Synthesis and photoelectrochemical properties of a fullerene-azothiophene dyad

In this paper we describe the synthesis, electrochemistry and photophysical behavior of a fullerene-based donor-acceptor dyad (2) in which the donor unit is an azothiophene dye. Dyad 2, prepared in one step starting from C-60, commercially available N-methylglycine and thienylazobenzeneamine 1, can be selectively excited in the visible region where the dye has an absorption maximum at 567 nm. Implementation of electrochemical and photophysical data reveals that both intramolecular energy- and electron-transfer are thermodynamically feasible processes. Steady-state luminescence of dyad 2 in CH2Cl2 shows a quenching of the dye singlet excited state ((1)dye*) and evidence of the fullerene singlet excited state (C-1(60)*) emission. Flash-photolytic experiments, on the other hand, exhibit characteristic differential absorption changes attributed to the C-60(.-)-dye(.+) charge-separated state. Interestingly, it has been estimated that the energy difference between C-1(60)* and the charge separated state is very small, leading to the hypothesis that a rapid exchange between the two states occurs. Sensitization of TiO2 with dyad 2 and model compound 3 is also reported and discussed.