Enhanced Triplet–Triplet Energy Transfer and Upconversion Fluorescence through Host–Guest Complexation

Metal-Free White Light-Emitting Fluorescent Material Based on Simple Pillar[5]arene-tripodal Amide System and Theoretical Insights on Its Assembly and Fluorescent Properties

The booming of host-guest assembly-based supramolecular chemistry provides abundant ways to construct functional systems and materials. Attracted by the important application prospect of white light emission and aggregation-induced emission (AIE) materials, herein, we report an efficient way for fabricating metal-free white light-emitting AIE materials through the supramolecular assembly of simple organic compounds: methoxyl pillar[5]arene (MP5) and tri-(pyridine-4-ylamido)benzene (TAP). By host-guest assembly, MP5 and TAP formed a supramolecular polymer (MP5-T); meanwhile, the MP5-T xerogel powder emitted white light at CIE coordinates (0.29 and 0.29). The supramolecular assembly and white light-emitting mechanisms were carefully investigated by experiments as well as quantum chemical calculations including density functional theory (DFT), reduced density gradient, electrostatic surface potential, independent gradient model, and frontier molecular orbital (highest-occupied molecular orbital-lowest-unoccupied molecular orbital) analyses. Interestingly, according to the experiments and calculations, the supramolecular assembly is critical in the white light-emitting phenomenon. Moreover, in this work, the quantum chemical calculations could not only support experimental phenomena but also provide deep understanding and visualized presentation of the assembly and emission mechanism. In addition, the obtained MP5-T solid powder could serve as a novel and easy means to make material for white light-emitting devices.

Molecular Engineering of Chromophores to Enable Triplet-Triplet Annihilation Upconversion

Triplet-triplet annihilation upconversion (TTA-UC) is an unconventional photophysical process that yields high-energy photons from low-energy incident light and offers pathways for innovation across many technologies, including solar energy harvesting, photochemistry, and optogenetics. Within aromatic organic chromophores, TTA-UC is achieved through several consecutive energy conversion events that ultimately fuse two triplet excitons into a singlet exciton. In chromophores where the singlet exciton is roughly isoergic with two triplet excitons, the limiting step is the triplet-triplet annihilation pathway, where the kinetics and yield depend sensitively on the energies of the lowest singlet and triplet excited states. Herein we report up to 40-fold improvements in upconversion quantum yields using molecular engineering to selectively tailor the relative energies of the lowest singlet and triplet excited states, enhancing the yield of triplet-triplet annihilation and promoting radiative decay of the resulting singlet exciton. Using this general and effective strategy, we obtain upconversion yields with red emission that are among the highest reported, with remarkable chemical stability under ambient conditions.

Entropic Mixing Allows Monomeric-Like Absorption in Neat BODIPY Films

Intermolecular interactions play a crucial role in materials chemistry because they govern thin film morphology. The photophysical properties of films of organic dyes are highly sensitive to the local environment, and a considerable effort has therefore been dedicated to engineering the morphology of organic thin films. Solubilizing side chains can successfully spatially separate chromophores, reducing detrimental intermolecular interactions. However, this strategy is also significantly decreasing achievable dye concentration. Here, five BODIPY derivatives containing small alkyl chains in the α-position were synthesized and photophysically characterized. By blending two or more derivatives, the increase in entropy reduces aggregation and therefore produces films with extreme dye concentration and, at the same time almost solution like absorption properties. Such a film was placed inside an optical cavity and the achieved system was demonstrated to reach the strong exciton-photon coupling regime by virtue of the achieved dye concentration and sharp absorption features of the film.

Examining the influence of bilayer structure on energy transfer and molecular photon upconversion in metal ion linked multilayers

