Linearly Arranged Multi-π-Stacked Structure for Efficient Through-Space Charge-Transfer Emitters.

Noncovalent spatial interaction has become an intriguing and important tool for constructing optoelectronic molecules. In this study, we linearly attached three conjugated units in a multi π-stacked manner by using just one trident bridge based on indeno[2,1-b]fluorene. To achieve this structure, we improved the synthetic approach through double C-H activation, significantly simplifying the preparation process. Due to the proximity of the C10, C11, and C12 sites in indeno[2,1-b]fluorene, we derived two novel donor|acceptor|donor (D|A|D) type molecules, 2DMB and 2DMFB, which exhibited closely packed intramolecular stacking, enabling efficient through-space charge transfer. This molecular construction is particularly suitable for developing high-performance thermally activated delayed fluorescence materials. With donor(s) and acceptor(s) constrained and separated within this spatially rigid structure, elevated radiative transition rates, and high photoluminescence quantum yields were achieved. Organic light-emitting diodes incorporating 2DMB and 2DMFB demonstrated superior efficiency, achieving maximum external quantum efficiencies of 28.6% and 16.2%, respectively.

Design and Synthesis of Red Through-Space Charge Transfer Thermally Activated Delayed Fluorescence Emitters with Donor/Acceptor/Donor Stacking.

Red through-space charge transfer thermally activated delayed fluorescence (TSCT TADF) materials named SAF36DCPP and SAF27DCPP with sandwiched structures were synthesized. Single crystals indicated that the intramolecular C-H···π interactions play a vital role in rigidifying the sandwiched structure, which results in a fluorescence yield of 63% for SAF36DCPP compared to 40% for SAF27DCPP. Organic light-emitting diodes with SAF36DCPP as the emitter realized a maximum external quantum efficiency of 16.12%.

Steric Modulation of Spiro Structure for Highly Efficient Multiple Resonance Emitters.

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.

Segregated Array Tailoring Charge-Transfer Degree of Organic Cocrystal for the Efficient Near-Infrared Emission beyond 760 nm.

Harvesting the narrow bandgap excitons of charge-transfer (CT) complexes for the achievement of near-infrared (NIR) emission has attracted intensive attention for its fundamental importance and practical application. Herein, the triphenylene (TP)-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4 TCNQ) CT organic complex is designed and fabricated via the supramolecular self-assembly process, which demonstrates the NIR emission with a maximum peak of 770 nm and a photoluminescence quantum yield (PLQY) of 5.4%. The segregated stacking mode of TP-F4 TCNQ CT complex based on the multiple types of intermolecular interaction has a low CT degree of 0.00103 and a small counter pitch angle of 40° between F4 TCNQ and TP molecules, which breaks the forbidden electronic transitions of CT state, resulting in the effective NIR emission. Acting as the promising candidates for the active optical waveguide in the NIR region beyond 760 nm, the self-assembled TP-F4 TCNQ single-crystalline organic microwires display an ultralow optical-loss coefficient of 0.060 dB µm-1 . This work holds considerable insights for the exploration of novel NIR-emissive organic materials via an universal "cocrystal engineering" strategy.

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.

Divergent Synthesis of Contorted Polycyclic Aromatics Containing Pentagons, Heptagon, and/or Azulene.

Divergent synthesis of four contorted aromatics containing pentagons, a heptagon, and/or an azulene from the same difluorenyl pentacenediene precursor were realized in one step. The subtle differences in molecular structure were confirmed by X-ray crystallography. The mechanisms for the control of different products, which involve a ring-expansion rearrangement, Scholl reactions, and/or Mallory cyclization were proposed on the basis of control experiments and DFT calculations. Such development adds good structure versatility and materials accessibility to the study of contorted aromatics.

A narrowband blue circularly polarized thermally activated delayed fluorescence emitter with a hetero-helicene structure.

A narrowband blue CP-TADF emitter with a rigid hetero-helicene structure (QAO-PhCz) was synthesized and characterized. QAO-PhCz exhibits good electroluminescence performance (EQE = 14.0%) and narrow FWHM. The enantiomers of QAO-PhCz display CPL and CPEL properties with |glum| and |gEL|values of up to 1.1 × 10-3 and 1.5 × 10-3, respectively.

Efficient and Spectrally Stable Blue Perovskite Light-Emitting Diodes Employing a Cationic π-Conjugated Polymer.

Metal halide perovskite semiconductors have demonstrated remarkable potentials in solution-processed blue light-emitting diodes (LEDs). However, the unsatisfied efficiency and spectral stability responsible for trap-mediated non-radiative losses and halide phase segregation remain the primary unsolved challenges for blue perovskite LEDs. In this study, it is reported that a fluorene-based π-conjugated cationic polymer can be blended with the perovskite semiconductor to control film formation and optoelectronic properties. As a result, sky-blue and true-blue perovskite LEDs with Commission Internationale de l'Eclairage coordinates of (0.08, 0.22) and (0.12, 0.13) at the record external quantum efficiencies of 11.2% and 8.0% were achieved. In addition, the mixed halide perovskites with the conjugated cationic polymer exhibit excellent spectral stability under external bias. This result illustrates that π-conjugated cationic polymers have a great potential to realize efficient blue mixed-halide perovskite LEDs with stable electroluminescence.

