University of Michigan researchers have presented a novel strategy for developing ultrafast triplet emitters by creating heterostructures that combine organic chromophores with transition metal dichalcogenides (TMDs). The key achievement is obtaining microsecond phosphorescence at room temperature from these hybrid structures, whereas the organic chromophores alone typically exhibit much slower millisecond phosphorescence under vibration-free conditions.
One of the big challenges in OLED technology has been dealing with how electrons behave when they get excited. When electricity excites these materials, you get two types of excited states: singles and triplets. Think of singles as being easy to get light from, while triplets are usually wasted energy.
Current OLED technology uses expensive materials containing metals like iridium to handle this triplet problem. These materials are great because they can convert those usually-wasted triplets into light very quickly – in millionths of a second (microseconds), which is perfect for displays. However, these materials are expensive and can wear out over time, especially for blue light.
What these researchers have done is pretty clever. Instead of putting expensive metals directly into the light-emitting molecules, they’ve created a sandwich structure. They take a simple organic molecule (which would normally only give you slow, inefficient light from triplets) and put it very close to a special type of material called a transition metal dichalcogenide (TMD) – think of it as a very thin sheet of metal-containing material.
When these two materials are placed close together – about a millionth of a hair’s width apart – something amazing happens. The TMD acts like a catalyst, helping the organic molecule convert its triplets into light much faster than it could on its own. Instead of taking milliseconds (thousandths of a second), which is too slow for displays, it now happens in microseconds (millionths of a second) – just like the expensive iridium-based materials.
The researchers focused on a model system combining diethyl 2,5-dihydroxy terephthalate (DDT) with various TMDs like MoS2, MoSe2, WS2, and WSe2. DDT was chosen because it contains aromatic carbonyl groups that can facilitate intersystem crossing for triplet population. While DDT normally shows only blue fluorescence at room temperature and very slow millisecond phosphorescence at 77K, the DDT/TMD hybrids exhibited bright green phosphorescence with lifetimes of only tens of microseconds at room temperature.
The dramatic enhancement in phosphorescence speed, thousands of times faster than normal, occurs through what the researchers term a “through-space spin-orbit proximity effect.” When DDT molecules align closely with the TMD surface, the heavy transition metal atoms in the TMD enhance spin-orbit coupling in the nearby DDT molecules. This was evidenced by detailed quantum chemical calculations showing that the presence of MoS2 causes significant changes in DDT’s excited state energies and dramatically increases spin-orbit coupling matrix elements between relevant states.
The researchers identified several crucial structural requirements for achieving this ultrafast emission. The TMD must have a 2H crystal structure containing heavy transition metals like Mo or W, as lighter metals such as Ti did not produce the effect. The organic chromophore needs an aromatic carbonyl core with ortho-hydroxyl groups to enable proper alignment and electronic coupling with the TMD surface. Additionally, the spacing between the organic molecule and TMD surface must be carefully controlled, with approximately 4.6 Å found to be optimal.
Through multiple experimental techniques, the researchers validated their findings comprehensively. Time-resolved photoluminescence demonstrated microsecond decay, while electron paramagnetic resonance revealed new triplet signatures. Raman spectroscopy showed electronic coupling between the components, and scanning tunneling microscopy confirmed proper molecular alignment. Detailed quantum chemical calculations provided theoretical understanding of the mechanism.
The significance of this work extends across several domains. It provides a new strategy for creating fast phosphorescent materials without requiring expensive noble metals like iridium or platinum. The microsecond emission timescale makes it practical for applications like displays operating at high refresh rates. Moreover, the approach separates the roles of color generation and spin-orbit coupling enhancement, allowing them to be optimized independently. These materials may also prove more stable than traditional organometallic phosphors since they don’t rely on potentially fragile metal-ligand bonds.
The researchers propose that this phenomenon occurs through multiple concurrent mechanisms. These include direct enhancement of spin-orbit coupling through proximity to heavy atoms, electronic state reordering enabling new intersystem crossing pathways, contribution from TMD defect states that can host triplet populations, and possible involvement of charge transfer states at the interface.
This study opens new directions for designing efficient triplet emitters by exploiting interfacial effects rather than traditional molecular design strategies.
Reference
Choi, J., Im, H., Heo, JM. et al. Microsecond triplet emission from organic chromophore-transition metal dichalcogenide hybrids via through-space spin orbit proximity effect. Nat Commun 15, 10282 (2024). https://doi.org/10.1038/s41467-024-51501-8
