Smectic Mesophases Research Articles

Towards 3D printing of lipids - The impact of surface polarity on lipid assembly under spatial confinement.

Molecular dynamics (MD) simulations in the MARTINI model are used to study the assembly of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) molecules under spatial confinement, such as during solvent evaporation from ultrasmall (femtoliter quantity) droplets. The impact of surface polarity on molecular assembly is discussed in detail. Our results reveal that solvent evaporation gives rise to the formation of well-defined stacks of lipid bilayers in a smectic alignment. These smectic mesophases form on both polar and nonpolar surfaces but with a notable distinction. On polar surfaces, the director of the stack is oriented perpendicular to the support surface. By contrast, the stacks orient at an angle on the nonpolar surfaces. The packing of head groups on surfaces and lipid molecular mobility exhibits significant differences as surface polarity changes. The role of glycerol in the assembly and stability is also revealed. The insights revealed from the simulation have a significant impact on additive manufacturing, biomaterials, model membranes, and engineering protocells. For example, POPC assemblies via evaporation of ultrasmall droplets were produced and characterized. The trends compare well with the bilayer stack models. The surface polarity influences the local morphology and structures at the interfaces, which could be rationalized via the molecule-surface interactions observed from simulations.

First Gemini-type fluorescent ionic liquid crystals: Synthesis, fluorescence emission and self-assembly in liquid crystals and gels

Although many Gemini-type ionic liquids crystals (ILCs) had been studied, the Gemini-type fluorescence ionic liquid was not presented so far. In this work, the first Gemini-type fluorescence ILCs based on dicyanodivinylbenzene unit and imidazole cations with –CH3, –C4H9 and –C6H13 chains were prepared. They emitted strong fluorescence in aggregated states based on the aggregation-induced emission effect. They displayed the reversible smectic mesophase in wide scope of temperature. Furthermore, the stable gels in DMSO-toluene (1:1) system were observed for these new ILCs and the gels possessed the excellent thermal reversible response. The novel ionic liquid with C6H13 chains presented the orderly rectangular columnar stacking in gel. The CVs studies suggested that these Gemini-type fluorescence ILCs possessed the stable electrochemical properties. This research not only reported the first Gemini-type ILCs bearing high fluorescence, good liquid crystalline and gel properties, but also presented a unique self-assembled change from a coated mesophase in liquid crystal to a rectangular columnar stacking in gel.

Pentacoordinated Liquid Crystalline Zn(II) Complex Organized in Smectic Mesophase: Synthesis, Structural and Electrochemical Properties

The synthesis and structural characterization of a new liquid crystalline coordination complex based on pentacoordinated Zn(II) metal centre with the coordination fulfilled by the tridentate chelating N^N^N 2,2′;6′,2″-terpyridine ligand and two monoanionic gallates decorated with several long alkyl chains is described. The mesomorphic properties were accurately investigated by small- and wide-angle X-ray scattering studies. Despite the bulky coordination around the metal centre, the complex self-organizes into a smectic phase and, based on the structural and geometrical parameters, a model for the supramolecular organization in the liquid crystalline phase is proposed. Electrochemical investigations showed the importance of the molecular structure of the coordination complex in enhancing its aqueous sensing capacities: the bulky organic ligands form an organic shell separating the metal centres and favouring the redox system through their reduction followed by stripping.

Preparation of Complex Ratio‐Dependent Nanomaterials from Polymerizable Hydrogen‐Bonded Liquid Crystal

A scalable approach to generating different nanomaterials from the same hydrogen-bondeded complex by photo-crosslinking is described. The hydrogen-bonded complex exhibits a smectic or columnar mesophase, depending on the molar ratio between the donor and acceptor. The mesophase structure is effectively retained after photo-crosslinking. Polymer nanosheets with negatively charged carboxylate groups can be prepared from the complex with the smectic mesophase after subsequent extraction and chemical treatment, and they show effective adsorption of Pd2+. Meanwhile, a nanoporous polymer is successfully achieved from the complex having the columnar mesophase by using the same technology of photo-crosslinking and solvent treatment. This work provides a new strategy to fabricate different nanomaterials from the same hydrogen-bonded complex system.

Impact of Surface Polarity on Lipid Assembly under Spatial Confinement.

