Influence of Surface and Structural Variations in Donor-Acceptor-Donor Sensitizers on Photoelectrocatalytic Water Splitting.

Conjugated organic chromophores composed of linked donor (D) and acceptor (A) moieties have attracted considerable attention for photoelectrochemical applications. In this work, we compare the optoelectronic properties and photoelectrochemical performance of two D-A-D structural isomers with thiophene-X-carboxylic acid (X denotes 3 and 2 positions) derivatives and 2,1,3-benzothiadiazole as the D and A moieties, respectively. 5,5'-(Benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-3-carboxylic acid), BTD1, and 5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-2-carboxylic acid), BTD2, were employed in the study to understand how structural isomers affect surface attachments within chromophore-catalyst assemblies and their influence on charge-transfer dynamics. Crystal structures revealed that varying the position of the -COOH anchoring group causes the molecules to either contort out of a plane (BTD1) or adopt a near-perfect planar conformation (BTD2). BTD1 and BTD2 were co-loaded with either a water oxidation catalyst, [Ru(2,6-bis(1-methylbenzimidazol-2-yl)pyridine)-(4,4'-((HO)2OPCH2)2-2,2'-bipyridine)(OH2)]2, RuCt2+, or proton reduction catalyst [Ni(P2PhN2C6H4CH2PO3H2)2]2+, NiCt2+, on oxide electrodes to facilitate photodriven water splitting reactions. Emission quenching measurements indicate that both BTD1 and BTD2 inject electrons into n-type SnO2|TiO2 electrodes and holes into p-type NiO semiconductors from their respective excited states at high efficiencies >60%. Photocurrent densities of chromophore-catalyst assemblies obtained using linear sweep voltammetry (LSV) show that BTD2-sensitized photoanodes generate significantly more photocurrent than BTD1-sensitized electrodes; however, both exhibit similar performances at the photocathode. Photoelectrocatyltic measurements demonstrate that both BTD1 and BTD2 performed similarly, generating Faradaic efficiencies of 39 and 38% at the anode or 61 and 79% at the cathode. Transient absorption measurements suggest that the differences between the LSV and photoelectrocatalytic measurements result from the differences in quantum yields of the photogenerated redox equivalents, which is also a reflection of the varying metal oxide surface conformation. Our findings suggest that BTD2 should be investigated further in photocathodic studies since it has the structural advantage of being incorporated into diverse types of chromophore-catalyst assemblies.

A Silicon-Based Heterojunction Integrated with a Molecular Excited State in a Water-Splitting Tandem Cell.

Semiconductor-based photocathodes with high light-absorption capability are of interest in the production of solar fuels, but many of them are limited by low efficiencies due to rapid interfacial back electron transfer. We demonstrate here that a nanowire-structured p-type Si (p-Si) electrode, surface-modified with a perylene-diimide derivative (PDI'), can undergo photoreduction of a surface-bound, water reduction catalyst toward efficient H2 evolution under a low applied bias. At the electrode interface, the PDI' layer converts green light into high-energy holes at its excited state for extraction of photogenerated electrons at the photoexcited p-Si. The photogenerated electrons at the reduced PDI' are subsequently transferred to the molecular H2-evolution catalyst. Involvement of the photoexcited PDI' enables effective redox separation between the electrons at the reduced catalyst and the holes at the valence band of p-Si. The heterojunction photocathode was used in a tandem cell by coupling with a dye-sensitized photoanode for solar-driven water splitting into H2 and O2.

Design and reactivity of pentapyridyl metal complexes for ammonia oxidation.

Oxidation of NH3 to N2 by pentapyridyl metal complexes via hydrogen atom abstraction was investigated computationally. Quantum chemical analysis reveals insights on orbital symmetry requirements for efficient NH3 oxidation. The most promising complex, [(PY5)Mo(NH3)]2+, was studied experimentally. It shows conversion of NH3 to N2 upon treatment with 2,4,6-tri-tert-butylphenoxyl radical.

Anionic guest-dependent slow magnetic relaxation in Co(ii) tripodal iminopyridine complexes.

We report the syntheses and magnetic property characterizations of four mononuclear cobalt(ii) complex salts featuring a tripodal iminopyridine ligand with external anion receptor groups, [CoL5-ONHtBu]X2 (X = Cl (1), Br (2), I (3) and ClO4 (4)). While all four salts exhibit anion binding through pendant amide moieties, only in the case of 1 is field-induced slow relaxation of magnetisation observed, whereas in the other salts this phenomenon is absent at the limits of our instrumentation. The effect of chloride inducing a seventh Co-N interaction and concomitant structural distortion is hypothesized as the origin of the observed dynamic magnetic properties observed in 1. Ab initio computational studies carried out on a 7-coordinate Co(ii) model species survey the complex interplay of coordination number and trigonal twisting on the sign and magnitude of the axial anisotropy parameter (D), and identify structural features whose distortions can trigger large switches in the sign and magnitude of magnetic anisotropy.

