Optical and electrochemical properties of hydrogen-bonded phenol-pyrrolidino[60]fullerenes

We report the photophysical and electrochemical properties of phenol-pyrrolidino[60]fullerenes 1 and 2, in which the phenol hydroxyl group is ortho and para to the pyrrolidino group, respectively, as well as those of a phenyl-pyrrolidino[60]fullerene model compound, 3. For the ortho analog 1, the presence of an intramolecular hydrogen bond is supported by 1H NMR and FTIR characterization. The redox potential of the phenoxyl radical-phenol couple in this architecture is 240 mV lower than that observed in the associated para compound 2. Further, the C60 excited-state lifetime of the hydrogen-bonded compound 1 in benzonitrile is 260 ps, while the corresponding lifetime for 2 is identical to that of the model compound 3 at 1.34 ns. Addition of excess organic acid to a benzonitrile solution of 1 gives rise to a new species, 4, with an excited-state lifetime of 1.40 ns. In nonpolar aprotic solvents such as toluene, all three compounds have a C60 excited-state lifetime of ∼1 ns. These results suggest that the presence of an intramolecular H-bond in 1 poises the potential of phenoxyl radical-phenol redox couple at a value that it is thermodynamically capable of reducing the photoexcited fullerene. This is not the case for the para analog 2 nor is it the case for the protonated species 4. This work illustrates that in addition to being used as light activated electron acceptors, pyrrolidino fullerenes are also capable of acting as built-in proton-accepting units that influence the potential of an attached donor when organized in an appropriate molecular design.Fil: Moore, Gary F.. Lawrence Berkeley National Laboratory; Estados Unidos. Arizona State University; Estados UnidosFil: Megiatto, Jackson D.. Arizona State University; Estados UnidosFil: Hambourger, Michael. Arizona State University; Estados Unidos. Appalachian State University (appstate);Fil: Gervaldo, Miguel Andres. Arizona State University; Estados Unidos. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Instituto de Investigaciones en Tecnologías Energéticas y Materiales Avanzados. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Tecnologías Energéticas y Materiales Avanzados; ArgentinaFil: Kodis, Gerdenis. Arizona State University; Estados UnidosFil: Moore, Thomas A.. Arizona State University; Estados UnidosFil: Gust, Devens. Arizona State University; Estados UnidosFil: Moore, Ana L.. Arizona State University; Estados Unido

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation

In the photosynthetic photosystem II, electrons are transferred from the manganese-containing oxygen evolving complex (OEC) to the oxidized primary electron-donor chlorophyll P680•+ by a proton-coupled electron transfer process involving a tyrosine-histidine pair. Proton transfer from the tyrosine phenolic group to a histidine nitrogen positions the redox potential of the tyrosine between those of P680•+ and the OEC. We report the synthesis and time-resolved spectroscopic study of a molecular triad that models this electron transfer. The triad consists of a high-potential porphyrin bearing two pentafluorophenyl groups (PF10), a tetracyanoporphyrin electron acceptor (TCNP), and a benzimidazole-phenol secondary electron-donor (Bi-PhOH). Excitation of PF10 in benzonitrile is followed by singlet energy transfer to TCNP (τ = 41 ps), whose excited state decays by photoinduced electron transfer (τ = 830 ps) to yield Bi-PhOH-PF 10•+-TCNP•-. A second electron transfer reaction follows (τ < 12 ps), giving a final state postulated as BiH+-PhO•-PF10-TCNP•-, in which the phenolic proton now resides on benzimidazole. This final state decays with a time constant of 3.8 μs. The triad thus functionally mimics the electron transfers involving the tyrosine-histidine pair in PSII. The final charge-separated state is thermodynamically capable of water oxidation, and its long lifetime suggests the possibility of coupling systems such as this system to water oxidation catalysts for use in artificial photosynthetic fuel production.Fil: Megiatto, Jackson D.. Arizona State University; Estados UnidosFil: Antoniuk Pablant, Antaeres. Arizona State University; Estados UnidosFil: Sherman, Benjamin D.. Arizona State University; Estados UnidosFil: Kodis, Gerdenis. Arizona State University; Estados UnidosFil: Gervaldo, Miguel Andres. Universidad Nacional de Río Cuarto; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: Moore, Thomas A.. Arizona State University; Estados UnidosFil: Moore, Ana L.. Arizona State University; Estados UnidosFil: Gust, Devens. Arizona State University; Estados Unido

A photo- and electrochemically-active porphyrin–fullerene dyad electropolymer

A hole- and electron-conducting polymer has been prepared by electropolymerization of aporphyrin–fullerene monomer. The porphyrin units are linked by aminophenyl groups to form a linear chain in which the porphyrin is an integral part of the polymer backbone. The absorption spectrum of a film formed on indium-tin-oxide-coated glass resembles that of a model porphyrin–fullerene dyad, but with significant peak broadening. The film demonstrates a first oxidation potential of 0.75 V vs. SCE, corresponding to oxidation of the porphyrin polymer, and a first reduction potential of -0.63 V vs. SCE, corresponding to fullerene reduction. Time-resolved fluorescence studies show that the porphyrin first excited singlet state is strongly quenched by photoinduced electron transfer to fullerene. Transient absorption investigations reveal that excitation generates mobile charge carriers that recombine by both geminate and nongeminate pathways over a large range of time scales. Similar studies on a related polymer that lacks the fullerene component show complex, laser-intensity-dependent photoinduced electron transfer behavior. The properties of the porphyrin–fullerene electropolymer suggest that it maybe useful in organic photovoltaic applications, wherein light absorption leads to charge separationwithin picoseconds in a “molecular heterojunction” with no requirement for exciton migration.Fil: Gervaldo, Miguel Andres. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Instituto de Investigaciones en Tecnologías Energéticas y Materiales Avanzados. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Tecnologías Energéticas y Materiales Avanzados; ArgentinaFil: Liddell, Paul A.. Arizona State University; Estados UnidosFil: Kodis, Gerdenis. Arizona State University; Estados UnidosFil: Brennan, Bradley J.. Arizona State University; Estados UnidosFil: Johnson, Christopher R.. Arizona State University; Estados UnidosFil: Bridgewater, James W.. Arizona State University; Estados UnidosFil: Moore, Ana L.. Arizona State University; Estados UnidosFil: Moore, Thomas A.. Arizona State University; Estados UnidosFil: Gust, Devens. Arizona State University; Estados Unido

Artificial Photosynthetic Reaction Center Exhibiting Acid-Responsive Regulation of Photoinduced Charge Separation.

Nonphotochemical quenching (NPQ) is a photoprotective regulatory mechanism employed by many photosynthetic organisms to dynamically modulate energy flow within the photosynthetic apparatus in response to fluctuating light conditions. Activated by decreases in lumen pH produced during periods of high photon flux, NPQ induces rapid thermal dissipation of excess excitation energy. As a result, the rate of charge separation (CS) decreases, thereby limiting the accumulation of potentially deleterious reactive intermediates and byproducts. Herein, a molecular triad that functionally mimics the effects of NPQ associated with an artificial photosynthetic reaction center is described. Steady-state absorption and emission, time-resolved fluorescence, and transient absorption spectroscopies have been used to demonstrate a 1 order of magnitude reduction in the CS quantum yield via reversible protonation of an excited-state-quenching molecular switch moiety. As in the natural system, the populations of unquenched and quenched states and therefore the overall yields of CS were found to be dependent on acid concentration