Adsorption and absorption behavior of cationic porphyrin on titania and clay nanosheets

ABSTRACT In this study, the adsorption and absorption behavior of porphyrin derivatives on inorganic nanosheets were investigated. The relationship between the adsorption orientation angle (θad) and the maximum absorption wavelength (λmax) of tetra (N-methyl-4-pyridinium)-porphyrin (p-TMPyP4+) derivatives adsorbed on clay nanosheet (Sumecton SA (SSA)) and titania nanosheet (TNS, Ti0.89□0.11O2-0.11) monolayer film prepared by the Langmuir-Blodgett method was examined by dichroic waveguide spectroscopy. The λmax of each TMPyP4+ was red-shifted by an adsorption on the nanosheets. For example, the λmax of p-TMPyP4+ in water was 422 nm, while that was shifted to 450 and 459 nm upon an adsorption on SSA and TNS, respectively. According to dichroic measurements, the θad of p-TMPyP4+ was estimated to be 13 and 3 degrees on SSA and TNS, respectively. Judging from the results for all porphyrin, a strong correlation between the λmax shift upon an adsorption and the θad on nanosheets was found out. In addition, it was turned out that the θad depends on the electrostatic interactions between porphyrin and anionic nanosheets. Thus, the λmax shift upon an adsorption can be controlled by the combination of porphyrin and nanosheet.