Planar Chiral [2.2]Paracyclophane-Based Thermally Activated Delayed Fluorescent Materials for Circularly Polarized Electroluminescence

Access to [2.2]Paracyclophane-Fused N-Heterocycles via Palladium-Catalyzed C-H Activation and Cyclization of [2.2]Paracyclophaneacylhydrazones with Vinyl Azides

A novel protocol for the synthesis of [2.2]paracyclophane-fused N-heterocycles via palladium-catalyzed C-H activation and cyclization of [2.2]paracyclophaneacylhydrazones with vinyl azides has been developed. High regioselectivity control was realized by selective activation/functionalization of the C-H bond of highly rigid [2.2]paracyclophanes. A broad range of [2.2]paracyclophane-fused isoquinolinophane derivatives were prepared in sole regioselectivity with good functional group tolerance, which would be difficult to synthesize by other approaches in a one-step manner.

Chiral Blue TADF Materials Enhance the Spin Transitions to Improve Emission Quantum Yield

Circularly polarized thermally activated delayed fluorescence materials not only possess high exciton utilization efficiency but also have the capability to emit circularly polarized light for potential information storage and sensing. In this work, chiral blue TADF enantiomers are prepared. The energy difference between singlet and triplet, ΔEST, increases with the strength of chirality. The chiral orbit-induced spin degeneracy elimination could enhance spin relaxation, where spin could flip easily to lead to an effective transition from triplet to singlet states. This induces a pronounced enhancement in fluorescence quantum yield. Furthermore, circularly polarized emission of chiral TADF materials under different external magnetic fields are studied. Magnetic field control of glum presents a mirror symmetry effect for chiral TADF enantiomers, which provides evidence for the transition between the photon spin and electron spin.

Chiral Resolution and Chiroptical Properties of Hindered Tetraphenylethylene Helicates

Hindered tetraphenylethylene (hTPE) helicates are resolved into two left-handed (M) and right-handed (P) isomers by linkage and removal of chiral auxiliary (1R,2S,5R)-menthol, furnishing gram-scale hTPE enantiomers via flash silica column chromatography. hTPE helicate enantiomers bearing electron-accepting cyano and electron-donating triphenylamine groups can emit deep-blue CPL signals with a fluorescence quantum yield surpassing 50%. Full-color and white-light emission were achieved by blending them with dyes in a poly(methyl methacrylate) (PMMA) film.

High-Performance Circular Polarization Multiple-Resonance TADF Molecules with Enhanced Long-Range Charge Transfer Based on Chiral Paracyclophane

Circularly polarized multiple-resonance thermally activated delayed fluorescence (CP-MR-TADF) materials have received widespread attention in recent years, but it remains a formidable challenge to design high-performance CP-MR-TADF emitters concurrently exhibiting high quantum efficiency, narrowband emission, and high dissymmetry factor (g). Here, we perform an in-depth theoretical investigation on the CP-MR-TADF materials based on [2.2] paracyclophane (pCp) derivatives. The MR-based materials with enhanced long-range charge transfer (LRCT) characteristics upon excitation show increased g values owing to the coaxial dominated transition components of the transition electric dipole moment (TEDM) and the transition magnetic dipole moment (TMDM) but inevitably result in the loss of narrowband emission performance. Furthermore, the newly designed molecules by fusing the peripheral benzene units of MR cores within the planar chiral pCp bridge maintain narrowband emissions and exhibit increased g values on the order of 1 × 10-3. These findings with rich physical insights on the structure-performance relation of chiral paracyclophane-based molecules should provide important clues for designing high-performance chiral materials.

Synthesis and Properties of Donor-Acceptor-Type Cyclobisbiphenylenecarbonyls

The scalable synthesis of figure-eight π-systems is challenging for the conventional bottom-up approach. We have recently reported that the oxidative inner-bond cleavage of commercially available dibenzo[g,p]chrysene efficiently furnishes a figure-eight π-acceptor, cyclobisbiphenylenecarbonyl (CBBC), in large quantity. Furthermore, its donor-acceptor-type derivative with four N-carbazolyl substituents at the meta-positions of the carbonyl groups exhibited thermally activated delayed fluorescence (TADF) and circularly polarized luminescence (CPL) with a high |gCPL| value of 1.0×10-2. Herein, we synthesized nine donor-acceptor-type CBBC derivatives by changing the donor substituents and their positions. Compared to previously reported carbazole-substituted CBBC, tetramethylcarbazole- and 9,10-dihydro-9,9-dimethylacridine-substituted derivatives exhibited enhanced emission quantum yields and accelerated reverse intersystem crossing. The functionalization of the para-positions of the carbonyl groups resulted in better tunability of emission colors rather than meta-functionalization, whose color variation ranges from light blue to red. The incorporation of bulky substituents at the meta-positions of the carbonyl groups induced the conformational change to a distorted ring structure. Investigation of the substituent effect on the chiroptical properties revealed that the introduction of less bulky donor units such as carbazole at the meta-positions of the carbonyl groups is effective in achieving high |gCPL| values.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Inner-Bond-Cleavage Approach to Figure-Eight Macrocycles from Planar Aromatic Hydrocarbons

