The realm of liquid crystals (LCs) has significantly benefited when amalgamated with the exciting features of nanotechnology. Various nanomaterials (NMs) have shown their potentiality in tailoring the display and non-display characteristics of LC materials. Carbon-based nanomaterials (CNMs) such as carbon nanotubes, graphene oxide, graphene flakes, graphene and carbon dots are found to exhibit synergistic interaction with LCs to provide interesting properties such as low switching threshold voltage, faster switching response, reduced ionic conductivity and the formation of localized pseudonematic domains, etc. The network of benzene rings in the CNMs is found to offer π–π stacking of electrons with a benzene-based core of LC molecules which could provide a susceptible pathway for the strong interaction between the LCs and the CNMs. However, the uniform and stable dispersion of CNMs in the LC matrix has been found to be a challenging step in preparation of CNM/LC composites. Herein, we present a detailed review on the recent research work based on CNM-dispersed LC composites with an emphasis on the methods to obtain uniform CNM/LC dispersion and the dielectric, electro-optical, orientation, photophysical properties, etc, of the composites. Moreover, we have critically discussed the pros and cons of these composites and proposed the future scope of research in this exciting field.

Six novel 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) display a columnar centered rectangular (Col_r) phase while 8c (n = 10, m = 1) and 9c (n = 10, m = 3) show a weak smectic (Sm) phase. Remarkably, the mesophase transforms from Col_r and Sm to columnar hexagonal (Col_h) on doping the pure compounds with electron-acceptor 2,4,7-trinitrofluorenone (TNF) molecules in a 2 : 1 TNF/compound ratio. The induction of stable Col_h mesomorphism is driven through charge-transfer interactions of electron-rich PA and TP moieties with electron-acceptor TNF molecules. The thermal behaviour of the compounds was 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 good alignment ability of 9a in the Col_r and complex 9a/TNF in the Col_h phase. Additional information about the arrangement of molecules in the Col_r and the Col_h 2D lattice is deduced by the construction of electron density maps (EDM). The pure non-symmetric dimers 8a–c and 9a–c were blue luminescent in the solution state and shifted to green in the 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 illustrates the restoration of the electroluminescence (EL) spectrum to narrow and a blue shifted photoluminescence (PL) spectrum, which indicates the suppression of aggregates. Emitters 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 the dopant concentration. The best EL performance is observed for the device fabricated with 8a (5 wt% in the CBP host) and 9a (3 wt% in the CBP host) with a maximum external quantum efficiency (EQE) of 2.1% and CIE coordinates of (0.16, 0.10). One of the studied complexes 8b/TNF exhibited an 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 electrons and holes, respectively, at room temperature. The observed ambipolar mobility of the complex compound reveals its potential application as an organic semiconducting material.

The finding of pure organic emitter materials is the need of the hour for the mass production of cost-effective and metal-free fluorescent organic light-emitting diodes (FOLEDs). In this paper, we report a new series of perylene tetraesters (PTEs) that exhibit the room temperature columnar (Col) mesophase and can act as efficient fluorescent emitter materials in OLEDs. The molecular design involves the attachment of triazole moieties with the PTE discotic core via click chemistry. Triazole groups were chosen as they can improve the electron transport as well as tune the luminescence behavior of discogens. All the PTE derivatives 3a–d exhibited ordered columnar rectangular (Col_ro) mesophases at ambient temperatures suitable for various device applications. The electron mobility of perylene tetraester derivative 3a was measured in the mesophase by time of flight (TOF) technique and found to be 0.014 cm² V⁻¹ s⁻¹. However, the balanced hole and electron charge transport behaviour was observed in fabricated hole-only and electron-only devices. Taking advantage of both charge transport and the luminescence nature of the PTE derivative in OLEDs, a series of devices were fabricated by utilizing 3a as a sole emitter and in the dispersed form at 1, 5 and 8 wt% with the CBP host and at 5 wt% in the SimCP2 host. A significantly high value of the external quantum efficiency (EQE) of 6.5% is obtained in doped devices with the CBP host at 5 wt% dopant (3a) concentration with CIE coordinates of (0.37, 0.53) corresponding to green color. The obtained high EQE value will certainly offer an important step forward to expand the application of smart DLC materials in OLEDs.