INFLUENCE OF FLUORO SUBSTITUTION ON PACKING OF STYRYLCOUMARINS AND THEIR PHOTOBEHAVIOUR IN THE CRYSTALLINE STATE. K. Vishnumurthy¹, K. Venkatesan¹ and T. N. Guru Row^1,2, ¹Department of Organic Chemistry, ²Solid State Structural Chemistry Unit Indian Institute of Science Bangalore 560 012, India

The correlation between reactive molecules in crystals and stereochemistry of the photoproducts has been a subject of intensive study(a). Crystals of styryl- coumarins and chloro - substituted derivatives yield centrosymmetric crystals leading to anti-HT photodimers. However, replacement of a hydrogen by fluorine as in 4-(4'-fluoro styryl) coumarin (1) and 4-(2'-fluoro styryl) coumarin (2) produces syn-HH dimers across the styrenic double bond(b). Whereas 4-(3'-fluoro styryl) coumarin (3) and 4-styryl, 6-fluoro coumarin (4) produces anti-HT dimers. The packing modes vary depending upon the position at which fluorine is substituted. In all these structures, there are no significant short intermolecular F...F, F...H, F...C and F...O contacts although in (3) there are a few C-H...F interactions. These observations suggest that long range electrostatic interactions might also play a significant role in these crystals.

(a) V. Ramamurthy and K. Venkatesan; Chem. Rev., 1987, 87, 433.

(b) K. Vishnumurthy, T. N. Guru Row and K. Venkatesan; J. Chem. Soc., Perkin Trans. 2 (in press).