorescence (TADF)-based white organic light-emitting diodes (WOLEDs) are highly attractive because the TADF emitters provide a promising alternative route to harvest triplet excitons. One of the major challenges is to achieve superior efficiency/color rendering index/color stability and low efficiency roll-off simultaneously. In this paper, high-performance hybrid WOLEDs are demonstrated by employing an efficient blue TADF emitter combined with red and green phosphorescent emitters. The resulting WOLED shows the maximum external quantum efficiency, current efficiency, and power efficiency of 23.0%, 51.0 cd A⁻¹, and 51.7 lm W⁻¹, respectively. Moreover, the device exhibits extremely stable electroluminescence spectra with a high color rendering index of 89 and Commission Internationale de L'Eclairage coordinates of (0.438, 0.438) at the practical brightness of 1000 cd m⁻². The achievement of these excellent performances is systematically investigated by versatile experimental and theoretical evidences, from which it is concluded that the utilization of a blue-green-red cascade energy transfer structure and the precise manipulation of charges and excitons are the key points. It can be anticipated that this work might be a starting point for further research towards high-performance hybrid WOLEDs.

Two host materials of {4-[diphenyl(4-pyridin-3-ylphenyl)silyl]phenyl}diphenylamine (p-PySiTPA) and {4-[[4-(diphenylphosphoryl)phenyl](diphenyl)silyl]phenyl}diphenylamine (p-POSiTPA), and an electron-transporting material of [(diphenylsilanediyl)bis(4,1-phenylene)]bis(diphenylphosphine) dioxide (SiDPO) are developed by incorporating appropriate charge transporting units into the tetraarylsilane skeleton. The host materials feature both high triplet energies (ca. 2.93 eV) and ambipolar charge transporting nature; the electron-transporting material comprising diphenylphosphine oxide units and tetraphenylsilane skeleton exhibits a high triplet energy (3.21 eV) and a deep highest occupied molecular orbital (HOMO) level (-6.47 eV). Using these tetraarylsilane-based functional materials results in a high-efficiency blue phosphorescent device with a three-organic-layer structure of 1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane (TAPC)/p-POSiTPA: iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6)/SiDPO that exhibits a forward-viewing maximum external quantum efficiency (EQE) up to 22.2%. This is the first report of three-organic-layer FIr6-based blue PhOLEDs with the forward-viewing EQE over 20%, and the device performance is among the highest for FIr6-based blue PhOLEDs even compared with the four or more than four organic-layer devices. Furthermore, with the introduction of bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzoimidazol-N,C³)iridium acetylacetonate [(fbi)₂Ir(acac)] as an orange emitter, an all-phosphor warm-white PhOLED achieves a peak power efficiency of 47.2 lm W⁻¹, which is close to the highest values ever reported for two-color white PhOLEDs.

To achieve high efficiencies in blue phosphorescent organic light-emitting diodes (PhOLEDs), the triplet energies (T₁) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S₁) of 2.80 eV and triplet energy of 2.71 eV can be used as the host material for the blue phosphor, [bis(4,6-difluorophenylpyridinato-N,C^2′)iridium(III)] tetrakis(1-pyrazolyl)borate (FIr6; T₁=2.73 eV). In both the photo- and electro-excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic-layer device, the maximum current efficiency of 37 cd A⁻¹ and power efficiency of 40 Lm W⁻¹ were achieved for the FIr6-based blue PhOLEDs.

