An Energy-Gradient Strategy for Triplet-Pair Separation in Singlet-Fission Oligomers

Abstract

Singlet fission (SF) offers a pathway to surpass the Shockley–Queisser limit by converting one photoexcited singlet exciton into two triplet excitons. However, in molecular assemblies, this process commonly yields a strongly exchange-coupled correlated triplet pair, 1(TT), where intertriplet interactions facilitate geminate recombination and back-transfer to the singlet manifold. Here we implement an energy-gradient strategy in linear diphenylhexatriene (DPH) hetero-oligomers to guide postfission dynamics toward triplet-pair separation while minimizing thermodynamic loss. The hetero-oligomer series is constructed from two DPH-based building blocks with matched chromophore frameworks but offset triplet energies: a TIPS-substituted unit (T-DPH) and an alkyl-substituted unit (P-DPH). By positioning T-DPH units at the termini of the hetero-oligomers, we encode a modest downhill triplet-energy gradient (∼60 meV) that drives the migration of the triplet population away from the initially formed 1(TT) configuration toward the chain periphery. Using femtosecond and nanosecond transient absorption spectroscopy, we track the kinetic evolution from the singlet manifold through 1(TT) formation to a spatially separated, weakly exchange-coupled (T··T) state, and ultimately to free triplets. Magnetic-field-dependent transient absorption and photoluminescence measurements provide supporting evidence for population of a weak-exchange triplet-pair regime. Comparison with a homotrimer (PPP-DPH3) lacking energetic asymmetry directly isolates the role of this design lever, demonstrating that a ∼60 meV offset is sufficient to accelerate 1(TT) → (T··T) separation. Thus, this energy-gradient strategy enhances harvestable free-triplet yield while retaining the high triplet energy required for device-relevant SF optoelectronics and provides a simple and transferable design principle for SF materials that favor productive triplet-pair separation over geminate loss.