Long-range two-dimensional exciton sensitization of triplet-pair emission

The spin-entangled triplet-pair state produced by singlet fission is promising for quantum information applications, but its optical darkness severely limits spectroscopic access and optical readout. Here we show that monolayer WS₂ can act as a long-range excitonic sensitizer for triplet-pair emission in the singlet-fission molecule 6,13-bis(triisopropylsilylethynyl)-tetramethyl pentacene (TCPS). Strong excitonic resonances and spin-orbit-split valleys in WS₂ mediate long-range excitonic energy transfer into the triplet-pair manifold, enhancing triplet-pair emission by more than two orders of magnitude. This enhancement arises from the exceptionally large exciton transition dipole moment of WS₂, which enables efficient dipole-dipole coupling across the hybrid interface. Temperature-dependent and time-resolved spectroscopy reveals ultrafast picosecond energy transfer. Distance-dependent measurements demonstrate efficient transfer beyond 100 nm, inconsistent with classical Förster scaling and instead indicative of radiative, near-field-assisted coupling with a shallow  ~ 1/d dependence. These results establish a mechanism for long-range triplet-pair sensitization in hybrid molecular/transition-metal dichalcogenide heterostructures and provide a route for coupling correlated multiexciton states to optically active excitonic materials.