Spin polarization in fullerene derivatives containing a nitroxide group. Observation of the intermediate photoexcited quartet state in radical triplet pair interaction (original) (raw)
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The Journal of Physical Chemistry A, 2001
Nitroxide free radical/[60]fullerene derivative liquid solutions photoexcited by visible laser pulses are investigated by time-resolved EPR. Both radical and triplet excited fullerene spin-polarized EPR signals are observed. Their time evolution is examined in terms of CIDEP effects due to electron spin polarization transfer from the initially polarized triplet to the nitroxide and to spin polarization generated by the radical triplet pair mechanism. Radical and triplet spin relaxation times and rate constants for the processes of polarization transfer and radical triplet pair mechanism are obtained.
The sign of the exchange interaction between triplet excited fullerene and nitroxide free radicals
Applied Magnetic Resonance, 2000
The sign of the exchange interaction J in a series of radical triplet pairs (RTPs), formed by a nitroxide free radical and a triplet excited fullerene, has been determined from the spin polarization of time-resolved electron paramagnetic resonance spectra. Radical and fullerene are linked together by covalent bonds in different geometries. It is shown that the sign of J depends on the overlap between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly occupied in the excited triplet state. When the overlap does not vanish, a negative contribution to J arises from the admixing of a charge transfer structure in the wave function of the excited doublet state D of the RTP, which does not take place in the excited quartet state Q. The mixing of D* and Q' states lowers the energy of the former spin state and gives antiferromagnetic coupling.
Electron Spin Polarization of Functionalized Fullerenes. Reversed Quartet Mechanism
The Journal of Physical Chemistry A, 2005
Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study two functionalized fullerenes consisting of a C 60 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 ( 2 T 1 and 4 T 1 ) states. This ISC is accompanied by ESP transfer to the ground state ( 2 S 0 ) by either electron-transfer reaction (in our case via charge transfer state, 2 CT, i.e., 2 T 1 f 2 CT f 2 S 0 ) or internal conversion, 2 T 1 f 2 S 0 .
1997
Fulleropyrrolidine derivatives containing a TEMPO group (FPNOs) are examined in toluene solution, between 200 and 270 K, by TR-EPR spectroscopy afler LASER excitation. Signals of the ground and exeited states are observed and their time profiles are recorded. In presente of ferrocene the time pattern of the pola¡ is modified, showing the occurrence of intermoleeular electron transfer from the donor ferrocene to the excited FPNOs. The rate constant k~r is obtained from the simulation of the EPR signal time evolution of the ground and excited states.
The Journal of Physical Chemistry, 1993
Time-resolved electron spin resonance (TR ESR) has been used to investigate the chemically induced dynamic electron polarization (CIDEP) generated by the interaction of stable free radicals with the triplet states of benzophenone, benzil, and 2-acetylnaphthalene. The stable radicals were mono-, di-, tri-, and tetranitroxyl free radicals possessing the 2,2,6,6-tetramethylpiperidineN-oxyl moiety. All of the stable radical systems investigated were found to be emissively polarized by interaction with the triplet states, and the phase of polarization was independent of the sign of zero-field splitting (D) of the interacting triplet molecule. Possibleand likely mechanisms of polarization transfer (creation) resulting from the interaction of photoexcited triplet molecules with nitroxyls in the strong electron exchange are discussed. The emissive CIDEP of nitroxyls observed in the interactions with triplet benzil, which has D > 0, provides strong support for the operation of the radical-triplet pair mechanism. Within the time scale of T R ESR experiments (-lO-'-lO" s) no significant variation in the shape of the CIDEP spectra of the nitroxyls was observed, either in viscous media or in micelles. It is concluded that intramolecular spin exchange (or conformational change) of polynitroxyls occurs much faster than the time resolution of the experiment,
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.
Chemical Physics, 2001
Photoinduced electron transfer was observed in a series of methylsulfanyl sexithiophene/fulleropyrrolidine composites deposited as ®lms. Paramagnetic states were observed by transient EPR spectroscopy in the microseconds time domain. The spectra displayed polarized lines with characteristic antiphase emission/absorption pattern of spin polarization and were assigned to spin correlated radical ion pairs (SCRP) formed by intermolecular electron transfer from sexithiophene donor to C 60 fullerene acceptor molecules. Also transient signals detected at selected magnetic ®elds showed phase eects that are typical of SCRPs. Spectrum simulation was obtained by allowing for a distribution of respective orientations of the dipolar axes and g-tensors of partners in a pair. Fitting parameters used for one composite were Dagb À135 lT and J agb 2X5 lT for dipolar and spin exchange coupling constants, corresponding to an interradical mean distance of 27.4 # A. Similar values were obtained for the other examined systems. Pair structure and dynamics suggest to ascribe the charge-separated state to a radical pair generated after a hopping of the electron±hole charges from a primary pair originated in neighbor molecular sites. Ó
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.
Polarizabilities, charge states, and vibrational modes of isolated fullerene molecules
Physical Review B, 1992
We have used our local-orbital cluster codes to perform detailed density-functional-based calculations on isolated C60 molecules. We present firstand second-electron affinities, which include all effects due to spin polarization and charge-induced geometrical relaxation. The effects due to the generalized gradient approximation are reported as well. The two Ag and single A"vibrational modes are presented and frequency shifts due to charging are estimated. By placing the fullerene molecules in a static electric field of variable strength, the molecular static polarizabilities are obtained. In comparison to isolated carbon atoms, we find enhancements in the linear polarizabilities due to the delocalized electrons at the Fermi level, but do not observe any large nonlinear static contributions. By including effects due to charge transfer, on-site geometric relaxation, and fullerene polarization, we introduce a simple potential that accounts for long-range interactions and predict Hubbard parameters as a function of alkali-dopant concentration.
Dynamics of photoinduced radical pairs in poly(3-dodecylthiophene)/fullerene composite
Physica E: Low-dimensional Systems and Nanostructures, 2007
Radical pair laser (660 nm) photoinduced in poly(3-dodecylthiophene)/[1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C 61 ] (P3DDT/ PCBM) composite were investigated by the X-band light-induced EPR method in a wide temperature region. Two lines with different magnetic parameters were attributed to non-interacting polaron and fullerene ion-radical. The susceptibility of these centres follow activation law and decrease drastically at T4180 K due to their geminate recombination. The spin-lattice and spin-spin relaxation times of these paramagnetic centres were determined separately by the steady-state saturation method. It was shown that two independent spin dynamics processes are mainly realized in illuminated P3DDT/PCBM composite, namely polaron diffusion along the polymer chains with activation energy of 0.071 eV and rotation of fullerene ion-radicals with activation energy of 0.024 eV.