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Azobenzene-functionalized molecular glasses for holographic applications
(2010)
- Holography is an optical imaging technique, with which an authentic copy of the original object can be created, even in the absence of the object itself. This means, that in contrast to conventional photography, the information of depth is not lost. Holography is based on writing an interference grating in a photosensitive volume element. Hereby two light sources are generating an interference pattern, which causes chemical or physical changes in the photosensitive material. By illumination of the stored diffraction grating, the original information can be reconstructed. One of the most important classes of photoaddressable chromophores utilized in holography are azobenzene compounds. Owing to the rich photochemistry of these chromophores, materials incorporating azobenzenes can be used as photoswitches, allowing fast and reversible control over the chemical, physical or optical properties of the entire system. Therefore, azobenzene-containing compounds are envisioned as smart light-responsive materials for various holographic applications. This thesis describes the synthesis and characterization of azobenzene-containing molecular glasses as well as their application as functional materials in specific holographic experiments. By utilizing a modular design principle, we were able to fine-tune their physical and photo-physical properties and optimize the molecular structure in view of the formation of surface relief nanostructures as well as inscription of holographic volume gratings. Understanding the formation of surface relief nanostructures and discovering ways of controlling the process is of importance, as uniform surface relief gratings (SRGs) with adjustable spacing and amplitude are of interest. Therefore a new series of azobenzene-containing molecular glasses based on a triphenylamine core has been synthesized and photo-physically characterized. A clear relationship between the chemical structure of these molecules and SRG build-up was established: the rate of formation and the maximal achievable amplitude of SRGs strongly depend on the optical susceptibility at the wavelength of the writing laser. Furthermore, we found that different polarizations of the laser beams also have a major influence. With this knowledge we were able to efficiently form SRGs with amplitude heights of up to 600 nm by tailoring the molecular structure of the material and selecting specific experimental conditions. Furthermore, it has been demonstrated that these surface patterns are stable enough to be transferred to a polymer surface with replica molding techniques. This concept has the potential to be practically applied for holographic optical elements. Holography is a most promising solution for optical data storage, as in contrast to conventional optical storage media, the entire volume of the medium is used instead of only a few thin layers. Unfortunately, current rewritable materials still exhibit certain challenges, most important, sufficiently fast writing times. Therefore, material concepts especially for improving the recording time as well as the long-term stability of holographic volume gratings are presented. By employing azobenzene-containing molecular glasses in blends with photoaddressable polymers, we were able to merge the excellent long-term stability of the polymer systems with the higher photo-physical sensitivity of the molecular glasses, thus creating a superior holographic material which combines the advantages of both material classes. In order to find a suitable blending material, we synthesized series of photochromic azobenzene-containing molecular glasses and screened them with respect to their photo-physical properties. The best combination of structural variations was chosen for the blending experiments. Already a blend comprising as less as ten wt% of molecular glass allowed us to decrease the holographic writing time of a photoaddressable block copolymer system by a factor of three while increasing the recording sensitivity by the factor five. In addition to molecular glasses with ordinary azobenzene chromophores we also examined low molecular weight materials functionalized with bisazobenzene moieties. This enabled us to achieve higher maximum refractive index modulations. Liquid-crystalline behaviour could be introduced with the incorporation of substituents at the bisazobenzene moiety. Subsequent investigations of the photo-physical properties revealed a long-term stable photo-orientation solely based on small molecular compounds, making such materials an interesting alternative to established systems. In summary, this thesis demonstrates that azobenzene-containing molecular glasses are a worthwile focus for research, as they are an amazingly versatile and adaptable class of materials suitable for a large number of different applications.
