Complexation of Mg-tetrabenzoporphyrin with pyridine enhances singlet oxygen generation and affects its partitioning into apolar microenvironments

Photophysical and singlet oxygen (¹O₂) sensitization processes of Mg-tetrabenzoporphyrin (MgTBP) were studied upon complexation of the central Mg ion with pyridine. The addition of pyridine affected the absorption spectrum of MgTBP due to complexation, shifting the Soret absorption peak by as much as 13 nm. Principal component analysis (PCA) was employed to resolve the spectrum of MgTBP from the mono-ligand and bi-ligand complexes, MgTBP(pyr)₁ and MgTBP(pyr)₂, respectively. We found that the quantum yield of singlet oxygen production (Φ_Δ) by MgTBP was also affected by complexation of pyridine. In chloroform, toluene, water and SDS micelles the bi-ligand complexes had a higher quantum yield than MgTBP, by as much as 70%. In those solvents in which the mono-liganded complex was isolated, we found that its singlet oxygen quantum yield was higher than that of MgTBP by up to 20%. This enhancement can be explained by a relatively more efficient intersystem crossing of the complexes. We also found that MgTBP is capable of intercalating into a non-polar microenvironment of SDS micelles or lecithin liposomes. The axial pyridine ligands were, however, shed-off when the molecule entered the lipid bilayer of liposomes while they remained complexed when entering the micelle environment. This probably arises from the tight and ordered liposomal lipid phase as compared with the more fluid micellar environment.