Synthesis and Exciton Dynamics of Donor-Orthogonal Acceptor Conjugated Polymers: Reducing the Singlet-Triplet Energy Gap

Abstract

The presence of energetically low-lying triplet states is a hallmark of organic semiconductors. Even though they present a wealth of interesting photophysical properties, these optically dark states significantly limit optoelectronic device performance. Recent advances in emissive charge-transfer molecules have pioneered routes to reduce the energy gap between triplets and "bright" singlets, allowing thermal population exchange between them and eliminating a significant loss channel in devices. In conjugated polymers, this gap has proved resistant to modification. Here, we introduce a general approach to reduce the singlet-triplet energy gap in fully conjugated polymers, using a donor-orthogonal acceptor motif to spatially separate electron and hole wave functions. This new generation of conjugated polymers allows for a greatly reduced exchange energy, enhancing triplet formation and enabling thermally activated delayed fluorescence. We find that the mechanisms of both processes are driven by excited-state mixing between π-π*and charge-transfer states, affording new insight into reverse intersystem crossing.Part of this work was funded by EU project 679789 – 455 CONTREX, EC H2020 SYNCHRONICS (643238), EC H2020 SOLEDLIGHT (643791) and EPSRC (EP/M005143/1) A.J.M. was supported by the EPSRC (EP/456M01083X). J.M.F. was supported by EPSRC (EP/K016288/1). F.C. is a Royal Society Wolfson Research Merit Award holder. H.L.S. was supported by the Winton Programme 457 for the Physics of Sustainability. We are grateful to the Imperial College High Performance Computing Service (doi: 10.14469/hpc/2232)