Doppelte Photoionisation von Dimethylaminobenzonitril in Lösung: ein Dreiquantenprozess mit chemischem Zwischenschritt

Goez, Martin; Zubarev, Valentin

doi:10.1002/1521-3757(20010803)113:15<2948::aid-ange2948>3.0.co;2-q

Cited by 4 publications

(1 citation statement)

References 17 publications

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“…525,812 Indirect evidence, interpreted in favor of the involvement of a 3 CT state in a deprotonated intermediate of DMABN, is based on the three-photon ionization of 1. 826 Due to the complex multistep mechanism, this conclusion calls for further direct experimental testing.…”

Section: Charge-transfer Triplet Statementioning

confidence: 99%

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

2003

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Section: Charge-transfer Triplet Statementioning

confidence: 99%

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

2003

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Effiziente Photoionisierung eines Norrish‐II‐ Diradikals

Goez

Zubarev

2006

Angewandte Chemie

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Efficient Photoionization of a Norrish II Diradical

Goez

Zubarev

2006

Angew Chem Int Ed

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The photochemistry of reactive intermediates in solution [1] is a fascinating subject because of its promise of new mechanisms and applications.[2] Photoionizations of excited singlet and triplet states are common; examples of photoionizations of radicals [3] and radical anions [4] are known. Here, we present the first report on the photoionization of a diradical and show that it is considerably more efficient than that of its precursor triplet.We performed laser flash photolysis at very high light intensities, where mechanisms involving more than one photon reveal themselves much more clearly than under the usual conditions. Our setup [4c] allows two-flash two-color experiments (355 or 266 nm, pulse width 6 ns; 308 nm, pulse width 30 ns; freely adjustable interpulse delay) and homogeneously illuminates a well-defined reaction volume, thus permitting quantitative determinations of absolute concentrations of transients. As the primary observable, the concentration of the hydrated electron e CÀ aq directly at the end of a laser pulse was measured through its optical absorbance. [5] Alkyl phenyl ketones M can be ionized with 308 nm light. [4d, 6] The usual mechanism is a sequential two-photon process as displayed in Scheme 1. Absorption of the first photon leads to an intermediate *M, normally the triplet state, which is then ionized by the second photon. Even at our high light intensities, the rate of the first step is limited by the excitation of M because of the small extinction coefficient at 308 nm; the subsequent dark reactions to give *M, such as intersystem crossing, are much faster.When the ketone bears a hydrogen atom in the g position, its triplet is rapidly transformed into a 1,4-diradical by an intramolecular hydrogen abstraction (the Norrish II reaction). [7] We were interested in how this classic photoreaction influences the photoionization. Figure 1 compares the electron yields, as functions of the laser intensity in single-flash experiments at 308 nm, for two model compounds in degassed aqueous base (nominal concentration of KOH, 0.4 m), gmethylvalerophenone as an efficient Norrish II substrate, [8] and propiophenone as the longest-chain alkyl phenyl ketone that is not yet capable of undergoing this reaction. It is evident that the Norrish II substrate produces substantially more electrons.The upward curvature of the intensity dependence bears out that the ionization of the Norrish II substrate is still biphotonic. With the mechanism of Scheme 1, the electron yield is determined by the (intensity-dependent) rates of both steps. As expected, the extinction coefficients of the two ground-state ketones, and thus the rates of production of *M for given laser intensity, are very similar. Hence, this step cannot explain the observed effect, and the different yields have to be ascribed to different properties of the photoionizable intermediate *M. It is natural to assume that they reflect the differences between the triplet (for propiophenone) and the Norrish II diradical (for g-methylvalerophenone) beca...

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Doppelte Photoionisation von Dimethylaminobenzonitril in Lösung: ein Dreiquantenprozess mit chemischem Zwischenschritt

Cited by 4 publications

References 17 publications

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Effiziente Photoionisierung eines Norrish‐II‐ Diradikals

Efficient Photoionization of a Norrish II Diradical

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