Photoinduced orientation of azo dyes in polymeric films. Characterization of molecular angular mobility

Dichroism of Poly(Methyl Methacrylate) Thin Films Doped with Disperse Orange 11 Molecules

“…If the angular distribution of the keto form is biased in a particular direction, then the material can be considered as anisotropic. This means that the absorption characteristics of this material are also angularly dependent [16]. However, the molecular anisotropy of the dye molecules plays an important role as well, in which the anisotropy of the keto forms can lead to a bias in their angular distribution.…”

“…On the other hand, the absorption (A ⊥ ) perpendicular to the polarisation direction of the pump light increases as a consequence of more molecules being aligned in this direction. Although the number of molecules parallel to the pump polarisation direction decreases while the number of molecules perpendicular increases, the total number of keto molecules is constant, one obtains [16] ∆A = −2∆A ⊥ . In other words, the depletion of the keto molecules away from the polarisation direction of the pump beam does not go to completion because of the thermal energy of the keto molecules that causes random motion leading to oppose the alignment process [15].…”

“…In other words, the depletion of the keto molecules away from the polarisation direction of the pump beam does not go to completion because of the thermal energy of the keto molecules that causes random motion leading to oppose the alignment process [15]. Switching the pump beam off leads to have the anisotropy relaxing due to angular diffusion of the molecules [16].…”

Photoinduced Orientation of Azobenzene Chromophores in Amorphous Polymers As Studied by Real-Time Visible and FTIR Spectroscopies

“…When the laser is turned off, the photoinduced orientation is relatively stable in polymers containing azobenzene side chains (80% of the orientation is preserved), 2,3 while it is less stable in azobenzene-doped polymer matrixes. 4 In this last case, Sekkat et al [5][6][7] have proposed the basis of a simple three-level theoretical model in order to explain the reorientation mechanisms of the "azo" molecules during the orientation (laser on) and the relaxation (laser off) processes. In this model, they have considered that the polarized light induces a selective optical pumping ("angular holeburning") followed by angular reorientations during the trans-cis isomerization and the cis-trans thermal back-relaxation.…”

“…This phenomenological approach explains the reorientation mechanisms of the azo molecules during photoisomerization cycles, and its predictions can be compared with experimental results of the time-dependent orientation and relaxation processes. [3][4][5][6] However, it is not straightforward to understand the time dependence of the photoinduced orientation in azobenzene amorphous polymers because several structural factors may be involved in the reorientation mechanisms. Among these factors, the nature of the neighbors of an azobenzene unit in a functionalized polymer may be quite important.…”