Light-harvesting Chromophore Research Articles

Near-atomic-resolution structure of J-aggregated helical light-harvesting nanotubes.

Cryo-electron microscopy has delivered a resolution revolution for biological self-assemblies, yet only a handful of structures have been solved for synthetic supramolecular materials. Particularly for chromophore supramolecular aggregates, high-resolution structures are necessary for understanding and modulating the long-range excitonic coupling. Here, we present a 3.3 Å structure of prototypical biomimetic light-harvesting nanotubes derived from an amphiphilic cyanine dye (C8S3-Cl). Helical 3D reconstruction directly visualizes the chromophore packing that controls the excitonic properties. Our structure clearly shows a brick layer arrangement, revising the previously hypothesized herringbone arrangement. Furthermore, we identify a new non-biological supramolecular motif-interlocking sulfonates-that may be responsible for the slip-stacked packing and J-aggregate nature of the light-harvesting nanotubes. This work shows how independently obtained native-state structures complement photophysical measurements and will enable accurate understanding of (excitonic) structure-function properties, informing materials design for light-harvesting chromophore aggregates.

Functional Coupling of Biohybrid Photosynthetic Antennae and Reaction Center Complexes: Quantitative Comparison with Native Antennae.

Light-harvesting (LH) complexes in photosynthetic organisms absorb photons within limited wavelength ranges over a broad solar spectrum. Extension of the LH wavelength has been realized by attaching artificial fluorophores to LH complexes (biohybrid LH complexes) for complementing the limited-wavelength regions. However, how efficiently such fluorophores in biohybrid LH complexes function to drive the photocatalytic reaction center (RC) has not been quantitatively evaluated, specifically in comparison with native LH antenna complexes. In this study, we prepared various biohybrid LH1-RC complexes (from Rhodopseudomonas palustris), to quantitatively evaluate the LH activity of the attached external chromophores through a photocurrent generation reaction by LH1-RC on an electrode. For a direct comparison of the LH activity among the LH chromophores that were examined, we introduced the k1 term, which represents the extent of the functional coupling of LH and the photochemical reactions in the RC. We determined that the hydrophobic fluorophore ATTO647N attached to LH1 possesses the highest LH activity among the examined hydrophilic fluorophores such as Alexa647, and its activity is comparable to that of native LH1(-RC). The LH activity of LH2 (from Rhodoblastus acidophilus strain 10050) and its biohybrid LH2s were examined for the comprehensive assessment of their LH activity.

Preparation of Xanthene-TEMPO Dyads: Synthesis and Study of the Radical Enhanced Intersystem Crossing.

We prepared a rhodamine-TEMPO chromophore-radical dyad (RB-TEMPO) to study the radical enhanced intersystem crossing (REISC). The visible light-harvesting chromophore rhodamine is connected with the TEMPO (a nitroxide radical) via a C-N bond. The UV-vis absorption spectrum indicates negligible electron interaction between the two units at the ground state. Interestingly, the fluorescence of the rhodamine moiety is strongly quenched in RB-TEMPO, and the fluorescence lifetime of the rhodamine moiety is shortened to 0.29 ns, from the lifetime of 3.17 ns. We attribute this quenching effect to the intramolecular electron spin-spin interaction between the nitroxide radical and the photoexcited rhodamine chromophore. Nanosecond transient absorption spectra confirm the REISC in RB-TEMPO, indicated by the detection of the rhodamine chromophore triplet excited state; the lifetime was determined as 128 ns, which is shorter than the native rhodamine triplet state lifetime (0.58 μs). The zero-field splitting (ZFS) parameters of the triplet state of the chromophore were determined with the pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra. RB-TEMPO was used as a photoinitiator for the photopolymerization of pentaerythritol triacrylate (PETA). These studies are useful for the design of heavy atom-free triplet photosensitizers, the study of the ISC, and the electron spin dynamics of the radical-chromophore systems upon photoexcitation.

Enhanced chemodynamic and photoluminescence efficiencies of Fe-O4 coordinated carbon dots via the core-shell synergistic effect.

