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.