Controlled Surface-Assembly of Nanoscale Leaf-Type Cu-Oxide Electrocatalyst for High Activity Water Oxidation

The controlled surface deposition of a robust and high-performance nanostructured copper-oxide (<b>CuO</b>_<b>x</b>-NLs) electrocatalyst for water oxidation is presented. The material exhibits a characteristic leaf-type morphology and self-assembles on a copper substrate by straightforward constant-current anodization. The oxygen onset occurs at about 1.55 V versus RHE (η = 320 mV), which is 400–500 mV less than for amorphous Cu-oxide films. A Tafel slope of 44 mV dec^–1 is obtained, which is the lowest observed relative to other copper-based materials. Long-term catalytic performance and stability tests of the electrocatalytic <b>CuO</b>_<b>x</b>-NLs sample show a stable current density of >17 mA cm^–2 for oxygen evolution, which was sustained for many hours

In-Silico Design of a Donor–Antenna–Acceptor Supramolecular Complex for Photoinduced Charge Separation

We investigate via density functional theory a series of donor–antenna–acceptor molecular rectifiers designed as modules for artificial photosynthesis devices. We consider triad modules containing phenothiazine (PTZ) as the electron donor and different derivatives of naphthalene diimide (NDI) as the antenna and secondary electron acceptor. The choice of the molecular components in the triad is guided by the redox and optical properties of each subunit. Using time-dependent DFT in combination with the long-range corrected xc-functional CAM-B3LYP we investigate how photoinduced charge transfer states are affected by systematic modifications of the triad molecular structure. In particular, we show how by controlling the length of the molecular bridges connecting the different charge separator subunits it is possible to control the driving force for the relaxation of the excitonic state into the full charge-separated state. On the basis of these findings we propose a supramolecular triad consisting of inexpensive and readily available molecular components that can find its implementation in artificial devices for solar energy transduction

A Dynamic View of Proton-Coupled Electron Transfer in Photocatalytic Water Splitting

Solar-driven water splitting is a key reaction step in a photoelectrochemical cell for solar fuel production. We propose a photoanode in which a TiO₂ substrate is functionalized with a supramolecular complex consisting of a fully organic naphthalene-diimide (NDI) dye covalently bound to a mononuclear Ru-based water oxidation catalyst. By performing ab initio Molecular Dynamics simulations, we elucidate microscopic details of water oxidation at the photoanode induced by visible light absorption. The fast photoinduced electron injection from the NDI into the semiconductor provides the driving force for the activation of the Ru catalyst. The proton-coupled electron transfer nature of this catalytic reaction path is unveiled through the explicit description of the water environment, which is essential to determine the proton diffusion channel and the free energy change along the reaction. The mechanistic insight into the photocatalytic processes obtained with our computational strategy can facilitate the design of new and efficient photoelectrochemical devices

Structure Determination of a Bio-Inspired Self-Assembled Light-Harvesting Antenna by Solid-State NMR and Molecular Modeling

The molecular stacking of an artificial light-harvesting antenna self-assembled from 3¹-aminofunctionalized zinc-chlorins was determined by solid-state NMR in combination with quantum-chemical and molecular-mechanics modeling. A library of trial molecular stacking arrangements was generated based on available structural data for natural and semisynthetic homologues of the Zn-chlorins. NMR assignments obtained for the monomer in solution were validated for self-assembled aggregates and refined with ¹H–¹³C heteronuclear correlation spectroscopy data collected from samples with ¹³C at natural abundance. Solid-state ring-current shifts for the ¹H provided spatial constraints to determine the molecular overlap. This procedure allows for a discrimination between different self-assembled structures and a classification of the stacking mode in terms of electric dipole alignment and π–π interactions, parameters that determine the functional properties of light-harvesting assemblies and conducting nanowires. The combination with quantum-mechanical modeling then allowed building a low-resolution packing model in silico from molecular stacks. The method allows for moderate disorder and residual polymorphism at the stack or molecular level and is generally applicable to determine molecular packing structures of aromatic molecules with structural asymmetry, such as is commonly provided by functionalized side chains that serve to tune the self-assembly process