In natural systems like photosynthetic organisms and photo-active enzymes, the spatial organization of chromophores is critical for efficient light harvesting and bio-catalysis. Inspired by nature, a novel modular nanoplatform with both biological imaging and therapeutic functions is constructed by taking advantage of the intrinsic core-shell structure of Fe-decorated carbon dots. Light-harvesting chromophores with deep-red photoluminescence are densely packed into the carbon core. Simultaneously, the atomically dispersed Fe3+ catalytic sites accounting for efficient conversion of H2O2 to ˙OH are discretely distributed on the shell. Precise control over their spatial distribution leads to the elegant integration and exciting interplay of the functional moieties. On the one hand, incorporating a catalysis shell enhances the emission of chromophores via synergistic shielding and rigidifying effects. On the other hand, visible light excitation of the chromophores significantly increases the catalytic activity and cytotoxicity against cancer cells, ascribed to the promotion of the charge transfer process. This nanoplatform exhibits excellent biocompatibility, bright red fluorescence, and light-regulated cytotoxicity for anti-cancer treatment, promising its applications in smart nanocatalytic medicines and efficient chemodynamic therapy.

Synthesis and Photoluminescence Properties of Alkylbromoporphyrin

<p>The usage of porphyrin as a light-harvesting chromophore is considered as one of the keys to obtaining low-cost and high-efficiency dye-sensitized solar cell (DSSC). In this paper, a novel porphyrin, 5,10,15-tris(nitrophenyl)-20-(p-(11-bromo)dodecoxyphenyl))porphyrin, having a long alkyl chain and three nitro groups was synthesized. The nitro groups serve as anchoring groups to TiO<sub>2</sub> surfaces and long alkyl chain prevents unwanted dye aggregation. The porphyrin was synthesized by condensation of <em>p</em>-(12-Bromododecoxy)benzaldehyde and pyrrole in propionic acid according to an adaptation of the general Rothemund method <a href="https://doi.org/10.1021/ja01265a096">[1]</a>. <em>p</em>-(12-Bromododecoxy)benzaldehyde was synthesized by nucleophilic substitution reaction between 4-hydroxybenzaldehyde and 1,12-dibromododecane in acetone. The reaction products were analyzed by <sup>1</sup>H-NMR and mass spectroscopy. The absorption and fluorescence spectra of the porphyrin were also recorded. As results, the absorption spectrum of the porphyrin consists of a strong Soret and four weak Q-band. Compared to 5,10,15-tris(nitrophenyl)-20-(p-(11-bromo)dodecoxyphenyl))porphyrin spectrum, there is no wavelength shifting because of the incorporation of the alkyl chain. The fluorescence spectrum of the porphyrin shows two characteristic emission bands and the intensity ratio of those emission bands is always constant when irradiated by different excitation wavelength related to Soret and Q-band.</p>

Key interactions with deazariboflavin cofactor for light-driven energy transfer in Xenopus (6\u20134) photolyase

Photolyases are flavoenzymes responsible for light-driven repair of carcinogenic crosslinks formed in DNA by UV exposure. They possess two non-covalently bound chromophores: flavin adenine dinucleotide (FAD) as a catalytic center and an auxiliary antenna chromophore that harvests photons and transfers solar energy to the catalytic center. Although the energy transfer reaction has been characterized by time-resolved spectroscopy, it is strikingly important to understand how well natural biological systems organize the chromophores for the efficient energy transfer. Here, we comprehensively characterized the binding of 8-hydroxy-7,8-didemethyl-5-deazariboflavin (8-HDF) to Xenopus (6–4) photolyase. In silico simulations indicated that a hydrophobic amino acid residue located at the entrance of the binding site dominates translocation of a loop upon binding of 8-HDF, and a mutation of this residue caused dysfunction of the efficient energy transfer in the DNA repair reaction. Mutational analyses of the protein combined with modification of the chromophore suggested that Coulombic interactions between positively charged residues in the protein and the phenoxide moiety in 8-HDF play a key role in accommodation of 8-HDF in the proper direction. This study provides a clear evidence that Xenopus (6–4) photolyase can utilize 8-HDF as the light-harvesting chromophore. The obtained new insights into binding of the natural antenna molecule will be helpful for the development of artificial light-harvesting chromophores and future characterization of the energy transfer in (6–4) photolyase by spectroscopic studies.

