Time-resolved fluorescence spectroscopy of quinine dication free and bound to polymethacrylic acid

The fluorescence decay and quenching by halides of quinine dication (QS) and derivatives in hydrogenated form (QSH) and bound to polymethacrylic acid in aqueous solution at low pH are investigated by time-resolved and steady-state fluorescence measurements. QS and QSH have very similar bi-exponential decay surfaces indicating that the peripherical double bond of the quinuclidine group play no role in the excited state deactivation. Experiments with the free probe in water/ethylene glycol solvent mixtures show that the QS fluorescence decay depends strongly on the solvent viscosity. Solvent friction and polarization drive the interconversion rate between different conformers in excited state. When QS is bound to a compact polymer coil, the decay becomes three-exponential, and an additional lifetime component of about 10 ns is recovered. This decay component is ascribed to a fraction of the probe placed on a high local viscous medium. The quenching process by halides follows the efficiency series I- > Br- > Cl- in all cases investigated. The quenching rate constants are higher for QS than for QSH or QS bound to polymer. In the case of iodide, there is a static quenching contribution which may be ascribed to a weak association between probe and quencher. The average free energy change in the association process is of the order of 2.5 kcal/mol. (C) 1999 Elsevier Science S.A. All rights reserved.