Hydrogen-Bond Promoted Intramolecular Electron Transfer to Photogenerated Ru(III): A Functional Mimic of TyrosineZ and Histidine 190 in Photosystem II

Photoinduced Electron Transfer between Dipolar Reactants: Solvent and Excitation Wavelength Effects

Electron transfer (ET) quenching in nonpolar media is not as well understood as in polar environments. Here, we investigate the effect of dipole-dipole interactions between the reactants using ultrafast broadband electronic spectroscopy combined with molecular dynamics simulations. We find that the quenching of the S1 state of two polar dyes, coumarin 152a and Nile red, by the polar N,N-dimethylaniline (DMA) in cyclohexane is faster by a factor up to 3 when exciting on the red edge rather than at the maximum of their S1 ← S0 absorption band. This originates from the inhomogeneous broadening of the band due to a distribution of the number of quencher molecules around the dyes. As a consequence, red-edge excitation photoselects dyes in a DMA-rich environment. Such broadening is not present in acetonitrile, and no excitation wavelength dependence of the ET dynamics is observed. The quenching of both dyes is markedly faster in nonpolar than polar solvents, independently of the excitation wavelength. According to molecular dynamics simulations, this is due to the preferential solvation of the dyes by DMA in cyclohexane. The opposite preferential solvation is predicted in acetonitrile. Consequently, close contact between the reactants in acetonitrile requires partial desolvation. By contrast, the recombination of the quenching product is slower in nonpolar than in polar solvents and exhibits much smaller dependence, if any, on the excitation wavelength.

Electrostatically tuned phenols: a scalable organocatalyst for transfer hydrogenation and tandem reductive alkylation of N-heteroarenes

One of the fundamental aims in catalysis research is to understand what makes a certain scaffold perform better as a catalyst than another. For instance, in nature enzymes act as versatile catalysts, providing a starting point for researchers to understand how to achieve superior performance by positioning the substrate close to the catalyst using non-covalent interactions. However, translating this information to a non-biological catalyst is a challenging task. Here, we report a simple and scalable electrostatically tuned phenol (ETP) as an organocatalyst for transfer hydrogenation of N-arenes using the Hantzsch ester as a hydride source. The biomimetic catalyst (1-5 mol%) displays potential catalytic activity to prepare diverse tetrahydroquinoline derivatives with good to excellent conversion under ambient reaction conditions. Kinetic studies reveal that the ETP is 130-fold faster than the uncharged counterpart, towards completion of the reaction. Control experiments and NMR spectroscopic investigations elucidate the role of the charged environment in the catalytic transformation.

Synthesis, crystallization and Hirshfeld surface analysis of transition metal carboxylate pentapyridines

Single X-ray diffraction studies and Hirshfeld surface analysis of three transition metal coordination complexes of Py5Me2COOH reveals the ligand's diverse coordination utility and the structure directing effect of the carboxylate moiety.

Photochemistry in Real Space: Batho- and Hypsochromism in the Water Dimer

The development of chemical intuition in photochemistry faces several difficulties that result from the inadequacy of the one-particle picture, the Born-Oppenheimer approximation, and other basic ideas used to build models. It is shown herein how real-space approaches can be efficiently used to gain valuable insights in photochemistry through a simple example of red and blue shift effects: the double hypso- and bathochromic shifts in the low-lying valence excited states of (H₂ O)₂ . It is demonstrated that 1) the use of these techniques allows the perturbative language used in the theory of intermolecular interactions, even in the strongly interacting short-range regime, to be maintained; 2) one and only one molecule is photoexcited in each of the addressed excited states and 3) the electrostatic interaction between the in-the-cluster molecular dipoles provides a fairly intuitive rationalisation of the observed batho- and hypsochromism. The methods exploited and illustrated herein are able to maintain the individuality and properties of the interacting entities in a molecular aggregate, and thereby they allow chemical intuition in general states, at any geometry and using a broad variety of electronic structure methods to be kept and built.

