TIME-RESOLVED FLUORESCENCE STUDIES ON COVALENTLY-LINKED PORPHYRIN NITROARENE COMPLEXES - CONFORMATIONAL CONTROL OF PHOTOINDUCED ELECTRON-TRANSFER REACTIONS

Time-resolved fluorescence studies were carried out on a series of fre e-base and zinc(II) derivatives of meso-tetraphenylporphyrins covalent ly linked to either 1,3-dinitrobenzene (DNB) or 1,3,5-trinitrobenzene (TNB) acceptor units. These acceptor units were linked at different si tes (at the ortho, meta or para positions of one of the phenyl groups of meso-tetraphenylporphyrin) to the donor porphyrins such that the re sulting isomeric intramolecular donor-acceptor complexes exhibit diffe rent centre-to-centre (ctc) distances and relative orientations. Biexp onential fluorescence decay profiles observed for several of these cov alently linked complexes were rationalized in terms of the presence of ''closed'' and ''extended'' conformers. Detailed analyses of the fluo rescence decay data have provided a comprehensive understanding of the photoinduced electron transfer (PET) reactions occurring in systems c ontaining zinc(II) porphyrin donors. It is observed that although DNB- linked zinc(II) complexes follow the trends predicted for the efficien cy of PET with respect to donor-acceptor distance, the TNB-linked zinc (II) porphyrins exhibit a behaviour which is dictated by steric effect s. Similarly, although the thermodynamic criteria predict a greater ef ficiency of charge separation in TNB-linked complexes compared with DN B-linked complexes, the reverse trend observed has been attributed to orientational effects. In the complexes containing free-base porphyrin donors, PET is expected to be less efficient from a thermodynamic vie wpoint. In a few of these cases, fluorescence quenching seems to occur by parallel mechanisms other than PET.