Emerging Light-Harvesting Materials Based on Organic Photovoltaic D/A Heterojunctions for Efficient Photocatalytic Water Splitting

Photocatalytic water splitting to hydrogen is a highly promising method to meet the surging energy consumption globally through the environmentally friendly means. As the initial step before photocatalysis, harvesting photons from sunlight is crucially important, thus making the design of photosensitizers with visible even near-infrared (NIR) absorptions get more and more attentions. In the past three years, organic donor/acceptor (D/A) heterojunctions with absorptions extending to 950 nm, have emerged as the new star light-harvesting materials for photocatalytic water splitting, demonstrating exciting advantages over inorganic materials in solar light utilization, hydrogen yielding rate, etc. This Minireview firstly gives a brief discussion about the principle processes and determining factors for photocatalytic water splitting with organic photovoltaic D/A heterojunction as photosensitizers. Thereafter, the current progress is summarized in details by introducing typical and excellent D/A heterojunction-based photocatalytic systems. Finally, not only the great prospects but also the most challenging issues confronted by organic D/A heterojunctions are indicated along with a perspective on the opportunities and new directions for future material explorations.     

可见光促进的偕二氟烯烃制备与应用研究进展

偕二氟烯烃化合物在医药、材料、精细化工等领域具有广泛的应用,同时也是一类非常重要的用于制备各类复杂的有机氟化合物的有机合成中间体.可见光催化反应具有条件温和、绿色清洁等特点,已经成为有机化学中非常重要的合成手段之一.本文综述了近年来可见光促进的偕二氟烯烃化合物的制备及其在有机合成化学中的应用.     

Recent development in near infrared light-responsive polymeric materials for smart drug-delivery systems

Stimuli-responsive drug delivery systems (DDS) may overcome the drawbacks of conventional chemotherapy for cancer treatment. In particular, light-responsive polymer-based DDS may ensure spatio and temporal control in drug delivery. In this regard, near infrared (NIR) light triggered drug nanocarriers present several advantages when compared to UV–visible light triggered nanocarriers. This review surveys the recent development on the design, synthesis, functions, and applications of NIR photo-sensitive compounds in the development of long-wavelength light-responsive nanocarriers. Diverse NIR light responsive groups such as coumarin (CM), ortho-nitrobenzyl (ONB), 2-diazo-1,2-naphthoquinone (DNQ) and spiropyran (SP) derivatives and their photo-cleavage reaction mechanisms are discussed, as well as the use of indocyanine green (ICG) and its photo-thermal application. The loading into polymeric nanocarriers of up converting nanoparticles (UCNPs) which can convert NIR light into UV or visible light is also discussed. The described DDS are classified on the basis on the photo responsive groups. In details, the behavior of different polymeric materials such as micelles, hydrogels bearing photo responsive groups linked to bioactive molecules which are released under NIR light irradiation is reviewed and discussed. A section relative to commonly used instrument setup for drug release studies by NIR light irradiation is also presented for better understanding how the light has been used to irradiate in various experimental situations.     

Anomalous Light-Induced Spin-State Switching for Iron(II) Spin-Crossover Molecules in Direct Contact with Metal Surfaces

Light-induced spin-state switching is one of the most attractive properties of spin-crossover materials. In bulk, low-spin (LS) to high-spin (HS) conversion via the light-induced excited spin-state trapping (LIESST) effect may be achieved with a visible light, while the HS-to-LS one (reverse-LIESST) requires an excitation in the near-infrared range. Now, it is shown that those phenomena are strongly modified at the interface with a metal. Indeed, an anomalous spin conversion is presented from HS state to LS state under blue light illumination for Fe^II spin-crossover molecules that are in direct contact with metallic (111) single-crystal surfaces (copper, silver, and gold). To interpret this anomalous spin-state switching, a new mechanism is proposed for the spin conversion based on the light absorption by the substrate that can generate low energy valence photoelectrons promoting molecular vibrational excitations and subsequent spin-state switching at the molecule–metal interface.     

