Triphenylamine (TPA) radical cations and related macrocycles

Photoinduced Stable Circularly Polarized Luminescent Radicals From a Triphenylamine-Attached Planar Chiral Pillar[5]Arene

Photoinduced organic radicals with unique luminescent properties are highly sought-after due to their important prospects in synthetic chemistry and materials science. However, the current development of organic free radicals, including photoinduced ones, is significantly limited and faces challenges related to stability and poor luminescence behavior. Taking advantage of the photoelectric activity of triarylamine, we herein describe an unusual luminescent radical, which can be rapidly generated by UV irradiation of a solid-state triarylamine-functionalized π-conjugated pillar[5]arene (EtP5NN) in air, accompanied by luminescent color switching from bluish-violet to sky-blue. The persistent radicals within EtP5NN with a half-life of 12.7 h suggest that the pillar[5]arene skeleton straightforwardly improves the stability of radicals. The sterically bulky triarylamine groups inhibit the racemization of planar chiral pillar[5]arene and allow the optical resolution of this system. The enhancement of circularly polarized luminescence (CPL) is triggered by UV irradiation of the enantiomers (pS/pR-EtP5NN) in the solid state.

Photoinduced Radical Luminescence of Diazapyrene Derivatives upon Complexation with Cucurbit[n]uril Hosts

Molecular design with an electron donor-acceptor (D-A) structure and host-guest complexation are effective strategies for stabilizing radicals. Herein, we report the design and synthesis of cationic diazapyrene derivatives featuring a D-A structure, as well as the influence of host-guest complexation on their photoinduced organic radical photophysical properties. Compared to the methylated diazapyrene 1, the other three derivatives 2-4, containing triphenylamine, coumarin, and naphthalene units as D groups, respectively, are more favorable for the generation of radicals. Binding studies reveal that cucurbit[8]uril (CB[8]) forms 1 : 1 inclusion complexes with derivatives 2-4, while CB[10] forms a 2 : 2 inclusion complex with 2 and supramolecular polymers with 3 and 4. The radical luminescence of derivatives 1-4 in aqueous solution is significantly promoted upon complexation with CB[8] or CB[10]. Additionally, the emission color of 3 shifts from cyan to yellow-green upon light irradiation in the presence of CB[8].

Synthesis and Photophysical Properties of Bipolar Compound: Triphenylamine/Diazafluorene-Carbazole

Carbazole and triphenylamine, are two well-known hole transporting units that are attached to electron transporting unit 4,5-diazafluorene in a fascinating way to bring out non-planar configuration of a molecule. The synthesized compound exhibits good thermal stability (Td > 515 °C) and high glass transition temperature (Tg, 191 °C). Optical bandgap (Egopt) obtained from solid state absorption spectra was calculated to be 2.93 eV. Solid state photoluminescence spectra displays the emission maxima at 473 nm. The emission characteristics of the compound observed in solvents of different polarity confirms the existence of intramolecular charge transfer in their excited state. Density functional theory studies reveal that HOMO and HOMO-1 localized on triphenylamine is spatially separated from LUMO of 4,5-diazafluorene, which manifest its bipolar character. The realization of long lived charge separated state upon photo-excitation from time resolved photoluminescence studies ascertains the charge transfer from triphenylamine to 4,5-diazafluorene. The experimental and theoretical analysis of the compound proved it to be a promising candidate for the fabrication of OLED devices.

Catalyst-Transfer Macrocyclization Protocol: Synthesis of π-Conjugated Azaparacyclophanes Made Easy

