Tunable Optical Molecular Thermometers Based on Metallacrowns.

The effect of ligands' energy levels on thermal dependence of lanthanide emission was examined to create new molecular nanothermometers. A series of LnGaLL metallacrowns (shorthand LnL), where Ln = Gd, Tb, or Sm (HL' = salicylhydroxamic acid (Hshi), 5-methylsalicylhydroxamic acid (Hmshi), 5-methoxysalicylhydroxamic acid (Hmoshi), and 3-hydroxy-2-naphthohydroxamic acid (Hnha)) and HL″ = isophthalic acid (Hiph), was synthesized and characterized. Within the series, ligand-centered singlet state (S) energy levels ranged from 23,300 to 27,800 cm, while triplet (T) energy levels ranged from 18,150 to 21,980 cm. We demonstrated that the difference between T levels and relevant energies of the excited G level of Sm (17,800 cm) and D level of Tb (20,400 cm) is the major parameter controlling thermal dependence of the emission intensity via the back energy transfer mechanism. However, when the energy difference between S and T levels is small (below 3760 cm), the S → T intersystem crossing (and its reverse, S ← T) mechanism contributes to the thermal behavior of metallacrowns. Both mechanisms affect Ln-centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for TbL and from 0.0267(7)% to 2.27(5)% for SmL. The maximal thermal dependence varies over a wide thermal range (ca. 150-350 K) based on energy gaps between relevant ligand-based and lanthanide-based electronic states. By mixing Tbmoshi with Smmoshi in a 1:1 ratio, an optical thermometer with a relative thermal sensitivity larger than 3%/K at 225 K was created. Other temperature ranges are also accessible with this approach.