Substituted acetophenones. Importance of activation energies in mixed state models of photoreactivity

Berger, Mitchell B.; McAlpine, E.; Steel, Colin

doi:10.1021/ja00484a039

Cited by 42 publications

(29 citation statements)

References 11 publications

(19 reference statements)

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“…In the 1,4-diaroylbutanes, kSq decreases as the IT,IT* triplet becomes lower in energy than the n,m* triplet. This behavior parallels the corresponding drops in observed rate constants for n,m* reactions (33,38) and strongly suggests that the selfquenching involves mainly the n,n* triplet. This conclusion reinforces the first one.…”

Section: Structural Dependence Of Ksqsupporting

confidence: 75%

Photochemistry of nonconjugated diketones: internal self-quenching and energy transfer

Wagner¹,

Frerking²

1995

Can. J. Chem.

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The triplet state behavior of nine a,o-dibenzoylalkanes indicates the occurrence of a rapid quenching interaction between the two carbonyl groups. This quenching is fastest (k = 3 x lo7 s-') in dibenzoylbutane, is slightly slower (-lo7 s-I) in dibenzoylethane, dibenzoylpentane, and 2.2-dibenzoylpropane, but is absent in 1,3-dibenzoy lpropane. It also occurs in several "mixed" 1,4-diaroylbutanes incorporating p-ethy lbenzoy 1 or p-methoxybenzoyl chromophores This internal self-quenching is interpreted as the intramolecular counterpart of the well-known bimolecular self-quenching of aryl ketones, although no exact mechanism can be proposed. Such internal quenching does not occur as rapidly, if at all, in three "turned around diketones: 6-@-acetylphenyl)valerophenone, 6-@-acetylphenoxy)valerophenone, and y-@-acety1phenoxy)butyrophenone. This fact, together with the varying rates of internal self-quenching in the dibenzoylalkanes, indicates the necessity for a very specific and close orientation of the two carbonyl groups for self-quenching. In the mixed diketones containing ap-alkylbenzoyl group, triplet excitation appears to be fully equilibrated between the two chromophores. However, in those containing a p-methoxybenzoyl group, excitation does not fully equilibrate before triplet decay, as evidenced by different quenching efficiencies for products from the two carbonyls. Analysis indicates intramolecular energy transfer rate constants 5 108 s-'. These are sufficiently lower than in other bichromophoric systems to suggest relatively slow energy hopping in the polymers of phenyl vinyl ketone.Key words: nonconjugated diketones, dibenzoylalkanes, self-quenching, energy transfer, triplet ketones.Resume : Le comportement de 1'Ctat triplet de neuf a,o-dibenzoylalcanes indique qu'il se produit une rapide interaction de dtsactivation entre les deux groupes carbonyles. Cette dksactivation se fait le plus rapidement (k = 3 x 107 S-l) dans le dibenzoylbutane; elle est 1Cgtrement plus lente (-lo7 S-l) dans les dibenzoylethane, dibenzoylpentane et 2,2-dibenzoylpropane, mais elle est totalement absente dans le 1,3-dibenzoylpropane. Elle se produit aussi dans plusieurs 14-diaroylbutanes cc mixtes n qui incorporent les chromophoresp-Cthylbenzoyle ou p-mCthoxybenzoyle. On interprbte cette autodCsactivation interne comme la contrepartie intramolCculaire de I'autodCsactivation bimolCculaire bien connue des arylcttones, m&me si aucun mCcanisme exact a Ct C proposC. Si elles se produisent, de telles dCsactivations internes ne se produisent pas non plus rapidement dans les trois dicttones cc qui sont retournCes n : 6-@-acCtylphCnyl)valCrophtnone, 6-@-acCty1phtnoxy)valCrophCnone et y-(p-acCtylphtnoxy)butyrophCnone. Ce fait, pris avec les vitesses variables de I'autodCsactivation interne dans les dibenzoylalcanes, indique qu'il est ntcessaire d'avoir une orientation spkcifique et rapprochCe des deux groupes carbonyles pour obtenir de I'autodCsactivation. Dans les cas des dicCtones mixtes contenant un groupe p-alkylbenzoyle, il...

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Section: Structural Dependence Of Ksqsupporting

confidence: 75%

Photochemistry of nonconjugated diketones: internal self-quenching and energy transfer

Wagner¹,

Frerking²

1995

Can. J. Chem.

