Through-Space Intervalence Charge Transfer as a Mechanism for

May 4, 2018 - Computational modeling of the optical data combined with application of Marcus–Hush theory to the IVCT bands for the mixed-valence ...
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Article Cite This: J. Am. Chem. Soc. 2018, 140, 6622−6630

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Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal−Organic Frameworks Carol Hua,† Patrick W. Doheny,† Bowen Ding,† Bun Chan,‡ Michelle Yu,† Cameron J. Kepert,† and Deanna M. D’Alessandro*,† †

School of Chemistry, The University of Sydney, New South Wales 2006, Australia Graduate School of Engineering, Nagasaki University, Nagasaki 852-8521, Japan

‡

S Supporting Information *

ABSTRACT: Understanding the nature of charge transfer mechanisms in 3-dimensional metal−organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn(II) frameworks containing cofacial thiazolo[5,4-d]thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus− Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both in situ and ex situ electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where throughspace mixed-valence interactions are operative.

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transfer.3−5 This interest has been driven, in part, by potential applications of these systems in technologically and industrially useful devices for applications in energy storage and conversion, including electrochromic devices,2,6−8 electrocatalysts,9−11 and battery materials.12−14 MOFs also provide a unique platform upon which fundamental electron transfer phenomena can be interrogated in 3-dimensional coordination space owing to their ordered, crystalline structures, in which the relative spatial orientation and location of the multiple metal and organic units are unambiguously defined. One means of interrogating the mechanisms of charge transfer in MOFs, with a view to exert greater control over their conductive properties, is through the exploitation of mixed valency. This is a widespread phenomenon in a plethora of physical, chemical, and biological systems ranging from gemstones and photosynthetic reaction centers in nature,15−17 to materials science applications such as molecular electronics for quantum computing.18,19 The common feature of mixed-

INTRODUCTION Metal−organic frameworks (MOFs) are a class of functional materials derived from the reaction of metal nodes with organic linkers to form supramolecular assemblies that exhibit features such as nanoscale porosity, large internal surface areas, welldefined cavities, and voids throughout their structures.1 Although MOFs are well-known for their adsorptive properties, the incorporation of redox-active metal ions or organic ligands can impart additional functionality via the chemical and structural changes associated with redox transformations.2 The synthesis of electroactive MOFs is often impeded by the intrinsically insulating nature of the organic components and the closed-shell nature of the metal ions (i.e., Zn2+, Zr4+) commonly used in the formation of such materials. An additional challenge is the inherent instability of MOFs to oxidation or reduction, where structural changes due to redox manipulation of either the metal or organic components can result in irreversible decomposition and loss of structure. Despite this, important strides in the experimental studies of electroactive and conductive MOF materials have driven a surge of interest in understanding their mechanisms of charge © 2018 American Chemical Society

Received: March 12, 2018 Published: May 4, 2018 6622

DOI: 10.1021/jacs.8b02638 J. Am. Chem. Soc. 2018, 140, 6622−6630

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Journal of the American Chemical Society

strong electronic coupling between the redox centers. While there has been extensive investigation of electron transfer in short-range systems, such as discrete complexes, there has been a distinct lack of exploration of longer-range and polymeric systems, such as MOFs. This can be attributed to the difficulties in obtaining experimental data for the often unstable intermediate mixed-valence states of solid-state materials. It has only been recently that in situ solid-state UV/vis/NIR spectroelectrochemical techniques have been developed that allow spectral data of the mixed-valence state of a bulk sample to be obtained.2,39 In addition, quantifying experimental parameters for theoretical analyses of IVCT from the electromagnetic spectra of mixed-valence species have, to date, proven problematic due to difficulties in quantifying the molar extinction coefficient from the electronic spectra of solid materials. Herein, we describe a new Zn(II) framework, [Zn2(BPPTzTz)2(tdc)2]n, denoted 1 (where BPPTzTz = 2,5bis(4-(pyridine-4-yl)phenyl)thiazolo[5,4-d]thiazole), containing cofacial thiazolo[5,4-d]thiazole units that exhibit the first example of the through-space IVCT phenomenon in a framework material. The origin of the mixed-valence interaction is elucidated computationally using density functional theory (DFT) methods, where the close proximity of the electroactive units, facilitated by aromatic stacking interactions, is crucial. Although alluded to in bacterial photosynthetic15,16,35 and discrete molecular systems,32−34 this mechanism has not previously been observed or characterized in framework materials, although aromatic stacking interactions and their associated structural effects have been previously demonstrated to facilitate intermetallic through-bond IVCT in mixed-valence Fe/Co tetranuclear complexes.40 Important considerations for the theoretical analysis of mixed-valence systems in the solid state are addressed, particularly determination of the extinction coefficient (ε) for the IVCT band of the mixed-valence species using Kubelka−Munk analysis, which enables application of Marcus−Hush theory. This connection between experiment and theory represents a significant advance in quantifying charge transfer interactions in MOFs. The combined experimental and computational approach is also used to probe the distance dependence of the through-space mixed valency, a prediction of Marcus−Hush theory,21 using the closely related framework [Zn4(BDPPTzTz)2(tdc)2]n (2) (where BDPPTzTz = 2,5-bis(4-(pyridine-4-yl)-3,5-dimethylphenyl)thiazolo[5,4-d]thiazole) in which the close approach of the electroactive ligands is precluded, in addition to computational simulations of the electronic spectra. This work provides a new window into charge-delocalized transfer mechanisms in 3-dimensional materials.

