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.