Studies in crystal engineering: Steering ability of fluorine in styrylcoumarins

In continuation of our studies on crystal engineering using fluorine as a s teering group, the photobehaviour of di and tri fluoro 4-styrylcoumarins ha s been examined. It is found that out of the five derivatives, four crystal lize into P-packing mode producing syn-HH photodimer upon irradiation where as the parent hydrocarbon produces an anti K-T dimer. The packing features of the photolabile crystals of 4-(4-fluorostyryl)-6-fluorocoumarin (1), 4-( 2,6-difluorostyryl) 6-fluorocoumarin (2) and the photodimer (3a) of 4-(2,6- fluorostyryl)-7-fluorocoumarin (3) have been determined by single crystal X -ray diffraction studies. The stereochemistry of the photodimer of 4-(2-flu orostyryl)-6-fluorocoumarin (4) is deduced based on preliminary X-ray cryst allographic data. However, 4-(2,6-difluorostyryl) coumarin (5) is photoiner t. The remarkable steering ability of fluorine is established with the mole cular packing in the crystal lattice leading to the formation of syn H-H di mer in the above four examples. (C) 1999 Elsevier Science Ltd. All rights r eserved.