A Charge‐Stabilizing,Multimodular, Ferrocene–Bis(triphenylamine)–Zinc‐porphyrin–Fullerene Polyad

A novel multimodular donor–acceptor polyad featuring zinc porphyrin, fullerene, ferrocene, and triphenylamine entities was designed, synthesized, and studied as a charge‐stabilizing, photosynthetic‐antenna/reaction‐center mimic. The ferrocene and fullerene entities, covalently linked to the porphyrin ring, were distantly separated to accomplish the charge‐separation/hole‐migration events leading to the creation of a long‐lived charge‐separated state. The geometry and electronic structures of the newly synthesized compound was deduced by B3LYP/3‐21G(*) optimization, while the energy levels for different photochemical events was established using data from the optical absorption and emission, and electrochemical studies. Excitation of the triphenylamine entities revealed singlet‐singlet energy transfer to the appended zinc porphyrin. As predicted from the energy levels, photoinduced electron transfer from both the singlet and triplet excited states of the zinc porphyrin to fullerene followed by subsequent hole migration involving ferrocene was witnessed from the transient absorption studies. The charge‐separated state persisted for about 8.5 μs and was governed by the distance between the final charge‐transfer product, that is, a species involving a ferrocenium cation and a fullerene radical anion, with additional influence from the charge‐stabilizing triphenylamine entities located on the zinc‐porphyrin macrocycle.     

Simple Synthesis of a Chlorin - Fullerene Dyad with a Novel Ring-Closure Reaction

To model the photosynthetic reaction center, the chlorin - fullerene dyad 1 was synthesized in a one-pot reaction from a linear tetrapyrrole and C(60). It is the first dyad of this type which contains a fullerene and a chlorin unit, the chromophore of naturally occurring photosynthetic systems.     

Functional in situ evaluation of photosynthesis-protecting carotenoids in mutants of the cyanobacterium Synechocystis PCC6803

Cyanobacteria possess different carotenoids as scavengers of reactive oxygen species. In Synechocystis PCC6803, zeaxanthin, echinenone, beta-carotene and myxoxanthophyll are synthesized. By disruption of the ketolase and hydroxylase genes, it was possible to obtain mutants devoid of either zeaxanthin, echinenone, or a combination of both carotenoids. With these mutants, their function in protecting photosynthetic electron transport under high light stress as well as chlorophyll and carotenoid degradation after initiation of singlet oxygen or radical formation was analyzed. Wild type Synechocystis is very well protected against high light-mediated photooxidation. Absence of echinenone affects photosynthetic electron transport to only a small extent. However, complete depletion of zeaxanthin together with a modification of myxoxanthophyll resulted in strong photoinhibition of overall photosynthetic electron transport as well as the photosystem II reaction. In the double mutant lacking both carotenoids the effects were additive. The light saturation curves of photosynthetic electron transport of the high light-treated mutants exhibited not only a lower saturation value but also smaller slopes. Using methylviologen or methylene blue as a radical or singlet oxygen generators, respectively, massive degradation of chlorophyll and carotenoids, indicative of photooxidative destruction of the photosynthetic apparatus, was observed, especially in the mutants devoid of zeaxanthin.     

The reaction of fullerene C(60) with 4,6-dimethyl-1,2,3-triazine: formation of an open-cage fullerene derivative.

A thermal reaction of fullerene C(60) with 4,6-dimethyl-1,2,3-triazine (4) in o-dichlorobenzene gave azacyclohexadiene-fused fullerene derivative 5, by the reaction with intermediate azete 11, and then, after flash chromatography over SiO(2), open-cage fullerene derivative 6 having an eight-membered ring orifice on the C(60) cage. Compound 6 is assumed to be formed via addition of diradical intermediate 13 to C(60). Compound 6 underwent a further photochemical reaction with singlet oxygen with the cleavage of one of the double bonds at the rim of the orifice to afford triketone derivative 8 having a 12-membered ring orifice.     

Photophysical Properties and Singlet Oxygen Generation Efficiencies of Water‐Soluble Fullerene Nanoparticles

As various fullerene derivatives have been developed, it is necessary to explore their photophysical properties for potential use in photoelectronics and medicine. Here, we address the photophysical properties of newly synthesized water‐soluble fullerene‐based nanoparticles and polyhydroxylated fullerene as a representative water‐soluble fullerene derivative. They show broad emission band arising from a wide‐range of excitation energies. It is attributed to the optical transitions from disorder‐induced states, which decay in the nanosecond time range. We determine the kinetic properties of the singlet oxygen (¹O₂) luminescence generated by the fullerene nanoparticles and polyhydroxylated fullerene to consider the potential as photodynamic agents. Triplet state decay of the nanoparticles was longer than ¹O₂ lifetime in water. Singlet oxygen quantum yield of a series of the fullerene nanoparticles is comparably higher ranging from 0.15 to 0.2 than that of polyhydroxylated fullerene, which is about 0.06.     

