Delayed Fluorescence from Inverted Singlet and Triplet Excited States for Efficient Organic Light-Emitting Diodes

Abstract

Hund’s multiplicity rule states that for a given electronic configuration, a higher spin state has a lower energy. Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, which has been consistent with numerous experimental observations over almost a century. Here, we report a fluorescent molecule that disobeys Hund’s rule, possessing a negative singlet–triplet energy gap of –11 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature due to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating potential implications for optoelectronic devices, including displays, lighting, and lasers.

On the origin of the inverted singlet–triplet gap of the 5th generation light-emitting molecules

“…Results obtained in experimental studies on chemical derivatives of this class of molecules agree with the theoretically predicted reverse order of the lowest singlet and triplet states. 3,6,11 The inverted singlettriplet gap, often abbreviated INVEST, makes these molecules interesting not only from fundamental research point of view, but also as promising candidate molecules for use in photophysical applications.…”

Origin of the Failure of Density Functional Theories in Predicting Inverted Singlet–Triplet Gaps

“… 30 Recently, Miyajima et al experimentally confirmed inverted singlet–triplet gaps in heptazine derivatives. 31 While many correlated wave function-based electronic structure methods have predicted inverted singlet–triplet gaps qualitatively correctly, one of the most widely used excited-state electronic structure method, LR-TDDFT, has failed to predict the inversion of singlet–triplet gaps. Computational studies, in the past, pointed toward the inability of LR-TDDFT to incorporate double excitations by going beyond adiabatic approximation 32 as the source of this error.…”

Origin of The Failure of Density Functional Theories in Predicting Inverted Singlet-Triplet Gaps

“…However, some N-doped triangulate molecules have been shown to violate Hund's rule by having inverted singlet-triplet gaps (ΔEST < 0) . 15,16 These materials benefit from the efficient reverse intersystem crossing process from T1 to S1 state, leading to substantial fluorescence rates, thus, in turn, suitable candidates for TADF materials.…”