Possible Impact of Heterogeneous Photocatalysis on the Global Chemistry of the Earth's Atmosphere

Photochemistry is recognized to be important for various physicochemical processes in the atmosphere, such as formation of the ozone layer and smogs, degradation of waste substances, etc. [1]. However, up to the present the emphasis in atmospheric photochemistry has been mainly on the study of photochemical reactions that occur with molecules directly excited by absorption of light quanta. However, the major components and impurities of the earth's atmosphere (such as nitrogen, oxygen, water, carbon dioxide, methane, methane halides, etc.) are totally transparent to most solar radiation. Electronically excited states of these molecules are formed only upon absorption of vacuum ultraviolet light quanta with energy hv ≥ 5 eV (i.e., with wavelength λ ≤ 200 nm). Only a small portion of the energy of solar light is found in this spectral region. In other words, most of the energy of the solar flux cannot participate in such direct photochemical reactions.     

Possible Impact of Heterogeneous Photocatalysis on the Global Chemistry of the Earth's Atmosphere

Photochemistry is recognized to be important for various physicochemical processes in the atmosphere, such as formation of the ozone layer and smogs, degradation of waste substances, etc. [1]. However, up to the present the emphasis in atmospheric photochemistry has been mainly on the study of photochemical reactions that occur with molecules directly excited by absorption of light quanta. However, the major components and impurities of the earth's atmosphere (such as nitrogen, oxygen, water, carbon dioxide, methane, methane halides, etc.) are totally transparent to most solar radiation. Electronically excited states of these molecules are formed only upon absorption of vacuum ultraviolet light quanta with energy hv ≥ 5 eV (i.e., with wavelength λ ≤ 200 nm). Only a small portion of the energy of solar light is found in this spectral region. In other words, most of the energy of the solar flux cannot participate in such direct photochemical reactions.     

有机激发三重态参与的光化学反应研究进展

光化学反应包括直接光解和间接氧化反应，其中间接氧化反应主要通过活性氧化性物种（reactive oxygen species, ROS）包括单线态氧（¹O₂）、过氧自由基（{\mathrm{O}}_{\mathrm{2}}^{-}）和羟基自由基（·OH）等进行。近年来，有机激发三重态（³C^*）作为一种特殊的氧化剂参与光化学反应引起了广泛关注。本文主要综述了有机三重态光敏剂的形成和光化学反应机制、³C^*参与的环境光化学反应、³C^*自由基寿命、反应速率及稳态浓度测定和激发三重态的应用5个方面，并对未来有机分子激发三重态的研究方向进行了展望，指出³C^*是很多挥发性有机化合物（VOCs）或半/中等挥发性有机化合物（S/IVOCs）在大气中重要的汇，自然水体中溶解性有机质（dissolved organic matter，DOM）的激发三重态（³DOM^*）是污染物降解的主要氧化剂。今后应扩大³C^*与有机物反应的研究范畴，测定不同体系中³C^*的稳态浓度、产生速率（量子产率）等，为污染物治理提供理论依据。     

Molecular strategies to read and write at the nanoscale with far-field optics

Diffraction prevents the focusing of ultraviolet and visible radiations within nanoscaled volumes and, as a result, the imaging and patterning of nanostructures with conventional far-field illumination. Specifically, the irradiation of a fluorescent or photosensitive material with focused light results in the simultaneous excitation of multiple chromophores distributed over a large area, relative to the dimensions of single molecules. It follows that the spatial control of fluorescence and photochemical reactions with molecular precision is impossible with conventional illumination configurations. However, the photochemical and photophysical properties of organic chromophores can be engineered to overcome diffraction in combination with patterned or reiterative illumination. These ingenious strategies offer the opportunity to confine excited chromophores within nanoscaled volumes and, therefore, restrict fluorescence or photochemical reactions within subdiffraction areas. Indeed, information can be "read" in the form of fluorescence and "written" in the form of photochemical products with resolution down to the nanometre level on the basis of these innovative approaches. In fact, these promising far-field optical methods permit the convenient imaging of biological samples and fabrication of miniaturized objects with unprecedented resolution and can have long-term and profound implications in biomedical research and information technology.     

