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Amphiphilic amino acids represent promising scaffolds for biologically active soft matter. In order to understand the bulk self-assembly of amphiphilic amino acids into thermotropic liquid crystalline phases and their biological properties a series of tyrosine ionic liquid crystals (ILCs) was synthesized, carrying a benzoate unit with 0–3 alkoxy chains at the tyrosine unit and a cationic guanidinium head group. Investigation of the mesomorphic properties by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (WAXS, SAXS) revealed smectic A bilayers (SmAd) for ILCs with 4-alkoxy- and 3,4-dialkoxybenzoates, whereas ILCs with 3,4,5-trisalkoxybenzoates showed hexagonal columnar mesophases (Colh ), while different counterions had only a minor influence. Dielectric measurements revealed a slightly higher dipole moment of non-mesomorphic tyrosine-benzoates as compared to their mesomorphic counterparts. The absence of lipophilic side chains on the benzoate unit was important for the biological activity. Thus, non-mesomorphic tyrosine benzoates and crown ether benzoates devoid of additional side chains at the benzoate unit displayed the highest cytotoxicities (against L929 mouse fibroblast cell line) and antimicrobial activity (against Escherichia coli DTolC and Staphylococcus aureus) and promising selectivity ratio in favour of antimicrobial activity.
A combination of neutron time-of-flight and neutron backscattering spectroscopy was used to investigate the molecular dynamics of Janus polynorbornenes (Janus poly(tricyclononenes)) on a microscopic level.
These Janus polynorbornenes, denoted as PTCNSiOR, have a semirigid backbone with −Si(OR)3 side groups attached to it. R represents the length of the alkyl side chain. Here side chain lengths of R = 3 (propyl) and R = 8 (octyl) were considered. It is worth mentioning that these polymers have some potential as active layers in gas separation membranes, especially for the separation of higher hydrocarbons. The combination of time-of-flight and backscattering will ensure a reasonably broad time window for analysis where the incoherent intermediate scattering function SInc(q,t) is considered. Previously, it was shown by X-ray investigations that the system undergoes a nanophase separation into alkyl side chain-rich domains surrounded by a backbone-rich matrix. For PTCNSiOPr (R = 3), the alkyl side-chain-rich domains are truly isolated in the backbone-rich matrix, whereas for PTCNSiOOc (R = 8) these domains percolate through the matrix. Further, it was also previously shown that the alkyl side-chain-rich domains undergo a glass transition. The advantage of neutron scattering experiments discussed here is that besides temporal also spatial information is obtained which will allow conclusions to be drawn about the type of molecular fluctuations. At the lowest measured temperature, the decay in Sinc(q,t) is due to the methyl group rotation. The methyl group dynamics is analyzed in terms of a modified jump-diffusion in a 3-fold potential and yields to a reasonable fraction of hydrogens which contribute to the methyl group rotation. At higher temperatures, the decay in SInc(q,t) is due to both the methyl group rotation and the segmental dynamics in the alkyl side-chain-rich domains. The segmental diffusion is modeled by a sublinear diffusion. For the analysis of the scattering function SInc(q,t) of PTCNSiOPr an elastic scattering due to the immobilized backbone-rich matrix must be taken into account. The analysis reveals that the segmental dynamics is confined by the finite size of alkyl chain-rich domains and that it is intrinsically heterogeneous in nature. Both effects are more pronounced for PTCNSiOPr in comparison to those of PTCNSiOOc.
Discotic ionic liquid crystals (DILCs) consist of self-assembled superdiscs of cations and anions that spontaneously stack in linear columns with high one-dimensional ionic and electronic charge mobility, making them prominent model systems for functional soft matter. Compared to classical nonionic discotic liquid crystals, many liquid crystalline structures with a combination of electronic and ionic conductivity have been reported, which are of interest for separation membranes, artificial ion/proton conducting membranes, and optoelectronics. Unfortunately, a homogeneous alignment of the DILCs on the macroscale is often not achievable, which significantly limits the applicability of DILCs. Infiltration into nanoporous solid scaffolds can, in principle, overcome this drawback. However, due to the experimental challenges to scrutinize liquid crystalline order in extreme spatial confinement, little is known about the structures of DILCs in nanopores. Here, we present temperaturedependent high-resolution optical birefringence measurement and 3D reciprocal space mapping based on synchrotron X-ray scattering to investigate the thermotropic phase behavior of dopamine-based ionic liquid crystals confined in cylindrical channels of 180 nm diameter in macroscopic anodic aluminum oxide membranes. As a function of the membranes’ hydrophilicity and thus the molecular anchoring to the pore walls (edge-on or face-on) and the variation of the hydrophilic−hydrophobic balance between the aromatic cores and the alkyl side chain motifs of the superdiscs by tailored chemical synthesis, we find a particularly rich phase behavior, which is not present in the bulk state. It is governed by a complex interplay of liquid crystalline elastic energies (bending and splay deformations), polar interactions, and pure geometric confinement and includes textural transitions between radial and axial alignment of the columns with respect to the long nanochannel axis. Furthermore, confinement-induced continuous order formation is observed in contrast to discontinuous first-order phase transitions, which can be quantitatively described by Landau-de Gennes free energy models for liquid crystalline order transitions in confinement. Our observations suggest that the infiltration of DILCs into nanoporous solids allows tailoring their nanoscale texture and ion channel formation and thus their electrical and optical functionalities over an even wider range than in the bulk state in a homogeneous manner on the centimeter scale as controlled by the monolithic nanoporous scaffolds.
