Efficient photocyclization of dithienylethene dimer, trimer, and tetramer: quantum yield and reaction dynamics

Multi-dithienylethene arrays, in which two, three, or four 1,2-bis(2,4-dimethylthiophen-3-yl)perfluorocyclopentenes are ethynylene-bridged, were synthesized. Upon irradiation with ultraviolet light the hexane solutions of the arrays turned violet-blue and the color disappeared by irradiation with visible light. The quantum yields of photocyclization reactions successively increased from 0.21 to 0.40 by increasing the number of the dithienylethene moieties in the arrays from one to four. Picosecond laser photolysis as well as the fluorescence depolarization experiment confirmed that efficient excited energy migration in the arrays from the photochemically inactive parallel conformer to the photoactive antiparallel conformer resulted in the high quantum yields.     

Novel optical and magnetic bistability and photoinduced transition in a one-dimensional halogen-bridged binuclear pt complex

In iodine-bridged binuclear Pt compounds, R(4)[Pt(2)(pop)(4)I]nH(2)O and R(')(2)[Pt(2)(pop)(4)I]nH(2)O (pop=P(2)O5H2-2), electronic structures on the PtPtI chains have been controlled between a diamagnetic charge-density-wave (CDW) state and a paramagnetic charge-polarization (CP) state by modification of the counterions (R, R') located between chains. In the R=(C2H5)2NH+2 compound, a pressure-induced CP to CDW transition with a drastic color change is identified. This transition is accompanied by a large hysteresis loop within which photoinduced transition between CDW and CP can be driven by selecting the excitation photon energy.     

Novel ET-coordinated copper(I) complexes: syntheses, structures, and physical properties (ET = BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene)

New molecular charge-transfer complexes of bis(ethylenedithio)tetrathiafulvalene (ET), (ET)Cu(2)Br(4) (1), (ET)(2)Cu(6)Br(10) (2), (ET)(2)[Cu(4)Br(6)ET] (3), (ET)(2)Cu(2)Br(4) (4), (ET)(2)Cu(3)Br(7)(H(2)O) (5), and (ET)(2)Cu(6)Br(10)(H(2)O)(2) (6), have been synthesized by diffusing reaction of ET and Cu(II)Br(2). Their crystal structures and physical properties have been investigated. X-ray analyses revealed that ET molecules coordinated to the copper ions with the sulfur atoms of the ethylenedithio groups in all compounds. The Cu-S distances are found in the range 2.268(5)-2.417(8) A, being close to the typical Cu(I)-S coordination bond distances. Strong d-pi interactions between d-electrons of the copper ions and pi-electrons of the ET molecules can be expected through the Cu-S coordination bonds. ET molecules behave as trans-bidentate ligands bonding to two different copper ions in 1 and 3, as cis-bidentate ligands in 2, 5, and 6, and as monodentate ligands in 4. In the crystal structure of 3, there are two types of ET molecules in the crystal structure, where the first type is a trans-bidentate ligand and the second forms a stacking structure by itself. Compounds 1, 2, 4, and 6 show semiconducting behavior down to low temperature (sigma(RT) = 1.6 x 10(-2) S cm(-1) and E(a) = 122 meV for 1, sigma(RT) = 2.1 S cm(-1) and E(a) = 21 meV for 2, sigma(RT) = 5.4 x 10(-4) S cm(-1) and E(a) = 239 meV for 4, and sigma(RT) = 5.1 x 10(-2) S cm(-1) and E(a) = 207 meV for 6). On the other hand, in 3, a metal-like region is observed along the b-axis and c-axis, due to the contribution of stacked ET molecules, and a metal-semiconductor transition occurs at 280 and 270 K, respectively. Also, 5 exhibits metallic conductivity in the temperature ranges 240-300 and 200-300 K along the b-axis and c-axis, respectively, despite the oxidation state of ET with 2+.     