Molecular dynamics (MD) simulations in the MARTINI model are used to study the assembly of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) molecules under spatial confinement, such as during solvent evaporation from ultrasmall (femtoliter quantity) droplets. The impact of surface polarity on molecular assembly is discussed in detail. To the best of our knowledge, this work represents the first of its kind. Our results reveal that solvent evaporation gives rise to the formation of well-defined stacks of lipid bilayers in a smectic alignment. These smectic mesophases form on both polar and nonpolar surfaces but with a notable distinction. On polar surfaces, the director of the stack is oriented perpendicular to the support surface. By contrast, the stacks orient at an angle on the nonpolar surfaces. The packing of head groups on surfaces and lipid molecular mobility exhibits significant differences as surface polarity changes. The role of glycerol in the assembly and stability is also revealed. The insights revealed from the simulation have a significant impact on additive manufacturing, biomaterials, model membranes, and engineering protocells. For example, POPC assemblies via evaporation of ultrasmall droplets were produced and characterized. The trends compare well with the bilayer stack models. The surface polarity influences the local morphology and structures at the interfaces, which could be rationalized via the molecule–surface interactions observed from simulations.

Liquid-Crystalline Thermally Activated Delayed Fluorescence: Design, Synthesis, and Application in Solution-Processed Organic Light-Emitting Diodes.

Realizing both high efficiency and liquid crystallinity in one molecule remains a challenge in thermally activated delayed fluorescence (TADF) emission. Herein, two isomeric compounds─m-DPSAc-LC and p-DPSAc-LC with different connection positions between donor and acceptor moieties─were synthesized and characterized. Diphenylsulfone (DPS) was used as the acceptor, acridine (Ac) was used as the donor, and biphenyl derivatives (LC) were employed as the mesogenic group. Both compounds showed a smectic mesophase evidenced by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and temperature-dependent small-angle X-ray scattering (SAXS). The compound p-DPSAc-LC clearly exhibited thermally activated delayed fluorescence due to the much more distorted geometry, whereas m-DPSAc-LC showed simple fluorescence. Compared to the parent TADF molecules without appended mesogenic groups (DPS-Ac), these liquid-crystalline emitters possessed higher hole mobilities and improved device performance. The OLEDs fabricated via solution processing using the liquid-crystalline compound p-DPSAc showed a maximum external quantum efficiency of ∼15% and as such is the first example of a liquid-crystalline TADF material in an OLED device.

Main-chain side-chain combined liquid crystalline polymers bearing azobenzene and biphenyl-based mesogens – segregation of the mesogens within two distinct layers

ABSTRACT Two different mesogenic units, namely biphenyl and azobenzene, were installed centrally within the polymer backbone and pendant alkylene segments; the purpose was to examine if chain folding of the backbone occurs so as to segregate the azobenzene and biphenyl units in adjacent layers of a smectic-type ordering. Two sets of isomeric main-chain side-chain liquid crystalline polymers (MCSC-LCPs), namely SC-AzoCx-MC-BPCx and SC-BPCx-MC-AzoCx (x=6 or 10), were synthesised; the former series has biphenyl in the backbone and azobenzene in the side-chain, while their locations are switched in the latter. It was observed that all four polymers were semicrystalline at room temperature and exhibited a smectic mesophase upon melting. DSC and X-ray scattering studies revealed that switching the locations of the azobenzene and biphenyls had little effect on; the phase transition temperatures and smectic layer spacings. A second series, namely SC-AzoCx-MC-BPCy and SC-BPCx-MC-AzoCy, where the length of alkylene spacers in the side-chain (Cx) and backbone (Cy) is different, was also studied; this was done to examine the influence of unequal alkylene segment lengths on the mesophase structure and interlayer spacing, and importantly, whether the zigzag folding-induced formation of a layered structure is retained, despite their unsymmetrical nature.

Preliminary study on the self-assembly of a pair of azobenzene liquid crystals derived from alanine

A series of azobenzene liquid crystals containing one amino acid residue were synthesized. They exhibit smectic mesophases. The compounds derived from alanine can self-assemble into twisted nanoribbons. The molecules organized in a monolayer structure within the nanoribbon. Circular dichroism characterization indicated that the neighboring liquid crystal cores packed in a chiral structure. FT-IR spectra indicated that the nanoribbons were caused by H-bonding and association of hydrocarbon chains.