Grafting six fluorene units to a benzene ring generates a new highly twisted core of hexakis(fluoren-2-yl)benzene. Based on the new core, six-arm star-shaped oligofluorenes from the first generation T1 to third generation T3 are constructed. Their thermal, photophysical, and electrochemical properties are studied, and the relationship between the structures and properties is discussed. Simple double-layer electroluminescence (EL) devices using T1–T3 as non-doped solution-processed emitters display deep-blue emissions with Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.08) for T1, (0.16, 0.08) for T2, and (0.16, 0.07) for T3. These devices exhibit excellent performance, with maximum current efficiency of up to 5.4 cd A⁻¹, and maximum external quantum efficiency of up to 6.8%, which is the highest efficiency for non-doped solution-processed deep-blue organic light-emitting diodes (OLEDs) based on starburst oligofluorenes, and is even comparable with other solution-processed deep-blue fluorescent OLEDs. Furthermore, T2- and T3-based devices show striking blue EL color stability independent of driving voltage. In addition, using T0–T3 as hole-transporting materials, the devices of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)/T0–T3/tris(8-hydroxyquinolinato)aluminium (Alq₃)/LiF/Al achieve maximum current efficiencies of 5.51–6.62 cd A⁻¹, which are among the highest for hole-transporting materials in identical device structure.

Two new polyfluorenes with dipicolylamine (DPA) pendant, PF-TDPA and PF-HDPA, are designed and synthesized by pre- and post-functionalization, respectively. PF-TDPA with a rigid side chain shows a selective fluorescence quenching upon the addition of Cu²⁺ in a mixture solution of tetrahydrofuran and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer. What is more, the PF-TDPA/Cu²⁺ complex can selectively detect histidine over other amino acids with a fluorescence recovery. In contrast, PF-HDPA with a flexible spacer exhibits a fluorescence quenching to Cu²⁺ but slightly fluorescence recovery after the addition of histidine. This indicates that the proper distance between the two DPA groups play an important role in the detection of histidine.

A new cationic cyano-substituted poly(p-phenylenevinylene) (N-CNPPV) is synthesized by Knoevenagel condensation. The water-soluble polymer shows different emission spectra in different solvents and displays unique fluorescent behaviors in the mixed solvents of water and THF. The new polymer can form a complex with ssDNA by adopting a more planar conformation, exhibiting red shift of emission wavelength and enhancement of fluorescence intensity. By investigating the fluorescent response of N-CNPPV to various surfactants, we demonstrate that the hydrophobic interaction and electrostatic interaction result in the selective response of N-CNPPV to ssDNA. This is the first report on selective fluorescence enhancement of conjugated polyelectrolyte induced by ssDNA.

Two hybrids based on 1,8-disubstituted carbazole, 1,8-OXDCz and 1,8-mBICz, have been designed and synthesized through a facile process. The incorporation of oxadiazole or N-phenylbenzimidazole moieties at the 1,8-positions of carbazole greatly improves its morphological stability, giving glass transition temperatures (T_g) as high as 138 and 154 °C, respectively. Blue phosphorescent organic light-emitting devices (PhOLEDs) with 1,8-mBICz exhibit almost the same performance as a similarly structured device based on the mCP host, and green PhOLEDs employing the new host material 1,8-OXDCz exhibit an ideal turn-on voltage (2.5 V at 1.58 cd m⁻²), a maximum current efficiency (η_c,max) of 73.9 cd A⁻¹, and a power efficiency (η_p,max) of 89.7 lm W⁻¹. These results are among the best performances of [Ir(ppy)₃]-based devices with simple device configurations.

A series of starburst oligomers (T1–T3) that contained a fully diarylmethene-bridged triphenylamine core and oligofluorene arms were designed and synthesized through Suzuki cross-coupling reactions. Their thermal, photophysical, and electrochemical properties were also investigated. These materials showed high glass transition, in the range of 123–129 °C, and good film-forming abilities. They displayed deep-blue emission both in solution and as thin films. Solution-processed devices based on these oligomers exhibited highly efficient deep-blue electroluminescence and the device performances were significantly enhanced with the extension of the oligofluorene arms. The double-layered device that contained T3 as an emitter showed a maximum current efficiency of 3.83 cd A⁻¹ and a maximum external quantum efficiency of 4.19 % with CIE coordinates of (0.16, 0.09), which are among the highest values for undoped deep-blue OLEDs that are based on solution-processable starburst oligomers.

