Showing papers in "Journal of Photochemistry and Photobiology C-photochemistry Reviews in 2015"

Visible-light activation of TiO2 photocatalysts: Advances in theory and experiments

Abstract: The remarkable achievement by Fujishima and Honda (1972) in the photo-electrochemical water splitting results in the extensive use of TiO 2 nanomaterials for environmental purification and energy storage/conversion applications. Though there are many advantages for the TiO 2 compared to other semiconductor photocatalysts, its band gap of 3.2 eV restrains application to the UV-region of the electromagnetic spectrum ( λ ≤ 387.5 nm). As a result, development of visible-light active titanium dioxide is one of the key challenges in the field of semiconductor photocatalysis. In this review, advances in the strategies for the visible light activation, origin of visible-light activity, and electronic structure of various visible-light active TiO 2 photocatalysts are discussed in detail. It has also been shown that if appropriate models are used, the theoretical insights can successfully be employed to develop novel catalysts to enhance the photocatalytic performance in the visible region. Recent developments in theory and experiments in visible-light induced water splitting, degradation of environmental pollutants, water and air purification and antibacterial applications are also reviewed. Various strategies to identify appropriate dopants for improved visible-light absorption and electron–hole separation to enhance the photocatalytic activity are discussed in detail, and a number of recommendations are also presented.

Review of material design and reactor engineering on TiO2 photocatalysis for CO2 reduction

Abstract: The continuous combustion of non-renewable fossil fuels and depletion of existing resources is intensifying the research and development of alternative future energy options that can directly abate and process ever-increasing carbon dioxide (CO2) emissions. Since CO2 is a thermodynamically stable compound, its reduction must not consume additional energy or increase net CO2 emissions. Renewable sources like solar energy provide readily available and continuous light supply required for driving this conversion process. Therefore, the use of solar energy to drive CO2 photocatalytic reactions simultaneously addresses the aforementioned challenges, while producing sustainable fuels or chemicals suitable for use in existing energy infrastructure. Recent progress in this area has focused on the development and testing of promising TiO2 based photocatalysts in different reactor configurations due to their unique physicochemical properties for CO2 photoreduction. TiO2 nanostructured materials with different morphological and textural properties modified by using organic and inorganic compounds as photosensitizers (dye sensitization), coupling semiconductors of different energy levels or doping with metals or non-metals have been tested. This review presents contemporary views on state of the art in photocatalytic CO2 reduction over titanium oxide (TiO2) nanostructured materials, with emphasis on material design and reactor configurations. In this review, we discuss existing and recent TiO2 based supports, encompassing comparative analysis of existing systems, novel designs being employed to improve selectivity and photoconversion rates as well as emerging opportunities for future development, crucial to the field of CO2 photocatalytic reduction. The influence of different operating and morphological variables on the selectivity and efficiency of CO2 photoreduction is reviewed. Finally, perspectives on the progress of TiO2 induced photocatalysis for CO2 photoreduction will be presented.

Photocatalytic reduction of CO2 using metal complexes

Abstract: Developing photocatalytic systems for CO 2 reduction will provide useful and energy-rich compounds and would be one of the most important focuses in the field of “artificial photosynthesis” and “solar fuels”. Such studies have been conducted in the past three decades from the perspective of basic science and for solving the shortage of fossil resources, which include both energy and carbon sources. More recently, focus has been placed on the mitigation of global warming through the reduction of atmospheric CO 2 . This review summarizes the enormous body of reported literature in this field, particularly studies that describe photocatalytic systems that use transition metal complexes as key players, i.e., as catalysts (Cat) and/or photosensitizers (PS). In addition, we briefly describe the evaluation of various photocatalytic systems, especially the performance of reductants (D) and solvents. Furthermore, we analyze the types of photocatalytic systems and classify each component in these systems according to their role: (1) PS, (2) Cat for CO 2 reduction catalysts, and (3) D. Briefly, we summarize the important features of each component and provide typical examples. The next section discusses the photocatalytic abilities of each of the three categories of photocatalytic systems: multicomponent systems comprising PS and Cat, supramolecular photocatalysts comprising a multinuclear complex, and hybrid systems constructed with metal-complex photocatalysts and inorganic materials, such as semiconductors or electrodes.