HYDROGEN BOND IN POTENTIALLY MESOGENIC MOLECULAR COMPLEXES OF 4-(PHENYLAZO)BENZOIC ACID AND 4-(PHENYLAZO)PHENOL WITH PYRIDINE DERIVATIVES

Using experimental and theoretical methods, it has been shown that hydrogen-bonded molecular complexes in two-component systems based on 4-pyridyl-4'-n-alkyloxybenzoates (n = 7, 12) with 4-(phenylazo)benzoic acid and 4-(phenylazo)phenol (composition of 1:1) are formed. Quantum chemistry methods (DFT (B3LYP)/cc-pVTZ) were used to determine the conformational properties of 4-(phenylazo)benzoic acid and 4-(phenylazo)phenol molecules. It has been shown that the introduction of 4-carboxy and 4-hydroxy groups into the azobenzene molecule has practically no effect on the energy characteristics of the trans-cis isomerization process. The energy, geometric, and electronic characteristics of intermolecular hydrogen bonds in the H-complexes have been estimated. The calculations showed that the H-complexes of 4-pyridyl-4-n-propyloxybenzoate with 4-(phenylazo)benzoic acid and 4-(phenylazo)phenol differ significantly from each other in geometric structure: the complex with 4-(phenylazo)benzoic acid – rod-shaped, complex with 4-(phenylazo)phenol – angular. A comparison of the energy characteristics of H-complexes, as well as the characteristics of hydrogen bonds, showed that a stronger hydrogen bond is formed in the complex with 4-(phenylazo)benzoic acid than in the complex with 4-(phenylazo)phenol. The results of modeling dimers of molecules of 4-(phenylazo)benzoic acid, 4-(phenylazo)phenol, reproducing intermolecular interactions in crystals, and their hydrogen-bonded complexes allow to conclude that in the process of self-organization of systems with a composition of 1:1, associates of 4-(phenylazo)benzoic acid molecules, associates of 4-(phenylazo)phenol molecules will be broken and H-complexes with 4-pyridyl-4'-alkyloxybenzoates will be formed instead. Changes in the experimental IR spectra recorded for the initial components and for systems with a composition of 1:1 confirm the formation of molecular complexes. The structural units of the studied systems based on 4-pyridyl-4'-alkyloxybenzoates with 4-(phenylazo)benzoic acid and 4-(phenylazo)phenol with a composition of 1:1 can be considered supramolecules, formed by hydrogen bonding. The structure of supermolecules and the presence of their geometric anisotropy indicate their potential mesogenicity. The sample of H-complexes of 4-(phenylazo)phenol and 4-pyridyl-4'-n-dodecyloxybenzoate (composition of 1:1) was studied for the manifestation of mesomorphic properties using the method of polarization thermomicroscopy. The textures registered in a polarizing microscope allow to conclude that the H-complex has a smectic mesophase in the temperature range of 101.4–109.4 °C.

Fast Photoswitchable Order-Disorder Transitions in Liquid-Crystalline Block Co-oligomers.

Optically driven ordering transitions are rarely observed in macromolecular systems, often because of kinetic limitations. Here, we report a series of block co-oligomers (BCOs) that rapidly order and disorder at room temperature in response to optical illumination, and the absence thereof. The system is a triblock where rigid azobenzene (Azo) mesogens are attached to each end of a flexible siloxane chain. UV-induced trans-to-cis Azo isomerization, and vice versa in the absence of UV light, drive disordering and ordering of lamellar superstructures and smectic mesophases, as manifested by liquefaction and solidification of the material, respectively. The impacts of chemical structure on BCO self-assembly and photoswitching kinetics are explored by in situ microscopy and X-ray measurements for different mesogen end groups (NO2 or CN), and different carbon chain lengths (0C or 12C) between the siloxane and the mesogen. The presence of the 12C spacer leads to hierarchical ordering with smectic layers of mesogens existing alongside larger length-scale lamellae, versus only smectic ordering without the spacer. These hierarchically ordered BCOs display highly persistent lamellar sheets that contrast with the tortuous, low-persistence "fingerprint"-type structures seen in conventional block copolymers. The reordering kinetics upon removal of UV illumination are extremely rapid (<5 s). This fast response is due to the electron-withdrawing NO2 and CN, which facilitate cis-to-trans isomerization via thermal relaxation at room temperature without additional stimuli. This work elucidates structure-property relationships in photoswitching BCOs and advances the possibility of developing systems in which ordered nanostructures can be easily optically written and erased.

