High-Lying Triplet Excitons of Thermally Activated Delayed Fluorescence Molecules

We investigate high-lying triplet excitons involving a transition from the highest-occupied molecular orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO) for 18 thermally activated delayed fluorescence (TADF) molecules, within the first-principles one-shot GW+Bethe–Salpeter method. On the basis of our exciton analysis using the exciton wave functions, detailed exciton features are discussed in terms of exciton size, exciton binding energy, electron–hole separation distance, exciton map, and the overlap strength between the electron and hole wave functions. Contrary to our expectation, no exciton that could be purely classified as a charge-transfer exciton is found in our exciton map; moreover, the energy difference between the lowest singlet exciton and the high-lying triplet exciton is nearly zero for some TADF molecules. Our simulation strongly suggests that the hot-exciton process involving a high-lying triplet exciton is more likely to occur in the TADF mechanism than the conventionally considered process between the lowest singlet and triplet excitons, and our results support those of recent experiments. We propose a new method for calculating the energy difference between singlet and triplet excitons from the expectation value of the exchange bare Coulomb interaction and demonstrate that the combined use with exciton map is efficient and accurate for screening TADF molecules

Reverse Stability of Oxyluciferin Isomers in Aqueous Solutions

We investigated the stability of oxyluciferin anions (keto, enol, and enolate isomers) in aqueous solution at room temperature by performing a nanosecond time scale first-principles molecular dynamics simulation. In contrast to all previous quantum chemistry calculations, which suggested the keto-type to be the most stable, we show that the enol-type is slightly more stable than the keto-type, in agreement with some recent experimental studies. The simulation highlights the remarkable hydrophobicity of the keto-type by the cavity formed at the oxyluciferin–water interface as well as a reduction in hydrophobicity with the number of hydrating water molecules. It is therefore predicted that the isomeric form in a hydrated cluster is size-dependent

First-Principles Investigation of Strong Excitonic Effects in Oxygen 1s X‑ray Absorption Spectra

We calculated the oxygen 1s X-ray absorption spectra (XAS) of acetone and acetic acid molecules in vacuum by utilizing the first-principles <i>GW</i>+Bethe–Salpeter method with an all-electron mixed basis. The calculated excitation energies show good agreement with the available experimental data without an artificial shift. The remaining error, which is less than 1% or 2–5 eV, is a significant improvement from those of time-dependent (TD) density functional methods (5% error or 27–29 eV for TD-LDA and 2.4–2.8% error or 13–15 eV for TD-B3LYP). Our method reproduces the first and second isolated peaks and broad peaks at higher photon energies, corresponding to Rydberg excitations. We observed a failure of the one-particle picture (or independent particle approximation) from our assignment of the five lowest exciton peaks and significant excitonic or state-hybridization effects inherent in the core electron excitations

Reverse Stability of Oxyluciferin Isomers in Aqueous Solutions

We investigated the stability of oxyluciferin anions (keto, enol, and enolate isomers) in aqueous solution at room temperature by performing a nanosecond time scale first-principles molecular dynamics simulation. In contrast to all previous quantum chemistry calculations, which suggested the keto-type to be the most stable, we show that the enol-type is slightly more stable than the keto-type, in agreement with some recent experimental studies. The simulation highlights the remarkable hydrophobicity of the keto-type by the cavity formed at the oxyluciferin–water interface as well as a reduction in hydrophobicity with the number of hydrating water molecules. It is therefore predicted that the isomeric form in a hydrated cluster is size-dependent