Photodecomposition of organic peroxides containing coumarin chromophore: Spectroscopic studies

Dmitry Polyansky, Douglas C. Neckers

Research output: Contribution to journalArticle

9 Citations (Scopus)

Abstract

The photodecomposition of coumarin-3-t-Bu peroxyester (1) and coumarin-3-carbonyl-m-chlorobenzoylperoxide (2) has been studied using nanosecond and femtosecond spectroscopy to elucidate the nature of transient species involved. Excitation of the coumarin chromophore leads to its singlet excited-state decaying with the rate 9 × 10 9 s -1 that results from a composite of emission, intersystem crossing, thermal relaxation, and -O-O- bond homolysis. Dissociation of the weak oxygen-oxygen bond proceeds from both triplet and singlet excited states. The nature of this combination of states is predissociative rather than dissociative as demonstrated by the relatively slow rates of oxygen-oxygen bond rupture. The decomposition of 1 and 2 leads to the formation of coumarin-3-carbonyloxyl radical (R1). The later was observed spectroscopically on the nanosecond time scale using both time-resolved FTIR and UV-vis transient techniques. R1 is consumed in two competitive processes: unimolecular decarboxylation and bi-molecular hydrogen atom transfer. The rates of these reactions are 4.3 × 10 5 s -1 and 1 × 10 6 M -1 s -1 respectively. The transition state geometries and energies of decarboxylation of R1 have been determined using DFT calculations and are compared with values for the benzoyloxyl radical. The decarboxylation of R1 proceeds via a transition state in which the carboxyl group is almost perpendicular (dihedral angle 114°) to the plane of the coumarin chromophore. The transition state of the benzoyloxyl radical, in contrast, is flat (0°). The varied transition state energies of the radicals (13.6 kcal/mol for coumarin carboxyl radical vs 8 kcal/mol for benzoyloxyl radical) correlate with different decarboxylation rates of these two species.

Original languageEnglish
Pages (from-to)2793-2800
Number of pages8
JournalJournal of Physical Chemistry A
Volume109
Issue number12
DOIs
Publication statusPublished - Mar 31 2005

Fingerprint

organic peroxides
photodecomposition
Peroxides
Chromophores
chromophores
decarboxylation
Oxygen
oxygen
Excited states
excitation
carboxyl group
Dihedral angle
Electron transitions
Discrete Fourier transforms
Electron energy levels
dihedral angle
coumarin
Hydrogen
hydrogen atoms
Spectroscopy

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Photodecomposition of organic peroxides containing coumarin chromophore : Spectroscopic studies. / Polyansky, Dmitry; Neckers, Douglas C.

In: Journal of Physical Chemistry A, Vol. 109, No. 12, 31.03.2005, p. 2793-2800.

Research output: Contribution to journalArticle

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