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

Abstract

Photoluminescent isotypical phosphonate-based metal–organic frameworks, [Ln(H 5 btp)]·2H 2 O [where Ln 3+ = Tb 3+ ( 1 ) or Gd 3+ ( 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 Ln 3+ 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 5 D 4 → 7 F 5 Tb 3+ 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 2,1 → S 0 and metal 5 D 4 → 7 F 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.