Crystal engineering: fluorine as a new steering group

Abstract

For the first time fluorine has been examined as a steering group for crystal engineering. In sharp contrast to the packing and photo behaviour of unsubstituted coumarin, molecules of both 6- and 7-fluorocoumarins are found to attain a packing mode which enables them to undergo [2 + 2] photodimerization in the solid state leading stereospecifically to a mirror symmetric dimer.