Revealing weak spin-orbit coupling effects on charge carriers in a $\pi$-conjugated polymer

TL;DR

This study uses EDMR and DFT calculations to quantify and observe weak spin-orbit coupling effects on charge carriers in a conjugated polymer, revealing their monomolecular nature.

Contribution

It provides the first quantification and direct observation of spin-orbit coupling effects on charge carriers in weakly SO-coupled hydrocarbons.

Findings

Spectral broadening due to SOC exceeds hyperfine effects.

DFT calculations reveal g-tensors for charge carriers.

Spin-related phenomena are monomolecular in nature.

Abstract

We measure electrically detected magnetic resonance (EDMR) on organic light-emitting diodes (OLEDs) made of the polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) at room temperature and high magnetic fields, where spectral broadening of the resonance due to spin-orbit coupling (SOC) exceeds that due to the local hyperfine fields. Density-functional-theory calculations on an open-shell model of the material reveal g-tensors of charge-carrier spins in the lowest unoccupied (electron) and highest occupied (hole) molecular orbitals. These tensors are used for simulations of magnetic resonance line-shapes. Besides providing the first quantification and direct observation of SOC effects on charge-carrier states in these weakly SO-coupled hydrocarbons, this procedure demonstrates that spin-related phenomena in these materials are fundamentally monomolecular in nature.