Efficient Photoinduced Electron Transfer in a Porphyrin Tripod-Fullerene Supramolecular Complex via π-π Interactions in Nonpolar Media

A novel porphyrin tripod (TPZn3) was synthesized via click chemistry. Three porphyrin moieties of TPZn3 are geometrically close and linked by a flexible linker. The electron-transfer oxidation of TPZn3 results in intramolecular pi-dimer formation between porphyrin moieties as indicated by electrochemical, vis-NIR, and ESR measurements. The cyclic voltammogram of TPZn3 exhibited stepwise one-electron oxidation processes of three porphyrin moieties in the range from 0.58 to 0.73 V (vs SCE in CH2Cl2)When TPZn3 was oxidized by tris(2,2'-bipyridyl)-ruthenium(III) ([Ru(bpy)(3)](3+)), the oxidized species (TPZn3)(n+) (0 < n <= 3) exhibited a charge resonance band in the NIR region due to the pi-dimer formation between porphyrin moieties. A supramolecular electron donor acceptor system was also constructed using TPZn3. The flexible conformation of TPZn3 makes it possible to capture a fullerene derivative containing a pyridine moiety (PyC60) inside the cavity by pi-pi interactions as well as the coordination bond between Zn2+ and the pyridine moiety. The formation of a 1:1 supramolecular complex of TPZn3 with PyC60 (TPZn3 PyC60) was indicated in the UV vis and H-1 NMR spectra in nonpolar solvents. The association constant of TPZn3 with PyC60 (1.1 x 10(5) M-1 in toluene) is much larger as compared with those of the corresponding monomer (MPZn) and dimer porphyrin (DPZn2). The dynamics of photoinduced electron transfer from the singlet excited state of TPZn3 to PyC60 was examined by laser flash photolysis measurements. The efficient intracomplex photoinduced electron transfer in TPZn3 PyC60 occurred in nonpolar solvents, resulting from the pi-pi a interactions between the porphyrin and fullerene moieties, together with intramolecular pi-bond formation between the porphyrin radical cation and the neutral porphyrin in TPZn3 center dot+.