Smectic E Research Articles

Carrier Transport Switching of Ferroelectric BTBT Derivative.

Alkylamide-substituted [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative of BTBT-NHCOC14H29 (1), which has ferroelectric N-H···O= hydrogen-bonding network of alkylamide group and two-dimensional (2D) electric structure of BTBT π-cores, was prepared to design the external electric field-responsive organic semiconductors. The short-chain derivative of BTBT-NHCOC3H7 (1') revealed the coexistence of a 2D electronic band structure based on the herringbone BTBT arrangement and the one-dimensional (1D) hydrogen-bonding chain. 1 formed a smectic E (SmE) liquid crystal phase above 412 K and showed ferroelectric hysteresis in the electric field-polarization (P-E) curves at 403-433 K. The remanent polarization (Pr) and coercive electric field (Ec) of 1 at 408 K, 0.1 Hz were 24.0 μC cm-2 and 5.54 V μm-1, respectively. By thermal annealing of thin-film 1 at 443 K, the molecular assembly structure of 1 changed from a monolayer to a bilayer structure with high crystallinity, resulting in conducting layers of BTBT parallel to the substrate surface. The organic field-effect transistor (OFET) device with thermally annealed thin-film 1 showed p-type semiconducting behavior with the hole mobility of 1.0 × 10-3 cm2 V-1 s-1. Furthermore, device 1 showed switching behavior of semiconducting properties by electric field poling and thermal annealing cycle. The electric field response of ferroelectrics modulated the molecular orientation and conduction properties of organic semiconductors, resulting in external electric field control of carrier transport properties.

Ferroelectric Organic Semiconductor: [1]Benzothieno[3,2-b][1]benzothiophene-Bearing Hydrogen-Bonding -CONHC14H29 Chain.

An alkylamide-substituted [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative of BTBT-CONHC14H29 (1) and C8H17-BTBT-CONHC14H29 (2) were prepared to design the multifunctional organic materials, which can show both ferroelectric and semiconducting properties. Single-crystal X-ray structural analyses of short-chain (-CONHC3H7) derivatives revealed the coexistence of two-dimensional (2D) electronic band structures brought from a herringbone arrangement of the BTBT π core and the one-dimensional (1D) hydrogen-bonding chains of -CONHC3H7 chains. The thin films of 1 and 2 fabricated on the Si/SiO2 substrate surface have monolayer and bilayer structures, respectively, resulting in conducting layers parallel to the substrate surface, which is suitable for a channel layer of organic field-effect transistors (OFETs). The thin film of 1 indicated a hole mobility μFET = 2.4 × 10-5 cm2 V-1 s-1 and threshold voltage VTh = - 29 V, whereas that of 2 showed a μFET = 2.1 × 10-2 cm2 V-1 s-1 and threshold voltage VTh = -9.7 V. Both 1 and 2 formed the smectic E (SmE) phase above 410 and 369 K, respectively, where the existence of a hole transport pathway was confirmed in the SmE phase. The ferroelectric hysteresis behavior was observed in bulk 1 and 2 in the polarization-electric field (P-E) curves at the SmE phase. 1 showed the remanent polarization Pr = 2.3 μC cm-2 and coercive electric field Ec = 5.2 V μm-1, whereas the Pr and Ec of 2 were 3.4 μC cm-2 and 7.0 V μm-1 at the conditions of 453 K and 1 Hz. Introduction of alkylamide units into the BTBT π core has the potential to develop the external stimulus-responsive organic semiconductors brought from both ferroelectricity and semiconducting properties.

Conformational Change of Alkyl Chains at Phase Transitions in Thin Films of an Asymmetric Benzothienothiophene Derivative.

Among many promising organic semiconducting materials, 2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) shows outstanding device performances for organic field-effect transistors. This compound has a highly ordered liquid crystalline state, i.e., the smectic E (SmE) phase. Although the transition from the crystalline state to the SmE phase is believed to accompany melting of the alkyl chains, no spectroscopic evidence has been found so far. In this study, the conformational change of the decyl chains in Ph-BTBT-C10 films across the phase transition is analyzed by temperature-dependent measurements in situ using infrared spectroscopy. The spectral analysis reveals that the polycrystalline film has latent conformational disorder (the gauche conformer), the rate of which becomes more pronounced with the heat treatment. As expected, melting of the decyl chains is observed above the transition temperature to the SmE phase. This study also highlights the discovery of some key bands sensitive to the phase transitions in liquid crystalline organic semiconductors.

