An isomerization-induced cage-breaking process in a molecular glass former below Tg

Abstract

A recent experimental [P. Karageorgiev, D. Neher, B. Schulz, B. Stiller, U. Pietsch, M. Giersig, L. Brehmer, Nature Mater. 4, 699 (2005)] study has found liquidlike diffusion below the glass-transition temperature in azobenzene-containing materials under irradiation. This result suggests that the isomerization-induced massive mass transport that leads to surface relief gratings formation in these materials, is induced by this huge increase of the matrix diffusion coefficient around the probe. In order to investigate the microscopic origin of the increase of the diffusion, we use molecular dynamics simulations of the photoisomerization of probe dispersed red 1 molecules dispersed inside a glassy molecular matrix. Results show that the increased diffusion is due to an isomerization-induced cage-breaking process. A process that explains the induced cooperative motions recently observed in these photoactive materials.

Athermal photofluidization of glasses

“…Absorption of a photon at 514 nm deposits an energy of hn ¼ 2.4 eV into a dMR azo core, enough to excite a single harmonic degree of freedomto T ¼ hn=k B ¼ 29,000 K. Some fraction of this energy appears in a form that locally tests the molecular orientational barrier distribution at the effective temperature of T CP B800 K. Fast spectroscopy [45][46][47] , quantum/molecular dynamic simulation 18,48 and molecular dynamic simulation 17 provide a semi-quantitative picture of this process, indicating that it is principally mechanical, with the photon energy appearing as a coherent force acting transiently on the environment of the absorbing molecule 49,50 , as follows. Upon photon absorption and electronic excitation, the azo core returns to the ground electronic manifold in a transition state configuration from which coherent change of the intra-molecular configuration takes place along one of several possible paths,for example, trans to cis or trans to trans, all of which reduce the internal potential energy by B2 eV (45 kcal per mole), representing almost all of the absorbed photon energy 18,48 .…”

“…Effectively complete bypass of the thermal process occurs at light intensity sufficiently low to produce no observable effects of average heating. The success of azobenzenes in converting photon energy to effective local fluidization can be attributed to their distinctive photo-induced change in molecular shape 16,17 , combined with, importantly, relatively slow (B10 ps) dynamics 18 of the resulting energy transfer to their local environment.…”

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

“…We use that molecule as a model system in this work, chosen because of its well established characteristics from a number of previous works. A detailed description of the simulation procedure can be found in previous works [17,38,47,58,72]. We model the medium molecule with the 4 centers of force coarse-grain potential function described in ref.…”

“…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.…”

Stimuli Thresholds for Isomerization-Induced Molecular Motions in Azobenzene-Containing Materials

“…The subject has attracted a large number of investigations due to the various possible applications ranging from biological applications [2][3][4] to the information storage 5-7 and nanotechnology [8][9][10][11][12][13][14][15] . This unexplained isomerization induced transport is also of fundamental interest due to a possible connection 16,17 with the glass-transition long standing problem [18][19][20][21] . If that transport property is without any doubt due to the photoisomerization of the dyes, the physical mechanisms that lead to that transport are still the object of conjectures.…”