Highly twisted pyrene derivatives for non-doped blue OLEDs
Introduction
Organic light-emitting diodes (OLEDs) have attracted considerable attention or their emission properties at wide viewing angle angles and their application to flat-panel displays [1], [2], [3], [4], [5], [6]. Blue, green and red emitters with have high emission efficiency and high color purity are required for full color display applications. Green and red emitters are usually used in phosphorescent systems, while blue emitters are still used in fluorescent systems, so developing effective blue OLEDs is an important part of developing these systems. The color purity of blue emitters can affect power consumption and is an important issue for researchers [7], [8], [9], [10], [11]. It is very difficult to develop color pure and efficient blue emitters due to the wide band gap required, regardless of the type of material.
Research has revealed that host-guest systems experience some problems such as phase separation upon heating, complexity due to addition of dopants, and the high cost of mass production, despite improved EL efficiency [12], [13]. Non-doped emitting layer systems may therefore have advantages over host-guest systems.
Pyrene has strong π electron delocalization energy and efficient fluorescence properties due to its large planar conjugated aromatic characteristics [14]. Some pyrene derivatives have been used in OLEDs to improve hole transport through their electron-rich characterization [15].
Recently, our group reported anthracene derivatives with a xylene group, with highly twisted and rigid non-planar structures, which prevents the close-packing of molecules in the solid state and increases efficiency through reduced vibronic coupling. These anthracene derivatives exhibited high efficiency and color-pure blue emission [11], [16].
In this study, we designed and developed new pyrene derivatives containing xylene units, specially, 1,6-bis(2,5-dimethyl-4-naphthalene-2-yl)phenyl)pyrene (BDNP) and 1,6-bis(2,5-dimethyl-4-phenyl)phenyl)pyrene (BDPP). BDNP and BDPP which have bulky, rigid non-coplanar structures due to steric tortional hindrance of the 2,5-dimethyl phenyl (xylene) unit, are expected to demonstrate efficient and color-pure blue emission.
Section snippets
Materials
All starting materials were purchased from Aldrich and TCI. Pd catalyst was purchased from Umicore. All starting materials were used without further purification. 3,4-Dibromothiophene-1,1-dioxide and 2,3-dibromoanthraquinone were prepared according to literature procedures [17]. All solvents were further purified prior to use.
Instruments
1H NMR spectra were recorded using a Bruker Avance 300 MHz FT-NMR spectrometer, and chemical shifts (ppm) were reported with tetramethylsilane (TMS) as an internal
Synthesis and characterization
Synthetic scheme for the two pyrene derivatives, BDNP and BDPP, are shown in Scheme 1. BDNP and BDPP were synthesized using a simple twofold by two times Suzuki coupling reaction. Phenyl boronic acid and naphthyl boronic acid were reacted with 1,4-dibromo-2,5-dimethylbenzene to give 1-bromo-2,5-dimethyl-4-phenylbenzene and 1-bromo-2,5-dimethyl-4-naphthylbenzene, respectively. The Suzuki coupling reaction was employed between monobromide and pyrene boronic ester to obtain the target compounds.
Conclusions
We designed and developed new deep blue light-emitting materials composed of a pyrene core with highly twisted xylene units. The non-coplanar structure due to steric hindrance of the introduced substituent resulted in very deep blue emissions with a maximum EL peak of 436 nm and narrow FWHM of 50 nm. In particular, the device using BDPP exhibited a peak EQE of 3.69% with CIE color coordinates (x, y: 0.15, 0.06).
Acknowledgment
The authors acknowledge the Industrial Strategic Technology Development Program (10045269, Development of Soluble TFT and Pixel Formation Materials/Process Technologies for AMOLED TV) funded by MOTIE/KEIT.
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These authors are equally contributors.