Lanthanide-doped metal-organic frameworks with multicolor mechanoluminescence

Metal-organic frameworks (MOFs) and mechanoluminescent (ML) materials have been considered as two types of promising materials that have their own application fields. It would be amazing to endow one material with the advantages of ML and MOFs, thus broadening their applications. However, there are quite few investigations on this topic, and the ML mechanism in ML-MOFs remains unclear. In this study, we proposed a strategy for developing ML-MOFs by doping lanthanide ions into the non-centrosymmetric SBD ([Sr(μ-BDC)(DMF)]∞) MOF, and successfully synthesized a series of lanthanide-doped MOFs Ln-SBD (Ln = Tb, Dy, Sm, Eu) and Tb1−xEux-SBD (x = 0.2, 0.4, 0.6, 0.8) with multicolor ML. The lanthanide ions were uniformly distributed in the matrix of the SBD-MOF, and occupied the Sr site. The MLMOFs exhibited intense multicolor ML emissions varying from green to yellow to red by changing the co-doping ratios and species of lanthanide ions. The similar ML and photoluminescence (PL) spectra indicated that the ML emission was assigned to the radiative transition from the excited states to the ground states of lanthanide ions. The radiative transition was induced by the electron bombardment process that originated from the piezoelectric effect of the non-centrosymmetric SBD host. In addition, a pioneering temperature sensing research based on ML was carried out, which is promising for realizing dual-functional detection of stress and temperature without excitation light sources. This study gives a unique insight for developing more versatile and interesting smart materials by combining the versatility of MOF with the ML emission, imparting additional values to both MOF and ML materials. Moreover, this study provides a general rule for selecting MOFs with an acentric structure as the host for ML materials.