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

In aqueous solutions of pH 10.1, inclusional complexation of tetrakis(4-sulfonatophenyl)porphyrin (TSPP) with alpha-cyclodextrin (alpha-CD), beta-CD, heptakis(2,3,6-tri-O-methyl)-beta-CD (TM-P-CD), and gamma-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. alpha-CD and beta-CD most likely form 1 :1 and 2: 1 host-guest inclusion complexes with TSPP. TM-beta-CD and gamma-CD form 2:1 and 1 :1 inclusion complexes with TSPP, respectively. An equilibrium constant for the formation of the 2: 1 TM-beta-CD-TSPP inclusion complex has been estimated to be 2.92 x 10(13) mol(-2) dm(6) from a simulation for the absorbance change of TSPP with the TM-beta-CD concentration. An equilibrium constant (K-1) for the formation of the 1 : 1 gamma-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 gamma-CD-TSPP inclusion complex. An induced circular dichroism spectrum of TSPP in the presence of alpha-CD or beta-CD exhibits a negative sign, whereas that in the presence of TM-beta-CD or gamma-CD exhibits a positive sign, indicating different inclusion modes. (C) 1999 Elsevier Science S.A. All rights reserved.