Crystal engineering studies on the salts of trans-4,4′-stilbenedicarboxylic acid in the context of solid state [2+2] cycloaddition reaction

A series of molecular salts of trans-4,4'-stilbenedicarboxylic acid (H2SDC) with various amines (cyclohexylamine 1, ethylenediamine 2, 1,3-diaminopropane 3, 1,4-diaminobutane 4, cystamine 5, guanidine 6, 4-aminopyridine 7, piperizine 8) have been synthesized and their structures have been determined by X-ray crystallography to understand the influence of various dominant non-covalent interactions in their crystal packing to attain energy minima in the solid state architectures. The well-defined patterns of these interactions have been analysed to assign the corresponding graph set notations and the concept of 'supramolecular synthon' has been justified. The presence of olefinic double bond and its orientations in these molecular salts make it interesting to study their photoreactivity towards UV light for [2 + 2] cycloaddition reaction. In the salt of 1,3-diaminopropane, the dianion was found to align perfectly parallel, congenial for solid state [2 + 2] cycloaddition reaction, undergoing quantitative dimerization under a UV lamp. Although the dianion was found to align perfectly parallel in salt 1, the photostability of this salt justifies Schmidt's distance criterion. The slip-stacked orientation of the dianion makes the molecular salt 4 photostable. A zigzag water cluster chain stabilised by various H-bonding interactions was identified in the molecular salt 6. The photoreactivity of the salts with 3,3'-dipropylamino amine 9 and with ammonia 10 are also accounted to discuss the possible relation between the chain length of the dications.