Temperature-dependent hole mobility in pyrene-thiophene-based room-temperature discotic liquid crystals.

π-Conjugated pyrene-thiophene-based room-temperature discotic liquid crystals armed with four peripheral aliphatic chains are reported to study their potential use in a hole-transporting organic semiconductor. The charge carrier mobility studies using the ToF method revealed room temperature hole mobility in the order of 10-4 cm2 V-1 s-1 for both mesogens. However, the mobility values for compound 1a were observed in the order of 10-3 cm2 V-1 s-1 at high temperatures. Such molecular systems can potentially be used in nonlinear organic electronic applications.

High Electrical Conductivity and Hole Transport in an Insightfully Engineered Columnar Liquid Crystal for Solution-Processable Nanoelectronics.

Discotic liquid crystals (DLCs) are widely acknowledged as a class of organic semiconductors that can harmonize charge carrier mobility and device processability through supramolecular self-assembly. In spite of circumventing such a major challenge in fabricating low-cost charge transport layers, DLC-based hole transport layers (HTLs) have remained elusive in modern organo-electronics. In this work, a minimalistic design strategy is envisioned to effectuate a cyanovinylene-integrated pyrene-based discotic liquid crystal (PY-DLC) with a room-temperature columnar hexagonal mesophase and narrow bandgap for efficient semiconducting behavior. Adequately combined photophysical, electrochemical, and theoretical studies investigate the structure-property relations, logically correlating them with efficient hole transport. With a low reorganization energy of 0.2 eV, PY-DLC exhibits superior charge extraction ability from the contact electrodes at low values of applied voltage, achieving an electrical conductivity of 3.22 × 10-4 S m-1 , the highest reported value for any pristine DLC film in a vertical charge transport device. The columnar self-assembly, in conjunction with solution-processable self-healed films, results in commendably elevated values of hole mobility (≈10-3 cm2  V-1 s-1 ). This study provides an unprecedented constructive outlook toward the development of DLC semiconductors as practical HTLs in organic electronics.

Tailoring Chiral Discotic Liquid Crystals: Mesophase Engineering through Alternative Approaches and Chain Lengths.

Hydrogen (H)-bonding is crucial in constructing superstructures in chemical (such as chiral discotic liquid crystals (DLCs)) as well as in biological systems due to its specific and directional nature. In this context, we achieved the successful synthesis of two branches of heptazine-based H-bonded complexes using distinct strategies. Hpz*-Es-Cn , we incorporated chiral alkyl tails (Hpz-chiral) onto the central C3 symmetric heptazine core, connected to achiral benzoic acid derivatives (Es-Cn acid) through H-bonding. In Hpz-Es-Cn -acid*, we used an achiral heptazine derivative (Hpz-Es-Cn ) linked to a chiral acid via H-bonding. On the other hand, based on the DSC results, we observed that Hpz*-Es-Cn complexes exhibited three distinct phases, whereas Hpz-Es-Cn -acid* complexes displayed only a single mesophase. In polarized optical microscopy (POM) observations, all the complexes displayed birefringence at room temperature, with the color of the POM images changing as the temperature varied. X-ray diffraction (XRD) studies at lower temperatures confirmed that Hpz*-Es-C8 exhibited the columnar rectangular (Colr ) phase, while Hpz*-Es-C10/12 exhibited the columnar oblique (Colob ) phase. However, all the H-bonded complexes exhibited the columnar hexagonal (Colh ) phase at higher temperatures. The chiroptical spectra recorded by Circular dichroism (CD) highlight the specific observations in the columnar phase of two complexes, Hpz*-Es-C10 and Hpz*-Es-C12 . This behavior has potential applications in various fields, including sensors, displays, and responsive materials.

Solution-processable organic light-emitting diodes utilizing electroluminescent perylene tetraester-based columnar liquid crystals.

Herein, we reveal a homologous series of liquid crystals involving perylene tetraesters as the core connected to the four trialkoxyphenyl units at the periphery using the triazole moiety as the linker. A thorough analysis using differential scanning calorimetry, polarized optical microscopy, and small- and wide-angle X-ray scattering studies confirm that all the mesogens 1a-c hold a stable enantiotropic columnar mesophase. Suitable molecular orbital levels and excellent material photophysical and thermal properties encouraged the study of their electroluminescent properties. Due to this, a well designed solution-processable organic light emitting diode device structure is configured as ITO (125 nm)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (35 nm)/host: x wt% emitter (x = 0.5, 1.0, 3.0, 5.0) (20 nm)/2,2'2''-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi) (40 nm)/lithium fluoride (LiF) (1 nm)/aluminium (Al) (200 nm) using compounds 1a-c as emitters. 4,4',4''-Tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) and 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) were chosen as two different host materials. The current density-voltage-luminance and current efficacy-luminance-power efficacy plots suggest that m-MTDATA is a better host than CBP. Amongst, device based on 1 wt% emitter 1c doped in the m-MTDATA host matrix displayed the best performance, with a maximum power efficacy of 17.2 lm W-1, current efficacy of 18.5 cd A-1, and external quantum efficiency of 6.3%.

