The Effect of Permanent Dipoles on Dark States in Molecular Dimers

TL;DR

This paper explores how permanent dipoles in molecular dimers influence dark states, revealing that excitation-dependent dipoles enable optical transitions and can enhance the robustness of dark states, with implications for photovoltaic efficiency.

Contribution

It demonstrates the role of permanent dipoles in enabling and stabilizing dark states in molecular dimers, a factor often neglected in quantum optics models.

Findings

Permanent dipoles enable optical transitions between bright and dark states.

Dark states formed via indirect coupling are more robust against energy fluctuations.

Implications for improved photovoltaic device design.

Abstract

Many organic molecules possess large permanent dipole moments that differ depending on the electronic state. These permanent dipoles influence both intermolecular coupling and interactions with the optical fields, yet they are often neglected in typical theoretical quantum optics treatments. Here, we investigate the optical properties and their effect on dark states of dimers possessing such permanent dipoles. We show that when monomers have excitation-dependent permanent dipoles, optical transitions between the bright and dark states of the dimer are enabled. We investigate how permanent dipoles allow for the existence of static driving terms between the ground and excited states of each monomer. In turn, these can cause the excited states of the monomers to couple indirectly to the zero excitation state of the dimer. This leads to interference between permanent and transition dipoles and can result in the formation of dark states that are entirely localised. Furthermore, dark states formed through indirect coupling exhibit enhanced robustness against energy level fluctuations, which may improve the efficiency of the design of photovoltaic devices.