Constructing high-efficiency orange-red thermally activated delayed fluorescence emitters by three-dimension molecular engineering

Thermally Activated Delayed Fluorescence-Guided Photodynamic Therapy Through Skeleton-Homologous Nanoparticles: a Rational Material Design for High-Efficient and High-Contrast Theranostics

Although photoluminescence imaging-guided photodynamic therapy (PDT) is promising for theranostics, it easily suffers from tissue autofluorescence and PDT photoproducts. To develop time-resolved imaging (TRI)-guided PDT with long-lived emission pathways, like thermally activated delayed fluorescence (TADF), is urgent but challenging, because of the triplet competition between radiative transition and reactive oxygen species (ROS) production. Herein, skeleton-homologous nanoparticles are designed and constructed to address this dilemma, thereby achieving in vivo TRI-guided PDT for the first time. This system is formed with a lipophilic TADF core (as a TRI probe) encapsulated by an amphiphilic photosensitizer shell (as the corona exposed to oxygen for PDT), both of which are derived from the same donor-acceptor skeleton to minimize phase separation in the single entity, and enable the same long-wavelength photoexcitation for TRI and PDT. The chloropropylamine group is helpful for endoplasmic reticulum targeting to enhance PDT upon minimizing the ROS transmission path. Synchronously, the TADF core exhibits a delayed fluorescence of 40 µs for a clear TRI. The NPs are eventually applied in vivo with a high signal-to-background ratio (45.25) and outstanding PDT effects in a mouse model of deep-seated kidney cancer. Such a material design is beneficial for developing high-efficient and high-contrast theranostic approaches.

A triply linked propellane-nanoring hybrid serving as a good host

A novel propellane-nanoring hybrid, TPPTI-[9]CMP, was synthesized by triply combining [9]cyclo-meta-phenylene ([9]CMP) with triperyleno[3,3,3]propellane triimides (TPPTI). This structure features two [9]CMP subunits, which necessarily fill the voids of TPPTI and promote assembly of the hybrid to form a porous superstructure held together by attractive dispersion between the [9]CMP subunits of neighboring molecules. In the structure, three large spatial cavities are formed, which allow efficient binding of up to three C60 within a single hybrid. Transient absorption spectroscopy revealed that TPPTI-[9]CMP and C60 interact with each other to form a stable complex and produce long-lived triplet states. Notably, the hybrid can adsorb ethane (C2H6) with very excellent selectivity over ethylene (C2H4), leading to a highly selective C2H6/C2H4 separation.

Regulating the Spatially Folded Arrangement of Donor and Acceptor Units To Achieve Efficient Orange-Red Thermally Activated Delayed Fluorescence

Achieving efficient long-wavelength organic light-emitting diodes (OLEDs) remains a challenge due to the energy gap law, which leads to increased non-radiative decay rates as the emission wavelength shifts to longer regions. Herein, a strategy of constructing folded three-dimensional architectures is proposed to explore new orange-red thermally activated delayed fluorescence (TADF) emitters with through-space charge transfer characteristics. Innovatively, naphthalene is selected as a bridge to connect O-bridged triphenylamine donor and planar dibenzo[a,c]phenazine acceptor respectively via simple Suzuki-Miyaura Coupling. In this way, a series of rigid orange-red emitters with "U"-shaped, folded "Z"-shaped and "W"-shaped configurations are elaborately constructed by modifying end groups, adjusting the numbers of naphthalene and donor, and regulating the linkage sites. The excited state natures and photophysical properties of the emitters can be effectively regulated and optimized by changing three-dimensional architectures. Finally, the prepared emitter QX36 achieves a lower non-radiative transition rate, a higher radiative rate and a higher photoluminescence quantum efficiency. Solution-processed OLEDs based QX36 present the excellent electroluminescent performance with a maximum external quantum efficiency (EQE) of up to 32.3 % and EQE of 20.6 % at 1000 cd m-2, which are the leading values of solution-processed orange-red OLEDs. This work demonstrates the promising potential of folded TADF based naphthalene backbone as emitters for future efficient solution-processed long-wavelength OLEDs.

