Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for Molecular Semiconductors and Metals Kristina Lekin,† Alicea A. Leitch,† Abdeljalil Assoud,† Wenjun Yong,‡ Jacques Desmarais,§ John S. Tse,§ Serge Desgreniers,∥ Richard A. Secco,‡ and Richard T. Oakley*,† †

Department Department § Department ∥ Department ‡

of of of of

Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

S Supporting Information *

ABSTRACT: In pursuit of closed-shell building blocks for single-component organic semiconductors and metals, we have prepared benzoquino-bis-1,2,3thiaselenazole QS, a heterocyclic selenium-based zwitterion with a small gap (λmax = 729 nm) between its highest occupied and lowest unoccupied molecular orbitals. In the solid state, QS exists in two crystalline phases and one nanocrystalline phase. The structures of the crystalline phases (space groups R3c and P21/c) have been determined by high-resolution powder X-ray diffraction methods at ambient and elevated pressures (0−15 GPa), and their crystal packing patterns have been compared with that of the related all-sulfur zwitterion benzoquino-bis-1,2,3-dithiazole QT (space group Cmc21). Structural differences between the S- and Se-based materials are interpreted in terms of local intermolecular S/Se···N′/O′ secondary bonding interactions, the strength of which varies with the nature of the chalcogen (S vs Se). While the perfectly two-dimensional “brick-wall” packing pattern associated with the Cmc21 phase of QT is not found for QS, all three phases of QS are nonetheless small band gap semiconductors, with σRT ranging from 10−5 S cm−1 for the P21/c phase to 10−3 S cm−1 for the R3c phase. The bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy Eact. For the R3c phase, band gap closure to yield an organic molecular metal with a σRT of ∼102 S cm−1 occurs at 6 GPa. Band gaps estimated from density functional theory band structure calculations on the ambient- and high-pressure crystal structures of QT and QS correlate well with those obtained experimentally.

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triplet or biradical singlet states,5,6 and internal salts.7 In all these systems, the small molecular HOMO−LUMO gap ΔE can be fine-tuned by substituent effects, but further reduction of the solid state energy gap Eg relative to ΔE requires broadening of bandwidths WVB and WCB of the associated resulting valence and conduction bands (Figure 1a). Increasing the width of an electronic energy band depends not only on increasing the magnitude of intermolecular hopping interactions but also the number of such interactions, that is, the overall dimensionality of the electronic structure. For example, in a crystal composed of planar molecules packed into superimposed π-stacks, electronic interactions are restricted to a single dimension (Figure 1b). In slipped πstack and herringbone arrays (Figure 1c,d), additional lateral hopping interactions may come into play, thereby increasing electronic dimensionality, but full two-dimensionality requires lamellar or “brick-wall” packing arrangements (Figure 1e) in which there are four equivalent nearest neighbors.8 The impact of increased dimensionality on bandwidths, optical gaps, and

INTRODUCTION In most closed-shell molecular solids, intermolecular interactions are weak. As a result, the optical energy gap Eg between the valence and conduction bands of the solid may be loosely equated with the separation ΔE between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of the isolated molecule.1 Under these circumstances, electrical conductivity by thermal activation of carriers is low, as the HOMO−LUMO gaps in most organic molecules are large. Charge carriers can nonetheless be generated by the use of electric fields, chemical doping, and/ or photoexcitation, and exploitation of these effects has led to the development of a host of materials with applications in optical, electronic, and information storage devices.2 Attempts to improve the native conductivity of organic solids and films through the design of molecules with small HOMO−LUMO gaps have also been pursued,3 in part because of the potential use of such materials as organic thermoelectrics,4 but also because of the possibility of achieving the elusive goal of a single-component, all-organic molecular metal. Popular targets have included anti-aromatic 4nπ-electron systems, which can exist as charge-separated zwitterions rather than open-shell © XXXX American Chemical Society

Received: February 23, 2018

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DOI: 10.1021/acs.inorgchem.8b00485 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Likewise, the crystal structures of the pyridine- and pyridonebridged zwitterionic bisdithiazoles 1 and 2 (Chart 1) were not Chart 1