valence systems is the presence of either metal or organic moieties with different formal oxidation states, giving rise to the rapid oscillation of charge between the centers and, correspondingly, the absorption of light in the electromagnetic spectrum (which explains the highly colored nature of these compounds). Marcus−Hush theory has been widely employed as a cornerstone of analysis for these systems, providing insight into both through-bond (inner sphere) and through-space (outer sphere) electron transfer through the pioneering experimental and theoretical work of Taube, Marcus, and Hush, among many others.20−24 To date, mixed valency has been characterized in frameworks that exhibit a through-bond intervalence charge transfer (IVCT) mechanism which has been invoked as the origin of electroactivity and conductivity in these materials. For example, mixed valency is the origin of the through-bond charge hopping mechanism in the historically important pigment Prussian blue (Fe4III[FeII(CN)6]3·xH2O) whose deep color and intrinsic semiconductivity arise from IVCT between the Fe(II) and Fe(III) nodes mediated by cyanido ligands.25,26 In contrast to the prototypical metal-to-metal IVCT of Prussian blue, Long and co-workers recently reported an Fe(III) framework that exhibits the first example of purely ligand-based IVCT originating from the through-bond interplay between multiple redox states of the 2,5-dihydroxybenzoquinone (dhbq) ligands.27,28 This material, [(NBu4)2FeIII2(dhbq)3] (where NBu4 = tetrabutylammonium and dhbq = dihydroxybenzoquinone), is the most highly conductive three-dimensional MOF known at the present time and exhibits ligand-based mixed valency which manifests in an IVCT band due to charge transfer between dhbq2− and dhbq3−. Other examples of through-bond mixed valency in frameworks with (6,3) net-type topologies include Harris’ 2D iron 2,5-dichloro-3,6-dihydroxo1,4-benzoquinone-based material29,30 and Abrahams and Robson’s (NEt4)2[Fe2(can)3] (where can is the dianion of chloranilic acid), where a strong near-infrared (NIR) IVCT band was observed in the latter case.29,31 By comparison, through-space IVCT behavior is extremely rare in both discrete and extended coordination materials, with the exception of the elegant rhenium-based discrete molecular rectangles reported by Dinolfo and Hupp.32−34 Through-space mixed valency is also well-known in biological systems such as the mixed-valence radical cation or “special pair” in bacterial photosynthesis.15,16,35 The ability to engineer 3D materials which display such through-space electronic delocalization is significant from both a practical and theoretical perspective, the former being a versatile platform from which to fabricate materials that exploit mixed valency to achieve both conductivity and multifunctionality. From a theoretical perspective, the presence of mixed valency leading to IVCT should offer new insights into fundamental aspects of charge transfer in 3D systems. Marcus−Hush theory has been widely applied to quantify the degree of electron delocalization in biological, mineralogical, and synthetic materials.36,37 For mixed-valence systems, the degree of electron delocalization can be classified into three main classes using the Robin and Day classification scheme.38 Class I compounds involve redox centers that do not interact with each other or where the interaction is too small to be detected; Class II compounds have weakly interacting redox centers, where the charge is primarily localized on one of the redox centers; and Class III describes systems where the charge is completely delocalized over the entire system due to the

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RESULTS AND DISCUSSION Synthesis and Structure. The two new ligands, BPPTzTz and BDPPTzTz, were synthesized via two-step reactions. The first step involved a Suzuki coupling between 4-pyridylboronic acid and either 4-bromobenzaldehyde (BPPTzTz) or 4-bromo3,5-dimethylbenzaldehyde (BDPPTzTz) to yield the pyridyl compound. The intermediate then underwent a condensation reaction with dithiooxamide in DMF to form the thiazolo− thiazole core. Two Zn(II) frameworks, [Zn2(BPPTzTz)2(tdc)2]n (1) and [Zn4(BDPPTzTz)2(tdc)2]n (2), were obtained upon heating the BPPTzTz or BDPPTzTz ligands with 2,5-thiophene dicarboxylic acid (tdc) and Zn(NO3)2·6H2O at 120 °C to 6623

DOI: 10.1021/jacs.8b02638 J. Am. Chem. Soc. 2018, 140, 6622−6630

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Journal of the American Chemical Society

Figure 1. Crystal structure of framework 1 showing the cofacial alignment of the BPPTzTz ligands. (a) View down the c axis, (b) view down the a axis, and (c) the “cofacial” pair of BPPTzTz ligands shown in the box in (a). Only one of the two interpenetrated networks is shown. Atom labeling: Zn = cyan, O = red, N = blue, S = yellow, C = black. The hydrogen atoms are omitted for clarity.

