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Photoinduced proton-coupled electron transfer and long-range two-proton transport via a Grotthuss-type mechanism are investigated in a biomimetic construct. The ultrafast, nonequilibrium dynamics are assessed via two-dimensional electronic vibrational spectroscopy, in concert with electrochemical and computational techniques. A low-frequency mode is identified experimentally and found to promote double proton and electron transfer, supported by recent theoretical simulations of a similar but abbreviated (non-photoactive) system. Excitation frequency peak evolution and center line slope dynamics show direct evidence of strongly coupled nuclear and electronic degrees of freedom, from which we can conclude that the double proton and electron transfer processes are concerted (up to an uncertainty of 24 fs). The nonequilibrium pathway from the photoexcited Franck-Condon region to the E2PT state is characterized by an ∼110 fs time scale. This study and the tools presented herein constitute a new window into hot charge transfer processes involving an electron and multiple protons.

Light-activated photosystem II (PSII) carries out the critical step of splitting water in photosynthesis. However, PSII is susceptible to light-induced damage. Here, results are presented from a novel microbial electro-photosynthetic system (MEPS) that uses redox mediators in conjunction with an electrode to drive electron transport in live

Proton-coupled electron transfer (PCET) reactions depend on the hydrogen-bond connectivity between sites of proton donors and acceptors. The 2-(2′-hydroxyphenyl) benzimidazole (BIP) based systems, which mimic the natural TyrZ-His190 pair of Photosystem II, have been useful for understanding the associated PCET process triggered by one-electron oxidation of the phenol. Substitution of the benzimidazole by an appropriate terminal proton acceptor (TPA) group allows for two-proton translocations. However, the prototropic properties of substituted benzimidazole rings and rotation around the bond linking the phenol and the benzimidazole can lead to isomers that interrupt the intramolecular hydrogen-bonded network and thereby prevent a second proton translocation. Herein, a strategic symmetrization of a benzimidazole based system with two identical TPAs yields an uninterrupted network of intramolecular hydrogen bonds regardless of the isomeric form. NMR data confirms the presence of a single isomeric form in the disubstituted system but not in the monosubstituted system in certain solvents. Infrared spectroelectrochemistry demonstrates a two-proton transfer process associated with the oxidation of the phenol occurring at a lower redox potential in the disubstituted system relative to its monosubstituted analogue. Computational studies support these findings and show that the disubstituted system stabilizes the oxidized two-proton transfer product through the formation of a bifurcated hydrogen bond. Considering the prototropic properties of the benzimidazole heterocycle in the context of multiple PCET will improve the next generation of novel, bioinspired constructs built by concatenated units of benzimidazoles, thus allowing proton translocations at nanoscale length., Proton-coupled electron transfer (PCET) reactions depend on the hydrogen-bond connectivity between sites of proton donors and acceptors.

Expanding proton-coupled electron transfer to multiproton translocations (MPCET) provides a bioinspired mechanism to transport protons away from the redox site. This expansion has been accomplished by separating the initial phenolic proton donor from the pyridine-based terminal proton acceptor by a Grotthuss-type proton wire made up of concatenated benzimidazoles that form a hydrogen-bonded network. However, it was found that the midpoint potential of the phenol oxidation that launched the Grotthuss-type proton translocations is a function of the number of benzimidazoles in the hydrogen-bonded network; it becomes less positive (i.e., a weaker oxidant) as the number of bridging benzimidazoles increases. Herein, we report a strategy to maintain the high redox potential necessary for oxidative processes relevant to artificial photosynthesis, e.g., water oxidation and long-range MPCET processes for managing protons. The integrated structural and functional roles of the benzimidazole-based bridge provide sites for substitution of the benzimidazoles with electron-withdrawing groups (e.g., trifluoromethyl groups). Such substitution increases the midpoint potential of the phenoxyl radical/phenol couple so that proton translocations over ∼11 Å become thermodynamically comparable to that of an unsubstituted system where one proton is transferred over ∼2.5 Å. The extended, substituted system maintains the hydrogen-bonded network; infrared spectroelectrochemistry confirms reversible proton translocations from the phenol to the pyridyl terminal proton acceptor upon oxidation and reduction. Theory supports the change in driving force with added electron-withdrawing groups and provides insight into the role of electron density and electrostatic potential in MPCET processes associated with these Grotthuss-type proton translocations.

In water-oxidizing photosynthetic organisms, excitation of the reaction-center chlorophylls (P680) triggers a cascade of electron and proton transfer reactions that establish charge separation across the membrane and proton-motive force. An early oxidation step in this process involves proton-coupled electron transfer (PCET) via a tyrosine-histidine redox relay (Yz-H190). Herein, we report the synthesis and structural characterization of two isomeric dyads designed to model this PCET process. Both are based on the same high potential fluorinated porphyrin (model for P680), linked to isomeric pyridylbenzimidazole-phenols (models for Yz-H190). The two isomeric dyads have different hydrogen bond frameworks, which is expected to change the PCET photooxidation mechanism. In these dyads, 1H NMR evidence indicates that in one dyad the hydrogen bond network would support a Grotthuss-type proton transfer process, whereas in the other the hydrogen bond network is interrupted. Photoinduced one-electron, two-proton transfer is expected to occur in the fully hydrogen-bonded dyad upon oxidation of the phenol by the excited state of the porphyrin. In contrast for the isomer with the interrupted hydrogen bond network, an ultrafast photoinduced one-electron one-proton transfer process is anticipated, followed by a much slower proton transfer to the terminal proton acceptor. Understanding the nature of photoinduced PCET mechanisms in these biomimetic models will provide insights into the design of future generations of artificial constructs involved in energy conversion schemes.