Photochemistry and photophysics of poly(organophosphazenes) and related compounds: A review. II. Bimolecular reactions

In the second part of this review we highlight the bimolecular reactions (hydrogen abstraction, and energy or electron transfer) that take place in the photochemistry of poly(organophosphazenes). Both inter-molecular interactions (i.e. between excited free chromophores and ground state groups attached to the phosphazenes, or between excited phosphazene substituents and external quenchers) and intra-molecular processes (i.e. between excited and ground state groups geminally attached to the same phosphorus or supported to different phosphorus along the polyphosphazene skeleton) are exploited. Suggestions are given on the possible practical application of these reactions in different photochemical domains, e.g. heterogeneous-phase photosensitization, photocrosslinking, photoconductivity, microelectronics, light-induced radical polymerization of vinyl monomers, etc.     

Photostability of Anthraquinone and Azo Dyes in N-Ethylacetamide (Nylon Model)

Measurements of the quantum yields of photodegradation of anthraquinone and azo dyes in N-ethylacetamide (nylon model) and triplet sensitisation of dye fading showed that the photochemical reactions are initiated by an upper excited n-π* triplet state. The primary photochemical reaction with both anthraquinone and azo dyes involves hydrogen abstraction from the amide so/vent. In some cases oxygen retards dye fading by reoxidation of the reduced structures. In other cases oxygen accelerates the photochemical reaction via free-radical initiated oxidation reactions. The quantum yields of dye fading are strongly wavelength dependent. On exposure to a simulated sunlight spectrum photodegradation is mostly caused by radiation in the region 300–400 nm. While unsubstituted anthraquinone is very photoreactive, amino-substituted anthraquinones are much more photostable in N-ethylacetam/de. Photostability of mono- and di-substituted anthraquinone derivatives increases with the electron-donating power of substituent groups. Azo dyes with increased conjugation such as diazo and naphthazo structures are more photostable than simple azobenzene derivatives. Certain electron-withdrawing substituents, which do not affect dye colour, improve the ‘oxidative’ photostability of both anthraquinone and azo dyes. A cobalt premetallised azo dye (C.I. Acid Red 182) is very photostable in N-ethylacetamide, showing the effect of metal chelates on photostability. The low values of quantum yields for the dye solutions are comparable with those of acid and disperse dyes in nylon films, indicating that N-ethylacetamide is a suitable nylon model for mechanistic studies of dye fading.     

Rigorous and simplified approach to the modelling of continuous photoreactors

Photochemical reactions where the absorption of radiant energy occurs by the reacting species are considered. In this case the radiant energy balance equation can not be decoupled from the mass balance of the reacting species and a highly non linear integro-differential problem arises.Mathematical difficulties have been usually bypassed by modelling the radiation field on the basis of simplified models of questionable physical reliability. A rigorous modelling of the radiation field has not yet been considered for such a class of photochemical processes.An approach based on a rigorous modelling is therefore presented here, the solution being obtained through a transient approach. Results are compared with those obtained on the basis of simplified models in order to investigate the range of the significant parameters where such simple models, which are less computer-time demanding, can be conveniently used.     

Hydrogen bonding in the electronic excited state

Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology.     

The Dynamic Roles of Interstitial and Surface Defects on Oxidation and Reduction Reactions on Titania

Defect sites in titania have a substantial effect on many thermal and photochemical reactions. Three common types of defects are titanium interstitials, oxygen vacancies and oxygen adatoms—all of which can react with organic molecules adsorbed on the surface. We look broadly at thermal reduction and photochemical oxidation reactions of oxygenates. In particular, we focus on the reductive coupling of benzaldehyde, the photo-oxidation of butyrophenone, the photostability of benzoic acid, and the photo-oxidative coupling of methanol to methyl formate. Methods used include temperature programmed reaction spectroscopy, scanning tunneling microscopy, and density functional theory.     