Thermally stable,photoactive polymerizable diazenes. Applications in polymer technology

New and useful nonpolymerizable and polymerizable diazene free radical initiators have been designed, based on molecular orbital and empirical linear free energy calculations. The design of the polymerizable diazenes has been based on four requirements: independent reactivities of azo and vinyl moieties, high thermal stability of the trans-diazene isomers, efficient convertability of the trans isomer to the cis isomer, and high thermal reactivity (to produce free radicals) of the cis-diazene isomers. The cis-diazene isomers are obtained from the trans isomers by irradiation with visible light. A new free radical initiator has been shown to be useful in preparing graft copolymers and in crosslinking polymer chains by visible light activation. Poly(styrene-co-2-(meta-styrylazo)-2-methoxypropane) (2) was prepared by the copolymerization of styrene monomer and 2-(meta-styrylazo)-2-methoxypropane (1), either thermally or via a free radical initiator. Copolymer 2 then was reacted with methyl methacrylate monomer in the presence of visible light, to produce poly (styrene-g-methyl methacrylate) (6). Poly(styrene-co-2-(meta-styrylazo)-2-methylpropane) (17), prepared by the copolymerization of styrene monomer and 2-(meta-styrylazo)-2-methylpropane (15), was successfully crosslinked at elevated temperatures. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3203–3213, 1999     

Photo Catalytic Degradation of Azo Dyes over Mn2+ Doped TiO2 Catalyst under UV/Solar Light:An Insight to the Route of Electron Transfer in the Mixed Phase of Anatase and Rutile

Mn²⁺ ion was doped into the TiO₂ matrix and its photocatalytic activity was evaluated for the degradation of a mono azo dye methyl orange (MO) and a di‐azo dye brilliant yellow (BY) under UV/solar light. X‐ray diffraction results revealed the phase transformation from anatase to rutile due to the inclusion of Mn²⁺ ion into the TiO₂ matrix. All the doped catalysts showed a red shift in the band gap to the visible region. The degradation reaction of the dyes was found to be dependent on its structure. It was found that mono azo dye degrades faster than di azo dye under UV/solar light. The rate constant under identical conditions calculated for the degradation of MO is 2.4 times (under UV light) and 4.5 times (under solar light) higher compared to BY. Among the photocatalysts studied, Mn²⁺(0.06 at.%)‐TiO₂ showed higher activity under both UV and solar light illumination. The synergestic effect in the bicrystalline framework of anatase and rutile effectively suppresses the charge carrier recombination and enhances the photocatalytic activity. The degradation reaction was followed by UV‐visible spectroscopy and the photoproducts formed were analyzed by GC‐MS techniques.     

Phototunable Liquid‐Crystalline Phases Made of Nanoparticles

The properties of liquid‐crystalline (LC) hybrid systems made of inorganic nanoparticles grafted with photosensitive azo compounds are presented. For materials with a large density of azo ligands at the surface, the LC structure can be reversibly melted by UV light, and the return to the LC state does not require the absorption of visible light. For systems with a lower density of azo ligands, UV light causes shortening of the distance between metal sublayers in the lamellar phase. Interestingly, the azo derivatives attached to the nanoparticle surface show very different kinetics of cis/trans conformational change as compared to the free molecules. The cis form of free ligands in solution is stable for days, whereas the isomerization of molecules attached to the nanoparticle surface to the trans form takes only a few minutes. Apparently, owing to the crowded environment, azo ligands immobilized at a metal surface behave as they would in the condensed state.     

Photocatalytic Water Splitting Under Visible Light with Particulate Semiconductor Catalysts

Photocatalytic water splitting (PWS) is the most promising technology to produce H₂ energy directly from renewable water and solar light. PWS has made a remarkable progress last decades under ultra-violet (UV) light, but there are many technical challenges remaining for PWS under visible light. Several approaches are taken in search of photocatalysts efficient for PWS under visible light: (i) to find new single phase materials, (ii) to decorate UV-active photocatalysts with a photosensitizer absorbing visible light, (iii) to tune the band gap energy by modifying cations or anions of UV-active photocatalysts with substitutional doping, and (iv) to fabricate multi-component photocatalysts by forming composites or solid solutions. This article discusses the above approaches based on our experimental results as well as data available in the literature. At the moment, the greatest challenge to the progress of visible light PWS is the low efficiency of light utilization. Finding new photocatalytic materials with unique structure and phase is still the key to the success. In addition, the synthesis of these materials with high crystallinity and high surface area is also important, because these properties exert great impact on the activity of the material of the same structure and phase. Finally, smart combination and modification of known materials could also be fruitful.     

