Molecular interactions of monosulfonate tetraphenylporphyrin (TPPS1) and meso-tetra(4-sulfonatophenyl)porphyrin (TPPS) with dimethyl methylphosphonate (DMMP)

The molecular interactions of monosulfonate tetraphenylporphyrin (TPPS₁) and meso-tetra(4-sulfonatophenyl)porphyrin (TPPS) with dimethyl methylphosphonate (DMMP) have been investigated by UV–vis and fluorescence spectroscopies. The association constants and interaction stoichiometries of the bindings were obtained through Benesi–Hildebrand (B–H) method. Particularly, both linear and nonlinear fitting procedures were performed to evaluate the possible 1:2 interactions. Experimental results showed that hydrogen-bonding interactions existed in both of the two systems, resulting in regular changes in the absorbance and fluorescence characteristics of the porphyrins. The association constants and stoichiometries determined from absorbance and fluorescence studies were in excellent agreement. Using a nonlinear fitting method, we demonstrated that the one-step 1:2 interaction took place in the TPPS₁–DMMP system, and the association constants were determined to be 71.4 M⁻¹ by absorbance measurements and 70.92 M⁻¹ by fluorescence measurements. The interaction stoichiometry of the TPPS–DMMP system was 1:1, and the association constants were determined to be 16.06 M⁻¹ by absorbance measurements and 16.03 M⁻¹ by fluorescence measurements. It was concluded that the interaction between TPPS₁ and DMMP was stronger than that between TPPS and DMMP.