TiO2 photocatalysis for the degradation of pollutants in gas phase: From morphological design to plasmonic enhancement

Abstract: TiO2-based photocatalysis has become a viable technology in various application fields such as (waste)water purification, photovoltaics/artificial photosynthesis, environmentally friendly organic synthesis and remediation of air pollution. Because of the increasing impact of bad air quality worldwide, this review focuses on the use and optimization of TiO2-based photocatalysts for gas phase applications. Over the past years various specific aspects of TiO2 photocatalysis have been reviewed individually. The intent of this review is to offer a broad tutorial on (recent) trends in TiO2 photocatalyst modification for the intensification of photocatalytic air treatment. After briefly introducing the fundamentals of photocatalysis, TiO2 photocatalyst modification is discussed both on a morphological and an electronic level from the perspective of gas phase applications. The main focus is laid on recent developments, but also possible opportunities to the field. This review is intended as a solid introduction for researchers new to the field, as well as a summarizing update for established investigators.

Photoelectrochemical biosensors: New insights into promising photoelectrodes and signal amplification strategies

Abstract: The photoelectrochemical (PEC) process is a promising low-cost approach to convert chemical energy to electricity under light illumination and applied potential. PEC biosensing has attracted huge attention because of its ability to detect biomolecules through the photocurrent generated from biomolecule oxidation. However, important factors in the mechanism of PEC biosensing, particularly photoexcited (charge) carrier generation and separation at nano-bio interfaces, are not well explored. Therefore, with the objective of emphasising the implications of photoexcited (charge) carrier transport, here we review recent efforts indicating the significance of electrode design to enhance the performance of PEC biosensor with semiconductor photocatalytic materials. Besides enzymatic PEC biosensors, the underlying beneficial mechanism of direct oxidisation of biomolecules onto a wide range of semiconductor photocatalyst surfaces by the photogenerated holes is briefly discussed. This review is primarily divided into three parts: materials, signal amplification, and promising device architectures, based on recent advances in PEC biosensors. In addition, this review outlines the strategies used to detect a wide range of bioanalytes. After a summary of PEC sensing architectures, the review concludes with an outlook and the current challenges in fabricating solar-light-driven and self-powered biosensors using nanostructured photocatalytic semiconductors. The PEC biosensing schemes presented in this review provide unambiguous operating guidelines of this subject to facilitate our understanding of the compatibility between semiconductor photocatalysts and bioanalytes.

Potential applications of porphyrins in photodynamic inactivation beyond the medical scope

Abstract: Although the discovery of light-activated antimicrobial agents had been reported in the 1900s, only more recently research work has been developed toward the use of photodynamic process as an alternative to more conventional methods of inactivation of micro(organisms). The photoprocess causes cell death through irreversible oxidative damage by reactive oxygen species produced by the interaction between a photosensitizing compound and a light source. With great emphasis on the environmental area, photodynamic inactivation (PDI) has been tested in insect eradication and in water disinfection. Lately, other studies have been carried out concerning its possible use in aquaculture waters or to the control of food-borne pathogens. Other potential applications of PDI in household, industrial and hospital settings have been considered. In the last decade, scientific research in this area has gained importance not only due to great developments in the field of materials chemistry but also because of the serious problem of the increasing number of bacterial species resistant to common antibiotics. In fact, the design of antimicrobial surfaces or self-cleaning materials is a very appealing idea from the economic, social and public health standpoints. Thus, PDI of micro(organisms) represents a promising alternative. In this review, the efforts made in the last decade in the investigation of PDI of (micro)organisms with potential applications beyond the medical field will be discussed, focusing on porphyrins, free or immobilized on solid supports, as photosensitizing agents.

