Synthesis and photochemical properties of octacinnamoylsubstituted tetraazaporphyrins

Abstract

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, ¹H and ¹³C 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 (CC) 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/R⁶ indicates a long-range dipole–dipole mechanism.