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A spin pH probe technique was used to study the influence of the channel diameter on the EPR spectra of pH-sensitive nitroxide radicals (NR) located in the channels of the mesoporous molecular sieves MCM-41 and SBA-15 with diameters ranging from 2.3 to 8.1 nm. From EPR spectra analysis and the results of the NR retention by the mesoporous molecular sieves upon washing with an aqueous KCl solution, the regularities of NR molecular location inside the channels were studied. The obtained dependence of the fraction of the radical molecules in the fast motional regime (with the rotational correlation times, τc = 2 × 10-11 s-9 × 10-11s) in the channels of the mesoporous molecular sieves as a function of pH indicates that both NR in the fast and slow motional regime (with τc = 8 × 10 -9s-7 × 10-10s) may be used for estimation of the solution acidity inside the channels and of the near-surface electrical potential. © 2013 Elsevier Inc. All rights reserved. This work was supported by the Research Program of the Ministry of the Education of the Russian Federation (2007-2011). E.G.K. thanks the Program of Cooperation “Mikhail Lomonosov” between the Ministry of Education of Russian Federation and the German Service of Academic Exchanges (DAAD). We are grateful to Dr. G. Chandrasekar for assistance in the synthesis of MMS and their characterization. Appendix A

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study two functionalized fullerenes consisting of a C60 moiety covalently linked to TEMPO radical via spacers of different length. Photoinduced electron spin polarization (ESP) reflecting a non-Boltzmann population within the energy levels of the spin system was observed in the electronic ground and excited states. Both fullerenes are characterized by a sign inversion of their TREPR spectra. A new mechanism of ESP generation was suggested to explain the experimental results. This mechanism, termed as the reversed quartet mechanism (RQM), includes the intersystem crossing process, which generates ESP in the excited trip-doublet and trip-quartet (2T1 and 4T1) states. This ISC is accompanied by ESP transfer to the ground state (2S0) by either electron-transfer reaction (in our case via charge transfer state, 2CT, i.e., 2T1 → 2CT → 2S0) or internal conversion, 2T1 → 2S0.

Enantiomerically pure (3 R ,4 S ) and (3 S ,4 R ) protected 4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-3-carboxylic acids were synthesized by reduction of the enamines resulting from the condensation of 3-carboxymethyl-1-oxyl-2,2,6,6-tetramethyl-4-piperidone with ( R ) or ( S )-α-methylbenzylamine. While NaBH 3 CN/CH 3 COOH reduction gave predominantly a mixture of the two possible cis -diastereomers, the use of NaBH 4 /(CH 3 ) 2 CHCOOH resulted in a mixture of only one trans - and one cis -diastereomer. Removal of the chiral auxiliary from the separated diastereoisomers by hydrogenolysis and regeneration of the nitroxide radical gave the desired β-amino esters. The ESR spectrum of the (3 R ,4 S )-enantiomer is also reported.

A nitroxide [60]fullerene adduct containing a pyrrolidine-1-oxyl group has been synthesized. Its orientational order in the nematic phase of the liquid crystal solvent 4,4′-azoxyanisole (PAA) has been measured from the variation of the EPR spectral parameters on passing from the isotropic to the nematic phase. Highly resolved EPR lines allow for precise evaluation of the shifts of the g, a N and a H values. Since the g and the hyperfine tensors are known, the order matrix could be obtained. This is compared with the one calculated with a theoretical model based on short range solute-solvent interactions, which predicts a considerable degree of orientation of the molecular axes, despite the almost spherical shape of the molecule. The agreement with experimental findings is quite good and it is further improved if a bent structure of the pyrrolidine ring is taken into account.

[60]fullerene bisadducts containing the C 2 ν -symmetrical 2,2,5,5-tetramethylpyrrolidine-1-oxyl moiety have been synthesized. Their EPR spectrum is typical of nitroxide biradicals with electron exchange interaction J of the same order of the nitrogen hyperfine coupling. From the EPR spectrum the absolute value of J was obtained, while the sign was measured by ENDOR. When the derivatives are photoexcited strong spin polarization is observed with A/E/A/E or E/A/E/A patterns, according to a positive or a negative sign of J . This behavior is accounted for by an intramolecular triplet–triplet interaction. The analogy with the polarization of transient spin correlated radical pairs is pointed out.

We investigated by W-band (94 GHz) transient electron paramagnetic resonance (TREPR) and photoinduced absorption (PIA) spectroscopy two fullerene derivatives bearing a nitroxide radical unit. After pulsed laser photoexcitation of the molecules in liquid toluene solution, complex EPR spectra are recorded, with lines in absorption and emission. The intrinsic higher spectral and temporal resolution of the W-band frequency leads to the assignment of all the lines in the spectrum and the determination of the sign and the absolute value of the exchange coupling between the fullerene in its photoexcited triplet state (ST = 1) and the radical (SR = 1/2). The two compounds with different fullerenenitroxide spacers show oppositeferromagnetic and antiferromagneticexchange couplings. The time evolution of the spectra and the polarization of the lines are interpreted in terms of several possible spin polarization mechanisms. The EPR measurements are complemented with PIA experiments.

