Qing-Xiao Tong

Blue-emitting organic electrofluorescence materials: progress and prospective

The intense interest in organic light-emitting devices (OLEDs) originates from their attractive prospects as the next generation display and lighting technologies. The development of blue emitters is of great significance in OLED applications as full-color displays and energy-saving lightings. The electrofluorescence using triplet energy for radiation has recently become a spotlight in the area of organic electronics. Various triplet-to-singlet conversion mechanisms have been established, including triplet–triplet annihilation (TTA), thermally activated delayed fluorescence (TADF) and “hot exciton” model with hybridized local and charge-transfer (HLCT) excited state, and they are expected to shed light on the development on blue OLEDs. This study revolves around the recent progress in blue electrofluorescence materials utilizing triplet excitons for radiation. Owing to the page limitation, special focus is placed on small molecule-based purely organic fluorophores with breakthrough device performances. We begin with the general information of blue-emitting organic electrofluorescence devices, and then give an overview of blue fluorescence OLEDs based on different electroluminescence mechanisms, which is followed by individual molecular design strategies towards high efficiency.

High-efficiency nondoped white organic light-emitting devices

High-efficiency nondoped white organic light-emitting devices (WOLEDs) were demonstrated by using both the intrinsic and exciplex emissions from a single electroluminescent material, 4,4′,4″-trispyrenylphenylamine (TPyPA). The simple device structure of indium tin oxide/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/TPyPA/4,7-diphenyl-1,10-phenanthroline/LiF∕Al exhibited a luminance of 10000cd∕m2 at a low driving voltage of 4.5V, and high current and power efficiencies of 9.4cd∕A and 9.0lm∕W, respectively. Such WOLED showed excellent color stability and purity with the Commission Internationale de L’Eclairage coordinates of (0.31, 0.35), which remained unchanged over a wide range of luminance from 100to20000cd∕m2.

An ESIPT fluorescent dye based on HBI with high quantum yield and large Stokes shift for selective detection of Cys

We report 3-(4,5-diphenyl-1H-imidazol-2-yl)naphthalen-2-ol (DPIN) as an interesting luminescent material displaying ESIPT with a large Stokes shift of ∼180 nm even in protic/polar solvents. Stable homo-dispersed nanoparticles formed by inter- and intramolecular H-bonds in aqueous media and the corresponding aggregation induced enhanced emission with a high quantum yield up to 0.45 were observed. Factors such as pH value and ions (cations and anions) showed a negligible effect on the fluorescence performance. A probe of 3-(4,5-diphenyl-1H-imidazol-2-yl)naphthalen-2-yl-acrylate (DPIN-A) based on this molecule was designed. The results revealed that it can be used for sensing of Cys with high selectivity and sensitivity.

A novel bipolar phenanthroimidazole derivative host material for highly efficient green and orange-red phosphorescent OLEDs with low efficiency roll-off at high brightness

A new bipolar fluorophore, N,N-diphenyl-4′-(9-(4′-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1′-biphenyl]-4-yl)-9H-fluoren-9-yl)-[1,1′-biphenyl]-4-amine (PPI-F-TPA), consisting of an electron-withdrawing phenanthro[9,10-d]imidazole (PI) chromophore and an electron-donating triphenylamine group, based on an indirect linkage, has been designed and synthesized. The sp3-hybridized C9 atom of the fluorene linkage efficiently interrupts molecular conjugation and inhibits π–π intermolecular interactions, resulting in efficient violet-blue emission, excellent thermal stability and high triplet energy. Equipped with balanced carrier mobility, PPI-F-TPA shows impressive performance as the emitting layer in non-doped OLEDs, which achieved an external quantum efficiency (EQE) of 3.11% with a CIE coordinate of (0.16, 0.05). Furthermore, the high triplet energy allows PPI-F-TPA to be used as a host for PhOLEDs. High performance green and orange-red PhOLEDs with the maximum EQEs, current efficiencies (CE) and power efficiencies (PE) of 15.6% and 12.5%, 57 cd A−1 and 27 cd A−1, 60 lm W−1 and 28.3 lm W−1, respectively, have been successfully obtained. More importantly, all the devices exhibit low efficiency roll-off; in particular, that of the orange-red PhOLEDs is extremely small. The orange-red PhOLED has a decay rate of EQE less than 1% at 1000 cd m−2, 13.6% at 10 000 cd m−2 and 29.5% even at 50 000 cd m−2, which is very rare among orange or orange-red PhOLEDs at such high brightness.

A meta-molecular tailoring strategy towards an efficient violet-blue organic electroluminescent material

In this paper, an efficient violet-blue emitter 4,4′′-bis(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-1,1′:3′,1′′-terphenyl (m-BBTPI) was designed and synthesized by linking two phenanthroimidazole units via the meta position of a freely rotatable phenyl bridge. The present design provides a suitable level of conjugation between the two phenanthroimidazole units such that fluorescence is strengthened over the single unit while a violet-blue emission can be maintained by limiting the amount of redshift. The new emitter m-BBTPI is also found to have good thermal stability, strong violet-blue emission and bipolar charge transporting properties. An electroluminescent device using m-BBTPI as a non-doped emission layer shows a low turn-on voltage (3.2 V), good colour purity (0.16, 0.06) as well as high current and power efficiencies (1.99 cd A−1, 1.81 lm W−1). These performance parameters are comparable to the state-of-the-art non-doped violet-blue OLEDs.