ments (Supporting Information Figure S3). The framework displays a small adsorption of N2 (0.76 mmol/g at 960 mbar), H2 (1.72 mmol/g at 1200 mbar), and CO2 (0.7 mmol/g at 1200 mbar) with a Brunauer−Emmett−Teller (BET) surface area of 53.4 ± 0.2 m2/g (Supporting Information Figure S4). To generate a material in which cofacial interactions between ligands were eliminated, the sterically bulky BDPPTzTz ligand was used to synthesize framework 2 which was solved and refined in the triclinic P-1 space group. The asymmetric unit of the framework consists of two BDPPTzTz ligands and four tdc ligands with four crystallographically distinct Zn(II) centers (Supporting Information Figure S5). In contrast to 1, the oxygen donors from the carboxylates on four tdc ligands bind in a monodentate fashion to two Zn(II) centers to yield a “paddlewheel” unit. Coordination of the tdc ligand with monodentate binding from each of the two carboxylate groups, as opposed to monodentate coordination from the same carboxylate group, results in a distance of 8.93 Å between the two BDPPTzTz ligands. The adjacent BDPPTzTz ligands are rotated with respect to each other indicating the presence of minimal π-stacking interactions. On the axial positions of the “paddlewheel” are nitrogen donors from the pyridyl ring of the BDPPTzTz ligand. The BDPPTzTz ligand has a 90° orientation of its pyridyl rings with respect to the central core. Framework 2 consists of two interpenetrating networks with pores down the c axis (Supporting Information Figure S5). The SQUEEZE41 function in PLATON42 gave an estimated 42% accessible pore spacea value that is significantly greater than that for 1, presumably due to the influence of the steric bulk afforded by the extra methyl groups in the BDPPTzTz ligand. The TGA of 2 showed a mass loss of 15% under 200 °C due to liberation of the DMF solvent from the pores of the framework (Supporting Information Figure S2). It is likely that steric hindrance provided by the methyl groups on the BDPPTzTz ligand prevents the loss of additional solvent from the pores.

yield orange and yellow prismatic crystals, respectively. The synthesis of the frameworks was repeated a number of times and was able to be repeated reliably. The structure of 1 was solved and refined in the orthorhombic space group Pcc2 (Figure 1). The asymmetric unit consists of two BPPTzTz ligands and two tdc coligands. The two Zn(II) centers are coordinated by the carboxylate moiety on the tdc coligand, where the oxygen donors are coordinated in a monodentate manner to each of the Zn(II) centers. Importantly, the tdc coligand holds two π-stacking BPPTzTz ligands in close proximity resulting in a ligand-to-ligand distance of 3.80 Å. The Zn(II) centers exhibit a trigonal bipyramidal coordination with three oxygen donors in the equatorial plane and two nitrogen donors from the pyridyl rings of the BPPTzTz ligand on the axial positions. The biphenyl moiety in the ligand exhibits a slight rotation of approximately 27° from each of the pyridyl rings, such that they are not coplanar. The dinuclear Zn(II) centers in 1 are linked by tdc ligands yielding a sheet-like structure, which is then extended into a 3D cubic α-polonium (pcu) network by the dipyridyl BPPTzTz ligand acting as a pillar and the binuclear units serving as sixconnecting nodes (Figures 1a and b). 1 consists of two interpenetrated networks but still exhibits significant channels when viewed down either the a or c axes, which is partly due to the π-stacking of the aromatic moieties and favorable S···N interactions between adjacent BPPTzTz ligands in the structure (Figure 1c). The pore void space was calculated in SQUEEZE41 to be 30%. The TGA of 1 exhibits a gradual mass loss until 300 °C due to liberation of the DMF in the pores of the framework, before a steep mass loss indicates decomposition of the framework (Supporting Information Figure S2). Gas adsorption studies with N2, H2, and CO2 were performed on the framework after activation (using a supercritical CO2 wash), following which the structural integrity of the material was maintained according to powder X-ray diffraction measure6624

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Figure 2. Solid-state spectroelectrochemistry at room temperature of 1. (a) Vis/NIR at applied potentials of 0 to −1.75 V in [(n-C4H9)4N]PF6/ CH3CN electrolyte. Photographs of the framework during the solid-state UV/vis/NIR spectroelectrochemistry experiment demonstrate the marked color change from yellow (neutral) to dark green (mixed-valence state). (b) X-band EPR at applied potentials of 0 to −2.0 V in [(n-C4H9)4N]PF6/ CH2Cl2 electrolyte (applied modulation amplitude of 2 G) showing the progressive evolution of a radical species.