The present Protocol describes the application of the catalyst-transfer macrocyclization (CTM) reaction, focusing on the synthesis of aza[1n]paracyclophanes (APCs). APCs are fully π-conjugated shape-persistent macrocycles with potential supramolecular chemistry and materials science applications. This method leverages the Pd-catalyzed Buchwald-Hartwig cross-coupling reaction to selectively form π-conjugated cyclic structures, a significant advancement due to its efficiency, versatility, and scalability. Overall, this Article highlights the following attributes of the CTM method: a) Efficiency and Yield: The CTM method works at mild temperatures (40 °C) and short reaction times (≥2 h), producing high yields of APCs (>75% macrocycles). It avoids the typical high-dilution conditions, making it more practical for large-scale applications. b) Versatility: The method allows the synthesis of APCs with diverse endocyclic and exocyclic functionalities and ring sizes (typically from 4- to 9-membered rings), expanding the chemical space for these compounds. This flexibility is crucial for tailoring APC properties for specific applications. c) Scalability and Reproducibility: Unlike many macrocyclization reactions, which require highly dilute conditions, CTM can perform under concentrated regimes (35-350 mM), making it more suitable for large-scale applications. d) Applications in Materials Science: APCs are noted for their potential in optoelectronic applications due to their π-conjugated structures, which are helpful in organic semiconductors, light-harvesting systems, and other advanced materials. This approach addresses the challenge of complicated multistep syntheses that have hindered the widespread integration of APCs into functional devices. A step-by-step guide to preparing exemplary APCs, including troubleshooting, is provided with photographic illustrations.

Ester-Bearing Calix[n]phenoxazines: Side Chain Enhanced Recognition and Redox-Responsive Reversible Host-Guest System

We report an enhanced recognition and redox-responsive reversible host-guest system based on ester-bearing calix[n]phenoxazines. The carbonyl groups, oriented toward the cavity, act as the extra binding sites to enhance the binding affinity, which is confirmed by NMR and FTIR experiments and single-crystal structure analysis. Due to the oxidizable nature of calix[n]phenoxazine, a redox-controlled reversible response is established. This research not only provides a strategy to enhance the binding affinity in calix-like macrocyclic arenes but also marks a major advance in the development of a macrocyclic arene-based reversibly responsive system.

Triphenylamine-Based Push-Pull Dyes for Chromogenic Detection of HSO4- Ion in Water: The Role of Anion in the Formation of Fluorescent Organic Nanoparticles

Optical detection of the HSO4- ion in pure aqueous medium is rare, owing to the very high Gibbs free energy of hydration and ambiguity to distinguish with the isostructural H2PO4- ion. Herein, a pair of triphenylamine-based push-pull dyes with different numbers of terminal pyridine fragments, connected via an acetylenic linker, were synthesized by Sonogashira cross-coupling reaction. These two dyes displayed highly selective (LOD = 15.1/8.3 ppb), dual-mode color-changing responses toward the HSO4- ion in pure aqueous medium without any interference. Despite the halochromic behavior, both compounds exhibited very distinct optical responses with the HSO4- ion. The mechanistic investigation indicated that HSO4- was engaged in a bifurcated intermolecular hydrogen bonding interaction (leading to proton transfer) with pyridine nitrogen atoms that altered the extent of intramolecular charge transfer (ICT). The self-assembly of such protonated species was found to be prominent when sulfate was present as the counteranion. The extent of self-assembly was found to be more prominent for the trisubstituted, Y-shaped quadrupolar derivative than that observed for the linear, monosubstituted one. Furthermore, the present system was utilized for the detection of HSO4- ions in commercially available samples with satisfactory responses.

Controlling Charged State Colors in Triphenylamine-Based Anodically Coloring Electrochromes

In this work, we have designed a series of anodically coloring electrochromic (ACE) molecules comprised of thioalkyl-substituted 3,4-ethylenedioxythiophenes coupled to triphenylamine units (EDOT-TPA) that vary in the position and degree of electron richness of substituents, which influences the molecules geometric, electrochemical, optoelectronic, and excited-state properties. We evaluated their redox properties and discovered that modulation of both the first and second oxidation potential, formation of the cation radical and dication, can be varied from 0.03 to 0.18 V and 0.32 to 0.46 V versus Fc/Fc+ respectively. For the first time in ACE-based molecular systems, we demonstrated the ability to vary the electrochemical potential separation between successive charge states, which is directly involved in the generation of color. We use the chemical oxidant, Fe(OTf)3, to visualize the saturation and contrast of the vibrantly colored cation radical solutions at 1 equivalent, followed by a second equivalents that opens a new and differing set in the color palette for the dication state. Optical transitions were probed during electrochemical oxidation using an optically transparent thin layer electrode (OTTLE) demonstrating selective control in generating successive charge states. We couple our findings with Density Functional Theory (TD-DFT) simulations to show how modulating electron richness and steric interactions control the optical transitions. Specifically, excited state analysis is performed to elucidate how substituent identity affects the neutral, cation radical, and dication transitions in the visible and near infrared, and thereby the resulting color.

