An ultrafast time-resolved fluorescence spectroscopy system for metal ion complexation studies with organic ligands

A dedicated spectrofluorimeter using ultrashort laser pulses as an excitation source was developed to measure the fluorescence properties of organic ligands for metal ion complexation with organic ligands. The laser system consists of an oscillator system for generation of femtosecond laser pulses, an amplifier system to increase the pulse energy of the generated pulses to about 2 mJ and an optical parametrical amplifier system to provide tunable laser pulses over a wide wavelength range (280 nm-10 microm). The laser pulses were applied to the sample and the emitted fluorescence was detected using a fast-gating intensified CCD camera-based spectrometer. To verify the performance of the laser, the well-known protonation constant [Pure Appl. Chem. 69 (1997) 329] of 2,3-dihydroxybenzoic acid was determined. The fluorescence lifetime of the excited species was determined as 375+/-32 ps in the pH range from 1.0 to 6.0, having a fluorescence emission maximum at 438 nm. The first protonation constant was determined from fluorescence data as log K(3)=3.17+/-0.05 at an ionic strength of 0.1 M and at 294 K exploiting the Stern-Volmer mechanism. The agreement of the protonation constant with literature data (log K(3)=3.10+/-0.20, I=0.1 M, T=298 K [Bull. Soc. Jpn. 44 (1971) 3459]) demonstrates the excellent performance of our system. Furthermore, we determined the complex formation constant log K(1)=-3.11+/-0.16 by measuring the fluorescence properties of the ligand for the 1:1 uranyldihydroxobenzoate complex in the pH range from 3.0 to 4.5 at ionic strength of 0.1 M and at 294 K. We also determined the complex formation constant via the fluorescence emission of the metal ion uranium(VI). The fluorescence of the uranyl ion is influenced by dynamic quenching of the non-dissociated ligand and by static quenching due to the complex formation. After correction of these effects using the determined fluorescence lifetime, the complex formation constant was calculated to be log K(1)=-3.99+/-0.44. A 1:1 metal:ligand stoichiometry was determined with both measurement methods. However, the difference of the obtained formation constants and the derived standard deviations indicate a superimposition of effects with the excited-state reactions of the ligand.