Low-temperature emission of singlet oxygen from the surface of transition metal oxides

Low-temperature emission of the singlet form of dioxygen ¹O₂ from the surface of the individual and mixed V and Mo oxides was detected by thermal desorption at 20–350°C. The amount of the desorbing ¹O₂ and the temperature range of ¹O₂ emission were found to depend on the composition of the catalyst; the low-temperature ¹O₂ form can be regenerated after its contact with oxygen.     

Quantum‐chemical study of the singlet oxygen emission

Intensity distribution in rotational lines of the 0–0 band of the a¹Δ_g → X³Σ transition in the oxygen molecule at λ = 1270 nm is studied by quadratic response (QR) method in a framework of multiconfiguration self‐consisted field (MCSCF) theory. The distance dependence of the transition magnetic moment and the (0–0)/(0–1) bands intensity ratio are calculated. A short review of previous theory of the red and infrared atmospheric oxygen bands and of their enhancement by collisions is presented to analyze and compare the new results. Enhancement of these bands in O₂ collisions with Li₂ and N₂ molecules is calculated by QR method. Diamagnetic species simulate solvent molecules of different optical polarizability. Specific influence of collisions on vibronic bands is stressed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

On the issue of structure-chemical features of silicon oxides and thermal emission of singlet oxygen

Special features of the chemical structure of silicon oxides were analyzed by the methods of vibrational and electronic spectroscopy and it was found that cationic (Si-Si) and anionic (O₂ and O₃) sublattices contain isolated individual Si-CO oscillators, including electron-excited oscillators, and homoatomic bonds. By means of chromatographic and sorption-calorimetric analyses inverse dependence of singlet oxygen specific thermal emission on specific surface area of samples was found that corresponds to volume generation of excited states.     

Reversible 1,4-cycloaddition of singlet oxygen to N-substituted 2-pyridones: 1,4-endoperoxide as a versatile chemical source of singlet oxygen

N-substituted pyridones (1) easily undergo singlet oxygenation to give exclusively the corresponding endoperoxides (2), which decompose to give pyridones again while liberating 1O2 in high yield.     

Stereochemical features of the physical and chemical interactions of singlet oxygen with enecarbamates

Oxazolidinone-substituted enecarbamates represent a mechanistically rich system for the study of stereoelectronic, steric, and conformational effects on stereoselectivity and mode selectivity in (1)O(2) [2 + 2] and ene reactions. Photooxygenation of these enecarbamates with (1)O(2) leads to diastereomerically pure dioxetanes that decompose to yield an oxazolidinone carbaldehyde and one of the two enantiomers of methyldesoxybenzoin in enantiomeric excess. Stereoselectivity originates at the allylic stereocenter, a result supported by quenching studies, computational analysis, and deuterium solvent isotope effects. [reaction: see text]     

Quantum chemical calculation of nickel and copper atomic valencies in crystalline oxides

A new quantum chemical definition is proposed of the full atomic valence, V_A, taking into account both the covalent and ionic parts of the chemical bonds formed by atoms in molecules and crystals. The full atomic valencies, covalencies, and charges on atoms are calculated for nickel–oxygen crystalline compounds in the CNDO band theory approximation. A comparison of the chemical bonding in nickel and copper crystalline oxides is given. © 1994 John Wiley & Sons, Inc.     

Measurement and characterization of singlet oxygen production in copper ion-catalyzed aerobic oxidation of ascorbic acid

Production of singlet molecular oxygen (¹O₂) in the aerobic oxidation of ascorbic acid catalyzed by copper ion was measured and characterized using [4′-(9-anthryl)-2,2′:6′,2″-terpyridine-6,6″-diyl]bis(methylenenitrilo)tetrakis(acetate)-Eu³⁺ (ATTA-Eu³⁺) as a highly sensitive and selective time-resolved luminescence probe for ¹O₂. The ¹O₂ produced in the reaction was further characterized and confirmed by (i) chemical trapping of ¹O₂ with 9,10-diphenylanthracene (DPA), the corresponding endoperoxide was detected by HPLC and (ii) spin trapping of ¹O₂ with 2,2,6,6-tetramethyl-4-piperidinol (TMP-OH), the corresponding free radical of TMP-OH oxide (TMPO) was detected by electron spin resonance (ESR) spectroscopy. The effects of deuterium oxide, sodium azide and histidine on the ¹O₂ signal were investigated. The mechanism investigation of ¹O₂ production implied that the ascorbic acid-Cu(I) complex formed in the reaction could be an important intermediate for the ¹O₂ production. The reaction of ascorbic acid with copper ion monitored by ¹H NMR and absorption spectroscopy demonstrated the formation of a copper ion-ascorbic acid complex. Except for Cu²⁺ and Cu⁺-ascorbic acid systems, no detectable ¹O₂ was produced in other transition metal cation-ascorbic acid systems in the studied range.     

