Photoionization of xanthone via its triplet state or via its radical anion

Goez, Martin; Hussein, Belal H. M.

doi:10.1039/b410179k

Cited by 33 publications

(37 citation statements)

References 29 publications

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“…Lastly, radiationless deactivation of 3 A* is of very minor importance, because we found quenching by the micelle to be ten times faster. [6] Using the polarity dependence of the transient absorption on a picosecond timescale, Scaiano et al showed that A as well as 3 A* are strongly bound to SDS micelles (binding constant approximately 6000), where they reside near the surface with the carbonyl group extending into the Stern layer, [15] and they measured a time constant of 0.6 ms, which they equated with the exit rate from the micelles. [16] Similar studies of the fate of 3 A* in the presence of electron and hydrogen donors are severely hampered by two facts.…”

Section: Resultsmentioning

confidence: 98%

“…Nevertheless, the photoreaction is completely reversible, and the system eventually stabilises by recovering the starting materials; this requires reverse electron or hydrogen transfer in the singlet state, which becomes possible either by (field-dependent) intersystem crossing to a singlet pair during the spin-correlated life, or by encounters of free radicals that have escaped from the pairs. As the extinction coefficient of DC + is small and MC is nonabsorbing, [6] we monitor the reaction by observing the xanthone-derived radicals (regardless of whether they are still contained in spincorrelated pairs or present as free radicals). Lastly, radiationless deactivation of 3 A* is of very minor importance, because we found quenching by the micelle to be ten times faster.…”

Section: Resultsmentioning

confidence: 99%

“…The xanthone-derived transients are very difficult to distinguish by UV/Vis spectroscopy on a nanosecond timescale under these circumstances. [6] However, we will show that magnetic-field effects [7] (MFEs) permit their unambiguous separation and provide clear answers to the questions posed above. This approach uses the same detection method, but focuses on absorption differences in the presence and in the absence of an external magnetic field, which reflect field-dependent product yields.…”

Section: Introductionmentioning

confidence: 97%

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Quenching Mechanisms and Diffusional Pathways in Micellar Systems Unravelled by Time‐Resolved Magnetic‐Field Effects

Goez

Henbest

Windham

et al. 2009

Chemistry A European J

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Magnetic-field effects (MFEs) are used to investigate the photoreaction of xanthone (A) and DABCO (D) in anionic (SDS) or cationic (DTAC) micelles at high pH (DABCO = 1,4-diazabicyclo[2.2.2]octane, SDS = sodium dodecyl sulfate, DTAC = dodecyl trimethyl ammonium chloride). From MFE experiments with nanosecond time resolution, the radical anion A(.)(-) can be observed without any interference from the much more strongly absorbing triplet (3)A*, the different quenching processes can be separated and their rates can be measured. Triplet (3)A* is quenched dynamically both by the SDS micelle (k(1) = 5.0x10(5) s(-1)) and by DABCO approaching from the aqueous phase (k(2) = 2.0x10(9) M(-1) s(-1)). Static quenching by solubilised DABCO (association constant with the SDS micelles, 1.5 M(-1)) also participates at high DABCO concentrations, but is chemically nonproductive and does not lead to MFE generation. The MFEs stemming from the radical ion pairs A(.)(-) D(.)(+) are about 40 times larger in the anionic micelles than in the cationic ones despite a higher yield of free radicals in the latter case. This can be rationalised by different diffusional dynamics: Because of the location of their precursors, A(.)(-) and D(.)(+) are formed at opposite sides of the micelle boundary. Subsequently, the negatively charged Stern layer of the SDS micelle traps the radical cation, which then undergoes surface diffusion, so both the recombination probability and the spin mixing are high; in contrast, the positive surface charge of the DTAC micelle forces the radical cation into the bulk of the solution, thus efficiently blocking a recombination.

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Section: Resultsmentioning

confidence: 98%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

See 1 more Smart Citation

Quenching Mechanisms and Diffusional Pathways in Micellar Systems Unravelled by Time‐Resolved Magnetic‐Field Effects

Goez

Henbest

Windham

et al. 2009

Chemistry A European J

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“…[1][2][3][4][5][6] It is well known that the polarity of a solvent greatly influences the energies of the electronic states as well as the photophysical properties in aromatic carbonyl systems. [7][8][9][10] An increase in the solvent polarity results in the stabilization of ππ * states, as well as a blue shift of n → π * transitions, as evidenced by a shift of the absorption maxima.…”

Section: Introductionmentioning

confidence: 99%

Dependence of the thioxanthone triplet-triplet absorption spectrum with solvent polarity and aromatic ring substitution

