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”

Abstract

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.

Electrochemical investigation of a novel metalloporphyrin intercalated layered niobate modified electrode and its electrocatalysis on ascorbic acid

“…2b shows the strongest Soret band at 426 nm and a weak Q bands between 550 and 700 nm (curve b). The 2 nm red shift of Soret band (comparing to curve a, the spectrum of Fe III TMPyP) is believed to be the result of flattening of the metalloporphyrin molecule on the surface of inorganic nanosheets [42], besides, the shift is very small, suggesting little aggregation of metalloporphyrin in the hybrid [32], and the matrix environment plays the major role in the spectrum characteristics [33].…”

A study on the interactions of cationic porphyrin with nano clay platelets in Layer-by-Layer (LbL) self assembled films

“…The 4 nm red shifting of Soret peak in aqueous clay dispersion (425 nm) with respect to the Soret peak at 421 nm in pure aqueous solution might be due to the changes in solvent polarity in aqueous clay dispersion.However, Takagi et. al [21]. reported that due to the adsorption of TMPyP molecules onto nano clay platelets in aqueous dispersion, a red shifting of about 10 -30 nm was observed in the peak position of Soret band.…”

“…reported that due to the adsorption of TMPyP molecules onto nano clay platelets in aqueous dispersion, a red shifting of about 10 -30 nm was observed in the peak position of Soret band. The spectral shift has been explained as due to the conformational change owing to the flattening of the meso-substituents of TMPyP molecules with respect to the plane of the porphyrin ring in the process of adsorption on the charged clay substrates[21]. The spectral shift depends on several factors.…”

Stabilization and Modification of Gold Nanocube Surfaces with Layered Silicate

“…Total cationic charges of TMPyP neutralized 20% of whole anionic charges on LS sheets. The spectra are shown in Figure 6b report, 18 the porphyrin with pyridyl meso substituents (circled with dotted lines in Figure 6a) shows a red shift of 2736 nm when adsorbed to LS surfaces; the shift is caused by the internal rotation of meso substituents with adsorption. In the present study, the observed red shift of 33 nm upon addition of TMPyP to AuCTACs with LS was similar to that observed for TMPyP on LS without gold particles, indicating that TMPyP is adsorbed to LS on AuCTACs.…”