A water-soluble TbIII complex as a temperature-sensitive luminescent probe

The water soluble [Tb(dipicCbz)(3)](3-)(dipicCbz = 4-(9H-carbazol-9-yl-)pyridine-2,6-dicarboxylato) complex was isolated and evaluated as a temperature sensor in water. The 1:3 (Tb-III:dipicCbz(2-)) stoichiometry in solution was confirmed by luminescence titration and high-resolution mass spectrometry. The quantum yield of sensitized emission is 3.8% +/- 0.4% at 25.0 +/- 0.1 degrees C, and the emission intensity depends on the temperature in the range of 5-70 degrees C with a relative thermal sensitivity of 3.4% degrees C-1 at 35 degrees C and temperature resolution < 0.01 degrees C in the range of 30-40 degrees C. The reversibility of this behavior was demonstrated for three heating-cooling cycles. Calculations of the energy gap between donor and acceptor show that the temperature dependence of the emission intensity is due to back-energy transfer from the Tb D-5(4) excited state to the triplet and twisted intramolecular charge transfer (TICT) states of the dipicCbz. The assignment of one of the energy levels as a TICT state was confirmed by the temperature-dependent behavior of the phosphorescence band.