Excitonic properties of pi-conjugated acene ethynylene-bridged polymers from a trivial to a non-trivial insulating phase

In recent papers (Refs. [1]-[2]) it was shown that it is possible to observe a topological phase transition in 1D acene ethynylene-bridged polymers from a trivial to a non-trivial insulating phase as a function of the monomer acene size. Combining tight-binding, DFT and GW calculations they showed that such quantum phase transition was accompanied by a ethylenic-to-cummulenic transition in the molecular bridges between the acenes, and that the presence of a topological transition could provide a route to novel organic metals.

However, the optical properties of such polymers and the impact of the topological transition on their excitonic excitations were not discussed. In this work we study the excitonic properties of infinitely extended pi-conjugated acene ethynylene-bridged polymers by means of  self-consistent GW calculations and the solution of the Bethe-Salpeter equation both in the trivial and non-trivial quantum phase. We find that the optical band gap can be mechanically tuned to a great extent in the non-trivial topological phase, as can the excitonic dispersions of the singlet and triplet excitons. Other excitonic properties such as binding energies and oscillator strengths are found to vary in a highly non-monotonic way across the transition, and the effective band inversion in the non-trivial phase leads to striking effects in the real-space wave functions of the excitons that may provide a range of novel opportunities for optoelectronic applications.

[1] B. Cirera et al., Nature Nanotechnology 15, 437-443 (2020).

[2] H. Gonzàlez-Herrero et al., arXiv:2105.00025 [cond-mat.mes-hall] (2021).