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“…15a, 18a, 23, 40–44 This has been attributed generally to the solvent effect associated with the high polarity, high ionizing and high H‐bonding ability of water. In the case of BDP, this solvent effect may mainly manifest itself as follows: i) by stabilization of the closely lying ππ* state and destabilization of the lowest nπ* state,23, 40–45 the high polarity and high H‐bonding capacity of water could introduce increased ππ* character into the original nπ* dominated BDP triplet state. This in turn could lead to a decreased reactivity for the nπ* based reactions (the deprotection–cyclization).…”

Section: Resultsmentioning

confidence: 99%

Femtosecond Transient Absorption and Nanosecond Time‐Resolved Resonance Raman Study of the Solvent‐Dependent Photo‐Deprotection Reaction of Benzoin Diethyl Phosphate

Kwok

et al. 2007

Chemistry A European J

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A combined femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR3) study was performed to directly detect the dynamics and elucidate the mechanism of the excited state deactivation and solvent-dependent photo-deprotection pathways for benzoin diethyl phosphate (BDP) in neat acetonitrile (MeCN) and 75 % H2O/25 % MeCN. Comparison of the TA spectral evolution observed in the two solvents provides explicit evidence that the photophysical deactivation of the BDP singlet excited state has little solvent dependence. The TA spectra also indicate the related internal conversion (IC) and intersystem crossing (ISC) processes occur rapidly on hundreds of femtoseconds and approximately 2-3 ps time scales, respectively. From this and in conjunction with a photochemistry study and ground state resonance Raman (RR) measurements, the TA results reveal that the phenacyl localized BDP triplet state (that is mainly npi* nature) is the common and immediate precursor to the photo-deprotection reaction in both solvents. However, the triplet deprotection follows different pathways in neat MeCN versus the largely water containing solvent. The deprotection reaction in MeCN was determined to occur with a approximately 11 ns time constant and the reaction was found to be an unimolecular process leading to elimination of the diethyl phosphoric acid apparently concurrent with cyclization to yield the benzofuran product. In the water mixed solvent, the triplet reaction was observed to proceed with a approximately 15 ns time constant and the reaction leads to not only the deprotection-cyclization but also a heterolytic dissociation to release the diethyl phosphate anion through a branching and competing mechanism. The ns-TR3 spectra combined with relevant DFT calculations have been used to characterize the dynamics, structure and vibrational frequencies to help identify the important intermediates as well as to explore the reaction pathway leading to formation of the solvolysis product in the largely water solvent. A consecutive mechanism has been revealed for the heterolysis-solvolysis reaction in the water mixed solvent. The present work provides direct and irrevocable evidence for the dynamics and mechanistic description of the overall photophysics and deprotection related photochemistry for BDP.

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“…It is generally considered that the reactivity arises from mixing of the nπ* character into the ππ* state 1. A mechanism that involves the thermal excitation to a closely lying np* state has also been suggested 10–12. These arguments are not based on direct experimental evidence on the ordering of the excited triplet states and therefore are not conclusive; conventional spectroscopic techniques have not been effective in observing close‐lying excited triplet states of aromatic carbonyl compounds 13.…”

Section: Photoreduction Activity and Band Intensity Ratios Of Substitmentioning

confidence: 99%

Ordering, Interaction, and Reactivity of the Low‐Lying nπ* and ππ* Excited Triplet States of Acetophenone Derivatives

et al. 2010

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Im Gleichgewicht: Die niedrig liegenden nπ*‐ und ππ*‐Triplettzustände substituierter Acetophenone wurden mit zeitauflösender Nahinfrarotspektroskopie direkt beobachtet. Die effektiven Reaktivitäten der ππ*‐T1‐Zustände von Acetophenonen ergeben sich aus der thermischen Besetzung der höheren nπ*‐T2‐Zustände. Die T1‐ und T2‐Zustände der Acetophenone befinden sich im thermischen Gleichgewicht.

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Substituted acetophenones. Importance of activation energies in mixed state models of photoreactivity

Cited by 42 publications

References 11 publications

Photochemistry of nonconjugated diketones: internal self-quenching and energy transfer

Photochemistry of nonconjugated diketones: internal self-quenching and energy transfer

Femtosecond Transient Absorption and Nanosecond Time‐Resolved Resonance Raman Study of the Solvent‐Dependent Photo‐Deprotection Reaction of Benzoin Diethyl Phosphate

Ordering, Interaction, and Reactivity of the Low‐Lying nπ* and ππ* Excited Triplet States of Acetophenone Derivatives

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