Photosynthesis powers our planet and is a source of inspiration for developing artificial constructs mimicking many aspects of the natural energy transducing process. In the complex machinery of photosystem II (PSII), the redox activity of the tyrosine Z (Tyrz) hydrogen-bonded to histidine 190 (His190) is essential for its functions. For example, the Tyrz–His190 pair provides a proton-coupled electron transfer (PCET) pathway that effectively competes against the back-electron transfer reaction and tunes the redox potential of the phenoxyl radical/phenol redox couple ensuring a high net quantum yield of photoinduced charge separation in PSII. Herein, artificial assemblies mimicking both the structural and redox properties of the Tyrz–His190 pair are described. The bioinspired constructs contain a phenol (Tyrz model) covalently linked to a benzimidazole (His190 model) featuring an intramolecular hydrogen bond which closely emulates the one observed in the natural counterpart. Incorporation of electron-withdrawing groups in the benzimidazole moiety systematically changes the intramolecular hydrogen bond strength and modifies the potential of the phenoxyl radical/phenol redox couple over a range of ~ 250 mV. Infrared spectroelectrochemistry (IRSEC) demonstrates the associated one-electron, one-proton transfer (E1PT) process upon electrochemical oxidation of the phenol. The present contribution provides insight regarding the factors controlling the redox potential of the phenol and highlights strategies for the design of futures constructs capable of transporting protons across longer distances while maintaining a high potential of the phenoxyl radical/phenol redox couple.

The essential role of a well-defined hydrogen-bond network in achieving chemically reversible multiproton translocations triggered by one-electron electrochemical oxidation/reduction is investigated by using pyridylbenzimidazole-phenol models. The two molecular architectures designed for these studies differ with respect to the position of the N atom on the pyridyl ring. In one of the structures, a hydrogen-bond network extends uninterrupted across the molecule from the phenol to the pyridyl group. Experimental and theoretical evidence indicates that an overall chemically reversible two-proton-coupled electron-transfer process (E2PT) takes place upon electrochemical oxidation of the phenol. This E2PT process yields the pyridinium cation and is observed regardless of the cyclic voltammogram scan rate. In contrast, when the hydrogen-bond network is disrupted, as seen in the isomer, at high scan rates (μ1000 mV s-1) a chemically reversible process is observed with an E1/2 characteristic of a one-proton-coupled electron-transfer process (E1PT). At slow cyclic voltammetric scan rates (

Upon photoinitiated electron transfer, charge recombination limits the quantum yield of photoredox reactions for which the rates for the forward reaction and back electron transfer are competitive. Taking inspiration from a proton-coupled electron transfer (PCET) process in Photosystem II, a benzimidazole-phenol (BIP) has been covalently attached to the 2,2'-bipyridyl ligand of [Ir(dF(CF3)ppy)2(bpy)][PF6] (dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; bpy = 2,2'-bipyridyl). Excitation of the [Ir(dF(CF3)ppy)2(BIP-bpy)][PF6] photocatalyst results in intramolecular PCET to form a charge-separated state with oxidized BIP. Subsequent reduction of methyl viologen dication (MV2+), a substrate surrogate, by the reducing moiety of the charge separated species demonstrates that the inclusion of BIP significantly slows the charge recombination rate. The effect of ∼24-fold slower charge recombination in a photocatalytic phthalimide ester reduction resulted in a greater than 2-fold increase in reaction quantum efficiency.

Although photoinduced proton-coupled electron transfer (PCET) plays an essential role in photosynthesis, a full understanding of the mechanism is still lacking due to the complex nonequilibrium dynamics arising from the strongly coupled electronic and nuclear degrees of freedom. Here we report the photoinduced PCET dynamics of a biomimetic model system investigated by means of transient IR and two-dimensional electronic-vibrational (2DEV) spectroscopies, IR spectroelectrochemistry (IRSEC), and calculations utilizing long-range-corrected hybrid density functionals. This collective experimental and theoretical effort provides a nuanced picture of the complicated dynamics and synergistic motions involved in photoinduced PCET. In particular, the evolution of the 2DEV line shape, which is highly sensitive to the mixing of vibronic states, is interpreted by accurate computational modeling of the charge separated state and is shown to represent a gradual change in electron density distribution associated with a dihedral twist that occurs on a 120 fs time scale.

Photosynthesis powers our planet and is a source of inspiration for developing artificial constructs mimicking many aspects of the natural energy transducing process. In the complex machinery of photosystem II (PSII), the redox activity of the tyrosine Z (Tyr

Summary The photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that comprises the Mn4CaO5 cluster, with participation of the redox-active tyrosine residue (YZ) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between YZ and D1-His190 likely renders YZ kinetically and thermodynamically competent leading to highly efficient water oxidation. However, a detailed understanding of the proton-coupled electron transfer (PCET) at YZ remains elusive owing to the transient nature of its intermediate states involving YZ⋅. Herein, we employ a combination of high-resolution two-dimensional 14N hyperfine sublevel correlation spectroscopy and density functional theory methods to investigate a bioinspired artificial photosynthetic reaction center that mimics the PCET process involving the YZ residue of PSII. Our results underscore the importance of proximal water molecules and charge delocalization on the electronic structure of the artificial reaction center., Graphical Abstract, Highlights • Structural factors are critical in the design of artificial photosynthetic systems • Correlation between hyperfine couplings of the N atoms and electron spin density • Spin density distribution affected by charge delocalization and explicit waters • Spin density modulation by electronic coupling as observed with P680 and YZ in PSII, Spectroscopy; Organic Reaction; Computational Chemistry

Conjugated porphyrin arrays are heavily investigated as efficient molecular systems for photosynthesis and photocatalysis. Recently, a series of one-, two-, and six-zinc-porphyrin arrays, noncovalently linked through benzene-based hubs, have been synthesized with the aim of mimicking the structure and function of the bacteriochlorophyll "special pair" in photosynthetic reaction centers. The excitonically coupled porphyrin subunits are expected to activate additional excited state relaxation channels with respect to the monomer. Here, we unveil the appearance of such supramolecular electronic interactions using ultrafast transient absorption spectroscopy with sub-25 fs time resolution. Upon photoexcitation of the Soret band, we resolve energy trapping within ∼150 fs in a delocalized dark excitonic manifold. Moreover, excitonic interactions promote an additional fast internal conversion from the Q-band to the ground state with an efficiency of up to 60% in the hexamer. These relaxation pathways appear to be common loss channels that limit the lifetime of the exciton states in noncovalently bound molecular aggregates.