Ferreira¹,

Cavalheiro²,

Neumann³

2006

J. Braz. Chem. Soc.

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Foram estudadas as absorções triplete-triplete (TT) e de transientes de tioxantonas substituídas e não substituídas em diferentes solventes, com a finalidade de avaliar o efeito do solvente, assim como dos substituintes no anel aromático. Os espectros determinados alguns microssegundos depois da excitação mostram absorções de três transientes principais: o estado triplete (600-650 nm), o radical cetila da tioxantona (~450 nm) e uma superposição de ambos (~300 nm). A quantidade de radicais formados em solventes não-hidroxílicos é muito menor do que em álcoois. O máximo da absorção TT mostra uma boa correlação com o parâmetro E T (30) dos solventes.The triplet-triplet (TT) and transient absorptions of non-substituted and substituted thioxanthones has been studied in different solvents in order to ascertain the effect of the solvent, as well as the substituents on the aromatic ring. Spectra taken after a couple of μs after the flash show three main transient absorptions due to the triplet state (600-650 nm), the thioxanthone ketyl radical (~450 nm) and an overlap of both (~300 nm). The amount of radicals formed in non hydroxylic solvents is much lower than in alcohols. The maxima of the TT absorption peaks show a good correlation with the E T (30) solvent parameter. Keywords: thioxanthone, triplet-triplet absorption IntroductionDue to their role in photochemical and photobiological reactions, the triplet states of aromatic ketones continue to receive attention from both experimental and theoretical points of view. [1][2][3][4][5][6] It is well known that the polarity of a solvent greatly influences the energies of the electronic states as well as the photophysical properties in aromatic carbonyl systems. [7][8][9][10] An increase in the solvent polarity results in the stabilization of ππ * states, as well as a blue shift of n → π * transitions, as evidenced by a shift of the absorption maxima. 11,12 Similar shifts have been observed in the triplet energy levels as a function of solvent polarity. 13 Among the aromatic carbonyl compounds, xanthones and thioxanthones (TX) were extensively studied from a solvent effect point of view.Thioxanthones are efficiently and extensively used as photoinitiators in dentistry resins [14][15][16] and other curing and photopolymerization systems. 17,18 Singlet and triplet excited states are involved in the photoinitiation. 19 This process can be strongly dependent on the polarity of the medium showing a distinct solvent effect on its properties and its triplet-triplet absorption spectra (TTA). 1,3 Actually, the polarity of mixtures used in industrial applications are not well defined but the academic comprehension of polarity effect on the excited states gives support to studies leading to the optimization of the photoinitiators' efficiency. 20,21 In this work we report on the changes in the transient absorption spectra due to different solvents, as well as substitutions on the aromatic ring of thioxanthones. ExperimentalEthyl acetate (AcOEt, Mallinckrodt) used in this work was spec...

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“…So, it would give rise to significant overlap [34]. The energetics of radical ion pair formation through electron transfer (et) can be estimated with Eq.…”

Section: Transient Absorption Spectroscopymentioning

confidence: 99%

Xanthone-photosensitized detoxification of the veterinary anthelmintic fenbendazole

Jornet

Castillo

Sabater

et al. 2013

Journal of Photochemistry and Photobiology A: Chemistry

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(1) is a common veterinary anthelmintic, toxic to water living microorganisms. Fluorescence quantum yields of 1 were found to be 0.11 in acetonitrile, 0.068 in methanol, 0.034 in cyclohexane, and 0.013 in water. The singlet excited state energy was ca. 96 kcal mol −1 in all solvents. The phosphorescence spectrum of 1 in ethanol at 77 K displayed a maximum at 450 nm, leading to a triplet energy of 75 kcal mol −1 . Experimental excited state energies agree well with the results of DFT calculations at the time-dependent B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level. Laser flash photolysis (LFP) of 1 at 266 nm led to transients absorbing in the 300-700 nm range, ascribed to radical cation 1 •+ , which were also observed upon 355 nm LFP of xanthone (XA) in the presence of 1. Solar-simulated photolysis revealed XA-enhanced photodegradation of 1 and led to decreased toxicity, as shown by Daphnia magna assays.

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Photoionization of xanthone via its triplet state or via its radical anion

Cited by 33 publications

References 29 publications

Quenching Mechanisms and Diffusional Pathways in Micellar Systems Unravelled by Time‐Resolved Magnetic‐Field Effects

Quenching Mechanisms and Diffusional Pathways in Micellar Systems Unravelled by Time‐Resolved Magnetic‐Field Effects

Dependence of the thioxanthone triplet-triplet absorption spectrum with solvent polarity and aromatic ring substitution

Xanthone-photosensitized detoxification of the veterinary anthelmintic fenbendazole

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