Weakly bound oxygen complexes X-O2 as a new source of singlet oxygen

Singlet Oxygen ¹O₂ in Photocatalysis on TiO₂. Where Does It Come from?

The Journal of Physical Chemistry C

Features of the near-infrared phosphorescence of singlet oxygen ¹O₂ photosensitized by TiO₂ in solutions have been investigated. The short-lived phosphorescence of ¹O₂ with spectral maximum at approximately 1300 nm is observed to follow excitation by pulsed laser radiation at 355 nm of TiO₂ suspended in CCl₄ and water H₂O. The shorter lifetime and spectral shift of this phosphorescence, as compared with phosphorescence of ¹O₂ sensitized by phenalenone in these solvents, are attributed to the adsorbed state of emitting singlet oxygen. The observed laser pulse energy dependence indicates the two-photon and three-photon nature of this phosphorescence initiation in CCl₄ and H₂O, respectively. The formation of ¹O₂ in CCl₄ is assumed to be provided by the sequence of one-photon excitation of TiO₂, giving rise to superoxide anion O₂^– and one-photon photodetachment of electron from O₂^–. The photodetachment of electron from O₂^– in water H₂O is a two-photon process because of higher solvation energy of superoxide anion in water. The processes that govern the rise and decay of singlet oxygen phosphorescence sensitized by TiO₂ are discussed. The factors affecting the yield of singlet oxygen provided by electron photodetachment are also considered.

Reply to Comments on “Singlet Oxygen ¹O₂ in Photocatalysis on TiO₂. Where Does It Come from?”

The Journal of Physical Chemistry C

In the Comment the authors Y. Nosaka and A. Y. Nosaka present arguments against the mechanism of singlet oxygen production which was suggested in the earlier published paper (Demyanenko et al., J. Phys. Chem. C 2019, 123, 2175) to explain the experimental data for the Near IR (NIR)-luminescence observed to appear after laser excitation at 355 nm of TiO₂ suspension in air-saturated solvents CCl₄ and H₂O. The main arguments of the Comment are directed against Assignment of the NIR Emission and Electron Photodetachment Mechanism suggested by Demyanenko et al.. Particularly the Comment claims that the red shift (near 30 nm) and bandwidth (50-60 nm) of the NIR luminescence observed in paper by Demyanenko et al. are too large to be assigned to the luminescence of singlet oxygen ¹O₂. In the Reply it is shown that several arguments of the Comment are based on the incorrect statements. The current interpretation of the nature of the red shift and bandwidth of the luminescence of singlet oxygen governed by interaction with the environment is considered. It is shown that the characteristics of NIR luminescence (rather large red shift and bandwidth as compared with the literature data for the luminescence of ¹O₂ in solutions) observed in experiments by Demyanenko et al. are in a qualitative agreement with expected tendencies. All the arguments of the Comment are considered in detail and extended reply is given.

An Automated Apparatus for Measuring Spectral Dependences of the Mass Spectra and Velocity Map Images of Photofragments

Instruments and Experimental Techniques

REMPI detection of singlet oxygen ¹O₂ arising from UV‑photodissociation of van der Waals complex isoprene-oxygen C₅H₈‑O₂

Chemical Physics Letters

Singlet oxygen photogeneration from X–O₂ van der Waals complexes: double spin-flip vs. charge-transfer mechanism

Physical Chemistry Chemical Physics

The channel of singlet oxygen O₂(¹Δ_g) photogeneration from van der Waals complexes of oxygen X–O₂ has been investigated to discriminate between two possible mechanisms based on charge-transfer (CT) or double spin-flip (DSF) transitions. The results obtained in this work for complexes with X = ethylene C₂H₄, 1,3-butadiene C₄H₆, deuterated methyl iodide CD₃I, benzene C₆H₆ and water H₂O and for those investigated previously indicate the DSF mechanism as a source of singlet oxygen. The formation of O₂ (¹Δ_g) is observed only when the energy of exciting quantum is sufficient for DSF transition. Universally detected low vibrational excitation of O₂(¹Δ_g) arising in the photodissociation of van der Waals complexes X–O₂ indicates the DSF mechanism as its source. For complex of ethylene C₂H₄–O₂ ab initio calculations of vertical energy ΔE_vert for DSF and CT transitions have been carried out. The positive results of singlet oxygen formation from C₂H₄–O₂ can be explained by the DSF but not by the CT mechanism.

Singlet oxygen O₂(a¹Δ_g) formation via UV-excitation of isoprene-oxygen C₅H₈–O₂ encounter complexes in gas phase

Chemical Physics Letters

Quantum Yield and Mechanism of Singlet Oxygen Generation via UV Photoexcitation of O₂–O₂ and N₂–O₂ Encounter Complexes

The Journal of Physical Chemistry A

The mechanism and spectral dependence of the quantum yield of singlet oxygen O₂(a ¹Δ_g) photogenerated by UV radiation in gaseous oxygen at elevated pressure (32–130 bar) have been experimentally investigated within the 238–285 nm spectral region overlapping the range of the Wulf bands in the absorption spectrum of oxygen. The dominant channel of singlet oxygen generation with measured quantum yield up to about 2 is attributed to the one-quantum absorption by the encounter complexes O₂–O₂. This absorption gives rise to oxygen in the Herzberg III state O₂(A′ ³Δ_u), which is assumed to be responsible for singlet oxygen production in the relaxation process O₂(A′ ³Δ_u, υ) + O₂(X ³Σ_g^–) → O₂({a ¹Δ_g" {b ¹Σ_g⁺}) + O₂({a ¹Δ_g, υ = 0" {b ¹Σ_g⁺, υ = 0}) with further collisional relaxation of b to a state. This mechanism is deduced from the analysis of the avoiding crossing locations on the potential energy surface of colliding O₂–O₂ pair. The observed drop of the O₂(a ¹Δ_g) yield near spectral threshold for O₂ dissociation is explained by the competition between above relaxation and reaction giving rise to O₃ + O (O + O + O₂) supposed in literature. The quantum yield of O₂(a ¹Δ_g) formation from encounter complex N₂–O₂ measured at λ = 266 nm was found to be the same as that for O₂–O₂.