Enhanced Electron Transfer Ability via Coordination in Block Copolymer/Porphyrin/Fullerene Micelle

Inspired by structures of antenna-reaction centers in photosynthesis,the complex micelle was prepared from zinc tetra-phenyl porphyrin (ZnTPP),fullerene derivative (PyC60) and poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-bPCL).The core-shell structure made the hydrophobic donor-acceptor system work in aqueous.In micellar core,coordination interaction occurred between ZnTPP and PyC60 molecules which ensured the enhanced energy migration from the donor to the acceptor.The enhanced interaction between porphyrin and fullerene was confirmed by absorption,steady-state fluorescence and transient fluorescence.The generation of singlet oxygen and superoxide radical was detected by iodide method and reduction of nitro blue tetrazolium,respectively,which confirmed that electron transfer reaction in the complex micellar core occurred.Moreover,the complex micelle exhibited effective electron transfer performance in photodebromination of 2,3-dibromo-3-phenylpropionic acid.The complex micellar structure endowed the donor-acceptor system with improved stability under irradiation.This strategy could be helpful for designing new electron transfer platform and artificial photosynthetic system.     

Photosynthetic reaction center mimicry of a "special pair" dimer linked to electron acceptors by a supramolecular approach: self-assembled cofacial zinc porphyrin dimer complexed with fullerene(s)

Biomimetic bacterial photosynthetic reaction center complexes have been constructed using well-defined self-assembled supramolecular approaches. The "special pair" donor, a cofacial porphyrin dimer, was formed via potassium ion induced dimerization of meso-(benzo-[15]crown-5)porphyrinatozinc. The dimer was subsequently self-assembled with functionalized fullerenes via axial coordination and crown ether-alkyl ammonium cation complexation to form the donor-acceptor pairs, mimicking the noncovalently bound entities of the photosynthetic reaction center. The adopted self-assembly methodology yielded supramolecular complexes of higher stability, with defined geometry and orientation. Efficient forward electron transfer from the singlet excited zinc porphyrin dimer to the fullerene entity and relatively slow reverse electron transfer, important steps in the photosynthetic light energy conversion have been achieved in these novel biomimetic model systems.     

Synthese von Sulfolenobilinen und deren Cyclisierung zu Chlorinatozink‐Fulleren‐Dyaden

Synthesis of Sulfolenobilins and Their Cyclization Directed to Chlorinatozinc‐Fullerene Dyads A novel chlorinatozinc‐fullerene dyad 18 was synthesized to model the photosynthetic reaction center. The synthetic key step for the formation of the dyad 18 is an unusual one‐pot reaction of the (sulfolenobilinato)nickel rac‐ 16a , b with concomitant generation of the chlorin macrocycle and linkage to the [5,6]fullerene‐C₆₀‐I_h. This one‐pot reaction is a complex cascade of single reaction steps with a total yield of 32% and an average yield of 83% for the individual steps. The chlorinatozinc‐fullerene dyad is so far one of three examples that contain chlorin moieties, the chromophores in naturally occurring photosynthetic systems.     

Magnetic field effect on singlet oxygen production in a biochemical system

The yield of singlet oxygen sensitized by chemically modified, carotenoidless bacterial photosynthetic reaction centres and the ensuing oxidative damage are both shown to be magnetic field-dependent.     

Intramolecular Charge Transfer of Carotene‐porphyrin‐fullerene Triad: Sequential or Superexchange Cechanism

