TR-EPR Spectra Research Articles

CIDEP Studies of Fullerene-Derived Radical Adducts

Photolyses of solutions containing organomercury compounds (HgR2) in the presence of C60 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 3C60 (A polarization) and electron-spin-polarized adducts •C60R (E/A + E polarization) are observed. The CW EPR spectra of the •C60R radicals under steady-state 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 C60 to form •C60R. The latter combine to form the dimers, [C60R]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 3C60 by excitation of C60 and •C60R 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 •C60R radicals (geminate or free) formed from the photocleavage of [C60R]2 dimers. The additional E polarization observed at later times (>1 μs) is proposed to result from the interaction of 3C60 with •C60R 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 •C60R 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 [C60R]2 dimers of 226 kJ/mol is established by the observation of the CIDEP of •C60R radicals when 532-nm excitation is employed. The role of multiple adducts in the observed FT TR EPR spectra is discussed.

Substituent and Matrix Effects on the Excited Triplet States of 1,4-Naphthoquinones

Phosphorescence and TREPR spectra have been measured for 2-methyl-1,4-naphthoquinone (MNQ), 2,3-dimethyl-1,4-naphthoquinone (DMNQ), and 2-methoxynaphthoquinone (MeONQ) in several matrices at low temperatures. The |D| value of ZFS parameter decreases with increasing the electron-donating character of the substituent group. For MNQ, the T1 state is clearly assigned to be 3nπ* in character. The broad phosphorescence spectra and minor effects of the matrix polarity on the ZFS parameters lead to the conclusion that the character of the T1 states are mainly 3ππ* in DMNQ and MeONQ. The T1 states of the present naphthoquinone derivatives have more or less mixed character of the 3nπ* and 3ππ* states.