Calorimetric and Spectroscopic Evidence of Chain-Melting in Smectic E and Smectic A Phases of 4-Alkyl-4′-isothiocyanatobiphenyl (nTCB)

Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation

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.

Effect of the linking bridge type on the self-assembly behaviour of 2',3'-difluoroterphenyl derivatives

The self-assembly behaviour and the crystallization kinetics of two liquid crystal compounds containing 2',3'-difluorosubstituted terphenyl as the mesogenic core have been described. Calorimetric studies show that the linking bridge type affects the polymorphism of smectic phases. The CH2O.3 compound with the -CH2O- linking bridge connected with a longer methylene spacer possesses the chiral smectic phase with antiferroelectric properties (SmCA* phase), while the COO.6 liquid crystal with the -COO- linking bridge connected with a shorter alkyl chain forms the chiral smectic phase with ferroelectric properties (SmC* phase). Both compounds crystallize upon slow cooling, while fast cooling causes the vitrification of the conformationally disordered crystal phase. Dielectric measurements reveal the complex relaxation dynamics in the identified thermodynamic states. DFT calculations allow us to estimate the nature of relaxation processes.

Reversible Molecular Conformation Transitions of Smectic Liquid Crystals for Light/Bias-Gated Transistor Memory

In recent years, organic photonic field-effect transistors have made remarkable progress with the rapid development of conjugated polycrystalline materials. Liquid crystals, with their smooth surface, defined layer thickness, and crystalline structures, are commonly used for these advantages. In this work, a series of smectic liquid crystalline molecules, 2,9-didecyl-dinaphtho-thienothiophene (C10-DNTT), 2,7-didecyl-benzothieno-benzothiopene (C10-BTBT), 3,9-didecyl-dinaphtho-thiophene (C10-DNT), and didecyl-sexithiophene (C10-6T), have been used in photonic transistor memory, functioning as both hole-transport channels and electron traps to investigate systematically the reasons and mechanisms behind the memory behavior of smectic liquid crystals. After thermal annealing, C10-BTBT and C10-6T/C10-DNTT are homeotropically aligned from the smectic A and smectic X phases, respectively. The 3D-ordered structure of these smectic-aligned crystals contributed to efficient photowriting and electrical erasing processes. Among them, the device performance of C10-BTBT was particularly significant, with a memory window of 21 V. The memory ratio could reach 1.5 × 106 and maintain a memory ratio of over 3 orders after 10,000 s, contributing to its smectic A structure. Through the research, we confirmed the memory and light/bias-gated behaviors of these smectic liquid crystalline molecules, attributing them to reversible molecular conformation transitions and the inherent structural inhomogeneity inside the polycrystalline channel layer.

Molecular aggregation in liquid-crystalline layers crucially affects their physics: smectic A (SmA)-nematic (N) phase transition

Recently, two molecular packing modes of the alkyl chain in smectic A (SmA) liquid crystal phases were revealed: normal and tilted types, which are named by the orientation relative to the layer normal. This study reveals the relationship between the packing mode and thermodynamic order of the SmA-nematic (N) phase transition. Two normal type and three tilted type mesogens were subject to thermodynamic and structural experiments. The DSC results showed that the SmA-N phase transitions of the normal and tilted types are of the second and first order, respectively. The analysis of the intensities of reflections in wide-angle X-ray diffraction related to the periodicity of the SmA layer yielded the distribution of the mass centers of molecules along the normal to the SmA layers. The resultant distribution offered a rationale for the correlation of the thermodynamic order of the SmA-N phase transition and molecular packing modes in the SmA phases based on the Meyer-Lubensky theory.

Solution-Processable Indenofluorenes on Polymer Brush Interlayer: Remarkable N-Channel Field-Effect Transistor Characteristics under Ambient Conditions