Figure-eight-shaped nonplanar π-systems adopt distinctive chiral D2-symmetric structures, which are ideal for realizing efficient circularly polarized luminescence (CPL). However, the short-step and enantioselective synthesis of figure-eight π-systems represents a considerable challenge for the conventional bottom-up synthetic strategy. Herein, we report that the oxidative cleavage of the internal double bond of a commercially available polycyclic aromatic hydrocarbon, i.e., dibenzo[g,p]chrysene (DBC), catalytically affords a figure-eight electron-accepting macrocycle, i.e., cyclobisbiphenylenecarbonyl (CBBC), with high scalability (up to 3.3 g) and excellent enantioselectivity (94% ee). This inner-bond-cleavage approach also applies to larger PAHs, affording highly distorted molecular frameworks that comprise two figure-eight subunits. Furthermore, we demonstrate that the peripheral functionalization of CBBC with carbazole afforded donor-acceptor-type emitter, which shows thermally activated delayed fluorescence and emits CPL with a g value of 1.0 × 10-2. This g value is ten times higher than those of previously reported chiral TADF-active emitters for circularly polarized organic light-emitting diodes. These results demonstrate that oxidative inner-bond cleavage is a powerful synthetic strategy for creating innovative materials that incorporate molecules with figure-eight structures.

Tetraborated Intrinsically Axial Chiral Multi-resonance Thermally Activated Delayed Fluorescence Materials

Chiral multi-resonance thermally activated delayed fluorescence (CP-MR-TADF) materials hold promise for circularly polarized organic light-emitting diodes (CP-OLEDs) and 3D displays. Herein, we present two pairs of tetraborated intrinsically axial CP-MR-TADF materials, R/S-BDBF-BOH and R/S-BDBT-BOH, with conjugation-extended bidibenzo[b,d]furan and bidibenzo[b,d]thiophene as chiral sources, which effectively participate in the distribution of the frontier molecular orbitals. Due to the heavy-atom effect, sulfur atoms are introduced to accelerate the reverse intersystem crossing process and increase the efficiency of molecules. R/S-BDBF-BOH and R/S-BDBT-BOH manifest ultra-pure blue emission with a maximum at 458/459 nm with a full width at half maximum of 27 nm, photoluminescence quantum yields of 90 %/91 %, and dissymmetry factors (|gPL|) of 6.8×10-4/8.5×10-4, respectively. Correspondingly, the CP-OLEDs exhibit good performances with an external quantum efficiency of 30.1 % and |gEL| factors of 1.2×10-3.

Two Different Chiral Groups Based Thermally Activated Delayed Fluorescence Materials for Circularly Polarized OLEDs

Circularly polarized organic light-emitting diodes (CP-OLEDs) hold significant promise for applications in 3D displays due to the ability to generate circularly polarized luminescence (CPL) directly. In this study, two pairs of circularly polarized thermally activated delayed fluorescence (CP-TADF) enantiomers, named RR/SS-ONCN and RS/SR-ONCN, were synthesized by integrating two distinct chiral groups into the dicyanobenzene unit. The RR/SS-ONCN and RS/SR-ONCN enantiomers show CPL properties with dissymmetry photoluminescence factors (|gPL|) of 1.3×10-3 and 2.0×10-3 in doped films, respectively. Notably, RR/SS-ONCN exhibit higher |gPL| values than that of RS/SR-ONCN, especially in doped films, indicating that when the configurations of the two chiral groups are identical, the |gPL| value of the CP-TADF materials can be enhanced, demonstrating a certain stacking effect. Moreover, the corresponding CP-OLEDs demonstrate good performances, achieving maximum external quantum efficiencies of up to 21.9 % and notable CP electroluminescence with |gEL| factors of up to 1.0×10-3.

