Charge-transfer phenomena in holographic recording material: dichromated poly(acrylic acid)

Dichromated poly(acrylic acid) (DCPAA) has been used as real time holograph ic recording material. We report here the photochemical behavior of DCPAA w hen irradiated under conditions representative of those used for hologram r ecording. Under UV-vis spectroscopy a photoredox process (Cr-VI --> Cr-III) was observed whose efficiency depends on the presence of dimethylformamide (DMF) used as an electron donor. ESR spectroscopy upon irradiation permits detection of the formation of Cr(V) of photochemical origin. We also ident ified chromium(V) of thermal origin (in the dark), organic macroradicals, a nd chromium(III). The formation of CO2 and carboxylate ions was clearly sho wn by IR spectroscopy. The photoreaction is assumed to go through an acido- basic reaction between dichromate in the excited state and poly(acrylic aci d). The resulting unstable chromium polyacrylate undergoes a redox process that gives rise to chromium(V) and a macroradical R-COOdegrees from which C O2 is formed. The presence of DMF (10% by volume) makes the overall reactio n faster by a factor of approximately 3.5-4. The similar shape of the curve s giving chromium(V) formation as a function of irradiation time and the cu rves giving the diffraction efficiencies of the hologram recorded in DCPAA as a function of exposure energy; with and without DIM, strongly argues in favor of a direct involvement of chromium(V) in the quality of the resultin g hologram.