Figure 1. Evolution of (a) the HOMO−LUMO gap ΔE of an isolated closed-shell molecule into the optical band gap Eg between the valence and conduction bands of a molecular solid, with bandwidths WVB and WCB. Common stacking arrangements for planar molecules: (b) onedimensional π-stack, (c) slipped π-stack, (d) herringbone, and (e) twodimensional brick-wall arrays.

carrier mobility is now well-recognized,9 and there is considerable interest in the synthesis of new organic semiconductors with such architectures.10 Attempts to modify the shapes of organic molecules to generate a particular solid state architecture remain very much empirical in nature. That being said, the use of secondary bonding interactions (SBIs)11 or supramolecular synthons12 to guide the direction of local molecular approaches has afforded impressive results in terms of the generation of specific onedimensional (1D) and two-dimensional (2D) supramolecular motifs. However, while SBIs may orient a molecule in a desired manner, it is vital that the molecule itself possess an intrinsically small optical gap.13 The challenge therefore is to fulfill both conditions simultaneously, to find the right molecule and the right crystal structure. Under these circumstances, improvements may be possible using “chemical pressure”, such as by the incorporation of heavy heteroatoms, whose more diffuse valence orbitals afford more effective intermolecular orbital overlap. The replacement of sulfur with selenium in TTF-based charge transfer salts14 and, more recently, in heterocyclic thiazyl radicals and their dimers15 has clearly demonstrated the effectiveness of this heavy atom approach as a means of improving bandwidth and charge transport. In addition to the use of chemical pressure to increase bandwidth, the application of physical pressure to reduce and even close the band gap of closed-shell, single-component molecular crystals has also been explored. However, with few exceptions,16 early attempts at pressure-induced metallization of heavy atom closed-shell heterocycles did not augur well. For example, while enhancement of the conductivity of S-,17 Se-,18 and Te-based19 polyacenes has been observed, metal-like behavior generally required pressures in excess of 20 GPa, where covalent bonds begin to rupture.20 In hindsight, the lack of response of these systems to pressure may have been related to their herringbone packing patterns, which are not conducive to well-developed band structures. Given these findings, the comment by Cui et al. 17 that “the pressure-induced metallization of a single-component π-molecular crystal while maintaining the initial molecular structure appears to be very difficult” is not unreasonable. In spite of these caveats, attempts to find the right molecule with the right crystal structure, such that metallization can be induced at low pressure and without rearrangement,16b have continued. Some time ago we explored the use of anti-aromatic thiazyl and selenazyl heterocycles,21 but 1D π-stacked or herringbone packing patterns proved to be hard to avoid.

suitable for well-developed band electronic structures.22 Recently, however, we observed that replacement of the pyridone spacer in 2 with a quinone bridge, to afford benzoquino-bis-1,2,3-dithiazole 3, led to a dramatic change in solid state packing and hence electronic properties.23 In this latter material, hereafter termed QT, the polar carbonyl groups combine with nitrogen lone pairs to generate multicenter SBIs by binding to electropositive sulfurs on neighboring molecules. The net result is the formation of planar ribbon-like arrays 4 that pack into π-stacked layers, with each molecule offset from its neighbors above and below to create a crystallographically perfect 2D brick-wall packing pattern. The resulting spreading of the valence and conduction bands associated with this motif, coupled with an intrinsically small optical gap (λmax = 732 nm) for the molecule itself, affords a small band gap semiconductor with a σRT of ∼10−3 S cm−1 at ambient pressure. Moreover, with compression to just 8 GPa, the residual band gap of QT can be closed completely, yielding an organic metal with a σRT of >10 S cm−1. Having characterized the sulfur-based zwitterion QT, we sought to investigate the effect of selenium incorporation, in the hope that the “chemical pressure” occasioned by an isomorphous S/Se replacement would facilitate closure of the band gap and formation of a metallic state. Several modifications to QT are possible, depending upon the site(s) of selenium insertion, but as a first step, we focused on the partially substituted variant benzoquino-bis-1,2,3-thiaselenazole 5, hereafter termed QS. On the basis of our experience with related S/Se heterocycles, selenium can be introduced in many cases without a change in solid state structure,15 and we were hopeful that QS would follow suit. To our surprise, however, it did not. Herein, we describe the preparation and structural characterization of three different phases of QS, none of which displays the brick-wall packing pattern adopted by QT. Highpressure conductivity and crystallographic measurements reveal significant differences in the crystal packing and performance for these phases, with one affording a metallic state with a σRT of ∼102 S cm−1 under an applied pressure of just 6 GPa. The structures of the observed QT and QS phases are interpreted in terms of the dominant structure-making SBIs, and their charge transport properties are discussed in light of density functional theory (DFT) molecular and band electronic structure calculations. B