Intramolecular charge transfer assisted multi-resonance thermally activated delayed fluorescence emitters for high-performance solution-processed narrowband OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been actively employed in high-resolution solution-processed organic light emitting diodes (OLEDs) due to their excellent color purity. Nonetheless, they are always confronted with intrinsic slow spin flip of triplet excitons, impeding the electroluminescence properties, especially in non-sensitized OLEDs. Herein, we constructed intramolecular charge transfer (ICT) assisted MR-TADF emitters by grafting donor-acceptor-type moieties with a meta- or para-substitution as a pendant on an organoboron multi-resonance core. The newly designed MR-TADF emitters not only maintain short range charge transfer characteristics in emissive states without sacrificing color purity but the accelerated spin flips facilitated by the ICT process at a high-lying state are also confirmed by ultrafast spectroscopy and theoretical calculation, achieving over a 10-fold increase in the reverse intersystem crossing rate compared with unsubstituted counterpart emitters. In sensitizer-free solution-processed OLEDs, a cutting-edge external quantum efficiency of 27.8% can be achieved together with reduced efficiency roll-offs and an attractive full width at half maximum of 29 nm, representing a breakthrough in efficiency for solution-processed MR-TADF based narrowband OLEDs.

Sulfur Lone Pairs Open Avenues for π* → n Orange-to-Red TADF and OLEDs

It is always important and fascinating to explore new organic emitters that exploit unconventional pathways to unveil their emission with unique properties, such as thermally activated delayed fluorescence (TADF). In this study, we report that the rarely explored sulfur lone pair (n) is a promising alternative, where the correlated π* → n emission can be used to attain strong TADF and thus practical OLEDs. The designed strategy incorporates several key concepts (Figure 1a), in which the persulfide aromatic spirocycle enhances spin-orbit coupling, thereby increasing the intersystem crossing rate. Next, molecules with a twisted donor-acceptor configuration bridged by spiro[4.4]nonane as well as spatially orthogonal sulfur lone pairs and π* features significantly reduce the singlet-triplet gap. Finally, the rigid spirocyclic backbone inhibits nonradiative transitions. The proof-of-concept is given by compound 1, which achieves nπ* thermally activated delayed fluorescence (TADF) maximized at 635 nm with photoluminescence quantum yields as high as 52% in CH2Cl2. Wet-processed OLEDs containing 1 achieved a maximum external quantum efficiency of 6.4% at a brightness of 189 cd m-2 (∼600 nm), opening an alternative 1nπ* route for practical π* → n TADF OLEDs.

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.

Cascade Effect of a Dimerized Thermally Activated Delayed Fluorescence Dendrimer

Thermally activated delayed fluorescence (TADF) emitters with a high horizontal orientation are highly essential for improving the external quantum efficiency (EQE) of organic light-emitting diodes; however, pivotal molecular design strategies to improve the horizontal orientation of solution-processable TADF emitters are still scarce and challenging. Herein, a phenyl bridge is adopted to connect the double TADF units, and generate a dimerized TADF dendrimer, D4CzBNPh-SF. Compared to its counterpart with a single TADF unit, the proof-of-the-concept molecule not only exhibits an improved horizontal dipole ratio (78 %) due to the π-delocalization-induced extended molecular conjugation, but also displays a faster reversed intersystem crossing rate constant (6.08×106 s-1) and a high photoluminescence quantum yield of 95 % in neat film. Consequently, the non-doped solution-processed device with D4CzBNPh-SF as the emitter achieves an ultra-high maximum EQE of 32.6 %, which remains at 26.6 % under a luminance of 1000 cd/m2. Furthermore, when using D4CzBNPh-SF as a sensitizer, the TADF-sensitized fluorescence device exhibits a high maximum EQE of 30.7 % at a luminance of 575 cd/m2 and a full width at half maximum of 36 nm.

