Intrinsic Molybdenum-Based POMOFs with Impressive Gas Adsorptions and Photochromism

Novel molybdenum(VI/V) POM-based self-constructed frameworks [Mo^VI ₁₂ O₂₄ (μ₂ -O)₁₂ (trz)₆ (H₂ O)₆ ] ⋅ 6Hma ⋅ 18H₂ O (1, Htrz=1H-1,2,3-triazole, ma=methylamine), [Mo^VI ₇ O₁₄ (μ₂ -O)₈ (trz)₅ (H₂ O)] ⋅ 7Hma ⋅ 5H₂ O (2), Na₃ [Mo^V ₆ O₆ (μ₂ -O)₉ (Htrz)₃ (trz)₃ ] ⋅ 7.5H₂ O (3) and [Mo^V ₈ O₈ (μ₂ -O)₁₂ (Htrz)₈ ] ⋅ 30H₂ O (4) have been covalently decorated with tri-coordinated deprotonated/protonated 1,2,3-triazoles. Channels with an inner diameter of 7.5 Å were found in 1, whereas a tunnel composed of stacking molecules with an inner diameter of 4.1 Å along the b-axis exists in 2; it is occupied by free disordered methylamines, showing selective adsorption of O₂ and CO₂ at 25 °C. Obvious downfield shifts were observed by ¹³ C NMR spectroscopies for methylamines inside the confined channels in 1 and 2. There are diversified pores in 3 and 4, which are formed by the molecules themselves and intermolecular accumulations. Adsorption tests indicate that 3 and 4 are fine adsorption materials for CH₄ and CO₂ under low pressure that rely on the environments built by the POMs. Correspondingly, 1 and 2 display reversible photoresponsive thermochromism that is subtlety influenced by the channels. The polyoxometalate organic frameworks (POMOFs) with multiple functional adsorptions are easy to assemble. Their photo-/thermoresponse properties offer a new pathway for the self-constructions of one-off hybrid materials that possess the good properties of both POMs and MOFs.