Pierre C. Roberge

Methylene blue sensitized gelatin as a photosensitive medium for conventional and polarizing holography

The properties of thin gelatin films sensitized with a dye (methylene blue) when used as a holographic material are discussed. On illumination the excited dye molecule changes its structure giving a colorless molecule. The higher rate of bleaching and therefore the higher diffraction efficiency of the developed phase gratings are obtained in an oxygen-free atmosphere. Furthermore, the light absorption probability of the dye molecules depends on its position with respect to the electric vector of the light, with the result that the exposed part of the dye becomes dichroic when illuminated with linearly polarized light. This result allows the use of this material for polarizing holography.

Red sensitivity of dichromated gelatin films

Dichromated gelatin films present sensitivity to red light without any special sensitization. This sensitivity is due to an absorption appearing in dichromated gelatin films and attributed mainly to a charge–transfer transition which results in an electron transfer from gelatin to chromium. To characterize holographically this recording material, both reflection and transmission gratings have been obtained using the 632.8-nm radiation from a He–Ne laser.

Photophysics of aryl substituted 1,3,4-oxadiazoles. I. SCF CI calculations in different molecular configurations and assignment of the T1→Tn spectrum of 2,5-di-(1-naphthyl)-1,3,4-oxadiazole

Abstract An SCF MO calculation followed by configuration interaction has been performed for 2,5-di-(1-naphthyl)-1,3,4-oxadiazole as a function of the dihedral angles. The use of the configuration analysis method shows an important contribution of a resonance exciton state of the binaphthyl type to the lowest triplet state. As the molecule becomes more nearly planar, there is an increase in the charge-transfer character of the lowest triplet, which is directly responsible for the two absorption bands in the long wavelength part of the T-T spectrum. The supermolecule αNND is well depicted in terms of a binaphthyl fragment interacting with an oxadiazole ring.

Photophysics of aryl substituted 1,3,4-oxadiazoles: The triplet state of 2.5-diphenyl-1,3,4-oxadiazole

Abstract A theoretical approach using a SCF Cl method has been used to analyse T 1 T n absorption spectrum of 2,5-diphenyl-1,3,4-oxadiazole. In its fist triplet state in a non-polar solvent, this molecule is strongly non-planar. This conclusion is corroborated by comparison of the fluorescence and phosphorescence spectra. A change of geometry, towards a more planar conformation, appears to occur in an alcoholic solution. it is suggested that n,π autocomplexes are formed in the triplet state. Conflicting results published for the TT absorption must be attributed to an impurity.

Photophysics of aryl substituted 1,3,4-oxadiazoles. II. SCF Cl calculations in different molecular configurations and assignment of the T1 → Tn spectra of αNPD and βNPD

Abstract An SCF MO calculation followed by configuration interaction has been performed for 2-(1-naphthyl)-5-phenyl-1,3,4-oxadiazole (gaNPD) and for 2-(2-naphthyl)-5-phenyl-1,3,4-oxadiazole (βNPD) as a function of the dihedral angles between the different fragments of the supermolecules. An analysis of the experimental T-T spectra in terms of the theoretical results indicates that two types of transition are observed. A transition originating in the naphthalene fragment accounts for the absorption in the blue part of the spectrum while a charge-transfer transition is mainly responsible for the absorption towards the red. It appears that, in the first triplet state, the naphthyl and oxadiazole rings are coplanar in αNPD while they are not in βNPD. A more suitable geometry for hydrogen bonding in the α-naphthyl oxadiazoles could account for such a behavior.

Radiolysis of vinyl iodide. Part 3.—Formation of polymer in carbon tetrachloride solutions

The polymerization by 60Co gamma-rays of vinyl iodide in carbon tetrachloride solutions was studied. The infra-red spectrum of the polymer shows the existence of a CCl3 group per macromolecule independently of the monomer concentration in solutions from which the polymer is formed. Moreover, the polymerization shows a relatively constant M.W., around 7500, a negative temperature coefficient and an intensity exponent of ∼1.0. It is assumed that initiation of polymerization is due to hot CCl3 radicals or CCl+3 ions, while termination is by deactivating transfer on solvent and on monomer.

Radiolysis of vinyl iodide. Part 2.—Solutions of vinyl iodide in carbon tetrachloride

Radiolysis of solutions of vinyl iodide in carbon tetrachloride enables a mechanism of formation of products to be specified. Ethylene is formed within spurs in reactions involving vinyl radicals and hydrogen iodide, while acetylene is formed in three different processes involving molecular decomposition of vinyl iodide, and hydrogen-abstraction reactions by either iodide or chlorine atoms from vinyl radicals. Hydrogen iodide is formed in hydrogen-abstraction reaction by atomic iodine from vinyl radical. Hydrogen is formed in the molecular decomposition and by hot hydrogen atom abstraction. Hydrogen chloride is formed mainly by hot chlorine atom abstraction of hydrogen from vinyl iodide. Competition between the dissociative electron attachment processes of CCl4 and C2H3I strongly affects the hydrogen chloride formation. Vinyl chloride is formed within spurs in reactions involving vinyl radicals and HCl or Cl atoms. There is no evidence of electronic excitation transfer in the present system, and charge transfer seems improbable.

Radiolysis of liquid vinyl iodide by 60Co γ-rays

The 60Co γ-irradiation of pure liquid vinyl iodide yields the following products, the corresponding G values being given in brackets: hydrogen (0.077), acetylene (3.89), ethylene (1.46), 1,3-butadiene (0.15), hydrogen iodide (0.63), iodine (2.48), and also two minor non-identified compounds. There is no formation of polymers even after prolonged irradiation.