Metal ion linked multilayers is a unique motif to spatially control and geometrically restrict molecules on a metal oxide surface and is of interest in a number of promising applications. Here we use a bilayer composed of a metal oxide surface, an anthracene annihilator molecule, Zn(II) linking ion, and porphyrin sensitizers to probe the influence of the position of the metal ion binding site on energy transfer, photon upconversion, and photocurrent generation. Despite being energetically similar, varying the position of the carboxy metal ion binding group (i.e. ortho, meta, para) of the Pt(II) tetraphenyl porphyrin sensitizer had a large impact on energy transfer rates and upconverted photocurrent that can be attributed to differences in their geometries. From polarized attenuated total reflectance measurements of the bilayers on ITO, we found that the orientation of the first layer (anthracene) was largely unperturbed by subsequent layers. However, the tilt angle of the porphyrin plane varies dramatically from 41° to 64° to 57° for the para-, meta-, and ortho-COOH substituted porphyrin molecules, which is likely responsible for the variation in energy transfer rates. We go on to show using molecular dynamics simulations that there is considerable flexibility in porphyrin orientation, indicating that an average structure is insufficient to predict the ensemble behavior. Instead, even a small subset of the population with highly favorable energy transfer rates can be the primary driver in increasing the likelihood of energy transfer. Gaining control of the orientation and its distribution will be a critical step in maximizing the potential of the metal ion linked structures.

Rotaxane-Branched Dendrimers with Enhanced Photosensitization

During the past few decades, fabrication of functional rotaxane-branched dendrimers has become one of the most attractive yet challenging topics within supramolecular chemistry and materials science. Herein, we present the successful fabrication of a family of new rotaxane-branched dendrimers containing up to 21 platinum atoms and 42 photosensitizer moieties through an efficient and controllable divergent approach. Notably, the photosensitization efficiencies of these rotaxane-branched dendrimers gradually increased with the increase of dendrimer generation. For example, third-generation rotaxane-branched dendrimer PG3 revealed 13.3-fold higher ¹O₂ generation efficiency than its corresponding monomer AN. The enhanced ¹O₂ generation efficiency was attributed to the enhancement of intersystem crossing (ISC) through the simple and efficient incorporation of multiple heavy atoms and photosensitizer moieties on the axles and wheels of the rotaxane units, respectively, which has been validated by UV-visible and fluorescence techniques, time-dependent density functional theory calculations, photolysis model reactions, and apparent activation energy calculations. Therefore, we develop a new promising platform of rotaxane-branched dendrimers for the preparation of effective photosensitizers.

Organic Linkers Enable Tunable Transfer of Migrated Energy from Upconversion Nanoparticles

Energy transfer plays a pivotal role in applying lanthanide-doped upconversion nanoparticles (UCNPs) as optical probes for diverse applications, particularly in biology and medicine. However, achieving tunable energy transfer from UCNPs to different acceptors remains a daunting challenge. Here, we demonstrate that using small organic molecules as linkers, the energy transfer from UCNPs to acceptors can be modulated. Specifically, organic linkers can enable efficient energy transfer from NaGdF₄:Yb/Tm@NaGdF₄ core-shell UCNPs to different acceptors. Moreover, the organic linker-mediated energy transfer can be facilely tuned by simply changing organic linkers. Based on our mechanistic investigations, the extraction of Gd³⁺ migrated energy from UCNPs by organic linkers and the subsequent energy injection from linkers to acceptors should be the two key processes for controlling the energy transfer. The tunable energy transfer from UCNPs allows us to design novel applications, including sensors and optical waveguides, based on UCNPs. These findings may open up new ways to develop UCNP-based bioapplications and advance further fabrication of hybrid upconversion nanomaterials.

Peptide assembly assisted triplet-triplet annihilation photon upconversion in non-deoxygenated water

Triplet-triplet annihilation upconversion (TTA-UC) has great potential in many fields. However, a stable TTA-UC system with adjustable UC efficiency in non-deoxygenated water is still in urgent demand. Here, the first example of short peptide-tuned UC luminescence in water is reported. With only a small amount of peptides, UC chromophores can assemble into tetrahedral microrods with adjustable size and UC efficiency. Successful TTA-UC luminescence of these microrods in water is achieved due to the regular and dense organization of molecular upconversion chromophores tuned by peptides, which allows rapid triplet exciton migration, avoids aggregation-induced quenching and screens molecular oxygen to upconversion chromophores.