Engineering couplings for exciton transport using synthetic DNA scaffolds

Summary Control over excitons enables electronic energy to be harnessed and transported for light harvesting and molecular electronics. Such control requires nanoscale precision over the molecular components. Natural light-harvesting systems achieve this precision through sophisticated protein machinery, which is challenging to replicate synthetically. Here, we introduce a DNA-based platform that spatially organizes cyanine chromophores to construct tunable excitonic systems. We synthesized DNA-chromophore nanostructures and characterized them with ensemble ultrafast and single-molecule spectroscopy and structure-based modeling. This synthetic approach facilitated independent control over the coupling among the chromophores and between the chromophores and the environment. We demonstrated that the coupling between the chromophores and the environment could enhance exciton transport efficiency, highlighting the key role of the environment in driving exciton dynamics. Control over excitons, as reported here, offers a path toward the development of designer nanophotonic devices.

Enhanced DNA repair by DNA photolyase bearing an artificial light-harvesting chromophore.

Photolyases are flavoenzymes responsible for the repair of carcinogenic DNA damage caused by ultraviolet radiation. They harbor the catalytic cofactor flavin adenine dinucleotide (FAD). The light-driven electron transfer from the excited state of the fully-reduced form of FAD to the DNA lesions causes rearrangement of the covalent bonds, leading to the restoration of intact nucleobases. In addition to the catalytic chromophore, some photolyases bear a secondary chromophore with better light absorption capability than FAD, acting as a light-harvesting chromophore that harvests photons in sunlight efficiently and transfers light energy to the catalytic center, as observed in natural photoreceptor proteins. Inspired by nature, we covalently and site-specifically attached a synthetic chromophore to the surface of photolyase using oligonucleotides containing a modified nucleoside and a cyclobutane-type DNA lesion, and successfully enhanced its enzymatic activity in the light-driven DNA repair. Peptide mapping in combination with theoretical calculations identified the amino acid residue that binds to the chromophore, working as an artificial light-harvesting chromophore. Our results broaden the strategies for protein engineering and provide a guideline for tuning of the light perception abilities and enzymatic activity of the photoreceptor proteins.

Long‐Lived Triplet Excited State Accessed with Spin–Orbit Charge Transfer Intersystem Crossing in Red Light‐Absorbing Phenoxazine‐Styryl BODIPY Electron Donor/Acceptor Dyads

Orthogonal phenoxazine‐styryl BODIPY compact electron donor/acceptor dyads were prepared as heavy atom‐free triplet photosensitizers (PSs) with strong red light absorption (ϵ=1.33×105 M−1 cm−1 at 630 nm), whereas the previously reported triplet photosensitizers based on the spin‐orbit charge transfer intersystem crossing (SOCT‐ISC) mechanism show absorption in a shorter wavelength range (<500 nm). More importantly, a long‐lived triplet state (τT=333 μs) was observed for the new dyads. In comparison, the triplet state lifetime of the same chromophore accessed with the conventional heavy atom effect (HAE) is much shorter (τT=1.8 μs). Long triplet state lifetime is beneficial to enhance electron or energy transfer, the primary photophysical processes in the application of triplet PSs. Our approach is based on SOCT‐ISC, without invoking of the HAE, which may shorten the triplet state lifetime. We used bisstyrylBodipy both as the electron acceptor and the visible light‐harvesting chromophore, which shows red‐light absorption. Femtosecond transient absorption spectra indicated the charge separation (109 ps) and SOCT‐ISC (charge recombination, CR; 2.3 ns) for BDP‐1. ISC efficiency of BDP‐1 was determined as ΦT=25 % (in toluene). The dyad BDP‐3 was used as triplet PS for triplet‐triplet annihilation upconversion (upconversion quantum yield ΦUC=1.5 %; anti‐Stokes shift is 5900 cm−1).

