Temperature dependence of the photochemistry of aryl alkyl ketones

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The photodecomposition of several alkylbenzoylformates has been examined using quantum yield and laser flash photolysis techniques. The Norrish Type II reaction is a major triplet deactivation path in all substrates having γ-hydrogen atoms. Representative triplet lifetimes are 670, 500, and 310 ns, for the methyl, ethyl, and isopropyl esters, respectively, in 1:4 chlorobenzene: n-heptane at 298 K. Product studies suggest that several competing reactions are involved in the decay of the 1,4-biradical; some of these lead to highly absorbing (and presumably quenching) products, that make it essential to extrapolate quantum yields to "zero conversion" conditions.

Section: Frc 6 Arrhenius Plot For the Intramolecular Hydrogen Abstrcontrasting

confidence: 54%

Section: Frc 6 Arrhenius Plot For the Intramolecular Hydrogen Abstrmentioning

confidence: 76%

Section: Frc 6 Arrhenius Plot For the Intramolecular Hydrogen Abstrmentioning

confidence: 99%

See 1 more Smart Citation

Photochemistry of alkyl esters of benzoylformic acid

Encinas¹,

Lissi²,

Zanocco³

et al. 1984

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“…It should, however, be noted that these comparisons do leave room to some reservations, since the surface of silica gel is quite hydrophilic (19), in contrast with the hydrophobicity of the Silicalite channels. A critical dependence of the phosphorescence emission intensity of P-phenylpropiophenone and its derivatives on cavity size has also been observed in recent experiments in which this ketone was complexed with cy~lodextrins.~ Deactivation in the case of valerophenone (see reaction [I]) requires a conformation similar to that required in the case of P-phenylpropiophenone; however, the absence of an aromatic ring on the hydrocarbon chain permits larger mobility, thus making the process possible, even if kinetically it may be slower than in solution, where triplet valerophenone lives less than 10 ns (20). Quite clearly, spatial requirements in the case of valerophenone are less stringent than for P-phenylpropiophenone.…”

Section: Discussionmentioning

confidence: 61%

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

Casal¹,

Scaiano²

1985

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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]

“…It is well established that the mechanism for intramolecular P-aryl quenching of carbonyl triplets, like the Nonish Type I and I1 reactions (19), requires n, n* triplet character (6) and adequate mobility so that the conformation for quenching can be achieved (see Chart 1). Conformational restrictions, imposed either by changes in the molecular structure (4), or by media (e.g., by inclusion in zeolite channels (7) or cyclodextrins (9, 10)) can lead to significant increases in triplet lifetime.…”

Section: Discussionmentioning

confidence: 99%

Effect of methyl substitution on the intramolecular triplet deactivation of p-methoxy-β-phenylpropiophenone

Boch¹,

Bohne²,

Netto‐Ferreira³

et al. 1991

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. Can. J. Chem. 69, 2053Chem. 69, (1991. The photochemistry of several methyl-substituted p-methoxy-P-phenylpropiophenones (1-5) has been examined by nanosecond laser flash photolysis using 337-nm laser excitation. The ketone triplets decay by intramolecular P-aryl quenching of the carbonyl in a well-known process that is believed to involve charge transfer. Surprisingly, methyl substitution at the P-methylene group, which is not a participant in the deactiviation mechanism, causes a dramatic decrease in the triplet lifetime; the effect is attributed to different conformational preferences in ketones 1-3 and is rationalized on the basis of MMX calculations. Methyl substitution in the P-aryl ring affects the ease of oxidation of this ring and as a result induces changes in the kinetics for intramolecular charge transfer. This conclusion is further supported by electrochemical studies of Faisant appel B la photolyse Cclair au niveau de la nanoseconde utilisant une excitation laser de 337 nm, on a CtudiC la photochimie de plusieurs p-methoxy-P-phCnylpropiophCnones substituCes par des groupes mCthyles (1-5). Les triplets des cetones dkcroissent par un pitgeage intramolCculaire du carbonyle par un P-aryle par un processus bien connu qui impliquerait une transfert de charge. I1 est surprenant qu'une substitution par un methyle sur le groupe mkthylbne en P qui ne participe pas au mCcanisme de dksactivation provoque une diminution dramatique du temps de vie du triplet; on attribue I'effect B des prefkrences conformationnelles diffkrentes dans les cCtones 1-3 et on les rationalise B I'aide de calculs MMX.Une substitution par un groupement mCthyle sur le cycle P-aryle affecte la facilitC avec laquelle on peut oxyder ce cycle et, par consCquence, induit des changements dans la cinCtique du transfert de charge intramolCculaire. Cette conclusion est en accord avec les etudes Clectrochimiques des citones 1, 4 et 5.