The development of solution-processable n-type molecular semiconductors that exhibit high electron mobility (μe ≥ 0.5 cm2/(V·s)) under ambient conditions, along with high current modulation (Ion/Ioff ≥ 106-107) and near-zero turn on voltage (Von) characteristics, has lagged behind that of other semiconductors in organic field-effect transistors (OFETs). Here, we report the design, synthesis, physicochemical and optoelectronic characterizations, and OFET performances of a library of solution-processable, low-LUMO (-4.20 eV) 2,2'-(2,8-bis(3-alkylthiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile small molecules, β,β'-Cn-TIFDMTs, having varied alkyl chain lengths (n = 8, 12, 16). An intriguing correlation is identified between the solid-isotropic liquid transition enthalpies and the solubilities, indicating that cohesive energetics, which are tuned by alkyl chains, play a pivotal role in determining solubility. The semiconductors were spin-coated under ambient conditions on densely packed (grafting densities of 0.19-0.45 chains/nm2) ultrathin (∼3.6-6.6 nm) polystyrene-brush surfaces. It is demonstrated that, on this polymer interlayer, thermally induced dispersive interactions occurring over a large number of methylene units between flexible alkyl chains (i.e., zipper effect) are critical to achieve a favorable thin-film crystallization with a proper microstructure and morphology for efficient charge transport. While C8 and C16 chains show a minimal zipper effect upon thermal annealing, C12 chains undergo an extended interdigitation involving ∼6 methylene units. This results in the formation of large crystallites having lamellar stacking ((100) coherence length ∼30 nm) in the out-of-plane direction and highly favorable in-plane π-interactions in a slipped-stacked arrangement. Uninterrupted microstructural integrity (i.e., no face-on (010)-oriented crystallites) was found to be critical to achieving high mobilities. The excellent crystallinity of the C12-substituted semiconductor thin film was also evident in the observed crystal lattice vibrations (phonons) at 58 cm-1 in low-frequency Raman scattering. Two-dimensional micrometer-sized (∼1-3 μm), sharp-edged plate-like grains lying parallel with the substrate plane were observed. OFETs fabricated by the current small molecules showed excellent n-channel behavior in ambient with μe values reaching ∼0.9 cm2/(V·s), Ion/Ioff ∼ 107-108, and Von ≈ 0 V. Our study not only demonstrates one of the highest performing n-channel OFET devices reported under ambient conditions via solution processing but also elucidates significant relationships among chemical structures, molecular properties, self-assembly from solution into a thin film, and semiconducting thin-film properties. The design rationales presented herein may open up new avenues for the development of high-electron-mobility novel electron-deficient indenofluorene and short-axis substituted donor-acceptor π-architectures via alkyl chain engineering and interface engineering.

Aggregation structure of chiral cubic liquid crystals revealed by X-ray diffraction utilizing a new algorithm

Chiral aggregation structure spontaneously formed by achiral rodlike molecules, a long-time unsolved problem in liquid crystal science, has been clarified by applying a new crystallographic algorithm recently developed while utilizing aggregation characteristics of this type. Bicontinuously interwoven networks characterize it similarly to the neighboring Gyroid phase in a phase diagram against the alkyl chain length and temperature. However, the network connectivity is significantly different from the bicontinuous networks that have been either known for related compounds or assumed for this phase. The network is compatible with the homochiral arrangement of rodlike molecules with successive twists by a proper angle between adjacent junctions.

Vibrational dynamics of ethosuximide polymorphs. Infrared absorption and inelastic neutron scattering spectroscopy and model calculations

Commercially available and administered to the patients ethosuximide is a racemic mixture of two enantiomers, each of them exist in different conformations. The presence of the species mentioned are proven by the title experimental methods aided by DFT model calculations. Results of the latter are matched against spectroscopic data by the clustering window analysis. One type of hydrogen bonds exist in the solid forms of ethosuximide NH⋯O, leading to the polymorphic variety of the substance studied.

Three Rings Schiff Base Ester Liquid Crystals: Experimental and Computational Approaches of Mesogenic Core Orientation Effect, Heterocycle Impact

Three rings 2-hydroxypyridine liquid crystalline compounds have been prepared and fully characterized. The mesomorphic behavior of the prepared compounds has been investigated in terms of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Moreover, a comparative study between the prepared compounds and previously reported analogs has been discussed in terms of the orientation and position of the mesogenic core, in addition to the direction of the terminal alkyl chains. Furthermore, a detailed computational approach has been studied to illustrate the effect of geometrical and dimensional parameters on the type of the enhanced texture and the mesomorphic range and stability. The results of the DFT study revealed that the orientation of the mesogen could affect the mesomorphic behavior and this has been attributed in terms of the degree of the polarizability of the linking groups. This result has been confirmed by calculation of the net dipole moment and the molecular electrostatic potential that show how the mesogen orientation and position could impact the molecular charge separation. Finally, the effect of the pyridyl group has been also investigated in terms of the calculated aromaticity index and the π-π stacking.

