Hongji Jiang

Cruciform p-n diblock conjugated oligomers for electroluminescent applications

The optoelectronic properties of the cruciform p–n diblock oligomers 1–3 can be independently tuned by the oligofluorene and oxadiazole branches to increase the charge mobility.

Synthesis and characterization of heteroatom substituted carbazole derivatives: potential host materials for phosphorescent organic light-emitting diodes

A series of carbazole derivatives based on 9-ethyl-carbazole substituted at the 3-position with diphenylamine, carbazole, phenoxazine, phenothiazine, and phenothiazine-S,S-dioxide units were successfully developed, and a comparative study of their thermal and photoelectrical properties was carried out by differential scanning calorimetric measurements, thermogravimetric analysis, UV-vis absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, and phosphorescent spectroscopy. All the synthesized compounds were found to have high thermal decomposition temperature in the range of 304–344 °C (except for C-SO, 145 °C) and showed a remarkable improvement when compared with 9-ethyl-carbazole (180 °C). The emission properties can be effectively tuned by systematically changing the substituent units. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and triplet energy (ET) of the synthesized compounds were determined both theoretically by quantum chemical calculations and experimentally by examination. Their energy levels can also be tuned by the change of the moieties connected to the 3-position through coupling reaction. The experimental values of ionization potentials range from 5.37 to 5.46 eV. Some of these compounds exhibit high ET, and C-SO achieves the highest ET value of 2.79 eV, which exhibits a marked improvement over that of 4,4′-bis(9-carbazolyl)-biphenyl (2.6 eV). Green phosphorescent devices using tris(2-phenylpyridine)iridium as a guest and C-O as a host show excellent performances. Remarkably, a device with C-O as host exhibits the best performance with a maximum luminance of 10270 cd m−2, a maximum current efficiency of 34.8 cd A−1, and a maximum power efficiency of 26.0 lm W−1. These results illustrate that the introduction of heteroatoms into carbazole can result in a significant change in their optoelectronic characteristics. The low LUMO, high HOMO, suitable ET and thermal stability of the carbazole derivatives endow them with the potential to be green, red, and even blue host materials for phosphorescent organic light-emitting diodes.

Efficient synthesis and photovoltaic properties of highly rigid perylene-embedded benzothiazolyls

Perylene-embedded benzothiazolyls, designed by 2D-fusing of dithienylbenzothiadiazoles (DTBT), have been synthesized and characterized for organic photovoltaic (OPV) applications. A new polymer based on the newly-developed electron-accepting building block exhibits excellent solubility, stability, and photoabsorbability with optimal bandgaps and frontier orbitals, achieving a power efficiency up to 3.22%.

Synthesis and characterization of a novel spirocyclic aromatic derivative: unique roles of phenothiazine

In order to investigate the explicit effects of phenothiazine on the general properties of a spirocyclic aromatic derivative with a sterically perpendicular configuration for electro-optical applications, a novel spirocyclic aromatic derivative SPIS, based on fluorene and phenothiazine, was synthesized and fully characterized by 1H NMR, 13C NMR, elemental analysis and matrix assistant laser desorption/ionization time-of-flight mass, respectively. The photophysical properties, thermal stability and energy levels of compound SPIS were further compared with those of 2,7-dibromo-9,9,-diethyl fluorene. The experimental results indicated that compound SPIS displayed a high thermal stability with a decomposition temperature of 386 °C at 5% mass loss and a melting temperature of 221 °C, which was a notable improvement compared to those of 2,7-dibromo-9,9,-diethyl fluorene. Compound SPIS took on a band gap of about 2.63 eV with blue-green emission in the thin solid film. The introduction of phenothiazine into the spirocyclic aromatic derivative can lower the lowest unoccupied molecular orbital level to −3.05 eV, while increasing the highest occupied molecular orbital level to −5.68 eV relative to those of 2,7-dibromo-9,9,-diethyl fluorene. In conclusion, it is obviously illustrated that the introduction of a rigid segment of phenothiazine into the skeleton of a spirocyclic aromatic derivative can improve the thermal stability, and effectively adjust the photophysical properties and energy levels of fluorene-based monomers over a wide range.