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

Abstract

The phase behaviour and mesomorphism of poly(4-(6-propenoyloxyhexyloxy)benzoic acid) (PPOHBA) and 4-pentyloxybenzoic acid (POBA) is studied using variabletemperature Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction. PPHOBA exhibits a smectic C phase and POBA, a nematic phase. The temperature dependence of the Fermi resonance bands associated with the hydroxyl groups and of the carbonyl stretching region in the FTIR spectra indicates that there is a dynamic equilibrium between monomers and open and closed dimers formed by hydrogen bonding between benzoic acid moieties. The nematic phase observed for POBA is linked to the anisotropic cyclic dimer, while an abrupt increase in the concentration of monomer drives isotropisation. In PPOHBA, hydrogen-bonded supramesogens promote smectic behaviour, while hydrogen-bonded crosslinks stabilise the lamellae. The increased viscosity arising from this dynamic crosslinking is offset by the flexibility of the acrylate backbone and alkyl spacers.

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

“…Similar changes have been reported for other liquid crystalline benzoic acids, and accounted for in terms of a dynamic equilibrium between cyclic and open dimers, and the monomeric species, as sketched in Fig. 7 [41,48]. Thus, the bands at around 1680 cm À1 and 1700 cm À1 contain contributions associated with cyclic and open hydrogen-bonded dimers, respectively, whereas the band seen at about 1730 cm À1 is associated with the monomeric species also refereed to as free acid [49e51].…”

“…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].…”

“…11. Three peaks have been used to fit contributions arising from OOBA at y $ 1720 cm À1 (free molecules), 1700 cm À1 (open dimers) and 1680 cm À1 (cyclic dimers) [48], and two bands at 1710 and 1690 cm À1 are assigned to the carbonyl groups in the OOBA/BuABOnOPyr complex [41,52]. The use of two bands to account for the contribution of the complex in these spectra rather just a single band, arises from the double minimum potential energy surface most commonly used to describe the unionised hydrogen bond between an acid and pyridyl fragments [53].…”

Mesophase behaviour of azobenzene-based angular supramolecular hydrogen-bonded liquid crystals

“…FTIR spectroscopy FTIR spectroscopy was chosen to confirm the formation of intermolecular hydrogen bond between the acid components (In) and pyridine-based derivatives (IIm) because of its sensitivity to the change of molecular conformation via inter-or intramolecular interactions. [41][42][43] The IR absorption spectra for all 4-n-alkoxyphenylazo benzoic acids [38,39] homologues (In) were found to be very similar indicating that the mesomeric shift of the acid C = O group is not affected by the change in the alkoxy chain length (n). This was also the case for the IR spectra obtained for 4ʹ-2ʹʹ-pyridylazophenyl-4-n-alkoxyphenyl benzoates (IIm), where similar IR spectra were observed for all derivatives (IIm).…”

An Optical Sensor Based on a Photonic Polymer Film to Detect Calcium in Serum

“…[ 25 ] A highly intense peak centered at 1696 cm −1 was observed due to the stretching vibration of hydrogen bonded C O. The broad nature of this peak suggests the presence of more than one type of H-bonded carboxylic acids, [ 26 ] i.e., H-bonded MAAs, H-bonded benzoic acids, and mixed MAA-benzoic acid complexes. It should be noted that since the molar concentration of MAA with respect to each benzoic acid is double, not all the carboxylic acid moieties of the MAA molecules are bound to the polymer network.…”