Static pyrene excimer formation at hydrophobic zirconium phosphate surfaces: The salt effect

Abstract

4-(1-Pyrene)butylamine hydrochloride (PBAC) binds to layered zirconium phosphate (BAZrP) in the presence of butylamine hydrochloride, with a high affinity. Electronic absorption and emission spectra of the pyrenyl chromophore change dramatically upon binding to the layered phosphate and provide spectroscopic signatures for the microenvironment of the pyrenyl chromophore. For example, BAZrP-promoted excimer emission from PBAC was observed only in a narrow range of BAZrP and PBAC concentrations. The intensity of the excimer emission was very high at low concentrations of BAZrP and decreased dramatically at high concentrations of BAZrP. The intensity of the excimer emission also depended upon the ionic strength of the medium. For example, addition of sodium chloride to a suspension of PBAC and BAZrP dramatically enhanced the excimer formation. This result is in contrast to our previous observations with 9-anthracenemethylamine hydrochloride bound to BAZrP where a decrease in the anthracene excimer emission was noted with increased NACl concentration. The strong role of electrostatic and hydrophobic interactions on the binding of the probe as well as on the efficiency of excimer formation are clearly indicated by the current observations. Fluorescence quenching studies with iodide anion show that the intensity of excimer emission is influenced by both the changes in the ionic strength as well as the iodide quenching effect. The relative magnitudes of these two effects depended upon the concentration of the iodide. Excimer emission was not observed when PBAC binds to DNA or to polyelectrolytes, highlighting the unique role of the layered phosphates on the probe excited state dynamics.