Triarylmethyl Radicals Research Articles

Synthesis and Structural Characterization of Carbazole‐Tailored Luminescent Triarylmethyl Radical and its Stable Cation

Synthesis and Structural Characterization of Carbazole‐Tailored Luminescent Triarylmethyl Radical and its Stable Cation

Synthesis and Structural Characterization of Carbazole-Tailored Luminescent Triarylmethyl Radical and its Stable Cation.

The development of novel luminescent radicals, characterized by their unique doublet emission, endows a significant challenge. In this study, we reported the synthesis of a luminescent neutral radical, BCzAnM-R, tailored by two carbazolyl groups and an anthryl group to achieve a nonalternant structure. It exhibited near-infrared emission with a peak at 1020 nm in toluene. Interestingly, its corresponding cation, BCzAnM-C, was synthesized through an unconventional SnCl2-mediated reduction-aromatization-oxidation reaction in one-pot and gram-scale. The cation demonstrated remarkable stability for up to weeks in ambient conditions and facilitated the silica-gel chromatography isolation as an organic salt with SnCl3-as the counter ion. The carbazolyl groups effectively modulate molecular structures, photophysical properties, and stabilities. Notably, BCzAnM-R represents the first luminescent triarylmethyl radical with two carbazolyl groups directly attached to the central carbon.

Twisting Between Topological Phases in 1D Conjugated Polymers via a Multiradical Transition State

AbstractIn recent years, it has become possible via on‐surface bottom‐up synthesis to engineer the topological character of carbon nanostructures. Graphene nanoribbons and 1D conjugated polymers (1DCPs) have thus tailored so as to host either topologically trivial or non‐trivial phases. Molecular design is the primary means to set the topological class of these nanomaterials. However, external control over topology is also demonstrated via electric fields or top‐down hydrogenation. Inspired by the connection between topology and π‐conjugation, here it is demonstrated via first‐principles calculations that aryl ring twist angles also serve as topological knobs. Focusing on rationally designed 1DCPs composed of triarylmethyl (TAM) units, it is shown that rotation of certain aryl rings enables a transition from the trivial to the topologically non‐trivial phase. Accordingly, fixing a particular twist angle configuration (e.g., via chemical functionalization) is equivalent to robustly setting a targeted topological phase. It is also found that in considered 1DCPs, the quantum phase transition occurs without the electronic band gap closing, due to a multiradical antiferromagnetic phase emerging at the transition point. All in all, this study highlights the potential of aryl ring twisting for engineering topological properties in carbon nanomaterials and establishes TAM 1DCPs as exotic topological 1D systems.

On the Effect of Donor Strength on the Photoluminescence Performance in Mono‐Substituted N‐Donor Triarylmethyl Radicals

AbstractThe functionalization of light‐emitting triarylmethyl radicals with electron‐donating moieties can significantly increase their photoluminescence quantum yield ϕ. As luminophores in light‐emitting diodes, such open‐shell radicals can be used to overcome the problem of spin‐statistics inherent to conventional closed‐shell emitters. However, so far the functionalization of triarylmethyl radicals with donors of varying strength is limited by the restricted reactivity of the triarylmethyl radical, constraining optimization of performance to empirical trial and error approaches. Here, the reliable reactivity of N‐heterocyclic donors is used in radical‐mediated aromatic substitutions, allowing to systematically investigate the effect of donor strength on the emission characteristics of triarylmethyl radicals. As a single descriptor proxy to the donor strength, the ionization energy IE of the donor moiety is employed, which is determined by density functional theory calculations. A systematic bathochromic shift of the emission wavelength λmax is observed for increasing donor strength, while maximum ϕ values are obtained for medium‐strength donors. These effects are rationalized with a simple model based on Marcus theory supported by quantum chemical calculations and electron paramagnetic resonance. This allows to understand the effect of the donor strength on both λmax and ϕ, enabling the design of improved light‐emitting radicals in the future.

Triple para-Functionalized Cations and Neutral Radicals of Enantiopure Diaza[4]helicenes.

