Mixed Valency as a Strategy for Achieving Charge Delocalization in

Nov 10, 2017 - further illustrated by the electronic properties; Berlin green exhibits a slight ..... In 2011, Robson et al. synthesized a series of 3...
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Mixed Valency as a Strategy for Achieving Charge Delocalization in Semiconducting and Conducting Framework Materials Ryuichi Murase, Chanel F. Leong, and Deanna M. D’Alessandro*

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School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia ABSTRACT: The fundamentally important phenomenon of mixed valency has been discussed in detail over the past 50 years, predominantly in the context of dinuclear complexes, which are used as model systems for understanding electron delocalization in more complex biological and physical systems. Very recently, mixed valency has been shown to be an important mechanism for charge transfer, leading to delocalization and conductivity in two- and threedimensional framework materials such as metal−organic frameworks and related systems including covalent organic frameworks and semicrystalline semiconducting metal−organic graphenes. This Viewpoint provides a current perspective on the field of mixed-valence frameworks, where the property is either intrinsic or generated postsynthetically via an external stimulus. Aspects of the spectroscopy and applications of these materials are also discussed, highlighting the future potential for exploiting mixed valency in extended solid-state systems.

1. INTRODUCTION Mixed-valence compounds comprise two or more electroactive moieties that exist in more than one formal oxidation state. Mixed valency often results in electronic coupling between the sites with different formal charges, leading to the rapid oscillation of charge between them, a process known as intervalence charge transfer (IVCT).1 In the simplest case of coupling between two sites (A and B), the degree of charge delocalization is often described by the Robin and Day classification scheme: (1) in class I, materials contain electrons localized at sites A and B; (2) in class II, IVCT between A and B is accessible given a small optical or thermal activation energy; (3) in class III, there is full delocalization and IVCT corresponds to a transition within the molecular orbital manifold.2 A consequence of the IVCT process is a characteristic absorption band, often observed in the infrared (IR), mid-IR-to-near-infrared (NIR), or visible regions of the optical spectrum.3,4 This band often accounts for the intense coloration of mixed-valence compounds, including many naturally occurring gemstones and minerals. For example, IVCT gives minerals such as the iron(II,III)-based vivianite as well as the historically significant pigment Prussian blue their color, both of which can be considered as Robin and Day class II materials.1 In three-dimensional (3D) materials such as Prussian blue, mixed valency has been shown to play an important role in affording long-range network semiconductivity by virtue of through-bond electron-transport pathways.5−8 Moving beyond discrete systems and inorganic materials, the incorporation of mixed valency into porous metal−organic frameworks (MOFs) offers promise for the development of functional, modular platforms. Through the judicious choice of redox-non-innocent metal ions and organic ligands, as well as by exploitation of the porosity of MOFs via host−guest chemistry and postsynthetic modification, new avenues for emergent properties are available.9 This opens up the prospect of applications such as tunable conductors, optoelectronics, © 2017 American Chemical Society

electrochromic devices, and chemiresistive sensors, among others. The area of mixed-valence MOFs has already demonstrated immense promise as a strategy to promote conductivity in typically insulating porous materials.6 Despite these efforts, fundamental insights into the electronic structures of these network materials and theoretical analyses of their mixed-valence character using Marcus−Hush theory, which is widely used for discrete mixed-valence systems, remain largely unexplored.10 Both through-bond and through-space mixedvalence interactions9,11 can be envisaged in framework materials, and indeed research into conductive frameworks, which engender mixed valency, is gaining momentum. This Viewpoint discusses the emergent property of semiconductivity and conductivity in MOFs arising from mixed valency between proximal, interacting redox-active centers. Key structure−function relationships giving rise to mixed valency, IVCT, and semiconductivity in these materials will be explored. Mixed valency can be an intrinsic property of the framework material or can be induced by external stimuli (often a chemical or electrochemical stimulus), which are used to switch the redox states. Also included in this perspective are examples of twodimensional (2D) and 3D coordination polymers, which may not necessarily engender permanent porosity (a requirement of MOFs as defined by IUPAC) or were not probed for their adsorption capacities as the original aim.12 The scope encompasses mixed valency in class II and III MOFs, and thus frameworks incorporating non-interacting electroactive moieties, one-dimensional (1D) mixed-valence chains,13 covalent organic frameworks (COFs)14 and polyoxometallates15 will not be covered in depth, although relevant examples will be presented throughout. Variability in conductivity measurement techniques Received: August 23, 2017 Published: November 10, 2017 14373