Thermodynamic vs. kinetic control of excited-state proton transfer reactions

Abstract: The “Excited-State Intramolecular Proton Transfer” (ESIPT) reactions in a number of organic fluorophores are among the fastest basic chemical reactions known so far and their rates can be observed even on femtosecond time scale. Accordingly, the reactant concentration, as monitored by its emission, should be negligibly small. In sharp contrast to this conventional wisdom, however, the coexistence of the reactant and the product of this reaction is so frequently observed in condensed media. We then discuss two possible origins of these effects: when the ESIPT reaction is perturbed and hence is slow on the time scale of emission (kinetic control) or when the reverse reaction repopulating the reactant state is fast and leads to the excited-state equilibrium (thermodynamic control). Upon reviewing a great number of ESIPT prototypical systems, we summarize and discuss different criteria for distinguishing these cases based on the steady-state and time-resolved spectroscopic studies and derive correlations between reversibility of these reactions and the solvent-dependent effects observed in fluorescence spectra.

Artificial photosynthesis: Where are we now? Where can we go?

Abstract: Widespread implementation of renewable energy technologies, while preventing significant increases in greenhouse gas emissions, appears to be the only viable solution to meeting the world's energy demands for a sustainable energy future The final energy mix will include conservation and energy efficiency, wind, geothermal, biomass, and others, but none more ubiquitous or abundant than the sun Over several decades of development, the cost of photovoltaic cells has decreased significantly with lifetimes that exceed 25 years and there is promise for widespread implementation in the future However, the solar input is intermittent and, to be practical at a truly large scale, will require an equally large capability for energy storage One approach involves artificial photosynthesis and the use of the sun to drive solar fuel reactions for water splitting into hydrogen and oxygen or to reduce CO2 to reduced carbon fuels An early breakthrough in this area came from an initial report by Honda and Fujishima on photoelectrochemical water splitting at TiO2 with UV excitation Significant progress has been made since in exploiting semiconductor devices in water splitting with impressive gains in spectral coverage and solar efficiencies An alternate, hybrid approach, which integrates molecular light absorption and catalysis with the band gap properties of oxide semiconductors, the dye-sensitized photoelectrosynthesis cell (DSPEC), has been pioneered by the University of North Carolina Energy Frontier Research Center (UNC EFRC) on Solar Fuels By utilizing chromophore-catalyst assemblies, core/shell oxide structures, and surface stabilization, the EFRC recently demonstrated a viable DSPEC for solar water splitting

Synergetic photoelectrocatalytic reactors for environmental remediation: A review

Abstract: Integrating electrochemistry with photocatalytic technology, photoelectrocatalysis has been identified as a superior candidate to debottleneck photocatalytic processes. Photoelectrocatalysis involves a photocatalytic system to which an external positive bias is applied, which can significantly increase the rates of photocatalytic reactions by driving the photo-generated electron–hole pairs in opposite directions, reducing their recombination rates. The design of a cost-efficient photoelectrocatalytic reactor plays a critical role in the ultimate acceptance of this promising technology in industry for environmental remediation as well as other applications. In this study, photoelectrocatalysis and associated novel reactor designs reported in recent years are reviewed and discussed. Some of the topics which are discussed in this study include various reactor configurations with different illumination sources, photocatalyst utilization modes, and electrodes as well as composite systems incorporating solar cells in addition to microbial and photocatalytic fuel cells. Future efforts are suggested to push the industrial application of photoelectrocatalysis out of its infancy.

Enhancing photodynamic therapy of refractory solid cancers : Combining second-generation photosensitizers with multi-targeted liposomal delivery

