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 Ln 2 Ga 8 L 8 ' L 8 ″ metallacrowns (shorthand Ln 2 L 8 ' ), where Ln = Gd 3+ , Tb 3+ , or Sm 3+ (H 3 L' = salicylhydroxamic acid (H 3 shi), 5-methylsalicylhydroxamic acid (H 3 mshi), 5-methoxysalicylhydroxamic acid (H 3 moshi), and 3-hydroxy-2-naphthohydroxamic acid (H 3 nha)) and H 2 L″ = isophthalic acid (H 2 iph), was synthesized and characterized. Within the series, ligand-centered singlet state (S 1 ) energy levels ranged from 23,300 to 27,800 cm -1 , while triplet (T 1 ) energy levels ranged from 18,150 to 21,980 cm -1 . We demonstrated that the difference between T 1 levels and relevant energies of the excited 4 G 5/2 level of Sm 3+ (17,800 cm -1 ) and 5 D 4 level of Tb 3+ (20,400 cm -1 ) is the major parameter controlling thermal dependence of the emission intensity via the back energy transfer mechanism. However, when the energy difference between S 1 and T 1 levels is small (below 3760 cm -1 ), the S 1 → T 1 intersystem crossing (and its reverse, S 1 ← T 1 ) mechanism contributes to the thermal behavior of metallacrowns. Both mechanisms affect Ln 3+ -centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for Tb 2 L 8 ' and from 0.0267(7)% to 2.27(5)% for Sm 2 L 8 ' . 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 Tb 2 moshi 8 ' with Sm 2 moshi 8 ' 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.