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Spectroscopic, redox, and photochemical behavior of a self-assembled donor-acceptor triad formed by axial coordination of zinc meso-tetraphenylporphyrin, Zn(TPP) with fulleropyrrolidine bearing two pyridine moieties (C₆₀(Py)₂) was investigated. The UV-visible and ¹H NMR spectral studies revealed supramolecular 1:2 triad formation between C₆₀(Py)₂ and Zn(TPP). The determined overall formation constant, K was as large as 1.45 × 10⁴mol^-2 in o-dichlorobenzene (o-DCB). The ¹H NMR studies revealed axial coordination of both the pyridine entities of C₆₀(Py)₂ to the central metal ion of the zinc porphyrins. Electrochemical studies of the isolated self-assembled triad in o-DCB established the molecular stoichiometry of the supramolecular complex possessing one C₆₀(Py)₂ and two Zn(TPP) entities. The excited state electron transfer reactions were monitored by both steady-state emission as well as transient absorption techniques. In o-DCB, upon coordination of the pyridine entities of C₆₀(Py)₂ to two Zn(TPP), the main quenching pathway involved charge-separation from the singlet excited Zn(TPP) to the C₆₀ moiety. However, in a coordinating solvent like PhCN, intermolecular electron-transfer takes place from the triplet excited state of Zn(TPP) to the C₆₀ moiety.

Self-exchange electron transfer rates between π-radical cations of zinc porphyrins and the neutral metalloporphyrins have been determined from the line-width broadening in the ESR spectra in different solvents at various temperatures. Fine tuning of the substituent on the porphyrin ring and the proper choice of the solvent have enabled us to observe negative activation enthalpies for the self-exchange electron transfer reactions. The observation of negative activation enthalpies indicates that the self-exchange electron transfer occurs via the charge-transfer π-complexes formed between zinc porphyrin radical cations and the neutral zinc porphyrins. The complete delocalization of the unpaired electron over two porphyrin moieties is observed in the radical cation of a zinc porphyrin dimer, 5,5'-bis(10,20-bis(3,5-di-tert-butylphenyl)porphyrinatozinc(II)). This is regarded as the extreme limit of the rapid self-exchange electron transfer between zinc porphyrin radical cation and the neutral form.

The nitrogen of pyridylnaphthalenediimide (PyNIm) coordinates to the metal center of zinc tetraphenylporphyrin (ZnTPP) to form a donor-acceptor complex: ZnTPP-PyNIm. Formation of the ZnTPP-PyNIm complex was probed by UV-vis, fluorescence and NMR spectra. The fluorescence of ZnTPP is strongly quenched and the fluorescence lifetime is shortened significantly in the complex. The transient absorption spectrum of the charge-separated state (ZnTPP^•+-PyNIm^•-) is successfully detected by laser flash photolysis measurements of the ZnTPP-PyNIm system in benzonitrile. The charge-separated state of the complex produced by the photoinduced electron transfer has the longest lifetime, (450 μs) at 288 K, ever reported for donor-acceptor systems linked covalently or non-covalently in solution. However, when benzonitrile is replaced by benzene, the triplet excited state (³ZnTPP*), rather than the charge-separated state, is formed upon laser excitation of the ZnTPP-PyNIm complex, due to the lower energy of (³ZnTPP*) compared to the charge-separated state in benzene.

Zinc porphyrin-fullerene-zinc porphyrin triad, in which two zinc porphyrin (ZnP) moieties and a fullerene (C₆₀) moiety are linked by flexible bonds and which is intended to be a working model of the photosynthetic antenna-reaction centre, has been newly synthesized. Its photophysical properties have been investigated by both time-resolved emission and transient absorption techniques. Excitation of the zinc porphyrin moiety of the triad induced charge separation, generating the radical ion pair, in which the electron localizes on the C₆₀ moiety and the hole localizes on the zinc porphyrin moiety. In polar solvents, the charge-separated states decayed with lifetimes of 300-600 ns returning to the ground state. Compared with ZnP-C₆₀ dyad, ZnP-C₆₀-ZnP triad showed longer lifetimes for the radical ion pair due to the conformation of the two ZnP moieties. The effects of the coordinating reagents on the zinc atom have been studied, with the expectation of conformational change of the two ZnP moieties with respect to C₆₀.

