1
|
Phani Kumar BVN, Lobo NP, Mattea C, Stapf S. Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation. J Phys Chem B 2024; 128:3997-4007. [PMID: 38616575 DOI: 10.1021/acs.jpcb.3c08259] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/16/2024]
Abstract
The molecular dynamics study of thermotropic mesogens exhibiting the crystal phases is valuable in unraveling the complex global (collective) and local (noncollective) motions executed by liquid crystal molecules, which would further advance the existing knowledge on orientationally disordered crystalline (ODIC) phases. Toward the fulfillment of such a task, a combined nuclear magnetic resonance (NMR) relaxometry approach employing the fast field cycling (FFC) NMR (10 kHz-30 MHz) and high-field pulsed NMR (400 MHz) techniques is utilized to sample the broad frequency range offered by molecular motions in the crystal phase of 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The validity of the observed relaxation data is tested and interpreted by the Bloembergen-Purcell-Pound (BPP) model involving the superposition of four mutually independent Lorentzian spectral densities, reflecting molecular dynamical processes on different time scales. The salient feature of the detailed analysis reveals that the lengthening of temporal dynamics in the crystal phase due to molecular rotations by jumps, which are of intermolecular origin, is evident and further supports the presence of collective-like local dynamics. The analysis does permit decoupling of the molecular reorientations about their short axes (∼100 ns) as well as long axes (∼50 ns) and methyl group rotations (∼0.5 ns) on distinct time scales. The activation energies for reorientations about the short axes and methyl group rotations are found to be 27.3 ± 2.7 and 15.8 ± 1.1 kJ/mol, respectively. The fast methyl rotations in the crystal phase of 6CHBT obtained from FFC NMR are further well complemented by high-field NMR, where 1H NMR line shapes are relatively narrow when compared to those of the nematic phase.
Collapse
Affiliation(s)
- Bandaru V N Phani Kumar
- Department of Technical Physics II/Polymer Physics, Institute of Physics, Ilmenau University of Technology, PO Box 100565, D-98684 Ilmenau, Germany
- NMR, Centre for Analysis, Testing, Evaluation & Reporting Services (CATERS), CSIR-Central Leather Research Institute, Chennai 600020, India
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
| | - Nitin P Lobo
- NMR, Centre for Analysis, Testing, Evaluation & Reporting Services (CATERS), CSIR-Central Leather Research Institute, Chennai 600020, India
- Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India
| | - Carlos Mattea
- Department of Technical Physics II/Polymer Physics, Institute of Physics, Ilmenau University of Technology, PO Box 100565, D-98684 Ilmenau, Germany
| | - Siegfried Stapf
- Department of Technical Physics II/Polymer Physics, Institute of Physics, Ilmenau University of Technology, PO Box 100565, D-98684 Ilmenau, Germany
| |
Collapse
|
2
|
Merkel K, Loska B, Arakawa Y, Mehl GH, Karcz J, Kocot A. How Do Intermolecular Interactions Evolve at the Nematic to Twist–Bent Phase Transition? Int J Mol Sci 2022; 23:ijms231911018. [PMID: 36232324 PMCID: PMC9570452 DOI: 10.3390/ijms231911018] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2022] [Revised: 09/15/2022] [Accepted: 09/18/2022] [Indexed: 11/30/2022] Open
Abstract
Polarized beam infrared (IR) spectroscopy provides valuable information on changes in the orientation of samples in nematic phases, especially on the role of intermolecular interactions in forming the periodically modulated twist–bent phase. Infrared absorbance measurements and quantum chemistry calculations based on the density functional theory (DFT) were performed to investigate the structure and how the molecules interact in the nematic (N) and twist–bend (NTB) phases of thioether dimers. The nematic twist–bend phase observed significant changes in the mean IR absorbance. On cooling, the transition from the N phase to the NTB phase was found to be accompanied by a marked decrease in absorbance for longitudinal dipoles. Then, with further cooling, the absorbance of the transverse dipoles increased, indicating that transverse dipoles became correlated in parallel. To investigate the influence of the closest neighbors, DFT calculations were performed. As a result of the optimization of the molecular cores system, we observed changes in the square of the transition dipoles, which well corresponds to absorbance changes observed in the IR spectra. Interactions of molecules dominated by pairing were observed, as well as the axial shift of the core to each other.