Fluorescence quenching by photoinduced electron transfer between 7-methoxycoumarin and guanine base facilitated by hydrogen bonds: an in silico study

In this study, the effects of hydrogen bond (H-bond) formation on fluorescence quenching of 7-methoxycoumarin (7MC) via photo-induced electron transfer from a guanine base (Gua) are investigated using a combined quantum mechanics/molecular mechanics simulation. The electronic structure is calculated by the floating occupation molecular orbital complete active space configuration interaction modification on a semiempirical method. Then the full multiple spawning method is employed for the dynamics simulations on multiple electronic states. The methods employed here are validated by simulating direct dynamics of 7MC (without Gua) and compared with available experimental results. Our computational results are in good agreement with the previously reported experimental results in terms of spectroscopic properties of 7MC. In the case of a H-bonded 7MC-Gua complex, the results from constrained dynamics simulations and single-point calculations suggest that the electron transfer occurs on the second excited state and it depends not only on the H-bond length but also on the intermolecular planarity between 7MC and Gua. Moreover, a proton coupled electron transfer can occur at ≈1 Å of H-bond length, where a proton from Gua is also transferred together with the electron to 7MC. The obtained simulations are expected to be greatly beneficial for designing effective fluorescently labeled nucleotide probes as well as providing information for precise fluorescence signal interpretation.

Ultrafast Transient Absorption Spectra of Photoexcited YOYO-1 molecules call for additional investigations of their fluorescence quenching mechanism

In this report, we observed that YOYO-1 immobilized on a glass surface is much brighter when dried (quantum yield 16±4% in the ambient air) or in hexane than in water (quantum yield ~%).YOYO-1 is a typical cyanine dye that has a photo-isomerization reaction upon light illumination. In order to understand this quenching mechanism, we use femtosecond transient absorption spectroscopy to measure YOYO-1's electron dynamics after excitation directly. By deconvoluting the hot-ground-state absorption and the stimulated emission, the dynamics of electronic relaxation and balance are revealed. The results support the intermolecular charge transfer mechanism better than the intramolecular relaxation mechanism that has been widely believed before. We believe that the first step of the relaxation involves a Dexter charge transfer between the photo-excited YOYO-1 molecule and another guest molecule that is directly bound to the YOYO-1 giving two radicals with opposite signs of charges. The charges are recombined either directly between these two molecules, or both molecules start to rotate and separate from each other. Eventually, the two charges recombined non-radiatively via various pathways. These pathways are reflected on the complicated multi-exponential decay curves of YOYO-1 fluorescence lifetime measurements. This charge transfer mechanism suggests that (1) electrical insulation may help improve the quantum yield of YOYO-1 in polar solutions significantly and (2) a steric hindrance for the intramolecular rotation may have a less significant effect.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Deciphering the incognito role of water in a light driven proton coupled electron transfer process

Light induced multisite electron proton transfer in two different phenol (simple and phenol carrying an intramolecularly hydrogen bonded base) pendants on acridinedione dye (ADD) and an NADH analogue was studied by following fluorescence quenching dynamics in an ultrafast timescale. In a simple phenol derivative (ADDOH), photo-excited acridinedione acquires an electron from phenol intramolecularly, coupled with the transfer of a proton to solvent water. But in a phenol carrying hydrogen bonded base (ADDDP), both electron and proton transfer occur completely intramolecularly. The sequence of this electron and proton transfer process was validated by discerning the pH dependency of the reaction kinetics. Since photo-excited ADDs are stronger oxidants, the sequential electron first proton transfer mechanism (ETPT) was observed in ADDOH and hence there is no change in the PCET reaction kinetics kETPT ∼ 6.57 × 109 s-1 in the entire pH range (pH 2-12). But the phenol carrying hydrogen bonded base (ADDDP) unleashes concerted electron proton transfer where the PCET reaction rate decreases upon decreasing the pH below its pKa. Noticeably, the concerted EPT process in ADDDP mimics the donor side of photosystem II and it occurs by two distinct pathways: (i) through direct intramolecular hydrogen bonding between the phenol and amine, kDEPT ∼ 12.5 × 1010 s-1 and (ii) through the bidirectional hydrogen bond extended by the water molecule trapped in between the proton donor and acceptor, which mediates the proton transfer and serves as a proton wire, kWMEPT ∼ 2.85 × 1010 s-1. These results unravel the incognito role played by water in mediating the proton transfer process when the structural elements do not favor direct hydrogen bonding between the proton donor and acceptor in a concerted PCET reaction.

