Probing the local structure: macromolecular combs in external fields

Recent experimental methods allow to monitor the response of macromolecules to locally applied fields, complementing usual, mesoscopic techniques. Based on the Rouse-model and its extension to generalized Gaussian structures (GGS), we follow here the stretching of comb macromolecules under local fields. This leads to a wealth of informations about the structure: Namely, given the inhomogeneous architecture of combs, the dynamics and amount of stretching depend strongly on the position of the monomer on which the external fields act. We discuss both the theoretical and the experimental implications of our findings, given that micromanipulations can be supplemented by fluorescence measurements, which are very sensitive to changes in the intramolecular distances.Comment: 16 pages, 5 pdf figures, to appear in Chem. Phy

Fluorescence Dynamics of Coumarin C522 on Reduced-Charge Montmorillonite in Aqueous Dispersion

Solvation is an important phenomenon, especially in association with heterogeneous phase interactions. Coumarin C522, C14H12NO2F3, is used as a fluorophore probe to study the interaction between coumarin and a reduced-charge montmorillonite (RCM) surface. Such hydrophilic and hydrophobic interactions are of interest for sorption processes in confined environments. The prepared RCM series with 0.00, 0.12, 0.26, 0.43, 0.66, and 0.97 Li+ molar fractions provide different surface charges. The aqueous dispersion of the C522/water/RCM system is studied by using steady-state and time-resolved fluorescence spectroscopies. Both the Stokes shift and the dynamics of the solvation process varied as a function of surface charge. Steady-state fluorescence spectroscopy reveals that the C522 Stokes shift varies from 5115 cm−1 for the 0.00 Li+ molar fraction to 3988 cm−1 for the 0.97 Li+ molar fraction. Time-resolved fluorescence spectroscopy determines that the decay time T(1) varies from 1.0 ps for the 0.00 Li+ molar fraction to 3.6 ps for the 0.97 Li+ molar fraction. Within the range of a few picoseconds, the dynamics of the water solvation shell may be described with H-bond rearrangement, modified with the different RCM surface charges. Two models illustrating the interactions between C522 and RCM in water are proposed which qualitatively describe the dynamics. To the best of our knowledge, this experiment is the first measurement of solvation dynamics on a montmorillonite structure surface using ultrafast laser fluorescence spectroscopy