Strategies to design bipolar small molecules for OLEDs: donor-acceptor structure and non-donor-acceptor structure

Pure hydrocarbon host materials based on spirofluorene with excellent performances for green phosphorescent light-emitting devices

Four novel pure hydrocarbon spirofluorene-based compounds were fully investigated for green electrophosphorescent devices.

Quantum chemical design of carbazole- and pyridoindole-based ambipolar host materials for blue phosphorescent OLEDs

Structure–property relationship of carbazole- and pyridoindole-type host materials for blue PhOLEDs using quantum chemical calculations.

Synthesis and investigation of intra-molecular charge transfer state properties of novel donor-acceptor-donor pyridine derivatives: the effects of temperature and environment on molecular configurations and the origin of delayed fluorescence

A novel series of donor-acceptor-donor (D-A-D) structured pyridine derivatives were synthesised and detailed photo-physical investigations were made using mainly steady-state and time-resolved spectroscopy techniques at varying temperatures. The investigations showed that the molecules have solvent polarity and temperature dependent excited-state configurations, confirmed in two different polarity solvents (295-90 K), i.e. methyl cyclohexane (MCH) and 2-methyltetrahdrofurane (2-MeTHF). In MCH, the investigations revealed dual fluorescence over the temperature range of 295-90 K. At 295 K, the ground-state configuration of the molecules has a partially twisted geometry as determined by DFT calculation, yet the emission originates totally from a locally excited (LE) state, however once the temperature is lowered to 90 K, the twisted molecular configuration is stabilised, and the emission originates from a fully-relaxed intramolecular charge transfer state (ICT), this is contrary to the systems where structural reorganisation stabilises ICT and this is frozen out at low temperatures. The DFT calculations revealed different ground state molecular configurations due to the presence of different electron-donating groups, e.g. the molecule including anthracene groups has a near 90° twisted geometry whereas the triphenylamine including molecule has a pyramidal geometrical folding, therefore, the decrease in temperature restricts the donor degree of rotational freedom. In 2-MeTHF solution, the fluorescence spectrum of both molecules is always of ICT character, but gradually red-shifts through the fluid to glass transition temperature (∼135 K), in this case, the fluorescence occurs after structural and solvent-shell relaxations, however, upon cooling below 135 K, the spectra dramatically shift back to blue giving rise to strong emission from an ICT excited-state (but not the LE state) where the molecules have unrelaxed geometries. This significant change in the nature of the emitting species was explained with specific solvent-solute dynamic interactions in the vicinity of the solvation shell and the effect of thermal excitation of molecular vibrational modes of the C-C bond linking donor and acceptor units. Finally, we confirmed that the molecules have ICT ground-state geometry in the solid-state phase (spin-coated films), and the time-resolved decay dynamics were investigated comparing the spin-coated films (at RT and 25 K) and MCH solutions (at 295 K and 90 K).

Regioselective Substitution at the 1,3- and 6,8-Positions of Pyrene for the Construction of Small Dipolar Molecules

This article presents a novel asymmetrical functionalization strategy for the construction of dipolar molecules via efficient regioselective functionalization along the Z-axis of pyrene at both the 1,3- and 6,8-positions. Three asymmetrically substituted 1,3-diphenyl-6,8-R-disubsituted pyrenes were fully characterized by X-ray crystallography, photophysical properties, electrochemistry, and density functional theory calculations.

One-Step Borylation of 1,3-Diaryloxybenzenes Towards Efficient Materials for Organic Light-Emitting Diodes

The development of a one-step borylation of 1,3-diaryloxybenzenes, yielding novel boron-containing polycyclic aromatic compounds, is reported. The resulting boron-containing compounds possess high singlet-triplet excitation energies as a result of localized frontier molecular orbitals induced by boron and oxygen. Using these compounds as a host material, we successfully prepared phosphorescent organic light-emitting diodes exhibiting high efficiency and adequate lifetimes. Moreover, using the present one-step borylation, we succeeded in the synthesis of an efficient, thermally activated delayed fluorescence emitter and boron-fused benzo[6]helicene.

Novel Carbazol-Pyridine-Carbonitrile Derivative as Excellent Blue Thermally Activated Delayed Fluorescence Emitter for Highly Efficient Organic Light-Emitting Devices

A novel blue thermally activated delayed fluorescence (TADF) emitter, CPC (2,6-di(9H-carbazol-9-yl)-4-phenylpyridine-3,5-dicarbonitrile), was designed and synthesized. By directly linking carbazole (to serve as electron-donor) and pyridine-3,5-dicarbonitrile (to serve as electron-acceptor), and distributing cyanogroups and carbazole groups at the para-position of pyridine core, CPC successfully achieves an extremely small singlet-triplet splitting and fairish photoluminescence quantum yield, thus can act as the highly efficient blue TADF emitter. The optimized organic light-emitting diode (OLED) based on 13 wt % CPC doped in mCP (1,3-bis(9H-carbazol-9-yl)benzene) host exhibits maximum current efficiency, power efficiency, and external quantum efficiency of 47.7 cd A(-1), 42.8 lm W(-1), and 21.2%, respectively, which are the best results in reported blue TADF-based devices up to date and even comparable with the best reported blue phosphorescent OLEDs.