Designing molecular platforms for controlling proton and electron movement in artificial photosynthetic systems is crucial to efficient catalysis and solar energy conversion. The transfer of both protons and electrons during a reaction is known as proton-coupled electron transfer (PCET) and is used by nature in myriad ways to provide low overpotential pathways for redox reactions and redox leveling, as well as to generate bioenergetic proton currents. Herein, we describe theoretical and electrochemical studies of a series of bioinspired benzimidazole-phenol (BIP) derivatives and a series of dibenzimidazole-phenol (BI2P) analogs with each series bearing the same set of terminal proton-accepting (TPA) groups. The set of TPAs spans more than 6 pKa units. These compounds have been designed to explore the role of the bridging benzimidazole(s) in a one-electron oxidation process coupled to intramolecular proton translocation across either two (the BIP series) or three (the BI2P series) acid/base sites. These molecular constructs feature an electrochemically active phenol connected to the TPA group through a benzimidazole-based bridge, which together with the phenol and TPA group form a covalent framework supporting a Grotthuss-type hydrogen-bonded network. Infrared spectroelectrochemistry demonstrates that upon oxidation of the phenol, protons translocate across this well-defined hydrogen-bonded network to a TPA group. The experimental data show the benzimidazole bridges are non-innocent participants in the PCET process in that the addition of each benzimidazole unit lowers the redox potential of the phenoxyl radical/phenol couple by 60 mV, regardless of the nature of the TPA group. Using a series of hypothetical thermodynamic steps, density functional theory calculations correctly predicted the dependence of the redox potential of the phenoxyl radical/phenol couple on the nature of the final protonated species and provided insight into the thermodynamic role of dibenzimidazole units in the PCET process. This information is crucial for developing molecular “dry proton wires” with these moieties, which can transfer protons via a Grotthuss-type mechanism over long distances without the intervention of water molecules., Experimental and theoretical methods characterize the thermodynamics of electrochemically driven proton-coupled electron transfer processes in bioinspired constructs involving multiple proton translocations over Grotthus-type proton wires.

Bioinspired constructs consisting of benzimidazole-phenol moieties bearing N-phenylimines as proton-accepting substituents have been designed to mimic the H-bond network associated with the TyrZ-His190 redox relay in photosystem II. These compounds provide a platform to theoretically and experimentally explore and expand proton-coupled electron transfer (PCET) processes. The models feature H-bonds between the phenol and the nitrogen at the 3-position of the benzimidazole and between the 1H -benzimidazole proton and the imine nitrogen. Protonation of the benzimidazole and the imine can be unambiguously detected by infrared spectroelectrochemistry (IRSEC) upon oxidation of the phenol. DFT calculations and IRSEC results demonstrate that with sufficiently strong electron-donating groups at the para-position of the N-phenylimine group (e.g., -OCH3 substitution), proton transfer to the imine is exergonic upon phenol oxidation, leading to a one-electron, two-proton (E2PT) product with the imidazole acting as a proton relay. When transfer of the second proton is not sufficiently exergonic (e.g., -CN substitution), a one-electron, one-proton transfer (EPT) product is dominant. Thus, the extent of proton translocation along the H-bond network, either ~1.6 Å or ~6.4 Å, can be controlled through imine substitution. Moreover, the H-bond strength between the benzimidazole NH and the imine nitrogen, which is a function of their relative pKa values, and the redox potential of the phenoxyl radical/phenol couple are linearly correlated with the Hammett constants of the substituents. In all cases, a high potential (~1 V vs SCE) is observed for the phenoxyl radical/phenol couple. Designing and tuning redox-coupled proton wires is important for understanding bioenergetics and developing novel artificial photosynthetic systems. Fil: Odella, Emmanuel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Arizona State University; Estados Unidos. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Departamento de Química; Argentina Fil: Mora, Sabrina Jimena. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Arizona State University; Estados Unidos Fil: Wadsworth, Brian L.. Arizona State University; Estados Unidos Fil: Huynh, Mioy T.. University of Yale; Estados Unidos Fil: Goings, Joshua J.. University of Yale; Estados Unidos Fil: Liddell, Paul A.. Arizona State University; Estados Unidos Fil: Groy, Thomas L.. Arizona State University; Estados Unidos 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; Argentina Fil: Sereno, Leonides Edmundo. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Departamento de Química; Argentina Fil: Gust, Devens. Arizona State University; Estados Unidos Fil: Moore, Thomas A.. Arizona State University; Estados Unidos Fil: Moore, Gary F.. Arizona State University; Estados Unidos Fil: Hammes-Schiffer, Sharon. University of Yale; Estados Unidos Fil: Moore, Ana L.. Arizona State University; Estados Unidos

International audience; Three Pd(II) phthalocyanine-carotenoid dyads featuring chromophores linked by amide bonds were prepared in order to investigate the rate of triplet-triplet (T-T) energy transfer from the tetrapyrrole to the covalently attached carotenoid as a function of the number of conjugated double bonds in the carotenoid. Carotenoids having 9, 10 and 11 conjugated double bonds were studied. Transient absorption measurements show that intersystem crossing in the Pd(II) phthalocyanine takes place in 10 ps in each case and that T-T energy transfer occurs in 126, 81 and 132 ps in the dyads bearing 9, 10 and 11 double bond carotenoids, respectively. To identify the origin of this variation in T-T energy transfer rates, density functional theory (DFT) was used to calculate the T-T electronic coupling in the three dyads. According to the calculations, the primary reason for the observed T-T energy transfer trend is larger T-T electronic coupling between the tetrapyrrole and the 10-double bond carotenoid. A methyl group adjacent to the amide linker that connects the Pd(II) phthalocyanine and the carotenoid in the 9 and 11-double bond carotenoids is absent in the 10-double bond carotenoid, and this difference alters its electronic structure to increase the coupling.

