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José Vidal-Gancedo

Publications and source records attributed to José Vidal-Gancedo.

18 recordsLinked to original sources
PubMed2006

Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals.

Five bis(tert-butylnitroxide) diradicals connected by a silole (7 a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2(1) with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ray crystal structure determination, electronic spectroscopy, variable-temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6-31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b, 7 c, and 7 d are quite stable both in solution and in the solid state, 7 a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a pi-conjugated Kekulé structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a "Bleaney-Bowers" model that gives values of J = -142.0 cm(-1) for 7 a, -1.8 cm(-1) for 7 b, -1.3 cm(-1) for 7 c, -4.2 cm(-1) for 7 d, and -248.0 cm(-1) for 12. The temperature dependence of the EPR half-field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7 b, 7 c, and 7 d with DeltaE(T-S) values of 3.48, 2.09, and 8 cm(-1), respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12. Similarities between the DeltaE(T-S) and J values (DeltaE(T-S) = -2 J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a "Karplus-Conroy"-type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5-positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon pi system of the silole.

Butanes↗
PubMed2006

Remarkable interplay of redox states and conformational changes in a sterically crowded, cross-conjugated tetrathiafulvalene vinylog.

Derivatives of 9-[2-(1,3-dithiol-2-ylidene)ethylidene]thioxanthene have been synthesized using Horner-Wadsworth-Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with thioxanthen-9-ylidene-acetaldehyde (5). Further reactions lead to the sterically crowded cross-conjugated "vinylogous tetrathiafulvalene" derivative 9-[2,3-bis-(4,5-dimethyl-1,3-dithiol-2-ylidene)-propylidene]thioxanthene (10). X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry, and simultaneous electrochemistry and electron paramagnetic resonance spectroscopy, combined with theoretical calculations performed at the B3LYP/6-31G(d) level, elucidate the interplay of the electronic and structural properties in these molecules. For compound 10, multistage redox behavior is observed: the overall electrochemical process can be represented by 10-->10(.+)-->10(2+)-->10(4+) with good reversibility for the 10-->10(.+)-->10(2+) transformations. At the tetracation stage there is the maximum gain in aromaticity at the dithiolium and thioxanthenium rings. Theory predicts that for 10, 10(.+), and 10(2+) the trans isomers are more stable than the cis isomers (by ca. 2-18 kJ mol(-1)), whereas for 10(4+) the cis isomer becomes more stable than the trans isomer (by ca. 25 kJ mol(-1)) [trans and cis refer to the arrangement of the two dithiole moieties with respect to the central ==C(R)--C(H)== fragment]. These data explain the detection in cyclic voltammograms of both trans and cis isomers of 10 and 10(.+) during the reduction of 10(4+) at fast scan rates (>100 mV s(-1)) when the cis-trans isomerization is not completed within the timescale of the experiment. The X-ray structure of the charge-transfer complex (CTC) of 10 with 2,4,5,7-tetranitrofluorene-9-dicyanomethylenefluorene (DTeF) [stoichiometry: 10(.+)(DTeF)(2) (.-)2 PhCl] reveals a twisted conformation of 10(.+) (driven by the bulky thioxanthene moiety) and provides a very rare example of segregated stacking of a fluorene acceptor in a CTC.

Journal Article↗
PubMed2006

The interplay of inverted redox potentials and aromaticity in the oxidized states of new pi-electron donors: 9-(1,3-dithiol-2-ylidene)fluorene and 9-(1,3-dithiol-2-ylidene)thioxanthene derivatives.

Derivatives of 9-(1,3-dithiol-2-ylidene)fluorene (9) and 9-(1,3-dithiol-2-ylidene)thioxanthene (10) have been synthesised using Horner-Wadsworth-Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with fluorenone and thioxanthen-9-one. X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR), combined with theoretical calculations performed at the B3P86/6-31G** level, elucidate the interplay of the electronic and structural properties in these molecules. These compounds are strong two-electron donors, and the oxidation potentials depend on the electronic structure of the oxidised state. Two, single-electron oxidations (E(1)ox < E(1)ox) were observed for 9-(1,3-dithiol-2-ylidene)fluorene systems (9). In contrast, derivatives of 9-(1,3-dithiol-2-ylidene)thioxanthene (10) display the unusual phenomenon of inverted potentials (E(1)ox > E(1)ox) resulting in a single, two-electron oxidation process. The latter is due to the aromatic structure of the thioxanthenium cation (formed on the loss of a second electron), which stabilises the dication state (10(2+)) compared with the radical cation. This contrasts with the nonaromatic structure of the fluorenium cation of system 9. The two-electron oxidation wave in the thioxanthene derivatives is split into two separate one-electron waves in the corresponding sulfoxide and sulfone derivatives 27-29 owing to destabilisation of the dication state.