DNA-organized artificial LHCs - testing the limits of chromophore segmentation.

DNA-organized multi-chromophoric systems containing phenanthrene and pyrene derivatives exhibit a highly efficient excitation energy transfer from phenanthrene (donor) to pyrene (acceptor). The energy transfer also occurs if the phenanthrene antenna is interrupted by intervening DNA base pairs. Artificial light-harvesting complexes composed of up to five phenanthrene-DNA alternations with fluorescence quantum yields as high as 68% are described.

Revealing structural involvement of chromophores in algal light harvesting complexes using symmetry-adapted perturbation theory

The attribution of quantum beats observed in the time-resolved spectroscopy of photosynthetic light-harvesting antennae to nontrivial quantum coherences has sparked a flurry of research activity beginning a decade ago. Even though investigations into the functional aspects of photosynthetic light-harvesting were supported by X-ray crystal structures, the non-covalent interactions between pigments and their local protein environment that drive such function has yet to be comprehensively explored. Using symmetry-adapted perturbation theory (SAPT), we have comprehensively determined the magnitude and compositions of these non-covalent interactions involving light-harvesting chromophores in two quintessential photosynthetic pigment-protein complexes — peridinin chlorophyll-a protein (PCP) from dinoflagellate Amphidinium carterae and phycocyanin 645 (PC645) from cryptophyte Chroomonas mesostigmatica. In PCP, the chlorophylls are dispersion-bound to the peridinins, which in turn are electrostatically anchored to the protein scaffold via their polar terminal rings. This might be an evolutionary design principle in which the relative orientation of the carotenoids towards the aqueous environment determines the arrangement of the other chromophores in carotenoid-based antennas. On the other hand, electrostatics dominate the non-covalent interactions in PC645. Our ab initio simulations also suggest full protonation of the PC645 chromophores in physiological conditions, and that changes to their protonation states result in their participation as switches between folded and unfolded conformations.

Supplementation with Phycocyanobilin, Citrulline, Taurine, and Supranutritional Doses of Folic Acid and Biotin-Potential for Preventing or Slowing the Progression of Diabetic Complications.

Oxidative stress, the resulting uncoupling of endothelial nitric oxide synthase (eNOS), and loss of nitric oxide (NO) bioactivity, are key mediators of the vascular and microvascular complications of diabetes. Much of this oxidative stress arises from up-regulated nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity. Phycocyanobilin (PhyCB), the light-harvesting chromophore in edible cyanobacteria such as spirulina, is a biliverdin derivative that shares the ability of free bilirubin to inhibit certain isoforms of NADPH oxidase. Epidemiological studies reveal that diabetics with relatively elevated serum bilirubin are less likely to develop coronary disease or microvascular complications; this may reflect the ability of bilirubin to ward off these complications via inhibition of NADPH oxidase. Oral PhyCB may likewise have potential in this regard, and has been shown to protect diabetic mice from glomerulosclerosis. With respect to oxidant-mediated uncoupling of eNOS, high-dose folate can help to reverse this by modulating the oxidation status of the eNOS cofactor tetrahydrobiopterin (BH4). Oxidation of BH4 yields dihydrobiopterin (BH2), which competes with BH4 for binding to eNOS and promotes its uncoupling. The reduced intracellular metabolites of folate have versatile oxidant-scavenging activity that can prevent oxidation of BH4; concurrently, these metabolites promote induction of dihydrofolate reductase, which functions to reconvert BH2 to BH4, and hence alleviate the uncoupling of eNOS. The arginine metabolite asymmetric dimethylarginine (ADMA), typically elevated in diabetics, also uncouples eNOS by competitively inhibiting binding of arginine to eNOS; this effect is exacerbated by the increased expression of arginase that accompanies diabetes. These effects can be countered via supplementation with citrulline, which efficiently enhances tissue levels of arginine. With respect to the loss of NO bioactivity that contributes to diabetic complications, high dose biotin has the potential to “pinch hit” for diminished NO by direct activation of soluble guanylate cyclase (sGC). High-dose biotin also may aid glycemic control via modulatory effects on enzyme induction in hepatocytes and pancreatic beta cells. Taurine, which suppresses diabetic complications in rodents, has the potential to reverse the inactivating impact of oxidative stress on sGC by boosting synthesis of hydrogen sulfide. Hence, it is proposed that concurrent administration of PhyCB, citrulline, taurine, and supranutritional doses of folate and biotin may have considerable potential for prevention and control of diabetic complications. Such a regimen could also be complemented with antioxidants such as lipoic acid, N-acetylcysteine, and melatonin—that boost cellular expression of antioxidant enzymes and glutathione—as well as astaxanthin, zinc, and glycine. The development of appropriate functional foods might make it feasible for patients to use complex nutraceutical regimens of the sort suggested here.

