Philippe Turek

Preparation and study of a lithium phthalocyanine radical: optical and magnetic properties

Abstract A metallophthalocyanine radical, PcLi, was prepared by electrochemical oxidation of the corresponding dianion. The redox properties in solution show that PcLi is both easily reduced and easily oxidized (ΔE = Ered1/2-E1/2ox = 0.83 V). This property is of the utmost importance for obtaining intrinsic molecular semiconductors. The optical properties are similar to those for oxidized phthalocyanines and involve two low-lying π-π transitions at 430 and 810 nm. Despite the tendency to aggregation, the ESR spectrum of the isolated PcLi molecule has been obtained. It is consistent with an unpaired electron delocalized over the whole phthalocyanine ring.

“Chain-Like” Trimetallic Ruthenium Complexes with C7 Carbon-Rich Bridges: Experimental and Theoretical Investigations of Electronic Communication Tuning in Five Distinct Oxidation States

In this work, we report the synthesis and the electronic properties of the unique highly conjugated molecular wires trans-[Cl-(dppe)(2)Ru=C=C=(Ph)C-CH=(CH(3))C-C[triple bond]C-(X)(2)Ru-C=C-C(CH(3))=CH-C(Ph)=C=C=Ru(dppe)(2)Cl](n+) (n = 2, X = dppe ([3a](OTf)(2)) and dppm ([3b](OTf)(2)) with three similar metal centers spanned by two odd-numbered unsaturated C(7) chains providing a 28 A long conjugated path and displaying five well-separated redox states (n = 0-4). Successive one-electron transfer steps were studied by means of cyclic voltammetry, EPR and UV-vis-NIR-IR spectroelectrochemistry. The electronic and physical properties of the different states were further rationalized with the help of DFT-based calculations. Upon one-electron reduction (n = 1), the single electron is delocalized over the two carbon chains through the central metal atom to an extent driven by the rotations within and between the chains. The second reduction (n = 0) involves the whole carbon-rich conjugated path of the molecule in a spin polarized scheme: one electron is delocalized over each chain, and the two electrons are antiferromagnetically coupled with a coupling on the order of kT. Interestingly, oxidation processes strongly involve both the metal atoms and the bridging ligands. The combined investigations reveal that the mono-oxidized system (n = 3) presents a spin density uniformly distributed between the metal atoms and the carbon atoms of the chains, whereas in the second oxidation state (n = 4) the compounds show a strong antiferromagnetic coupling on the order of 4 kT between the two single electrons localized in two distinct delocalized spin orbitals implying all the carbon atoms of the bridges and the three metal atoms. Thus, for the first time, electronic communication was fully evidenced and tuned in homonuclear trimetallic oligomeric carbon-rich systems in either an oxidation or a reduction process.

Role of hydrogen bonds in the propagation of ferromagnetic interactions in organic molecular solids. Part 1.—The p-hydroxyphenyl α-nitronyl aminoxyl radical case

The role of hydrogen bonds in the obtention of magnetic molecular materials is investigated in a purely organic openshell molecular solid. The structural and magnetic properties of the 2-(4-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-yloxyl 3-oxide radical (1) are studied in detail. The intermolecular hydrogen bonds in 1 are found to be able to control the crystal packing (i.e. the relative disposition of the radical molecules within the crystal lattice) and, at the same time, to propagate ferromagnetic intermolecular interactions along certain directions of the solid. Molecular orbital (MO) calculations, structural studies about the crystal packing, static magnetic susceptibility measurements, electron paramagnetic resonance (EPR) solution spectra and a detailed EPR study of an oriented single crystal of the title compound are presented and discussed. All these studies point strongly towards the fact that this material behaves as a quasi-two-dimensional ferromagnet, in which hydrogen bonds of O–H ⋯ O–N and C–H ⋯ O–N types are found to be responsible for the ransmission of the ferromagnetic interactions within the (a, b) crystallographic plane.

