Laser flash photolytic generation of radical ions of carotenoids in organic solvents. Studies of their subsequent fates, including formation of stable carotenoid sigma dimer radical anion (CAR)2[rad]–

Ali El-Agamey, Thor B. Melø, K. Razi Naqvi, Maha A. El-Hagrasy, Kei Ohkubo, Shunichi Fukuzumi

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Abstract

In this study, time-resolved laser flash photolysis technique (LFP) was used to generate and study extraordinary stable transients formed following the decay of radical anions of canthaxanthin (CAN) and astaxanthin (ASTA). The radical anions were obtained via one-electron reduction of CAN and ASTA by acetone ketyl radical (AC[rad]−) in various solvents. At room temperature, the decay of CAN[rad]−max = 870 nm) and ASTA[rad]−max = 1000 nm), in acetonitrile (ACN), led to the formation of indefinitely stable colored transients (blue-green for CAN and navy blue for ASTA) absorbing at shorter wavelengths (λmax = 690 nm and 720 nm, respectively) relative to their corresponding radical anions. Rapid quenching by oxygen was observed indicating the radical nature of these transients, which was further confirmed by EPR spectroscopy. By employing EPR spectroscopy, LFP technique, and UV–visible absorption measurements, these extraordinary stable transients were identified as sigma-dimer radical anions ((CAR)2[rad]−) formed via the dimerization of the parent carotenoid (CAR) with its radical anion (CAR[rad]−). The suggested structures of these dimers were postulated using DFT calculations, which indicated that the unpaired electron is delocalized over the entire molecule. This result is extremely important because of the paucity of reports on dimer radical anions in solution.

Original languageEnglish
Article number113707
JournalJournal of Photochemistry and Photobiology A: Chemistry
Volume425
DOIs
StatePublished - 1 Mar 2022

Bibliographical note

Funding Information:
Dr El-Agamey is grateful to the Egyptian Ministry of Higher Education and Deanship of Scientific Research at King Faisal University (Nasher Track; grant No.: 186172) for financial support and to Dr Hati, A.G. for his continuous help during this research and to Mr Tony Heslop (Market Development Manager, BASF) for providing HHEMP and to Ms Marium Elagamy for her help with the TOC design and to Department of Physics at NTNU for laboratory facilities and hospitality. The authors are grateful to DSM Nutritional Products for supplying the carotenoids used in this work.

Publisher Copyright:
© 2021

Keywords

  • Acetonitrile
  • DFT calculations
  • EPR spectroscopy
  • Extraordinary stability
  • Laser flash photolysis
  • Sigma-dimer radical anions

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