Organic semiconductors can be designed and constructed in π‐stacked structures instead of the conventional π‐conjugated structures. Through‐space interaction (TSI) occurs in π‐stacked optoelectronic ...materials. Thus, unlike electronic coupling along the conjugated chain, the functional groups can stack closely to facilitate spatial electron communication. Using π‐stacked motifs, chemists and materials scientists can find new ways for constructing materials with aggregation‐induced emission (AIE), thermally activated delayed fluorescence (TADF), circularly polarized luminescence (CPL), and room‐temperature phosphorescence (RTP), as well as enhanced molecular conductance. Organic optoelectronic devices based on π‐stacked molecules have exhibited very promising performance, with some of them exceeding π‐conjugated analogues. Recently, reports on various organic π‐stacked structures have grown rapidly, prompting this review. Representative molecular scaffolds and newly developed π‐stacked systems could stimulate more attention on through‐space charge transfer the well‐known through‐bond charge transfer. Finally, the opportunities and challenges for utilizing and improving particular materials are discussed. The previous achievements and upcoming prospects may provide new insights into the theory, materials, and devices in the field of organic semiconductors.
Unlike traditional covalent bond‐connected conjugated molecules, π‐stacked small molecules have special advantages in organic semiconductors. This review mainly focuses on the research development of π‐stacked molecular systems and introduces the new characteristics brought by the special molecular configuration and its application in organic semiconductors.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Significant effort has been made to develop novel material systems to improve the efficiency of near‐infrared organic light‐emitting diodes (NIR OLEDs). Of those, fluorescent chromophores are mostly ...studied because of their advantages in cost and tunability. However, it is still rare for fluorescent NIR emitters to present good color purities in the NIR range and to have high external quantum efficiency (EQE). Here, a wedge‐shaped D‐π‐A‐π‐D emitter APDC‐DTPA with thermally activated delayed fluorescence property and a small single‐triplet splitting (ΔEst) of 0.14 eV is presented. The non‐doped NIR device exhibits excellent performance with a maximum EQE of 2.19% and a peak wavelength of 777 nm. Remarkably, when 10 wt% of APDC‐DTPA is doped in 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene host, an extremely high EQE of 10.19% with an emission peak of 693 nm is achieved. All these values represent the best result for NIR OLEDs based on a pure organic fluorescent emitter with similar device structure and color gamut.
A near‐infrared (NIR) thermally activated delayed fluorescence material, APDC‐DTPA, containing acenaphtho1,2‐bpyrazine‐8,9‐dicarbonitrile unit as acceptor and diphenylamine as donor unit is developed. A non‐doped device based on APDC‐DTPA exhibits a maximum external quantum efficiency (EQE) of 2.19% with an emission peak at 777 nm. A maximum EQE of up to 10.19% is achieved in a doped NIR device (λEL = 693 nm).
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In this work, two novel thermally activated delayed fluorescence (TADF) emitters, 2tDMG and 3tDMG, are synthesized for high‐efficiency organic light‐emitting diodes (OLEDs), The two emitters have a ...tilted face‐to‐face alignment of donor (D)/acceptor (A) units presenting intramolecular noncovalent interactions. The two TADF materials are deposited either by an evaporation‐process or by a solution‐process, both of them leading to high OLED performance. 2tDMG used as the emitter in evaporation‐processed OLEDs achieves a high external quantum efficiency (EQE) of 30.8% with a very flat efficiency roll‐off of 7% at 1000 cd m−2. The solution‐processed OLEDs also display an interesting EQE of 16.2%. 3tDMG shows improved solubility and solution processability as compared to 2tDMG, and thus a high EQE of 20.2% in solution‐processed OLEDs is recorded. The corresponding evaporation‐processed OLEDs also reach a reasonably high EQE of 26.3%. Encouragingly, this work provides a novel strategy to address the imperious demands for OLEDs with high EQE and low roll‐off.
