Re-evaluation of the steady-state self-quenching constant of quinine bisulphate from fluorescence measurements in transmission geometry

Abstract

In the present work we show that a recent methodology developed by us to acquire emission spectra and fluorescence quantum yields of highly absorbing samples in transmission configuration, constitutes a very simple and robust alternative to determine self-quenching constants, KSQ. We measured the absorption and the steady-state emission spectra of quinine bisulphate, QBS, solutions ranging between 1.5 × 10-5 and 1.5 × 10-1 M. From these data, we calculated the expected emission spectra, affected by re-absorption, for all QBS concentrations. For higher concentrations, the re-absorption in the excitation/detection direction reaches values up to 6% of the total emitted intensity. The KSQ of the dye was reevaluated from the concentration dependence of the quotients between the calculated and the experimental integrated emission spectra. The obtained value, KSQ = 18.4 ± 0.1 M-1, shows no significant differences with those obtained from steady-state and average lifetimes by other authors, pointing out the diffusional nature of the self-quenching phenomenon. The present work helps clarify some ambiguous aspects concerning the photophysics of QBS, stressing that re-absorption phenomena must be considered in QBS concentrated solutions for accuracy measurements.