The simultaneous, concerted transfer of electrons and protonselectron-proton transfer (EPT)-is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4′-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H þ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated þ H─B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.electron transfer | proton-coupled electron transfer P roton-coupled electron transfer (PCET), in which electrons and protons are both transferred, is at the heart of many energy conversion processes in chemistry and biology (1-6). PCET reactions can occur by sequential two-step transfers (e.g., electron transfer followed by proton transfer, ET-PT, or proton transfer followed by electron transfer, PT-ET) or by concerted electron-proton transfer (EPT) (1, 2). EPT pathways are important in avoiding high-energy intermediates, playing an integral role in photosynthesis and respiration, for example.Photo-driven EPT (photo-EPT), with electron and proton transfers occurring simultaneously during the optical excitation process, would appear to be ruled out on fundamental grounds, because electronic excitation occurs rapidly on the time scale for nuclear motions, including proton transfer. Using a combination of femtosecond pump-probe and coherent Raman techniques, we have observed simultaneous electron-proton transfer induced by intramolecular charge transfer (ICT) excitation in two different hydrogen-bonded adducts formed between an organic dye (A─O─H) and an external base (:B). One is formed between a para-nitrophenyl-phenol and an amine base, and the other between a coumarin derivative and an imidazole base (Fig. 1).The shift in electron density away from the hydroxyl group to the intramolecular ...

Knowledge of electronic structures and transport mechanisms in dye-sensitized semiconductors is motivated by their ubiquity in photoelectrochemical cells. In this work, optical spectroscopies are used to uncover the elementary dynamics initiated by light absorption at such molecule–semiconductor interfaces (e.g., electron transfer and nuclear relaxation). These processes are explored in a family of ruthenium bipyridyl complexes in aqueous solutions, wherein phosphonate groups are used to bind the molecules to TiO2 nanocrystalline films. The complexes differ in (i) the number of phosphonate groups and (ii) the presence (or absence) of a methylene bridge between the molecule and the TiO2 surface. A resonance Raman intensity analysis suggests that the electronic excitations possess very little charge transfer character for all complexes. That is, the electronic orbitals involved in light absorption are essentially localized to the molecules. Because the electronic resonances are molecular in character, the photophysics are most appropriately viewed as sequences in which light absorption precedes electron transfer. Transient absorption measurements conducted on the dye-sensitized films show that electron injection processes initiating directly from the photoexcited singlet states of the molecules occur in 100 fs or less. In contrast, the electron transfer rates slow down by at least a factor of 10 when intersystem crossing in the molecule precedes electron injection into TiO2. For ruthenium complexes linked to TiO2 with methylene bridges, intersystem crossing is more efficient than singlet electron injection because of attenuated molecule–TiO2 couplings; electron transfer primarily initiates in triplet states for these systems. Overall, the fundamental connections drawn in this work between molecular structure and photophysical behavior contribute to the general understanding of photoelectrochemical cells based on related molecule–semiconductor systems.

Femtosecond transient absorption spectroscopy is used to characterize the first photoactivation step in a chromophore/water oxidation catalyst assembly formed through a "layer-by-layer" approach. Assemblies incorporating both chromophores and catalysts are central to the function of dye-sensitized photoelectrosynthesis cells (DSPECs) for generating solar fuels. The chromophore, [Rua(II)](2+) = [Ru(pbpy)2(bpy)](2+), and water oxidation catalyst, [Rub(II)-OH2](2+) = [Ru(4,4'-(CH2PO3H2)2bpy)(Mebimpy)(H2O)](2+), where bpy = 2,2'-bipyridine, pbpy = 4,4'-(PO3H2)2bpy, and Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine), are arranged on nanocrystalline TiO2 via phosphonate-Zr(IV) coordination linkages. Analysis of the transient spectra of the assembly (denoted TiO2-[Rua(II)-Zr-Rub(II)-OH2](4+)) reveal that photoexcitation initiates electron injection, which is then followed by the transfer of the oxidative equivalent from the chromophore to the catalyst with a rate of kET = 5.9 × 10(9) s(-1) (τ = 170 ps). While the assembly, TiO2-[Rua(II)-Zr-Rub(II)-OH2](4+), has a near-unit efficiency for transfer of the oxidative equivalent to the catalyst, the overall efficiency of the system is only 43% due to nonproductive photoexcitation of the catalyst and nonunit efficiency for electron injection. The modular nature of the layer-by-layer system allows for variation of the light-harvesting chromophore and water oxidation catalyst for future studies to increase the overall efficiency.

