Superexchange Charge Transport in Loaded Metal Organic Frameworks.

In the past, nanoporous metal-organic frameworks (MOFs) have been mostly studied for their huge potential with regard to gas storage and separation. More recently, the discovery that the electrical conductivity of a widely studied, highly insulating MOF, HKUST-1, improves dramatically when loaded with guest molecules has triggered a huge interest in the charge carrier transport properties of MOFs. The observed high conductivity, however, is difficult to reconcile with conventional transport mechanisms: neither simple hopping nor band transport models are consistent with the available experimental data. Here, we combine theoretical results and new experimental data to demonstrate that the observed conductivity can be explained by an extended hopping transport model including virtual hops through localized MOF states or molecular superexchange. Predictions of this model agree well with precise conductivity measurements, where experimental artifacts and the influence of defects are largely avoided by using well-defined samples and the Hg-drop junction approach.

Looking Inside the Perchlorinated Trityl Radical/Metal Spinterface through Spectroscopy.

We report on a spectroscopic multitechnique approach to study the metal/radical spinterface formed by a perchlorinated trityl radical derivative and either gold or silver. The spectroscopic fingerprint of their paramagnetic properties could be determined by comparison with their diamagnetic precursor and by DFT calculations. Thanks to the presented approach, we could gain unprecedented insight into the radical-metal interaction and how this latter perturbs the spin polarization and consequently the magnetoelectronic properties of the radical adlayer. Knowledge of the factors influencing the spinterface is an essential tool toward the tailoring of the properties of spin-based electronic devices.

Electric transport properties of surface-anchored metal-organic frameworks and the effect of ferrocene loading.

Understanding of the electric transport through surface-anchored metal-organic frameworks (SURMOFs) is important both from a fundamental perspective as well as with regards to possible future applications in electronic devices. To address this mostly unexplored subject, we integrated a series of representative SURMOF thin films, formed by copper nodes and trimesic acid and known as HKUST-1, in a mercury-drop-based tunneling junction. Although the transport properties of these SURMOFs are analogous to those of hybrid metal-organic molecular wires, manifested by a very low value of the tunneling decay constant (ß ≈ 0.006 Å(-1)), they are at the same time found to be consistent with a linear increase of resistance with film thickness. Upon loading of SURMOF pores with ferrocene (Fc), a noticeable increase in transport current was observed. A transport model and ab initio electronic structure calculations were used to reveal a hopping transport mechanism and to relate the changes upon Fc loading to those of the electronic and vibrational structures of the SURMOF films.

An ordered organic radical adsorbed on a Cu-doped Au(111) surface.

The tri-para-carboxylic polychlorotriphenylmethyl radical adsorption on a Cu/Au(111) surface has been investigated in ultra-high vacuum environment. The presence of copper favours the formation of metal-organic assemblies, which are analysed using scanning tunnelling microscopy (STM) and high resolution electron energy loss spectroscopy (HREELS). DFT methods indicate that the unpaired electron survives after adsorption, thus creating systems which could be regarded as potential candidates for spintronics applications.

Chiral Conformation at a Molecular Level of a Propeller-Like Open-Shell Molecule on Au(111).

A key stage in engineering molecular functional organizations is represented by controlling the supramolecular assembly of single molecular building blocks, tectons, into ordered networks. Here, we show how an open-shell, propeller-like molecule has been deposited under UHV conditions on Au(111) and its supramolecular organization characterized by scanning tunneling microscopy (STM). Racemic islands were observed at room temperature, and their chirality was imaged at the molecular level at low temperature. Modeling further suggests that the observed chirally alternating ordering dominated by intermolecular interactions is energetically favored. Electron paramagnetic resonance and ultraviolet photoemission spectroscopy evidences suggest that the supramolecular networks may preserve the open-shell character of the tecton. These results represent a fundamental step forward toward the engineering of purely organic spintronic devices.

