WITHDRAWN (NO REG) 6/14/22 EQ08.01.01 Advancing the Synthesis of Metal-Chalcogenide Quantum Dots to Achieve Upconversion Photochemistry via Nanocrystal-Sensitized Triplet Fusion

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.^1,2,3 Once there, pairs of these long-lived excitations can combine via triplet fusion (triplet-triplet annihilation) to generate shorter-wavelength fluorescence.<br/>To advance triplet-fusion upconversion, we are using optical spectroscopy to deepen our understanding of nanocrystal synthesis and probe the movement of energy and charge within and between organic and inorganic semiconductors.<br/>For instance, we have uncovered that a pre-nucleation cluster intermediate has historically frustrated efforts to synthesize low-dispersity ensembles of small (d&lt;4 nm) PbS nanocrystals, and showed that Lewis basic additives can restore one-step growth and yield markedly narrower heterogeneous linewidths in reactions that run to completion.⁴ We are expanding from this insight to build mechanistic understanding of the synthesis⁵ and surface⁶ of metal-chalcogenide nanocrystals.<br/>We then harnessed the ultra-small (d~1.7 nm, <i>hν</i>_peak,abs=2.2eV) PbS quantum dots that we can now controllably produce to sensitize ‘red-to-blue’ triplet-fusion upconversion in solution.⁷ We show that the long (&gt;µs) photoluminescence lifetimes of these particles enable max-efficiency upconversion at lower 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.<br/>References:<br/>1. Wu, Congreve, MWBW <i>et al.</i> (Bulovic, Bawendi, Baldo) <i>Nature Photon</i>. 10:31 (2016)<br/>2. Huang <i>et al.</i> (Bardeen, Tang) <i>Nano Lett.</i> 15:5552 (2015)<br/>3. Mongin, <i>et al. </i>(Castellano) <i>Science</i> 351(6271):369-372 (2016<br/>4. Green, et al. (MWBW) <i>Chem. Mater.</i> 32(9):4803–4094 (2020)<br/>5. Yarur Villanueva, et al. (MWBW) <i>ACS Nano </i>(ASAP, 2021) 10.1021/acsnano.1c06730<br/>6. Green, <i>et al.</i> (MWBW) <i>ACS Appl. Nano. Mater. </i>4(6):5655–5664<i> </i>(2021)<br/>7. Imperiale, et al. (MWBW) <i>Chemical Science</i>. (Advanced Article, 2021) 10.1039/D1SC04330G