The Halogen Substitution Effect on the para Position of the Tetra-meso Phenyl Porphyrin’s pKa

Synthetic porphyrins are compounds formed by four pyrrole rings connected by methine (=C-) bridges, which have found applications in solar cells and photodynamic therapy. These complex compounds undergo substitutions at the methine bridges, influencing their basicity due to two primary factors: nonplanar distortions caused by steric interactions and the electronic effect of the substituents composing the porphyrin’s overall composition. This study examines the basicity of meso-tetra(p-bromophenyl) porphyrin (TPP-Br), meso-tetra(p-chlorophenyl) porphyrin (TPP-Cl), and meso-tetra(p-fluorophenyl) porphyrin (TPP-F) through titrations in toluene, comparing them to meso-tetraphenylporphyrin (TPP) using incremental additions of trifluoroacetic acid (TFA). Protonation was followed through the use of UV spectroscopy, enabling the determination of their apparent pK_a values, which were calculated using a modified Henderson-Hasselbalch equation.