Correlation of the EPR properties of perchlorotriphenylmethyl radicals and their efficiency as DNP polarizers.

Water soluble perchlorinated trityl (PTM) radicals were found to be effective 95 GHz DNP (dynamic nuclear polarization) polarizers in ex situ (dissolution) (13)C DNP (Gabellieri et al., Angew Chem., Int. Ed. 2010, 49, 3360). The degree of the nuclear polarization obtained was reported to be dependent on the position of the chlorine substituents on the trityl skeleton. In addition, on the basis of the DNP frequency sweeps it was suggested that the (13)C NMR signal enhancement is mediated by the Cl nuclei. To understand the DNP mechanism of the PTM radicals we have explored the 95 GHz EPR characteristics of these radicals that are relevant to their performance as DNP polarizers. The EPR spectra of the radicals revealed axially symmetric g-tensors. A comparison of the spectra with the (13)C DNP frequency sweeps showed that although the solid effect mechanism is operational the DNP frequency sweeps reveal some extra width suggesting that contributions from EPR forbidden transitions involving (35,37)Cl nuclear flips are likely. This was substantiated experimentally by ELDOR (electron-electron double resonance) detected NMR measurements, which map the EPR forbidden transitions, and ELDOR experiments that follow the depolarization of the electron spin upon irradiation of the forbidden EPR transitions. DFT (density functional theory) calculations helped to assign the observed transitions and provided the relevant spin Hamiltonian parameters. These results show that the (35,37)Cl hyperfine and nuclear quadrupolar interactions cause a considerable nuclear state mixing at 95 GHz thus facilitating the polarization of the Cl nuclei upon microwave irradiation. Overlap of Cl nuclear frequencies and the (13)C Larmor frequency further facilitates the polarization of the (13)C nuclei by spin diffusion. Calculation of the (13)C DNP frequency sweep based on the Cl nuclear polarization showed that it does lead to an increase in the width of the spectra, improving the agreement with the experimental sweeps, thus supporting the existence of a new heteronuclear assisted DNP mechanism.

Novel guests for porous columnar thin films: the switchable perchlorinated trityl radical derivatives.

TiO(2) and SiO(2) porous thin films consisting of tilted nanocolumns prepared by glancing angle evaporation (GLAD) have been infiltrated with guest derivatives belonging to the family of perchlorinated trityl radicals, novel guest molecules presenting an open-shell electronic configuration associated with paramagnetism, fluorescence, and electroactivity. The main driving forces for infiltration from aqueous solutions of the carboxylate-substituted radical derivatives are the electrostatic interactions between their negative charge and the net positive charges induced on the film pores. Positive charges on the internal surface of the films were induced by either adjusting the radical solution pH at values lower than the point of zero charge (PZC) of the oxide or passivating the nanocolumns oxide surface with a positively charged aminosilane. The infiltrated composite thin films are robust and easy to handle thanks to the physical protection exerted by the film columns. They also keep the multifunctionality of the used guests, as confirmed by electron paramagnetic resonance (EPR), UV-vis spectroscopy, and fluorescence spectroscopy. To prove the electroactivity of the infiltrated porous films, a porous TiO(2) host layer was supported onto conductive indium tin oxide (ITO). By application of an appropriate redox potential, the guest radical molecules have been reversibly switched from their open-shell electronic configuration to their diamagnetic state and hence changed their optical properties. On the basis of these results, it is herein proposed that the appropriate surface functionalization of the pore internal surface of GLAD thin films can be used to prepare novel radical-oxide composite thin films usable for the development of robust switchable electrically driven photonic and magnetic devices.

Polychlorinated trityl radicals for dynamic nuclear polarization: the role of chlorine nuclei.

Polychlorinated trityl radicals bearing carboxylate substituents are water soluble persistent radicals that can be used for dynamic nuclear polarization. In contrast to other trityl radicals, the polarization mechanism differs from the classical solid effect. DFT calculations performed to rationalize this behaviour support the hypothesis that polarization is transferred from the unpaired electron to chlorine nuclei and from these to carbon by spin diffusion. The marked differences observed between neutral and anionic forms of the radical will be discussed.

