Hidden photophysics is elucidated in the very well‐known thermally activated delayed fluorescence (TADF) emitter, DMAC‐TRZ. A molecule that, based on its structure, is considered not to have more than one structural conformation. However, based on experimental and computational studies, two conformers, a quasi‐axial (QA) and a quasi‐equatorial (QE) are found, and the effect of their co‐existence on both optical and electrical excitation isexplored. The relative small population of the QA conformer has a disproportionate effect because of its strong local excited state character. The energy transfer efficiency from the QA to the QE conformer is high, even at low concentrations, dependent on the host environment. The current accepted triplet energy of DMAC‐TRZ is shown to originate from the QA conformer, completely changing the understanding of DMAC‐TRZ. The contribution of the QA conformer in devices helps to explain the good performance of the material in non‐doped organic light‐emitting diodes (OLEDs). Moreover, hyperfluorescence (HF) devices, using v‐DABNA emitter show direct energy transfer from the QA conformer to v‐DABNA, explaining the relatively improved Förster resonance energy transfer efficiency compared to similar HF systems. Highly efficient OLEDs where green light (TADF‐only devices) is converted to blue light (HF devices) with the maximum external quantum efficiency remaining close to 30% are demonstrated.

Although there have been several studies on the mechanism of thermally activated delayed fluorescence (TADF), providing useful models to understand the behaviour of TADF molecules, some photophysical features cannot be explained. Here, we investigate the hidden phenomena taking place in the very well-known TADF emitter, DMAC-TRZ. A molecule that, based on its structure, was considered not to fulfil the required criteria to have more than one ground and excited state conformation. However, based on experimental and computational studies, we have found two conformers, a quasi-axial (QA) and a quasi-equatorial (QE), and explored the effect of their co-existence on both optical and electrical excitation. Although the relative population of the QA conformer appears to be small, its effect is disproportionate because it has high local excited state character. The energy transfer efficiency from the QA to the QE conformer is shown to be high, even at low concentrations, and changes depending on the hosting environment. The currently known triplet energy of DMAC-TRZ quoted from experiment is shown to originate from the QA conformer, completely changing the understanding we have so far for this donor-acceptor molecule. The contribution of the QA conformer in devices has been explored and its presence explains the good performance of the material in neat emissive layer devices. Moreover, hyperfluorescnece (HF) devices, using v-DABNA as the terminal emitter show direct energy transfer from the QA conformer to v-DABNA, explaining the relative improved Förster resonance energy transfer (FRET) efficiency compared to similar HF systems. Using this approach, we demonstrate highly efficient organic light emitting diodes (OLEDs) were green light (TADF only devices) is converted to blue light (HF devices) with the maximum external quantum efficiency remaining high and close to 30%.

Recent experimental studies of a spiro-linked anthracenone (A)–naphthalene (N) compound (AN) in butyronitrile solution [Dobkowski et al., J. Phys. Chem. A 2019, 123, 6978] proposed an excited-state energy dissipation pathway {1ππ*(N)+1ππ*(A)}→1nπ*(A)→3nπ*(A)→3ππ*(N). However, a detailed theoretical study employing combined density functional theory and multireference configuration interaction methods, performed in the present work, suggests that the photoexcitation decay follows a different pathway. In butyronitrile solution, the intersystem crossing (ISC) follows the well-established El-Sayed rule and involves the 3ππ*(A) state which is found to be the lowest excited triplet state localized on the anthracenone moiety. Because the Dexter triplet excitation energy transfer (TEET) to the first excited triplet state of the naphthalene subunit is forbidden in C2v symmetry, it is mandatory to go beyond the Condon approximation in modeling this process. Non-adiabatic coupling matrix elementswere computed to obtain a TEET rate different from zero. Our calculations yield time constants of 5 ps for the 1nπ*(A)→3ππ*(A) ISC and of 3 ps for the subsequent 3ππ*(A)→3ππ*(N) TEET in butyronitrile whereas the energy dissipation involving the 3nπ*(A) state as an intermediate occurs on a much longer time scale.