Journal Article↗
PubMed2006

Influence of bridge topology and torsion on the intramolecular electron transfer.

To study molecules able to act as good "molecular wires", intramolecular electron transfer between two triphenylmethyl redox centers connected by bridges with different topologies and substituents have been studied in solution, both by UV-Vis-NIR and EPR spectroscopies. The synthetic methodology used allows a complete control of the geometry of polychlorotriphenylmethyl diradicals 1 and 2, which have para and meta topologies, respectively, as well as of their E/Z isomerism. This fact is used to show the influence of the different topologies in the ease of electron transfer, which is larger for the para than for the meta isomer where a small or negligible electronic coupling is observed. The related diradical 3 that have the same topology as the para isomer 1 but bearing two substituents on the central phenyl ring shows similar ease of electron transfer, that the para isomer 1.

Journal Article↗
PubMed2005

Highly selective chromogenic and redox or fluorescent sensors of Hg2+ in aqueous environment based on 1,4-disubstituted azines.

Two new chemosensors that exhibit high affinity and high selectivity for Hg2+ in aqueous environment which operate through two different channels, optic/redox and optic/fluorescent, are reported. The optical change in sensing can be used even for a "naked-eye" detection of Hg2+ ions, whereas the fluorescent response can be modulated by varying the solvent polarity.

Journal Article↗
PubMed2004

Alkaline side-coordination strategy for the design of nickel(II) and nickel(III) bis(1,2-diselenolene) complex based materials.

The deprotonated form of the pyrazine-2,3-diselenol (pds) ligand, pds(2-), reacts with Ni(II) inorganic salts to form the nickel compounds [Ni(II)(pds)(2)](nBu(4)N)(2) (1), [Ni(II)(pds)(2)]Na(2).2H(2)O (2), and [Ni(III)(pds)(2)](2)Na(2).4H(2)O (3), depending on the reaction conditions. They are characterized by NMR, EPR, UV-vis, and IR spectroscopies, elemental analysis, cyclic voltammetry, and X-ray crystallography. The crystal structure of compound 3 shows the formation of segregated stacks of Ni(pds)(2-) units, with a strong dimerization along the stacks. The stacked fashion of the crystal packing was expected since the supramolecular forces of the alkaline side coordination to the pyrazine moieties dominate, as happens in the recently reported analogous copper system [Cu(III)(pds)(2)]Na.2H(2)O. The structure of 2 further emphasizes the alkaline coordination as the dominating supramolecular event, and an orthogonal array of 2D layers is observed. The absence of alkaline cations in complex 1 is reflected in a crystal packing with isolated complex Ni(pds)(2)(2-) units. The dimerization found in the paramagnetic Ni(III) complex 3 promotes a very strong antiferromagnetic interaction, leading to a singlet ground state.

Journal Article↗
PubMed2004

A new multifunctional ferrocenyl-substituted ferrocenophane derivative: optical and electronic properties and selective recognition of Mg2+ ions.

The synthesis, characterisation, and X-ray structure of a new strained asymmetric diferrocene derivative (2) is reported. Compound 2 acts as a highly specific electrochemical and optical Mg(2+) ion sensor, as revealed by spectroscopic and electrochemical techniques. Thus, in the presence of Mg(2+), a new redox peak appears in the cyclic voltammogram (CV) that is anodically shifted compared to the E(1/2) of the free receptor (DeltaE(1/2)=340 mV). Diferrocene derivative 2 also gives a highly visual response upon addition of Mg(2+), namely a change of colour from orange to deep purple. In addition, compound 2 does not show any significant sensing activity in the presence of Ca(2+) or alkaline ions. On protonation, it is converted into the stable diferrocenylcarbenium salt 4, in which two different modes of stabilisation of the alpha-carbocationic centre are clearly demonstrated by a combination of (1)H NMR and (57)Fe Mössbauer spectroscopic measurements. Finally, by a partial (chemical or electrochemical) oxidation, compound 2 forms the asymmetric mixed-valence species 2(+), which can be isolated as the solid salt 6 by using CF(3)SO(3) (-) as a counterion. This mixed-valence species shows a fast intramolecular electron-transfer process, as ascertained by several spectroscopic techniques.

Electrons↗
PubMed2004

Novel fused D-A dyad and A-D-A triad incorporating tetrathiafulvalene and p-benzoquinone.

