Order and Dynamics of a Liquid Crystalline Dendrimer by Means of ²H NMR Spectroscopy

Abstract

A complete Deuterium NMR study performed on partially deuterated liquid crystalline carbosilane dendrimer is here reported. The dendrimer under investigation shows a SmA phase in a large temperature range from 381 to 293 K, and its mesophasic properties have been previously determined. However, in this work the occurrence of a biphasic region between the isotropic and SmA phases has been put in evidence. The orientational order of the dendrimer, labeled on its lateral mesogenic units, is here evaluated in the whole temperature range by means of (2)H NMR, revealing a peculiar trend at low temperatures (T < 326 K). This aspect has been further investigated by a detailed analysis of the (2)H NMR spectral features, such as the quadrupolar splitting, the line shape, and the line-width, as a function of temperature. In the context of a detailed NMR analysis, relaxation times (T(1) and T(2)) have also been measured, pointing out a slowing down of the dynamics by decreasing the temperature, which determines from one side the spectral changes observed in the NMR spectra, on the other the observation of a minimum in the T(1).

NMR relaxation of the orientation of single segments in semiflexible dendrimers

“…Nuclear magnetic resonance (NMR) spectroscopy is a very effective method for studying the local dynamics. [3][4][5][6][7][8][9][10][11][12] Recent works 11,12 have demonstrated that one can address via NMR different shells of Frechet-type dendrimers and thus monitor their segment mobilities separately. In particular, one can differentiate between the inner and the terminal segments.…”

Nuclear magnetic resonance: a powerful tool to study liquid crystals

“…The analysis of the spectral shape of 2 H NMR spectra of liquid crystals provided an excellent and powerful method to investigate the supramolecular structure of liquid crystals, such as in frustrated phases (TGBA, TGBC* and so on) [24] and ferroelectric (SmC*) phases [25]. Dynamic properties, such as molecular reorientational diffusion, translational self-diffusion and collective motions are mainly investigated by specific NMR approaches, such as NMR diffusometry [26] and NMR relaxometry [27] (which requires specific equipment), or by analysing the spin-spin (T 2 ) and spin-lattice (T 1 ) relaxation times (mainly 1 H and 2 H) through specific models [28][29][30]. In few cases, for instance if molecules are distributed in not homogeneously aligned structures, 2D exchange NMR methods can also be applied [31].…”

The role of NMR in the study of partially ordered materials: Perspectives and challenges

“…Despite their spherical shape in solution or in the isotropic phase, LC dendrimers arrange in highly anisotropic structures when entering the LC mesophases. The microsegregation between the cores and the terminal units observed in smectic phases of carbosilane dendrimers, as well as the ordering properties of these LC dendrimers in the presence of external fields, were investigated by 2 H and 1 H NMR [76][77][78].…”