A thermally activated delayed fluorescence emitter, 2tDMG, is designed and synthesized based on the donor (D)/acceptor (A) spatially intramolecular noncovalent interaction. The D/A units are connected via a rigid linker, thereby confining them into a close‐packed coplanar configuration for small singlet–triplet splitting energy. 2tDMG achieves a high external quantum efficiency of 30.8% with a low efficiency roll‐off in evaporation‐processed organic light‐emitting diodes (OLEDs).
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A multiple resonance thermally activated delayed fluorescence (MR‐TADF) molecule with a fused, planar architecture tends to aggregate at high doping ratios, resulting in broad full width at half ...maximum (FWHM), redshifting electroluminescence peaks, and low device efficiency. Herein, we propose a mono‐substituted design strategy by introducing spiro‐9,9′‐bifluorene (SBF) units with different substituted sites into the MR‐TADF system for the first time. As a classic steric group, SBF can hinder interchromophore interactions, leading to high device efficiency (32.2–35.9 %) and narrow‐band emission (≈27 nm). Particularly, the shield‐like molecule, SF1BN, seldom exhibits a broadened FWHM as the doping ratio rises, which differs from the C3‐substituted isomer and unhindered parent emitter. These results manifest an effective method for constructing highly efficient MR‐TADF emitters through a spiro strategy and elucidate the feasibility for steric modulation of the spiro structure in π‐framework.
By incorporating a three‐dimensional spiro unit into multiple resonance thermally activated delayed fluorescence emitters, the device efficiency is increased to nearly 1.5 times that of the unhindered emitter. Notably, the linkage pattern with spatial interaction and hindrance can maintain the narrow FWHM and curb unfavorable redshifts at a high doping ratio.
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Charge-transfer (CT) complexes, formed by electron transfer from a donor to an acceptor, play a crucial role in organic semiconductors. Excited-state CT complexes, termed exciplexes, harness both ...singlet and triplet excitons for light emission, and are thus useful for organic light-emitting diodes (OLEDs). However, present exciplex emitters often suffer from low photoluminescence quantum efficiencies (PLQEs), due to limited control over the relative orientation, electronic coupling and non-radiative recombination channels of the donor and acceptor subunits. Here, we use a rigid linker to control the spacing and relative orientation of the donor and acceptor subunits, as demonstrated with a series of intramolecular exciplex emitters based on 10-phenyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine. Sky-blue OLEDs employing one of these emitters achieve an external quantum efficiency (EQE) of 27.4% at 67 cd m
with only minor efficiency roll-off (EQE = 24.4%) at a higher luminous intensity of 1,000 cd m
. As a control experiment, devices using chemically and structurally related but less rigid emitters reach substantially lower EQEs. These design rules are transferrable to other donor/acceptor combinations, which will allow further tuning of emission colour and other key optoelectronic properties.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Rational manipulation of frontier orbital distribution and singlet‐triplet splitting is crucial to exploit the luminescent properties of organic molecules. To realize ultra‐blue luminescence, both ...blue‐shifted wavelength peak (λpeak) and narrow full‐width at half‐maximum (FWHM) are required. Herein, a new thermally activated delayed fluorescence (TADF) skeleton by inserting the diphenyl methylene intramolecular‐lock to adjust the torsion angles and restrict the intramolecular relaxation is developed. Two rigid emitters, incorporating phenoxazine (PXZN‐B) and acridine (DMACN‐B) as donors and mesitylboron as an acceptor, exhibit narrow FWHMs (<50 nm) with deep‐blue (0.133, 0.147) and violet‐blue emission (0.151, 0.045), respectively. In particular, the Commission Internationale de l'Eclairage (CIE) coordinates of a DMACN‐B‐based device closely approach the Rec.2020 standard (0.131, 0.046). Moreover, both of the organic light‐emitting diodes (OLEDs) based on PXZN‐B and DMACN‐B show TADF character, with high external quantum efficiencies (EQEs) exceeding 10%. Furthermore, owing to the large orbital overlap, these TADF emitters own a fast S1–S0 transition rate exceeding 108 s–1, thereby exhibiting marked amplified spontaneous emission (ASE) with low thresholds. Therefore, the intramolecular‐lock strategy provides not only innovation for realizing high‐efficiency deep‐blue TADF emission with high color purity but also an avenue for a TADF‐based ASE and lasing application.
