Structural transformations in a crystal during the photochemical reaction of 2-benzyl-5-benzylidenecyclopentanone

The studies presented are the first example in which the substrate and product molecules were monitored separately over many stages during the chemical reaction progress in a crystal to understand the reaction path of the molecules in detail during the whole transformation. The BBCP, 2-benzyl-5-benzylidenecyclopentanone, single crystal was irradiated without any destruction over several steps. After each of step the X-ray structure with, a different substrate/product ratio and with a disorder was determined. The study revealed that the photochemical [2+2]-cycloaddition reaction of BBCP in a crystal does not exhibit cooperative character and can be regarded as of the first-order. During the transformation the product molecules do not adopt the position of the molecules in the final pure dimer crystal, but move gradually and smoothly in this direction. The movement of the molecules was also observed for the substrate. This remarkable behaviour of the product and reactant molecules bears a rotational component. The distance between reaction centres of adjacent BBCP substrate molecules decreases fluently with the progress of the reaction, but it does not get smaller than a sum of van der Waals radii.