Tailoring intersystem crossing in phosphorus corroles through axial chalcogenation: a detailed theoretical study

Intersystem crossing (ISC) of visible-light absorbing metal-free corrole macrocycles can be greatly tuned by means of suitable chemical functionalization. Axially chalcogenated phosphorus corrole derivatives (XPCs; X = O, S, Se) are expected to show large spin-orbit coupling (SOC) via the heavy-atom effect and therefore a much improved ISC. Excited-state deactivation of XPCs including PC is studied using time-dependent optimally tuned range-separated hybrid functionals combined with a polarizable continuum model with toluene as a dielectric medium to account for polar solvent effects. PC and all XPCs are dynamically stable and also show favourable thermodynamic formation feasibility as confirmed by Gibbs free energy analysis. In spite of the relatively smaller contribution of P and X to the frontier molecular orbitals compared to the tetrapyrrolic ring, SOC is considerably improved due to the heavy-atom effect. While PC shows a one-order larger ISC rate of ∼107 s-1 than fluorescence, competitive fluorescence and ISC rates of ∼107 s-1 are found for OPC. In contrast, both SPC and SePC exhibit significantly larger ISC rates of ∼109 s-1 and ∼1013 s-1, respectively, with much smaller fluorescence rates of ∼107 s-1. Importantly, the first report of anti-Kasha's emission in metal-free corroles is predicted for OPC with a radiative rate of ∼109 s-1. Furthermore, calculated phosphorescence and ISC rates from the near-degenerate lowest excited triplets to the ground-state suggest millisecond to microsecond triplet lifetimes, signalling towards long-lived excited triplet formation. Overall, all three XPCs including PC could act as triplet photosensitizers and especially both SPC and SePC are predicted to be the highly efficient ones.

Reverse Intersystem Crossing Boosting Sensitizer for Ultra-High Efficiency Blue Organic Light-Emitting Diode

In designing thermally activated delayed fluorescence (TADF) emitters, a high reverse intersystem crossing (RISC) rate with a high photoluminescence quantum yield is essential. Herein, two blue TADF molecules, 2',5'-di(9H-carbazol-9-yl)-3',6'-bis(3,6-ditert-butyl-9H-carbazol-9-yl)-[1,1':4',1″-terphenyl]-4,4″-dicarbonitrile (CzTCzPhBN) and 2',5'-bis(3,6-ditert-butyl-9H-carbazol-9-yl)-3',6'-bis(3,6-diphenyl-9H-carbazol-9-yl)-[1,1':4',1″-terphenyl]-4,4″-dicarbonitrile (PhCzTCzPhBN) with a high RISC rate, were developed through donor engineering. CzTCzPhBN and PhCzTCzPhBN showed a high RISC rate of 4.00 × 105 and 16.62 × 105 s-1, respectively, with a high photoluminescence quantum yield of 80.1 and 84.9%, which resulted in high external quantum efficiency of 27.0 and 27.8% with color coordinates (0.148, 0.170) and (0.150, 0.230) in blue TADF organic light-emitting diodes, respectively. The high RISC rate and device efficiency inspired two TADF molecules to be used as sensitizers in hyperfluorescence devices. The hyperfluorescence devices showed ultra-high external quantum efficiency of 30.7 and 36.4% with color coordinates (0.125, 0.164) and (0.127, 0.193), respectively.

