Electron Spin Resonance (ESR) is a technique used to study materials with unpaired electrons. It detects the magnetic fields produced by these electrons when they are exposed to microwave radiation in the presence of a magnetic field. This interaction provides valuable information about the electronic structure and dynamics of the material. ESR is widely used in various fields, including chemistry, physics, and biology, to investigate free radicals and transition metal complexes. By analyzing the resulting spectra, researchers can gain insights into molecular interactions, reaction mechanisms, and the behavior of materials under different conditions.