Electron injection dynamics in high-potential porphyrin photoanodes

Cofacial porphyrin organic cages. Metals regulating excitation electron transfer and CO2 reduction electrocatalytic properties

Herein, we introduce a comprehensive study of the photophysical behaviors and CO2 reduction electrocatalytic properties of a series of cofacial porphyrin organic cages (CPOC-M, M = H2, Co(ii), Ni(ii), Cu(ii), Zn(ii)), which are constructed by the covalent-bonded self-assembly of 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP) and chiral (2-aminocyclohexyl)-1,4,5,8-naphthalenetetraformyl diimide (ANDI), followed by post-synthetic metalation. Electronic coupling between the TFPP donor and naphthalene-1,4 : 5,8-bis(dicarboximide) (NDI) acceptor in the metal-free cage is revealed to be very weak by UV-vis spectroscopic, electrochemical, and theoretical investigations. Photoexcitation of CPOC-H2, as well as its post-synthetic Zn and Co counterparts, leads to fast energy transfer from the triplet state porphyrin to the NDI unit according to the femtosecond transient absorption spectroscopic results. In addition, CPOC-Co enables much better electrocatalytic activity for CO2 reduction reaction than the other metallic CPOC-M (M = Ni(ii), Cu(ii), Zn(ii)) and monomeric porphyrin cobalt compartment, supplying a partial current density of 18.0 mA cm-2 at -0.90 V with 90% faradaic efficiency of CO.

First Example of a Heptazine-Porphyrin Dyad; Synthesis and Spectroscopic Properties

We have prepared the first example of a porphyrin linked to an heptazine photoactive antenna. The two entities, linked with an alkyl spacer, demonstrate the activity of both active moieties. While they behave electrochemically independantly, on the other hand the spectroscopy shows the existence of energy transfer between both partners.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Applications of Fluorous Porphyrinoids: An Update†

Porphyrins and related macrocycles have been studied broadly for their applications in medicine and materials because of their tunable physicochemical, optoelectronic and magnetic properties. In this review article, we focused on the applications of fluorinated porphyrinoids and their supramolecular systems and summarized the reports published on these chromophores in the past 5-6 years. The commercially available fluorinated porphyrinoids: meso-perfluorophenylporphyrin (TPPF₂₀ ) perfluorophthalocyanine (PcF₁₆ ) and meso-perfluorophenylcorrole (CorF₁₅ ) have increased photo and oxidative stability due to the presence of fluoro groups. Because of their tunable properties and robustness toward oxidative damage these porphyrinoid-based chromophores continue to gain attention of researchers developing advanced functional materials for applications such as sensors, photonic devices, component for solar cells, biomedical imaging, theranostics and catalysts.

Nanotechnology for catalysis and solar energy conversion

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure-property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.

Nature of fluorine interactions in ‘wheel and axle’ topology based hexa-coordinated Sn(iv)-porphyrins: an experimental and theoretical analysis

The experimental and theoretical investigations on Sn(iv)-tetrapyridylporphyrins demonstrate that ‘Gulliver effect’ has to be taken into consideration in explaining the genesis of F-based intermolecular interactions.

Fluorinated ZnII Porphyrins for Dye-Sensitized Aqueous Photoelectrosynthetic Cells

Three perfluorinated Zn^II porphyrins were evaluated as n-type sensitizers in photoelectrosynthetic cells for HBr and water splitting. All the dyes are featured by the presence of pentafluorophenyl electron-withdrawing groups to increase the ground-state oxidation potential and differ for the nature and position of the π-conjugate linker between the core and anchoring group tasked to bind the metal oxide, in order to assess the best way of coupling with the semiconductor. A phenyl-triazole moiety was used to link the carboxylic anchoring group onto the meso position, while an ethynyl-phenyl linker was chosen to bridge carboxylic and cyanoacrylic groups onto the β-pyrrolic position. A combination of electrochemical, computational, and spectroscopic investigations confirmed the strong electron-withdrawing effect of the perfluorinated porphyrin core, which assures all the investigated dyes of the high oxidation potential required to the coupling with water oxidation catalysts (WOC). Such an electron-poor core, however, affects the charge separation character of the dyes, as demonstrated by the spatial distribution of the excited states, leading to a nonquantitative charge injection, although tilting of the molecules on the semiconductor surface could bring the porphyrin ring closer to the semiconductor, offering additional charge-transfer pathways. Indeed, all the dyes demonstrated successful in the splitting of both aqueous HBr and water, with the best results found for the SnO₂/TiO₂ photoanode sensitized with the β-substituted porphyrin equipped with a cyanoacrylic terminal group, achieving 0.4 and 0.1 mA/cm² photoanodic currents in HBr and water under visible light, respectively. The faradaic yield for oxygen evolution in the presence of an Ir^IV catalyst was over 95%, and the photoanode operation was stable for more than 1000 s. Thus, the perfluorinated porphyrins with a cyanoacrylic anchoring group at the β-position should be considered for further development to improve the charge-transfer character.

Surface anchored self-assembled reaction centre mimics as photoanodes consisting of a secondary electron donor, aluminium(iii) porphyrin and TiO2 semiconductor

Vertically assembled photoanodes, consisting of aluminum(iii) porphyrin, an electron donor, and semiconductor TiO₂, have been fabricated and their photophysical properties investigated.

