Unusual Dynamic Behavior in the Isotropic Phase of Banana Mesogens Detected by ²H NMR Line Width and T₂ Measurements

Abstract

In this work the first experimental observation of a peculiar behavior in the isotropic phase of liquid crystals by means of 2H NMR is reported. In particular, two five-ring banana-shaped mesogens, the 1,3-phenylenebis{4,4'-(11-undecenyloxy)benzoyloxy}benzoate (Pbis11BB) and its 4-chloro homologue (ClPbis11BB), selectively deuterium labeled on their central rings, are the subject of our investigation. The dynamic behavior of the two liquid crystals was studied in their isotropic phases and in the nematic phase of ClPbis11BB by means of 2H NMR line width and spin-spin relaxation time (T2) analysis. The results obtained reveal that the unusual line broadening observed in the 2H NMR spectra in the isotropic phase, even far above the isotropic phase-mesophase transition, has a homogeneous nature, thus indicating the presence of reorientational motions much slower than in conventional isotropic liquid-crystalline phases.

Rheological properties of bent-core liquid crystals

“…According to ref. 22 H relaxation times reveal a peculiar slow dynamic behavior, in accordance with prior DLS measurements, 11 1 H relaxation times seem to be affected only by fast dynamics. To explain these, both of our smectic cluster model and Domenici's entanglement model 22 were considered, but the data could not conclusively distinguish between these two.…”

“…This behavior can be clearly attributed to the bent molecular shape. We note that previous papers 10, 22 already suggested clusters based on electrohydrodynamic instability studies and NMR studies, respectively. According to ref.…”

Banana‐Shaped Molecules Peculiarly Oriented in a Magnetic Field:2H NMR Spectroscopy and Quantum Mechanical Calculations

“…In the case of banana-shaped molecules, however, complex molecular aggregates may form, in which molecules are not completely free to rotate and molecular dynamics is scaled to the low-frequency range with respect to common calamitic liquid crystals. [9,13,18,20] In this frame, the relation between the molecular and mesophase magnetic susceptibility anisotropy is much stronger.…”

“…Several speculations about the origin of phase chirality have been proposed, which link the molecular structure and symmetry to the peculiar molecular packing of such bent molecules within different mesophases. [7] In this context, NMR spectroscopy can play a significant role, as demonstrated by several studies based on both deuterium NMR [8][9][10] and carbon-13 NMR measurements. [11] Another aspect of great interest is the fact that banana-shaped liquid crystals (BLCs) have a particular behaviour even in the nematic phase.…”

“…Moreover, the orientational order parameter S, referred to the local aromatic core of bent molecules, was found to be very high (up to 0.85), and the 2 H spinspin relaxation times, as well as the experimental line widths, indicate that molecules are affected by fast reorientational motions. When the same BLC molecules are in the bulk, the 2 H NMR spectra are affected by much slower motions [9,13,16,25,26] and therefore show completely different features. These experimental findings support the idea that BLC molecules have a peculiar packing within the nematic phase under the effect of a magnetic field, as proposed in ref.…”