Water-Dispersible Triplet-Triplet Annihilation Photon Upconversion Particle: Molecules Integrated in Hydrophobized Two-Dimensional Interlayer Space of Montmorillonite and Their Application for Photocatalysis in the Aqueous Phase

Green incident light (λ = ∼500 nm) is converted to blue light (λ = 400-450 nm) in air using bulky alkylammonium (DMDOA⁺), 9,10-diphenylanthracene (DPA), and Ru(dmb)₃²⁺ (dmb = 4,4'-dimethyl-2,2'-bipyridine) intercalated in a layered clay compound called "montmorillonite" [MMT-DMDOA⁺-DPA-Ru(dmb)₃²⁺]. The two-dimensional interstitial space has an interlayer spacing of a few nanometers. Emitter DPA is present in this interlayer spacing, having an intermolecular distance of approximately 3.0 nm at a high concentration. Sensitizer Ru(dmb)₃²⁺ is relatively dilute, having an intermolecular distance of 47 nm. The emission decay measurements and quantitative evaluation of the emission intensity demonstrate that blue light emission is induced by sequential processes, which consist of a triplet-triplet (T-T) energy transfer reaction from Ru(dmb)₃²⁺ to DPA and T-T annihilation of DPA molecules. From thermogravimetry and Fourier transform infrared spectra measurements, we observe that the cointercalated alkylammonium acts as a waterproof agent to prevent quenching of the molecules in the excited triplet states by H₂O. Finally, we demonstrate a photocatalytic decomposition of Rhodamine B dissolved in H₂O-containing MMT-DMDOA⁺-DPA-Ru(dmb)₃²⁺ and Pt-deposited WO₃ photocatalyst, where wavelength of incident light (λ > 440 nm) is longer than the absorption edge of WO₃ photocatalyst. The mechanism of photocatalytic decomposition is the following: (i) the incident long wavelength light is upconverted to 400-450 nm light by MMT-DMDOA⁺-DPA-Ru(dmb)₃²⁺, and then, (ii) WO₃ photocatalyst is excited by the generated 400-450 nm light, and finally, (iii) Rhodamine B is decomposed on the Pt cocatalyst induced by the holes in a valence band of WO₃.

Fulleropillar[4]arene: The Synthesis and Complexation Properties

A multihydroquinone ether dialdehyde derivative 2 was incidentally obtained through an unexpected ring opening of pillar[4]arene[1]quinone 1. And the Prato reaction of 2 with [60]fullerene led to [60]fullerene bisadducts, from which trans-4 cyclic regioisomer 3 was isolated and characterized. The fulleropillar[4]arene 3 showed a larger cavity and can accommodate a viologen derivative C₁₂V²⁺ with a much stronger affinity than permethyl pillar[5]arene (MP5) and pillar[4]arene[1]quinone 1.

Enhanced triplet state generation through radical pair intermediates in BODIPY-quantum dot complexes

Generation of triplet excited states through radical pair intermediates has been extensively studied in molecular complexes. Similar schemes remain rare in hybrid structures of quantum dot-organic molecules, despite intense recent interest of quantum dot sensitized triplet excited state generation. Herein, we demonstrate that the efficiency of the intersystem crossing from the singlet to the triplet state in boron dipyrromethene (BODIPY) can be enhanced in CdSe quantum dot-BODIPY complexes through a radical pair intermediate state consisting of an unpaired electron in the quantum dot conduction band and that in oxidized BODIPY. By transient absorption spectroscopy, we show that the excitation of BODIPY with 650 nm light leads to the formation of a charge separated state by electron transfer from BODIPY to CdSe (with a time constant of 6.33 ± 1.13 ns), competing with internal conversion to the ground state within BODIPY, and the radical pair state decays subsequently by back charge recombination to generate a triplet excited state (with a time constant of 158 ± 28 ns) or the ground state of BODIPY. The overall quantum efficiency of BODIPY triplet excited state generation was determined to be (27.2 ± 3.0)%. The findings of efficient triplet state formation and intermediate radical pair states in this hybrid system suggest that quantum dot-molecule complexes may be a promising platform for spintronics applications.

Photon Upconverted Circularly Polarized Luminescence via Triplet-Triplet Annihilation

Circularly polarized luminescent materials are of increasing attention due to their potential applications in advanced optical technologies, such as chiroptical devices and optical sensing. Recently, in all reported circularly polarized luminescent materials, high-energy excitation results in low-energy or downconverted circularly polarized luminescence (CPL) emission. Although photon upconversion-i.e., the conversion of low-energy light into higher-energy emission, with a wide variety of applications-has been widely reported, the integration of photon upconversion and CPL in one chiral system to achieve higher-energy CPL emission has never been reported. Herein, a brief review is provided of recent achievements in photon-upconverted CPL via the triplet-triplet annihilation mechanism, focusing on the amplified dissymmetry factor g_lum through energy transfer process and dual upconverted and downconverted CPL emission through chirality and energy transfer process.

