Spin correlated radical ion pairs generated by photoinduced electron transfer in composites of sexithiophene/fullerene derivatives: a transient EPR study

Photoinduced electron transfer was observed in a series of methylsulfanyl sexithiophene/fulleropyrrolidine composites deposited as films. Paramagnetic states were observed by transient EPR spectroscopy in the microseconds time domain. The spectra displayed polarized lines with characteristic antiphase emission/absorption pattern of spin polarization and were assigned to spin correlated radical ion pairs (SCRP) formed by intermolecular electron transfer from sexithiophene donor to C-60 fullerene acceptor molecules. Also transient signals detected at selected magnetic fields showed phase effects that are typical of SCRPs, Spectrum simulation was obtained by allowing for a distribution of respective orientations of the dipolar axes and g-tensors of partners in a pair. Fitting parameters used for one composite were D/g beta = -135 muT and J/g beta = 2.5 muT for dipolar and spin exchange coupling constants, corresponding to an interradical mean distance of 27.4 Angstrom. Similar values were obtained for the other examined systems. Pair structure and dynamics suggest to ascribe the charge-separated state to a radical pair generated after a hopping of the electron-hole charges from a primary pair originated in neighbor molecular sites.