Photoinduced Electron Transfer of Porphyrin Isomers: Impact of Molecular Structures on Electron‐Transfer Dynamics

Porphyrins have been investigated for a long time in various fields of chemistry owing to their excellent redox and optical properties. Structural isomers of porphyrins have been synthesized, namely, porphycene, hemiporphycene, and corrphycene. Although the number of studies on these structural isomers is limited, they exhibit interesting properties suitable for various applications such as photovoltaic devices, photocatalysts, and photodynamic therapy. In the present review, we summarized their photoinduced electron‐transfer processes, which are key steps of various photofunctions. Their electrochemical and photophysical properties are summarized as basic properties for the electron transfer. Furthermore, differences among these isomers in the electron‐transfer processes are clarified, and its origin has been discussed on the basis of their molecular structures.     

Proton‐Coupled Electron Transfer Induced by Near‐Infrared Light

A proton‐coupled electron transfer reaction induced by near‐infrared light (>710 nm) has been achieved using a dye that shows intense NIR absorption property and electron/proton‐accepting abilities. The developed system generated long‐lived radical species and showed high reversibility and robustness. Mechanistic investigations suggested that the rate‐determining step of the reaction involves the proton transfer process.     

Electron Transfer and Multi‐Electron Accumulation in ExBox4+

Molecules capable of accepting and storing multiple electrons are crucial components of artificial photosynthetic systems designed to drive catalysts, such as those used to reduce protons to hydrogen. ExBox⁴⁺, a boxlike cyclophane comprising two π‐electron‐poor extended viologen units tethered at both ends by two p‐xylylene linkers, has been shown previously to accept an electron through space from a photoexcited guest. Herein is an investigation of an alternate, through‐bond intramolecular electron‐transfer pathway involving ExBox⁴⁺ using a combination of transient absorption and femtosecond stimulated Raman spectroscopy (FSRS). Upon photoexcitation of ExBox⁴⁺, an electron is transferred from one of the p‐xylylene linkers to one of the extended viologen units in ca. 240 ps and recombines in ca. 4 ns. A crystal structure of the doubly reduced species ExBox²⁺ was obtained.     

Dehydrogenative Silylation of Alkenes for the Synthesis of Substituted Allylsilanes by Photoredox,Hydrogen‐Atom Transfer,and Cobalt Catalysis

A synergistic catalytic method combining photoredox catalysis, hydrogen‐atom transfer, and proton‐reduction catalysis for the dehydrogenative silylation of alkenes was developed. With this approach, a highly concise route to substituted allylsilanes has been achieved under very mild reaction conditions without using oxidants. This transformation features good to excellent yields, operational simplicity, and high atom economy. Based on control experiments, a possible reaction mechanism is proposed.     

Carborane Dyads for Photoinduced Electron Transfer: Photophysical Studies on Carbazole and Phenyl‐o‐carborane Molecular Assemblies

o‐Carborane‐based donor–acceptor dyads comprising an o‐carboranyl phenyl unit combined with N‐carbazole ( 1 ) or 4‐phenyl‐N‐carbazole ( 2 ) were prepared, and their dyad characters were confirmed by steady‐state photochemistry and photodynamic experiments as well as electrochemical studies. The absorption and electrochemical properties of the dyads were essentially the sum of those of the carbazole and o‐carboranyl phenyl units; this indicates negligible interaction between the carbazole and o‐carborane units in the ground state. However, the emission spectra of 1 and 2 indicated that carbazole fluorescence was effectively quenched and a new charge‐transfer (CT) emission was observed in solvents, varying from hexane to acetonitrile, which exhibited large Stoke shifts. The CT emission properties of o‐carborane‐based dyads were further analyzed by using Lippert–Mataga plots to show that unit charge separation occurred to form a charge‐separated species in the excited state, namely, 1?2 . This excited‐state species was confirmed by nanosecond transient absorption spectra and spectroelectrochemical measurements; the photoexcitation of carbazole generated the CT state in which a radical cation and anion were formed at the carbazole and o‐carborane units, respectively, within a few nanoseconds. DFT calculations corroborated the presence of this CT species and showed localized populations of the highest singly occupied molecular orbital on 2 in the reduced anionic state. As a result, molecular assemblies formed by linking the carbazole group with the o‐carborane cage through a phenylene or multi‐phenylene spacer revealed that the photoinduced electron‐transfer process occurred intramolecularly.     