Recent Progress of BODIPY Dyes With Aggregation-Induced Emission

With the development of organic optoelectronic materials and bioimaging technology, to exploit organic luminescent materials with high luminescent efficiency in aggregation-state has become a research hotspot. BODIPYs have become one of the research objects of this kind of material because of their obvious advantages. This review focuses on the design and synthesis of AIE-type BODIPYs, the mechanism of AIE properties and their applications in recent years. Through classification, analysis, and summary, this review aims to explore the structure-activity relationship of AIE-type BODIPYs and to provide ideas for the further design and potential applications of AIE-active fluorescent materials.

A building-block design for enhanced visible-light switching of diarylethenes

Current development of light-responsive materials and technologies imposes an urgent demand on visible-light photoswitching on account of its mild excitation with high penetration ability and low photo-toxicity. However, complicated molecular design and laborious synthesis are often required for visible-light photoswitch, especially for diarylethenes. Worse still, a dilemma is encountered as the visible-light excitation of the diarylethene is often achieved at the expense of photochromic performances. To tackle these setbacks, we introduce a building-block design strategy to achieve all-visible-light photochromism with the triplet-sensitization mechanism. The simply designed diarylethene system is constructed by employing a sensitizer building-block with narrow singlet-triplet energy gap (ΔE_ST) to a diarylethene building-block. A significant improvement on the photochromic efficiency is obtained as well as an enhanced photo-fatigue resistance over those under UV irradiation. The balance between the visible-light excitation and decent photochromism is thus realized, promoting a guiding principle for the visible-light photochromism.

Precise Manipulation of Temperature-Driven Chirality Switching of Molecular Universal Joints through Solvent Mixing

Three chiral bicyclic pillar[5]arene derivatives termed as molecular universal joints (MUJs), were synthesized and separated enantiomerically. These MUJs showed temperature-driven chirality switching in certain solvents. Herein, it is demonstrated that temperature-driven chirality switching could also be realized by mixing two miscible organic solvents, in each of which chirality inversion is not accomplishable. Additionally, solvent mixing drastically varied the inversion temperature of the MUJs, for example, from far below zero to room temperature. Moreover, the temperature-driven S_p /R_p to R_p /S_p chirality switching direction could be reversed by the solvent mixing and it was critically controlled by the mixing ratios of the two solvents. These observations allowed precise manipulation of the chirality switching behavior of the MUJs. Such a chirality switching was ascribed to the influences of solvent and temperature on the in-out equilibrium of the side rings, which is delicately controlled by several processes, including the solvation/desolvation and the inclusion/exclusion of the side rings and solvent molecules. Crucially, the solvent mixing introduced new supramolecular processes, in particular the desolvation of solvent molecules from the mixed solvent system and the solvation of the side ring by the mixed solvent, which significantly disturbed the original in-out equilibrium of MUJs and drastically switched the entropy and enthalpy changes of conformational interconversion.

An azine-containing bispillar[5]arene-based multi-stimuli responsive supramolecular pseudopolyrotaxane gel for effective adsorption of rhodamine B

An azine-containing bispillar[5]arene was designed and synthesized by the reaction of aldehyde functionalized-pillar[5]arene and hydrazine. Then, a novel bispillar[5]arene-based supramolecular pseudopolyrotaxane has been successfully prepared via host-guest interaction. Interestingly, by taking advantage of the host-guest interactions, π-π stacking interactions and hydrogen bonding interactions, the multi-stimuli-responsive gel-sol phase transitions of such a supramolecular pseudopolyrotaxane gel were successfully realized under different stimuli, such as acid, temperature, concentration, and competitive guests. Moreover, this supramolecular system could effectively adsorb dye molecule rhodamine B. It is worth noting that this supramolecular pseudopolyrotaxane gel prepared in cyclohexanol solution (BP5·G·C) could be used as an adsorbent material for adsorbing rhodamine B with adsorption efficiency of 98.4%. Meanwhile, the adsorption efficiency was 97.6% for supramolecular pseudopolyrotaxane gel prepared in DMSO-H2O (v : v, 8 : 2) binary solution (BP5·G·D), also indicating the superior adsorption effect of BP5·G·D toward the dye molecule rhodamine B.

