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1.
Artigo em Inglês | MEDLINE | ID: mdl-32930562

RESUMO

The spin-spin interactions between unpaired electrons in organic (poly)radicals, especially nitroxides, are of largely inves-tigated 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, TEMPO and polymers functionalized with nitroxides have been described as successful redox mediators, however, the study of spin-spin interactions effect in such an area is absent. This communication deals with the preparation and study of a novel family of discrete polynitroxide molecules, with the same number of radical units but different arrangement to study how intramolecular spin-spin interactions affect on their electrochemical potential and their use as redox mediators.

2.
Pharmaceutics ; 12(8)2020 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-32823903

RESUMO

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.

3.
Dalton Trans ; 49(29): 10011-10016, 2020 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-32643714

RESUMO

The chance to have persistent organic radicals in combination with metals has attracted much interest since it offers the possibility of having new functional molecules with multiple open-shell elements. In this study, we report the synthesis of two tripodal tris(2-pyridyl)methylamine ligands (TPMA) functionalized with nitronyl nitroxide persistent radicals. The newly formed ligands have been used to coordinate zinc(ii), copper(ii), iron(ii) and cobalt(ii). The resulting complexes have been investigated by means of electron paramagnetic resonance (EPR), ESI-MS, FT-IR spectroscopy and X-ray diffraction. An electron reduction of the N-O radical moiety has been observed, depending on the metal used for the formation of the complex and the reaction conditions. We have observed small differences in the EPR spectra depending on the meta or para position of the radical moiety in the complex structure and some antiferromagnetic interactions between the paramagnetic M(ii) ions and the radical species.

4.
Chemphyschem ; 2018 May 10.
Artigo em Inglês | MEDLINE | ID: mdl-29744989

RESUMO

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.

5.
J Am Chem Soc ; 139(2): 686-692, 2017 01 18.
Artigo em Inglês | MEDLINE | ID: mdl-27997166

RESUMO

Charge transfer/transport in molecular wires over varying distances is a subject of great interest. The feasible transport mechanisms have been generally accounted for on the basis of tunneling or superexchange charge transfer operating over small distances which progressively gives way to hopping transport over larger distances. The underlying molecular sequential steps that likely take place during hopping and the operative mechanism occurring at intermediate distances have received much less attention given the difficulty in assessing detailed molecular-level information. We describe here the operating mechanisms for unimolecular electron transfer/transport in the ground state of radical-anion mixed-valence derivatives occurring between their terminal perchlorotriphenylmethyl/ide groups through thiophene-vinylene oligomers that act as conjugated wires of increasing length up to 53 Å. The unique finding here is that the net transport of the electron in the larger molecular wires is initiated by an electron-hole dissociation intermediated by hole delocalization (conformationally assisted and thermally dependent) forming transient mobile polaronic states in the bridge that terminate by an electron-hole recombination at the other wire extreme. On the contrary, for the shorter radical-anions our results suggest that a flickering resonance mechanism which is intermediate between hopping and superexchange is the operative one. We support these mechanistic interpretations by applying the pertinent biased kinetic models of the charge/spin exchange rates determined by electron paramagnetic resonance and by molecular structural level information obtained from UV-vis and Raman spectroscopies and by quantum chemical modeling.

6.
Chem Commun (Camb) ; 52(91): 13397-13400, 2016 Nov 08.
Artigo em Inglês | MEDLINE | ID: mdl-27787532

RESUMO

Herein we report a water-induced single-crystal to single-crystal transformation that involves the formation of hetero-bimetallic paddlewheel clusters in coordination polymers. Through this transformation, which involves the cleavage and formation of different coordination bonds, two different Cu(ii)-Zn(ii) and Cu(ii)-Ni(ii) paddlewheel units exhibiting a 1 : 1 metal ratio were created.

