Wen-Hao Yu

Tetraphenylethene–triphenylene oligomers with an aggregation-induced emission effect and discotic columnar mesophase

Tetraphenylethene derivatives not only exhibit “aggregation-induced emission” (AIE) effects, but also can form a mesophase with a turbine-like rigid core. Here we report the synthesis of three tetraphenylethene–triphenylene oligomers 7, 8E and 8Z by a CuSO4/sodium ascorbate catalyzed alkyne–azide cycloaddition reaction (CuAAC), and the successful separation of the cis- and trans-isomers by column chromatography. The structure and conformation of the oligomers were fully characterized. The photophysical properties were studied by using UV-vision and photoluminescent (PL) spectroscopy. The morphology of the aggregates was investigated by scanning electron microscopy (SEM), and the thermal stability and mesomorphic properties were investigated using thermal gravimetric analysis (TGA), polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The results revealed that the oligomers possess an AIE effect, but display no mesomorphism. However, 7 exhibits a hexagonal columnar mesophase when mixed with 2,4,7-trinitrofluorenone (TNF), forming a charge transfer complex.

Synthesis and mesomorphism of ether–ester mixed tail C3-symmetrical truxene discotic liquid crystals

Truxene discotic liquid crystals are interesting for their polymesomorphism and as organic semiconductor candidates. We developed an efficient synthetic method for C3-symmetrical truxene derivative 2,7,12-trihydroxy-3,8,13-trimethoxytruxene 5, and further a series of ether–ester mixed tail truxene discogens HTn, TR(OMe)3[OOCC6H2(OCnH2n+1)3]3 were prepared. The truxene discogens HTn (n = 4–8, 10, 12) were characterised by using 1H and 13C nuclear magnetic resonance, and high-resolution mass spectrometry. Their thermal stability was studied with thermal gravimetric analysis and their liquid crystal properties were investigated by using polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. They exhibit a hexagonal columnar (Colh) mesophase in fairly wide temperature ranges.

Tuning Hydrogen-Bonding with Amide Groups for Stable Columnar Mesophases of Triphenylene Discogens

Tuning hydrogen-bonding of discotic liquid crystal can result in more ordered columnar mesophases and higher charge carrier mobility materials. In this paper, triphenylene derivatives with one, two and three amide groups with structure of C18H6(OR)5(OCH2CONHBu), C18H6(OC5H11)4(OCH2CONHBu)2 and symmetrical and asymmetrical C18H6(OR)3(OCH2CONHBu)3 (R = C n H2n+1, n = 4–9) respectively were designed and synthesized. The results showed that each intra-columnar H-bond forming amide (–CONH–) group caused an average increase of 22°C in clearing point from the non-H-bonding ester group (–COO–) triphenylene analogous. This was the first quantitative result of H-bonding on the stabilization of columnar mesophase.

New phosphorescent platinum(II) complexes: lamellar mesophase and mechanochromism

Several new square planar platinum(II) complexes based on modified 2-phenylpyridine derivatives as the main ligand and picolinic acid as the auxiliary ligand were synthesized and their photophysical properties, and mechanochromic and liquid crystalline behavior were investigated. It is found that only the Pt–L1 complexes, in which one flexible long chain is attached to each end of 2-phenylpyridine, display monotropic smectic liquid crystalline behavior. Moreover, mechanochromism has been observed by grinding the solid complexes. It seems that the liquid crystalline and mechanochromic properties of these complexes are relative to the number of flexible alkoxy chains in the cyclometalated ligand.

Synthesis and characterisation of 3-armed dendritic molecules with triphenylbenzene or triphenyltriazine as core and triphenylene derivative as shells

Two series 3-armed dendritic molecules with their 1,3,5-tris(4-methyloxyphenyl) benzene or 2,4,6-tris(4- hydroxylphenyl)-1,3,5-s-triazine core and triphenylene derivative shells connected by soft hydrocarbon chains have been synthesised and fully characterised. Whether liquid crystal (LC) or photoluminescence materials have been tuned by changing the length of the soft chain and they have been studied by POM, DSC, XRD, UV-Vis and PL. The results show that all the new dendritic compounds display UV to blue fluorescence in solvents such as DCM, THF and acetone. The TC3 compound bearing 1,3,5-tris(4-hydroxylphenyl) benzene core exhibits LC property with rectangular columnar phase (Colr) on cooling.

