Shape-responsive host-guest chemistry: metal-free tetra cationic porphyrin nonplanarity promoted by clay mineral interactions assessed by theoretical simulations

Abstract

Distortions in the porphyrin core from planarity can trigger a unique structure-property relationship, imparting its basicity, chemical stability, redox potential, and excited-state energetics, among other properties. The colour change promoted by such distortion is signed by redshifts in their electronic absorption spectra. The adsorption of guest meso-substituted free base porphyrins species on inorganic hosts such as clay minerals (layered aluminium or magnesium silicates) promotes colour changes, whose origin is not consensus. In this work, an extensive theoretical study was reported based on the Density Functional Theory (DFT) to model the interactions between tetracationic porphyrins comprising the meso-substituted groups N-methyl-4-pyridyl (p-TMPyP) and N-methyl-3-pyridyl (m-TMPyP), and montmorillonite (MMT) of the ideal formula [(Al1.67Mg0.33)Si4O10(OH)2]-0.33. The following conditions were evaluated: (i) p-TMPyP adsorbed or intercalated into MMT host structure, (ii) m-TMPyP intercalated into MMT; (iii) water effects in the intercalation process. The electrostatic interaction between the porphyrin and the MMT siloxane surface leads to a change in the conformation of p-TMPyP, comprising a rotation of substituent in the periphery of the macrocycle ring and a twist of the porphyrin plane. The nonplanarity of the intercalated p-TMPyP guest produces enough robust Bronsted basic sites to abstract H+ ions from intercalated water molecules, giving a dication. The macrocycle distortion can decrease the -conjugation, enhancing the localisation of the lone pair in the imine nitrogen atom. On the other hand, m-TMPyP shows slight deformations of the core macrocycle and minor changes in the dihedral angles of the meso-substituent group compared to its isomer and no protonation reaction when intercalated. Hence, the clay microenvironment is an exciting approach to induce alterations in the conformations of porphyrins, bringing about nonplanarity, and an example of a shape-responsive system based on guest-host chemistry.