TECHNICAL NOTE

Abstract— –The use of a proximity type 12 µm microchannel plate photomultiplier (Hamamatsu R1564U) is demonstrated for the accurate determination of picosecond lifetimes of xanthione, stiff stilbene, cryptocyanine, pinacyanol chloride, erythrosin and l, l'-diethyl-2,2'-dicarbocyanine iodide in solution at room temperature. Single curve and global analyses with reference convolution give statistically acceptable mono-exponential fits to all obtained decays. The count rate and wavelength dependence of the measured excitation pulse profile are discussed. Matching the count rates of sample and reference is recommended for reliable picosecond lifetime determinations.     

A novel route to dipeptide and its derivative by means of the palladium catalyzed facile cleavage of 1-(1-methoxycarbonyl)alkyl-3-substituted-4-arylazetidin-2-ones

1-(1-Methoxycarbonyl)alkyl-3-substituted-4-arylazetidin-2-ones were found to be cleaved exclusively at the NC⁴ bond by the hydrogenolysis on palladium catalyst to give the corresponding dipeptide and its derivative in excellent yields.     

A theoretical study on anomalous temperature dependence of pKw of water

pH, with its well-known value of 7 at ambient condition, is a most basic property of water, with wide implications in chemistry and biology. The pH value is determined by the tendency of autoionization of water molecules into ion pairs, H(+) and OH(-), and is expected to vary extensively with the water condition, which determines the stability of the ion pairs. When temperature rises from the normal to the supercritical region, the pH of water experimentally exhibits complex, nonmonotonic temperature dependence, that is, it first decreases from 7 and then increases rapidly. Accurate theoretical evaluation of pH and microscopic understanding of this anomalous behavior have proven to be a challenging task because the hydration of these ions, especially for OH(-), is very difficult to reproduce. In the present study a molecular simulation is performed to understand this peculiar temperature dependence. The imbalance between the ion-water and the water-water molecular interaction strengths and the concomitant water density enhancement in the hydration shell, observed in the supercritical liquids, serve to put a subtle balance to produce this temperature dependence of the pH value. It is found that the large charge transfers from H(+) and OH(-) to the surrounding water molecules take place. In these transfers, not only water molecules in the neighboring hydration shell but also those in the outer hydration shell play a significant role. The coordination number of water molecules around OH(-) is found to be 4.5 at 300 K, which decreases slowly with temperature, for example, 4 at 800 K, in the present calculation.     

Selective allylic hydroxylation of octahydronaphthalene derivatives with a bridgehead double bond using electrochemical method with iron picolinate complexes

The combination of electrolysis and the Fe(III)(PA)3/O2/CH3CN system was investigated for allylic hydroxylation of octahydronaphthalene derivatives. Substrates with a bridgehead double bond gave the allylic alcohol with alpha-preference, while non-bridgehead olefin did not react smoothly. This system is a useful tool for alpha-selective allylic hydroxylation of octahydronaphthalene derivatives with a bridgehead double bond as model compounds for the AB fused ring of cholesterols.     

Photovoltaic properties of self-assembled monolayers of porphyrins and porphyrin-fullerene dyads on ITO and gold surfaces

A systematic series of ITO electrodes modified chemically with self-assembled monolayers (SAMs) of porphyrins and porphyrin-fullerene dyads have been designed to provide valuable insight into the development of artificial photosynthetic devices. First the ITO and gold electrodes modified chemically with SAMs of porphyrins with a spacer of the same number of atoms were prepared to compare the effects of energy transfer (EN) quenching of the porphyrin excited singlet states by the two electrodes. Less EN quenching was observed on the ITO electrode as compared to the EN quenching on the corresponding gold electrode, leading to remarkable enhancement of the photocurrent generation (ca. 280 times) in the porphyrin SAMs on the ITO electrode in the presence of the triethanolamine (TEA) used as a sacrificial electron donor. The porphyrin (H(2)P) was then linked with C(60) which can act as an electron acceptor to construct H(2)P-C(60) SAMs on the ITO surface in the presence of hexyl viologen (HV(2+)) used as an electron carrier in a three electrode system, denoted as ITO/H(2)P-C(60)/HV(2+)/Pt. The quantum yield of the photocurrent generation of the ITO/H(2)P-C(60)/HV(2+)/Pt system (6.4%) is 30 times larger than that of the corresponding system without C(60): ITO/H(2)P-ref/HV(2+)/Pt (0.21%). Such enhancement of photocurrent generation in the porphyrin-fullerene dyad system is ascribed to an efficient photoinduced ET from the porphyrin singlet excited state to the C(60) moiety as indicated by the fluorescence lifetime measurements and also by time-resolved transient absorption studies on the ITO systems. The surface structures of H(2)P and H(2)P-C(60) SAMs on ITO (H(2)P/ITO and H(2)P-C(60)/ITO) have been observed successfully in molecular resolution with atomic force microscopy for the first time.     

Vibrational structures in the photoelectron spectrum of formic acid

He I photoelectron spectra of formic acid and its deuterium substituted derivatives have been measured. The first, second and fifth bands of each molecule exhibit very fine vibrational structures. The characters of molecular orbitals from which the photoelectron bands originate are discussed by the use of results of vibrational analyses and molecular orbital calculations.     

The Effect of Ge, Ni, and Fe Doping on the Ga Activity in the CoGa B2 Phase

Gallium activity in the B2 (CsCl-type) phase of ternary Co–Ga–X (X = Ge, Ni, and Fe) alloys was measured by the EMF method with a stabilized zirconia solid electrolyte. The temperature range was 1050–1250 K and the concentrations of the added elements were 2–6 at-% Ge and Ni, and 1–3 at-% Fe. The reference electrodes were Fe,Fe_xO or Ga,Ga₂O₃. The effect on the activity of gallium in the B2 phase with the addition of other elements was found to be the largest with Ge and the smallest in the case of Ni.