Paulo N. Martinho

Spin transition in arrays of gold nanoparticles and spin crossover molecules.

We investigate if the functionality of spin crossover molecules is preserved when they are assembled into an interfacial device structure. Specifically, we prepare and investigate gold nanoparticle arrays, into which room-temperature spin crossover molecules are introduced, more precisely, [Fe(AcS-BPP)2](ClO4)2, where AcS-BPP = (S)-(4-{[2,6-(dipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl)ethanethioate (in short, Fe(S-BPP)2). We combine three complementary experiments to characterize the molecule-nanoparticle structure in detail. Temperature-dependent Raman measurements provide direct evidence for a (partial) spin transition in the Fe(S-BPP)2-based arrays. This transition is qualitatively confirmed by magnetization measurements. Finally, charge transport measurements on the Fe(S-BPP)2-gold nanoparticle devices reveal a minimum in device resistance versus temperature, R(T), curves around 260-290 K. This is in contrast to similar networks containing passive molecules only that show monotonically decreasing R(T) characteristics. Backed by density functional theory calculations on single molecular conductance values for both spin states, we propose to relate the resistance minimum in R(T) to a spin transition under the hypothesis that (1) the molecular resistance of the high spin state is larger than that of the low spin state and (2) transport in the array is governed by a percolation model.

Cooperative Spin Transition in a Mononuclear Manganese(III) Complex

Mind the gap: A complete, cooperative spin transition for a mononuclear Mn(III) complex is reported with an 8 K hysteresis window. Raman spectra collected at a single temperature in warming and cooling modes confirm the electronic bistability within the hysteresis loop. The source of the cooperativity is a disconnection in the hydrogen-bonded 1D chains that connect adjacent cations owing to an order-disorder transition in the PF(6)(-) counterion.

Synthesis, crystal structure and EPR spectroscopic analysis of novel copper complexes formed from N-pyridyl-4-nitro-1,8-naphthalimide ligands

The mono-dentate, pyridyl containing, nitro naphthalimide ligands N-(4-pyridyl)-4-nitro-1,8-naphthalimide (L₁) and N-(3-pyridyl)-4-nitro-1,8-naphthalimide (L₂) were prepared and complexed with a selection of copper salts [Cu(OAc)2, Cu(CF3SO3)2 and Cu(ClO4)2]. Crystallographic studies were undertaken and revealed that dinuclear acetate bridged complexes resulted from reactions with Cu(OAc)2, while mononuclear systems resulted from reactions with Cu(CF3SO3)2 and Cu(ClO4)2. Despite the differing coordination environments the naphthalimide based ligands provided a range of interesting π-based interactions in the form of π···π, anion···π, nitro···π, solvent···π and C=O···π associations. Solid state EPR spectra were in agreement with the coordination environments observed from crystallography.

Electrochemical studies and potential anticancer activity in ferrocene derivatives

Abstract Several ferrocene derivatives (five mononuclear and two binuclear), including the new N-(p-chlorophenyl)-carboxamidoferrocene (1), were synthesized and their anticancer activity investigated. Two of them, 3 and 7, bearing a benzimidazole backbone were the most active against HeLa cells achieving IC50 values of ~5 μM along with 4 with a dipyridylamine ligand (~6 μM). Complex 6, also with a benzimidazole backbone, displayed slightly higher values (~11 μM). Cyclic voltammetry studies show that while the non-cytotoxic ferrocene derivatives 1, 2, and 5 follow a ferrocene-based redox behavior, derivatives 3, 4, 6, and 7 exhibit a more complex mechanism. These complex mechanisms are consistent with a more effective cytotoxic activity. Mössbauer spectroscopy parameters reflect a very small influence of the substituents.

The influence of molecular mobility on the properties of networks of gold nanoparticles and organic ligands

Summary We prepare and investigate two-dimensional (2D) single-layer arrays and multilayered networks of gold nanoparticles derivatized with conjugated hetero-aromatic molecules, i.e., S-(4-{[2,6-bipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl)thiolate (herein S-BPP), as capping ligands. These structures are fabricated by a combination of self-assembly and microcontact printing techniques, and are characterized by electron microscopy, UV–visible spectroscopy and Raman spectroscopy. Selective binding of the S-BPP molecules to the gold nanoparticles through Au–S bonds is found, with no evidence for the formation of N–Au bonds between the pyridine or pyrazole groups of BPP and the gold surface. Subtle, but significant shifts with temperature of specific Raman S-BPP modes are also observed. We attribute these to dynamic changes in the orientation and/or increased mobility of the molecules on the gold nanoparticle facets. As for their conductance, the temperature-dependence for S-BPP networks differs significantly from standard alkanethiol-capped networks, especially above 220 K. Relating the latter two observations, we propose that dynamic changes in the molecular layers effectively lower the molecular tunnel barrier for BPP-based arrays at higher temperatures.

Asymmetric binuclear Ni(II) and Cu(II) Schiff base metallopolymers

New asymmetric Cu(II)/Cu(II) and Ni(II)/Ni(II) binuclear Schiff base complexes were synthesised via a one-pot reaction introducing two different aldehyde building blocks (R = H, t-Bu). These complexes were successfully electropolymerised to afford new metallopolymers using different sweep scan rates (50, 100 and 200 mV s−1) between 0 and 1.4 V. Surface coverage – (Γ) values were determined assuming ligand-based conduction and are of about 10−7 mol cm−2 for both polymers. Atomic force microscopy (AFM) images showed morphological differences between Cu(II) and Ni(II) polymers with root mean square roughness (Rq) higher for the former. DFT studies support the ligand based electropolymerisation mechanism, based on the electron density of the cation at the para position of the phenolate ring, and suggest that copper dimers prefer slipped square planar geometry, with one Cu–O weak bond between adjacent units, rather than stack as in Ni(II) dimers, in agreement with the AFM observations.