PHOTOCHEMISTRY OF FLUORO-SUBSTITUTED AND CHLOROSUBSTITUTED TRANS-STILBENE-4-CARBOXYLIC-ACIDS AND THEIR DERIVATIVES IN THE CRYSTALLINE PHASE

trans-1-(2-, 3- or luorophenyl)-2-[4-(hydroxycarbonyl)-phenyl]-ethene (2'-, 3'- or 4'-fluoro-trans-stilbene-4-carboxylic-acid), trans-1-(2-, 3- or hlorophenyl)-2-[4-(hydroxycarbonyl)-phenyl]-ethene (2'-, 3'- or 4'-chloro-trans-stilbene-4-carboxylic-acid) and their alkaline and al kaline earth salts and methylesters were studied to determine their ph otochemical reactivity in the crystalline phase. Most of these compoun ds dimerize on excitation at the longest wavelength absorption band vi a stereospecific (2pi(s) + 2pi(s)) cycloaddition to the substituted cy clobutane system; some are photochemically inert. The configurations o f the product molecules were analysed and are discussed with respect t o the structure of the reactant molecules and their packing modes in t he crystalline phase.