Ultrafast Observation of a Photoredox Reaction Mechanism: Photoinitiation in Organocatalyzed Atom-Transfer Radical Polymerization

Daisuke Koyama, Harvey J A Dale, Andrew J Orr-Ewing*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)

35 Citations (Scopus)
337 Downloads (Pure)

Abstract

Photoredox catalysis has driven a revolution in the field of organic chemistry, but direct mechanistic insights into reactions of genuine synthetic utility remain relatively scarce. Herein we report ultrafast time-resolved spectroscopic observation of a bimolecular organocatalyzed photoredox reaction, from catalyst photoexcitation through to photoinduced electron transfer (PET) and intermediate formation, using transient vibrational and electronic absorption spectroscopy with sub-picosecond time resolution. Specifically, the photochemical dynamics of initiation in organocatalyzed atom-transfer radical polymerization (O-ATRP) are elucidated for two complementary photoredox organocatalysts (N,N-diaryl-5,10-dihydrophenazines). Following photoexcitation, a dissociative bimolecular electron transfer is observed from the first excited singlet state of both photocatalysts to methyl 2-bromopropionate in dichloromethane, toluene, and dimethylformamide. The photocatalyst excited donor state, ground state, and radical cation are tracked in real time alongside the debrominated radical fragment. Our work challenges previously proposed mechanisms of initiation in O-ATRP and indicates that PET from short-lived excited singlet states can exert control of polymer molecular weight and dispersity by suppressing the steady-state concentration of the reactive debrominated radical. More broadly, we aim to demonstrate the potential of ultrafast absorption spectroscopy to observe directly transient, open-shell intermediates in mechanistic studies of photoredox catalysis.

Original languageEnglish
Pages (from-to)1285-1293
Number of pages9
JournalJournal of the American Chemical Society
Volume140
Issue number4
Early online date19 Jan 2018
DOIs
Publication statusPublished - 31 Jan 2018

Keywords

  • Photoredox
  • Catalysis

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