Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor‐Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

Zhang, Xue; Zhao, Xiaoyu; Ye, Kaiyue; Zhao, Jianzhang

doi:10.1002/chem.202203737

Cited by 15 publications

(25 citation statements)

References 87 publications

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“…For instance, the LE state, the electron exchange energy (J) for the unpaired electrons of the CS states ( 1 CS/ 3 CS state energy gap is 7 2J), the 1 LE/ 3 LE states energy gap of the NI moiety are all not changed by the unique molecular structure modification method [51]. This approach is useful for studying of molecules showing complicated, entangled photophysical processes upon photoexcitation, for instance the electron donor-acceptor type of TADF emitters [44,46].…”

Section: Ph-o Ni-ptz-c5-o)mentioning

confidence: 99%

“…Femtosecond transient absorption spectroscopy (fs-TA) was rarely used for the study of the photophysics of the TADF emitters [12,[34][35][36][37], however, it suffers from the limitation of the time window of detection (up to a few ns). Nanosecond transient absorption spectra were used for study of the TADF mechanisms [38][39][40][41][42][43][44], but the examples are limited. Recently, time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used for study of the photophysical mechanisms of the TADF emitters [33,39,[44][45][46].…”

Section: Introductionmentioning

confidence: 99%

“…Nanosecond transient absorption spectra were used for study of the TADF mechanisms [38][39][40][41][42][43][44], but the examples are limited. Recently, time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used for study of the photophysical mechanisms of the TADF emitters [33,39,[44][45][46]. However, the examples are rare.…”

Section: Introductionmentioning

confidence: 99%

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Tuning the photophysical property of naphthalimide-phenothiazine dyads by variation of the electron donating and accepting capability: synthesis and study of the intersystem crossing and thermally-activated delayed fluorescence

Cao¹,

Liu²,

Zhang³

et al. 2023

Preprint

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A series of naphthalimide (NI)-phenothiazine (PTZ) electron donor-acceptor dyads were prepared to study the thermally activated delayed fluorescence (TADF) property of the dyads, from a point of view of detection of the various transient species. The photophysical properties of the dyads were tuned by changing the electron donating and the electron withdrawal capability of the PTZ and NI moieties, respectively, by oxidation of the PTZ unit, or by using different arylamines to attach with the NI unit. This tuning effect was manifested in the UV-vis absorption and fluorescence emission spectra, e.g., the change of the charge transfer absorption bands. TADF was observed for the dyads containing native PTZ unit, and the prompt and delayed fluorescence lifetimes changed when different arylamine was used. In polar solvent, no TADF was observed. For the dyads with the PTZ unit oxidized, no TADF was observed as while. Femtosecond transient absorption spectra show that the charge separation takes ca. 0.6 ps, and admixture of locally excited (3LE) state and charge separated (1CS/3CS) states formed (in n-hexane). The subsequent charge recombination from the 1CS state takes ca. 7.92 ns. Upon oxidation of the PTZ unit, the beginning of charge separation is at 178 fs, and formation of 3LE state takes 4.53 ns. Nanosecond transient absorption (ns-TA) spectra show that both 3CS and 3LE states were observed for the dyads showing TADF, whereas only 3LE or 3CS state was observed for the systems lacking of TADF. This is a rare but unambiguous experimental evidence that the spin-vibronic coupling of 3CS/ 3LE states is crucial for TADF. Without the mediating effect of the 3LE state, no TADF is resulted, even the long-lived 3CS state is populated (lifetime τCS ≈ 140 ns). This experimental result confirms the 3CS → 1CS reverse intersystem crossing (rISC) is slow, without coupling with an approximate 3LE state. These studies are useful for in-depth understanding of the photophysical mechanisms of the TADF emitters, as well as for molecular structure design of new electron donor-acceptor TADF emitters.

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Section: Ph-o Ni-ptz-c5-o)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Tuning the photophysical property of naphthalimide-phenothiazine dyads by variation of the electron donating and accepting capability: synthesis and study of the intersystem crossing and thermally-activated delayed fluorescence

Cao¹,

Liu²,

Zhang³

et al. 2023

Preprint

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“…This is also solid experimental evidence that having 1 CS, 3 CS, and 3 LE states sharing similar energy is essential for TADF since the 3 CS → 1 CS process is too slow to produce any TADF. 41 Moreover, it should be noted that the prompt fluorescence lifetime of PTZ-Ph-AQ, 62 ns, is exceptionally long for electron donor−acceptor type TADF emitters. 43,44,83 Note that the E T1 of the 2-phenylanthraquinone is 2.33 eV, 77 and for the native AQ, the T 1 state energy is ca.…”

Section: Molecular Structure Design and Synthesismentioning

confidence: 99%

“…53−57 However, direct experimental evidence for these theories is rare. 41 In order to study the above-mentioned photophysical processes in compact electron donor−acceptor dyads, herein, we prepared a series of phenothiazine (PTZ)−anthraquinone (AQ) dyads (Scheme 1), with p-phenylene or p-biphenylene linkers between the two units, to tune the electronic coupling magnitude as well as the energy of the CS state. Methyl groups were furthermore attached on the phenylene linker to impose conformation restriction.…”

Section: Introductionmentioning

confidence: 99%

Observation of Long-Lived Charge-Separated States in Anthraquinone–Phenothiazine Electron Donor–Acceptor Dyads: Transient Optical and Electron Paramagnetic Resonance Spectroscopic Studies

