Manipulation of Molecular Aggregation States to Realize Polymorphism, AIE, MCL, and TADF in a Single Molecule

Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor-Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

The photophysical processes involved in the electron donor-acceptor thermally activated delayed fluorescence (TADF) emitters are complicated and controversial. The recent consensus is that at least three states are involved, i. e. the singlet charge transfer state (¹ CT), the triplet localized excited state (³ LE) and the triplet CT state (³ CT). It is clear the very often used steady state and time-resolved luminescence spectroscopic methods are unable to present direct evidence for the dark states, i. e. the ³ LE and ³ CT states, as well as the interconversion of these states. Concerning this aspect, the femtosecond-nanosecond transient absorption spectroscopic methods are in particular interests. Both the emissive state and the dark state can be detected in these spectra, and interconversion of the states involved in TADF process can be also revealed. This review article focuses on the recent development of using the transient absorption spectra to study the photophysics of the TADF emitters.

Dynamic B/N Lewis Pairs: Insights into the Structural Variations and Photochromism via Light-Induced Fluorescence to Phosphorescence Switching

Ultralong afterglow emissions due to room-temperature phosphorescence (RTP) are of paramount importance in the advancement of smart sensors, bioimaging and light-emitting devices. We herein present an efficient approach to achieve rarely accessible phosphorescence of heavy atom-free organoboranes via photochemical switching of sterically tunable fluorescent Lewis pairs (LPs). LPs are widely applied in and well-known for their outstanding performance in catalysis and supramolecular soft materials but have not thus far been exploited to develop photo-responsive RTP materials. The intramolecular LP M1BNM not only shows a dynamic response to thermal treatment due to reversible N→B coordination but crystals of M1BNM also undergo rapid photochromic switching. As a result, unusual emission switching from short-lived fluorescence to long-lived phosphorescence (rad-M1BNM, τ_RTP =232 ms) is observed. The reported discoveries in the field of Lewis pairs chemistry offer important insights into their structural dynamics, while also pointing to new opportunities for photoactive materials with implications for fast responsive detectors.

Optimal Dihedral Angle in Twisted Donor-Acceptor Organic Emitters for Maximized Thermally Activated Delayed Fluorescence

The twisted donor-acceptor (D-A) organic formwork with a large dihedral angle (θ_DA ) is usually adopted to narrow the singlet-triplet energy gap for obtaining excellent thermally activated delayed fluorescence (TADF) emitters. However, the dependence of overall TADF properties on θ_DA has not been systematically investigated to this day. Taking new designed CzBP, CzBP-1M and CzBP-2M via introducing methyl as investigated models, it is found that (i) with increasing θ_DA , the charge transfer component in S₁ is larger than that in T₁ in varying degrees, leading to non-monotonic spin-orbit couplings; (ii) the electron-vibration couplings between S₁ and T₁ states become the largest when θ_DA approaching 80°, facilitating phonon-driven up-conversion; (iii) the overall TADF rate reaches a peak at θ_DA ≈80°. By this, the TADF on/off switching is realized via methyl moiety for regulating θ_DA from theoretical prediction to experimental confirmation. Importantly, the θ_DA near 80° would be a good descriptor for screening excellent D-A type TADF emitters.

Solid-State Emission and Aggregate Emission of Aroyl- S,N-Ketene Acetals Are Controlled and Tuned by Their Substitution Pattern

Aroyl-S,N-ketene acetals are a novel highly diverse class of aggregation-induced emission fluorogens (AIEgens) with a plethora of interesting properties. An expanded compound library of more than 110 dyes set the stage for the first qualitative control and tuneability of all aspects of their photophysical properties. The interplay of substituents not only allows tuning and prediction of the emission color, but also of the intensity, and quantum yields both in solids and in the aggregated state; these can be rationalized by scrutinizing intermolecular interactions in the crystalline solid state.

Highly Efficient Sky-Blue π-Stacked Thermally Activated Delayed Fluorescence Emitter with Multi-Stimulus Response Properties

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.

A Tri-state Fluorescent Switch with "Gated" Solid-state Photochromism Induced by an External Force

Mr. Chen Qian, Dr. Zhimin Ma, Mr. Jianwei Liu, Mrs. Xue Zhang, Prof. Shitao Wang and Prof. Zhiyong Ma. In this article, we report a newly designed molecule composed of a dihydroazulene (DHA) group and a phenothiazine (PTZ) moiety, which achieves aggregation-induced emission enhancement (AIEE), mechanochromism and "gated" solid-state photochromism upon stimulation by an external force. Grinding loosens intermolecular interactions in the crystal and causes a red-shift of fluorescence from 570 nm to 600 nm. Meanwhile, the ring-opening reaction of DHA unit is activated by grinding and a remarkable photochromism could be observed from the grinded powder. The reddish emission of the grinded powder peaked at 600 nm weakened gradually and finally became dark, and a new absorption band at 470 nm emerged in the absorption spectra. Time-dependent density functional theory (TD-DFT) calculation results reveal that the intramolecular intramolecular charge-transfer (ICT) process is replaced by a locally excited (LE) emission on the DHA group, which leads to the quenching of fluorescence. Its impressive photochromic property inspired us to a simple but effective way to develop an encryption system which can let the correct information be displayed upon external stimulation.

