Photoinduced energy and electron transfer in fullerene-oligophenyleneethynylene systems: dependence on the substituents of the oligomer unit.

The photophysical properties of fullerene hybrid systems in which disymmetrically substituted linear oligophenyleneethynylene (OPE) substituents have been attached to C(60) through a pyrrolidine ring are discussed. These hybrid systems differ in both the length of the conjugated OPE backbone and in the type of terminating groups employed, i.e. tri-isopropylsilane (-Si(iPr)(3)) and N,N-di-n-butylaniline (PhN(nBu)(2)). The terminating group is found to be crucial in determining the fate of light absorbed by the hybrid. In CH(2)Cl(2) and benzonitrile, the PhN(nBu)(2) terminated hybrids undergo electron transfer with charge separation lasting as long as 390 ns in the more polar medium, as detected via near-infrared transient absorption spectroscopy. Under the same conditions the Si(iPr)(3) terminated hybrids show ultrafast OPE --> C(60) singlet energy transfer (k = 10(9)-10(10) s(-1)) followed by regular deactivation of the C(60) moiety, as determined via UV-VIR-NIR steady state and time-resolved spectroscopy. Only in polar benzonitrile such systems can undergo electron transfer to some extent (40% yield). The results here presented can be readily explained in light of the electrochemical properties of the hybrids. The low oxidation potentials of the PhN(nBu)(2) terminated systems allow the formation of low lying charge separated states ( approximately 1.45 eV) which, in Si(iPr)(3) terminated analogues, are shifted substantially upward ( approximately 1.90 eV) and become hardly accessible via direct excitation or sensitization of the C(60) singlet level (1.72 eV). These results, when examined in light of the performance of photovoltaic devices using these hybrids as active materials, show a nice structure-activity relationship supporting the appeal of the so-called molecular approach to photovoltaic devices.