Motonori Watanabe

The synthesis, crystal structure and charge-transport properties of hexacene

Acenes can be thought of as one-dimensional strips of graphene and they have the potential to be used in the next generation of electronic devices. However, because acenes larger than pentacene have been found to be unstable, it was generally accepted that they would not be particularly useful materials under normal conditions. Here, we show that, by using a physical vapour-transport method, platelet-shaped crystals of hexacene can be prepared from a monoketone precursor. These crystals are stable in the dark for a long period of time under ambient conditions. In the crystal, the molecules are arranged in herringbone arrays, quite similar to that observed for pentacene. A field-effect transistor made using a single crystal of hexacene displayed a hole mobility significantly higher than that of pentacene. This result suggests that it might be instructive to further explore the potential of other higher acenes.

Phenothiazine derivatives as organic sensitizers for highly efficient dye-sensitized solar cells

A series of organic dyes containing a phenothiazine central unit were synthesized and were used effectively in the fabrication of dye-sensitized solar cells (DSSCs). A cyanoacrylate moiety was added at the C(3) position of the phenothiazine as an electron acceptor, and a triarylamine moiety was attached at the C(7) position as an electron donor. The DSSCs made with these dyes displayed remarkable quantum efficiency, ranging from 4.2–6.2% under an AM 1.5 solar condition (100 mW cm−2). A variety of substituents, i.e., methyl, hexyl and triphenylamino groups, were added at the N(10) of phenothiazine in order to optimize the incident photon-to-current conversion efficiency. Along the main chromophore a thiophenylene group was inserted at different positions to examine its influence on the properties of devices. The best performance was found in compound NSPt-C6, in which a hexyl group was attached at the N(10) of phenothiazine and a thiophenylene at the C(7) position. It displayed a short-circuit current (Jsc) of 14.42 mA cm−2, an open-circuit voltage (Voc) of 0.69 V, and a fill factor (ff) of 0.63, corresponding to an overall conversion efficiency of 6.22%. Their photophysical properties were analyzed with the aid of a time-dependent density functional theory (TDDFT) model with the B3LYP functional. Their photovoltaic behavior was further elucidated by the electrochemical impedance spectroscopy.

Organic dyes containing oligo-phenothiazine for dye-sensitized solar cells

A series of organic dyes containing oligo-phenothiazine were synthesized and used effectively on the fabrication of dye-sensitized solar cells (DSSCs). In these compounds the phenothiazine moiety functions both as an electron donor and as a π-bridge. These materials exhibit considerably high values of open-circuit voltage (Voc) ranging from 0.78–0.83 V under an AM1.5 solar condition (100 mW cm−2). Two kinds of substituents, i.e., hexyl and hexyloxyphenyl groups, were added onto the N(10) of phenothiazine for comparison. The best device displayed a short-circuit current (Jsc) of 14.3 mA cm−2, an open-circuit voltage (Voc) of 0.83 V, a fill factor (FF) of 0.65, corresponding to an overall conversion efficiency of 7.78%. Their photophysical properties were analyzed with the aid of a time-dependent density functional theory (TDDFT) model with the B3LYP functional. The electronic nature of the devices was further elucidated by using electrochemical impedance spectroscopy.

Spacer effects in metal-free organic dyes for visible-light-driven dye-sensitized photocatalytic hydrogen production†

Metal-free organic dyes containing benzo[b]phenothiazine were synthesized and effectively used for dye-sensitized visible-light-driven photocatalytic hydrogen production. The materials exhibited high stability and hydrogen production when numerous π-conjugated bridges were inserted as spacers between the donor and the anchor moiety. Photocatalytic hydrogen production was investigated in a TiO2/dye/Pt structure using triethanolamine as the sacrificial reagent. Compound dye 3, which had the longest spacer between the donor and the acceptor, exhibited the best hydrogen production performance of the series examined in this study. It displayed a turnover number (TON) of 4460, a turnover frequency of 278 after 16 h, and a photo-quantum efficiency of 1.65% at 420 nm. Furthermore, it showed the longest electron injection lifetime because its coordination structure was considered to be vertically standing on the TiO2 surface by theoretical calculations. On the other hand, dye 1 showed the lowest hydrogen production performance with a TON of 483 and very short electron injection lifetime. This observation is confirmed by the computation results, which showed the lying geometry of 1 with monodentate coordination of the dye with respect to the TiO2 surface. This spacer effectproperty relationship study may provide a good strategy for the development of metal-free organic dyes for dye-sensitized photocatalytic water splitting.

Delocalization of Positive Charge in π-Stacked Multi-benzene Rings in Multilayered Cyclophanes

In the present study, delocalization of a positive charge in π-stacked multi-benzene rings in multilayered para- and meta-cyclophanes, in which benzene rings are connected by propyl chains to form a chromophore array with the face-to-face structure, was investigated by means of transient absorption spectroscopy during the pulse radiolysis using dichloroethane as a solvent. The local excitation and charge resonance (CR) bands were successfully observed. It was revealed that the CR band shifted to the longer wavelength side with the number of the benzene rings. The stabilization energy estimated from the peak position of the CR band showed the efficient charge delocalization over the cyclophanes. Furthermore, the CR bands showed the slight spectral change attributable to the change in distribution of the conformers. The substantially long lifetime of the CR band can be explained on the basis of the smaller charge distribution on the outer layers of the multilayered cyclophanes.