Collapse
Affiliation(s)
- Katarzyna Merkel
- Institute of Materials Engineering, Faculty of Science and Technology, University of Silesia, ul. 75 Pułku Piechoty, 41-500 Chorzów, Poland
| | - Barbara Loska
- Institute of Materials Engineering, Faculty of Science and Technology, University of Silesia, ul. 75 Pułku Piechoty, 41-500 Chorzów, Poland
| | - Yuki Arakawa
- Department of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan
| | - Georg H. Mehl
- Department of Chemistry, University of Hull, Hull HU6 7RX, UK
| | - Jakub Karcz
- Faculty of Advanced Technologies and Chemistry, Military University of Technology, 00-908 Warszawa, Poland
| | - Antoni Kocot
- Institute of Materials Engineering, Faculty of Science and Technology, University of Silesia, ul. 75 Pułku Piechoty, 41-500 Chorzów, Poland
- Correspondence: ; Tel.: +48-32-3497630
| |
Collapse
|
3
|
Kocot A, Loska B, Arakawa Y, Mehl GH, Merkel K. Study of the Experimental and Simulated Vibrational Spectra Together with Conformational Analysis of Thioether Cyanobiphenyl-Based Liquid Crystal Dimers. Int J Mol Sci 2022; 23:ijms23148005. [PMID: 35887352 PMCID: PMC9316788 DOI: 10.3390/ijms23148005] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2022] [Revised: 07/15/2022] [Accepted: 07/18/2022] [Indexed: 11/16/2022] Open
Abstract
Infrared spectroscopy (IR) and quantum chemistry calculations that are based on the density functional theory (DFT) have been used to study the structure and molecular interactions of the nematic and twist-bend phases of thioether-linked dimers. Infrared absorbance measurements were conducted in a polarized beam for a homogeneously aligned sample in order to obtain more details about the orientation of the vibrational transition dipole moments. The distributions to investigate the structure and conformation of the molecule dihedral angle were calculated. The calculated spectrum was compared with the experimental infrared spectra and as a result, detailed vibrational assignments are reported.
Collapse
Affiliation(s)
- Antoni Kocot
- Institute of Materials Engineering, Faculty of Science and Technology, University of Silesia, ul. 75. Pułku Piechoty, 41-500 Chorzów, Poland; (A.K.); (B.L.)
| | - Barbara Loska
- Institute of Materials Engineering, Faculty of Science and Technology, University of Silesia, ul. 75. Pułku Piechoty, 41-500 Chorzów, Poland; (A.K.); (B.L.)
| | - Yuki Arakawa
- Department of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan;
| | - Georg H. Mehl
- Department of Chemistry, University of Hull, Hull HU6 7RX, UK;
| | - Katarzyna Merkel
- Institute of Materials Engineering, Faculty of Science and Technology, University of Silesia, ul. 75. Pułku Piechoty, 41-500 Chorzów, Poland; (A.K.); (B.L.)
- Correspondence: ; Tel.: +48-32-349-7630
| |
Collapse
|
4
|
Lehmann M, Maisch S, Scheuring N, Carvalho J, Cruz C, Sebastião PJ, Dong RY. From molecular biaxiality of real board-shaped mesogens to phase biaxiality? On the hunt for the holy grail of liquid crystal science. SOFT MATTER 2019; 15:8496-8511. [PMID: 31633147 DOI: 10.1039/c9sm01007f] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
In the search of the predicted biaxial nematic phase, a series of shape-persistent board-shaped mesogens with maximum molecular biaxiality and a dipole along the minor molecular axis were designed to form nematic (N) mesophases. One compound exhibits a wide nematic temperature range, which can be supercooled to room temperature. A comprehensive variable temperature X-ray study on aligned samples reveals patterns being dominated by the form factor of very small aggregates, from which the aspect ratio of the lead compound with length (L) : breadth (B) : width (W) of 10.73 : 3.16 : 1.23 could be obtained. The ratio is close to the predicted optimum molecular biaxiality by Straley's hard particle model. Hence variable temperature proton relaxation studies on this mesogen were carried out over a wide frequency range. The global fit of the frequency dispersions at five temperatures with a motional model requires in addition to the usual rotation/reorientation contribution, two independent director fluctuations contributions: one for the conventional nematic order director (n) fluctuations and the other for the minor director (m) fluctuations (normal to n). The correlation length of the minor directors determined by NMR could extend to 5-8 molecules in the W direction, but only to the nearest neighbour in the B direction, as found by X-ray diffraction. Both X-ray and NMR studies indicate that these new types of lead structure are extremely promising to find the long sought-after biaxial N mesophase.