Here, we report a water-induced supramolecular polymer built from confined structural water and an intrinsic amphiphilic macrocyclic self-assembly in a nanophase separated structure. The newly designed crown ether macrocycle, featuring strong hydrophilic hydrogen bonding receptor selenoxide and a synergistical hydrophobic selenium-substituted crown core, confines the structural water in a segregated, interdigitated architecture. Although water molecules typically freeze around 0 °C, the confined structural water in this supramolecular polymer remains in a liquid-like state down to 80 °C. Previous studies suggest that multiple crown ether units are needed to generate structural water. However, here, one unit is sufficient to control the formation and disappearance of structural water and consequent supramolecular polymerization. Typically, the DC conductivity of water shows Arrhenius temperature dependency (lnσDC ∝ 1/T). In contrast, this new crown unit maintains water in confined states, which exhibit a Vogel/Fulcher/Tammann behavior (lnσDC ∝ 1/(T-T0)) at temperatures above the glass transition temperature. Moreover, this water-induced supramolecular polymer exhibits remarkable adhesion properties to hydrophilic surfaces and maintains tough adhesion at low temperatures. These findings show how a single small macrocycle can govern the complex structure and functionality of water in supramolecular systems.
Ionic Liquid Crystals are ionic liquids that exhibit liquid crystalline mesomorphism together with ionic conductivity. As known confined liquid crystal mesophases can show an anomalous dynamics and phase behavior. Investigations considering the factors controlling the macroscopic properties of ILCs in confinement are scare in the literature. This study reports the molecular mobility, and the phase transition behavior of a guanidinium based columnar ILC confined in the nanopores of self-ordered anodic aluminum oxide membranes of various pore diameters (25 – 180 nm) using Broadband Dielectric Spectroscopy (BDS), calorimetry and X-ray scattering. It is aimed to reveal in which way the pore size as well as the pore surface wettability (hydrophobic or hydrophilic) alters the molecular dynamics, and phase transition behavior for this system. These properties are crucial for applications. The DSC investigations reveal: (i) the phase transition temperature for the transition from the plastic crystalline to the crystalline-liquid state has non-monotonic dependence versus the inverse pore diameter and (ii) the transition from the liquid crystalline to the isotropic phase is suppressed for all nanoconfined samples. This transition suppressed in the thermal signal was evidenced by BDS and X-ray scattering. It is discussed as a continuous phase transition taking place in the pores instead of a discontinuous first order transition as observed for the bulk. BDS investigations show different relaxation processes for the bulk and the nanoconfined ILC. Molecular origins for various relaxation processes are discussed and suggested. It is further shown that the self-assembly of this ILC is dynamic in nature which might apply for other ILCs too. The obtained results will have implications for the nanoscale applications of ionic liquid crystals.
This study explores the molecular mobility, phase behavior, and electrical conductivity of dihydroxyphenylalanine-based ionic liquid crystals (DOPAn, with alkyl side chains n = 12, 14, 16) featuring cyclic guanidiniumchloride headgroups, in both bulk and nanoconfined states. Using broadband dielectric spectroscopy, differential scanning calorimetry, and fast scanning calorimetry, the research uncovers a complex interplay between molecular structure, self-assembly, and molecular mobility. In bulk, DOPAn shows a phase sequence from plastic crystalline to hexagonal columnar and isotropic phases, driven by superdisc formation and columnar organization. Multiple relaxation processes are identified: localized side-chain dynamics (γ-relaxation), ionic headgroup or core motions (α1-relaxation), and cooperative alkyl domain fluctuations (α2-relaxation). Conductivity decreases with increasing side chain length. Under nanoconfinement in anodic aluminum oxide membranes, phase behavior changes: the Colh–Iso transition is suppressed, and a new α3-relaxation appears, linked to dynamics in an adsorbed interfacial layer. DC conductivity drops by up to four orders of magnitude due to confinement effects, altered molecular orientation, and phase transitions—especially the emergence of a nematic-like state in DOPA16. These findings highlight the importance of molecular design, pore geometry, and surface chemistry in tuning ionic liquid crystal properties for advanced applications in nanofluidics, ion transport, and responsive materials.
