LED‐Illuminated NMR Spectroscopy: A Practical Tool for Mechanistic Studies of Photochemical Reactions

Abstract

This Concept article highlights the development of a novel analytical tool, LED-NMR (a combination of in situ light illumination using a light-emitting diode and NMR spectroscopy) and its variant UVNMR (LED-NMR coupled with UV/Vis absorption spectroscopy), as well as their applications in the mechanistic investigation of light-induced transformations. The utility of these new tools has been demonstrated by providing rich kinetic and structural data of reaction species offering mechanistic insights into photochemical and photocatalytic reactions. Furthermore, NMR actinometry has been recently developed as a practical and simple method for quantum yield measurements. Quantum yield is an important parameter in photoinduced processes, but is rarely measured in practice because of the barriers associated with traditional actinometry. These new tools and techniques streamline measurements of the quantum efficiency while affording informative mechanistic insights into photochemical reactions. We anticipate these techniques will enable chemists to further advance the rapidly emerging photochemistry field. adoption of LED-NMR will make this powerful technique a standard method in the field of photochemistry.

LED‐NMR Monitoring of an Enantioselective Catalytic [2+2] Photocycloaddition

“…Many photocatalytic reactions conducted on preparative scale exhibit a linear relationship between photon flux and reaction rate because the solutions are optically dense, absorbing >99 % of the incident light . In such a photon‐limited regime, photocatalytic reaction rates would be controlled solely by the absorption of photons and should thus be zero‐order in the photocatalyst . This expectation is not consistent with the data in our reaction, as we observe a first‐order dependence on photocatalyst concentration under identical conditions.…”

“…This expectation is not consistent with the data in our reaction, as we observe a first‐order dependence on photocatalyst concentration under identical conditions. We conclude therefore that these experiments fall outside of the photon‐limited regime, and thus both photon flux and photocatalyst concentration have an influence on the reaction rate law. Indeed, even at the highest photocatalyst concentration, a simple Beer's law calculation shows that only 20 % of the photons will be absorbed over a 3 cm pathlength within the NMR tube (see Table S1 in the Supporting Information).…”

“…Recent work has provided convenient methods based on in situ , light‐coupled NMR spectroscopy . These methods are typically applied under photon‐limited conditions, allowing a single point initial rate measurement to be used to calculate quantum yield . Because we were unable to reach a photon‐limited regime, we combined our experimental initial rates with a correction for the fraction of light absorbed (see SI), and were able to estimate the quantum yields of the [2+2] cycloaddition at the different LED intensities shown in Figure C .…”

Evidence for Photocatalyst Involvement in Oxidative Additions of Nickel-Catalyzed Carboxylate O-Arylations

“…To switch from the photon-limited to the photon-unlimited regime, we needed to increase the ratio of photons and/or PC to nickel. Although numerous photocatalytic reactions with metal centers have been tracked with in situ apparatuses, 33 to our knowledge only one study has described finding a region in which the metal’s access to excitation is carefully controlled to be unimpeded. 34 In the work of Lehnherr et al, light directly excited a metal catalyst, with no photocatalytic intermediary.…”

Process analytical technology (PAT) as a versatile tool for real-time monitoring and kinetic evaluation of photocatalytic reactions

“…Thus, photocatalytic reactions that involve either a reactive or inert gas-atmosphere could only be studied at low performance including mass transport limitation or are simply not feasible. 35 Process analytical technology (PAT) combine in situ realtime analysis with chemometric data evaluation for the monitoring and evaluation of chemical processes. 36 The use of in situ techniques like UV/Vis, IR or Raman spectroscopy enables in-process monitoring of the relevant information without physical sampling.…”