2-Octyl thiophene based three ring mesogens: solid state (13)C NMR and XRD investigations

Strikingly different molecular organization and molecular order of tetracatenar mesogens in columnar mesophases revealed by XRD and 13C NMR

For a successful design of functional mesogens, it is paramount to understand factors that contribute to molecular organisation such as molecular shape, the non-covalent interactions of the constituent moieties as well as nanosegregation of incompatible molecular parts. In this study on four tetracatenar mesogens, we show that by a slight change in the length of the terminal chain, the molecular organization changes from lamellar to columnar phase and that the orientational order experiences profound change between the lamellar, the center rectangular columnar and the hexagonal columnar mesophases. We consider here, mesogens that exhibit lamellar and columnar mesophases with five phenyl rings in the central rod-like core which are subjected to XRD and high resolution solid state ¹³C NMR investigations in their mesophases. The XRD studies indicate that the lower homologs exhibit a lamellar mesophase while the higher homologs show either a centre rectangular columnar phase or a 2D hexagonal columnar mesophase. ¹³C NMR investigations reveal interesting and strikingly different molecular orientations in each of these phases. For example, values of order parameters of one of the phenyl rings in the core region of the mesogens vary from 0.75 and 0.77 for the lamellar mesogens to 0.45 and 0.17 for the centre rectangular columnar and the hexagonal columnar mesogens respectively. While these values indicate that the mesogenic molecules are oriented along the magnetic field as expected in the lamellar phases, the very low order parameter in the hexagonal columnar phase arises due to molecules distributed azimuthally in layers and undergoing motion about the columnar axis which itself is oriented orthogonal to the magnetic field. Such cutting edge information extracted from the combined use of XRD and ¹³C NMR studies on tetracatenar mesogens is expected to be of significant use for the study of π-conjugated polycatenar systems where functional properties depend on the molecular orientation and order.

13 C NMR investigations of molecular order of rod-like, bent-core, and thiophene mesogens

In this review, methods to obtain the orientational order of topologically variant molecular mesogens using by one- and two-dimensional (2D) solid-state ¹³ C nuclear magnetic resonance (NMR) spectroscopy are described. Besides ¹³ C chemical shifts, the ¹³ C─¹ H dipolar couplings measured from 2D-separated local field (SLF) technique are used for computing the order parameters of a variety of mesogens. The investigated molecules are composed of a variable number of rings in the core, that is, core ranging from simply one ring to five rings. Among the mesogens investigated, a special focus has been placed on mesogens with thiophene rings, which are gaining popularity as liquid crystalline organic semiconductors. The replacement of a phenyl ring by thiophene in the core has a dramatic influence on molecular topology, as observed from the measured order parameters. The review highlights the advantages of the 2D SLF method for understanding the local dynamics and for mapping the topology of mesogens through the measured order parameters. SLF NMR studies of as many as 24 molecular mesogens that vary in terms of the molecular structure as well as topology are covered in the review. Order parameters of the rings have been estimated from the ¹³ C─¹ H dipolar couplings in the nematic, smectic A, smectic C, and tilted hexatic phases as well as in B₁ and B₂ mesophases of various mesogens. It is anticipated that, in the years to come, the 2D SLF method would provide advanced molecular information on structurally complex mesogens that are emerging in liquid crystal science through the incessant efforts of synthetic chemists. The mini review covers the orientational order of topologically variant molecular mesogens determined by 1D and 2D solid-state ¹³ C NMR spectroscopy. Accordingly, rod-like, bent-core, and thiophene mesogens were subjected to 2D SLF measurements to get the order parameters from which the topology was established. The replacement of phenyl ring by thiophene and its influence on order parameters as well as on molecular topology is also discussed.

Effect of alkyl chain and linking units on mesophase transitions and molecular order of rod-like thiophene mesogens:13C NMR investigation

This study provides insight into the influence of alkyl chains and linking units on the mesophase properties of thiophene-based mesogens and on the orientational constraints of the core and terminal alkyl chain.

Molecular Order and Mesophase Investigation of Thiophene-Based Forked Mesogens

Thiophene-based rodlike molecules constructed from a three phenyl ring core and terminal dialkoxy chains recognized as forked mesogens are synthesized, and their mesophase properties as well as the molecular order are investigated. The synthesized forked mesogens would serve as model compounds for tetracatenar or biforked mesogens. On the basis of the position of the thiophene link with the rest of the core, 2-substituted and 3-substituted mesogens are realized in which the length of the terminal alkoxy chains is varied. The mesophase properties are evaluated using a hot-stage polarizing microscope and differential scanning calorimetry. For both homologues, the appearance of either nematic phase alone or in conjunction with smectic C phase is noticed depending on the length of the terminal alkoxy chains. The existence of layer ordering characteristic of the smectic C phase is confirmed for a representative mesogen using variable-temperature powder X-ray diffraction. High-resolution solid-state (13)C NMR measurements of C12 homologues of the two series reveal orientational order parameters of all rings of the core as well as terminal chains in the liquid crystalline phase. For both homologues, because of the asymmetry of ring I, the order parameter value is higher in contrast to ring II, ring III, and the thiophene ring. The chemical shifts and (13)C-(1)H dipolar couplings of OCH2 carbons of the terminal dodecyloxy chains provide contrasting conformations, reflecting the orientational constraints. Furthermore, the investigations also reveal that the mesophase range and the tendency for layer ordering are higher for 3-substituted mesogens compared to 2-substituted homologues.

(13)C NMR Studies, Molecular Order, and Mesophase Properties of Thiophene Mesogens

Three-ring mesogens with a core comprising thiophene linked to one phenyl ring directly and to the other via flexible ester are synthesized with terminal alkoxy chains to probe the mesophase properties and find the molecular order. The phenyl thiophene link in the core offers a comparison of the mesophase features with the molecular shape of the mesogen. The synthesized mesogens display enantiotropic polymesomorphism and accordingly nematic, smectic A, smectic C and smectic B mesophases are perceived depending upon the terminal chain length. For some of the homologues, monotropic higher order smectic phases such as smectic F and crystal E are also witnessed. The existence of polymesomorphism are originally observed by HOPM and DSC and further confirmed by powder X-ray diffraction studies. For the C8 homologue, high resolution solid state (13)C NMR spectroscopy is employed to find the molecular structure in the liquid crystalline phase and using the 2D SLF technique, the (13)C-(1)H dipolar couplings are extracted to calculate the order parameter. By comparing the ratio of local order of thiophene as well as phenyl rings, we establish the bent-core shape of the mesogen. Importantly, for assigning the carbon chemical shifts of the core unit of aligned C8 mesogen, the (13)C NMR measured in mesophase of the synthetic intermediate is employed. Thus, the proposed approach addresses the key step in the spectral assignment of target mesogens with the use of (13)C NMR data of mesomorphic intermediate.

Intramolecular charge transfer interactions and molecular order of rod like mesogens

Photophysical studies, VT-XRD and¹³C solid state NMR investigation of three ring based dimethylamino mesogens reveal intramolecular charge transfer, smectic A_dmesophase and molecular order.