Two Birds with One Stone: Polymerized Thermally Activated Delayed Fluorescence Small Molecules

Thermally activated delayed fluorescence (TADF) polymers show a great potential in low-cost, large-area and flexible full-color flat-panel displays. One of the most promising design rules is based on TADF+Linker, where a small molecular TADF unit is bonded to each other by a simple linker. Unlike the expensive vacuum deposition for small molecules, these polymerized TADF small molecules (Poly-TADF-SMs) are capable of cost-effective solution processing. Meanwhile, the good luminescent property of small molecular TADF emitters can be well inherited by Poly-TADF-SMs so as to bridge the efficiency gap between small molecules and polymers. Herein, we will highlight the recent progress of Poly-TADF-SMs, together with emphasis on their molecular design, photophysical and electroluminescence properties.

A general supramolecular strategy for fabricating full-color-tunable thermally activated delayed fluorescence materials

Developing a facile and feasible strategy to fabricate thermally activated delayed fluorescence materials exhibiting full-color tunability remains an appealing yet challenging task. In this work, a general supramolecular strategy for fabricating thermally activated delayed fluorescence materials is proposed. Consequently, a series of host-guest cocrystals are prepared by electron-donating calix[3]acridan and various electron-withdrawing guests. Owing to the through-space charge transfer mediated by multiple noncovalent interactions, these cocrystals all display efficient thermally activated delayed fluorescence. Especially, by delicately modulating the electron-withdrawing ability of the guest molecules, the emission colors of these cocrystals can be continuously tuned from blue (440 nm) to red (610 nm). Meanwhile, high photoluminescence quantum yields of up to 87% is achieved. This research not only provides an alternative and general strategy for the fabrication of thermally activated delayed fluorescence materials, but also establishes a reliable supramolecular protocol toward the design of advanced luminescent materials.

Adjusting the Electron-Withdrawing Ability of Acceptors in Thermally Activated Delayed Fluorescence Conjugated Polymers for High-Performance OLEDs

Constructing high-performance solution-processed organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) conjugated polymers remains a challenging issue. The electron-withdrawing ability of acceptors in TADF units significantly affects the TADF properties of the conjugated polymers. Herein, we have designed three TADF conjugated polymers, in which phenoxazine donors and anthracen-9(10H)-one acceptors are incorporated into the polymeric backbones and side chains, respectively, and the carbazole derivative is copolymerized as the host. By incorporating different heteroatoms, such as nitrogen, oxygen, or sulfur, with slightly different electronegativities into anthracen-9(10H)-one, the effect of the electron-withdrawing ability of the acceptor on the performance of conjugated TADF polymer-based OLEDs is thus systematically studied. It is found that the introduction of a nitrogen atom can enhance the spin-orbital coupling and RISC process due to the modulated energy levels and nature of the excited states. As a result, the solution-processed OLEDs based on the prepared polymer p-PXZ-XN display an excellent comprehensive performance with an EQEmax of 17.6%, a low turn-on voltage of 2.8 V, and a maximum brightness of 14750 cd m-2. Notably, the efficiency roll-off is quite low, maintaining 15.1% at 1000 cd m-2, 12.1% at 3000 cd m-2, and 6.1% at 10000 cd m-2, which ranks in the first tier among the reported TADF conjugated polymers. This work provides a guideline for constructing high-efficiency TADF polymers.

Thermally Activated Delayed Fluorescence Emitters Based on a Special Tetrahedral Silane Core

Based on the tetraphenylsilane skeleton, a new class of thermally activated delayed fluorescence (TADF) molecules have been designed and synthesized. Benefiting from the unique tetrahedron architecture of tetraphenylsilane, the intermolecular distance between TADF units can be enlarged and thus weakened the aggregation-induced quenching of triplet excitons. By adjusting the numbers of TADF subunits, the spin-orbit coupling processes can be controlled, leading to efficient up-conversion processes. The related OLEDs are fabricated through the solution processing technology, and pure-blue and green electroluminescence were observed with maximum external quantum efficiencies (EQEmax) of 6.6 and 13.8% as well as Commission Internationale de l'Eclairage coordinates of (0.14, 0.15) and (0.25, 0.45), respectively. This study provides a new idea for designing color-tunable TADF emitters through spatial structure regulation.