Modulation of absorbance and emission is key for the design of chiral chromophores. Accessing a series of compounds absorbing and emitting (circularly polarized) light over a wide spectral window and often toward near-infrared is of practical value in (chir)optical applications. Herein, by late-stage functionalization on derivatives bridging triaryl methyl and helicene domains, we have achieved the regioselective triple introduction of para electron-donating or electron-withdrawing substituents. Extended tuning of electronic (e.g., E1/2red -1.50 V → -0.68 V) and optical (e.g., emission covering from 550 to 850 nm) properties is achieved for the cations and neutral radicals; the latter compounds being easily prepared by mono electron reductions under electrochemical or chemical conditions. While luminescence quantum yields can be increased up to 70% in the cationic series, strong Cotton effects are obtained for certain radicals at low energies (λabs ∼ 700-900 nm) with gabs values above 10-3. The open-shell electronic nature of the radicals was further characterized by electron paramagnetic resonance revealing an important spin density delocalization that contributes to their persistence.

Bright Fluorescent p-Phenylene-bridged Triarylmethyl Highly Stable Diradical.

Two units of highly stable luminescent triarylmethyl radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM), were bridged by p-phenylene linker. The photoluminescence quantum yield (PLQY) of PyBTM-PhPyBTM was at most 0.4 % in various organic solvents. Adding two mesityl groups on the terminals did not improve the PLQY. In the MesPyBTM-PhPyBTM-Mes, the mesityl group did not worked as an electron donor unlike the previously reported monoradical MesPyBTM. However, adding two n-hexyl groups on the bridging p-phenylene did greatly improve it, and the PLQY of the PyBTM-(Hex2 Ph)PyBTM was 7 % in dichloromethane and acetone, and 12 % in poly(methyl methacrylate) (PMMA) film. Twisting p-phenylene linker by hexyl groups hindered the π-conjugation and suppressed the non-radiative decay of the excited state.

SOX71, A Biocompatible Succinyl Derivative of the Triarylmethyl Radical OX071 for In Vivo Quantitative Oxygen Mapping Using Electron Paramagnetic Resonance.

This study aimed to develop a biocompatible oximetric electron paramagnetic resonance (EPR) spin probe with reduced self-relaxation, and sensitivity to oxygen for a higher signal-to-noise ratio and longer relaxation times at high oxygen concentration, compared to the reference spin probe OX071. SOX71 was synthesized by succinylation of the twelve alcohol groups of OX071 spin probe and characterized by EPR at X-Band (9.5GHz) and at low field (720MHz). The biocompatibility of SOX71 was tested in vitro and in vivo in mice. A pharmacokinetic study was performed to determine the best time frame for EPR imaging. Finally, a proof-of-concept EPR oxygen imaging was performed on a mouse model of a fibrosarcoma tumor. SOX71 was synthesized in one step from OX071. SOX71 exhibits a narrow line EPR spectrum with a peak-to-peak linewidth of 66 mG, similar to OX071. SOX71 does not bind to albumin nor show cell toxicity for the concentrations tested up to 5mM. No toxicity was observed after systemic delivery via intraperitoneal injection in mice at twice the dose required for EPR imaging. After the injection, the probe is readily absorbed into the bloodstream, with a peak blood concentration half an hour, post-injection. Then, the probe is quickly cleared by the kidney with a half-life of ~ 45min. SOX71 shows long relaxation times under anoxic condition (T1e = 9.5µs and T2e = 5.1µs; [SOX71] = 1mM in PBS at 37°C, pO2 = 0mmHg, 720MHz). Both the relaxation rates R1e and R2e show a decreased sensitivity to pO2, leading to twice longer relaxation times under room air conditions (pO2 = 159mmHg) compared to OX071. This is ideal for oxygen imaging in samples with a wide range of pO2. Both the relaxation rates R1e and R2e show a decreased sensitivity to self-relaxation compared to OX071, with a negligible effect of the probe concentration on R1e. SOX71 was successfully applied to image oxygen in a tumor. SOX71, a succinylated derivative of OX071 was synthesized, characterized, and applied for in vivo EPR tumor oxygen imaging. SOX71 is highly biocompatible, and shows decreased sensitivity to oxygen and self-relaxation. This first report suggests that SOX71 is superior to OX071 for absolute oxygen mapping under a broad range of pO2 values.

