Annealing temperature effects on the azodye photoisomerization processes of layer-by-layer self-assembled poly(p-phenylenevinylene)/Congo red thin films

In the present paper the layer-by-layer self-assembly (LbL) technique was used to obtain poly(p-phenylenevinylene) (PPV) films exchanging the counter-ion Cl of the poly(xylylidene tetrahydrothiophenium) (PTHT ) precursor polymer by Congo Red (CR) azodye. The objective of the present paper is to study the azodye photoisomerization trans→cis→trans process as a function of the PPV thermal annealing procedure. The LbL PTHT/CR films were thermally converted into LbL PPV/CR films at low (110 °C) and conventional (230 °C) temperatures, and characterized through both absorption and emission techniques. We found that the unexpected emission band above 650 nm, attributed to the azodye, for LbL PPV/CR film thermally converted at lower temperature (110 °C) is an evidence of the considerable decrease of CR degradation in the polymeric matrix. Emission anisotropy and selective photoluminescence were carried out to characterize qualitatively the exciton energy transfer.