Radical Dendrimers: A Family of Five Generations of Phosphorus Dendrimers Functionalized with TEMPO Radicals

Fluorescent and Magnetic Radical Dendrimers as Potential Bimodal Imaging Probes

Dual or multimodal imaging probes have emerged as powerful tools that improve detection sensitivity and accuracy in disease diagnosis by imaging techniques. Two imaging techniques that are complementary and do not use ionizing radiation are magnetic resonance imaging (MRI) and optical fluorescence imaging (OFI). Herein, we prepared metal-free organic species based on dendrimers with magnetic and fluorescent properties as proof-of-concept of bimodal probes for potential MRI and OFI applications. We used oligo(styryl)benzene (OSB) dendrimers core that are fluorescent on their own, and TEMPO organic radicals anchored on their surfaces, as the magnetic component. In this way, we synthesized six radical dendrimers and characterized them by FT-IR, ¹H NMR, UV-Vis, MALDI-TOF, SEC, EPR, fluorimetry, and in vitro MRI. Importantly, it was demonstrated that the new dendrimers present two properties: on one hand, they are paramagnetic and show the ability to generate contrast by MRI in vitro, and, on the other hand, they also show fluoresce emission. This is a remarkable result since it is one of the very few cases of macromolecules with bimodal magnetic and fluorescent properties using organic radicals as the magnetic probe.

Metal-Free Radical Dendrimers as MRI Contrast Agents for Glioblastoma Diagnosis: Ex Vivo and In Vivo Approaches

Simultaneously being a nonradiative and noninvasive technique makes magnetic resonance imaging (MRI) one of the highly required imaging approaches for the early diagnosis and follow-up of tumors, specifically for brain cancer. Paramagnetic gadolinium (Gd)-based contrast agents (CAs) are the most widely used ones in brain MRI acquisitions with special interest when assessing blood–brain barrier (BBB) integrity, a characteristic of high-grade tumors. However, alternatives to Gd-based contrast agents (CAs) are highly required to overcome their established toxicity. Organic radicals anchored on a dendrimer macromolecule surface (radical dendrimers) are promising alternatives since they also exhibit paramagnetic properties and can act as T₁ CAs like Gd-based CAs while being organic species (mitigating concerns about toxic metal accumulation). Here, we studied the third generation of a water-soluble family of poly(phosphorhydrazone) radical dendrimers, with 48 PROXYL radical units anchored on their branches, exploring their potential of ex vivo and in vivo contrast enhancement in brain tumors (in particular, of immunocompetent, orthotopic GL261 murine glioblastoma (GB)). Remarkably, this radical species provides suitable contrast enhancement on murine GL261 GB tumors, which was comparable to that of commercial Gd-based CAs (at standard dose 0.1 mmol/kg), even at its 4 times lower administered dose (0.025 mmol/kg). Importantly, no signs of toxicity were detected in vivo. In addition, it showed a selective accumulation in brain tumor tissues, exhibiting longer retention within the tumor, which allows performing imaging acquisition over longer time frames (≥2.5 h) as opposed to Gd chelates. Finally, we observed high stability of the radicals in biological media, on the order of hours instead of minutes, characteristic of the isolated radicals. All of these features allow us to suggest that the G3-Tyr-PROXYL-ONa radical dendrimer could be a viable alternative to metal-based MRI contrast agents, particularly on MRI analysis of GB, representing, to the best of our knowledge, the first case of organic radical species used for this purpose and one of the very few examples of these types of radical species working as MRI CAs in vivo.

Tethered Blatter Radical for Molecular Grafting: Synthesis of 6-Hydroxyhexyloxy, Hydroxymethyl, and Bis(hydroxymethyl) Derivatives and Their Functionalization

Synthetic access to 7-CF₃-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl radicals containing 4-(6-hydroxyhexyloxy)phenyl, 4-hydroxymethylphenyl or 3,5-bis(hydroxymethyl)phenyl groups at the C(3) position and their conversion to tosylates and phosphates are described. The tosylates were used to obtain disulfides and an azide with good yields. The Blatter radical containing the azido group underwent a copper(I)-catalyzed azide-alkyne cycloaddition with phenylacetylene under mild conditions, giving the [1,2,3]triazole product in 84% yield. This indicates the suitability of the azido derivative for grafting Blatter radical onto other molecular objects via the CuAAC "click" reaction. The presented derivatives are promising for accessing surfaces and macromolecules spin-labeled with the Blatter radical.