Magnetic and porous molecule-based materials.

In this chapter, we give an overview of the recent state-of-the-art research of porous and magnetic molecule-based materials. The subject is introduced by a section devoted to the fundamentals of magnetism in molecular magnets, with special attention to the design strategies to prepare molecular magnetic materials. We will then focus on the two main families of materials combining porosity and magnetism: the purely organic and the metal-organic porous magnetic materials. For both families, a selection of the most representative examples has been made. A complete section is devoted to magnetic and porous materials with flexible frameworks, an area of emerging importance in this field, because of their wide range of applications. Finally, we conclude with a brief overview on the most recent approaches for the future development of these materials.

Nanosized trigonal prismatic and antiprismatic CuII coordination cages based on tricarboxylate linkers.

3D coordination cages have been synthesized via the supramolecular 3 + 2 self-assembly of macrocyclic dicopper molecular clips and tricarboxylate linkers, the nature of which allows the introduction of different functionalities to the 3D cages.

PELDOR spectroscopy with DOPA-beta2 and NH2Y-alpha2s: distance measurements between residues involved in the radical propagation pathway of E. coli ribonucleotide reductase.

Escherichia coli ribonucleotide reductase (RNR) catalyzes the reduction of nucleotides to 2'-deoxynucleotides. The active enzyme is a 1:1 complex of two homodimeric subunits, alpha2 and beta2. The alpha2 is the site of nucleotide reduction, and beta2 harbors a diferric tyrosyl radical (Y122*) cofactor. Turnover requires formation of a cysteinyl radical (C439*) in the active site of alpha2 at the expense of the Y122* in beta2. A docking model for the alpha2beta2 interaction and a pathway for radical transfer from beta2 to alpha2 have been proposed. This pathway contains three Ys: Y356 in beta2 and Y731/Y730 in alpha2. We have previously incorporated 3-hydroxytyrosine and 3-aminotyrosine into these residues and showed that they act as radical traps. In this study, we use these alpha2/beta2 variants and PELDOR spectroscopy to measure the distance between the Y122* in one alphabeta pair and the newly formed radical in the second alphabeta pair. The results yield distances that are similar to those predicted by the docking model for radical transfer. Further, they support a long-range radical initiation process for C439* generation and provide a structural constraint for residue Y356, which is thermally labile in all beta2 structures solved to date.

Hydrogen bonding affects the persistency of alkyl peroxy radicals.

The effect of 1,1,1,3,3,3-hexafluoro-2-phenylpropan-2-ol (HFPP) on the persistency of sec-alkyl peroxy radicals has been investigated. The formation of a hydrogen-bonding complex between HFPP and the radical resulted in a large increase in the lifetime of the radical. This variation was accompanied by a decrease in the g-value. An enthalpy change of -3.4 kcal mol-1 was estimated for the formation of the hydrogen-bonded complex.

Electronic and hydrogen bonding effects on the chain-breaking activity of sulfur-containing phenolic antioxidants.

A kinetic and thermodynamic investigation of phenols para-substituted with thiyl (SR), sulfinyl (SOR), and sulfonyl (SO(2)R) groups and ortho-substituted with thiyl groups is reported. The effect of the sulfur substituents on the O-H bond dissociation enthalpy values, BDE(O-H), was measured by means of the EPR radical equilibration technique and the reactivity toward peroxyl radicals, k(inh), of these phenolic antioxidants was determined by inhibited autoxidation studies. An inverse correlation between these two parameters was found. A p-SMe substituent decreased the BDE(O-H) value to a lesser extent than a p-OMe group (-3.6 vs -4.4 kcal/mol), whereas the effect of the same groups in an ortho position showed an opposite trend (-0.85 vs -0.2 kcal/mol). The latter result is explained in terms of the different strength of the intramolecular hydrogen bond between the OH proton and the sulfur or oxygen substituents in ortho derivatives. ESI-MS analysis of the products formed by reacting the sulfides with peroxyl radicals from the azoinitiator AIBN revealed the formation of a complex mixture of products, which may play an important role in determining the overall antioxidant activity of the parent compounds.