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RESULTS AND DISCUSSION Synthesis. Direct replacement of sulfur with selenium at the 2-position of dithiazolylium (DTA) salts using selenium dioxide in refluxing acetic acid was first reported 40 years ago.24 The method has since been adapted to incorporate selenium into neutral dithiazoles,25 dithiadiazolylium,26 and bisdithiazolylium salts.27 Here we have employed a similar approach (Scheme 1)

sulfoxide (DMSO), dimethylacetamide (DMA), and 1,3dimethyl-2-imidazolidinone (DMI), affording deep green solutions with a λmax near 729 nm (Figure S1). When these solutions were cooled, small amounts of crystalline material could be isolated, but infrared analysis indicated the presence of solvent in the lattice. In one case, using DMI as solvent, singlecrystal structure analysis (see below) established a solvate of composition QS·2DMI. This solvate displays no measurable conductivity; that is, σRT < 10−7 S cm−1. To avoid solvate formation, we returned to the use of less polar nitriles for the deprotonation reaction and explored the effect of changes in reaction temperature and the strength of the base. Success was eventually achieved by using the weak base 3-cyanopyridine to deprotonate a nearly saturated solution of [QS][HONf] in boiling acetonitrile. This afforded the zwitterion in the form of analytically pure, black microcrystals that were structurally characterized by PXRD methods (see below). This material, hereafter termed the R3c phase, could also be produced by switching the solvent from acetonitrile to propionitrile. Thus, while [QS][HONf] is not soluble in propionitrile at room temperature, deprotonation of a slurry of the salt at reflux temperature with Proton Sponge yielded black microcrystals of the R3c phase. Somewhat to our surprise, R3c QS was also formed in high phase purity (Figures S2, S3, and S4) when crystals of the DMI solvate QS·2DMI were heated overnight at 120 °C and 10−2 Torr. Produced by any of the methods described above, R3c QS displays a conductivity σRT of ∼10−3 S cm−1. A second, analytically pure crystalline phase of QS, structurally characterized by PXRD and hereafter termed the P21/c phase, could be generated by the action of 3cyanopyridine on a slurry of [QS][HONf] in acetonitrile at room temperature. Here the combination of (i) a low temperature, (ii) the low solubility of the salt, and (iii) a weak base led to long reaction times (3−5 days). The results were not always consistent; sometimes mixtures of the P21/c and R3c phases were produced. However, persistence eventually afforded phase pure samples of P21/c QS suitable for PXRD and charge transport measurements. This material displays a conductivity lower than those of the R3c and nano phases, with a σRT of ∼10−5 S cm−1. Crystallography. We have determined the crystal structures of the protonated salt [QS][HONf] and the DMI solvate QS·2DMI by single-crystal X-ray diffraction, while highresolution PXRD methods were used to characterize the two microcrystalline phases of the native zwitterion QS. The effect of pressure on these two phases was explored over the range of 0−15 GPa by high-pressure (HP) PXRD using synchrotron radiation and diamond anvil cell techniques. The nano-QS phase was also examined by PXRD methods, at ambient and high pressure (≤8 GPa). Structural solutions for the two crystalline phases were generated using simulated annealing methods based on a molecular model extracted from the molecular geometry of the zwitterion in QS·2DMI. During the initial Rietveld refinements, a rigid-body constraint was employed, but in the final Rietveld refinements, only the unit cell parameters were optimized (Figures S5 and S6). Crystal data from the single-crystal and powder refinements are listed in Table S1; ORTEP drawings and selected intramolecular metrics for the DMI solvate and the protonated salt are shown in Figure 2. Crystal packing is described below. [QS][HONf]. The crystal packing of [QS][HONf], space group Pbca, consists of QS molecules protonated on oxygen

