TbCo and Tb0.5Dy0.5Co layered cyanido-bridged frameworks for construction of colorimetric and ratiometric luminescent thermometers

Abstract

Heterometallic cyanido-bridged networks are versatile molecular platforms for diverse optical, magnetic and electronic functionalities. We present a pioneering synthetic route which employs polycyanidometallates in the preparation of unique d–f coordination systems revealing multi-stimuli responsive multi-coloured photoluminescence with potential application in colorimetric and ratiometric temperature sensing. We report two layered cyanido-bridged frameworks, bimetallic {[Tb^III(4-OHpy)₂(H₂O)₃][Co^III(CN)₆]}·0.5H₂O (1) (4-OHpy = 4-hydroxypyridine), and trimetallic {[Tb^III_0.5Dy^III_0.5(4-OHpy)₂(H₂O)₃][Co^III(CN)₆]}·0.5H₂O (2), along with their structural and full physicochemical characterization. They exhibit room temperature visible photoluminescence within an extensive colour range, including white light emission, tunable through the applied lanthanide ions, and switchable by excitation with light through selective excitation of the green emissive Tb^III, yellow emissive Dy^III, blue luminescent 4-OHpy, and red luminescent [Co(CN)₆]³⁻ components. The emission properties of 1 and 2, including the energy transfer from organic ligands and cyanide complexes towards lanthanide ions, are strongly modulated by the change of temperature in the broad 120–300 K range. As a result, colorimetric luminescent temperature sensing exploiting a wide emission colour range was achieved for both 1 and 2. Moreover, 2 reveals the temperature dependent ratio between the intensities of sharp emission lines of Tb^III and Dy^III which can be used as a thermometric parametric for ratiometric luminescent detection of temperature. The related thermometer perfomance was examined for various excitation wavelengths, and the best parameters of relative thermal sensitivity, S_r > 1% K⁻¹, with the maximal value 2.2(3)% K⁻¹, and temperature uncertainty, δT < 1 K, are detected for 270 nm excitation in the range 120–200 K.