Long-Lived Photoinduced Charge Separation in Inclusion Complexes Composed of a Phenothiazine-Bridged Cyclic Porphyrin Dimer and Fullerenes

Abstract

C-60, [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM), lithium-cation-encapsulated C-60 (Li+@C-60), and [6,6]-diphenyl-C-62-bis(butyric acid methyl ester) (bis-PCBM) were included into a phenothiazine-bridged cyclic free-base porphyrin dimer (H-4-Ptz-CPDPy(TEO)) in a polar solvent (benzonitrile) with large association constants of 1.3 x 10 (6), 6.4 x 10 (5), 3.2 x 10 (6), and 2.5 x 10 (5) M-1, respectively. Based on the electrochemical data, the lowest energy levels of the charge-separated (CS) states for the inclusion complexes of H-4-Ptz-CPDPy(TEO) with C-60, PCBM, Li+@C-60, and bis-PCBM (designated as C-60 subset of H-4-Ptz-CPDPy(TEO), PCBM subset of H-4-Ptz-CPDPy(TEO), Li+@C-60 subset of H-4-Ptz-CPDPy(TEO), and bis-PCBM subset of H-4-Ptz-CPDPy(TEO)) composed of the phenothiazine donor and fullerene acceptors were determined to be 1.30, 1.40, 0.66, and 1.51 eV, respectively. Both C-60 subset of H-4-Ptz-CPDPy(TEO) and PCBM subset of H-4-Ptz-CPDPy(TEO) underwent electron transfer upon photoexcitation of the porphyrin and fullerene chromophores, and the resultant photoinduced CS states comprised the phenothiazine cation and the fullerene anions with lifetimes of 0.71 ms determined by time-resolved transient absorption spectra. Li+@C-60 subset of H-4-Ptz-CPDPy(TEO) also afforded a similar CS state with a lifetime of 0.56 ms. These lifetimes are the longest values ever reported for the CS states of phenothiazinefullerene complexes in solution. The spin states of these long-lived CS states were assigned to be triplet by ESR spectroscopy. The remarkably long CS lifetimes are attributable mainly to the lower CS energies than the triplet energies of the phenothiazine, fullerenes, and porphyrin moieties and the spin-forbidden slow back-electron-transfer processes. On the other hand, the photoinduced CS state of bis-PCBM subset of H-4-Ptz-CPDPy(TEO) was quenched rapidly by fast back electron transfer due to the relatively high CS energy comparable to the triplet energies of the porphyrin and fullerene.