Abstract: Contemporary photodynamic therapy (PDT) for the last-line treatment of refractory cancers such as nasopharyngeal carcinomas, superficial recurrent urothelial carcinomas, and non-resectable extrahepatic cholangiocarcinomas yields poor clinical outcomes and may be associated with adverse events. This is mainly attributable to three factors: (1) the currently employed photosensitizers exhibit suboptimal spectral properties, (2) the route of administration is associated with unfavorable photosensitizer pharmacokinetics, and (3) the upregulation of survival pathways in tumor cells may impede cell death after PDT. Consequently, there is a strong medical need to improve PDT of these recalcitrant cancers. An increase in PDT efficacy and reduction in clinical side-effects may be achieved by encapsulating second-generation photosensitizers into liposomes that selectively target to pharmacologically important tumor locations, namely tumor cells, tumor endothelium, and tumor interstitial spaces. In addition to addressing the drawbacks of clinically approved photosensitizers, this review addresses the most relevant pharmacological aspects that dictate clinical outcome, including photosensitizer biodistribution and intracellular localization in relation to PDT efficacy, the mechanisms of PDT-induced cell death, and PDT-induced antitumor immune responses. Also, a rationale is provided for the use of second-generation photosensitizers such as diamagnetic phthalocyanines (e.g., zinc or aluminum phthalocyanine), which exhibit superior photophysical and photochemical properties, in combination with a multi-targeted liposomal photosensitizer delivery system. The rationale for this PDT platform is corroborated by preliminary experimental data and proof-of-concept studies. Finally, a summary of the different nanoparticulate photosensitizer delivery systems is provided followed by a section on phototriggered release mechanisms in the context of liposomal photosensitizer delivery systems.

Aggregation-induced phosphorescence enhancement (AIPE) based on transition metal complexes—An overview

Abstract: Aggregation-induced emission (enhancement) (AIE(E)) is an extraordinary phenomenon in the field of photochemistry and offers a new platform for researchers to investigate the light-emitting process from nanoaggregates. Non- or weakly emissive molecules are induced to emit efficiently through nanoaggregate formation, which are referred to as aggregation-induced emission (AIE) and aggregation-induced emission enhancement (AIEE), respectively. A large number of AIE/AIEE active organic molecules have now been developed and reviews on them appeared. Comparatively little attempt is made on the AIEE activity based on organometallic and coordination complexes. In the case of most of the transition metal complexes intersystem crossing from the excited S1 to T1 state is close to unity and predominant emission process is phosphorescence. This review concentrates on the design, synthesis, and photophysical behavior and important applications of aggregation-induced phosphorescence enhancement (AIPE)-active molecules based on transition metal complexes. Researchers have incorporated a rotating or isomerizable or long alkyl chain unit in the ligand or cyclometalated ligand, to generate a family of AIPE active members. Several transition metal complexes including Re(I), Ir(III), Pt(II), Au(I), Zn(II) and Cu(I) have been demonstrated to exhibit a significant AIPE phenomenon in the presence of appropriate stimuli. The phosphorescence intensity and quantum yield could be enhanced by adding poor solvents to induce nanoaggregate formation to restrict the intramolecular rotation or isomerization. The characteristics of highly phosphorescent aggregates differentiate them from conventional chromophores and make them ideal candidates for high-tech applications in the field of chemosensors, bioprobes, stimuli-responsive nanomaterials and optoelectronic materials, etc.

Tunable one-dimensional photonic crystals from soft materials

Abstract: Photonic crystals are periodic dielectric nanostructures that can affect the propagation of light. Polymer-based photonic crystals have attracted great attentions for their potential application as sensors or optical switches due to their stimuli-responsive properties. This review summarizes the recent developments in one-dimensional (1-D) polymer-based photonic crystals, including the inspiration of the material from nature, principles for design and fabrication, mechanism of color tuning, and their tunable structural color in responsive to various stimuli. A number of fabrication methods, either by bottom–up or top–down approaches for 1-D polymeric photonic crystals have been overviewed.