Recent advances in circularly polarized luminescence of planar chiral organic compounds

Circularly polarized luminescence (CPL), as an important chiroptical phenomenon, can not only directly characterize excited-state structural information about chiroptical materials but also has great application prospects in 3D optical displays, information storage, biological probes, CPL lasers and so forth. Recently, chiral organic small molecules with CPL have attracted a lot of research interest because of their excellent luminescence efficiency, clear molecular structures, unique flexibility and easy functionalization. Planar chiral organic compounds make up an important class of chiral organic small molecular materials and often have rigid macrocyclic skeletons, which have important research value in the field of chiral supramolecular chemistry (e.g., chiral self-assembly and chiral host-guest chemistry). Therefore, research into planar chiral organic compounds has become a hotspot for CPL. It is time to summarize the recent developments in CPL-active compounds based on planar chirality. In this feature article, we summarize various types of CPL-active compounds based on planar chirality. Meanwhile, we overview recent research in the field of planar chiral CPL-active compounds in terms of optoelectronic devices, asymmetric catalysis, and chiroptical sensing. Finally, we discuss their future research prospects in the field of CPL-active materials. We hope that this review will be helpful to research work related to planar chiral luminescent materials and promote the development of chiral macrocyclic chemistry.

Recent Advances of Chiral Isolated and Small Organic Molecules: Structure and Properties for Circularly Polarized Luminescence

This paper explores recent advancements in the field of circularly polarized luminescence (CPL) exhibited by small and isolated organic molecules. The development and application of small CPL molecule are systematically reviewed through eight different chiral skeleton sections. Investigating the intricate interplay between molecular structure and CPL properties, the paper aims at providing and enlighting novel strategies for CPL-based applications.

Integration of fine-tuned chiral donor with hybrid long/short-range charge-transfer for high-performance circularly polarized electroluminescence

The synergistic integration of a fine-tuned chiral donor with a hybrid long/short-range charge-transfer mechanism offers an accessible pathway to construct highly efficient circularly polarized emitters. Consequently, a notable dissymmetry factor of 1.6 × 10-3, concomitantly with a record-setting maximum external quantum efficiency of 37.4%, is synchronously realized within a single embodiment.

Liquid-crystalline circularly polarised TADF emitters for high-efficiency, solution-processable organic light-emitting diodes

Achieving a high emission efficiency and a large luminescence asymmetry factor (glum) in a single molecule exhibiting circularly polarised thermally activated delayed fluorescence (CP-TADF) remains a formidable challenge. In this work, a proof-of-concept, liquid-crystalline CP-TADF molecule is proposed to realise high glum by taking advantage of the order inherent in liquid crystals. Employing a chiral dinaphthol-based CP-TADF molecule as the emissive unit, a pair of liquid-crystalline CP-TADF molecules (R/S-4) is synthesised via the introduction of six mesogenic moieties. The enantiomers show intense emission at about 520 nm which has clear TADF and liquid-crystalline characteristics. Both enantiomers display symmetrical electronic circular dichroism (CD) and circular polarisation luminescence (CPL) signals as thin films. Impressively, relatively large glum values of 0.11 are realised for the films. Solution-processed devices were fabricated using R/S-4 as the dopants, with the TADF molecule CzAcSF as the sensitiser. The OLEDs so prepared show a very high maximum external quantum efficiency of 21.2%, revealing a novel strategy for realising large glum values in CP-TADF.

Introduction of a Chiral Biphenanthrene-Diol Unit to Achieve Circularly Polarized Thermally Activated Delayed Fluorescence

Circularly polarized luminescence (CPL) materials are promising candidates for future display technology. However, such highly efficient emitters suffer from the issues of difficult chiral separation and low photoluminescence quantum yield (PLQY). In this work, the chiral 4,4'-biphenanthrene-3,3'-diol (BIPOL) unit was introduced into a thermally activated delayed fluorescence (TADF) framework for the first time. We presented two series of enantiomers, R/S-o-DCzBPNCN and R/S-p-DCzBPNCN, and the synthesis of enantiopure BIPOL can be prepared via normal column chromatography. Notably, o-DCzBPNCN showed narrow singlet-triplet gap of 0.05 eV, efficient TADF, and high PLQYs of 82 % in doped films. In addition, R/S-o-DCzBPNCN exhibited high luminescence dissymmetry factor (gPL ) values of -1.94×10-2 /+1.91×10-2 in doped films. The strategy of BIPOL introduction offers a new approach to organic emitters with stereospecific synthesis, TADF, and chiroptical properties.