DOI: 10.1021/acs.inorgchem.7b02090 Inorg. Chem. 2017, 56, 14373−14382

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Inorganic Chemistry

topology.36 Interestingly, phase I of this compound is mixedvalent because of the existence of both TCNQ0 and TCNQ− species in the de novo compound, as confirmed by IR and crystallographic analysis. Conductivity measurements found that phase I was 1000-fold more conductive (2.5 × 10−1 S cm−1) than phase II (1.3 × 10−5 S cm−1) because of the delocalized stacks of TCNQ in phase I, offering another mode of electron transfer as well as through the metal−ligand backbone. These related polymorphs are electrochemically interchangeable, making them applicable as bistable switches.39 Frameworks constructed from DCNQI display some of the highest conductivities known at the present time in framework chemistry. The framework Cu(2,5-DMDCNQI) (2,5DMDCNQI = 2,5-dimethyl-N,N′-dicyanoquinonediimine) exhibits metal-like behavior, whereby the single-crystal conductivity increases from 800 to 5 × 105 S cm−1 upon cooling from 295 to 3.5 K.31 Further spectroscopic studies revealed that the highly delocalized nature of this material and the high conductivity could be attributed to the coexistence of CuI/II and 2,5DMDCNQI0/− in the framework.46 In 2000, a ruthenium-paddlewheel-based framework, [{Ru2(O2CCF3)4}2(μ-TCNQ)]·3C7H8, comprised of mixedvalence RuII/III dimers that were pillared axially by two TCNQ ligands to form a 2D sheet, was reported by Dunbar et al.38 Although the conducting properties of this framework were not reported, a crystallographic and spectroscopic analysis revealed that the partial charge state (ρ) of TCNQ was −0.42. While the electron-rich Ru ions would provide favorable π−d overlap toward TCNQ, it was also envisaged that the ionization potential of the diruthenium tetraacetate paddlewheel units could be altered by varying the spectator ligands. Following this work, a 3D framework utilizing a similar secondary building unit was reported with a modified TCNQ ligand. 47 [{Ru 2 (mCH3PhCO2)4}2(BTDA-TCNQ)]·3.4CH2Cl2·1.6(p-Cltoluene) (m-CH3PhCO2 = m-methylbenzoate; BTDA-TCNQ = bis(1,2,5-thiadiazolo)tetracyanoquinodimethane; p-Cltoluene = pchlorotoluene), in comparison to the aforementioned TCNQ and DCNQI frameworks, exhibited lower conductivity (8.0 × 10−5 S cm−1 at 300 K), possibly because BTDA-TCNQ exists in the dianion state. However, upon cooling, a sudden increase in the conductivity to ca. 1.3 × 10−3 S cm−1 was observed, which was tentatively attributed to the closing of the electronic band gap via charge fluctuation between the ruthenium-based dinuclear nodes and BTDA-TCNQ2− ligands. While 1D frameworks lie outside the scope of this Viewpoint, the covalently linked transition-metal chain-type materials, often referred to as “MX-chains”, are a fundamentally interesting and important example of mixed-valence materials.13,48,49 These 1D chains vary in their architecture from simple single chains to quadruple-stranded building blocks. One of the more complex examples is the four-legged PtII/IV compound reported by Kitagawa et al.,49,50 [Pt(en)(bpy)]4(NO3)8·5H2O (en = ethylenediamine; bpy = 4,4′-bipyridine), which possesses a squaregrid-type structure with alternating PtII−I−−PtIV chains that are bridged by bpy ligands. In this case, individual [Pt(en)(bpy)4]8+ chains are electronically delocalized, and because of the irregular electronic states that exist within the platinate−iodide chains, an unusual charge-density-wave (CDW) phenomenon was observed. Two IVCT bands were observed in the single-crystalpolarized optical spectrum with an optical band gap of 0.76 eV. The valence configuration in this compound was found to be strongly correlated to its optical spectrum, which revealed that

and computational methods is also outside the scope but warrants more consistency in the field.16,17

