On the Multiplicity of the Phosphorescent State of Organic Molecules

Azumi, Tohru; O'Donnell, C. M.; McGlynn, S. P.

doi:10.1063/1.1728019

Cited by 84 publications

(48 citation statements)

References 22 publications

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“…( Compare sections 4 and 6 and refs. [2,4,5,[139][140][141] The amount of the energy separation E(S1-T1) has a particularly strong impact on the emission properties of the TADF materials. Accordingly, this feature represents one of the main topics of this study.…”

Section: (T1)mentioning

confidence: 99%

Cu(I) complexes – Thermally activated delayed fluorescence. Photophysical approach and material design

Czerwieniec

Leitl

Homeier

et al. 2016

Coordination Chemistry Reviews

490

653

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Cu(I) complexes often show transitions of distinct metal-to-ligand charge transfer (MLCT) character. This can lead to small energy separations between the lowest singlet S1 and triplet T1 state. Hence, thermally activated delayed fluorescence (TADF) and, if applied to electroluminescent devices, singlet harvesting can become highly effective. In this contribution, we introduce the TADF mechanism and identify crucial parameters that are necessary to optimize materials' properties, in particular, with respect to short emission decay times and high quantum yields at ambient temperature. In different case studies, we present a photophysical background for a deeper understanding of the materials' properties. Accordingly, we elucidate strategies for obtaining high quantum yields. These are mainly based on enhancing the intrinsic rigidity of the complexes and of their environment. Efficient TADF essentially requires small energy separations E(S1-T1) with preference below about 1000 cm 1 (≈ 120 meV). This is achievable with complexes that exhibit small spatial HOMO-LUMO overlap. Thus, energy separations below 300 cm 1 (≈ 37 meV) are obtained, giving short radiative TADF decay times of less than 5 s. In a case study, it is shown that the TADF properties may be tuned or the TADF effect can even be turned off. However, very small E(S1-T1) energy separations are related to small radiative rates or small

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Section: (T1)mentioning

confidence: 99%

Cu(I) complexes – Thermally activated delayed fluorescence. Photophysical approach and material design

Czerwieniec

Leitl

Homeier

et al. 2016

Coordination Chemistry Reviews

490

653

View full text Add to dashboard Cite

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“…(Compare Figure ) In this situation, the decay time τ( T ) of the luminescent molecule is described by a Boltzmann‐type relation (compare also refs. ) [Eq. ]:

true τ ((), T) = \frac{3 + e x p (- \frac{Δ E (() {normalS}_{1} - {normalT}_{1})}{k_{B} T})}{3 k ((), {normalT}_{1}) + k ((), {normalS}_{1}) e x p (- \frac{Δ E (S_{1} - T_{1})}{k_{normalB} T})}

…”

Section: Tadf Molecular Parameters and Diversity Of Materialsmentioning

confidence: 99%

TADF Material Design: Photophysical Background and Case Studies Focusing on Cu^Iand Ag^IComplexes

et al. 2017

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The development of organic light emitting diodes (OLEDs) and the use of emitting molecules have strongly stimulated scientific research of emitting compounds. In particular, for OLEDs it is required to harvest all singlet and triplet excitons that are generated in the emission layer. This can be achieved using the so-called triplet harvesting mechanism. However, the materials to be applied are based on high-cost rare metals and therefore, it has been proposed already more than one decade ago by our group to use the effect of thermally activated delayed fluorescence (TADF) to harvest all generated excitons in the lowest excited singlet state S . In this situation, the resulting emission is an S →S fluorescence, though a delayed one. Hence, this mechanism represents the singlet harvesting mechanism. Using this effect, high-cost and strong SOC-carrying rare metals are not required. This mechanism can very effectively be realized by use of Cu or Ag complexes and even by purely organic molecules. In this investigation, we focus on photoluminescence properties and on crucial requirements for designing Cu and Ag materials that exhibit short TADF decay times at high emission quantum yields. The decay times should be as short as possible to minimize non-radiative quenching and, in particular, chemical reactions that frequently occur in the excited state. Thus, a short TADF decay time can strongly increase the material's long-term stability. Here, we study crucial parameters and analyze their impact on the TADF decay time. For example, the energy separation ΔE(S -T ) between the lowest excited singlet state S and the triplet state T should be small. Accordingly, we present detailed photophysical properties of two case-study materials designed to exhibit a large ΔE(S -T ) value of 1000 cm (120 meV) and, for comparison, a small one of 370 cm (46 meV). From these studies-extended by investigations of many other Cu TADF compounds-we can conclude that just small ΔE(S -T ) is not a sufficient requirement for short TADF decay times. High allowedness of the transition from the emitting S state to the electronic ground state S , expressed by the radiative rate k (S →S ) or the oscillator strength f(S →S ), is also very important. However, mostly small ΔE(S -T ) is related to small k (S →S ). This relation results from an experimental investigation of a large number of Cu complexes and basic quantum mechanical considerations. As a consequence, a reduction of τ(TADF) to below a few μs might be problematic. However, new materials can be designed for which this disadvantage is not prevailing. A new TADF compound, Ag(dbp)(P -nCB) (with dbp=2,9-di-n-butyl-1,10-phenanthroline and P -nCB=bis-(diphenylphosphine)-nido-carborane) seems to represent such an example. Accordingly, this material shows TADF record properties, such as short TADF decay time at high emission quantum yield. These properties are based (i) on geometry optimizations of the Ag complex for a fast radiative S →S rate and (ii) on restricting the extent of geometry reorganiza...

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“…compare Refs. [105,156,157]) and has also been applied to Pt(2-thpy) 2 [65]. From this investigation one obtains a decay rate of k II, 0 = (2.78 ± 0.2) ¥ 10 5 s -1 (t II = 3.6 µs) for substate II.…”

Section: Spin-lattice Relaxation At T = 13 Kmentioning

confidence: 93%

“…(Compare, for example Refs. [105][106][107][108][109][110][111][112]). In several respects the situation is similar for organometallic compounds like Pd(2-thpy) 2 and related complexes.…”

Section: Triplet Populations and Decaysmentioning

confidence: 99%

Transition Metal and Rare Earth Compounds

2000

Topics in Current Chemistry

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The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.Cover design: Friedhelm Steinen-Broo, Barcelona; MEDIO, Berlin Typesetting: Fotosatz-Service K6hler GmbH, 97084 Wiirzburg SPIN: 10719407 02/3020 ra -5 4 3 2 1 0 -Printed on acid-free paper

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On the Multiplicity of the Phosphorescent State of Organic Molecules

Cited by 84 publications

References 22 publications

Cu(I) complexes – Thermally activated delayed fluorescence. Photophysical approach and material design

Cu(I) complexes – Thermally activated delayed fluorescence. Photophysical approach and material design

TADF Material Design: Photophysical Background and Case Studies Focusing on Cu^Iand Ag^IComplexes

Transition Metal and Rare Earth Compounds

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On the Multiplicity of the Phosphorescent State of Organic Molecules

Cited by 84 publications

References 22 publications

Cu(I) complexes – Thermally activated delayed fluorescence. Photophysical approach and material design

Cu(I) complexes – Thermally activated delayed fluorescence. Photophysical approach and material design

TADF Material Design: Photophysical Background and Case Studies Focusing on CuIand AgIComplexes

Transition Metal and Rare Earth Compounds

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Product

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TADF Material Design: Photophysical Background and Case Studies Focusing on Cu^Iand Ag^IComplexes