Synthesis, enantioseparation and photophysical properties of planar-chiral pillar[5]arene derivatives bearing fluorophore fragments

Planar chiral pillar[5]arene derivatives (P5A-DPA and P5A-Py) bearing bulky fluorophores were obtained in high yield by click reaction. The photophysical properties of both compounds were investigated in detail. P5A-DPA with two 9,10-diphenylanthracene (DPA) pigments grafted on the pillar[5]arene showed a high fluorescence quantum yield of 89.5%. This is comparable to the monomer DPA-6, while P5A-Py with two perylene (Py) pigments grafted on the pillar[5]arene showed a significantly reduced quantum yield of 46.4% vs 78.2% for the monomer Py-6. The oxygen-through-annulus rotation of the phenolic units was inhibited for both compounds due to the bulky chromophore introduced, and the resolution of the enantiomers was achieved due to the bulky size of the fluorophores. The absolute configuration of the enantiomers was determined by circular dichroism (CD) spectra. The solvent-induced aggregation behavior was investigated with the enantiopure P5A-DPA and P5A-Py. It was found that the CD signals were enhanced by aggregation. P5A-DPA showed aggregation-induced emission enhancement, while P5A-Py showed aggregation-induced emission quenching, accompanied by excimer emission when aggregating in water and THF mixed solution.

Resolution and Racemization of a Planar-Chiral A1/A2-Disubstituted Pillar[5]arene

Butoxycarbonyl (Boc)-protected pillar[4]arene[1]-diaminobenzene (BP) was synthesized by introducing the Boc protection onto the A1/A2 positions of BP. The oxygen-through-annulus rotation was partially inhibited because of the presence of the middle-sized Boc substituents. We succeeded in isolating the enantiopure RP (RP, RP, RP, RP, and RP)- and SP (SP, SP, SP, SP, and SP)-BP, and studied their circular dichroism (CD) spectral properties. As the Boc substituent is not large enough to completely prevent the flip of the benzene units, enantiopure BP-f1 underwent racemization in solution. It is found that the racemization kinetics is a function of the solvent and temperature employed. The chirality of the BP-f1 could be maintained in n-hexane and CH2Cl2 for a long period at room temperature, whereas increasing the temperature or using solvents that cannot enter into the cavity of BP-f1 accelerated the racemization of BP-f1. The racemization kinetics and the thermodynamic parameters of racemization were studied in several different organic solvents.

Effects of Temperature and Host Concentration on the Supramolecular Enantiodifferentiating [4 + 4] Photodimerization of 2-Anthracenecarboxylate through Triplet-Triplet Annihilation Catalyzed by Pt-Modified Cyclodextrins

Visible-light-driven photocatalytic supramolecular enantiodifferentiating dimerization of 2-anthracenecarboxylic acid (AC) through triplet-triplet annihilation (TTA), mediated by the Schiff base Pt(II) complex (Pt-1, Pt-2, and Pt-3) was studied. The host concentration and the temperature effects on the stereoselectivity were comprehensively investigated. Increasing the concentration of sensitizers/hosts significantly enhanced the conversion of the photoreaction but led to reduced enantioselectivities of the chiral photodimers 2 and 3 when the photoreaction was triggered by a 532 nm laser, which was in contrast with the results obtained by direct irradiation of AC with a 365 nm light-emitting diode (LED) lamp, due to the aggregation of the sensitizer/host in water. The cyclization of AC through triplet-triplet annihilation displayed significant temperature dependency when Pt-3 was employed as the sensitizer/host. Increasing the temperature from 0 °C to 30 °C with 5% equiv. of Pt-3 led to a great increase of the ee of 2 from 2.1% to 31.6%. However, hardly any temperature dependency was observed when the photodimerization was mediated by other sensitizers and/or hosts, or the photoreaction was triggered directly with a 365 nm LED lamp.