Rational Design of Porous Conjugated Polymers and Roles of Residual Palladium for Photocatalytic Hydrogen Production

Developing highly efficient photocatalyts for water splitting is one of the grand challenges in solar energy conversion. Here, we report the rational design and synthesis of porous conjugated polymer (PCP) that photocatalytically generates hydrogen from water splitting. The design mimics natural photosynthetics systems with conjugated polymer component to harvest photons and the transition metal part to facilitate catalytic activities. A series of PCPs have been synthesized with different light harvesting chromophores and transition metal binding bipyridyl (bpy) sites. The photocatalytic activity of these bpy-containing PCPs can be greatly enhanced due to the improved light absorption, better wettability, local ordering structure, and the improved charge separation process. The PCP made of strong and fully conjugated donor chromophore DBD (M4) shows the highest hydrogen production rate at ∼33 μmol/h. The results indicate that copolymerization between a strong electron donor and weak electron acceptor into the same polymer chain is a useful strategy for developing efficient photocatalysts. This study also reveals that the residual palladium in the PCP networks plays a key role for the catalytic performance. The hydrogen generation activity of PCP photocatalyst can be further enhanced to 164 μmol/h with an apparent quantum yield of 1.8% at 350 nm by loading 2 wt % of extra platinum cocatalyst.

Ultrafast transient absorption revisited: Phase-flips, spectral fingers, and other dynamical features.

We rebuild the theory of ultrafast transient-absorption/transmission spectroscopy starting from the optical response of an individual molecule to incident femtosecond pump and probe pulses. The resulting description makes use of pulse propagators and free molecular evolution operators to arrive at compact expressions for the several contributions to a transient-absorption signal. In this alternative description, which is physically equivalent to the conventional response-function formalism, these signal contributions are conveniently expressed as quantum mechanical overlaps between nuclear wave packets that have undergone different sequences of pulse-driven optical transitions and time-evolution on different electronic potential-energy surfaces. Using this setup in application to a simple, multimode model of the light-harvesting chromophores of PC577, we develop wave-packet pictures of certain generic features of ultrafast transient-absorption signals related to the probed-frequency dependence of vibrational quantum beats. These include a Stokes-shifting node at the time-evolving peak emission frequency, antiphasing between vibrational oscillations on opposite sides (i.e., to the red or blue) of this node, and spectral fingering due to vibrational overtones and combinations. Our calculations make a vibrationally abrupt approximation for the incident pump and probe pulses, but properly account for temporal pulse overlap and signal turn-on, rather than neglecting pulse overlap or assuming delta-function excitations, as are sometimes done.

New biosynthetic pathway for pink pigments from uncultured oceanic viruses.

The pink open-chain tetrapyrrole pigment phycoerythrobilin (PEB) is employed by marine cyanobacteria, red algae and cryptophytes as a light-harvesting chromophore in phycobiliproteins. Genes encoding biosynthesis proteins for PEB have also been discovered in cyanophages, viruses that infect cyanobacteria, and mimic host pigment biosynthesis with the exception of PebS which combines the enzymatic activities of two host enzymes. In this study, we have identified novel members of the PEB biosynthetic enzyme families, heme oxygenases and ferredoxin-dependent bilin reductases. Encoding genes were found in metagenomic datasets and could be traced back to bacteriophage but not cyanophage origin. While the heme oxygenase exhibited standard activity, a new bilin reductase with highest homology to the teal pigment producing enzyme PcyA revealed PEB biosynthetic activity. Although PcyX possesses PebS-like activity both enzymes share only 9% sequence identity and likely catalyze the reaction via two independent mechanisms. Our data point towards the presence of phycobilin biosynthetic genes in phages that probably infect alphaproteobacteria and, therefore, further support a role of phycobilins outside oxygenic phototrophs.

