Bistability in Fc-PTM crystals: the role of intermolecular electrostatic interactions

Fc-PTM is a valence tautomeric radical, where the ferrocene (Fc) group, a good electron donor, is linked by an ethylenic spacer to a perchlorotriphenylmethyl radical (PTM(*)), a good electron acceptor. In solution this compound exists mainly in the neutral Fc-PTM(*) form which can be photoexcited through an intramolecular electron transfer to the zwitterionic Fc(+*)-PTM(-) form. By contrast, in crystals of Fc-PTM at room temperature both the neutral and the zwitterionic forms coexist, pointing to a true bistability phenomenon. We rationalize these findings accounting for the role of intermolecular electrostatic interactions in Fc-PTM crystals. In fact the energy of the zwitterionic Fc(+*)-PTM(-) form is lowered in the crystal by attractive electrostatic intermolecular interactions and the cooperative nature of these interactions explains the observed coexistence of neutral Fc-PTM(*) and zwitterionic Fc(+*)-PTM(-) species. The temperature evolution of Mossbauer spectra of Fc-PTM is quantitatively reproduced adopting a bottom-up modeling strategy that combines a molecular model, derived from optical spectra of Fc-PTM in solution, with a model for intermolecular electrostatic interactions, supported by quantum-chemical calculations. Fc-PTM then offers the first experimental demonstration of bistability induced by electrostatic interactions in crystals of valence tautomeric donor-acceptor molecules.     

Preparation of uniform rich cholesterol unilamellar nanovesicles using CO2-expanded solvents

Nanoscopic and uniform unilamellar vesicles, rich in cholesterol, have been prepared by a new procedure, named "DELOS-SUSP", which is based on the depressurization of a cholesterol solution in CO2-expanded acetone into an aqueous solution containing a surfactant. The CO2 is used here as a cosolvent medium, allowing the straightforward preparation of vesicular systems with controlled size distribution, uniform shape, and stability unachievable by conventional mixing technologies. The resulting nanoscopic vesicular systems dispersed in water were characterized using dynamic light scattering, zeta potential, turbidity, and cryogenic transmission electron microscopy. The influence of operational parameters of this new methodology on the physicochemical characteristics of the vesicular systems is also reported.     

Solvent tuning from normal to inverted marcus region of intramolecular electron transfer in ferrocene-based organic radicals

The solvent dependence of spectroscopic data of two neutral paramagnetic donor-acceptor dyads, based on a polychlorinated triphenylmethyl radical acceptor unit linked through a vinylene pi-bridge to a ferrocene (compound 1) or a nonamethylferrocene donor (compound 2) unit, is described. Both compounds exhibit broad absorptions in the near-IR region, with band maxima appearing around 1000 and 1500 nm for 1 and 2, respectively. These bands correspond to the excitation of a neutral DA ground state to the charge-separated D+A- state, indicative of an intramolecular electron-transfer process. Compounds 1 and 2 show two reversible one-electron redox processes associated with the oxidation of the ferrocene and the reduction of the polychlorotriphenylmethyl radical subunits. The solvent dependence of the redox potentials was also investigated, allowing the determination of the redox asymmetries DeltaG degrees of both dyads. The latter values, along with the experimental Eopt spectroscopic data, allow us to estimate, using the total energy balance Eopt = lambda + DeltaG degrees , the reorganization energy values, lambda, and their solvent polarity dependence. Since DeltaG degrees and lambda are of the same order of magnitude but exhibit opposite trends in their solvent polarity dependence, a unique shift from the normal to the inverted Marcus region with the change in solvent polarity is found. The kinetics of the charge recombination step of the excited charge-separated D+A- state was studied by picosecond transient absorption spectroscopy, which allows us to observe and monitor for the first time the charge-separated D+A- state, thereby confirming unambiguously the photoinduced electron-transfer phenomena.     

