Influence of steric hindrance on the solvent-dependent absorption spectral behavior of highly brominated free base porphyrin and its Zn(H) complex

Electronic absorption spectra of highly brominated free base porphyrin, 2,3,7,8,12,13,17,18-octabromo-tetrakis(2',6'-dibromo-3',5'-dimethoxyphenyl)porphyrin, H2T(3',5'-DMP)PBr16 and its Zn(II) complex were examined in various solvents. These derivatives exhibit red-shifted absorption spectral features in polar solvents relative to that observed in less polar or nonpolar solvents. The extent of red shifts observed in highly brominated porphyrins is less than those reported for the corresponding 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin, H2TPPBr8 and its Zn(II) complex. The crystal structure of the H2T(3',5'-DMP)PBr16 showed saddle-shaped geometry with steric hindrance of the bromo groups on the opposite phenyl rings which provide limited open access to the core. The decreased red shift of the absorption spectral bands of MT(3',5'-DMP) PBr16 (M = 2H, Zn(II)) derivatives when compared to unhindered MTPPBr8 is possibly due to the steric hindrance offered by the ortho-bromo groups rather than the electronic effects of the porphyrin it-system. Copyright (c) 2007 Society of Porphyrins & Phthalocyanines.