A series of solution-processable small molecules PO1–PO4 were designed and synthesized by linking N-phenylnaphthalen-1-amine groups to a phenyl phosphine oxide core through a π-conjugated bridge, and their thermal, photophysical, and electrochemical properties were investigated. The phosphine oxide linkage can disrupt the conjugation and allows the molecular system to be extended to enable solution processability and high glass transition temperatures (159–181 °C) while preserving the deep-blue emission. The noncoplanar molecular structures resulting from the trigonal-pyramidal configuration of the phosphine oxide can suppress intermolecular interactions, and thus these compounds exhibit strong deep-blue emission both in solution and the solid state with high photoluminescent quantum yield (PLQY) of 0.88–0.99 in dilute toluene solution. Solution-processed nondoped organic light-emitting diodes featuring PO4 as emitter achieve a maximum current efficiency of 2.36 cd A⁻¹ with CIE coordinates of (0.15, 0.11) that are very close to the NTSC blue standard. Noticeably, all devices based on these small-molecular fluorescent emitters show striking deep-blue electroluminescent color stability and extremely low efficiency roll-off.

A series of water-soluble copolymers, named N-PFPx (x = 10, 25, and 50), and their model compound 6,6',6'',6'''-(2,2'-(2-hydroxy-5-methoxy-1,4-phenylene)bis(9H-fluorene-9,9,2-triyl))tetrakis(N,N,N-trimethylhexan-1-aminium) bromide (FMOPF) were synthesized by Suzuki coupling reaction of fluorene derivatives and p-methoxyphenol. For the polymers with relatively low contents of methoxyphenol (N-PFP10 and N-PFP25), the absorption and fluorescence spectra could be mainly ascribed to the polyfluorene, whereas for the polymer N-PFP50 and the model compound FMOPF, the absorption and fluorescence spectra could be assigned to the fluorene-alt-methoxyphenol. All the conjugated polymers (CPs), regardless of the content of methoxyphenol moieties, exhibit good sensitivity to hypochlorite/hypochlorous acid (HClO) because of the superquenching effect of CPs. On the contrary, the absorption changes of the polymers N-PFPx on the addition of hypochlorite/HClO depend on the content of methoxyphenol moieties. As the content of methoxyphenol moieties increase, the changes of absorption spectra become more intense. Considering the sensitivity and selectivity, the polymer N-PFP10 and N-PFP25 have been demonstrated to be good polymeric fluorescent probes to hypochlorite under the aqueous condition. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

A series of tetraarylsilane compounds, namely p-BISiTPA (1), m-BISiTPA (2), p-OXDSiTPA (3), m-OXDSiTPA (4), are designed and synthesized by incorporating electron-donating arylamine and electron-accepting benzimidazole or oxadiazole into one molecule via a silicon-bridge linkage mode. Their thermal, photophysical and electrochemical properties can be finely tuned through the different groups and linking topologies. The para-disposition compounds 1 and 3 display higher glass transition temperatures, slightly lower HOMO levels and triplet energies than their meta-disposition isomers 2 and 4, respectively. The silicon-interrupted conjugation of the electron-donating and electron-accepting segments gives these materials the following advantages: i) relative high triplet energies in the range of 2.69–2.73 eV; ii) HOMO/LUMO levels of the compounds mainly depend on the electron-donating and electron-accepting groups, respectively; iii) bipolar transporting feature as indicated by hole-only and electron-only devices. These advantages make these materials ideal universal hosts for multicolor phosphorescent OLEDs. 1 and 3 have been demonstrated as universal hosts for blue, green, orange and white electrophosphorescence, exhibiting high efficiencies and low efficiency roll-off. For example, the devices hosted by 1 achieve maximum external quantum efficiencies of 16.1% for blue, 22.7% for green, 20.5% for orange, and 19.1% for white electrophosphorescence. Furthermore, the external quantum efficiencies are still as high as 14.2% for blue, 22.4% for green, 18.9% for orange, and 17.4% for white electrophosphorescence at a high luminance of 1000 cd m⁻². The two-color, all-phosphor white device hosted by 3 acquires a maximum current efficiency of 51.4 cd A⁻¹, and a maximum power efficiency of 51.9 lm W⁻¹. These values are among the highest for single emitting layer white PhOLEDs reported till now.