The Diradical-Dication Strategy for BODIPY- and Porphyrin-Based Dyes with Near-Infrared Absorption Maxima from 1070 to 2040 nm

Four stable boron dipyrromethene (BODIPY)- and porphyrin-based bis-arylamine diradical dications were synthesized by two-electron oxidation of their neutral molecules. The two BODIPY-based dications have open-shell singlet ground states. UV/Vis absorption spectra of all four dications showed large redshifts in the NIR region compared to their neutral precursors with absorption maxima at 1274 and 1068 nm for the two BODIPY-based dications and 1746 and 2037 nm for the two porphyrin-based dications. Thus, two new types of NIR dyes with longer wavelengths are provided by the diradical-dication strategy, which can be applied for the generation of other NIR dyes with a range of different chromophores and auxochromes.

Reactive Porphyrin Adsorption on TiO2 Anatase Particles: Solvent Assistance and the Effect of Water Addition

The surface functionalization of metal oxide nanoparticles with complex organic molecules can lead to optoelectronically very different material properties, depending on whether adsorption occurs at the solid-gas or solid-liquid interface. Here, we report on two different approaches to decorate anatase TiO₂ nanoparticle powders with 2 H-tetraphenylporphyrin (2HTPP) molecules: (i) porphyrin adsorption in dispersions of organic liquids and (ii) gas-phase functionalization where evaporated porphyrin molecules attach to dehydrated particle surfaces in the absence of solvent molecules. In the latter case, a bottom-up approach is pursued to explore both the impact of organic solvent molecules and the impact of spurious water on the surface chemistry of porphyrin-sensitized TiO₂ nanoparticles. Vis diffuse reflectance and photoluminescence emission spectroscopy provide clear evidence for the promotion of interfacial reorganization processes of the adsorbate species by coadsorbed solvent molecules in liquids. Moreover, traces of spurious water were found to induce protonation-deprotonation reactions on the adsorbed porphyrins with a strong impact on the optical properties of the resulting hybrid materials.

Artificial photosynthesis by light absorption, charge separation, and multielectron catalysis

We highlight recent novel approaches in the field of artificial photosynthesis. We emphasize the potential of a highly modular plug-and-play concept that we hope will persuade the community to explore a more inclusive variety of multielectron redox catalysis to complement the proton reduction and water oxidation half-reactions in traditional solar water splitting systems.

Synthesis and coordination studies of 5-(4′-carboxyphenyl)-10,15,20-tris(pentafluorophenyl)porphyrin and its pyrrolidine-fused chlorin derivative

An efficient strategy was developed to obtain carboxyphenyl porphyrin, chlorins and metal complexes, with potential applications in photonics and biology.

Interrelationship between TiO2 nanoparticle size and kind/size of dyes in the mechanism and conversion efficiency of dye sensitized solar cells

Photocurrent efficiency measurements and other experimental results demonstrate that the best TiO₂ nanoparticle size depends on the intrinsic properties of the dye and the best size changes with dye type.

Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells

The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO₂ by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO₂ reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO₂ reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

Femtosecond studies: observing transition states and other ultra-short-lived species

To celebrate 2015 as the 'International Year of Light', this article offers a short survey of the progress made since the award of the Nobel Prize of 1999 to Professor Ahmed Zewail for his pioneering work on taking the timescale for observation of light-induced events down to the femtosecond level. Developments have included the extension of studies (i) to larger molecules, leading up to biological systems; (ii) the increased range of detection methods of transient species from the UV-Vis to the infrared region; (iii) the introduction of Raman spectroscopy to augment IR studies; (iv) examination of combination events to supplement dissociation events; (v) the interrogation of transient structures by X-ray absorption spectroscopy; (vi) the study of reactions taking place at solid surfaces.

Iron(III) Fluorinated Porphyrins: Greener Chemistry from Synthesis to Oxidative Catalysis Reactions

Iron(III) fluorinated porphyrins play a central role in the biomimetics of heme enzymes and enable cleaner routes to the oxidation of organic compounds. The present work reports significant improvements in the eco-compatibility of the synthesis of 5,10,15,20-tetrakis-pentafluorophenylporphyrin (H₂TPFPP) and the corresponding iron complex [Fe(TPFPP)Cl], and the use of [Fe(TPFPP)Cl] as an oxidation catalyst in green conditions. The preparations of H₂TPFPP and [Fe(TPFPP)Cl] typically use toxic solvents and can be made significantly greener and simpler using microwave heating and optimization of the reaction conditions. In the optimized procedure it was possible to eliminate nitrobenzene from the porphyrin synthesis and replace DMF by acetonitrile in the metalation reaction, concomitant with a significant reduction of reaction time and simplification of the purification procedure. The Fe(III)porphyrin is then tested as catalyst in the selective oxidation of aromatics at room temperature using a green oxidant (hydrogen peroxide) and green solvent (ethanol). Efficient epoxidation of indene and selective oxidation of 3,5-dimethylphenol and naphthalene to the corresponding quinones is observed.

Novel porphyrin-preparation, characterization, and applications in solar energy conversion

Accelerated inner charge transfer in porphyrins promotes a broad light-harvesting ability up to 840 nm and a conversion efficiency of 9.2%.

Are the orientation and bond strength of the RCO2(-)···M link key factors for ultrafast electron transfers?

The photo-induced electron transfers in the "straight up" ionic assemblies [Pd3(2+)]···MCP and [Pd3(2+)]···DCP···[Pd3(2+)] ([Pd3(2+)]* → MCP or DCP) are ultrafast (<85 fs) indicating that it is not necessary to have a strong coordination bond or a bent geometry to obtain fast electron injection in porphyrin-containing DSSCs.