Naphtalimide-Based Bipolar Derivatives Enabling High-Efficiency OLEDs

Organic light-emitting diodes (OLEDs) have revolutionized the world of technology, making significant contributions to enhancing our everyday lives. With their exceptional display and lighting capabilities, OLEDs have become indispensable in various industries such as smartphones, tablets, televisions, and automotives. They have emerged as a dominant technology, inspiring continuous advancements, and improvements. Taking inspiration from the remarkable advancements in OLED advancements, we have successfully developed naphtalimide-based compounds, namely RB-08, RB-09, RB-10, and RB-11. These compounds exhibit desirable characteristics such as a wide bandgap, high decomposition temperatures (306-366 °C), and very high glass transition temperatures (133-179 °C). Leveraging these exceptional properties, we have harnessed these compounds as green emitters in the aforementioned devices. Among the various fabricated OLEDs, the one incorporating the RB-11 emitter has exhibited superior performance. This specific configuration achieved maximum power efficacy of 7.7 lm/W, current efficacy of 7.9 cd/A, and external quantum efficiency of 3.3%. These results highlight the outstanding capabilities of our synthesized emitter and its potential for further advancements in the field.

Regulating the Nature of Triplet Excited States of Thermally Activated Delayed Fluorescence Emitters

ConspectusCharacterized by the reverse intersystem crossing (RISC) process from the triplet state (T₁) to the singlet state (S₁), thermally activated delayed fluorescence (TADF) emitters, which produce light by harvesting both triplet and singlet excitons without noble metals, are considered to be third-generation organic electroluminescent materials. Rapid advances in molecular design criteria, understanding the photophysics underlying TADF, and applications of TADF materials as emitters in organic light-emitting diodes (OLEDs) have been achieved. Theoretically, enhanced spin-orbit coupling (SOC) between singlet and triplet states can result in a fast RISC process and thus a high light-emitting efficiency according to Fermi's golden rule. Therefore, regulating the nature of triplet excited states by elaborate molecular design to improve SOC is an effective approach to high-efficiency TADF-based OLEDs. Generally, on one hand, the increased local excited (LE) populations of the excited triplet state can significantly improve the nature flips between S₁ and T₁. On other hand, the reduced energy gap between S₁ and the lowest triplet with a charge transfer (CT) characteristic can also enhance their vibronic coupling. Consequently, it is vital to determine how to regulate the nature of triplet excited states by molecular design to guide the material synthesis, especially for polymeric emitters.In this Account, we focus on modulating the strategy of triplet excited states for TADF emitters and an in-depth understanding of the photophysical processes, leading to optimized OLED device performance. We include several kinds of strategies to control the nature of triplet excited states to guide the synthesis of small-molecule and polymer TADF emitters: (1) Modulating the electronic distribution of conjugated polymeric backbones by copolymerizing the electron-donating host: accordingly, the nature of excited states can be changed, especially for triplets. Meanwhile, the utilization of excitons can be systematically improved by adjusting the electronic structure of triplet states with long-range distribution in the conjugated polymeric backbones. (2) Halogenating acceptors of TADF units: the introduced halogen atoms would reestablish the electronic distribution of the triplet and relocate the hole orbits, resulting in a CT and LE hybrid nature of a triplet transformed into a LE-predominant state, which favors the RISC process. (3) Stereostructure regulation: by constructing a diverse arrangement of three-dimensional spatial configurations or conjugated architectures, the nature of the triplet can also be finely tuned, such as hyperbranched structures with multiple triplet-singlet vibration couplings, half-dendronized-half-encapsulated asymmetric systems, trinaphtho[3,3,3] propeller-based three-dimensional spatial interspersed structures, intramolecular close-packed donor-acceptor systems, and so on. We hope that this Account will provide insights into new structures and mechanisms for achieving high-performance OLEDs based on regulating the nature of triplet excited states.