Isotropic reorientations of fullerene C70 triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance (original) (raw)
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The Journal of Chemical Physics, 2009
Spin-lattice relaxation times T 1 of photoexcited triplets 3 C 70 in glassy decalin were obtained from electron spin echo inversion recovery dependences. In the range 30-100 K, the temperature dependence of T 1 was fitted by the Arrhenius law with an activation energy of 172 cm −1. This indicates that the dominant relaxation process of 3 C 70 is described by an Orbach-Aminov mechanism involving the higher triplet state t 2 which lies 172 cm −1 above the lowest triplet state t 1. Chemical modification of C 70 fullerene not only decreases the intrinsic triplet lifetime by about ten times but also increases T 1 by several orders of magnitude. The reason for this is the presence of a low-lying excited triplet state in 3 C 70 and its absence in triplet C 70 derivatives. The presence of the higher triplet state in C 70 is in good agreement with the previous results from phosphorescence spectroscopy.
The Journal of Physical Chemistry, 1993
Detailed analysis of time-resolved electron paramagnetic resonance (EPR) line shapes of photoexcited triplet states of c 6 0 in isotropic and anisotropic matrices has been carried out. Two main motional models have been tested over a very wide temperature range. For most temperatures, the best fit to the experimental line shapes was obtained with a model of discrete jumps between two configurations, which can be formally defined as an exchange between the longitudinal (Z) dipolar axis and one of the two transverse axes (Xu). This exchange process is probably related to two distorted Jahn-Teller (JT) configurations of the molecule. For very high temperatures, the best fit was obtained with a recently developed model of Brownian rotational diffusion. In the latter cases, relating to the liquid phase of the solvent, the triplets appear to be in thermal spin equilibrium, and their kinetics can be described by a special form of transient nutations. In the former cases, related to the glass and amorphous phases (toluene) or nematic phase (E-7), the triplets are clearly spin polarized. For very low temperatures the motional (jump) rate is practically temperature independent. This seems to be due to the nature of the motion, Le., exchange between JT configurations, rather than actual molecular motion.
The Journal of Physical Chemistry A, 2008
Spin-polarized echo-detected electron paramagnetic resonance (EPR) spectra and the transversal relaxation rate T 2-1 of the photoexcited triplet state of fullerene C 60 molecules were studied in o-terphenyl, 1-methylnaphthalene, and decalin glassy matrices. The model is composed of a fast (correlation time ∼10-12 s) pseudorotation of 3 C 60 in a local anisotropic potential created by interaction of the fullerene molecule with the surrounding matrix molecules. In simulations, this potential is assumed to be axially symmetric around some axis of a preferable orientation in a matrix cage. The fitted value of the potential was found to depend on the type of glass and to decrease monotonically with a temperature increase. A sharp increase of the T 2-1 temperature dependence was found near 240 K in glassy o-terphenyl and near 100 K in glassy 1-methylnaphthalene and decalin. This increase probably is related to the influence on the pseudorotation of the onset of large-amplitude vibrational molecular motions (dynamical transition in glass) that are known for glasses from neutron scattering and molecular dynamics studies. The obtained results suggest that molecular and spin dynamics of the triplet fullerene are extremely sensitive to molecular motions in glassy materials.