Hierarchical Self-Assembly of Organic Core/Multi-Shell Microwires for Trichromatic White-Light Sources.

White-light-emissive organic micro/nanostructures hold exotic potential applications in full-color displays, on-chip wavelength-division multiplexing, and backlights of portable display devices, but are rarely realized in organic core/shell heterostructures. Herein, through regulating the noncovalent interactions between organic semiconductor molecules, a hierarchical self-assembly approach of horizontal epitaxial-growth is demonstrated for the fine synthesis of organic core/mono-shell microwires with multicolor emission (red-green, red-blue, and green-blue) and especially organic core/double-shell microwires with radial red-green-blue (RGB) emission, whose components are dibenzo[g,p]chrysene (DgpC)-based charge-transfer (CT) complexes. In fact, the desired lattice mismatching (≈2%) and the excellent structure compatibility of these CT complexes facilitate the epitaxial-growth process for the facile synthesis of organic core/shell microwires. With the RGB-emissive substructures, these core/double-shell organic microwires are microscale white-light sources (CIE [0.34, 0.36]). Besides, the white-emissive core/double-shell microwires demonstrate the fascinating full-spectrum light transportation from 400 to 700 nm. This work indeed opens up a novel avenue for the accurate construction of organic core/shell heterostructures, which provides an attractive platform for the organic integrated optoelectronics.

Fully Bridged Triphenylamine Derivatives as Color-Tunable Thermally Activated Delayed Fluorescence Emitters.

Three emissive bridged-triphenylamine derivatives are designed and synthesized by incorporating carbon (DQAO), oxygen (OQAO), and sulfur (SQAO) atoms with two carbonyl groups. The fully bridged geometry and unique frontier molecular orbital distribution reveal its potential as narrowband thermally activated delayed fluorescence emitters. DQAO-, OQAO-, and SQAO-based organic light-emitting diodes exhibit the maximum external quantum efficiency (EQEmax) of 15.2%, 20.3%, and 17.8% for blue, green, and yellow, respectively.

Multi-Layer π-Stacked Molecules as Efficient Thermally Activated Delayed Fluorescence Emitters.

Multi-layer π-stacked emitters based on spatially confined donor/acceptor/donor (D/A/D) patterns have been developed to achieve high-efficiency thermally activated delayed fluorescence (TADF). In this case, dual donor moieties and a single acceptor moiety are introduced to form two three-dimensional (3D) emitters, DM-BD1 and DM-BD2, which rely on spatial charge transfer (CT). Owing to the enforced face-to-face D/A/D pattern, effective CT interactions are realized, which lead to high photoluminescence quantum yields (PLQYs) of 94.2 % and 92.8 % for the two molecules, respectively. The resulting emitters exhibit small singlet-triplet energy splitting (ΔEST ) and fast reverse intersystem crossing (RISC) processes. Maximum external quantum efficiencies (EQEs) of 28.0 % and 26.6 % were realized for devices based on DM-BD1 and DM-BD2, respectively, which are higher than those of their D/A-type analogues.

Highly efficient luminescence from space-confined charge-transfer emitters.

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-2 with only minor efficiency roll-off (EQE = 24.4%) at a higher luminous intensity of 1,000 cd m-2. 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.

Evolution of pure hydrocarbon hosts: simpler structure, higher performance and universal application in RGB phosphorescent organic light-emitting diodes.

In the field of phosphorescent organic light-emitting diodes (PhOLEDs), designing high-efficiency universal host materials for red, green and blue (RGB) phosphors has been quite a challenge. To date, most of the high-efficiency universal hosts reported incorporate heteroatoms, which have a crucial role in the device performance. However, the introduction of different kinds of heterocycles increases the design complexity and cost of the target material and also creates potential instability in the device performance. In this work, we show that pure aromatic hydrocarbon hosts designed with the 9,9'-spirobifluorene scaffold are high-efficiency and versatile hosts for PhOLEDs. With external quantum efficiencies of 27.3%, 26.0% and 27.1% for RGB PhOLEDs respectively, this work not only reports the first examples of high-efficiency pure hydrocarbon materials used as hosts in RGB PhOLEDs but also the highest performance reported to date for a universal host (including heteroatom-based hosts). This work shows that the PHC design strategy is promising for the future development of the OLED industry as a high-performance and low-cost option.

Zn(II)-benzotriazolate clusters based amide functionalized porous coordination polymers with high CO2 adsorption selectivity.

Two new porous coordination polymers (PCPs) based on different nanosized C3 symmetry ligands and Zn(II)-benzotriazolate clusters have been synthesized solvothermally. Both of the desolvated complexes show selective uptake of CO2 over CH4 and N2 at ambient temperature.