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

Abstract

The photophysical properties of fullerene hybrid systems in which disymmetrically substituted linear oligophenyleneethynylene (OPE) substituents have been attached to C₆₀ 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 CH₂Cl₂ 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 → C₆₀ singlet energy transfer (k = 10⁹–10¹⁰ s⁻¹) followed by regular deactivation of the C₆₀ 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 (∼1.45 eV) which, in Si(iPr)₃ terminated analogues, are shifted substantially upward (∼1.90 eV) and become hardly accessible via direct excitation or sensitization of the C₆₀ 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.