Tuning Excited‐State Reactivity by Proton‐Coupled Electron Transfer

The reactivity, and even reaction pathway, of excited states can be tuned by proton‐coupled electron transfer (PCET). The triplet state of benzophenone functionalized with a Brønsted acid (³*BP‐COOH) showed a more powerful oxidation capability over the simple triplet state of benzophenone (³*BP). ³*BP‐COOH could remove an electron from benzene at the rate of 8.0×10⁵ m ^?1 s^?1, in contrast to the reactivity of ³*BP which was inactive towards benzene oxidation. The origin of this great enhancement on the ability of the excited states to remove electrons from substrates is attributed to the intramolecular Brønsted acid, which enables the reductive quenching of ³*BP by concerted electron–proton transfer.     

Sequence‐Dependent Photocurrent Generation through Long‐Distance Excess‐Electron Transfer in DNA

Given its well‐ordered continuous π stacking of nucleobases, DNA has been considered as a biomaterial for charge transfer in biosensors. For cathodic photocurrent generation resulting from hole transfer in DNA, sensitivity to DNA structure and base‐pair stacking has been confirmed. However, such information has not been provided for anodic photocurrent generation resulting from excess‐electron transfer in DNA. In the present study, we measured the anodic photocurrent of a DNA‐modified Au electrode. Our results demonstrate long‐distance excess‐electron transfer in DNA, which is dominated by a hopping mechanism, and the photocurrent generation is sequence dependent.     

A Computational Study of the Mechanism of Hydrogen Evolution by Cobalt(Diimine‐Dioxime) Catalysts

Cobalt(diimine‐dioxime) complexes catalyze hydrogen evolution with low overpotentials and remarkable stability. In this study, DFT calculations were used to investigate their catalytic mechanism, to demonstrate that the initial active state was a Co^I complex and that H₂ was evolved in a heterolytic manner through the protonation of a Co^II? hydride intermediate. In addition, these catalysts were shown to adjust their electrocatalytic potential for hydrogen evolution to the pH value of the solution and such a property was assigned to the presence of a H⁺‐exchange site on the oxime bridge. It was possible to establish that protonation of the bridge was directly involved in the H₂‐evolution mechanism through proton‐coupled electron‐transfer steps. A consistent mechanistic scheme is proposed that fits the experimentally determined electrocatalytic and electrochemical potentials of cobalt(diimine‐dioxime) complexes and reproduces the observed positive shift of the electrocatalytic potential with increasing acidity of the proton source.     

Stereoselective Triplet‐Sensitised Radical Reactions of Furanone Derivatives

The stereo‐ and regioselectivity of triplet‐sensitised radical reactions of furanone derivatives have been investigated. Furanones 7 a , b were excited to the ³ππ* state by triplet energy transfer from acetone. Intramolecular hydrogen abstraction then occurred such that hydrogen was transferred from the tetrahydropyran to the β position of the furanone moiety. Radical combination of the tetrahydropyranyl and the oxoallyl radicals led to the final products 8 a , b . In the intramolecular reaction, overall, a pyranyl group adds to the α position of the furanone. The effect of conformation was first investigated with compounds 9 a , b carrying an additional substituent on the tether between the furanone and pyranyl moiety. Further information on the effect of conformation and the relative configuration at the pyranyl anomeric centre and the furanone moiety was obtained from the transformations of the glucose derivatives 12 , 14 , 17 and 18 . Radical abstraction occurred at the anomeric centre and at the 5′‐position of the glucosyl moiety. Computational studies of the hydrogen‐abstraction step were carried out with model structures. The activation barriers of this step for different stereoisomers and the abstraction at the anomeric centre and at the 6‘‐position of the tetrahydropyranyl moiety were calculated. The results of this investigation are in accordance with experimental observations. Furthermore, they reveal that the reactivity and regioselectivity are mainly determined in the hydrogen‐abstraction step. Intramolecular hydrogen abstraction (almost simultaneous electron and proton transfer) in ³ππ* excited furanones only takes place under restricted structural conditions in a limited number of conformations that are defined by the relative configuration of the substrates. It is observed that in the biradical intermediate, back‐hydrogen transfer occurs leading to the starting compound. In the case of glucose derivatives, this reaction led to epimerisation at the anomeric centre.     

