Extracting Design Principles for Efficient Thermally Activated Delayed Fluorescence (TADF) from a Simple Four-State Model

Silva, Piotr de; Kim, Changhae Andrew; Zhu, Tianyu; Voorhis, Troy Van

doi:10.26434/chemrxiv.7854968

Cited by 28 publications

(39 citation statements)

References 13 publications

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“…Interestingly, and independently of any of the method selected, a negative singlettriplet energy difference ∆E ST (roughly up to -0.3 eV) is predicted at all levels for compounds 2T-a, 2T-b, 3T-a, 3T-b, 4T-c, 4T-d, 4T-e, 4T-f, 4T-i, and 4T-j. Note that an inverted ∆E ST is in agreement with previous (simplified) models involving four spin-mixed diabatic states representing pure charge transfer and local excitations, 77 as well as with former findings in exciplex (dimer) systems. 78 For 4T-a and 4T-h we obtain a very small ∆E ST around 0.0 eV, and only compounds 4T-b and 4T-g gave rise to ∆E ST (positive) values around 0.3-0.4 eV.…”

Section: Application Of Ab Initio Methodssupporting

confidence: 91%

Negative Singlet–Triplet Excitation Energy Gap in Triangle-Shaped Molecular Emitters for Efficient Triplet Harvesting

et al. 2021

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The full harvesting of both singlet and triplet excitons can pave the way towards more efficient molecular light-emission mechanisms (i.e., TADF or Thermally Activated Delayed Fluorescence) beyond the spin statistics limit. This TADF mechanism benefits from low (but typically positive) singlet-triplet energy gaps or ∆E ST . Recent research has suggested the possibility of inverting the order of the energy of lowest singlet and triplet excited-states, thus opening new pathways to foster light emission without any energy barrier through triplet to singlet conversion, which is systematically investigated here by means of theoretical methods. To this end, we have selected a set of heteroatom-substituted triangle-shaped molecules (or triangulenes) for which ∆E ST < 0 is predicted. We successfully rationalize the origin of that energy inversion, and the reasons for which theoretical methods might produce qualitatively inconsistent predictions depending on how they treat n-tuple excitations (e.g., the large contribution of double excitations for all the ground-and excited-states involved). Unfortunately, the TD-DFT method is unable to deal with the physical effects driving this behaviour, which prompted us to the use here of more sophisticated ab initio methods such as SA-CASSCF, SC-NEVPT2, SCS-CC2, and SCS-ADC(2).

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Section: Application Of Ab Initio Methodssupporting

confidence: 91%

Negative Singlet–Triplet Excitation Energy Gap in Triangle-Shaped Molecular Emitters for Efficient Triplet Harvesting

et al. 2021

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“…For instance, it has been experimentally demonstrated that intersystem crossing processes can be greatly affected by suppressing vibrational modes 21 , suggesting that vibronic effects must be taken into account in the description of the phenomenon. These observations have also been corroborated by different theoretical works [22][23][24][25] , which have suggested that vibronic couplings between different singlet and triplet states may be responsible for rISC rates that are greater than those predicted by standard first-order perturbation theory. For this reason, different methods have been used to estimate (r)ISC rates that include vibrational effects (see, for instance, Refs.…”

Section: Introductionsupporting

confidence: 62%

A Unified Framework for Photophysical Rate Calculations in TADF Molecules

Sousa¹,

Silva

2021

Preprint

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One of the challenges in organic light emitting diodes research is finding ways to increase device efficiency by making use of the triplet excitons that are inevitably generated in the process of electroluminescence. One way to do so is by thermally activated delayed fluorescence, a process in which singlet excitons undergo up-conversion to singlet states, allowing them to relax radiatively. The discovery of this phenomenon has ensued a quest for new materials that are able to effectively take advantage of this mechanism. From a theoretical standpoint, this requires the capacity to estimate the rates of the various processes involved in the photophysics of candidate molecules, such as intersystem crossing, reverse intersystem crossing, fluorescence and phosphorescence. Here we present a method that is able to, within a single framework, compute all these rates and predict the photophysics of new molecules. We apply the method to two TADF molecules and show that results compare favorably with other theoretical approaches and experimental results. Finally, we use a kinetic model to show how the calculated rates act in concert to produce different photophysical behavior.

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“…17 On the other hand, to realize TADF characteristics, vibrations between electron-donor (D) and the acceptor (A) moieties are necessary. 17,18 As we presented before, it is possible to slow down the rotation between donor and acceptor moiety in order to observe the RTP process by attaching bulky, steric hindering groups. 19 The limiting value here is the temperature and decay lifetime.…”

Section: Introductionmentioning

confidence: 99%

Heavy-Atom-Free Room-Temperature Phosphorescent Organic Light-Emitting Diodes Enabled by Excited States Engineering

Higginbotham

Okazaki

Silva

et al. 2021

ACS Appl. Mater. Interfaces

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Room temperature phosphorescence materials offer great opportunities for applications in optoelectronics, due to their unique photophysical characteristics. However, purely organic emitters that can realize distinct electrophosphorescence are rarely exploited. Herein a new approach for designing heavy-atom-free organic room temperature phosphorescence emitters for organic light-emitting diodes is presented. The subtle tuning of the energy diagrams of singlet and triplet excited states by appropriate choice of host matrix allows tailored emission properties and switching of emission channels between thermally activated delayed fluorescence and room temperature phosphorescence. Moreover, an efficient and heavy-atom-free room temperature phosphorescence organic light-emitting diodes using the developed emitter is realized.

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Extracting Design Principles for Efficient Thermally Activated Delayed Fluorescence (TADF) from a Simple Four-State Model

Cited by 28 publications

References 13 publications

Negative Singlet–Triplet Excitation Energy Gap in Triangle-Shaped Molecular Emitters for Efficient Triplet Harvesting

Negative Singlet–Triplet Excitation Energy Gap in Triangle-Shaped Molecular Emitters for Efficient Triplet Harvesting

A Unified Framework for Photophysical Rate Calculations in TADF Molecules

Heavy-Atom-Free Room-Temperature Phosphorescent Organic Light-Emitting Diodes Enabled by Excited States Engineering

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