Wan-Sinn Yam

Synthesis and mesomorphic properties of 7‐acyloxy‐3‐(4‐acyloxyphenyl)‐4H‐1‐benzopyran‐4‐one

A new homologous series of 7‐acyloxy‐3‐(4‐acyloxyphenyl)‐4H‐1‐benzopyran‐4‐ones was synthesized and characterized by elemental analysis and spectroscopic techniques along with thermal behaviour study using differential scanning calorimetry. Texture observation was performed under the polarizing optical microscope from which the derivatives were found to exhibit different molecular organizations as exemplified by various mesophases. Whilst the compounds containing butanoyloxy and hexanoyloxy groups behave as nematogens, the other compounds with heptanoyloxy and longer side chains show a SmC phase. The octadecanoyloxy derivative was the only member showing a monotropic SmC phase. The difference in phase behaviour is discussed based on the molecular structure and the presence of a lateral dipole which can be associated with the intermolecular interaction within the mesophase.

Non-symmetric chiral isoflavone dimers: synthesis, characterisation and mesomorphic behaviour

A new series of non-symmetric chiral isoflavone-based liquid crystalline dimers, α-(2-methylbutyl-4′-(4″-phenyloxy)benzoate)-ω-(3-(4′-decyloxyphenyl)-4H-1-benzopyran-4-one-7-oxy)alkanes, with 3–12 carbon atoms in the alkyloxy spacer, have been synthesised. A pronounced odd–even effect for the phase transition temperatures upon varying the spacer length was observed. The short dimers exhibited monolayer smectic A (SmA) and smectic C (SmC*) phases while for longer homologues a chiral nematic (N*) phase was found. The temperature range of the nematic phase was broadened with elongation of the alkyl spacer. Stabilisation of the nematic phase resulted from competition between the monolayer and intercalated smectic structures. The SmA–SmC* phase transition was second order for all studied compounds with a cross over to the de Vries type behaviour for the shortest homologue.

New para-substituted non-symmetric isoflavones for their fast photo-switching ability: Synthesis and their liquid crystal characterization

The first example of non-symmetric isoflavone-based fast photo-switchable liquid crystals with different functional groups at the terminal position were synthesized and characterized. Polarizing optical microscopy study revealed that the compounds showed least ordered nematic phase. Optical photo switching study exhibited very fast photoisomerization effect in solution. The E-Z and Z-E conversion occurred around 3-5s and 40-700 s respectively. This is also the first example of para-substituted non-symmetric isoflavone liquid crystals exhibiting very fast photo switching property in solution. Argument based on non-symmetrical behaviour might be the reason for the observed behaviour.

Synthesis and Phase Behavior of New Isoflavone Derivatives: Crystal Structure of 7-Hexyloxy-3-[4′-(3-methylbutyloxy)phenyl]-4H-1-benzopyran-4-one

This article describes the synthesis and mesomorophic behavior of a novel homologous series of 7-alkyloxy-3-[4′-(3-methylbutyloxyphenyl)-4H-1-benzopyran-4-one. The title compounds were made up of central isoflavone core with branched and linear alkyloxy terminal chains at C-4′ and C-7, respectively. The influences of linear alkyloxy terminal chain in different length of OR (where R = CnH2n+1; even number of n ranging from 4 to 18) were discussed. The thermal behavior especially the phase transition and respective enthalpy values of the compounds thus synthesized were analyzed using differential scanning calorimetry. The occurrence of mesophases under the polarized light has suggested the molecular orientation and arrangement of the title compounds. The molecular structure of compound 7-hexyloxy-3-[4′-(3methylbutyloxyphenyl)]-4H-1-benzopyran-4-one in crystal phase was confirmed by single-crystal X-ray diffraction of which the space group is P-1(#2) with the lattice parameters a = 6.100(5) Å, b = 11.704(10) Å, c = 17.082(17) Å, α = 80.85(4)°, β = 85.56(4)°, γ = 72.48(3)°, and V = 1147.6(18) Å3. The elongated alkyloxy terminal chains were found to be fully stretched in solid phase. All present compounds except the derivative with R = C4H9 were smectogenic.

Chemical constituents and biological activities of the genus Subergorgia.

The genus Subergorgia (coelenterata, Gorgonacea, Subergorgiidae) is distributed in the Indo-pacific region. Previous investigations on the various species of the genus have revealed the presence of a number of new compounds including alkaloids, sesquiterpenes, diterpenes, and steroids. Certain biological activities particularly cytotoxic activity have been observed for the isolated constituents and compositions derived from the coral. This review covers the secondary metabolites reported from the genus Subergorgia and their biological properties.

