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Room temperature Discotic Liquid Crystalline Triphenylene-Pentalkynylbenzene dyads as an emitter in blue OLEDs and their Charge transfer complexes with ambipolar charge transport behaviour

Abstract

A novel six non-symmetric discotic liquid crystalline (DLC) dimers composed of pentaalkynylbenzene (PA) and triphenylene (TP) moieties were synthesized by using the CuI-Et3N catalyzed click reaction between terminal TP alkyne and PA azide. All the compounds exhibit mesomorphic behaviour at room temperature. Pure compounds 8a-b (n=6, 8 & m=1) and 9a-b (n=6, 8 & m=3) displays columnar centered rectangular (Colr) phase while 8c (n=10, m=1) and 9c (n=10, m=3) shows weak smectic (Sm) phase. Remarkably, the mesophase transforms from Colr and Sm to columnar hexagonal (Colh) on doping the pure compounds with electron-acceptor 2,4,7-trinitrofluorenone (TNF) molecule in 2:1 TNF/compound ratio. The induction of stable Colh mesomorphism is driven through charge-transfer interactions of electron-rich PA and TP moiety with electron-acceptor TNF molecule. The thermal behaviour of the compounds were studied through polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies. The performed grazing incidence small and wide angle X-ray scattering (GISAXS/GIWAXS) study showed the good alignment ability of 9a in Colr and complex 9a/TNF in Colh phase. Additional information about the arrangement of molecules in the Colr and Colh 2D lattice is deduced by the construction of electron density maps (EDM). As the pure non-symmetric dimers 8a-c and 9a-c were blue luminescent in solution state and shifted to green in solid state because of aggregation. Hence, the fabrication of doped OLED devices by using 8a and 9a as an emitter with the configuration ITO/PEDOT:PSS/Host:Emitter/TPBi/LiF/Al, illustrate the restoration of electroluminescence (EL) spectrum to narrow and blue shifted photoluminescence (PL) spectrum which indicates the suppression of aggregates. The emitter 8a and 9a are doped with three different hosts: (carbazolyl)-1,1’-biphenyl (CBP), 4’,4’’-tri(N-carbazolyl)triphenylamine (TCTA) and 2,7-Bis(carbazol-9-yl)-9,9-spirobifluorene (Spiro-2CBP) by varying dopant concentration. The best EL performance is observed for the device fabricated with 8a (5 wt% on CBP host) and 9a (3 wt% on CBP host) with maximum external quantum efficiency (EQE) of 2.1% with CIE coordinates of (0.16, 0.10). One of the studied complex 8b/TNF exhibited ambipolar charge carrier mobility of 1.78 × 10-3 cm2 V-1 s-1 and 1.25 × 10-3 cm2 V-1 s-1, for electron and hole, respectively at room temperature. The observed ambipolar mobility of complex compound reveals its potential application as organic semiconducting material.

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Publication details

The article was accepted on 11 Apr 2019 and first published on 13 Apr 2019


Article type: Paper
DOI: 10.1039/C9TC01178A
Citation: J. Mater. Chem. C, 2019, Accepted Manuscript

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    Room temperature Discotic Liquid Crystalline Triphenylene-Pentalkynylbenzene dyads as an emitter in blue OLEDs and their Charge transfer complexes with ambipolar charge transport behaviour

    I. Bala, W. Yang, S. P. Gupta, J. De, R. A. K. Yadav, D. P. Singh, D. K. Dubey, J. Jou, R. Douali and S. K. Pal, J. Mater. Chem. C, 2019, Accepted Manuscript , DOI: 10.1039/C9TC01178A

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