Effect of the aromatic substituent on the para-position of pyridine-bis(oxazoline) sensitizers on the emission efficiency of their EuIII and TbIII complexes

Abstract

Two efficient lanthanide ion sensitizers 2,6-bis(oxazoline)-4-phenyl-pyridine (PyboxPh, 1) and 2,6-bis(oxazoline)-4-thiophen-2-yl-pyridine (Pybox2Th, 2) were synthesized. 1 crystallizes in the monoclinic space group P21/c with cell parameters a = 16.3794(4) Å, b = 7.2856(2) Å, c = 11.7073(3) Å, β = 97.229(1)° and V = 1385.97(6) ų. 2 crystallizes in the monoclinic space group P21/n with cell parameters a = 5.9472(2), b = 16.0747(6), c = 14.3716(5) Å, β = 93.503(1)° and V = 1371.35(8) ų. Photophysical characterization of 1 shows that its triplet state energy is located at 22 250 cm⁻¹ and efficient energy transfer is observed for Eu^III and Tb^III. Solutions of [Ln(PyboxPh)₃]³⁺ in dichloromethane display an emission efficiency of 37.2% for LnEu and 24.0% for LnTb. The excited state lifetimes for Eu^III and Tb^III are 2.227 ms and 723 μs, respectively. The triplet state energy of 2 is located at 19 280 cm⁻¹ and is therefore too low to efficiently sensitize Tb^III emission. However, the sensitization of Eu^III is effective, with an emission quantum yield of 14.5% and an excited state lifetime of 714 μs. This shows that the derivatization of the chelator is strongly influenced by the aromatic substituents on the para-position of the pyridine ring. New isostructural 1 : 1 complexes of PyboxPh with Eu^III (3) and Tb^III (4) were also isolated and crystallize in the triclinic space group P with cell parameters a = 9.1845(2) Å, b = 10.3327(2) Å, c = 11.9654(2) Å, α = 98.419(1)°, β = 108.109(1)°, γ = 91.791(1)°, V = 1064.08(4) ų and a = 7.8052(1) Å, b = 11.8910(1) Å, c = 14.2668(2) Å, α = 72.557(1)°, β = 86.355(1)°, γ = 77.223(1)°, V = 1231.95(3) ų, respectively.