Menzel, Jan PNoble, Dr BenjaminLauer, AndreaCoote, MichelleBlinco, James PBarner-Kowollik, Christopher2020-08-302020-08-300002-7863http://hdl.handle.net/1885/209117The 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.C.B.-K. acknowledges key continued support by the Queensland University of Technology (QUT), the Australian Research Council (ARC) in the form of a Laureate Fellowship as well as by the Karlsruhe Institute of Technology in the context of the STN program of the Helmholtz association. M.L.C. acknowledges financial support from the ARC as well as generous allocations of supercomputing time on the National Facility of the Australian National Computational Infrastructure. J.P.M. acknowledges funding for his Ph.D. studies by the Queensland University of Technology (QUT). Funding from the German Research Council (DFG) supporting the Ph.D. studies of A.L. is additionally acknowledged.application/pdfen-AU© 2017 American Chemical Society201710.1021/jacs.7b08047