Construction of multi-hydroxyl/ketone lanthanide metal–organic frameworks for understanding mechanochromic luminescence and high proton conductivity

Abstract

Two types of lanthanide metal–organic frameworks (LnMOFs) [Zr₁₆-BPDC-Eu_1−xTb_x] and [Zr₁₆-Eu_1−xTb_x] (x = 0.17, 0.25, 0.5) were designed and constructed through the directed assembly of multi-hydroxyl/ketone Zr-oxo clusters and Ln³⁺ ions in a single-crystal-to-single-crystal (SCSC) structural transformation manner. [Zr₁₆-BPDC-Eu_1−xTb_x] is the first type of LnMOF to be found with mechanochromic luminescence (MCL) properties; for example, the emission color of [Zr₁₆-BPDC-Eu_0.83Tb_0.17] can change from yellow to orange-red when stimulated by mechanical forces, compared to which the luminescence of [Zr₁₆-Eu_0.83Tb_0.17] only becomes dark yellow. The proton conduction properties of [Zr₁₆-BPDC-Eu] and [Zr₁₆-Eu] were investigated because of the multi-hydroxyl/ketone character in their structures, and the proton conductivity of [Zr₁₆-Eu] was found to be 6.17 × 10⁻² S cm⁻¹ at 358 K and 98% relative humidity, which is the highest among hydroxyl/ketone-function MOFs. The LnMOFs with MCL and high proton conductivity pave the way to the development of more valuable properties and applications of LnMOFs.