Control of excited states of organic luminescent molecules has been fundamentally important in their light-related applications such as LEDs, solar cells, sensors, photocatalysts, etc. When neutral and closed-shell organic molecules are optically or electrically excited, spin multiplicity of the excited state will be either singlet or triplet. In general, without any heavy atoms, the singlet excited state is bright and the triplet excited state is dark because of a spin selection rule. Therefore, reverse intersystem crossing from triplet excited states to singlet excited states makes light emission efficient. Energetically low and long-lived triplet excited states are involved in unique photophysical characteristics: singlet exciton fission, triplet–triplet annihilation, delayed fluorescence, etc. We designed and synthesized the organic luminescent molecules showing unique triplet excited states–related phenomena [1].

In molecular excited states, an energy gap between singlet (S1) and triplet (T1) excited states is positive, i.e. energy level of S1 is higher than that of T1. It is because of exchange energy of the two electrons in the excited state. However, here, we report fluorescent molecules that have a negative S1– T1 energy gap of –11 ± 2 meV [4]. The energy inversion of the S1 and T1 excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature owing to the emissive singlet character of the lowest-energy excited state. OLEDs using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating its potential implications for optoelectronic devices, including displays, lighting, and lasers.

[1]Aizawa, Pu et al., "Delayed fluorescence from inverted singlet and triplet excited states", Nature 2022, 609, 502.