Binuclear copper­(II) porphyrins in which two copper­(II) porphyrin macrocycles are doubly fused at the meso-beta positions are shown to be active electrocatalysts for the hydrogen evolution reaction (2H+ + 2e– → H2). Structural characterization, including use of electron paramagnetic resonance and X-ray photoelectron spectroscopies, verifies the fused species contains two copper­(II) metal centers in its resting state. In comparison to the nonfused copper­(II) porphyrin complex, the fused species is reduced at significantly less applied bias potentials (ΔE 1/2 ∼ 570 mV for the first reduction process). Electrochemical characterization in the presence of substrate protons confirms the production of hydrogen with near-unity Faradaic efficiency, and kinetic analysis shows the catalyst achieves a maximum turnover frequency above 2 000 000 s–1. The enhancement in catalytic performance over analogous nonfused copper­(II) porphyrins indicates extended macrocycles provide an advantageous structural motif and design element for preparing electrocatalysts that activate small molecules of consequence to renewable energy.

A novel synthetic method is used to prepare metalloporphyrin-modified gallium phosphide photocathodes for solar-driven hydrogen evolution from water.

We report on the electrocatalytic and optical properties of cobaloxime hydrogen production catalysts assembled on a polymer-modified nanostructured indium tin oxide (nanoITO) electrode. The hybrid construct is assembled using built-in ligand sites (pyridyl groups) of the surface-attached polymer to direct, template, and assemble cobaloxime units. The conductive nature of the nanoITO substrate allows direct electrochemical measurements of the CoIII/CoII and CoII/CoI redox couples of the cobaloxime–polyvinylpyridine assembly recorded in organic electrolyte solutions, confirming the polymer interface used in this work does not preclude formation of reduced cobalt species. Electrochemical measurements using modified and nonmodified nanoITO electrodes in buffered aqueous solutions indicate the immobilized cobaloxime units remain catalytically active. The relatively large surface area of the nanostructured support, coupled with its visual transparency, also permits optical characterization of the modified electrodes. In general, the cobaloxime–polymer assembly possesses optical and electronic properties similar to those of the non-surface-attached counterpart, albeit with enhanced chemical reversibility. We propose that the unique encapsulating environments of surface-grafted polymeric architectures can provide a molecular strategy for improving the chemical stability of surface-immobilized catalysts. The modular nature of the attachment chemistry used in this work should allow application to a range of catalysts, polymers, and transparent conducting oxide surfaces. Thus, the construct sets the stage for an improved understanding of structure–function relationships governing the optoelectronic and catalytic properties of surface-immobilized catalyst–polymer assemblies.

Hybrid materials that link light capture and conversion technologies with the ability to drive reductive chemical transformations are attractive as components in photoelectrosynthetic cells. We show that thin-film polypyridine surface coatings provide a molecular interface to assemble cobalt porphyrin catalysts for hydrogen evolution onto a visible-light-absorbing p-type gallium phosphide semiconductor. Spectroscopic techniques, including grazing angle attenuated total reflection Fourier transform infrared spectroscopy, confirm that the cobalt centers of the porphyrin macrocycles coordinate to pyridyl nitrogen sites of the organic surface coating. The cobalt porphyrin surface concentration and fraction of pyridyl sites coordinated to a cobalt center are quantified using complementary methods of ellipsometry, inductively coupled plasma mass spectrometry, and X-ray photoelectron spectroscopy. In aqueous solutions under simulated solar illumination the modified cathode is photochemically active for hydrogen production, generating the product gas with near-unity Faradaic efficiency at a rate of ≈10 μL min cm when studied in a three-electrode configuration and polarized at the equilibrium potential of the H/H couple. This equates to a photoelectrochemical hydrogen evolution reaction activity of 17.6 H molecules s Co, the highest value reported to date for a molecular-modified semiconductor. Key features of the functionalized photocathode include (1) the relative ease of synthetic preparation made possible by application of an organic surface coating that provides molecular recognition sites for immobilizing the cobalt porphyrin complexes at the semiconductor surface and (2) the use of visible light to drive cathodic fuel-forming reactions in aqueous solutions with no added organic acids or sacrificial chemical reductants.

New opportunities for organizing and controlling molecular components arise with the use of a stabilizing organic layer composed of grafted polymer chains at a semiconductor surface. We highlight recent advances in our research efforts to use polymer brush coatings containing pendent ligands that direct and assemble molecular catalysts for fuel production to visible-light-absorbing substrates. We illustrate how the polymeric interface can be varied to control the structure and photoelectrochemical response of gallium phosphide (100) electrodes containing surface-immobilized pyridyl or imidazole ligands with attached cobaloximes for hydrogen production. Surface sensitive spectroscopic methods, including X-ray photoelectron spectroscopy, grazing angle total reflectance Fourier transform infrared spectroscopy, and ellipsometry provide structural information regarding the nanoscale molecular connectivity and mesoscale dimensions of the cobaloxime-containing polymer grafts. At the macroscale, three-electrode photoelectrochemical testing of the cobaloxime-modified electrodes under simulated solar lighting conditions in pH neutral aqueous solutions show up to a 3-fold increase of hydrogen production as compared to results obtained using polymer-grafted electrodes without attached cobaloximes tested under nearly identical conditions.