Photochemical Properties of Mono‑, Tri‑, and Penta-Cationic Antimony(V) Metalloporphyrin Derivatives on a Clay Layer Surface

Abstract

Three types of mono-, tri-, and penta-cationic antimony­(V) porphyrin derivatives (Sb^VPors) were synthesized, and their photochemical properties on the anionic clay were systematically investigated. Sb^VPor derivatives are dihydroxo­(5,10,15,20-tetraphenylporphyrinato)­antimony­(V) chloride ([Sb^V(TPP)­(OH)₂]⁺Cl^–), dihydroxo­[5,10-diphenyl-15,20-di­(<i>N</i>-methyl-pyridinium-4-yl)­porphyrinato]­antimony­(V) trichloride ([Sb^V(DMPyP)­(OH)₂]³⁺3Cl^–), and dihydroxo­[5,10,15,20-tetrakis­(<i>N</i>-methyl-pyridinium-4-yl)­porphyrinato]­antimony­(V) pentachloride ([Sb^V(TMPyP)­(OH)₂]⁵⁺5Cl^–). The photochemical behaviors of three cationic Sb^VPors with and without clay were examined in aqueous solution. For all Sb^VPor, aggregation behaviors were not observed in the clay complexes even at high density adsorption conditions. The transition probabilities and fluorescence quantum yields of Sb^VPor showed a tendency to be increased by the complex formation with clay. The less cationic Sb^VPor/clay complex showed the larger fluorescence quantum yield. The more cationic Sb^VPor/clay complex showed the longer fluorescence lifetime. These effects of complex formation with clay on the photochemical properties of Sb^VPors were discussed using the molecular potential energy curves of the porphyrin ground state and excited state. It is concluded that two types of effects work in the Sb^VPor/clay system: effect i (structure resembling effect) is that the most stable structure becomes relatively similar between the ground and excited states, mainly by hydrophobic interactions between the porphyrin molecule and the clay surface, and effect ii (structure fixing effect) is that sharpened potential energy curves of clay complexes can lead to the increase of activation energy for the internal conversion from excited state to a high vibration level of ground state, mainly by electrostatic interactions between cationic porphyrin and anionic clay. Like this, the unique effects of the clay surface on the photochemical behavior of dyes were observed and the mechanisms were rationally discussed