Co-reporter:Sanami Yazaki ; Masahiro Funahashi ; Junko Kagimoto ; Hiroyuki Ohno ;Takashi Kato
Journal of the American Chemical Society 2010 Volume 132(Issue 22) pp:7702-7708
Publication Date(Web):May 13, 2010
DOI:10.1021/ja101366x
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 π-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 °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 π-conjugated liquid crystals containing imidazolium moieties with and without electroactive thin layers on the electrodes.
Co-reporter:Fapei Zhang, Masahiro Funahashi, Nobuyuki Tamaoki
Organic Electronics 2010 Volume 11(Issue 3) pp:363-368
Publication Date(Web):March 2010
DOI:10.1016/j.orgel.2009.11.014
We have fabricated the flexible field-effect transistors based on the liquid crystalline phenylterthiophene derivative via a solution process. A dielectric layer of polyvinyl alcohol (PVA) are deposited on a polyimide sheet substrate by a spin-coating method and then the thin film of 5-propyl-5′′-(4-pentylphenyl)-2,2′:5′,2′′-terthiophene is fabricated from its chlorobenzene solution on the dielectric layer by spin-coating or solution sheared deposition method. The fabricated devices exhibit good electrical performance with a hole mobility of up to 0.05 cm2 V−1 s−1 as well as excellent mechanical flexibility. The performance does not degrade at the external strain reached 3%. Our results demonstrate that liquid crystalline semiconductors are promising materials for low-cost flexible electronic devices.
Co-reporter:Aya Matsui;Toru Tsuji ;Takashi Kato
Chemistry - A European Journal 2010 Volume 16( Issue 45) pp:13465-13472
Publication Date(Web):
DOI:10.1002/chem.200902440
Abstract
Side-chain liquid-crystalline siloxane polymers bearing terthiophene moieties as mesogenic pendant groups have been synthesized. An alkenylterthiophene derivative was treated with poly(hydrogenmethylsiloxane) and poly(dimethylsiloxane-co-hydrogenmethylsiloxane)s in Me2SiO/MeHSiO ratios of 1:1 and 7:3, respectively, in the presence of the Karstedt catalyst, to produce pale yellow polymers. The degrees of introduction of the mesogenic unit were 100, 50, and 30 %, respectively. The polymers exhibit ordered smectic phases at room temperature. The copolymers with dimethylsiloxane units form smectic phases as a consequence of nanosegregation between the mesogenic units and siloxane backbones with the alkylene spacers. Time-of-flight measurement reveals that the hole mobility exceeds 1×10−2 cm2 V−1 s−1 in the ordered smectic phase of the copolymer with a degree introduction of the mesogenic units of 50 %. This value is comparable to that of the highly ordered mesophases of low-molecular-weight derivatives of phenylnaphthalene and terthiophene. Because of the segregation behavior induced by the flexible backbone, a closer molecular packing structure favorable for fast carrier transport may be formed in the smectic phase of the copolymer in spite of the low density of the mesogenic groups.
Co-reporter:Fapei Zhang, Masahiro Funahashi, Nobuyuki Tamaoki
Organic Electronics 2009 Volume 10(Issue 1) pp:73-84
Publication Date(Web):February 2009
DOI:10.1016/j.orgel.2008.10.020
The authors fabricated thin films by solution processes using liquid-crystalline (LC) semiconductors, 5-alkyl-5’’-(4-hexyltetrafluorophenyl)-2,2’:5’,2’’-terthiophene (2–5). Films of 5-propyl-5’’-(4-hexyltetrafluorophenyl)-2,2’:5’,2’’-terthiophene (2) show similar molecular packing as their non-fluorinated counterparts. However, the degree of molecular packing ordering from X-ray diffraction measurement is higher, and the films exhibit a more crystal-like structure. Moreover, fluorination has a remarkable effect on their mesomorphic behaviors. Films of 2 consist of large size LC domains (in the range of 100 μm) at room temperature. Thin-film transistors (TFTs) of 2 show p-type operation with good hole mobility up to 0.027 cm2/Vs as well as improved operation stability under ambient conditions and high on/off ratio. Tetrafluorophenyl substitution leads to lowering of HOMO energy by 0.15 eV for 2 and 0.35 eV for 5, resulting in operation stability. Variable-temperature current-voltage measurements indicate intrinsic carrier transport in films of 2.
Co-reporter:Masahiro Funahashi
Polymer Journal 2009 41(6) pp:459-469
Publication Date(Web):April 8, 2009
DOI:10.1295/polymj.PJ2008324
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 π-conjugated system of the liquid-crystal 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 × 10−2 cm2 V−1 s−1 and 107, respectively.
Co-reporter:Masahiro Funahashi Dr.;Fapei Zhang Dr.;Nobuyuki Tamaoki Dr.;Jun-ichi Hanna Dr.
ChemPhysChem 2008 Volume 9( Issue 10) pp:1465-1473
Publication Date(Web):
DOI:10.1002/cphc.200800206
Abstract
5-Hexyl-5′′-hexynyl-2,2′:5′,2′′-terthiophene exhibits the smectic E phase below 200 °C and does not crystallize when it is cooled to −100 °C. Between 200 and −100 °C, non-dispersive transport is observed for holes and electrons with time-of-flight spectroscopy. Over the entire temperature range, the electron mobility is approximately twice as high as that of the hole. The hole and electron transport characteristics in the smectic phase below 0 °C are explained by the Gaussian disorder model, which was proposed for amorphous organic semiconductors. The disorder parameters, σ and Σ, are almost the same for holes and electrons. However, the pre-exponential parameter μ0 for the electron is twice as large as that for the hole, which can be attributed to the difference in the extension of the LUMO of the molecules. The energetic disorder σ is primarily determined by the disorder in the orientation of the permanent dipoles of liquid crystal molecules.