Recent advances in photosynthetic energy conversion

Abstract: Photosynthesis is one of the first natural processes evolved by cyanobacteria, algae and green plants to trap light and CO 2 in the form of reduced carbon compounds while simultaneously oxidizing water to oxygen. The photosynthetic energy conversion forms the basis for all the existing life today. The photosynthetic energy is being harnessed in many ways using modern technologies for the production of fuels using photosynthetic organisms, generation of direct electricity using photosystems/photosynthetic organisms in photo-bioelectrochemical cells or through photovoltaic systems. While the production of energy rich carbon fuels (ethanol, propanol) from photosynthetic organisms has already been accomplished due to advancement in understanding microbial physiology and metabolism, the photosynthetic hydrogen production as well as direct electricity generation from light is still at its infancy. Recent advances include combining photosystem complexes with hydrogenases for hydrogen production, using isolated thylakoids, photosystems on nanostructured electrodes such as gold nanoparticles, carbon nanotubes, ZnO nanoparticles for electricity generation. Many challenging optimizations on the immobilization methods, catalyst stability and isolation procedures, electron transfer strategies have acquired momentum leading to the production of more stable and higher current densities and power densities in photosynthetic devices. Further, the use of whole cell microorganisms (cyanobacteria, microalgae) rather than their isolated counterparts has produced promising results. The photosynthetic energy conversion has an enormous potential for renewable energy generation in a sustainable and environment friendly manner.

Molecular cathode and photocathode materials for hydrogen evolution in photoelectrochemical devices

Abstract: Storage of solar energy in the form of readily available easy-to-handle fuels is the main bottleneck toward the development of a carbon-neutral alternative energy. Taking inspiration from natural systems, artificial photosynthesis is a technology to be for efficiently converting the tremendous solar energy received every day on Earth into chemical energy, i.e. fuels. In particular, hydrogen production through light-driven water splitting is the subject of numerous investigations. We focus here on the construction of electrodes and photoelectrodes achieving H2 evolution, as components of photoelectrochemical (PEC) cells. In such devices, H2 evolution at the cathode or photocathode is combined with water oxidation to oxygen at the photoanode or anode. We review here the various molecular-based materials developed in this context with emphasis on those specifically exploiting the properties of Earth-abundant elements.

Molecular complexes in water oxidation: Pre-catalysts or real catalysts

Abstract: Artificial photosynthesis is considered a promising method to produce clean and renewable energy by sunlight. To accomplish this aim, development of efficient and robust catalysts for water oxidation and hydrogen production is extremely important. Owing to the advantages of easily modified structures and traceable catalytic processes, molecular water oxidation catalysts (WOCs) attract much attention during the past decade. However, the transformation of molecular WOCs to metal oxides/hydroxides or metal ions may occur under the harsh catalytic conditions, making the identification of true active species difficult. In this article, recent progress on molecular complexes acting as real catalysts or precursors toward water oxidation was briefly reviewed. We summarized the commonly used physical techniques and chemical methods that enable to distinguish homogeneous catalysts from heterogeneous catalysts. The factors that affect the nature of WOCs, such as reaction conditions, transition metal centers, and supporting ligands were discussed as well.

Natural and artificial light-harvesting systems utilizing the functions of carotenoids

Abstract: Carotenoids are essential pigments in natural photosynthesis. They absorb in the blue–green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and so expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet–singlet energy transfer and so carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. Carotenoids also act to protect photosynthetic organisms from the harmful effects of excess exposure to light. In this case, triplet–triplet energy transfer from (bacterio-)chlorophyll to carotenoid plays a key role in this photoprotective reaction. In the light-harvesting pigment–protein complexes from purple photosynthetic bacteria and chlorophytes, carotenoids have an additional role, namely the structural stabilization of those complexes. In this article we review what is currently known about how carotenoids discharge these functions. The molecular architecture of photosynthetic systems will be outlined to provide a basis from which to describe the photochemistry of carotenoids, which underlies most of their important functions in photosynthesis. Then, the possibility to utilize the functions of carotenoids in artificial photosynthetic light-harvesting systems will be discussed. Some examples of the model systems are introduced.

