A high sensitive ion pairing probe (the interaction of pyrenetetrasulphonate and methyl viologen): Salt and temperature dependences and applications

The interaction between pyrenetetrasulphonate (PTS) and methyl viologen (MV2+) leads to a 1:1 charge transfer complex (CTC) in the concentration range below mmol L−1 of the ligands. Quantum mechanical calculations show the 1:1 complex having the planar moiety of PTS and the charges of the sulfonate groups stabilized by the twisted rings of the positively charged MV2+ species. The peculiar nature of PTS includes high fluorescence quantum yield (~1), clear specular UV-vis spectra and fluorescence emission images, as well similar S2←S0 and S3←S0 transitions as those of S1←S0, all of them exhibiting well resolved vibrational structure. MV2+ has well known electron-accepting properties that favor the complexation. These features were studied as a function of salt concentration and temperature dependences allowing a detailed comprehension of static and dynamic association processes. Quantum mechanical calculations show the 1:1 stabilization of PTS/MV2+. In addition the effect of urea on the CTC equilibrium is presented, as expected the additive acts towards the non-complexed species (solvated free ions). The fluorescence quenching of MV2+over PTS highlights is one of the applications of this effect for giant vesicles characterization.