Photoactivation and Detection of Photoexcited Molecules and Photochemical Products

Aerobic photoactivation of photosensitizing dye molecules can lead to the formation of oxygen radicals, singlet oxygen and other partially reduced oxygen species, collectively called reactive oxygen species (ROS), which are responsible for photodynamic damage and the accompanying cytotoxicity. This review briefly describes basic photophysical phenomena involved in the formation of electronically excited states and photochemical processes that play a key role in the generation of ROS. Physicochemical properties of the excited states of the photosensitizing dye molecules and of ROS, particularly their chemical reactivity with selected substrate molecules, as well as major spectroscopic and analytical methods used for the detection and characterization of reactive intermediates involved in photodynamic phenomena, are critically discussed in this paper.     

光化学反应器

光化学反应过程由于具有选择性好且可在常温常压下进行等特点而在施工领域有着良好的应用前景，光化学反应器作为光化学生产过程的核心设备，在光化学工艺的应用中具有十分重要的地位。本文从工程应用的角度出发，介绍了常见的光化学反应器的类型，总结了描述光化学反应器行为的模型，并讨论了有关光化学反应器的设计计算及放大等问题。     

光化学微反应技术的基础及研究进展

光化学转化是有效利用光能实现化学反应的重要途径,微反应技术为提高其过程效率提供了一个强有力的平台。本文首先指出微反应器相比于传统釜式光反应器,在光强分布、过程放大、光能利用效率等诸多方面存在明显的优势,能够实现光化学反应过程的高效强化。简要地介绍了光化学转化及光化学微反应技术的基本特征,然后系统地综述了光化学微反应器的设计构建及其在有机合成、聚合等方面的应用,并详细介绍了自动化控制的光化学微反应系统及应用。重点介绍了微反应技术在紫外光、可见光辐照下的光化学合成进展及其过程放大。最后,对光化学微反应技术的研究进展进行总结,并对其发展趋势进行了展望。     

Studies leading to the development of a single-electron transfer (SET) photochemical strategy for syntheses of macrocyclic polyethers, polythioethers, and polyamides

Organic photochemists began to recognize in the 1970s that a new mechanistic pathway involving excited-state single-electron transfer (SET) could be used to drive unique photochemical reactions. Arnold's seminal studies demonstrated that SET photochemical reactions proceed by way of ion radical intermediates, the properties of which govern the nature of the ensuing reaction pathways. Thus, in contrast to classical photochemical reactions, SET-promoted excited-state processes are controlled by the nature and rates of secondary reactions of intermediate ion radicals. In this Account, we discuss our work in harnessing SET pathways for photochemical synthesis, focusing on the successful production of macrocyclic polyethers, polythioethers, and polyamides. One major thrust of our studies in SET photochemistry has been to develop new, efficient reactions that can be used for the preparation of important natural and non-natural substances. Our efforts with α-silyl donor-tethered phthalimides and naphthalimides have led to the discovery of efficient photochemical processes in which excited-state SET is followed by regioselective formation of carbon-centered radicals. The radical formation takes place through nucleophile-assisted desilylation of intermediate α-silyl-substituted ether-, thioether-, amine-, and amide-centered cation radicals. Early laser flash photolysis studies demonstrated that the rates of methanol- and water-promoted bimolecular desilylations of cation radicals (derived from α-silyl electron donors) exceeded the rates of other cation radical α-fragmentation processes, such as α-deprotonation. In addition, mechanistic analyses of a variety of SET-promoted photocyclization reactions of α-silyl polydonor-linked phthalimides and naphthalimides showed that the chemical and quantum efficiencies of the processes are highly dependent on the lengths and types of the chains connecting the imide acceptor and α-silyl electron donor centers. We also observed that reaction efficiencies are controlled by the rates of desilylation at the α-silyl donor cation radical moieties in intermediate zwitterionic biradicals that are formed by either direct excited-state intramolecular SET or by SET between the donor sites in the intervening chains. It is important to note that knowledge about how these factors govern product yields, regiochemical selectivities, and quantum efficiencies was crucial for the design of synthetically useful photochemical reactions of linked polydonor-acceptor substrates. The fruits of these insights are exemplified by synthetic applications in the concise preparation of cyclic peptide mimics, crown ethers and their lariat- and bis-analogs, and substances that serve as fluorescence sensors for important heavy metal cations.     

