Submolecular Plasticization Induced by Photons in Azobenzene Materials

Abstract

We demonstrate experimentally for the first time that the illumination of azobenzene derivatives leads to changes in molecular environment similar to those observed on heating but that are highly heterogeneous at the submolecular scale. This localized photoplasticization, which can be associated with a free volume gradient, helps to understand the puzzling phenomenon of photoinduced macroscopic material flow and photoexpansion upon illumination far below the glass transition temperature (T(g)). The findings stem from the correlation of infrared (IR) spectral band shifts measured upon illumination with those measured at controlled temperatures for two amorphous DR1-functionalized azo derivatives, a polymer, pDR1A, and a molecular glass, gDR1. This new approach reveals that IR spectroscopy can be used as an efficient label-free molecular-scale thermometer that allows the assignment of an effective temperature (T(eff)) to each moiety in these compounds when irradiated. While no band shift is observed upon illumination for the vibrational modes assigned to backbone moieties of pDR1A and gDR1 and a small band shift is found for the spacer moiety, dramatic band shifts are recorded for the azo moiety, corresponding to an increase in T(eff) of up to nearly 200 °C and a molecular environment that is equivalent to thermal heating well above the bulk T(g) of the material. An irradiated azo-containing material thus combines characteristic properties of amorphous materials both below and above its bulk T(g). The direct measurement of T(eff) is a powerful probe of the local environment at the submolecular scale, paving the way toward better rationalization of photoexpansion and the athermal malleability of azo-containing materials upon illumination below their T(g).

Light-induced dynamic shaping and self-division of multipodal polyelectrolyte-surfactant microarchitectures via azobenzene photomechanics

“…The onset of fluidization is consistent with previous reports on UV-induced amorphization of azobenzene-based polymer films17, and softening in pure crystals of azobenzene molecules1041424344 and azobenzene-doped elastomers283145, which has been attributed to the higher free volume requirements of the cis bent conformation compared with the trans isomer2746. We therefore postulate that blue light-induced fluidization facilitates the co-alignment of kinetically-trapped, randomly arranged mesostructured domains in the trans -azoTAB:PAA particles such that the irregular particles of trans -azoTAB:PAA are reconfigured into hexagonal platelets with well-defined low energy faces.…”

Light-Triggered Formation of Surface Topographies in Azo Polymers

“…Recent research also shows an influence of temperature upon irradiation. This leads to photothermal effects described as photo-softening [42,[110][111][112]. A full understanding of the driving mechanism in the creation of surface structures in azo polymers is sought to improve or fabricate new surface topography changing coatings.…”

“…The realignment process is either powerful enough to move the polymer chains and/or fluidize the polymer, leading to the formation of an anisotropic fluid state [41]. It is known that during the light absorption of the azobenzene, localized heat is generated which can enable additional mobility [42]. This light-induced realignment process creates typical SRG topographies with the maxima located in the low intensity regions of the intensity interference pattern.…”

Folding time dependence of the motions of a molecular motor in an amorphous medium

“…Indeed, characteristics of the glass-transition physics like cooperative motions called dynamic heterogeneities were reported to be generated during the motion of various motors [38][39][40][41][42]. The photo-fluidization and softening of the host material, or transient liquid-like behaviors, were also reported by different groups experimentally and by simulations [38,[43][44][45][46] with particular motors. These behaviors and possible cage-breaking processes induced by the motor [47,48] suggest that molecular motors small stimuli can be used to probe the physics of the glasstransition.…”