Jinshan Wang

Fluorescent Organic Nanoparticles Based on Branched Small Molecule: Preparation and Ion Detection in Lithium-Ion Battery

Fluorescent organic nanoparticles (FONs) as a new class of nanomaterials can provide more advantages than molecule based probes. However, their applications in specific metal ion detection have rarely been exploited. We design and synthesize a branched small-molecule compound with triazole as a core and benzothiadiazole derivative as branches. By a facile reprecipitation method, nanoparticles (NPs) of this compound can be prepared in aqueous solutions, which can show high selectivity and sensitivity to Fe(III) ions based on fluorescence quenching. In addition, the fluorescence intensity of these NPs is resistant to pH changes in solutions. Such characters of this kind of NPs can be utilized in Fe(3+) impurity detection in a promising cathode material (LiFePO4) for lithium ion batteries. When exposed to Fe(3+), both the triazole and benzothiadiazole groups contribute to the fluorescence quenching of NPs, but the former one plays a more important role in Fe(3+) impurity detection. The sensing mechanism has also been investigated which indicates that a Fe-organic complex formation may be responsible for such sensing behavior. Our findings demonstrate that specific metal ion detection can be realized by FONs and have extended the application field of FONs for chemical sensing in aqueous solutions.

Fluorescence Resonance Energy Transfer in a Binary Organic Nanoparticle System and Its Application

Fluorescent organic nanoparticles have a much better photostability than molecule-based probes. Here, we report a simple strategy to detect chemicals and biomolecules by a binary nanoparticle system based on fluorescence resonance energy transfer (FRET). Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO, energy donor) and poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV, energy acceptor) are utilized to prepare the binary nanoparticle system through a reprecipitation method. Since the FRET process is strongly distance-dependent, a change in the interparticle distance between the two kinds of nanoparticles after introduction of analytes will alter the FRET efficiency. The response of the binary nanoparticle system to cationic polyelectrolytes was investigated by monitoring the FRET efficiency from PFO to MEH-PPV nanoparticles and the fluorescence color of the nanoparticle solutions. Furthermore, the cationic polyelectrolyte pretreated binary nanoparticle system can be used to detect DNA by desorption of nanoparticles from the polyelectrolytes chains and the detection concentration can go down to 10(-14) M. Thus, the binary nanoparticle system shows great promise for applications in chemical sensing or biosensing.

Exploring the application of conjugated polymer nanoparticles in chemical sensing: detection of free radicals by a synergy between fluorescent nanoparticles of two conjugated polymers

Two classical conjugated polymers, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), commonly used in organic optoelectronic devices are endowed with a new function for radical sensing. The synergetic effect between MEH-PPV and PFO nanoparticles (NPs) plays an important role in the detection of hydroxyl radicals and sulfate anion radicals. When exposed to free radicals, MEH-PPV NPs are subjected to attack by the radicals and undergo molecular structure changes. Thus, a strong-polarity shell can be formed on the radical treated MEH-PPV NPs. When such radical treated MEH-PPV NPs come into close contact with PFO NPs, a new phenomenon whereby PFO alters its fluorescence emission intensity of the 0–2 transition relative to the 0–0 transition band is observed. The relative intensity ratio of these two transition bands can serve as an index for the hydroxyl radical concentration. Therefore, radical detection can be realized by measuring the solid state fluorescence, which is highly desired in off-site laboratory determination since solid samples are more convenient for storage and transport than solutions. Our results can open a way for the application of conjugated polymer nanoparticles in chemical/biological sensing.

Improved efficiency in polymer light-emitting diodes using metal-enhanced fluorescence

Metal-enhanced fluorescence was realized in the emissive layer of organic electroluminescent devices. Core-shell Au nanoparticles (Au@SiO2) doped into the emissive layer of polymer light-emitting diodes (PLEDs) were used to enhance the luminous efficiency by a factor of 1.6 relative to the undoped reference devices (from 6.3 cd/A to 10.0 cd/A). The silica shell outside the Au nanoparticles was used to ensure that there was sufficient distance between the Au nanoparticles and the fluorescent polymer material to avoid quenching of the excitons. In addition, sufficient overlap of the energy of the localized surface plasmon resonance of the Au nanoparticles and the energy of the excitons formed in the emissive layer was guaranteed. These led to an enhanced PLED efficiency. This research provides a way to obtain high performance organic electroluminescent devices.