Thermal and photostabilization studies on carbohydrates functionalized with azo chromophoric system 2‐[(E)‐(3‐hydroxynaphthalen‐2‐yl)diazenyl]benzoic acid

In the present work, 2‐[(E)‐(3‐hydroxynaphthalen‐2‐yl)diazenyl]benzoic acid, an azo chromophoric system was prepared and incorporated onto carbohydrates such as starch and cellulose by dicyclohexylcarbodiimide coupling. The products were characterized by UV‐visible, fluorescence, FT‐IR and NMR spectroscopic methods. The results of the studies show that incorporation of the chromophoric system onto the polymeric core enhanced the light absorption, emission and light stabilization properties of the chromophoric system. The light fastening properties of chromophoric system and the carbohydrate‐bound photochromic systems were compared. It shows that light stabilization of the chromophoric system greatly enhanced on attaching to the polymeric core materials. Thermal stability of the chromophoric system also enhanced on attaching to the biopolymeric core systems. Copyright © 2013 John Wiley & Sons, Ltd.     

ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

Improving the photochemical properties of molecular photoswitches is crucial for the development of light‐responsive systems in materials and life sciences. ortho‐Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ‐electron‐withdrawing F atoms ortho to the N?N unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para‐electron‐withdrawing groups (EWGs) work in concert with ortho‐F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho‐fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.     

Epoxy-based azo molecular glasses with four-arm architecture: Preparation,characterization and holographic recording

A series of epoxy-based azo molecular glasses with four-arm architecture was synthesized, characterized and applied to holographic recording.     

Photoluminescence switching of azobenzene-conjugated Pt(II) terpyridine complexes by trans-cis photoisomerization

Pt(II) complexes with a terpyridylazobenzene ligand (tpyAB) were newly synthesized, and their photoluminescence properties by trans-cis isomerization of the azo moiety were investigated. In these complexes, upon excitation with 366-nm light in polar solvents such as DMF, DMSO, and propylene carbonate, trans-to-cis isomerization with significant UV-vis spectral changes occurred almost completely. Cis-to-trans isomerization was observed both by irradiation with visible light and by heat. The reduction peaks due to the terpyridine and the azo group in the cyclic voltammograms of the Pt complexes were shifted in the positive direction by trans-to-cis isomerization. Emission spectral changes due to trans-cis isomerization were observed for both the tpyAB and the Pt complexes. The significant differences in the emission properties of the complex compared to tpyAB include the observation that both the excitation and emission wavelengths were shifted to lower energy, located in the visible region. Moreover, the change in emission intensity between the trans and cis forms was more significant upon excitation with UV light, because the trans form of the complexes showed absolutely no emission. Accordingly, the azobenzene-conjugated Pt(II) terpyridine complexes promise to be doubly photofunctional materials, showing complete off-on switching of emission linked to the trans-cis conformation change.     

Visible‐light‐driven Efficient Photocatalytic Reduction of Organic Azides to Amines over CdS Sheet–rGO Nanocomposite

CdS sheet–rGO nanocomposite as a heterogeneous photocatalyst enables visible‐light‐induced photocatalytic reduction of aromatic, heteroaromatic, aliphatic and sulfonyl azides to the corresponding amines using hydrazine hydrate as a reductant. The reaction shows excellent conversion and chemoselectivity towards the formation of the amine without self‐photoactivated azo compounds. In the adopted strategy, CdS not only accelerates the formation of nitrene through photoactivation of azide but also enhances the decomposition of azide to a certain extent, which entirely suppressed formation of the azo compound. The developed CdS sheet‐rGO nanocomposite catalyst is very active, providing excellent results under irradiation with a 40 W simple household CFL lamp.     

Structural Design Principle of Small‐Molecule Organic Semiconductors for Metal‐Free,Visible‐Light‐Promoted Photocatalysis

Herein, we report on the structural design principle of small‐molecule organic semiconductors as metal‐free, pure organic and visible light‐active photocatalysts. Two series of electron‐donor and acceptor‐type organic semiconductor molecules were synthesized to meet crucial requirements, such as 1) absorption range in the visible region, 2) sufficient photoredox potential, and 3) long lifetime of photogenerated excitons. The photocatalytic activity was demonstrated in the intermolecular C?H functionalization of electron‐rich heteroaromates with malonate derivatives. A mechanistic study of the light‐induced electron transport between the organic photocatalyst, substrate, and the sacrificial agent are described. With their tunable absorption range and defined energy‐band structure, the small‐molecule organic semiconductors could offer a new class of metal‐free and visible light‐active photocatalysts for chemical reactions.     