The effect of the framework structure on the chemical properties of the vanadium oxide species incorporated within zeolites

V-silicalite catalysts (VS-1 and VS-2) prepared by hydrothermal synthesis have been studied by ESR, XAFS (XANES and EXAFS) and photoluminescence spectroscopy. The in situ characterization of these V-silicalites shows that vanadium is present within the zeolitic framework as a highly dispersed tetrahedrally coordinated V-oxides, VO₄ unit, having a short V=O bond length. Photoluminescence spectroscopy in static and dynamic mode, as well as XAFS studies allow to detect in the V-silicalites different V species than that present in V-HMS or V/SiO₂, in terms of V=O bond length, vibrational energy, bond angle and lifetime of the excited triplet state. It is suggested that the combined contribution of the neighboring Si---OH group attached to the VO₄ unit and the zeolitic rigid framework structure of V-silicalites cause a more significant and pronounced effect on the chemical properties of the VO₄ unit than the flexible structure of V-HMS or V/SiO₂. Moreover, the dynamic quenching of the phosphorescence by the addition of reactant molecules such as NO or propane indicates that the V species in the excited triplet state can be expected to be the active sites for the photocatalytic reactions.     

Environmentally friendly photosensitizer: starch modified with chlorophyll‐type chromophores

A novel water‐soluble polymeric photosensitizer (SPheo) based on starch and containing pheophorbide (Pheo), a chlorophyll‐derived chromophore, was synthesized and its photophysical and photochemical properties were studied. Pheo chromophores attached to the polymeric chains of starch absorb light of the UV‐visible spectral region. The clustering of hydrophobic Pheo chromophores results in the formation of hydrophobic microdomains in aqueous solution where organic molecules can be solubilized. SPheo polymer efficiently photosensitizes reactions mediated by energy and/or electron transfer from the electronically excited chromophores to the molecules of organic compounds solubilized in polymeric microdomains or residing in water. Copyright © 2007 Society of Chemical Industry     

The role of far UV radiation in the photografting process

Summary With low density polyethylene (LDPE) film as substrate, polyethyleneterephthalate (PET) film as filter, and an high pressure mercury (HPM 15) lamp as UV radiation source, the function of far UV radiation was examined. The results show that when the far UV (200–300 nm) was eliminated, the rate of polymerization of acrylic acid in the interlayer between two LDPE films initiated by benzophenone (BP) dramatically decreased, and the grafting efficiency became close to zero. The decisive effect of the far UV is further confirmed in UV-VIS spectra measuring the hydrogen abstraction reaction of the excited BP. For polymerization systems containing allylic hydrogens, this effect is smaller, while hydroxycyclohexyl phenyl ketone (HHPK) and benzoyldimethylketal (BDK), which are typical photocleaving initiators, show little sensitivity to the far UV. Based on an energy graph of the excited states, the bond energies and the relevant photochemical reactions a tentative interpretation of the results has been made.     