Solution-processed oxadiazole-based electron-transporting layer for white organic light-emitting diodes

A novel alcohol-soluble electron-transporting small-molecule material comprising oxadiazole and arylphosphine oxide moieties, ((1,3,4-oxadiazole-2,5-diyl)bis(4,1-phenylene))bis(diphenylphosphine oxide) (OXDPPO), has been synthesized and characterized. Its single crystal structure, together with the photophysical, electrochemical and thermal properties, has been investigated. This material not only possesses a wide bandgap with a low HOMO level but also exhibits a strong π–π stacking with a distance of 3.35 A. Moreover, this compound shows excellent thermal stability with a high glass transition temperature of 104 °C and a decomposition temperature of 384 °C. The unique solubility in 2-propanol makes it a good candidate for fabricating fully solution-processed multilayer organic light-emitting diodes (OLEDs). Efficient solution-processed white OLEDs have been fabricated with this compound as an electron-transporting layer (ETL). It was found that this ETL can greatly balance the electrons and holes in devices with the high work-function metal cathode (Al) and an increase in luminous efficiency of ∼70-fold can be achieved. The maximum luminous efficiency of devices with an ETL/Al configuration is even higher than that of devices using a Ca cathode.

Solution processed blue phosphorescent organic light emitting diodes using a Ge-based small molecular host

Two kinds of host materials, 4,4′-(diphenylgermanediyl)bis(N,N-diphenylaniline) and bis(4-(9H-carbazol-9-yl)phenyl)diphenylgermane (DCzGe), for blue phosphorescent organic light emitting diodes (PhOLEDs) were designed by incorporating electron donating groups (carbazole and triphenylamine) into tetraphenylgermane, which is a new type of core moiety that has never been studied for use in this field. This molecular structure endows the compounds with a wide energy bandgap, high thermal/morphological stability and good solution processability. Based on the theoretic calculations, DCzGe was selected and synthesized as a host material which demonstrates a wide bandgap (Eg: 3.56 eV) and a high triplet energy (ET: 3.02 eV). It also exhibits a high glass transition temperature (110 °C), which is beneficial for resisting the Joule heat in devices. All solution processed, blue emitting PhOLEDs were fabricated by using a mixed host combining DCzGe and an electron-transporting material, with a maximum luminance of 10 000 cd m−2 and a maximum current efficiency of 15.2 cd A−1. Furthermore, the devices showed a very low current efficiency roll-off, which remained as high as 15.2 cd A−1 at the luminance of 1000 cd m−2, and the roll-off is only 2.6% even at the higher luminance of 2000 cd m−2.

An air-stable microwire radial heterojunction with high photoconductivity based on a new building block

Organic semiconductor materials with one-dimensional (1D) radial (core–shell) heterojunction structures are highly desired for their expected excellent optoelectronic properties. However, currently, such structures are still in a fledgling period for optoelectronic applications due to the absence of both good materials and suitable preparation methods. Here we have synthesized a p-type organic semiconductor based on a new electron-donating unit (dithienopyrazine) and utilized it as a shell material to construct organic 1D radial p–n heterojunctions. This p-type compound shows a higher oxidation potential and is more resistant to photooxidation in air than its analogs with the commonly-used benzodithiophene unit. Moreover, we prepared organic microwires with radial heterojunctions via a solution-processed method by self-assembly of our p-type material on the surface of n-type cores. Thus, photoconductive devices based on an individual microwire with the radial heterojunction can be fabricated and demonstrate a high photoconductivity. Our work provides a path for preparing 1D radial heterojunctions suitable for optoelectronic applications.

