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 s exithiophene/fulleropyrrolidine composites deposited as films. Paramagnetic states were observed by transient EPR spectroscopy in the microseconds tim e domain. The spectra displayed polarized lines with characteristic antipha se emission/absorption pattern of spin polarization and were assigned to sp in correlated radical ion pairs (SCRP) formed by intermolecular electron tr ansfer 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 fo r a distribution of respective orientations of the dipolar axes and g-tenso rs 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 coup ling constants, corresponding to an interradical mean distance of 27.4 Angs trom. Similar values were obtained for the other examined systems. Pair str ucture and dynamics suggest to ascribe the charge-separated state to a radi cal pair generated after a hopping of the electron-hole charges from a prim ary pair originated in neighbor molecular sites. (C) 2001 Elsevier Science B.V. All rights reserved.