Nuclear magnetic resonance (NMR) spectroscopy, as a non-destructive and non-invasive method, enables structural identification and quantitative analysis of the sample of interest and thus represents a very powerful tool that is used every day in organic synthesis. Monitoring the course of a chemical reaction and analyzing its kinetic parameters using NMR spectroscopy is routinely performed for reactions that can be initiated ex situ, by increasing the temperature in situ, or sufficiently slow reactions where sample manipulation time is not a limiting factor. For photochemical reactions, this is usually not the case.

Great progress was made in the field of organic photochemistry, and the application of photocatalysis as a complementary method to thermal reactions is becoming more and more frequent, resulting in new molecular structures otherwise unavailable within the framework of classical organic synthesis. Along with the development of such photochemical and photocatalytic strategies, there is a need to study the mechanisms of such conversions in a way that would bypass the need for ex situ manipulation, i.e. by taking an aliquot of the reaction mixture for a specific analysis.

The solution for such reactions is an integrated LED-NMR device (Figure 1) that uses the insensitivity of optical fibers to strong magnetic fields, which allows the introduction of a light source into the very center of the magnetic field inside the magnet of the NMR spectrometer. The integration of classical NMR spectroscopy and in situ photochemistry thus achieved allows structural characterization of highly reactive and short-lived chemical species and intermediates, which can determine the kinetics of the reaction transformation, and thus the mechanism of a particular photochemical transformation.

Specific settings of the LED-NMR instrument also allow for so-called "on-off" experiments where at certain time intervals the reaction mixture is analyzed in periods of “dark” and “light”. In this way, background processes that occur in the dark, which are directly or indirectly related to the steps in the reaction mechanism that are initiated by light, can be monitored and determined. Accurate identification of the processes that occur during light exposure and their separation from those that occur in the dark is very important for finding the most favorable reaction conditions for the desired transformation and suppression of unwanted processes.