Strukturoptimierung in photoresponsiven Polymeren: Vom Moleküldesign zu funktionellen Materialien

The aim of the thesis is the improvement of photo responsive polymers for the development of functional materials. The molecular framework for this is the structural motif of coumarin dimers. Many applications are known through the underlying photochemical [2+2] cycloaddition or reversion of the class of molecules, where the properties of matter can be altered with light. Most of these applications require a high degree of reversibility in the process. However, this high reversibility is usually not achieved, so the materials can only be used in a limited extent according to their intended purpose. The findings of this work contribute to improve the reversibility of the photoreactions in responsive polymers. For this aim, firstly influences of the exact molecular structure on the processes of the photoreactions of small molecules are determined with increasing dimensionality of the investigated matter. The focus is primarily on the cycloreversion reaction, since the determining factors for this reaction are less well known than for the cycloaddition. By variation in different positions of the molecular backbone, structures were selected, whose cycloreversion reaction can be triggered by one- and two-photon absorption processes. Suitable structural motifs are identified for application in reversibly photo switchable polymers, which are characterized by efficient photoreactions with simultaneously high chemical stability. High chemical stability is a necessary condition, as it is the only way to ensure the stability of the structure even after integration into photo-responsive polymers. Head-to-tail configurations of coumarin functionalized in the 7-position emerge as the preferred structural motif, since this variant represents the best compromise between the described requirements. After identification of the central photoactive element, the structural motif is chemically functionalized for integration into macromolecules. The novel approach of the intramolecular coumarin dimer (ICD) structure is used, in which two single chromophores are covalently linked in the same molecule. Due to the intramolecular variant and the isomer pure dimer structure, both the cycloaddition for formation and the cycloreversion run very efficiently. On the one hand, this can be used for multi-gram scale synthesis; on the other hand, it also ensures a high degree of reversibility: By using the ICD structure and the best possible conservation through low mobility, the cleavage and recyclization of the structure within polymeric materials can be carried out over many irradiation cycles of alternating wavelengths without significant loss in photoactivity. The intramolecular dimer structure enables new potential applications, since properties can be switched independently by using the structure. In all variants known in the literature, photo switching to change certain properties (e.g., optical) always results in a change in the linkage of the polymer network, so that properties cannot be switched independently of one another. By using the ICD structure, the properties are decoupled, since the cyclo-reversion does not lead to reconfiguration of the polymer network. As an example of application, a change of the refractive index in the visible wavelength range and the spatially resolved switching of fluorescence are shown as typical optical properties. In addition to the high reversibility of the ICD structure within polymeric materials, it is also possible to cleave the structure via the non-linear process of two-photon absorption. This extends the application potential extensively, as the structure can thus also be used in three-dimensional materials. As an example of application, the experiments present 3D printing of a strongly UV-absorbing polymer matrix in which the ICD structure is covalently integrated. Via the use of two-photon absorption with visible light within the printable polymer material, cleavage of the ICD structure can be selectively triggered without causing degradation reactions of the matrix. Together with the presented changes of the optical properties, a large application potential is given, which can be further increased e.g., with advanced printing and illumination techniques to improve the achievable (spatial) resolution. In summary, the use of the ICD structure and associated photoreactions via one- and two-photon absorption processes is very well suited for use in photoactive materials with high reversibility.