2. RESULTS AND DISCUSSION 2.1. Intrinsically Mixed-Valence Frameworks. Prior to the origins of coordination polymers and MOFs, the first synthetic mixed-valence framework material, Prussian blue, was discovered in the 1700s.18 Although this material has existed invaluably in the hands of artisans and in industry as a pigment for over a century, its structure and chemical importance has only more recently been elucidated.19 Moreover, Prussian blue analogues have gained commercial interest in electrochromic devices20 and host−guest chemistry21,22 among others. Within the scope of this Viewpoint, Prussian blue and its congeners are archetypal examples of mixed-valence systems that help to illustrate how mixed valency is manifested in the physical and spectroscopic properties of materials. The crystal structure of Prussian blue consists of iron(II) ferricyanides octahedrally linked by Fe I I I units to form the 3D framework FeIII4[FeII(CN)]3.23 The material is classified as a class II mixed-valence compound in accordance with the Robin and Day scheme.24 The electronic absorption spectrum of Prussian blue possesses a band in the low-energy visible region, which is attributed to an IVCT transition between FeII and FeIII units. Mixed valency in this compound has been linked to its interesting electronic and optical properties. For example, a spectroelectrochemical (SEC) investigation has revealed that the in situ oxidation of Prussian blue to FeIII4[FeIII(CN)]3 (Berlin green) results in a red shift of the metal-to-metal charge-transfer band. Meanwhile, the reduction product FeII4[FeII(CN)]3 (Everitt’s salt) undergoes the loss of the characteristic IVCT transition.25,26 The physical changes in the redox states of Prussian blue are further illustrated by the electronic properties; Berlin green exhibits a slight increase in the conductivity from Prussian blue due to better energy matching between high- and low-spin FeIII ions, while Everitt’s salt is an insulator.27,28 Although the electronic properties of Prussian blue may not rival those of metals or organic charge-transfer complexes, the substitution of iron for heavier transition metals with more diffuse orbitals (e.g., Ru, Os) yields a more delocalized system.29 Experimental studies have revealed that the compound K1.2Ru3.6[Ru(CN)6]3·16H2O showed a considerably higher electrical conductivity (5.69 × 10−3 S cm−1) than that of the parent compound because of increased framework delocalization.5 Structure−property relationship studies on Prussian blue analogues have aided in understanding the importance and interplay of metal-based mixed valency. However, the closed-shell nature of the cyanido ligand and the localized valence electrons of the metals prohibit full electron delocalization because charge transfer is governed by charge hopping between sites that are relatively charge localized. Another strategy used to induce long-range delocalization is the incorporation of donor and/or acceptor moieties within a framework backbone. A fundamental and applied perspective of donor−acceptor frameworks is described in an excellent review by Miyasaka.30 A series of frameworks that fall into this category are those derived from polycyanido-bridging ligands. A number of frameworks synthesized from N,N′-dicyanoquinonediimine (DCNQI) 31−35 and 7,7,8,8-tetracyanoquinodimethane (TCNQ)36−45 analogues display ligand-based mixed valency because of their redox-active properties. Of notable mention is the framework Cu(TCNQ), which possesses two structural phases: phase I consisting of columnar π-stacking TCNQs bridged by CuI ions and phase II, a framework with the PtS 14374

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Inorganic Chemistry

Figure 1. (a) Redox states of dhbq. (b) Crystal structure of (NEt4)2[Fe2(can)3] framework. (c) Diffuse-reflectance spectra of (NEt4)2[Fe2(can)3] (black), (NEt4)2[Zn2(can)3] (blue), and chloranilic acid (green). The inset shows the Gaussian fit of the NIR IVCT band of (NEt4)2[Fe2(can)3]. Adapted with permission from ref 70. Copyright 2017 American Chemical Society.

