ISOMERS OF BETA-SUBSTITUTED DI-NITROTETRAPHENYLPORPHYRINS AND TRI-NITROTETRAPHENYLPORPHYRINS AND THEIR COPPER(II) DERIVATIVES - STRUCTURE, OPTICAL AND ELECTROCHEMICAL REDOX PROPERTIES

Controlled nitration of meso-tetraphenylporphyrinato copper(II) using fuming nitric acid leads to the formation of different beta-substitute d nitro derivatives. Five isomers of dinitro and three isomers of trin itrotetraphenylporphyrins have been isolated and characterised using F AB/MS, H-1 NMR, UV-VIS and IR spectroscopies. The single crystal X-ray data on the copper(II) derivative of one of the isomers of trinitrote traphenylporphyrin revealed 'saddle' conformation of the porphyrin cor e resulting from the nitro group substitution at the beta-pyrrole carb ons. The electrochemical redox behaviour of the free-base porphyrins a nd their copper(II) derivatives revealed that successive substitution of nitro groups at the pyrrole carbons shifts the one-electron ring ox idations anodically while the ring reduction occurs at a less cathodic potential relative to the unsubstituted porphyrin. The isomer specifi c shifts in the redox potentials bear a direct relationship with the r elative energy levels of HOMO/LUMO obtained from AM1 calculations.