Wavelength Dependence of Light-Induced Cycloadditions

Jan P. Menzel, Benjamin B. Noble, Andrea Lauer, Michelle L. Coote, James P. Blinco, Christopher Barner-Kowollik

Research output: Contribution to journalArticlepeer-review

59 Citations (Scopus)

Abstract

The wavelength-dependent conversion of two rapid photoinduced ligation reactions, i.e., the light activation of o-methylbenzaldehydes, leading to the formation of reactive o-quinodimethanes (photoenols), and the photolysis of 2,5-diphenyltetrazoles, affording highly reactive nitrile imines, is probed via a monochromatic wavelength scan at constant photon count. The transient species are trapped by cycloaddition with N-ethylmaleimide, and the reactions are traced by high resolution mass spectrometry and nuclear magnetic resonance spectroscopy. The resulting action plots are assessed in the context of Beer-Lambert's law and provide combined with time-dependent density functional theory and multireference calculations an in-depth understanding of the underpinning mechanistic processes, including conical intersections. The π → π∗ transition of the carbonyl group of the o-methylbenzaldehyde correlates with a highly efficient conversion to the cycloadduct, showing no significant wavelength dependence, while conversion following the n → π∗ transition proceeds markedly less efficient at longer wavelengths. The influence of absorbance and reactivity has critical consequences for an effective reaction design: At high concentrations of o-methylbenzaldehydes (c = 8 mmol L-1), photoligations with N-ethylmaleimide (possible for λ ≤ 390 nm) are ideally performed at 330 nm, whereas at high light penetration regimes at lower concentrations (c = 0.3 mmol L-1), 315 nm irradiation leads to the highest conversion. Activation and trapping of 2,5-diphenyltetrazoles (possible for λ ≤ 322 nm) proceeds best at a wavelength shorter than 295 nm, irrespective of concentration.

Original languageEnglish
Pages (from-to)15812-15820
Number of pages9
JournalJournal of The American Chemical Society
Volume139
Issue number44
DOIs
Publication statusPublished - 8 Nov 2017
Externally publishedYes

Keywords

  • Absorption
  • Light
  • Nanoelectromechanical systems
  • Quantum mechanics
  • Lasers

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