Plasmonic antenna effects on photochemical reactions

Efficient solar energy conversion has been vigorously pursued since the 1970s, but its large-scale implementation hinges on the availability of high-efficiency modules. For maximum efficiency, it is important to absorb most of the incoming radiation, which necessitates both efficient photoexcitation and minimal electron-hole recombination. To date, researchers have primarily focused on the latter difficulty: finding a strategy to effectively separate photoinduced electrons and holes. Very few reports have been devoted to broadband sunlight absorption and photoexcitation. However, the currently available photovoltaic cells, such as amorphous silicon, and even single-crystal silicon and sensitized solar cells, cannot respond to the wide range of the solar spectrum. The photoelectric conversion characteristics of solar cells generally decrease in the infrared wavelength range. Thus, the fraction of the solar spectrum absorbed is relatively poor. In addition, the large mismatch between the diffraction limit of light and the absorption cross-section makes the probability of interactions between photons and cell materials quite low, which greatly limits photoexcitation efficiency. Therefore, there is a pressing need for research aimed at finding conditions that lead to highly efficient photoexcitation over a wide spectrum of sunlight, particularly in the visible to near-infrared wavelengths. As characterized in the emerging field of plasmonics, metallic nanostructures are endowed with optical antenna effects. These plasmonic antenna effects provide a promising platform for artificially sidestepping the diffraction limit of light and strongly enhancing absorption cross-sections. Moreover, they can efficiently excite photochemical reactions between photons and molecules close to an optical antenna through the local field enhancement. This technology has the potential to induce highly efficient photoexcitation between photons and molecules over a wide spectrum of sunlight, from visible to near-infrared wavelengths. In this Account, we describe our recent work in using metallic nanostructures to assist photochemical reactions for augmenting photoexcitation efficiency. These studies investigate the optical antenna effects of coupled plasmonic gold nanoblocks, which were fabricated with electron-beam lithography and a lift-off technique to afford high resolution and nanometric accuracy. The two-photon photoluminescence of gold and the resulting nonlinear photopolymerization on gold nanoblocks substantiate the existence of enhanced optical field domains. Local two-photon photochemical reactions due to weak incoherent light sources were identified. The optical antenna effects support the unprecedented realization of (i) direct photocarrier injection from the gold nanorods into TiO(2) and (ii) efficient and stable photocurrent generation in the absence of electron donors from visible (450 nm) to near-infrared (1300 nm) wavelengths.     

Theoretical considerations on the sensitivities of electron beam resists

A theoretical study has been carried out in order to explain the sensitivities of electron beam and X-ray resists. A preliminary investigation reveals that the behavior of these resists, on irradiation by high energy radiation, may be considered to be the electronically-excited species in the polymer. To elucidate the chemical reactions in the excited states the adiabatic potential curves are calculated by the INDO/S procedure, which considers all the valence electrons and all the singly excited electronic configurations. Polyethylene and polyisobutylene were chosen as representative of crosslinkable and degradable polymers, respectively, since there is a parallelism between the beam sensitivity of resists and the effects of high energy radiation on polymers. Polyisobutylene has many antibonding curves favorable for the main chain scission in the excited states and polyethylene does not except for one improbable state. It was concluded that degradability is explainable by the ease of bond fission in the excited states; the crosslinkability is considered to be nondegradable property.     

Photochemical synthesis of oligothiophene thin films and nano-patterns in condensed multilayer films of 2,5-diiodothiophene—Effects of surface chemistry of substrates

This paper reviews photochemical reactions of 2,5-diiodothiophene multilayers on various solid substrates that lead to production of oligothiophene thin films and micro-patterns with a thickness relevant to nanotechnology applications. Upon UV absorption, the CI bond of 2,5-diiodothiophene dissociates generating a thienyl radical and iodine atom. The radicals generated in multilayers can react with other radicals or intact monomers to form dimers. Since the CI bonds are present at the ends of the coupling reaction product, further photodissociation and coupling reactions can take place forming oligomeric species. On inert substrates, the average conjugation length of the product is about 3–4 thienyl units. Various pattern generation schemes can be incorporated with this photochemical reaction. Examples of masked irradiation, wettability pre-patterning, and controlled clustering of thermal desorption process are demonstrated. On copper surfaces, the average conjugation length of the produced oligothiophene is increased to 6–7 units, which is long enough for technical applications. This increase is due to Ullmann coupling reactions at the buried interface between the copper and adsorbed film. The mechanism for this buried surface reaction is elucidated from the thickness dependence of the conjugation length and Ullmann coupling side product.