Observation of "de Vries-like" properties in bent-core molecules.

"de Vries" liquid crystals, defined by a maximum layer shrinkage of ≤1% from the smectic A to C phase transition, are an integral component of ferroelectric liquid crystal (FLC) displays. Bona fide de Vries materials described in the literature are primarily perfluorinated, polysiloxane and polysilane-terminated rod-like (or calamitic) LCs. Herein, for the first time, we report a series of newly designed achiral unsymmetrical bent-core molecules with terminal alkoxy chains exhibiting similar properties to "de Vries" LCs. The new molecular structure is based on the systematic distribution of four phenyl rings attached via ester and imine linkers having 3-amino-2-methylbenzoic acid as the central core with a bent angle of 147°. Detailed microscopic investigations in differently aligned (planar as well as homeotropic) cells along with SAXS/WAXS studies revealed that the materials exhibited a SmA-SmC phase sequence along with the appearance of the nematic phase at higher temperatures. SAXS measurements divulged the layer spacings (d-spacings) and hence, the layer shrinkage was calculated ranging from 0.19% to 0.68% just below the SmA-SmC transition. The variation of the calculated molecular tilt angle (α) derived from the temperature-dependent SAXS data, followed the power law with exponent values 0.29 ± 0.01 and 0.25 ± 0.01 for compounds 1/10 and 1/12, respectively. The experimental values obtained were very close to the theoretically predicted values for the materials with de Vries-like properties. The analysis of temperature-dependent birefringence studies based on the prediction of the Landau theory, showed a dip across the SmA-SmC phase transition typical of compounds exhibiting the de Vries characteristics. The collective results obtained suggest "de Vries" SmA as a probable model for this bent-core system which may find applications in displays.

Correction: Luminescent columnar discotics as highly efficient emitters in pure deep-blue OLEDs with an external quantum efficiency of 4.7.

Correction for 'Luminescent columnar discotics as highly efficient emitters in pure deep-blue OLEDs with an external quantum efficiency of 4.7%' by Joydip De et al., Soft Matter, 2022, DOI: 10.1039/d1sm01558c.

Luminescent columnar discotics as highly efficient emitters in pure deep-blue OLEDs with an external quantum efficiency of 4.7.

Development of materials that serve as efficient blue emitters in solution-processable OLEDs is challenging. In this study, we report three derivatives of C3-symmetric 1,3,5-tris(thien-2-yl)benzene-based highly luminescent room temperature columnar discotic liquid crystals (DLCs) suitable as solid-state emitters in OLED devices. When employed in solution-processed OLEDs, one of the derivatives having the highest photoluminescence quantum yield exhibited a maximum EQE of 4.7% and CIE chromaticity of (0.16, 0.05) corresponding to the ultra deep-blue emission. The finding is sufficiently significant in the field of DLC-based deep blue emitters.

Molecular Engineering for the Development of a Discotic Nematic Mesophase and Solid-State Emitter in Deep-Blue OLEDs.

A unique strategy for the attainment of a discotic nematic (ND) mesophase is reported consisting of a central benzene core to which are attached two 4-alkylphenyl and two 4-pentylbiphenyl moieties diagonally via alkynyl linkers. The rotational nature and incompatibility of unequal phenylethynyl units led to the disruption of π-π interactions within cores that aids to the realization of ND phase and favors high solid-state emission. When used in OLEDs, compounds act as an efficient solid-state pure deep-blue emitter with Commission Internationale de L'Eclairage (CIEx,y) coordinates of (0.16, 0.07).

Luminescent Conductive Columnar π-Gelators for Fe(II) Sensing and Bio-Imaging Applications.

The high demand and scarcity of luminescent, photoconductive, and transparent gels necessitate its finding as they are potential components in photonic devices such as solar cell concentrators where optical losses via scattering and reabsorption require to be minimized. In this direction, we have reported highly transparent, blue luminescent as well as photoconductive gels exhibiting the hole mobility of 10-3 cm2/V s at ambient temperature as investigated by the time-of-flight technique. The π-driven self-standing supergels were formed using triazole-modified phenylene-vinylene derivatives as gelators in a nonpolar solvent. Different microscopic studies revealed its entangled network of interwoven fibrilar self-assembly and anisotropic order in the gel state. Supramolecular assembly of xerogels, studied by small- and wide-angle X-ray scattering (SAXS/WAXS) suggesting their local columnar hexagonal (Colh) superstructure, is beneficial for conducting gels. Rheological measurements direct the stiffness and robustness of the organogels. In addition, the gelators were developed as a sensing platform for the ultrasensitive detection of Fe(II) ions at ppb level. 1H nuclear magnetic resonance (NMR) titrimetric studies revealed that the interaction of the H-atom of triazole units with Fe(II) is responsible for quenching of blue fluorescence. Also, one of the gelators was successfully applied in bio-imaging using the pollen grains of the Hibiscus rosa-sinensis plant.

