Clay-Porphyrin Systems: Spectroscopic Evidence of TMPyP Protonation, non-Planar Distortion and Meso Substituent Rotation

Abstract

The interaction of the water-soluble 5,10,15,20-tetrakis(l -methyl-4-pyridyl)-21H,23H-porphine (TMPyP) with different 2:1 phyllosilicates was examined by Raman and UV-visible spectroscopies. The clay samples were saturated with the tetracationic porphyrin and isolated from the aqueous suspension. A red shift of the Soret band was observed for all the clay-TMPyP systems in the order vermiculite < Laponite < mica-smectite (Syn-1) < montmorillonite (SWy-2). Furthermore, three components were observed for the Soret band (at ~425, 455 and 488 nm). Raman spectra of the isolated solids excited at 457.9 nm, 488.0 nm and 514.5 nm suggest the occurrence of porphyrin protonation, nonplanar distortion and rotation of the meso substituent. Based on the vibrational data, an acidity scale was proposed for the clays: vermiculite < Laponite < SWy-2 < Syn-1. The relative contribution of the protonated spectra is larger at 457.9 nm than at 488.0 nm, suggesting that the peak at 455 nm corresponds to the protonated species. In Laponite, the relative intensity of the meso substituent band at ~1635 cm^-1 indicates that the dihedral angle formed between the porphyrin and the methyl-pyridyl rings decreased in the non-protonated porphyrin as a consequence of intercalation. Raman data are thus consistent with the presence of at least two porphyrin species in resonance at 457.9 nm: the protonated and a more planar non-protonated porphyrin. At 488.0 nm the number of enhanced modes increases suggesting a decrease in the porphyrin symmetry. This allows assignment of the absorption band centered at 488 nm to a non-planar porphyrin conformation.