Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

Kaminski, Jeremy M.; Rodriguez-Serrano, Angela; Dinkelbach, Fabian; Miranda-Salinas, Hector; Monkman, Andrew P.; Marian, Christel M.

doi:10.1039/d1sc07101g

Cited by 16 publications

(20 citation statements)

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“…Large DF contributions are also observed in many exciplex TADF molecules 29 and especially intramolecular through space TADF systems, where the D and A are physically decoupled spatially but TADF is still driven by second-order vibrational coupled SOC and efficient. 30 In combination with the strongly decoupled nature of the D and A localized excited (LE) states (as demonstrated above in singlet and triplet emission channels), we argue that the CT states in ACRSA are also intramolecular through space charge transfer-like rather than the expected through-bond conjugation. This realization gives deep and novel insight into the nature of "spiro-conjugation" 31 that provides orbital overlap of D and A, in that the spiro-center does not contribute beyond tethering the D and A fragments at distances and orientations that allow for a through-space interaction.…”

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confidence: 51%

“…The delayed CT emission is 10-fold more intense than the prompt CT emission (Figure S12 of the Supporting Information). Large DF contributions are also observed in many exciplex TADF molecules and especially intramolecular through space TADF systems, where the D and A are physically decoupled spatially but TADF is still driven by second-order vibrational coupled SOC and efficient …”

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confidence: 99%

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Donor, Acceptor, and Molecular Charge Transfer Emission All in One Molecule

Franca

Danos

Monkman

2023

J. Phys. Chem. Lett.

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The molecular photophysics in the thermally activated delayed fluorescence (TADF) spiro-acridine–anthracenone compound, ACRSA, is dominated by the rigid orthogonal spirocarbon bridging bond between the donor and acceptor. This critically decouples the donor and acceptor units, yielding photophysics, which includes (dual) phosphorescence and the molecular charge transfer (CT) states giving rise to TADF, that are dependent upon the excitation wavelength. The molecular singlet CT state can be directly excited, and we propose that supposed “spiro-conjugation” between acridine and anthracenone is more accurately an example of intramolecular through-space charge transfer. In addition, we show that the lowest local and CT triplet states are highly dependent upon spontaneous polarization of the environment, leading to energy reorganization of the triplet states, with the CT triplet becoming lowest in energy, profoundly affecting phosphorescence and TADF, as evident by a (thermally controlled) competition between reverse intersystem crossing and reverse internal conversion, i.e., dual delayed fluorescence (DF) mechanisms.

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confidence: 51%

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confidence: 99%

Donor, Acceptor, and Molecular Charge Transfer Emission All in One Molecule

Franca

Danos

Monkman

2023

J. Phys. Chem. Lett.

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“…Long residence times in the excited triplet state make TADF emitters particularly vulnerable to photochemical degradation or triplet–triplet annihilation . One strategy to accelerate the rISC process is to devise donor–acceptor compounds with an energetically proximate locally excited (LE) state that mediates the interconversion of 1 CT and 3 CT states by spin–vibronic coupling. ,− Another strategy involves the implementation of transition metals with d 10 configuration in the electronic ground state to increase SOC. In these complexes, low-lying metal-centered (MC) d–d* excitations that cause metal–ligand bond dissociation in d 6 and d 8 systems and eventually lead to the degradation of the OLED dye are absent.…”

Section: Introductionmentioning

confidence: 99%

Finding Design Principles of OLED Emitters through Theoretical Investigations of Zn(II) Carbene Complexes

2022

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In this work, Zn(II) carbene complexes carrying a dianionic 1,2-dithiolbenzene (dtb) or 1,2-diolbenzene (dob) ligand were investigated regarding their suitability as organic light-emitting diode (OLED) emitter. For the optimization of the complexes, density functional-based methods were used and frequency analyses verified the obtained structures as minima. All calculations were carried out including a polarizable continuum model to mimic solvent−solute interactions. Multireference configuration interaction methods were used to determine excitation energies, spin-orbit couplings, and luminescence properties. Rate constants of spinallowed and spin-forbidden transitions were calculated according to a Fermi golden rule expression. Using carbene ligands with varying σ-donor and π-acceptor strengths, the luminescence is found to be tunable from yellow to orange/red to deep red/nearinfrared. The calculated intersystem crossing (ISC) time constants indicate thermally activated delayed fluorescence (TADF) to be the main decay channel. In contrast to many d 10 coinage metal complexes, a parallel orientation of dtb or dob and the carbene ligand is found to be highly favorable. For the complexes with a cyclic (alkyl)(amino) carbene (CAAC) or cyclic (amino)(aryl) carbene (CAArC) ligand, the S 1 and T 1 states have ligand-to-ligand charge-transfer (LLCT) character and are energetically close. The complex with a classical N-heterocyclic carbene (NHC) ligand has S 1 and T 1 states with mixed ligand-to-metal charge-transfer (LMCT)/LLCT character and is a very rare example in which the zinc ion contributes to the excitation.

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“…[23] The triplet value of the individual donor and acceptor units have also been reported to have much higher values (TD,A > 3 eV). [34] In addition, TpAT-tFFO which is incapable of having a QA conformer, has been reported to have a 3 CT character lowest triplet state, [39] and its value depends on the distance between the D-A units. This experimentally has been difficult to prove and the lowest lying triplet energy was calculated to be above 3 eV.…”

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confidence: 99%

Unexpected quasi-axial conformer in thermally activated delayed fluorescence DMAC-TRZ. Demonstrating how to turn green OLEDs into blue.

Stavrou

Franca

Böhmer

et al. 2022

Preprint

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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%.

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Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

Abstract: Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three...

Cited by 16 publications

References 42 publications

Donor, Acceptor, and Molecular Charge Transfer Emission All in One Molecule

Donor, Acceptor, and Molecular Charge Transfer Emission All in One Molecule

Finding Design Principles of OLED Emitters through Theoretical Investigations of Zn(II) Carbene Complexes

Unexpected quasi-axial conformer in thermally activated delayed fluorescence DMAC-TRZ. Demonstrating how to turn green OLEDs into blue.

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