Dynamic aspects of the behavior of aromatic ketone triplets in anionic micelles. A laser flash photolysis study

Scaiano, J. C.; Selwyn, J. C.

doi:10.1139/v81-342

Cited by 44 publications

(36 citation statements)

References 16 publications

(19 reference statements)

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“…Furthermore, the apparent rate of self-quenching of the 1-azaxanthone triplet in the SDS micelles is almost 3-fold larger compared to aqueous solutions (Table ) which points out to a redistribution favoring the micellar phase as previously suggested. Interestingly, the lifetime of the 1-azaxanthone triplet extrapolated to zero ketone concentration is considerably shorter than for xanthone but considerably longer compared to benzophenone …”

Section: Resultsmentioning

confidence: 93%

The Photochemistry of 1-Azaxanthone in Aqueous Solutions and in Micellar Environments

Martínez

Scaiano

1998

J. Phys. Chem. A

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The reactivity and spectroscopic characteristics of the 1-azaxanthone triplet state in aqueous solutions indicate that inversion of states in the triplet manifold occurs in this solvent. Both the reactivity and spectroscopy of 1-azaxanthone suggest a predominantly π,π* character for the lowest triplet state in aqueous solutions, in contrast with organic solvents where 1-azaxanthone has n,π* character, even in polar media. The pH-dependent properties of the 1-azaxanthone triplet and its ketyl radical were examined. Despite the high reactivity of 1-azaxanthone toward photoreduction in organic solvents, in micellar systems the generation of ketyl radicals is inefficient. Studies of magnetic field effects on radical-pair behavior reveal similar characteristics to those observed with other carbonyl compounds, where geminate reaction and micellar exit are competing processes.The photochemical behavior of aromatic compounds in aqueous solutions can sometimes be quite different from that observed in organic solvents. For example, photoionization becomes an important photochemical pathway in a highly polar environment such as water, and several aromatic compounds are known to yield hydrated electrons by either monophotonic or biphotonic mechanisms. 1-5 One major factor that limits photochemical studies of organic compounds in aqueous solutions is their poor solubility in this medium, despite its unique properties and obvious biological relevance.We had previously reported the photochemical and photophysical properties of 1-azaxanthone in organic solvents. 6 Part of our interest in this ketone reflects its potential use as a probe to study guest/host interactions and radical-pair dynamics in supramolecular systems.1-Azaxanthone proved to be efficiently photoreduced; its photoreduction rate is at least an order of magnitude faster compared to the rates of other related ketones such as xanthone or benzophenone. 6 Furthermore, it is sparingly soluble in water thus making it an attractive molecule for the study of radicalpair dynamics and magnetic field effects in micellar environments.1-Azaxanthone has two basic sites (i.e., the pyridyl nitrogen and the carbonyl oxygen). 7 This allows for potential differences in the photochemical and spectroscopic properties of its different prototropic species; both factors are extremely important to consider when working in aqueous environments. We report here on the photochemical and photophysical properties of 1-azaxanthone in aqueous solutions and in micellar systems, as well as the entry-exit dynamics for micelles of different sizes. The photochemical properties of the protonated (i.e., pyridinium cation) form of 1-azaxanthone have also been examined. These studies provide the basis for its potential application in other supramolecular systems such as cyclodextrins, zeolites, or clays. Experimental Section1-Azaxanthone was purchased from Lancaster and recrystallized twice from ethanol. Water was purified through a Millipore Milli Q system and used for the preparation of all buffers and...

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Section: Resultsmentioning

confidence: 93%

The Photochemistry of 1-Azaxanthone in Aqueous Solutions and in Micellar Environments

Martínez

Scaiano

1998

J. Phys. Chem. A

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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: 99%

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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“…This probably reflects the easy access of acetophenone to the channel framework of Silicalite. Further, it is also possible that energy migration and self-quenching processes that are well established in micellar solution (26) may play an important role.…”

Section: Discussionmentioning

confidence: 99%

Intrazeolite photochemistry. II. Evidence for site inhomogeneity from studies of aromatic ketone phosphorescence

Casal¹,

Scaiano²

1985

Can. J. Chem.

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This paper is dedicated to the memory of Li Xiao-BaiH. L. CASAL and J. C. SCAIANO. Can. J. Chem. 63, 1308 (1985. The luminescent properties of several aromatic ketones included in the hydrophobic zeolite Silicalite have been examined. Acetophenone, benzophenone, and P-phenylpropiophenone show readily detectable phosphorescence; by contrast, valerophenone does not luminesce, but undergoes the Norrish Type 11 reaction. Thus, irradiated samples of valerophenone in Silicalite show phosphorescence due to the accumulation of acetophenone. In the case of P-phenylpropiophenone the triplet lifetime is ca. 100 000 times longer than the solution value, suggesting severe conformational restrictions. Co-inclusion of acetophenone with various substrates and oxygen quenching studies indicate that Silicalite has at least two distinct classes of inclusion sites. In one of them energy transfer processes are rapid and efficient, suggesting a cooperative effect in the inclusion of ketones in these regions of the Silicalite framework.H. L. CASAL et J. C. SCAIANO. Can. J. Chem. 63, 1308Chem. 63, (1985. On Ctudie les propriCtCs luminescentes de plusieurs cttones aromatiques contenues dans la zColite hydrophobe Silicalite. L'acCtophCnone, la benzophknone et la P-phtnylpropiophCnone prksentent une phosphorescence que I'on peut dCceler facilement; par ailleurs, la valCrophenone n'est pas luminescente, mais elle subit une reaction de type Norrish 11. Ainsi, les tchantillons irradiCs de valkrophCnone dans la Silicalite ont une phosphorescence qui provient d'une accumulation d'acCtophknone. Dans le cas de la P-phtnylpropiophenone, le temps de vie du triplet est environ 100 000 fois plus long que la valeur en solution et ceci suggkre la prCsence d'importantes contraintes conformationnelles. La co-inclusion de I'acCtophCnone avec differents substrats et des Ctudes de pikgage d'oxygkne indiquent que la Silicalite a au moins deux classes distinctes de sites d'inclusion. Dans I'un d'eux, le processus de transfert dlCnergie est rapide et efficace; ceci suggZre un effet d'ensemble lors de I'inclusion des cCtones dans ces regions du squelette de la Silicalite.[Traduit par le journal]

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Dynamic aspects of the behavior of aromatic ketone triplets in anionic micelles. A laser flash photolysis study

Cited by 44 publications

References 16 publications

The Photochemistry of 1-Azaxanthone in Aqueous Solutions and in Micellar Environments

The Photochemistry of 1-Azaxanthone in Aqueous Solutions and in Micellar Environments

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

Intrazeolite photochemistry. II. Evidence for site inhomogeneity from studies of aromatic ketone phosphorescence

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