Rocco Angelone

Synthesis and electrooptical characterization of polysiloxanes containing indolyl groups acting as photoconductive substrates for photorefractive materials

Abstract Synthesis and electrooptical analysis of polysiloxane and poly-N-vinyl derivatives containing indolyl groups are reported. The indole group and some of its methyl derivatives have been taken into account in order to evaluate their behaviour, with respect to that of the widely employed carbazole group, when used as photoconductive centres attached to a polymer chain. The obtained data show that photoconductivity and traps formation mechanism can be inferred as functions of the physico-chemical parameters (electric dipole moment and ionization potential) of the different groups and polymers involved. To this end such parameters have been carefully computed for a series of pyrrole, indole and carbazole derivatives. The 2,3-dimethylindole derivative appears to be particularly promising due to its electrooptical behaviour in the red absorption region where measurements have been accomplished and are shown to be consistent with the theoretical predictions.

New Nuclear Quadrupole Resonance Frequencies at 14N in Saturated Cyclic Amines

The values of the nuclear quadrupole coupling constants e2Qq of 14N and the asymmetry parameters η in a series of saturated six‐, five‐, four‐, and three‐membered cyclic amines and of their N‐methyl derivatives have been measured at various temperatures, starting from liquid nitrogen temperature. Several resonance lines which could not be observed at 77°K, were detected at higher temperatures. By applying the Townes and Dailey theory to the experimental results, the electron populations of the atomic orbitals used by the nitrogen atom for the bonds with the carbon and hydrogen atoms have been evaluated. The values of the valence angles of nitrogen in the various kinds of molecules have also been evaluated. A particularly interesting result has been obtained for the four‐ and three‐membered cyclic amines.

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.

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.

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.

Zeeman Effect Study of the Nuclear Quadrupole Resonance of 81Br in 4,4′‐Dibromodiphenylether and 4,4′‐Dibromodiphenylthioether

The Zeeman effect of the 81Br NQR line was studied on single crystals of 4,4′‐dibromodiphenylether and 4,4′‐dibromodiphenylthioether. The relative orientations of the electric‐field‐gradient tensors acting upon the Br nuclei in the same molecule have been determined. The angle between the two Z‐axis directions is 129°0′ ± 5′ in the former compound and 107°15′ ± 4′ in the latter. The angle between the normal to the plane determined by the two Z axes in a molecule and the X axis of each gradient is 33°30′ ± 1° and 37°25′ ± 30′, respectively. These values are in general agreement with known data from x‐ray diffraction although the differences are significant. An accurate comparison is not, however, possible since carbon‐atom coordinates are not reported in the literature.

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.

A very efficient and stable supramolecular organic blend having a very high value of the optical gain for photorefractivity applications

A derivative of 2-methylindole, 3-[2-(4-nitrophenyl)ethenyl]-1-allyl-2-methylindole, NPEMI-A, has been studied for its photoconductivity and photorefractivity behaviour. The neat material was studied together with its blends with the organic polymer poly-(2,3-dimethyl-N-vinylindole), PVDMI. It was possible to carry out measurements with the content of the chromophore NPEMI-A changing from zero to 100 wt. %. No opacity hint was observed. The photorefractive optical gain Γ2 was obtained as a function of the chromophore content inside the different films. Differential Scanning Calorimetry measurements (DSC) were also carried out to obtain the whole change of the glass transition temperature Tg as a function of the amount of chromophore contained in the blends. From the experimental trend of Tg it was possible to gather a meaningful quantitative estimate of the value of the electrostatic interactions inside the blends. The importance of the value of Tg, and hence of the electrostatic interactions, in determining the extent of the photorefractivity has been clearly put in evidence. This was done by comparing the results obtained for NPEMI-A (Γ2 = 210.1 cm−1) with other ones previously discussed for a molecule similar to NPEMI-A but having a 2-ethylhexyl group, NPEMI-E (Γ2 = 2,027 cm−1), instead of an allyl group. Three different contributions to the photorefractivity of NPEMI-A, namely the Pockels, the Kerr and the Collaborative ones, could be distinguished from the peculiar trend of Γ2 as a function of the composition of the blends.