Electroactive Properties. Solid-state cyclic voltammograms on the BPPTzTz ligand in [(n-C4H9)4N]PF6/CH3CN revealed two processes in the cathodic region and no processes in the anodic region (Supporting Information Figure S6). The processes observed at −1.69 and −2.01 V vs Fc/Fc+ can be assigned to two sequential one-electron reduction processes of the thiazolo−thiazole core in the BPPTzTz ligand, involving a one-electron quasi reversible reduction to the radical anion followed by a further one-electron irreversible reduction to the dianion. Similar processes were observed for BDPPTzTz; however, an additional cathodic process was observed. The peaks at −1.49 and −1.82 V vs Fc/Fc+ were assigned to reduction of the TzTz core, with the peak at −2.40 V vs Fc/Fc+ due to reduction of the pyridyl moieties on the ligand (Supporting Information Figure S7). The reduction of the pyridyl rings for BDPPTzTz within the electrochemical window for acetonitrile may be a consequence of weakly electrondonating methyl groups which lower the potential required for reduction. The electrochemistry of frameworks 1 and 2 is dominated by ligand-based processes (Figures S9−S11). In the anodic region, a broad process at ∼1.0 V vs Fc/Fc+ was observed for both frameworks and can be assigned to oxidation of the thiophene ring in the tdc coligand. In contrast, the presence of the methyl groups in BDPPTzTz resulted in a marked difference in the electrochemical potentials for reduction of the ligand when compared to BPPTzTz. Two broad reductive processes were observed in the solid-state cyclic voltammogram for 1 in the cathodic region at −1.38 and −2.03 V vs Fc/Fc+. These processes can be assigned to the corresponding one- and twoelectron reductions of the BPPTzTz ligand (Supporting Information Figure S8). In contrast, three reductive processes were present for 2, where the weakly electron-donating methyl groups facilitate reduction of the ligand within the framework (Figure S9). The processes observed at −1.15 and −2.03 V vs Fc/Fc+ in 2 are due to the thiazolo−thiazole core, while the process at −2.32 V vs Fc/Fc+ is assigned to reduction of the pyridyl rings within the BDPPTzTz ligand. These reduction processes correspond well to those observed for the ligand itself.

The electrochemistry of 1 was also examined in [(n-C4H9)4N]PF6/CH2Cl2 electrolyte (Supporting Information Figure S10) for use in the solid-state EPR spectroelectrochemical experiment (vide inf ra) due to the lower dielectric constant of dichloromethane compared with acetonitrile. At scan rates of 100 and 500 mV/s, two quasi reversible processes were observed in the cathodic region at −1.26 and −1.40 vs Fc/Fc+. These processes are ascribed to one-electron reduction of the thiazolo−thiazole core to its radical anion. The framework was also found to have good electrochemical properties in 0.1 M KCl/H2O electrolyte, where the material underwent multiple cycles with minimal changes in the CV, demonstrating its structural stability in both aqueous and nonaqueous media (Supporting Information Figure S11). Modulating Redox State. The neutral BPPTzTz and BDPPTzTz ligands displayed one main band in their electronic absorption spectra above 20 000 cm−1 which can be attributed to the π → π* transition of the aromatic phenyl and pyridyl groups in the ligand. As the potential was decreased from 0 to −1.6 V during the solid-state UV/vis/NIR spectroelectrochemical experiment for the BPPTzTz ligand, three bands formed at 14 000, 17 500, and 23 000 cm−1 along with an associated color change from yellow to dark yellow (Supporting Information Figure S12). The bands at ∼14 000 and 17 500 cm−1 can be assigned to the radical anion of the thiazolo−thiazole core. The features in the solid-state UV/vis/NIR spectrum of 1 predominantly arise from absorptions of the BPPTzTz ligand (Supporting Information Figure S12). The bands above 20 000 cm−1 can be assigned to π → π* transitions of the aromatic moieties within the BPPTzTz ligands. Similar to the ligands themselves, 1 displays a lower energy absorption band than 2. Upon changing the potential from 0 to −1.75 V for 1, new bands form at 15 000, 17 000, and 23 500 cm−1, with an associated color change from yellow to dark green (Figure 2a, Supporting Information Figure S13). Unexpectedly, a new broad band appeared in the NIR region of the spectrum, which is absent upon reduction of the ligand itself. The broad band in the NIR region can be deconvoluted to give two Gaussian bands centered at 6576 and 8202 cm−1. As the potential is further changed to −1.85 V, the NIR band decreases, 6625

