Ibuprofen molecular aggregation by direct back-face transmission steady-state fluorescence.

Direct back-face transmission steady-state fluorescence was successfully applied to the study of aggregation of ibuprofen and ibuprofenate anion in solution taking advantage of its own fluorescence. The analysis of the experimental data involves the use of the differential reabsorption model to account for re-absorption phenomenon and the closed association model to describe aggregation. The fluorescence quantum yield of ibuprofenate increases when it aggregates in the presence of sodium, but it markedly decreases when 1-butyl-3-methylimidazolium is used as counterion. The proposed methodology allows the accurate determination of the critical aggregation concentrations and the mean aggregation numbers. Results were supported by complementary techniques such as time-resolved fluorescence, 1H-NMR and small-angle neutron and X-ray scattering. The developed technique constitutes a promising strategy to characterize the aggregation of poorly fluorescent surfactants that aggregates at high concentrations and hence at high absorbance values, conditions in which the most common right-angle configuration for fluorescence acquisition is troublesome due to re-absorption.

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 1.5 × 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 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.

Steady-State Fluorescence of Highly Absorbing Samples in Transmission Geometry: A Simplified Quantitative Approach Considering Reabsorption Events.

A simplified methodology to acquire steady-state emission spectra and quantum yields of highly absorbing samples is presented. The experimental setup consists of a commercial spectrofluorometer adapted to transmission geometry, allowing the detection of the emitted light at 180° with respect to the excitation beam. The procedure includes two different mathematical approaches to describe and reproduce the distortions caused by reabsorption on emission spectra and quantum yields. Toluene solutions of 9,10-diphenylanthracence, DPA, with concentrations ranging between 1.12 × 10-5 and 1.30 × 10-2 M, were used to validate the proposed methodology. This dye has significant probability of reabsorption and re-emission in concentrated solutions without showing self-quenching or aggregation phenomena. The results indicate that the reabsorption corrections, applied on molecular emission spectra and quantum yields of the samples, accurately reproduce experimental data. A further discussion is performed concerning why the re-emitted radiation is not detected in the experiments, even at the highest DPA concentrations.