A novel design of aggregation-induced emission (AIE) active columnar (Col) luminomesogens is reported, and they are demonstrated to act as highly efficient deep-blue emitters in organic light-emitting diodes (OLEDs). All derivatives exhibit Col liquid crystalline (LC) behavior at room temperature over a wide temperature range and desirable alignment properties, which is very important in using them as materials for organic electronic devices. These new AIE active luminomesogens were found to act as highly efficient emitters in OLEDs and unveiled a maximum external quantum efficiency of 4.0% for the first time in Col LCs with Commission International de l’E’clairage coordinates of (0.17, 0.07), which closely matches the National Television System Committee (NTSC) standard, corresponding to pure deep blue color. The detailed supramolecular assembly of the compounds has been characterized by modeling in the mesophase derived from small- and wide-angle X-ray scattering results.

Room-Temperature Columnar Liquid Crystals as Efficient Pure Deep-Blue Emitters in Organic Light-Emitting Diodes with an External Quantum Efficiency of 4.0.

Copolymers with graft architectures possess interesting material properties distinct from their linear polymer counterparts. The effects of multidimensional architectures on the optoelectronic and physical properties of all-conjugated graft copolymers are not well-known, thus providing a large incentive for their study. In order to readily access these materials (hypothesized to have “comb” architectures), it is extremely important to investigate the methods used in their synthesis. Here we study the graft-to synthesis of comb copolymers composed of polythiophene backbones and polyselenophene side chains and identify the opportunities and challenges associated with copolymer formation. Azide-functionalized polythiophene “backbones” and acetylene-terminated polyselenophene “side chains” were synthesized in a controlled fashion using Kumada catalyst-transfer polycondensation (KCTP) polymerization and grafted together using copper-catalyzed azide–alkyne click chemistry (CuAAC). 1H NMR, GPC, and FTIR results ...

Exploring the Graft-To Synthesis of All-Conjugated Comb Copolymers Using Azide–Alkyne Click Chemistry

To investigate the effect of unusual substitutions in the phenothiazine core toward the development of deep blue emitters, two compounds have been designed and synthesized by integrating electron donor and electron acceptor units in a nonconventional fashion. Compared with the conventional strategy, we brought two acceptor units in para-relationship to each other on the phenothiazine backbone. Noticeably, emission was shifted hypsochromically to 430 nm in compound 3 from 480 nm, as observed for compound 2. Theoretical studies also provided a deep insight into the excitation and emission properties of the studied compounds. Frontier molecular orbitals and natural transition orbitals revealed that the extent of conjugation in compound 3 was much limited compared to 2. Single crystal X-ray studies helped to predict the packing modes of these compounds in relation to the substitution pattern. The butterfly angle in phenothiazine was found to be the lowest in compound 3. The phenothiazine ring was found to attain a plane perpendicular to the plane of the rest of molecule in 3 which was responsible to delimit the intermolecular stacking at the molecular level. The solution processed devices gave external quantum efficiencies of 2.7% and 1.6% with Commission Internationale de L'Eclairage (CIE) coordinates of 0.16, 0.13 and 0.16, 0.09 for devices using 2 and 3 as the dopant in the host matrix respectively. These results indicate that the introduction of acceptors in para-relationship on the phenothiazine core is a promising design strategy towards deep blue emitters.

Trend breaking substitution pattern of phenothiazine with acceptors as a rational design platform for blue emitters

In this work, we employ the Corey–Fuchs reaction followed by coupling reactions to develop a series of π-conjugated aggregation-induced emission (AIE) small-molecule luminophores (DT2A, DT3A and DT4A) through a 2,2-(2,2-diphenylethene-1,1-diyl)dithiophene (DPDT) core with different amounts and different strengths of TPA peripheral moieties. Interestingly, these molecules give obviously higher solid fluorescent quantum efficiency and AIE phenomena. In particular, the thin film of DT3A exhibited the highest fluorescent quantum efficiency of ca. 25% and the DT2A showed the highest αAIE of 78. Moreover, the excellent optical waveguide applications of DT2A and DT4A were achieved because of their excellent self-assembly properties. Meanwhile, piezofluorochromic behavior with a large red shift of 35 nm only appeared when DT4A was ground using a pestle, because DT4A inserted more TPA and changed the twisting conformation. The piezofluorochromic behavior can be recovered to its original color by fuming with solvent. Finally, DT2A and DT4A were utilized as fluorescent probes to detect nitroaromatic/nitroaliphatic compounds and showed evident fluorescence quenching. These results indicate a huge potential to develop bright AIEgens based on DPDT core units and also provide insights into understanding how piezofluorochromism, optical waveguides and explosive detection properties are influenced by alternating the spatial symmetry of AIE materials with different numbers of TPA terminal groups.

