What Changes in Topochemistry when Going from Small Molecule Dimerizations to Polymerizations in Single Crystals?

This publication promotes the increased necessity for strain management in topochemical reactions with an enlarged structural extension of the molecular products formed, i. e., when going from small molecule dimerizations, through linear polymerizations to the formation of 2D polymers. Further, it promotes to combine the trap model for photon absorption with concrete molecular scale consequences of this absorption on topochemical transformations and briefly discusses the expected consequences topological dimensionality of the forming (macro)molecular products has on trap location. The time appears ripe for going in this direction because local information concerning structural changes within single crystals is now accessible by the 3D-ΔPDF method. This method greatly facilitates the analysis of diffuse X-ray scattering providing access to concrete values of pair distribution functions and, thus, factual information on which and how distances change near a reaction site. Although only based on a first case where distance changes could be quantified in a lateral polymerization, the thoughts put forward may ignite more research towards a full understanding of all the action that occurs when a photochemically triggered topochemical reaction takes place. The 3D-ΔPDF method is so attractive for this purpose because it provides otherwise inaccessible local information in pair correlation functions rather than average structure information, which is used through the ubiquitous Bragg scattering.