Minoru Yamaji

Superconductivity in alkali-metal-doped picene

Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds, boron-doped diamond and—most prominently—iron arsenides such as LaO1–xFxFeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (Tc) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (Kxpicene) shows Tc values of 7 K and 18 K, depending on the metal content. The drop of magnetization in Kxpicene solids at the transition temperature is sharp (<2 K), similar to the behaviour of Ca-intercalated graphite. The Tc of 18 K is comparable to that of K-intercalated C60 (ref. 4). This discovery of superconductivity in Kxpicene shows that organic hydrocarbons are promising candidates for improved Tc values.

Air-assisted High-performance Field-effect Transistor with Thin Films of Picene

A field-effect transistor (FET) with thin films of picene has been fabricated on SiO2 gate dielectric. The FET showed p-channel enhancement-type FET characteristics with the field-effect mobility, mu, of 1.1 cm2 V-1 s-1 and the on-off ratio of >10(5). This excellent device performance was realized under atmospheric conditions. The mu increased with an increase in temperature, and the FET performance was improved by exposure to air or O2 for a long time. This result implies that this device is an air (O2)-assisted FET. The FET characteristics are discussed on the basis of structural topography and the energy diagram of picene thin films.

Trap states and transport characteristics in picene thin film field-effect transistor

Transport characteristics and trap states are investigated in picene thin film field-effect transistor under O2 atmosphere on the basis of multiple shallow trap and release (MTR) model. The channel transport is dominated by MTR below 300 K. It has been clarified on the basis of MTR model that the O2-exposure induces a drastic reduction in shallow trap density to increase both the field-effect mobility μ and on-off ratio. We also found that the O2-exposure never caused an increase in hole carrier density. Actually, a very high μ value of 3.2 cm2 V−1 s−1 is realized under 500 Torr of O2.

Laser photolysis studies of electron transfer between triplet naphthoquinones and amines

Abstract Electron transfer between triplet naphthoquinones ( 3 Nap ∗ ; 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone and 2,3-dimethyl-1,4-naphthoquinone in the triplet state) and N , N -dimethylaniline (DMA) or triethylamine (TEA) in acetonitrile (MeCN) and a mixture of MeCN and H 2 O (4:1, v/v) was studied by laser photolysis techniques at 295 K. By analysis of the transient absorption spectrum observed after the completion of electron transfer, the absorption spectra and coefficients of the naphthoquinone anion radicals (Nap .− ) were determined on the basis of those of the DMA cation radical (DMA .+ ) in MeCN and MeCNH 2 O (4:1, v/v). The values of the rate constants ( k et ) for electron transfer between 3 Nap ∗ and DMA were found to be close to those for diffusion-controlled processes as predicted by the Rehm-Weller theory on electron transfer. Using the determined molar absorption coefficients of Nap .− , the efficiencies ( Γ rad ) for the information of Nap .− and DMA .+ were shown to be unity since the radical ion pair produced ( 3 [Nap .− + DMA .+ ]) readily dissociates into Nap .− and DMA .+ due to the triplet spin multiplicity.

Laser flash photolysis studies on hydrogen atom abstraction from phenol by triplet naphthoquinones in acetonitrile

By means of ns laser flash photolysis, the absorption spectra and molar absorption coefficients (Iµ) of the 1,4-naphthosemiquinone radical (NQH˙) and 2,3-dimethyl-1,4-naphthosemiquinone radical (DMNQH˙) have been determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 7100 dm3 mol–1 cm–1 at 368 nm in acetonitrile (MeCN) and MeCN–H2O (4 : 1 v/v) at 295 K. On the basis of the determined absorption spectra and Iµ values, hydrogen atom abstraction (HA) by triplet 1,4-naphthoquinone (NQ) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) from phenol (PhOH) in MeCN was studied by laser photolysis techniques. It was found that HA by triplet NQ and DMNQ (3NQ* and 3DMNQ*) from PhOH occurred in a collision process with quenching rate constants, kq= 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. On the basis of the obtained values of kq and quantum yields (ΦHA), the efficiencies (ψHA) for HA by both 3NQ* and 3DMNQ* from PhOH were obtained to be unity. The rate constants (kHA) for HA of 3NQ* and 3DMNA* were determined to be 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. The difference in kHA may be derived from (1) the steric hindrance by the methyl groups of DMNQ and (2) the degree of 3(π,π*) character mixed in with the 3(n,π*) of NA and DMNQ. The triplet–triplet absorption spectra of NQ and DMNQ were identified, and their absolute molar absorption coefficients (IµT–Tλ) were determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 5200 dm3 mol–1 cm–1 at 368 nm, respectively, in MeCN on the basis of the quantum yields of intersystem crossing (Φisc; 0.74 for NQ, 0.98 for DMNQ) evaluated by thermal lensing techniques. The deactivation mechanism of 3NQ* and 3DMNQ* in the absence of PhOH was found to be self-quenching by NQ and DMNQ in MeCN with the rate constants, kSQ= 9.7 × 108 and 1.2 × 108 dm3 mol–1 s–1, respectively.

Facile synthesis of picene from 1,2-di(1-naphthyl)ethane by 9-fluorenone-sensitized photolysis.

