Christine Pizzocaro

Photochemical behavior in dichromated gelatin

The photochemical behavior of dichromated gelatin and of different aminoacids-chromium (VI) systems was investigated. The photoredox process that takes place between chromium (VI) (complexed or not) and the organic moiety leads to the formation of chromium (V) which is then reduced to chromium (III) likely complexed to the medium. In chromium (VI)- aminoacids systems, the quantum yield (Phi) , that measures the efficiency of a photon at a given wavelength, strongly depends on the reductive properties of the aminoacid. (Phi) is proportional to aminoacid concentration. In irradiated DCG films, the formation of chromium (V) is observed and this intermediate species appears to be surprisingly stable in the polymeric matrix.

Photochemical formation of chromium (V) in dichromated materials: a quantitative and comparative approach

We have been working on dichromated photosensitive materials for several years. After investigating the photochemical process in various systems, it appears that the chemical structure of the polymeric matrix plays an essential role in the progress of the reaction that takes place upon irradiation by the laser beams. If the primary photochemical process is always the electron transfer from the matrix to the metallic cation giving rise to chromium (V) and a macroradical, the fate of chromium (V) in the system strongly depends on the chemical structure of the polymer. In dichromated poly(acrylic acid) DCPAA, chromium (V) is an unstable species only detectably by ESR spectroscopy upon irradiation: it disappears after stopping the irradiation very fast. On the contrary, chromium (V) is surprisingly stable in poly(vinyl alcohol) and in gelatin. The first spectral evolution corresponds to the only reduction of chromium (VI) into chromium (V) as evidenced by the presence of an isosbestic point. The subsequent reduction of chromium (V) is a very slow process: the complete transformation into chromium (III) is only achieved after several days. As a result, we were able to estimate the UV-visible spectrum of chromium (V) in the matrix and for the first time, it was possible to quantitatively follow the formation of this species and to draw some conclusion about the complexation sites in gelatin.

Photochemical process in dichromated photosensitive material: dichromated (polyvinyl alcohol-polyacrylic acid)-- a model for dichromated gelatin?

Gelatin (G) can be described as a peptidic chain with different pendent groups among which, hydroxy groups-OH, carboxylic groups-COOH and amino groups -NH2. Works performed on polyacrylic acid (PAA), a poymeric chain with only pendent-COOH group and on polyvinyl alcohol (PVA) with only -OH group revelaed the strong influence of the nature of the chemical structure on the photochemical behavior of dichromated photopolymers, DCG, DCPAA and DCPVA. Actually, the stability and the state of complexation of the different chromium species was completely opposite in the two matrices: the stabilization of chromium (V) resulting from the photochemical charge transfer in DCPVA, by complexation with PVA is in contrast to what was observed in DCPAA where chromium (V) was highly instable. Regarding chromium (III), the final reduction chromium species, it is complexed in PAA and not in PVA. The primary proces is identical in DCG and in DCPVA, so PVA appears as a good model of the first step of gelatin behavior. But the material obtained after irradiation and treatment of DCG only contains chromium strongly complexed by gelatin. The experiments performed on films of DC with only a few percents of PAA gave evidence for the strong influence of the presence of the carboxylic groups on the photochemical behavior: the rate of the formation and the stability of chromium species, mainly chromium (V), involved in the process. Chromium (V) appears to play a key role in the photosensitive properties of dichromated materials. The diffraction efficiencies of holograms recorded in DCPAA or DCPVA resemble the profiles of chromium (V) evolution.

Primary photochemical process in chromium doped polymeric material: interactivity with holography

A comparative investigation of the photochemical behavior of dichromated gelatin (DCG), dichromated polyacrylic acid (DCPAA) and dichromated polyvinylalcohol (DCPVA) gives evidence for the similarity of the photochemicalbehaviour involved when recording holograms in DCG and DCPVA. Chromium (V), the chromium species resulting from the photoredox process between the starting chromium (VI) (dichromate) and the polymeric chain, is stable, likely strongly complexed in gelatin and polyvinylalcohol. The quantum yields of the reaction are high and similar for both reactions. On the contrary, even though chromium (V) is also formed during the primary process in DCPAA, this species is quite unstable and disappears as soon as the irradiation is stopped. The behavior observed in DCG can be assigned to the presence of OH groups on the polymeric chain, these groups being the only groups present in polyvinylalcohol.

Charge transfer phenomenon in holographic recording material: dichromated polyacrylic acid

Dichromated polyacrylic acid (DCPAA) has been used as real time holographic recording material. We report here the photochemical behavior of DCPAA when irradiated under conditions similar to those used for hologram recording. A photoredox process was observed whose efficiency depends on the presence of an electron donor, dimethylformamide (DMF). By ESR spectroscopy upon irradiation four different species were detected and identified: (1) chromium (V) of thermal origin (in the dark); (2) chromium (V) of photochemical origin; (3) an organic macroradical; (4) chromium (III). DMF appeared to possess a strong influence on the formation of these species.

Dichromated (PVA-PAA) material: optimization of the holographic response

The holographic properties of mixed dichromated [polyvinylalcohol (PVA)/polyacrylic acid (PAA)] were investigated and compared to the pure photosensitive materials DCPVA and DCPAA. It appeared that the addition of 25% in weight of PVA in PAA strongly increase the diffraction efficiency (DE) of the resulting hologram. On the contrary, the addition of PAA in PVA only slightly increases DE values. In the last case, the addition of a complexing agent such as dimethylformamide DMF improves the hologram quality.

Metal-doped photosensitive materials: correlation between photoreactivity and holography

Dichromated polyacrylic acid (DCPAA) and dichromated polyvinylalcohol (DCPVA) have been investigated with a double approach: photochemical behavior and holographic results. The study focused on two particular systems: the effect of the addition of nitrilotriacetic acid (NTA) or dimethylformamide to the photosensitive material. From the photochemical mechanism it was possible to assess that the complexing ability of both compounds plays a major role in the optimization of the holographic properties of the material. On the contrary, the presence of an electron donor like NTA in DCPAA is less favorable. A close correlation seems to be present between the photochemical behavior and the results obtained in holography.