Solvent effects on some new meso-aryl substituted octabromoporphyrins

A series of porphyrins with tolyl and naphthyl substituents at the meso positions, their octabromoderivatives (OBP) with Br substituents at β-pyrrole positions are synthesised and characterised by chemical analysis. 1H NMR and electronic spectral studies. It is seen that all the OBPs exhibit pronounced red shifts in both the Soret and Q bands of their electronic spectra compared to their nonbrominated form in various polar and nonpolar solvents, the energy difference Δv̄ being in the range 2300-2700 cm-1. The high energy B band of naphthyl porphyrins (both brominated and nonbrominated) are found to be more red-shifted than that of tolyl porphyrins, owing to the noticeable mesomeric effect of the naphthyl groups. Detailed spectral studies reveal that while none of the nonbrominated porphyrin show solvent-dependent change in their B and Q bands, all the OBPs manifest significant shifts depending on the nature of solvents. Solvent-solute interaction can be considered to be of strong dipole-dipole nature for OBPs with polar solvents and of π-π type with aromatic non-polar solvents. In the brominated form we find two categories of porphyrins exhibiting distinctly different absorption phenomena in aromatic solvents. The OBPs having meso-groups not shielding the porphyrin π-framework exhibit additional absorption peaks (split Soret peaks and broadened Q bands) in some aromatic solvents. This could be explained in terms of π-π type donor-acceptor (DA) complex formation between such bromoporphyrins (acceptor) and the aromatic solvent molecules (donor) that is not possible for OBPs that have bulky meso groups that block the approach of aromatic solvent molecules close to the porphyrin framework.