Toward an Understanding of How the Optical Property of Water-Soluble Cationic Polythiophene Derivative Is Altered by the Addition of Salts: The Hofmeister Effect

Abstract

Hofmeister ion-specific effects on optical properties of a water-soluble cationic polymer, poly(3-alkoxy-4-methylthiophene) (PMNT), are investigated by means of absorption, resonance Raman spectroscopy, and molecular dynamic simulations. It is found that the ionochromism of conjugated polyelectrolytes PMNT obeys Hofmeister series with high optical sensitivity, while the spectral changes result from the different electrostatic interactions and the conformational change of the cationic PMNT in different salt solutions. As a result, UV–vis absorption spectra exhibit almost no shift of absorption of PMNT in the presence of SO42–, F–, etc., whereas a red-shifted absorption of PMNT with I–, SCN– is clearly observed. To gain a deeper understanding of the nature of these anion-dependent chromic phenomena, ab initio calculations and molecular dynamics (MD) simulations are carried out to present the microscopic insights, that the Hofmeister effect occurs at the PMNT/water interface through the direct (hydrophobic, hydrophilic, and electrostatic) interactions between the anions and PMNT backbone. It is found that the salting-in anions like I– strongly suppress the hydrophobic collapse of PMNT backbone, leading to more extended and ordered PMNT backbone with red-shifted absorption, and the salting-out anions like F– strongly avoid the hydrophobic PMNT backbone, keeping a random-coiled configuration of PMNT backbone without obvious absorption changes in KF solution. The existence of ordered and disordered backbone configurations is further verified by monitoring the main in-plane skeleton Raman modes (C═C and C–C stretch) of PMNT in various salt solutions. The results presented here could provide a fundamental understanding of salt effects on chemical and biological processes occurring at the macromolecular/water interface, and then may potentially stimulate many chemical and biological applications.