Direct Excitation Ln(III) Luminescence Spectroscopy to Probe the Coordination Sphere of Ln(III) Catalysts, Optical Sensors and MRI Agents

The production of Ln(III) luminescence by direct excitation of the electronic transitions in the f-manifold is a powerful tool for the study of Ln(III) coordination environment. Direct excitation methods require powerful (mJ per pulse) laser-based instrumentation because the electronic transitions between states with a 4fn configuration are electric dipole (parity) and spin forbidden. In addition, the luminescence of Ln(III) ions is quenched in aqueous solution by non-radiative processes largely through OH oscillators. This makes it challenging to study direct excitation Ln(III) luminescence under biologically relevant conditions. A modern spectroscopic system based on a Nd:YAG pump laser coupled with an optical parametric oscillator (master oscillator power oscillator (MOPO)) offers a tunable output in the UV, visible, and IR regions of the spectrum with approximately 50 mJ pulse-1 at 580 nm, the wavelength region at which Eu(III) is directly excited. This type of system facilitates the study of Eu(III) at 1000-fold lower detection limits than previously reported and also makes it possible to study excitation experiments of several members of the Ln(III) series over a single day. In this chapter we cover several types of direct excitation luminescence experiments for the characterization of Ln(III) complexes. Information about the complex speciation in solution is available through monitoring the 7F0→5D0 transition of Eu(III). Europium excitation spectroscopy is useful for the study of changes in inner-sphere ligand coordination as well for monitoring the ionization of ligands. Luminescence lifetime measurements provide information on the number of OH or NH oscillators in the Ln(III) coordination sphere. Luminescence resonance energy transfer is useful for determining the proximity of two lanthanide ions if they are separated by 10 Å or less. The multiple emission peaks of the Ln(III) are useful to glean further information on changes in the coordination sphere. Examples of these luminescence experiments are given for Ln(III) macrocyclic complexes as catalysts, optical sensors and MRI contrast agents.