Specificity of Counterion Binding to a Conjugated Polyelectrolyte: A Combined Molecular Dynamics and NOESY Investigation.

Gregor Hostnik,Guillaume Mériguet,Črtomir Podlipnik,Janez Cerar

doi:10.1021/acs.macromol.9b02161

Abstract

Poly(thiophen-3-ylacetic acid) (PTAA) is a representative of conjugated polyelectrolytes which are used in many optoelectronics devices. The performance of these devices is affected by the polymer conformation, which, among others, depends on the nature of the counterion. In this study, the binding of tetrabutylammonium counterions (TBA+) on PTAA was determined using a combination of nuclear Overhauser effect spectroscopy (NOESY) and molecular dynamics (MD) simulation. It was found that TBA+ ions specifically bind on the hydrophobic main chain of PTAA, while, according to MD simulations, alkali counterions predominantly bind in the vicinity of negatively charged carboxylic groups located on side chains. The MD trajectories were used to compute the relaxation matrices and the NOESY spectra. With the help of these latter calculations, the changes of intensities in experimental NOESY spectra upon binding of TBA+ ions to PTAA were interpreted.

Highlights

Poly(thiophen-3-ylacetic acid), due to its conjugated polymer main chain and ionizable carboxylic side groups, belongs to the class of conjugated polyelectrolytes
To investigate the specificity of binding of ions onto a conjugated polyelectrolyte, Poly(thiophen-3-ylacetic acid) (PTAA), we use a combination of molecular dynamics (MD) simulations and nuclear magnetic resonance (NMR) spectroscopy, especially nuclear Overhauser effect spectroscopy (NOESY) experiments
The analysis of RDFs, obtained from MD simulation trajectories, showed a marked specificity for the location of binding that differs for alkali counterions which bind in the vicinity of the PTAA carboxylate group and TAA counterions which bind in the vicinity of the thiophene ring of the main polyion chain

Summary

Introduction

Poly(thiophen-3-ylacetic acid) (denoted further by PTAA), due to its conjugated polymer main chain and ionizable carboxylic side groups (see Figure 1), belongs to the class of conjugated polyelectrolytes. One of the crucial properties of polyelectrolytes is their conformation.[7] Changes in conformation of conjugated polymers are often reflected in their spectral properties.[8,9] The conformation of a conjugated polyion is influenced by counterions present in the system. This effect can be exploited in design of different optoelectronic devices and sensors. On the basis of these results, we proposed that the higher fraction of bound counterions and reversed dependence on the hydration enthalpy is a consequence of a different binding mechanism, very likely based on the hydrophobic interactions between PTAA and TAA ions

Methods

Results

Conclusion