Effect of protonation induced distortions on the spectral properties and electronic structure of Octaphenylporphyrins: UV-vis, electrochemical, VT-NMR and ab initio studies

The extent of conformational alteration of the porphyrin macrocycle during the course of protonation was observed, both experimentally and theoretically. For synthesized β-substituted octaphenylporphyrin (OPP) ring systems this macrocyclic distortions have been investigated by spectroscopic and computational analysis. Protonation induced nonplanarity in macrocyclic system is experimentally observed in a single step during spectrophotometric titration. The progress of the protonation by TFA was monitored i.e. the addition of proton to the core of the ring was monitored by UV/Vis spectroscopy, which was also determined logβ value. The spectral changes demonstrate the conformational flexibility of these macrocyclic systems correlating with the photophysical properties. The degree of nonplanarity exhibited incremental progress in response to protonation, with a specific order observed: H2TPP < H2OPP < H2OPPBr4 < H2OPPBr3NO2. Furthermore, our study delved into the structural aspects, frontier molecular orbitals and ground state energies of H4P(CF3COO)2 absorption bands computed at the TD-B3LYP/6-31G level of theory, anticipated a red shift in both the Soret and Q0,0 bands. This theoretical prediction aligns seamlessly with the experimental findings from spectrophotometric titrations, providing validation to our computation insights. In summary, this comprehensive study elucidates the extent of conformational changes occurring in β-substituted OPP ring systems upon protonation and deprotonation.