Intensity and lifetime ratiometric luminescent thermometer based on a Tb(III) coordination polymer

A three-dimensional terbium(III) coordination polymer of formula [Tb(bttb)_0.5(2,5-pzdc)_0.5] (1) [H₄bttb = 1,2,4,5-tetrakis(4'-carboxyphenyl)benzene and H₂-2,5-pzdc = 2,5-pyrazinedicarboxylic acid] was obtained under hydrothermal conditions. The bttb^4- tetraanion in 1 adopts the bridging and chelating-bridging pseudo-oxo coordination modes while the 2,5-pzdc^2- dianion exhibits a rather unusual bis-bidentate bridging pseudo-oxo coordination mode, both ligands being responsible for the stiffness of the resulting 3D structure. Solid-state photoluminescent measurements illustrate that 1 exhibits remarkable green luminescence emission, the most intense band occurring in the region of 550 nm (⁵D₄ → ⁷F₅) with lifetimes at the millisecond scale. Thermometric performances of 1 reveal a maximum relative sensitivity (S_m) of 0.76% K^-1 at 295 K (δT = 0.05 K), constituting a Tb^III ratiometric solid luminescent thermometer over the physiological temperature range. Variable-temperature static (dc) magnetic susceptibility measurements for 1 in the temperature range 2.0-300 K show the expected behavior for the depopulation of the splitted m levels of the ⁷F₇ ground state of the magnetically anisotropic terbium(III) ion plus a weak antiferromagnetic interaction through the carboxylate bridges. No significant out-of-phase magnetic susceptibility signals were observed for 1 in the temperature range 2.0-10.0 K, either in the absence or presence of a static dc magnetic field.