Application of a bodipy-C70 dyad in triplet-triplet annihilation upconversion of perylene as a metal-free photosensitizer

A bodipy-C70 dyad was synthesized and applied in triplet-triplet annihilation (TTA) upconversion of perylene as a novel metal-free organic photosensitizer. The photophysical processes were investigated by the methods of steady-state UV-Vis absorption and fluorescence spectroscopy, nanosecond time-resolved transient absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The bodipy-C70 dyad showed an increased molar extinction coefficient up to 82 300 mol-1 cm-1 at 518 nm compared with the C70 monomer. With photo-excitation of the bodipy moiety at 532 nm, the intramolecular singlet-singlet energy transfer between bodipy and C70 units was efficient with a quantum yield of nearly 100%, and the lowest triplet state of the dyad was subsequently populated via ISC of the C70 moiety, with a lifetime of ca. 80 μs in toluene. Electrochemical investigation suggested that the intramolecular electron transfer of the excited dyad was thermodynamically prohibited in toluene due to the positive ΔGCS for charge-separation. With the presence of perylene in solution as the triplet energy acceptor and emitter, the TTA upconverted fluorescence was observed with a maximum quantum yield of 10.3%. The overall upconversion capability of 4417 M-1 cm-1 exceeded that of C70 approximately two-fold. Moreover, the bodipy-C70 dyad also exhibited an enhanced optical stability under intense irradiation. All data indicated that the dyad was another ideal photosensitizer for TTA upconversion of perylene in the fullerene derivative family.

Single-Molecule-Based Electroluminescent Device as Future White Light Source

During the last two decades, spectacular development of light-emitting diodes (LEDs) has been achieved owing to their widespread application possibilities. However, traditional LEDs suffer from unavoidable energy loss because of the down conversion of photons, toxicity due to the involvement of rare-earth materials in their production, higher manufacturing cost, and reduced thermal stability that prevent them from all-inclusive applications. To address the existing challenges associated with current commercially available white LEDs, herein, we report a broad-band emission originating from an intrinsic lanthanide-free single-molecule-based LED. Self-assembly of a butterfly-shaped strontium-based compound {[Sr(H₂btc)₂(MeOH)(H₂O)₂]·2H₂O} (1) was achieved through the reaction of Sr(NO₃)₂ with 1,2,3-benzenetricarboxylic acid hydrate (1,2,3-H₃btc) under hydrothermal conditions. A white LED based on this single molecule exhibited a remarkable broad-band luminescence spectrum with Commission Internationale de l'Eclairage (CIE) coordinates at (0.33, 0.32) under 30 mA current injection. Such a broad luminescence spectrum can be attributed to the simultaneous existence of several emission lines originating from the intramolecular interactions within the structure. To further examine the nature of the observed transitions, density functional theory (DFT) calculations were carried out to explore the geometric and electronic properties of the complex. Our study thus paves the way toward a key step for developing a basic understanding and the development of high performance broad-band light-emitting devices with environment-friendly characteristics based on organic-inorganic supramolecular materials.

Semi-Rigid Molecular-Clip-Based Molecular Crystal Gearshift

A new version of molecular clip, with a semi-rigid symmetrical crab-type architecture and flexible cavity size, has been successfully designed and synthesized via a one-pot Friedel-Crafts alkylation reaction. The X-ray single-crystal diffraction data provide a simple and intuitive explanation, not only for its well-preorganized and regulated conformation but also for its selective and tunable guest-binding capability. For the first time, the newly designed molecular clip was demonstrated to be not only a controllable variable-speed nonporous adsorption material in solution iodine capture, but also capable of on-off switching in volatile iodine capture. The presented new concept of molecular crystal gearshift directly from the molecular clip crystals represents an important advance in the development of synthetic receptor chemistry, which will exert a significant influence on small-molecule crystallography.

Photon-upconverting chiral liquid crystal: significantly amplified upconverted circularly polarized luminescence

In this work, we demonstrate a room-temperature chiral liquid crystal which shows remarkable photon upconverted circularly polarized luminescence (UC-CPL). Circularly polarized luminescent materials with higher dissymmetry factor (g lum) underpin the basis for future applications. Since most chiral organic molecules have only a small g lum, it is vital to explore a new pathway to amplify the g lum of organic compounds. Here, by dispersing a chiral emitter and a triplet donor into a room-temperature nematic liquid crystal, highly efficient triplet-triplet annihilation-based photon upconversion (TTA-UC) and UC-CPL were observed in the induced chiral nematic liquid crystal (N*LC). Moreover, this system could simultaneously amplify the promoted circularly polarized luminescence (CPL) and the upconverted circularly polarized luminescence. The dissymmetry factors g lum of CPL and UC-CPL have been amplified by three orders of magnitude and one order of magnitude, respectively.

