Electrocatalytic Oxidation of 4-Aminophenol Molecules at the Surface of an FeS2 /Carbon Nanotube Modified Glassy Carbon Electrode in Aqueous Medium.

FeS2 /carbon nanotube (CNT) nanocomposites were synthesized and immobilized on the surface of a glassy carbon electrode (GCE) in order to investigate the electrocatalytic conversion of 4-aminophenol (4-AP) into p-quinone in an aqueous medium. The reformed electronic properties (in terms of lowering of band-gap energy and charge-transfer resistance), as well as improved surface area, result in an enhanced redox reaction of 4-AP in the presence of FeS2 -CNT NCs compared to that with FeS2 alone. The 4-AP molecules undergo coupled two-proton and two-electron transfer quasi-reversible redox reactions with a symmetry factor of 0.55 and standard rate constant (k°) of 0.8 cm s-1 . Here, quinone imine is generated as an intermediate which is later converted into quinone in an irreversible hydrolysis reaction. The best catalytic performance can be obtained at the pH value of 7.0.

Time-resolved high-frequency EPR studies on magnesium and zinc tetraphenylporphines in their lowest excited triplet states.

The lowest excited triplet (T(1)) states of magnesium and zinc tetraphenylporphines (MgTPP and ZnTPP) were studied by time-resolved (TR) high-frequency/high-field W-band electron paramagnetic resonance (hf-EPR) spectroscopy in rigid glasses at low temperatures. Inspections of the TR-hf-EPR spectra of the spin-polarized triplets revealed that the zero field splitting (ZFS) parameters, D and E, for MgTPP and ZnTPP triplets were nearly the same. At the same time, their g-tensors were found to be different. These results are interpreted quantitatively in terms of spin-orbit couplings (SOCs) and angular momenta among the excited states, giving a magnitude of SOC in the T(1) state of ZnTPP. For the first time, both the TR-hf-EPR spectra and corresponding time profiles were acquired on the ZnTPP's triplet at room temperature in liquid paraffin solution with the populations of the electron spin states being in Boltzmann equilibrium. Because of relatively fast paramagnetic relaxation in rotating triplet at room temperature, the spectra and time profiles were free from the effects of microwave saturation that allowed for the direct measurement of the absolute intersystem crossing ratios P(x):P(y):P(z) 0.085:0.085:0.83. All of these results have demonstrated advantages and new perspectives of the W-band EPR spectroscopy.

Spin coupling in the supramolecular structure of a new tetra(quinoline-TEMPO)yttrium(III) complex.

The newly synthesized tetra(quinoline-TEMPO)yttrium(III) potassium salt shows interesting structural features at the molecular and supramolecular levels, revealed by the analysis of the X-ray diffraction data. The magnetic susceptibility and EPR data corroborated with structural considerations showed that the exchange and dipolar spin coupling interactions are taking place at the nodes assembling the supramolecular 2D structure. The Y(III) center shows antiprismatic octacoordination, close to the idealized D2 symmetry. The diamagnetic transition metal plays no role in mediating the radical interactions since the TEMPO-type fragments are remote from the chelating moieties of the ligand. In turn, significant interaction occurs on the nodes consisting in the quasi-rectangular coordination of potassium counterions by the spin-bearing TEMPO groups coming from four distinct complex units. The antiferromagnetic susceptibility was consistently modeled by a spin Hamiltonian based on the rectangle topology of four spins S = 1/2. The fitted exchange parameters are Ja = -5.1 cm-1 and Jb = -3.4 cm-1 for the edges, imposing Jd = 0 for the diagonal. These values are in excellent agreement with the ab initio results Ja = -4.83 cm-1, Jb = -3.44 cm-1, Jd = -0.07 cm-1 obtained in a CASSCF(12,8) calculation. Based on the reliability of the ab initio results we were able to select the presented J parameters among several versions of multiple solutions with acceptable goodness of the fit. A methodological caveat about the artifacts of the automatic use of best fit parameters, in the absence of supplementary criteria, in the context of relative blindness of magnetic susceptibility modeling, is raised. The details of the EPR spectrum at 10 K are also consistent, in the frame of dipolar approximation, with the model of four interacting spins at the nodes of the supramolecular assembling.

Novel excited quintet state in porphyrin: bis(quinoline-TEMPO)-yttrium-tetraphenylporphine complex.

New mono- and bis[4-(3-hydroxy-2-methyl-4-quinolinoyloxy)-2,2,6,6-tetramethylpiperidin-1-oxyl](meso-tetraphenylporphyrinato)yttrium(III) complexes have been synthesized, and the properties of the excited states generated by photoexcitation of porphyrin were studied by time-resolved (TR) and pulsed two-dimensional electron paramagnetic resonance (EPR) spectroscopy. A TR-EPR spectrum was observed in the quartet (S=3/2) or quintet (S=2) states generated from interactions of one or two radicals with the photoexcited triplet state of the porphyrin. The zero-field splitting D values of these states were analyzed in terms of those of the triplet and the radical-triplet pair. The spin states of the excited states were definitely assigned by measuring the mutation frequencies with pulsed EPR.