Charge-Transfer States Enhance Triplet Yield in Silicon Quantum Dot-Pyrene Hybrid Nanostructure

Photon upconversion based on triplet-triplet annihilation can utilize incoherent sub-bandgap photons for energy conversion and photocatalysis. Though quantum dots (QDs), with their tunable bandgaps, make attractive triplet photosensitizers, energy transfer must compete with other exciton-depleting pathways that stem from various trap states. Here, we show that strong electronic coupling can be introduced through conjugated bridges between Si QDs and surface-bound pyrene acceptors by creating a transient hole-transfer intermediate from Si to attached pyrenes, which enhances triplet energy transfer. The hybrid achieved an upconversion quantum yield of 10.8% (with a theoretical maximum of 50%) with DPA as the emitter, which is the highest reported for Si QD-based upconversion. By using a combination of cyclic voltammetry, time-resolved electron paramagnetic resonance, and transient absorption spectroscopies, we show that the alkyne and vinyl bridges hybridize QD and molecular states, facilitating hole-then-electron transfer that competes effectively with exciton quenching. Specifically, the alkyne-linked Si QD: AP hybrid exhibits hole transfer with a time constant of 1.7 ns and an efficiency of 27%, highlighting linker conjugation as an effective strategy for overcoming unfavorable kinetics and energetics.