Selective Expression of a Carbazole-Phenothiazine Derivative Leads to Dual-mode AIEE, TADF and Distinctive Mechanochromism

In this article, we report a newly designed D-A-D' derivative (CNCzPTZ), which displays selective expression of chromophores. This enables CNCzPTZ with solvatochromism, rare dual-mode AIEE properties, solid-state dual-emissions with phosphorescence and distinctive mechanochromism.CNCzPTZ exhibits dual-mode AIEE properties, since the emission band abruptly shifts from 550 nm to 500 nm as the water fraction increases. In the crystalline state, CNCzPTZ demonstrated dual emission bands of 478 nm and 538 nm.CNCzPTZ shows distinctive mechanochromic property in the solid state due to the planarization.

Donor/Acceptor Pairs Created by Electrostatic Interaction: Design, Synthesis, and Investigation on the Exciplex Formed Within the Pair

Exciplexes formed between donors and acceptors have been widely explored but isolating them from each other and tuning the interaction between the donor and acceptor have remained challenges. Here, we report donor/acceptor (D/A) pairs created by electrostatic interaction between a carbazole-based anionic donor and a 1,3,5-triazine-based cationic acceptor and the exciplex formed within the pair. In a diluted film, the D/A pair affords an isolated exciplex which shows thermally activated delayed fluorescence (TADF). By changing the anchoring position of the imidazolium cation in the cationic acceptor, interactions between the donor and acceptor can be changed. Compared to the conventional exciplex formed in a neat film, the isolated exciplex exhibits a substantially higher luminescence efficiency. The D/A pairs show intriguing mechanochromic luminescence and mechanical grinding-induced/reinforced TADF in the solid state and promising performances as emitters in organic light-emitting diodes.

Surfactant-Induced Interfacial Aggregation of Porphyrins for Structuring Color-Tunable Liquids

Locking nonequilibrium shapes of liquids into targeted architectures by interfacial jamming of nanoparticles is an emerging area in material science. 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (H₆ TPPS) shows three different aggregation states that present an absorption imaging platform to monitor the assembly and jamming of supramolecular polymer surfactants (SPSs) at the liquid/liquid interface. The interfacial interconversion of H₆ TPPS, specifically H₄ TPPS^2- dissolved in water, from J- to an H-aggregation was induced by strong electrostatic interactions with amine-terminated polystyrene dissolved in toluene at the water/toluene interface. This resulted in color-tunable liquids due to interfacial jamming of the SPSs formed between H₄ TPPS^2- and amine-terminated polystyrene. However, the formed SPSs cannot lock in nonequilibrium shapes of liquids. In addition, a self-wrinkling behavior was observed when amphiphilic triblock copolymers of PS-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) were used to interact with H₄ TPPS^2- . Subsequently, the SPSs formed can lock in nonequilibrium shapes of liquids.

Reversibly Switchable Phase-Dependent Emission of Quinoline and Phenothiazine Derivatives towards Applications in Optical Sensing and Information Multicoding

Three new quinoline and di-tert-butyl phenothiazine isomeric derivatives were synthesized and characterized towards applications for oxygen sensing and optical information multicoding. The compounds with phenylene linker showed outstanding phase-dependent reversibility between ON/OFF states (low and high emission intensity, drastic shifting of emission colors, short- and long-lived fluorescence) in systematic grinding/fuming cycles, as required for multichannel memory devices based on optical information multicoding. The conformational diversity of the phenothiazine unit resulted in dual emission of the doped films implemented by the different luminescence mechanisms with peaks located at 414/530, 416/540, and 440/582 nm. The presence of a phenylene linker and thus two rotational degrees of freedom resulted in quenching of the delayed fluorescence of quasi-equatorial conformers in the solid state. The compound containing no phenylene bridge was characterized by two different driving photoluminescence mechanisms of the doped films: short fluorescence of the quasi-axial conformer and thermally activated delayed fluorescence of the quasi-equatorial form. This compound showed oxygen sensitivity with a Stern-Volmer constant of 7.5×10^-4  ppm^-1 .

