Shiyan Chen

Organic thin film transistors based on stable amorphous ladder tetraazapentacenes semiconductors

Three ladder linear 5,7,12,14-tetraazapentacenes (1–3) have been synthesized by an improved condensation reaction with high yields. These tetraazapentacenes show higher thermal and optical stability in solution and solid films compared to pentacene. Organic thin film transistors (OTFTs) based on amorphous films of 5,7,12,14-tetraazapentacene are investigated in this contribution. A field-effect mobility of a stable amorphous OTFT in air up to 2 × 10−2 cm2 V−1 s−1 has been achieved. The high stability and high mobility in the amorphous state of ladder compounds 1–3 combined with the easy synthesis make them a new family of promising candidate for organic electronic devices.

Synthesis and characterization of a quinoxaline compound containing polyphenylphenyl and strong electron-accepting groups, and its multiple applications in electroluminescent devices

We report a novel electroluminescent material, 6,7-dicyano-2,3-di-[4-(2,3,4,5-tetraphenylphenyl)phenyl]quinoxaline (CPQ), which can be used as a multifunctional material in organic light-emitting diodes (OLEDs), serving as an effective emitting, electron-transporting, and color-tuning layer. OLEDs employing CPQ as both the emissive and electron transporting layer give bright bluish-green light emission and show a high luminous efficiency of 4.7 cd A−1. In addition, multicolor OLEDs based on CPQ and various hole transport materials (HTMs) were fabricated. We successfully achieved an effective color tuning of emission, from bluish-green through yellowish-green and yellow to reddish-orange, in OLEDs by varying HTMs. Our results provide a simple and effective approach to construction of highly efficient and multicolored OLEDs.

Photophysical properties of polyphenylphenyl compounds in aqueous solutions and application of their nanoparticles for nucleobase sensing

Abnormal photoluminescent properties of polyphenylphenyl compounds 6,7-dicyano-2,3-di-[4-(2,3,4,5-tetraphenylphenyl)phenyl]quinoxaline (CPQ) and 6,7-dimethyl-2,3-di-(4-(2,3,4,5-tetraphenylphenyl)phenyl)quinoxaline (MPQ) in aqueous solutions have been investigated. Photoluminescent emissions of CPQ and MPQ in tetrahydrofuran (THF)–water mixtures do not change monotonously with increasing their effective concentration in solution. This phenomenon is different from both aggregation-induced emission quenching and aggregation-induced emission enhancement, and can be ascribed to the results of combinational effects of intramolecular rotation, intermolecular hydrogen bonds, and solvent viscosity and hydration. We also observed that organic nanoparticles of CPQ and MPQ are formed in the aqueous solutions. Based on detection of the fluorescence of CPQ nanoparticles in aqueous solutions when introducing nucleobases, we report for the first time the application of organic nanoparticles for nucleobase sensing and found that CPQ nanoparticles can recognize the nucleobases with a sensitivity of guanine > adenine > thymine ≥ cytosine. These findings elucidate the photoluminescent behavior of polyphenylphenyl compounds in aqueous solutions and provide an approach to apply fluorescent organic nanoparticles for biosensing instead of metal and inorganic nanoparticles.

Highly efficient blue electrophosphorescent devices with a new series of host materials: polyphenylene-dendronized oxadiazole derivatives

A new series of oxadiazole derivatives TPO, PPO, MTPO, MPPO, BTPO, and BPPO have been designed and synthesized in high yields by a convenient synthetic procedure. A single crystal structure of TPO exhibited that this polyphenylene-dendronized material has a sterically crowded structure which efficiently prevents π–π stacking and endows it with good thermal stability. The photophysical properties of the materials show strong emission between 376 and 390 nm in the films. In addition, good reversible reductive waves in the cyclic voltammograms implied that these materials might have good electron transport properties. Blue electrophosphorescent devices fabricated using these materials as the host materials and iridium(III)bis[4,6-di-fluorophenyl-pyridinato-N,C2′]picolinate as the blue phosphorescent dopant emitter show very high efficiencies. A maximum luminance of 4484 cd m−2 and an external quantum efficiency of 6.20% were achieved under ambient conditions.

A non-planar pentaphenylbenzene functionalized benzo[2,1,3]thiadiazole derivative as a novel red molecular emitter for non-doped organic light-emitting diodes

A novel non-planar pentaphenylbenzene functionalized benzo[2,1,3]thiadiazole derivative (BPTBTD) has been designed and synthesized with only three steps, and applied into non-doped organic light-emitting diodes (OLEDs) as a red molecular emitter. The molecule exhibits good solubility in common organic solvents, excellent thermal stability and film-forming properties. The most important feature is that photoluminescence of BPTBTD shows excellent stablity in different solvents, at different concentrations and in different solid states. Red organic light-emitting diodes were fabricated in a facile non-doped configuration with different thicknesses of emission layers. Saturated red-emission was observed with an emission peak at 621 nm, a maximum external quantum efficiency of 1.0% and a maximum brightness of 1572 cd m−2. Especially, the properties of devices with thick emission layers are two times better than those of the devices with thin emission layers. These results indicate that the non-planar pentaphenylphenyl functional groups substantially suppressed the tendency for molecular aggregation in thin solid films, leading to a very stable photoluminescence and higher efficiency of devices with thicker layers.

Fabrication and characterization of molecular scale field-effect transistors

Molecular electronics are considered one of the most promising ways to meet the challenge of micro-electronics facing its scaling down pathway. Molecular devices, especially molecular scale field-effect transistors (MSFET), are key building blocks for molecular electronics. Three major hurdles to device fabrication are yet to be overcome: electrode pairs must be fabricated with a controllable gap size commensurate with the functional molecule size of interest; the molecules of interest must be arranged between the electrodes with precise location and orientation control; and stable, conducting contacts must be made between the molecules and the electrodes. We have combined “top-down” and “bottom-up” approaches to solve these problems. Using photolithography and molecular lithography with self-assembled mono/multiple molecule layer(s) as a resist, we fabricated electrode structures with a controllable molecular-scale gap between source and drain electrodes and a third terminal of a buried gate. For our device, we synthesized a thiolated phthalocyanine derivative molecule, {di-[1-(S-acetylthio)-4-ethynylphenyl]-di-(tert-butyl)phthalocyanato}copper(II), with acetylthio groups on both ends, conjugated with ethynylphenyl groups. The synthesized end-thiolated molecules were assembled between the tailored molecular gap of the as-fabricated FET electrode structures in solution via Au–S bonding, forming stable contacts between the electrodes and the molecules, and a 3 terminal MSFET device was formed. Electrical measurements show that the device has characteristics of a typical FET device. The field-effect mobility of the as-fabricated MS-FET is 0.16 cm2 V−1 s−1.