Li-Rong Zhu

Synthesis and liquid crystal properties of a new class of calamitic mesogens based on substituted 2,5‐diaryl‐1,3,4‐thiadiazole derivatives with wide mesomorphic temperature ranges

Liquid crystals based on substituted 2,5‐diaryl‐1,3,4‐thiadiazole derivatives (1a–1f, 3a and 3b) and 1,3,4‐oxadiazole analogues (2a–2f, 4a and 4b) were synthesised and characterised by 1H, 13C nuclear magnetic resonance, Fourier transform infrared, mass spectrometry, high‐resolution mass spectrometry techniques and elemental analyses. The X‐ray crystal structure of 1e revealed that it contains tilted lamellar arrangement of molecules in the crystalline solid. The liquid crystal properties have been investigated by polarised‐light optical microscopy, differential scanning calorimetry and in‐situ variable‐temperature X‐ray diffraction. All compounds (except 2e and 2f) exhibited thermotropic liquid crystal behaviours with various mesophases (smectic A and C, nematic N or soft crystal E phases). Notably, the 1,3,4‐thiadiazole derivatives consistently have wider mesomorphic temperature ranges than those of the respective 1,3,4‐oxadiazole analogues. The solutions of all compounds in CH2Cl2 individually displayed one or two absorption bands with λ max values at 297–355 nm and emitted with λ max values at 363–545 nm and quantum yields of 0.12–0.73. Structure–property relationships of these compounds are discussed in the contexts of their molecular structures and weak intermolecular interactions.

Synthesis and mesomorphic behaviour of heterocycle‐based liquid crystals containing 1,3,4‐oxadiazole/thiadiazole and thiophene units

A new class of thiophene‐based 1,3,4‐oxadiazole derivatives ( n ‐OXD‐R) and the corresponding 1,3,4‐thiadiazole derivatives ( n ‐THD‐R) was synthesised and characterised by means of 1H NMR, 13C NMR, MS and HRMS techniques. Liquid crystal properties were investigated by differential scanning calorimetry, polarising optical microscopy and variable temperature X‐ray diffraction measurements. All the 1,3,4‐thiadiazole compounds ( n ‐THD‐R) exhibited enantiotropic mesophases (smectic A, smectic C and nematic phases) with wide mesomorphic temperature ranges (68.5–109.5°C). In contrast, only the oxadiazole compounds 8‐OXD‐Cl, 9‐OXD‐Cl and 10‐OXD‐Cl, bearing electron‐withdrawing terminal group and longer alkoxy chain, displayed an enantiotropic smectic A phase with narrow mesomorphic temperature ranges (4.1–10.9°C). The effects of central heterocyclic rings, the terminal groups and the length of the terminal alkoxy on the mesomorphic behaviour are discussed.

Synthesis, single crystal structure and liquid crystalline properties of bent-shaped 2,5-diaryl 1,3,4-oxadiazoles

Two bent-shaped 1,3,4-oxadiazole-based compounds, namely 2-[4-(2-(4-methylphenyl)-(E)-1-ethenyl)]phenyl-5-(4-pentyloxyphenyl)-1,3,4-oxadiazole 5a and 2-[4-(2-(4-fluorophenyl)-(E)- ethenyl-5-(4-pentyloxyphenyl)-1,3,4-oxadiazole 5b, were synthesised and their liquid crystalline properties were studied in this paper. Compound 5a exhibited an enantiotropic nematic mesophase, while 5b displayed an enantiotropic smectic A phase. No banana-shaped mesophases were found in these mesogens, although they adopt a bent-shaped molecular structure as confirmed by the single X-ray diffraction crystallography.

Synthesis and liquid crystalline properties of substituted 2,5-diaryl 1,3,4-oxadiazole derivatives without flexible chains

A series of new compounds based on aromatically 2,5‐disubstituted 1,3,4‐oxadiazoles without flexible chains, formulated as p‐R–C6H4–(OC2N2)–(p‐C6H4)2–R′ with (i) R = CH3O, R′ = CH3O, CH3S, F, H (Ia–Id), (ii) R = CH3S, R′ = CH3O, CH3S, F, H (IIa–IId) and (iii) R = F, R′ = CH3O, CH3S, F, H (IIIa–IIId) (p‐C6H4 and OC2N2 represent a p‐phenylene spacer and a 1,3,4‐oxadiazole ring, respectively), were synthesised and characterised by 1H and 13C NMR, MS and HRMS techniques. Mesomorphic properties were investigated using differential scanning calorimetry and polarizing optical microscopy. All of the target compounds (except Id, IId, IIIc and IIId) exhibited an enantiotropic nematic mesophase with high melting temperatures. The liquid crystalline properties of these compounds were influenced greatly by polarity, steric factors and positions of the terminal groups. The effect of the terminal groups on the liquid crystal properties is discussed.

Microwave-assisted Synthesis, Structure, and Tunable Liquid-Crystal Properties of 2,5-Diaryl-1,3,4-Thiadiazole Derivatives through Peripheral n-Alkoxy Chains

A series of substituted 2,5-diaryl-1,3,4-thiadiazole derivatives are prepared by microwave-assisted synthesis in the absence of an organic solvent. All derivatives are well characterized by (1)H and (13)C NMR, MS, and elemental analyses. The X-ray crystal structure of 2,5-di-(4-decyloxyphenyl)-1,3,4-thiadiazole reveals the tilt lamellar arrangement of rod-shaped molecules, which are stabilized by a variety of weak non-covalent interactions. The liquid crystalline properties are studied by polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), and in situ variable temperature X-ray diffraction (VTXRD). By variations in the peripheral n-alkoxy chains, the calamitic mesogens exhibit enantiotropic smectic (SmC and/or SmA) mesophases with wide mesomorphic temperature ranges, whilst the disc-like mesogens form hexagonal columnar mesophase (Col(h)) at room temperature. The bulk electrical conductivity values of the smectic mesophases of 1-3 are in the range of 10(-3)-10(-4) S cm(-1), which are slightly higher than that of their solid films. In contrast, the solid film made from 2,5-di-(3,4,5-trioctyloxyphenyl)-1,3,4-thiadiazole shows poor conductivity (2x10(-7) S cm(-1)).