Red-to-blue photon up-conversion with high efficiency based on a TADF fluorescein derivative

A new photon up-conversion system with a TADF fluorescein derivative as a photosensitizer was developed to achieve a quite large anti-Stokes shift from red to blue with a fairly high up-conversion emission quantum yield.

Room-temperature phosphorescent γ-cyclodextrin-cucurbit[6]uril-cowheeled [4]rotaxanes for specific sensing of tryptophan

The room temperature phosphorescence of iodine-substituted-γ-CD-CB[6]-cowheeled[4] rotaxanes was quenched specifically by tryptophan among plasma amino acids.

Electrochemically triggered upconverted luminescence for light-emitting devices

Electrochemically triggered upconverted luminescence through triplet–triplet energy transfer (TTET) and subsequent triplet–triplet annihilation upconversion (TTA-UC) is observed for the first time.

2-Azaallyl Anions, 2-Azaallyl Cations, 2-Azaallyl Radicals, and Azomethine Ylides

This review covers the use of 2-azaallyl anions, 2-azaallyl cations, and 2-azaallyl radicals in organic synthesis up through June 2018. Particular attention is paid to both foundational studies and recent advances over the past decade involving semistabilized and nonstabilized 2-azaallyl anions as key intermediates in various carbon-carbon and carbon-heteroatom bond-forming processes. Both transition-metal-catalyzed and transition-metal-free transformations are covered. Azomethine ylides, which have received significant attention elsewhere, are discussed briefly with the primary focus on critical comparisons with 2-azaallyl anions in regard to generation and use.

Solid-State Photon Upconversion Materials: Structural Integrity and Triplet-Singlet Dual Energy Migration

Triplet-triplet annihilation-based photon upconversion (TTA-UC) is a process wherein longer-wavelength light (lower-energy photons) is converted into shorter-wavelength light (higher-energy photons) under low excitation intensity in multichromophore systems. There have been many reports on highly efficient TTA-UC in solution; however, significant challenges remain in the development of solid-state upconverters in order to explore real-world applications. In this Perspective, we discuss the advantages and challenges of different approaches for TTA-UC in solvent-free solid systems. We consider that the energy migration-based TTA-UC has the potential to achieve ideal materials with high UC efficiency at weak solar irradiance. While the UC performance of such systems is still limited at this moment, we introduce recently developed important concepts to improve it, including kinetic/thermodynamic donor dispersion in acceptor assemblies, defectless crystals, and triplet-singlet dual energy migration. Future integration of these concepts into a single material would realize the ideal TTA-UC system.

Supramolecular Polymer-Based Fluorescent Microfibers for Switchable Optical Waveguides

We report the switchable optical waveguide microfibers based on fluorescent supramolecular polymer for the first time. The pillar[5]arene-based supramolecular polymeric microfibers were prepared easily from the viscous solution of bispillar[5]arene host (bisP5A) and diphenylanthracene-derived guest (GD). The resulting microfibers  act as an active optical waveguide material with long propagation distance (400 μm) and low optical propagation loss (0.01 dB/μm). When photoresponsive dithienylethene-derived guest (GDTE) was added, the resulting ternary microfibers show switchable optical waveguide by the noninvasive control of UV/vis light with negligible fatigue over four cycles. This convenient preparation method is also applied for the quadruple-hydrogen-bonded fluorescent supramolecular polymeric microfibers which imply good light propagation property with an optical loss coefficient of 0.02 dB/μm.

New perspectives for triplet-triplet annihilation based photon upconversion using all-organic energy donor & acceptor chromophores

It is recognized that metal organic complexes that serve as sensitizers can present various degrees of challenges viz. synthesis and stability for photonic applications such as triplet-triplet annihilation based photon upconversion (TTA-PUC). Presently, researchers, including our group, are turning their attention toward purely organic triplet sensitizers, which can be handled more easily for photon management science. In this review, we surveyed recently developed all-organic chromophoric systems that were devised and used for TTA-PUC research. Knowing that TTA-PUC research has mainly been focused on the design and synthesis of the triplet sensitizers, we detailed the underlying photophysics and thermodynamics that served as the starting point for the synthesis of the purely organic chromophores in question. Accordingly, this review details triplet sensitizers that operate on (i) spin-orbit coupling or heavy atom effect, (ii) Baird-type aromaticity and antiaromaticity, (iii) open-shell characteristics or doublet excited state and (iv) thermally activated delayed fluorescence.