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The proton hyperfine tensors of five nitroxide radicals have been obtained by ENDOR spectroscopy in frozen solution. The spectra are interpreted by computing the dipolar hyperfine interaction and simulating the spectra. EPR spectra in solution of the same radicals have been simulated by taking into account the effects of the proton hyperfine tensors. We have been able to reproduce accurately the line broadening effects of the proton hyperfine structures inside each nitrogen hyperfine component and we have determined the correlation times for the rotational motion. In the case of the radical Tempol, our analysis allows discrimination between the effects due to the protons of the axial and equatorial methyl groups. On the basis of experimental evidence we can attribute the larger isotropic hyperfine coupling constant to the axial methyl protons. The possible use of the present results for interpreting the spectra of other nitroxide radicals is discussed.

Time-resolved electron paramagnetic resonance spectra of two single spin-labeled and two double spin-labeled C60 derivatives in frozen solution are recorded with pulsed laser excitation and 100 ns time resolution. Quartet and quintet excited species are detected which arise from the electron spin coupling of the triplet excited fullerene moiety with the unpaired spin(s) of the nitroxide label(s). Despite the similar molecular structure, in both series of single and double labeled derivatives a different behavior was found, which is due to substantial difference of the energy of exchange coupling.

Indolinonic and quinolinic aromatic nitroxides have been shown to efficiently inhibit free radical mediated oxidation reactions in biological systems. Since all antioxidants also possess pro-oxidant activity, possibly through a hydrogen abstraction process from suitable substrates, the relative hydrogen abstraction abilities of these compounds were evaluated also in relation to their structure. Therefore, different hydrogen donors were reacted with indolinic, indolinonic and quinolinic nitroxides and the rates of hydrogen abstraction were determined using UV-Vis spectroscopy. From the data obtained, a structure-activity relationship was found for indolinonic and indolinic nitroxides. In addition, hydrogen abstraction abilities of these compounds were much greater when compared to that of the aliphatic nitroxide TEMPO, although the antioxidant activities of these two classes of compounds are very similar.

The modulation induced by different solvents and by pH on the nitrogen isotropic hyperfine coupling consts. (AN) of 2,2,5,5-tetramethyl-3-carboxypyrrolidine (3-carboxy-PROXYL,CP) and 2,2,6,6-tetramethyl-4-carboxypiperidine (4-carboxy-TEMPO,CT) nitroxides was studied through ab initio quantum mech. computations and by EPR spectroscopy. The geometry of the neutral and deprotonated forms and the corresponding dissocn. consts. are computed by using the hybrid PBE0 functional. With these structures, accurate AN values were obtained by an integrated post-Hartree-Fock/PBE0 approach. Specific solvent effects are modeled by including a few solvent mols. (strongly bound to the solute), whereas bulk contributions are taken into account by the polarizable continuum model (PCM). Both EPR spectra and quantum mech. computations point out that AN increases with increasing solvent polarity and, esp., H-bond ability. The AN values obtained by the authors' computational model both for the neutral and deprotonated forms in aq. soln. are in good agreement with EPR values. However, the computed pKa values(4.97 and 5.25 for CP and CT, resp.) are significantly larger than the value reported in the literature for CP (3.4). New potentiometric and spectroscopic measurements lead to values (4 and 4.30 for CP and CT, resp.) in better agreement with quantum mech. computations and chem. intuition. Also the calcd. magnetic titrn. curves in aq. soln. are in quite good agreement with the authors' new EPR curves.

A series of biradicals consisting of a C60 linked to two nitroxide addends have been studied by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The series includes all four trans bisadducts and the equatorial isomer. After a visible laser pulse, the bisadducts in toluene liquid solution show chemically induced dynamic electron polarization (CIDEP) effects with a complex pattern of narrow lines in emission and enhanced absorption. From the knowledge of the sign of exchange interaction between the two nitroxide moieties, it was possible to account for the CIDEP effect in terms of an intramolecular triplet-triplet annihilation. A broad signal attributed to the quintet excited state was observed at short time delay after the laser pulse. The TR-EPR spectra recorded at low temperature in glassy matrix are assigned to an excited triplet state localized on the C60, with a small interaction with the spins of the two nitroxide fragments. The assignment is based on spectral simulation and analysis of the transient nutation frequency. The small exchange interaction between excited triplet C60 and nitroxide is shown to depend on the nature of the nitroxide addend.

The spin polarization of excited triplet states caused by mutual annihilation of triplet states was detected by time-resolved electron paramagnetic resonance (EPR) of triplet excitons in two molecular crystals, anthracene-tetracyanobenzene and phenazine-tetracyanoquinodimenthane. The time profile of the two EPR lines of the triplet exciton spectrum have been studied in the time range up to 200 μs after a laser pulse. Besides the initial polarization of the lines, due to intersystem crossing, a long-lasting polarization process is detected which is attributed to the triplet-triplet annihilation. The investigation of the dependence of the phenomenon on the intensity of the light pulse and on the orientation of the crystals in the magnetic field is presented.