The Mechanism of the Porphyrin Spectral Shift on Inorganic Nanosheets: The Molecular Flattening Induced by the Strong Host-Guest Interaction due to the Size-Matching Rule

The mechanism inducing the unique absorption spectral shifts of porphyrin molecules upon adsorption on the clay surface was experimentally confirmed to be the flattening of the meso substituent with respect to the plane of the porphyrin ring. We investigated the spectral shift systematically by using seven types of porphyrin derivatives, differing in their center metal, meso substituent, and number of cationic sites. The aggregation, which usually induces the spectral shift, is suppressed in our clay/porphyrin systems. The adsorption strengths of porphyrin molecules on the clay surface were estimated as the relative adsorption equilibrium constants K-rel. The seven types of porphyrins studied had different values of K-rel, which can be explained by the steric effect and the Coulomb interaction due to the differences in the molecular structure. The obtained K-rel values have interesting information of the photochemical property of the clay/porphyrin complexes. The absorption spectral shifts of the porphyrin Soret bands between those in the bulk solution and those on the clay surface are well related to K-rel values. Like this, by the systematic experiments using the series of porphyrin derivatives, it was directly confirmed that the flattening of the molecule is the dominant mechanism for the spectral shift on the surface of inorganic nanosheets.