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.

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Light‐induced spin‐polarized transient EPR spectra are reported for several water‐soluble copper porphyrins. The spectra are assigned to the doublet ground state, with emissive spin polarization resulting from photoexcitation and subsequent electronic relaxation. In contrast to other systems for which polarization of a doublet ground state has been observed, the exchange interactions in the copper porphyrins are strong and the geometry is fixed. It is proposed that intersystem crossing from the photoexcited trip‐doublet to the trip‐quartet state can lead to net polarization of the spin system and that this polarization is maintained during electronic decay, possibly via charge‐transfer and exciplex states. The intensity of the observed spin polarization is essentially independent of the molecular orientation in the external field, but is strongly dependent on the nature of the charged peripheral groups. Possible reasons for this behavior are discussed.

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We present a Fourier transform EPR study of photoexcited rubidium-tetrahydrofuran solutions (Rb/THF), in the absence and presence of thecomplexing chelate 222 cryptand (Kp), Rb/THF/Kp. The temporal behavior of the EPR signals is interpreted in terms of two components participating in the complex processes, i.e., the spin-polarized photoelectrons and the solvated electrons that are coupled to the alkali-metal cations and, when present, to the cryptand molecules. The spin polarization mechanism is due to the radical pair mechanism, which is induced by charge transfer to solvent as a result of the photoabsorption. Spin relaxation times, measured directly in these systems, are discussed in terms of a site-exchange narrowing mechanism and a spin-rotation coupling mechanism in Rb/THF solutions and by collision-induced electron transfer in Rb/THF/Kp solutions.