7.
J Phys Chem Lett ; 7(12): 2234-9, 2016 Jun 16.
Artigo em Inglês | MEDLINE | ID: mdl-27231856

RESUMO

The diradical acceptor-donor-acceptor triad 1(••), based on two polychlorotriphenylmethyl (PTM) radicals connected through a tetrathiafulvalene(TTF)-vinylene bridge, has been synthesized. The generation of the mixed-valence radical anion, 1(•-), and triradical cation species, 1(•••+), obtained upon electrochemical reduction and oxidation, respectively, was monitored by optical and ESR spectroscopy. Interestingly, the modification of electron delocalization and magnetic coupling was observed when the charged species were generated and the changes have been rationalized by theoretical calculations.

8.
Chemistry ; 22(5): 1805-15, 2016 Jan 26.
Artigo em Inglês | MEDLINE | ID: mdl-26743879

RESUMO

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.

9.
Chemphyschem ; 16(15): 3302-7, 2015 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-26489060

RESUMO

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.


Assuntos
Óxidos N-Cíclicos/química , Magnetismo , Modelos Moleculares , Difração de Raios X
10.
Org Lett ; 16(20): 5402-5, 2014 Oct 17.
Artigo em Inglês | MEDLINE | ID: mdl-25280239

RESUMO

The synthesis and characterization of a novel trityl-TEMPO biradical and the investigation of its properties as Dynamic Nuclear Polarization (DNP) polarizing agent are reported. Comparison with a structurally related monoradical (PTM-TEMPE) or mixtures of the two monoradical components reveals that the biradical has a much higher polarization efficiency and a faster polarization buildup. This offers the possibility of faster recycling further contributing to its efficiency as a polarizing agent.

11.
J Chem Phys ; 140(16): 164903, 2014 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-24784306

RESUMO

We present a complete Raman spectroscopic study in two structurally well-defined diradical species of different lengths incorporating oligo p-phenylene vinylene bridges between two polychlorinated triphenylmethyl radical units, a disposition that allows sizeable conjugation between the two radicals through and with the bridge. The spectroscopic data are interpreted and supported by quantum chemical calculations. We focus the attention on the Raman frequency changes, interpretable in terms of: (i) bridge length (conjugation length); (ii) bridge conformational structure; and (iii) electronic coupling between the terminal radical units with the bridge and through the bridge, which could delineate through-bond spin polarization, or spin delocalization. These items are addressed by using the "oligomer approach" in conjunction with pressure and temperature dependent Raman spectroscopic data. In summary, we have attempted to translate the well-known strategy to study the electron (charge) structure of π-conjugated molecules by Raman spectroscopy to the case of electron (spin) interactions via the spin delocalization mechanism.


Assuntos
Conformação Molecular , Análise Espectral Raman , Compostos de Tritil/química , Compostos de Vinila/química , Elétrons , Temperatura
12.
Chemistry ; 19(49): 16656-64, 2013 Dec 02.
Artigo em Inglês | MEDLINE | ID: mdl-24281812

RESUMO

This work presents a joint theoretical and experimental characterisation of the structural and electronic properties of two tetrathiafulvalene (TTF)-based acceptor-donor-acceptor triads (BQ-TTF-BQ and BTCNQ-TTF-BTCNQ; BQ is naphthoquinone and BTCNQ is benzotetracyano-p-quinodimethane) in their neutral and reduced states. The study is performed with the use of electrochemical, electron paramagnetic resonance (EPR), and UV/Vis/NIR spectroelectrochemical techniques guided by quantum-chemical calculations. Emphasis is placed on the mixed-valence properties of both triads in their radical anion states. The electrochemical and EPR results reveal that both BQ-TTF-BQ and BTCNQ-TTF-BTCNQ triads in their radical anion states behave as class-II mixed-valence compounds with significant electronic communication between the acceptor moieties. Density functional theory calculations (BLYP35/cc-pVTZ), taking into account the solvent effects, predict charge-localised species (BQ(.-)-TTF-BQ and BTCNQ(.-)-TTF-BTCNQ) as the most stable structures for the radical anion states of both triads. A stronger localisation is found both experimentally and theoretically for the BTCNQ-TTF-BTCNQ anion, in accordance with the more electron-withdrawing character of the BTCNQ acceptor. CASSCF/CASPT2 calculations suggest that the low-energy, broad absorption bands observed experimentally for the BQ-TTF-BQ and BTCNQ-TTF-BTCNQ radical anions are associated with the intervalence charge transfer (IV-CT) electronic transition and two nearby donor-to-acceptor CT excitations. The study highlights the molecular efficiency of the electron-donor TTF unit as a molecular wire connecting two acceptor redox centres.