thylpiperidine dithiocarbamate).57 While a detailed study of the electronic and spectroscopic properties was not undertaken, future work to explore the mechanisms of charge transfer as well as structure−electronic property relationships in this and related frameworks would be valuable. An underutilized strategy in the design of conductive framework materials is the use of bridging ligands that facilitate through-bond electron transfer via either a superexchange or redox-mediated mechanism. In the former case, oxalate-based frameworks form strong π−d orbital interactions through their bis-chelating mode of coordination to metals.58−60 Additionally, counterions employed to template the self-assembly of 2D and 3D structures can also be used to affect the properties of the framework. The mixed-valence honeycomb frameworks of the form A[FeIIFeIII(C2O4)3] [A− = N(n-C3H7)4, N(n-C4H9)4, N(nC5H11)4, P(n-C4H9)4, P(C6H5)4, N(n-C4H9)3(C6H5CH2), (C6H5)3PNP(C6H5)3, As(C6H5)4] have been found to possess counteranion-dependent magnetic properties.59 Spectroscopic and electronic studies were not reported; however, it could be speculated that these frameworks possess interesting intraframework and intersheet interactions. A framework templated by the molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), [BEDT-TTF]3[MnCr(C2O4)3], was found to possess nearmetallic conductivity (250 S cm−1).61 Although the framework was not in a mixed-valence form, the BEDT-TTF molecules existed in a partially oxidized state (with a charge of +0.34), which was responsible for the high conductivity. For framework materials, instigating electronic communication between distant metal nodes requires linkages by ligands that possess highest occupied molecular orbitals close in energy to the lowest unoccupied molecular orbital of the metal ion. A series of MOFs exemplifying such interactions are those derived from the anilate family of ligands of the form (C6O4X2)2− (X = H, F, Cl). These are a reemerging class of compounds that display intrinsic mixed valency, particularly when incorporated with Fe. As shown in Figure 1a, the parent deprotonated ligand 2,5dihydroxy-1,4-benzoquinone (dhbq2−) possesses multiple oxidation states.62,63 Frameworks can be synthesized by the incorporation of the air-stable dianion species or the fully reduced tetraoxolene form, although to the best of our knowledge, only one example has been reported that incorporates the tetranion.64

both in-phase and out-of-phase CDWs were present for [Pt(en)(bpy)]4(NO3)8·5H2O. In more recent developments, Dincă and co-workers demonstrated the importance of mixed valency in achieving electrical conductivity.51 Within four isostructural series of MOFs with formulas [M2(DOBDC)(DMF)2] (M = MgII, MnII, FeII, CoII, NiII, CuII, ZnII; H4DOBDC = 2,5-dihydroxybenzene1,4-dicarboxylic acid; DMF = N,N′-dimethylformamide), [M2(DSBDC)(DMF)2] (M = MnII, FeII; H4DSBDC = 2,5disulfhydrylbenzene-1,4-dicarboxylic acid), [M2Cl2(BTDD)(DMF)2] (M = MnII, FeII, CoII, NiII; H2BTDD = bis(1H-1,2,3triazolo[4,5-b][4′,5′-i]dibenzo[1,4]dioxin), and [M(1,2,3-triazolate)2] (M = MgII, MnII, FeII, CoII, CuII, ZnII, CdII), the iron analogues were found to exhibit significantly higher conductivities and lower activation energies in comparison to the other first-row transition metals. The observed discrepancy was ascribed to the higher charge mobility afforded by mixed-valence FeII/III versus other transition metals that possess lower charge densities. Cu-based frameworks often generate intrinsically mixedvalence species because of the ease of accessibility of the CuI/II/III oxidation states, and a number of semiconducting 2D and 3D frameworks have been reported.15,52−54 Similar instances of metal redox transformations have been observed in a framework reported by Kitagawa and co-workers.55 CuII[CuII(pdt)2] (pdt = pyrazine-2,3-dithiolate) displays charge bistability, where CuII ions can disproportionate to CuI[CuIII(pdt)2], resulting in IVCT. This compound showed semiconducting behavior (6 × 10−4 S cm−1 at 300 K), which was attributed to the bistability of Cu. Kuroda-Sowa and co-workers reported the framework [CuI4CuII2Br4(Pyr-dtc)4]·CHCl3n (Pyr-dtc− = pyrrolidine dithiocarbamate), consisting of CuIBr chains bridged by CuII(Pyrdtc)2 units to yield a three-dimensional open framework.56 The absorption spectrum of this framework exhibited a broad IVCT band in the vis−NIR range with an estimated optical band gap of 1.01 eV. Furthermore, investigation of the electronic properties revealed a room temperature conductivity of 5.2 × 10−7 S cm−1 by impedance spectroscopy and a charge-carrier mobility of ∼0.4 cm2 V−1 s−1 via flash-photolysis time-resolved microwave conductivity measurements. From the same group, similar Cu mixed valency was observed for the 2D framework [Cu3ICuIIBr3(3,5-Dmpip-dtc)2] (3,5-Dmpip-dtc− = 3,5-dime14375