Artificial photosynthetic constructs can in principle operate more efficiently than natural photosynthesis because they can be rationally designed to optimize solar energy conversion for meeting human demands rather than the multiple needs of an organism competing for growth and reproduction in a complex ecosystem. The artificial photosynthetic constructs described in this Account consist primarily of covalently linked synthetic chromophores, electron donors and acceptors, and proton donors and acceptors that carry out the light absorption, electron transfer, and proton-coupled electron transfer (PCET) processes characteristic of photosynthetic cells. PCET is the movement of an electron from one site to another accompanied by proton transfer. PCET and the transport of protons over tens of angstroms are important in all living cells because they are a fundamental link between redox processes and the establishment of transmembrane gradients of proton electrochemical potential, known as proton-motive force (PMF), which is the unifying concept in bioenergetics. We have chosen a benzimidazole phenol (BIP) system as a platform for the study of PCET because with appropriate substitutions it is possible to design assemblies in which one or multiple proton transfers can accompany oxidation of the phenol. In BIP, oxidation of the phenol increases its acidity by more than ten pK

Presently, the world is experiencing an unprecedented crisis associated with the CO2 produced by the use of fossil fuels to power our economies. As evidenced by the increasing levels in the atmosphere, the reduction of CO2 to biomass by photosynthesis cannot keep pace with production with the result that nature has lost control of the global carbon cycle. In order to restore control of the global carbon cycle to solar-driven processes, highly efficient artificial photosynthesis can augment photosynthesis in specific ways and places. The increased efficiency of artificial photosynthesis can provide both renewable carbon-based fuels and lower net atmospheric levels of CO2, which will preserve land and support the ecosystem services upon which all life on Earth depends. The development of artificial photosynthetic antennas and reaction centers contributes to the understanding of natural photosynthesis and to the knowledge base necessary for the development of future scalable technologies. This review focuses on the design and study of molecular and hybrid molecular-semiconductor nanoparticle based systems, all of which are inspired by functions found in photosynthesis and some of which are inspired by components of photosynthesis. In addition to constructs illustrating energy transfer, photoinduced electron transfer, charge shift reactions and proton coupled electron transfer, our review covers systems that produce proton motive force.

Benzimidazole phenol-porphyrin dyads have been synthesized to study proton-coupled electron transfer (PCET) reactions induced by photoexcitation. High-potential porphyrins have been chosen to model P680, the photoactive chlorophyll cluster of photosynthetic photosystem II (PSII). They have either two or three pentafluorophenyl groups at the meso positions to impart the high redox potential. The benzimidazole phenol (BIP) moiety models the Tyrz-His190 pair of PSII, which is a redox mediator that shuttles electrons from the water oxidation catalyst to P680•+. The dyads consisting of a porphyrin and an unsubstituted BIP are designed to study one-electron one-proton transfer (E1PT) processes upon excitation of the porphyrin. When the BIP moiety is substituted with proton-accepting groups such as imines, one-electron two-proton transfer (E2PT) processes are expected to take place upon oxidation of the phenol by the excited state of the porphyrin. The bis-pentafluorophenyl porphyrins linked to BIPs provide platforms for introducing a variety of electron-accepting moieties and/or anchoring groups to attach semiconductor nanoparticles to the macrocycle. The triads thus formed will serve to study the PCET process involving the BIPs when the oxidation of the phenol is achieved by the photochemically produced radical cation of the porphyrin. Fil: Mora, Sabrina Jimena. Arizona State University; Estados Unidos. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Heredia, Daniel Alejandro. Universidad Nacional de Río Cuarto. Instituto para el Desarrollo Agroindustrial y de la Salud. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto para el Desarrollo Agroindustrial y de la Salud; Argentina Fil: Odella, Emmanuel. Arizona State University; Estados Unidos. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Departamento de Química; Argentina Fil: Vrudhula, Uma. Arizona State University; Estados Unidos Fil: Gust, Devens. Arizona State University; Estados Unidos Fil: Moore, Thomas A.. Arizona State University; Estados Unidos Fil: Moore, Ana L.. Arizona State University; Estados Unidos

Proton-coupled electron transfer (PCET) combines the movement of fundamental charged species to form an essential link between electron- and proton-transport reactions in bioenergetics and catalysis in general. The length scale over which proton transport may occur within PCET processes and the thermodynamic consequences of the resulting proton chemical potential to the oxidation reaction driving these PCET processes have not been generally established. Here we report the design of bioinspired molecules that employ oxidation-reduction processes to move reversibly two, three, and four protons via a Grotthuss-type mechanism along hydrogen-bonded networks up to ∼16 A in length. These molecules are composed of benzimidazole moieties linking a phenol to the final proton acceptor, a cyclohexylimine. Following electrochemical oxidation of the phenol, the appearance of an infrared band at 1660 cm-1 signals proton arrival at the terminal basic site. Switching the electrode potential to reducing conditions reverses the proton translocation and resets the structure to the initial species. In addition to mimicking the first step of the iconic PCET process used by the Tyrz-His190 redox relay in photosystem II to oxidize water, this work specifically addresses theoretically and experimentally the length scale over which PCET processes may occur. The thermodynamic findings from these redox-driven, bioinspired "proton wires" have implications for understanding and rationally designing pumps for the generation of proton-motive force in artificial and reengineered photosynthesis, as well as for management of proton activity around catalytic sites, including those for water oxidation and oxygen reduction.

A simple 3-step synthesis of a novel pyrazine-containing heteroaromatic building block for use as a scaffold for producing multi-chromophoric sensitizers for use in dye sensitized solar cells has been developed. This moiety features two esterified carboxylic acid groups whose hydrolysis provides moieties for anchoring to nanoparticulate metal oxides and two bromine atoms that facilitate attachment of chromophores via palladium mediated cross-coupling methods. The utility of the building block for the synthesis of complex dye motifs is demonstrated by the preparation of a novel panchromatic dye featuring a BODIPY chromophore and a strongly electron donating porphyrin chromophore using both Suzuki and copper-free Sonogashira cross-coupling methods. The building block could be used either to attach two different or identical dye moieties to the metal oxide, or to attach an electron injection dye plus an auxiliary absorber, photoprotective agent, secondary redox moiety, etc.

