Studies in crystal engineering: effect of fluorine substitution in crystal packing and topological photodimerization of styryl coumarins in the solid state

Abstract

Styryl coumarins generally yield centrosymmetric (α-mode, anti-HT) photodimers when subjected to irradiation in the solid state. However, the substitution of fluorine dramatically alters the packing mode and steers the molecules 4-(4-fluorostyryl)coumarin 1 and 4-(2-fluorostyryl)coumarin 2 to form a stereospecific photodimer, β-mode, syn-HH across the styrenic double bond (yield 78-85%). The stereochemistry of the photodimer 2a has been established by X-ray crystallography. There is no evidence for the presence of C-H-…F interactions. The true nature of the weak atom-atom interactions called into play when fluorine is substituted is not clear. It is observed that the fluoro substituted compounds have greater crystal density than the corresponding unsubstituted ones.