Recent Applications of Triarylmethyl (TAM) Derivatives as Electron Paramagnetic Resonance (EPR) Spin Labels in Biomacromolecular Structural Studies

Recent Applications of Triarylmethyl (TAM) Derivatives as Electron Paramagnetic Resonance (EPR) Spin Labels in Biomacromolecular Structural Studies

13C Hyperpolarization of Viscous Liquids by Transfer of Solid-Effect 1H Dynamic Nuclear Polarization at High Magnetic Field.

Dynamic nuclear polarization (DNP) is routinely used as a method for increasing the sensitivity to nuclear magnetic resonance (NMR). Recently, high-field solid-effect DNP in viscous liquids on 1H nuclei was demonstrated using narrow-line polarizing agents. Here we expand the applicability of DNP in viscous media to 13C nuclei. To hyperpolarize 13C nuclei, we combined solid-effect 1H DNP with a subsequent transfer of the 1H polarization to 13C via insensitive nuclei enhanced by polarization transfer (INEPT). We demonstrate this approach using a triarylmethyl radical as a polarizing agent and glycerol-13C3 as an analyte. We achieved 13C enhancement factors of up to 45 at a magnetic field of 9.4 T and room temperature.

EPR Viscometric Measurements Using a 13C-Labeled Triarylmethyl Radical in Protein-based Biotherapeutics and Human Synovial Fluids.

The viscosity measurements are of clinical significance for evaluation of the potential pathological conditions of biological lubricants such as synovial fluids of joints, and for formulation and characterization of peptide- and protein-based biotherapeutics. Due to inherent potential therapeutic activity, protein drugs have proven to be one of the most efficient therapeutic agents in treatment of several life-threatening disorders, such as diabetes and autoimmune diseases. However, home-use applications for treating chronic inflammatory diseases, such as diabetes and rheumatoid arthritis, necessitate the development of high-concentration insulin and monoclonal antibodies formulations for patient self-administration. High protein concentrations can affect viscosity of the corresponding drug solutions complicating their manufacture and administration. The measurements of the viscosity of new insulin analogs and monoclonal antibodies solutions under development is of practical importance to avoid unwanted highly viscous, and therefore, painful for injection drug formulations. Recently, we have demonstrated capability of the electron paramagnetic resonance (EPR) viscometry using viscosity-sensitive 13C-labeled trityl spin probe (13C1-dFT) to report the viscosity of human blood, and interstitial fluids measured in various organs in mice ex-vivo and in anesthetized mice, in vivo. In the present work, we demonstrate utility of the EPR viscometry using 13C1-dFT to measure microviscosity of commercial insulin samples, antibodies solution, and human synovial fluids using small microliter volume samples (5-50 μL). This viscometry analysis approach provides useful tool to control formulations and administration of new biopharmaceuticals, and for evaluation of the state of synovial fluids of importance for clinical applications.

Solid-Effect Dynamic Nuclear Polarization in Viscous Liquids at 9.4 T Using Narrow-Line Polarizing Agents.

Dynamic nuclear polarization (DNP) is a hyperpolarization method that is widely used for increasing the sensitivity of nuclear magnetic resonance (NMR) experiments. DNP is efficient in solid-state and liquid-state NMR, but its implementation in the intermediate state, namely, viscous media, is still less explored. Here, we show that a 1H DNP enhancement of over 50 can be obtained in viscous liquids at a magnetic field of 9.4 T and a temperature of 315 K. This was accomplished by using narrow-line polarizing agents in glycerol, both the water-soluble α,γ-bisdiphenylen-β-phenylallyl (BDPA) and triarylmethyl radicals, and a microwave/RF double-resonance probehead. We observed DNP enhancements with a field profile indicative of the solid effect and investigated the influence of microwave power, temperature, and concentration on the 1H NMR results. To demonstrate potential applications of this new DNP approach for chemistry and biology, we show hyperpolarized 1H NMR spectra of tripeptides, triglycine, and glypromate, in glycerol-d8.

Synthesis and structure elucidation of triarylmethyl radicals with anthryl substitution.