Approaches for achieving highly efficient exciplex-based organic light-emitting devices

We studied the performance of exciplex-based organic light-emitting devices (OLEDs) made of different electron transporting materials (ETMs) with similar electron affinities to minimize the effect of the lowest unoccupied molecular orbital levels. A strong correlation was observed between the intensity of exciplex emission and the choice of ETMs. The intensity of exciplex emission relied on interfacial charge accumulation densities at organic/organic contacts, which in turn determined device color and efficiency. Contrary to common belief, highly efficient exciplex-based OLEDs can be achieved, provided that the involved organic materials have high carrier mobility, high photoluminescence quantum yield, and suitable electron energy levels.

Efficient green organic light-emitting devices with a nondoped dual-functional electroluminescent material

An efficient nondoped green organic light-emitting device was demonstrated by using a dual-functional electroluminescent material, 4,4′,4″-tris[8-(7,10-diphenylfluoranthenyl)] phenylamine (TDPFPA). TDPFPA was shown to be a good hole transporting [with a mobility of (1.1–1.2)×10−4cm2V−1s−1 at (1.8–5.6)×105Vcm−1] and efficient fluorescent material with an exceptionally high glass transition temperature of 237°C. The device with a simple structure of indium tin oxide/TDPFPA/4,7-diphenyl-1,10-phenanthroline/LiF∕Al showed green emission with Commission Internationale de L’Eclairage coordinates of (0.24, 0.54), a current efficiency of 9.9cd∕A, and power efficiency of 10.6lm∕W.

High-Performance Blue OLEDs Based on Phenanthroimidazole Emitters via Substitutions at the C6- and C9-Positions for Improving Exciton Utilization.

Donor-acceptor (D-A) molecular architecture has been shown to be an effective strategy for obtaining high-performance electroluminescent materials. In this work, two D-A molecules, Ph-BPA-BPI and Py-BPA-BPI, have been synthesized by attaching highly fluorescent phenanthrene or pyrene groups to the C6- and C9-positions of a locally excited-state emitting phenylamine-phenanthroimidazole moiety. Equipped with good physical and hybridized local and charge-transfer properties, both molecules show high performances as blue emitters in nondoped organic light-emitting devices (OLEDs). An OLED using Ph-BPA-BPI as the emitting layer exhibits deep-blue emission with CIE coordinates of (0.15, 0.08), and a maximum external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 4.56 %, 3.60 cd A(-1) , and 3.66 lm W(-1) , respectively. On the other hand, a Py-BPA-BPI-based, sky-blue OLED delivers the best results among nondoped OLEDs with CIEy values of < 0.3 reported so far, for which a very low turn-on voltage of 2.15 V, CIE coordinates of (0.17, 0.29), and maximum CE, PE, and EQE values of 10.9 cd A(-1) , 10.5 lm W(-1) , and 5.64 %, were achieved, respectively. More importantly, both devices show little or even no efficiency roll-off and high singlet exciton-utilizing efficiencies of 36.2 % for Ph-BPA-BPI and 39.2 % for Py-BPA-BPI.

Pd-Au@carbon dots nanocomposite: Facile synthesis and application as an ultrasensitive electrochemical biosensor for determination of colitoxin DNA in human serum

In this study, a green and fast method was developed to synthesize high-yield carbon dots (CDs) via one-pot microwave treatment of banana peels without using any other surface passivation agents. Then the as-prepared CDs was used as the reducing agent and stabilizer to synthesize a Pd-Au@CDs nanocomposite by a simple sequential reduction strategy. Finally, Pd-Au@CDs nanocomposite modified glassy carbon electrode (Pd-Au@CDs/GCE) was obtained as a biosensor for target DNA after being immobilized a single-stranded probe DNA by a carboxyl ammonia condensation reaction. Under the optimal conditions, the sensor could detect target DNA concentrations in the range from 5.0×10-16 to 1.0×10-1°molL-1. The detection limit (LD) was estimated to be 1.82×10-17molL-1, which showed higher sensitivity than other electrochemical biosensors reported. In addition, the DNA sensor was also successfully applied to detect colitoxin DNA in human serum.

Ambipolar D–A type bifunctional materials with hybridized local and charge-transfer excited state for high performance electroluminescence with EQE of 7.20% and CIEy ∼ 0.06

Construction of donor–acceptor (D–A) molecules with a highly hybridized local and charge-transfer (HLCT) excited state has been shown to be an effective strategy to achieve the maximum electroluminescence (EL) efficiency through the synchronous harvest of high photoluminescence (PL) efficiency and exciton utilization. Herein, two novel D–A-structured bifunctional (emissive and hole-transporting) materials, PPI-2TPA and PPI-2NPA, have been designed and synthesized for application in deep-blue OLEDs. As revealed by theoretical calculations and comprehensive photophysical experiments, both of them exhibit significant HLCT excited-state characteristics and ambipolar properties. Using them as emitting layers (EML) in multilayer non-doped devices presents true deep-blue Commission Internationale de l’Eclairage (CIE) coordinates of ca. (0.15, 0.06), accompanied by record-setting performance with maximum external quantum efficiencies (EQEs) of 7.20% for PPI-2TPA and 6.33% for PPI-2NPA. Remarkably, the simple bilayer devices fabricated using them as non-dopant EML and hole-transporting layers (HTLs) still deliver EQEs as high as 4.69% and 4.10% with little changes in color purity (PPI-2TPA: CIE (0.150, 0.063) and PPI-2NPA: (0.152, 0.063)). To the best of our knowledge, this performance is the highest among the reported non-doped devices in this color gamut, irrespective of whether the two newly formed molecules functioned as EML or EML and HTL simultaneously.