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Journal of the American Chemical Society supporting its assignment as an intervalence charge transfer (IVCT) band that arises upon formation of a mixed-valence state of the material (Supporting Information Figure S13). Upon returning the potential to 0 V, the framework is able to be “reset” to its neutral state, with a color change from dark green to yellow and a spectrum reminiscent of that for the starting material demonstrating both electrochemical reversibility and structural stability of the material to redox modulation. An in situ solid-state EPR spectroelectrochemical experiment43−45 on 1 (Figure 2b) corroborated the formation of a radical state upon reduction. The properties of 2 in which no cofacial interactions are present were of interest to understand the nature of the ligandto-ligand interactions in 1 which give rise to mixed valency. As the potential was reduced from 0 to −1.4 V in 2, a color change from a pale to dark yellow was observed. This was similar to observations for the BDPPTzTz ligand itself, which darkened from yellow to dark yellow at −2.0 V with increases in intensity of bands at 18 690, 21 730, and 24 290 cm−1. These processes can be assigned to the formation of the radical anion state of the thiazolo−thiazole core and were fully reversible upon returning the potential to 0 V (Supporting Information Figure S14). In a similar manner to the UV/vis/NIR spectroelectrochemical experiment on the BDPPTzTz ligand, reduction of 2 at −1.4 V led to an increase in intensity of the bands at 15 450, 20 880, and 24 000 cm−1 (Supporting Information Figures S16 and S17). The reductive process was spectrally reversible, with the starting spectrum obtained upon returning the applied potential to 0 V. The redox processes in 2 are therefore dominated by those of the ligand. Notably there is the lack of a band in the NIR as observed for 1. This can be explained upon inspection of the solid-state structures of each of the zinc frameworks, where there is an absence of cofacial interactions between the BDPPTzTz ligands in 2 which are located at a distance of 8.93 Å apart, precluding through-space mixed valency and IVCT. Computational Modeling. Single-point TD-DFT calculations using the BMK\6-311G(d) procedure46 on the neutral BPPTzTz ligand revealed a single band at 22 700 cm−1 due to a transition between the HOMO and LUMO+1 orbitals (Supporting Information, Figure S18), which is consistent with the experimental spectrum (23 500 cm−1, Figure 2a) and supports the assignment as a π → π* transition. The same calculation was performed on the cofacial dimer pair of BPPTzTz ligands extracted from the X-ray crystal structure of 1. For the singly reduced dimer, using the atomic coordinates of the neutral material, bands centered at 7319 and 9292 cm−1 were predicted for the SOMO to LUMO+1 and SOMO to LUMO+2 transitions, respectively (Figure 3a, Supporting Information Figure S18). Visual inspection of the molecular orbitals shown in Figure 3a reveals that the lower energy NIR transition occurs from a SOMO almost fully localized on one BPPTzTz molecule of the dimer pair to the LUMO+1 orbital predominantly localized on the adjacent BPPTzTz, i.e., an IVCT transition. The second transition in the NIR arises from the SOMO orbital localized on one BPPTzTz ligand to the LUMO+2 orbital localized mainly on the same ligand, but also with substantial IVCT character. Gaussian deconvolution of the NIR band obtained from the experimental spectrum of the mixed-valence species in Figure 2a indicated two major underlying components at 6576 and 8202 cm−1 (Figure 3b) which are consistent with the predictions from the DFT calculations (Supporting Information Table S4 contains

Figure 3. Comparison of the computationally predicted and experimental IVCT bands in the mixed-valence form of 1. (a) Molecular orbitals involved in the transitions in the NIR region for the cofacial mixed-valence dimer (BPPTzTz0/•−)2 extracted from the crystal structure of 1. (b) The two lowest energy Gaussian components of the NIR band assigned to the mixed-valence form of 1, [Zn2(BPPTzTz0/•−)2(tdc)2]n, obtained upon deconvolution of the experimental spectrum. The curve fit (black line) obtained upon deconvolution (dotted lines) is overlaid with the experimental spectrum (red line).

details of the deconvoluted spectrum). Transitions in the visible region at 19 107 and 19 911 cm−1 were also predicted via computational calculations (vs 15 000 and 17 000 cm−1 determined experimentally) due to the generation of an anionic radical with associated SOMO to LUMO+6 and HOMO to LUMO transitions, respectively. In light of these results, the prominent band observed in the NIR region for the mixed-valence state of framework 1 arises primarily due to IVCT between BPPTzTz in its radical anion state, BPPTzTz•−, and the adjacent, formally neutral BPPTzTz0 ligand. Calculations performed on the discrete neutral and singly reduced ligands reveal that the angular offset of the outer pyridyl rings decreases relative to the planar core upon reduction. We postulate that a similar effect occurs in the framework, whereby reduction of one ligand induces a resonance effect in the other that decreases the stacking offset and increases the overall donor−acceptor orbital overlap, thereby facilitating the IVCT behavior. Such an effect has previously been postulated for stacked porphyrinic ligands in molecular rectangles displaying similar through-space IVCT properties.34 The distance dependence of IVCT transitions is well-known in biological systems and in synthetically designed dinuclear complexes which have been used as models for exploring mixed valency.15,16,21,35,47 In the present work, the relative distance between the DPPTzTz ligands in the dimer pair was adjusted computationally to reflect a geometry in which orbital overlap would be increased and another in which the distance between the pair would be so great that orbital overlap would be nonexistent (Figure 4, see also Supporting Information Figures S19−S26). TD-DFT calculations revealed that the NIR 6626

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Kubelka−Munk theory.48 This involved first quantifying the molar extinction coefficient of the UV/vis π → π* band in the neutral framework using KBr pellets containing various concentrations of the framework (see Supporting Information and Figures S30−S32 for details of the analysis). The extinction coefficients for the two NIR bands at 6576 and 8202 cm−1 were subsequently determined as 54 and 106 M−1 cm−1, respectively, and the electronic coupling constants, Hab, as 123 and 273 cm−1, respectively, through application of Marcus−Hush theory.20 Given that both transitions contain some intramolecular charge transfer character localized on a given BPPTzTz ligand, these values should be considered as upper limits. The Robin and Day classification scheme for electron transfer38,49,50 provides a description of the extent of electronic delocalization: since 2Hab ≪ νmax, the mixed-valence framework falls within class II of the classification scheme, indicating that delocalization is confined to each dimer pair. Chemical Reduction. 1 was chemically reduced with increasing quantities of LiNP in THF, yielding color changes from the initial bright yellow solid to dark yellow (0.082 equiv), light brown (0.158 equiv), and black, respectively (0.952 and 2.61 equiv). The actual amounts of Li intercalated into the framework were determined by ICP-MS (Supporting Information Table S5). Comparison of the PXRD patterns of the reduced samples with the neutral material confirmed that the framework was stable to chemical reduction (Supporting Information Figure S34). As the amount of reductant was increased past 0.158 equiv, a broad transition in the NIR region of the spectrum was observed. Auto-oxidation of the samples occurred in air, with a decrease in the NIR band as the material regained its neutral state with retention of the structure (Supporting Information Figures S35−S36). An increase in the EPR signal for 1 was observed with an increase in the extent of chemical reduction due to the formation of the BPPTzTz•− radical anion (Supporting Information Figure S37). The signal exhibiting axial symmetry was simulated using Easyspin51 to yield values of gz = 2.0074 and gx = gy = 2.0044, suggesting that the radical is in close proximity to the 14N atoms in the thiazolo[5,4-d]thiazole core. The radical is presumably also interacting with the sulfur atoms, but as the EPR active isotope of sulfur (33S) is only 7.6% abundant, it is not expected to contribute significantly to the signal. This confirms that chemical reduction of 1 results in the reduction of the thiazolo−thiazole core to yield the corresponding radical anion and corresponds well with the results obtained from the EPR spectroelectrochemical measurements on 1 (Figure 2b).

