[2+2] Photodimerization and photopolymerization of diphenylhexatriene crystals utilizing perfluorophenyl-phenyl stacking interactions

Irradiation of the crystal of 1-perfluorophenyl-6-phenyl-1,3,5-hexatriene (1), the 1:1 cocrystal of 1,6diphenyl-1,3,5-hexatriene (2) and 1,6-bis(perfluorophenyl)-1,3,5-hexatriene (3) (2/3), and crystal 3 gave a mixture of dimer, trimer and higher oligomers that was soluble in common organic solvents. The highest molecular weight was 5000-8000 (the degree of polymerization = 15-20). The order of reactivity was 1 > 2/3 >> 3. The reaction of I was relatively efficient compared to typical organic crystals. The conversion reached 100% after 3 h irradiation. In each case, the regio- and stereoselectivity in the photodimerization was high, whereas in the formation of trimer and higher oligomers, the selectivity was much lower. The main dimer was spectroscopically identified to be the face-to-face dimer formed by the [2 + 2] cycloaddition at the 1,2-position of the triene double bonds. The photoproducts from 1 and 2/3 were amorphous, as evidenced by powder X-ray diffraction and polarizing optical microscopy. This is probably due to the nonplanar and bulky structures of the cyclobutane products. The photodimerization and polymerization are considered to be non-topochemical reactions. (C) 2008 Elsevier B.V. All rights reserved.