Time-resolved ¯uorescence spectroscopy of quinine dication free and bound to polymethacrylic acid Claudio J. Rocha a , Marcelo H. Gehlen a,* , Rosangela da Silva b , Paulo M. Donate b a Universidade de Sa Äo Paulo, Instituto de Quõ Âmica de Sa Äo Carlos, Sa Äo Carlos-SP, Brazil b Faculdade de Filoso®a Cie Ãncia e Letras de Ribeira Äo Preto, Departamento de Quimica, 14040-901, Ribeira Äo Preto-SP, Brazil Received 18 November 1998; accepted 22 February 1999 Abstract The ¯uorescence 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 ¯uorescence 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 ¯uorescence 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 ef®ciency 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. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Time-resolved ¯uorescence spectroscopy; Quinine; Polymethacrylic acid 1. Introduction The photophysics of the ¯uorescence standard quinine sulfate has been the subject of several experimental inves- tigations in time-resolved emission spectroscopy due to its more complex decay kinetics than a single exponential relaxation. The ®rst evidence of a bi-exponential decay of the dication in water was reported by O'Connor et al. [1] by pulse decay measurements and later con®rmed by Barrow and Lentz [2] using phase and modulation method. These results have promoted further experimental investigations by Pant and coworkers using quinine and its derivatives in several different conditions where solvent polarity and viscosity, protonation degree of the ¯uorophore, tempera- ture, excitation and emission wavelength were changed [3± 8]. For quinine dication, the bi-exponential behavior was ascribed to an interplay between two different emitting states formed by an intramolecular charge transfer from the 6-methoxy group to the quinoline ring upon excitation [7,8]. This process leads to changes in the charge distribu- tion density and geometry of the ¯uorophore which com- bined with solvent relaxation gives a multi-exponential decay behavior. Thus, solvent viscosity and polarity affect dramatically the ¯uorescence decay kinetics of quinine dication, since those factors drive the interconversion rate between the two main conformers in the excited state [8]. The quenching of the quinine dication by iodide, bromide and chloride has been reported [9±13]. This process has been Journal of Photochemistry and Photobiology A: Chemistry 123 (1999) 129±136 *Corresponding author. Tel.: +55-16-274-9208; fax: +55-16-273-9205; e-mail: marcelog@iqsc.sc.usp.br 1010-6030/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S1010-6030(99)00053-2