Two stable triarylmethyl radicals with one or two anthryl substitutions are synthesized in gram scale, and are isolated in the crystalline state. Detailed structural elucidation with X-ray crystallographic analysis and DFT calculations revealed that the twisted structure is more energetically favorable than the folded structure, and consequently, the spin density is mainly localized at the methyl carbon. The spin distribution leads to unique physical properties, making them promising open-shell organic materials.

A new 13C trityl-based spin label enables the use of DEER for distance measurements

Triarylmethyl (TAM)-based labels, while still underutilized, are a powerful class of labels for pulsed-Electron Spin Resonance (ESR) distance measurements. They feature slow relaxation rates for long-lasting signals, high stability for cellular experiments, and narrow spectral features for efficient excitation of the spins. However, the typical narrow line shape limits the available distance measurements to only single-frequency experiments, such as Double Quantum Coherence (DQC) and Relaxation Induced Dipolar Modulation Enhancement (RIDME), which can be complicated to perform or hard to process. Therefore, widespread usage of TAM labels can be enhanced by the use of Double Electron-Electron Resonance (DEER) distance measurements. In this work, we developed a new spin label, 13C1-mOX063-d24, with a 13C isotope as the radical center. Due to the resolved hyperfine splitting, the spectrum is sufficiently broadened to permit DEER-based experiments at Q-band spectrometers. Additionally, this new label can be incorporated orthogonally with Cu(II)-based protein label. The orthogonal labeling scheme enables DEER distance measurement at X-band frequencies. Overall, the new trityl label allows for DEER-based distance measurements that complement existing TAM-label DQC and RIDME experiments.

Stable Triarylmethyl Radicals and Cobalt(II) Ions Based 1D/2D Coordination Polymers.

The incorporation of organic radicals into coordination polymers was considered as a promising strategy to promote metal-ligand exchange interactions, but there are only a very limited number of stable organic radical-based ligands that can serve well such a purpose. Herein, we report two new tris(2,4,6-trichlorophenyl)methyl (TTM) radical-based ligands L1 and L2 with two and three imidazole substituents, respectively. The imidazole unit serves as a coordination site and it can also stabilize the TTM radical by intramolecular donor-acceptor interaction. Coordination of L1 and L2 with cobalt(II) ions gave the corresponding one- (CoCP-1) and two-dimensional (CoCP-2) coordination polymers, the structures of which were confirmed by X-ray crystallographic analysis. Magnetic measurements and theoretical calculations suggest antiferromagnetic coupling between the paramagnetic cobalt(II) ions and the radical ligands. Our study provides a rational design for stable organic radical-based ligands and further demonstrated the feasibility of a metal-radical approach toward magnetic materials.

Research Progress on Triarylmethyl Radical-Based High-Efficiency OLED.

Perchlorotrityl radical (PTM), tris (2,4,6-trichlorophenyl) methyl radical (TTM), (3,5-dichloro-4-pyridyl) bis (2,4,6 trichlorophenyl) methyl radical (PyBTM), (N-carbazolyl) bis (2,4,6-trichlorophenyl) methyl radical (CzBTM), and their derivatives are stable organic radicals that exhibit light emissions at room temperature. Since these triarylmethyl radicals have an unpaired electron, their electron spins at the lowest excited state and ground state are both doublets, and the transition from the lowest excited state to the ground state does not pose the problem of a spin-forbidden reaction. When used as OLED layers, these triarylmethyl radicals exhibit unique light-emitting properties, which can increase the theoretical upper limit of the OLED’s internal quantum efficiency (IQE) to 100%. In recent years, research on the luminescent properties of triarylmethyl radicals has attracted increasing attention. In this review, recent developments in these triarylmethyl radicals and their derivatives in OLED devices are introduced.

The simplest structure of a stable radical showing high fluorescence efficiency in solution: benzene donors with triarylmethyl radicals.

