| dc.description.abstracteng | An investigation into the self-assembly and the host-guest chemistry of Pd<sup>II</sup>-based metal-organic architectures was presented in this dissertation.
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In Chapter 2, the formation of an inclusion complex of the hexamolybdate anion [Mo<sub>6</sub>O<sub>19</sub>]<sup>2−</sup> inside a cationic Pd<sub>2</sub>L<sub>4</sub> coordination cage in solution was described. Interestingly, a structure conversion was observed by recrystallization after an excess of hexamolybdate guest was added to the 1:1 inclusion complex. The X-ray structure of {[Mo<sub>6</sub>O<sub>19</sub>]@L3+2H} reveals that one [Mo<sub>6</sub>O<sub>19</sub>]<sup>2−</sup> anion is wrapped in a chiral, cyclic arrangement by three ligands, in the absence of any Pd<sup>II</sup> cations. Additionally, two of the six pyridines of the ligands are protonated. The strategies for the formation of the inclusion complex and the compound after structure conversion are explained and comprehensively characterized both in solution and in the solid state.
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In the following Chapter 3, a conversion of a cubic Pd<sub>6</sub>L<sup>1</sup><sub>12</sub> into a square-cuboid Pd<sub>6</sub>L<sup>2</sup><sub>8</sub> box was described. Through a mathematical derivation based on the arm lengths and angles of tripodal ligands, we found out under which geometric condition a structure with reduced symmetry such as a square-cuboid box could be formed. After computational assisted design, a 90°-angled bis-pyridyl ligand L<sup>1</sup> and a tripodal tris-pyridyl ligand L<sup>2</sup> were synthesized. The complexation reaction of the precisely designed ligands with Pd<sup>II</sup> cations, led to clean quantitatively formation of a cubic Pd<sub>6</sub>L<sup>1</sup><sub>12</sub> and a square-cuboid Pd<sub>6</sub>L<sup>2</sup><sub>8</sub>, respectively. Especially attractive in the structure of the square-cuboid box is, that all six faces, top and bottom two squares and four side rectangles, are constructed cleanly from simple Pd(pyridine)<sub>4</sub> complexes.
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Moreover, in Chapter 4, a light-triggered photochromic coordination cage Pd<sub>2</sub>L<sub>4</sub> based on photoswitchable dithienylethene (DTE) units and square-planar-coordinated Pd<sup>II</sup> ions is introduced. All four ligands exhibit reversible interconversion between a flexible “open-ring” form and a rigid “closed-ring” form under alternating irradiation wavelengths. This light-driven interconversion of the cages provides full dynamic and reversible control over the uptake and release of suitable guests, such as the spherical anion dodecafluorododecaborate [B<sub>12</sub>F<sub>12</sub>]<sup>2−</sup>. The high photo responsiveness of the cage itself, and the alterable binding affinity will provide wide applications in fields such as controlled drug release and supramolecular catalysis.
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Finally in Chapter 5, a similar light-switchable bidentate pyridyl ligand based on the same DTE units as described in Chapter 4 was used to trigger a clean structural interconversion between coordination self-assembled structures with tremendous change in shape, size and nuclearity. The open-formed ligand could form three- and four-membered rings with stoichiometric amount of Pd<sup>II</sup>. Whereas the closed-formed rigid ligand, was observed to form large Pd<sub>24</sub>L<sub>48</sub> rhombicuboctahedral spheres with about 7 nm diameter and a molecular weight of 31802 Da including all counter anions. By altering the wavelength of the light, the interconversion can be fully reversed. Due to the significant differences between the photoswitched conformations, the kinetics of the interconversion between the triangular ring and rhombicuboctahedral sphere exhibit substantial dissimilarities. These results provide a platform to investigate the stimuli-responsive structural reorganization processes, to design artificial multicomponent complexes closer to the massive scale of biological self-assemblies, to understand the interactions between macromolecules, such as protein-protein interactions.
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Overall, special emphasis has been paid in this work on the construction of lower symmetric self-assemblies by rational ligand design, the light-responsive structure conversion process, and the host-guest chemistry as well as the synthesis and characterization of new self-assemblies. Although the complexity of these systems does not approach that of many complicated processes in nature, the results described in this work showed that the future of self-assembled systems is still attractive. These simpler systems have played very important roles in developing and expanding the supramolecular chemistry field. By starting from such simple systems step by step, where the introduction of different functionalities could be more selectively, the interactions between molecules can be studied more precisely and individually, useful applications on the basis of more complex structures will be widely exploited.
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