AIE-active mechanoluminescent discotic liquid crystals for applications in OLEDs and bio-imaging.

A multifunctional molecular design of fluorescent discotic liquid crystal (DLC) consisting of a tetraphenylethylene core is reported, which is found to serve as an excellent solid-state emitter in OLED devices with EQE of 4.4% and tunable mechanochromism. X-ray diffraction studies unveiled that change in supramolecular self-assembly is the physical origin of mechanochromism. The luminescent DLC molecule has been shown to act as a highly selective probe for labelling acidic cellular compartments (such as lysosomes) in bio-imaging using HeLa cells.

High hole mobility in room temperature discotic liquid crystalline tetrathienoanthracenes.

Tetrathienoanthracene (TTA), a new discotic core fragment, is explored that shows a remarkably high hole mobility (µh) of 4.22 cm2 V-1 s-1 at room temperature when used in space-charge limited current (SCLC) devices. Strong co-facial interactions (π-π, SS) among TTA cores along with a high tendency of the derivative to align homeotropically in the columnar mesophase over a large area in SCLC cells contributed to the top-class charge carrier mobility.

Effect of adsorbed polyelectrolytes on the interactions and elasticity of charged surfactant bilayers.

We present studies on the structure of complexes of the cationic, bilayer-forming surfactant, didodecyldimethylammonium bromide (DDAB), and the anionic polyelectrolyte sodium polyacrylate (PAANa). In the presence of uncomplexed polyelectrolyte in the coexisting aqueous solution, these complexes are found to exhibit a swelling transition followed by a deswelling transition on increasing the salt concentration. Lamellar structures with low periodicities occur at both low and high salt concentrations, which are stabilized by polymer bridging and van der Waals attraction, respectively. The swollen complex found at intermediate salt concentrations forms the sponge phase. Our results reveal that polyelectrolyte adsorption on bilayers has a profound effect on inter-bilayer interactions. The polymer-induced interaction changes from being attractive to repulsive as the surface coverage increases on increasing the salt concentration. Our results also confirm that polymer adsorption alters the elastic moduli of the bilayer, in agreement with earlier theoretical predictions.

High Hole Mobility and Efficient Ambipolar Charge Transport in Heterocoronene-Based Ordered Columnar Discotics.

Heterocoronene, a new redox-active core fragment, is utilized for the synthesis of room-temperature columnar discotic liquid crystals (DLCs). Three wedge-shaped side chains having different lengths of alkyl tails are introduced at the periphery of the heterocoronene core to prepare three kinds of discotic molecules, 1 (R = C10H21), 2 (R = C12H25), and 3 (R = C14H29). X-ray diffraction (XRD) analysis confirmed the packing variation in the columnar lattices regulated by alkyl chains of discrete length and steric bulk. When used in space charge limited current devices, compound 1 exhibits a high hole mobility value of 8.84 cm2/V s at ambient temperature, whereas compounds 2 and 3 show efficient ambipolar charge transport behavior with maximum hole (µh) and electron (µe) mobilities of 0.70 and 3.59 cm2/V s, respectively, for compound 3. The mobility values (µh = 8.84 cm2/V s for 1 and µe = 3.59 cm2/V s for 3) are remarkable and the highest ever disclosed for any DLC-based organic semiconductor, promising to deliver a good balance between mobility and processability in devices. The grazing incidence small- and wide-angle X-ray scattering experiments are employed to quantify the extent of alignment in the film state, which correlates with the observed trend of mobility values.

Polar Switching and Cybotactic Nematic Ordering in 1,3,4-Thiadiazole-Based Short-Core Hockey Stick-Shaped Fluorescent Liquid Crystals.

We report here the synthesis and thermotropic properties of novel short-core hockey stick-shaped liquid crystalline molecules based on the 1,3,4-thiadiazole core. Polar switching behavior is observed in the cybotactic nematic and smectic mesophases for the bent-core thiadiazole derivatives. The presence of the lateral methoxy moiety in the outer phenyl ring of the four-ring molecules facilitates the formation of spontaneous ordering in the nematic phase observed via X-ray diffraction measurements. Anomalous temperature dependence of spontaneous polarization on cooling is explained by the possible antiferroelectric packing of the molecules that require higher electric field for switching. The compounds exhibited a strong absorption band at ∼356 nm and a blue emission band at ∼445 nm with a good quantum yield of φf ∼0.39. The mega Stokes shift is observed and depends on the nature of the solvent.

