Synthesis and characterization of fluorene‐based oligomers and polymers incorporating <i>N</i>‐arylphenothiazine‐<i>S,S</i>‐dioxide units

Kamtekar, Kiran T.; Dahms, Katja; Batsanov, Andrei S.; Jankus, Vygintas; Vaughan, Helen L.; Monkman, Andrew P.

doi:10.1002/pola.24527

Cited by 21 publications

(16 citation statements)

References 45 publications

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“…19−26 Particularly, phenothiazine contains two sp 3 -hybridized heteroatoms (S, N) in the heterocyclic structure, which leads to a nonplanar butterfly-shaped geometry. 27 This molecular configuration endows the 3,7-position and 10-position of phenothiazine with significantly different steric hindrances. As a result, the D−A torsion angle should be of difference when introducing the same acceptor to the 3,7-position and 10-position of phenothiazine.…”

Section: ■ Introductionmentioning

confidence: 99%

Tailoring Excited-State Properties and Electroluminescence Performance of Donor–Acceptor Molecules through Tuning the Energy Level of the Charge-Transfer State

et al. 2015

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Three donor−acceptor (D−A) compounds (PTZ-10-AnP, PTZ-10P-AnP, and PTZ-3-AnP) were designed and synthesized through linking the same D and A units with different architectures. The structural change, mainly referring to the torsion angle and distance between D and A, gives rise to the significantly different CT level positions for these compounds. The CT state of PTZ-10-AnP locates at the lowest excited state, while that of PTZ-10P-AnP stays at the higher excited state. For PTZ-3-AnP, the CT energy is close to that of the locally π−π* excited state, and a hybridized local and charge-transfer (HLCT) state dominates the S 1 state. Photophysical experiment data of these compounds demonstrate that the energy level of the charge-transfer (CT) state plays a decisive role in the emissive state properties of donor−acceptor (D−A) compounds. In addition, the exciton utilization efficiency of the PTZ-3-AnP device exceeds the limit of the spin statistics, which enlightens the molecular design toward harvesting triplet excitons in fluorescent organic light-emitting diodes (OLEDs). ■ INTRODUCTIONRecently, substantial progress has been made in the field of organic light-emitting diodes (OLEDs), that is, employing triplet excitons through a reverse intersystem crossing (RISC) process in fluorescent OLEDs (FOLEDs), which will certainly bring about exceeding the spin statistical limit of singlet excitons. 1−10 This phenomenon is mainly concentrated in donor−acceptor (D−A) molecules with unique excited-state properties. The combination of D and A moieties often results in the formation of a charge-transfer (CT) excited state. 11−14 The CT state along with the π−π* state constitute the energy level system in the D−A compound, which determines the optical and electronic properties of the compound. Generally, CT states possess weak binding energy due to their spacially separated transition orbitals. 15 This intrinsic merit facilitates the RISC process of triplet excitons. 16 On the other hand, the separated transition orbitals of CT states can cause undesirably low radiative transition rates and photoluminescence efficiency. 17,18 Consequently, for D−A molecules, their CT energy levels have crucial effects on the final emissive state properties. To deeply understand the relationship between CT energy level and excited-state properties, a systematical modulation for CT energy level in D−A molecules is required.As for molecular design, phenothiazine has been widely used as a donor moiety because of its low ionization potential (around 5.0 eV) and strong electron-donating ability. 19−26 Particularly, phenothiazine contains two sp 3 -hybridized heteroatoms (S, N) in the heterocyclic structure, which leads to a nonplanar butterfly-shaped geometry. 27 This molecular configuration endows the 3,7-position and 10-position of phenothiazine with significantly different steric hindrances. As a result, the D−A torsion angle should be of difference when introducing the same acceptor to the 3,7-position and 10-position of phenothiazine. Through c...

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Section: ■ Introductionmentioning

confidence: 99%

Tailoring Excited-State Properties and Electroluminescence Performance of Donor–Acceptor Molecules through Tuning the Energy Level of the Charge-Transfer State

et al. 2015

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“…[16a] We also have shown that TF in exciplex based OLEDs can compete with TADF leading to less efficient triplet recycling . Another issue is the choice of an appropriate host having a high triplet level, similar to those used in phosphorescent devices . It is therefore vital to understand the details of the TADF mechanism in solid state films and OLED devices, so that new more efficient materials sets can be designed for efficient blue TADF OLEDs in the future.…”

Section: Introductionmentioning

confidence: 99%

Highly Efficient TADF OLEDs: How the Emitter–Host Interaction Controls Both the Excited State Species and Electrical Properties of the Devices to Achieve Near 100% Triplet Harvesting and High Efficiency

