A new concept of charge stabilization via delocalization of the pi-cation radical species over the donor macrocycle substituents in a relatively simple donor-acceptor bearing multimodular conjugates is reported. The newly synthesized multimodular systems were composed of three covalently linked triphenylamine entities at the meso position of the porphyrin ring and one fulleropyrrolidine at the fourth meso position. The triphenylamine entities were expected to act as energy transferring antenna units and to enhance the electron donating ability of both free-base and zinc(II) porphyrin derivatives of these pentads. Appreciable electronic interactions between the meso-substituted triphenylamine entities and the porphyrin pi-system were observed, and as a consequence, these moieties acted together as an electron-donor while the fullerene moiety acted as an electron-acceptor in the multimodular conjugates. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier highest occupied molecular orbital (HOMO) over the triphenylamine entities in addition to the porphyrin macrocycle. Free-energy calculations suggested that the light-induced processes from the singlet excited state of porphyrins are exothermic in the investigated multimodular conjugates. The occurrence of photoinduced charge-separation and charge-recombination processes was confirmed by the combination of time-resolved fluorescence and nanosecond transient absorption spectral measurements. Charge-separated states, on the order of a few microseconds, were observed as a result of the delocalization of the pi-cation radical species over the porphyrin macrocycle and the meso-substituted triphenylamine entities. The present study successfully demonstrates a novel approach of charge-stabilization in donor-acceptor multimodular conjugates.

An elegant method of self-assembly for modification of a TiO(2) surface using coordinating ligands followed by immobilization of variety of sensitizers and a dyad is reported. This highly versatile method, in addition to testing the photoelectrochemical behavior of different zinc tetrapyrroles, allowed the use of fairly complex structures involving more than one donor entity. Utilization of the zinc porphyrin-ferrocene dyad markedly improved the current-voltage performance of the photoelectrochemical cell through an electron transfer-hole migration mechanism. Incident photon-to-current efficiency values up to 37% were obtained for the electrode modified with the dyad, signifying the importance of photocells built on the basis of biomimetic principles for efficient harvesting of solar energy.

New molecular triads composed of closely spaced ferrocene-boron dipyrrin-fullerene, 1 and triphenylamine-boron dipyrrin-fullerene, 2 are synthesized, and photoinduced electron transfer leading to charge stabilization is demonstrated using a femtosecond transient spectroscopic technique.

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), (Ph 2 N) 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 π-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 (Ph 2 N) 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, (Ph 2 N) 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 [(Ph 2 N) 4 ZnP polymer]-fullerene hybrid film.