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A series of liquid‐crystalline (LC) π‐‐conjugated oligothiophenes bearing three or two alkoxy chains at their extremities has been designed and synthesized. These polycatenar oligothiophenes form various LC nanostructures including smectic, columnar, and micellar cubic phases. These properties depend on the number and length of the terminal alkoxy chains. The hole mobilities for the oligothiophenes have been measured. The layered smectic and columnar structures are capable of transporting holes, leading to mobilities of up to 0.01 cm2 V−1 s−1. The columnar LC assemblies have also been explored to produce linearly polarized light‐emission. Fine red polarized fluorescence is observed from a uniaxially aligned film of the oligothiophenes. The redox properties of the oligothiophenes both in solutions and in films have been examined. The oligothiophenes exhibit electrochromism upon applying an oxidative potential. The present design strategy is useful for fabricating a variety of functional electro‐active molecular assemblies.

plastics or metal foils. This is associated with the possibility of realizing efficient and reliable TFTs exhibiting high mobility, and opens new doors to so-called ªinvisible electronic circuitsº which will be highly important for the next generation of invisible and flexible electronics; e.g., as switching for addressing organic light-emitting matrices. ExperimentalThe ZnO films (doped and undoped) were deposited onto sodalime glass substrates by radiofrequency (rf, 13.56 MHz) magnetron sputtering using a ceramic-oxide target of ZnO from Super Conductor Materials Inc. with a purity of 99.99 % and 2 in (5.08 cm) in diameter. The sputtering was carried out at room temperature, and the argon deposition pressure was 0.15 Pa. The distance between the substrate and the target was 10 cm, and the rf power was changed between 50 W and 175 W. The deposition rate was varied between 15± 30 nm min ±1 . The film thickness was measured using a surface profilometer (Dektak 3 from Sloan Tech). Electrical resistivity was measured as a function of temperature in the range of 300±500 K using thermally evaporated aluminum electrodes in a coplanar configuration. X-ray diffraction measurements were performed at room temperature in air using the Cu Ka line (Rigaku DMAX III-C diffractometer). Surface morphologies were analyzed using a field-emission scanning electron microscope (Hitachi S-1400). The optical transmittance measurements were performed with a Shimadzu UV-vis 3100 PC double-beam spectrophotometer in the wavelength range from 200 nm to 2500 nm. Organic semiconductors were first utilized for the industrial fabrication of organic photoconductive coatings for xerographic photoreceptors (OPCs), [1] and have found recent application in organic light-emitting diodes (OLEDs).[2]Recently, increasing attention has been paid to organic fieldeffect transistors (OFETs).[3] For OPCs and OLEDs, thin films of amorphous organic semiconductors are used, which are characterized by a low carrier mobility, 10 ±6 to 10 ±3 times smaller than those of crystalline materials. [4] In the OFET applications, however, the amorphous materials are not available because the field-effect transistor requires a high mobility for fast switching or a high current density. Hence, polycrystalline materials, e.g., vacuum-evaporated pentacene, have been studied extensively for high mobility thin-film transistors (TFTs). In fact, a high mobility of up to 8 cm 2 V ±1 s ±1 has already been achieved. [5] In practical applications of polycrystalline materials, however, a new problem of ªgrain boundariesº that originates from molecular alignment in crystalline materials, and which is not the case in amorphous materials, emerges: the grain boundaries cause electrically active localized states and spoil long-term stability by adsorbing ambient contaminants. Therefore, its control is a key issue in realizing practical applications of polycrystalline materials. On the other hand, there is an approach that avoids this boundary effect. In the last ten years, it has been rev...

New molecular materials combining ionic and electronic functions have been prepared by using liquid crystals consisting of terthiophene-based mesogens and terminal imidazolium groups. These liquid crystals show thermotropic smectic A phases. Nanosegregation of the pi-conjugated mesogens and the ionic imidazolium moieties leads to the formation of layered liquid-crystalline (LC) structures consisting of 2D alternating pathways for electronic charges and ionic species. These nanostructured materials act as efficient electrochromic redox systems that exhibit coupled electrochemical reduction and oxidation in the ordered bulk states. For example, compound 1 having the terthienylphenylcyanoethylene mesogen and the imidazolium triflate moiety forms the smectic LC nanostructure. Distinct reversible electrochromic responses are observed for compound 1 without additional electrolyte solution on the application of double-potential steps between 0 and 2.5 V in the smectic A phase at 160 degrees C. In contrast, compound 2 having a tetrafluorophenylterthiophene moiety and compound 3 having a phenylterthiophene moiety exhibit irreversible cathodic reduction and reversible anodic oxidation in the smectic A phases. The use of poly(3,4-ethylenedioxythiophene)-poly(4-styrene sulfonate) (PEDOT-PSS) as an electron-accepting layer on the cathode leads to the distinct electrochromic responses for 2 and 3. These results show that new self-organized molecular redox systems can be built by nanosegregated pi-conjugated liquid crystals containing imidazolium moieties with and without electroactive thin layers on the electrodes.