Towards a Better Understanding of the Magnetic Interactions withinm-Phenyleneα-Nitronyl Imino Nitroxide Based Biradicals

An extensive investigation of the magnetic properties of three series of biradicals (bis-nitronyl nitroxides diNN-R, bis-imino nitroxides diIN-R and mixed INNN-R, where R is either hydrogen, a triple bond or trimethylsilylacetylenic group) has been carried out to give clear values of the intramolecular interactions through the m-phenylene coupling unit with alpha-nitronyl nitroxides (NN) or alpha-imino nitroxides (IN). An EPR study of the molecules in the isolated state is validated by ab initio calculations, which show the respective influence of spin polarisation and molecular conformation on the singlet-triplet gaps. All these results indicate that the triplet state is the ground state for such biradicals, except when the imidazolyl cycles are orthogonal to the phenyl ring. The magnetic properties of the biradicals in the solid state can be rationalised by examination of the short contacts produced between the ONCNO and ONCN groups. EPR studies on single crystals of the H-substituted series have confirmed the presence of a structural distortion for diNN-H whereas diIN-H and INNN-H do not exhibit such a peculiarity. The magnetic behaviour of diIN-H is described well by a four-spins model, with a strong intermolecular antiferromagnetic interaction of -90 K, whereas in the case of the two other compounds, a supplementary contact involves more complex interactions between the dimers. The compound diNN-tmsa exhibits a ferromagnetic intermolecular interaction of +11 K within the dimers, and this could be attributed to the relative disposition of the imidazolyl rings. Compound diNN-tr reveals a chain-like behaviour, whereas diIN-tr shows a predominant antiferromagnetic interaction within the dimers. The values for the intramolecular interactions in the solid state are in good agreement with those found for the isolated molecules.

Nitroxo spin-labelled calix[4]arene podands and cryptands: Allosteric regulation of spin-spin exchange interaction

Abstract Hybrid calix[4]arene podands and cryptands, each bearing two or four stable nitronyl-nitroxide radicals and two bipyridine subunits, exhibit through-space exchange interaction, the magnitude of which can be modulated by coordination of Zn 2+ cations at the bipyridine sites.

Magnetic Properties of Gold Nanoparticles: A Room-Temperature Quantum Effect

Gold nanoparticles elicit a huge research activity in view of ntheir applications in diagnostics,[1, 2] therapy,[3] drug or gene delivery,[ n4] sensing[5, 6, 7] and imaging.[8] Gold nanoparticles also display ninteresting catalytic[9, 10] and optical[11, 12, 13, 14] properties. nThis Communication focuses on the least understood and so nfar unused property of gold: its becoming magnetic when prepared nin the form of nanoparticles. All these desirable properties, nbound together in one nanometric piece of matter, possibly nself-organized thanks to its ligands, make functionalized ngold nanoparticles a treasurable entity for nanosciences. The nex nihilo magnetic properties of functionalized gold (and other ndiamagnetic metals, such a Ag or Cu) nanoparticles, that is, ntheir ferromagnetic-like behavior, are well documented, nthough still poorly understood.[15] This unexpected property nopens new perspectives in materials science, in particular for nthe design of metamaterials. One may also envisage applications nin information storage and processing: nanometric magnetic nparticles with no obvious temperature limitation and possibly nself-organizing are currently much sought-after by the ncomputer industry and developing a room-temperature magnetic semiconductor is paramount for the realization of spintronics ntechnologies. nHerein, we wish to present the results of our own investigations ninto the magnetic properties of functionalized gold nanoparticles. nWe have made attempts at understanding this magnetic nbehavior using both traditional techniques (e.g. superconducting nquantum interference device, SQUID, magnetometry) nand other methods less common in this field, such as zerofield n197Au NMR (nuclear magnetic resonance) and SANS (smallangle nneutron scattering). We also directly probed the local nmagnetic field at the surface of gold nanoparticles using paramagnetic nTEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] radicals nand ESR (electron spin resonance) spectrometry. Surprisingly, nnone of these experiments provided a clearer picture in nfine. These “negative” results led us to pondering whether or nnot the explanation could be elsewhere. Our hypothesis is that nthe magnetism of gold (and possibly other metals) could very nwell originate in self-sustained persistent currents. We shall ndemonstrate hereafter that this hypothesis is indeed very plausible nand would actually reconcile all of the experimental data nreported to date.Striking results are often obtained when SQUID magnetometry nis performed on functionalized Au nanoparticles, such as ndodecanethiol-coated ones. Rather than being diamagnetic, as nexpected, the nanoparticles can be found to be para- or ferromagnetic nat room temperature and above. When hysteresis is nobserved, the magnetization curve looks like that of a soft ferromagnet nand exhibits a remnant magnetization MR and a coercive nfield HC, though both are rather weak. These parameters nhave been observed to have values that vary by orders of nmagnitude from sample to sample[15] (see Figure ESI-1 of the nSupporting Information). Very often, the magnetization does nnot saturate. Diamagnetic samples are more diamagnetic than nthe bulk metal. Also, the magnetic observables show little dependence non temperature between 2 and 400 K. The measurements nreported so far have been performed by totally independent ngroups, on systems that were synthesized using nknown chemical procedures. Figure 1 compares the magnetization nof bulk gold with that of two diamagnetic samples of ngold nanoparticles. It can be seen that nanoparticles have na much larger absolute diamagnetic susceptibility than massive ngold. nFigure 2 compares two samples of gold nanoparticles, exhibiting na paramagnetic behavior and a ferromagnetic-like one. nThere is a weak but clear hysteresis, and the magnetization ndoes not really saturate even at high field values.