Figure 4. Distance dependence of the through-space IVCT interaction. (a) Schematic representation of the cofacial pair of BPPTzTz ligands showing reduction to the mixed-valence state which facilitates a through-space IVCT interaction. (b) Simulations of the UV/vis/NIR spectra for the cofacial dimer (BPPTzTz0/•−)2 in a −1 reduced state. The distance between the BPPTzTz ligands was initially the crystallographic distance of 3.80 Å, where the two major components of the NIR band were reproduced. Moving the ligands closer (3.5 Å) also confirmed the appearance of two major underlying components, whereas increasing the interligand separation to 6.0 Å led to a loss of the through-space IVCT transition, with the appearance of an intraligand transition only.

transitions corresponding to IVCT in the dimer pair did indeed have a strong distance dependence, with the relative intensity of the underlying IVCT transitions increasing at distances closer than the crystallographic distance (see also Supporting Information Figures S27−S29). Conversely, when the dimer pair was set at a distance >5 Å, IVCT transitions were nonexistent in the calculated spectra, with only an intramolecular transition localized on DPPTzTz appearing in the NIR region of the calculated spectrum (see Supporting Information Figure S18). These results are consistent with the well-known distance dependence for IVCT21 and corroborate the absence of an IVCT band in framework 2 where the ligands are separated by >8 Å. Intervalence Charge Transfer. To quantify the extent of electronic delocalization in the mixed-valence framework, Marcus−Hush theory was employed based on the parameters of the two lowest energy NIR bands which were found to have significant IVCT character. The energies (νmax = 6576 and 8202 cm−1) and bandwidths-at-half-height (Δν1/2 = 1465 and 2939 cm−1) for these two transitions were readily determined from the deconvolution to the experimental spectrum for the mixedvalence form of 1, [Zn2(BPPTzTz0/•−)2(tdc)2]n, as shown in Figure 3. The molar extinction coefficients for the two band maxima were derived by application of an extended form of

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CONCLUSIONS This work has demonstrated a new potential through-space mechanism for charge delocalization in MOFs. Electrochemical or chemical reduction of framework 1 generates a mixedvalence state that is characterized by new IVCT bands in the NIR spectrum due to charge transfer between cofacial BPPTzTz ligands which are in formally radical monoanionic and neutral states. The extinction coefficients for the IVCT transitions were determined using a Kubelka−Munk analysis, enabling application of Marcus−Hush theory to quantify the extent of delocalization. We find that the system is a localized example of a mixed-valence compound which falls within Robin and Day class II. The distance dependence of the through-space mixed-valence interaction was identified using a closely related framework, 2, in which the cofacial interaction between ligands 6627

DOI: 10.1021/jacs.8b02638 J. Am. Chem. Soc. 2018, 140, 6622−6630

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Journal of the American Chemical Society