An “intramolecular‐lock” is proposed as part of the thermally activated delayed fluorescence (TADF) molecular design for manipulating torsion angles and wave function distributions. The quasi‐planar TADF emitters lead to ultrapure violet‐blue TADF electroluminescence with CIE‐(0.151, 0.045), approaching the Rec. 2020 standard. Furthermore, a TADF‐based amplified spontaneous emission with low thresholds is triggered, which paves the way for future TADF‐based lasing application.
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Organic materials with multi‐stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π‐stacked thermally activated delayed fluorescence (TADF) ...material with multi‐stimulus response (MSR) properties, named SDMAC, was designed and synthesized using distorted 9,9‐dimethyl‐10‐phenyl‐9,10‐dihydroacridine as a donor. SDMAC possesses a rigid π‐stacked configuration with intramolecular through‐space interactions and exhibits aggregation‐induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties of SDMAC can be used in displays and lighting. Using SDMAC as an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light‐emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP‐TADF OLEDs based on the through‐space charge transfer strategy. The CP organic light‐emitting diodes (CP‐OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.
An efficient thermally activated delayed fluorescence (TADF) emitter has been developed that possesses a rigid π‐stacked configuration with intramolecular through‐space interactions. This emitter exhibits solvatochromism, piezochromism, aggregation‐induced emission enhancement (AIEE), and circularly polarized luminescence (CPL) under different external stimuli.
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Multiple resonances induced thermally activated delayed fluorescence (MR‐TADF) has great advantages in high color purity display. Up to now, current MR‐TADF emitters are only based on the ...boron‐nitrogen‐containing fragment. Reported herein is a novel class of MR‐TADF emitter, quinolino3,2,1‐deacridine‐5,9‐dione (QAO), realized by the opposite resonance effect of the carbonyl and the nitrogen atoms, which is also the smallest TADF emitter reported so far. The QAO‐based pure blue organic light‐emitting diode achieves a maximum external quantum efficiency (EQEmax) of 19.4% with a small full width at half maximum of 39 nm. Moreover, tert‐butyl modified QAO can be employed as an efficient electron acceptor to construct an efficient yellow‐green evaporation and solution process feasible TADF emitter, 7‐(tert‐butyl)‐3,11‐bis(9,9‐dimethylacridin‐10(9H)‐yl)quinolino3,2,1‐deacridine‐5,9‐dione (QAO‐DAd), with significantly improved performance as compared to the previous flexible diketone‐based emitters.
A novel class of multiple resonance induced thermally activated delayed fluorescence (MR‐TADF) emitter (QAO) is developed, by using the opposite resonance effect of the carbonyl group and the nitrogen atom. Moreover, QAO can be employed as an efficient electron acceptor to construct traditional donor–acceptor–donor type TADF emitter, QAO‐DAd, and then achieve high performances in evaporation and solution‐processed devices.
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Reported here are C1‐linked spiro‐bifluorene dimers. A comprehensive study is carried out to analyze the electronic properties of these highly twisted structures. This work shows that the C1‐position ...enables the design of pure hydrocarbon materials, with a high triplet energy, for hosting blue phosphors in efficient phosphorescent OLEDs (PhOLEDs). To date, this work describes the highest performance of blue PhOLEDs ever reported for pure hydrocarbons (external quantum efficiency of ca. 23 %), thus highlighting the potential of the C1‐spirobifluorene scaffold in organic electronics.
Phosphorescence with a twist: The highly twisted C1‐linked spiro‐bifluorene dimers are reported. This work shows that the C1‐position enables the design of pure hydrocarbon materials, with a high triplet energy, for hosting blue phosphors in highly efficient phosphorescent OLEDs (PhOLEDs).
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Efficient sky‐blue organic light‐emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) display a three orders of magnitude increase in lifetime, which is superior to those ...of controlled phosphorescent OLEDs used in this study. The combination of electro‐oxidation and photo‐oxidation of the TADF emitters in their triplet excited‐states is suppressed through molecule design and device engineering.
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