Synthesis of cholesterol containing unsymmetrical dimers: a new series of liquid crystals

ABSTRACT We report an efficient and practical procedure for the synthesis of cholesterol containing unsymmetrical liquid crystals dimers. Liquid crystalline and thermal properties were studied by POM, TGA, DSC, and SAXS analysis. All final products (3a-d) presented liquid crystal properties. The presence of smectic (compounds 3a, 3b, and 3d) and chiral nematic (3 c) mesophases evidenced by polarised optical microscopy (POM) and further confirmed by small-angle X-ray scattering (SAXS) analyses. The compound 3 c remains supercooled until room temperature and presented enantiotropic mesomorphism at a low temperature. This characteristic of organisation and mesomorphism at room temperature makes compound 3 c an ideal candidate for technological applications. The SAXS analyses further suggest possibly the presence of smectic clusters within the nematic liquid crystalline phase. All compounds presented high thermal stability at around 300°C when tested for their thermal properties. The highest occupied molecular orbital (HOMO) energies were estimated from cyclic voltammetry (CV) and used together with the optical absorption measurements to determine the band gap and lowest unoccupied molecular orbital (LUMO) energies. The values obtained were: HOMO (–5.81 to – 6.03 eV), LUMO (–1.53 to – 2.12 eV), and band gap (3.88 to 4.28 eV).

Self‐Assembly of All‐DNA Rods with Controlled Patchiness

AbstractDouble‐stranded DNA (dsDNA) fragments exhibit noncovalent attractive interactions between their tips. It is still unclear how DNA liquid crystal self‐assembly is affected by such blunt‐end attractions. It is demonstrated that stiff dsDNA fragments with moderate aspect ratio can specifically self‐assemble in concentrated aqueous solutions into different types of smectic mesophases on the basis of selectively screening of blunt‐end DNA stacking interactions. To this end, this type of attractions are engineered at the molecular level by constructing DNA duplexes where the attractions between one or both ends are screened by short hairpin caps. All‐DNA bilayer and monolayer smectic‐A type of phases, as well as a columnar phase, can be stabilized by controlling attractions strength. The results imply that the so far elusive smectic‐A in DNA rod‐like liquid crystals is a thermodynamically stable phase. The existence of the bilayer smectic phase is confirmed by Monte‐Carlo simulations of hard cylinders decorated with one attractive terminal site. This work demonstrates that DNA blunt‐ends behave as well‐defined monovalent attractive patches whose strength and position can be potentially precisely tuned, highlighting unique opportunities concerning the stabilization of nonconventional DNA‐based lyotropic liquid crystal phases assembled by all‐DNA patchy particles with arbitrary geometry and composition.

Born to be a Liquid Crystal: The role of fluorinated chain in the design and synthesis of new mesogens

The title of this article is devoted to the collection of compounds synthesized in this work, in which all the molecules had an astonished performance as “characters of a scientific movie” about liquid crystals (LCs). Just like movie stars, all compounds synthesized have shone magnificently on the LCs stage. The plot of this movie was designed by preparing 3,5-diarylisoxazoles carrying a semi-perfluorinated alkyl chain and a polar-terminated one with flexible spacer group. From the beginning to the end of the synthetic route, all the intermediates and the products exhibited smectic mesophases. The performance as LCs actors was analyzed by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The cohesive energy density (CED) was calculated for some of them to analyze the effects of microphase segregation on mesomorphic behavior.

Synthesis and mesomorphic properties of new organosiloxane chiral calamitic liquid crystals

A new chiral calamitic compound consisting of a phenyl core that is connected through ester linkers to a benzoate carrying a chiral (S)-2-methylbutoxy group at one side and a biphenyl carboxylate with an undecenyloxy chain at other side has been synthesized. Additionally, the siloxane substituted derivatives and a dimer with these two identical chiral calamitics linked by a disiloxane spacer have been obtained via hydrosilylation reaction to study the effect of combining the chiral moiety and siloxane segments within the same structure on liquid crystalline properties. The chiral calamitic compound and derivatives with a siloxane end-group or a siloxane spacer have been characterized using classical spectroscopic methods (1H NMR, 13C NMR, 29Si NMR and MS) and elemental analysis (EA). The liquid crystalline properties of all new compounds were investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray scattering and electro-optic methods. With the introduction of a siloxane end-group to the vinyl-terminated calamitic compound exhibiting enantiotropic chiral nematic phase (N*) and chiral tilted smectic phase (SmC*), the occurrence of stable smectic phase was strongly promoted. In a similar way, the dimer with two identical calamitic mesogens connected by a disiloxane spacer exhibited a wider smectic mesophase interval in addition to the presence of N* mesophase. The new mesogens exhibit a ferroelectric switching with PS of around 200 nC cm−2 in their chiral tilted smectic (SmC*) mesophase range. The transition temperatures of organosiloxanes are considerably lower than that of olefinic precursor.