A Fluorescent Organic Probe for Naked-Eye Detection of Chloromethanes

An organic fluorescent probe (OFP-TAR) with a propeller-like structure was designed and synthesized. The photoluminescence of OFP-TAR in solution exhibited a significant red shift with the increase of solvent polarity, enabling a transition of fluorescence emission from blue (445 nm) to yellow (540 nm). The organic thin-film materials based on OFP-TAR/PMMA exhibit significant color changes upon exposure to CH2Cl2, CHCl3, and CCl4, with their maximum fluorescence wavelengths measured at 445, 471, and 494 nm respectively. The device facilitates the visual detection of chloromethanes and is capable of enduring more than 7 cycles of testing. These materials can also be prepared as binary-coded microarray data storage devices or applied in the field of anti-counterfeiting. The quantum yields of guest-loaded crystals CH2Cl2@OFP-TAR, CHCl3@OFP-TAR and CCl4@OFP-TAR are observed as 19.13 %, 8.79 %, and 0.83 % respectively, which are consistent with the tendency of OFP-TAR in CH2Cl2 (47.30 %), CHCl3 (34.27 %) and CCl4 (3.10 %). The fluorescence properties of OFP-TAR, OFP-TAR/PMMA, guest-loaded and guest-free crystals provided insights into the special response mechanism of OFP-TAR towards different chloromethanes.

Tandem mass spectrometry of π-expanded triphenylamine and N-heterotriangulene scaffolds: Radical cation versus silver(I) adduct

Triphenylamine (TPA) and N-heterotriangulene (N-HTA) scaffolds with up to three oligophenyl extensions are investigated by electrospray ionization (tandem) mass spectrometry (ESI-[MS/]MS). Due to their low oxidation potentials, all molecules readily form radical cations in the electrospray process. The energy-resolved collision-induced dissociation behaviour of the molecular ions is contrasted to that of the silver(I) adducts. Complexation with Ag(I) leads to the expected [1:1] and [2:1] complexes (MAg+ and M2Ag+); however, even [1:2] complexes (MAg2 2+) can be detected for molecules with two and three large π-expansions to allow stabilization of two charges. The TPA scaffolds decompose only at high collision energies through the loss of peripheral tert-butyl groups. A general mechanism for this is proposed commencing with a methyl loss and followed by the release of isobutene and butyl radical moieties. The N-HTA-based scaffolds are considerably less stable and molecular ions fragment at low collision energies. This is caused by the facile loss of methyl radicals from the dimethylmethylene-bridged triangulene core. In contrast, complexation with Ag+ leads to a dramatic stabilization. Most interestingly, dissociation eventually proceeds via the loss of neutral AgCH3, which is indicative of strong bidentate, tweezer-like bonding of Ag+ to the molecules.

Stable and Fully-Oxidized Methylene-Bridged Macrocyclic Phenothiazine Polyradical Cations

Macrocyclic arenes represent one of the most important and intensively investigated entities in supramolecular chemistry. However, research on the redox activities of macrocyclic arenes, especially their isolable and crystalline polyradical analogues, has been rarely reported. Here, we present the synthesis, redox activity, and application of methylene-bridged macrocyclic phenothiazines, where polyradical cations are successfully isolated and unambiguously characterized for the first time. This research provides an effective method for preparing polyradical macrocycles, which expands the scope of investigation into macrocyclic arenes and their potential applications.

