Chromophores in LBK films

The structure with the deformed hairy rods is preserved when multilayers are deposited on solid supports, as evidenced by X ray reflectometry revealing a marked bilayered structure (see Figure 6.16). Photoisomeriza-tion of the a egated azobenzene chromophores in LBK films is possible and leads to significant structural changes. The bilayered structure is completely... 

LBK films are highly ordered and densely packed supramolecular assemblies. The packing of the chromophores in LBK films influences their photo-reaction. Therefore, LBK films have to be tailored by the choice of the amphiphilic molecules and the transfer conditions to provide a matrix for the chromophores that has sufficient flexibility and free volume for the photoreaction to occur. On the other hand, the LBK film s structure can be tailored in such a way that the photoreaction of the chromophore enhances the structure or the properties of the entire film. In this way, photoactive films can be designed to show a much more sophisticated photoresponse than that of a chromophore in an inert matrix. 

Besides azobenzene, spirobenzopyran, and salicylidene derivatives, other photochromic dyes have been used in LBK films. For example, anthocyanine dyes 5 (see Figure 6.5) have been used to obtain monolayers that change the area at constant pressure upon irradiation. Diphenyldiacetylerie chromophores 6 (Figure 6.5) have shown photoinduced anisotropy upon polar-... 

The peak shift due to aggregation is observed not only in LBK films containing azobenzene chromophores, but also for other chromophores with extended Ji-systems, such as viologen polymers. For monolayers of the poly(p-phenylene sulfonate) 9/ dioctadecyldimethylammonium bromide (DODA) complex, the peak shift due to aggregation results in a piezo-chromic effect—that is, upon compression of the monolayer, a significant shift of the poly(p-phenylene sulfate) A band is observed (see Figure 6.9). This photochromic effect has been shown to be based on the improved 7t-Jt interaction upon compression of the monolayer. ... 

Furthermore, the means of binding the alkyl tail to the azobenzene influences the rigidity of the monolayers and the trans to cis photoisomerization. If the alkyl chain is bound to the azobenzene unit via an ether linkage (as in 13), the alkyl chain can adopt a better-ordered arrangement in which the chromophores are aggregated to an higher extent. So, in LBK films of 13, the hindrance of the isomerization is more pronounced than in LBK films of azobenzene amphiphiles having the alkyl chain directly bound to the chromophore (as in 7). ... 

The corresponding fatty acid could not be photoisomerized in the LBK film. By attaching the azobenzene chromophore to the hydrophilic backbone, however, the free volume in LBK films was increased and photoisomerization was possible (i.e., 50 to 70% cts-isomer compared to 0% for the nontethered azobenzene amphiphile and 90% cis-isomer in solution). However, concomitant with the increased free volume, there is a decrease in the orientational order of the chromophores. These polymers have been widely used as command surfaces to control the orientation of liquid crystals and to investigate the photomechanical effect. ... 

The number of chromophores in the LBK film can be enhanced by-mixing the poly(glutamate) (37) with a low molecular wei t azobenzene dye. Both chromophores, the covalently bound and the admixed dye, can be isomerized in LBK films by irradiation with UV light. The trans to cis isomerization reduces significantly the interaction between the bound and the admixed chromophores, and phase separation occurs. Upon repeated isomerization cycles, the low molecular weight dye is expelled from the LBK film and forms crystals on top of the film. This can be easily detected by polarized light microscopy. ... 

As mentioned previously, the interactions of the chromophores with extended tt-system in densely packed solids like LBK films significantly influence the optical properties. In particular the aggregation causes a significant peak shift, the direction and extent of which depends on the number of aggregated chromophores and their distance from each other in the ag egate. Furthermore, a particular influence on the extent and direction of the peak shift is found for the orientation of the chromophores to each other i.e., the angle of inclination 6 as defined in Figure 6.6. This phenomenon can be described by Me Rae and Kasha s molecular exciton model or the more elaborated extended dipole model by Kuhn et... 

LBK films of the azobenzene containing fatty acid 7 show a similar behavior. When compressed and transferred in the trans-fotm, a peak shift due to aggregation is observed and the photoisomerization is hindered. The chromophores are very densely packed, as established by STM measurements. When compressed and deposited in the ds-form, i.e., under illumination with UV light, there is no aggregation, and the cis to trans isomerization is unrestricted. Alternate irradiation under constant surface area causes changes in the surface pressure. 

These examples and investigations on azobenzene moieties in polymers show that the photochromic behavior is mainly Gontrolled by the free volume distribution around the chromophore. Tb obtain LBK films in which azobenzene moieties can undergo photoisomerization, therefore, the free volume around the azobenzene chromophore must be controlled precisely to allow for the molecular rearrangement inherent in the reversible tran to cis photo-isomerization. This is possible by (1) mixing with other amphiphiles, (2) adjusting the architecture of the amphiphile, or (3) attaching the chromophore to a polymer either by coulomb interaction or covalently. 

In polyurethane 33, the azobenzene moieties are separated from each other along the polymer backbone by isophorone units and have the free volume necessary for isomerization and molecular reorientation. This polymer s azobenzene moieties can be photoisomerized readily in mixed LBK films, as demonstrated by measuring the optically induced birefringence that originates from the photoreorientation of the chromophore upon polarized irradiation see Section 6.5.2). 

In conclusion regarding the results obtained with polymer-bound azobenzene chromophores, establishing liquid crystalline phases that combine order and flexibility in the side chain region of the polymer seems to be the best way to obtain materials with a strong photoresponse. This is because a high density of chromophores can be combined with sufficient flexibility that the chromophores still can be photoisomerized in the dense packing of an LBK film. When a low density of chromophores is acceptable, however, the chromophores can be diluted along the polymer chain to reduce their interaction and secure sufficient free volume. 

Employing UV-vis spectroscopy in combination with electrochemical methods, Wang et al. have shown that for an LBK film of azobenzene amphiphile, indeed only those chromophores that are parallel to the electric field vector of the incident light are isomerized by polarized irradiation. Fmthermore, the authors have shown that the steric requirements of the ds-isomer favor a back reaction of the cis-isomer into a new orientation of the trans-isomer ... 

Illumination of LBK films of copolymers 34 (1 2.4) in the liquid crystalline state at 63 C, with polarized light at 457 nm, results in a photo-induced optical anisotropy. That is, the chromophores can be reoriented, which indicates that efficient photoisomerization is possible. Photoreorientation is also possible at room temperature after the original structure of the LBK film has been broken up by irradiation with UV light (that is, after a photostationary state with a high content of cfs-isomer has been established). ... 

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