Constructing small molecular AIE luminophores through a 2,2-(2,2-diphenylethene-1,1-diyl)dithiophene core and peripheral triphenylamine with applications in piezofluorochromism, optical waveguides, and explosive detection

Blue organic luminescent materials are highly desired for high-quality full-color displays, but are still quite insufficient. Herein, two new luminescent molecules, 4-(10-(3-(9H-carbazol-9-yl)phenyl)-2,6-di-tert-butylanthracen-9-yl)benzonitrile (mCz-TAn-CN) and 4-(2,6-di-tert-butyl-10-(3,5-di(9H-carbazol-9-yl)phenyl)anthracen-9-yl)benzonitrile (m2Cz-TAn-CN), are designed and synthesized. Their photophysical properties, electronic structures, electrochemical behaviors, thermal stabilities and electroluminescence performances are comprehensively studied. The non-doped organic light-emitting diode (OLED) based on mCz-TAn-CN provides stable deep-blue light [Commission Internationale de l’Eclairage (CIEx,y) = 0.14, 0.12] and a maximum excellent external quantum efficiency (ηext,max) of 7.03%, contributed by the triplet–triplet annihilation process. The doped OLED of m2Cz-TAn-CN radiates bluer light (CIEx,y = 0.14, 0.09) with an outstanding ηext,max of 7.28%. These results demonstrate that both new molecules are promising candidates for application in deep-blue OLEDs.

Highly efficient deep-blue fluorescent OLEDs based on anthracene derivatives with a triplet–triplet annihilation mechanism

Four new cross-conjugated small molecules based on a central benzo[1,2-d:4,5-d′]bisoxazole moiety possessing semi-independently tunable HOMO and LUMO levels were synthesized and the properties of these materials were evaluated experimentally and theoretically. The molecules were thermally stable with 5% weight loss occurring well above 350 °C. The cruciforms all exhibited blue emission in solution ranging from 433–450 nm. Host–guest OLEDs fabricated from various concentrations of these materials using the small molecule host 4,4′-bis(9-carbazolyl)-biphenyl (CBP) exhibited deep blue-emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15 ≤ x ≤ 0.17, 0.05 ≤ y ≤ 0.11), and maximum luminance efficiencies as high as ∼2 cd A−1. These results demonstrate the potential of benzobisoxazole cruciforms as emitters for developing high-performance deep blue OLEDs.

/pdf/benzobisoxazole-cruciforms-a-tunable-cross-conjugated-4h93io6pd1.pdf

Benzobisoxazole cruciforms: a tunable, cross-conjugated platform for the generation of deep blue OLED materials

Six 2,4,6,8-tetrarylbenzo[1,2- d:4,5- d']bisoxazoles (BBOs) were synthesized: three bearing phenyl substituents at the 2- and 6-positions and three bearing perfluorophenyl groups at those positions. The influence of perfluoro-aryl group substitution on the physical, optical, and electronic properties of 2,4,6,8-tetrarylbenzo[1,2- d:4,5- d']bisoxazoles (BBO) was evaluated using both experimental and theoretical methods. The density functional theory (DFT) model was found to be well-matched to the experimental optical data, as evidenced by the UV-vis spectra. Both cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS) were used to determine the position of the HOMO with varying results. The values obtained by CV were deeper than those obtained via UPS and correlated well with the theoretical calculations. However, the UPS values were more consistent with the expected outcomes for a system with segregated frontier molecular orbitals (FMOs). The UPS results are also supported by the electrostatic potential maps, which indicate that the electron density within the LUMO and HOMO is nearly completely localized along the 2,6- or 4,8-axis, respectively. The summation of the results indicates that strongly electron-withdrawing groups can be used to selectively tune the LUMO level with minor perturbation of the HOMO, something that is challenging to accomplish in typical donor-acceptor systems.

Evaluating the Impact of Fluorination on the Electro-optical Properties of Cross-Conjugated Benzobisoxazoles

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Benzobisoxazole cruciforms: a tunable, cross-conjugated platform for the generation of deep blue OLED materials

Evaluating the Impact of Fluorination on the Electro-optical Properties of Cross-Conjugated Benzobisoxazoles