The structure and electronic state of photoexcited fullerene linked with a nitroxide radical based on an analysis of a two-dimensional electron paramagnetic resonance nutation spectrum

An electron paramagnetic resonance (EPR) study of 3,4-fulleropyrrolidine-2-spiro-4′-[2′,2′,6′,6′- tetramethyl]piperidine-1′-oxyl (1) was performed on the photoexcited quartet state in toluene glass. The spectrum of the |S,Ms〉=|3/2,±3/2〉⇔|3/2,±1/2〉 transitions was observed selectively by using a two-dimensional (2D) nutation method and analyzed with a spectral simulation in a randomly oriented system. A position of the nitroxide moiety was determined with respect to the zero-field splitting (zfs) axes of excited triplet fullerene (³C₆₀) by taking into account of the dipolar-dipolar interaction between the radical and ³C₆₀, the hyperfine coupling, the anisotropic g-value of the nitroxide radical, and the zfs of the ³C₆₀ moiety. It was found that none of the zfs axes of the ³C₆₀ moiety coincide with the local C₂ axis of the molecule which is defined by the position of addition. A symmetry of the electronic structure in ³C₆₀ is discussed on the basis of the result.