DOI: 10.1021/acs.inorgchem.7b02090 Inorg. Chem. 2017, 56, 14373−14382

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Inorganic Chemistry In 2011, Robson et al. synthesized a series of 3D frameworks of the general formula (NBu4)2[MII2(dhbq)3] (MII = Mn, Fe, Co, Ni, Zn, Cd) with (10,3)-a topology.65 Subsequently, the iron derivative was investigated by Darago et al. because of its unique spectroscopic character.66 (NBu4)2[Fe2(dhbq)3] was elucidated to be in a mixed-valence state, whereby dhbq was found to reside in its dianion and trianion forms. As a consequence of the ligandbased mixed valency, the electronic spectrum of this compound displayed an IVCT transition in the low-energy region (νmax = 7000 cm−1) characteristic of a Robin and Day class II/III compound, the first for 3D frameworks. This, together with the high conductivity (0.16 S cm−1), was indicative of significant framework delocalization. Furthermore, it was suggested that the mechanism of charge transport was via electron hopping between the dhbq2−/3− moieties. Following in this work, a number of 2D honeycomb-type (6,3) networks have been reported, all of which were found to feature ligand-based mixed valency.67−70 Notably, (6,3) frameworks of dhbq analogues synthesized in combination with iron are significantly more conductive MOFs, which signifies the importance of d−π orbital matching toward optimizing electronic and optical properties. For example, the framework of (Me2NH2)2[Fe2L3]·2H2O· 6DMF (LH2 = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) with (6,3) topology also exhibits ligand-based mixed valency.69 While this framework possesses a topology different from that of the aforementioned (10,3)-a framework, the room temperature conductivity of this framework (1.4(7) × 10−2 S cm−1) is within an order of magnitude difference. This may suggest that topological influences on framework delocalization may be of less significance to the design of conductive frameworks than local orbital matching considerations. In addition to their mixed-valence character, the virtue of tunability and porosity of quinone-based frameworks makes them ideal candidates for batteries and gas sorption applications. Indeed, the number of studies focusing on mixed valency in these frameworks are, to date, still limited. However, the structural diversity and tunability of functional groups in these frameworks may provide insight into the key factors that govern the properties that arise from mixed valency. A unique subset of MOFs are those materials described as nanosheets or semiconducting metal−organic graphenes (sMOGs).71−79 These semicrystalline 2D nanosheets are characterized by their highly delocalized, π-conjugated structure, and they possess some of the highest conductivities for framework materials known at the present time. In addition, a number of these frameworks exhibit porosity that has been exploited in applications such as capacitors. Examples of these systems include compounds of the type [M3(BHT)2] (where M = Ni and Cu and BHT = benzenehexathiol), which have received significant attention because of their metallic behavior.72,77 Nanosheets of [Ni3(BHT)2] were found to exhibit a conductivity of 0.15 S cm−1. This was attributed to the mixed valency of nickel bis(dithiolene) units existing in the 0 or −1 states in a 26:74 ratio, respectively, with a ligand-based radical residing on the S. Additionally, the Cu variant reported by Huang and co-workers displayed one of the highest conductivities recorded to date, with a room temperature conductivity of 1.36 × 103 S cm−1 and an activation of energy of just 2.06 meV, although the oxidation state of the Cu and ligand were ambiguous. Computational studies of BHT nanosheets have also been undertaken to further understand the electronic properties of these materials80 as well as their potential application as oxygen reduction catalysts.76