Electronic and Magnetic Properties of a Three-Arm Nonconjugated Open-Shell Macromolecule

Nonconjugated radical polymers (i.e., macromolecules with aliphatic backbones that have stable open-shell sites along their pendant groups) have arisen as an intriguing complement to π-conjugated polymers in organic electronic devices and may prove to have superior properties in magneto-responsive applications. To date, however, the design of nonconjugated radical polymers has primarily focused on linear homopolymer, copolymer, and block polymer motifs even though conjugated dendritic macromolecules (i.e., polyradicals) have shown significant promise in terms of their response under applied magnetic fields. Here, we address this gap in creating a nonconjugated, three-arm radical macromolecule with nitroxide open-shell sites using a straightforward, single-step reaction, and we evaluated the electronic and magnetic properties of this material using a combined computational and experimental approach. The synthetic approach employed resulted in a high-purity macromolecule with a well-defined molecular weight and narrow molecular weight distribution. Moreover, epoxide-based units were implemented in the three-arm radical macromolecule design, and this resulted in a nonlinear radical macromolecule with a low (i.e., below room temperature) glass transition temperature and one that was an amorphous material in the solid state. These properties allowed thin films of the three-arm radical macromolecule to have electrical conductivity values on par with many linear radical polymers previously reported, and our computational efforts suggest the potential of higher generation open-shell dendrimers to achieve advanced electronic and magnetic properties. Importantly, the three-arm radical macromolecule also demonstrated antiferromagnetic exchange coupling between spins at temperatures < 10 K. In this way, this effort puts forward key structure-property relationships in nonlinear radical macromolecules and presents a clear path for the creation of next-generation macromolecules of this type.

On eccentricity-based entropy measures for dendrimers

The eccentricity-based entropy inspired by Shannon's entropy approach is the information-theoretic quantity to figure out the structural information of complex networks. The investigation for advance biomedical utilization of dendrimers has improved the synthesis of radical based molecules. Categorically, attaining radical dendrimers has initiated their use in different fields such as anti-tumor agents and as magnetic resonance imaging. The use of radical dendrimers has increased the possibility of establishing new kinds of devices based on para-magnetic axioms of organic radicals. In this article, we discussed dendrimer based on cyclotriphosphazene ( N 3 P 3 ) which has balanced edge groups and these are examined by EPR temperature spectrum. Firstly, we computed eccentricity-based indices and then we computed eccentricity based entropies by developing an acquaintance between these indices and their entropies. Moreover we presented our computed result numerically and graphically which leads to good importance of our contribution.

Polyphosphorhydrazone-Based Radical Dendrimers

The search for new biomedical applications of dendrimers has promoted the synthesis of new radical-based molecules. Specifically, obtaining radical dendrimers has opened the door to their use in various fields such as magnetic resonance imaging, as anti-tumor or antioxidant agents, or the possibility of developing new types of devices based on the paramagnetic properties of organic radicals. Herein, we present a mini review of radical dendrimers based on polyphosphorhydrazone, a new type of macromolecule with which, thanks to their versatility, new metal-free contrast agents are being obtained, among other possible applications.

Synthesis, crystal structures and magnetic properties of a P-stereogenic ortho-(4-amino-tempo)phosphinic amide radical and its CuII complex

The synthesis of phosphinic amides containing one 4-amino-TEMPO substituent at the ortho position has been achieved through copper(i) catalyzed cross-coupling reactions of ortho-iodophosphinic amides with 4-amino-TEMPO. The method has been extended to the preparation of the first example of a P-stereogenic ortho-(4-amino-tempo)phosphinic amide radical 10. The reaction of 10 with Cu(hfac)2 afforded the P-stereogenic CuII complex 19. The crystal structure of both chiral compounds is reported. The molecular structure of 10 consists of a supramolecular zig-zag chain formed by intermolecular hydrogen bonds between the NH group of the phosphinic amide moiety and the nitroxide oxygen atom. In complex 19, the ligand acts as a bridge between two CuII ions coordinated to the oxygen atoms of the P[double bond, length as m-dash]O and N-O· groups leading to the formation of a polymeric helicate chain in which the metal ions exist in a distorted octahedral geometry. The magnetic behavior of ligand 10 is characterized by very weak intermolecular antiferromagnetic interactions, whereas ferro- and anti-ferromagnetic interactions are present in complex 19.

