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

Abstract

A novel porphyrin tripod (TPZn(3)) was synthesized via "click chemistry". Three porphyrin moieties of TPZn(3) are geometrically close and linked by a flexible linker. The electron-transfer oxidation of TPZn(3) results in intramolecular pi-dimer formation between porphyrin moieties as indicated by electrochemical, vis-NIR, and ESR measurements. The cyclic voltammogram of TPZn(3) exhibited stepwise one-electron oxidation processes of three porphyrin moieties in the range from 0.58 to 0.73 V (vs SCE in CH(2)Cl(2)). When TPZn(3) was oxidized by tris(2,2'-bipyridyl)-ruthenium(III) ([Ru(bpy)(3)](3+)), the oxidized species (TPZn(3))(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 TPZn(3). The flexible conformation of TPZn(3) makes it possible to capture a fullerene derivative containing a pyridine moiety (PyC(60)) inside the cavity by pi-pi interactions as well as the coordination bond between Zn(2+) and the pyridine moiety. The formation of a 1:1 supramolecular complex of TPZn(3) with PyC(60) (TPZn(3)-PyC(60)) was indicated in the UV-vis and (1)H NMR spectra in nonpolar solvents. The association constant of TPZn(3) with PyC(60) (1.1 x 10(5) M(-1) in toluene) is much larger as compared with those of the corresponding monomer (MPZn) and dimer porphyrin (DPZn(2)). The dynamics of photoinduced electron transfer from the singlet excited state of TPZn(3) to PyC(60) was examined by laser flash photolysis measurements. The efficient intracomplex photoinduced electron transfer in TPZn(3)-PyC(60) occurred in nonpolar solvents, resulting from the pi-pi interactions between the porphyrin and fullerene moieties, together with intramolecular pi-bond formation between the porphyrin radical cation and the neutral porphyrin in TPZn(3)(*+).