Speciation, Luminescence, and Alkaline Fluorescence Quenching of 4-(2-methylbutyl)aminodipicolinic acid (H2MEBADPA)

4-(2-Methylbutyl)aminodipicolinic acid (H2MEBADPA) has been synthesized and fully characterized in terms of aqueous phase protonation constants (pKa\u27s) and photophysical measurements. The pKa\u27s 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 MEBADPA2– is appreciably emissive. The fluorescence of MEBADPA2– 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 pKas and quenching constants, the fluorescent intensity of MEBADPA2– has been successfully modeled as a function of pH

Ground-state proton transfer of 7-hydroxyquinoline confined in biologically relevant water nanopools

The ground-state reverse proton transfer of 7-hydroxyquinoline catalyzed by water confined in AOT reverse micelles has been investigated by measuring time-resolved transient-absorption spectra and kinetic profiles. The transfer time is profoundly retarded in water nanopools compared with that in bulk water (26 ??s) although it diminishes with the size increase of the water nanopool. The spectral-shift time of tautomeric transient absorption agrees well with the proton transfer time. The probe molecule is subject to the local gradient of polarity, whose magnitude is altered with the sizes of water nanopools. Accordingly, the observations made in this study indicate the multidimensional character of reaction coordinates, in which solvent polarization coupled to charge transfer plays a seminal role in the control of overall proton-transfer dynamics. The retardation of proton transfer in water nanopools is ascribed to the increased formation energy of a charge-transferred optimal configuration, which is prerequisite to facile intrinsic proton transfer via tunneling.close201