Azido- and amino-substituted dipicolinates for the sensitization of the luminescent lanthanides EuIII and TbIII

Dipicolinate-based ligands substituted with azido- or amino- functional groups were synthesized and characterized. Crystal structures of EN3, CN3, CNH2, and ENH2 were obtained, and the ability of these compounds to sensitize LnIII luminescence (LnIII = EuIII and TbIII) was explored. Luminescence efficiencies of EuIII and TbIII were determined and are ϕEu = 0.11%, 0.15%, and 13.8% for [Ln(CN3)3]3+, [Ln(EN3)3]3+, [Ln(CNH2)3]3+, respectively, and ϕTb = 0.30%, 0.27%, and 28.9% for [Ln(CN3)3]3+, [Ln(EN3)3]3+, [Ln(ENH2)3]3+. The highest luminescence efficiencies are ϕEu = 14.4% for [Eu(ENH2)3]3+ and ϕTb = 29.7% for [Tb(CNH2)3]3+. Similar 1S and 3T excited state energies were observed for all ligands and did not explain the observed discrepancies in luminescence efficiency of their TbIII and EuIII complexes. Further analysis revealed that the azido-derivatives were not photostable, which likely contributed to the low luminescence efficiency.