Host-guest interactions control the fundamental processes in porous materials for many applications such as gas storage and catalysis. The study of these processes, however, is not trivial, even if the material is crystalline. In particular, metal-organic frameworks (MOFs) represent a complex situation since guest molecules can interact with different parts of the organic linkers and the metal clusters and may alter the details of the pore structure and system properties. A prominent example is the so-called retrofitted MOF material TCNQ@Cu3(BTC)2 that has attracted a lot of attention due to its electronic properties induced by the host-guest interactions. Only recently, structural evidence has been presented for a bridging binding mode of TCNQ to two Cu paddlewheel units; however, many issues regarding the redox chemistry of Cu3(BTC)2 and TCNQ are currently unsolved. Herein, we report a powerful spectroscopic approach to study the host-guest chemistry of this material. Combining IR spectroscopy in the presence of CO and EPR spectroscopy, we found that the intrinsic Cu(I) defects of the host react with the guest, forming TCNQ radical anions. This chemistry has profound implications, in particular, with respect to the performance of TCNQ@Cu3(BTC)2 as an electronic conductor. A decreasing availability of open Cu(II) sites with increasing TCNQ loading proves the coordinative binding of TCNQ to the paddlewheel nodes, and a heterogeneous structure is formed with different TCNQ arrangements and pore environments at low TCNQ loadings. Finally, the combined use of spectroscopic characterization techniques has proven to be, in general, a powerful approach for studying the complex chemistry of host-guest materials.
Keywords: CO probe IR spectroscopy; EPR spectroscopy; defect chemistry; metal−organic frameworks; redox-active guests.