Novel columnar metallomesogens based on cationic platinum(II) complexes without long peripheral chains

Phosphorescent cationic platinum(II) complexes were obtained and among them the complexes without any peripheral flexible chains around the platinum(II) center show thermotropic columnar liquid crystal properties.

Monosubstituted ferrocene liquid crystals containing click triazole with a wide nematic phase temperature range

ABSTRACT New monosubstituted ferrocenomesogens of the 1,4-substituted-1,2,3-triazole series 3a-3d and the 5-halogen-1,4-substituted-1,2,3-triazole series 4a-4d were successfully synthesised through a facile and versatile synthetic route using click reactions with moderate yields, in different atmospheres, and catalysed by different amounts of CuX (X = Br, I). Two series of compounds exhibited a typical nematic liquid crystal texture, and the 5-halogen-1,4-substituted-1,2,3-triazole series 4 had a wider mesomorphic temperature range than the corresponding series 3 due to the effect of the halogen atom substituent on the triazole. These compounds showed similar absorption spectra, but the fluorescence emission spectra of 4 were obviously redshift relative to 3. Cyclic voltammetry investigations revealed that the redox-active ferrocenyl groups of 3 and 4 follow one-electron transfer processes. GRAPHICAL ABSTRACT

Influence of peripheral alkyl chain length on the mesomorphic behaviours of hexasubstituted triphenylene 2,3-dicarboxylic esters

ABSTRACT A comparative study of our established synthetic approaches to hexasubsituted triphenylenes 2,3-dicarboxylic esters containing four identical β-alkoxy and two adjacent β-alkoxycarbonyl side chains shows that the phase behaviours of small-sized discotic liquid crystals can be tailored over a wide range by simply varying the length of the peripheral alkyl chains. All the prepared esters in two series were observed to form a single hexagonal columnar phase, except for Tp4-1 having four β-butyloxy and two adjacent β-methoxycarbonyl chains which displays two columnar mesophase behaviours with a transition from the columnar plastic phase to hexagonal columnar phase. A significant difference between the two mesophase was observed in the variable temperature X-ray diffraction studies, and the mesophase assignment was also confirmed by polarising optical microscopy and differential scanning calorimetry. Moreover, the prepared esters in each series display the general trend of decreasing clearing temperature upon increasing alkoxy or alkoxycarbonyl chains length. The intermediate triphenylene 2,3-dicarboxylic acids were also found not only to exhibit columnar hexagonal mesophase over a narrower temperature range by maintaining high melting and clearing points but also to form organogel on mixing with toluene or dichloromethane with the assistance of hydrogen bonding. GRAPHICAL ABSTRACT

Synthesis and Mesomorphism of Asymmetric Triphenylene Discotic Liquid Crystals Bearing with Mono-Amido-Based Alkyl Soft Chains

A series of triphenylene discotic liquid crystals with amido-based alkyl chains in different lengths were synthesized and characterized. The liquid crystal properties were studied with differential scanning calorimetry and polarizing optical microscopy. The influences of amide alkyl chain lengths on the mesomorphism were discussed. Based on the results, we concluded that the triphenylene discogens with an amido-based alkyl chain could form intermolecular hydrogen bonds inside their columnar stacks, and hence, exhibit higher melting and clearing points, and wider mesophase ranges.

Synthesis and characterization of new H-shaped triphenylene discotic room-temperature liquid crystal tetramers by a copper-free click reaction

Herein, four new H-shaped triphenylene discotic liquid crystal tetramers have been designed and synthesized using a copper-free [3+2] cycloaddition reaction between the triphenylene dimer and diazide compounds. To probe the molecular self-assembly and mesophase properties, the length and rigid spacers in diazide have been tailored. The tetramers 7 with a soft spacer display a room-temperature enantiotropic DLC mesophase, and X-ray diffraction confirms that they exhibit an unusual mesophase of a lamellar–hexagonal columnar phase (Colhl). The results of the photophysical tests show that all the new H-shaped DLC tetramers display relatively high intensity photoluminescence properties.