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.

Biased Bowl-Direction of Monofluorosumanene in the Solid State.

A new curved π-conjugated molecule 1-fluorosumanene (1) was designed and synthesized that possesses one fluorine atom on the benzylic carbon of sumanene. This compound can exhibit bowl inversion in solution, leading to the formation of two diastereomers, 1endo and 1exo, with different dipole moments. Experimental and theoretical investigation revealed an energetical relationship among 1exo, 1endo, and solvent to realize the various endo:exo ratios in the single crystals of 1 depending on the crystallization solvent. Significantly, the molecular dynamics (MD) simulations revealed that 1exo positively worked for the elongation of the stacking structure and the final endo:exo ratio was affected by the relative stability difference between 1endo and 1exo derived by solvation. Such an arrangeable endo:exo ratio of 1 realized the preparation of unique materials showing a different dielectric response from the same molecule 1 just by changing the crystallization solvent.

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.

Supramolecular Complexation and Collective Optical Properties Induced by Linking Two Methyl Salicylates via a σ-Bridge.

Supramolecular complexes or polymers, formed by noncovalent intermolecular forces such as π-π and dipole-dipole interactions, have the potential to render collective optical properties brought about by excitons spreading over multiple molecules, as seen in J-aggregates. In this respect, molecules with a large π-system and dipole moment are advantageous. However, we report here that methyl salicyate (MS) dyad-type molecules, synthesized by connection of two MSs via a σ-bridge, are effective for forming stable aggregates with collective optical properties. The self-association of MS-dyads occurs in a CHCl3 solution at a high concentration of over 10-2 M, which is recognized by the appearance of an absorption band (λmax = 464 nm) bathochromically shifted beyond 8300 cm-1 from the band in the dilute solution (λmax = 334 nm). Upon excitation of this band, an intense green fluorescence is observed without aggregation-caused quenching. The absorption and fluorescence bands, both of which have well-resolved vibronic progressions, are in a near-mirror image relationship, yielding a small Stokes shift of 600 cm-1. A reasonable explanation for these characteristic optical properties is provided from theoretical considerations on the aggregate model constructed based on the results of single-crystal X-ray analysis. The 1H NMR measurements suggest that unconnected MSs also form aggregates at high concentrations, although the absorption measurements do not provide any evidence for this. It is thus presumed that the connection of MSs stabilizes the MS stacking structure of the aggregates, leading to the generation of an excited state delocalized over multiple molecules.

Dynamics of Chiral Cations in Two-Dimensional CuX4 and PbX4 Perovskites (X = Cl and Br).

Chiral organic ammonium cations ((R)-2-methylphenethylammonium (R-MPhA) and (R)-3,7-dimethyloctylammonium (R-DMOA)) cations were combined with [MX4]2- anions (M = Cu and Pb, X = Cl and Br) to form two-dimensional (2D) perovskites: (R-MPhA)2CuCl4 (1a), (R-MPhA)2CuBr4 (1b), (R-DMOA)2CuCl4 (2a), (R-DMOA)2CuBr4 (2b), (R-DMOA)2PbCl4 (2c), and (R-DMOA)2PbBr4 (2d). The point shearing of the MX4 octahedron formed 2D perovskite layers, which were sandwiched by the bilayer molecular assembly of chiral organic ammonium cations. We found that the flexible and polar organic R-MPhA and R-DMOA cations in the 2D perovskites played an important role in the phase transition behavior and dielectric responses. Salts 2a-2d showed similar solid-solid (S1-S2) phase transitions, for which the temperatures decreased in the order of CuCl4 (2a) > PbCl4 (2c) > CuBr4 (2b) > PbBr4 (2d). The occupation volume of one R-DMOA per MX4 octahedron determined the dynamic crystalline space for the motional freedom of chiral ammonium in the 2D perovskite layer. Although thermally activated dielectric fluctuations were observed in salts 2a, 2b, and 2c, only an order-disorder-type dielectric phase transition was observed in salt 2d. Interband optical transitions were observed in the CuCl4 and CuBr4 2D perovskites, whereas sharp exciton absorptions were observed in the 2D PbCl4 and PbBr4 layers in perovskite salts 2c and 2d.

Nanoscale Effects in One-Dimensional Columnar Supramolecular Ferroelectrics.

Organic ferroelectrics have been actively developed with the goal of fabricating environmentally friendly and low-cost memory devices. The remanent polarization of hydrogen-bonded organic ferroelectrics approaches that of the inorganic ones. Nanoscale fabrication of organic ferroelectrics is an essential aspect of high-density memory devices. A pyrene derivative with four tetradecylamide (-CONHC14 H29 ) chains (1) formed an amide-type N-H⋅⋅⋅O hydrogen-bonded one-dimensional (1D) column, which demonstrated ferroelectricity in the discotic hexagonal columnar (Colh ) liquid crystalline phase through the inversion of the orientation of the hydrogen-bonded chains. On the contrary, similar chiral pyrene derivatives bearing 3,7-dimethyl-1-octhylamide chains (S-2 and R-2) did not indicate the Colh phase and ferroelectricity. Homogeneous mixed liquid crystals (1)1-x (S-2)x (i.e., between the ferroelectric 1 and the non-ferroelectric S-2) enable the control of the nanoscale aggregation state of the organic ferroelectrics, resulting in a nanoscale effect of the 1D supramolecular ferroelectrics. Ferroelectric mixed liquid crystals (1)1-x (S-2)x were observed at x≦0.03, where one S-2 molecule was inserted after every thirty-three 1 molecule in the mixed liquid crystal (1)33 (S-2). An average (1)34 length of approximately 12 nm was required to maintain the 1D ferroelectricity, which was similar to the nanoscale limit of inorganic ferroelectrics, such as hafnium oxide thin film (≈15 nm).