Photochemistry and photophysics of lanthanide complexes

Abstract

Luminescent lanthanide complexes have proven to be a productive area of research due in essence to their unique emission properties enabling their use in a variety of applications. A major drawback to using lanthanides is their low extinction coefficients, so that the antenna effect is often used to overcome this problem. In this way, the antenna chromophore absorbs incoming radiation and transfers the energy to the ion, leading to indirect excitation of the lanthanide. Understanding this mechanism of energy transfer is of fundamental importance both for the theory of the photophysical processes, and for the development of more efficient materials. In this thesis, many of the areas of both the population and the deactivation of the lanthanide excited states are discussed. The time resolved data of the Tm(^3+) ion in a variety of solvents and chelate systems have been measured, and the effects of deuteration on the decay kinetics investigated. The mechanism of energy transfer is followed by detailed kinetic measurements of a complex with a benzophenone containing chromophore. The calculation of the pure radiative lifetime allowed the efficiency of each of the steps involved in the sensitisation process to be determined for the first time. Changing the coordination environment around the ion, even slightly, by the addition of a single CH(_2) group into the arm of the ligand affects the oscillator strength of the hypersensitive transitions in both the Eu(^3+) and Tb(^3+) complexes, and influences the efficiency of energy transfer. Intramolecular sensitisation of Eu(^3+) ions by acetophenone containing ligands was investigated in a series with increasing electron donating ability of the chromophore. It was found that the efficiency of energy transfer increases with the polarity of the antenna, but that in the case of a dimethylamino substituted ligand, the solvent also has a profound effect on the intensity of Eu(^3+) emission. Energy transfer to bound Eu(^3+) and Tb(^3+) ions in aqueous micellar systems is demonstrated, with the process dependent on the hydrophobic properties of the chromophore and the properties of the surfactant. Photoinduced electron transfer reactions are found to occur in ethoxybenzyl substituted complexes with the emission from the chromophore and from the lanthanide ion dependent upon the lanthanide reduction potential and the AG value of both the forward and backward electron transfer reactions.