High-Performance Organic Materials for Dye-Sensitized Solar Cells: Triarylene-Linked Dyads with a 4-tert-Butylphenylamine Donor

A series of organic dyes were prepared that displayed remarkable solar-to-energy conversion efficiencies in dye-sensitized solar cells (DSSCs). These dyes are composed of a 4-tert-butylphenylamine donor group (D), a cyanoacrylic-acid acceptor group (A), and a phenylene-thiophene-phenylene (PSP) spacer group, forming a D-π-A system. A dye containing a bulky tert-butylphenylene-substituted carbazole (CB) donor group showed the highest performance, with an overall conversion efficiency of 6.70%. The performance of the device was correlated to the structural features of the donor groups; that is, the presence of a tert-butyl group can not only enhance the electron-donating ability of the donor, but can also suppress intermolecular aggregation. A typical device made with the CB-PSP dye afforded a maximum photon-to-current conversion efficiency (IPCE) of 80% in the region 400-480 nm, a short-circuit photocurrent density J(sc) =14.63 mA cm(-2), an open-circuit photovoltage V(oc) =0.685 V, and a fill factor FF=0.67. When chenodeoxycholic acid (CDCA) was used as a co-absorbent, the open-circuit voltage of CB-PSP was elevated significantly, yet the overall performance decreased by 16-18%. This result indicated that the presence of 4-tert-butylphenyl substituents can effectively inhibit self-aggregation, even without CDCA.

Synthesis, structure, and electronic and photophysical properties of two- and three-layered [3.3]Paracyclophane-based donor-acceptor systems (1)

The synthesis, structural, redox, and photophysical properties of the two- and three-layered donor-acceptor (D-A) type [3.3]paracyclophanes ([3.3]PCPs) are described. The synthesis of the two- and three-layered [3.3]PCPs 1 and 2 containing 2,1,3-benzothiadiazole (BTD) as an acceptor was achieved by the (p-ethylbenzenesulfonyl)methyl isocyanide coupling method. The cyclic voltammograms of 1 and 2 along with those of respective dione precursors 5 and 7 clearly indicate that the presence of the -CH(2)COCH(2)- bridge interferes with the electronic interactions between the BTD and the benzene rings, suggesting the importance of the through-bond interaction in the ground state. In sharp contrast, the UV/vis spectra of 1 and 5 as well as those of 2 and 7 exhibit similar bands regardless of the presence of the -CH(2)COCH(2)- or -CH(2)CH(2)CH(2)- bridges, indicating that the charge-transfer (CT) interaction is mainly responsible for the through-space interaction. The two-layered PCPs, 5 and 1, show broad structureless fluorescence bands at the same position of 468 nm, while those of the three-layered PCPs, 7 and 2, appear at 501 and 496 nm, respectively, with lower quantum yields compared to those of the two-layered PCPs probably due to the stronger intramolecular CT interaction of the three-layered PCPs in the ground state.

Solution-Processed Optoelectronic Properties of Functionalized Anthradifuran

Acenes are promising molecules for their applications in optoelectronic devices such as organic field-effect transistors (OFET), organic light-emitting diodes (OLED), and organic photovoltaic devices (OPV), and so forth. Heteroacenes containing a fused heterocyclic aromatic ring are more versatile than the corresponding parent acenes owing to their higher thermalas well as photochemical stability. A wide variety of fused heteroacenes have been reported in the literature, such as selenophene, pyridine, pyrazine, and 2,1,3benzothiaziazole, and so forth. Among them, thiophenecontaining heteroacenes have attracted the most attention, such as aceno ACHTUNGTRENNUNG[2,3-b]thiophene,[3] 2,2’-binaphtho ACHTUNGTRENNUNG[2,3-b]thiophene, thienoACHTUNGTRENNUNG[3,2-b]thiophene[5] derivatives, and benzo[2a,6] , naphtho, and anthra[2,3-b :6,7-b’]dithiophene (ADT), for their applications in OFETs. In the family of chalcogen elements, oxygen is the smallest with the highest electronegativity. Acenes that contain furan rings are known to show a lower degree of aromaticity compared to thiophene and selenophene. With respect to the electronic properties, a lower degree of aromaticity generally leads to a lower potential energy of the HOMO (highest occupied molecular orbital). This behavior of furanoacene could influence the OFET characteristics. Recently, a few examples of furan-containing heteroacenes have been reported as components for OFET devices, for example, naphtho[2,1-b ;6,5b’]difuran derivatives have good hole mobilities, and naphtho[2,3-b:7,6-b’]difuran displayed not only a good hole mobility but also significant OPV properties. The molecular scaffold of benzofuran has been proven to be useful in the design of optoelectronic materials, such as in OLEDs and OLETs. The syntheses of the anti-form of anthra[2,3b :6,7-b’]-difuran (anti-ADF) and its diphenyl derivatives have been previously reported in the literature. Although the application of nonsubstituted anti-ADF in an OFET was unsuccessful, its diphenyl-substituted derivative showed a good hole transporting property. In this study, two new ADF derivatives have been synthesized. Two alkylethynylsilyl substituents were added to the ring moieties of the ADF molecule to improve their stability and solubility in organic solvents. Their physical properties and their applications in both OFET and OPV are discussed. The synthetic procedure for the alkylethynylsilyl-functionalized ADF is shown in Scheme 1. An aldol-type reaction of

Optical Resolution, Absolute Configuration, and Chiroptical Properties of Three-Layered [3.3]Paracyclophane

A racemic mixture of three-layered [3.3]paracyclophane ([3.3]PCP), 1, has been resolved into two enantiomers, and their absolute configuration was determined from a comparison of experimental chiroptical properties and density functional theory (DFT) calculations. A simple model comprising two p-xylenes and 1,2,4,5-tetramethylbenzene (durene) was used to explain the origin of the chiroptical properties of the three-layered cyclophane system.

A soluble precursor of hexacene and its application in thin film transistors

A soluble precursor of hexacene is prepared from a cycloaddition of hexacene and diethyl ketomalonate in high yield. It can be used to make hexacene thin-films through spin-coating for the fabrication of organic field effect transistors.