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K-band electron spin resonance (ESR) technique was employed to study films of regioregular poly(3-hexylthiophene) (P3HT) and P3HT/PCBM ([6,6]-phenyl- C61-butyric acid methyl ester) blends to estimate their structural macroscopic anisotropy. As for nematic liquids (or liquid crystals) our consideration was based on the approach that the free energy of self-organised polymer molecules of P3HT is a function of molecular orientation and therefore chains in polymer films exhibit some degree of orientational order. The lamellar molecular orientation of the films was confirmed by angular-dependent ESR spectroscopy of polarons, which were considered as a localised paramagnetic centre with an unpaired carbon π-electron of the thiophene ring. The additional ESR signal initiated by the UV/visible CW Xe-lamp illumination of the films at air atmosphere was attributed to the negative polaron (trapped photo-electron) on the polymer chain, as well as to the radical due to chain degradation. © 2009 Elsevier Ltd. All rights reserved.

Photo-induced electron transfer (PET) between a poly(3-dodecylthiophene) (P3DDT) and maleic anhydride (MA) in their blend in liquid solution of tetrahydrofuran was observed by the time-resolved electron spin resonance (TRESR) under UV flash photolysis at 308 nm. The observed spectra were identified as free-radical signals of positive polarons on polymer chains and MA anion radicals. Their emissive chemical induced dynamic electron polarization (CIDEP) originated mainly from excited triplet states (triplet mechanism of CIDEP). Analysis of radical spectra integral intensity distribution shows at the influence of radical pair mechanism (RPM) of CIDEP with the positive sign of the exchange interaction constant (J > 0). The last is attributed to Coulombic interaction between geminately formed polarons and MA anion radicals. The spin relaxation times of radicals were determined by fitting the time evolution of the TRESR signal at near-resonance positions of the field using the Bloch equations and direct Fourier transform analysis. © 2008 Elsevier B.V. All rights reserved.

Degradation processes of N-methylmorpholine-N-oxide monohydrate (NMMO), cellulose and cellulose/NMMO solutions were studied by high performance liquid chromatography (HPLC) and electron spin resonance (ESR) spectroscopy. Kinetics of radical accumulation processes under UV (λ = 248 nm) excimer laser flash photolysis was investigated by ESR at 77 K. Beside radical products of cellulose generated and stabilized at low temperature, radicals in NMMO and cellulose/NMMO solutions were studied for the first time in those systems and attributed to nitroxide type radicals ∼CH2- NO•∼-CH2∼ and/or ∼CH2 NO•-CH3∼ at the first and methyl •CH3 and formyl -CHO radicals at the second step of the photo-induced reaction. Kinetic study of radicals revealed that formation and recombination rates of radical reaction depend on cellulose concentration in cellulose/NMMO solutions and additional ingredients, e.g., Fe(II) and propyl gallate. HPLC measurements showed that the concentrations of ring degradation products, e.g., aminoethanol and acetaldehyde, are determined by the composition of the cellulose/NMMO solution. Results based on HPLC are mainly maintained by ESR that supports the assumption concerning a radical initiated ring-opening of NMMO. © Springer Science+Business Media B.V. 2007.

We present evidence of strong Shubnikov-de-Haas magnetoresistance oscillations in a polarization-doped degenerate three-dimensional electron slab in an Al$_{x}$Ga$_{1-x}$N semiconductor system. The degenerate free carriers are generated by a novel technique by grading a polar alloy semiconductor with spatially changing polarization. Analysis of the magnetotransport data enables us to extract an effective mass of $m^{\star}=0.19 m_{0}$ and a quantum scattering time of $\tau_{q}= 0.3 ps$. Analysis of scattering processes helps us extract an alloy scattering parameter for the Al$_{x}$Ga$_{1-x}$N material system to be $V_{0}=1.8eV$.