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

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 S-5(2)-->F-5(5) 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.