CRYSTALLOGRAPHIC INTERPRETATION OF THE TOPOCHEMICAL BEHAVIOR OF ALKYL ALPHA-CYANO-4-[2-(4-PYRIDYL)ETHENYL]CINNAMATES IN THE CRYSTALLINE STATE - ENHANCEMENT OF PHOTOPOLYMERIZABILITY BY COMPLEX-FORMATION

The photochemical behavior of methyl, ethyl, and propyl alpha-cyano-4-[2-(4-pyridyl)ethenyl]-cinnamates (1a-1c) was investigated in the crystalline state. The behavior was highly diversified and was found to be strongly dependent on even a slight modification of the ester alkyl group in these monomers and on the solvent used for recrystallization. Irradiation of crystal 1a gave a beta-homo-type dimer and oligomers. On the other hand, irradiation of monomers 1b and 1c gave either an alpha-homo-type dimer or a linear polymer; 1b led to a polymer (MBAR(n) = 3100) through an "even-numbered polymerization mechanism", whereas 1c led to a photostable dimer crystal in quantitative yield. Two dimers (2b and 2c) crystallized as complexes with solvents when the dimers were recrystallized from solutions of conventional solvents, such as alcohols, butyl acetate, and p-xylene. Those dimer complexes consisting of a dimer with ethanol or propanol showed a higher photoreactivity compared with the corresponding dimer alone. Regarding photoirradiation, these complexes gave linear high polymers having the same type repeating structure as that obtained directly from 1b. The topochemical behavior of the monomers (1a-1c) and of the dimer complexes (2b-EtOH and 2c-PrOH) was completely interpreted from their crystal structures. Furthermore, the crystal structure required for the formation of a high polymer is discussed in terms of the length of the monomer repeating unit in the stack in those crystals.