We report the synthesis, optical and electrochemical properties, as well as the fabrication of light-emitting devices for a new twistacene 1,2,3,4,6,13-hexaphenyl-7 : 8,11 : 12-bisbenzo-pentacene (HBP 1). Its structure, determined by X-ray crystallography, confirmed that this material has a twisted topology with the torsion angle as high as 23.0 . HBP 1 showed bright green emission both in solution and solid state. The HOMO-LUMO gap of HBP 1 calculated from the difference between the half-wave redox potentials (E 1/2 ox ¼ +0.74 eV and E 1/2 red ¼ À1.93 eV) is 2.67 eV, which is in good agreement with the band gap, 2.64 eV, derived from the UV-Vis absorption data. Organic light emitting devices using HBP 1 as the emitters have been fabricated. The results revealed that twistacenes are promising materials to enhance the efficiency of OLEDs.
We report a hybrid, quantum dot (QD)-based, organic light-emitting diode architecture using a noninverted structure with the QDs sandwiched between hole transporting layers (HTLs) outperforming the reference device structure implemented in conventional noninverted architecture by over five folds and suppressing the blue emission that is otherwise observed in the conventional structure because of the excess electrons leaking towards the HTL. It is predicted in the new device structure that 97.44% of the exciton formation takes place in the QD layer, while 2.56% of the excitons form in the HTL. It is found that the enhancement in the external quantum efficiency is mainly due to the stronger confinement of exciton formation to the QDs.
An asymmetric twistacene, 1',4'-diphenyl-naphtho-(2'.3':1.2)-pyrene-6'-nitro-7'-methyl carboxylate (tetracene 2), was synthesized by using benzyne-trapping chemistry. Its structure, determined by X-ray crystallography, confirmed that this material has a twisted topology with torsion angles as high as 23.8(3) degrees. Organic light-emitting devices using tetracene 2 as either charge-transporting materials or emitters have been fabricated. The results indicate that this material has bipolar transporting behavior in these devices.
We studied the quenching mechanisms responsible for the low efficiency of thin film phosphorescence by a specially designed organic light-emitting diode with an emission layer consisting of a few repeating cells made of a sequentially evaporated host and guest. Variation of the thickness of the guest layer in each cell enables the study of the effect of molecule aggregation on the quantum efficiency. On the other hand, variation of the thickness of the host layer reveals a new long-range quenching mechanism involving a Förster-like dipole-dipole interaction. The quantitative analysis shows that the external quantum efficiency as a function of the host layer thickness follows the characteristic of the long-range Förster process. Our study provides a new understanding of quenching mechanisms in phosphorescent material and extends the existing knowledge on long-range energy transfer.
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