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.