New supramolecular hydrogen-bonded liquid crystals based on 4-alkylbenzenesulfonic acids and 4-pyridyl 4′-alkyloxybenzoates: Quantum chemical modeling and mesomorphic properties

• The new supramolecular liquid crystals with aromatic sulfonic acids were obtained. • Structural unit of systems under study is hydrogen-bonded complex. • Proton transfer occurs between sulfonic group and pyridyl fragment. • The results were confirmed by quantum chemistry, FTIR spectroscopy. • H-complexes formed by mesogen and nonmesogen have mesomorphic properties. The results of quantum-chemical modeling of the self-assembly process in the system based on 4-alkylbenzenesulfonic acids and 4-pyridyl 4′-alkyloxybenzoates are presented. It has been shown that the interaction of the pyridyl fragment of 4-pyridyl 4′-alkyloxybenzoat and the sulfonic group of 4-alkylbenzenesulfonic acid leads to the strongest H-complexes. The potential function of proton transfer from the sulfonic group to the pyridyl fragment was simulated both in the gas phase and in a solvent (ethanol). It was found that an H-complex with a hydrogen bond of the O∙∙∙H–N type is formed between the compounds under study. FTIR spectra of the 1(4-alkylbenzenesulfonic acid) : 1(4-pyridyl 4′-alkyloxybenzoate) systems confirm the results of quantum chemical calculations. The mesomorphic properties of the 1(4-methylbenzenesulfonic acid) : 1(4-pyridyl 4′-heptyloxybenzoate) supramolecular complex were studied using DSC and polarizing microscopy. It is shown that the H-complex formed by 4-methylbenzenesulfonic acid and 4-pyridyl 4′-heptyloxybenzoate has a smectic mesophase with expanded temperature range (59º) in comparison with the 4-pyridyl 4′-heptyloxybenzoate (28º).

Polar Alkoxy Group and Pyridyl Effects on the Mesomorphic Behavior of New Non-Symmetrical Schiff Base Liquid Crystals

A series of non-symmetrical Schiff base liquid crystals were prepared and investigated. Schiff bases of p-alkyloxy aniline derivatives and 4-phenyl pyridine-4′-carbaldehyde were synthesized. The terminal alkoxy groups substituting aniline are of varied chain length, namely C6, C8, and C16. The structures of the compounds were confirmed via 1H NMR and 13C NMR spectroscopy. Different mesophases of the samples were thermally and optically characterized by differential thermal analysis (DSC) and polarized optical microscopy (POM). All samples revealed enantiotropic smectic B (SmB) and smectic A (SmA) mesophases. The results obtained were further correlated with the density functional theory (DFT) theoretical calculations. The results are compared to a series of compounds bearing biphenyl moiety in their mesogens. The thermal stabilities of the different mesophase reduced upon the increment of the alkoxy chain length. The temperature ranges of both the smectic mesophases of new compounds bearing the 4-phenyl pyridine moiety are generally expanded higher than the other series. In addition, the total mesophase range is greater in the new compounds when compared to their biphenyl analogues. The DFT results were investigated in terms of the molecular geometries and the frontier molecular orbitals as well as the charge distribution mapping to show and illustrate the difference in the mesomorphic properties.

A Highly Robust Ionotronic Fiber with Unprecedented Mechanomodulation of Ionic Conduction.

Stretchable ionic conductors are appealing for tissue-like soft electronics, yet suffer from a tardy mechanoelectric response due to their poor modulation of ionic conduction arising from intrinsic homogeneous soft chain network. Here, a highly robust ionotronic fiber is designed by synergizing ionic liquid and liquid crystal elastomer with alternate rigid mesogen units and soft chain spacers, which shows an unprecedented strain-induced ionic conductivity boost (≈103 times enhanced as stretched to 2000% strain). Such a surprisingly high enhancement is attributed to the formation of microphase-separated low-tortuosity ion-conducting nanochannels guided by strain-induced emergence of aligned smectic mesophases, thus allowing for ultrafast ion transport that resembles the role of "swimming lanes." Intriguingly, the boosting conductivity even reverses Pouillet's Law-dictated resistance increase at certain strains, leading to unique waveform-discernible strain sensing. Moreover, the fiber retains thermal actuation properties with a maximum of 70% strain changes upon heating, and enables integrated self-perception and actuation. The findings offer a promising molecular engineering route to mechanically modulate the ion transport behavior of ionic conductors toward advanced ionotronic applications.