Nanomaterials with Supramolecular Assembly Based on AIE Luminogens for Theranostic Applications

One of the major pursuits of biomedical science is to develop advanced strategies for theranostics, which is expected to be an effective approach for achieving the transition from conventional medicine to precision medicine. Supramolecular assembly can serve as a powerful tool in the development of nanotheranostics with accurate imaging of tumors and real-time monitoring of the therapeutic process upon the incorporation of aggregation-induced emission (AIE) ability. AIE luminogens (AIEgens) will not only enable fluorescence imaging but will also aid in improving the efficacy of therapies. Furthermore, the fluorescent signals and therapeutic performance of these nanomaterials can be manipulated precisely owing to the reversible and stimuli-responsive characteristics of the supramolecular systems. Inspired by rapid advances in this field, recent research conducted on nanotheranostics with the AIE effect based on supramolecular assembly is summarized. Here, three representative strategies for supramolecular nanomaterials are presented as follows: a) supramolecular self-assembly of AIEgens, b) the loading of AIEgens within nanocarriers with supramolecular assembly, and c) supramolecular macrocycle-guided assembly via host-guest interactions. Meanwhile, the diverse applications of such nanomaterials in diagnostics and therapeutics have also been discussed in detail. Finally, the challenges of this field are listed in this review.

Mechanochromic Delayed Fluorescence Switching in Propeller-Shaped Carbazole-Isophthalonitrile Luminogens with Stimuli-Responsive Intramolecular Charge-Transfer Excited States

Herein, the universal design of high-efficiency stimuli-responsive luminous materials endowed with mechanochromic luminescence (MCL) and thermally activated delayed fluorescence (TADF) functions is reported. The origin of the unique stimuli-triggered TADF switching for a series of carbazole-isophthalonitrile-based donor-acceptor (D-A) luminogens is demonstrated based on systematic photophysical and X-ray analysis, coupled with theoretical calculations. It was revealed that a tiny alteration of the intramolecular D-A twisting in the excited-state structures governed by the solid morphologies is responsible for this dynamic TADF switching behavior. This concept is applicable to the fabrication of bicolor emissive organic light-emitting diodes using a single TADF emitter.

Precise Molecular Design for High-Performance Luminogens with Aggregation-Induced Emission

Precise design of fluorescent molecules with desired properties has enabled the rapid development of many research fields. Among the different types of optically active materials, luminogens with aggregation-induced emission (AIEgens) have attracted significant interest over the past two decades. The negligible luminescence of AIEgens as a molecular species and high brightness in aggregate states distinguish them from conventional fluorescent dyes, which has galvanized efforts to bring AIEgens to a wide array of multidisciplinary applications. Herein, the useful principles and emerging structure-property relationships for precise molecular design toward AIEgens with desirable properties using concrete examples are revealed. The cutting-edge applications of AIEgens and their excellent performance in enabling new research directions in biomedical theranostics, optoelectronic devices, stimuli-responsive smart materials, and visualization of physical processes are also highlighted.

Crystal-State Photochromism and Dual-Mode Mechanochromism of an Organic Molecule with Fluorescence, Room-Temperature Phosphorescence, and Delayed Fluorescence

A D-A-D' type pure organic molecule, named ODFRCZ, has unique triple-emission character covering fluorescence, phosphorescence, and delayed fluorescence (DF). The phosphorescence of ODFRCZ has a rather long lifetime of about 350 ms at room temperature. One dimer of ODFRCZ with enhanced parallel molecular packing acts more effectively to prompt ISC processes, which further generates room-temperature phosphorescence (RTP), owing to the larger transition dipole moment and closer energy level between S₁ and T_n . ODFRCZ is a rare example of an organic RTP molecule that shows dual-stimuli responsiveness of dual-mode mechanochromism (fluorescence red-shift and RTP/DF on-off switch) and reversible crystal-state photochromism. This work may broaden the knowledge for stimuli-responsive RTP organic molecules and lay the foundation for their wide-scale applications.

Recent Advancements in and the Future of Organic Emitters: TADF- and RTP-Active Multifunctional Organic Materials

Organic emitting compounds that are based on π-conjugated skeletons have emerged as promising next-generation materials for application in optoelectronic devices. In this Minireview, recent advances in the development of organic emitters that irradiate room-temperature phosphorescence and/or thermally activated delayed fluorescence with extraordinary luminescence properties, such as aggregation-induced emission, mechanochromic luminescence, and circularly polarized luminescence, are discussed.

Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

Thermally activated delayed fluorescence (TADF) is one of the most intriguing and promising discoveries towards realization of highly-efficient organic light emitting diodes (OLED) utilizing small molecules as emitters. It has the capability of manifesting all excitons generated during the electroluminescent processes, consequently achieving 100 % of internal quantum efficiency. Since the report of the first efficient OLED based on a TADF small molecule in 2012 by Adachi et al., the quest for optimal TADF materials for OLED application has never stopped. Various TADF molecules bearing different design concepts and strategies have been designed and produced, with the aim to boost the overall performances of corresponding OLEDs. In this minireview, the general principles of TADF molecular design based on three basic categories of TADF species: twisted intramolecular charge transfer (TICT), through-space charge transfer (TSCT) and multi-resonance induced TADF (MR-TADF) are discussed in detail. Several key aspects with respect to each category, as well as some effective methods to enhance the efficiency of TADF materials and corresponding OLEDs from the molecular engineering perspectives, are summarized and discussed to exhibit a general landscape of TADF molecular design to a wide variety of scientific researchers within this particular disciplinary area.