Long-Lived Charge Separation in a Dyad of Closely-Linked Subphthalocyanine-Zinc Porphyrin Bearing Multiple Triphenylamines

Photoinduced intramolecular events of the newly synthesized multimodular system composed of three triphenylamine (TPA) entities covalently substituted at the meso positions of the zinc porphyrin ring (ZnP), which is linked with the dodecafluorosubphthalocyanine SubPc(F)(12) at its axial position with the B-O bond have been examined. Appreciable electronic interactions between the meso-substituted TPA entities and the ZnP pi-system were observed, and as a consequence, ZnP(TPA)(3) acts as an electron-donor, whereas the SubPc(F)(12) moiety acts as an excellent electron-acceptor unit in the multimodular conjugate, ZnP(TPA)(3)-SubPc(F)(12). The computational studies performed by the DFT-B3LYP at the 6-31G level revealed delocalization of the highest occupied molecular orbital (HOMO) over the (TPA)(3) entities and the porphyrin macrocycle, while the lowest unoccupied molecular orbital (LUMO) is localized on Subpc(F)(12). Free-energy calculations suggested that the light-induced processes from the excited states of ZnP(TPA)(3) are exothermic in both polar benzonitrile and nonpolar toluene and benzene. The occurrence of fast and efficient charge-separation processes (similar to 10(12) s(-1)) via the singlet excited state of ZnP(TPA)(3) was confirmed by the femtosecond transient absorption spectral measurements in polar and nonpolar solvents. The delocalization of the pi-cation radical species over the donor ZnP(TPA)(3), the lower energy of the radical-ion pair, the particular characteristics of the axial B-O bond, and the triplet radical-ion pair character rationalize the charge stabilization of (ZnP(TPA)(3))(center dot+)-(SubPc(F)(12))(center dot-) with extremely long lifetime (370 mu s) compared with the reported phthalocyanine-based compounds.