MESOSUBSTITUTED OCTABROMOPORPHYRINS - SYNTHESIS, SPECTROSCOPY, ELECTROCHEMISTRY AND ELECTRONIC-STRUCTURE

The free-base, copper(II) and zinc(II) derivatives of 5,10,15,20-tetra arylporphyrin (aryl = phenyl, 4-methylphenyl or 4-chlorophenyl) and th e corresponding brominated 2,13,17,18-octabromo-5,10,15,20-tetraarylpo rphyrin derivatives have been synthesized and their spectral and redox properties compared by UV/VIS, H-1 NMR, ESR and cyclic voltammetric m ethods. Substitution with the electron-withdrawing bromine groups at t he pyrrole carbons has a profound influence on the UV/VIS and H-1 NMR spectral features and also on the redox potentials of these systems. O n the other hand, electron-withdrawing chloro or electron-donating met hyl groups at the para positions of the four phenyl rings have only a marginal effect on the spectra and redox potentials of both the bromin ated and the non-brominated derivatives. The ESR data for the copper(I I) derivatives of ail these systems reveal that substitution at either the beta-pyrrole carbons and/or the para positions of the meso-phenyl groups does not significantly affect the spin-Hamiltonian parameters that describe the metal centre in each case. Collectively, these obser vations suggest that the highest-occupied (HOMO) and lowest-unoccupied molecular orbitals (LUMO) of the octabromoporphyrins involve the porp hyrin pi-ring system as is the case with the non-brominated derivative s.-Investigations have been carried out to probe the electronic struct ures of these systems by three different approaches involving spectral and redox potential data as well as AMI calculations. The results obt ained suggest that the electron-withdrawing beta-bromine substituents stabilize the LUMOs and, to a lesser degree, the HOMOs and that the ex tent of these changes can be fine-tuned, in a subtle way, by substitut ing at the meso-aryl rings of a given porphyrin.