If this stabilization overcomes the exchange energy, Δ E ST could be a negative value (Fig. Because the Pauli exclusion principle restricts the accessible double-excitation configurations in T 1, an effective admixture of such configurations stabilizes S 1 relative to T 1. Recent theoretical studies have also suggested the possibility of negative Δ E ST in these molecules by accounting for double-excitation configurations in which two electrons of occupied orbitals have been promoted out to virtual orbitals 15, 16, 17, 18, 19 (Supplementary Fig. Although there is general agreement that Δ E ST must be positive, potentially negative Δ E ST has been discussed in nitrogen-substituted phenalene analogues, such as cyclazine and heptazine, during the past two decades 12, 13, 14, 15, 16, 17, 18, 19, 20, 21. Δ E ST is simply equal to twice the positive exchange energy if the lowest-energy singlet and triplet excited states (S 1 and T 1) have the same single-excitation configuration 11. Numerous observations of positive Δ E ST in molecular excited states are generally understood by the exchange interaction, the quantum-mechanical effect involving Pauli repulsion, which stabilizes triplet states relative to singlet states 11. Herein, we demonstrate experimental evidence of the existence of highly fluorescent organic molecules that disobey Hund’s rule and possess negative Δ E ST for constructing efficient OLEDs. Alternatively, an ideal case would be thermodynamically favourable downconversion with negative Δ E ST, which is not expected if applying Hund’s multiplicity rule to the lowest-energy excited state. The research community has thus focused on minimizing the singlet–triplet energy gap (Δ E ST) to accelerate the upconversion by thermal activation 7. These bimolecular annihilations lead to the decrease in device efficiency under high current densities, known as efficiency roll-off in OLEDs 8, 9, and also generate high-energy excitons that are suspected to cause chemical degradation of materials, particularly in blue OLEDs 10. Although the concept of TADF has the advantage of eliminating the need for transition metals, the resultant temporally delayed fluorescence typically has a time constant in the microsecond or even millisecond range, which is long enough for detrimental bimolecular annihilations, such as triplet–triplet annihilation and triplet–polaron annihilation, to compete with delayed fluorescence. This class of materials has energetically close singlet and triplet excited states, in which ambient thermal energy upconverts the triplet states into the singlet states through reverse intersystem crossing (RISC). The other uses organic molecules that exhibit thermally activated delayed fluorescence (TADF) 5, 6, 7. The first relies on organometallic complexes with transition metals, such as iridium and platinum, which induce a large spin–orbit coupling to allow triplet states to emit photons as phosphorescence 2, 3, 4. To overcome this issue, two strategies for harvesting the ‘dark’ triplet excited states as photons have been established. This spin statistics limits the internal quantum efficiency of OLEDs and leads to the energy loss owing to the spin-forbidden nature of triplet excited states to emit photons. In the case of OLEDs, recombination of charge carriers leads to the formation of singlet and triplet excited states in a 1:3 ratio. The spin multiplicity of molecular excited states plays a crucial role in organic optoelectronic devices.
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