Synthesis, photophysics, and photoresponse of fullerene-based azoaromatic dyads

The synthesis and photophysical characterization of a series of fullerene-based, donor-acceptor dyads is presented, along with a description of their behavior as single molecular components in photovoltaic cells. The spectroscopic and photophysical properties of the dyads, investigated by steady-state fluorescence spectroscopy, pico- and nanosecond transient optical spectroscopy and time-resolved electron paramagnetic resonance (EPR) spectroscopy, revealed that the dyads undergo multiple-step energy transfer from the donor singlet excited state to the fullerene triplet excited state, which in turn decays to the donor triplet state. The inefficient formation of a charge-separated state, both in solution and in the solid state, translates into a poor photovoltaic performance of dyads 2 b-4 b if compared to that of dyad 1 b, in which photoinduced electron transfer is operative in the solid state. In addition, the results of the photophysical investigation suggested that the performance of the solar cells was also limited by the low-lying donor triplet excited state that acts as a photoexcitation energy sink.