EPR distance measurements support a model for long-range radical initiation in E. coli ribonucleotide reductase.

The class I E. coli ribonucleotide reductase, composed of homodimers of R1 and R2, catalyzes the conversion of nucleoside diphosphates to deoxynucleoside diphosphates. The reduction process involves the tyrosyl radical on R2 that generates a transient thiyl radical on R1 over a proposed distance of 35 A. A mechanism-based inhibitor, 2'-azido-2'-deoxyuridine-5'-diphosphate, that reduces the tyrosyl radical on R2 and forms a nitrogen-centered radical on R1 has provided a method to measure the diagonal distance between the two subunits. PELDOR and DQC paramagnetic resonance methods give rise to a distance of 48 A, similar to that calculated from a docking model of the R1 and R2 structures.

Antioxidant activity of hydroxystilbene derivatives in homogeneous solution.

The antioxidant activity of the cis and trans isomers of several analogues of resveratrol and pterostilbene has been investigated, especially with regard to the effect of the stereochemistry about the olefinic double bond. The antioxidant power of these compounds was estimated by measuring the rate constants for their reactions with peroxyl radicals and, with two of them, the bond dissociation enthalpy (BDE) of the phenolic O-H bond which is cleaved in the inhibition reaction. The present data show that in homogeneous solution the various hydroxystilbenes investigated behave as mild antioxidants with the notable exceptions of the trans isomer of 4 and 6, whose activities are only slightly lower than that of alpha-tocopherol (vitamin E). The rate constants of the inhibition reaction show that the antioxidant activity of the cis-hydroxystilbene is in all the examined cases worse, by a factor ranging between 2 and 6, than that of the corresponding trans isomers. This lower reactivity depends on enthalpy factors as it can be inferred by the experimental values of the O-H bond dissociation enthalpy in the two geometric isomers of 3',5'-di-tert-butyl-4'-hydroxy-3,5-dimethoxystilbene showing that the strength of the O-H bond in the cis isomer is larger by 1.8 kcal/mol. DFT calculations provide a rationalization of this result, indicating that, although the cis geometry implies a destabilization with respect to the trans species of both phenoxyl radical and parent hydroxystilbene, the destabilization of the radical is larger because the folding of the structure strongly reduces the delocalization of the unpaired electron on the styryl group. A comparison of these results with previously reported data on the proapoptotic activity of these stilbenoids suggests that these two properties are not correlated.

Hydrogen-bonding effects on the properties of phenoxyl radicals. An EPR, kinetic, and computational study.

The effect of 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) on the properties of phenoxyl radicals has been investigated. HFP produces large variations of the phenoxyl hyperfine splitting constants indicative of a large redistribution of electron spin density, which can be accounted for by the increased importance of the mesomeric structures with electric charge separation. The conformational rigidity of phenoxyl radicals with electron-releasing substituents is also greatly enhanced in the presence of HFP, as demonstrated by the 2 kcal/mol increase in the activation energy for the internal rotation of the p-OMe group in the p-methoxyphenoxyl radical. By using the EPR equilibration technique, we have found that in phenols the O-H bond dissociation enthalpy (BDE) is lowered in the presence of HFP because it preferentially stabilizes the phenoxyl radical. In phenols containing groups such as OR that are acceptors of H-bonds, the interaction between HFP and the substituent is stronger in the phenol than in the corresponding phenoxyl radical because the radical oxygen behaves as an electron-withdrawing group, which decreases the complexating ability of the substituent. In phenols containing OH or NH(2) groups, EPR experiments performed in H-bond accepting solvents showed that the interaction between the solvent and the substituent is much stronger in the phenoxyl radical than in the parent phenol because of the electron-withdrawing effect of the radical oxygen, which makes more acidic, and therefore more available to give H-bonds, the OH or NH(2) groups. These experimental results have been confirmed by DFT calculations. The effect of HFP solvent on the reactivity of phenols toward alkyl radicals has also been investigated. The results indicated that the decrease of BDE observed in the presence of HFP is not accompanied by a larger reactivity. The origin of this unexpected behavior has been shown by DFT computations. Finally, a remarkable increase in the persistency of the alpha-tocopheroxyl radical has been observed in the presence of HFP.