Суханов

Xiao

et al. 2023

J. Phys. Chem. B

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We prepared a series of phenothiazine (PTZ)− anthraquinone (AQ) electron donor−acceptor dyads to study the relationship between molecular structures and the possibility of charge transfer (CT) and intersystem crossing (ISC). As compared to the previously reported PTZ-AQ dyad with a direct connection of two units via a C−N single bond, the PTZ and AQ units are connected via a p-phenylene or p-biphenylene linker. Conformation restriction is imposed by attaching ortho-methyl groups on the phenylene linker. UV−vis absorption spectra indicate electronic coupling between the PTZ and AQ units in the dyads without conformation restriction. Different from the previously reported PTZ-AQ, thermally activated delayed fluorescence (TADF) is observed for the dyads containing one phenylene linker (PTZ-Ph-AQ and PTZ-PhMe-AQ). The prompt fluorescence lifetime in cyclohexane is exceptionally long (τ PF = 62.0 ns, population ratio: 99.2%) and 245.0 ns (93.5%) for PTZ-Ph-AQ and PTZ-PhMe-AQ, respectively (normally τ PF <20 ns); the delayed fluorescence lifetimes for these two dyads were determined as τ DF = 2.4 μs (6.5%) and 7.6 μs (0.8%), respectively. For the dyad containing a biphenylene linker (PTZ-Ph 2 Me-AQ), no TADF was observed. Charge-separated (CS) states were observed for PTZ-Ph-AQ and PTZ-PhMe-AQ, and the lifetimes were determined as 7.0 and 1.3 μs, respectively, indicating the triplet spin multiplicity of the CS state. The 3 CS state lifetimes are shortened to 100 ns and 440 ns for the two dyads, respectively, in the polar solvent acetonitrile. For dyads with a longer linker, i.e., PTZ-Ph 2 Me-AQ, the CS state lifetime is not sensitive to solvent polarity (τ CS = 1.8 and 1.3 μs in cyclohexane and acetonitrile, respectively). In reference dyads, where the PTZ unit is oxidized to sulfoxide, no CT absorption band and TADF were observed, which is attributed to the increased CS state energy (>3 eV) becoming higher than that of the AQ triplet ( 3 AQ*) state (ca. 2.7 eV). These experimental evidence show that the presence of 1 CS, 3 CS, and 3 LE (LE: locally excited) states sharing similar energy is essential for the occurrence of TADF. Population of the long-lived 3 CS state (with a lifetime of a few μs) does not produce by itself TADF, because the ISC process of 1 CS→ 3 CS is nonsufficient. Femtosecond transient absorption spectra show that charge separation (CS) occurs readily (<5 ps) for most dyads, even in nonpolar solvents. Nanosecond pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that either a spin correlated radical pair (SCRP) is formed, with the electron exchange energy 2J = +2.14 mT, or radical pairs with stronger interaction, |2J| > 6.57 mT. These studies are useful for in-depth understanding of the CS and ISC in compact electron donor−acceptor dyads and for design of efficient TADF emitters.

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Y‐Shaped D‐A Type Thioxanthone Derivatives: Mechano‐Induced Thermally Activated Delayed Fluorescence and High‐Contrast Mechano‐Responsive Luminescence

Liao

et al. 2023

Chemistry A European J

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High-contrast mechano-responsive luminescence (MRL) materials with mechano-induced emission enhancement properties are fascinating candidates but few, for applications in rewritable media and recording devices.Here, an interesting design strategy of "Y-shape" donoracceptor (DÀ A) type molecules for high-contrast MRL materials was presented, based on substituted diphenylamine donor and planar acceptor. Interestingly, their DÀ A torsion angles are small in crystals but increased after ground, resulted in planar and twist intramolecular charge transfer (PICT and TICT) states, respectively. Therefore, high-contrast MRL switching between weak blue (450 nm) fluorescence and bright yellow (552 nm) thermally activated delayed fluorescence (TADF) can be achieved for compound TXDO (4,4'-dimeth-oxydiphenylamine donor), which photoluminescence quantum yield increased from 2.8 % to 54.7 % after ground. Most importantly, the two independent DÀ A conjugation dihedral angles are actually independent in the "Y-shape" molecules. Especially for compound TXDT (4,4'-di-tert-butyldiphenylamine donor), its crystal exhibited both PICT and TICT processes inside, resulted from the different dihedral angles of 11.8°and 35.5°, respectively. The TXDT crystal thus showed dual-peak emission, including both TICT fluorescence and PICT room-temperature phosphorescence. Therefore, this strategy of "Y-shape" DÀ A type molecules provide a new approach to design advanced luminescent materials with mechano-induced TADF feature, for high-contrast MRL and single-component white luminescence.

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Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor‐Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

Cited by 15 publications

References 87 publications

Tuning the photophysical property of naphthalimide-phenothiazine dyads by variation of the electron donating and accepting capability: synthesis and study of the intersystem crossing and thermally-activated delayed fluorescence

Tuning the photophysical property of naphthalimide-phenothiazine dyads by variation of the electron donating and accepting capability: synthesis and study of the intersystem crossing and thermally-activated delayed fluorescence

Observation of Long-Lived Charge-Separated States in Anthraquinone–Phenothiazine Electron Donor–Acceptor Dyads: Transient Optical and Electron Paramagnetic Resonance Spectroscopic Studies

Y‐Shaped D‐A Type Thioxanthone Derivatives: Mechano‐Induced Thermally Activated Delayed Fluorescence and High‐Contrast Mechano‐Responsive Luminescence

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