Free volume in the smectic-E phase of 4-hexyl-4' isothiocyanatobiphenyl studied by positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy has been used to study 4-hexyl-4'-isothiocyanatobiphenyl. Changes of the orthopositronium lifetime parameters with temperature have been observed for the supercooled smectic-E phase. The measurements confirm that positronium is created and annihilates in a layer of a lower electron density containing alkyl chains of molecules. The two-state bond-lattice model of glass transition explains the thermal activation of the centers where orthopositronium is created and annihilates when the glass of the smectic-E phase softens. However, the subsequent cold crystallization of the softened regions also influences the orthopositronium lifetime and intensity, which complicates the picture seen by positrons. The measurements during isothermal crystallization suggest that it progresses in two stages. The first stage can be described by the Avrami equation with the Avrami exponent close to unity, which indicates low-dimensional crystallization. Similarly to liquid n alkanes, the application of pressure is equivalent to temperature lowering with the similar equivalence relationship between pressure and temperature, which seems to confirm the structure of the smectic-E phase with sublayers containing alkyl chains in a molten state. The dependence of the orthopositronium lifetime on pressure for the smectic-E phase may be described by the bubble model where the positronium bubble is approximated with a finite square potential well with the depth of U=1.45eV.

Preparation, Electronic and Liquid Crystalline Properties of Electron-Accepting Azaacene Derivatives

A series of electron-accepting azaacene-type materials 1-4 with different kinds and degrees of intermolecular interactions were synthesized. Simple modification of the terminal substituents significantly modulated the photophysical and electrochemical properties. The degree of weak intermolecular interaction determined the emergence of a liquid crystalline (LC) phase for each compound. Dipole-dipole interaction, π-π interaction, and van der Waals interaction all contributed to stabilize the LC phase of 1 and 2. The introduction of strong hydrogen bonding interaction enabled the formation of a highly ordered LC phase in 4. Charge-transport properties of 1, 2, and 4 were also investigated.

Thermoresponsive luminescence properties of polyfluorinated bistolane-type light-emitting liquid crystals

We developed and characterized four polyfluorinated bistolane derivatives. These compounds, which possess either two alkoxy substituents or an alkoxy group and a bromine atom in their two molecular terminals, were synthesized from readily available 4-alkoxy-1-ethynylbenzene with a facile three-step procedure. Their thermodynamic and photophysical properties were evaluated in detail, and they were found to display both liquid-crystalline (LC) and photoluminescence properties. Remarkably, the photoluminescence behaviors dramatically changed during the thermal phase transition between the crystal and LC phases. Thus, these polyfluorinated bistolanes may be promising candidates for thermoresponsive luminous molecules.

Structure and molecular packing in smectic BCr and Ad phases of Schiff base liquid crystal compounds through the analyses of layer spacing, entropy and crystal structure

Single-crystal structural analyses and heat capacity measurements were performed on two Schiff base liquid crystal compounds, 5CBAA (4-chlorobenzylidene-4'-pentyloxyaniline) and 5ABCA (4-pentyloxybenzylidene-4'-chloroaniline). The alkyloxy-chain of a 5CBAA molecule was conformationally ordered in the crystal at room temperature. While that of 5ABCA was partially disordered in the room temperature phase but ordered in a low-temperature phase at 100 K. The structural phase transition involving the disordering of the conformation was observed at 107 K in the heat capacity of 5ABCA. Both compounds showed two liquid crystalline phases, SmB_Cr and SmA_d. The net entropy change associated with the chain disordering was essentially the same in them despite the difference in the orientation of their central -CH[double bond, length as m-dash]N- moiety. The layer-spacings of SmB_Cr and SmA_d phases were analyzed for their chain-length dependence in both series of mesogens (nCBAA and nABCA), as well as in the case of nBBAA (4-bromobenzylidene-4'-alkyloxyaniline). The results reveal that these smectic structures are composed of alternately stacked core- and chain-layers with an antiparallel arrangement of cores and a bent-form of molecules.

Examination of molecular packing in orthogonal smectic liquid crystal phases: a guide for molecular design of functional smectic phases

The reported layer spacings (d_smectic) of six homologues of mesogens exhibiting orthogonal smectic phases (SmE, SmB, and SmA phases) are reexamined. The slopes of the linear dependences on chain length (n, the number of carbon atoms in the hydrocarbon chain) are clearly categorized into two groups: 1.9 Å (CH₂)^-1 and 1.4 Å (CH₂)^-1. It is clarified that in the former the molecules take a rod-like form (rod-form; category-I), whereas in the latter the molecules are bent around the connection between the core and chain moieties (bent-form; category-II). The average relative positions of adjacent molecules within the smectic structures are deduced from the intercept of the linear functions of d_smectic against n. The relation between and the features of molecules belonging to the two categories are discussed for molecular design of functional smectic liquid crystals.

