Nuclear magnetic resonance (NMR) spectroscopy is a non-destructive and non-invasive method mainly used for structural identification and quantitative analysis, but also for reaction monitoring and kinetic studies. The latter are limited to reactions that can be initiated ex situ, by increasing the temperature in situ, or for sufficiently slow reactions where the time for sample manipulation is not a limiting factor, which usually not the case for photochemical reactions.

Organic photochemistry has experienced a revival in the last decade and photocatalysis is now increasingly used as a complementary method to thermal reactions, leading to new molecules that are not available in classical organic synthesis. In parallel with the development of such photochemical and photocatalytic strategies, there is a need to study the mechanisms of such transformations in a way that eliminates the need for ex situ manipulation.

The solution for such reactions is an integrated LED-NMR device (Figure 1) that exploits the insensitivity of optical fibres to strong magnetic fields, which allows the introduction of a light source exactly to the centre of the magnetic field in the NMR spectrometer. The integration of classical NMR spectroscopy and in situ photochemistry enables the structural characterisation of highly reactive and short-lived chemical species and intermediates, allowing the study of reaction kinetics and thus the mechanism of a particular photochemical transformation to be determined.

Additionally, the specific LED-NMR settings allow for so-called "on-off" experiments, in which the reaction mixture is analysed at specific time intervals in periods of "darkness" and "light". In this way, it is possible to monitor and determine the background processes taking place in the dark, that are directly or indirectly related to the steps in the reaction mechanism triggered by light. Accurately identifying the processes that take place in the light and separating them from those that occur in the dark is very important for finding the most favourable reaction conditions for the desired transformation and suppressing undesirable processes.