Highly Stable Mixed‐Lanthanide Metal–Organic Frameworks for Self‐Referencing and Colorimetric Luminescent pH Sensing

Abstract A strategy utilizing mixed‐lanthanide metal–organic frameworks (MOFs) for self‐referencing and colorimetric luminescent pH sensing has been developed. The mixed‐lanthanide MOFs Eu_xTb_1−xHdpda (x=0.097, 0.205, 0.303, H₄dpda=4‐(3,5‐dicarboxyphenyl)pyridine‐2,6‐dicarboxylic acid) exhibit excellent stability in phosphate buffer solution with pH values varying from 3.90 to 7.50. Density functional theory (DFT) calculations indicate that the deprotonation of uncoordinated carboxyl groups in ligands results in the elevation of the triplet excited state (T₁) energy level of H₄dpda, thus increasing the energy transfer probability from the T₁ to ⁵D₄ level of Tb³⁺. As a result, the emission intensity of Tb³⁺ in Eu_xTb_1−xHdpda increases gradually while that of Eu³⁺ is almost not changed with the pH varying from 3.90 to 7.50. The intensity ratios of Eu³⁺ to Tb³⁺ can be correlated to the pH value, suggesting they are promising self‐referencing luminescent pH sensors. Furthermore, the significant pH‐dependent emission color change enables them to act as a luminescent colorimetric pH sensor for in situ and real‐time pH imaging. Because the energy transfer between the ligand and lanthanide ion can be easily tuned by selecting an appropriate ligand to further optimize the pH response range and sensitivity, such a strategy opens a new perspective for the development of self‐referencing luminescent pH sensors.