Ruthenium (II) and Iridium (III) Complexes of N-Heterocyclic Carbene and Pyridinol Derived Bidentate Chelates: Synthesis, Characterization, and Reactivity

We report the synthesis and characterization of new ruthenium(II) and iridium(III) complexes of a new bidentate chelate, NHCR'-pyOR (OR = OMe, OtBu, OH and R' = Me, Et). Synthesis and characterization studies were done on the following compounds: four ligand precursors (1-4); two silver complexes of these NHCR'-pyOR ligands (5-7); six ruthenium complexes of the type [η6-(p-cymene)Ru(NHCR'-pyOR)Cl]X with R' = Me, Et and R = Me, tBu, H and X = OTf-, PF6- and PO2F2- (8-13); and two iridium complexes, [Cp*Ir(NHCMe-pyOtBu)Cl]PF6 (14) and [Cp*Ir(NHCMe-pyOH)Cl]PO2F2 (15). The complexes are air stable and were isolated in moderate yield. However, for the PF6- salts, hydrolysis of the PF6- counter anion to PO2F2- during t-butyl ether deprotection was observed. Most of the complexes were characterized by 1H and 13C-NMR, MS, IR, and X-ray diffraction. The ruthenium complexes [η6-(p-cymene)Ru(NHCMe-pyOR)Cl]OTf (R = Me (8) and tBu (9)) were tested for their ability to accelerate CO2 hydrogenation and formic acid dehydrogenation. However, our studies show that the complexes transform during the reaction and these complexes are best thought of as pre-catalysts.

Photogeneration and Quenching of Tryptophan Radical in Azurin

Tryptophan and tyrosine can form radical intermediates that enable long-range, multistep electron transfer (ET) reactions in proteins. This report describes the mechanisms of formation and quenching of a neutral tryptophan radical in azurin, a blue-copper protein that contains native tyrosine (Y108 and Y72) and tryptophan (W48) residues. A long-lived neutral tryptophan radical W48• is formed upon UV-photoexcitation of a zinc(II)-substituted azurin mutant in the presence of an external electron acceptor. The quantum yield of W48• formation (Φ) depends upon the tyrosine residues in the protein. A tyrosine-deficient mutant, Zn(II)Az48W, exhibited a value of Φ = 0.080 with a Co(III) electron acceptor. A nearly identical quantum yield was observed when the electron acceptor was the analogous tyrosine-free, copper(II) mutant; this result for the Zn(II)Az48W:Cu(II)Az48W mixture suggests there is an interprotein ET path. A single tyrosine residue at one of the native positions reduced the quantum yield to 0.062 (Y108) or 0.067 (Y72). Wild-type azurin with two tyrosine residues exhibited a quantum yield of Φ = 0.045. These data indicate that tyrosine is able to quench the tryptophan radical in azurin.

Photoinduced PCET in ruthenium-phenol systems: thermodynamic equivalence of uni- and bidirectional reactions

Six termolecular reaction systems comprised of Ru(4,4′-bis(trifluoromethyl)-2,2′-bipyridine)32+, phenols with different para substituents, and pyridine in acetonitrile undergo proton-coupled electron transfer (PCET) upon photoexcitation of the metal complex. Five of these six phenols are found to release in concerted fashion an electron to the ruthenium photooxidant and a proton to the pyridine base. The kinetics for this concerted bidirectional PCET process and its relationship to the reaction free energy were compared to the driving-force dependence of reaction kinetics for unidirectional concerted proton–electron transfer (CPET) between the same phenols and Ru(2,2′-bipyrazine)32+, a combined electron/proton acceptor. The results strongly support the concept of thermodynamic equivalence between separated electron/proton acceptors and single-reagent hydrogen-atom acceptors. A key feature of the explored systems is the similarity between molecules employed for bi- and unidirectional CPET.