Organic Polyradicals as Redox Mediators: Effect of Intramolecular Radical Interactions on Their Efficiency

The spin-spin interactions between unpaired electrons in organic (poly)radicals, especially nitroxides, are largely investigated and are of crucial importance for their applications in areas such as organic magnetism, molecular charge transfer, or multiple spin labeling in structural biology. Recently, 2,2,6,6-tetramethylpiperidinyloxyl and polymers functionalized with nitroxides have been described as successful redox mediators in several electrochemical applications; however, the study of spin-spin interaction effect in such an area is absent. This communication reports the preparation of a novel family of discrete polynitroxide molecules, with the same number of radical units but different arrangements to study the effect of intramolecular spin-spin interactions on their electrochemical potential and their use as oxidation redox mediators in a Li-oxygen battery. We find that the intensity of interactions, as measured by the d₁/d electron paramagnetic resonance parameter, progressively lowers the reduction potential. This allows us to tune the charging potential of the battery, optimizing its energy efficiency.

Radical Dendrimers Based on Biocompatible Oligoethylene Glycol Dendrimers as Contrast Agents for MRI

Finding alternatives to gadolinium (Gd)-based contrast agents (CA) with the same or even better paramagnetic properties is crucial to overcome their established toxicity. Herein we describe the synthesis and characterization of entirely organic metal-free paramagnetic macromolecules based on biocompatible oligoethylene glycol dendrimers fully functionalized with 5 and 20 organic radicals (OEG Gn-PROXYL (n = 0, 1) radical dendrimers) with the aim to be used as magnetic resonance imaging (MRI) contrast agents. Conferring high water solubility on such systems is often a concern, especially in large generation dendrimers. Our approach to overcome such an issue in this study is by synthesizing dendrimers with highly water-soluble branches themselves. In this work, we show that the highly water-soluble OEG Gn-PROXYL (n = 0, 1) radical dendrimers obtained showed properties that convert them in good candidates to be studied as contrast agents for MRI applications like diagnosis and follow-up of infectious diseases, among others. Importantly, with the first generation radical dendrimer, a similar r1 relaxivity value (3.4 mM-1s-1) in comparison to gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) used in clinics (3.2 mM-1s-1, r.t. 7T) has been obtained, and it has been shown to not be cytotoxic, avoiding the toxicity risks associated with the unwanted accumulation of Gd in the body.

Crossover from a Solid Effect to Thermal Mixing 1H Dynamic Nuclear Polarization with Trityl-OX063

Trityl-OX063 is a narrow-line, water-soluble, and biocompatible polarizing agent, widely used for dynamic nuclear polarization (DNP) amplified NMR of ¹³C, but not of the abundant ¹H nuclear spin, for which the ineffective solid effect (SE) is expected to be operational. Surprisingly, we observed a crossover from SE to thermal mixing (TM) DNP of ¹H with increasing Trityl-OX063 concentration at 7 T. We experimentally ascertained diagnostic signatures of TM-DNP that have only been theoretically predicted: (i) an electron paramagnetic resonance (EPR) spectrum that maintains an asymmetrically broadened EPR line from strong e-e couplings and (ii) hyperpolarization, i.e., cooling of select electron-spin populations, manifested in a characteristic pump-probe electron double-resonance spectrum under DNP conditions. Low microwave power requirements, high polarization transfer rates, and efficient DNP at high magnetic fields are the key benefits of TM-DNP.

On Irregularity Measures of Some Dendrimers Structures

A graph is said to be a regular graph if all its vertices have the same degree, otherwise, it is irregular. Irregularity indices are usually used for quantitative characterization of the topological structure of non-regular graphs. In numerous applications and problems in material engineering and chemistry, it is useful to be aware that how irregular a molecular structure is? Furthermore, evaluations of the irregularity of underline molecular graphs could be valuable for QSAR/QSPR studies, and for the expressive determines of chemical and physical properties, such as enthalpy of vaporization, toxicity, resistance, Entropy, melting and boiling points. In this paper, we think over the following four irregularity measures: the irregularity index by Albertson, σ irregularity index, the total irregularity index and the variance of vertex degrees. By way of graph structural estimation and derivations, we determine these irregularity measures of the molecular graphs of different classes of dendrimers.