Thermally induced bilayered crystals in a solution-processed polycrystalline thin film of phenylterthiophene-based monoalkyl smectic liquid crystals and their effect on FET mobility

Herein, a series of asymmetric monoalkyl terthiophene derivatives, Ph-(Tp)₃-Cn, (6 ≤ n ≤ 18), were synthesized to study the phase transition from monolayered crystal to bilayered crystal, leading to a significant increase of OFET mobility.

Positronium formation and annihilation in liquid crystalline smectic-E phase revisited

The results of the positron lifetime measurements of the quenched smectic-E (Sm-E) phase of 4-butyl-4'-isothiocyano-1,1'-biphenyl (4TCB) are revisited. The sites of positronium formation and annihilation, according to the model with nanosegregated layered structure of the Sm-E phase and molten state of alkyl chains of molecules, are identified in the sublayer containing alkyl chains of molecules. The possibility of vitrification of the Sm-E phase for 4TCB consisting in freezing of the alkyl chain motions is considered as a cause of the thermally activated creation of sites where o-Ps is formed and annihilates in the quenched Sm-E phase. The description of the temperature dependence of ortho-positronium intensity is performed using the glass transition model which assumes that the molecules occupy two thermodynamic states: solidlike or liquidlike regarding mobility of their alkyl chains. The equilibrium temperature between solidlike and liquidlike domains of the model obtained from positron lifetime measurements coincides with the exothermic effect in the temperature dependence of the heat capacity.

A possible critical point for nematic order on the basis of Landau free energy having dual instabilities for nano-segregated smectic liquid crystals

Landau expansion of free energy assuming dual instabilities for the nano-segregated SmA phase is analyzed. In addition to known phase sequences (on cooling, disordered isotropic liquid → nematic phase → smectic phase, and disordered isotropic liquid → smectic phase), a new sequence (disordered isotropic liquid → density wave with subsidiary nematic order → smectic phase) and the existence of a critical point are demonstrated in the case where the instability for density wave formation occurs at a higher temperature.

Phase Behavior and Dynamics of the Liquid Crystal 4'-butyl-4-(2-methylbutoxy)azoxybenzene (4ABO5*)

The results of dielectric relaxation spectroscopy and polarizing microscope observation of the 4'-butyl-4-(2-methylbutoxy)azoxybenzene (abbreviated as 4ABO5*) are presented. Numerical analysis of the dielectric spectra results points to complex dynamics of 4ABO5* molecules in isotropic, cholesteric, and crystalline phases. Two well-separated maxima on the imaginary part of dielectric permittivity and the third low frequency relaxation process, hidden in the conductivity region, were detected and described in cholesteric and crystalline phases. Temperature dependence of mean relaxation times characterizing flip-flop motions and rotation around long axes, observed in all phases, is of the Arrhenius type.

Positron annihilation studies of 4-n-butyl-4'-isothiocyanato-1,1'-biphenyl

Positron annihilation lifetime spectroscopy (PALS) measurements were performed between 93 and 293 K in order to study the supercooled smectic-E (Sm-E) phase of 4-n-butyl-4'-isothiocyanato-1,1'-biphenyl (4TCB), the ordered molecular crystal of 4TCB, and the phase transition between the Sm-E phase and the ordered molecular crystal of 4TCB. The phase transition was well reflected in the abrupt increase of the ortho-positronium (o-Ps) lifetime and intensity. The value of the o-Ps lifetime in the Sm-E liquid crystalline phase of 4TCB, i.e., 2.21 ns at room temperature, was explained by the formation of bubbles induced by Ps atoms, which are created due to a liquidlike state of the butyl chains of 4TCB molecules in the Sm-E phase. The temperature dependence of the o-Ps intensity for the supercooled Sm-E phase can be explained by thermal generation of sites where bubbles are formed; an activation energy equal to 0.30±0.02 eV was estimated. This value was compared with the activation energies of molecular motions. The o-Ps lifetime in the ordered molecular crystal was interpreted as originating from the annihilation of o-Ps confined in molecular vacancy-type imperfections in the crystal lattice. The value of the o-Ps pickoff annihilation between 1.8 and 1.9 ns is in accordance with the size of the molecular vacancy for the 4TCB crystal lattice. Its intensity is lower than 5%. The isothermal crystallization of the 4TCB Sm-E phase was observed by PALS. The low-dimensional crystal growth was concluded from the Avrami equation fitted to the time dependence of the o-Ps intensity, which resulted in an Avrami exponent equal to 1.73.