Novel fused donor-acceptor dyad (TTF-Q or D-A) and acceptor-donor-acceptor triad (Q-TTF-Q or A-D-A) incorporating the donor tetrathiafulvalene (TTF) and the acceptor p-benzoquinone (Q) have been synthesized. The solution UV-vis spectra of these molecules display a low-energy absorption band that is attributed to an intramolecular charge transfer between both antagonistic units. The presence of reversible oxidation and reduction waves for the donor and acceptor moieties was shown by cyclic voltammetry, in agreement with the ratio TTF/quinone(s) units. The successive generation from these compounds of the cation radical and anion radical obtained upon (electro)chemical oxidation and reduction, respectively, was monitored by optical and ESR spectroscopies. The anion radical Q-TTF-Q(-.) triad was demonstrated to be a class II mixed-valence system with the existence of a temperature-dependent intramolecular electron transfer. The crystallographic tendency of these fused systems to overlap in mixed stacks of alternating A-D-A units is also discussed.

Journal Article↗
PubMed2004

Supramolecular photomagnetic materials: photoinduced dimerization of ferrocene-based polychlorotriphenylmethyl radicals.

New ferrocenyl Schiff-base polychlorotriphenylmethyl radicals have been synthesized and characterized. The imino group of one such radical undergoes an irreversible trans to cis structural isomerization induced by light. Such photoinduced isomerization has been monitored by UV/Vis and ESR spectroscopy and also monitored by HPLC. ESR frozen solution experiments at low temperature revealed that the cis isomer dimerizes, showing a strong antiferromagnetic interaction. Although numerous photochromic supramolecular systems have been described, such a photoinduced self-assembly process represents the first example of a one-way photoswitchable magnetic system in which a conversion between a doublet and a singlet ground state species is promoted by a photoinduced dimerization process driven by the formation of hydrogen bonds. DFT calculations on the minimized structure and on the rotational barriers have been performed to establish the origin of such behavior. The effect of the substituents and the media polarity on the photoisomerization of this imine chromophore have also been studied. It has been observed that the efficiency of the process is markedly dependent on the presence and characteristics of electron-donor and electron-acceptor substituents of the ferrocenyl Schiff-base polychlorotriphenylmethyl radicals as well as on the polarity of the solvent.

Journal Article↗
PubMed2003

Multistability in a BEDT-TTF based molecular conductor.

The low dimensional organic conductor (BEDT-TTF)(2)Br(1.3)I(1.1)Cl(0.6) [BEDT-TTF = bis(ethylenedithio)tertathiafulvalene] is shown to be a unique molecular solid that exists in three crystalline polymorphic forms (alpha'-, alpha' "-, beta' '-phase) and, surprisingly, is able to adopt the same metal-like beta' '-phase at both low (T < 185 K) and high (T > 395 K) temperatures. Several crystals of the alpha'- and alpha' "-phases have been studied using three different techniques: dc-conductivity measurements, ESR spectroscopy, and X-ray diffraction analysis. All these techniques show the existence of the reversible semiconductor <--> metal (alpha' " <--> beta' ') phase transition at both high and low temperatures as well as the alpha' <--> alpha' " phase transition at high temperatures. The phase transitions of these polymorphs are characterized by huge hysteresis and dramatic changes in the transport and magnetic properties. Based on ab initio calculations, it is suggested that dipole-dipole interactions can play a key role in the rich polymorphism of this molecular solid.

Journal Article↗
PubMed2002

Radical para-benzoic acid derivatives: transmission of ferromagnetic interactions through hydrogen bonds at long distances.

Investigation of the transmission of magnetic interactions through hydrogen bonds has been carried out for two different benzoic acid derivatives which bear either a tert-butyl nitroxide (NOA) or a poly(chloro)triphenylmethyl (PTMA) radical moiety. In the solid state, both radical acids formed dimer aggregates by the complementary association of two carboxylic groups though hydrogen bonding. This association ensured that atoms with most spin density are separated from one another by more than 15 A. Thus, no competing through-space magnetic exchange interactions are expected in these dimers and, hence, they provide good models to investigate whether noncovalent hydrogen bonds play a role in the long-range transmission of magnetic interactions. The nature of the magnetic exchange interaction and their strengths within similar dimer aggregates in solution was assessed by electron spin resonance (ESR) spectroscopy. In the case of radical NOA, low-temperature ESR experiments showed a weak ferromagnetic interaction between the two radicals in the dimer aggregates (which have the same geometry as in the solid state). In contrast, the corresponding solution ESR study performed with radical PTMA did not lead to any conclusive results, as aggregates were formed by noncovalent interactions other than hydrogen bonds. However, the bulkiness of the poly(chloro)triphenylmethyl radical prevented interdimer contacts in the solid state between regions of high spin density. Hence, solid-state measurements of the alpha phase of PTMA radical provided evidence of the intradimer interaction to confirm the transmission of a weak ferromagnetic interaction through the carboxylic acid bridges, as found for the NOA radical. Moreover, crystallization of the PTMA radical in presence of ethanol to form the beta phase of PTMA radical prevented the dimer formation; this resulted in the suppression of this interaction and provides further evidence of the magnetic exchange mechanism through noncovalent hydrogen bonds at long distances.