A crossover from rod-shaped to bent-shaped in symmetric isoflavone liquid crystal trimers exhibiting unusual mesomorphic behaviour

A new series of symmetric isoflavone-based liquid crystalline trimers, 1,3-bis{[3-(4′-decyloxyphenyl)-4H-1-benzopyran-4-one-7-oxy]alkyloxy}benzenes with 3–12 methylene units in both lateral spacers have been synthesized and characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. As expected, the phase transition temperatures strongly alternate as the number of methylene units in the spacers changes. An unusual mesomorphic behaviour was observed wherein short and long trimers exhibited smectic A (SmA) and/or smectic C (SmC) phase(s) while homologues with intermediate spacer length were purely nematogenic (N) in a broad temperature range. Formation and stabilization of the uniaxial nematic phase is caused by change of molecular geometry from rod-shaped for short homologues to strongly bent-shaped for long homologues.

Synthesis, anisotropic behaviour and structural changes in some para-substituted isoflavones: 4′-substituted-7-(4″-decyloxybenzoyloxy)-4H-1-benzopyran-4-ones

A new series of 4′-substituted-7-(4″-decyloxybenzoyloxy)-4H-1-benzopyran-4-one esters have been synthesized and their mesomorphic properties have been studied along with the spectroscopic and optical-oriented technique in which the smectic layer spacing of compound with fluorine substituent has been well substantiated by X-ray diffraction. Phase transition temperatures and the respective enthalpy values of the title compounds are obtained from differential scanning calorimetry, whilst texture observation is performed under polarizing optical microscopy. The thermal data indicate that all title compounds are thermotropic exhibiting either monomorphism or variants of polymorphism. Crystal paramorphism is also observed for some of the title compounds. The variation in transition temperatures as a consequence of structural changes of the title compounds is discussed.

A Comparative Study of the Volatile Constituents of Southeast Asian Coffea arabica, Coffea liberica and Coffea robusta Green Beans and their Antioxidant Activities

Abstract A comparative study on the volatile constituents of green beans of three commercially grown coffees in Southeast Asia, namely Coffea arabica, Coffea robusta and Coffea liberica, and their antioxidant properties was carried out. Volatile constituents of the green beans were isolated by hydrodistillation followed by extraction of the distillates with dichloromethane. These compounds were analysed using capillary gas chromatography and gas chromatography-mass spectrometry. The volatile compositions of the green beans of these three species were remarkably similar, being characterized by high levels of phenolic compounds, principally p-vinylguaiacol, and carboxylic acids. The volatile extracts were examined for antioxidant activity using total phenols, DPPH radical scavenging and FRAP assays which showed that the volatile extract from C. liberica possessed the highest antioxidant capacity, followed by those of C. arabica and C. robusta, respectively.

Synthesis and Phase Behaviour of Some New Isoflavone Derivatives

An overview on new isoflavones comprising 7- and 3-disubsituted-4H-1-benzopyran-4-ones has been carried out. The derivatives thus obtained exhibit different molecular orientation and phase sequences as evident by texture observation under polarized light as well as differential scanning calorimetry. Typical examples from these newly derived isoflavones include those compounds possessing butanoyloxy and hexanoyloxy groups at both 7- and 3-position in benzopyran-4-ones which favour the nematic phases. The compounds attached by heptanoyloxy and longer side chains at both 7- and 3-positions show SmC phase. The monotropic SmC phase has been observed for the compound with octadecanoyloxy moiety only. The analogues 7-decanoyloxy-3-(4′-substitutedphenyl)- 4H-1-benzopyran-4-one possessing different substituents at 4′-position exhibiting various polymorphism. The results on the structural changes, the electronic polarizability, the presence of lateral dipole associated with the intermolecular interaction within the mesophase and the crystal structure at solid phase in the derivatives containing 3-methylbutyloxy group at 4′-position will also be discussed.

Asymmetric N-heterocyclic carbene benzimidazolium salts and their silver(I) complexes: potential as ionic liquid crystals

ABSTRACT The synthesis and characterisation of a homologous series of monodentate benzimidazolium salts, 1–4 and their mononuclear silver(I)–NHC (where NHC = N-heterocyclic carbene) complexes, 5–8, are reported. The benzimidazolium salts were prepared from the N-alkylation of 1-methyl-benzimidazole with alkyl halides of varying carbon chain lengths. The mono silver(I)-NHC complexes, 5–8, were prepared by the reaction of the benzimidazolium salts with Ag2O. All the synthesised compounds were fully characterised by 1H-nuclear magnetic resonance (1H-NMR), 13C-NMR and fourier-transform infrared (FTIR) spectroscopy. The molecular structures of compounds 3·PF6, 4·PF6, 7 and 8 were elucidated through single-crystal X-ray diffraction analyses. We postulate that the attachment of long alkyl chains to the heterocyclic core of 1-methyl benzimidazole could induce mesophase formation. The liquid crystalline behaviour of the benzimidazolium salts was investigated by polarised optical microscope and differential scanning calorimetry. Salts 3 and 4 were found to be thermotropic liquid crystals which exhibited a smectic A phase. However, upon complexation with silver(I) ions, all the Ag(I)–NHC complexes are found to be non-mesogenic. Graphical Abstract