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]-phenyl-C₆₁-butyric acid methyl ester (PCBM), a soluble derivative of C₆₀. 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 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_=2.0034(1) and g_⊥=2.0024(1). 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 gₓ=2.0003(1), gy=2.0001(1), and gz=1.9982(1). Different spin-lattice relaxation of both species gives rise to a rapid passage effect for the positive polaron spectrum.