Excited-state proton-transfer dynamics between 7-hydroxy-4-(trifluoromethyl)coumarin and 1-methylimidazole base in toluene were studied using ultrafast pump-probe and time-resolved emission methods. Charge-transfer excitation of the hydroxycoumarin shifts electron density from the hydroxyl group to the carbonyl, resulting in an excited state where proton transfer to the base is highly favored. In addition to its the photoacid characteristics, the shift in the hydroxycoumarin electronic distribution gives it characteristics of a photobase as well. The result is a tautomerization process occurring on the picosecond time scale in which the 1-methylimidazole base acts as a proton-transfer shuttle from the hydroxyl group to the carbonyl.

We report a detailed kinetic analysis of ultrafast interfacial and intraassembly electron transfer following excitation of an oligoproline scaffold functionalized by chemically linked light-harvesting chromophore [Ru(pbpy) 2 (bpy)] 2+ (pbpy = 4,4′-(PO 3 H 2 ) 2 -2,2′-bipyridine, bpy = 2,2′-bipyridine) and water oxidation catalyst [Ru-(Mebimpy)(bpy)OH 2 ] 2+ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine). The oligoproline scaffold approach is appealing due to its modular nature and helical tertiary structure. They allow for the control of electron transfer distances in chromophore− catalyst assemblies for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The proline chromophore−catalyst assembly was loaded onto nanocrystalline TiO 2 with the helical structure of the oligoproline scaffold maintaining the controlled relative positions of the chromophore and catalyst. Ultrafast transient absorption spectroscopy was used to analyze the kinetics of the first photoactivation step for oxidation of water in the assembly. A global kinetic analysis of the transient absorption spectra reveals that photoinduced electron injection occurs in 18 ps and is followed by intra-assembly oxidative activation of the water oxidation catalyst on the hundreds of picoseconds time scale (k ET = 2.6 × 10 9 s −1 ; τ = 380 ps). The first photoactivation step in the water oxidation cycle of the chromophore−catalyst assembly anchored to TiO 2 is complete within 380 ps.

Solid-phase peptide synthesis has been applied to the preparation of phosphonate-derivatized oligoproline assemblies containing two different Ru(II) polypyridyl chromophores coupled via "click" chemistry. In water or methanol the assembly adopts the polyproline II (PPII) helical structure, which brings the chromophores into close contact. Excitation of the assembly on ZrO2 at the outer Ru(II) in 0.1 M HClO4 at 25 °C is followed by rapid, efficient intra-assembly energy transfer to the inner Ru(II) (k(EnT) = 3.0 × 10(7) s(-1), implying 96% relative efficiency). The comparable energy transfer rate constants in solution and on nanocrystalline ZrO2 suggest that the PPII structure is retained when bound to ZrO2. On nanocrystalline films of TiO2, excitation at the inner Ru(II) is followed by rapid, efficient injection into TiO2. Excitation of the outer Ru(II) is followed by rapid intra-assembly energy transfer and then by electron injection. The oligoproline/click chemistry approach holds great promise for the preparation of interfacial assemblies for energy conversion based on a family of assemblies having controlled compositions and distances between key functional groups.