Obtaining highly efficient single-emissive-layer orange and two-element white organic light-emitting diodes by the solution process

By attaching two electron-withdrawing trifluoromethyl (CF3) groups to the 2-phenylbenzothiazole cyclo-metalated ligand, a bis-trifluoromethyl-functionalized orange-emitting phosphorescent iridium(III) complex bis-(6-(trifluoromethyl)-2-(4-(trifluoromethyl)phenylbenzothiozolato))iridium(acetylacetonate) [(F3BT-CF3P)2Ir(acac)] was successfully synthesized. The optical, electrochemical and electroluminescence (EL) properties of this new complex were studied. The experimental results support the theoretical expectation that incorporating electron-withdrawing trifluoromethyl groups at the 4-site of the phenyl ring directly bonded to the metal center, and at the 6-site of 2-phenylbenzothiazole, cause a bathochromic shift in the emission peak and bring the emission color much closer to long-wavelength orange light. Moreover, such trifluoromethyl substituents can hinder the π–π stacking or self-polarization effect occurring from the aggregation of the molecules. The new iridium complex gives an unchanged luminescence spectrum, regardless of whether it is in solution, in untreated film or in film doped at different concentrations. Using this iridium complex as a dopant emitter, solution-processed single emissive layer orange and two-element white OLEDs with good performance can be obtained. Highly efficient orange electroluminescence was obtained with a maximum efficiency of 10.5 cd A−1 and CIE coordinates (0.48, 0.51). When combined with a commercial sky-blue phosphorescent emitter, (CF3BT–CF3P)2Ir(acac) can be utilized to achieve two-element white OLEDs that exhibited a high efficiency of 28.3 cd A−1. Such OLEDs retain high efficiency at a luminance suitable for lighting (e.g. 5000 cd m−2).

Design, synthesis and characterization of a new blue phosphorescent Ir complex

Being incompatible with host materials in a physically blended emitting layer, phosphorescent dyes are prone to form aggregates induced by Joule heat in devices under work. In this work, a new and efficient blue phosphorescent dye Cz-C8-FIrpic was designed and synthesised by incorporating 9-phenyl-9H-carbazole into a commonly used blue emissive iridium complex bis(4,6-(difluorophenyl)pyridine-N,C2′)picolinate (FIrpic) via an alkyl chain linkage. This phosphorescent dye exhibits similar photophysical properties to the units of FIrpic and 9-phenyl-9H-carbazole in solutions. In solid films of Cz-C8-FIrpic, the energy transfer from 9-phenyl-9H-carbazole to FIrpic units is effective. The Cz-C8-FIrpic doped emissive layer was investigated by AFM, STEM-EDS, transient photoluminescence decay curves and molecular dynamics simulations. The results show that in the Cz-C8-FIrpic doped film the phase aggregation of FIrpic units is less severe than that in the typically used FIrpic film. In addition, the optimized Cz-C8-FIrpic based device achieved a maximum luminance of 25 142 cd m−2, a maximum EQE of 8.5% and a maximum current efficiency of 22.5 cd A−1 which is about 15% higher than that of the control device based on FIrpic. We conclude that grafting a typically used dye to functional groups with alkyl chains is useful to restrict phase separation in physically blended emitting layers, and thus can achieve high electroluminescence performances.

Piezoelectric properties and structural phase transitions of naturally polarized (Na0.75K0.25Bi)0.5TiO3 crystal

Electrical properties of a naturally polarized (Na0.75K0.25Bi)0.5TiO3 crystal have been investigated over a broad range of temperatures. Noticeable piezoelectric resonance signals have been observed up to 540°C. Based on the piezoelectric measurement, we suggest a R3c-P4bm-Pm3¯m sequence of structural phase transitions in this crystal. Moreover, at temperatures above 520°C, mobile sodium ions in the crystal are proposed to contribute to the electrical conduction; they have an activation energy of 2.06eV. This migration of Na+ at high temperatures can explain the naturally generated piezoelectricity in (Na0.75K0.25Bi)0.5TiO3 crystal.