Scrutinizing the Pore Chemistry and the Importance of Cu(I) Defects in TCNQ-Loaded Cu₃(BTC)₂ by a Multitechnique Spectroscopic Approach

Abstract

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.

In Situ Spectroscopy of Calcium Fluoride Anchored Metal–Organic Framework Thin Films during Gas Sorption

“…Thep eak shift (2179 to 2173 cm À1 )s hows that relatively high coverage of CO is reached at low CO pressures, [23] which is expected for thin films.The second peak at 2142 cm À1 is not always reported in the literature,a si ti so ften lost in the more intense surrounding peaks.H owever,B ordiga et al observed the formation of this band specifically at higher coverages and ascribed it to "liquid-like" state CO bands,which are similar to CO in the condensed phase. [23,24] Thep eak at 2128 cm À1 is ascribed to Cu + •••CO adducts.T he origin of the Cu + is yet unclear:whilst some groups claim it is due to Cu 2 Oimpurities originating from either the Cu acetate precursors or thermal reduction of the Cu 2+ paddlewheel in the activation steps prior to catalysis or sorption, [22,23,[25][26][27] other groups state that it originates from defective Cu + /Cu 2+ dimeric paddlewheel species,w hich are reversible redox sites. [28,29] Recent work Figure 4.…”

“…Theh igher temperature results in the reduction of the Cu 2+ paddlewheel species,p resumably due to oxidative decarboxylation as reported by Wçll et al [30] Arguably the pretreatment at 250 8 8Ca lso induces decomposed impurities, likely copper oxides,asthe Cu + •••CO band around 2120 cm À1 becomes very broad. [27] To minimalize differences that the pretreatment of the MOF might induce on the IR bands resulting from CO adsorption, we compare our SURMOF to bulk which were both pretreated at 100 8 8C.…”

Spatio‐Chemical Heterogeneity of Defect‐Engineered Metal–Organic Framework Crystals Revealed by Full‐Field Tomographic X‐ray Absorption Spectroscopy

“…Thev isualization of heterogeneities,i ncluding the detection of secondary phases of reduced copper coordination, within entire defect-engineered MOF crystals,a llowed us to provide an unprecedented point of view towards explaining the structure-property relationship of defect-engineered MOFs.B ased on these observations,w es uggest that some of the properties of defect-engineered MOFs that affect gas sorption and catalytic properties,such as changes in porosity and copper coordination, are in part ar esult of or emerge from these secondary phases. [10,42,50] While the observations made here point to ap ost-synthesis framework collapse in regions of increased defect or defective linker density,w hat causes ad efective linker zonation in the first place is still in need of an explanation. Theabsence of apreferential spatial distribution of secondary coordination polymer [51] clusters with regards to crystallographic directions in defect-engineered HKUST-1 denies,t oacertain degree,a nexplanation based on preferential step interactions.Asecond explanation is ag radual Ostwald ripening [18] process and the diffusion of defective linkers along ap otential gradient, leading to local defective linker aggregation and an eventual lattice collapse.…”

Heterometallic Actinide‐Containing Photoresponsive Metal‐Organic Frameworks: Dynamic and Static Tuning of Electronic Properties

“…Herein, we tailor optoelectronic properties of heterometallic An‐MOFs, M IV ‐MOFs (M=U and Th), through the design of heterometallic nodes, integration of stimuli‐responsive organic moieties, and inclusion of redox‐active guests (Scheme 1). Two types of guest molecules, iodine and 7,7,8,8‐tetracyanoquinodimethane (TCNQ), have been selected for tuning MOF electronic structures due to two different reasons: synthetic limitations imposed by scaffold stability and redox activity as discussed below [32–38] . We consider static and dynamic tailoring of electronic properties through second metal integration (“irreversible” modifications, i.e., static) and photochromic linker installation allowing alternation of an electronic profile as a function of an excitation wavelength (i.e., dynamic).…”