Radical-Doped Metal-Organic Framework: Route to Nanoscale Defects and Magnetostructural Functionalities

Nanosized structural defects in metal-organic frameworks (MOFs) attract growing attention and often remarkably enhance functional properties of these materials for various applications. In this work, a series of MOFs [Cu₂(TPTA)_{1- x}(BDPBTR) _x] (H₄TPTA, [1,1':3',1″-terphenyl]-3,3'',5,5''-tetracarboxylic acid; H₄BDPBTR, 1,3-bis(3,5-dicarboxyphenyl)-1,2,4-benzotriazin-4-yl radical)) with a new stable radical linker doped into the structure has been synthesized and investigated using Electron Paramagnetic Resonance (EPR). Mixed linkers H₄TPTA and H₄BDPBTR were used to bridge copper(II) paddle-wheel units into a porous framework, where H₄BDPBTR is the close structural analogue of H₄TPTA. MOFs with various x = 0-0.4 were investigated. EPR studies indicated that the radical linker binds to the copper(II) units differently compared to diamagnetic linker, resulting in the formation of nanosized structural defects. Moreover, remarkable kinetic phenomena were observed upon cooling of this MOF, where slow structural rearrangements and concomitant changes of magnetic interactions were induced. Thus, our findings demonstrate that doping of structurally mimicking radical linkers into MOFs represents an efficient approach for designing target nanosized defects and introducing new magnetostructural functionalities for a variety of applications.