Arturo Colligiani

Some aspects of the reactivity of rhodium(I) and rhodium(II) tricyclohexylphosphine derivatives studied by ESR spectroscopy II. Formation of a paramagnetic Rh(II) carbonyl species by treatment of RhCl2(PCy3)2 in the solid state with carbon monoxide

Abstract The behaviour of RhCl(PCy 3 ) 2 in the solid state towards O 2 , Co, and H 2 was investigated. Whereas no reaction occurred with dioxygen and dihydrogen, treatment with carbon monoxide led to the formation of RhCl 2 (CO)(PCy 3 ) 2 , which is the first example of a paramagnetic carbonyl Rh(II) derivative. The product is rather labile but the life-time in the solid state was long enough to allow the spectroscopic study of the decay process. ESR and IR spectra indicated a disproportionation reaction to Rh(I) and Rh(II) carbonyl species.

Zeeman effect on the nuclear quadrupole resonance of 81Br in single crystals of methyl 4-bromobenzoate, 4,4′-dibromodiphenyl-ether and 4,4′-dibromodiphenylsulphide at 77 K

The Zeeman effect on the N.Q.R. of methyl 4-bromobenzoate, 4,4′-dibromodiphenylether, 4,4′-dibromodiphenylsulphide has been studied at 77 K on single crystals, using the ‘geometric method’. The apparatus used in the measurements, which allows one to maintain any temperature between room temperature and 77 K, is described, and the possible significance of the slight variations in the molecular arrangement found in the present measurements and in those performed at room temperature is discussed.

Five-coordinated metal complexes of bis(2-hydroxy-1-naphthylideneimine-3-propyl)amine and their reactivity towards dioxygen. Part I. An electrochemical investigation on manganese(II), iron(II), cobalt(II), nickel(II) and copper(II) complexes

Abstract The electrochemical behaviour in aprotic solvent of the complexes {M[bis-(2-hydroxy-l-naphthylideneimine-3-propyl)amine]}, where M = Mn(II), Co(II), Fe(II), Ni(II) and Cu(II) is reported. The complexes were prepared and characterized by elemental analysis, infrared and visible spectroscopy and magnetic susceptibility measurements. In addition the reactivity towards dioxygen of the Mn(II), Fe(II) and Co(II) derivatives was investigated, mainly by cyclic voltammetry and gas-volumetric uptake measurements. The results indicate that the Co(II) complexes are able to add dioxygen reversibly, while Mn(II) and Fe(II) compounds undergo an irreversible oxygenation process. The pathway of the dioxygenation processes is tentatively interpreted on the basis of the electrochemical responses. The results confirm that the location of the oxidation potential allows one to predict whether a compound is able to react with dioxygen, but it is not sufficient to predict whether the dioxygenation reaction proceeds reversibly.

Unconditionally stable indole-derived glass blends having very high photorefractive gain: the role of intermolecular interactions.

The photorefractivity of an indole derivative and of its polymer blends has been studied at room temperature. The indole derivative 3-[2-(4-nitrophenyl)ethenyl]-1-(2-ethylhexyl)-2-methylindole (NPEMI-E) is a typical low-molecular-weight glass-forming molecule having peculiar nonlinear optics characteristics. It is unconditionally soluble in the photoconductive poly-(N-vinyl-2,3-dimethylindole) so that all the possible blends can be studied for a weight percent (wt. %) content of NPEMI-E ranging from zero to 100. A very high and sharp maximum of the photorefractive optical gain Gamma(2) approximately 2000 cm(-1) was obtained for a NPEMI-E wt. % content of about 90. On the basis of recently published theoretical calculations, we have made the hypothesis that the rapid change of Gamma(2) can also be ascribed to a correspondingly quick variation of the molecular electro-optic parameters of the dissolved chromophore for some well distinguished values of its concentration in the polymer matrix. Differential scanning calorimetry measurements were made and the results carefully analyzed with the aim of obtaining information on the intermolecular interactions. These last measurements also allowed rationalizing the unconditionally stable glass appearance of the obtained blends. Measurements of spectroscopic ellipsometry were also made on blends with different NPEMI-E content.

Some aspects of the reactivity of rhodium(I) and rhodium(II) tricyclohexyl-phosphine derivatives studied by ESR spectroscopy. I. Formation of paramagnetic RhIIO−2 species autooxidation of the coordinated PCy3 in the solid state

Abstract The reactivity of RhCl(PCy 3 ) 2 in the solid state towards O 2 , CO, and H 2 was investigated by ESR and IR spectroscopies and compared with that in solution. The ‘decomposition’ reaction occuring upon exposure to air produces superoxo species by dioxygen coordination. This leads to auto-oxidation of the coordinated PCy 3 , which is partially dissociated from the complex. This behaviour is quite different from that in solution, where a dioxygen adduct is formed. The ability of the complex in activating O 2 in the solid state has been related to the existence of polynuclear species. No substantial difference has been found for the reactivity of the complex in solution and in the solid state towards CO and H 2 , except that the treatment with CO allowed to evidence Rh(II) impurities contained in the product.

