Subphthalocyanines: tuneable molecular scaffolds for intramolecular electron and energy transfer processes

A series of four subphthalocyanine-C(60) fullerene dyads have been prepared through axial functionalization of the macrocycle with m-hydroxybenzaldehyde and a subsequent dipolar cycloaddition reaction. The subphthalocyanine moiety has been peripherally functionalized with substituents of different electronic character, namely fluorine or iodine atoms and ether or amino groups, thus reaching a control over its electron-donating properties. This is evidenced in cyclic voltammetry experiments by a progressive shift to lower potentials, by ca. 200 mV, of the first oxidation event of the SubPc unit in the dyads. As a consequence, the energy level of the SubPc(*)(+)-C(60)(*)(-) charge-transfer state may be tuned so as to compete with energy transfer deactivation pathways upon selective excitation of the SubPc component. For instance, excitation of those systems where the level of the radical pair lies high in energy triggers a sequence of exergonic photophysical events that comprise (i) nearly quantitative singlet-singlet energy transfer to the C(60) moiety, (ii) fullerene intersystem crossing, and (iii) triplet-triplet energy transfer back to the SubPc. On the contrary, the stabilization of the SubPc(*)(+)-C(60)(*)(-) radical pair state by increasing the polarity of the medium or by lowering the donor-acceptor redox gap causes charge transfer to dominate. In the case of 1c in benzonitrile, the thus formed radical pair has a lifetime of 0.65 ns and decays via the energetically lower lying triplet excited state. Further stabilization is achieved for dyad 1d, whose charge-transfer state would lie now below both triplets. The radical pair lifetime consequently increases in more than 2 orders of magnitude with respect to 1c and presents a significant stabilization in less polar solvents, revealing a low reorganization energy for this kind of SubPc-C(60) systems.