Singlet-Triplet Inversions in Through-Bond Charge-Transfer States

02 August 2024, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Molecules where the lowest excited singlet state is lower in energy than the lowest triplet are highly promising for a number of organic materials applications as efficiency limitations stemming from spin statistics are overcome. All molecules known to possess such singlet-triplet inversions exhibit a pattern of spatially alternating but non-overlapping HOMO and LUMO orbitals, meaning the lowest excited states are of a local character. Here, we demonstrate that derivatives of the bicyclic hydrocarbon calicene exhibit Hund’s rule violations in charge-transfer (CT) states between its rings. These CT states can be tuned with substituents, so that the first excited singlet and triplet state are energetically inverted. This provides a conceptual connection between the emerging fields of inverted gap molecules and existing molecular design rules for state-of-the art thermally-activated delayed fluorescence materials.

Keywords

inverted singlet-triplet gap
Hund's rule violation
organic light-emitting diode
thermally-activated delayed fluorescence
aromaticity

Supplementary materials

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Supporting Information
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Additional figures and data tables
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XYZ files
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XYZ coordinate files of optimized structures
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