Visible‐Light‐Driven Halogen Bond Donor Based Molecular Switches: From Reversible Unwinding to Handedness Inversion in Self‐Organized Soft Helical Superstructures

Visible‐light‐driven molecular switches endowing reversible modulation of the functionalities of self‐organized soft materials are currently highly sought after for fundamental scientific studies and technological applications. Reported herein are the design and synthesis of two novel halogen bond donor based chiral molecular switches that exhibit reversible photoisomerization upon exposure to visible light of different wavelengths. These chiral molecular switches induce photoresponsive helical superstructures, that is, cholesteric liquid crystals, when doped into the commercially available room‐temperature achiral liquid crystal host 5CB, which also acts as a halogen‐bond acceptor. The induced helical superstructure containing the molecular switch with terminal iodo atoms exhibits visible‐light‐driven reversible unwinding, that is, a cholesteric–nematic phase transition. Interestingly, the molecular switch with terminal bromo atoms confers reversible handedness inversion to the helical superstructure upon irradiation with visible light of different wavelengths. This visible‐light‐driven, reversible handedness inversion, enabled by a halogen bond donor molecular switch, is unprecedented.     

Microwave-assisted synthesis of near-infrared fluorescent sphingosine derivatives

Microwave-assisted synthesis of near-infrared fluorescent sphingosine derivatives is described, and the utility of the probes demonstrated by co-localization studies with visible wavelength fluorescent sphingosine derivatives.     

Visible Light-Driven Molecular Switches and Motors: Recent Developments and Applications

Inspired by human vision, a diverse range of light-driven molecular switches and motors have been developed for fundamental understanding and application in material science and biology. Recently, the design and synthesis of visible light-driven molecular switches and motors have been actively pursued. This emerging trend is partly motivated to avoid the harmful effects of ultraviolet light, which was necessary to drive the classical molecular switches and motors at least in one direction, impeding their employment in biomedical and photopharmacology applications. Moreover, visible light-driven molecular switches and motors are demonstrated to enable benign optical materials for advanced photonic devices. Therefore, during the past several years, visible light-driven molecular switches based on azobenzene derivatives, diarylethenes, 1,2-dicyanodithienylethenes, hemithioindigo derivatives, iminothioindoxyls, donor-acceptor Stenhouse adducts, and overcrowded alkene based molecular motors have been judiciously designed, synthesized, and used in the development of functional materials and systems for a wide range of applications. In this Review, we present the recent developments toward the design of visible light-driven molecular switches and motors, with their applications in the fabrication of functional materials and systems in material science, bioscience, pharmacology, etc. The visible light-driven molecular switches and motors realized so far undoubtedly widen the scope of these interesting compounds for technological and biological applications. We hope this Review article could provide additional impetus and inspire further research interests for future exploration of visible light-driven advanced materials, systems, and devices.     

The coupling of alkylboronic acids and esters with Baylis–Hillman derivatives by Lewis base/photoredox dual catalysis

Utilizing Lewis base/photoredox dual catalysis, carbon radicals generated from either alkylboronic acids or esters were coupled with Baylis–Hillman derivatives under visible light irradiation. This protocol provides a mild and operationally simple method for the synthesis of a variety of α,β-unsaturated carbonyl compounds in a broad scope of the substrates. The mechanism of Lewis base activation and reductive quenching cycle was probably involved.     

可见光响应光催化剂研究进展*

本文综述了近年来可见光响应光催化剂的研究进展,从窄禁带半导体开发、表面光敏化和离子掺杂三个方面,详细探讨了该类光催化剂的构成原理、设计思想、制备方法、目前效果及缺点,展望了该领域的研究前景.     

A DFT study on photoinduced surface catalytic coupling reactions on nanostructured silver: selective formation of azobenzene derivatives from para-substituted nitrobenzene and aniline

We propose that aromatic nitro and amine compounds undergo photochemical reductive and oxidative coupling, respectively, to specifically produce azobenzene derivatives which exhibit characteristic Raman signals related to the azo group. A photoinduced charge transfer model is presented to explain the transformations observed in para-substituted ArNO(2) and ArNH(2) on nanostructured silver due to the surface plasmon resonance effect. Theoretical calculations show that the initial reaction takes place through excitation of an electron from the filled level of silver to the lowest unoccupied molecular orbital (LUMO) of an adsorbed ArNO(2) molecule, and from the highest occupied molecular orbital (HOMO) of an adsorbed ArNH(2) molecule to the unoccupied level of silver, during irradiation with visible light. The para-substituted ArNO(2)(-)˙ and ArNH(2)(+)˙ surface species react further to produce the azobenzene derivatives. Our results may provide a new strategy for the syntheses of aromatic azo dyes from aromatic nitro and amine compounds based on the use of nanostructured silver as a catalyst.