1]Benzothieno[3,2-b][1]benzothiophene-based liquid crystalline organic semiconductor for solution-processed organic thin film transistors

Liquid crystalline organic semiconductors have drawn tremendous interest due to potentials of applications in organic thin-film transistors (OTFTs) with their high electrical performance and solution-processability. However, alkylated liquid crystals often encounter poor thermal durability and mobility drop at smectic E (SmE) phase. Here, we introduce new solution-processable [1]benzothieno[3,2-b][1]benzothiophene-based small molecules with different types of alkyl side chains for OTFTs. All BTBT derivatives showed smectic phase; dialkylated 2-octyl-7-(5′-octyl-[2,2′-bithiophen]-5-yl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene (O-2Th-BTBT) and 2-(5′-(2-ethylhexyl)-[2,2′-bithiophen]-5-yl)-7-octylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (E-2Th-BTBT) suffered mobility degradation at SmE phase. However, monoalkylated 2-([2,2′-bithiophen]-5-yl)-7-octylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (2Th-BTBT) did not exhibit SmE phase over temperature range, resulting in high carrier mobility up to 0.31 cm2/Vs through molecular bilayer transition by thermal annealing. Physicochemical and liquid crystal properties were analyzed, and thin film characterization by atomic force microscopy and X-ray diffraction was discussed.

Precise orientation control of a liquid crystal organic semiconductor via anisotropic surface treatment

We report a three-dimensional (3D) molecular orientation control of a liquid crystal organic semiconductor (LC-OSC) based on the long-range ordering characteristic of an LC material. To this end, a synthetic LC-OSC molecule, MeOPh-BTBT-C8, with a fluidic nematic (N) phase that is essential for alignment control over a large area and a smectic E (SmE) phase showing high ordering, was prepared. A simple flipping of a sandwich cell made of the LC-OSC material between the top and bottom substrates that have uniaxial–planar degenerated alignment as well as crossed rubbing directions responds to the given surface anchoring condition and temperature gradient. Optical observation of the alignment-controlled LC-OSC was carried out by polarized optical microscopy (POM), and the corresponding charge carrier mobility was also measured by fabricating organic field-effect transistors (OFETs). Our platform offers a facile approach for multidirectional and multifunctional organic electronic devices using the stimulus–response characteristics of LC materials.

Carrier transport properties in highly ordered smectic liquid crystal phase

AbstractWe have designed and synthesized a new liquid crystalline phenylbenzothienobenzothiphene derivative, Ph‐BTBT‐4S‐3, in order to investigate the carrier transport properties in highly ordered smectic liquid crystal phase at a wide temperature range lower than room temperature enough to analyze them. Ph‐BTBT‐4S‐3 exhibits smectic E (SmE) phase, which is the one of highly ordered smectic liquid crystal phases, at the wide temperature range from 200°C to under −130°C. Thanks to its wide temperature range of SmE phase, we could reveal the carrier transport properties in the low temperature range, which were well explained on the basis of Gaussian disorder model having a Gaussian width, σ, of 87 meV, which was larger than those of typical nonpolar liquid crystals of 40−60 meV and was attributed to additional dipole of a thioether group in the side chain.

Solution-processable liquid crystalline chrysene semiconductors with wide band gap: Self-organization and carrier transport properties

