PHOTOPHYSICS OF HALOGENATED PORPHYRINS

A series of free-base octaalkylporphyrins having halogen substituents at different sites on the nucleus has been examined by photophysical methods. In general, halogenation decreases the fluorescence yield and lifetime, increases triplet formation and decreases the resultant triplet lifetime. The magnitude of these effects, which increases in the order F < Cl < Br < l and is dependent on the site of halogenation (benzylic position of alkyl < beta-position of pyrrole < meso position) can be explained in terms of a modified spin-orbit coupling theory. Multiple halogenation at meso positions, however, can cause secondary effects, due to steric crowding (bromine) or electron redistribution (fluorine). Photophysical data derived from the full set of meso-fluorinated porphyrins can be described quantitatively in terms of combined spin-orbit coupling and electronic perturbation in which a fluorine substituent deactivates adjacent meso sites but activates the site opposite the substituent.