High-frequency (95 GHz) electron paramagnetic resonance study of the photoinduced charge transfer in conjugated polymer-fullerene composites

Light-induced electron paramagnetic resonance (LEPR) measurements are reported in composites of poly(2-methoxy-5-(3-,7-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and ${[6,6]-\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l}-\mathrm{C}}_{61}-\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{y}\mathrm{r}\mathrm{i}\mathrm{c}$ acid methyl ester (PCBM), a soluble derivative of ${\mathrm{C}}_{60}.$ Under illumination of the sample, two paramagnetic species are formed due to photoinduced charge transfer between conjugated polymer and fullerene. One is the positive polaron ${\mathrm{P}}^{+}$ on the polymer backbone and the other is the radical anion on the methanofullerene. Using high-frequency (95 GHz) LEPR it was possible to separate these two contributions to the spectrum on the basis of their g factors, and moreover to resolve the g anisotropy for both radicals. The positive polaron on the conjugated polymer chain possesses axial symmetry with g values $g_{\Vert }=2.0034\mathrm{}\left(1\right)\mathrm{}$ and $g_{\perp }=2.0024\mathrm{}\left(1\right).$ EPR on low doped polymer gave extra proof for the assignment to the positive polaron. The negatively charged methanofullerene has a lower, rhombic symmetry with $g_x=2.0003\mathrm{}\left(1\right),$ $g_y=2.0001\mathrm{}\left(1\right),$ and $g_z=1.9982\mathrm{}\left(1\right).$ Different spin-lattice relaxation of both species gives rise to a rapid passage effect for the positive polaron spectrum.