Donor-radical acceptor systems have recently attracted much attention as efficient doublet emitters that offer significant advantages for applications such as OLEDs. We employed an alkylbenzene (mesityl group) as the simplest donor to date and added it to a diphenylpyridylmethyl radical acceptor. The (3,5-difluoro-4-pyridyl)bis[2,6-dichloro-4-(2,4,6-trimethylphenyl)phenyl]methyl radical (Mes2F2PyBTM) was prepared in only three steps from commercially available reagents. A stable radical composed of only one pyridine ring, four benzene rings, methyl groups, halogens, and hydrogens showed fluorescence of over 60% photoluminescence quantum yield (PLQY) in chloroform, dichloromethane, and PMMA. The key to high fluorescence efficiency was benzene rings perpendicular to the diphenylpyridylmethyl radical in the doublet ground (D0) state. The relatively low energy of the β-HOMO and the electron-accepting character of the radical enabled the use of benzenes as electron donors. Furthermore, the structural relaxation of the doublet lowest excited (D1) state was minimized by steric hindrance of the methyl groups. The reasons for this high efficiency include the relatively fast fluorescence transition and the slow internal conversion, both of which were explained by the overlap density between the D1 and D0 states.

Understanding Charge Transport in Triarylmethyl-Based Spintronic Nanodevices

In this Article, we explore the potential of triarylmethyl (TAM) oligomers as devices for organic molecular spintronic applications. We use the Landauer formalism to compute global spin transport a...

Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles.

High-valent iron halide corroles were examined to determine their reactivity with carbon radicals and their ability to undergo radical rebound-like processes. Beginning with Fe(Cl)(ttppc) (1) (ttppc = 5,10,15-tris(2,4,6-triphenylphenyl)corrolato3-), the new iron corroles Fe(OTf)(ttppc) (2), Fe(OTf)(ttppc)(AgOTf) (3), and Fe(F)(ttppc) (4) were synthesized. Complexes 3 and 4 are the first iron triflate and iron fluoride corroles to be structurally characterized by single crystal X-ray diffraction. The structure of 3 reveals an AgI-pyrrole (η2-π) interaction. The Fe(Cl)(ttppc) and Fe(F)(ttppc) complexes undergo halogen transfer to triarylmethyl radicals, and kinetic analysis of the reaction between (p-OMe-C6H4)3C• and 1 gave k = 1.34(3) × 103 M-1 s-1 at 23 °C and 2.2(2) M-1 s-1 at -60 °C, ΔH⧧ = +9.8(3) kcal mol-1, and ΔS⧧ = -14(1) cal mol-1 K-1 through an Eyring analysis. Complex 4 is significantly more reactive, giving k = 1.16(6) × 105 M-1 s-1 at 23 °C. The data point to a concerted mechanism and show the trend X = F- > Cl- > OH- for Fe(X)(ttppc). This study provides mechanistic insights into halogen rebound for an iron porphyrinoid complex.

Structural and Magnetic Studies on Nickel(II) and Cobalt(II) Complexes with Polychlorinated Diphenyl(4-pyridyl)methyl Radical Ligands.

New magnetic metal complexes with organic radical ligands, [M(hfac)2(PyBTM)2] (M = NiII, CoII; hfac = hexafluoroacetylacetonato, PyBTM = (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical), were prepared and their crystal structures, magnetic properties, and electronic structures were investigated. Metal ions in [M(hfac)2(PyBTM)2] constructed distorted octahedral coordination geometry, where the two PyBTM molecules ligated in the trans configuration. Magnetic investigation using a SQUID magnetometer revealed that χT increased with decreasing temperature from 300 K in the two complexes, indicating an efficient intramolecular ferromagnetic exchange interaction taking place between the spins on PyBTM and M with J/kB of 21.8 K and 11.8 K for [NiII(hfac)2(PyBTM)2] and [CoII(hfac)2(PyBTM)2]. The intramolecular ferromagnetic couplings in the two complexes could be explained by density functional theory calculations, and would be attributed to a nearly orthogonal relationship between the spin orbitals on PyBTM and the metal ions. These results demonstrate that pyridyl-containing triarylmethyl radicals are key building blocks for magnetic molecular materials with controllable/predictable magnetic interactions.

Photostable Luminescent Triarylmethyl Radicals and Their Metal Complexes: Photofunctions Unique to Open-shell Electronic States

Stable organic radicals with unpaired electrons exhibit vivid physical characteristics such as magnetic and electron-conducting properties, while being assumed as non-emissive chemical species. This highlight review introduces the luminescence properties of radicals and radical-ligated metal complexes as emerging functions of open-shell molecular systems. Their unique photofunctions will expand the utility of radicals in molecular photonics.