Antioxidant activity of o-bisphenols: the role of intramolecular hydrogen bonding.

A kinetic and thermodynamic investigation on the antioxidant activity of 2,2'-methylenebis(6-tert-butyl-4-methylphenol) (2), 2,2'-ethylidenebis(4,6-di-tert-butylphenol) (3), and 4,4'-methylenebis(2,6-di-tert-butylphenol) (4) are reported. EPR studies of the equilibration between 3 or 4 and a reference phenol, and the corresponding phenoxyl radicals, allowed us to determine the O-H bond dissociation enthalpy (BDE) of the O-H bond as 81.2 and 81.1 kcal/mol in 3 and 4, respectively. Despite this similarity, the absolute rate constants for the reaction with peroxyl radicals, determined by autoxidation studies under controlled conditions, indicate that the o-bisphenols 2 and 3 behave as excellent antioxidants while the p-bisphenol 4 is less effective by a factor of 64 and 22, respectively. FT-IR spectroscopy and product studies suggest that the very good antioxidant activity of the o-bisphenols largely arises from both the reduced steric crowding about the hydroxyl group and the stabilization of the aroxyl radical due to the formation of an intramolecular hydrogen bond between the residual OH and the oxygen radical center.

Hydroxylamines as oxidation catalysts: thermochemical and kinetic studies.

Bond dissociation enthalpies (BDE) of hydroxylamines containing alkyl, aryl, vinyl, and carbonyl substituents at the nitrogen atom have been determined by using the EPR radical equilibration technique in order to study the effect of the substituents on the O-H bond strength of these compounds. It has been found that substitution of an alkyl group directly bonded to the nitrogen atom with vinyl or aryl groups has a small effect, while substitution with acyl groups induces a large increase of the O-H BDE value. Thus, dialkyl hydroxylamines have O-H bond strengths of only ca. 70 kcal/mol, while acylhydroxylamines and N-hydroxyphthalimide (NHPI), containing two acyl substituents at nitrogen, are characterized by BDE values of ca. 80 and 88 kcal/mol, respectively. Since the phthalimide N-oxyl radical (PINO) has been recently proposed as an efficient oxidation catalyst of hydrocarbons or other substrates, the large BDE value found for the parent hydroxylamine (NHPI) justifies this proposal. Kinetic studies, carried out in order to better understand the mechanism of the NHPI-catalyzed aerobic oxidation of cumene, are consistent with a simple kinetic model where the rate-determining step is the hydrogen atom abstraction from the hydroxylamine by cumylperoxyl radicals.

Determination of the substituent effect on the O-H bond dissociation enthalpies of phenolic antioxidants by the EPR radical equilibration technique.

The bond dissociation enthalpies (BDE) of several phenols containing electron-withdrawing substituents in the para position have been determined by means of the EPR radical equilibration technique. It has been found that CN, NO(2), CHO, COOR, and COOH induce an increase of the BDE value of the O-H bond, thus producing a worsening of the antioxidant activity of phenols, while Cl, Ph, and CH[double bond]CHPh show an opposite effect. The contributions of these substituents for the calculation of the BDE values in polysubstituted phenols by using the group additivity rule have also been derived. It is shown that this rule provides quite reliable predictions of bond strengths, so that the method can be conveniently used to estimate new data on substituted phenols.