Molecule that breaks the nearly 100-year-old Hund’s Rule opens path for more efficient organic LEDs

HzTFEX2 exhibiting blue fluorescence in solution (left) and its molecular structure (right).

As part of a team led by researchers at the RIKEN Center for Emergent Matter Science, ICReDD members co-authored a study reporting a material that breaks the nearly 100-year-old Hund’s Rule and rewrites the textbook on fluorescent materials. The breakthrough may lead to the development of highly efficient organic LEDs, which are used in various applications, including smartphone displays and lighting.

Electrons in an atom or molecule occupy orbitals, and each orbital can hold two electrons of opposing spin direction. Hund’s rule specifies that if there are open orbitals available, it requires less energy for electrons to occupy separate orbitals and have the same spin direction as each other (a “triplet state”) than for them to adopt a configuration where they  have opposed spin directions (a “singlet state”).

Researchers report here on a material for which Hund’s rule is broken and the singlet state is lower in energy than the triplet state. The excited state of conventional electroluminescent materials is dominated by the triplet state, which doesn’t emit light, and can’t convert to the higher energy, light-emitting singlet state. This results in a major loss of energy when running devices. For the novel material reported here, HzTFEX2, the singlet is lower in energy, so the triplet state can quickly convert to the singlet state, enabling much greater light emission efficiency.

To identify candidate molecules, researchers first computationally screened tens of thousands of ligand combinations on a heptazine backbone. The results were filtered for blue light fluorescence and synthetic feasibility before performing higher level calculations and choosing molecules for experimental evaluation.

Experimental characterization showed that HzTFEX2 exhibited delayed blue fluorescence with unique properties, including a short, sub-microsecond emission time constant and decreasing fluorescence lifetime as temperature decreased. An OLED made using HzTFEX2 exhibited a impressive 17% external quantum efficiency, showing the potential applicability of the new material in LEDs. Other materials based on heptazine were also shown to break Hund’s rule, opening up a new class of materials to explore.