CIDEP Studies of Fullerene-Derived Radical Adducts

Abstract

Photolyses of solutions containing organomercury compounds (HgR 2 ) in the presence of C 60 fullerene have been investigated by Fourier transform time-resolved EPR (FT TR EPR) and continuous-wave EPR (CW EPR) techniques. By FT TR EPR, both electron-spin-polarized 3 C 60 (A polarization) and electron-spin-polarized adducts • C 60 R (E/A + E polarization) are observed. The CW EPR spectra of the • C 60 R radicals under steadystate irradiation also exhibit some electron-spin polarization. The chemically induced dynamic electron polarization (CIDEP) in the FT TR EPR experiments is explained by the following series of steps. Photolysis initially causes cleavage of the organomercury compounds into radicals that add to C 60 to form • C 60 R. The latter combine to form the dimers, [C 60 R] 2 , which are thermally stable and accumulate in the samples. In all of the reported experiments, a certain quantity of dimers is produced by photolysis before the EPR spectra are acquired. In the FT TR EPR experiments, laser excitation produces 3 C 60 by excitation of C 60 and • C 60 R by photocleavage of the dimers. The observed E/A CIDEP patterns at short (<1 µs) delays after the laser flash are proposed to be a result of the creation of polarization through the radical-pair mechanism (RPM) resulting from the interactions of two• C 60 R radicals (geminate or free) formed from the photocleavage of [C 60 R] 2 dimers. The additional E polarization observed at later times (>1 µs) is proposed to result from the interaction of 3 C 60 with • C 60 R radicals, creating E polarization through the radical-pair-triplet mechanism (RPTM). The polarization observed in the CW EPR experiments is attributed to the maintenance of polarization through the radical lifetime because of the extremely long spin-lattice relaxation of the • C 60 R radicals. The latter conclusion is consistent with the very small (50 mG) line widths of the adduct radicals. An upper limit for the bond energy of the [C 60 R] 2 dimers of 226 kJ/mol is established by the observation of the CIDEP of • C 60 R radicals when 532-nm excitation is employed. The role of multiple adducts in the observed FT TR EPR spectra is discussed.

Study of the contact charge transfer behavior between cryptophanes (A and E) and fullerene by absorption, fluorescence and 1H NMR spectroscopy

“…For these reasons, the preparations of donor-acceptor linked molecular systems have been extensively studied in order that the complicated CT process can be better understood and more efficient CT devices can be developed [7,8].Among the wide variety of donor-acceptor systems, fullerene (C 60 ) is particularly appealing as electron acceptor because of its three-dimensional structure, delocalized -electrons within the spherical carbon framework, small reorganization energy, low reduction potential, and absorption extending over most of the visible region. These unique physical and chemical properties make C 60 a promising candidate for investigating its electron transfer processes with electron-donor compounds [9][10][11][12][13][14]. So far many CT complexes have been found between fullerenes and electron donors including cyclotriveratrylene (CTV), crown ethers, pyrrolidine derivatives and amine derivatives [15][16][17][18].…”

Radical Additions

“…• , have provided a valuable insight into the electronic situation on the fullerene surface and other physical phenomena of these species [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The organic radical species R • , which is to be added to the fullerene, can be generated in situ either photochemically or thermally by established free radical reactions from suitable precursor molecules.…”

“…Dimerization at C-3 or C-3′ seems to be most likely for steric reasons [2,3]. Excitation of the alkylfullerene dimers at 532 nm leads quantitatively to the alkylfullerene radicals [16]. Excitation of the alkylfullerene dimers at 532 nm leads quantitatively to the alkylfullerene radicals [16].…”

Chemiluminescence upon the dimerization of EtC60 · fullerenyl radicals

“…Of all the components of solution 1, it is the dimer that has the weakest (according to the estimation, 9 53.8 kcal mol -1 ) EtC 60 -C 60 Et bond. Therefore, we at tribute the generation of the ESR and CL signals to the photolytic decomposition of the dimer to two EtC 60 • radi cals (reaction (1)) followed by the recombination of EtC 60 • to form the dimer molecules, some of which are generated in the excited state (reaction (2)).…”