Intramolecular electron-transfer process of porphyrin-fullerene dyad linked by phenyl buta-1,3-diynyl-phenyl unit (ZnP-sp-C₆₀) was studied by laser flash photolysis. Picosecond fluorescence lifetime and transient absorption measurements revealed that photoinduced charge-separation takes place via the excited singlet state (¹ZnP*) with the rate constant of (1-2) × 10¹⁰s⁻¹. For the charge recombination, about a half of the radical-ion pair decayed quickly with 2.9 × 10⁹s⁻¹ as evaluated from picosecond transient absorption measurements, whereas the remaining half was long-lived with slow decay (1.6 × 10⁶s⁻¹) as estimated from nanosecond transient absorption measurements. The lifetime of the radical-ion pair of ZnP-sp-C₆₀ was longer than those of directly connected dyads with a buta-1,3-diynyl bridge and buta-1,3-diynyl-phenyl bridge by the insertion of an extra phenyl group in addition to the pyrrodino ring.

Magnesium tetrapyrrole, the natural choice of metal tetrapyrrole in photosynthesis, as a photosensitizer in dye sensitized solar cell applications is performed by constructing solar cells using metal-ligand axial coordination approach on TiO₂ surface modified with 4-carboxyphenyl imidazole and compared with cells constructed using traditionally used zinc porphyrin as sensitizer. Studies involving optical absorption, steady-state and time-resolved fluorescence, and differential pulse voltammetry, suggested that the employed magnesium tetraphenylporphyrin (MgTPP) to be a better photosentizer compared to zinc tetraphenylporphyrin (ZnTPP) for dye sensitized solar cell applications under the employed self-assembly conditions. Consequently, the constructed solar cells using MgTPP outperformed the cells constructed using ZnTPP in all aspects. That is, the open circuit potential, short circuit current, fill-factor, incident photon-to-current conversion efficiency, and overall efficiency of the solar cell were found to be better for the cells built using MgTPP photosensitizer. In order to further improve the performance of the solar cells, efforts were made to increase the fill-facotor by adding polar acetonitrile to the mediator solvent media made out of dichlorobenzene. Moderate additions of acetonitrile improved the performance of the solar cells. However, the performance of DSSC constructed using pure acetonitrile was poor due to acetonitrile competatively binding to MgTPP instead of imidazolde on the TiO₂ surface. Electrochemical impedance spectroscopy studies suggested that the significant decrease in the resistance at platinum electrolyte interface facilitating better iodide/iodine mediation and dye regeneration are main contributing factors for this improved cell performance.

We report herein the synthesis and characterization of novel cobalt chlathrochelate-zinc porphyrin assemblies. X-ray data and electrochemical studies support the formation and maintenance of these structures both in solid and solution states. Light induced charge accumulation at the cobalt center was realized in the presence of a sacrificial electron donor. The photogenerated Co(I) species was stable in the ms time scale in aqueous medium.

Mechanistic aspects of photoinduced charge separation in supramolecular triads, constructed using covalently linked zinc porphyrin-ferrocene(s) dyads — self-assembled via axial coordination to either pyridine or phenylimidazole appended fulleropyrrolidine (Fc_x-ZnP:PyC₆₀ or Fc_x-ZnP:ImC₆₀; x = 1 or 2), has been investigated using femtosecond pump-probe transient spectroscopy. Upon photoexcitation of ZnP, charge separation from ferrocene to ¹ZnP* to yield the initial Fc⁺-ZnP^•-:C₆₀ radical ion-pair or charge separation from ¹ZnP* to C₆₀ to yield the initial Fc-ZnP^•+:C₆₀^•- radical ion-pair, depending upon the ferrocene-zinc porphyrin intermolecular distance, was observed. These radical ion-pairs resulted in the formation of ultimate distantly separated Fc⁺-ZnP:C₆₀^•- radical ion-pairs either via an electron migration (former case) or hole shift (latter case) process. Kinetics of charge separation as a function of spacer connecting the ferrocene and porphyrin, and spacer between the porphyrin and fullerene is reported. In agreement with our earlier study (J. Phys. Chem. B 2004; 108: 11333–11343), the Fc⁺-ZnP:C₆₀^•- radical ion-pair persisted beyond the monitoring time window of our instrument, suggesting charge stabilization in these supramolecular triads.

