Electrochemical and spectroscopic behavior of iron(III) porphyrazines in Langmuir-Schafer films.

Thin films of a newly synthesized iron(III) porphyrazine, LFeOESPz ( L = ClEtO, OESPz = ethylsulfanylporphyrazine), have been deposited by the Langmuir-Schafer (LS) technique (horizontal lifting) on ITO or gold substrates. Before deposition, the floating films have been investigated at the air-water interface by pressure/area per molecule (pi/ A) experiments, Brewster angle microscopy (BAM) and UV-vis reflection spectroscopy (RefSpec). The complex reacts with water subphase (pH 6.2) forming the mu-oxo dimer, which becomes the predominant component of the LS films ( LS-Fe) as indicated by optical, IR, XPS, and electrochemical data. LS-Fe multilayers exhibit, between open circuit potential (OCP) and +0.90 V (vs SCE), two independent peak pairs with formal potentials, E surf (I) and E surf(II) of +0.56 V and +0.78 V, respectively. According to dynamic voltammetric and coulometric experiments the peak pair at +0.56 V is attributed to one-electron process at the iron(III) centers on the monomer, while the peak pair at +0.78 V is associated to a four-electron process involving mu-oxo-dimer oligomers. LS-Fe films prove to be quite stable electrochemically between OCP and +0.90 V. The electrochemical stability decreases, however, when the potential range is extended both anodically and cathodically outside these limits, due to formation of new species. Upon incubation with TCA solutions, LS-Fe films show remarkable changes in the UV-vis spectra, which are consistent with a significant mu-oxo dimer --> monomer conversion. Addition of TCA to the electrochemical cell using a LS-Fe film as working electrode, results in a linear increase of a cathodic current peak near -0.40 V as the TCA concentration varies in the 0.1-2.0 mM range. This behavior is interpreted in terms of TCA inducing a progressive change in the composition of the LS-Fe films in favor of the monomeric iron(III) porphyrazine, which is responsible for the observed increase in the cathodic current near -0.40 V.

Nanoaggregates of copper porphyrazine in floating layers and Langmuir-Schaefer films.

Aggregation behavior of unsubstituted copper porphyrazine (CuPaz) on the water surface was studied by analysis of compression curves, Brewster angle microscopy (BAM), and optical spectroscopy. The structure and stability of the CuPaz aqua aggregates in the floating layers are determined by hydration degree that depends on initial surface concentration and surface pressure. Langmuir-Schaefer (LS) films of CuPaz were prepared by deposition of the variously structured floating layers and studied by X-ray scattering technique and optical spectroscopy. Stable and labile structures were detected and compared with the floating CuPaz aqua aggregates. Conditions of formation of the stable four-stacked nanoaggregates in LS films were determined. A model comprising both nucleation of CuPaz on the water surface and structural transformations in the solid films is proposed.

Growth and characterization of films containing fullerenes and water soluble porphyrins for solar energy conversion applications.

Thin films consisting of two fulleropyrrolidine derivatives 1 or 2 and a water-soluble porphyrin, TPPS4, were prepared by the Langmuir-Schäfer (LS, horizontal lifting) method. In particular, a solution of the fulleropyrrolidine in chloroform and dimethyl sulfoxide was spread on the water surface, while the porphyrin (bearing peripheral anionic sulfonic groups) was dissolved into the aqueous subphase. To the best of our knowledge, such a versatile method for film fabrication of fullerene/porphyrin mixed composite films has never been used by other researchers. Evidence of the effective interactions between the two components at the air-water interface was obtained from the analysis of the floating layers by means of surface pressure vs area per molecule Langmuir curves, Brewster angle microscopy, and UV-visible reflection spectroscopy. The characterization of the LS films by UV-visible spectroscopy reveals that in each case the two constituents behave as strongly interacting pi systems. The use of polarized light suggests the existence of a preferential direction of the TPPS4 macrocyclic rings with an edge-on arrangement with respect to the substrate surface, regardless which fulleropyrrolidine derivative is in the composite film. Atomic force microscopy investigations give evidence of morphologically flat layers even for LS transfer at low surface pressures. Photoaction spectra were recorded from films deposited by only one horizontal lifting onto indium-tin-oxide (ITO) electrodes, and the observed photocurrent increased notably with increasing transfer surface pressure for both 1/TPPS4 and 2/TPPS4 composite films. IPCE values are larger for 2/TPPS4 systems in comparison with 1/TPPS4 composite layers. Finally, a nonconventional approach to photoinduced phenomena is proposed by differential spectroscopy in the FT-IR attenuated total reflectance (ATR) mode.

