Peculiarities and mechanism of surface topography changes in photochromic cholesteric oligomer-based films

Detailed investigations of surface topography peculiarities for two cholesteric mixtures with photovariable helix pitch are presented. The mixtures were prepared by doping cholesteric cyclosiloxane with two chiral-photochromic substances possessing different handedness. Both chiral-photochromic dopants are susceptible to UV light-induced E-Z isomerization and changing of their helical twisting power. UV irradiation allows one to change the helix pitch values of the mixtures in a wide spectral range (~350–740 nm). AFM investigation reveals the specific peculiarities of the surface topography of the mixtures’ films (formation of “fingerprint like” topography or circular, spiralized domains depending on helix pitch). Quantitative analysis of the geometry of the domains (size and cross sections) allows one to find the correlations between photo- and thermally induced helix pitch values and surface features of the films. The handedness of the dopants does not affect surface relief topography. A mechanism has been suggested to explain the topographical changes under helix untwisting, which occurs upon slow cooling of mixtures films.