Distance Dependence of Bidirectional Concerted Proton–Electron Transfer in Phenol‐Ru(2,2′‐bipyridine)32+ Dyads

Proton‐coupled electron transfer (PCET) was investigated in three covalent donor–bridge–acceptor molecules with different bridge lengths. Upon photoexcitation of their Ru(bpy)₃²⁺ (bpy=2,2′‐bipyridine) photosensitizer in acetonitrile, intramolecular long‐range electron transfer from a phenolic unit to Ru(bpy)₃²⁺ occurs in concert with release of the phenolic proton to pyrrolidine base. The kinetics of this bidirectional concerted proton–electron transfer (CPET) reaction were studied as a function of phenol–Ru(bpy)₃²⁺ distance by increasing the number of bridging p‐xylene units. A distance decay constant (β) of 0.67±0.23 Å^?1 was determined. The distance dependence of the rates for CPET is thus not significantly steeper than that for ordinary (i.e., not proton coupled) electron transfer across the same bridges, despite the concerted motion of oppositely charged particles into different directions. Long‐range bidirectional CPET is an important reaction in many proteins and plays a key role in photosynthesis; our results are relevant in the context of photoinduced separation of protons and electrons as a means of light‐to‐chemical energy conversion. This is the first determination of β for a bidirectional CPET reaction.     

Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase

The hydrogenation of benzaldehyde to benzyl alcohol on carbon‐supported metals in water, enabled by an external potential, is markedly promoted by polarization of the functional groups. The presence of polar co‐adsorbates, such as substituted phenols, enhances the hydrogenation rate of the aldehyde by two effects, that is, polarizing the carbonyl group and increasing the probability of forming a transition state for H addition. These two effects enable a hydrogenation route, in which phenol acts as a conduit for proton addition, with a higher rate than the direct proton transfer from hydronium ions. The fast hydrogenation enabled by the presence of phenol and applied potential overcompensates for the decrease in coverage of benzaldehyde caused by competitive adsorption. A higher acid strength of the co‐adsorbate increases the intensity of interactions and the rates of selective carbonyl reduction.     

Highly Dispersed Platinum Nanoparticles on TiO2 Prepared by Using the Microwave‐Assisted Deposition Method: An Efficient Photocatalyst for the Formation of H2 and N2 from Aqueous NH3

A simple and practical technique to synthesize nanosized platinum particles loaded on TiO₂ (Pt–TiO₂) by using a microwave (Mw)‐assisted deposition method has been exploited in the development of a highly efficient photocatalyst for the formation of H₂ and N₂ gases from harmful nitrogen‐containing chemical wastes, for example, aqueous ammonia (NH₃). Upon Mw irradiation, a platinum precursor can be deposited quickly on the TiO₂ surface from an aqueous solution of platinum and subsequent reduction with H₂ affords the nanosized platinum metal particles with a narrow size distribution (Mw‐Pt–TiO₂). Characterization by CO adsorption, platinum L_III‐edge X‐ray absorption fine structure analysis, and TEM analysis revealed that the size of the metal nanoparticles strongly depended on the preparation methods. Smaller platinum nanoparticles were obtained by the Mw heating method than those obtained by conventional preparation techniques, such as photoassisted deposition (PAD), impregnation (Imp), and equilibrium adsorption (EA) deposition by conventional convective heating. The H₂ and N₂ formation rates increased with increasing dispersity of platinum. Pt–TiO₂ prepared by the Mw heating method exhibited a specifically high H₂ formation activity in the photocatalytic decomposition of aqueous NH₃ in a nearly stoichiometric 3:1 (H₂/N₂) molar ratio under inert conditions. The present Mw heating method is applicable to a variety of anatase‐type TiO₂ species possessing different specific surface areas to provide small and highly dispersed platinum nanoparticles with a narrow size distribution.     

Dynamics of Excess‐Electron Transfer through Alternating Adenine:Thymine Sequences in DNA

This paper presents the results of an investigation into the sequence‐dependent excess‐electron transfer (EET) dynamics in DNA, which plays an important role in DNA damage/repair. There are many published studies on EET in consecutive adenine:thymine (A:T) sequences ( Tn ), but those in alternating A:T sequences ( ATn ) remain limited. Here, two series of functionalized DNA oligomers, Tn and ATn , were synthesized with a strongly electron‐donating photosensitizer, a trimer of ethylenedioxythiophene ( 3 E ), and an electron acceptor, diphenylacetylene ( DPA ). Laser flash photolysis experiments showed that the EET rate constant of AT3 is two times lower than that of T3 due to the lack of π‐stacking of Ts in AT3 . Thus, it was indicated that excess‐electron hopping is affected by the interaction between LUMOs of nucleotides.     

