Photophysical studies of m-terphenyl-sensitized visible and near-infrared emission from organic 1∶1 lanthanide ion complexes in methanol solutions

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Abstract
The luminescence properties of several 1∶1 lanthanide ion (Sm3+, Tb3+, Dy3+, Pr3+, Nd3+, Ho3+, Tm3+ and Yb3+) complexes based on the m-terphenyl-containing ligands 1–3 have been studied in methanol solutions. The organic complexes show their typical luminescence in the visible (Sm3+, Tb3+, Dy3+ and Pr3+) and in the near-infrared (Nd3+, Er3+ and Yb3+) region of the electromagnetic spectrum. The degree of shielding of the lanthanide ions from high-energy quenching modes of the solvent by the acyclic ligand 3 is less than the shielding by the macrocyclic ligands 1 and 2. Not only the high-energy vibrational modes of the solvent quench the luminescent state, but also the C–H modes of the organic ligand, and even O–D and C–D modes can contribute significantly to the quenching. In general, the high-energy vibrational O–H and C–H modes are most efficient in luminescence quenching, but the quenching is strongly dependent on the magnitude of the energy gap between the lowest luminescent state and a lower-lying state. Luminescence at longer wavelengths can be quenched relatively easily because of the smaller energy gaps, rendering all quenching pathways, especially quenching by the remaining C–H modes in the partially deuterated ligand, efficient. When the energy gap is resonant with (an overtone of) a vibrational mode, i.e. O–H, C–H, O–D or C–D, the luminescence is very efficiently quenched by these modes and can even be extinguished. For instance: Ho3+ luminescence was not observed because the 5S2→5F5 transition is resonant with the C–H vibrational mode, deuteration is less effective than expected for Pr3+ because the energy gap is resonant with the first overtone of the C–D vibration, and Nd3+ is efficiently quenched by the deuterated solvent because the energy gap is resonant with the first overtone of the O–D vibration.

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