Photophysical properties of porphyrin tapes

The novel fused Zn(II)porphyrin arrays (Tn, porphyrin tapes) in which the porphyrin macrocycles are triply linked at meso-meso, beta-beta, beta-beta positions have been investigated by steady-state and time-resolved spectroscopic measurements along with theoretical MO calculations. The absorption spectra of the porphyrin tapes show a systematic downshift to the IR region as the number of porphyrin pigments increases in the arrays. The fused porphyrin arrays exhibit a rapid formation of the lowest excited states (for T2, approximately 500 fs) via fast internal conversion processes upon photoexcitation at 400 nm (Soret bands), which is much faster than the internal conversion process of approximately 1.2 ps observed for a monomeric Zn(II)porphyrin. The relaxation dynamics of the lowest excited states of the porphyrin tapes were accelerated from approximately 4.5 ps for the T2 dimer to approximately 0.3 ps for the T6 hexamer as the number of porphyrin units increases, being explained well by the energy gap law. The overall photophysical properties of the porphyrin tapes were observed to be in a sharp contrast to those of the orthogonal porphyrin arrays. The PPP-SCI calculated charge-transfer probability indicates that the lowest excited state of the porphyrin tapes (Tn) resembles a Wannier-type exciton closely, whereas the lowest excited state of the directly linked porphyrin arrays can be considered as a Frenkel-type exciton. Conclusively, these unique photophysical properties of the porphyrin tapes have aroused much interest in the fundamental photophysics of large flat organic molecules as well as in the possible applications as electric wires, IR sensors, and nonlinear optical materials.     

Polymeric phosphonium salts as a novel class of cationic biocides. V. Synthesis and antibacterial activity of polyesters releasing phosphonium biocides

Polyesters were prepared which retained phosphonium biocides as counter ions of sodium sulfonate moieties incorporated into the polymers, and surface antibacterial activity of the polyester films against Staphylococcus aureus and Escherichia coli was explored. These films exhibited a high surface antibacterial activity against S. aureus and E. coli, particularly against S. aureus, and the activity was affected by the structure and the compositional ratio of the phosphonium salts. Amount of the released phosphonium salts was very small, so that liberation of the phosphonium biocides can be expected to occur over a long period. Morphological changes of the cells of S. aureus and E. coli in contact with the polyester films were evaluated by scanning electron microscopy. It was found that the surface antibacterial activity of the polyester films was rather bacteriostatic than bactericidal as evidenced by no morphological changes of the bacterial cells in contact with the phosphonium biocides © 1993 John Wiley & Sons, Inc.     

Radiation-induced solid-state polymerization of trioxane under high pressure

The in-source polymerization of trioxane in the solid state was investigated over a wide range of temperature and pressure, i.e., from 30 to 140°C and up to 7000 kg/cm², respectively. In the polymerization that was carried out slightly below the melting point under pressure, the higher the pressure, the higher the rate of polymerization. It was confirmed that the maximum rate of solid-state polymerization of trioxane occurs near the melting points, even under elevated pressure. The rate of polymerization decreased with increasing pressure at constant temperature. The shape of the time–conversion curves may be classified into two types, i.e., one which is typical of high pressure and low temperature, and the other which is typical of low pressure and high temperature. Changes in the shape of the conversion—intrinsic viscosity curves occurred coincidentally. Thus, three regions for the different “polymerization characteristic” were determined as functions of polymerization temperature and pressure. Explanations are given for the above-mentioned polymerization characteristic.     

Kinetics of polymer translocation through a pore

We theoretically study kinetics of a polymer threading through a pore embedded in a flat membrane. We numerically solve three coupled kinetic equations for the number n(1) of polymer segments in one side of the membrane and expansion factors of the polymer chain in each side of the membrane. We find the time evolution n(1) proportional to t(1/(1+nu)) at late stages and the translocation time tau(t) is scaled as tau(t) proportional to 1+nu) for large number n of the polymer segments, where nu is the effective size exponent of the radius of gyration of the polymer. When the polymer is translocated into a region with a good solvent condition (nu=3/5), we obtain n(1) proportional to t(5/8) and tau(t) proportional to n(8/5).     

Trans-cis Photoisomerization of 1-Methyl-4-(4′-hydroxystyryl)pyridinium in inclusion complexes ofβ-cyclodextrin and its derivatives

The effects of -cyclodextrin (-CyD), heptakis(2,6-di-O-methyl)--cyclodextrin (DMCyD) and heptakis(2,3,6-tri-O-methyl)--cyclodextrin (TMCyD) ontrans-cis photoisomerization of 1-ethyl-4-(4-hydroxystyryl)pyridinium (POH) have been studied in aqueous solutions. The ratio of [cis]/[trans] for POH in the photostationary state at pH 8.54 was remarkably reduced by the presence of CyD or DMCyD. The reduction of the [cis]/[trans] ratio in the photostationary state was explained in terms of the shift of the equilibrium of POH ₊ ^trans PO _trans + H^– toward PO _trans formation. The binding constants of CyD and DMCyD for PO _trans were 2.00- and 1.36-fold larger than those for POH ₊ ^trans , respectively. The binding constants of TMCyD for both species are much smaller than those of CyD and DMCyD. This result indicates that PO _trans , which has a betain structure, forms stable complexes with CyD and DMCyD with its hydrophobic parts inside and the charged parts outside the CyD cavities.     

