Porphyrin-beta-Oligo-Ethynylenephenylene-[60]Fullerene Triads: Synthesis and Electrochemical and Photophysical Characterization of the New Porphyrin-Oligo-PPE-[60]Fullerene Systems

The synthesis and electrochemical and photophysical studies of new electron donor-acceptor arrays, bearing porphyrins covalently linked to fullerene, are described. In the reported investigation, phenyleneethynylene subunits were chosen as a linking bridge to guarantee a high conjugation degree between the donor (i.e., porphyrin), the molecular bridge (i.e., oligo-phenyleneethynylenes), and the acceptor (i.e., fullerene). To enhance the electronic interactions through the extended pi-system, the molecular bridge has been directly linked to the beta-pyrrole position of the porphyrin ring, generating a new example of donor-bridge-acceptor systems where, for the first time, the meso-phenyl ring of the macrocycle is not used to hold the "bridge" between porphyrin and fullerene moieties. This modification allows altering the chemical and physical properties of the tetrapyrrole ring. Steady-state and time-resolved fluorescence studies together with transient absorption measurements reveal that in nonpolar media (i.e., toluene) transduction of singlet excited-state energy governs the excited-state deactivation, whereas in polar media (i.e., tetrahydrofuran) charge transfer prevails generating a long-lived radical ion pair state. The lifetimes hereof range from 300 to 700 ns. The study also sheds light onto the wirelike behavior of the oligo-phenyleneethynylene bridges, for which a damping factor (beta) of 0.11 +/- 0.05 angstrom(-1) has been determined in the current study.