Scheme 1

to introduce selenium regioselectively into the protonated cation [QT]H+, prepared by the condensation of diaminodihydroxybenzene (DADHB) with sulfur monochloride.23 The choice of counteranion proved to be crucial; attempts to generate the trifluoromethanesulfonate (triflate, OTf−) salt [QS][HOTf] starting from [QT][HOTf] were thwarted by the extremely low solubility of the product. Success was achieved by switching to the longer chain nonafluorobutanesulfonate (nonaflate, ONf−) anion; this afforded a product that was slightly more tractable in organic media. We explored different solvents and temperatures, notably, (i) acetonitrile at 110 °C in a pressure vessel and (ii) acetic acid (HOAc) at reflux, and the progress of the reaction was monitored by mass spectrometry. While the yield for the acetonitrile route was higher, the reaction took at least 4 days to ensure complete insertion of selenium into both DTA rings. By contrast, and despite a slightly lower yield, the reaction performed in boiling acetic acid was complete in just 15 min. Produced by either route, [QS][HONf] can be recrystallized from acetonitrile, in which it is sparingly soluble, with ∼0.5 g L−1 dissolving (at the boil) to afford a deep blue solution with a λmax of 648 nm (Figure S1). Despite the low solubility of [QS][HONf] in organic solvents, its deprotonation as a slurry in acetonitrile was readily effected at ambient temperature using Proton Sponge. This afforded the neutral zwitterion QS as an analytically pure, black powder with a room-temperature conductivity σRT of ∼10−4 S cm−1. However, in contrast to the parent all-sulfur zwitterion QT, which sublimes cleanly at 240 °C and 10−3 Torr, QS could not be sublimed without decomposition. Furthermore, powder X-ray diffraction (PXRD) analysis (see below) indicated a nanocrystalline rather than crystalline morphology. This material, hereafter termed the nano-QS phase, is nonetheless soluble in hot, strongly polar solvents such as dimethyl C


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witnessed by its presence here and in the structures to be described below. In the case presented here, the molecular ribbons of [QS]H+ cations assemble into antiparallel slipped πstack layers in the ab plane with a mean interplanar separation of 3.376 Å. The resulting 2D arrays of cations are interspersed by layers of charge-balancing ONf− anions (Figure 3b). QS·2DMI. In the crystal structure of the DMI solvate, space group C2/c, each zwitterion lies on a 2-fold axis with two equivalent DMI molecules located on either side in an approximately coplanar arrangement; the carbonyl oxygens of the two DMI molecules are coordinated to it by short Se1···O3′ (2.832 Å) and S1···O3′ (2.838 Å) contacts (Figure 4). The Figure 2. ORTEP drawings of QS (in its DMI adduct) and the [QS]H+ cation (in its ONf− salt), with atom numbering and selected metrics in Angstroms. For averaged values, numbers in parentheses are the greater of the difference and the standard deviation.

and hydrogen-bonded to the sulfonate groups of the ONf− anions. The cations are linked into ribbon-like arrays running parallel to the b axis by close four-center (Se···N′)2 contacts (Figure 3a) that are well inside the standard van der Waals

Figure 4. (a) Intermolecular contacts in QS·2DMI and (b) view of the unit cell, showing superimposed ABBABB π-stacking of zwitterions and solvent molecules.

steric bulk of the DMI molecules severely reduces intermolecular contacts between zwitterions. Thus, while there are still four-center (Se···N′)2 interactions with outrigger Se···O′ contacts linking neighboring zwitterions laterally, they are all significantly longer (Figure 3a) than those in [QS][HONf], and the resulting ribbons are staggered, with the mean planes of neighboring molecules offset by 1.507 Å. In addition, there is no overlaid π-stacking of zwitterions, as in the protonated salt. Instead, the zwitterion (A) and two DMI molecules (B) assemble as approximately superimposed ABBABB π-stacks. Within these columns, the zwitterions are essentially isolated from one another, as a result of which intermolecular overlap is negligible. With no apparent pathway for charge transport, the absence of any measurable conductivity (noted above) is not surprising. R3c QS. The high-resolution PXRD pattern (Figure 5a) of the microcrystalline material generated by treatment of hot solutions of [QS][HONf] in MeCN with Proton Sponge was indexed as belonging to the R3c space group. The resulting crystal structure was determined and refined to afford a unit cell (Figure 6a) displaying a flower petal-like pattern created from twisted π-stacks that spiral about the 31 axes. The packing can be further described in terms of double-layer, ribbon-like arrays