High-performance, green, orange, and red top-emitting organic light-emitting diodes (TOLEDs) with p–i–n homojunction are demonstrated. An excellent ambipolar host, 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD), which has good thermal and morphological stabilities, a high triplet energy level, and equally high electron and hole mobilities, is chosen as the organic host material for the homojunction devices. By electrical doping, the carrier injection and transporting characteristics are greatly improved. The optical structure is optimized in view of light emission of different colors to enhance the color purity and improve the view characte­ristics. As a result, high efficiency p–i–n homojunction TOLEDs with saturated intrinsic emission of the emitting materials and angular independence of the emission are realized. The performances of these p–i–n homojunction TOLEDs are even higher than the multi-layer heterojunction bottom-emitting devices using the same emitting layers.

A series of bipolar transport host materials: 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) (1), 2,5-bis(4-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (p-CzOXD) (2), 2,5-bis(3-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (m-CzOXD) (3) and 2-(2-(9H-carbazol-9-yl)phenyl)-5-(4-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (op-CzOXD) (4) are synthesized through simple aromatic nucleophilic substitution reactions. The incorporation of the oxadiazole moiety greatly improves their morphological stability, with T_d and T_g in the range of 428–464 °C and 97–133 °C, respectively. The ortho and meta positions of the 2,5-diphenyl-1,3,4-oxadiazole linked hybrids (1 and 3) show less intramolecular charge transfer and a higher triplet energy compared to the para-position linked analogue (2). The four compounds exhibit similar LUMO levels (2.55–2.59 eV) to other oxadiazole derivatives, whereas the HOMO levels vary in a range from 5.55 eV to 5.69 eV, depending on the linkage modes. DFT-calculation results indicate that 1, 3, and 4 have almost complete separation of their HOMO and LUMO levels at the hole- and electron-transporting moieties, while 2 exhibits only partial separation of the HOMO and LUMO levels possibly due to intramolecular charge transfer. Phosphorescent organic light-emitting devices fabricated using 1–4 as hosts and a green emitter, Ir(ppy)₃ or (ppy)₂Ir(acac), as the guest exhibit good to excellent performance. Devices hosted by o-CzOXD (1) achieve maximum current efficiencies (η_c) as high as 77.9 cd A⁻¹ for Ir(ppy)₃ and 64.2 cd A⁻¹ for (ppy)₂Ir(acac). The excellent device performance may be attributed to the well-matched energy levels between the host and hole-transport layers, the high triplet energy of the host and the complete spatial separation of HOMO and LUMO energy levels.

