Exciton Diffusion in Highly-Ordered One Dimensional Conjugated Polymers: Effects of Back-Bone Torsion, Electronic Symmetry, Phonons and Annihilation

Abstract

Many optoelectronic devices based on organic materials require rapid and long-range singlet exciton transport. Key factors controlling exciton transport include material structure, exciton-phonon coupling and electronic state symmetry. Here, we employ femtosecond transient absorption microscopy to study the influence of these parameters on exciton transport in one-dimensional conjugated polymers. We find that excitons with 2¹A_g^- symmetry and a planar backbone exhibit a significantly higher diffusion coefficient (34 ± 10 cm² s^-1) compared to excitons with 1¹B_u⁺ symmetry (7 ± 6 cm² s^-1) with a twisted backbone. We also find that exciton transport in the 2¹A_g^- state occurs without exciton-exciton annihilation. Both 2¹A_g^- and 1¹B_u⁺ states are found to exhibit subdiffusive behavior. Ab initio <i>GW</i>-BSE calculations reveal that this is due to the comparable strengths of the exciton-phonon interaction and exciton coupling. Our results demonstrate the link between electronic state symmetry, backbone torsion and phonons in exciton transport in π-conjugated polymers.