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

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) angstrom, b = 7.2856(2) angstrom, c = 11.7073(3) angstrom, beta = 97.229(1)degrees and V = 1385.97(6) angstrom(3). 2 crystallizes in the monoclinic space group P21/n with cell parameters a = 5.9472(2), b = 16.0747(6), c = 14.3716(5) angstrom, beta = 93.503(1)degrees and V = 1371.35(8) angstrom(3). 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 Ln=Eu and 24.0% for Ln=Tb. The excited state lifetimes for Eu-III and Tb-III are 2.227 ms and 723 mu s, 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 mu s. 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 (1) over bar with cell parameters a = 9.1845(2) angstrom, b = 10.3327(2) angstrom, c = 11.9654(2) angstrom, alpha = 98.419(1)degrees, beta = 108.109(1)degrees, gamma = 91.791(1)degrees, V = 1064.08(4) angstrom(3) and a = 7.8052(1) angstrom, b = 11.8910(1) angstrom, c = 14.2668(2) angstrom, alpha = 72.557(1)degrees, beta = 86.355(1)degrees, gamma = 77.223(1)degrees, V = 1231.95(3) angstrom(3), respectively.