Thermal evolution of the chemical structure and properties of silicon oxides

High-resolution IR spectroscopic study has shown that stepwise heating of samples of silicon oxides in the temperature range from 50 to 1000°C is accompanied by accumulation of electronically excited states in the skeletal Si-O bond system, as well as by formation of associates in the cationic and anionic sublattices. Stationary association process and cooperative interaction of the excited states give rise to IR luminescence and diamagnetic response which increases as the temperature rises. Accumulation of the excited states in the chemical structure and their relaxation, including that occurring due to thermal desorption of singlet oxygen, are characterized by nonmonotonic temperature dependence of the heat capacity.     

Chemistry of singlet oxygen. XVIII. Rates of reaction and quenching of alpha-tocopherol and singlet oxygen

Abstract —α-Tocopherol scavenges singlet oxygen (produced by methylene blue photosensitization in methanol) by a combination of chemical reaction (4.6 times 10⁷M^-1s^-1) and quenching (6.2 times 10⁸M^-1 s^-1). The total rate of scavenging (6.7 times 10⁸ M^-1s^-1) makes it an effective protective agent against photooxidation mediated by singlet oxygen.     

Photosensitized generation of singlet oxygen

This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.     

Calcium peroxide diperoxohydrate as a storable chemical generator of singlet oxygen for organic synthesis

Calcium peroxide diperoxohydrate (CaO(2).2H(2)O(2)) is an environmentally friendly generator of singlet oxygen ((1)O(2), (1)Delta(g)) that can be used in organic synthesis as an alternative to the regular photochemical method. This compound produces (1)O(2) in various solvents and can be easily recovered by filtration for further regeneration. Both monitoring of (1)O(2) luminescence at 1270 nm and specific trapping have shown that CaO(2).2H(2)O(2) can be stored for several days at -80 degrees C and that the yield of (1)O(2) is equal to 25%. Oxidation of typical organic substrates in methanol or THF through [4 + 2] or [2 + 2] cycloaddition and ene reaction have been carried out on a preparative scale with total conversion and selectivity.     

Dimethylsulfoxide as a kinetic booster for the chemical generation of singlet oxygen in methanol

The kinetic booster effect of dimethylsulfoxide on the chemical generation of singlet oxygen, ¹O₂, from the disproportionation of hydrogen peroxide catalyzed by molybdate ions in methanol has been evidenced by detection of the IR luminescence of ¹O₂ at 1270 nm and by ⁹⁵Mo NMR spectroscopy. DMSO interacts rapidly, through a direct oxygen transfer with the stable tetraperoxomolybdate , leading to DMSO₂ and to the unstable triperoxomolybdate , which releases ¹O₂. The procedure was applied to accelerate the dark singlet oxygenation of β-citronellol and α-terpinene.     

ESCA studies of copper oxides and copper molybdates

Valence band, $\text{Cu}\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{,}\text{O}\text{1s,}\text{Mo}\text{3d}$, and Cu L₃M₄₅M₄₅ photoelectron and X-ray-induced Auger spectra were recorded for metallic copper, Cu₂O, CuO, Cu₂Mo₃O₁₀, Cu₆Mo₄O₁₅, CuMoO₄, Cu₃Mo₂O₉, and Cu_3.85Mo₃O₁₂. $\text{Cu}\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ binding energy is 0.9 eV lower for Cu⁺-containing molybdates than for Cu₂O and 0.7 eV higher for Cu²⁺-containing molybdates with respect to that of CuO. Calculation of net chemical shift demonstrates the influence of Madelung potential on the binding energy of core electrons. On the basis of differences in binding energy it was possible to distinguish between various Cu-containing phases and to follow the surface redox processes of copper molybdates which, as it was seen, follow the same reactions as in the bulk processes. Auger spectra suggest the presence of a very thin layer of “surface phase” common for all five studied molybdates and independent of bulk structure and composition.