Flattening of TMPyP Adsorbed on Laponite. Evidence in Observed and Calculated UV−vis Spectra

Abstract

The +4 charged dye tetramethylpyridylporphyrin, TMPyP (≡H2TMPyP4+), and the metallo ZnTMPYP are water-soluble. The cationic TMPy groups are nearly perpendicular to the plane of the porphine. When the dyes were applied to the layered inflatable clay mineral Laponite a ∼30 nm red shift in the spectrum of both TMPyP−'s adsorbed on outer surfaces and a ∼60 nm shift in the dyes trapped in interlamellar galleries were found. The negatively charged clay surface may be very acidic. In previous works, the red shifts were interpreted in terms of diprotonation of TMPyP. In this work, several possible causes of the observed red shifts were rigorously examined. These included diprotonation, redox reactions, solvent effects, π-electron interaction with the oxygen plane, and electronic changes due to the flattening of TMPyP on the clay. To facilitate in a deductive manner a possible explanation for the observed metachromasy, the charged, flat Laponite was replaced with its neutral analogue: talc and porous silica. The electronic structure of the porphine ring is classically treated by means of the 4-level model. In charged porphyrins, such as TMPyP, the modelization is much complicated. Here, semiempirical quantum chemical PM3 modeling in a vacuum confirmed the perpendicularity of the TMPy groups. The dihedral angles were constrained toward planarity and the resulting red shifts in the spectra were calculated, then a classical (four-level) adaptation was applied. It then appears that the ∼30° twists from the vertical would cause a ∼30 nm red shift in the Soret band with a ∼3 kcal/mol rise in ground-state energy. Then ∼46° twists would cause a ∼60 nm red shift in the Soret band with a ∼11 kcal/mol rise in ground-state energy. Hence, sterically induced hindrance in adsorbed TMPyP had a profound influence on the absorption spectra, confirming our conclusions from the experimental part.