Measurements of picosecond lifetimes by time correlated single photon counting method: The effect of the refraction index of the solvent on the instrument response function

The article concerns the possibility of precise picosecond lifetime measurements by the method of the time correlated single photon counting (TCSPC). The lifetimes $\mathrm{(\tau )}$ of ${\mathrm{S}}_2$ fluorescence of xanthione measured in different solvents have shown that it is possible to obtain $\tau$ as short as a few picoseconds. The values of $\tau$ varying from $\text{∼5\hspace{0.5em}}\mathrm{to}\text{\hspace{0.5em}∼38\hspace{0.5em}}\mathrm{ps}$ in solvents whose refraction indices range from 1.34 to 1.50 have been proposed as picosecond standards in lifetime measurements. However, in order to obtain reliable results some conditions must be met: the effect of the absorbing and emitting impurities must be eliminated, the instrument response function (IRF) must be very stable and correctly determined, and because of a much lower time resolution of the TCSPC method relative to that of the upconversion method, the procedure of measurements and numerical analysis of results proposed in our work should be applied. In the study reported the effect of impurities was eliminated by applying high performance liquid chromatography to choose the excitation and emission wavelength for which the emission of impurities is negligible for fluorescence decay measurements of the sample (S) and reference (R). It has been shown that the IRF in TCSPC depends on the refraction index of the solvent used. In order to avoid the systematic errors related to different IRF for S and R, the calculations of fluorescence lifetimes should be performed for S and R in solvents of the possibly most similar refraction indices. The effect of the solvents difference in the refraction indices on the shape of IRF has been shown and then on the fluorescence lifetimes below $\text{20\hspace{0.5em}}\mathrm{ps}$ . The proposed procedure of determination of single picosecond lifetimes with the use of TCSPC is particularly suitable when it is necessary to use a very low concentration ${\text{(⩽10}}^{\text{−5}}\hspace{0.5em}\mathrm{M})$ of the compound studied in order to avoid dimers and oligomers formation (in the upconversion method concentration ${\text{>10}}^{\text{−4}}\hspace{0.5em}\mathrm{M}$ is generally used). A very good statistics of the TCSPC method—much better than in the other time-resolved methods—together with the use of the procedure of measurements—the procedure of numerical analysis and procedure of elimination of emission of impurities proposed in our article—permit elimination of the systematic errors (usually $\text{±3\hspace{0.5em}}\mathrm{ps}$ ) in the lifetimes determined for monoexponential decays and obtaining a highly reliable description of the fluorescence decay kinetics also when the fluorescence decay is nonexponential.