New insights into the thermal stability of Mn12 clusters: the case of complex [Mn12O12(O2CC[triple bond]-CH)16(H2O)4] x 3H2O and its thermolysis derived [Mn3(O2CC[triple bond]CH)6(H2O)4] x 2H2O complex

Two novel Mn12 derivatives [Mn12O12(O2CC[triple bond]CH)16(H2O)4] x 3H2O (1) and [Mn12(O2CC[triple bond]CC6H5)16(H2O)4] x 3H2O (2) have been prepared and characterized. Magnetic measurements confirm that both function as single-molecule magnets (SMM), showing frequency-dependent out-of-phase AC susceptibility signals and magnetization hysteresis curves. Thermal stability studies of both complexes were first conducted in the solid state. While complex 1 undergoes a sudden exothermal decomposition at T(onset) = 118 degrees C, complex 2 exhibits a higher stability. Thermolysis reaction of 1 was hence assessed in solution to yield dark red crystals of a two-dimensional Mn(II)-based co-ordination polymer [Mn3(O2CC[triple bond]CH)6(H2O)4] x 2H2O (3), which corresponds to an extended sheet-like structure that crystallizes in the monoclinic space group P2(1)/n; a = 9.2800(2) angstroms, b = 9.4132(2) angstroms, c = 14.9675(3) angstroms, beta = 99.630(1) degrees, and Z = 2. Finally, the magnetic properties of complex 3 have been studied on an oriented single crystal over two different orientations of the reciprocal vector versus the external field.     

Advances on the nanostructuration of magnetic molecules on surfaces: the case of single-molecule magnets (SMM)

SMMs exhibit slow magnetization relaxation rates characteristic of nanodomain particles whose origin is however on individual molecules. For this reason, they have attracted much interest due to their potential applications in high-density information storage devices and quantum computing applications, where for instance, each molecule can be used as a magnetic bit of information. However, for this to become a reality, several basic studies such as their deposition on surfaces are still highly required. Here we will revise all the experimental approximations that have been so far reported for their addressing, nanostructuration and study on surfaces, from the use of stamps as templates to their anchorage to gold surface through the use of thiol-based ligands. It is also important to emphasize that the results and methodologies described along this review are applicable not only to SMMs but to any molecular material.     

Self‐Assembled Architectures with Segregated Donor and Acceptor Units of a Dyad Based on a Monopyrrolo‐Annulated TTF–PTM Radical

An electron donor–acceptor dyad based on a polychlorotriphenylmethyl (PTM) radical subunit linked to a tetrathiafulvalene (TTF) unit through a π‐conjugated N‐phenyl–pyrrole–vinylene bridge has been synthesized and characterized. The intramolecular electron transfer process and magnetic properties of the radical dyad have been evaluated by cyclic voltammetry, UV/Vis spectroscopy, vibrational spectroscopy, and ESR spectroscopy in solution and in the solid state. The self‐assembling abilities of the radical dyad and of its protonated non‐radical analogue have been investigated by X‐ray crystallographic analysis, which revealed that the radical dyad produced a supramolecular architecture with segregated donor and acceptor units in which the TTF subunits were arranged in 1D herringbone‐type stacks. Analysis of the X‐ray data at different temperatures suggests that the two inequivalent molecules that form the asymmetric unit of the crystal of the radical dyad evolve into an opposite degree of electronic delocalization as the temperature decreases.     

Benzodicarbomethoxytetrathiafulvalene derivatives as soluble organic semiconductors

A series of new tetrathiafulvalene (TTF) derivatives bearing dimethoxycarbonyl and phenyl or phthalimidyl groups fused to the TTF core (6 and 15-18) has been synthesized as potential soluble semiconductor materials for organic field-effect transistors (OFETs). The electron-withdrawing substituents lower the energy of the HOMO and LUMO levels and increase the solubility and stability of the semiconducting material. Crystal structures of all new TTF derivatives are also described, and theoretical DFT calculations were carried out to study the potential of the crystals to be used in OFET. In the experimental study, the best performing device exhibited a hole mobility up to 7.5 × 10(-3) cm(2) V(-1) s(-1)).     

Selective picomolar detection of mercury(II) using optical sensors

The rational design of a mercury(II) ligand consisting in a 1-(4'-oxyphenyl)-4(1'-pyrenyl)-2,3-diaza-1,3-butadiene receptor unit, optimizes the sensitivity and reliability of a SPR sensor by the formation of a well packed SAM over the gold surface. SPR analysis allows detecting mercury(II) concentrations in aqueous systems in the picomolar range, meliorating on three orders of magnitude the EU mercury(II) detection limit in drinkable water.