Ethane-bridged zinc porphyrin dimers in Langmuir-Shäfer thin films: structural and spectroscopic properties.

This work reports on the structural and spectroscopic properties of ethane-bridged Zn porphyrin dimers (1) in Langmuir-Schäfer (LS) thin films by combining scanning force microscopy (SFM) with film balance, UV-vis absorption, fluorescence, and nanosecond laser flash photolysis measurements. Results show that depending on the surface pressure the Langmuir films of pure 1 can be arranged in two different condensed phases, whereas SFM of the LS films shows characteristic fractal networks constituted by nanoscopic aggregates. The spectral findings agree with a picture in which 1 is apparently present in the anti conformation but aggregated in a sort of H-type structure whose optical features resemble those of the syn conformer. This type of structure is not responsive to light stimuli. By diluting 1 in arachidic acid the porphyrin aggregation is significantly minimized with 1 exhibiting almost exclusively the anti conformation. As a result the LS films become photoresponsive, showing fluorescence emission and triplet-triplet transient absorption.

Piezoelectric sensor functionalised by a self-assembled bipyridinium derivative: characterisation and preliminary applications in the detection of heavy metal ions.

The synthesis of a molecule, 1-(11-dodecylsulfanyl-undecyl)-[4,4']bipyridinium bromide (1), suitable at the same time to form a covalent bond with gold electrodes of a piezoelectric quartz crystal and to interact with heavy metal ions in aqueous solutions, has been successfully accomplished. A commercial quartz crystal microbalance instrument has been modified in order to perform a Flow-Injection Analysis. The behaviour of the system follows the Kanazawa-Gordon equation as demonstrated by measurements with glucose solutions. The self-assembled layer of 1 onto a gold electrode has been characterised by Atomic Force Microscopy before and after the sensing tests. The sensing performances of the modified gold electrode were investigated by monitoring the frequency variation induced by the presence of heavy metal ions, such as lead, cadmium and mercury, in aqueous media. The explored concentrations ranged between 10(-4) and 10(-2) M and the corresponding frequency variations ranged between 10 and 50 Hz. All responses observed were fast, reproducible and reversible. In particular, the response to mercury appears significantly higher in comparison with the other analytes. To the best of our knowledge, this contribution represents the first example of sensing layer based on bipyridinium receptor showing reversible and, at some extent, differentiated response towards heavy metal ions.

Langmuir-Schäfer films of an amphiphilic ruthenium complex bearing an "almost naked" multicharged headgroup.

In this paper the preparation and characterization of Langmuir-Schäfer (LS) films of a novel amphiphilic dipolar complex, [Ru(II)(NH(3))(5)(N-dodecyl-4,4'-bpy)](PF(6))(3) (1), are reported. Preparation of these films, otherwise precluded utilizing standard Langmuir procedures, is achieved by using a subphase at relatively high ionic strength, by addition of NH(4)PF(6). The morphology and the spectroscopic features of the floating films are investigated by Brewster angle microscopy and UV-vis reflection spectroscopy at the water-air interface, respectively, whereas LS films are characterized by absorption spectroscopy and atomic force microscopy. The overall results indicate the existence of aggregates of 1 and formation of homogeneous, densely packed layers. The presented approach could represent a general method to achieve Langmuir-Blodgett films of amphiphilic metal complexes having an "almost naked" multicharged headgroup.