Supramolecular photochemistry concepts highlighted with select examples

Abstract: ‘Supramolecular photochemistry’ (SP) deals with a study of the properties of molecules in their excited states where the medium plays a significant role. While ‘molecular photochemistry’ (MP) deals with studies in isotropic solution, the SP deals with reactant molecules that interact weakly with their surroundings. The surroundings in general are highly organized assemblies such as crystals, liquid crystals, micelles, and host–guest structures. The behavior of exited molecules in SP unlike in isotropic solution is controlled not only by their inherent electronic and steric properties but also by the immediate surroundings. The weak interactions that control the chemistry include van der Walls, hydrophobic, C H⋯π, π⋯π and several types of hydrogen bonds. In this review the uniqueness of SP compared to MP is highlighted with examples chosen from reactions in crystals, micelles and host–guest assemblies. In spite of distinctly different structures (crystals, micelles, etc.) the influence of the medium could be understood on the basis of a model developed by G.M.J. Schmidt for photoreactions in crystals. The principles of reaction cavity model are briefly outlined in this review. There are a few important features that are specific to SP. For example, highly reactive molecules and intermediates could be stabilized in a confined environment; they enable phosphorescence to be observed at room temperature and favor chiral induction in photochemical reactions. Using such examples the uniqueness of SP is highlighted. The future of SP depends on developing efficient and unique catalytic photoreactions using easily available reaction ‘containers’. In addition, their value in artificial photosynthesis should be established for SP to occupy a center stage in the future.

Use of biomolecular scaffolds for assembling multistep light harvesting and energy transfer devices

Abstract: The development of biologically templated artificial light harvesting antennae and energy transfer devices is a highly active research area with exceptional challenges. Natural energy harvesting complexes have exquisite spectrally- and spatially-tuned systems with high redundancy to maximize their ability to gather, channel, and distribute electromagnetic radiation. Attempting to mimic these highly efficient systems requires at the very least (sub)nanoscale precision in the positioning of light sensitive molecules, the latter of which must also possess carefully selected photophysical properties; in essence, these two fundamental properties must be exploited in a synergistic manner. First, the scaffold must be highly organized, ideally with multiple symmetrical components that are spatially arranged with nanoscale accuracy. Second, the structure must be amenable to chemical modification in order to be (bio)functionalized with the desired light sensitive moieties which have expanded greatly to now include organic dyes, metal chelates, fluorescent proteins, dye-doped and noble metal nanoparticles, photoactive polymers, along with semiconductor quantum dots amongst others. Several families of biological scaffolding molecules offer strong potential to meet these stringent requirements. Recent advances in bionanotechnology have provided the ability to assemble diverse naturally derived scaffolds along with manipulating their properties and this is allowing us to understand the capabilities and limitations of such artificial light-harvesting antennae and devices. The range of scaffold or template materials that have been used varies from highly symmetrical virus capsids to self-assembled biomaterials including nucleic acids and small peptides as well as a range of hybrid inorganic–biological systems. This review surveys the burgeoning field of artificial light-harvesting and energy transfer complexes that utilize biological scaffolds from the perspective of what each has to offer for optimized energy transfer. We highlight each biological scaffold with prominent examples from the literature and discuss some of the benefits and liabilities of each approach. Cumulatively, the available data suggest that DNA is the most versatile biological material currently available, though it has challenges including precise dye placement and subsequent dye performance. We conclude by providing a perspective on how this field will progress in both the short and long term, with a focus on the transition to applications and devices.

Photoactivation of the cryptochrome/photolyase superfamily

Abstract: The cryptochrome/photolyase superfamily is a class of flavoproteins that can regulate the growth and development in plants, as well as the circadian clock and the potential magnetic navigation in animals, primarily by absorbing UV-A and blue light. It is generally agreed that these functions depend on the photochemical reaction of the flavin adenine dinucleotide (FAD) chromophore, non-covalently binding to cryptochromes or photolyases. Irradiation can initiate either photoreduction between FAD and certain electron donors or electron jumping in FAD, thereby leading to the generation of intermediates that activate the protein. This signaling process is known as photoactivation. Subsequently, the activated protein will interact with downstream receptors to transfer the photo and magnetic signals. Based on in-depth research on photoactivation, two photo-cycle mechanisms for the photoreception/photosignaling of the cryptochrome/photolyase superfamily, i.e., the photolyase model and the phototropin model, have been proposed. There is no apparent alternative to the photo-cycle of cyclobutane pyrimidine dimer (CPD) or (6-4) photolyase following the photolyase model. However, the mechanism is not clear for the photoactivation of cryptochromes and CRY-DASH, a new subcategory of photolyase. Since the photoactivation process is the first step for the physiological function of proteins, more and improved research efforts in this field have been widely developed. This review first briefly presents the structure, the photoactivation, and the repair mechanism of CPD and (6-4) photolyase. Next, we review in detail the photoactivation of cryptochromes and CRY-DASH by analyzing the current status of research, as well as the contradictions in the resting redox states of FAD, intermediates in photoreactions, the photo-cycle of FAD, the signaling state of proteins, and the necessity of given tryptophans for protein activity. Based on these studies, the correlations of photoactivation and photo-cycle mechanisms, as well as the correlations of photoactivation and magnetoreception of proteins, are discussed. Finally the crucial open questions regarding the photoactivation mechanisms of the cryptochrome/photolyase superfamily are outlined, considering the hypothesis for a cryptochrome-based model of avian magnetoreception.

