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This organic molecule shines bright by breaking the rules

A new heptazine analog could power organic light-emitting diodes by defying Hund’s rule

by Ariana Remmel
September 17, 2022 | A version of this story appeared in Volume 100, Issue 33

The molecular structure of a heptazine analogue called HzTFEX<sub>2</sub>. The core is three fused six-membered rings that contain seven nitrogen atoms. This makes a triangle-shaped molecule with a trifluoromethyl ether at the top point and a dimethylbenzene moiety at each of the bottom two points.
This heptazine derivative emits blue light by defying the laws of quantum mechanics.

A new organic light-emitting diode (OLED) fluoresces blue by turning a textbook quantum principle on its head, according to a new study (Nature 2022, DOI: 10.1038/s41586-022-05132-y).

In OLED devices, electric current excites electrons in organic molecules to a higher energy state, and the molecules fluoresce visible light as the electrons relax back to ground state. The electronic configuration of these excited states are dictated by quantum mechanics, but one principle has stymied advances in OLED technology: Hund’s rule holds that for a given electron configuration, the state with the most unpaired electrons spinning in parallel—called the triplet state—will have a lower energy than a state in which more of these unpaired electrons have opposite spins. This higher-energy configuration is called the singlet state, and only electrons moving from the excited singlet state to the ground state fluoresce light that’s useful for OLEDs. But many of these electrons get trapped in the intermediate triplet state long enough to extinguish fluorescence and kill an OLED’s efficiency, especially when it comes to generating blue light.

Naoya Aizawa, a chemist at the University of Osaka, and his colleagues wondered if they could overcome this challenge by finding exceptions to Hund’s rule where the triplet state is higher energy than the singlet state. That would clear a path for electrons languishing in a triplet state to easily move into a singlet state to emit light. Recent studies suggested that some heptazine​​​​-​​​based molecules could do the trick.

To investigate, Aizawa and his team created computational simulations to screen 34,596 heptazine analogs for electron configurations where the triplet and singlet state energies could be inverted. Out of 5,264 promising candidates, the researchers synthesized two heptazine analogs that their calculations predicted would fluoresce blue light.

They found that a molecule called HzTFEX2 broke Hund’s rule with inverted singlet and triplet states that allow the molecule to emit blue light as predicted. When they built a prototype OLED device using HzTFEX2, the researchers calculated 85% quantum efficiency versus the maximum 25% efficiency of traditional OLEDs constrained by Hund’s rule, Aizawa says.

Cody Schlenker, a University of Washington chemist who wasn’t involved in the study, says this work shows that questioning conventional wisdom can lead to technological breakthroughs. “These inverted singlet-triplet materials will spur a lot of exciting new developments in the field” of OLEDs, he says.



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