New insights into the transitional behaviour of methyl-6-O-(n-dodecanoyl)-α-D-glucopyranoside using variable temperature FTIR spectroscopy and X-ray diffraction

Abstract

The liquid crystalline behaviour of methyl-6-O-(n-dodecanoyl)-α-D-glucopyranoside, 1, has been characterised using X-ray diffraction and variable temperature Fourier transform infrared (FTIR spectroscopy). 1 exhibits a monotropic interdigitated smectic A phase consisting of bilayers in which the alkyl chains are overlapped. The crystal-isotropic transition is accompanied by a pronounced decrease in the strength of the hydrogen bonding network involving the sugar groups resulting in a marked change in the environment of the alkyl chains. The isotropic phase consists of disordered smectic-like domains stabilised via hydrogen bonding between the sugar groups. At the transition to the smectic A phase, a subtle change in hydrogen bonding is observed which is manifested by a change in the temperature dependence of the OH stretching peak position in the FTIR spectrum. On crystallisation, the strong hydrogen bonding network is re-established accompanied by a change in the conformational distribution of the alkyl chains. A model is proposed in which a combination of hydrogen bonding (enthalpic effects) and conformational arrangements (entropic effects) promotes initially the formation of smectic-like domains in the isotropic phase and subsequently stabilises the smectic A phase by inhibiting the microphase separation leading to the crystal phase.

Hydrogen bonding and liquid crystallinity of low molar mass and polymeric mesogens containing benzoic acids: a variable temperature Fourier transform infrared spectroscopic study

“…Similar behaviour is also seen for bands around 700 cm À1 region associated with C-H rocking, r, modes. [22] This contrasting behaviour supports the view that the methylene groups can be found in two differing chemical environments in the crystal phase and also that most of the methylene groups may be found either near alkyl chains or near aromatic rings of the neighbouring molecules. This is wholly consistent with the reported crystal structure, see Figure 7 [52] in which hydrophilic and hydrophobic domains exist.…”

“…Similar behaviour has been reported for other liquid crystalline systems. [22,23,51] We also observe changes in bands in the 1480-1400 cm À1 region, associated with inplane bending, d, of the aliphatic CH groups, see Figure 5 inset. At the low-frequency end of this region (1450-1400 cm À1 ), the bands assigned earlier to C-H bonds in close proximity to the ether bond increase in intensity on increasing temperature, showing step changes at the phase transitions.…”

“…[16][17][18] Hydrogen bonding also plays a key role in driving liquid crystalline behaviour in a diverse range of other materials including commercially important high-performance fibres, [19] protonic conductors [20,21] and liquid crystal sugars. [22][23][24][25] Supramolecular SCLCPs have attracted particular attention and these have been assembled not only using hydrogen bonding (see, for example, [7,26,27]), but also by utilising other types of non-covalent interactions such as ionic bonding [28] and quadrupolar interactions. [29] In approaching the design of a supramolecular SCLCP, it is useful first to consider the molecular architecture of a conventional SCLCP, which comprises three structural elements: a polymer backbone, a flexible spacer and a mesogenic group.…”

The role of hydrogen bonding in the phase behaviour of supramolecular liquid crystal dimers

“…We attribute this to the tendency of these mixtures to spontaneously align as evidenced by the observation of homeotropic textures when viewed through the polarised microscope. We saw similar, but more extreme behaviour, for hydrogen bonded liquid crystal sugars [34,35]. By contrast, the C-type band does not appear to change significantly on cooling, and this may reflect the different nature of the vibrations associated with this resonance band [48].…”

“…5. The decrease associated with crystallisation may be attributed to orientation effects discussed in detail elsewhere [34,35,46,47]. The observation of the B-type Fermi bands on cooling from the isotropic phase through the nematic and smectic C phases indicates extensive hydrogen bonding even at high temperatures.…”