Selective Topochemical Photoreaction of Crystallized 2,3-Bis(2-phenylethenyl)-4,5-dicyanopyrazines

Topochemical photocycloaddition of morphologically isomeric yellow and orange crystals of bis(2-phenylethenyl) dicyanopyrazine 1 were conducted in solution, in the vapor deposited thin film and in the single crystals. Photoreaction of the yellow crystal gave syn head-to-tail dimer and its polymer, but the orange crystal did not react at all. The selective topochemical photocycloaddition of 1 was discussed based upon the lattice contraction and the dynamic movement of the molecular packing arrangements.