1. Prediction of spin-flip rates

Reverse intersystem crossing (RISC), the uphill spin-flip process from a triplet to a singlet excited state, plays a key role in a wide range of photochemical applications. Understanding and predicting the kinetics of such processes in vastly different molecular structures would facilitate the rational material design. We have proposed a theoretical expression that successfully reproduces experimental RISC rate constants ranging over five orders of magnitude in twenty different molecules. We have demonstrated that the spin flip occurs across the singlet–triplet crossing seam involving a higher-lying triplet excited state where the semi-classical Marcus parabola is no longer valid. Nat. Commun. 11, 3909 (2020). Highlighted in ChemStation (ケムステ)

2. Inverted singlet and triplet excited states

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 singlet and triplet excited states. We have discovered a highly fluorescent molecule that disobeys Hund’s rule and possesses a negative singlet–triplet energy gap of –11 ± 2 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 lowering the temperature due to the emissive singlet character of the lowest-energy excited state. OLEDs using the molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating its potential implications for optoelectronic devices including displays, lightings, and lasers. Nature 609, 502–506 (2022). Highlighted in Nature N&V.Highlighted in ChemStation (ケムステ)