A facile formation of picene was achieved by photosensitization of 1,2-di(1-naphthyl)ethane using 9-fluorenone as a sensitizer. This sensitized photoreaction is the first photochemical cyclization of ethylene-bridged naphthalene moieties to afford the picene skeleton. 5,8-Dibromopicene, prepared by this procedure using 1,2-di[1-(4-bromonaphthyl)]ethane as the substrate, was readily converted to novel functionalized picenes by conventional substitution and cross-coupling reactions.

LASER PHOTOLYSIS INVESTIGATION OF INDUCED QUENCHING IN PHOTOREDUCTION OF BENZOPHENONE BY ALKYLBENZENES AND ANISOLES

The quenching processes of triplet benzophenone (3BP*) by alkylbenzenes (AB) and anisole derivatives (AD) in benzene (Bz) and a mixture of acetonitrile (ACN) and water (4:1 v/v) have been studied on the basis of rate constants and efficiencies determined by nanosecond laser flash photolysis at 355 nm at 295 K. It was found that (1) the deactivation of 3BP* by ADs in ACN–H2O (4:1 v/v) was governed by electron transfer (ET) to produce the benzophenone anion (BP-) and corresponding cation (AD+) radicals with efficiencies, αET<1 whereas no chemical species were formed in Bz; and (2) photoreduction of 3BP* by ABs resulted in benzophenone ketyl radical (BPK) formation by benzylic hydrogen abstraction (HA) with efficiencies αHA<1 in Bz and ACN–H2O (4:1 v/v). The residual efficiency (αIQ: 1-αET or 1-αHA) was attributed to a bimolecular process with no photochemical product, which was named ‘induced-quenching (IQ)’. The quenching rate constants (kq) of 3BP* by ADs and ABs were less than the diffusion limits of both Bz and ACN–H2O (4:1 v/v). The net bimolecular rate constants for the ET, HA and IQ processes were estimated from the kq values and efficiencies. The rate constants (kET and kIQ) of ET and IQ with AD versus the oxidation potential (Eox) of AD followed Rehm–Weller behaviour while logarithmic rate constants (kHA and kIQ) of HA and IQ by ABs increased linearly with a decrease in the Eox of AB. It was suggested, for the deactivation mechanism of 3BP* by ABs and ADs (RH), that (1) the IQ process was intersystem crossing (ISC) enhanced by the partial charge–transfer (CT) character of the triplet exciplexes, 3(BPδ-···RHδ+)cage*; (2) radical ion formation by ET might be accomplished in a polar solvent by further CT interaction in the exciplex; (3) the process of BPK formation was inferred to be H-atom transfer in the exciplex, where the more protic H-atom was readily mobile, rather than ET followed by proton transfer and (4) the loss of efficiencies of photochemical-product formation was derived not from back ET but from the IQ process, inherent to photoreactions, via triplet exciplexes. The deactivation processes of 3BP* by RH are illustrated in Scheme 1.

Photoreduction of iron(III) tetraphenylporphyrin in ethanol studied by laser flash photolysis: effects of concentration on quantum yields

Chloroiron(III) tetraphenylporphyrin, CIFeIIITPP, in deaerated ethanol is photoreduced to yield iron(II) tetraphenylporphyrin, FeIITPP, upon irradiation with UV light. The 355 nm laser photolysis studies have confirmed that the initial yields for the formation of FeIITPP are markedly decreased with an increase in the concentration of CIFeIIITPP: ϕ= 0.015 at 5.35 × 10–6 mol dm–3 CIFeIIITPP and ϕ= 0.01 at 1.0 × 10–4 mol dm–3 CIFeIIITPP. The effects of the concentration on the quantum yields were interpreted in terms of the formation of photoreactive iron(III) porphyrin at low concentrations of CIFeIIITPP.

EMISSION PROPERTIES OF 1,4-ANTHRAQUINONE

Emission, excitation and absorption spectra of 1,4-anthraquinone (1,4-AQ) have been measured in a degassed CCl4 fluid solution at different temperatures as well as in a glassy matrix at 77 K. It is shown that 1,4-AQ exhibits weak emission in CCl4 at temperatures near room temperature. Based on the quantitative analysis of the emission yield measured as a function of temperature, the observed emission is attributed to the fluorescence from the S1(n, π∗) state which is in thermodynamic equilibrium with the T1(π, π∗) state.

Photochemistry of 1,4-anthraquinone studied by steady-state and laser-flash photolysis

Abstract Photochemical reactions of 1,4-anthraquinone (1,4-AQ) whose lowest triplet state (T1) is of the π, π∗ type have been investigated in solution by means of steady-state and nanosecond laser flash photolyses. The absorption spectrum of triplet 1,4-AQ in the non-polar solvent is determined for the first time. The triplet state 1,4-AQ is quenched by the ground-state 1,4-AQ in CCl4 to generate a dimerized product. In cycloalkanes, the final products obtained upon photoirradiation are adducts of the solvents to 1,4-AQ initiated with the 1,4-AQ ketyl radical. The formation of the 1,4-AQ ketyl radical originates from H-atom abstraction from cycloalkanes via the T2 (n, π∗) state which is in thermodynamic equilibrium with the T1 (π, π∗) state. In cycloalkenes and styrene, triplet 1,4-AQ gives the [2+2] cycloadducts forming the four-membered rings without H-atom abstraction.