Physical Characterization and Instrumentation. Crystallography of 1 and 2. A yellow block-like crystal of 1 or 2 was mounted on an Agilent SuperNova (Dual, Cu at zero, Atlas) diffractometer employing monochromated Cu Kα radiation generated from a sealed X-ray tube. For 1, cell constants were obtained from a least-squares refinement against 37 094 reflections located between 6.82 and 147.34° 2θ. Data were collected at 150(2) K with ω scans to 129.98° 2θ with subsequent computations carried out with the WinGX graphical user interface.53 For 2, cell constants were obtained from a least-squares refinement against 56 985 reflections located between 7.83 and 151.12° 2θ. Data were collected at 150(2) K with ω scans to 152.00° 2θ with subsequent computations carried out with the WinGX graphical user interface.53 The structures of 1 and 2 were solved in the orthorhombic Pcc2 space group (#27) and the P-1 space group (#2), respectively, by direct methods within SHELXT54 and extended and refined within SHELXL-2014/7.55 The non-hydrogen atoms in the asymmetric unit were modeled with anisotropic displacement parameters, and a riding atom model with group displacement parameters was used for the hydrogen atoms in both 1 and 2. For 1, the absolute structure was established with the Flack parameter refining to −0.006(9).56−58 Solid-State Electrochemistry. Solid-state electrochemical measurements were performed using a Bioanalytical Systems Electrochemical Analyzer. Argon was bubbled through 0.1 M electrolyte solutions to thoroughly degas the electrochemical cell. The CVs in organic solvents were recorded using a glassy carbon working electrode (1.5 mm diameter), a platinum wire auxiliary electrode, and a Ag/Ag+ wire quasi reference electrode. The CVs in KCl/H2O were recorded using a Ag/ AgCl aqueous reference electrode. The sample was mounted on the glassy carbon working electrode by dipping the electrode into a paste made of the powder sample in CH3CN. Ferrocene was added as an internal standard upon completion of each experiment. All potentials are quoted in V vs Fc+/Fc. Solid-State UV/vis/NIR Spectroscopy. UV/vis/NIR spectra were obtained on the samples at room temperature using a CARY5000 spectrophotometer equipped with a Harrick Praying Mantis accessory over the wavenumber range 5000−40 000 cm−1. BaSO4 or MgO was used for the baseline. Spectra are reported as the Kubelka−Munk transform, where F(R) = (1 − R)2/2R (R is the diffuse reflectance of the sample as compared to BaSO4 or MgO). Air sensitive, chemically reduced samples were loaded into a Harris ambient pressure dome equipped with SiO2 windows in a glovebox under argon. Solid-State EPR Spectroelectrochemistry. The procedure and cell setup used were as previously reported for a solution state EPR spectroelectrochemical experiment.43−45 A three-electrode assembly consisting of simple narrow wires (A−M Systems) as electrodes was used. Teflon-coated platinum (0.20 and 0.13 mm coated and uncoated diameters, respectively) and silver wires (0.18 and 0.13 mm coated and uncoated diameters, respectively) were used for the working and quasi reference electrodes, respectively, while a naked platinum wire (0.125 mm) attached to a small piece of rolled platinum mesh (∼5 mm × 4 mm) was used for the counter electrode. The bottom 1 cm of the Teflon-coated wires was stripped (using an Eraser International Ltd., RT2S fine wire stripper). The sample of interest was wrapped in a small piece of platinum mesh (∼5 mm × 3 mm) lengthwise and twisted to ensure the sample remained immobilized. The exposed end of the working electrode was attached to the platinum mesh such that this electrode was positioned lowest and the redox product of interest was generated at the bottom of the tube (and was well separated from the counter electrode). The electrodes were soldered to a narrow three-core flexible wire/cable. The cell used was made by flame sealing the tip of a glass pipet. The potential was controlled with a portable μAutolab II potentiostat and the EPR spectra obtained using an EMX Micro X-band EPR spectrometer equipped with a 1.0 T electromagnet. The EPR signals obtained were referenced against Strong Pitch to obtain the g-factor value, and the response of the EPR sample cavity and that of the sample holder and wire−setup was checked prior to each experiment. In all cases the samples were EPR silent prior to sample reduction unless otherwise stated. A 0.1 M [(n-C4H9)4N]PF6/ CH3CN supporting electrolyte solution was used for all experiments.

was precluded, and no IVCT transitions were found. DFT calculations on a model cofacial dimer pair of BPPTzTz ligands corroborated the assignment of the IVCT bands in 1 and demonstrated the distance dependence of the mixed-valence interaction. While the nature of this through-space charge transfer mechanism is localized within each cofacial pair of ligands, systems displaying this cofacial structural motif have been of significant interest, for example, as models for natural biological systems such as the special pair radical cation in bacterial photosynthesis.15,16,35 Moreover, elaboration of this through-space mixed-valence interaction could lead to longrange delocalization, essential for conductivity in MOFs. Current work in our laboratory is probing cofacial interactions in a wider range of frameworks of relevance to both fundamental explorations of charge delocalization and applications of the materials in molecular electronics. It is hopeful that further studies may lead to practical applications by combining our approach with, for example, the successful molecular engineering strategies pioneered to date in the field.52

