Suppressing depolarization by tail substitution in an organic supramolecular ferroelectric

Theory for nonlinear conductivity switching in semiconducting organic ferroelectrics

In this work, the ferroelectric and semiconducting properties of the organic semiconducting ferroelectric benzotrithiophene tricarboxamide (BTTTA), and especially their nonlinear coupling, are theoretically investigated. BTTTA is an exponent of a small class of semiconducting organic ferroelectrics for which experiments have established a surprising polarization direction dependence of the bulk conductivity at finite fields. First, molecular dynamics (MD) simulations are used to investigate the occurrence and, under the influence of an external electric field, the inversion of the macroscopic electric dipole that forms along the axis of supramolecular columns of BTTTA. The MD results are consistent with the experimentally observed ferroelectric behavior of the material. Building on the MD results, a QM/MM scheme is used to investigate the charge carrier mobility in the quasi-1D BTTTA stacks in the linear and non-linear regimes. Indeed, at finite electric fields, a clear resistance switching effect was observed in the form of a hole mobility that is a factor ∼2 larger for antiparallel orientations of the polarization and field than for a parallel orientation. This phenomenon can be understood as a microscopic ratchet that is based on the non-equilibrium interaction between the (oriented) dipoles and the (direction of the) charge transport.

Positions of Chiral Alkoxy Groups Responsible for Ferroelectricity in a Columnar Liquid Crystal Phase of Diphenylureas with Six Alkoxy Groups

The columnar polarization direction of ferroelectric columnar liquid crystals can be switched by applying an external electric field, and the polarization direction can be maintained, even after the electric field is removed. If the polarization direction of each column in ferroelectric columnar liquid crystals can be switched and maintained, then ultrahigh-density memory devices can be generated. Recently, we found that the columnar phase of N,N'-bis(3,4,5-tri(S)-citronellyloxyphenyl)urea (Urea-(S)-cit) shows ferroelectricity, whereas that of N,N'-bis(3,4,5-tridecyloxyphenyl)urea (Urea-10) does not. However, the mechanisms by which the six chiral alkoxy groups in Urea-(S)-cit generate ferroelectricity have not been determined. In this study, we regioselectively synthesized four diphenylurea compounds containing (S)-citronellyloxy and decyloxy groups, i.e., N,N'-bis(3,5-di((S)-citronellyloxy)-4-decyloxyphenyl)urea (1), N,N'-bis(4-((S)-citronellyloxy)-3,5-didecyloxyphenyl)urea (2), N,N'-bis(3-((S)-citronellyloxy)-4,5-didecyloxyphenyl)urea (3), and N,N'-bis(3,4-di((S)-citronellyloxy)-5-decyloxyphenyl)urea (4), and investigated which chiral alkoxy group at which position is strongly responsible for the ferroelectricity. The chiral alkoxy groups at 3- and 5-positions of the phenyl groups were clarified to play a significant role in the generation of ferroelectricity. Furthermore, a comparison of these four compounds based on circular dichroism spectroscopy and second harmonic generation experiments revealed the relationship between the helical structure order and the stability of the polarized structure.

Ferro- and ferrielectricity and negative piezoelectricity in thioamide-based supramolecular organic discotics

Amide-based discotic supramolecular organic materials are of interest for fundamental understanding of cooperative self-assembly and collective dipole switching mechanisms as well as for practically relevant ferroelectric and piezoelectric properties. Here, we show how replacing amides (dipole moment of ∼3.5 D) with thioamides (∼5.1 D) as dipolar moieties in the archetypal C₃-symmetric discotic molecule BTA leads to ferroelectric materials with a higher remnant polarization and lower coercive field. The thioamide-based materials also demonstrate a rare negative piezoelectricity and a previously predicted, yet never experimentally observed, polarization reversal via asymmetric intermediate states, that is, ferrielectric switching.

Dynamics of proton, ion, molecule, and crystal lattice in functional molecular assemblies

Dynamic molecular processes, such as short- or long-range proton (H⁺) and ion (M⁺) motions, and molecular rotations in electrical conducting and magnetic molecular assemblies enable the fabrication of electron-H⁺ (or M⁺) coupling systems, while crystal lattice dynamics and molecular conformation changes in hydrogen-bonded molecular crystals have been utilised in external stimuli responsive reversible gas-induced gate opening and molecular adsorption/desorption behavior. These dynamics of the polar structural units are responsible for the dielectric measurements. The H⁺ dynamics are formed from ferroelectrics and H⁺ conductors, while the dynamic M⁺ motions of Li⁺ and Na⁺ involve ionic conductors and coupling to the conduction electrons. In n-type organic semiconductors, the crystal lattices are modulated by replacing M⁺ cations, with cations such as Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. The use of polar rotator or inversion structures such as alkyl amides, m-fluoroanilinium cations, and bowl-shaped trithiasumanene π-cores enables the formation of ferroelectric molecular assemblies. The host-guest molecular systems of ESIPT fluorescent chromic molecules showed interesting molecular sensing properties using various bases, where the dynamic transformation of the crystal lattice and the molecular conformational change were coupled to each other.

