Effect of Protonation-Induced Distortions on the Spectral Properties and Electronic Structure of Octaphenylporphyrins through UV-Vis and VT-NMR Spectral, Electrochemical and Ab Initio Studies

The extent of conformational change of the porphyrin macrocycle during the course of protonation was observed, both experimentally and theoretically. For synthesised β-substituted octaphenylporphyrin (OPP) ring systems, these macrocyclic distortions have been investigated by spectrophotometric titrations, electrochemical, variable temperature nuclear magnetic resonance (VT-NMR) and computational analysis. Protonation-induced non-planarity in a macrocyclic system is experimentally observed in a single step during spectrophotometric titration. The addition of trifluoroacetic acid (TFA) eventually formed dicationic form while tetrabutylammonium hydroxide (TBAOH) was employed for deprotonation study for titration in toluene. The progress of the protonation 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. VT-NMR experiments were carried out in dichloromethane over a temperature range of 193–298 K, providing dynamic behaviour of these systems. Furthermore, our study delved into the structural aspects, frontier molecular orbitals and ground state energies of H₄P(CF₃COO)₂ and absorption bands computed at the TD-B3LYP/6-31G level of theory, anticipated a red shift in both the Soret and Q_0,0 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 octaphenylporphyrin (OPP) ring systems upon protonation and deprotonation.