Two zinc porphyrins substituted at the meso-positions with different donor units (denoted as ZnP1 and ZnP2) have been designed and synthesized to construct new self-assemblies (denoted as ZnPx-ZnPA) with anchor porphyrin (ZnPA) dyads formed by the coordination bonds of Zn-to-ligand approach. Then these assemblies were absorbed on the semiconducting TiO₂ electrode surfaces by the carboxylic groups of anchor porphyrin (ZnPA) to build dye sensitized solar cells. Their spectral properties, electrochemical, theoretical calculations and photovoltaic properties were systematically investigated. Interestingly, these self-assemblies devices compared to monomer anchoring porphyrin device showed higher $J_{sc}$ and $\eta$. Especially, the dyad of ester-based zinc porphyrin (ZnP2) assembly device has the highest photoelectric conversion efficiency. The assembled modes of the assemblies immobilized on TiO₂ electrode surfaces were also verified by transmission electron microscopy (TEM).

Tetrapyrrole building blocks are invaluable constituents in the construction of molecular architectures for use in biomimicry, functional materials, and biomedicine. The reaction of dipyrromethane and the triisopropylsilyl-protected 3,5-diethynylbenzaldehyde afforded the corresponding trans-${\mathrm{\text{A}}}_2$-porphyrin (free base) bearing four ethynes. Subsequent meso-bromination, Suzuki coupling, and protecting group removal afforded a porphyrin building block bearing four ethynes and one benzylamine. The reaction of dipyrromethane and 3,5-bis(propargyloxy)benzaldehyde afforded the corresponding trans-${\mathrm{\text{A}}}_2$-porphyrin (free base) bearing four ethynes. The reaction of 5-(3,5-bis(propargyloxy)phenyl)dipyrromethane and the Eschenmoser (1,9-dimethylaminomethyl) derivative of a 5-($p$-substituted aryl)dipyrromethane was used to create two trans-AB-porphyrins (zinc chelates). The $p$-substituent of the aryl group was cyano or an acetal moiety. Hydrolysis of the acetal and a click reaction with m-PEG24-azide gave the bis(PEGylated)porphyrin-carboxaldehyde. The porphyrins present readily derivatizable functional groups in a compact architecture.

As a non-invasive cancer therapy method, photodynamic therapy (PDT) shows tremendous promise in clinical cancer treatment. Light-activated singlet oxygen production of photosensitizers (PSs) is the prerequisite for cancer PDT, and the use of organic photosensitizers is always limited by visible light-based activation, hydrophilicity, biocompatibility, selectivity and quantum yield of singlet oxygen. Currently, both zinc porphyrin- and BODIPY-based structures have been widely used in the development of PDT PSs. Here, we developed a novel naphthalimide bridged zinc porphyrin/BODIPY molecule (Por-BDP-1) with two poly(ethylene glycol) (PEG) chains, in which D-A structure was constructed between the naphthalimide group and porphyrin group. After self-assembly into nanoparticles, Por-BDP-1 NPs (Diameter: 122.4 nm) could quench fluorescence in 600–700 nm, bind with calf thymus-DNA, and produce singlet oxygen during light-irradiation (laser: 680 nm, 1.0 W/cm${}^2)$. In addition, Por-BDP-1 NPs effectively killed HeLa cells with a IC${}_{50}$ value = 44.8 μg/mL and showed a lower dark toxicity under the same conditions. All our results demonstrated that our naphthalimide bridged zinc porphyrin/BODIPY nano-photosensitizer is a promising nanoagent for PDT in the clinic.