Red-shifted optical absorption in films of azo-polyurea - polystyrene blends: Structural correlations and implications

Abstract

We report the optical absorption characteristics in thin film coatings of a blend of two polymers: azo-polyurea (azo-PU) and polystyrene (PS). It is shown using UV–visible spectroscopy that the absorption spectrum of the resultant blend undergoes a distinct and decisive redshift towards the visible wavelength regime, accompanied by broadening and overlapping of the characteristic absorption peaks. Light exposure studies reveal that the red-shifted optical absorption induces trans to cis isomerization of the blended polymer films upon exposure to visible wavelengths, which would otherwise occur typically only by stimulus from ultraviolet wavelengths. The extent of redshift in the absorption maxima and broadening in the absorption spectrum of the polymer blend films are found to be sensitive to the amounts of the constituent polymers. The changes in spectral configuration are understood in terms of lattice structural changes that occur in azo-PU chains due to the incorporation of PS, in particular the disarrangement in hydrogen bonding. It is suggested that the modified optical absorption characteristics would endow significant advantages, such as the possibility of replacing UV with visible light in applications requiring cis → trans and trans → cis isomerization, as well as photoswitchability due to trans-cis-trans cycling.