Elucidation of the dynamic quenching behavior of pyridinium porphyrin derivatives on clay nanosheet

Abstract

In this study, we investigated the adsorption and photophysical behaviors of meso-tetra(N-methyl-4-pyridyl)porphyrin (m-TMPyP) and meso-tetra(N-butyl-4-pyridyl)porphyrin (m-TBPyP) molecules on the surface of a synthetic silicate nanosheet. The λ_max of m-TMPyP and m-TBPyP on the synthetic clay nanosheet were 429 and 440 nm under the non-aggregation state, respectively. The spectroscopic analysis revealed that a flattening of the molecular structure of m-TBPyP upon adsorption is stronger than m-TMPyP. Compared to m-TMPyP, the fluorescence quenching of m-TBPyP was suppressed to some extent, especially at adsorption densities below 30 % vs. cation exchange capacity of the nanosheet, due to the steric effects of its longer alkyl chain reducing molecular contact. However, m-TBPyP suffered a 10 % self-fluorescence quenching rate at saturation adsorption as same as the case of m-TMPyP. Interestingly, mixing two dyes in a 1:1 ratio suppressed self-fluorescence quenching even at saturation adsorption. This indicates that the neighboring of same porphyrin, leading to the quenching, was suppressed in the mixture system. Thus, the arrangement of m-TMPyP and m-TBPyP is uniform and prevents molecular collisions between same species. The uniform arrangement and no-fluorescence quenching of m-TMPyP (energy donor) and m-TBPyP (acceptor) on clay nanosheet could facilitate efficient photochemical energy transfer between them. As a result, by suppressing self-fluorescence quenching, the efficient energy transfer between m-TMPyP and m-TBPyP was observed. It was determined that the energy transfer efficiency (η_ET) is 85 %, with a self-fluorescence quenching efficiency (ϕ_q) of 0 %. These findings are beneficial to design more efficient systems for artificial light-harvesting systems by optimizing molecular interactions on clay surfaces.