Investegations on the Antenna Effect in Kinetically Stable Lanthanide Complexes

Lanthanide based probes used in bioimaging and in bioassays are generally based on the antenna principle, where an organic chromophore is attached to a lanthanide complex and used as a light harvesting agent. Upon excitation the organic chromophore transfers the excited state energy to the lanthanide centre, thus making it luminesce. A fundamental understanding of the underlying principles involved in the design of efficient lanthanide probes based on the antenna principle is already in place. This thesis has focused on adding information to the guidelines involving the design of the molecular structure, by investigating the energy migration pathways involved in detail. The lanthanide probes are used in solution and resolving the solution state structure is therefore of vital importance, if a better understanding of the energy migration pathways and photophysical properties of the probes are to be obtained. This has been a continuous task throughall of the studies done in this thesis. In the first study of the thesis, the narrow line like emission spectrum of the lanthanide ions were used to investigate remote substituent effects in responsive lanthanide complexes, to establish the boundaries of these effects. A contribution was made to this field by investigating how a wellknown binding event could be transmitted by the lanthanide emission. The responsive systems were made by linking two different crown ethers to kinetically inert lanthanide complexes. The resulting complexes were investigated using 1H NMR and optical spectroscopy. Titrations with potassium chloride observing the lanthanide centred emissions were used to investigate the response of the systems. The shape of the lanthanide emission spectra was shown to be unperturbed by the binding of potassium, while the binding was reported by an overall increa se in intensity of the lanthanide centred emission. The solution structure of the lanthanide complexes was found to be determining for the observed physicochemical properties of these systems.In the second study of the thesis, kinetically inert DO3A complexes with a 7-methoxy-coumarin antenna chromophore were made, and used to investigate the excited state energy migration pathways leading away from the chromophore excited state. By contrasting the photophysical properties of complexes containing metal centres with and without accessible excited states, the contributions from the heavy atom effect, photoinduced electron transfer quenching, excited state energy transfer and molecular conformations could be separated. Furthermore, by studying the VIphotophysical properties of the antenna chromophore, the solution structure of the complexes could be monitored directly. The 7-methoxy-coumarin-DO3A complexes were also used in a study to reveal subtle differences between the solution chemistries of trivalent 4f and 5f elements. The data collected in the study, indicated that there is a significant difference in the way the photophysics of the antenna chromophore were perturbed by the 4f and 5f elements. In contrast, the size of the metal centre did not appear to be determining for the physicochemical properties of the kinetically inert complexes.In the last study of the thesis, a series of 2-methyl-1-azathioxanthones were synthesised andinvestigated. The motivation behind the study was based on the fact that thioxanthones are efficient triplet sensitizers, and thus promising antennas for lanthanide complexes. Three different 2-methyl-1-azathioxanthones were synthesised targeting efficient triplet formation following absorption in the visible range of the spectrum. The substituent effect on the core structure was investigated by optical spectroscopy and by applying theoretical models. Their potential use as lanthanide antennas were evaluated by their photophysical properties including mapping of the excited state kinetics and by a collisional quenching experiment. The three potential antennas were titrated with lanthanide ions, to determine whether or not excited state energy transfer would happen upon the collision between chromophore and lanthanide ion.