Removing shortcomings of linear molecules to develop high efficiencies deep-blue organic electroluminescent materials
Graphical abstract
Introduction
Development of deep-blue emitters for OLEDs (organic light-emitting diodes) has always been a hot topic of research [1], [2]. Because efficient deep-blue emitters are indispensable for high performance white OLEDs and critical for improving color gamut as well as reducing power consumption of full-color displays [3], [4]. However, compared to the other two primary colors, green and red, progress on the development of blue emitter is lagging behind [5], [6].
Ideal deep-blue materials should possess outstanding photophysical and electrical properties, including deep-blue emission, high photoluminescent quantum yield (ϕPL) and good carriers injection/transport ability [6], [7]. It is reported that ϕPL increase with the molecular conjugation length in similar molecular systems [8], [9], [10]. For example, biphenyl shows a ϕPL of only 2% in solution, while when it linearly extended to p-quinquephenyl the corresponding figure is 91% [10]. Similar phenomena are observed in solid state. Fluorophore TPE presented a ϕPL of only 14.63%, but high ϕPL of 50% can be achieved in conjugation-extended DPBPPE [11]. Further extending molecule to be BTPE also leads to higher ϕPL [12]. The excellent electroluminescent materials building block PPI exhibits a ϕPL of 40% in thin film of itself. But the ϕPL of its “two-in-one” analogue BPPI is more than double, up to 85% [13]. In terms of electrical properties, linear extension of π system is conducive to synergistically raising the highest occupied molecular orbital (HOMO) and lowering the lowest unoccupied molecular orbital (LUMO). Thus linear system is able to facilitate the injection/transport of hole and electron [14]. In addition to the above-mentioned attractive merits, linear molecules tend to present high proportion of horizontal dipole in thin-film, which is beneficial to improve their devices efficiencies with enhancing out-coupling efficiencies [15], [16]. However, molecular emission will be red-shifted with the extension of molecular conjugation. Another drawback in solid state is that linear molecular structure increase the probability of intermolecular π-π stacking [16], [17], which often results in spectral bathochromic shifts and even considerably quenched fluorescence.
With the aim of tackling the disadvantages of linear molecule, herein, we used molecular twisting and substitution of bulky groups in a linear molecular system of biphenantro[9, 10-d]imidazole, designed and synthesized two linear molecule, named 2,2'-(2-methyl-[1,1'-biphenyl]-4,4'-diyl)bis(1-phenyl-1H-phenantro[9, 10-d]imidazole) and 2,2'-(2-methyl-[1,1'-biphenyl]-4,4'-diyl)bis(1-(4-(tert-butyl)phe-nyl)-1H-phenanthro [9,10-d]imidazole) (BiPI-1 and BiPI-2, Fig. 1), as deep-blue emitters for OLED applications. A methyl group was added to the biphenyl bridge which links the two phenantroimidazole (PI) fluorophores, aiming at adding an appropriate torsional angle between the two PI planes without breaking the linear π conjugated system. High conjugation extent maintains the high ϕPL of the materials and the twisted structure should help in avoiding π-π stacking in solid state. A bulky tert-butyl group (BiPI-2) is added to each of the fluorophore unit to further suppress intermolecular interaction. With the two materials as emitters, non-doped OLEDs achieved standard deep-blue emissions with the same Commission internationale de l'éclairage (CIE) coordinates of (0.15, 0.08) and high electroluminescent efficiencies of maximum current efficiencies (CE) of 4.62 and 3.38 cd A−1, power efficiencies (PE) of 4.55 and 3.44 l m W−1 and external quantum efficiencies (EQE) of 6.18% and 4.52% for BiPI-1- and BiPI-2-based device respectively. In the following sections, we would like to report the synthesis methods, thermal stabilities, photophysical and electrical properties in detail.
Section snippets
General methods
All reagents and solvents were purchased from commercial sources and used as received without further purification unless otherwise stated. 1H NMR was recorded with a Varian Gemin-400 spectrometer. Mass spectra were recorded on a PE SCIEX API-MS spectrometer. Elemental analysis (C, H, N) was performed using a Vario EL III CHNS elemental analyzer. UV–vis absorption and photoluminescence (PL) spectra were measured on a Perkin-Elmer Lambda 950 UV/vis Spectrometer and a PerkinElmer LS50
Synthesis and characterization
Two simple reactions are employed to synthesize these new compounds (Scheme 1), including a Suzuki coupling reaction and a “one-pot” reaction [20], [21] for PI construction, with easy purification processes and satisfying yields. Characterization information are summarized in experimental section.
Thermal property
To check the thermal stabilities of BiPI-1 and BiPI-2, TGA and DSC measurements were performed under a N2 atmosphere. As presented in Fig. 2 and Table 1, these two materials all possess a high
Conclusion
In conclusion, using molecular twisting and bulky side group substitution, we successfully develop two deep-blue emitters, BiPI-1 and BiPI-2. These materials retain the excellent photophysical properties of planar analogue and remarkably supress redshifts in solid state emission. Besides, the two materials presented outstanding bipolar feature in electrical aspect, Especially, BiPI-1 can simultaneously balance both optical and electrical needs, that is, deep-blue emission and high carrier
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Project No. 51473138 and 51273108) and the National Basic Research Program of China (973 Program No. 2013CB834803).
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These authors contributed equally to this work.