"Development and optimization of bibenzimidazole transition metal chromophores for luminescent ion sensing and photocatalysis"

The recognition and fixation of highly reactive systems by binding within defined spatial geometries is a fundamental principle of the enlivened nature. In the last four decades this biologically tenet was transferred to artificial systems and the yet emergent area of supramolecular chemistry developed to a driving force. The latter defined itself “over and above the molecule”, that is the chemistry of tailored intermolecular interactions. Thereby, not only the interplay of molecules, but also that of characteristic molecule entities – with its inherent features – is defined as “supramolecular”.[1,2] The stunning development of this research area is inseparably related to the increasing knowledge of preparative methods for the synthesis and characterization of complex host-guest structures.[3] Accessible and comprehensively investigated supramolecular host-compounds of today range from “classic” crown ethers[4] over calixarenes[5] and imines[6–11] to photosensible hosts.[1,12,13] The mentioned compound classes bear a receptor function for second-sphere interaction with substrates, and hence enable the selective coordination of anionic, cationic and neutral guests. These features provide the basis for the development of the research area of artificial molecular recognition. Especially, transition metal chromophores based on polypyridyl ruthenium(II), osmium(II) or iridium(III) complexes have become fundamental building blocks in numerous molecular optical sensors.[14–18] This is mainly due to the outstanding photophysical properties, i.e. broad light absorption and emission features across the visible regime of the light spectrum. Furthermore, the complexes are featured with long-lived triplet metal-to-ligand charge transfer (3MLCT) states.[14,19–27] The focal point of this work was the synthesis and characterization of luminescent complex compounds for the usage as supramolecular sensors and for application in the heterogeneous light-induced water reduction. Along these lines, the principal chromophore scaffolds investigated in this thesis are primarily derivatives of bis(2-phenylpyridine) iridium(III) α-diimine and in the second instance bis(2,2’-bipyridine) ruthenium(II) α-diimine. In the first part of the thesis the iridium(III) model complex of 5,5’,6,6’-tetramethyl-2,2’-bibenzimidazole and its homodinuclear complex bearing a simple unsubstituted bibenzimidazole motif are introduced. The structure-property relations of the iridium(III) bibenzimidazole complex are discussed and compared to related ruthenium(II) complexes. Especially the acid-base chemistry is outstanding with a strongly luminescent character in all protonation states of the complex. This feature is of pivotal importance for this and further studies, as it could provide a spectroscopic tool for direct observation of proton release of these molecules. Inspired by the special features of the iridium(III) model complex, the interaction of the latter with basic anions was investigated. Hydrogen bond donor features of the iridium(III) complex were experimentally verified and implications for luminescent sensing could be drawn. In a second step the present supramolecular sensor strategy was optimized using the intermolecular quencher 3,5-dinitrobenzoate (DNBA). Strong H-bond supported ion pair bonding with the electron accepting dinitro-benzoate anion switches the luminescence “off”. The luminescence of the sensor system is switched back “on” when benzoate is replaced by competing H-bonded small anions, therefore leading to enhanced sensitivity of the sensor system. Additionally, to the best of my knowledge, the presented system exceeds all ruthenium(II) biimidazole-type sensors known in literature so far, regarding an increase in luminescence intensity of e.g. 450 % for chloride and as the change in the luminescence intensity can easily be followed with the naked eye. For ruthenium(II) complexes, pioneering work was performed by Ye and co-workers who presented a number of Ru(II) imidazole-like complexes acting as promising anion sensors, however limited by small selectivity and sensitivity. In order to improve the literature known ruthenium(II) based sensors, sterically demanding substituents were introduced in the ligand framework of the bibenzimidazole. Thereby, the secondary binding site of the complexes are spatially confined which leads to a functional dimming of the emission intensity. Subsequently, the recognition properties towards small anions of the complexes could be directed selectively, accompanied by a significant enhancement in luminescence sensitivity. In a conclusive project, the heterodinuclear bibenzimidazole bridged iridium(III)/ ruthenium(II) complex Ir-BBI-Ru was synthesized to investigate the synergy effects of the sole chromophore units tested before. The comprehensive investigation of the basic properties of the complex and its applicability as photosensitizer in the light-induced hydrogen production are presented