Comprehensive Photophysical and Nonlinear Spectroscopic Study of Thioanisolyl-Picolinate Triazacyclononane Lanthanide Complexes

Detailed photophysical studies of luminescent lanthanide complexes are presented and elaborated using a newly developed thioanisolyl-picolinate antenna and the related tacn macrocyclic ligand. The new ligand proved to sensitise Nd(III), Sm(III), Eu(III) and Yb(III) emission. Eu(III) complex showed complete energy transfer, yielding high quantum yield (44%) and brightness, while the Tb(III) analogue underwent a thermally activated back-energy transfer, resulting in a strong oxygen quenching of the triplet excited state. Transient absorption spectroscopy measurements of Gd(III), Tb(III) and Eu(III) compounds confirmed the sensitization processes upon the charge-transfer antenna excitation. The triplet excited state lifetime of the Tb(III) complex was 5-times longer than that of the Gd(III) analogue. In contrast, the triplet state was totally quenched by the energy transfer to the 4f-metal ion in the Eu(III) species. Nonlinear two-photon absorption highlighted efficient biphotonic sensitization in Eu(III) and Sm(III) complexes. In case of the Nd(III) compound, one-photon absorption in 4f-4f transitions was predominant, despite the excitation at the antenna two-photon band. This phenomenon was due to the Nd(III) 4f-4f transitions overlapping with the wavelength-doubled absorption of the complex.