Synthesis of soluble aromatic polyesteramides by stepwise copolycondensation of bisphenols and aromatic diamines with diphenyl chlorophosphate in pyridine

The reaction promoted by diphenyl chlorophosphate (DPCP) in pyridine was successfully applied to the preparation of soluble aromatic copolyesteramides of high molecular weights directly from aromatic dicarboxylic acids, bisphenols, and a wide range of mol % aromatic diamines. Dropwise addition of a mixture of bisphenols and diamines (more favorably of bisphenols and then diamines) to the mixture of dicarboxylic acids activated by DPCP led the reactions homogeneously even with high mol % of diamines to produce copolymers of good solubility. This improved copolymer solubility was roughly estimated by sequence distribution of polyamide and polyester units in the copolymers, which was studied in a model reaction and in the copolycondensations by simultaneous and stepwise addition of bisphenols and diamines.     

Porphyrin boxes constructed by homochiral self-sorting assembly: optical separation, exciton coupling, and efficient excitation energy migration

meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-meso-linked dimers Dn assemble spontaneously, in noncoordinating solvents such as CHCl3, into tetrameric porphyrin squares Sn and porphyrin boxes Bn, respectively. Interestingly, formation of Bn from Dn proceeds via homochiral self-sorting assembly, which has been verified by optical separations of B1 and B2. Optically pure enantiomers of B1 and B2 display strong Cotton effects in the CD spectra, which reflect the length of the pyridyl arm, thus providing evidence for the exciton coupling between the noncovalent neighboring porphyrin rings. Excitation energy migration processes within Bn have been investigated by steady-state and time-resolved spectroscopic methods in conjunction with polarization anisotropy measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the F?rster-type incoherent energy hopping model by assuming a number of hopping sites of N = 4 and an exciton coherence length of L = 2. Consequently, the excitation energy hopping rates between the zinc(II) diporphyrin units have been estimated for B1 (48 ps)(-1), B2 (98 +/- 3 ps)(-1), and B3 (361 +/- 6 ps)(-1). Overall, the self-assembled porphyrin boxes Bn serve as a well-defined three-dimensional model for the light-harvesting complex.     

Determination of ammonium ion in a flow-injection system with a gas-diffusion membrane : Selection of Optimal Conditions for the pH Indicator

The spectrophotometric determination of ammonium ion in water by flow-injection analysis with a membrane-separator and a pH indicator for detection is studied in detail. The relations derived facilitate the selection of appropriate solution compositions or the prediction of sensitivity. It is shown that 1.5×10^?5 M bromocresol purple (pH 6.8) as acceptor solution gives the maximal sensitivity in the flow system with a laboratory-made separation unit. Application of ultrasonic radiation in the separation step and the use of different flow rates for the donor and acceptor streams may result in increased permeation of ammonia and a correspondingly high sensitivity. By modifying the acceptor solution so that the sensitivity is decreased, more concentrated samples such as urine can be analyzed by direct injection without prior dilution. In this procedure, the sample rate was 60 h^?1 for ammonium concentration of more than 10^?5 M and 30–40 h^?1 for concentrations in the range 3×10^?7?10^?5 M. The detection limit was about 3×10^?7 M.     

α-Carboxylation reaction of carbonyl compounds with bromomagnesium ureide-carbon dioxide adducts

Bromomagnesium ureide-carbon dioxide adducts, models of the carboxylated biotin complex, undergo caboxylation of a variety of carbonyl compounds in good yield.     

Thermo-optic properties of B2O3 doped Li2O-Al2O3-SiO2 glass-ceramics

Reduction in the temperature coefficient of the optical path length, dS/dT of Li₂O-Al₂O₃-SiO₂ glass-ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ?, and the thermal expansion coefficient, α. The dS/dT value of 2.6 mol% B₂O₃-doped glass-ceramic was 12.5  × 10⁻⁶/°C, which was 0.9 ×  10⁻⁶/°C smaller than that of B₂O₃-free glass-ceramic. On the other hand, reduction in dS/dT through B₂O₃ doping was not confirmed in precursor glasses. Results showed that reduction in dS/dT of the glass-ceramic through B₂O₃ doping is caused by the reduction in ?. The reduction in ? from B₂O₃ doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ? value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases dS/dT. The dS/dT value of B₂O₃-doped glass-ceramic crystallized under 196 MPa was 11.7 ×  10⁻⁶/°C. That value was slightly larger than that of silica glass. The α value of this glass-ceramic was smaller than that of silica glass.