Small-angle X-ray scattering of isotactic poly(4-methyl-1-pentene)

Although the crystalline phase of poly(4-methyl-1-pentene) (P4MP) is less dense than the amorphous phase, the density difference is very small at room temperature. This makes it difficult to study the fine structure of P4MP by small-angle x-ray scattering (SAXS). It has therefore not been clear whether a so-called superlattice, or long-spacing structure, is formed in P4MP as in other synthetic polymers. However, it is found that raising the temperature increases the density difference sufficiently to make SAXS study feasible, and the existence of the long-spacing structure of P4MP is indicated. Annealing studies reveal two characteristic temperatures of 170 and 200°C: above 170°C the long-spacing structure is formed with an abrupt increase of crystallinity, and at 200°C the structure is further developed.     

Effect of molecular weight distribution on the structure and mechanical properties of ultradrawn,ultrahigh-molecular-weight polyethylene cast from solution. I. Thermoluminescence and NMR

Ultradrawing of ultrahigh-molecular-weight polyethylene (UHMW-PE) films produced by gelation/crystallization from solution was investigated using thermoluminescence (TL), DSC, and pulsed NMR. The TL from an x-irradiated UHMW-PE gel film is considerably affected by ultradrawing. The TL glow peak is correlated with the α_c dispersion of the crystalline phase in the UHMW-PE gel film. The TL integrated intensity is weak for folded-chain crystals but strong for extended-chain crystals. An extra higher TL peak, observed for samples drawn 150x, is related to the appearance of some highly extended tie-chains in the interfacial region in the process of ultradrawing. On the basis of the above observations, the effects of molecular weight distribution on the morphology of UHMW-PE gel film are examined. It is concluded that B-PE (Broad MWD) having a broader MWD has better drawability than N-PE; but at a given draw ratio, higher modulus and tensile strength are realized for N-PE (Narrow MWD).     

Calorimetric and thermoanalytical assessment of mechanically activated PbCO3

Excess enthalpy values, ΔH*, were obtained through the change in the heats of dissolution into Na—EDTA (ΔH*_L) as well as of decomposition to PbO (ΔH*_D). They tallied fairly well with each other, although a slight systematic discrepancy depending on the conditions of vibro-milling was recognized. A good correlation between ΔH* and the relative non-crystallinity was observed when the material was activated under the same conditions with a varying grinding time. Changes in the grinding conditions, such as in amplitude or atmosphere, brought about a considerable shift of the correlation between ΔH* and the non-crystallinity. Thus, under the same degree of non-crystallinity, materials ground in cyclohexane stored a larger amount of energy than those ground in air. These differences in the correlation were discussed by referring the degree of dispersion, the distribution of active sites, and the effect of temperature.     

Properties and structures of films drawn from polyethylene single-crystal mats

The properties and structures of polyethylene films drawn from mats of single crystals were investigated in comparison with films drawn from bulk polymer. The most important result obtained is that the structural reorganization stimulated by mechanical stress and annealing occurs with much greater difficulty in the mat-drawn film. The chief mechanical characteristics of the film are a very low extensibility and a very high modulus. Structural characteristics, such as the double-orientation of the crystalline region and the morphology of the crystals, do suffer no substantial changes during annealing at high temperatures. This stability of the structure can be related to the characteristic fine structure of the mat-drawn film in which there are a much larger number of tie chains, connecting neighboring crystals, than in the bulk-drawn specimen. Relaxation of the amorphous region and a notable increase in long spacing take place in the mat-drawn sample as in ordinary bulk-drawn samples. However, the morphological changes of the crystals accompanied by the folding back of chains seem to take place with great difficulty during annealing even in the vicinity of the melting point.     

Thickening of crystals during rapid heating. I. Linear polyethylene and nylon-6

The change of long spacing in polyethylene and nylon-6 during heating at 18°C/min. was measured with an apparatus devised for rapid measurement of the small-angle x-ray diffraction. The apparatus made it possible to obtain a diffraction profile in 1.8 sec. Long spacings increased notably at high temperatures near the melting point, as has been observed during low-rate heating. However, the increase in long spacing during heating was very small in a methoxymethylated nylon-6 sample. This suggests that the increase in the long spacing at high temperature is due to the thickening of crystals. The long spacing in polyethylene samples in the vicinity of the melting point is apparently independent of thermal history.