Critical role of intermediate electronic states for spin-flip processes in charge-transfer-type organic molecules with multiple donors and acceptors

Noda, Haruhiko; Chen, Xiankai; Nakanotani, Hajime; Hosokai, Takuya; Miyajima, Momoka; Notsuka, Naoto; Kashima, Yuuki; Brédas, Jean Luc; Adachi, Chihaya

doi:10.1038/s41563-019-0465-6

Cited by 311 publications

(299 citation statements)

References 44 publications

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“…The quantities used for the best fits are shown in Table . The determined Δ E a value for 4CzIPN is almost the same as one reported by Noda et al, who determined Δ E a from the k ISC values in the temperature range from 250 to 300 K. Theoretically, it is expected that the direct ISC between S 1 and T n takes place with a slow and temperature‐independent rate in 4CzIPN . Therefore, the observed temperature dependence is ascribed to T n .…”

Section: Resultssupporting

confidence: 86%

“…In flavin in aqueous solution, for example, the ISC between S 1 and T 2 seems to be forbidden according to the El‐Sayed rule but it efficiently proceeds via the intersection at a rate of k ISC = 10 8 s −1 . Noda et al have proposed that the (reverse) ISC in 4CzIPN is accelerated when the temperature is high enough for excitons to surmount the potential barrier that is formed by the potential surfaces of S 1 and T n In such a case, the determined activation energy of k ISC corresponds to the height of the potential barrier measured from the potential minimum of S 1 . In Figure , we show two possible potential energy surfaces with large and small geometrical differences between the potential minima of S 1 and T n .…”

Section: Resultsmentioning

confidence: 99%

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Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters

Kobayashi

Kawate

Niwa

et al. 2019

Physica Status Solidi (a)

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For a better understanding of the exciton decay process in thermally activated delayed fluorescence (TADF) emitters, the intersystem crossing rate, k ISC , is one of the important physical constants that have to be determined. Herein, a method to calculate the k ISC value from photoluminescence (PL) measurements is reconsidered. The k ISC value can be determined at very low temperatures where delayed fluorescence (DF) is completely suppressed, as well as around room temperature where triplet excitons mainly decay into the ground state by emitting DF. However, there is a temperature range where the k ISC value cannot be determined accurately because the influences of nonradiative decay paths can be neither ignored nor corrected. For such a temperature range, an alternative approach, which utilizes the temperature dependence of an observed PL decay rate, is presented. In this way, k ISC values from 300 to 10 K are determined for thin films of two TADF emitters, i.e., 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene and 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene, which are known as 4CzIPN and 2CzPN, respectively.

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Section: Resultssupporting

confidence: 86%

Section: Resultsmentioning

confidence: 99%

Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters

Kobayashi

Kawate

Niwa

et al. 2019

Physica Status Solidi (a)

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“…In contrast, remarkably strong SOC can be expected when the spin-flip processes arise between the CT and energetically close-lying locally excited (LE) states with different spin multiplicities owing to the orbital angular momentum change between the two states 27,39 . Several groups have reported that k RISC is accelerated by the strong SOC matrix element between the 1 CT (S 1 ) and 3 LE (T 2 ) states, caused by non-adiabatic vibronic coupling (Ĥ VC ) between the 3 CT (T 1 ) and 3 LE (T 2 ) states [40][41][42] . In this study, we demonstrate that a small modulation in excited states of an aromatic organic molecule, mediated by SOC between 1 CT and 3 LE states, greatly affects the rate constants of a TADF molecule.…”

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confidence: 99%

Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff

et al. 2020

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Aromatic organic deep-blue emitters that exhibit thermally activated delayed fluorescence (TADF) can harvest all excitons in electrically generated singlets and triplets as light emission. However, blue TADF emitters generally have long exciton lifetimes, leading to severe efficiency decrease, i.e., rolloff, at high current density and luminance by exciton annihilations in organic light-emitting diodes (OLEDs). Here, we report a deep-blue TADF emitter employing simple molecular design, in which an activation energy as well as spin-orbit coupling between excited states with different spin multiplicities, were simultaneously controlled. An extremely fast exciton lifetime of 750 ns was realized in a donor-acceptor-type molecular structure without heavy metal elements. An OLED utilizing this TADF emitter displayed deep-blue electroluminescence (EL) with CIE chromaticity coordinates of (0.14, 0.18) and a high maximum EL quantum efficiency of 20.7%. Further, the high maximum efficiency were retained to be 20.2% and 17.4% even at high luminance.

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“…It has been widely demonstrated that a small singlet–triplet energy difference (Δ E ST ) is the necessary condition to harvest nonradiative triplets into radiative singlets, providing the basic information on energy parameter effects. On the other hand, spin–orbital coupling (SOC) has been considered as the key mechanism to flip the spins of triplets, enabling the triplet‐to‐singlet conversion toward developing delayed fluorescence . However, the polarization parameter has been reminded as an unaddressed issue to be related to spin and energy parameters (SOC and Δ E ST ).…”

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confidence: 99%

Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High‐Efficiency Exciplex Light‐Emitting Diodes

et al. 2019

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Excited states in organic light-emitting diodes (OLEDs) are inevitably formed with both singlets and triplets under electrical excitation. Singlets and triplets are allowed and forbidden to recombine, respectively, due to spin selection rule. It has been shown that the triplets can be almost 100% converted into singlets in thermally activated delayed fluorescence (TADF) molecules based on the design of chemically combining donor and acceptor moieties to enable intramolecular chargetransfer states. [1][2][3][4] Recently, various TADFmolecule-based OLEDs with extremely high external quantum efficiency (EQE) exceeding 35% have been successfully demonstrated. [5,6] Similarly, high EQEs can also be conveniently realized by physically mixing donor and acceptor components to form intermolecular charge-transfer states in exciplex systems, where the nonradiative triplets are also largely converted into radiative singlets. The advantages and versatile applications of exciplex systems for giving high-efficiency OLEDs have been highlighted recently. [7] More significantly, OLEDs with exciplex-forming systems as emitting layer have been reported to achieve EQE higher than 19%, [8][9][10] manifesting their bright and promising prospects in OLED technology based on physically Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge-transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex lightemitting diodes are reported. Magneto-photoluminescence studies reveal that the triplet-to-singlet conversion in exciplexes involves an artificially generated spin-orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge-transfer occurs with forming electric dipoles (D +• →A −• ), providing the ionic polarization to generate SOC in exciplexes. By having different singlet-triplet energy differences (ΔE ST ) in 9,9′-diphenyl-9H,9′H-3,3′-bicarbazole (BCzPh):3′,3′″,3′″″-(1,3,5-triazine-2,4,6-triyl) tris(([1,1′-biphenyl]-3-carbonitrile)) (CN-T2T) (ΔE ST = 30 meV) andBCzPh:bis-4,6-(3,5-di-3-pyridylphenyl)-2-methyl-pyrimidine (B3PYMPM) (ΔE ST = 130 meV) exciplexes, the SOC generated by the intermolecular charge-transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE max (21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE ST , and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D +• →A −• ) size, enhanced SOC, increased orbital polarization, and decreased ΔE ST can simultaneously occur to cooperatively operate the triplet-to-singlet conversion.

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Critical role of intermediate electronic states for spin-flip processes in charge-transfer-type organic molecules with multiple donors and acceptors

Cited by 311 publications

References 44 publications

Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters

Intersystem Crossing Rate in Thermally Activated Delayed Fluorescence Emitters

Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff

Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High‐Efficiency Exciplex Light‐Emitting Diodes

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