A new triphenylamine/oxadiazole hybrid, namely m-TPA-o-OXD, formed by connecting the meta-position of a phenyl ring in triphenylamine with the ortho-position of 2,5-biphenyl-1,3,4-oxadiazole, is designed and synthesized. The new bipolar compound is applicable in the phosphorescent organic light-emitting diodes (PHOLEDs) as both host and exciton-blocking material. By using the new material and the optimization of the device structures, very high efficiency green and yellow electrophosphorescence are achieved. For example, by introducing 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) to replace 2, 9-dimethyl-4,7-diphenyl-1, 10-phenanthroline (BCP)/tris(8-hydroxyquinoline)aluminium (Alq₃) as hole blocking/electron transporting layer, followed by tuning the thicknesses of hole-transport 1, 4-bis[(1-naphthylphenyl)amino]biphenyl (NPB) layer to manipulate the charge balance, a maximum external quantum efficiency (η_EQE,max) of 23.0% and a maximum power efficiency (η_p,max) of 94.3 lm W⁻¹ are attained for (ppy)₂Ir(acac) based green electrophosphorescence. Subsequently, by inserting a thin layer of m-TPA-o-OXD as self triplet exciton block layer between hole-transport and emissive layer to confine triplet excitons, a η_EQE,max of 23.7% and η_p,max of 105 lm W⁻¹ are achieved. This is the highest efficiency ever reported for (ppy)₂Ir(acac) based green PHOLEDs. Furthermore, the new host m-TPA-o-OXD is also applicable for other phosphorescent emitters, such as green-emissive Ir(ppy)₃ and yellow-emissive (fbi)₂Ir(acac). A yellow electrophosphorescent device with η_EQE,max of 20.6%, η_c,max of 62.1 cd A⁻¹, and η_p,max of 61.7 lm W⁻¹, is fabricated. To the author’s knowledge, this is also the highest efficiency ever reported for yellow PHOLEDs.

A series of 9,9′-spirobifluorene/oxadiazole hybrids with various linkages between two components, namely SBF-p-OXD (1), SBF-m-OXD (2), and SBF-o-OXD (3) are designed and synthesized through Suzuki cross-coupling reactions. The incorporation of a rigid and bulky spirobifluorene moiety greatly improves their thermal and morphological stability, with T_d (decomposition temperature) and T_g (glass transition temperature) in the ranges of 401–480 °C and 136–210 °C, respectively. 2 and 3 with meta- and ortho-linkage display higher triplet energy and blue-shifted absorption and emission than their para-linked analogue 1 owing to the decreasing π-conjugation between the two components. Their HOMO and LUMO energy levels depend on the linkage modes within the range of 5.57–5.64 eV and 2.33–2.49 eV, respectively. Multilayer deep red electrophosphorescent devices with 1, 2, 3 as hosts were fabricated and their EL efficiencies follow the order of 3 (o)>2 (m)>1 (p), which correlates with their triplet energy and the separation of HOMO and LUMO distributions at molecular orbitals. The maximum external quantum efficiencies of 11.7 % for green and 9.8 % for deep red phosphorescent organic light-emitting diodes (OLEDs) are achieved by using 2 and 3 as host materials, respectively.

Two new bipolar compounds, N,N,N′,N′-tetraphenyl-5′-(1-phenyl-1H-benzimidazol-2-yl)-1,1′:3′,1′′-terphenyl-4,4′′-diamine (1) and N,N,N′,N′-tetraphenyl-5′-(1-phenyl-1H-benzimidazol-2-yl)-1,1′:3′,1′′-terphenyl-3,3′′-diamine (2), were synthesized and characterized, and their thermal, photophysical, and electrochemical properties were investigated. Compounds 1 and 2 possess good thermal stability with high glass-transition temperatures of 109–129 °C and thermal decomposition temperatures of 501–531 °C. The fluorescence quantum yield of 1 (0.52) is higher than that of 2 (0.16), which could be attributed to greater π conjugation between the donor and acceptor moieties. A nondoped deep-blue fluorescent organic light-emitting diode (OLED) using 1 as the blue emitter displays high performance, with a maximum current efficiency of 2.2 cd A⁻¹ and a maximum external efficiency of 2.9 % at the CIE coordinates of (0.17, 0.07) that are very close to the National Television System Committee’s blue standard (0.15, 0.07). Electrophosphorescent devices using the two compounds as host materials for green and red phosphor emitters show high efficiencies. The best performance of a green phosphorescent device was achieved using 2 as the host, with a maximum current efficiency of 64.3 cd A⁻¹ and a maximum power efficiency of 68.3 lm W⁻¹; whereas the best performance of a red phosphorescent device was achieved using 1 as the host, with a maximum current efficiency of 11.5 cd A⁻¹, and a maximum power efficiency of 9.8 lm W⁻¹. The relationship between the molecular structures and optoelectronic properties are discussed.