New insight into photo-induced electron transfer with a simple ubiquinone-based triphenylamine model

A ubiquinone-based triphenylamine system was designed as a simple model to study the photo-induced electron transfer (PET).

A DFT/TD-DFT study of thiazolidinedione derivative in dimethylformamide: cooperative roles of hydrogen bondings, electronic and vibrational spectra

The time-dependent density functional theory (TDDFT) method has been applied to investigate the thiazolidinedione (TZD) derivative A and its hydrogen-bonded complexes with dimethylformamide (DMF) (A-DMF and A-2DMF). The calculation results showed that the excited-state hydrogen bondings of O-H⋯O=C and N-H⋯O=C are strengthened and weakened in the hydrogen-bonded trimer A-2DMF, and their cooperation effect caused a blue shift in the electronic spectrum of A-2DMF. This modulation mechanism of the hydrogen-bond strengthening and weakening and its role in influencing the spectroscopy property of the TZD derivative A in DMF have been analyzed and showed in details.

A Co(ii)–Ru(ii) dyad relevant to light-driven water oxidation catalysis

The covalent anchoring of a Co(ii) water oxidation catalyst to a Ru(ii) photosensitizer with potential application in artificial photosynthesis.

Proton-coupled electron transfer with photoexcited metal complexes

Proton-coupled electron transfer (PCET) plays a crucial role in many enzymatic reactions and is relevant for a variety of processes including water oxidation, nitrogen fixation, and carbon dioxide reduction. Much of the research on PCET has focused on transfers between molecules in their electronic ground states, but increasingly researchers are investigating PCET between photoexcited reactants. This Account describes recent studies of excited-state PCET with d(6) metal complexes emphasizing work performed in my laboratory. Upon photoexcitation, some complexes release an electron and a proton to benzoquinone reaction partners. Others act as combined electron-proton acceptors in the presence of phenols. As a result, we can investigate photoinduced PCET involving electron and proton transfer in a given direction, a process that resembles hydrogen-atom transfer (HAT). In other studies, the photoexcited metal complexes merely serve as electron donors or electron acceptors because the proton donating and accepting sites are located on other parts of the molecular PCET ensemble. We and others have used this multisite design to explore so-called bidirectional PCET which occurs in many enzymes. A central question in all of these studies is whether concerted proton-electron transfer (CPET) can compete kinetically with sequential electron and proton transfer steps. Short laser pulses can trigger excited-state PCET, making it possible to investigate rapid reactions. Luminescence spectroscopy is a convenient tool for monitoring PCET, but unambiguous identification of reaction products can require a combination of luminescence spectroscopy and transient absorption spectroscopy. Nevertheless, in some cases, distinguishing between PCET photoproducts and reaction products formed by simple photoinduced electron transfer (ET) (reactions that don't include proton transfer) is tricky. Some of the studies presented here deal directly with this important problem. In one case study we employed a cyclometalated iridium(III) complex. Our other studies with ruthenium(II) complexes and phenols focused on systematic variations of the reaction free energies for the CPET, ET, and proton transfer (PT) steps to explore their influence on the overall PCET reaction. Still other work with rhenium(I) complexes concentrated on the question of how the electronic structure of the metal-to-ligand charge transfer (MLCT) excited states affects PCET. We used covalent rhenium(I)-phenol dyads to explore the influence of the electron donor-electron acceptor distance on bidirectional PCET. In covalent triarylamine-Ru(bpy)₃²⁺/Os(bpy)₃²⁺-anthraquinone triads (bpy = 2,2'-bipyridine), hydrogen-bond donating solvents significantly lengthened the lifetimes of photogenerated electron/hole pairs because of hydrogen-bonding to the quinone radical anion. Until now, comparatively few researchers have investigated this variation of PCET: the strengthening of H-bonds upon photoreduction.

Improving the efficiency of water splitting in dye-sensitized solar cells by using a biomimetic electron transfer mediator

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.

