New insights into the role of hydrogen bonding on the liquid crystal behaviour of 4-alkoxybenzoic acids: a detailed IR spectroscopy study

Hydrogen bonding is an efficient alternative to covalent bonding as a way to stabilise liquid crystallinity, by yielding symmetric and non-symmetric complexes with increased molecular anisotropy. In designing new hydrogen-bonded liquid crystals, HBLCs, it is crucial to account for the competing hydrogen bonds that can lead to different supramolecular species coexisting in a temperature-dependent equilibrium. Thus, as part of a systematic development of this area, in the present work we study with detail the relationships between the phase behaviour and hydrogen bonding in a series of 4-n-alkoxybenzoic acids, nOBAs, which are widely used as components in HBLCs. Five acids with alkyl chain lengths of n = 1, 4, 5, 7 and 8 have been investigated using Fourier transform infrared spectroscopy, FTIR, in a broad range of temperatures under two different experimental configurations: sandwiched between potassium bromide, KBr, windows and dispersed in KBr discs. The nematic phase is correlated with the amounts of closed dimers between acid molecules, through the formation of strong hydrogen bonds. Moreover, high concentrations of open dimers are found in samples sandwiched between KBr slides, which are linked to the appearance of smectic-like aggregates that perturb the local order of the nematic phase. The results are interpreted in terms of the ability of the 4-alkoxybenzoic acids to align due to surface interactions, which are less acute in samples dispersed in the discs. These effects must be taken into account in order to correctly interpret the information about the supramolecular species present in the samples, and thus to better understand the relationships between hydrogen bond strength and mesomorphism in HBLCs.