Exploring the stereochemistry of In(III)-A3B-type porphyrins axially ligated with p-PDA: synthesis, electronic properties and DFT calculations

Abstract

In(III)-A3B-type porphyrins containing para-phenylenediamine (p-PDA) as an axial moiety have been efficiently synthesized and characterized spectroscopically to procure the structures of the synthesized complexes. The axial coordination of the amine moiety with In(III) porphyrins brings about a modest shift in the 1H NMR signals and the electronic absorptions in CH3OH, CH3COCH3, CHCl3, and CH3COOC2H5 reflect the hyperchromicity of p-PDA-In(III) porphyrins in comparison to non-ligated metal chelates. All the synthesized macrocyclic porphyrins have negative solvatochromism with the blue shift in the electronic absorption spectra in more polar solvents like CH3OH than those in the less polar CHCl3 as solvent. The photoluminescence spectra show maximum excitonic peaks at 623, 620, and 615 nm for p-phenylenediamine-In(III)-5,10,15-trikis(p-methylphenyl)-20-(p-nitrophenyl)porphyrin [p-PDA-In-tPP1], p-phenylenediamine-In(III)-5,10,15-trikis(p-methoxyphenyl)-20-(p-nitrophenyl)porphyrin [p-PDA-In-tPP2] and p-phenylenediamine-In(III)-5,10,15-trikis(p-dimethylaminophenyl)-20-(p-nitrophenyl)porphyrin [p-PDA-In-tPP3], respectively, which confirmed that the In(III) porphyrins are considered to be sitting atop (SAT) or out of the plane (OOP) type complexes. Further, the stability of the complexes was determined through thermogravimetric analysis, and the structural evolutions through different phases of the synthesized porphyrins were deduced from the powder X-ray diffraction. To get more insight into the molecular structures and their complexes, we carried out the quantum mechanical calculations using DFT-based dispersion which showed that from OH− to p-PDA substitution, the BDE energies drop considerably.