Efficient Circularly Polarized Electroluminescence from Achiral Luminescent Materials

Circularly polarized electroluminescence (CP-EL) is generally produced in organic light-emitting diodes (OLEDs) based on special CP luminescent (CPL) materials, while common achiral luminescent materials are rarely considered to be capable of direct producing CP-EL. Herein, near ultraviolet CPL materials with high photoluminescence quantum yields and good CPL dissymmetry factors are developed, which can induce blue to red CPL for various achiral luminescent materials. Strong near ultraviolet CP-EL with the best external quantum efficiencies (η_ext s) of 9.0 % and small efficiency roll-offs are achieved by using them as emitters for CP-OLEDs. By adopting them as hosts or sensitizers, commercially available yellow-orange achiral phosphorescence, thermally activated delayed fluorescence (TADF) and multi-resonance (MR) TADF materials can generate intense CP-EL, with high dissymmetry factors and outstanding η_ext s (30.8 %), demonstrating a simple and universal avenue towards efficient CP-EL.

Inverted and Amplified CP-EL Behavior Promoted by AIE-Active Chiral Co-Assembled Helical Nanofibers

It is well-known that high-performance circularly polarized organic light-emitting diodes (CP-OLEDs) remain a formidable challenge to the future application of circularly polarized luminescent (CPL)-active materials. Herein, the design of a pair of AIE-active chiral enantiomers (L/D-HP) is described to construct chiral co-assemblies with an achiral naphthalimide dye (NTi). The resulting co-assemblies emit an inverted CPL signal compared with that from the L/D-HP enantiomers. After thermal annealing at 120 °C, the inverted CPL signal of this kind of L/D-HP-NTi with a 1:1 molar ratio shows regular and ordered helical nanofibers arranged through intermolecularly ordered layered packing and is accompanied with a further amplified effect (|g_em | = 0.032, λ_em  = 535 nm). Significantly, non-doped CP-OLEDs based on a device emitting layer (EML) of L/D-HP-NTi exhibits a low turn-on voltage (V_on ) of 4.7 V, a high maximum brightness (L_max ) of 2001 cd m^-2 , and moderate maximum external quantum efficiency (EQE_max ) of 2.3%, as well as excellent circularly polarized electroluminescence (CP-EL) (|g_EL | = 0.023, λ_em  = 533 nm).

Research Progress of Intramolecular π-Stacked Small Molecules for Device Applications

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.

Arene-perfluoroarene interaction induced chiroptical inversion and precise ee% detection of chiral acids in a benzimidazole-involved ternary coassembly

Flexible regulation of chirality and handedness of chiral functional materials and quantitative sensing of natural chiral compounds remains a considerable challenge. Herein, an achiral fluorescent 1-pyrenecarboxylic acid-benzimidazole derivative (PBI) was synthesized and its chiroptical properties upon coassembly with chiral acids (TA and MA) and octafluoronaphthalene (OFN) through hydrogen bonds between benzimidazole and chiral acids as well as an arene-perfluoroarene (AP) interaction between a pyrene moiety and OFN were systemically studied. The binary assemblies of PBI/TA and PBI/MA displayed opposite chiroptical properties including circular dichroism (CD) and circularly polarized luminescence (CPL) signals. Interestingly, the handedness of CPL was further inverted in ternary assemblies due to the synergistic effect of the AP interaction and hydrogen bonds. Besides, the highly accurate chiral sensing of chiral acids via binary assemblies was successfully achieved with a high correlation coefficient. This work provides a simple method for regulating the handedness of chiroptical active materials and quantitative sensing of chiral acids through orthogonal multiple component coassemblies.

Advances in circularly polarized electroluminescence based on chiral TADF-active materials

This review summarizes the development status of chiral TADF-active materials with CPEL, covering chiral perturbed TADF molecules, intrinsically chiral TADF molecules, and TADFsensitized fluorescent enantiomers.

Controlling the thermally activated delayed fluorescence of axially chiral organic emitters and their racemate for information encryption

A pair of axially chiral organic enantiomers were facilely prepared through a one-pot sequential synthesis. They exhibit circularly polarized luminescence activities and have thermally activated delayed fluorescence (TADF) and aggregation-induced emission enhancement properties. Meanwhile, these two enantiomers present remarkable and reversible thermochromism in the crystalline state, enabling dual-colour TADF switching between orange and red. However, when they form cocrystals, the resulting racemate shows opposite thermochromic behaviors. These intriguing results probably emanate from their different optical activities, leading to distinct molecular packing modes and molecular conformation variations. Moreover, information encryption based on thermochromism of organic enantiomers and their racemate has been presented for the first time. This work may expand the application scope of chiral organic luminogens and pave a new way to construct intelligent luminescent systems.

A pair of axially chiral organic enantiomers with circularly polarized thermally activated delayed fluorescence and aggregation-induced emission enhancement properties show opposite thermochromic behaviors to their racemate.

