Electronic absorption, fluorescence, and circular dichroism spectroscopic studies on the inclusion complexes of tetrakis(4-sulfonatophenyl)porphyrin with cyclodextrins in basic aqueous solutions

Abstract

In aqueous solutions of pH 10.1, inclusional complexation of tetrakis(4-sulfonatophenyl)porphyrin (TSPP) with α-cyclodextrin (α-CD), β-CD, heptakis(2,3,6-tri- O-methyl)-β-CD (TM-β-CD), and γ-CD has been examined by means of absorption, fluorescence, and induced circular dichroism spectra. Upon the addition of CD to a basic solution of TSPP, its absorption peak is shifted to longer wavelengths and the fluorescence peak is slightly shifted to longer wavelengths, with a band sharpening. α-CD and β-CD most likely form 1 : 1 and 2 : 1 host–guest inclusion complexes with TSPP. TM-β-CD and γ-CD form 2 : 1 and 1 : 1 inclusion complexes with TSPP, respectively. An equilibrium constant for the formation of the 2 : 1 TM-β-CD–TSPP inclusion complex has been estimated to be 2.92 × 10 13 mol −2 dm 6 from a simulation for the absorbance change of TSPP with the TM-β-CD concentration. An equilibrium constant ( K 1) for the formation of the 1 : 1 γ-CD–TSPP inclusion complex has been evaluated to be 1800 ± 100 mol −1 dm 3 from the absorbance change. This K 1 value is nearly the same as that (1600 ± 200 mol −1 dm 3) obtained from the fluorescence intensity change, supporting the formation of the 1 : 1 γ-CD–TSPP inclusion complex. An induced circular dichroism spectrum of TSPP in the presence of α-CD or β-CD exhibits a negative sign, whereas that in the presence of TM-β-CD or γ-CD exhibits a positive sign, indicating different inclusion modes.