Fullerenes, C60, modified with long alkyl chains form long-range ordered lamellar mesophases permitting a high C60 content. The mesomorphic fullerenes feature reversible electrochemistry and a comparably high electron carrier mobility making them attractive components for fullerene-based soft materials.

A new electrochromic molecule comprised of pi-conjugated and ionic moieties has been designed and synthesized. It forms a nanosegregated smectic phase in which ion-conductive layers of imidazolium salts are located between hole transport layers of phenylterthiophene moieties. Electrochromism is observed in the bulk liquid crystal state of this compound without an electrolyte solution. In this nanosegregated smectic phase, an electrical double layer is formed rapidly at the electrode. Consequently holes are injected from the electrode, resulting in oxidation of the pi-conjugated moieties.

Since the 1990s, we have witnessed remarkable progress in organic semiconductor technology.[1] In particular, reasonably high carrier mobilities, exceeding those of amorphous silicon, were observed in thin-film transistors fabricated from a single crystal of rubrene. [2] In general, it is difficult to fabricate single crystals of aromatic compounds; therefore, zone-melt and Bridgeman crystal-growth [3] or vacuum crystal-growth techniques [4] are indispensable. Polycrystalline thin films are relatively easy to fabricate and suitable for practical devices. High carrier mobilities-of the order of 1 cm 2 V -1 s -1 -have been observed in field-effect transistor (FET) devices based on polycrystalline pentacene thin films. [5] However, defects and domain boundaries affect the carrier transport in aromatic polycrystalline thin films; therefore, the crystal growth under the vacuum process is rigorously controlled.[6]Device fabrication with a more practical solution process has been investigated. As well as conjugated polymers, [7] precursor methods in which thin films fabricated using soluble precursors are transformed to polycrystalline thin films by thermal treatment, [8] and solution-processable pentacene and anthradithiophene derivatives, which form polycrystalline thin films using a spin-coat method, have been investigated.[9]The field-effect mobilities in these studies are of the order of 10 -2 cm 2 , and the carrier mobility is increased up to 0.1 ≈ 1 cm 2 V -1 s -1 by thermal treatment. [8,9] The optimum mobility is lower than those of the FET devices fabricated using vacuum deposition; the device characteristics strongly depend upon the film morphology, because the organic semiconductor thin films fabricated by the solution process have many defects and exhibit low carrier mobility.Liquid-crystalline semiconductors are also possible materials for solution-processable semiconductors. The bulk carriertransport properties have been investigated in discotic columnar, [10,11] smectic, [12][13][14] nematic, [15,16] and cholesteric [17] phases by using the time-of-flight (TOF) technique and pulse-radiolysis time-resolved microwave conductivity. Recently, high hole drift mobilities exceeding 0.1 cm 2 V -1 s -1 have been reported in the highly ordered smectic phases of alkynylquaterthiophene and dithienylbenzene derivatives; [18,19] furthermore a high microscopic band mobility exceeding 1 cm 2 V -1 s -1 has also been observed in the columnar phase of hexabenzocoronene derivatives.[20]Liquid-crystal materials are generally soluble in organic solvents because of their alkyl chains; therefore, solution-processable liquid-crystalline semiconductors have recently been synthesized and applied to light-emitting diode (LED) [21] and FET devices, [22,23] although, FET devices can also be fabricated by vacuum deposition of liquid-crystalline semiconductors. [24] In discotic systems, the zone-cast technique has been utilized for FET devices that exhibited p-type operation with a carrier mobility of the order of 10 -2 cm 2 V -1 s -1 ...

Carrier transport in liquid-crystal phases, design of liquid-crystalline semiconductors with high carrier mobilities, and their application to field-effect transistors are reviewed. High carrier mobility has been observed in the smectic phases of oligothiophene derivatives as well as in the columnar phases of triphenylene and hexabenzocoronene derivatives. The mobility increases with the molecular ordering in the mesophases and extension of the -conjugated system of the liquidcrystal molecules. Asymmetrically substituted liquid-crystalline oligothiophene derivatives retain the highly ordered smectic phases below room temperature. The liquid crystals are solution-processable and can be used in high-performance field-effect transistors. The transistors exhibit high hole mobility and on/off ratio of 4 Â 10 À2 cm 2 V À1 s À1 and 10 7 , respectively.