Highly tunable optics across a topological transition in organic polymers

TL;DR

This paper demonstrates that topological phase transitions in organic polymers can be used to tune excitonic properties, enabling potential applications in energy transport and singlet fission.

Contribution

It reveals a topological excitonic phase transition in ethynylene bridged polyacene polymers, linking topological changes to excitonic and energy transport properties.

Findings

Topological $Z_2$ phase transition occurs in the polymer spectrum.

The transition causes a real-space exciton wave function exchange.

Flat electronic structures lead to negatively dispersing triplet excitons.

Abstract

Controllable topological phase transitions are appealing as they allow for tunable single particle electronic properties. Here, by using state-of-the-art manybody perturbation theory techniques, we show that the topological $Z_2$ phase transition occurring in the single particle spectrum of the recently synthetized ethynylene bridged polyacene polymers is accompanied by a topological excitonic phase transition: the band inversion in the non-trivial phase yields real-space exciton wave functions in which electrons and holes exchange orbital characters with respect to the trivial phase. The topological excitonic phase transition results in a broad tunability of the singlet-triplet splittings, opening appealing perspectives for the occurrence of singlet fission. Finally, the flatness of the single-particle electronic structure in the topological non trivial phase leads to negatively dispersing triplet excitons in a large portion of the Brillouin zone, opening a route for spontaneously coherent energy transport at room temperature.