Photochemical Behavior and Formation of Surface Relief Grating on Self-Assembled Polyion/Dye Composite Film

Abstract

Holographic surface relief gratings (SRGs) were fabricated on composite films assembled by electrostatic layer-by-layer (ELBL) deposition of a polyelectrolyte, poly(dimethyl diallylammonium chloride) (PDAC), and an azo dye, Congo Red (CR). Surface modulation and first-order diffraction efficiency of the SRG were found to increase with the thickness of the PDAC/CR films. Polarized absorption spectra indicated an oriented growth of CR on the PDAC film. Analysis of the film thickness, FTIR, and FT-Raman results confirmed that the electrostatic attraction between CR and PDAC, as well as the π−π interaction between CR chromophores resulting in the formation of J aggregates, lead to formation of PDAC/CR composite films. Photochemical changes of the PDAC/CR films after irradiation were investigated by UV−vis absorption, FTIR, and FT-Raman spectroscopy. The results indicate that in addition to trans ⇔ cis photoisomerization of CR in the composite film, an irreversible photochemical degradation of CR also simultaneously occurs. Recording SRG on PDAC/CR films by s- and p-polarized beams show different behavior compared to spin-coated films of polymers containing functionalized azo chromophores. Our results indicate that the volume collapse due to the photodegradation of CR in the polymeric matrix, as well as gradient force-induced migration due to trans ⇔ cis isomerization cycling of CR contribute to the formation of SRG on the composite films. This approach provides a methodology to fabricate SRGs for optical information storage applications by using the facile ELBL technique to assemble commercially available azo dyes and polyelectrolytes.