Crucial Role of Nuclear Dynamics for Electron Injection in a Dye–Semiconductor Complex

We investigate the electron injection from a terrylene-based chromophore to the TiO₂ 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 Hückel 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

On the Morphology of a Discotic Liquid Crystalline Charge Transfer Complex

Discotic liquid crystalline (DLC) charge transfer (CT) complexes, which combine visible light absorption with rapid charge transfer characteristics within the CT complex, can have a great potential for photovoltaic applications when they can be made to self-assemble in a bulk heterojunction arrangement with separate channels for electron and hole conduction. However, the morphology of some liquid crystalline CT complexes has been under debate for many years. In particular, the liquid crystalline CT complex built from the electron acceptor 2,4,7-trinitro-9-fluorenone (TNF) and discotic molecules has been reported to have the TNF “sandwiched” either between the discotic molecules within the same column or between the columns within the aliphatic tails of the discotic molecules. We present a detailed structural study of the prototypic 1:1 mixture of the discotic 2,3,6,7,10,11-hexakis­(hexyloxy)­triphenylene (HAT6) and TNF. Nuclear magnetic resonance (NMR) line widths and cross-polarization rates are consistent with the picosecond time scale anisotropic thermal motions of the HAT6 and TNF molecules previously observed. By computational integration of Rietveld refinement analyses of neutron diffraction patterns with density experiments and short-range structural constraints from heteronuclear 2D NMR, we determine that the TNF molecules are vertically oriented between HAT6 columns. The data provide the insight that a morphology of separate hole conducting channels of HAT6 molecules can be realized in the liquid crystalline CT complex

Structural Variability in Wild-Type and <i>bchQ bchR</i> Mutant Chlorosomes of the Green Sulfur Bacterium <i>Chlorobaculum tepidum</i>

The self-aggregated state of bacteriochlorophyll (BChl) <i>c</i> molecules in chlorosomes belonging to a <i>bchQ bchR</i> mutant of the green sulfur bacteria <i>Chlorobaculum tepidum</i>, which mostly produces a single 17²-farnesyl-(<i>R</i>)-[8-ethyl,12-methyl]­BChl <i>c</i> homologue, was characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy and high-resolution electron microscopy. A nearly complete ¹H and ¹³C chemical shift assignment was obtained from well-resolved homonuclear ¹³C–¹³C and heteronuclear ¹H–¹³C NMR data sets collected from ¹³C-enriched chlorosome preparations. Pronounced doubling (1:1) of specific ¹³C and ¹H resonances revealed the presence of two distinct and nonequivalent BChl <i>c</i> components, attributed to all <i>syn-</i> and all <i>anti</i>-coordinated parallel stacks, depending on the rotation of the macrocycle with respect to the 3¹-methyl group. Steric hindrance from the 20-methyl functionality induces structural differences between the <i>syn</i> and <i>anti</i> forms. A weak but significant and reproducible reflection at 1/0.69 nm^–1 in the direction perpendicular to the curvature of cylindrical segments observed with electron microscopy also suggests parallel stacking of BChl <i>c</i> molecules, though the observed lamellar spacing of 2.4 nm suggests weaker packing than for wild-type chlorosomes. We propose that relaxation of the pseudosymmetry observed for the wild type and a related BChl <i>d</i> mutant leads to extended domains of alternating <i>syn</i> and <i>anti</i> stacks in the <i>bchQ bchR</i> chlorosomes. Domains can be joined to form cylinders by helical <i>syn–anti</i> transition trajectories. The phase separation in domains on the cylindrical surface represents a basic mechanism for establishing suprastructural heterogeneity in an otherwise uniform supramolecular scaffolding framework that is well-ordered at the molecular level