Collapse
Affiliation(s)
- Matthias Lehmann
- Institute of Organic Chemistry, University of Würzburg, Germany.
| | | | | | | | | | | | | |
Collapse
|
5
|
Shi J, Sidky H, Whitmer JK. Novel elastic response in twist-bend nematic models. SOFT MATTER 2019; 15:8219-8226. [PMID: 31495852 DOI: 10.1039/c9sm01395d] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
Abstract
Bent-shaped liquid crystals have attracted significant attention recently due to their novel mesostructure and the intriguing behavior of their elastic constants, which are strongly anisotropic and have an unusual temperature dependence. Though theories explain the onset of the twist-bend nematic phase (NTB) through spontaneous symmetry breaking concomitant with transition to a negative bend (K3) elastic constant, this has not been observed as yet in experiments. There, the small bend elastic constant has a strongly non-monotonic temperature dependence, which first increases after crossing the isotropic (I)-nematic (N) transition, then dips near the nematic (N)-twist-bend (NTB) transition before it increases again as the transition is crossed. The molecular mechanisms responsible for this exotic behavior are unclear. Here, we utilize density of states algorithms in Monte Carlo simulation applied to a variant of the Lebwohl-Lasher model which includes bent-shaped-like interactions to analyze the mechanism behind elastic response in this novel mesostructure and understand the temperature dependence of its Frank-Oseen elastic constants.
Collapse
Affiliation(s)
- Jiale Shi
- Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA.
| | | | | |
Collapse
|
6
|
Merkel K, Welch C, Ahmed Z, Piecek W, Mehl GH. Dielectric response of electric-field distortions of the twist-bend nematic phase for LC dimers. J Chem Phys 2019; 151:114908. [PMID: 31542029 DOI: 10.1063/1.5114824] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
Wide band dielectric spectroscopy of bent-shaped achiral liquid-crystal dimers 1″-n″-bis(4-cyanobiphenyl-4'-yl) n-alkanes (CBnCB n = 7, 9, 11) has been investigated in a frequency range 0.1 Hz-100 MHz using planar-aligned cells of sample thicknesses ranging from 2 to 10 (μm) over a temperature range that covers both nematic and twist bend nematic phases. Two peaks in the dielectric spectrum in the higher frequency range are assigned to the molecular relaxation processes. The peak at the highest frequency, ∼40 to 80 MHz, is assigned to an internal precessional rotation of a single unit of the dimer around the director. The mode in the next lower frequency range of 2-10 MHz is assigned to the spinning rotation of the dimer around its long axis. This involves fluctuations of the dipole moment of the bent-shaped conformation that is directed along its arrow direction of the bow shape formed by the dimer. The peak in the frequency range 100 kHz-1 MHz can be assigned to the collective fluctuations of the local director with reference to the helical axis of the NTB structure. The dependence of its frequency on temperature is reminiscent of the soft mode observed at the SmA* to SmC* phase transition. This result clearly corresponds to the electro-clinic effect-the response of the director to the applied electric field in an electro-optic experiment. The lowest frequency mode, observed in the frequency range of 0.1 Hz-100 Hz, is identified with the Goldstone mode. This mode is concerned with the long range azimuthal angle fluctuations of the local director. This leads to an alternating compression and expansion of the periodic structure of the NTB phase.
Collapse
Affiliation(s)
- K Merkel
- Faculty of Computer Science and Material Science, Institute of Technology and Mechatronics, University of Silesia in Katowice, Katowice, Poland
| | - C Welch
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Z Ahmed
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - W Piecek
- Faculty of Advanced Technologies and Chemistry, Military University of Technology, Warszawa, Poland
| | - G H Mehl
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| |
Collapse
|
7
|
Carvalho J, Cruz C, Figueirinhas JL, Tamba MG, Kohlmeier A, Mehl GH. Proton and Deuterium NMR Study of the CBC9CB Dimer System. J Phys Chem B 2019; 123:1442-1451. [DOI: 10.1021/acs.jpcb.8b11526] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- J. Carvalho
- Department of Physics, and CeFEMA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
| | - C. Cruz
- Department of Physics, and CeFEMA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
| | - J. L. Figueirinhas
- Department of Physics, and CeFEMA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
| | - M. G. Tamba
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - A. Kohlmeier
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - G. H. Mehl
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| |
Collapse
|