Jankus

Data

Graves

et al. 2014

Adv Funct Materials

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303

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Article type: Full PaperHighly efficient TADF OLEDs; how the emitter-host interaction controls both the excited state species and electrical properties of the devices to achieve near 100% triplet harvesting and high efficiency.Vygintas Jankus, * (OLEDs). Molecules that have a charge transfer (CT) excited state can potentially achieve this through the mechanism of thermally activated delayed fluorescence 2 (TADF). Here, it is shown that a D-A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. OLEDs based on one of our previously studied D-A-D molecules in a host TAPC achieves >14% external electroluminescence yield and shows nearly 100% efficient triplet harvesting. In these devices it is unambiguously established that the triplet states are harvested via TADF, but more interestingly these results are found to be independent of whether the emitter is the ICT state or the D-A-D/host exciplex. IntroductionArtificial lighting is an essential part of our lives, which consumes 19% of the planet's electricity usage, and cheaper ways of creating energy, and more efficient devices that use less electricity, are needed to reduce energy costs, and greatly cut CO 2 production. Ultraefficient lighting will play an important role in achieving this. In particular, white organic light emitting diodes (OLEDs) could become an integral part of the new lighting technologies; however, alternatives to Ir based phosphors, and especially much improved deep blue OLED emitters, are needed in order to give high quality, efficient white OLEDs that are not reliant on scarce rare-earth metals.Two types of excited states are created when charge recombines in an OLED -singlet and triplet excitons, but only the singlets directly give light, which fundamentally limits external OLED efficiency to 5%. [1] Thus the efficiency can be increased fourfold if the non-emissive triplets can be utilised. Currently, phosphorescent heavy metal complexes are used to 'harvest' the triplet states and generate light. [2][3][4] Unfortunately, pushing the metal-to-ligand charge transfer excited state of these complexes into the blue opens a non-radiative pathway via the 3 metal d-orbitals, which limits efficiency [5] , and makes the complexes thermally and photochemically unstable. [6] The production of singlets via triplet-triplet annihilation (TTA)i.e. triplet fusion (TF), has also been demonstrated in OLEDs, [7,8] but it has been shown that triplet annihilation can contribute to both an increase and a loss in yield in OLEDs, [9] and the maximum theoretical external quantum efficiency (EQE) 12.5%, when accounting for emission arising from TF, has not been reached. However, deep blue emission is essential to achieve the required colour rendering and efficiency in white OLEDs for lighting applications, therefore, other processes that can convert triplets to singlets must be found.An alternative way to convert t...

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“…One major issue in improving the efficiency is finding suitable host polymers for deep blue phosphorescent dopants, as these require high enough triplet energy to confine triplet excitons on the phosphorescent dopant. To date there have been several reported polymers with high triplet levels, but none have achieved a triplet energy greater than 2.6 eV,8–12 which is not high enough to prevent quenching of blue phosphorescence, emitting typically at 2.7–2.8 eV depending on the blue wavelengths required for different lighting applications or NTCS display colour gamuts. Poly(vinycarbazole) (PVK) has been extensively used as a hole transporting host for green emitting devices13 reaching up to 31 cd A −1 14 or even up to ∼40 cd A −1 current efficiency and ∼12% external quantum efficiency15 and many people are turning to PVK as a host for blue phosphorescent dopants 16–17.…”

Section: Introductionmentioning

confidence: 99%

Is Poly(vinylcarbazole) a Good Host for Blue Phosphorescent Dopants in PLEDs? Dimer Formation and Their Effects on the Triplet Energy Level of Poly(N‐vinylcarbazole) and Poly(N‐Ethyl‐2‐Vinylcarbazole)

Jankus

Monkman

2011

Adv Funct Materials

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The detailed measurement and analysis of the delayed emission from poly(vinylcarbazole) (PVK) and poly( N -ethyl-2-vinyl-carbazole) (P2VK) thin fi lms is described. PVK has rapidly become a "polymer of choice" for hosting phosphorescent dopants in PLEDs, especially blue emitters. In this respect it is important to have a full understanding of the triplet properties of this host. It is concluded that in fi lms, the electronic 0-0 peak energy of PVK phosphorescence is found at 2.88 eV (14 K). With an increase of temperature, > 44 K, increasing emission from new long lived, lower energy species, previously ascribed to "trap states" in the literature, is observed. Increasing temperature enables thermally assisted triplet exciton hopping to these trap states. Critically it is shown that some of these triplet trap species are ground state triplet dimers in origin for both PVK (2.46 eV) and P2VK (2.1 eV), and not all of them are of excimer nature as previously thought. These species can quench the emission of blue heavy metal complexes doped in PVK and drastically effect performance over lifetime if the dimer formation increases over time and at elevated operating temperature. It is therefore concluded that PVK might not be such an ideal host material for blue phosphorescent emitters.

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Synthesis and characterization of fluorene‐based oligomers and polymers incorporating N‐arylphenothiazine‐S,S‐dioxide units

Cited by 21 publications

References 45 publications

Tailoring Excited-State Properties and Electroluminescence Performance of Donor–Acceptor Molecules through Tuning the Energy Level of the Charge-Transfer State

Tailoring Excited-State Properties and Electroluminescence Performance of Donor–Acceptor Molecules through Tuning the Energy Level of the Charge-Transfer State

Highly Efficient TADF OLEDs: How the Emitter–Host Interaction Controls Both the Excited State Species and Electrical Properties of the Devices to Achieve Near 100% Triplet Harvesting and High Efficiency

Is Poly(vinylcarbazole) a Good Host for Blue Phosphorescent Dopants in PLEDs? Dimer Formation and Their Effects on the Triplet Energy Level of Poly(N‐vinylcarbazole) and Poly(N‐Ethyl‐2‐Vinylcarbazole)

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