Physiology, Biochemistry, and Applications of F420- and Fo-Dependent Redox Reactions.

5-Deazaflavin cofactors enhance the metabolic flexibility of microorganisms by catalyzing a wide range of challenging enzymatic redox reactions. While structurally similar to riboflavin, 5-deazaflavins have distinctive and biologically useful electrochemical and photochemical properties as a result of the substitution of N-5 of the isoalloxazine ring for a carbon. 8-Hydroxy-5-deazaflavin (Fo) appears to be used for a single function: as a light-harvesting chromophore for DNA photolyases across the three domains of life. In contrast, its oligoglutamyl derivative F420 is a taxonomically restricted but functionally versatile cofactor that facilitates many low-potential two-electron redox reactions. It serves as an essential catabolic cofactor in methanogenic, sulfate-reducing, and likely methanotrophic archaea. It also transforms a wide range of exogenous substrates and endogenous metabolites in aerobic actinobacteria, for example mycobacteria and streptomycetes. In this review, we discuss the physiological roles of F420 in microorganisms and the biochemistry of the various oxidoreductases that mediate these roles. Particular focus is placed on the central roles of F420 in methanogenic archaea in processes such as substrate oxidation, C1 pathways, respiration, and oxygen detoxification. We also describe how two F420-dependent oxidoreductase superfamilies mediate many environmentally and medically important reactions in bacteria, including biosynthesis of tetracycline and pyrrolobenzodiazepine antibiotics by streptomycetes, activation of the prodrugs pretomanid and delamanid by Mycobacterium tuberculosis, and degradation of environmental contaminants such as picrate, aflatoxin, and malachite green. The biosynthesis pathways of Fo and F420 are also detailed. We conclude by considering opportunities to exploit deazaflavin-dependent processes in tuberculosis treatment, methane mitigation, bioremediation, and industrial biocatalysis.

The influence of fluoroalkyl chains in redox electrolytes for energy conversion

We discovered an unprecedented fundamental interaction originated from the fluoroalkyl chains at the interface between light harvesting chromophore and charge transport redox electrolyte in dye-sensitized solar cells (DSSCs). A fluoroalkylated Ru complexes dye (FN719) was synthesized and upon anchoring on nanoparticulate TiO2 electrodes, FN719 exhibits 5 times better photostability in humid air than that of the conventional N719 due to the hydrophobicity of the fluoroalkyl chains. Moreover, the lipophobicity of fluoroalkylated chains in organic electrolyte causes a size-selective steric hindrance with respect to the redox couple I−/I3−, leading to a more stalled diffusion of the bulky I3− towards dyes and TiO2 photoanode than that of the lean I−. As such, without sacrificing the forward electron transport and quantum efficiency, the recombination in FN719-based DSSCs is suppressed to be 1.3 times slower than N719-based DSSCs studied by the photovoltage transients and electrochemical impedance spectroscopy; and the adverse I3− exchange with SCN− on dye is also inhibited as manifested by mass spectrometry. The J-V measurements suggest improved efficiency in FN719-based cells (9.2%) than similarly fabricated N719-based cells (8.5%).

Manipulating Excited-State Dynamics of Individual Light-Harvesting Chromophores through Restricted Motions in a Hydrated Nanoscale Protein Cavity.

