Excited-state Violations of Hunds Rule in Molecular Emitters

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Marc Hamilton Folkmann Garner


DKK 970,000




Internationalisation Fellowships


Non-emissive triplet states limit the technological applications of fluorescent molecules. Recently, few cases of molecules have been found where Hund's rule of maximum multiplicity is violated because singlet excited states have lower energy than triplet states. Such molecules may have extremely desirable fluorescent properties, but currently only one class of molecules have been identified. It is the goal of this project to identify new types of molecules where the excited states violate Hund's rule.


Hund's rule is a widely accepted fundamental rule in quantum chemistry and atomic physics, which governs the energetic order of electronic states of molecules. Until recently, practically no violations were known in experimentally relevant molecules. We will explore how (un)common deviations from Hund's rule are in excited states of molecules, and we aim to improve the basic understanding of these violations. If we identify new exceptions, the broad applicability of Hund's rule may eventually need to be redefined.


In this project we will simulate the electronic properties of molecules using state-of-the-art computational chemistry methods. We suspect violations of Hund's rule may have been overlooked historically because many computational methods cannot accurately predict the energy difference between the singlet and triplet excited states. By identifying new cases of violations of Hund's rule, we aim to improve the basic theoretical understanding of the phenomenon. This understanding can form the basis for making criteria that will allow us to screen many different types of molecules that may have these unusual excited-state properties.

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