Mechanism of Intersystem Crossing of Thermally Activated Delayed Fluorescence Molecules

Ogiwara, Toshinari; Wakikawa, Yusuke; Ikoma, Tadaaki

doi:10.1021/acs.jpca.5b02253

Cited by 120 publications

(144 citation statements)

References 19 publications

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“…Hyperfine coupling can, in principle, interconvert 1 CT to 3 CT and back,16 followed by internal conversion from 3 CT to 3 LE. However, ISC due to weak hyperfine interactions can only occur when the exchange energy, i.e., approximately the energy difference between 1 CT and 3 CT states, is smaller than the typical hyperfine energy of a large organic system, typically 1 to 20 μeV.…”

Section: Resultsmentioning

confidence: 99%

The Role of Local Triplet Excited States and D‐A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

et al. 2016

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Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ‐DBTO2, showing thermally activated delayed fluorescence (TADF), with near‐orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that DPTZ‐DBTO2 has minimal singlet–triplet energy splitting due to its near‐rigid molecular geometry. However, the electronic coupling between the local triplet (3LE) and the charge transfer states, singlet and triplet, (1CT, 3CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet 1CT state couples directly to the near‐degenerate 3LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in DPTZ‐DBTO2 shifts below (the static) 3LE, leading to decreased TADF efficiencies. The relatively large energy difference between the 1CT and 3LE states and the extremely low efficiency of the 1CT to 3CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between 1CT and 3LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.

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Section: Resultsmentioning

confidence: 99%

The Role of Local Triplet Excited States and D‐A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

et al. 2016

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“…Moreover, phenoxazine has been less extensively explored when compared with carbazole and fluorene in OLED materials. [18][19][20][21][22] To the best of our knowledge, this is the first time that the dimethyl TXO2 acceptor unit has been incorporated into a D-A-D molecule: it has a strongly folded and constrained structure which is in contrast to the very flexible diphenylsulfone 23 and the planar and rigid dibenzothiophene-S,S-dioxide units, 5 which are both well-established in TADF molecules. In comparison with dibenzothiophene-S,S-dioxide, the conjugation between the two phenyl rings of TXO2 is disrupted by the dimethylmethylene bridge.…”

Section: Introductionmentioning

confidence: 95%

Engineering the singlet–triplet energy splitting in a TADF molecule

et al. 2016

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The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The key to engineering an efficient TADF emitter is to achieve a small energy splitting between a pair of molecular singlet and triplet states. This work makes important contributions towards achieving this goal. By studying the new TADF emitter 2,7-bis(phenoxazin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DPO-TXO2) and the donor and acceptor units separately, the available radiative and non-radiative pathways of DPO-TXO2 have been identified. The energy splitting between singlet and triplet states was clearly identified in four different environments, in solutions and solid state. The results show that DPO-TXO2 is a promising TADF emitter, having ∆EST = 0.01 eV in zeonex matrix. We further show how the environment plays a key role in the fine tuning of the energy levels of the 1 CT state with respect to the donor 3 LED triplet state, which can then be used to control the ∆EST energy value. We elucidate the TADF mechanism dynamics when the 1 CT state is located below the 3 LE triplet state which it spin orbit couples to, and we also discuss the OLED device performance with this new emitter, which shows maximum external quantum efficiency (E.Q.E) of 13.5% at 166 cd/m 2 .

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“…[11][12][13][14] In addition to these exciting applications of TADF, the fundamental processes in TADF are also attracting considerable interest in both experimental and theoretical fields. [15][16][17][18][19][20][21][22][23][24][25][26] In this case, the goal is to achieve a sufficient understanding of these processes to realize a high RISC rate (k RISC ) by judicious designing and optimizing the most suitable chemical structures, emission environments, and so on. Increasing the k RISC suppresses triplet state quenching and is, therefore, beneficial for reducing roll-off in OLEDs and organic laser diodes.…”

Section: Introductionmentioning

confidence: 99%

Solvent-dependent investigation of carbazole benzonitrile derivatives: does the LE3−CT1 energy gap facilitate thermally activated delayed fluorescence?

et al. 2018

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, "Solvent-dependent investigation of carbazole benzonitrile derivatives: does the 3 LE − 1 CT energy gap facilitate thermally activated delayed fluorescence?," J. Photon. Energy 8(3), 032102 (2018), doi: 10.1117/1.JPE.8.032102. Abstract. The photophysical properties of six types of carbazole benzonitrile (CzBN) derivatives are investigated in different solvents to examine the thermally activated delayed fluorescence (TADF) activation via reducing the energy gap between the singlet charge-transfer and triplet locally excited states, ΔE STðLEÞ . Relative to the ΔE STðLEÞ values for the CzBN derivatives in the low polarity solvent toluene (ε ∼ 2), a reduction of ΔE STðLEÞ for the CzBN derivatives in the polar solvent acetonitrile (ε ∼ 37) was confirmed while maintaining fairly constant ΔE ST values. Notably, TADF activation was observed in acetonitrile for some CzBN derivatives that are TADF inactive in toluene. A numerical analysis of various rate constants revealed the cause of TADF activation as an increase in the reverse intersystem crossing rate and a suppression of the non-radiative decay rate of the triplet states. The positive effect of ΔE STðLEÞ was limited, however, as an excessive decrease in ΔE STðLEÞ facilitates the nonradiative deactivation of the triplet states, leading to a loss of the TADF efficiency. This paper shows that ΔE STðLEÞ provides a measure of TADF activation and that appropriate regulation of ΔE STðLEÞ is required to achieve high TADF efficiency.

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Mechanism of Intersystem Crossing of Thermally Activated Delayed Fluorescence Molecules

Cited by 120 publications

References 19 publications

The Role of Local Triplet Excited States and D‐A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

The Role of Local Triplet Excited States and D‐A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

Engineering the singlet–triplet energy splitting in a TADF molecule

Solvent-dependent investigation of carbazole benzonitrile derivatives: does the LE3−CT1 energy gap facilitate thermally activated delayed fluorescence?

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