Rational Design of Naphthol Groups Functionalized Bipolar Polymer Cathodes for High Performance Alkali-Ion Batteries

Redox-active organic compounds gather significant attention for their potential application as electrodes in alkali ion batteries, owing to the structural versatility, environmental friendliness, and cost-effectiveness. However, their practical applications of such compounds are impeded by insufficient active sites with limited capacity, dissolution in electrolytes, and sluggish kinetics. To address these issues, a naphthol group-containing triarylamine polymer, namely poly[6,6'-(phenylazanediyl)bis(naphthol)] (poly(DNap-OH)) is rationally designed and synthesized, via oxidative coupling polymerization. It is capable of endowing favorable steric structures that facilitate fast ion diffusion, excellent chemical stability in organic electrolytes, and additional redox-active sites that enable a bipolar redox reaction. By exploiting these advantages, poly(DNap-OH) cathodes demonstrate remarkable cycling stability in both lithium-ion batteries (LIBs) and potassium-ion batteries (PIBs), showcasing enhanced specific capacity and redox reaction kinetics in comparison to the conventional poly(4-methyltriphenylamine) cathodes. Overall, this work offers insights into molecular design strategies for the development of high-performance organic cathodes in alkali-ion batteries.

Phenyl Derivatives Modulate the Luminescent Properties and Stability of CzBTM-Type Radicals

The distinctive electron structures of luminescent radicals offer considerable potential for a diverse array of applications. Up to now, the luminescent properties of radicals have been modulated through the introduction of electron-donating substituents, predominantly derivatives of carbazole and polyaromatic amines with more and more complicated structures and redshifted luminescent spectra. Herein, four kinds of (N-carbazolyl)bis(2,4,6-tirchlorophenyl)-methyl (CzBTM) radicals, Ph2CzBTM, Mes2CzBTM, Ph2PyIDBTM, and Mes2PyIDBTM, were synthesized and characterized by introducing simple phenyl and 2,4,6-trimethylphenyl groups to CzBTM and PyIDBTM. These radicals exhibit rare blueshifted emission spectra compared to their parent radicals. Furthermore, modifications to CzBTM significantly enhanced the photoluminescence quantum yields (PLQYs), with a highest PLQY of 21% for Mes2CzBTM among CzBTM-type radicals. Additionally, the molecular structures, photophysical properties of molecular orbitals, and stability of the four radicals were systematically investigated. This study provides a novel strategy for tuning the luminescent color of radicals to shorter wavelengths and improving thermostability.

One-Step Catalyst-Transfer Macrocyclization: Expanding the Chemical Space of Azaparacyclophanes

In this paper, we report on a one-step catalyst-transfer macrocyclization (CTM) reaction, based on the Pd-catalyzed Buchwald-Hartwig cross-coupling reaction, selectively affording only cyclic structures. This route offers a versatile and efficient approach to synthesize aza[1n]paracyclophanes (APCs) featuring diverse functionalities and lumens. The method operates at mild reaction temperatures (40 °C) and short reaction times (∼2 h), delivering excellent isolated yields (>75% macrocycles) and up to 30% of a 6-membered cyclophane, all under nonhigh-dilution concentrations (35-350 mM). Structural insights into APCs reveal variations in product distribution based on different endocyclic substituents, with steric properties of exocyclic substituents having minimal influence on the macrocyclization. Aryl-type endocyclic substituents predominantly yield 6-membered macrocycles, while polycyclic aromatic units such as fluorene and carbazole favor 4-membered species. Experimental and computational studies support a proposed mechanism of ring-walking catalyst transfer that promotes the macrocycle formation. It has been found that the macrocyclization is driven by the formation of cyclic conformers during the oligomerization step favoring an intramolecular C-N bond formation that, depending on the cycle size, hinges on either preorganization effect or kinetic increase of the reductive elimination step or a combination of the two. The CTM process exhibits a "living" behavior, facilitating sequential synthesis of other macrocycles by introducing relevant monomers, thus providing a practical synthetic platform for chemical libraries. Notably, CTM operates both under diluted and concentrated regimes, offering scalability potential, unlike typical macrocyclization reactions usually operating in the 0.1-1 mM range.

Catalytic Aminium Radical-Cation Salt (Magic Blue)-Initiated SN2-Type Nucleophilic Ring-Opening Transformations of Aziridines

A simple and atom economic protocol for the construction of C-X/C-C bonds via catalytic aminium radical-cation salt (Magic Blue)-initiated SN2-type nucleophilic ring-opening transformations of racemic and nonracemic aziridines with different hetero and carbon nucleophiles to afford various amino ethers, thioethers, and amines in up to 99% yield, and with perfect enantiospecificity for some substrates but reduced ee with others (for nonracemic aziridines), is developed. This aminium radical-cation salt-initiated, SN2-type nucleophilic ring-opening strategy, along with various cyclization protocols, is employed to synthesize various biologically significant compounds.

