Molecular Packing Dictates One-Step vs Two-Step Triplet Sensitization for Photon Upconversion at the Organic/2D Semiconductor Interface

Near-infrared (NIR) photon upconversion via triplet–triplet annihilation (TTA) in emerging organic/2D semiconductor heterostructures holds great promise for light harvesting and optoelectronic applications. However, the mechanism and dynamics of the key triplet sensitization step at the organic/inorganic interface, in particular, the role of molecular packing and configuration, remain unexplored and overlooked. Herein, using rubrene/WSe 2 heterostructures with a WSe 2 monolayer as a near-IR sensitizer and rubrene of orthorhombic (Orth), monoclinic (Mono), and amorphous phases, as annihilators, we show NIR-to-visible photon upconversion in both heterostructures but distinctly different triplet sensitization mechanisms and pathways: one-step direct exciton Dexter energy transfer in the Mono heterostructure but two-step indirect charge-transfer-mediated triplet sensitization in the Orth heterostructure. Specifically, ultrafast hole transfer (∼5.3 ps) followed by delayed electron transfer (∼67 ps) drives two-step indirect triplet energy transfer (TET) in Orth heterostructures with a high TET quantum yield (Φ TET ) of 86%. In contrast, Mono heterostructures exhibit a slow (∼20 ps) one-step direct Dexter TET with a Φ TET of 47%. Further ultraviolet photoelectron spectroscopy (UPS) and optical measurements reveal very different interfacial band alignment modulated by different molecular π-stackings despite exactly the same molecule: small intermolecular displacement and strong intermolecular coupling in Orth rubrene elevates its highest occupied molecular orbital (HOMO) level, efficiently driving sequential charge transfer pathway, while weak coupling occurs in the Mono phase restricting TET to the Dexter mechanism. This work firmly unravels molecular packing as an overlooked critical factor in governing triplet sensitization pathways and efficiencies at the organic/2D semiconductor interface, providing design and optimization principles for solid-state photon upconversions.