Assuntos
Compostos Heterocíclicos/química , Derivados de Benzeno/química , Espectroscopia de Ressonância de Spin Eletrônica , Transporte de Elétrons , Elétrons , Fleroxacino/análogos & derivados , Fleroxacino/química , Modelos Moleculares , Naftoquinonas/química , Nitrilos/química , Oxirredução
13.
Org Lett ; 15(14): 3490-3, 2013 Jul 19.
Artigo em Inglês | MEDLINE | ID: mdl-23829655

RESUMO

The synthesis of the 3Gc0T zero generation dendrimer with a cyclotriphosphazene core functionalized with nitroxyl radicals in its six branches has been performed. The radical units have been used as probes to determine the orientation of the six branches in solution experimentally by Electron Paramagnetic Resonance (EPR) spectroscopy compared with the structure obtained in the solid state by X-ray diffraction. The orientation of the dendrimer branches is the same in solution as in the solid state.


Assuntos
Óxidos N-Cíclicos/química , Óxidos N-Cíclicos/síntese química , Dendrímeros/química , Compostos Organofosforados/química , Espectroscopia de Ressonância de Spin Eletrônica/métodos , Difração de Raios X
14.
Chemphyschem ; 14(8): 1670-5, 2013 Jun 03.
Artigo em Inglês | MEDLINE | ID: mdl-23553876

RESUMO

We show that electron transfer from the perchlorotriphenylmethide anion (PTM(-)) to Y@C82(C2v) is an instantaneous process, suggesting potential applications for using PTM(-) to perform redox titrations of numerous endohedral metallofullerenes. The first representative of a Y@C82-based salt containing the complex cation was prepared by treating Y@C82(C2v) with the [K(+)([18]crown-6)]PTM(-) salt. The synthesis developed involves the use of the [K(+)([18]crown-6)]PTM(-) salt as a provider of both a complex cation and an electron-donating anion that is able to reduce Y@C82 C2v). For the first time, the molar absorption coefficients for neutral and anionic forms of the pure isomer of Y@C82(C2v) were determined in organic solvents with significantly different polarities.


Assuntos
Fulerenos/química , Ítrio/química , Ânions/química , Espectroscopia de Ressonância de Spin Eletrônica , Transporte de Elétrons , Sais/química
15.
J Am Chem Soc ; 135(18): 6958-67, 2013 May 08.
Artigo em Inglês | MEDLINE | ID: mdl-23517105

RESUMO

An extensive investigation of aggregation phenomena occurring in solution for a family of electron donor-acceptor derivatives, based on polychlorotriphenylmethyl radicals (PTM) linked via a vinylene-bridge to tetrathiafulvalene (TTF) units, is presented. A large set of temperature and/or concentration dependent optical absorption and electron spin resonance (ESR) spectra in a solution of dyads bearing different number of electrons and/or with a hydrogenated PTM residue offer reliable information on the formation of homo dimers and mixed valence dimers. The results shed light on the reciprocal influence of intramolecular electron transfer (IET) within a dyad and the intermolecular charge transfer (CT) occurring in a dimer between the TTF residues and are rationalized based on a theoretical model that describes both interactions.


Assuntos
Compostos Heterocíclicos/química , Compostos de Tritil/química , Radicais Livres/química , Estrutura Molecular , Soluções
16.
J Am Chem Soc ; 135(7): 2620-7, 2013 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-23339278

RESUMO

We report experimental evidence indicating that the nature of the interaction established between HAT(CN)(6), a well-known strong electron acceptor aromatic compound, with mono- or polyatomic anions switches from the almost exclusive formation of reversible anion-π complexes, featuring a markedly charge transfer (CT) or formal electron-transfer (ET) character, to the quantitative and irreversible net production of the anion radical [HAT(CN)(6)](•-) and the dianion [HAT(CN)(6)](2-) species. The preferred mode of interaction is dictated by the electron donor abilities of the interacting anion. Thus, weaker Lewis basic anions such as Br(-) or I(-) are prone to form mainly anion-π complexes. On the contrary, stronger Lewis basic F(-) or (-)OH anions display a net ET process. The ET process can be either thermal or photoinduced depending on the HOMO/LUMO energy difference between the electron donor (anion) and the electron acceptor (HAT(CN)(6)). These ET processes possibly involve the intermediacy of anion-π complexes having strong ET character and producing an ion-pair radical complex. We hypothesize that the irreversible dissociation of the pair of radicals forming the solvent-caged complex is caused by the reduced stability (high reactivity) of the radical resulting from the anion.