We simulate a homologous series of guanidinium-based columnar ionic liquid crystals (ILCs) using coarse-grained molecular dynamics (MD) simulations with the Martini force field. We systematically vary the length of alkyl side chains, ILC-n (n = 8, 12, 16), and compare our results with previous experimental findings. Experimentally, ILC-8 exhibits a narrow mesophase window and weak columnar order, while ILC-12 and ILC-16 display a broad mesophase window and high columnar order. The MD simulations show that ILC-8 forms a percolated structure, whereas the longer chain analogues self-assemble into columns, with columnar assembly becoming more prominent as the side chain length increases, in qualitative agreement with the experiments. Furthermore, the intercolumnar distance increases monotonically with increasing side chain length and decreases with increasing temperature. Finally, we find that the diffusion coefficient and ionic conductivity decrease substantially with increasing chain length, consistent with experimental observations. We attribute this decrease in mobility to the formation of hexagonally ordered columns, which restrict transport more than percolated networks.
Electrical conductivity and multiple glassy dynamics of crow-ether based columnar liquid crystals
(2020)
The phase behavior of two unsymmetrical triphenylene crown ether-based columnar liquid crystals (CLCs) bearing different lengths of alkyl chains, KAL465 and KAL468, was investigated using differential scanning calorimetry (DSC). A plastic crystalline (Cry), columnar liquid crystalline (Colh) and an isotropic phase were observed along with two glass transitions in the Cry phase. The molecular mobility of the KAL compounds was further studied by a combination of broadband dielectric spectroscopy (BDS) and advanced calorimetric techniques. By the BDS investigations, three dielectric active relaxation processes were observed for both samples. At low temperatures, a γ-process in the Cry state was detected and is assigned to the localized fluctuations taking place in the alkyl chains. An α2-process takes place at higher temperatures in the Cry phase. An α3 process was found in the Colh mesophase. The advanced calorimetric techniques consist of fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS) employing temperature modulated DSC and FSC (TMDSC and TMFSC). The advanced calorimetric investigations revealed that besides the α2 process in agreement with BDS, a second dynamic glass transition (α1-process) is present which is not observed by dielectric spectroscopy. The results are in good agreement with the glass transitions detected by DSC for this process. The temperature dependences of the relaxation rates of the α1 , α2 and α3 processes are all different. Therefore, different molecular assignments for the relaxation processes are proposed. In addition to the relaxation processes, a conductivity contribution was explored by BDS for both KAL compounds. The conductivity contribution appears in both Cry and Colh phases, where the conductivity increases by ca. one order of magnitude at phase transition from the crystalline to the hexagonal phase.
The application of fast scanning calorimetry (FSC) with heating rates in the range from 10 K/s to 10,000 K/s in soft matter research is discussed through two examples. In the first part, FSC is applied to polymers of intrinsic microporosity (PIMs). No glass transition could be measured for these polymers by conventional calorimetry before their degradation. By decoupling the time scales of chemical degradation and the glass transition, it could be shown for the first time that PIMs undergo a glass transition. In the second part, FSC is applied to ionic liquid crystals to investigate their molecular mobility. In order to cover a broad dynamical range, FSC is combined with temperature-modulated differential scanning calorimetry, and temperature-modulated FSC.
Molecular mobility and conductivity of four bent shaped tetramethylated guanidinium based ionic liquid crystals (ILCs) with varying head group configuration (cyclic or acyclic) and alkyl chain length is investigated by a combination of broadband dielectric spectroscopy (BDS) and specific heat spectroscopy (SHS). Two dielectrically active processes observed in the plastic crystalline phase at low and high temperatures are denoted as γ and α1 relaxation. The former is assigned to localized fluctuations of methyl groups including nitrogen atoms in the guanidinium head groups. SHS investigations reveal one calorimetrically active process termed as α2 relaxation process. The temperature dependencies of the relaxation rates of α1 and α2 are similar for the cyclic ILC while for the acyclic counterpart they are different. Possible molecular assignments for the α1 and α2 relaxation are discussed in detail. Alongside relaxation processes, a significant conductivity contribution was observed for all ILCs, where the absolute value of DC conductivity increases by 4 orders of magnitude at the transition from the crystalline to the hexagonal columnar phase. The increase is traced to the change in the underlying conduction mechanism from the delocalized electrical conduction in the Cry phase to ionic conduction in the quasi 1D ion columns formed in the hexagonal columnar mesophase.