Figure 3. (a) Ribbon-like arrays of cations in [QS][HONf], linked by six-center (Se···N′/O′)2 SBIs, with ONf− anions above and below. (b) Antiparallel layers of π-stacked cations separated by ONf− anions, viewed parallel to the b axis (the direction of propagation of the ribbons).

separation.28 These centrosymmetric four-center secondary bonding interactions (SBIs) are ubiquitous in the crystal structures of both neutral and charged selenazole rings.29 Their electronic nature, that is, the relative importance of electrostatic, dispersion, and covalent (n → σ* hole) contributions, has been the subject of much debate.30 In the case presented here, the central four-center (Se···N′)2 SBI is augmented by a pair of “outrigger” Se···O′ interactions. The resulting six-center (Se···N′/O′)2 unit appears to be extremely robust,31 as D


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Figure 5. (a) Observed and calculated PXRD patterns for the R3c QS at 0 GPa (λ = 0.509176 Å). (b) Fractional changes in unit cell parameters as a function of pressure. Figure 7. (a) Coordination sphere of a single zwitterion in R3c QS and (b) a single ruffled ribbon. Dashed lines are intermolecular Se/S···O′ (green) and Se···N′ (blue) contacts.

ruffled ribbon network with neighboring zwitterions locked into an ABCABC sequence (Figure 7b).32 With the application of pressure, there is a steady evolution of the cell parameters out to 15 GPa (Figure 5b), the large response parallel to the c axis indicating a greater ease of compression of the layered π-stacks. A sample data set at 6.1 GPa (Figure S6) was solved and the structural solution refined (Table S1) to allow electronic band structure calculations. P21/c QS. The monoclinic phase of QS was the most difficult to isolate in a pure (microcrystalline) form, and identification of its space group proved to be a challenge, requiring consideration of powder diffraction data collected at both ambient and high pressure. While the overall cell dimensions of this phase are appealingly similar to those of the Cmc21 phase of QT,33 the space group symmetry is much lower; it is not C-centered and not even orthorhombic. Initial attempts to index the ambient-pressure data (Figure 8a) in terms of an orthorhombic cell led to several plausible primitive possibilities, but subsequent structural refinements provided less than satisfactory solutions. The need for a monoclinic cell, with space group P21/c and Z = 4, was eventually confirmed by the high-pressure measurements, which heralded a steady divergence of the β angle from 90° with increasing pressure. As

Figure 6. (a) Unit cell of R3c QS, with flower petal-like clusters about 31 axes (triangle at Se2) and across glide planes (box at Se1). (b) Ruffling of layered ribbons, with twisted π-stacks running parallel to the c axis.

of zwitterions running parallel to the a and b axes, with individual ribbons being ruffled, as illustrated in Figure 6b. The resulting ABABAB π-stacking, in which successive zwitterions rock to and fro across c glides, has the effect of suppressing πinteractions within the stacks. At the same time, however, interstack interactions are enhanced, giving rise to a 2D network of intermolecular Se···Se′ contacts that link the “flower petals” along the z direction through neighbors related by glide planes (Se1···Se1′, 3.724 Å) and 31 axes (Se2···Se2′, 3.470 Å). Despite the apparent complexity of packing within the unit cell (Z = 18), the local intermolecular interactions in R3c QS are reminiscent of those observed in [QS][HONf] and QS· 2DMI described above. Indeed, the coordination geometry of the zwitterion is strikingly similar to that found in the DMI solvate, with the carbonyl groups of two different zwitterions playing the role of solvent on either side of the central molecule, while two more zwitterions are linked above (left and right) by highly distorted six-center (Se···N′/O′)2 bridges (Figure 7a). In the rhombohedral lattice, this slightly nonplanar unit propagates parallel to the a and b axes to afford a 2D

Figure 8. (a) Observed and calculated PXRD patterns for the P21/c phase of QS at 0 GPa (λ = 0.509176 Å). (b) Fractional changes in unit cell parameters as a function of pressure. E


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Inorganic Chemistry with the R3c phase, there are no discontinuities out to 15 GPa in the cell parameters (Figure 8b) that might suggest a phase change or sample degradation. A sample data set at 5.7 GPa was solved (Figure S6) and the structural solution refined (Table S1) to provide a reference point for electronic band structure calculations (see below). Notwithstanding the difference in space groups, cursory comparison of the unit cells of P21/c QS and Cmc21 QT (Figure 9a,b) suggests an appealing but illusory resemblance in

