We demonstrate that a structurally rigid, weakly coupled molecular dimer can replace traditional monomeric annihilators for triplet fusion upconversion (TUC) in solution by observing emitted photons (λ = 540 nm) from a norbornyl-bridged tetracene homodimer following excitation of a triplet sensitizer at λ = 730 nm. Intriguingly, steadystate spectroscopy, kinetic simulations, and Stern−Volmer quenching experiments show that the dimer exhibits qualitatively different photophysics than its parent monomer: it is less effective at diffusion-mediated triplet exciton transfer, but it fuses extracted triplets more efficiently. Our results support the development of composite triplet-fusion platforms that go beyond diffusion-mediated triplet extraction, ultimately circumventing the concentration dependence of solution-phase TUC.

PbS nanocrystals are critical materials for infrared optoelectronics, but the persistent challenge in synthesizing small nanocrystals with narrow line widths demands improved mechanistic understanding. Here, we show that the conventional hot-injection synthesis of PbS nanocrystals per Hines exhibits two-step kinetics involving an intermediate species. The intermediate is small, lead-rich, and has characteristic, reproducible, visible-wavelength emissionall consistent. with a PbS prenucleation cluster (PNC). We then demonstrate that high-pK a amines disrupt the PNC, accelerating nanocrystal nucleation and enabling the synthesis of PbS nanocrystals with diameters as small as ⌀ ∼ 1.7 nm and distinct ensemble absorption peaks (hν = 2.2 eV, λ = 560 nm) in reactions allowed to run to completion. We show that the basicity of the amine additive controls the average size of nanocrystals at reaction completion, which we understand by incorporating metastable PNCs into reaction models that partition monomers between nanocrystal nucleation and nanocrystal growth. This conceptual advance permits the routine synthesis of ultrasmall PbS NCs with excitonic absorption line widths that are up to 25% narrower than previously reported for comparable sizes (⌀: 1.7–3 nm, λpeak,abs: 560–885 nm, hνpeak,abs: 2.2–1.4 eV). This reduced electronic dispersity will enhance device performance, and the underlying insight is further evidence of the exquisite ability of metal-complexing additives to direct the bottom-up syntheses of nanostructured materials.

We demonstrate the use of ultra-small PbS quantum dots as endothermic sensitizers for red-to-blue triplet-fusion upconversion, achieving nanocrystal-sensitized upconversion photochemistry.

Ligand-exchange procedures are ubiquitous in the functionalization of colloidal nanocrystals for applications in biological imaging, photocatalysis, and photonic/optoelectronic devices. However, the rich interactions between functional ligands and the nanocrystal surface offer a vast opportunity to achieve emergent self-assembled structures. Here, using 1 H NMR as a probe and L-type-promoted Z-type ligand displacement as a tool to study PbS nanocrystals, we demonstrate that 9-anthracene carboxylic acid (9-ACA) ligands strongly segregate to the highestenergy binding sites at the conclusion of X-for-X exchanges. These weaker sites are associated with nanocrystal facet-edges, and linewidth analysis corroborates that 9-ACA replaces the most conformationally dynamic native ligands. The templated assembly of this bulky model fluorophore at the nanocrystal surface is an opportunity to enhance energy transport and contrasts sharply with conventional aliphatic ligands, where we find that exchanges are isotropic. Our results indicate that ligand−ligand interactions and the spatial correlation of nanocrystal binding-site heterogeneity can be leveraged to produce functionalized particles with tailored, anisotropic ligand morphologies. This opportunity to promote clustering could influence the design of photoactive ligands for multiexcitation processes such as incoherent photon conversion.

The use of excess PbCl2 in the synthesis of PbS nanocrystals has become a convenient route to produce narrow-line-width infrared emitters. However, these materials have found limited adoption in optoelectronic deviceseven compared to PbS nanocrystals prepared with lead oleate. Here, using both transmission electron microscopy and small-angle X-ray scattering, we show that excess PbCl2 results in larger-diameter PbS nanocrystals for the same excitonic features, which is consistent with the formation of an intrinsic insulating shell. We observe further differences in excess-lead-chloride nanocrystals consistent with a shell, including lattice strain and smaller Stokes shifts for intermediate sizes (⌀: 4.8–6.8 nm) that match the passivation/rigidification predicted for a chloride-terminate surface. Our results clarify and rationalize the divergent properties of PbS nanocrystals prepared using different synthetic methodologies, give guidance for device implementation, and offer a new target for synthetic control.