Excimer Formation in a Terbium Metal-Organic Framework Assists Luminescence Thermometry

Photoluminescent isotypical phosphonate-based metal organic frameworks, [Ln(H(5)btp)]center dot 2H(2)O [where Ln(3+) = Tb3+ (1) or Gd3+ (2) and H(8)btp = [1,1'-biphenyl]-3,3',5,5'-tetrayltetrakis(phosphonic acid)] prepared under solvo(hydro)thermal conditions exhibit a three-dimensional supramolecular structure built up from phosphonate linkers bridging Li3+ ions. Single-crystal X-ray diffraction of 1 revealed hydrogen bonds along the a-axis between the crystallization water molecules and the phosphonate groups. The network features lozenge-shaped tubular channels with intermolecular interactions between adjacent organic ligand molecules. Crystal packing forces bring together two ligand biphenyl groups in a parallel arrangement, forming an excimer. A ratiometric luminescent thermometer operative close to room temperature was built based on the excimer and the D-5(4) -> F-7(5) Tb3+ emissions, exhibiting a maximum relative thermal sensitivity of 1.26%-K-1 and a minimum temperature uncertainty of 0.75 K at 319 K. This performance is similar to that of a thermometer based on the ligand singlet S-2,S-1 -> S-0 and metal D-5(4) -> F-7(5) transitions. Thus, the same MOF material provides two separate temperature data sets. This work shows the possibility of using excimer emissions in ratiometric luminescence thermometry.