Self-assembly of conformation-adaptive dihydrophenazine-based coordination cages

In this study, we designed and synthesized three conformation-adaptive Pd2L4- and Pd3L6-type coordination cages based on three dihydrophenazine-based ligands with different lengths. Interestingly, the shorter ligands L1 and L2 self-assembled into Pd2L4-type coordination cages while the longer ligand L3 formed Pd3L6-type one, mainly driven by the anion template effect. All coordination cages were confirmed through single-crystal X-ray diffraction, and their structural conformations underwent great changes compared with those of their corresponding ligands. Moreover, the conformational changes also significantly affected their photophysical and electrochemical properties which were distinct from their parent ligands.

Redox-mediated electrochemiluminescence enhancement for bead-based immunoassay

Electrochemiluminescence (ECL) is a highly sensitive mode of detection utilised in commercialised bead-based immunoassays. Recently, the introduction of a freely diffusing water-soluble Ir(iii) complex was demonstrated to enhance the ECL emission of [Ru(bpy)3]2+ labels anchored to microbeads, but a comprehensive investigation of the proposed 'redox-mediated' mechanism was not carried out. In this work, we select three different water-soluble Ir(iii) complexes by virtue of their photophysical and electrochemical properties in comparison with those of the [Ru(bpy)3]2+ luminophore and the TPrA co-reactant. A systematic investigation of the influence of each Ir(iii) complex on the emission of the Ru(ii) labels on single beads by ECL microscopy revealed that the heterogeneous ECL can be finely tuned and either enhanced up to 107% or lowered by 75%. The variation of the [Ru(bpy)3]2+ ECL emission was correlated to the properties of each Ir(iii)-based mediator, which enabled us to decipher the mechanism of interaction and define guidelines for the future design of novel Ir(iii) complexes to further enhance the ECL emission of bead-based immunoassays. Ultimately, we showcase the potential of this technology for practical sample analysis in commercial instruments by assessing the enhancement of the collective ECL intensity from a bead-based system.

Structure-property-function relationships of stabilized and persistent C- and N-based triaryl radicals

Structurally similar C- and N-based triaryl radicals are among the most commonly used structural motifs in stable, open-shell, organic molecules. The application of such species is associated with their stability, properties and structural design. This study summarizes the basic stabilization and persistence principles of C- and N-based triaryl radicals and highlights recent advances in design strategies of radicals tailored for specific applications.

Recent advances on the construction of long-wavelength emissive supramolecular coordination complexes for photo-diagnosis and therapy

Recently, metal-based drugs have attracted relentless interest in the biomedical field. However, their short excitation/emission wavelengths and unsatisfactory therapeutic efficiency limit their biological applications in vivo. Currently, the second near-infrared window (NIR-II, 1000-1700 nm) provides more accurate imaging and therapeutic options. Thus, there has been a constant focus on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. Fortunately, supramolecular coordination complexes (SCCs) formed by the coordination-driven self-assembly of NIR-II emissive ligands can address the above issues. Importantly, metal receptors with chemotherapeutic properties in SCCs can bind to luminescent ligands, thus becoming a versatile therapeutic platform for chemotherapy, imaging and phototherapy. In this context, we systematically summarize the evolution of NIR-II emissive SCCs for biomedical applications and discuss future challenges and prospects.