Tuning Spin-Spin Interactions in Radical Dendrimers

Two generations of polyphosphorhydrazone (PPH) dendrimers were synthesized and fully functionalized with TEMPO radicals via acrylamido or imino group linkers to evaluate the impact of the linker substitution on the radical-radical interactions. A drastic change in the way that the radicals interacted among them was observed by EPR and CV studies: while radicals in Gn -imino-TEMPO dendrimers presented a strong spin-spin interaction, in the Gn -acrylamido-TEMPO ones they acted mainly as independent radicals. This shows that these interactions could be tuned by the solely substitution of the radical linker, opening the perspective of controlling and modulating the extension of these interactions depending on each application. The chemical properties of the linker strongly influence the spin-spin exchange between pendant radicals.

Recent progress in the electron paramagnetic resonance study of polymers

This review article provides an overview of the contemporary research based on a tailor-made technique to understand the paramagnetic behavior of different polymer classes.

Nitroxide radical polymers – a versatile material class for high-tech applications

A comprehensive summary of synthetic strategies for the preparation of nitroxide radical polymer materials and a state-of-the-art perspective on their latest and most exciting applications.

Synthetic Access to a Pure Polyradical Architecture: Nucleophilic Insertion of Nitronyl Nitroxide on a Cyclotriphosphazene Scaffold

An optimized nucleophilic synthetic approach featuring mild conditions and microwave energy was utilized to circumvent the classical Ullman procedure and access a polynitronyl nitroxide radical easily and in pure form. The simultaneous controlled introduction of preformed nitronyl nitroxide radicals on a cyclotriphosphazene core leads to a novel polyphosphazene monomer which is suitable for both n- and a p-type redox-active material in organic rechargeable batteries as demonstrated by electrochemistry. Additionally, absorption spectra and square-wave voltammetry were utilized to quantify the number of nitronyl nitroxide radical units on the cyclotriphosphazene scaffold.

Synthesis, characterization and the paramagnetic properties of bottle-brush copolymers with shielding TEMPO radicals

Bottle-brush copolymers PHEMA-g-[(PAA-g-TEMPO)-b-PMA] and PHEMA-g-(PAA-g-TEMPO) with shielding TEMPO radicals were synthesized. The paramagnetic properties in bulk and solution were studied, and the results confirmed that the presence of the outer segment played an important role on the EPR spectra of copolymers.

Synthesis and Straightforward Quantification Methods of Imino Nitroxide-Based Hexaradical Architecture on a Cyclotriphosphazene Scaffold

The synthesis of a homogeneous neutral hexaradical architecture consisting of six imino nitroxide radical moieties covalently bonded on a cyclotriphosphazene scaffold was reported. The synthesis of hexaradical imino nitroxide compounds follows the Ullman procedure involving the condensation of 2,3-bis(hydroxylamino)-2,3-dimethylbutane with hexa-(4-formylphenoxy)cyclotriphosphazene (3) followed by oxidation of the condensation product hexa-[4-(1-hydroxy-4,4,5,5-tetramethyl-2-imidazoline-2-yl)phenoxy]cyclotriphosphazene (2) by NaIO₄. Characterization of hexaradical was performed by X-ray and SQUID in solid state and by EPR, absorption spectroscopy, and electrochemistry in solution. CV of 1 shows an oxidation peak at 1.184 V (vs SCE) and a reduction peak at -0.883 V, both characteristics of the presence of phenyl imino nitroxide (7) moieties, suggesting that the contribution of the cyclotriphosphazene core is negligible. Attention was particularly focused on developing methods, UV-vis spectroscopy and square-wave voltammetry, to quantify the number of radicals in a way to confirm easily and rapidly the polyradicals' structure.

Covalently Functionalized Graphene by Radical Polymers for Graphene-Based High-Performance Cathode Materials

Polymer-functionalized graphene sheets play an important role in graphene-containing composite materials. Herein, functionalized graphene sheets covalently linked with radical polymer, graphene-graft-poly(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl methacrylate) (G-g-PTMA), were prepared via surface-initiated atom transfer radical polymerization (SI-ATRP). A composite cathode with G-g-PTMA as major active material and reduced graphene oxide (RGO) as conductive additive was fabricated via a simple dispersing-depositing process, and this composite cathode exhibited a relatively high specific capacity up to 466 mAh g(-1) based on the mass of PTMA, which is much higher than the theoretical capacity of PTMA. This extraordinary electrochemical performance is attributed to the fast one-electron redox reaction of G-g-PTMA and surface Faradaic reaction of RGO boosted by G-g-PTMA, which suggested that G-g-PTMA sheets play a dual role in the composite materials, that is, on the one hand provided the fast one-electron redox reaction of PTMA and on the other hand worked as nanofiller for facilitating the surface Faradaic reaction-based lithium storage of RGO.