Iridium oxide (IrOx) is one of the best water splitting electrocatalysts, but its active site details are not well known. As with all heterogeneous catalysts, a strategy for counting the number of active sites is not clear, and understanding their nature and structure is remarkably difficult. In this work, we performed a combined study using optical spectroscopy, magnetic resonance and electrochemistry to characterize the interaction of IrOx nanoparticles (NPs) with a probe molecule, catechol. The catalyst is heterogeneous given that the substrate is in a different phase, but behaves as a homogeneous catalyst from the point of view of electrochemistry since it remains in colloidal suspension. We find two types of binding sites: centers A which bind catechol irreversibly making up 21% of the surface, and centers B which bind catechol reversibly making up 79% of the surface. UV-vis absorption spectroscopy shows that the A sites are responsible for the characteristic blue color of the NPs. Electrochemical experiments indicate that the B sites are catalytically active and we give the number of active sites per nanoparticle. We conclude by performing a survey of ligands used in solar cell architectures and show which ones bind well to the surface and which ones inhibit the catalytic activity when doing so, presenting quantitative guidelines for the correct handling of IrOx nanoparticles during their incorporation into multifunctional solar energy harvesting architectures. We suggest ligands binding on the surface oxygen atoms allow for large bound ligand densities with no detrimental effect on the catalytic activity.

The Marcus theory of electron transfer predicts a bell-shaped dependence of the reaction rate on the reaction free energy. The top of the "inverted parabola" corresponds to zero activation barrier when the electron-transfer reorganization energy and the reaction free energy add up to zero. Although this point has traditionally been reached by altering the chemical structures of donors and acceptors, the theory suggests that it can also be reached by varying other parameters of the system including temperature. We find here dramatic evidence of this phenomenon from experiments on a fullerene-porphyrin dyad. Following photoinduced electron transfer, the rate of charge recombination shows a bell-shaped dependence on the inverse temperature, first increasing with cooling and then decreasing at still lower temperatures. This non-Arrhenius rate law is a result of a strong, approximately hyperbolic temperature variation of the reorganization energy and the reaction free energy. Our results provide potentially the cleanest confirmation of the Marcus energy gap law so far since no modification of the chemical structure is involved.

In order to investigate the possibility of a fullerene acting as an electron and/or singlet energy relay between a donor chromophore and an acceptor, a triad consisting of a fullerene (C60) covalently linked to both a porphyrin energy and electron donor (P) and a β-tetracyanoporphyrin energy and electron acceptor (CyP) was synthesized. Steady state and time-resolved spectroscopic investigations show that the porphyrin first excited singlet state donates singlet excitation and an electron to the fullerene and also donates singlet excitation to the CyP. All three processes differ in rate constant by factors of ≤1.3, and all are much faster than the decay of (1)P-C60-CyP by unichromophoric processes. The fullerene excited state accepts an electron from P and donates singlet excitation energy to CyP. The P(•+)-C60(•-)-CyP charge-separated state transfers an electron to CyP to produce a final P(•+)-C60-CyP(•-) state. The same state is formed from P-C60-(1)CyP. Overall, the final charge-separated state is formed with a quantum yield of 85% in benzonitrile, and has a lifetime of 350 ps. Rate constants for formation and quantum yields of all intermediate states were estimated from results for the triad and several model compounds. Interestingly, the intermediate P(•+)-C60(•-)-CyP charge-separated state has a lifetime of 660 ps. It is longer lived than the final state in spite of stronger coupling of the radical ions. This is ascribed to the fact that recombination lies far into the inverted region of the Marcus rate constant vs thermodynamic driving force relationship.

β-Cyanoporphyrins have high positive potentials for oxidation and absorb light at longer wavelengths than most porphyrins, making them potential candidates for sensitizers in photoelectrosynthetic cells for water oxidation. In order to begin to evaluate this potential, two Zn(II) tetra-β-cyanoporphyrins have been synthesized and evaluated as sensitizers in dye sensitized solar cells using I−/I3− as the redox mediator. To prepare such specialized β-cyanoporphyrins, a new synthetic method has been developed. This approach involves reaction of Zn(CN)2 with β-brominated zinc porphyrins in the presence of tris-(dibenzylideneacetone)dipalladium. The tetra-cyanation reaction is complete under milder conditions as compared to those usually employed in previous methods and gives improved yields of up to ∼50%. The procedure allows for the cyanation of porphyrins with relatively sensitive functional groups. Examples of its application to a range of substituted tetra-arylporphyrins are reported, and the absorption and electrochemical properties of the compounds prepared are given. The results from using two of the molecules as sensitizers in dye sensitized solar cells are presented. It was found that the porphyrins produced no photocurrents in nanoparticulate TiO2-based cells, but both molecules produced photocurrents in SnO2-based cells, and are potential candidates for sensitizers in photoelectrosynthetic cells for water oxidation.

Combining a dye-sensitized photoelectrochemical cell for hydrogen production based on a SnO2 photoanode in series with a dye-sensitized photovoltaic solar cell using a TiO2 photoanode yields a tandem system for the generation of H2 from hydroquinone using only light energy and no applied electrical bias. To target distinct portions of the solar spectrum, a more blue-absorbing free base porphyrin and a more red-absorbing Si phthalocyanine chromophore are used at the two photoelectrodes. With incorporation of a suitable water oxidation catalyst, a similar approach could enable tandem photochemical water splitting without the need for p-type semiconductors.

A zinc and a free base phthalocyanine-fulleropyrrolidine dyad in which the chromophores are linked by a phenylethynyl group have been prepared using a new synthetic route, and their photoelectrochemical properties have been investigated. The zinc dyad is readily soluble in a variety of solvents, and its spectroscopic properties have been determined in toluene and benzonitrile. In toluene, excitation of the zinc phthalocyanine is followed by rapid establishment of an equilibrium between the phthalocyanine and fullerene excited states. These excited states decay mainly to the ground state and the respective triplet states. The fullerene triplet then transfers its energy to form the phthalocyanine triplet. About 20% of the phthalocyanine excited states lead to formation of a charge-separated state. In benzonitrile, the same decay pathways are observed, but photoinduced electron transfer is much faster, and generates the charge separated state with a quantum yield of ≥85%. The charge separated state has a lifetime of 2.8 ns in toluene and 94 ps in benzonitrile.