Cobalt Phthalocyanine Immobilized on Graphene Oxide: An Efficient Visible‐Active Catalyst for the Photoreduction of Carbon Dioxide

New graphene oxide (GO)‐tethered–Co^II phthalocyanine complex [CoPc–GO] was synthesized by a stepwise procedure and demonstrated to be an efficient, cost‐effective and recyclable photocatalyst for the reduction of carbon dioxide to produce methanol as the main product. The developed GO‐immobilized CoPc was characterized by X‐ray diffraction (XRD), FTIR, XPS, Raman, diffusion reflection UV/Vis spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP‐AES), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). FTIR, XPS, Raman, UV/Vis and ICP‐AES along with elemental analysis data showed that Co^II–Pc complex was successfully grafted on GO. The prepared catalyst was used for the photocatalytic reduction of carbon dioxide by using water as a solvent and triethylamine as the sacrificial donor. Methanol was obtained as the major reaction product along with the formation of minor amount of CO (0.82 %). It was found that GO‐grafted CoPc exhibited higher photocatalytic activity than homogeneous CoPc, as well as GO, and showed good recoverability without significant leaching during the reaction. Quantitative determination of methanol was done by GC flame‐ionization detector (FID), and verification of product was done by NMR spectroscopy. The yield of methanol after 48 h of reaction by using GO–CoPc catalyst in the presence of sacrificial donor triethylamine was found to be 3781.8881 μmol g^?1 cat., and the conversion rate was found to be 78.7893 μmol g^?1cat. h^?1. After the photoreduction experiment, the catalyst was easily recovered by filtration and reused for the subsequent recycling experiment without significant change in the catalytic efficiency.     

Cobalt Single‐Atom Catalysts with High Stability for Selective Dehydrogenation of Formic Acid

Metal–organic framework (MOF)‐derived Co‐N‐C catalysts with isolated single cobalt atoms have been synthesized and compared with cobalt nanoparticles for formic acid dehydrogenation. The atomically dispersed Co‐N‐C catalyst achieves superior activity, better acid resistance, and improved long‐term stability compared with nanoparticles synthesized by a similar route. High‐angle annular dark‐field–scanning transmission electron microscopy, X‐ray photoelectron spectroscopy, electron paramagnetic resonance, and X‐ray absorption fine structure characterizations reveal the formation of Co^IIN_x centers as active sites. The optimal low‐cost catalyst is a promising candidate for liquid H₂ generation.     

Interfacial Impregnation Chemistry in the Synthesis of Cobalt Catalysts Supported on Titania

The interfacial chemistry of the impregnation step involved in the synthesis of cobalt catalysts supported on titania was investigated with regard to the mode of interfacial deposition of the aqua complex [Co(H₂O)₆]²⁺ on the “titania/electrolyte solution” interface, the structure of the inner‐sphere complexes formed, and their relative interfacial concentrations. Several methodologies based on the application of deposition experiments and electrochemical techniques were used in conjunction with diffuse‐reflectance spectroscopy and EPR spectroscopy. These suggested the formation of mononuclear/oligonuclear inner‐sphere complexes on deposition of the [Co(H₂O)₆]²⁺ ions at the “titania/electrolyte solution” interface. The joint application of semiempirical quantum‐mechanical calculations, stereochemical considerations, and modeling of the deposition data revealed the exact structure of these complexes and allowed their relative concentrations at various Co^II surface concentrations to be determined. It was found that the interface speciation depends on the Co^II surface concentration. Mononuclear complexes are formed at the compact layer of the “titania/electrolyte solution” interface for low and medium Co^II surface concentrations. Formation of mono‐hydrolyzed Ti₂O–TiO and the dihydrolyzed TiO–TiO disubstituted configurations is very probable. In the first configuration one water ligand of the [Co(H₂O)₆]²⁺ ion is substituted by a bridging surface oxygen atom and another by a terminal surface oxygen atom. In the second configuration two water ligands of the [Co(H₂O)₆]²⁺ ion are substituted by two terminal surface oxygen atoms. Binuclear and trinuclear inner‐sphere complexes are formed, in addition to the mononuclear ones, at relatively high Co^II surface concentrations.     

Energy Transfer in Dye‐Coupled Lanthanide‐Doped Nanoparticles: From Design to Application

Surface modification with organic dye molecules is a useful strategy to manipulate the optical properties of lanthanide‐doped nanoparticles (LnNPs). It enables energy transfer between dyes and LnNPs, which provides unprecedented possibilities to gain new optical phenomena from the dye–LnNPs composite systems. This has led to a wide range of emerging applications, such as biosensing, drug delivery, gene targeting, information storage, and photon energy conversion. Herein, the mechanism of energy transfer and the structural‐dependent energy‐transfer properties in dye‐coupled LnNPs are reviewed. The design strategies for achieving effective dye–LnNP functionalization are presented. Recent advances in these composite nanomaterials in biomedicine and energy conversion applications are highlighted.