Ultrafast Singlet Fission and Long-Lived Triplet Generation in Conjugated Polymers through Torsional-Modulated Intrachain Electronic Coupling

The implementation of singlet fission (SF) in solar energy conversion critically depends on the fast SF rate and long-lifetime triplet simultaneously. This requires a delicate balance of interchromophore electronic coupling that enables both the generation and subsequent separation of the triplet pair state. Conjugated polymers emerge as attractive SF candidates by offering the ease of independent modification of intra- and interchain electronic interactions. An in-depth understanding of the structure-property relationship is crucial to design high-performing SF polymers yet remains elusive. In the present work, two SF-conjugated copolymers constructed by connecting fission-capable benzodipyrrolidone (BDPP) units with two planar spacers with comparable size (difluorostilbene and 1,4-diethynylbenzene groups) but distinct dihedron torsion, namely, PB-2F and PB-B, were synthesized and thoroughly characterized. Ultrafast intramolecular SF is shown to occur in both solution and solid-state film. The varied dihedron angle between the adjacent BDPP units affects interchromophore coupling both along and across the polymer chains, producing triplet efficiently which lasts for several microseconds in the highly twisted PB-2F film. Our work demonstrates the effect of the linker on SF dynamics, offering valuable insights into the design of new fission-capable polymeric materials.