A series of fluorene-based oligomers with novel spiro-annulated triarylamine structures, namely DFSTPA, TFSTPA, and TFSDTC, are synthesized by a Suzuki cross-coupling reaction. The spiro-configuration molecular structures lead to very high glass transition temperatures (197–253 °C) and weak intermolecular interactions, and consequently the structures retain good morphological stability and high fluorescence quantum efficiencies(0.69–0.98). This molecular design simultaneously solves the spectral stability problems and hole-injection and transport issues for fluorene-based blue-light-emitting materials. Simple double-layer electroluminescence (EL) devices with a configuration of ITO/TFSTPA (device A) or TFSDTC (device B)/ TPBI/LiF/Al, where TFSTPA and TFSDTC serve as hole-transporting blue-light-emitting materials, show a deep-blue emission with a peak around 432 nm, and CIE coordinates of (0.17, 0.12) for TFSTPA and (0.16, 0.07) for TFSDTC, respectively, which are very close to the National Television System Committee (NTSC) standard for blue (0.15, 0.07). The maximum current efficiency/external quantum efficiencies are 1.63 cd A⁻¹/1.6% for device A and 1.91 cd A⁻¹/2.7% for device B, respectively. In addition, a device with the structure ITO/DFSTPA/Alq₃/LiF/Al, where DFSTPA acts as both the hole-injection and -transporting material, is shown to achieve a good performance, with a maximum luminance of 14 047 cd m⁻², and a maximum current efficiency of 5.56 cd A⁻¹. These values are significantly higher than those of devices based on commonly usedN,N′-di(1-naphthyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (NPB) as the hole-transporting layer (11 738 cd m⁻² and 3.97 cd A⁻¹) under identical device conditions.

A novel aryl-bridged triphenylamine derivative, 7-t-butyl-5,5,9,9-tetraaryl-13b-aza-naphtho[3,2,1-de]anthracene (ABTPA) was designed and synthesized. The alternating copolymers of ABTPA/dihexylfluorene (P1) and triphenylamine (TPA)/dihexylfluorene (P2) were synthesized by Suzuki coupling reaction. P1 shows excellent thermal stability with a decomposition temperature of 440 °C and a glass-transition temperature of 326 °C. The HOMO energy levels of the two polymers are very close (−5.15 eV for P1 and −5.13 eV for P2). The maximum absorption peak of P1 is red shifted by 23 nm with respect to P2, because the incorporation of ABTPA units into the PF backbone enhances the electronic conjugation degree compared with the case of TPA units. The rigidity and the steric hindrance of the ABTPA in P1 result in a small Stokes shift and almost the same emission spectra of P1 between its film and solution. A PLED with simple configurations of ITO/P1/TNS (tetranaphthalen-2-yl-silane)/Alq₃ (tris(8-hydroxyquinolinolato)aluminum)/Al emits a blue light with emission peak at 436 nm, and exhibits a maximum current efficiency of 1.89 cd/A and a maximum luminance of 4183 cd/m², which is superior to the device with P2 as emissive layer under the identical conditions. These results indicate that ABTPA unit could be a very promising candidate to replace TPA unit and find widely application in organic/polymeric optoelectronic materials. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3651–3661, 2009

Two novel pyrene-armed calix[4]arenes by triazole connection were synthesized using "click" chemistry. Compound 1 with two pyrene subunits appended to the lower rims of the calix[4]arene shows ratiometric fluorescence response toward Zn²⁺, and selective fluorescence quenching toward heavy metal ions such as Cu²⁺, Hg²⁺ and Pb²⁺; while compound 2 with one pyrene subunit exhibits significant fluorescence quenching toward Cu²⁺ and moderate quenching behaviour toward Hg²⁺. By utilizing the different fluorescence behavior of 1 toward Zn²⁺ and Cu²⁺, inhibition (INH) and not or (NOR) logic gates were established.