Structural and photophysical properties of Tb³⁺-tetra-1,3-diketonate complexes controlled by calix[4]arene-tetrathiacalix[4]arene scaffolds

The present work highlights the key aspects of the influence of calix[4]arene and tetrathiacalix[4]arene scaffolds on the structural and photophysical properties of Tb3+ complexes with tetra-1,3-diketone derivatives of the macrocycles in DMF solutions. The equilibrium forms of Tb3+ complexes with unsubstituted and functionalized by acetylacetonyl groups at the upper rim of calix[4]arenes and thiacalix[4]arenes are revealed from UV-, NMR, MALDI TOF mass spectroscopy, quantum-chemical calculations at the DFT level and luminescence spectroscopy data. In alkaline DMF solutions, the ligands form predominantly 1 : 1 complexes with Tb3+ ions. However, the replacement of a calix[4]arene-scaffold by a thiacalix[4]arene-scaffold in the tetra-1,3-diketone derivatives shifts the equilibrium forms of Tb3+ complexes from monomeric to the dimeric ones. DFT calculations in combination with experimental data reveal the most reliable structures of complexes. The quantitative analysis of the photophysical parameters in correlation with the structural features of the complexes highlights the specific inner-sphere environment of Tb3+ ions in the dimeric complexes with the thiacalix[4]arene derivatives as a reason for greater sensitization of Tb3+-centered luminescence.