We describe and evaluate a method for computationally predicting reduction potentials of a diverse group of organic molecules by linearly correlating calculated lowest unoccupied molecular orbital energies with ground state reduction potentials measured in acetonitrile. The approach is shown to provide a unique combination of extreme computational simplicity and excellent accuracy across a diverse range of organic structures and a wide window of reduction potentials. A disparate set of molecules (74 compounds belonging to six distinct structural families, comprised of molecules containing C, H, N, O, F, Cl, and Br, with functional groups including esters, ketones, halides, nitriles, quinones, alkenes, arenes, heteroarenes, and pyridinium and higher benzologs, all containing conjugated pi systems, spanning a 3.5-V range of reduction potentials) was used to build the correlations. This methodology was found to be computationally inexpensive compared with other approaches and to permit the useful prediction of reduction potentials of additional molecules of diverse structural types not included in the families used to determine the correlation parameters. The effects of varying the basis set used in the B3LYP electronic structure calculations and including solvent (compared with calculations in gas phase) were also examined. It was found that the inclusion of a continuum solvent model in the calculations was required for accurate results, particularly when including cationic species in the correlations (although when only neutral molecules were examined, reasonable results could even be obtained in vacuo). Several of the best correlations were used to predict the reduction potentials of seven much larger and structurally diverse chromophores that were not included in the correlation data set. Strong correlations (r 2 values>0.99) with very good predictive abilities (root mean square deviation

A new sensitizer motif for dye sensitized solar cells (DSSC) has been developed. A heteroaromatic moiety containing a pyrazine ring links two porphyrin chromophores to the metal oxide surface via two carboxylic acid attachment groups. A test DSSC sensitized with the new molecule was 3.5 times more efficient than a similar cell sensitized by a single porphyrin model compound. The open circuit photovoltage was increased by a modest factor of 1.3, but the photocurrent increased by a factor of 2.7. Most of the increase is attributed to a reduced rate of charge recombination of the charge separated state formed by photoinduced electron transfer from the excited sensitizer to the TiO2, although some of the difference is due to increased light absorption resulting from more dye on the photoanode. Increased light absorption due to the pyrazine-containing group may also play a role. The design illustrated here could also be used to link complementary sensitizers or antenna moieties in order to increase spectral coverage.

We present a direct comparison of two-photon spectra of various carotenoid-tetrapyrrole dyads and phthalocyanines (Pc) as well as chlorophylls (Chl) in the spectral range between 950 and 1360 nm, corresponding to one-photon spectra between 475 and 680 nm. For carotenoids (Car) with 8, 9, or 10 conjugated double bonds, the two-photon absorption cross section of states below the optical allowed carotenoid S2 is at least about 3-10 times higher than that of Pc or chlorophyll a and b at 550/1100 nm. A quantitative comparison of spectra from Pc with and without carotenoids of eight and nine conjugated double bonds confirms energy transfer from optically forbidden carotenoid states to Pc in these dyads. When considering that less than 100% efficient energy transfer reduces the two-photon contribution of the carotenoids in the spectra, the actual Car two-photon cross sections relative to Chl/Pc are even higher than a factor of 3-10. In addition, strong spectroscopic two-photon signatures at energies below the optical allowed carotenoid S2 state support the presence of additional optical forbidden carotenoid states such as S*, Sx, or, alternatively, contributions from higher vibronic or hot S1 states dominating two-photon spectra or energy transfer from the carotenoids. The onset of these states is shifted about 1500-3500 cm-1 to lower energies in comparison to the S2 states. Our data provides evidence that two-photon excitation of the carotenoid S*, Sx, or hot S1 states results in energy transfer to tetrapyrroles or chlorophylls similar to that observed with the Car S1 two-photon excitation.

International audience; The association of different metals in stable, well-defined molecular assemblies remains a great challenge of supramolecular chemistry. In such constructs, the emergence of synergism, or cooperative effects between the different metal centers is particularly intriguing. These effects can lead to uncommon reactivity or remarkable physico-chemical properties that are not otherwise achievable. For example, the association of alkaline or alkaline-earth cations and transition metals is pivotal for the activity of several biomolecules and human-made catalysts that carry out fundamental redox transformations (water oxidation, nitrogen reduction, water-gas shift reaction, etc.). In many cases the precise nature of the interactions between the alkaline-earth cations and the redox-active transition metals remains elusive due to the difficulty of building stable molecular heterometallic assemblies that associate transition metals and alkaline or alkaline-earth cations in a controlled way. In this work we present the rational design of porphyrin-based ligands possessing a second binding site for alkaline-earth cations above the porphyrin macrocycle primary complexation site. We demonstrate that by using a combination of crown ether and carboxylic acid substituents suitably positioned on the periphery of the porphyrin, bitopic ligands can be obtained. The binding of calcium, a typical alkaline-earth cation, by the newly prepared ligands has been studied in detail and we show that a moderately large binding constant can be achieved in protic media using ligands that possess some degree of structural flexibility. The formation of Zn-Ca assemblies discussed in this work is viewed as a stepping stone towards the assembly of well defined molecular transition metal-alkaline earth bimetallic centers using a versatile organic scaffold.

Photosynthesis, a process catalysed by plants, algae and cyanobacteria converts sunlight to energy thus sustaining all higher life on Earth. Two large membrane protein complexes, photosystem I and II (PSI and PSII), act in series to catalyse the light-driven reactions in photosynthesis. PSII catalyses the light-driven water splitting process, which maintains the Earth’s oxygenic atmosphere1. In this process, the oxygen-evolving complex (OEC) of PSII cycles through five states, S0 to S4, in which four electrons are sequentially extracted from the OEC in four light-driven charge-separation events. Here we describe time resolved experiments on PSII nano/microcrystals from Thermosynechococcus elongatus performed with the recently developed2 technique of serial femtosecond crystallography. Structures have been determined from PSII in the dark S1 state and after double laser excitation (putative S3 state) at 5 and 5.5 Å resolution, respectively. The results provide evidence that PSII undergoes significant conformational changes at the electron acceptor side and at the Mn4CaO5 core of the OEC. These include an elongation of the metal cluster, accompanied by changes in the protein environment, which could allow for binding of the second substrate water molecule between the more distant protruding Mn (referred to as the ‘dangler’ Mn) and the Mn3CaOx cubane in the S2 to S3 transition, as predicted by spectroscopic and computational studies3, 4. This work shows the great potential for time-resolved serial femtosecond crystallography for investigation of catalytic processes in biomolecules.

