Tunable Optical Molecular Thermometers Based on Metallacrowns

Abstract

The effect of ligands' energy levels on thermal dependence of lanthanide emission was examined to create new molecular nanothermometers. A series of Ln2Ga8L8′L8″metallacrowns (shorthand Ln2L8′), where Ln = Gd3+, Tb3+, or Sm3+(H3L′ = salicylhydroxamic acid (H3shi), 5-methylsalicylhydroxamic acid (H3mshi), 5-methoxysalicylhydroxamic acid (H3moshi), and 3-hydroxy-2-naphthohydroxamic acid (H3nha)) and H2L″ = isophthalic acid (H2iph), was synthesized and characterized. Within the series, ligand-centered singlet state (S1) energy levels ranged from 23,300 to 27,800 cm-1, while triplet (T1) energy levels ranged from 18,150 to 21,980 cm-1. We demonstrated that the difference between T1levels and relevant energies of the excited 4G5/2level of Sm3+(17,800 cm-1) and 5D4level of Tb3+(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 S1and T1levels is small (below 3760 cm-1), the S1→ T1intersystem crossing (and its reverse, S1→ T1) mechanism contributes to the thermal behavior of metallacrowns. Both mechanisms affect Ln3+-centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for Tb2L8′and from 0.0267(7)% to 2.27(5)% for Sm2L8′. 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 Tb2moshi8′with Sm2moshi8′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