Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.

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Here we study how the introduction of nitrogen into poly(pphenylene) type materials affects their ability to act as hydrogen evolution photocatalysts. Direct photocatalytic water splitting is an attractive strategy for clean energy production, but understanding which material properties are important, how they interplay, and how they can be influenced through doping remains a significant challenge, especially for polymers. Using a combined experimental and computational approach, we demonstrate that introducing nitrogen in conjugated polymers results in either materials that absorb significantly more visible light but worse predicted driving force for water/sacrificial electron donor oxidation, or materials with an improved driving force that absorb relatively less visible light. The latter materials are found to be much more active and the former much less. The trade-off between properties highlights that the optimization of a single property in isolation is a poor strategy for improving the overall activity of materials.

We compare, for a range of conjugated polymers relevant to water-splitting photocatalysis, the predictions for the redox potentials associated with charge carriers and excitons by a totalenergy ΔDFT approach to those measured experimentally. For solidstate potentials, of the different classes of potentials available experimentally for conjugated polymers, the class measured under conditions which are the most similar to those during water splitting, we find a good fit between the ionization potentials predicted using ΔB3LYP and those measured experimentally using photoemission spectroscopy (PES). We also observe a reasonable fit to the more limited data sets of excited-state ionization potentials, obtained from two-photon PES, and electron affinities, measured by inverse PES, respectively. Through a comparison of solid-state potentials with gas phase and solution potentials for a range of oligomers, we demonstrate how the quality of the fit to experimental solid-state data is probably the result of benign error cancellation. We discuss that the good fit for solid-state potentials in vacuum suggests that a similar accuracy can be expected for calculations on solid-state polymers interfaced with water. We also analyze the quality of approximating the ΔB3LYP potentials by orbital energies. Finally, we discuss what a comparison between experimental and predicted potentials teaches us about conjugated polymers as photocatalysts, focusing specifically on the large exciton-binding energy in these systems and the mechanism of free charge carrier generation.

ABSTRACT:We investigate the electron injection from a terrylenebased chromophore to the TiO 2 semiconductor bridged by a recently proposed phenyl-amide-phenyl molecular rectifier. The mechanism of electron transfer is studied by means of quantum dynamics simulations using an extended Huckel Hamiltonian. It is found that the inclusion of the nuclear motion is necessary to observe the photoinduced electron transfer. In particular, the fluctuations of the dihedral angle between the terrylene and the phenyl ring modulate the localization and thus the electronic coupling between the donor and acceptor states involved in the injection process. The electron propagation shows characteristic oscillatory features that correlate with interatomic distance fluctuations in the bridge, which are associated with the vibrational modes driving the process. The understanding of such effects is important for the design of functional dyes with optimal injection and rectification properties. E nvironmentally friendly and cost-effective dye-sensitized solar cells (DSSCs) are a potential alternative to siliconbased photovoltaics for solar energy conversion, can be equipped with molecular catalysts for direct conversion and storage of solar energy into fuel, and have been the subject of extensive research.1−5 In DSSC devices a semiconductor electrode is functionalized with a (molecular) chromophore absorbing visible light. The photoexcitation induces an interfacial electron transfer (ET) from the dye into the semiconductor conduction band (CB), while an electrolyte shuttles electrons from the counter electrode to regenerate the oxidized dye.Despite considerable improvement achieved in recent years, DSSC performances have not reached energy conversion yield and efficiency levels that are necessary to render them competitive with silicon-based solar cells. Higher efficiencies can be attained by increasing the light-harvesting properties of the solar cell, by decreasing internal energy losses, or by optimizing the conditions for fast electron injection.A major obstacle hampering the performance of DSSC devices is the internal losses from recombination between the electron injected into the semiconductor and the hole on the oxidized dye. 6 In an effort to design more efficient devices, several groups have investigated the effect of specific cell parameters for enhancing electron injection and reducing recombination losses, such as the length and nature of the bridges, 7−10 the chemical structure of the anchor groups 11 and the redox potential of the electrolyte. 12 In addition, detailed investigations of the molecular mechanisms at the dye/ semiconductor interface 13−18 have revealed the importance of nonadiabatic dynamics for charge separation and recombination. 19−21In an approach to prevent back electron transfer, Batista and coworkers recently proposed the use of a molecular rectifier 22 composed of two phenyl rings coupled through an amide bond (see AM molecule in Scheme 1a). This molecule acts as a rectifier when used as a molecular w...

Using first-principles molecular dynamics (AIMD) and constrained density functional theory (CDFT) we identify the pathway of primary electron transfer in the R. Sphaeroides reaction center from the special pair excited state (P*) to the accessory bacteriochlorophyll (BA). Previous AIMD simulations on the special pair (PLPM) predicted a charge-transfer intermediate formation through the excited-state relaxation along a reaction coordinate characterized by the rotation of an axial histidine (HisM202). To account for the full electron transfer we extend the model to include the primary acceptor BA. In this extended model, the LUMO is primarily localized on the acceptor BA and extends over an interstitial water (water A) that is known to influence the rate of electron transfer (Potter et al. Biochemistry 2005 280, 27155-27164). A vibrational analysis of the dynamical trajectories gives a frequency of 30-35 cm(-1) for a molecular motion involving the hydrogen-bond network around water A, in good agreement with experimental findings (Yakovlev et al. Biochemistry, 2003, 68, 603-610). In its binding pocket water A can act as a switch by breaking and forming hydrogen bonds. With CDFT we calculate the energy required to the formation of the charge-separated state and find it to decrease along the predicted anisotropic reaction coordinate. Furthermore, we observe an increased coupling between the ground and charge-separated state. Water A adapts its hydrogen-bonding network along this reaction coordinate and weakens the hydrogen bond with HisM202. We also present AIMD simulations on the radical cation (P(•+)) showing a weakening of the hydrogen bond between HisL168 and the 3(1)-acetyl of PL. This work demonstrates how proton displacements are crucially coupled to the primary electron transfer and characterizes the reaction coordinate of the initial photoproduct formation.

Solar-driven water splitting is a key reaction step in a photoelectrochemical

One of the key challenges in designing light-driven artificial photosynthesis devices is the optimisation of the catalytic water oxidation process. For this optimisation it is crucial to establish the catalytic mechanism and the intermediates of the catalytic cycle, yet a full description is often difficult to obtain using only experimental data. Here we consider a series of mononuclear ruthenium water oxidation catalysts of the formand its derivatives). The proposed catalytic cycle and intermediates are examined using density functional theory (DFT), radiation chemistry, spectroscopic techniques and electrochemistry to establish the water oxidation mechanism. The stability of the catalyst is investigated using Online Electrochemical Mass Spectrometry (OLEMS). The comparison between the calculated absorption spectra of the proposed intermediates with experimental ones, as well as free-energy calculations with electrochemical data, provides strong evidence for the proposed pathway: a water oxidation catalytic cycle involving four proton-coupled electron transfer (PCET) steps. The thermodynamic bottleneck is identified in the third PCET step involving the O-O bond formation. The good agreement between the optical and thermodynamic data with DFT predictions further confirms the general applicability of this methodology as a powerful tool in the characterisation of water oxidation catalysts and for the interpretation of experimental observables.2