The effect of derivatization and temperature on the photoluminescence of 1–2 nm size silicon particles of different origin is investigated in an attempt to understand the effect of surface on the particles’ photoluminescence. To this purpose, silicon nanoparticles were synthesized by electrochemical (top-down) and wet chemical (bottom-up) procedures. Further derivatization by silylation or sylanization yielded particles with ≡Si—C≡, ≡Si—O—Si≡, and ≡Si—O—C≡ groups at the interface. A detailed analysis of the corresponding excitation–emission matrices strongly indicates that different surface atomic arrangements contribute to the energy gap. In particular, particles with ≡Si—O—Si≡ groups at the interface show photoluminescence independent of the crystalline structure and on their further surface derivatization with different organic molecules. The lifetime and spectrum shape of all synthesized particles are invariable to changes in temperature in the range 270–330 K despite a significant reduction in the photoluminescence intensity being observed with increasing temperature; such behavior supports a thermal equilibrium between dark and bright conformations of the particles. The observed results are of importance for optimizing the use of silicon nanoparticles as optical sensors and therapeutic agents in biological systems. Fil: Romero, Juan José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina Fil: Llansola Portolés, Manuel Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina Fil: Dell'Arciprete, Maria Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina Fil: Rodriguez, Hernan Bernardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina Fil: Moore, Ana L. . Arizona State University; Estados Unidos Fil: Gonzalez, Monica Cristina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina

An efficient route to meso-β doubly connected fused porphyrin dimers was developed. Synthesis of the dimers incorporated two successive C–C bond-forming steps selectively coupling unsubstituted meso- and β-positions. Using Cu(BF4)2 as an oxidant in nitromethane solvent, the radical coupling of Cu(II) -porphyrins occurred in high yield and without side-products, allowing chromatography-free purification. Efficient demetalation of the product yielded free-base derivatives and the possibility to incorporate other metals into the macrocycles. The absorption and electrochemical properties vary with the inserted metal, showing broad UV-visible-NIR absorption and multiple one-electron oxidations/reductions in a relatively narrow electrochemical window.

We report on the photosensitization of titanium dioxide nanoparticles (TiO2 NPs) synthesized inside AOT (bis(2-ethylhexyl) sulfosuccinate sodium salt) reverse micelles following photoexcitation of perylene derivatives with dicarboxylate anchoring groups. The dyes, 1,7-dibromoperylene-3,4,9,10-tetracarboxy dianhydride (1), 1,7-dipyrrolidinylperylene-3,4,9,10-tetracarboxy dianhydride (2), and 1,7-bis(4-tert-butylphenyloxy)perylene-3,4,9,10-tetracarboxy dianhydride (3), have considerably different driving forces for photoinduced electron injection into the TiO2 conduction band, as estimated by electrochemical measurements and quantum mechanical calculations. Fluorescence anisotropy measurements indicate that dyes 1 and 2 are preferentially solubilized in the micellar structure, creating a relatively large local concentration that favors the attachment of the dye to the TiO2 surface. The binding process was followed by monitoring the hypsochromic shift of the dye absorption spectra over time for 1 and 2. Photoinduced electron transfer from the singlet excited state of 1 and 2 to the TiO2 conduction band (CB) is indicated by emission quenching of the TiO2-bound form of the dyes and confirmed by transient absorption measurements of the radical cation of the dyes and free carriers (injected electrons) in the TiO2 semiconductor. Steady state and transient spectroscopy indicate that dye 3 does not bind to the TiO2 NPs and does not photosensitize the semiconductor. This observation was rationalized as a consequence of the bulky t-butylphenyloxy groups which create a strong steric impediment for deep access of the dye within the micelle structure to reach the semiconductor oxide surface. Fil: Hernández, Laura. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Godin, Robert. McGill University; Canadá Fil: Bergkamp, Jesse J.. Arizona State University; Estados Unidos Fil: Llansola Portolés, Manuel Jose. Arizona State University; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Sherman, Benjamin D.. Arizona State University; Estados Unidos Fil: Tomlin, John. Arizona State University; Estados Unidos Fil: Kodis, Gerdenis. Arizona State University; Estados Unidos Fil: Méndez Hernández, Dalvin D.. Arizona State University; Estados Unidos Fil: Bertolotti, Sonia Graciela. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Chesta, Carlos Alberto. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Mariño Ochoa, Ernesto. Tecnológico de Monterrey; México Fil: Moore, Ana L.. Arizona State University; Estados Unidos Fil: Moore, Thomas A.. Arizona State University; Estados Unidos Fil: Cosa, Gonzalo. McGill University; Canadá Fil: Palacios, Rodrigo Emiliano. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Charge transport within films of several new types of electropolymerized porphyrin and porphyrin-fullerene dyad polymers was studied in order to obtain information on the suitability of these organic semiconductors for applications in solar energy conversion, sensor devices, etc. The films, prepared by electropolymerization on a conductive substrate, were immersed in acetonitrile and studied using chronocoulometric and cyclic voltammetric electrochemical methods. The charge diffusion coefficients were found to be dependent upon the electrolytic medium. Electrolyte anion size plays a significant role in determining the rate of migration of charge through the polymers, demonstrating that migration of positive charge is accompanied by migration of negative counterions. Bulkier anions markedly decrease the charge diffusion coefficient. This strong dependence suggests that anion mobility is the rate-limiting process for diffusional charge transport within the porphyrin polymer films and that the largest rates obtained are lower limits to the intrinsic cation mobility. With electrolytes containing the relatively small perchlorate anion, charge diffusion coefficients of the porphyrin polymers were similar to those reported for polyaniline under acidic conditions. The charge diffusion coefficient for a zinc porphyrin polymer was found to decrease 2 orders of magnitude in the presence of pyridine, suggesting that metal-containing porphyrins polymer films may have sensor applications. Cation (hole) mobilities previously reported in the literature for porphyrin-containing polymers with chemical structures quite different from those investigated here were much smaller than those found for the polymers in this study, but further investigation suggests that the differences are due to choice of electrode size and material.

