Charge-Separated Reactive Intermediates from the UV Photodissociation of Chlorobenzene in Solution

Tony Kao, Andrew J Orr-Ewing*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

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Although ultraviolet-induced photochemical cleavage of carbon-halogen bonds in gaseous halocarbons is mostly homolytic, the photolysis of chlorobenzene in solution has been proposed to produce a phenyl cation, c-C6H5+, which is a highly reactive intermediate of potential use in chemical synthesis and N2 activation. Any evidence for such a route to phenyl cations is indirect, with uncertainty remaining about the possible mechanism. Here, ultrafast transient absorption spectroscopy of UV-excited (λ = 240 and 270 nm) chlorobenzene solutions in fluorinated (perfluorohexane) and protic (ethanol and 2,2,2-trifluoroethanol) solvents reveals a broad electronic absorption band centred at 540 nm that is assigned to an isomer of chlorobenzene with both charge-separated and triplet-spin carbene character. This spectroscopic feature is weaker, or absent, when experiments are conducted in cyclohexane. The intermediate isomer of chlorobenzene has a solvent-dependent lifetime of 30 – 110 ps, determined by reaction with the solvent or quenching to a lower-lying singlet state. Evidence is presented for dissociation to ortho-benzyne, but the intermediate could also be a precursor to phenyl cation formation.
Original languageEnglish
Pages (from-to)6934 - 6943
Number of pages10
JournalJournal of Physical Chemistry A
Issue number39
Early online date23 Sept 2022
Publication statusPublished - 6 Oct 2022

Bibliographical note

Funding Information:
We are grateful to EPSRC (EP/V026690/1) for financial support and to William Whitaker and Connah Harris (University of Bristol) who measured the transient IR spectra shown in Figure S8 using the LIFEtime Facility at the STFC Rutherford Appleton Laboratory. We thank Dr. Andrew Jupp (University of Birmingham) for valuable discussions at the start of this project.

Publisher Copyright:
© 2022 American Chemical Society. All rights reserved.


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