Journal Article↗
PubMed2001

A New Photomagnetic Molecular System Based on Photoinduced Self-Assembly of Radicals This work was supported by grants from DGI (project MAT 2000-1388-C03-01), CIRIT (project 2000 SGR00114), the 3MD Network of the TMR program of the EU (contract ERBFMRX CT980181), and the ESF program Molecular Magnets. D.R.-M. thanks the Generalitat de Catalunya for a postdoctoral grant, and I.R. thanks the CSIC-Carburos Metálicos and ESF for fellowships.

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Journal Article↗
PubMed1999

Synthesis, X-ray Structure, and Electrochemical Oxidative Coupling Reactions of 1,5- and 2,6-Bis(1,4-dithiafulven-6-yl)naphthalenes.

Novel pi-extended tetrathiafulvalene (TTF) derivatives (12a-c, 13a-c, 15a-c) in which the two 1,3-dithiole units are connected through a naphthalene spacer have been prepared in high yields by Wittig-Horner olefination reaction from dialkoxy-substituted diformylnaphthalenes (11a,b, 14) and differently substituted phosphonate esters (10a-c). The electrochemical study revealed a similar behavior for the novel electron donor molecules (12a-c, 13a-c, 15a-c) regardless of the position of the 1,3-dithiole rings on the naphthalene core. The extended donors undergo an efficient electrooxidation process affording new oligomeric extended TTF species which exhibit lower oxidation potential values than their precursor donors. EPR experiments confirm the presence of the cation radical derived from the oligomeric TTF vinylogues and support an ECE process. The structural study has been carried out by X-ray analysis of 12a and semiempirical PM3 calculations and reveals a distorted geometry from the planarity with the naphthalene moiety forming an angle of approximately 35 degrees with the thiafulvalene rings. A good agreement was found between the experimental and calculated values, thus validating the PM3 method. The chemical oxidation of the synthesized donors (12a-c, 13a-c, 15a-c) with strong electron acceptors give rise to charge transfer complexes (CTC) which were characterized by UV-vis, FTIR, and EPR spectroscopy.

Journal Article↗
PubMed1996

Synthesis, Characterization, and Theoretical Study of Sulfur-Containing Donor-Acceptor DCNQI Derivatives with Photoinduced Intramolecular Electron Transfer.

The donor-acceptor compounds N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]dithiin-6,11-quinonediimine (9a) and N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]oxathiin-6,11-quinonediimine (10a) and their methyl-substituted derivatives (9b and 10b-d, respectively) have been prepared, and their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. The (1)H-NMR spectra evidence the existence of a syn/anti isomerism in solution. Both experimental and theoretical data suggest that the preferred configuration of the N-CN groups corresponds to a syn isomer for 9 and to an anti isomer for 10. The X-ray analysis performed for 9b reveals that molecules are not planar and pack in vertical stacks showing an overlap between donor and acceptor moieties of adjacent molecules. In agreement with X-ray data, theoretical calculations predict that both for 9 and 10 the acceptor DCNQI moiety is folded and adopts a butterfly-type structure and the donor moiety is bent along the line passing through the heteroatoms. The energy difference between planar and butterfly structures is calculated to be < 3 kcal/mol at the ab initio 6-31G level. The UV-vis spectra present a broad absorption in the visible which corresponds to a photoinduced intramolecular electron transfer from the high-energy HOMO furnished by the donor moiety to the low-energy LUMO located on the DCNQI fragment. Cyclic voltammetry displays one oxidation peak to the cation and two one-electron reduction waves to the anion and dianion. Theoretical calculations show the planarization of the acceptor/donor moiety induced by reduction/oxidation. The formation of stable radical anions is corroborated by the intense EPR signals recorded for reduced 9. The assignment of the hyperfine coupling constants of the EPR spectra is consistent with the existence of a preferred syn configuration.

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