Managing the Redox Potential of PCET in Grotthuss-Type Proton Wires.

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.

Concerted Electron-Nuclear Motion in Proton-Coupled Electron Transfer-Driven Grotthuss-Type Proton Translocation.

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.

Electrochemically Driven Photosynthetic Electron Transport in Cyanobacteria Lacking Photosystem II.

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 Synechocystis (ΔpsbB) cells lacking PSII. MEPS-generated, light-dependent current increased with light intensity up to 2050 µmol photons m-2 s-1, which yielded a delivery rate of 113 µmol electrons h-1 mg-chl-1 and an average current density of 150 A m-2 s-1 mg-chl-1. P700+ re-reduction kinetics demonstrated that initial rates exceeded wildtype PSII-driven electron delivery. The electron delivery occurs ahead of the cytochrome b6f complex to enable both NADPH and ATP production. This work demonstrates an electrochemical system that can drive photosynthetic electron transport, provides a platform for photosynthetic foundational studies, and has the potential for improving photosynthetic performance at high light intensities.

Dual Singlet Excited-State Quenching Mechanisms in an Artificial Caroteno-Phthalocyanine Light Harvesting Antenna.

Under excess illumination, photosystem II of plants dissipates excess energy through the quenching of chlorophyll fluorescence in the light harvesting antenna. Various models involving chlorophyll quenching by carotenoids have been proposed, including (i) direct energy transfer from chlorophyll to the low-lying optically forbidden carotenoid S1 state, (ii) formation of a collective quenched chlorophyll-carotenoid S1 excitonic state, (iii) chlorophyll-carotenoid charge separation and recombination, and (iv) chlorophyll-chlorophyll charge separation and recombination. In previous work, the first three processes were mimicked in model systems: in a Zn-phthalocyanine-carotenoid dyad with an amide linker, direct energy transfer was observed by femtosecond transient absorption spectroscopy, whereas in a Zn-phthalocyanine-carotenoid dyad with an amine linker excitonic quenching was demonstrated. Here, we present a transient absorption spectroscopic study on a Zn-phthalocyanine-carotenoid dyad with a phenylene linker. We observe that two quenching phases of the phthalocyanine excited state exist at 77 and 213 ps in addition to an unquenched phase at 2.7 ns. Within our instrument response of ∼100 fs, carotenoid S1 features rise which point at an excitonic quenching mechanism. Strikingly, we observe an additional rise of carotenoid S1 features at 3.6 ps, which shows that a direct energy transfer mechanism in an inverted kinetics regime is also in effect. We assign the 77 ps decay component to excitonic quenching and the 3.6 ps/213 ps rise and decay components to direct energy transfer. Our results indicate that dual quenching mechanisms may be active in the same molecular system, in addition to an unquenched fraction. Computational chemistry results indicate the presence of multiple conformers where one of the dihedral angles of the phenylene linker assumes distinct values. We propose that the parallel quenching pathways and the unquenched fraction result from such conformational subpopulations. Our results suggest that it is possible to switch between different regimes of quenching and nonquenching through a conformational change on the same molecule, offering insights into potential mechanisms used in biological photosynthesis to adapt to light intensity changes on fast time scales.

Tuning the redox potential of tyrosine-histidine bioinspired assemblies.

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.

Multi PCET in symmetrically substituted benzimidazoles.

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.

PCET-Based Ligand Limits Charge Recombination with an Ir(III) Photoredox Catalyst.

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.

Electron-Nuclear Dynamics Accompanying Proton-Coupled Electron Transfer.

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.

Role of Intact Hydrogen-Bond Networks in Multiproton-Coupled Electron Transfer.

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 (∼1000 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 (<1000 mV s-1) oxidation of the phenol results in an overall chemically irreversible two-proton-coupled electron-transfer process in which the second proton-transfer step yields the pyridinium cation detected by infrared spectroelectrochemistry. In this case, we postulate an initial intramolecular proton-coupled electron-transfer step yielding the E1PT product followed by a slow, likely intermolecular chemical step involving a second proton transfer to give the E2PT product. Insights into the electrochemical behavior of these systems are provided by theoretical calculations of the electrostatic potentials and electric fields at the site of the transferring protons for the forward and reverse processes. This work addresses a fundamental design principle for constructing molecular wires where protons are translocated over varied distances by a Grotthuss-type mechanism.

HYSCORE and DFT Studies of Proton-Coupled Electron Transfer in a Bioinspired Artificial Photosynthetic Reaction Center.

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.