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METHODS

Synthetic Protocols. 2,5-Bis(4-(pyridin-4-yl)phenyl)thiazolo[5,4d]thiazole (BPPTzTz). Dithiooxamide (0.130 g, 1.12 mmol) and 4-(4formylphenyl)pyridine (0.270 g, 1.46 mmol) were dissolved in DMF (1.5 mL) and heated at reflux overnight to yield a dark yellow suspension. The reaction mixture was cooled to room temperature before the product was collected by filtration and washed with water. Upon recrystallization with DMF, the pure product was obtained as brown crystals (190 mg, 58%). 1H NMR (TFA, 300 MHz): δ 9.62 (br s, 4H), 9.15 (br s, 4H), 9.05 (d, 3JH−H = 6.5 Hz, 4H), 8.85 (d, 3JH−H = 6.5 Hz, 4H) ppm. 13C{1H} NMR (TFA, 75 MHz): δ 174.8 (Cq), 160.8 (Cq), 150.3 (Cq), 143.4 (CH), 140.1 (Cq), 136.0 (Cq), 131.4 (CH), 130.7 (CH), 127.1 (CH) ppm. Elemental Analysis: Found C, 69.50; H, 3.10; and N, 12.25%. Calculated for C26H16N4S2: C, 69.62; H, 3.60; N, 12.49%. ESI-MS (ESI+, CH3CN): 449.03 (Calculated [M + H]+ = 449.09, 100%) amu. 2,5-Bis(3,5-dimethyl-4-(pyridine-4-yl)phenyl)thiazolo[5,4-d]thiazole (BDPPTzTz). Dithiooxamide (70.0 mg, 0.600 mmol) and 3,5dimethyl-4-(4-pyridinyl)-benzaldehyde (0.250 g, 1.18 mmol) were dissolved in DMF (10.0 mL) and heated at 120 °C for 3 days to yield a yellow suspension. The reaction mixture was cooled to room temperature and filtered. The crude product was washed with water before being recrystallized with DMF to yield the product as yellow crystals (134 mg, 45%). 1H NMR (CDCl3, 200 MHz): δ 8.80 (d, 3 JH−H = 5.8 Hz, 4H, phenyl), 7.82 (s, 4H, pyridyl), 7.44 (d, 3JH−H = 5.8 Hz, 4H, phenyl), 2.14 (s, 6H, CH3) ppm. 13C{1H} NMR (CDCl3, 75 MHz): δ 169.3 (Cq), 162.9 (Cq), 151.3 (Cq), 150.5 (CH), 149.1 (Cq), 141.8 (Cq), 136.8 (CH), 133.7 (Cq), 125.9 (CH), 21.1 (CH3) ppm. Elemental Analysis: Found C, 71.69; H, 4.76; and N, 11.60%. Calculated for C30H24N4S2: C, 71.40; H, 4.79; N, 11.10%. ESI-MS (ESI+, CH3CN): 505.14 (Calculated [M + H]+ = 505.14, 100%) amu. [Zn2(BPPTzTz)2(tdc)2]n Framework (1). Zn(NO3)2·6H2O (33.3 mg, 0.112 mmol), BPPTzTz (25 mg, 0.0557 mmol), and 2,5thiophenedicarboxylic acid (19.2 mg, 0.112 mmol) were dissolved in DMF (10.0 mL) and heated at 120 °C for 15 h to yield an orange solid (32.0 mg, 90%). Elemental Analysis: Found C, 61.20; H, 3.33; N, 5.98; and S, 13.23%. Calculated for C66H40O4N4S6Zn2·2DMF: C, 60.80; H, 3.83; N, 5.91; S, 13.52%. [Zn4(BDPPTzTz)2(tdc)4]n Framework (2). Zn(NO3)2·6H2O (11.8 mg, 0.0397 mmol), the BDPPTzTz ligand (10.0 mg, 0.0198 mmol) and 2,5-thiophenedicarboxylic acid (6.82 mg, 0.0396 mmol), was dissolved in DMF (6.0 mL) and heated at 120 °C for 3 days to yield prismatic yellow crystals (16 mg, 82%). Elemental Analysis: Found C, 51.24; H, 4.37; N, 7.23; and S, 9.87%. Calculated for C84H72O16N8S8Zn4·5DMF: C, 50.97; H, 4.62; N, 7.80; S, 10.99%. 6628

DOI: 10.1021/jacs.8b02638 J. Am. Chem. Soc. 2018, 140, 6622−6630

Article

Journal of the American Chemical Society Computational Methods. Density functional theory (DFT) calculations were carried out using the Gaussian09 software.59 In order to interpret the experimentally observed IVCT bands, timedependent density functional theory (TD-DFT) excited-state calculations were performed using the BMK46 functional and 6311G(d) basis set with the effects of solvation taken into account using the PCM continuum solvation model60 using the solvent parameters for acetonitrile to elucidate the electronic transitions responsible for the observed bands in both the neutral and reduced states. The geometries used for the excited-state calculations of the dimer model were taken from the crystallographic coordinates extracted from the crystal structure. The orbital transitions responsible for the calculated excited states were assigned using the dominant contribution to each excited state and visual inspection of the associated molecular orbitals (Supporting Information Figures S18−S22).

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ASSOCIATED CONTENT

* Supporting Information S

. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b02638. Additional experimental details, crystallographic data and structure refinement details, thermogravimetric analysis, powder X-ray diffraction, gas adsorption, electrochemical and spectral (UV/vis/NIR and EPR) data, molecular orbitals of calculated TD-DFT transitions, Calculated [TD-BMK/6-31G(d)] UV/vis spectra, distance dependence of the calculated transtitions, Kubelka−Munk and Marcus−Hush analyses of the IVCT band (PDF) CIF file for C64 H32 N8 O8 S6 Zn2 (CIF) CIF file for C33 H31 N5 O S2 (CIF) CIF file for C42 H28 N4 O8 S4 Zn2 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Patrick W. Doheny: 0000-0003-1705-8850 Bun Chan: 0000-0002-0082-5497 Deanna M. D’Alessandro: 0000-0002-1497-2543 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Australian Research Council, SydneyNano, and the Vibrational Spectroscopy Core Facility at the University of Sydney. This research was undertaken with the assistance of resources and services from the Australian Synchrotron Access Program and the National Computational Infrastructure (NCI), which are supported by the Australian Government. We are very grateful to Emeritus Professor Noel Hush (University of Sydney) for his invaluable advice and encouragement throughout this work.

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REFERENCES

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