Synthesis and photochemical properties of octacinnamoyl-substituted tetraazaporphyrins

New metal-free and metal-containing 2,3,7,8,12,13,17,18-octakis [(4-methoxycinnamoyl)oxyalkylthio]tetraazaporphyrins with different polymethylene chain lengths (3, 6, 8 or 11 methylene groups) were synthesized by esterification of 2, 3,7,8,12,13,17, 18-octakis(hydroxyalkylthio)tetraazaporphyrins, obtained from 1,2-dicyano-1,2-bis(hydroxyalkylthio)ethylenes, with 4-methoxycinnamic acid, The structures of the compounds were confirmed by UV-VIS, IR, H-1 and C-13 NMR spectroscopies. None of the derivatives shows liquid crystalline behaviour; their solid phase is of amorphous or microcrystalline consistency. On UV irradiation the dominating processes of the cinnamoyl groups are E,Z photoisomerization in solution and cross-linking reactions via the (C=C) bonds in spin-coated films, whereas photodegradation is the more (in solution) or less (in films) effective reaction of the. tetraazaporphyrin chromophore. Further irradiation of the spin-coated films results in insoluble transparent films with a high content of tetraazaporphyrin moieties. All of the processes determined show a dependence upon the length of the polymethylene chains. The efficiency of the fluorescence, the photoisomerization and the cross-linking reactions of the cinnamoyl group are lower with decreasing chain length, whereas the photodegradation of the tetraazaporphyrin is increased, This contrary behaviour suggests the existence of energy-transfer processes from the cinnamoyl chromophore to the tetraazaporphyrin chromophore. In solution, the dependence of the efficiency on the distance with 1/R(6) indicates a long-range dipole-dipole mechanism.