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

摘要

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 π-dimer formation between porphyrin moieties as indicated by electrochemical, vis-NIR, and ESR measurements. The cyclic voltammogram of TPZn_3 exhibited stepwise one-etectron oxidation processes of three porphyrin moieties in the range from 0.58 to 0.73 V (vs SCE in CH_2Cl_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 π-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 π-π 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 ~1H NMR spectra in nonpolar solvents. The association constant of TPZn_3 with PyC_(60) (1.1 × 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 π-π interactions between the porphyrin and fullerene moieties, together with intramolecular π-bond formation between the porphyrin radical cation and the neutral porphyrin in TPZn_3~(·+).