High performance full color OLEDs based on a class of molecules with dual carrier transport channels and small singlet-triplet splitting

Two deep blue emitting materials PPI-PPITPA and PPI-PPIPCz with dual carrier transport properties and small singlet-triplet splitting features are designed and synthesized. PPI-PPITPA and PPI-PPIPCz were used not only as non-doped emitting layers to fabricate highly efficient deep blue OLEDs, but also as hosts to construct high performance green, yellow and red phosphorescent OLEDs.

Bipolar host with multielectron transport benzimidazole units for low operating voltage and high power efficiency solution-processed phosphorescent OLEDs

Two soluble bipolar host materials (mCP-BPBI and CP-QPBI), comprising different proportions of hole-transporting carbazole and electron-transporting benzimidazole, were synthesized. Their thermal, physical, and electrochemical properties were characterized. The designated bulky star-shaped structures efficiently suppress the direct intramolecular interaction between the donor and acceptor subunits to give high triplet energies. Through computational studies, varying the ratio of hole- and electron-transporting moieties could significantly change the carrier injection/transporting abilities and charge balance properties of the host materials. Indeed, CP-QPBI with more benzimidazole units shows extremely enhanced current density at the same voltage when compared to mCP-BPBI. The operating voltage of solution-processed phosphorescent light-emitting diodes with CP-QPBI as host were dramatically reduced by ∼3 V compared with the similar devices of mCP-BPBI. At the same time, the power efficiencies were improved for 2-2.5 times at the corresponding voltage. Importantly, both blue and green devices maintain their high efficiencies even at brightness up to 1000 cd m(-2), which clearly demonstrates that the new strategy applied to improve electron-transporting ability and charge-balance property of the solution-processable host material by tuning the ratio of donor and acceptor unit is profitable.

Carbazole-functionalized polyphenylene-decorated solid state emissive D–A–D molecules: reduced donor–acceptor interaction and enhanced emission in the solid state

Reducing intermolecular donor–acceptor interactions through steric effects can prevent ACQ and lead to enhanced emission in the solid state.

Design, synthesis and electronic properties of push–pull–push type dye

Alkyne conjugated push–pull–push type dye where alkyne π-spacer and push units monitor the electronic properties of the dye.

Enhanced electron affinity and charge balance property of a bipolar material: highly efficient solution-processed deep blue electrofluorescent and green electrophosphorescent devices

The slight move in the ratio of the donor and acceptor group can slightly affect the optoelectronic property as a whole, and consequently modulate the electron affinity, charge balance property and exciton recombination efficiency.

Deep-blue electroluminescence from nondoped and doped organic light-emitting diodes (OLEDs) based on a new monoaza[6]helicene

Electroluminescent properties of a new monoaza[6]helicene as deep-blue light emitters in nondoped and doped OLEDs were investigated.

Structurally simple phenanthroimidazole-based bipolar hosts for high-performance green and red electroluminescent devices

Two structurally simple and easily synthesized phenanthroimidazole-based bipolar-transporting host materials realized efficient green and red phosphorescent OLEDs with low efficiency roll-off.

Phenothiazine dioxide based high triplet energy host materials for blue phosphorescent organic light-emitting diodes

Donor–acceptor type high triplet energy host materials were derived from a phenothiazine dioxide based electron transport moiety and a carbazole based hole transport moiety.

N-annelated perylenes as effective green emitters for OLEDs

New carbazole-substituted N-annelated perylenes with different linking topologies of the chromophores were synthesized and their physical properties were studied with various experimental techniques.

A versatile efficient one-step approach for carbazole–pyridine hybrid molecules: highly efficient host materials for blue phosphorescent OLEDs

A series of carbazole–pyridine hybrid hosts were prepared by a convenient catalyst free C–N coupling reaction for high-efficiency blue electrophosphorescence.

Yellow electrophosphorescent devices with hosts containing N1-(naphthalen-1-yl)-N1,N4-diphenylnaphthalene-1,4-diamine and tetraphenylsilane units

Two novel host materials SiP and SiM with a high glass transition temperature were synthesized and the yellow phosphorescent OLEDs show maximum current and power efficiency of 40.81 cd A⁻¹ and 33.60 lm W⁻¹ with low efficiency roll-off.