Abstract High-performance solution-processable organic semiconductors (OSCs) with wide bang gap (≥3.3eV) are highly required for fabricating transparent organic thin-film transistors (OTFTs) with the inexpensive and low-temperature printed technique. Herein, in this study, we reported the newly synthesized chrysene-based OSCs (di-CR-C12, mono-CR-C12) with considerably large band gap of 3.70 eV, superior liquid crystallinity and solution-processed OTFTs mobility as high as 0.5 cm2/V. Such large band gap resulted in complete optical transparency of thin film in the visible-light region and good photostability. Highly ordered smectic E (SmE) phase of mono-CR-C12 was the first reported case of the monoalkylated single core structure demonstrating advantageous capacity of CR backbone in stablizing Sm mesophase. Remarkably, high hole mobility exceeding 0.1 cm2/V in the bulk phase was determined by time-of-flight (TOF) technique intrinsically revealing their satisfactory carrier transport characteristics. Good solubility of mono-CR-C12 enables solution-processed OTFTs with Au nanoparticles deposited at room-temperature and under ambient conditions, exhibiting good device performance with mobility up to 0.5 cm2/V. All results demonstrate that they are very promising candidates as active component for transparent electronics based on OTFTs.

Liquid crystal and crystal structures of a phenyl-benzothienobenzothiophene derivative

ABSTRACTWe have investigated liquid crystal and crystal structures of a smectic E (SmE) liquid crystalline organic-semiconductor, 2-decyl-7-phenyl-benzothienobenzothiophene (Ph-BTBT-10). X-ray diffraction study revealed that the bulk and thin film crystals prepared from SmE phase exhibited the herringbones structure within molecular layer of ab-plane, which inherited from the SmE phase, and that the crystal structure of these crystals were almost same as that of single crystal. The high mobility in field effect transistors fabricated with crystalline thin films of Ph-BTBT-10 was discussed in relation to the transfer integral estimated from the lattice structure of single crystal of Ph-BTBT-10.

The effect of thermal annealing on the layered structure of smectic liquid crystalline organic semiconductor on polyimide gate insulator and its OFET performance

The electrical performance of organic field-effect transistors (OFETs) depends on the quality of thin-film organic semiconductors, which is significantly affected by solution-processing conditions and additional processes. We investigated the effects of post-thermal annealing on the thin-film morphologies of liquid crystalline organic semiconductors on polyimide gate insulator surface and the FET performances of the films. The active material selected for the OFETs was 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10), which shows a highly ordered smectic E (SmE) mesophase and polycrystalline thin films feature very high mobility. We aimed to produce well defined molecular orientation and crystal structure in thin-film Ph-BTBT-C10 on polyimide gate insulator not typical SiO2 gate insulator via thermal annealing process. Uniform bilayer- or monolayer-structured polycrystalline thin films were obtained on polymer gate insulator after thermal annealing at a SmE (over 148°C) and SmA (over 213°C) liquid crystalline phase temperature, respectively. The OFET using bilayer-structured thin film showed high performance with a mobility of 2.27cm2/Vs.

Solution-Processed, Low-Voltage Polycrystalline Organic Field-Effect Transistor Fabricated Using Highly Ordered Liquid Crystal With Low- Gate Dielectric

Low-voltage operable organic field-effect transistors (OFETs) of 2-decyl-7-phenyl-[1] benzothieno[3,2-b][1] benzothiophene (Ph-BTBT-10) with low- $k$ polystyrene/SiO2 hybrid gate dielectric are fabricated using highly ordered smectic E (SmE) liquid crystalline phase. SmE phase helps organic molecules form a well-aligned crystalline thin film during spin-coating process and gives it high thermal stability. Low- $k$ gate dielectric is especially favorable for practical device applications, because it decreases parasitic capacitance in the device circuitry, leading to low-power dissipation. Despite the polycrystalline nature of Ph-BTBT-10 and low- $k$ gate dielectric, the solution-processed, bottom-gate, bottom-contact Ph-BTBT-10 OFET exhibits saturation mobility up to 4.9 cm2/( $\text {V}\cdot \text {s}$ ) with the subthreshold swing of 79 mV/decade, which is very close to the theoretical limit of 60 mV/decade at room temperature. These outstanding characteristics can be attributed to the asymmetric molecule structure of Ph-BTBT-10, which forms a bilayer structure with quasi-2-D growth mode that allows for excellent carrier transport properties with little effect of grain boundaries.

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.

Induced smectic phases of stoichiometric liquid crystal mixtures.

