A stock solution
of the receptor R-Zn2+ (1.0 × 10−3 M) was also prepared: CH3CN/N-
2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES; 2:8, v/v;
pH = 7.8). A 2 mL stock solution of R-Zn2+ was placed in a quartz cell
of 1 cm width, and a tested cation solution was added in an
incremental fashion. Corresponding fluorescence spectra were
recorded at 298 K. Each titration was repeated at least two times to
get a consistent value. For all measurements, λexc = 360 nm and the
emission wavelength was monitored at 400−700 nm. There was no
considerable change in the shape of the emission spectra except a
significant enhancement of the initial fluorescence intensity of R-Zn2+
upon the gradual addition of an anion solution. Analysis of the
normalized fluorescence intensity (I0/I) as a function of increasing
guest concentration ([G]) was well described by the Stern−Volmer
equation I0/I = 1 + KSV[G]. KSV was calculated from the slope of the
Stern−Volmer plot.