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A reorientation phenomenon that takes place inside confined smectic-C (SmC) domains in a phase-segregated Langmuir film is reported. The process involves a sign reversal of the splay texture following the transient displacement of the central defect.

An experimental phenomenon of collective light-induced reoriental processes at the air or water interface is described. The way in which a cycle of weak on/off linearly polarized illumination can totally reverse the radially symmetric pattern of orientations of stretched amphiphilic molecules assembled in condensed smectic-C-like droplets in a Langmuir monolayer is demonstrated.

The article focuses on a study related to photoswitchable orientational patterns of confined domains in monolayers. Soft materials prepared from supramolecular or self-assembling strategies provide researchers with a diversity of potential technological benefits. Leaving aside obvious implications for the design of sensors and devices, model cooperative molecular systems may provide the means to unveil how noncovalent interactions in complex chemical systems can lead to the emergence of adaptive and collective responses to external stimuli.

We study the self-assembly of novel azobenzene-based chiral surfactants at the air/water interface, and find that while the pure enantiomers lack the ability to organize in ordered mesophases, the racemic mixture spontaneously forms a hexatic phase at low lateral pressures, which we detect by means of Brewster angle microscopy. This work provides a unique example of heterochiral recognition in which the racemic monolayer is not only condensed with respect to the pure enantiomers, but causes an ordered mesophase to form. Although hexatic order vanishes at high surface pressures, long-range orientational order is regained for all compositions upon monolayer collapse, which proceeds through the formation of birefringent trilayers with a well-defined lateral microstructure, as revealed by atomic force microscopy.

Preparation of Langmuir monolayers of a mixture of trans- and cis-isomers of an azobenzene derivative, 4-[4-[(4-octylphenyl)azo]phenoxy]butanoic acid, results in the segregation of birefringent trans-isomer domains embedded in an isotropic medium of cis-isomers. Brewster angle microscopy observations allow us to identify different textures inside the domains depending on surface pressure, temperature, and domain size. The evolution of the monolayer in the dark, from initial droplets formed after spreading to a stable stripe texture, is described. The dynamics of domain coalescence and some morphological transitions induced by temperature and surface pressure changes are also discussed. A simple theoretical model is included to supplement some of these experimental observations.

The monolayer behavior of the azobenzene derivative 8Az3COOH is shown to depend significatively on temperature and on the isomeric (trans-cis) composition. For pure trans monolayers, important temperature effects in the mesoscopic organization (as revealed by means of Brewster angle microscopy, BAM) are observed for the low-pressure phase in the studied temperature range (10 degrees C < T < 40 degrees C). Mixed trans-cis monolayers show that both isomers are virtually immiscible, leading to a phase segregation into birefringent, nearly pure trans droplets surrounded by an isotropic, nearly pure cis matrix. The existence of well-defined anchoring conditions at the droplet boundaries leads to highly symmetric textures, amenable of quantitative BAM image analysis, which helps to better visualize mesoscopic changes induced by variations in the control parameters (temperature, surface pressure or irradiation).