As an excellent artificial photosynthetic reaction center, the carotene (C)‐porphyrin (P)‐fullerene (F) triad was extensively investigated experimentally. To reveal the mechanism of the intramolecular charge transfer (ICT) on the mimic of photosynthetic solar energy conversion (such as singlet energy transfer between pigments, and photoinduced electron transfer from excited singlet states to give long‐lived charge‐separated states), the ICT mechanisms of C‐P‐F triad on the exciton were theoretically studied with quantum chemical methods as well as the 2D and 3D real space analysis approaches. The results of quantum chemical methods reveal that the excited states are the ICT states, since the densities of HOMO are localized in the carotene or porphyrin unit, and the densities of LUMO are localized in the fullerene unit. Furthermore, the excited states should be the intramolecular superexchange charge transfer (ISCT) states for the orbital transition from the HOMO whose densities are localized in the carotene to the LUMO whose densities are localized in the fullerene unit. The 3D charge difference densities can clearly show that some excited states are ISCT excited states, since the electron and hole are resident in the fullerene and carotene units, respectively. From the results of the electron‐hole coherence of the 2D transition density matrix, not only 3D results are supported, but also the delocalization size on the exciton can be observed. These phenomena were further interpreted with non‐linear optical effect. The large changes of the linear and non‐linear polarizabilities on the exciton result in the charge separate states, and if their changes are large enough, the ICT mechanism can become the ISCT on the exciton.     

Conformationally constrained macrocyclic diporphyrin-fullerene artificial photosynthetic reaction center

Photosynthetic reaction centers convert excitation energy from absorbed sunlight into chemical potential energy in the form of a charge-separated state. The rates of the electron transfer reactions necessary to achieve long-lived, high-energy charge-separated states with high quantum yields are determined in part by precise control of the electronic coupling among the chromophores, donors, and acceptors and of the reaction energetics. Successful artificial photosynthetic reaction centers for solar energy conversion have similar requirements. Control of electronic coupling in particular necessitates chemical linkages between active component moieties that both mediate coupling and restrict conformational mobility so that only spatial arrangements that promote favorable coupling are populated. Toward this end, we report the synthesis, structure, and photochemical properties of an artificial reaction center containing two porphyrin electron donor moieties and a fullerene electron acceptor in a macrocyclic arrangement involving a ring of 42 atoms. The two porphyrins are closely spaced, in an arrangement reminiscent of that of the special pair in bacterial reaction centers. The molecule is produced by an unusual cyclization reaction that yields mainly a product with C(2) symmetry and trans-2 disubstitution at the fullerene. The macrocycle maintains a rigid, highly constrained structure that was determined by UV-vis spectroscopy, NMR, mass spectrometry, and molecular modeling at the semiempirical PM6 and DFT (B3LYP/6-31G**) levels. Transient absorption results for the macrocycle in 2-methyltetrahydrofuran reveal photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene to form a P(?+)-C(60)(?-)-P charge separated state with a time constant of 1.1 ps. Photoinduced electron transfer to the fullerene excited singlet state to form the same charge-separated state has a time constant of 15 ps. The charge-separated state is formed with a quantum yield of essentially unity and has a lifetime of 2.7 ns. The ultrafast charge separation coupled with charge recombination that is over 2000 times slower is consistent with a very rigid molecular structure having a small reorganization energy for electron transfer, relative to related porphyrin-fullerene molecules.     

Synthesis of [60]fullerene derivatives bearing five-membered heterocyclic wings and an investigation of their photophysical kinetic properties

The preparation, characterization and photophysical properties of six new stable [6,6]-closed fullerene cycloadducts bearing five-membered heterocycles are described. The modified [60]fullerenes are obtained by a simple and rapid synthesis via a Bingel-type reaction with tetrazole and oxadiazole malonate derivatives. The photophysical kinetics of these new fullerene derivatives in toluene solution under ultraviolet illumination (375 nm, UVA) are studied by electron paramagnetic resonance and free-radical spin-trapping using α-phenyl-N-tert-butyl nitrone as a spin-trap. The results are compared with pure [60]fullerene and [6,6]-phenyl C₆₁ butyric acid methyl ester (C₆₀-PCBM). It is concluded that for all six new compounds as well as pure [60]fullerene and PCBM both superoxide and singlet oxygen are produced in the first stages of UVA illumination following the type I and II mechanisms, respectively. In all cases singlet oxygen is produced as the primary dominant species; however, the type I mechanism always occurs in parallel with type II. In the end, the superoxide is self-dismuted into hydroxyl radicals, thus yielding PBN-OH spin adducts (g = 2.007 and a_hf (¹⁴N) = 1.54 mT). The kinetic reaction constants and their efficiencies in the production of reactive oxygen species at 375 nm and per mW of absorbed power are determined. The experimental results are consistent with an autocatalytic reaction model in which the system evolutes under UVA illumination, with superoxide catalyzing the conversion of singlet oxygen into more superoxide.     

