Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications

Photoredox Catalysis by 21-Thiaporphyrins: A Green and Efficient Approach for C-N Borylation and C-H Arylation

Photoredox catalysis provides a green and sustainable alternative for C-H activation of organic molecules that eludes harsh conditions and use of transition metals. The photocatalytic C-N borylation and C-H arylation mostly depend on the ruthenium and iridium complexes or eosin Y and the use of porphyrin catalysts is still in infancy. A series of novel 21-thiaporphyrins (A2B2 and A3B type) were synthesized having carbazole/phenothiazine moieties at their meso-positions and screened as catalysts for C-N borylation and C-H arylation. This paper demonstrates the 21-thiaporphyrin catalyzed C-N borylation and het-arylation of anilines under visible light. The method utilizes only 0.1 mol % of 21-thiaporphyrin catalyst under blue light for the direct C-N borylation and het-arylation reactions. A variety of substituted anilines were used as source for expensive and unstable aryl diazonium salts in the reactions. The heterobiaryls and aryl boronic esters were obtained in decent yields (up to 88 %). Versatility of the 21-thiaporphyrin catalyst was tested by thiolation and selenylation of anilines under similar conditions. Mechanistic insight was obtained from DFT studies, suggesting that 21-thiaporphyrin undergo an oxidative quenching pathway. The photoredox process catalyzed by 21-thiaporphyrins offers a mild, efficient and metal-free alternative for the formation of C-C, C-S, and C-Se bonds in aryl compounds; it can also be extended to borylation reaction.

Photoelectrochemical signal polarity transition mediated by quercetin for the detection of neuron-specific enolase

A photoelectrochemical (PEC) biosensor with a wide linear detection range was developed for the sensitive detection of neuron-specific enolase (NSE), which was achieved by applying a photocurrent polarity transition strategy mediated by quercetin. The coupling reaction between Cr(VI) and quercetin drives the signal polarity from anodic to cathodic. When only quercetin is present in the test solution, photogenerated electrons are transferred to the electrode to generate anodic photocurrent. However, in the presence of the target, the signal probe released Cr(VI), which interacted with quercetin, and the electron transfer direction was changed to achieve signal polarity conversion. Meanwhile, protoporphyrin-sensitized Bi:SrTiO3 nanocubes were used as matrix photoactive materials to provide basic photocurrent. The doping of Bi element would adjust the bandgap of SrTiO3, and the organic-inorganic composite material exhibits good photostability and chemical stability that can maintain stable photoelectric properties over a long period of time. Such a novel signal polarity transition strategy greatly broadened the sensor detection to the range of 0.00007-170 ng mL-1 and obtained a relatively low detection limit (25 fg mL-1), which greatly improved the detection sensitivity and accuracy of the biosensor.

Metal Complexes for Dye-Sensitized Photoelectrochemical Cells (DSPECs)

Since Mallouk's earliest contribution, dye-sensitized photoelectrochemical cells (DSPECs) have emerged as a promising class of photoelectrochemical devices capable of storing solar light into chemical bonds. This review primarily focuses on metal complexes outlining stabilization strategies and applications. The ubiquity and safety of water have made its splitting an extensively studied reaction; here, we present some examples from the outset to recent advancements. Additionally, alternative oxidative pathways like HX splitting and organic reactions mediated by a redox shuttle are discussed.

Wetting-regulated gas-involving (photo)electrocatalysis: biomimetics in energy conversion

(Photo)electrolysis of water or gases with water to species serving as industrial feedstocks and energy carriers, such as hydrogen, ammonia, ethylene, propanol, etc., has drawn tremendous attention. Moreover, these processes can often be driven by renewable energy under ambient conditions as a sustainable alternative to traditional high-temperature and high-pressure synthesis methods. In addition to the extensive studies on catalyst development, increasing attention has been paid to the regulation of gas transport/diffusion behaviors during gas-involving (photo)electrocatalytic reactions towards the goal of creating industrially viable catalytic systems with high reaction rates, excellent long-term stabilities and near-unity selectivities. Biomimetic surfaces and systems with special wetting capabilities and structural advantages can shed light on the future design of (photo)electrodes and address long-standing challenges. This article is dedicated to bridging the fields of wetting and catalysis by reviewing the cutting-edge design methodologies of both gas-evolving and gas-consuming (photo)electrocatalytic systems. We first introduce the fundamentals of various in-air/underwater wetting states and their corresponding bioinspired structural properties. The relationship amongst the bubble transport behavior, wettability, and porosity/tortuosity is also discussed. Next, the latest implementations of wetting-related design principles for gas-evolving reactions (i.e. the hydrogen evolution reaction and oxygen evolution reaction) and gas-consuming reactions (i.e. the oxygen reduction reaction and CO2 reduction reaction) are summarized. For photoelectrode designs, additional factors are taken into account, such as light absorption and the separation, transport and recombination of photoinduced electrons and holes. The influences of wettability and 3D structuring of (photo)electrodes on the catalytic activity, stability and selectivity are analyzed to reveal the underlying mechanisms. Finally, remaining questions and related future perspectives are outlined.

