Unusual solvent dependent optical absorption spectral properties of free-base perhaloporphyrins

A pair of free-base perhaloporphyrins, 2,3,7,8,12,13,17,18-octabromo- and octachloro-5,10,15,20-tetraphenylporphyrins (H2OBTPP and H2OCTPP) were examined in solvents of varying polarity to delineate the effect of solvent on the optical absorption features. The perhaloporphyrins exhibit strong solvent-dependent absorption spectral features in contrast to 5,10,15,20-tetraphenylporphyrin, H2TPP. A dramatic red-shift of Soret, B (>25 nm) and visible, Q (>50 nm) were observed for perhaloporphyrins in polar solvents relative to non-polar solvents. This has been ascribed to the enhanced distortion of the macrocyclic ring induced by hydrogen-bonding interaction between the porphyrin core pyrrolic N–H or pyrrolenine nitrogens with the solvent molecules.