From the SmA to the Hexatic, Including the SmC*, SmC*_A and SmC*_re Phases: A 2H NMR Relaxation Study

Abstract

The molecular dynamics of a ferroelectric liquid crystal, denoted ZLL 7/*, is investigated by means of (2)H NMR relaxation. The spin-lattice (T(1Q) and T(1Z)) and spin-spin (T(2)) relaxation times of two isotopomers of ZLL 7/*, labeled on the phenyl and biphenyl fragments, are measured and their behavior upon passing from the SmA to the hexatic phase, through the ferroelectric SmC*, antiferroelectric SmC*(A), and re-entrant ferroelectric SmC*(re) phases, is discussed. A comparison between the measured T(2) and T(2)*, directly related to the experimental linewidth, provides information on the heterogeneity of the system, thus allowing confirmation of previous hypotheses concerning the structural and ordering properties of the SmC*(A) and SmC*(re) phases. The possibility to look at different sites of the core of the ZLL 7/* smectogen reveals a peculiar sensitivity of the phenyl moiety with respect to the biphenyl fragment, which may be justified by its vicinity to the chiral centers. Interestingly, the trend of the longitudinal relaxation times is characterized by a minimum that corresponds to the SmC*(A) and SmC*(re) phases, which is reproducible for the two isotopomers and at several Larmor frequencies. A quantitative analysis of T(1Q) and T(1Z) is performed in the SmA and SmC* phases, for which the narrowing regime approximation is valid. A multifrequency approach is applied to self-consistently determine the diffusion coefficients for the overall molecular motions, namely spinning and tumbling, and the internal rotations around the para axes of the phenyl and biphenyl fragments. The effect of the magnetic field in unwinding the helical structure of the SmC* phase (for H>9 T) allows observation of a sensitive change in the rotational diffusion coefficients in the frustrated unwound SmC* phase with respect to the SmC* phase.

NMR studies of the ferroelectric SmC* phase

“…Three widely investigated samples are the smectogens 10B1M7 [12,[21][22][23][24][25][26], 11EB1M7 [14,[27][28][29][30][31][32], and ZLL7/* [15,[32][33][34][35]38], where two isotopomers selectively deuterium labelled on the phenyl and biphenyl moieties were available (see Figure 1). This allowed us to determine the local orientational order of the two fragments and the relative angle a between them, as presented in Section 4.2.…”

“…Structural, orientational order and dynamic properties of 8BEF5 [10,17,18], MBHB [11,[19][20][21], 10B1M7 [12,[21][22][23][24][25][26], 11EB1M7 [14,[27][28][29][30][31][32], ZLL7/* [15,[32][33][34][35] and M10/** [16,36] samples were recently investigated by 2 H-NMR and 13 C-NMR spectroscopy. 2 H-NMR experiments were performed on different NMR spectrometers, which allowed us to study the influence of different magnetic fields strengths (from 4.7 to 9.04 T) on the supramolecular structure of chiral smectic phases of some of the compounds investigated, namely MBHB, 11EB1M7 and ZLL7/* [19,32,37,38].…”

Paranematic–nematic phase transition in liquid crystalline elastomers: a2H-NMR study

“…In fact, the unusual broadening of the 2 H-NMR signal in the isotropic phase of liquid crystals has also been observed in other LC systems, such as bentcore liquid crystals [33,34], and this feature is related to slow reorientation motions due to the presence of locally organized (and oriented) clusters [33][34][35]. The measure of T 2 and the comparison with the T Ã 2 is a very valuable method to study the motional processes in the slow-intermediate regime of motions [32][33][34][35][36][37], due to the high sensitivity of T 2 to motions in these regimes [30]. Also in the present case, the line broadening observed for LSCE systems could be fully justified in terms of dynamic motions due to the correspondence between the measured T 2 and T Ã 2 .…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…To list a few recent 13 C relaxation works, these include 10B1M7 [217], 4-(1-methylheptyloxycarbonyl)phenyl-4 0 -octyloxybiphenyl-4-carboxylate (MHPOBC) [218][219][220], 4-(2-methyloctanoyl)phenyl 4 0 -nonylbiphenyl-4-carboxylate (MONBIC) [216], 4-(1-methylhexyloxycarbonyl) phenyl 4 0 -octyloxybiphenyl-4-carboxylate (MH(6)POBC) [221]. In general, the 13 C T À1 1 rates of the aromatic carbons increase with decreasing temperature in a manner similar to the observed spectral densities for the aromatic deuterated sites (see for instant, ZLL 7/* [189]). The 13 C T À1 1 rates also increase along the flexible chain for sites moving toward the aromatic core.…”

“…In particular, these studies were carried out at several 2 H Larmor frequencies of the order of tens of MHz. Both T 1 and T 2 trends of the two aromatic moieties are generally sensitive to different frequency regimes of motions, reflecting closely to the supramolecular structures in different mesophases [189].…”