Supramolecular Donor-Acceptor Hybrid of Electropolymerized Zinc Porphyrin with Axially Coordinated Fullerene: Formation, Characterization, and Photoelectrochemical Properties

Photoelectrochemical behavior of self-assembled C-60 via axial coordination to an electrochemically polymerized zinc porphyrin film was systematically investigated to unravel the importance of the coordinated fullerene in improving the photocurrent and photovoltage generation of the resulting donor-acceptor dyad. For this, tetrakis(4-(N,N-diphenylamino)-phenyl)porphyrinato-zinc(II), (Ph2N)(4)ZnP, bearing electropolymerizable triphenylamine peripheral substituents was first electropolymerized to form a film on the electrode surface. The resulting formation of the electrochemically active and dense film was confirmed by using an electrochemical quartz crystal microbalance (EQCM) and by atomic force microscopy imaging. The optical absorption and emission studies revealed the characteristic absorption and emission bands of zinc porphyrin that suggested preservation of the pi-electron system of the porphyrin monomer in the polymer. Further, the fullerene, derivatized with an axially coordinating imidazole ligand, was allowed to self-assemble via axial coordination to the zinc center of the (Ph2N)(4)ZnP polymer film. The simultaneously performed piezoelectric microgravimetry and cyclic voltammetry studies using EQCM allowed us to prove this coordination and to evaluate the redox potential of the donor, (Ph2N)(4)ZnP, and acceptor, C-60, moiety. The fluorescence emission results, along with the free energy calculations, suggested the occurrence of vectorial photoinduced electron transfer from the singlet-excited ZnP in the polymer to the axially coordinated fullerene moiety. Systematic photoelectrochemical studies revealed cathodic photocurrent generation, a result unlike most of the dye-sensitized photoelectrochemical cells reported in the literature. Moreover, the coordinated fullerene to the zinc porphyrin film improved the photocurrent and photovoltage generation of the photoelectrochemical cell. An incident photon-to-current conversion efficiency (IPCE) of nearly 2% at the Soret region of maximum absorption was determined for the [(Ph2N)(4)ZnP polymer]-fullerene hybrid film.