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Principles Of Fluorescence Spectroscopy

Dye-sensitized solar cells (DSSC) have attracted considerable attention in recent years as they offer the possibility of low-cost conversion of photovoltaic energy This Review focuses on recent advances in molecular design and technological aspects of metal-free organic dyes for applications in dye-sensitized solar cells Special attention has been paid to the design principles of these dyes and on the effect of various electrolyte systems Cosensitization, an emerging technique to extend the absorption range, is also discussed as a way to improve the performance of the device In addition, we report on inverted dyes for photocathodes, which constitutes a relatively new approach for the production of tandem cells Special consideration has been paid to the correlation between the molecular structure and physical properties to their performance in DSSCs

Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: From Structure: Property Relationships to Design Rules

Ozonation of drinking water: part I. Oxidation kinetics and product formation.

Photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles. Such a charge distribution has direct influence in dictating the energetics of the composite by shifting the Fermi level to more negative potentials. The transfer of electrons to Au nanoparticles has now been probed by exciting TiO2 nanoparticles under steady-state and laser pulse excitation. Equilibration with the C60/C60- redox couple provides a means to determine the apparent Fermi level of the TiO2−Au composite system. The size-dependent shift in the apparent Fermi level of the TiO2−Au composite (20 mV for 8-nm diameter and 40 mV for 5-nm and 60 mV for 3-nm gold nanoparticles) shows the ability of Au nanoparticles to influence the energetics by improving the photoinduced charge separation. Isolation of individual charge-transfer steps from UV-excited TiO2 → Au → C60 has provided mechanistic and kinetic information on the role of metal in semiconductor-assisted photocatalysis and siz...

Catalysis with TiO2/Gold Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Bronsted acids, organocatalysts, enzymes, and transition metal complexes.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00018

Dual Catalysis Strategies in Photochemical Synthesis

We present a compilation of spectral parameters associated with triplet–triplet absorption of organic molecules in condensed media. The wavelengths of maximum absorbance and the corresponding extinction coefficients, where known, have been critically evaluated. Other data, for example, lifetimes, energies and energy transfer rates, relevant to the triplet states of these molecules are included by way of comments but have not been subjected to a similar scrutiny. Work in the gas phase has been omitted, as have theoretical studies. We provide an introduction to triplet state processes in solution and solids, developing the conceptual background and offering an historical perspective on the detection and measurement of triplet state absorption. Techniques employed to populate the triplet state are reviewed and the various approaches to the estimation of the extinction coefficient of triplet–triplet absorption are critically discussed. A statistical analysis of the available data is presented and recommendations for a hierarchical choice of extinction coefficients are made. Data collection is expected to be complete through the end of 1984. Compound name, molecular formula and author indexes are appended.

Triplet-triplet absorption spectra of organic molecules in condensed phases

The pathogenesis of Alzheimer's disease is strongly associated with the formation and deposition of β-amyloid peptide (βAP) in the brain. This peptide contains a methionine (Met) residue in the C-terminal domain, which is important for its neurotoxicity and its propensity to reduce transition metals and to form reactive oxygen species. Theoretical studies have proposed the formation of βAP Met radical cations as intermediates, but no experimental evidence with regard to formation and reactivity of these species in βAP is available, largely due to the insolubility of the peptide. To define the potential reactions of Met radical cations in βAP, we have performed time-resolved UV spectroscopic and conductivity studies with small model peptides, which show for the first time that (i) Met radical cations in peptides can be stabilized through bond formation with either the oxygen or the nitrogen atoms of adjacent peptide bonds; (ii) the formation of sulfur−oxygen bonds is kinetically preferred, but on longer ti...

Free Radical Reactions of Methionine in Peptides: Mechanisms Relevant to β-Amyloid Oxidation and Alzheimer's Disease

Adsorption of a cationic dye, cresyl violet, on SnO2 and SiO2 nanoclusters and nanocrystalline thin films results in the formation of H-aggregates. These dyes are photochemically and electrochemically active and extend the photoresponse of large bandgap semiconductors such as SnO2. Photocurrent generation in dye capped nanocrystalline films of SnO2 has been demonstrated with visible light excitation. A photon-to-photocurrent generation efficiency around 1% has been observed at 510 nm. Back electron transfer between the photoinjected electron and the oxidized sensitizer plays an important role in controlling the efficiency of net electron transfer. Transient absorption and microwave absorption measurements of the dye aggregate capped SnO2 films suggest that the back electron transfer is multiexponential and most is completed within a few hundred nanoseconds. The activation energy of the back electron transfer process is very low (∼1.7 kJ/mol).

Dye capped semiconductor nanoclusters. role of back electron transfer in the photosensitization of sno2 nanocrystallites with cresyl violet aggregates

The rate constants for the quenching of the excited states of metal ions and complexes in homogeneous fluid solution are reported in this compilation. Values of kq for dynamic, collisional processes between excited species and quenchers have been critically evaluated, and are presented with the following information, among others, from the original publications, when available: description of the solution medium, temperature at which kq was determined, experimental method, range of quencher concentration used, lifetime of the excited state in the absence of quencher, activation parameters, quenching mechanism. Data collection is complete through the end of 1986, and covers the coordination compounds of 26 metals, including the ions and complexes of the inner‐ and outer‐transition metals, and porphyrin complexes of nontransition metals. Data for 261 excited states quenched by more than 400 inorganic quenchers and 600 organic quenchers have been extracted from almost 500 publications. The introduction to th...

Rate Constants for the Quenching of Excited States of Metal Complexes in Fluid Solution

Radiation chemical methods were used to investigate the reactions of glycine anions, H2NCH2CO2- (Gly-), with •OH, (CH3)2C•OH, and •CH3 radicals. A major and most significant product from all of these processes is CO2. Pulse-radiolysis revealed that the initial step in the •OH-induced mechanism is oxidation of the amino group, producing +H2N•-CH2-CO2- and HN•-CH2-CO2- with yields of 63% and 37%, respectively. The amino radical cation, +H2N•-CH2-CO2-, suffers fast (≤100 ns) fragmentation into CO2 + •CH2NH2. The other primary radical, HN•-CH2-CO2-, can also be converted into the decarboxylating +H2N•-CH2-CO2- by reaction with proton donors such as phosphate (H2PO4-/k = 7.4 × 107 M-1 s-1, and HPO42-/k = 2.5 × 105 M-1 s-1) or the glycine zwitterion, Gly± (k = 3.9 × 105 M-1 s-1), but only on a much longer (typically μs to ms) time scale (k ≈ 4 × 105 M-1 s-1). Competitively, the HN•-CH2-CO2- transforms into a carbon-centered radical H2N-C•H-CO2- either by an intramolecular 1,2-H-atom shift (k = (1.2 ± 1.0) × 103...

Gordon L. Hug

Papers

Triplet-triplet absorption spectra of organic molecules in condensed phases

Free Radical Reactions of Methionine in Peptides: Mechanisms Relevant to β-Amyloid Oxidation and Alzheimer's Disease

Dye capped semiconductor nanoclusters. role of back electron transfer in the photosensitization of sno2 nanocrystallites with cresyl violet aggregates

Rate Constants for the Quenching of Excited States of Metal Complexes in Fluid Solution

Glycine Decarboxylation: The Free Radical Mechanism