Radical ion pairs in sexithiophene/fullerene blends studied by transient EPR spectroscopy

Radical ion pairs generated by Photoinduced Electron Transfer in methylsulfanyl sexithiophene/fullerene derivatives deposited as films are studied by transient EPR spectroscopy. The spin polarization pattern of the transient EPR spectrum consists of characteristic antiphase emission/absorption lines typical of spin correlated radical pairs with electron dipolar and spin exchange interactions. The mean distance of 2.8 nm between the two unpaired electrons of the pair is obtained from the dipolar coupling constant value of -0.12 mT extracted from spectrum simulation. The charge separated state has a lifetime of 10 mu s at 120 K. A mechanistic model is proposed for the generation and decay processes involving the observed intermediate radical ion pair. The Gaussian spectrum lineshape for positive and negative charges is interpreted in terms of localized electron hole pairs