We revealed the detailed structures of induced smectic liquid crystal (LC) phases composed of a binary mixture of charge-transfer (CT) LC substances. Although neither of the constituents had highly ordered smectic phases, the mixture exhibited smectic-E (SmE) or smectic-B (SmB) phases when mixed at ratios of 1 : 1 and 2 : 3, respectively. The results of polarized optical microscopy, differential scanning calorimetry, X-ray diffraction, and infrared spectroscopy indicated that the induced smectic phases were stabilized by an exquisite balance between the CT interactions, dipolar interactions, and excluded volume effects. We proposed a possible model for the molecular arrangements in the SmE and SmB phases, which consistently explained the experimental results including the stoichiometric ratios.

Phase Behaviors of Side-Chain Liquid Crystalline Polyacetylenes with Different Length of Spacer: Where Will the Decoupling Effect Appear?

Phase structures and transitions of a series of biphenyl containing side-chain liquid crystalline polyacetylenes (SCLCPAs) with different length of spacer and alkyl tail, poly((m + 2)-{[(4′-alkyloxy-4-biphenylyl)carbonyl]oxy}-1-yne) [P-m,7 (m = 4, 5, 8), P-m,5 (m = 9), P-m,5L (m = 3, 9), m is the number of methylene units in the spacer], were studied by differential scanning calorimeter, polarized light microscopy, one- and two-dimensional wide-angle X-ray diffraction. It is revealed that phase structures of the samples are strongly dependent on the lengths of spacer and alkyl tail. The samples with short spacer (m ≤ 4) render the molecular shape of sheet-like due to the strong coupling between the backbone and the biphenyl groups on side-chains. They form highly ordered smectic phase with an additional ordering on the subnanometer scale (SmXo) at low temperatures. For m > 5, P-m,7 exhibits the low-temperature phase of smectic B (SmB), while P-9,5 and P-9,5L form smectic E (SmE). As the long spacers adopt...

Liquid crystals for organic thin-film transistors

Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200 °C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10 cm2 V−1 s−1) after thermal annealing at 120 °C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

Reassessment of structure of smectic phases: Nano-segregation in smectic E phase in 4-n-alkyl-4′-isothiocyanato-1,1′-biphenyls

Based on new diffraction data from aligned samples of smectic E (SmE) phase of 4-n-alkyl-4'-isothiocyanato-1,1'-biphenyls, systematics against the alkyl chain length n is analyzed. In order to perform the analysis, the molecular form factor approximated by a box-shaped distribution is calculated while taking the rounding of the distribution at corners into account. The analysis clearly shows the nano-segregated layered structure, which does not fit to the traditional structural view of SmE phase but does fit to the model the present authors proposed recently. Some implications of this conclusion are discussed in relation to the importance of the molten state of alkyl chains in most of real mesogens revealed previously through thermodynamic analyses.

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

To confirm the molten state of the alkyl chain in soft crystalline phase, smectic E (SmE) phase, thermodynamic and spectroscopic analyses were performed on 4-n-alkyl-4'-isothiocyanatobiphenyl (nTCB, n: the number of carbon atoms in the alkyl group). DSC results of 11TCB and 12TCB, having extra smectic A phase besides smectic E phase, show that their chain-length dependence of entropies of transition (Δ(trs)S) from the ordered crystalline (OC) phase to the SmE phase matches the trend found for nTCB (n = 4-10), while no chain-length dependence is observed in Δ(trs)S at the SmE-to-SmA and SmA-to-isotropic liquid (IL) phase transitions in 11TCB and 12TCB. Temperature dependences of FT-IR spectra of six compounds (n = 2, 3, 5, 8, 10, and 12) were recorded. The CH stretching modes of the chain exhibited more pronounced change at the transition from the OC to the SmE phase than at the transition from the SmE phase to the IL or SmA phase. These results indicate that the alkyl chain is molten in the SmE phase as in IL. The disordering process of nTCB molecules from the OC to IL via anisotropic mesophases is discussed in terms of entropy.

