The interplay of thermally activated delayed fluorescence (TADF) and room temperature organic phosphorescence in sterically-constrained donor–acceptor charge-transfer molecules

Ward, Jonathan S.; Nobuyasu, Roberto S.; Batsanov, Andrei S.; Data, Przemysław; Monkman, Andrew P.; Dias, Fernando B.; Bryce, Martin R.

doi:10.1039/c5cc09645f

Cited by 221 publications

(249 citation statements)

References 27 publications

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“…Therefore, the two coupling terms driving this dynamics are the SOC and the vibronic coupling elements. This explains recent experimental results which demonstrated that steric hindrance of D–A dihedral angle switches the main pathway from TADF to phosphorescent . This steric hindrance is equivalent to removing the vibronic coupling term, which is shown herein to be strongest along modes exhibiting a distortion of the D–A dihedral angle.…”

Section: Figuresupporting

confidence: 80%

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The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

2016

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Factors influencing the rate of reverse intersystem crossing (k rISC) in thermally activated delayed fluorescence (TADF) emitters are critical for improving the efficiency and performance of third‐generation heavy‐metal‐free organic light‐emitting diodes (OLEDs). However, present understanding of the TADF mechanism does not extend far beyond a thermal equilibrium between the lowest singlet and triplet states and consequently research has focused almost exclusively on the energy gap between these two states. Herein, we use a model spin‐vibronic Hamiltonian to reveal the crucial role of non‐Born‐Oppenheimer effects in determining k rISC. We demonstrate that vibronic (nonadiabatic) coupling between the lowest local excitation triplet (3LE) and lowest charge transfer triplet (3CT) opens the possibility for significant second‐order coupling effects and increases k rISC by about four orders of magnitude. Crucially, these simulations reveal the dynamical mechanism for highly efficient TADF and opens design routes that go beyond the Born‐Oppenheimer approximation for the future development of high‐performing systems.

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Section: Figuresupporting

confidence: 80%

“…It is also important within a device context for the roll‐off efficiency of TADF OLEDs . Further supporting the breakdown of the thermal equilibrium representation, Ward et al . recently showed that different D–A–D molecules with very similar energy gaps (Δ E

_{S_{1} - T_{1}}

) exhibit large variations in k rISC .…”

Section: Figurementioning

confidence: 82%

The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

2016

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“…The lower energy, broad induced features are confirmed as induced CT absorption. Furthermore, the rigidity and polarity of the host has a major effect on rISC and hence TADF, also indicates that intramolecular motion of the D–A–D molecule is critical in the TADF process, especially radiative emission37. Three distinct regimes for TADF are identified dependent on the relative energetic positions of the CT and 3 LE states.…”

Section: Discussionmentioning

confidence: 95%

Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence

et al. 2016

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Knowing the underlying photophysics of thermally activated delayed fluorescence (TADF) allows proper design of high efficiency organic light-emitting diodes. We have proposed a model to describe reverse intersystem crossing (rISC) in donor–acceptor charge transfer molecules, where spin–orbit coupling between singlet and triplet states is mediated by one of the local triplet states of the donor (or acceptor). This second order, vibronically coupled mechanism describes the basic photophysics of TADF. Through a series of measurements, whereby the energy ordering of the charge transfer (CT) excited states and the local triplet are tuned in and out of resonance, we show that TADF reaches a maximum at the resonance point, substantiating our model of rISC. Moreover, using photoinduced absorption, we show how the populations of both singlet and triplet CT states and the local triplet state change in and out of resonance. Our vibronic coupling rISC model is used to predict this behaviour and describes how rISC and TADF are affected by external perturbation.

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“…This additional loss channel in 3,7-DPTZ-DBTO 2 is responsible for the reduced device performance compared to 2,8-DPTZ-DBTO 2 because CT excitations are created directly via charge-recombination and so either the quasi-equatorial CT or quasi-axial CT state is populated statistically during charge recombination in a device. This explains why in other D–A–D systems with both PTZ donors in the H-extra conformer no DF at all is observed because of this large S-T gap and low-lying axial triplet ‘trap'27. In general, a further result of the required vibronic coupling of the 1 CT and 3 LE states for efficient rISC and ISC means that when the S–T gap is very small, near zero, the CT and 3 LE states mix very effectively and the lifetime of the CT state increases through this strong state mixing.…”

Section: Discussionmentioning

confidence: 98%

Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters

et al. 2017

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Regio- and conformational isomerization are fundamental in chemistry, with profound effects upon physical properties, however their role in excited state properties is less developed. Here two regioisomers of bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide, a donor–acceptor–donor (D–A–D) thermally-activated delayed fluorescence (TADF) emitter, are studied. 2,8-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide exhibits only one quasi-equatorial conformer on both donor sites, with charge-transfer (CT) emission close to the local triplet state leading to efficient TADF via spin-vibronic coupling. However, 3,7-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide displays both a quasi-equatorial CT state and a higher-energy quasi-axial CT state. No TADF is observed in the quasi-axial CT emission. These two CT states link directly to the two folded conformers of phenothiazine. The presence of the low-lying local triplet state of the axial conformer also means that this quasi-axial CT is an effective loss pathway both photophysically and in devices. Importantly, donors or acceptors with more than one conformer have negative repercussions for TADF in organic light-emitting diodes.

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The interplay of thermally activated delayed fluorescence (TADF) and room temperature organic phosphorescence in sterically-constrained donor–acceptor charge-transfer molecules

Cited by 221 publications

References 27 publications

The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules

Revealing the spin–vibronic coupling mechanism of thermally activated delayed fluorescence

Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters

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