b S Supporting Information U nderstanding proton-coupled electron transfer (PCET) and concerted electronÀproton transfer (EPT) and their roles in chemistry and biology continues to evolve. 1À4 PCET is key in energy half reactions such as water oxidation, 2H 2 O f O 2 + 4H + + 4e À , and CO 2 reduction, CO 2 + 8e À + 8H + f CH 4 + 2H 2 O. Concerted transfer of electrons and protons in EPT is important in avoiding high-energy protonated intermediates. 5À7 An example occurs in oxidation of tyrosine (TyrOH) by Os(bpy) 3 3+ , Os(bpy) 3 3+ + TyrOH f Os(bpy) 3 2+ + TyrOH +• . Electron transfer is disfavored by 0.66 eV, whereas multiple site-EPT, with electron transfer to Os(bpy) 3 3+ and proton transfer to HPO 4 2À added as a base, eq 1, is favored by 0.06 eV. 8There is evidence that molecular excited states can also participate in EPT, a notable example in biology being green fluorescent protein (GFP). 9,10 The appearance of EPT in excited-state reactions has been examined to a limited extent, 5,11À13 but the phenomenon, and its implications for excited-state reactivity and energy conversion, remain to be documented comprehensively. It is of interest for possible application in the photoproduction of energetic intermediates capable of undergoing reactions more complex than electron or energy transfer. The underlying reactivity could be of value in possible applications in energy conversion and artificial photosynthesis, for example. 4,5,7,11,14,15 An example is reduction of the lowest, bpz-based metal-toligand charge transfer (MLCT) excited state of Ru(bpy) 2 (bpz) 2+ (bpz is 2,2 0 -bipyrazine) by hydroquinone (H 2 Q) to give the highly reduced transient PCET intermediate, Ru(bpy) 2 (bpzH • ) 2+ which is capable of undergoing net H-atom transfer, eq 2. 5In an extension of the previous results, we report here hydroquinone reduction of the lowest MLCT excited state of fac-[Re(bpy)(CO) 3 (4,4 0 -bpy)] + . The results are significant in revealing competitive pathways for excited-state reduction to give the potential H-atom transfer intermediate fac-[Re(bpy)-(CO) 3 (4,4 0 -bpyH • )] + . Evidence has been obtained for competitive quenching by electron transfer, followed by proton transfer (ET-PT) and by preassociation and concerted electronÀproton transfer (EPT) in a photoEPT reaction. This is similar to previous work showing complex competition between static and dynamic pathways. 16À18 Preparation and characterization of 1 as the PF 6 À salt are described in the Supporting Information (SI). Its absorption spectrum in CH 3 CN ( Figure S1 of the SI) includes intense, ligandlocalized π f π* bands in the UV and a broad shoulder at 330À430 nm from overlapping dπ(Re) f π*(bpy),π*(4,4 0 -bpy) ABSTRACT: The emitting metal-to-ligand charge transfer (MLCT) excited state of fac-[Re I (bpy)(CO) 3 (4,4 0 -bpy)] + (1) (bpy is 2,2 0 -bipyridine, 4,4 0 -bpy is 4,4 0 -bipyridine), [Re II (bpy À• )(CO) 3 (4,4 0 -bpy)] + *, is reductively quenched by 1,4-hydroquinone (H 2 Q) in CH 3 CN at 23 ( 2°C by competing pathways to give a common el...

Herein we report energy transfer studies in a series of Ru(II) and Os(II) linked coiled-coil peptides in which the supramolecular scaffold controls the functional properties of the assembly. A general and convergent method for the site-specific incorporation of bipyridyl Ru(II) and Os(II) complexes using solid-phase peptide synthesis and the copper-catalyzed azide-alkyne cycloaddition is reported. Supramolecular assembly positions the chromophores for energy transfer. Using time-resolved emission spectroscopy we measured position-dependent energy transfer that can be varied through changes in the sequence of the peptide scaffold. High level molecular dynamics simulations were used in conjunction with the spectroscopic techniques to gain molecular-level insight into the observed trends in energy transfer. The most efficient pair of Ru(II) and Os(II) linked peptides as predicted by molecular modeling also exhibited the fastest rate of energy transfer (with k(EnT) = 2.3 × 10(7) s(-1) (42 ns)). Additionally, the emission quenching for the Ru(II) and Os(II) peptides can be fit to binding models that agree with the dissociation constants determined for the peptides via chemical denaturation.