New bipolar host materials for realizing blue phosphorescent organic light-emitting diodes with high efficiency at 1000 cd/m2

New host molecules such as 9-(6-(9H-carbazol-9-yl)pyridin-3-yl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (pPCB2CZ) and 9-(6-(9H-carbazol-9-yl)pyridin-2-yl)-6-(9H-carbazol-9-yl)-9H-pyrido[2,3-b]indole (mPCB2CZ) were designed and synthesized for blue phosphorescent organic light-emitting diodes (PhOLEDs). The glass transition temperatures of two host molecules were measured higher than 120 °C, and the identical triplet energies were determined to be 2.92 eV for both molecules. The bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III) (FIrpic)-doped mPCB2CZ-based PhOLED exhibited practically useful driving voltage of 4.8 V in a simple organic three layer device configuration which has a smaller number of interfaces in conventional multilayer PhOLEDs. Also, the high quantum efficiency of 23.7% is reported at the practically useful brightness value of 1000 cd/m2.

Rational design of aggregation-induced emission luminogen with weak electron donor-acceptor interaction to achieve highly efficient undoped bilayer OLEDs

In this work, two tailored luminogens (TPE-NB and TPE-PNPB) consisting of tetraphenylethene (TPE), diphenylamino, and dimesitylboryl as a π-conjugated linkage, electron donor, and electron acceptor, respectively, are synthesized and characterized. Their thermal stabilities, photophysical properties, solvachromism, fluorescence decays, electronic structures, electrochemical behaviors, and electroluminescence (EL) properties are investigated systematically, and the impacts of electron donor-acceptor (D-A) interaction on optoelectronic properties are discussed. Due to the presence of a TPE unit, both luminogens show aggregation-induced emission, but strong D-A interaction causes a decrease in emission efficiency and red-shifts in photoluminescence and EL emissions. The luminogen, TPE-PNPB, with a weak D-A interaction fluoresces strongly in solid film with a high fluorescence quantum yield of 94%. The trilayer OLED [ITO/NPB (60 nm)/TPE-PNPB (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] utilizing TPE-PNPB as a light emitter shows a peak luminance of 49 993 cd m(-2) and high EL efficiencies up to 15.7 cd A(-1), 12.9 lm W(-1), and 5.12%. The bilayer OLED [ITO/TPE-PNPB (80 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)] adopting TPE-PNPB as a light emitter and hole transporter simultaneously affords even better EL efficiencies of 16.2 cd A(-1), 14.4 lm W(-1), and 5.35% in ambient air, revealing that TPE-PNPB is an eximious p-type light emitter.

High-efficiency fluorescent organic light-emitting devices using sensitizing hosts with a small singlet-triplet exchange energy

Materials with small singlet-triplet splits (ΔEST s) are introduced as sensitizing hosts to excite fluorescent dopants, breaking the trade-off between small ΔEST and high radiative decay rates. A highly efficient orange-fluorescent organic light-emitting diode (OLED) is prepared, showing a maximum external quantum efficiency of 12.2%.

C,C'-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes: synthesis, structures, photophysics and electrochemistry

Six new C,C'-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes, 1,A-R2-1,A-C2B10H10, where R represents the group 2-(1,3-Et2-1,3,2-N2BC6H4) or 2-(1,3-Ph2-1,3,2-N2BC6H4) and A is 2, 7 or 12, were synthesized from o-, m-, and p-dicarbadodecaboranes (carboranes) by lithiation and subsequent treatment with the respective 2-bromo-1,3,2-benzodiazaboroles. UV-visible and fluorescence spectra of all carboranes display low energy charge transfer emissions. While such emissions with Stokes shifts between 17,330 and 21,290 cm(-1) are typical for C,C'-bis(aryl)-ortho-carboranes, the observed low-energy emissions with Stokes shifts between 8320 and 15,170 cm(-1) for the meta- and para-isomers are unusual as high-energy emissions are typical for meta- and para-dicarbadodecaboranes. Fluorescence quantum yields (φF) for the novel 1,7- and 1,12-bis(benzodiazaborolyl)-carboranes depend on the substituents at the nitrogen atoms of the heterocycle. Thus, the para-carborane with N-ethyl substituents 1,12-(1',3'-Et2-1',3',2'-N2BC6H4)2-1,12-C2B10H10 has a φF value of 41% in cyclohexane solution and only of 9% in the solid state, whereas the analogous 1,12-(1',3'-Ph2-1',3',2'-N2BC6H4)2-1,12-C2B10H10 shows quantum yields of 3% in cyclohexane solution and 72% in the solid state. X-ray crystallographic, computational and cyclic voltammetry studies for these carboranes are also presented.

High quantum efficiency in solution and vacuum processed blue phosphorescent organic light emitting diodes using a novel benzofuropyridine-based bipolar host material

High quantum efficiency in solution and vacuum processed blue phosphorescent organic light emitting diodes are achieved using a new benzofuropyridine based bipolar host material. High quantum efficiencies of 18.0% and 23.0% are obtained in soluble and vacuum evaporable blue devices.

High-performance organic thin-film transistor based on a dipolar organic semiconductor

A merocyanine dye with an outstandingly large dipole moment of 14 Debye affords thin-film transistors with 0.18 cm(2) V(-1) s(-1) hole mobility and a 10(6) on/off ratio. These results suggest that molecules that lack symmetry and possess large dipole moments can perform excellent charge carrier transport contrary to established molecular semiconductor design strategies.