Modulating the electronic properties of divalent lanthanoid complexes with subtle ligand tuning

Five new compounds of formula [Ln^II(Metpa)₂](BPh₄)₂ (Ln = Eu, n = 0 (1-Eu), n = 2 (2-Eu) and n = 3 (3-Eu); Ln = Yb, n = 0 (1-Yb) and n = 2 (2-Yb); tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to successive methylation of the 6-position of the pyridine rings of Metpa) have been synthesized and their structural, photophysical and electrochemical properties investigated. The Ln^II ions in the five complexes possess cubic coordination geometry and exhibit only small structural differences, due to the lengthening of the Ln-N bonds to accommodate the additional steric bulk associated with increasing methylation of the Metpa ligands. Photophysical studies indicate moderate shifts in absorbance, emission and excitation bands associated with the 4f⁷ ↔ 4f⁶5d¹ (Eu^II) and 4f¹⁴ ↔ 4f¹³5d¹ (Yb^II) transitions, while electrochemistry reveals modulation of the redox potential of the Ln^II to Ln^III oxidation. There is a strong correlation between Ln-N bond lengths and both the photophysical transition energies and metal redox-potentials, revealing how subtle ligand changes and ligand field effects can be used to modulate the electronic properties of complexes of divalent lanthanoid ions. Utilization of these insights may ultimately afford design and property tuning strategies for future functional molecular complexes based on divalent lanthanoid metals.