Fluorescent liquid crystalline compounds with 1,3,4-oxadiazole and benzo[b]thiophene units

A series of fluorescent liquid crystalline compounds containing 1,3,4-oxadiazole and benzo[b]thiophene units have been prepared. In CH2Cl2 solution, these compounds displayed a fluorescent emission with λmax at 422–426 nm and quantum yields of 41–48%. Most of them exhibited thermotropic liquid crystalline properties with nematic and/or smectic A phases with excellent thermal stability. This work revealed that longer terminal alkoxy chains would be detrimental to the formation of mesophases for such kind of compounds. The effect of the terminal groups on mesomorphic and spectroscopic property is discussed.

Synthesis and liquid crystalline property of H-shaped 1,3,4-thiadiazole dimers

Two H-shaped liquid crystalline 1,3,4-thiadiazole dimers D1 and D2 and the corresponding monomers M1 and M2 were synthesised and characterised by 1H/13C nuclear magnetic resonance (NMR) and high-resolution mass spectrometry. The thermal properties of the dimers and monomers were investigated by polarised optical microscopy, differential scanning calorimetry and thermogravimetric analysis. All of these compounds exhibited liquid crystalline behaviours with excellent thermal stabilities. The dimers displayed an enantiotropic or a monotropic nematic phase, while the monomers showed nematic and/or smectic A phases enantiotropically. The nature of the mesophases was determined by the molecular shapes and the terminal groups. Notably, the H-shaped dimers exhibited much lower clearing temperatures than the corresponding rod-like monomers.

Self-assembled liquid crystals formed by hydrogen bonding between non-mesogenic 1,3,4-oxadiazole-based pyridines and substituted benzoic acids

A series of new supramolecular liquid crystalline complexes have been prepared readily through intermolecular hydrogen bonding between non-mesogenic 1,3,4-oxadiazole-based pyridines and substituted benzoic acids. The liquid crystalline properties of the H-bonded complexes were studied by infrared spectrometry, polarising optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. All the H-bonded complexes exhibited liquid crystalline behaviours with various mesophases such as nematic (N), smectic A and hexagonal columnar mesophases, which are determined by the electronic nature of terminal groups and the number of alkoxy chains of the H-acceptors and H-donors. The rod-like H-bonded complexes 2a/3a, 2a/3b and 2a/3c with an electron-withdrawing end group (NO2, F or CN) fascinate to form an enantiotropic smectic A phase, whereas complex 2a/3d bearing an electron-donating methoxy end group tends to display a monotropic N phase. In contrast, the disc-shaped complexes 2b/3e and 2c/3e with six peripheral decyloxy chains are room temperature liquid crystals exhibiting columnar mesophase. All the complexes 2a/3a, 2a/3b, 2a/3c and 2a/3d in CH2Cl2 solutions exhibited an intense broad absorption peak at ca. 303 nm, and emitted a strong blue emission with λmax at ca. 390 nm and good photoluminescence quantum yields (ΦFL = 0.56–0.81).

Synthesis, mesomorphic behaviour and photo-luminescent property of new mesogens containing 1,3,4-oxadiazole fluorophore

A series of 1,3,4-oxadiazole derivatives, namely cholesteryl 4-(4-(5-(4-(alkoxy)phenyl)-1,3,4-oxadiazol-2-yl)phenylethynyl)benzoate (Ch-OXD- n , n = 6, 8, 10) and methyl 4-(4-(5-(4-(alkoxy)phenyl)-1,3,4-oxadiazol-2-yl)phenylethynyl)benzoate (Me-OXD- n , n = 6, 8, 10) were synthesised and characterised by means of 1H NMR, 13C NMR, MS and HRMS. The phase behaviours of these two series of compounds have been investigated by polarising microscopic and calorimetric studies. All compounds Ch-OXD- n exhibited a cholesteric mesophase with wide mesomorphic temperature range, while the compounds Me-OXD- n displayed nematic and/or smectic A mesophases with relatively narrow temperature ranges. Both Ch-OXD- 8 and Me-OXD- 8 in chloroform individually exhibited an intense absorption band (λmax = 330 nm) and a strong blue fluorescence emission (λmax = 404 nm) with good photoluminescence quantum yields.

Microwave-assisted synthesis and liquid crystal properties of 1,3,4-thiadiazole-based liquid crystals

A series of 2-(4-alkoxyphenyl)-5-p-tolyl-1,3,4-thiadiazole derivatives 2a–2h were synthesised efficiently under microwave irradiation and solvent-free conditions. The thermal properties were determined using polarised optical microscopy (POM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). All these compounds exhibited enantiotropic liquid crystal properties with wide mesomorphic temperature ranges and good thermal stability. Compounds 2a–2f with short alkoxy chain (n = 1–6) all exhibited an enantiotropic nematic mesophase, while both nematic and smectic C mesophases were observed in compounds 2g (n = 7) and 2h (n = 8) with longer alkoxy spacer. The effect of the length of the alkoxy chain on liquid crystal properties is discussed in this paper.