A fullerene core to probe dendritic shielding effects

Dendrimers with a C₆₀ core have been obtained by cyclization of dendritic bis-malonate derivatives at the carbon sphere. The resulting bis-methanofullerene derivatives have been characterised by electrospray (ES) and/or MALDI-TOF mass spectrometries. UV-VIS absorption spectra, fluorescence spectra, and fullerene singlet excited state lifetimes have been determined in solvents of different polarity (toluene, dichloromethane, acetonitrile). These data suggest a tighter core/periphery contact upon increase of solvent polarity and dendrimer size. In all the investigated solvents, the fullerene triplet lifetimes are steadily increased with the dendrimer volume, reflecting lower diffusion rates of O₂ inside the dendrimers along the series. Measurements of quantum yields of singlet oxygen sensitization indicate that longer lived triplet states generate lower amounts of singlet oxygen (¹O₂^∗) in dichloromethane but not in apolar toluene suggesting a tighter contact between the dendritic branches and the fullerene core in CH₂Cl₂. In acetonitrile, the trend in singlet oxygen production is peculiar. Effectively, enhanced singlet oxygen production is monitored for the largest dendrimer. This reflects specific interactions of excited ¹O₂^∗ molecules with the dendritic wedges, as probed by singlet oxygen lifetime measurements, possibly as a consequence of trapping effects.     

Energy transfer followed by electron transfer in a supramolecular triad composed of boron dipyrrin, zinc porphyrin, and fullerene: a model for the photosynthetic antenna-reaction center complex

The first example of a working model of the photosynthetic antenna-reaction center complex, constructed via self-assembled supramolecular methodology, is reported. For this, a supramolecular triad is assembled by axially coordinating imidazole-appended fulleropyrrolidine to the zinc center of a covalently linked zinc porphyrin-boron dipyrrin dyad. Selective excitation of the boron dipyrrin moiety in the boron dipyrrin-zinc porphyrin dyad resulted in efficient energy transfer (k(ENT)(singlet) = 9.2 x 10(9) s(-)(1); Phi(ENT)(singlet) = 0.83) creating singlet excited zinc porphyrin. Upon forming the supramolecular triad, the excited zinc porphyrin resulted in efficient electron transfer to the coordinated fullerenes, resulting in a charge-separated state (k(cs)(singlet) = 4.7 x 10(9) s(-)(1); Phi(CS)(singlet) = 0.9). The observed energy transfer followed by electron transfer in the present supramolecular triad mimics the events of natural photosynthesis. Here, the boron dipyrrin acts as antenna chlorophyll that absorbs light energy and transports spatially to the photosynthetic reaction center, while the electron transfer from the excited zinc porphyrin to fullerene mimics the primary events of the reaction center where conversion of the electronic excitation energy to chemical energy in the form of charge separation takes place. The important feature of the present model system is its relative "simplicity" because of the utilized supramolecular approach to mimic rather complex "combined antenna-reaction center" events of photosynthesis.     

Preparation and photophysical studies of a fluorous phase-soluble fullerene derivative

We report the first synthesis of a well-characterized "Teflon ponytail" fullerene adduct (3) via the Hirsch-Bingel reaction with a malonate bearing two perfluorinated alkyl chains. This C3 tris-adduct shows excellent solubility in perfluorinated solvents, such as FC-72 and FC-75. Compound 3 was found to be an efficient sensitizer for singlet oxygen formation in fluorous media, which has potential in biphasic systems and in photobiology.     

Enhanced electron transfer ability <Emphasis Type="Italic">via</Emphasis> coordination in block copolymer/porphyrin/fullerene micelle

Inspired by structures of antenna-reaction centers in photosynthesis, the complex micelle was prepared from zinc tetra-phenyl porphyrin (ZnTPP), fullerene derivative (PyC₆₀) and poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-b-PCL). The core-shell structure made the hydrophobic donor-acceptor system work in aqueous. In micellar core, coordination interaction occurred between ZnTPP and PyC₆₀ molecules which ensured the enhanced energy migration from the donor to the acceptor. The enhanced interaction between porphyrin and fullerene was confirmed by absorption, steady-state fluorescence and transient fluorescence. The generation of singlet oxygen and superoxide radical was detected by iodide method and reduction of nitro blue tetrazolium, respectively, which confirmed that electron transfer reaction in the complex micellar core occurred. Moreover, the complex micelle exhibited effective electron transfer performance in photodebromination of 2,3-dibromo-3-phenylpropionic acid. The complex micellar structure endowed the donor-acceptor system with improved stability under irradiation. This strategy could be helpful for designing new electron transfer platform and artificial photosynthetic system.     