Improved thermal stability in organic FET fabricated with a soluble BTBT derivative

We have synthesized a new benzothienobenzothiophene (BTBT) derivative, 2-octylthienylbenzothienobenzothiophene having a highly ordered liquid crystal phase of smectic E (SmE) in the temperature range next to its crystal phase, fabricated its polycrystalline thin films by spin-coating technique, and applied them to field effect transistors (FETs). Thanks to the liquid crystal phase as a precursor of polycrystalline thin films, the resulting films were molecularly flat and uniform. The thermal stability of the thin films was very much improved compared with dialkyl BTBT derivatives because of the solid-like nature of the SmE phase up to 180°C. The FETs fabricated with films after solvent vapor annealing with toluene showed high FET mobility of 1.4±0.3cm2/Vs. As expected, the FETs were not destroyed by thermal load up to 150°C. The FET mobility was recovered to 0.54cm2/Vs by the solvent vapor annealing, although the FET mobility was decreased by the thermal load. Highly ordered liquid crystalline phases provide us with easy fabrication of quality polycrystalline thin films and high thermal stability of the polycrystalline thin films.

Multicriticality in smectic liquid crystals

A phenomenological model permitting a unified description of different types of smectic liquid crystalline phases including the hexatic-B phase has been developed. The model describes five different liquid crystalline phases: nematic (N), smectic-A (SmA), smectic-C (SmC), hexatic-B (HexB) and smectic-E (SmE). We observe various multicritical points among different smectic phases. General arguments are presented for the topologies of the phase diagrams in the vicinity of the SmA–SmC–HexB and N–SmA–SmC bicritical points and in the vicinity of the SmA–HexB–SmE, SmA–SmC–SmE and SmC–HexB–SmE triple points. The existence of the SmA–HexB, SmA–SmC and SmC–HexB tricritical points near the SmA–HexB–SmE, SmA–SmC–SmE and SmC–HexB–SmE triple points is discussed. Calculations based on this model agree qualitatively with the experiments.

Hierarchical Supramolecular Ordering with Biaxial Orientation of a Combined Main-Chain/Side-Chain Liquid-Crystalline Polymer Obtained from Radical Polymerization of 2-Vinylterephthalate

The liquid-crystalline (LC) phase structures and transitions of a combined main-chain/side-chain LC polymer (MCSCLCP) 1 obtained from radical polymerization of a 2-vinylterephthalate, poly(2,5-bis{[6-(4-butoxy-4'-oxybiphenyl) hexyl]oxycarbonyl}styrene), were studied using differential scanning calorimetry, one- and two-dimensional wide-angle X-ray diffraction (1D and 2D WAXD), and polarized light microscopy. We have found that 1 with sufficiently high molecular weight can self-assemble into a hierarchical structure with double orderings on the nanometer and subnanometer scales at low temperatures. The main chains of 1, which are rodlike as a result of the "jacketing" effect generated by the central rigid portion of the side chains laterally attached to every second carbon atom along the polyethylene backbone, form a 2D centered rectangular scaffold. The biphenyl-containing side chains fill the space between the main chains, forming a smectic E (SmE)-like structure with the side-chain axis perpendicular to that of the main chain. This biaxial orientation of 1 was confirmed by our 2D WAXD experiments through three orthogonal directions. The main-chain scaffold remains when the SmE-like packing is melted at elevated temperatures. Further heating leads to a normal smectic A (SmA) structure followed by the isotropic state. We found that when an external electric field was applied, the main-chain scaffold greatly inhibited the motion of the biphenyls. While the main chains gain a sufficiently high mobility in the SmA phase, macroscopic orientation of 1 can be achieved using a rather weak electric field, implying that the main and side chains with orthogonal directions can move cooperatively. Our work demonstrates that when two separate components, one offering the "jacketing" effect to the normally flexible backbone and the other with mesogens that form surrounding LC phases, are introduced simultaneously into the side chains, the polymer obtained can be described as an MCSCLCP with a fascinating hierarchically ordered structure.

X‐ray studies on the crystalline E phase of the 4‐n‐alkyl‐4′‐isothiocyanatobiphenyl homologous series (nBT, n = 2–10)

X‐ray diffraction measurements were carried out on nine members of the isothiocyanatobiphenyl homologous series (nBT) in their smectic E (SmE) phase, with the aim of estimating the layer thickness and deriving orthorhombic unit cell parameters for the SmE phase. The data obtained are discussed in relation to molecular reorientation motion around the short axis in that phase.