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 exam ined in solvents of varying polarity to delineate the effect of solvent on the optical absorption features. The perhaloporphyrins exhibit strong solve nt-dependent absorption spectral features in contrast to 5,10,15,20-tetraph enylporphyrin, H2TPP. A dramatic red-shift of Soret, B (> 25 nn) and visibl e, Q (> 50 nm) were observed for perhaloporphyrins in polar solvents relati ve to non-polar solvents. This has been ascribed to the enhanced distortion of the macrocyclic ring induced by hydrogen-bonding interaction between th e porphyrin core pyrrolic N-H or pyrrolenine nitrogens with the solvent mol ecules. (C) 2001 Elsevier Science B.V. All rights reserved.