Temperature Mediated Balance of Singlet Fission and Symmetry-Breaking Charge-Transfer in a Polycrystalline Perylene Bisimide Thin Film

Singlet fission (SF) and symmetry-breaking charge transfer (SBCT) are competing, and often concurrent, pathways observed in photoexcited organic semiconductor systems: a photoexcited molecule electronically coupled to a neighbor can yield two triplet excited states (SF) or a charge-separated radical ion pair (SBCT). The branching ratio of SF and SBCT is determined largely by the intermolecular electronic coupling, which depends on the relative orientation of neighboring molecules. In a solid thin film, this orientation is defined by the molecular packing, which is typically a static condition established during film formation, and difficult to change with external stimuli. Here, we show that intermolecular coupling in a solid thin film of an asymmetrically substituted perylene bisimide (mXPBI) can be reversibly modulated by temperature alone. Oblique π-stacking in mXPBI produces Davydov-split excitonic states with mixed Coulombic and charge-transfer (CT) coupling character. Temperature-dependent X-ray diffraction reveals modest changes in intermolecular distance and relative molecular orientation. Time-dependent density functional theory calculations demonstrate that these subtle changes to the molecular packing alter the balance between long-range Coulombic and short-range CT coupling, which is expected to influence the competition between SF and SBCT pathways. Transient absorption spectroscopy confirms this prediction, showing that the branching ratio between triplet (SF) and SBCT shifts from approximately 30:70 at 300 K to 50:50 at 100 K. These results demonstrate that excitonic coupling of mXPBI can be reversibly tuned by temperature in the solid state, thereby shifting the balance of competitive photophysical pathways.