Luminescence properties of Eu3+, Tb3+, and Gd3+ complexes of the hexadentate N-donor podand tris-[3-(2-pyridyl) pyrazol-lyl]hydroborate

The photophysical properties of Gd3+ Eu3+ and Tb3+ complexes of a podand ligand tris[3-(2-pyridyl)-pyrazolyl]hydroborate (L−) have been investigated in five solvents. [Gd(L)(NO3)2] (Gd-A) exhibits only the typical ligand-centered spectroscopy, i.e. luminescence and triplet-triplet absorption, whereas Eu-A and Tb-A show only the characteristic metal centered luminescence. The results obtained show that the podand does not fully surround the metal ion and the ligand-to-metal energy transfer efficiency is high for Tb-A because a large energy gap prevents back energy transfer, but it is small for Eu-A, because of the presence of low energy charge-transfer excited states. Also the three bis-podand complexes [LnL2]+ (Ln-B) have been investigated, but only in CH2Cl2 for solubility reasons.