Chemical Physics, 1996
The EPR spectra of excited triplet states of two fullerene derivatives, C6oO and C6oC2H4N(CH3), in the glassy matrices of toluene and of polymethylmethacrylate (PMMA) and as films were examined at low temperature and the magnetic parameters were measured. In films, species with zfs parameters smaller than those in toluene were found and they were assigned to triplet excitations visiting more than one molecule in the crystallites that form the film. When CoaC2H4N(CH3) was excited in PMMA grown by in situ thermal polymerization a new triplet species was originated characterized by a dipolar splitting constant D about three times larger than that measured in toluene. Evidence was gained that such species participates to cross-linking in the polymer. The effect of PMMA matrix on the molecular dynamics of 3C70 was also investigated. It was found that a dynamical model holds based on the pseudorotation of the molecule around the axis of larger dipolar splitting. Motion is activated with activation energy of 210 + 50 cm -~ a value comparable with that obtained for pseudorotation of 3C70 in cyclohexane. 0301-0104/96/$15.00 (~) 1996 Elsevier Science B.V. All rights reserved SSDI 0301-0104(95)00384-3
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2011
X-band echo-detected electron paramagnetic resonance (ED EPR) spectra of triplet state of fullerene C 70 generated by continuous light illumination were found to correspond below 30 K to a non-equilibrium electron spin polarization. Above 30 K spectra are characteristic of Boltzmann equilibrium. Spectra were simulated fairly well with zero-field splitting parameters D = 153 MHz and E and distributed within the range of 6-42 MHz. The origin of E distribution is attributed to the Jahn-Teller effect, which in glassy matrix is expected to depend on the local surrounding of a fullerene molecule (a so-called E-strain). In the center of ED EPR spectra a narrow hole was observed. With increase of the microwave pulse turning angle this hole transforms into a single narrow absorptive line. Numerical simulations by density matrix formalism confirm that central hole originates from a simultaneous excitation of both allowed electron spin transitions of the triplet (T 0 ↔ T + and T 0 ↔ T −), because of their degeneracy at this spectral position. Also explanations are given why this hole has not been observed in the previously reported experiments on continuous wave EPR and on ED EPR under laser pulse excitation.
The Journal of Physical Chemistry A, 2008
A previous study of C 70 in deuterated chlorobenzene generated evidence suggesting C 70 was experiencing unique reorientational behavior at given temperatures. The present study explores the possibility that this behavior is present across other solvents. The 13 C spin-lattice relaxation rates for four carbon resonances in C 70 were analyzed in benzene-d 6 , chlorobenzene-d 5 , and o-dichlorobenzene-d 4 , and as a function of temperature, to probe the reorientational dynamics of this fullerene. Anisotropic behavior was observed at the lowest (283 K) and highest temperatures (323 K), isotropic diffusion was seen between 293 and 303 K, and quasi-isotropic at 313 K. When anisotropic motion was present, diffusion about the figure axis was seen to be higher than diffusion of the figure axis. Experimentally obtained diffusion coefficients generated reorientational correlation times that were in excellent agreement with experimental values. Theoretical predictions generated by a modified Gierer-Wirtz model provided acceptable predictions of the diffusion constants; with D X usually being more closely reproduced and D Z values generally being underestimated. Overall, the results indicate that the factors affecting rotational behavior are complex and that multiple solvent factors are necessary to characterize the overall motion of C 70 in these solvents. Although a solvent's viscosity is normally sufficient to characterize the tumbling motion, the spinning motion is less sensitive to solvent viscosity but more responsive to solvent structure. The balance and collective influence of these factors ultimately determines the overall rotational behavior.
Journal of Molecular Structure, 1995
In this article we report the effect of dynamic disorder in the vicinity of orientational phase transition (Tc=250-260 K) in solid C6c on reduction of molecular local symmetry. We probe these phenomena by ellipsometry at the wavelength 520 nm, which corresponds to orbitally forbidden molecular absorption of tbllerene. The temperature dependence of optical constants of solid fullerene has been compared with its luminescent properties and depolarization of the reflected light. It has been shown that all data exhibit hysteresis-like behavior nearby Tc.
Magnetic resonance study of fullerene-like glassy carbon
Diamond and Related Materials, 2007
Multi-shell fullerene-like nanoparticles of glassy carbon were synthesized by a low-temperature pyrolysis of sucrose. These samples were studied by magnetic resonance spectroscopy. 13 C NMR spectrum of glassy carbon shows a complicated line the most intensive component of which originated from sp 2 carbons. Measured values of nuclear spin-lattice relaxation times are discussed with a model of system containing intrinsic paramagnetic centers, as it is supported by EPR. EPR reveals extrinsic and intrinsic magnetism in the samples. The former is probably due to para-and ferromagnetic impurities entrapped during the synthesis. The latter is attributed to the carbon system (e.g., dangling bonds). The carbon-originated EPR signal has Lorentzian lineshape and g-factor of 2.0030 ± 0.0001. Its line width as well as electron spin-lattice relaxation time was found to be extremely sensitive to partial oxygen pressure and humidity. The intrinsic magnetism obeys the Curie law at 130-400 K. The total amount of carbon-originated paramagnetic defects exceeds 2 × 10 19 spin/g. They are suggested to exist in all layers of imperfect fullerene (onion)-like particle around their breaches which are large enough allowing oxygen to penetrate freely into the particles.