Influence of Supramolecular Interactions on Electron-Transfer Photochromism of the Crystalline Adducts of 4,4′-Bipyridine and Carboxylic Acids

We have studied the electron-transfer photochromism of the crystalline adducts of 4,4'-bipyridine (Bpy) and carboxylic acids and revealed the key structural parameters that decide whether the photochromism can happen for the first time. Experimental and theoretical analyses on nine known examples showed that the hydrogen bonds, instead of pi-pi stacking interactions, are the defining factor to the photochromism. Only the presence of N-H center dot center dot center dot O hydrogen bonds can fulfill the electron transfer from the carboxylate group to the Bpy part, although both the N center dot center dot center dot O separations of O-H center dot center dot center dot N and N-H center dot center dot center dot O hydrogen bonds are suitable for the so-called through-space electron transfer. These results can not only help to screen out the photochromic species from the known hundreds of Bpy-carboxylic acid adducts deposited in the Cambridge Crystallographic Data Centre (CCDC) database but also guide the design and syntheses of new adducts using diverse N-heterocyclic aromatic molecules and carboxylic acids.