Phonon-induced localization of excitons in molecular crystals from first principles

TL;DR

This study uses first-principles methods to understand how phonons influence exciton localization in molecular crystals, revealing the roles of zero-point motion, thermal effects, and anharmonicity in exciton behavior.

Contribution

It provides a detailed microscopic analysis of phonon-induced exciton localization in pentacene, incorporating all orders of exciton-phonon coupling and anharmonic effects using advanced computational techniques.

Findings

Zero-point nuclear motion causes strong exciton localization.

Thermal motion further localizes Wannier-Mott excitons.

Anharmonic effects lead to temperature-dependent localization.

Abstract

The spatial extent of excitons in molecular systems underpins their photophysics and utility for optoelectronic applications. Phonons are reported to lead to both exciton localization and delocalization. However, a microscopic understanding of phonon-induced (de)localization is lacking, in particular how localized states form, the role of specific vibrations, and the relative importance of quantum and thermal nuclear fluctuations. Here we present a first-principles study of these phenomena in solid pentacene, a prototypical molecular crystal, capturing the formation of bound excitons, exciton-phonon coupling to all orders, and phonon anharmonicity, using density functional theory, the \emph{ab initio} $GW$-Bethe-Salpeter equation approach, finite difference, and path integral techniques. We find that for pentacene zero-point nuclear motion causes uniformly strong localization, with thermal motion providing additional localization only for Wannier-Mott-like excitons. Anharmonic effects drive temperature-dependent localization, and while such effects prevent the emergence of highly delocalized excitons, we explore the conditions under which these might be realized.