Frameworks similar in structure to the semicrystalline graphitic-type frameworks are compounds synthesized using the π-extended 2,3,6,7,10,11-hexaiminotriphenylene (HITP) ligand. Nanosheets of [Ni3(HITP)2] were found to exhibit properties similar to those of the aforementioned frameworks, such as high conductivity (2 S cm−1) and a NIR transition in the absorption spectrum but also microporosity due the extended linker.74 Electrochemical studies revealed a purely capacitative response upon cycling cathodically between 0.02 and −0.6 V, creating a window of opportunity for this system to be investigated toward energy storage.79 Moreover, the excellent electronic properties of [Ni3(HITP)2] allowed for the pelletized compound to act solely as the active electrode material without the need for conductive or emulsion additives. The gravimetric capacitance of this material was 111 F g−1 with only 10% loss of this capacitance over 10000 cycles. Although these materials display outstanding electronic and host−guest properties, the mixed valency is, as yet, poorly understood. Thus, characterization methods such as electrochemistry and spectroelectrochemistry, along with computational studies, may yield more comprehensive insight into the mechanism of electron transfer. While the majority of the frameworks discussed thus far have been derived from various metal−ligand combinations, COFs are a class of semicrystalline materials exclusively composed of organic molecules that also have the potential to exhibit mixedvalence behavior.81,82 Mixed valency in COFs generally stems from the π−π interactions between aromatic components to generate partial charge-transfer states. While examples in the literature remain scarce, the TTF-COF reported by Yaghi and co-workers exemplifies such interactions in a COF.14 Tetrathiafulvalene moieties in the de novo framework were found to exist in the mixed-valence state, as confirmed by electron paramagnetic resonance (EPR) spectroscopy, and a broad NIR band was observed in the electronic spectrum, diagnostic of intermolecular charge transfer. The TTF-COF was a semiconductor (1.2 × 10−6 S cm−1), however, upon I2 doping of this material, the conductivity increased to 2.8 × 10−3 S cm−1 because of optimization of the degree of charge transfer in the TTF stacks. Similar COFs with induced mixed valency have also been reported such as the 2D [(HOTP)2(TDB)3] framework (TCOF 1; HOTP = 2,3,6,7,10,11-hexaoxytriphenylene and TDB = 2,5-thiophenediboronate), which can be intercalated with TCNQ,83 or the triarylamine-based COFs, where generation of the mixed-valence radical amine was studied via in situ SEC methods.84 2.2. Mixed Valency via Postsynthetic Modification. Intrinsically mixed-valence MOFs are relatively rare because a key prerequisite is that their constituent redox-active ligands and/or metals synergistically facilitate redox isomerism. In this regard, postsynthetic modifications via chemical or electrochemical methods offer a convenient approach to generating mixed valency. Installing charge carriers to increase electron or hole mobility in intrinsically insulating or semiconducting MOFs is a proven strategy to alter the electronic and optical properties to bring about long-range conductivity. Pioneering work on this postsynthetic modification strategy was demonstrated by Allendorf and co-workers, who intercalated TCNQ into [Cu3(BTC)2] (HKUST-1; BTC3− = benzene-1,3,5tricarboxylate), leading to a 6-fold increase in its conductivity (to 0.07 S cm−1).10,85 UV−vis studies of TCNQ@Cu3(BTC)2 revealed significant charge-transfer interactions between Cu and TCNQ molecules, which were manifested as a new absorption band in the NIR region. Additionally, the degree of 14376

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Figure 2. (a) Single network of the as-synthesized [Fe(dca)TTF(py)4]·ClO4·CH2Cl2·2CH3OH (above) and the I2 intercalated [Fe(dca)TTF(py)4]· 0.5I3·ClO4·CH2Cl2·CH3OH·C6H12 (below). The insets show photographs of corresponding crystals. (b) Solid-state spectroelectrochemistry of [Fe(dca)TTF(py)4]·ClO4·CH2Cl2·2CH3OH. Adapted with permission from ref 96. Copyright 2017 John Wiley and Sons.

