Speciation, luminescence, and alkaline fluorescence quenching of 4-(2-methylbutyl)aminodipicolinic acid (h ₂mebadpa)

Abstract

4-(2-Methylbutyl)aminodipicolinic acid (H 2MEBADPA) has been synthesized and fully characterized in terms of aqueous phase protonation constants (pK a's) and photophysical measurements. The pK a's were determined by spectrophotometric titrations, utilizing a fully sealed titration system. Photophysical measurements consisted of room temperature fluorescence and frozen solution phosphorescence as well as quantum yield determinations at various pH, which showed that only fully deprotonated MEBADPA 2- is appreciably emissive. The fluorescence of MEBADPA 2- has been determined to be quenched by hydroxide and methoxide anions, most likely through base-catalyzed excited-state tautomerism or proton transfer. This quenching phenomenon has been quantitatively explored through steady-state and time-resolved fluorescence measurements. Utilizing the determined pK as and quenching constants, the fluorescent intensity of MEBADPA 2- has been successfully modeled as a function of pH. © 2011 American Chemical Society.