Dye-Sensitized Nonstoichiometric Strontium Titanate Core-Shell Photocathodes for Photoelectrosynthesis Applications

A core-shell approach that utilizes a high-surface-area conducting core and an outer semiconductor shell is exploited here to prepare p-type dye-sensitized solar energy cells that operate with a minimal applied bias. Photocathodes were prepared by coating thin films of nanocrystalline indium tin oxide with a 0.8 nm Al₂O₃ seeding layer, followed by the chemical growth of nonstoichiometric strontium titanate. Films were annealed and sensitized with either a porphyrin chromophore or a chromophore-catalyst molecular assembly consisting of the porphyrin covalently tethered to the ruthenium complex. The sensitized photoelectrodes produced cathodic photocurrents of up to -315 μA/cm² under simulated sunlight (AM1.5G, 100 mW/cm²) in aqueous media, pH 5. The photocurrent was increased by the addition of regenerative hole donors to the system, consistent with slow interfacial recombination kinetics, an important property of p-type dye-sensitized electrodes.

Electron-Withdrawing Boron Dipyrromethene Dyes As Visible Light Absorber/Sensitizers on Semiconductor Oxide Surfaces

The synthesis, characterization, and electrochemical and photophysical properties of the phosphonate-derivatized carbazole (CBZ) and boron dipyrromethene (BODIPY) chromophores in the dyes, BODIPY(CBZ)₂PO₃H₂ (8) and BODIPY(Tol)₂PO₃H₂ (7), are described. The oxide-bound dyes have been explored as light absorbers in dye-sensitized photoelectrosynthesis cell (DSPEC) applications. The BODIPY-CBZ phosphonate ester (6) features a broad, intense UV-visible absorption spectrum with absorptions at 297 and 650 nm that arise from mixed transitions at the CBZ and BODIPY units. Electrochemical measurements on BODIPY(CBZ)₂Br (4) in 0.1 M [nBu₄N][PF₆] in dichloromethane, vs normal hydrogen electrode (NHE), reveal reversible oxidations at 1.19 and 1.41 V and a reversible reduction at -0.59 V. On indium tin oxide (ITO) and TiO₂, a reversible one-electron oxidation appears for 7 at 0.86 and 0.90 V vs NHE in dichloromethane, respectively, which demonstrates the redox stability on metal oxide surfaces. The results of nanosecond transient absorption measurements on SnO₂/TiO₂ electrodes provide direct evidence for excited-state electron injection into the conduction band of TiO₂ following 590 nm excitation. A longer lifetime for 8⁺ compared to 7⁺ is consistent with extensive intramolecular charge separation between the CBZ and BODIPY units on the surface. Photoelectrochemical studies on 8 on a SnO₂/TiO₂ photoanode resulted in sustained photocurrents with current maxima of ∼200 μA/cm² with hydroquinone added as a reductant under 1 sun (AM1.5 100 mW·cm^-2) illumination at pH 4.5 in 0.1 M acetate buffer and 0.4 M LiClO₄. On mixed SnO₂/TiO₂ electrode surfaces, with the added catalyst [Ru(Mebimpy)((4,4'-(OH)₂PO-CH₂)₂bpy)(OH₂)]²⁺ and chromophores 7 and 8, addition of 0.1 M benzyl alcohol resulted in sustained photocurrents of 12 and 35 μA/cm², consistent with oxidation to benzaldehyde.

