Electron spin polarization of functionalized fullerenes. Reversed quartet mechanism

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study two functionalized fullerenes consisting of a C₆₀ 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 ( ²T₁ and ⁴T₁) states. This ISC is accompanied by ESP transfer to the ground state (²S₀) by either electron-transfer reaction (in our case via charge transfer state, ²CT, i.e., ²T₁ → ²CT → ²S₀) or internal conversion, ²T₁ → ²S₀.