D-π*-A type planar chiral TADF materials for efficient circularly polarized electroluminescence

Planar chiral organic fluorescent materials that exhibit high chiral stability, high efficiency and circularly polarized luminescence (CPL) currently remain an unresolved issue despite their promising applications in optical encryption and 3D-display. Herein, a pair of new donor-chiral π-acceptor (D-π*-A) type planar chiral thermally activated delayed fluorescence (TADF) enantiomers, namely R/S-PXZ-PT, are developed. Such a D-π*-A type structure completely suppresses the racemisation of the planar chirality, making it possible to prepare circularly polarized organic light-emitting diodes (CP-OLEDs) by vacuum deposition processing. Moreover, this design perfectly integrates the chiral unit into the luminescent unit to achieve intense CPL activity with luminescence asymmetry factors (g_lum) of ±1.9 × 10^-3. Notably, the enantiomer-based devices exhibit a yellow coloured emission with a maximum external quantum efficiency (EQE) of 20.1%, and mirror-image circularly polarized electroluminescence signals with electroluminescence dissymmetry factors (g_EL) of +1.5 × 10^-3/-1.3 × 10^-3. This work not only enriches the diversity of chiral TADF molecular design, but also provides a new perspective for the development of highly-efficient CP-OLEDs with stable planar chiral TADF materials.

Synthesis and Chiroptical Properties of Planar Chiral Azahelicenes Based on [2.2]Paracyclophane

Three pairs of planar chiral heteroarenes were synthesized using palladium-catalyzed Buchwald-Hartwig coupling and hypervalent iodine-mediated oxidative cyclization from optically pure 4-amino[2.2]paracyclophane. Among them, an enantiomer of planar chiral azahelicene was found to have circularly polarized luminescence activity that was remarkably stronger than that of planar chiral heteroarenes.

High-Efficiency Circularly Polarized Electroluminescence from TADF-Sensitized Fluorescent Enantiomers

A couple of fluorescent enantiomers, which are suitable for the emitters of high-efficiency TADF-sensitized CP-OLEDs, have been developed. The enantiomers show configurational stability, high PLQY of 98 %, large k_r of 7.8×10⁷  s^-1 , and intense CPL activities with |g_lum | values of about 2.5×10^-3 . Notably, by using matchable TADF sensitizer, the enantiomers were then exploited as emitter to fabricate CP-OLEDs. The TADF-sensitized CP-OLEDs not only show mirror-image CPEL activities with g_EL values of +1.8×10^-3 and -1.4×10^-3 , but also display fast start-up featuring with low V_T of 3.0 V as well as driving voltage of 4.8 V at 10 000 cd m^-2 . Meaningfully, the TADF-sensitized fluorescent devices show high EQE_max of 21.5 % and extremely low efficiency roll-off, whose EQEs are 21.2 % and 15.3 % at 1000 and 10 000 cd m^-2 , respectively. The obtained EQEs are comparable to those of CP-TADF emitters, which provides a promising perspective to break through the EL efficiency limit of CP-FL emitters.

Molecular Design and Synthesis of Dicarbazolophane-Based Centrosymmetric Through-Space Donors for Solution-Processed Thermally Activated Delayed Fluorescence OLEDs

Conjugation-extended carbazolophane donors, dicarbazolophanes (DCzp), were designed and synthesized using a multifold stepwise Pd-catalyzed Buchwald-Hartwig amination/ring cyclization process. Furthermore, elaboration of the DCzp core is possible with the introduction of pendant carbazole derivative groups. This provides a way to tune the optoelectronic properties of the thermally activated delayed fluorescence (TADF) compounds DCzpTRZtBu, dtBuCzDCzpTRZtBu, and dMeOCzDCzpTRZtBu. Solution-processed organic light-emitting diodes (OLEDs) were fabricated and achieved a maximum external quantum efficiency (EQE_max) of 8.2% and an EQE of 7.9% at 100 cd/m².

Planar Chiral B-N Heteroarenes Based on [2.2]Paracyclophane as Circularly Polarized Luminescence Emitters

Planar chiral boron-nitrogen heteroarenes based on [2.2]paracyclophane were successfully synthesized in a few steps as a new family of circularly polarized luminescence emitters. It represents the first case of boron-nitrogen heteroarenes with planar chirality. Those compounds have been demonstrated to exhibit strong circularly polarized luminescence signals and high quantum yields, in both solution and doped film (with g_lum up to 5.0 × 10^-3 and Φ_solution up to 73%).

Planar chiral boron difluoride complexes showing circularly polarized luminescence

A novel family of boron difluoride complexes based on [2.2]paracyclophane have been designed and facilely synthesized.