Hydrogen bonding in the electronic excited state

Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology.

Photochemical conversion of solar energy

Energy is the most important issue of the 21st century. About 85% of our energy comes from fossil fuels, a finite resource unevenly distributed beneath the Earth's surface. Reserves of fossil fuels are progressively decreasing, and their continued use produces harmful effects such as pollution that threatens human health and greenhouse gases associated with global warming. Prompt global action to solve the energy crisis is therefore needed. To pursue such an action, we are urged to save energy and to use energy in more efficient ways, but we are also forced to find alternative energy sources, the most convenient of which is solar energy for several reasons. The sun continuously provides the Earth with a huge amount of energy, fairly distributed all over the world. Its enormous potential as a clean, abundant, and economical energy source, however, cannot be exploited unless it is converted into useful forms of energy. This Review starts with a brief description of the mechanism at the basis of the natural photosynthesis and, then, reports the results obtained so far in the field of photochemical conversion of solar energy. The "grand challenge" for chemists is to find a convenient means for artificial conversion of solar energy into fuels. If chemists succeed to create an artificial photosynthetic process, "... life and civilization will continue as long as the sun shines!", as the Italian scientist Giacomo Ciamician forecast almost one hundred years ago.

Oligo-Asp tag/Zn(II) complex probe as a new pair for labeling and fluorescence imaging of proteins

To accomplish the selective labeling of a specific protein in complicated biological systems, a peptide tag incorporated into the protein and a complementary small molecular probe are required. Although a variety of peptide tag/probe pairs have been developed as molecular tools for protein analyses, the availability of pairs suitable for real-time imaging of proteins is still limited. We now report a new peptide tag/artificial probe pair composed of a genetically encodable oligo-aspartate sequence (D4 tag, (D4)n, n = 1-3) and the corresponding multinuclear Zn(II) complexes (Zn(II)-DpaTyrs). The strong binding affinity of the Zn(II)-DpaTyr probes with the D4 tag was a result of the multiple coordination bonds and the multivalent effect. It was measured quantitatively by isothermal titration calorimetry. The high affinity between the tag and the probe, indispensable for the selective protein labeling, enabled the pair to be used for the labeling and fluorescence imaging of a membrane-bound receptor protein tethering a triply repeated D4 tag ((D4)3) in an intact cell configuration without significantly affecting the receptor signal transduction.

Synthesis and Electron Transfer Studies of Ruthenium−Terpyridine-Based Dyads Attached to Nanostructured TiO2

A series of bis(terpyridine)RuII complexes have been prepared, where one of the terpyridines is functionalized in the 4'-position by a phosphonic or carboxylic acid group for attachment to TiO2. The other is functionalized, also in the 4'-position, by a potential electron donor. In complexes 1a, 3a, and 4a,b, this donor is tyrosine or hydrogen-bonded tyrosine, while in 2a it is carotenoic amide. The synthesis and photophysical properties of the complexes are discussed. On irradiation with visible light, the formation of a long-lived charge-separated state was anticipated, via primary electron ejection into the TiO2, followed by secondary electron transfer from the donor to the photogenerated RuIII. However, such a charge-separated state could be observed with certainty only with complex 2a. To explain the result, quantum chemical calculations were performed on the different types of complexes.

Water as a hydrogen bonding bridge between a phenol and imidazole. A simple model for water binding in enzymes

The X-ray crystal structure of the mono-hydrate of 2,2-bis(imidazol-1-ylmethyl)-4-methylphenol has been determined. Three hydrogen bonds hold water very tightly in the crystal, as determined by deuterium solid-state NMR. The hydrogen bond between the phenolic hydroxyl and water appears to have about the same strength as the direct hydrogen bond to imidazole, suggesting that the structure can be a good model for hydrogen bonds that are mediated by a water molecule in enzymes.

