The potential of pyridine-2,6-dicarboxylic acid based organolanthanide complexes for efficient near-infrared emission

We present detailed spectroscopic data obtained from Nd³⁺ pyridine-2,6-dicarboxylic acid based complexes in which the 4-position of the pyridine ring has been substituted with OH and Cl. In each case the ligands formed stable complexes with the Nd³⁺ ion without the requirement for any additional 'neutral' ligand to satisfy the 8-9 coordination requirement of the lanthanide ion. Photoluminescence is observed from both the ligand (centered ∼700 nm) and the Nd³⁺ ion (at ∼900 nm, 1064 nm, and 1320 nm due to the ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2, and ⁴F_13/2 → ⁴I_9/2 transitions respectively) following excitation in the low energy tail of the ligand π → π* absorption. The intensity of the ligand emission and sensitized Nd³⁺ emission was found to be dependent on the substituted 4-position of the pyridine ring. The origin of the observed phenomena are discussed in relation to the energy transfer process from ligand to Nd ³⁺ ion and the non-radiative relaxation of the sensitized Nd ³⁺ ion. These results suggest that further modification of the ligand through complete halogenation and/or addition of other functional groups may provide an attractive route to obtaining an efficient near-infrared emitting organolanthanide complex.