Effects of ring substituents on the type II photoreactions of phenyl ketones.  How interactions between nearby excited triplets affect chemical reactivity

Wagner, Peter J.; Kemppainen, Allen E.; Schott, Herbert

doi:10.1021/ja00798a027

Cited by 204 publications

(216 citation statements)

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“…3 Conversely, the 3 ππ * states are considered to be less reactive towards H-transfer. 8,25,27 On the other hand, if both triplet states are considered as a mixture of both states…”

Section: The Radical Formationmentioning

confidence: 99%

“…3 Conversely, the 3 ππ * states are considered to be less reactive towards H-transfer. 8,25,27 On the other hand, if both triplet states are considered as a mixture of both stateswhere the a and b coefficients represent the weight of the pure (ππ*) and (nπ*) states to each actual triplet, it could be considered that there is always a contribution of the nπ* state in the lower lying triplet. This would explain the H-abstraction in the case of alcohols, were the lower lying triplet is a ππ* state.…”

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confidence: 99%

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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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“…3 Conversely, the 3 ππ * states are considered to be less reactive towards H-transfer. 8,25,27 On the other hand, if both triplet states are considered as a mixture of both states…”

Section: The Radical Formationmentioning

confidence: 99%

mentioning

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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“…15 A p-OCH 3 substituent changes the nature of the lowest triplet state from a nπ* (unsubstituted valerophenone) to a ππ* state. Type II reactions are typically not observed from ππ* states.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…Type II reactions are typically not observed from ππ* states. 15 A p-OCH 3 -substituted derivative of 1M (p-OCH 3 -1M) should show negligible type II reaction (k H ) because of its ππ* triplet state. However, energy transfer from the p-OCH 3 acetophenone moiety to the cyclohexenone moiety (k ET ) should not be significantly affected by the p-OCH 3 substituent.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Study of a Two-Stage Photobase Generator for Photolithography in Microelectronics

Turro

Jockusch

et al. 2012

J. Org. Chem.

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The investigation of the photochemistry of a two-stage photobase generator (PBG) is described. Absorption of a photon by a latent PBG (1) (first step) produces a PBG (2). Irradiation of 2 in the presence of water produces a base (second step). This two-photon sequence (1 + hν → 2 + hν → base) is an important component in the design of photoresists for pitch division technology, a method that doubles the resolution of projection photolithography for the production of microelectronic chips. In the present system, the excitation of 1 results in a Norrish type II intramolecular hydrogen abstraction to generate a 1,4-biradiacal that undergoes cleavage to form 2 and acetophenone (Φ ∼ 0.04). In the second step, excitation of 2 causes cleavage of the oxime ester (Φ = 0.56) followed by base generation after reaction with water.

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“…The latter is in turn substantially less reactive than benzophenone itself (5). In the case of 2-phenylbenzophenone, it should be noted that the aromatic ring in the ortho position is not expected to be able to lead to an "all planar" configuration; this in turn, would be expected to enhance the carbonyl triplet reactivity, since the reactivity towards photoreduction is expected to decrease with increasing n,n* character of the triplet state (6).…”

Section: Introductionmentioning

confidence: 99%

Photochemistry of 2-phenylbenzophenone. An unusual bichromophoric molecule

Scaiano¹,

Nicodem²

1984

Can. J. Chem.

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2-Phenylbenzophenone is proposed to have two nearby triplets, a low lying π,π* state similar to that of biphenyl and a benzophenone-like triplet lying ~4 kcal/mol higher. The triplet lifetimes of this photostable ketone are extremely short, e.g. 0.56 and 2.5 ns at 300 K in toluene and isooctane, respectively, and are believed to be controlled by population of the benzophenone-like triplet and decay from this state via interaction of the carbonyl chromophore with the phenyl ring in the ortho position. The two triplets interconvert via ring rotation. The Arrhenius parameters for triplet decay in five solvents show some dependency with the temperature dependence of the solvent viscosity.

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Effects of ring substituents on the type II photoreactions of phenyl ketones. How interactions between nearby excited triplets affect chemical reactivity

Cited by 204 publications

References 2 publications

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

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

Study of a Two-Stage Photobase Generator for Photolithography in Microelectronics

Photochemistry of 2-phenylbenzophenone. An unusual bichromophoric molecule

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