Photoregulation of polymer conformation by photochromic moieties. II. Cationic and neutral moieties on an anionic polymer

Abstract— Photoregulation of the conformation of poly (methacrylic acid) (PMA) in the range 4·5 < pH > 6·5 has been achieved by means of the cationic photochromic ligand, p-phenyl-azophenyl trimethyl ammonium iodide (PTA), as well as by means of copolymerization of methacrylic acid (MA) with the neutral photochromic co-monomer, p-phenyl azomethacrylanilid (PM). At degrees of ionization α, corresponding to the conformational unfolding of PMA, the pK_app is 0·2 units higher when the ligand PTA is trans than when PTA is in the photostationary 80%“cis” state. Equilibrium dialysis shows that the binding of PTA is independent of the cis/trans ratio at high α (after the conformational unfolding), but at lower αtrans-PTA binds positively and cis-PTA negatively. In the transition region, the presence of non-polar trans-azo ligand is therefore held responsible for a shift of the conformational equilibrium toward the more compact, folded form, which is harder to ionize than the unfolded form. Because of the relation pK_app# pH + log [(1 –α)/α], the degree of ionization of PMA, and thus the polymer conformation, as reflected in the reduced viscosity, can be photochemically regulated in pH buffers. At pH # 5·5 (α# 0·20) a maximal relative shift of Δα/α= 20 per cent was found, with a corresponding relative shift in reduced viscosity of 20·5 per cent. In the photochromic copolymer a pK_app switch of similar magnitude was found. In this case the azo-moiety is permanently affixed to the PMA so that the photosensitivity of the pK_app in the range of the conformational unfolding is due to a shift in the conformational equilibrium induced by the presence of non-polar trans-azo and polar “cis” azo, respectively. In both cases only about 1 photochromic moiety per 100 monomer units is capable of appreciably shifting the conformational manifold of PMA and/or the degree of ionization in constant pH buffers, provided one stays close to the unfolding transition. Mention is made of the implications for photoregulation of the permeability of, and the potential across. model membranes made of photochromic PMA molecules.     

New synthesis method for nickel phosphide hydrotreating catalysts

Nickel phosphide particles on silica and alumina support were prepared from metal or metal oxide particles by treatment with phosphine and hydrogen at moderate temperature, resulting in small particle sizes equivalent to that of the precursor particle size.     

Reversible Electrochemical Modulation of a Catalytic Nanosystem

A catalytic system based on monolayer‐functionalized gold nanoparticles (Au NPs) that can be electrochemically modulated and reversibly activated is reported. The catalytic activity relies on the presence of metal ions (Cd²⁺ and Cu²⁺), which can be complexed by the nanoparticle‐bound monolayer. This activates the system towards the catalytic cleavage of 2‐hydroxypropyl‐p‐nitrophenyl phosphate (HPNPP), which can be monitored by UV/Vis spectroscopy. It is shown that Cu²⁺ metal ions can be delivered to the system by applying an oxidative potential to an electrode on which Cu⁰ was deposited. By exploiting the different affinity of Cd²⁺ and Cu²⁺ ions for the monolayer, it was also possible to upregulate the catalytic activity after releasing Cu²⁺ from an electrode into a solution containing Cd²⁺. Finally, it is shown that the activity of this supramolecular nanosystem can be reversibly switched on or off by oxidizing/reducing Cu/Cu²⁺ ions under controlled conditions.     

Counterion effect on the spin-transition properties of the cation [Fe(btzx)3]2+ (btzx=m-Xylylenebis(tetrazole))

The influence of the counteranion on the structure and the spin-transition properties of original 1D bis(tetrazole) Fe(II) systems, namely [Fe(btzx)(3)]X(2) (X=PF(6) (-) (1), CF(3)SO(3) (-) (2) and ClO(4) (-) (3); btzx=m-xylylenebis(tetrazole)) is studied. The X-ray crystal structures of compounds 1 and 2 are described in detail. These structures present a solvent molecule encapsulated within pockets formed by btzx ligands along the 1D coordination chains. Compound 2 is shown to be the first structurally characterised alternating HS-LS 1D spin-transition system (HS=high spin, LS=low spin). The magnetic susceptibility measurements of all three compounds are compared. The degree of completion and the transition temperature are both drastically influenced by the counterion used, while surprisingly, the cooperative nature of the transition is not affected by the choice of counterion. Compounds 1 and 2 are further studied by M?ssbauer spectroscopy and their distinct LIESST properties are compared.     

Effect of temperature on aluminum coordination in zeolites H-Y and H-USY and amorphous silica-alumina: an in situ Al K edge XANES study

In situ Al K edge XANES spectroscopy shows that the fraction of octahedrally coordinated aluminum in amorphous silica-alumina (ASA) and ultrastable Y zeolite (USY) decreases with increasing temperature under vacuum. In H-USY, about 10% of the aluminum remains octahedrally coordinated at 673 K, while, in ASA, virtually all the octahedrally coordinated aluminum is converted to tetrahedral coordination. In crystalline nonsteamed protonic zeolites, the fraction of octahedrally coordinated aluminum decreased to zero at 300 K. This is ascribed to the greater flexibility of the amorphous silica-alumina network in hosting water molecules and to the high concentration of silanol groups, which stabilize the hydrogen bonds. A large fraction of the nonframework aluminum in USY is amorphous silica-alumina.     

Charge transport in self-organized pi-stacks of p-phenylene vinylene oligomers

We have studied the mobility of charge carriers along self-organizing pi-stacks of hydrogen-bonded phenylene vinylene oligomers in solution, by time-resolved microwave conductivity measurements. The value deduced for the mobility along the stacks is 3 x 10(-3) and 9 x 10(-3) cm2/(V s) for holes and electrons, respectively. Additionally, we have calculated the mobility along the pi-stacks using a hopping model based on parameters from density functional theory. The mobility values obtained from these calculations are in good agreement with the experimental values if it is assumed that there are relatively large twist angles between neighboring molecules in the stack. It is shown that a significantly higher mobility can be attained if the twist angle between neighboring oligomers is reduced.