Assuntos
Compostos Aza/química , Crisenos/química , Elétrons , Nitrilos/química , Quinoxalinas/química , Ânions , Transporte de Elétrons , Modelos Moleculares
17.
Angew Chem Int Ed Engl ; 51(44): 11024-8, 2012 Oct 29.
Artigo em Inglês | MEDLINE | ID: mdl-23037801

RESUMO

An organic switch: An open-shell dyad, consisting of an electron acceptor perchlorotriphenylmethyl radical unit linked to an electron π-donor tetrathiafulvalene unit through a vinylene π-bridge, was synthesized (see picture). The self-assembly of the dyad in solution induced by its intramolecular electron transfer was studied.


Assuntos
Elétrons , Compostos Heterocíclicos/síntese química , Compostos Heterocíclicos/química , Estrutura Molecular , Temperatura
19.
Nat Chem ; 3(5): 359-64, 2011 May.
Artigo em Inglês | MEDLINE | ID: mdl-21505493

RESUMO

Bistable molecules that behave as switches in solution have long been known. Systems that can be reversibly converted between two stable states that differ in their physical properties are particularly attractive in the development of memory devices when immobilized in substrates. Here, we report a highly robust surface-confined switch based on an electroactive, persistent organic radical immobilized on indium tin oxide substrates that can be electrochemically and reversibly converted to the anion form. This molecular bistable system behaves as an extremely robust redox switch in which an electrical input is transduced into optical as well as magnetic outputs under ambient conditions. The fact that this molecular surface switch, operating at very low voltages, can be patterned and addressed locally, and also has exceptionally high long-term stability and excellent reversibility and reproducibility, makes it a very promising platform for non-volatile memory devices.


Assuntos
Magnetismo , Óptica e Fotônica , Eletroquímica , Reprodutibilidade dos Testes , Análise Espectral/métodos , Volatilização
20.
J Am Chem Soc ; 133(15): 5818-33, 2011 Apr 20.
Artigo em Inglês | MEDLINE | ID: mdl-21446654

RESUMO

Radical anions 1(-•)-5(-•), showing different lengths and incorporating up to five p-phenylenevinylene (PPV) bridges between two polychlorinated triphenylmethyl units, have been prepared by chemical or electrochemical reductions from the corresponding diradicals 1-5 which were prepared using Wittig-Horner-type chemistry. Such radical anions enabled us to study, by means of UV-vis-NIR and variable-temperature electron spin resonance spectroscopies, the long-range intramolecular electron transfer (IET) phenomena in their ground states, probing the influence of increasing the lengths of the bridges without the need of using an external bias to promote IET. The temperature dependence of the IET rate constants of mixed-valence species 1(-•)-5(-•) revealed the presence of two different regimes at low and high temperatures in which the mechanisms of electron tunneling via superexchange and thermally activated hopping are competing. Both mechanisms occur to different extents, depending on the sizes of the radical anions, since the lengths of the oligo-PPV bridges notably influence the tunneling efficiency and the activation energy barriers of the hopping processes, the barriers diminishing when the lengths are increased. The nature of solvents also modifies the IET rates by means of the interactions between the oligo-PPV bridges and the solvents. Finally, in the shortest compounds 1(-•) and 2(-•), the IET induced optically through the superexchange mechanism can also be observed by the exhibited intervalence bands, whose intensities decrease with the length of the PPV bridge.


Assuntos
Polivinil/química , Ânions/química , Eletroquímica , Transporte de Elétrons , Radicais Livres/química , Halogenação , Modelos Moleculares , Oxirredução
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