Solvent effects on optical excitations of poly para phenylene ethynylene studied by QM/MM simulations based on Many-Body Green's Functions Theory

TL;DR

This study uses advanced QM/MM simulations to investigate how solvents affect the optical excitations of poly para phenylene ethynylene, highlighting the dominance of conformational changes over electronic environment effects.

Contribution

It introduces a combined QM/MM approach with many-body Green's functions to differentiate conformational and electronic solvent effects on poly-PPE excitations.

Findings

Electronic environment effects are negligible compared to conformational dynamics.

Excited state energy and localization are sensitive to oligomer bends.

Solvent-induced conformational changes primarily influence optical properties.

Abstract

Electronic excitations in dilute solutions of poly para phenylene ethynylene (poly-PPE) are studied using a QM/MM approach combining many-body Green's functions theory within the $GW$ approximation and the Bethe-Salpeter equation with polarizable force field models. Oligomers up to a length of 7.5\,nm (10 repeat units) functionalized with nonyl side chains are solvated in toluene and water, respectively. After equilibration using atomistic molecular dynamics (MD), the system is partitioned into a quantum region (backbone) embedded into a classical (side chains and solvent) environment. Optical absorption properties are calculated solving the coupled QM/MM system self-consistently and special attention is paid to the effects of solvents. The model allows to differentiate the influence of oligomer conformation induced by the solvation from electronic effects related to local electric fields and polarization. It is found that the electronic environment contributions are negligible compared to the conformational dynamics of the conjugated PPE. An analysis of the electron-hole wave function reveals a sensitivity of energy and localization characteristics of the excited states to bends in the global conformation of the oligomer rather than to the relative of phenyl rings along the backbone.