Structural Understanding, Photoswitchability, and Supergelation of a New Class of Four Ring-Based Bent-Shaped Liquid Crystal.

Herein, we report a new type of azobenzene-based unsymmetrical bent-core molecules exhibiting photoswitchability in the liquid crystalline state, solid state, and solution state and in mixture upon UV irradiation and intense visible light. The compounds exhibited solid-state photochromism upon exposure to UV light, whereas in liquid crystalline state, reversible phase transitions were observed via both UV irradiation and intense visible light exposure. Crystal structure analysis reveals the basic structural understanding such as nonplanar bent molecular shape, antiparallel arrangement of the polar bent molecules, intra- and intermolecular hydrogen bonding, and different π-π interactions and interdigitation of long alkyl chains. The compounds are also found to act as supergelator toward various organic solvents. Hence, this is an excellent example of such potential bent-shaped liquid crystals that promise an immense perspective for device applications such as optical storage, molecular switches, etc.

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

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.

Reversibly photoswitchable alkoxy azobenzenes connected benzenetricarboxamide discotic liquid crystals with perpetual long range columnar assembly.

Liquid crystals (LCs) with photoswitchable groups are very interesting owing to their dual applications. In this regard, we report the synthesis of long chain alkoxy azobenzene incorporated benzenetricarboxamides 7a-c based room temperature columnar LCs. Apart from the light induced isomerization in the solution phase, the salient feature of these systems is the reversible photoisomerization even in the bulk state with perpetual columnar self-assembly at room temperature. Based on the observation of mesomorphic textures under polarised optical microscopy (POM) and grazing incidence small/wide angle X-ray scattering (GISAXS/GIWAXS) studies, the columnar assembly was found to be stable upon photoisomerization. However, subtle changes in height profile have been observed in AFM measurements after photoswitching. Interestingly, a temperature dependent change between rectangular and hexagonal mesophases in 7a has been observed. Upon extending the alkoxy chain length, only the hexagonal mesophase was observed. For comparison, the corresponding N-methylated derivative of 7a has also been synthesized. Despite the better photoswitching behaviour, due to the lack of planarity and H-bonding, 8a did not show any columnar mesophase.

Hydrogen-bond mediated columnar liquid crystalline assemblies of C3-symmetric heptazine derivatives at ambient temperature.

A new class of hydrogen (H) bonded fluorescent liquid crystals (FLCs) based on a newly discovered s-heptazine fluorophore discotic component have been synthesized. The tendency of the s-heptazine core to form H-bonded LCs has been explored for the first time. Interestingly, the pure heptazine derivatives (non-mesomorphic) on complexation with tri-alkoxy benzoic acids exhibit enantiotropic columnar mesomorphism over a wide range of temperatures including room temperature. This indicates the strength of the resulting H-bonded complexes. The H-bonded supramolecular complexes were studied through FT-IR, temperature dependent FT-IR and NMR studies and H-D exchange studies, and their thermal behaviour was deduced through polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies. Because of the inherently fluorescent pure heptazine derivative, the resulting complexes exhibit fluorescent behaviour in the solution state as well as in the solid state.

Phase Behavior of a New Class of Anthraquinone-Based Discotic Liquid Crystals.

Five novel columnar liquid crystalline compounds (4.1-4.5) consisting of a central anthraquinone core carrying four alkoxy chains (R = n-C6H13, n-C8H17, n-C10H21, n-C12H25, and 3,7-dimethyl octyl) with two diagonally opposite 1-ethynyl-4-pentylbenzene units were synthesized, and their phase transitions were investigated between changes in the molecular structure and their self-assembly into the columnar mesophases. Small and wide-angle X-ray scattering (SAXS/WAXS) studies were performed to deduce the exact nature of the mesophases, and their corresponding electron density maps were derived from the intensities of the peaks observed in the diffraction patterns. A comparison of compounds with different alkoxy chains indicated that the soft crystal columnar rectangular (Crcolrec) phase was stable at lower temperature for the shortest peripheral alkoxy chain (4.1; R = n-C6H13) and was found to exhibit the columnar hexagonal (Colh) phase and then the discotic nematic (ND) phase with increasing temperature. In contrast, increasing the peripheral chain length to n-C8H17 or the branched one (4.2 and 4.5) stabilized the Colh phase at lower temperature and showed the ND phase at higher temperature. Further increase in chain length (4.3 and 4.4; n-C10H21, n-C12H25) demonstrated the formation of the ND phase. Conductivity measurement in the Colh mesophase was found to be almost 10 times higher in magnitude than the corresponding Crcolrec phase. The HOMO-LUMO band gap of all the compounds was found to be in the range from 2.79 to 2.82 eV, which is quite less and comparable with the optical energy band gap.