Manipulating the photophysical properties of light-absorbing units is a crucial element in the design of biomimetic light-harvesting systems. Using a highly tunable synthetic platform combined with transient absorption and time-resolved fluorescence measurements and molecular dynamics simulations, we interrogate isolated chromophores covalently linked to different positions in the interior of the hydrated nanoscale cavity of a supramolecular protein assembly. We find that, following photoexcitation, the time scales over which these chromophores are solvated, undergo conformational rearrangements, and return to the ground state are highly sensitive to their position within this cavity and are significantly slower than in a bulk aqueous solution. Molecular dynamics simulations reveal the hindered translations and rotations of water molecules within the protein cavity with spatial specificity. The results presented herein show that fully hydrated nanoscale protein cavities are a promising way to mimic the tight protein pockets found in natural light-harvesting complexes. We also show that the interplay between protein, solvent, and chromophores can be used to substantially tune the relaxation processes within artificial light-harvesting assemblies in order to significantly improve the yield of interchromophore energy transfer and extend the range of excitation transport. Our observations have implications for other important, similarly sized bioinspired materials, such as nanoreactors and biocompatible targeted delivery agents.

Varying the electronic structure of surface-bound ruthenium(II) polypyridyl complexes.

In the design of light-harvesting chromophores for use in dye-sensitized photoelectrosynthesis cells (DSPECs), surface binding to metal oxides in aqueous solutions is often inhibited by synthetic difficulties. We report here a systematic synthesis approach for preparing a family of Ru(II) polypyridyl complexes of the type [Ru(4,4'-R2-bpy)2(4,4'-(PO3H2)2-bpy)](2+) (4,4'(PO3H2)2-bpy = [2,2'-bipyridine]-4,4'-diylbis(phosphonic acid); 4,4'-R2-bpy = 4,4'-R2-2,2'-bipyridine; and R = OCH3, CH3, H, or Br). In this series, the nature of the 4,4'-R2-bpy ligand is modified through the incorporation of electron-donating (R = OCH3 or CH3) or electron-withdrawing (R = Br) functionalities to tune redox potentials and excited-state energies. Electrochemical measurements show that the ground-state potentials, E(o')(Ru(3+/2+)), vary from 1.08 to 1.45 V (vs NHE) when the complexes are immobilized on TiO2 electrodes in aqueous HClO4 (0.1 M) as a result of increased Ru dπ-π* back-bonding caused by the lowering of the π* orbitals on the 4,4'-R2-bpy ligand. The same ligand variations cause a negligible shift in the metal-to-ligand charge-transfer absorption energies. Emission energies decrease from λmax = 644 to 708 nm across the series. Excited-state redox potentials are derived from single-mode Franck-Condon analyses of room-temperature emission spectra and are discussed in the context of DSPEC applications.

Synthesis and electrocatalytic water oxidation by electrode-bound helical peptide chromophore-catalyst assemblies.

Artificial photosynthesis based on dye-sensitized photoelectrosynthesis cells requires the assembly of a chromophore and catalyst in close proximity on the surface of a transparent, high band gap oxide semiconductor for integrated light absorption and catalysis. While there are a number of approaches to assemble mixtures of chromophores and catalysts on a surface for use in artificial photosynthesis based on dye-sensitized photoelectrosynthesis cells, the synthesis of discrete surface-bound chromophore-catalyst conjugates is a challenging task with few examples to date. Herein, a versatile synthetic approach and electrochemical characterization of a series of oligoproline-based light-harvesting chromophore-water-oxidation catalyst assemblies is described. This approach combines solid-phase peptide synthesis for systematic variation of the backbone, copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) as an orthogonal approach to install the chromophore, and assembly of the water-oxidation catalyst in the final step. Importantly, the catalyst was found to be incompatible with the conditions both for amide bond formation and for the CuAAC reaction. The modular nature of the synthesis with late-stage assembly of the catalyst allows for systematic variation in the spatial arrangement of light-harvesting chromophore and water-oxidation catalyst and the role of intrastrand distance on chromophore-catalyst assembly properties. Controlled potential electrolysis experiments verified that the surface-bound assemblies function as water-oxidation electrocatalysts, and electrochemical kinetics data demonstrate that the assemblies exhibit greater than 10-fold rate enhancements compared to the homogeneous catalyst alone.