Direct Triplet Exciton Generation from the Singlet Ground State in Organic Semiconductors Coupled with Lanthanide-Doped Nanoparticles

The ability to control triplet excitons is of paramount importance in and beyond the field of optoelectronics. As molecular triplet excitons are dark states, triplets can generally not be accessed directly nor harvested luminescently. Here we show that upon bringing organic molecules and polymers in close proximity to inorganic lanthanide-doped nanoparticles (LnNPs), the quantum mechanically forbidden singlet ground state to triplet excited state transition gains oscillator strength. This allows us to directly excite into the triplet excited state manifold from the singlet ground state.<br/>These absorptions (T1 &lt;- S0) can be readily measured via simple UV-Vis absorption spectroscopy and as such this method presents a straightforward way to measure triplet exciton energies of organic semiconductors. We show this strategy to be widely applicable to a range of organic semiconductors including acenes, organic donor polymers, organic acceptor molecules, and TADF molecules. Using a mixture of low-temperature absorption measurements and magneto-optic measurements we shine light on the mechanism underpinning this phenomenon.<br/>Furthermore, we use transient absorption spectroscopy to study the excited state dynamics of these directly generated triplet excitons. We find these triplets to have short lifetimes well-below 1 ns, allowing for efficient emission from these triplet-excitons. Current studies are underway looking into the possibilities of utilising these directly generated triplets for processes such as direct triplet-triplet annihilation.<br/>All in all, these results deepen our understanding of coupled organic-LnNPs systems and provide a new way to control triplet excitons, which is essential in and beyond the field of optoelectronics.