Primary reactions in the photochemistry of hexahalide complexes of platinum group metals: A minireview

Abstract: A B S T R A C T We review the results obtained for Pt IV Cl6 2� , Pt IV Br6 2� , Ir IV Cl6 2� , Ir IV Br6 2� , and Os IV Br6 2� complexes in aqueous and alcoholic solutions using ultrafast pump–probe spectroscopy, laser flash photolysis, ESR, and photoelectron spectroscopy. We discuss the correlations between the photophysics and the photochemistry of these complexes. The key reaction for Pt IV Cl6 2� is the inner-sphere electron transfer, which results in an Adamson radical pair that lives for several picoseconds, and the subsequent photoaquation in aqueous solutions and photoreduction in alcohols. The chlorine atom formed as the primary product escapes the solvent cage in aqueous solutions or oxidizes a solvent alcohol molecule via secondary electron transfer, producing secondary intermediates that react on the microsecond time scale. The photoexcitation of Pt IV Br6 2� results in the formation of pentacoordinated Pt IV intermediates, i.e. 3 Pt IV Br5 � and 1 Pt IV Br5 � , with characteristic lifetimes of approximately 1 and 10 ps, respectively. Subsequent reactions of these intermediates result in the complexation of a solvent molecule. Photoreduction is also possible in alcohols. Similar reactions occur with rather low quantum yields for Ir IV Cl6 2� , therefore, only the ground-state recovery could be monitored in ultrafast experiments, which occur on the 10-ps time scale. The photochemical behaviours of the Ir IV Br6 2� and Os IV Br6 2� complexes are similar to those of Ir IV Cl6 2� and Pt IV Br6 2� , respectively.

Influence of oxygen nonstoichiometry and doping with 2p-, 3p-, 6p- and 3d-elements on electronic structure, optical properties and photocatalytic activity of rutile and anatase: Ab initio approaches

Abstract: Among wide-energy-gap semiconductors, doped titanium dioxides (anatase or/and rutile polymorphs) are the most promising materials for designing photocatalysts active in the visible region of solar spectrum, for photodegradation of organic molecules and for photolysis of water. It has been established recently that doping of titanium dioxides with 2 p -, 3 p -, 6 p - and 3 d -elements significantly increases their photocatalytic activity. In this review we summarize calculation results on the electronic structure and optical properties of bulk stoichiometric and nonstoichiometric rutile and anatase and their doped compounds obtained by means of ab initio methods of condensed matter physics: the linearized muffin-tin orbital method, the linearized augmented plane-wave method, the plane-wave pseudopotential method, the coherent potential method, the Hartree–Fock method, etc. The possibilities and restrictions of the methods for accurate calculations of the electronic structure and optical properties of stoichiometric and nonstoichiometric titanium dioxides and titanium dioxides doped with 2 p -, 3 p -, 6 p - and 3 d -elements are discussed. As calculated with the included Coulomb correlation correction, within the hybrid potential approximation or with the self-interaction correction taken into account, the electronic structure and optical spectrum are in agreement with the experimental data. The results of the calculations correspond to the observed photocatalytic activity of rutile and anatase.