Photophysics and crystal structure of a europium(III) cryptate incorporating 3,3'-biisoquinoline-2,2'-dioxide.

Increased interest in the emission properties of lanthanide(III) (Eu and Tb) complexes containing ultraviolet and visible sensitizers is being driven by the desire to produce efficient and selective luminescent probes of biological structure. Of special interest are cryptates and other macrocyclic chelating ligands that efficiently encapsulate the lanthanide ions. These species also form relatively stable systems and in some cases are well protected from penetration of the first coordination sphere by solvent molecules and counterions. This work describes the X-ray structure and various spectroscopic measurements on a europium cryptate containing 3,3'-biisoquinoline-2,2'-dioxide (biqO2). This cryptate has been previously recognized for special stability and luminescence efficiency. The compound, (Eu:biqO2.2.2)(CF3SO3)3.CH3CN.H2O, forms rhombic crystals with the space group Pbca. Absorption, emission, and excitation spectra at 293, 77, and 4 K as well as luminescence decay time measurements are used to characterize the solid state and solutions. The ligand-to-metal energy-transfer mechanism and thermally activated back-energy-transfer processes are analyzed and compared to previously published results on related Eu(III) cryptate systems. Preliminary results on the use of high liquid pressure to perturb ligand singlet and triplet states and, as a consequence, probe the ligand-metal orbital interactions are also presented.