Evaluation of attractive interactions in the second coordination sphere of iron complexes containing pendant amines.

The interactions between pendant amines in the second coordination sphere and ligands in the first coordination sphere are important for understanding the structures and reactivity of complexes containing PR2NR'2 ligands, which have been shown to be highly active H2 oxidation/production catalysts. A series of [Fe(PPh2NBn2)2(X)(Y)]n+ complexes have been prepared and structurally characterized. These complexes have two different ligands with which the pendant amines of the diphosphine ligand can interact. The solid state structure of cis-Fe(PPh2NBn2)2Cl2 reveals that the six-membered rings adjacent to the P atoms are in a boat confirmation, resulting in close NP distances that suggests the P atoms have a greater affinity for the lone pair of electrons on the N atom than chloride ligands. Similarly, boat conformations are observed for both rings adjacent to the hydride ligands of trans-[HFe(PPh2NBn2)2(CH3CN)]+ and trans-HFe(PPh2NBn2)2Cl, resulting in short NH distances. Spectroscopic and computational studies of trans-[HFe(PPh2NBn2)2(CO)]+, trans-[HFe(PPh2NBn2)(PPh2NBn2H)(CO)]2+, and trans-[HFe(PPh2NBn2)2(H2)]+ indicate the complexes are more stable when the pendant amines in boat conformations are adjacent to the hydride ligand. These data suggest an attractor ordering of H- > CO > H2 > PR3 > Cl- ∼ CH3CN.

Modulating Hole Transport in Multilayered Photocathodes with Derivatized p-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting.

For solar water splitting, dye-sensitized NiO photocathodes have been a primary target. Despite marginal improvement in performance, limitations remain arising from the intrinsic disadvantages of NiO and insufficient catalysis. We report here a new approach to modifying NiO photocathodes with doped NiO bilayers and an additional layer of macro-mesoporous ITO. The trilayered electrode is functionalized with a surface-attached ruthenium polypyridyl dye and a covalently bridged nickel-based hydrogen evolution catalyst. The NiO film, containing a 2% K+-doped NiO inner layer and a 2% Cu2+-doped NiO outer layer, provides sufficient driving force for hole transport following hole injection by the molecular assembly. Upon light irradiation, the resulting photocathode generates hydrogen from water sustainably with enhanced photocurrents and a Faradaic efficiency of ∼90%. This approach highlights the value of modifying both the internal and surface structure of NiO and provides insights into a new generation of dye-sensitized photocathodes for solar-driven water splitting cells.

Intramolecular N-H···Cl hydrogen bonds in the outer coordination sphere of a bipyridyl bisurea-based ligand stabilize a tetrahedral FeLCl2 complex.

A bipyridyl-based anion receptor is utilized as a ligand in a tetrahedral FeCl2 complex and demonstrates secondary coordination sphere influence through intramolecular hydrogen bonding to the chloride ligands as evidenced by X-ray crystallography.

Anion dependence in the spin-crossover properties of a Fe(II) podand complex.

We report the syntheses, characterisations, and spin state behaviours of salts of the tripodal-ligated Fe(II) complex [FeL(6-OH)]X(2) (L(6-OH) = tris{4-[(6-methanol)-2-pyridyl]-3-aza-3-butenyl}amine, X = OTf(-) (1), Br(-) (2), I(-) (3), BPh(4)(-) (4)). Covalent linking of the ligand arms is imperative as a high-spin bis(tridentate) complex (5) is formed when a non-tethered ethyl iminopyridine ligand (L(2) = 4-[(6-methanol)-2-pyridyl]-3-aza-3-butenyl) is used. For salts 1-4, thermally-induced spin-crossover (SCO) is observed in the solid state, with dependence on anion and solvate molecules. Salts with larger anions show more complete SCO centred at higher temperatures (1 > 3 > 2); the triflate salt 1 (T(1/2) = 173 K) also shows the strongest cooperativity of the compounds examined. Hydrogen bonding appears to be critical to SCO in this family of salts: limiting interactions by use of tetraphenylborate produces a high-spin complex down to 5 K. In protic solvents such as methanol, spectra of [FeL(6-OH)](2+) are largely unchanged over a period of three days, but dissociate when interrogated with strong field bidentate ligands. Compounds 1-3, and 5 remain high spin in solution down to 180 K, consistent with the data obtained in the solid state.