Chiral Self-Sorting and the Realization of Ferroelectricity in the Columnar Liquid Crystal Phase of an Optically Inactive N,N'-Diphenylurea Derivative Possessing Six (±)-Citronellyl Groups

An axially polar-ferroelectric columnar liquid crystal (AP-FCLC) phase that exhibits both switching and maintenance of the macro-polarity in the column axis direction has been achieved in an N,N'-bis(3,4,5-trialkoxyphenyl)urea compound (rac-1) prepared from (±)-citronellyl bromide. Although it had been thought that chirality is necessary to achieve the AP-FCLC phase from our previous study, the optically inactive compound which is a mixture of 21 stereoisomers, generated an AP-FCLC phase. We confirmed its ferroelectricity and investigated the mechanism for realizing the AP-FCLC phase using optoelectronic experiments, X-ray diffraction, and circular dichroism spectroscopy. As a result, it was suggested that chiral self-sorting occurs in the columnar liquid crystal phase, in which molecules with a similar stereochemistry form a one-handed helical column, and columns with the same helicity gather together to form a chiral domain. Accordingly, we conclude that the optically inactive compound rac-1 also indicates ferroelectricity similar to that of an optically pure urea compound because of chiral self-sorting.

Understanding the remarkable difference in liquid crystal behaviour between secondary and tertiary amides: the synthesis and characterisation of new benzanilide-based liquid crystal dimers

A number of liquid crystal dimers have been synthesised and characterised containing secondary or tertiary (N-methyl) benzanilide-based mesogenic groups. The secondary amides all form nematic phases, and we present the first example of an amide to show the twist-bend nematic (NTB) phase. Only two of the corresponding N-methylated dimers formed a nematic phase and with greatly reduced nematic-isotropic transition temperatures. Characterisation using 2D ROESY NMR experiments, DFT geometry optimisation and X-ray diffraction reveal that there is a change in the preferred conformation of the benzanilide core on methylation, from Z to E. The rotational barrier around the N-C(O) bond has been measured using variable temperature 1H NMR spectroscopy. This dramatic change in shape accounts for the remarkable difference in liquid crystalline behaviour between these secondary and tertiary amide-based materials.

Dipolar relaxation in thin films of supramolecular stacks of benzenecarboxamides and insights to enhance their ferroelectric characteristics

The relationship between molecular structure and ferroelectric behaviour of thin films is explored in an all-organic supramolecular polymer material based on benzenecarboxamides, using atomistic molecular dynamics simulations. While increasing the number of amide groups around the phenyl core increases the dipole density of a molecule, increasing the length of the corresponding alkyl groups decreases the same. The interplay between these two contributions displays a rich behaviour on key material characteristics, in particular, the polarisation retention time. The latter is shown to be inversely proportional to the alkyl chain length, a consequence of weaker interactions between macrodipoles of stacks. Polarisation retention time was observed to be the highest in a molecule with five amide groups around the aromatic phenyl core which is explained as due to the large barrier for amide group rotation, which is one of the crucial channels for dipolar relaxation. Simulations also demonstrate that the barrier, however, does not affect the switchability of polarization, upon field reversal.

Mixed Columnar Assembly of Ferroelectric and Antiferroelectric Benzene Derivatives Bearing Multiple -CONHC14H29 Chains

The discotic hexagonal columnar (Col_h) liquid crystalline phases of simple benzene derivatives bearing -CONHC₁₄H₂₉ chains at the 1-, 3-, and 5-positions (3BC) and 1-, 2-, 4-, and 5-positions (4BC) display ferroelectricity and antiferroelectricity, respectively. The phase transition behavior, molecular assembly structures, dielectric response, and ferroelectric properties of their mixed crystals [(3BC)_1-x(4BC)_x] were evaluated to clarify the nanoscaling effect on the collective inversion of the one-dimensional (1D) N-H···O═ hydrogen bonding interaction observed in the (3BC)_∞ chain. A small quantity of 4BC doped into 3BC (x ≤ 0.03) maintained the ferroelectric polarization-electric field response (P-E) in the (3BC)_1-x(4BC)_x chains, where the antiferroelectric 4BC molecules in the ferroelectric 3BC column act as a pinning potential site for dipole inversion. On the contrary, a relatively large amount of 4BC doping (x ≥ 0.1) forms a domain separation state between the hydrogen-bonded (3BC)_∞ and (4BC)_∞ columns, in which the ferroelectric P-E hysteresis completely disappeared. The correlation length for the appearance of ferroelectricity in the 1D column was estimated to be ∼40 nm in the Col_h liquid crystalline phase of 3BC.

Ferroelectrically Switchable Axial Polarization in Columnar Liquid Crystalline Phases

Recently, ferroelectrically switchable columnar LCs have drawn a great deal of attention for their generation of rich polarization domains. Because of their unique dielectric and self-assembly properties, they are considered to be a promising material for the design of sensors and ultra-high memory devices. Herein, ferroelectrically switchable LCs by using ester, amide, and 1,2,3-triazole groups are reviewed. Most of them do not exhibit genuine ferroelectricity owing to the low energy barrier between the two polar states. The intermolecular interactions between polar groups strongly affect the switchability and stability of polarization. Therefore, it is challenging to balance these two competing factors to improve the ferroelectric function in columnar LCs. Overall, additional effort, including LC design and device fabrication, should be made to optimize the material performance for practical applications in the future.

Resistive switching in an organic supramolecular semiconducting ferroelectric

The combination of switchable dipolar side groups and the semiconducting core of the newly synthetized C3-symmetric benzotrithiophene molecule (BTTTA) leads to an ordered columnar material showing continuous tunability from injection- to bulk-limited conductivity modulation.