The Effect of Different Neutral Ligands on Photoluminescence and Electroluminescence Properties of Ternary Terbium Complexes

Three terbium complexes Tb(tba-PMP)3(TPPO) (A), Tb(tba-PMP)3(H2O) (B), and Tb(tba-PMP)3(Phen) (C)
(where tba-PMP, TPPO, and Phen stand for β-diketone 1-phenyl-3-methyl-4-(tert-butylacetyl)-5-pyrazolone,
triphenyl phosphine oxide, and 1,10-phenanthroline, respectively) with different neutral ligands were synthesized
and characterized, and the mechanism of how the neutral ligands affect photoluminescence (PL) and
electroluminescence (EL) properties of terbium complexes was studied. Experiments revealed neutral ligand
TPPO and Phen strongly affect the terbium complex PL intensity, TPPO enhanced the PL intensity of complex
A, while Phen reduced the PL intensity of complex C compared to that of complex B. Investigation indicated
this is caused by the different excited energy levels between tba-PMP, TPPO, and Phen, which were obtained
from their phosphorescence spectra measured with their corresponding gadolinium complexes Gd(tba-PMP)3(H2O)(EtOH), Gd(TPPO)2(NO3)3, and Gd(Phen)2(NO3)3. Compared to complex B, the energy absorbed by
TPPO can be efficiently transferred to tba-PMP and the central ion Tb3+ due to its excited singlet and triplet
energy levels matching that of tba-PMP and the 5D4 energy level of Tb3+, and consequently enhancing the
PL intensity of A, while on the contrary, negative energy transfer occurred between Phen and tab-PMP or
Tb3+ since the triplet energy level of Phen is lower than that of tba-PMP and the 5D4 energy level of Tb3+ as
well. Experimental results show terbium complex electroluminescence (EL) is greatly dependent on its PL
intensity, the performance achieved based on complexes A, B, and C being 9540 cd/m2 and 7.2 lm/W, 3230
cd/m2 and 1.17 lm/W, and 690 cd/m2 and 0.13 lm/W, respectively, with the power efficiency ratio A:B:C =
55:9:1, which was greatly enlarged compared to their PL intensity ratio A:B:C = 2.1:1.3:1.