ENDOR study of the displacements of the HCO2 - groups in the formation of secondary radicals in an X-irradiated single crystal of α-calcium formate

The results of a room temperature ENDOR investigation of the secondary radicals formed after X-irradiation in a single crystal of α-calcium formate are reported and discussed. 39 different proton hyperfine tensors have been determined, belonging to magnetically or structurally different radicals. The hyperfine tensors are typical of α-protons in π-radicals, with a spin density ρc = 0·75 on the central carbon atoms. The radicals are identified therefore -O-ĊH-O-CO2 - in twelve slightly different conformations. The mechanisms giving rise to the reactions of primary σ-radicals CO2 - with the neighbouring undamaged molecules through cooperative rotations around the Ca-O bonds are discussed. It is shown that the experimentally determined C-H directions are very well reproduced by rotations of the formate groups around a Ca-O bond.

An indole-based low molecular weight glass-former giving materials with high cooperative photorefractive optical gain

A derivative of 2-methylindole, namely 3-[2-(4-nitrophenyl)ethenyl]-1-(2-ethylhexyl)-2-metylindole (NPEMI-E) has been synthesized. Materials obtained from this molecule have been studied as thin films between two ITO layers. The study revealed that NPEMI-E collects in itself both photoconductivity and NLO characteristics. Differential Scanning Calorimetry (DSC) measurements showed the formation of stable glass films characterized by a Tg temperature lower than room temperature. Blends with the photoconductive poly-N-vinyl-2,3-dimethylindole (PVDMI) were also studied, giving again stable glass films independently of the wt.% contents of NPEMI-E. Photorefractivity measurements on both pure and blended NPEMI-E allowed to measure a value of the optical gain Γ = 627 cm-1 at an applied electric field E = 60 V/µm. This high value of Γ corresponds to a sharp maximum of the experimental trend of Γ as a function of the wt.% content of NPEMI-E. The corresponding content was wt.% = 91.5. The presence of this maximum induced us to make the hypothesis that, besides the well known reorientational contribution to the photorefractivity, a further mechanism (recently theoretically studied) is active in our blends. This mechanism arises in the interactions among the NLO polarized and polarizable moieties (cooperative effect). It can produce a rapid variation of some of the electrooptical parameters conditioning the extent of the photorefractivity. This can happen at a well defined mean intermolecular distance and hence at a well defined concentration of the NLO molecules.

NQR Zeeman study of 3-cyanopyridine

Abstract Zeeman measurements were performed on the NQR lines of 3-cyanopyridine at room temperature on a single crystal, and the efg principal axes were located. The Z axes at the two nitrogen nuclei pertaining Co the same molecule identified the aromatic plane accurately. The X axis of the cyano nitrogen and the Y axis of the pyridine one were found to be orthogonal to the molecular plane, all other axes lying in such plane. Two physically unequivalent sites were found; all pyridine axes of the two sites were almost (within 4–7°) parallel, and the angle among the Z axes of the two cyano groups was nearly exactly 120°. This strongly suggests that the molecules in the crystal are packed in parallel planes, the aromatic rings being referred by almost purely translational transformations, while the CN groups alternate on either side. The crystal symmetry was found to be monoclinic, although not far from rhombic. Because of the small S/N ratio, together with the low tl value for the cyano nitrogen and the near parallelism of the pyridine sites, methodological innovations had to be introduced, due to the failure of the more usual methods of detection and treatment of the experimental data.

Zeeman effect in the N.Q.R. of 81Br in single crystals of 2,4-dibromoanisole and 2,4,6-tribromoanisole

The nuclear quadrupole resonance frequencies of 2,4-dibromoanisole at 77 K and 296 K have been determined, as well as those of 2,4,6-tribromoanisole at 296 K. The Zeeman effect has been studied in both substances at 296 K using the so-called ‘geometric method’; this allowed us to find in both cases the value of the asymmetry parameter η of the electric field gradient tensors on the Br nuclei. An assignment of the resonance frequencies to the various bromine atoms is given on the basis of our results, which also give, using the Townes and Dailey theory, values for the electron populations of the σ and π orbitals.

Poly(1-vinylindole) and some of its methyl derivatives as substrates for photorefractive materials: their synthesis, optical, and electrical characterization

The best conditions for the synthesis of poly(1-vinylindole) (PVI) and some of its methyl derivatives have been investigated. The aim of the research was to verify if PVI could be used instead of poly(1- vinylcarbazole) (PVK) in polymeric blends having photoconductive and/or photorefractive behavior. All synthesized polymers are characterized by a glass transition temperature that is lower than that of PVK. It has been verified that the indole ring system, inclusive of its methyl substituted derivatives, lacking of the symmetry characteristics of carbazole, possesses an electric dipole moment that is constantly higher than that of PVK. A higher dipole moment can be an advantageous feature to improve the solubility within the polymetric matrix of the optically non-linear molecule, necessary for photorefractivity. Charge-transfer complexes with a fluorenone-like photosensitizer are efficiently formed by both PVI and its derivatives. A direct current measuring apparatus has been assembled by which photocurrents of few picoamperes can be detected and recorded. Preliminary reported results clearly show that PVI is a photoconductive material. Its photoconductivity is compared with that of PVK and of hybrid materials containing mixtures of carbazole and indole moieties, present in the measuring cell as pendant groups on a macromolecular chain, as single molecules, or both.