Experimental measurement of the van der Waals binding energy of X–O₂ clusters (X = Xe, CH₃I, C₃H₆, C₆H₁₂)

The Journal of Chemical Physics

Van der Waals binding energies for the X–O₂ complexes (X=Xe, CH₃I, C₃H₆, C₆H₁₂) are determined by analysis of experimental velocity map imaging data for O(³P₂) atoms arising from UV-photodissociation of the complex [A. V. Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. Several dissociation pathways have been observed, we focus on the channel corresponding to prompt dissociation of X–O₂ into X+2O(³P) fragments, which is present for complexes of O₂ with all partners X. Our method is based on analysis of the kinetic energy of all three photofragments, where the O atom kinetic energy was directly measured in the experiment and the kinetic energy of the X partner was calculated using momentum conservation, along with the measured angular anisotropy for O atom recoil. We exploit the fact that the clusters are all T-shaped or nearly T-shaped, which we also confirm by ab initio calculations, along with knowledge of the transition dipole governing radiative absorption by the complex. The effect of partitioning the kinetic energy between translation along the X–O₂ and O $-$ O coordinates on the angular anisotropy of the O atom recoil direction is discussed. Van der Waals binding energies of 110 ± 20 cm⁻¹, 280 ± 20 cm⁻¹, 135 ± 30 cm⁻¹, and 585 ± 20 cm⁻¹ are determined for Xe $-$ O₂, CH3I $-$ O₂, C₃H₆ $-$ O₂, and C₆H₁₂ $-$ O₂ clusters, respectively.

Weakly bound oxygen complexes X-O₂ as a new source of singlet oxygen

Publications

Singlet Oxygen ¹O₂ in Photocatalysis on TiO₂. Where Does It Come from?

Reply to Comments on “Singlet Oxygen ¹O₂ in Photocatalysis on TiO₂. Where Does It Come from?”

An Automated Apparatus for Measuring Spectral Dependences of the Mass Spectra and Velocity Map Images of Photofragments

REMPI detection of singlet oxygen ¹O₂ arising from UV‑photodissociation of van der Waals complex isoprene-oxygen C₅H₈‑O₂

Singlet oxygen photogeneration from X–O₂ van der Waals complexes: double spin-flip vs. charge-transfer mechanism

Singlet oxygen O₂(a¹Δ_g) formation via UV-excitation of isoprene-oxygen C₅H₈–O₂ encounter complexes in gas phase

Quantum Yield and Mechanism of Singlet Oxygen Generation via UV Photoexcitation of O₂–O₂ and N₂–O₂ Encounter Complexes

Experimental measurement of the van der Waals binding energy of X–O₂ clusters (X = Xe, CH₃I, C₃H₆, C₆H₁₂)

Weakly bound oxygen complexes X-O2 as a new source of singlet oxygen

Publications

Singlet Oxygen 1O2 in Photocatalysis on TiO2. Where Does It Come from?

Reply to Comments on “Singlet Oxygen 1O2 in Photocatalysis on TiO2. Where Does It Come from?”

An Automated Apparatus for Measuring Spectral Dependences of the Mass Spectra and Velocity Map Images of Photofragments

REMPI detection of singlet oxygen 1O2 arising from UV‑photodissociation of van der Waals complex isoprene-oxygen C5H8‑O2

Singlet oxygen photogeneration from X–O2 van der Waals complexes: double spin-flip vs. charge-transfer mechanism

Singlet oxygen O2(a1Δg) formation via UV-excitation of isoprene-oxygen C5H8–O2 encounter complexes in gas phase

Quantum Yield and Mechanism of Singlet Oxygen Generation via UV Photoexcitation of O2–O2 and N2–O2 Encounter Complexes

Experimental measurement of the van der Waals binding energy of X–O2 clusters (X = Xe, CH3I, C3H6, C6H12)

Weakly bound oxygen complexes X-O₂ as a new source of singlet oxygen

Singlet Oxygen ¹O₂ in Photocatalysis on TiO₂. Where Does It Come from?

Reply to Comments on “Singlet Oxygen ¹O₂ in Photocatalysis on TiO₂. Where Does It Come from?”

REMPI detection of singlet oxygen ¹O₂ arising from UV‑photodissociation of van der Waals complex isoprene-oxygen C₅H₈‑O₂

Singlet oxygen photogeneration from X–O₂ van der Waals complexes: double spin-flip vs. charge-transfer mechanism

Singlet oxygen O₂(a¹Δ_g) formation via UV-excitation of isoprene-oxygen C₅H₈–O₂ encounter complexes in gas phase

Quantum Yield and Mechanism of Singlet Oxygen Generation via UV Photoexcitation of O₂–O₂ and N₂–O₂ Encounter Complexes

Experimental measurement of the van der Waals binding energy of X–O₂ clusters (X = Xe, CH₃I, C₃H₆, C₆H₁₂)