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

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, K-SQ. We measured the absorption and the steady-state emission spectra of quinine bisulphate, QBS, solutions ranging between 1.5x10(-5) and 1.5x10(-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 K-SQ of the dye was re-evaluated from the concentration dependence of the quotients between the calculated and the experimental integrated emission spectra. The obtained value, K-SQ=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.