Near-infrared metal agents assisting precision medicine: from strategic design to bioimaging and therapeutic applications

Metal agents have made incredible strides in preclinical research and clinical applications in recent years, but their short emission/absorption wavelengths continue to be a barrier to their distribution, therapeutic action, visual tracking, and efficacy evaluation. Nowadays, the near-infrared window (NIR, 650-1700 nm) provides a more accurate imaging and treatment option. Thus, there has been ongoing research focusing on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. The design, characteristics, bioimaging, and therapy of NIR metal agents are covered in this overview of papers and reports published to date. To start with, we focus on describing the structure, design strategies, and photophysical properties of metal agents from the NIR-I (650-1000 nm) to NIR-II (1000-1700 nm) region, in order of molecular metal complexes (MMCs), metal-organic complexes (MOCs), and metal-organic frameworks (MOFs). Next, the biomedical applications brought by these superior photophysical and chemical properties for more accurate imaging and therapy are discussed. Finally, we explore the challenges and prospects of each type of NIR metal agent for future biomedical research and clinical translation.

Improving the Luminescence and Stability of Carbon-Centered Radicals by Kinetic Isotope Effect

The kinetic isotope effect (KIE) is beneficial to improve the performance of luminescent molecules and relevant light-emitting diodes. In this work, the influences of deuteration on the photophysical property and stability of luminescent radicals are investigated for the first time. Four deuterated radicals based on biphenylmethyl, triphenylmethyl, and deuterated carbazole were synthesized and sufficiently characterized. The deuterated radicals exhibited excellent redox stability, as well as improved thermal and photostability. The appropriate deuteration of relevant C-H bonds would effectively suppress the non-radiative process, resulting in the increase in photoluminescence quantum efficiency (PLQE). This research has demonstrated that the introduction of deuterium atoms could be an effective pathway to develop high-performance luminescent radicals.

Triarylamines as catalytic donors in light-mediated electron donor–acceptor complexes

EDA complexes with catalytic triarylamines allow C–H perfluoroalkylation of arenes and heteroarenes under visible light irradiation in pH- and redox-neutral conditions. A detailed photophysical characterization of the EDA complex is provided.

Catalysis in Single Crystalline Materials: From Discrete Molecules to Metal-Organic Frameworks

Catalysis is one of the key techniques for people's modern life. It has created numerous essential chemicals such as biomedicines, agricultural chemicals and unique materials. Heterogeneous catalysis is the new emerging method with reusable catalysts. Among heterogenous catalysis patterns developed so far, single crystalline catalysis has become the promising one owing to its high catalytic density and selectivity resulted by the inherent porosity, orderliness of the lattices and permeability. These crystalline catalysts could be used in various reactions such as photo-dimerization, Diels-Alder reaction, CO₂ transformation and so on. In this review, we highlighted the reported works about the single crystalline catalysts. Both discrete small molecules and metal-organic frameworks (MOFs) have been used to prepare single crystals for catalysis. For discrete molecules based crystalline catalysts, coordinated and covalent molecules have been used. There were more catalytic modes in crystalline MOF catalysts. Three patterns were identified in this review: single crystalline MOFs i) without catalytic sites, ii) with inherent catalytic features and iii) with introducing catalytic units by post synthetic modification. Based on these examples, this review committed to provide the inspirations for the further design and application of single crystalline materials.

Recent advances and perspectives on supramolecular radical cages

Supramolecular radical chemistry has been emerging as a cutting-edge interdisciplinary field of traditional supramolecular chemistry and radical chemistry in recent years. The purpose of such a fundamental research field is to combine traditional supramolecular chemistry and radical chemistry together, and take the benefit of both to eventually create new molecules and materials. Recently, supramolecular radical cages have been becoming one of the most frontier and challenging research focuses in the field of supramolecular chemistry. In this Perspective, we give a brief introduction to organic radical chemistry, supramolecular chemistry, and the emerging supramolecular radical chemistry along with their history and application. Subsequently, we turn to the main part of this topic: supramolecular radical cages. The design and synthesis of supramolecular cages consisting of redox-active building blocks and radical centres are summarized. The host-guest interactions between supramolecular (radical) cages and organic radicals are also surveyed. Some interesting properties and applications of supramolecular radical cages such as their unique spin-spin interactions and intriguing confinement effects in radical-mediated/catalyzed reactions are comprehensively discussed and highlighted in the main text. The purpose of this Perspective is to help students and researchers understand the development of supramolecular radical cages, and potentially to stimulate innovation and creativity and infuse new energy into the fields of traditional supramolecular chemistry and radical chemistry as well as supramolecular radical chemistry.