Mimicking the electron donor side of Photosystem II in artificial photosynthesis

This review focuses on our recent efforts in synthetic ruthenium-tyrosine-manganese chemistry mimicking the donor side reactions of Photosystem II. Tyrosine and tryptophan residues were linked to ruthenium photosensitizers, which resulted in model complexes for proton-coupled electron transfer from amino acids. A new mechanistic model was proposed and used to design complexes in which the mechanism could be switched between concerted and step-wise proton-coupled electron transfer. Moreover, a manganese dimer linked to a ruthenium complex could be oxidized in three successive steps, from Mn (2) (II,II) to Mn (2) (III,IV) by the photo-oxidized ruthenium sensitizer. This was possible thanks to a charge compensating ligand exchange in the manganese complex. Detailed studies of the ligand exchange suggested that at high water concentrations, each oxidation step is coupled to a proton-release of water-derived ligands, analogous to the oxidation steps of the manganese cluster of Photosystem II.

Synthesis and Characterization of Dinuclear Ruthenium Complexes Covalently Linked to RuII Tris-bipyridine: An Approach to Mimics of the Donor Side of Photosystem II

To mimic the electron-donor side of photosystem II (PSII), three trinuclear ruthenium complexes (2, 2a, 2b) were synthesized. In these complexes, a mixed-valent dinuclear Ru2(II,III) moiety with one phenoxy and two acetato bridges is covalently linked to a Ru(II) tris-bipyridine photosensitizer. The properties and photoinduced electron/energy transfer of these complexes were studied. The results show that the Ru2(II,III) moieties in the complexes readily undergo reversible one-electron reduction and one-electron oxidation to give the Ru2(II,III) and Ru2(III,III) states, respectively. This could allow for photooxidation of the sensitizer part with an external acceptor and subsequent electron transfer from the dinuclear ruthenium moiety to regenerate the sensitizer. However, all trinuclear ruthenium complexes have a very short excited-state lifetime, in the range of a few nanoseconds to less than 100 ps. Studies by femtosecond time-resolved techniques suggest that a mixture of intramolecular energy and electron transfer between the dinuclear ruthenium moiety and the excited [Ru(bpy)3]2+ photosensitizer is responsible for the short lifetimes. This problem is overcome by anchoring the complexes with ester- or carboxyl-substituted bipyridine ligands (2a, 2b) to nanocrystalline TiO2, and the desired electron transfer from the excited state of the [Ru(bpy)3]2+ moiety to the conduction band of TiO2 followed by intramolecular electron transfer from the dinuclear Ru2(II,III) moiety to photogenerated Ru(III) was observed. The resulting long-lived Ru2(III,III) state decays on the millisecond timescale.

Contribution of the intramolecular hydrogen bond to the shift of the pKa value and the oxidation potential of phenols and phenolate anions

Intramolecularly OHO[double bond, length as m-dash]C hydrogen bonded phenols, 2-HO-C6H2-3,5-(t-Bu)2-CONH-t-Bu (1-OH), 2-HO-C6H2-5-t-Bu-1,3-(CONH-t-Bu)2 (2-OH) and 2-HO-C6H2-3,5-(t-Bu)2-NHCO-t-Bu (4-OH), were synthesized and their phenolate anions were prepared as tetraethylammonium salts (-1O-(NEt4+), 2-O-(NEt4+) and 4-O-(NEt4+)) with intramolecular NHO(oxyanion) hydrogen bonds. 4-HO-C(6)H(2)-3,5-t-Bu(2)-CONH-t-Bu (3-OH) and its phenolate anion, 3-O-(NEt4+), were synthesized as non-hydrogen bonded references. The presence of intramolecular hydrogen bonds was established through the crystallographic analysis and/or (1)H NMR spectroscopic results. Intramolecular NHO(phenol) hydrogen bonds shift the pK(a) of the phenol to a more acidic value. The results of cyclic voltammetry show that the intramolecular OH...O=C hydrogen bond negatively shifts the oxidation potential of the phenol. In contrast, the intramolecular NHO(oxyanion) hydrogen bond positively shifts the oxidation potential of the phenolate anion, preventing oxidation. These contributions of the hydrogen bond to the pKa value and the oxidation potentials probably play an important role in the formation of a tyrosyl radical in photosystem II.