Thermally activated delayed fluorescence as a cycling process between excited singlet and triplet states: Application to the fullerenes

Baleizão, Carlos; Berberan‐Santos, Mário N.

doi:10.1063/1.2734974

Cited by 115 publications

(157 citation statements)

References 31 publications

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“…From figure 3a, the reverse intersystem crossing rate constant is determined using equation (4.1) [9,10,19] Figure 3b shows the variation of the steady-state delayed fluorescence I SS TADF at room temperature as a function of excitation dose, plotted in log/log scale, with a clear gradient 1, in agreement with equation (3.6) for the TADF mechanism.…”

Section: Resultssupporting

confidence: 50%

“…The efficiency of the TADF mechanism is controlled by two main parameters, the energy splitting between the singlet and triplet states ( E ST ) and the efficiency of non-radiative pathways available for the excited singlet and triplet states to decay [9,10]. Minimization of E ST has been achieved by designing materials that show intramolecular excited states with strong charge transfer character (ICT).…”

Section: Introductionmentioning

confidence: 99%

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Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

Dias

2015

Phil. Trans. R. Soc. A.

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The kinetics of thermally activated delayed fluorescence (TADF) is investigated in dilute solutions of organic materials with application in blue lightemitting diodes (OLEDs). A method to accurately determine the energy barrier ( E a ) and the rate of reverse intersystem crossing (k Risc ) in TADF emitters is developed, and applied to investigate the tripletharvesting mechanism in blue-emitting materials with large singlet-triplet energy gap ( E ST ). In these materials, triplet-triplet annihilation (TTA) is the dominant mechanism for triplet harvesting; however, above a threshold temperature TADF is able to compete with TTA and give enhanced delayed fluorescence. Evidence is obtained for the interplay between the TTA and the TADF mechanisms in these materials. IntroductionThermally activated delayed fluorescence (TADF) has recently become one of the favorite methods to overcome the 25% limitation on the internal quantum efficiency of organic light-emitting diodes (OLEDs) due to spin statistics [1,2]. The ratio (1 : 3) between singlet and triplet excited states created from charge recombination in OLEDs imposes that only 25% of the charge recombination events will give origin to emissive singlet states, the rest is wasted by other non-radiative processes [3]. This imposes a serious limitation for the application of organic materials in displays and lighting devices [4]. Different methods have been used when trying to overcome this limitation, including the use of organic phosphors containing Ir(III), Pt(II) or other heavy metals, which can convert dark triplet states into emissive 2015 The Author(s) Published by the Royal Society. All rights reserved. species with 100% efficiency due to the enhanced intersystem crossing via the heavy atom effect. These emitting complexes have to be dispersed in charge-transporting host materials with relatively higher energy triplet levels to avoid emission quenching. Finding appropriate host materials for emitters in the blue region is difficult. Also, the performance of blue-emitting phosphors is affected by substantial degradation, thus obtaining stable, long lifetime and deep blue-emitting phosphors has been difficult. Owing to these reasons, deep blue phosphorescent OLEDs with long operation lifetime have not yet been demonstrated [5]. Moreover, heavy metals are not abundant elements in nature, which will cause unnecessary stress on the fabrication costs of these devices.Using triplet-triplet annihilation (TTA) to up-convert non-emissive triplet states to emissive singlet states has also been attempted. In this case, the yield of singlet emissive states is highly dependent on the relative energy order of the excited singlet and triplet energy levels in the molecule, and the maximum total singlet yield is limited to 62.5%. OLEDs with TTA contribution as large as 40% have been demonstrated but the relative merit of this approach is still unsatisfactory [6,7].The TADF mechanism uses the thermal energy to assist reverse intersystem crossing and promote the up-conversion of lowe...

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Section: Resultssupporting

confidence: 50%

Section: Introductionmentioning

confidence: 99%

Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

Dias

2015

Phil. Trans. R. Soc. A.

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“…The (dominant) TADF contribution to the overall DPTZ‐DBTO2 emission is directly determined by comparing the emission in aerated and degassed solutions. In MCH, the recycled triplets3 contribute around 91% to the total DPTZ‐DBTO2 fluorescence.…”

Section: Resultsmentioning

confidence: 99%

“…In most cases, it is also observed that

k_{ISC}^{normalS} > > k_{RISC}

and

k_{ISC}^{normalT} > > k_{PH}

. Interconversion of the singlet and triplet emissive states then occurs many times before photon emission or nonradiative decay can take place 3. Very strong TADF is clearly occurring in DPTZ‐DBTO2 in MCH, and the yield of excited singlet states formed via reverse intersystem crossing,

Φ_{normalS}^{normalT}

, can be assumed to be close to 1.…”

Section: Resultsmentioning

confidence: 99%

“…Triplet state harvesting via this mechanism leads to thermally activated delayed fluorescence (TADF) or E‐type delayed fluorescence, and overcomes the spin statistical 25% limit on the singlet production yield arising from direct charge recombination 2. The efficiency of the RISC mechanism, and therefore the contribution of TADF to the overall emission, increases when two conditions are fulfilled: (i) the nonradiative internal conversion pathways, available for the excited singlet (S 1 ) and triplet (T 1 ) states, are suppressed and (ii) a small energy splitting between the lowest singlet and triplet states is achieved 3. These two conditions require molecular structures with strong intramolecular charge transfer (CT) character, where excited state molecular conformations break the conjugation between the donor (D) and acceptor (A) units 4…”

Section: Introductionmentioning

confidence: 99%

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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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Fluorescence Sensing of Temperature and Oxygen with Fullerenes

Berberan‐Santos

Baleizão

2012

Molecules at Work

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Thermally activated delayed fluorescence as a cycling process between excited singlet and triplet states: Application to the fullerenes

Cited by 115 publications

References 31 publications

Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

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

Fluorescence Sensing of Temperature and Oxygen with Fullerenes

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