partial charge transfer to TCNQ (0.3−0.4e−) was elucidated from the IR and Raman spectra, indicating charge delocalization within the framework backbone. Computational work on TCNQ@Cu3(BTC)2 provided evidence for the preferred TCNQ orientation, revealing the formation of a new conductance path as well as categorizing this material as a Robin and Day class III compound.24 Sholl and colleagues subsequently extended the study to investigate TCNQ infiltration into other Cu frameworks via computational methods.86 Further fundamental and experimental studies of HKUST-1 and its structural analogues have since been reported in an attempt to improve the electronic properties.87−89 An ntype semiconducting material has also been generated in a 2D metalloporphyrin framework via the intercalation of TCNQ.90 The as-synthesized framework [Cu2(OAc)4(CuTPyP)1/2]· CHCl3 (TPyP2− = 5,10,15,20-tetra-4-pyridyl-21H,23H-porphine) was found to be insulating (∼10−9 S cm−1), however, upon exposure to TCNQ, its conductivity increased to 1 × 10−6 S cm−1. The formation of a new absorption band in the NIR region was consistent with a charge-transfer complex between TCNQ and the metalloporphyrin. Guest intercalation as a strategy to induce mixed valency has been exploited in a number of frameworks. The heterometallic framework Cu[Ni(pdt) 2 ], which is isostructural to the aforementioned framework Cu[Cu(pdt)2],55 can be partially oxidized with iodine to yield a p-type semiconductor with a 4fold increase in the conductivity (1 × 10−4 S cm−1).91 Metalbased redox transformations have also been observed in the 3D framework (H3O)[CuI2(CN)(TTB)0.5]·1.5H2O (H4TTB = 1,2,4,5-tetrakis(2H-tetrazole-5-yl)benzene).77 Interestingly, a portion of CuI in this framework can be converted to CuII to generate the mixed-valence form via autooxidation or by heating of the as-synthesized species. The oxidation is accompanied by a drastic change in the absorption spectrum, whereby the oxidized form exhibits a broad band in the vis−NIR region. The redox properties of this framework were further exploited for heterogeneous catalysis, whereby the oxidized mixed-valence CuI/II framework was found to play a crucial role in aromatic C− H bond activation. Similarly to this work, redox tautomerisation has been observed in [Fe3O(H2O)2F0.81(OH)0.19{C6H3(CO2)3}2]·nH2O (MIL-100(Fe)).92 Partial reduction of FeIII to FeII was achieved by thermal activation of the compound, leaving bare-metal sites in the framework. Host−guest studies revealed that the increased Lewis

acidity of the FeII sites was responsible for selective adsorption of unsaturated gas molecules. Mixed valency has been invoked, but not explicitly discussed, on a number of occasions because the origin of conductivity increases in frameworks upon doping. For example, iodine doping has been used in TTF-based MOFs to modify or induce guest-emergent properties.93−97 The framework [Fe(dca)TTF(py)4]·ClO4·CH2Cl2·2CH3OH (dca = dicyanamide; TTF(py)4 = tetra(4-pyridyl)tetrathiafulvalene) undergoes a single-crystalto-single-crystal transformation upon iodine intercalation to generate [Fe(dca)TTF(py)4]·0.5I 3·ClO4 ·CH 2 Cl2 ·CH 3 OH· C6H12, as shown in Figure 2a.95 The intercalated framework displayed increased conductivity as well as altered spin-crossover properties, which were tentatively ascribed to the partial oxidation of TTF cores to the radical cation state, as indicated by solid-state spectroelectrochemistry (Figure 2b), X-ray photoelectron microscopy, and EPR studies. An important caveat here is that the formation of polyiodide chains can also occur, such that the observed conductivity enhancement may not be reflective of the framework itself. The mixed valency of the TTF cores has also been demonstrated to play a role in controlling the optoelectronic properties in the series of frameworks reported by Chen et al.98 The 3D structure [Cd2L(bpp)2(H2O)(C2O4)0.5]·ClO4·H2O (L = (dimethylthio)tetrathiafulvalenebicarboxylate; bpp = 1,3bis(4-pyridyl)propane) within this series was of particular interest because of the cofacially stacked TTF dimers in an ideal π−π stacking arrangement confined within the square cavities. The TTF dimers were found to exist in a partially charged state through EPR and optical studies. Photoelectrochemical studies of this framework revealed enhanced photocurrent generation, which was attributed to the more efficient photoinduced charge separation. A series of TCNQ coordination polymers with the formula [M(TCNQ)(bpy)] (M = Fe, Zn, Mn, Co, Cd) were studied for their guest-dependent spectroscopic and electronic properties.99,100 Upon immersion of the Fe analogue of this framework in nitrobenzene, the compound displayed a broad absorption spectrum with a low-energy transition centered at ∼850 nm, which was attributed to an IVCT band. IR spectroscopic studies also indicated the presence of reduced TCNQ species. Within the isostructural series, the Fe analogue was found to exhibit the highest conductivity of 1.7 × 10−8 S cm−1, while the Zn species was an insulator (