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