Rigidified tetrathiafulvalene–[60]fullerene assemblies: towards the control of through-space orientation between both electroactive units

Our recent works on fused TTF–C60 dyads, (TTF)n–C60 polyads and C60–TTF–C60 dumbbell triads in which the acceptor C60 is doubly tethered to the donor tetrathiafulvalene through a rigidified cyclohexene ring are presented. This approach was developed in order to control the relative orientation as well as the distance between both donor and acceptor entities. Thereby, through-space interactions which are of great importance for photoinduced electron- and/or energy-transfer processes are expected to dominate because of the special topology of the molecules. The two linked C60 and TTF chromophores in such adducts are not only in close proximity but also have optimal orbital orientations, thus facilitating these through-space electronic interactions. These new C60-based assemblies were synthesized by [4 + 2] Diels–Alder cycloaddition reactions. The different methodologies considered for their synthesis are discussed, their analytical, spectroscopic characterizations and electrochemical properties are also described. The selective electro-oxidation or reduction afforded the corresponding radical cation and radical anion which were characterized by EPR. These C60-based assemblies were studied for their nonlinear optical and optical limiting applications. Moreover, intramolecular photoinduced charge-separation and charge-recombination processes in a fused C60–TTF–C60 dumbbell triad which was designed to be soluble in organic solvents were investigated by time-resolved absorption and fluorescence techniques. Appreciable interaction between the C60 moiety and TTF moiety in the ground state was suggested by steady-state absorption spectra and the fluorescence spectra showed considerable interaction in the singlet excited state. The nanosecond transient absorption spectra displayed the formation of the charge-separated radical pair C60–TTF˙+–C60˙−, characterized by a lifetime of ca. 20 ns in benzonitrile.