Synthesis and magnetic properties of new mono- and binuclear iron complexes with salicyloylhydrazono dithiolane ligand.

We report here experimental evidence for the formation in the solid state of a new binuclear Fe (III) 2(mu-OMe) 2(HL) 4 complex (H 2L is 2-salicyloylhydrazono-1,3-dithiolane). The isostructural Mn (III) 2(mu-OMe) 2(HL) 4 complex has provided the strongest ferromagnetic interaction value (J approximately 20 cm (-1)) between Mn (III) ions to date. The new iron binuclear compound presented in this study shows antiferromagnetic intramolecular coupling, which agrees with the theoretical study that we previously proposed. During our synthetic work, we also observed an unexpected spontaneous reduction of the new Fe (III)(HL) 2Cl,S complex to the new Fe (II)(H 2L) 2Cl 2 high-spin mononuclear complex. This process has been checked by cyclo-voltammetry as well as pseudosteady voltammetry.

Diarylethene-Containing Carbon-Rich Ruthenium Organometallics: Tuning of Electrochromism

The association of a dithienylethene (DTE) system with ruthenium carbon-rich systems allows reaching sophisticated and efficient light- and electro-triggered multifunctional switches R-[Ru]-C≡C-DTE-C≡C-[Ru]-R, featuring multicolor electrochromism and electrochemical cyclization at remarkably low voltage. The spin density on the DTE ligand and the energetic stabilization of the system upon oxidation could be manipulated to influence the closing event, owing to the noninnocent behavior of carbon-rich ligands in the redox processes. A combination of spectroscopic (UV-vis-NIR-IR and EPR) and electrochemical studies, with the help of quantum chemical calculations, demonstrates that one can control and get a deeper understanding of the electrochemical ring closure with a slight modification of ligands remote from the DTE unit. This electrochemical cyclization was established to occur in the second oxidized state (EEC mechanism), and the kinetic rate constant in solution was measured. Importantly, these complexes provide an unprecedented experimental means to directly probe the remarkable efficiency of electronic (spin) delocalization between two trans carbon-rich ligands through a metal atom, in full agreement with the theoretical predictions. In addition, when no cyclization occurs upon oxidation, we could achieve a redox-triggered magnetic switch.

Spin Correlations in Organic Radical Magnets

Abstract The Electron Paramagnetic Resonance (EPR) technique is described as a valuable tool in order to probe the spin correlations in π-radical magnets. The peculiar features of the EPR response of low dimensional exchange coupled spin systems are recalled. The extreme sensitivity of the resonance field to both magnetic and dipolar interactions is emphasized. The example materials include phthalocyanine radical derivatives and nitronyl nitroxide radical derivatives. Both exhibit either anti ferromagnetic or ferromagnetic intermolecular interactions.

Electronic communication in "chain-like" trimetallic ruthenium complexes with two C7 carbon-rich conjugated bridges.

This paper describes the synthesis and properties of the first homotrinuclear metal complexes with large carbon-rich ligands that provide unique extended conduits for electron mobility.