Ultrafast Dynamics of Nonrigid Zinc-Porphyrin Arrays Mimicking the Photosynthetic "Special Pair".

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.

One Electron Multiple Proton Transfer in Model Organic Donor-Acceptor Systems: Implications for High Frequency EPR.

EPR spectroscopy is an important spectroscopic method for identification and characterization of radical species involved in many biological reactions. The tyrosyl radical is one of the most studied amino acid radical intermediates in biology. Often in conjunction with histidine residues, it is involved in many fundamental biological electron and proton transfer processes, such as in the water oxidation in photosystem II. As biological processes are typically extremely complicated and hard to control, molecular bio-mimetic model complexes are often used to clarify the mechanisms of the biological reactions. Here we present theoretical calculations to investigate the sensitivity of magnetic resonance parameters to proton-coupled electron transfer events, as well as conformational substates of the molecular constructs which mimic the tyrosine-histidine (Tyr-His) pairs found in a large variety of proteins. Upon oxidation of the phenol, the Tyr analogue, these complexes can perform not only one-electron one-proton transfer (EPT), but also one-electron two-proton transfers (E2PT). It is shown that in aprotic environment the gX-components of the electronic g-tensor are extremely sensitive to the first proton transfer from the phenoxyl oxygen to the imidazole nitrogen (EPT product), leading to a significant increase of the gX-value of up to 0.003, but are not sensitive to the second proton transfer (E2PT product). In the latter case the change of the gX-value is much smaller (ca. 0.0001), which is too small to be distinguished even by high frequency EPR. The 14N hyperfine values are also too similar to allow differentiation between the different protonation states in EPT and E2PT. The magnetic resonance parameters were also calculated as a function of the rotation angles around single bonds. It was demonstrated that rotation of the phenoxyl group results in large positive changes (>0.001) in the gX-values. Analysis of the data reveals that the main source of these changes is related to the strength of the H-bond between phenoxyl oxygen and the proton(s) on N1 and N2 positions of the imidazole.

Proton-coupled electron transfer across benzimidazole bridges in bioinspired proton wires.

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 pK a 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.

Proton-Coupled Electron Transfer Drives Long-Range Proton Translocation in Bioinspired Systems.

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 Šin 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.

Electronic Structure and Triplet-Triplet Energy Transfer in Artificial Photosynthetic Antennas.

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.

Controlling Proton-Coupled Electron Transfer in Bioinspired Artificial Photosynthetic Relays.

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 1 H-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 p Ka 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.

Proton-Coupled Electron Transfer in Artificial Photosynthetic Systems.

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 pKa units; thus, electrochemical oxidation of the phenol is associated with a proton transfer to the imidazole. This is an example of a PCET process involving transfer of one electron and one proton, known as electron-proton transfer (EPT). When the benzimidazole moiety of BIP is substituted at the 4-position with good proton acceptor groups such as aliphatic amines, experimental and theoretical results indicate that two proton transfers occur upon one-electron oxidation of the phenol. This phenomenon is described as a one-electron-two-proton transfer (E2PT) process and results in translocation of protons over ∼7 Švia a Grotthuss-type mechanism, where the protons traverse a network of internally H-bonded sites. In the case of the E2TP process involving BIP analogues with amino group substituents, the thermodynamic price paid in redox potential to move a proton to the final proton acceptor is ∼300 mV. In this example, the decrease in redox potential limits the oxidizing power of the resulting phenoxyl radical. Thus, unlike the biological counterpart, the artificial construct is thermodynamically incapable of effectively advancing the redox state of a water oxidation catalyst. The design of systems where multiple proton transfer events are coupled to an oxidation reaction while a relatively high redox potential is maintained remains an outstanding challenge. The ability to control proton transfer and activity at defined distances and times is key to achieving proton management in the vicinity of catalysts operating at low overpotential in myriad biochemically important processes. Artificial photosynthetic constructs with well-defined structures, such as the ones described in this Account, can provide the means for discovering design principles upon which efficient redox catalysts for electrolysis and fuel cells can be based.

Two-Photon Spectra of Chlorophylls and Carotenoid-Tetrapyrrole Dyads.

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.

Concerted One-Electron Two-Proton Transfer Processes in Models Inspired by the Tyr-His Couple of Photosystem II.

Nature employs a TyrZ-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays composed of different benzimidazole-phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the TyrZ-His pair have been prepared. When the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electrochemical oxidation of the phenol will take place. Also, theory predicts a decrease in the redox potential of the phenol by ∼300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Notably, these results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.

Understanding iridium oxide nanoparticle surface sites by their interaction with catechol.

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.