Synthesis and photophysical properties of two dual oligothiophene-fullerene linkage molecules as photoinduced long-distance charge separation systems

To further extend photoinduced charge separation previously observed for oligothiophene-fullerene dyads (nT-C₆₀), we have studied two novel dual oligothiophene-fullerene triads, 8T-4T-C₆₀ and 4T-ST-C ₆₀, where quaterthiophene (4T) and octithiophene (8T) are linked by a trimethylene chain and either one is attached to a fullerene (C₆₀). The cyclic voltammograms and electronic absorption spectra of these triad compounds indicated no electronic interactions among the three components. On the other hand, the emission spectra were markedly perturbed by electron transfer and/or energy transfer from the oligothiophene to fullerene. Detailed comparisons between the emission spectra of the triads (8T-4T-C₆₀ and 4T-8T-C₆₀) and the dyads (4T-C₆₀ and 8T-C₆₀) suggest that the additionally attached octithiophene or quaterthiophene in the triads is involved in the photophysical decay mechanism, and the 8T-4T-C ₆₀ triad undergoes photoinduced electron transfer leading to long-distance charge separation. This was actually corroborated by observation of the specific bands due to 8T^.+-4T-C60^.- species in the transient absorption spectra after photoexcitation of the octithiophene. The sandwich device based on the 8T-4T-C₆₀ triad produced a more effective photovoltaic response to visible light owing to the contribution of the additional octithiophene chromophore compared to that using the dyad 4T-C₆₀. On the other hand, the 4T-8T-C₆₀-based device demonstrated a rather poorer photovoltaic performance when compared to the ST-C₆₀ device.