|
The ability to efficiently up-convert broadband, low-intensity light would be an enabling technology for background-free biomedical imaging, volumetric 3D printing, and sensitizing silicon focal plane arrays to the short-wave infrared. Our approach uses colloidal quantum dots—size-tunable spin-mixing fluorophores—to absorb low-energy photons and sensitize the spin-triplet excitonic states of nearby conjugated molecules. Once there, pairs of these long-lived excitations can combine via triplet fusion (triplet-triplet annihilation) to generate shorter-wavelength fluorescence.
I will discuss our ongoing efforts to harness our recent advances in the synthesis of ultra-small PbS quantum dots (d~1.7 nm, hν_peak,abs=2.2eV) to sensitize ‘red-to-blue’ triplet-fusion upconversion in solution. We show that the long (>µs) photoluminescence lifetimes of these particles enable max-efficiency upconversion at modest light intensities (I_th=220 mW/cm2), overcoming a mildly endothermic sensitization scheme that maximizes the anti-Stokes shift (1.04 eV). This architecture facilitates the photo-initiated polymerization of methylmethacrylate using only long-wavelength light (λ: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry. Finally, from the quasi-equilibrium dynamics of triplet energy transfer, we infer that the chemical potential of photoexcited, ultra-small PbS quantum dots is surprisingly high—completing an advantageous suite of properties for upconversion photochemistry, but reinforcing questions regarding the emissive state.
|