SINGLET OXYGEN IS NOT PRODUCED IN PHOTOSYSTEM I UNDER PHOTOINHIBITORY CONDITIONS

Abstract— Excess illumination of photosynthetic systems brings about the complex functional and structural damage known as photoinhibition. According to the generally accepted and experimentally confirmed model, photoinhibition involves singlet oxygen production and subsequent oxidative damage in the photosystem II reaction center. However, it was recently suggested that singlet oxygen is not necessarily produced in photosystem II itself but rather in the non-heme iron-containing Fe-S centers of photosystem I (Chung, S.K. & J. Jung, Photochem. Photobiol. 61, 383–389, 1995). Contrary to this suggestion, our electron paramagnetic resonance spectroscopy experiments with the singlet oxygen trap 2,2,6,6-tetramethylpiperidine demonstrate that under photoinhibitory conditions, singlet oxygen is present in thylakoids and photosystem II core complex preparations but is not produced in photosystem I particles.     

Photoinduced electron transfer in a hexaphenylbenzene-based self-assembled porphyrin-fullerene triad

A hexaphenylbenzene-based zinc porphyrin dyad forms a 1:1 complex with a fullerene bearing two pyridyl groups via coordination of the pyridyl nitrogens with the zinc atoms. The fullerene is symmetrically located between the two zinc porphyrins. The binding constant for the complex is 7.3 x 10(4) M(-1) in 1,2-difluorobenzene. Photoinduced electron transfer from a porphyrin first excited singlet state to the fullerene occurs with a time constant of 3 ps, and the resulting charge-separated state has a lifetime of 230 ps. This self-assembled construct should form a basis for the construction of more elaborate model photosynthetic antenna-reaction center systems.     

Synthesis and characterization of water-soluble phenylene-vinylene-based singlet oxygen sensitizers for two-photon excitation

[reaction: see text] The synthesis and characterization of water-soluble singlet oxygen sensitizers with a phenylene-vinylene motif is presented. The principal motivation for this study was to better understand specific features of a water-soluble molecule that influence the photosensitized production of singlet oxygen upon nonlinear, two-photon excitation of that molecule. To achieve water solubility, sensitizers were synthesized with ionic as well as nonionic substituents. In the ionic approach, salts of N-methylated pyridine, benzothiazole, and 1-methyl-piperazine moieties were used, as were aryl-substituted sulfonic acid moieties. In the nonionic approach, aryl-substituted triethylene glycol moieties were used. Selected photophysical properties of the compounds synthesized were determined, including singlet oxygen quantum yields. Of the molecules examined, the most efficient singlet oxygen sensitizers had triethylene glycol units as the functional group that imparted water solubility. Molecules containing the ionic moieties did not make singlet oxygen in appreciable yield nor did they efficiently fluoresce. Rather, for these latter molecules, rapid charge-transfer-mediated non-radiative processes appear to dominate excited state deactivation.     

Synthesis,structure, and properties of novel open-cage fullerenes having heteroatom(s) on the rim of the orifice

A thermal liquid-phase reaction of fullerene C(60) with 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine afforded aza-open-cage fullerene derivative 5 having an eight-membered-ring orifice on the fullerene cage. Compound 5 was found to undergo oxidative ring-enlargement reactions with singlet oxygen under photo-irradiation to give azadioxo-open-cage fullerene derivatives 9 and 10, which have a 12-membered-ring orifice, in addition to a small amount of azadioxa-open-cage fullerene derivative 11, which has a 10-membered-ring orifice. A thermal reaction of 9 with elemental sulfur in the presence of tetrakis(dimethylamino)ethylene resulted in further ring-enlargement to give azadioxothia-open-cage fullerene derivative 15, which has a 13-membered-ring orifice. The structures of 5 and 15 were determined by X-ray crystallography, while those of 9, 10, and 11 were confirmed by the agreement of observed (13)C NMR spectra with those obtained by DFT-GIAO calculations. These reactions were rationalized based on the results of molecular orbital calculations. Following electrochemical measurements, compounds 9 and 10, which have two carbonyl groups on the rim of the orifice, were found to be more readily reduced than C(60) itself (the first reduction potential was found to be 0.2 V lower than that of C(60)), while the first reduction potentials of other open-cage fullerene derivatives, 5, 11, and 15, were nearly the same as that of C(60).