[2 + 2] photocycloaddition reaction as a tool to monitor the molecular motions in the solid state by mechanochemical grinding

Mechanochemistry, a burgeoning field in green chemistry, has been utilized frequently to synthesize various organic molecules, metal complexes, coordination polymers (CPs) and metal-organic frameworks (MOFs) in the solid state from the reactants with very little or no solvent. These mechanical grinding reactions also resulted in successful isolation of materials that are inaccessible otherwise from solution. On the contrary, single crystal X-ray crystallographic technique is routinely used to study the solid-state structural transformations driven by thermal and photochemical methods. In the absence of single crystals, [2 + 2] photocycloaddition reactions can easily be monitored by NMR spectroscopy along with other suitable physical and analytical techniques. During mechanical grinding, several structural changes have been found to take place with the loss of single crystalline nature. Here from our personal perspective, we reviewed how this [2 + 2] cycloaddition reactions have been used effectively to monitor the structural changes induced by mechanochemical grinding. These structural transformations are caused by the pedal motion of olefin bonds, conformational changes of molecular fragments, movements of molecules, change in the composition by absorbing water from the atmosphere, anisotropic expansion of volume, rotation of helical coordination polymers, dimensionality change, loss of coordinating and lattice solvents and catalytic role of template molecules on the [2 + 2] photocycloaddition reactivity.