The ability to efficiently up-convert low-intensity light would be enabling for volumetric 3D printing, background-free imaging, and sensitizing silicon focal-plane arrays to the short-wave infrared. A rising approach uses colloidal quantum dots to absorb low-energy photons and sensitize the spin-triplet states of nearby conjugated molecules. Once there, pairs of these long-lived excitations can combine via triplet fusion to generate shorter-wavelength fluorescence. We are advancing triplet-fusion upconversion by deepening our understanding of nanocrystal synthesis, and probing the movement of energy and charge in functionalized nanomaterials. For instance, we uncovered that a pre-nucleation intermediate had frustrated efforts to synthesize low-dispersity ensembles of small (d&lt;4 nm) PbS nanocrystals, and showed that amine additives can restore one-step growth and yield markedly improved homogeneity in reactions that run to completion. We continue to build mechanistic insight into metal-chalcogenide nanocrystals, including under-explored lead-free chromophores. We then harnessed the 'ultra-small' (d~1.9 nm, hνabs=2.0 eV) PbS quantum dots accessible from this refined synthesis to sensitize ‘red-to-blue’ triplet-fusion upconversion in solution. We showed that their long (&gt;µs) photoluminescence lifetimes help overcome a mildly endothermic sensitization scheme that maximizes the anti-Stokes shift (ΔE=1.04 eV) and minimizes non-specific excitation. This architecture facilitated the photo-initiated polymerization of methylmethacrylate using only red light (λ: 637 nm); a realization of nanocrystal-sensitized upconversion photochemistry. However, this might have been unanticipated, because the large 'Stokes shifts' of ultra-small nanocrystals imply an unacceptable loss of incident photon energy. However, from the quasi-equilibrium energy-transfer dynamics that we measure, we infer that the effective chemical potential of photoexcited ultra-small PbS quantum dots is surprisingly high. Though this completes an advantageous suite of properties for upconversion photochemistry, it is refocussing our attention on the multi-faceted role of the dynamic surface.

Nanocrystal-sensitized triplet-fusion–upconversion photochemistry

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ACS Fall 2022

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