Fig. 2: Mechanistic investigations.
From: Triplet state reactivity of iminium ions in organocatalytic asymmetric [2 + 2] photocycloadditions
a, Steady-state luminescence studies. UV–vis absorption spectroscopy to verify the formation of iminium ion 4 by formation of a new band in the visible region. Comparison of the fluorescence quantum yields ΦF and fluorescence lifetimes τ(S1) of compounds 4 and 5. λ is the wavelength of electromagnetic radiation. b, Transient absorption spectra of 4. c, Transient absorption spectra of 5. d, Kinetic decay measured at 700 nm of the triplet excited state of 4 compared with 5. OD, optical density. e, Control experiment using a triplet quencher. r.t., room temperature. f, Stereochemical outcome of the organocatalytic enantioselective [2 + 2] photocycloaddition starting from the two diastereoisomers 7-(E) and 7-(Z). Depending on the stereochemical outcome (stereodivergent versus stereoconvergent process), the involvement of a singlet or triplet excited state can be suggested. The epimerization of the additional stereogenic centre is ruled out by the unchanged d.r. value during the reaction course.
