Remotely Tuned Triplet Transfer via Ligand Proximity in Quantum Dot-Organic Spectral Converters

Quantum dot (QD)-organic hybrid materials offer promising spectral conversion platforms for applications such as photoredox catalysis, solar energy conversion, 3D printing, and bioimaging. In spectral upconversion, the overall triplet energy transfer (TET) efficiency remains limited due to inefficient secondary TET that occurs beyond the QD core, i.e., from triplet mediators to annihilators (TET2). Here, we reveal that the hydrocarbon ligands on nanoparticles can remotely govern this external TET2 that occurs entirely outside the core. By shrinking the native oleate ligand shell on PbSe QD sensitizers before attaching triplet mediator ligands, the NIR-to-visible upconversion performance can be significantly improved. Transient absorption spectroscopy confirms that the more compact ligand shell substantially accelerates TET2, boosting the highest transfer efficiency from 59.4% to 93.5%. We propose that the enhanced TET2 stems from shortened mediator-annihilator distances induced by reduced steric hindrance from the shorter, proximal hydrocarbon ligands, as confirmed by molecular dynamics simulations. The strategy proves versatile across multiple upconversion systems, including solid-state films, CdSe QD-based green-to-blue systems, and lanthanide-doped nanoparticle-sensitized hybrids. Furthermore, the same principle remains applicable to molecular singlet fission-based downconversion using QD as photon emitters, raising the highest photon-multiplication efficiency from 132% to 163%. Our work demonstrates that ligand shell proximity can remotely tune TET beyond the nanoparticle core, providing a general route to optimize inorganic-organic hybrid spectral up- and downconverters.