Silatrane Anchors for Metal Oxide Surfaces: Optimization for Potential Photocatalytic and Electrocatalytic Applications

Silatrane surface anchors are protected siloxanes that are known to bond firmly (from pH 2-11) to metal oxide electrodes under heating. However, these conditions are not always compatible with the other functionality present. A silatrane-containing porphyrin molecule and a silatrane-containing ruthenium complex have now been designed, synthesized and optimized conditions have been identified for surface binding. Two mild, room-temperature surface binding methods were explored: binding with or without an acidic pretreatment; these methods were compared to the traditional, harsher binding conditions involving strong heating. We find that a preacidified electrode gave comparable surface loadings at room temperature compared to sensitization by using the previous strong heating method. This was also true on TiO₂, SnO₂, and nanoITO electrodes and thus may be generalizable. The new, milder binding methods also resulted in excellent aqueous and electrochemical stability from pH 2-11. Using a water-insoluble porphyrin with a silatrane anchor further increased the aqueous stability of the deposit, aided by the insolubility of the porphyrin. Finally, X-ray photoelectron spectroscopy (XPS) data confirmed for the first time that the triethanolamine released from the silatrane on deprotection/binding in turn binds to TiO₂, SnO₂, and nanoITO electrodes. This undesired triethanolamine deposit was easily removed from the surface by electrochemical voltage cycling or with an aqueous acidic wash for 1 h.

Porphyrins as Photoredox Catalysts in Csp2-H Arylations: Batch and Continuous Flow Approaches

We have investigated both batch and continuous flow photoarylations of enol-acetates to yield different α-arylated aldehyde and ketone building blocks by using diazonium salts as the aryl-radical source. Different porphyrins were used as SET photocatalysts, and photophysical as well as electrochemical studies were performed to rationalize the photoredox properties and suggest mechanistic insights. Notably, the most electron-deficient porphyrin ( meso-tetra(pentafluorophenyl)porphyrin) shows the best photoactivity as an electron donor in the triplet excited state, which was rationalized by the redox potentials of excited states and the turnover of the porphyrins in the photocatalytic cycle. A two-step continuous protocol and multigram-scale reactions are also presented revealing a robust, cost-competitive, and easy methodology, highlighting the significant potential of porphyrins as SET photocatalysts.

Fabrication of carbon nanotube-multiporphyrin array composites as light-sensitizer for photocurrent generation, photochromism of viologen and catalytic degradation of methyl orange

Multiporphyrin arrays were assembled on the surface of MWNTs to produce light-sensitive nano-composites with improved opto-electric conversion efficiency, photochromic, and photocatalytic performance.

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.

Electrocatalytic reduction of CO2 with CCC-NHC pincer nickel complexes

CCC-NHC pincer Ni complexes electrocatalytically reduce CO₂ to CO and formate at the first reduction potential without producing or requiring molecular H₂.

Frontiers of water oxidation: the quest for true catalysts

Development of advanced analytical techniques is essential for the identification of water oxidation catalysts together with mechanistic studies.

Effects of a phosphonate anchoring group on the excited state electron transfer rates from a terthiophene chromophore to a ZnO nanocrystal

Relative to carboxyl-anchored chromophores, phosphonate-anchored dyes are bound more strongly but slow the excited state electron transfer to ZnO nanocrystals.

Interaction of a tricationic meso-substituted porphyrin with guanine-containing polyribonucleotides of various structures

The interaction of a tricationic water-soluble meso-(N-methylpyridinium)-substituted porphyrin, TMPyP³⁺, derived from classic TMPyP4, with double-stranded poly(G)  ⋅  poly(C) and four-stranded poly(G) polyribonucleotides has been studied in aqueous buffered solutions, pH 6.9, of low and near-physiological ionic strengths in a wide range of molar phosphate-to-dye ratios (P/D). To clarify the binding modes of TMPyP³⁺ to biopolymers various spectroscopic techniques, including absorption and polarized fluorescence spectroscopy, Raman spectroscopy, and resonance light scattering, were used. As a result, two competitive binding modes were revealed. In solution of low ionic strength outside binding of the porphyrin to the polynucleotide backbone with self-stacking prevailed at low P/D ratios (P/D  <  3.5). It manifested itself by the substantial quenching of porphyrin fluorescence. Also the formation of large-scale porphyrin aggregates was observed near the stoichiometric binding ratio. The spectral changes observed at P/D  >  30 including emission enhancement were supposed to be caused by the embedding of partially stacked porphyrin J-dimers into the polymer groove. TMPyP³⁺ binding to poly(G) induced a fluorescence increase 2.5 times as large as that observed for poly(G)  ⋅  poly(C). In solution of near-physiological ionic strength the efficiency of external porphyrin binding was reduced substantially due to the competitive binding of Na⁺ ions with the polymer backbone. The spectroscopic characteristics of porphyrin bound to polynucleotides at different conditions were compared with those for free porphyrin.