EPR studies of electron transfer processes in composites of substituted sexithiophenes with fullerene derivatives

We have examined by EPR spectroscopy a series of films made by composites of hexamethyl and tetramethylsulfanyl substituted sexithiophenes (T-6) with C-60 and C-60 derivatives. We have found that Photoinduced Electron Transfer (PET) from donor to acceptor molecules occurs in mixed films of C-60 with hexamethyl-T-6 and in composites of methylsulfanyl substituted T-6 With fullerene derivatives. Persistent electron transfer in the ground state was also observed. Spin polarization of the emissive EPR signal attributed to T-6(+) cation in hexamethyl-T-6/C-60 composite is consistent with the formation of the radicals from a non polarized triplet exciplex which gives rise to a loose radical pair after the PET process. In the mixed film of tetramethylsulfanyl-T-6 and C-60 polimerized with methyl methacrylate, time resolved EPR spectra reveal that a tightly bound radical pair is formed in which the partners are coupled by dipolar and exchange interactions. Such pair relaxes into a loose pair at later times.