Surface properties, thermal behavior, and molecular simulations of azo-polysiloxanes under light stimuli. Insight into the relaxation

Abstract

Light-sensitive materials as azo-polymers have interesting applications in microelectronics, biology, or energy storage, due to their capacity to change their supramolecular ordering. Light-induced modifications result in i) changes in the azobenzene-groups dipole moment due to the trans-cis isomerization, ii) thermal backward cistrans relaxation, and iii) re-ordering of the polymer chain conformation. Conjugation of these phenomena leads to mass transport in the solid state, which is still not well understood. Changes in the surface properties of non-substituted and p-CF 3 ,-CN,-NO 2 -substituted azophenoxy-polysiloxane films, before and after photo-excitation, were examined. Molecular simulations were performed to evaluate the dipole-moments of the trans and cis p-substituted azobenzene groups and provide a general vision of the spatial arrangement of polymer chains. The role of trans-cis back relaxation and that of the polymer matrix relaxation on the surface and bulk of the materials was discussed. Surface free energy determination of the non-substituted azophenoxy-polysiloxane suggested that the relaxation process is dominated by the thermal slow back-isomerization of the cis fraction and not by the motion of a glassy polymer matrix involving both the surface and bulk. A less clear situation was presented by the semi-crystalline polysiloxanes with electron-withdrawing groups substituted in the para-position. This was attributed mainly to the similar values of the dipole-moment of trans and cis azo-group, the former being slightly more dipolar than the latter.

Surface relief gratings induced by pulsed laser irradiation in low glass‐transition temperature azopolysiloxanes

“…As shown in Figure S4 in the Supporting Information, the ultraviolet–visible spectra corresponding to Azo–Psi and Azo–PCMS, reflected strong absorption maxima in the domain of 350 nm, which were attributed to the trans–cis isomerization (with the cis–trans absorption being present at 450 nm). As a result, in a microsecond scale time irradiation, a high excess of the cis isomer content was expected and was reported for the trans one (the cis–trans relaxation was less probable in the absence of visible light, with the process taking place in a scale time of hours). The trans–cis isomerization was not accomplished only by an important geometrical azo‐molecule modification but by a significant change in the dipole‐moment value, which increased from 1.6 to 4.5 D .…”

Rigid and flexible azopolymers modified with donor/acceptor groups. Synthesis and photochromic behavior

“…During the azobenzene isomerization, the wettability of the surface was also modified, as a result of geometry and dipole moment changes. Previous molecular modeling studies [32] evidenced a dipolemoment value for the trans-azophenol of 1.6 D that increases to 4.5 D in the cis configuration. These molecular modeling studies showed the displacement of the azo-groups at the film surface.…”

Study on Recognition of Metallic Ions by a Photo-Sensitivity Liquid Crystal of Ethylcyclohexyl Acyloxy Azobenzoic Acid

“…The liquid crystalline material has special physical, chemical and optical properties. Azo group is functional group of special optical activity [8][9][10]. If the liquid crystal compounds are combined with the optically active group of azo form.…”