Dynamics of intramolecular spin exchange interaction of a nitronyl nitroxide diradical in solution and on surfaces

In this paper we report the study of the dynamics of a thermally modulated intramolecular spin exchange interaction of a novel diradical nitronyl nitroxide-substituted disulfide in solution and when it is grafted on a gold surface. The structure of this diradical was designed to have flexible chains leading to intramolecular collisions and hence spin exchange interaction, and with an appropriate binding group to be grafted on the gold surface to study its behavior on the surface. In solution, this diradical shows a strong spin exchange interaction between both radicals which is modulated by temperature, but also gold nanoparticles (AuNPs) functionalized with this diradical permit investigation of such a phenomenon in surface-grafted radicals. The spin-labelled AuNP synthesis was optimized to obtain high coverage of spin labels to lead to high spin exchange interaction. The obtained AuNPs were studied by Electron Paramagnetic Resonance (EPR), UV-Vis, and IR spectroscopies, HR-TEM microscopy, Cyclic Voltammetry (CV), Energy Dispersive X-ray analysis (EDX) and Thermogravimetric Analysis (TGA). This inorganic-organic hybrid material also showed dipolar interactions between its radicals which were confirmed by the appearance in the EPR spectra of an |Δms| = 2 transition at half-field. This signal gives direct evidence of the presence of a high-spin state and permitted us to study the nature of the magnetic coupling between the spins which was found to be antiferromagnetic. Self-Assembled Monolayers (SAMs) of these radicals on the Au (111) substrate were also prepared and studied by contact angle, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Cyclic Voltammetry and EPR. The magnetic as well as the electrochemical properties of the hybrid surfaces were studied and compared with the properties of this diradical in solution. Analogies between the properties of AuNPs with high coverage of radicals and those of SAM were observed.

Magnetic and Electrochemical Properties of a TEMPO-Substituted Disulfide Diradical in Solution, in the Crystal, and on a Surface

A study of the magnetic and electrochemical properties of a TEMPO-substituted disulfide diradical in three different environments was carried out: in solution, in the crystal, and as a self-assembled monolayer (SAM) on an Au(111) substrate, and the relationship between them was explored. In solution, this flexible diradical shows a strong spin-exchange interaction between the two nitroxide functions that depends on the temperature and solvent. Structural, dynamic, and thermodynamic information has been extracted from the EPR spectra of this dinitroxide. The magnetic interactions in the crystal include intra- and intermolecular contributions, which have been studied separately and shown to be antiferromagnetic in both cases. Finally, we demonstrate that both the magnetic and electrochemical properties are preserved upon chemisorption of the diradical on a gold surface. The resulting SAM displayed anisotropic magnetic properties, and angle-resolved EPR spectra of the monocrystal allowed a rough determination of the orientation of the molecules in the SAM.

Synthesis, X-Ray Structure, Magnetic Properties, and a Study of Intra/Intermolecular Radical-Radical Interactions of a Triradical TEMPO Compound

A novel triradical compound with a P=S core and three branches functionalized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals is synthesized and characterized by IR, (1) H NMR, (31) P NMR, and EPR spectroscopy and MALDI-TOF mass spectrometry, and its chemical structure is confirmed by X-ray diffraction analysis. The triradical shows neither spin exchange interactions between its radical units nor detectable dipolar interactions. This is consistent with the separation between the radical units found in its X-ray diffraction structure, and discounts the existence of intramolecular interactions. This conclusion is confirmed by an EPR concentration study. The concentration at which intermolecular interactions start to appear is determined (5×10(-3)  m) and this concentration should be taken into account as a higher concentration limit when studies on intramolecular radical-radical interactions in polyradicals with similar structure are required. SQUID magnetometry analysis of the compound shows antiferromagnetic interactions between the spin carriers of different molecules; that is, antiferromagnetic intermolecular interactions.