Mesomorphic Behaviour and DFT Insight of Arylidene Schiff Base Liquid Crystals and Their Pyridine Impact Investigation

A new series of Schiff base liquid crystal have been prepared and studied. Schiff bases of p-alkyl aniline derivatives and 4-phenyl pyridine-4′-carbaldehyde were prepared. The terminal alkyl groups substituting aniline are of varied chain length, namely C8, C12 and C14. The structures of the compounds were elucidated by 1H NMR and 13C NMR. The mesomorphic thermal and optical characteristics of the samples were determined via differential thermal analysis (DSC) and polarization optical microscopy (POM). All compounds exhibit enantiotropic dimorphic mesophase behaviour, referred to as smectic X1 (SmX1) and smectic X2 (Sm A). Experimental results obtained for the mesophases were correlated with density functional theory (DFT) theoretical calculations. The results of the new series are further compared to two series of compounds bearing pyridine (two ring Schiff bases) and biphenyl, respectively, in their mesogens. The series of compounds of one pyridine ring are generally not mesomorphic. The results indicate that the alkyl chain length has a strong impact on the mesomorphic characteristics and thermal stabilities of the different mesophases. As a trend, the temperature ranges of both of smectic mesophases of all compounds are higher in new compounds bearing the 4-phenyl pyridine moiety. In addition, the total mesophase range is generally higher in the new compounds when compared to their biphenyl analogues. Finally, theoretical DFT calculations were performed to illustrate the experimental finding of the mesomorphic behaviour in terms of the molecular geometry and aromaticity, π–π stacking and LOL-π.

Synthesis and study the liquid crystalline behaviors of double Schiff bases bearing ester linkage as a central core

ABSTRACT Two series of 12 homologues, 4-(((4-substituentphenyl)imino)methyl)phenyl 4-((4-(alkoxy) benzylidene)amino)benzoate (Dn, X = –OCH3 and En, X = –-Br), have been synthesised. The two series have different alkoxy terminal chains (OC n H2 n +1, n = 3, 4, 6, 7, 9, 12) on one side of their structures while other side in the series (Dn and En) was substituted by methoxy and Bromo group, respectively. The structures of the synthesised derivatives have been identified using Fourier transform infrared spectrometry (FT-IR) and nuclear magnetic resonance for H and C (1H-NMR, 13C-NMR) and the elemental analysis (CHN). The texture of liquid crystalline phases of the mesogens has been observed by optical polarised microscopy (OPM) while the details of the thermal transition of compounds were investigated by differential scanning calorimetry (DSC). The obtained liquid crystalline results showed that the derivatives of series (Dn) display a broad range of nematic mesophase (N) while the mesogens of (En) series appeared nematic and smectic mesophases. All the studied homologues show a wide range of liquid crystalline mesophases, the stability of mesomorphic behaviour of these compounds was dependent mainly on the length of terminal alkoxy chain (the number of carbon atoms) chain and the effect of substituent groups (OCH3 and Br).

Shining rings: The effect of the rigid core and benzazole heterocycles on the properties of luminescent calamitic liquid crystals

We report the molecular designing of non-symmetrical luminescent liquid crystals based on the 1,2,3-benzotriazole (BTZ), 2,1,3-benzoxadiazole (BOX) and 2,1,3-benzothiadiazole (BTD) heterocycles. The compounds showed stable nematic and smectic mesophases in a wide range of temperatures (between 70 and 170 °C). Translational order parameter investigations based on X-ray diffraction experiments demonstrated high ordered layers, closely dependent of the molecular structure and the smectic organization. The molecules exhibited intense photoluminescence under UV–vis light excitation in solutions. The emission spectrum for BTZ revealed a maximum in the blue region (447 nm), while BOX and BTD emit in the yellow one (around 550 nm). The fluorescence quantum yields of the BTZ-based derivatives solution are up to 64%, significantly higher than BOX and BTD. Moreover, in solid films, the efficiencies remained in the same range of solution. DFT calculations indicate dihedral angles of 24-30° between the benzazole-phenyl rings, which may suppress π-stacking and consequently reduce fluorescence quenching in the solid phase. This work contributes to general knowledge, where asymmetric cores improve mesophase stabilization and photoluminescence preservation in the solid state compared to their symmetric pairs.