Electron spin resonance and electron nuclear double resonance of photogenerated polarons in polyfluorene and its fullerene composite

Abstract

Electron spin resonance ͑ESR͒ and electron-nuclear double resonance ͑ENDOR͒ of photogenerated polarons in poly͑9,9-dioctylfluorene͒ ͑PFO͒ and its composite with fullerene ͑C 60 ͒ using variable photoexcitation energy up to 4.1 eV are reported. For PFO, a light-induced ESR ͑LESR͒ signal ͑g = 2.003͒ is observed below 60 K, and its transient response and excitation spectrum indicate that the observed spins are photogenerated polarons on PFO. For the PFO-C 60 composite, two LESR signals of photogenerated positive polarons on PFO ͑g 1 = 2.003͒ and radical anions on C 60 ͑g 2 = 1.999͒, respectively, are observed below 120 K, which are caused by photoinduced electron transfer from PFO to C 60. A remarkable enhancement of the LESR signals in the excitation spectrum at ϳ2.8 eV is observed compared with the case of pure PFO. The bimolecularrecombination kinetics of photogenerated charge carriers in the composite are confirmed by the dependence of the LESR on excitation-light intensity and by the decay dynamics. Light-induced ENDOR ͑LENDOR͒ signals are clearly observed for excitation around 2.8 eV owing to the highly efficient photoinduced electron transfer in the composite. Broad LENDOR shifts directly reflect the spin-density distribution of the polarons in PFO. We have determined its maximum shift using LENDOR-induced ESR, and have evaluated the maximum spin density on the carbon site coupled to the proton as 0.032. This value is consistent with the theoretical result obtained by Pariser-Parr-Pople ͑PPP͒ model, where the spatial extent of the polarons is calculated as ϳ3 monomer units of PFO. The calculated LESR spectra of PFO based on the PPP model are consistent with the experimental spectra, which confirm the above spatial extension of the polaron in PFO.

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