A ground-state complex between methyl viologen and the fluorescent whitening agent 4,4′-bis(2-sulfostyryl)-biphenyl disodium salt: a fluorescence spectroscopy study

Abstract

This study explores the quenching of the dianionic fluorescent whitening agent, NFW, by various substances, including methyl viologen (MV), in water and in the presence of Beta-cyclodextrin (β-CD). Results of a fluorescence spectroscopic examination of the β-CD-NFW system are presented. It was found that NFW forms a 1:1 inclusion complex with β-CD with an association constant of 2540 ± 380 M<sup>-1</sup>. The included NFW fluorescent state is protected by the β-CD cavity from a range of water-based quenchers (neutral, anionic and cationic). Quenching proceeds near the diffusion-controlled limit in water for the quenchers tested with the exception of the dicationic MV. MV is an extremely efficient quencher of NFW fluorescence with a nominal K<sub>SV</sub> ~ 5.0x10<sup>3</sup> M<sup>-1</sup> in water alone, corresponding to a nominal k<sub>q</sub> of ~ 4x10<sup>12</sup> M<sup>-1</sup>s<sup>-1</sup>, which exceeds the diffusion-controlled limit in this solvent. The quenching efficiency of MV is strongly suppressed in the presence of 10 mM β-CD (K<sub>SV</sub> = 105 ± 12 M<sup>-1</sup>) and in the presence of NaCl (K<sub>SV</sub> = 106 ± 9 M<sup>-1</sup> at 0.5 M salt). In the absence of CD or salt, there is a strong contribution from static quenching in the MV system; the presence of these additives suppresses the static quenching. Various results suggest the static quenching is due to formation of a ground-state complex between the dianion NFW and the dication MV.