Photoelectrochemical water splitting directly converts solar energy to chemical energy stored in hydrogen, a high energy density fuel. Although water splitting using semiconductor photoelectrodes has been studied for more than 40 years, it has only recently been demonstrated using dye-sensitized electrodes. The quantum yield for water splitting in these dye-based systems has, so far, been very low because the charge recombination reaction is faster than the catalytic four-electron oxidation of water to oxygen. We show here that the quantum yield is more than doubled by incorporating an electron transfer mediator that is mimetic of the tyrosine-histidine mediator in Photosystem II. The mediator molecule is covalently bound to the water oxidation catalyst, a colloidal iridium oxide particle, and is coadsorbed onto a porous titanium dioxide electrode with a Ruthenium polypyridyl sensitizer. As in the natural photosynthetic system, this molecule mediates electron transfer between a relatively slow metal oxide catalyst that oxidizes water on the millisecond timescale and a dye molecule that is oxidized in a fast light-induced electron transfer reaction. The presence of the mediator molecule in the system results in photoelectrochemical water splitting with an internal quantum efficiency of approximately 2.3% using blue light.

Artificial photosynthesis comprises the design of systems for converting solar energy into useful forms based on the fundamental science underlying natural photosynthesis. There are many approaches to this problem. In this report, the emphasis is on molecule-based systems for photochemical production of fuels using sunlight. A few examples of typical components of artificial photosynthetic systems including antennas, reaction centres, catalysts for fuel production and water oxidation, and units for photoprotection and photoregulation are presented in order to illustrate the current state of the field and point out challenges yet to be fully addressed.

Photochromes are chromophores that are reversibly isomerized between two metastable forms using light, or light and heat. When photochromes are covalently linked to other chromophores, they can act as molecular photonic analogues of electronic transistors. As bistable switches, they can be incorporated into the design of molecules capable of binary arithmetic and both combinatorial and sequential digital logic operations. Small ensembles of such molecules can perform analogue signal modulation similar to that carried out by transistor amplifiers. Examples of molecules that perform multiple logic functions, act as control elements for fluorescent reporters, and mimic natural photoregulatory functions are presented.

Hydrogenases catalyze the interconversion of protons and hydrogen according to the reversible reaction: 2H(+) + 2e(-) ⇆ H(2) while using only the earth-abundant metals nickel and/or iron for catalysis. Due to their high activity for proton reduction and the technological significance of the H(+)/H(2) half reaction, it is important to characterize the catalytic activity of [FeFe]-hydrogenases using both biochemical and electrochemical techniques. Following a detailed electrochemical and photoelectrochemical study of an [FeFe]-hydrogenase from Clostridium acetobutylicum (CaHydA), we now report electrochemical and single-molecule imaging studies carried out on a catalytically active hydrogenase preparation. The enzyme CaHydA, a homologue (70% identity) of the [FeFe]-hydrogenase from Clostridium pasteurianum , CpI, was adsorbed to a negatively charged, self-assembled monolayer (SAM) for investigation by electrochemical scanning tunneling microscopy (EC-STM) techniques and macroscopic electrochemical measurements. The EC-STM imaging revealed uniform surface coverage with sufficient stability to undergo repeated scanning with a STM tip as well as other electrochemical investigations. Cyclic voltammetry yielded a characteristic cathodic hydrogen production signal when the potential was scanned sufficiently negative. The direct observation of the single enzyme distribution on the Au-SAM surface coupled with macroscopic electrochemical measurements obtained from the same electrode allowed the evaluation of a turnover frequency (TOF) as a function of potential for single [FeFe]-hydrogenase molecules.

A series of axial phenoxy substituted octabutoxy silicon phthalocyanines bearing ethyl carboxylic ester and diethyl phosphonate groups have been prepared from the corresponding phenols in pyridine. Axial bis-hydroxy silicon phthalocyanine was prepared using an adaptation of a reported protocol [1, 2] from the octabutoxy free-base phthalocyanine. The phenols bear either carboxylic ester or phosphonate groups, which upon deprotection can serve as anchoring groups for attaching the phthalocyanines to semiconducting metal oxides used in dye sensitized solar cells (DSSCs). All the phthalocyanines of the series absorb in the near infra-red region: 758–776 nm. The first oxidation potential for each phenoxy derivative occurs near 0.55 V vs. SCE as measured by cyclic voltammetry, with all falling within a 10 mV range. This indicates that these dyes will have sufficient energy in the photo-excited state to drive the reduction of protons to hydrogen. Taking into account the absorption and electrochemical potentials, these dyes are promising candidates for use in dual-threshold photo-electrochemical cells.

A molecular dyad consisting of a photochromic dihydroindolizine unit covalently linked to a porphyrin performs, when illuminated through a third-harmonic-generating crystal, the functions of both an AND and a NAND Boolean logic gate with shared all-optical inputs. The NAND gate is of particular interest as it is a so-called universal gate, and hence all other digital systems can be implemented by combinations of NAND gates. The functions of the AND and the NAND gates rely on changes in absorption and emission of the dyad in the visible spectral region upon isomerization of the photochromic unit. The change in absorption which forms the basis for the AND gate function is ascribed to the colorization/decolorization of the photochrome itself in response to the optical inputs. The variation in emission intensity which constitutes the NAND gate function is a result of the changes in redox properties of the photochrome that follow upon isomerization, such that only one of the two isomers is competent to quench the porphyrin emission by electron transfer.

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.

Colloidal solutions of iridium oxide hydrate (IrO2·nH2O) were formed using porphyrin stabilizers bearing malonate-like functional groups at each of the four meso positions of the porphyrin ring. Cyclic voltammetry and monitoring of solution oxygen concentrations under constant applied potential demonstrated the electrochemical catalytic activity of the porphyrin–IrO2·nH2O complexes for the oxidation of water to oxygen. Quenching of the porphyrin fluorescence in the complex implies strong interaction between the porphyrin and the IrO2·nH2O. These results mark a step toward developing a porphyrin-based photoanode for use in a photoelectrochemical water-splitting cell.