Having established the basic structural differences between P21/c QS and Cmc21 QT, we face the question of the existence of two phases rather than one. Why should a simple one-site S/ Se replacement at the molecular level cause such a major change in crystal packing? As a first step toward addressing this issue, we note that planar ribbons are present in the P21/c phase; they propagate along the directions of the cross-bracing, with neighboring molecules along the ribbons linked pairwise by inversion centers. The resulting pattern, illustrated in Figure 10, consists of dimers of zwitterions coupled laterally by

Figure 9. Unit cells of (a) P21/c QS and (b) Cmc21 QT, highlighting the top layer; dashed lines are intermolecular Se/S···O′ (green) and Se···N′ (blue) contacts. Side views showing (c) the ruffled ribbons in QS and (d) the rigorously planar arrays in QT.

Figure 10. Ribbons of zwitterions in P2 1 /c QS based on centrosymmetric dimers. Dashed lines are close intradimer Se···O′ (green) and Se···N′ (blue) interactions. Molecules shown edge-on above and below belong to cross-braced ribbons.

packing, with ribbons of zwitterions running parallel to the b and c axes and a labyrinth of lateral intermolecular S/Se···O′ and S/Se···N′ interactions (listed in Figure S7). However, the presence of C-centering in QT requires that the molecules in each layer be rigorously coplanar, with neighbors along the z direction related by c glides. By contrast, in P21/c QS, neighbors along z are still related by c glides, but with the loss of C-centering, there is no restriction on the value of their x coordinate. Consecutive molecules along z are thus free to rotate, left and right, to produce ruffled ribbons that may be characterized in terms of a tilt angle τ of 24.5° between the mean molecular plane and the a axis (Figure 9c). Ribbon ruffling in P21/c QS leads to a cross-braced slipped πstack architecture with an alternating ABAB sequence of zwitterions aligned into 1D stacks running parallel to the a axis (Figure 9c). Adjacent zwitterions within each stack are related by an inversion center, with an interplanar separation (3.448 Å) somewhat larger than in the 2D brick-wall pattern (Figure 9d) found in Cmc21 QT (3.164 Å). The high-pressure PXRD measurements (Figure 8b) indicate that axial contraction in P21/c QS is greatest along the a axis, as expected from the cross-braced structure, which can most easily undergo a “winerack” compression34−36 in this direction. Consistently, tilt angle τ steadily decreases with increasing pressure, reaching a value of 22.0° at 5.7 GPa and 21.0° at 11.8 GPa.

centrosymmetric six-center (Se···N′/O′)2 SBIs. The fact that variations on this dimer unit are found in the crystal structures of [QS][HONf], QS·2DMI, and R3c QS attests to its general structural importance, but in the case presented here, where it is perfectly wrapped around an inversion center, the dimer takes on an even more influential role as a molecular building block, with other ribbons emanating from it in a cross-braced fashion. In essence, we believe that it is the stability of this centrosymmetric unit that dictates the preference for a centric (P21/c) space group over the noncentric (Cmc21) alternative. To explore this possibility, we performed a series of DFT calculations to assess the energetics of various modes of association of both QT and QS. The results are presented below. Nano-QS. The PXRD pattern (λ = 0.509176 Å) at 0 GPa of nano-QS, prepared by deprotonation of a slurry of [QS][HONf] in cold acetonitrile with Proton Sponge, is shown in Figure 11a. Unlike the R3c and P21/c phases, this material shows little evidence of long-range order. Because of the paucity of reflections, attempts to index the data were unsuccessful. However, the presence of a strong and relatively sharp peak near 2θ = 8.79°, with two broader peaks at lower 2θ values, is consistent with a degree of short-range order or nanocrystallinity. F


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Figure 11. (a) PXRD pattern (λ = 0.509176 Å) of nano-QS at 0 GPa. The strong, well-resolved peak at 2θ = 8.79° is consistent with a d spacing of 3.33 Å. (b) Comparison of the pressure-induced change in d spacing in nano-QS with that of the respective π-stacking axis in Cmc21 QT, P21/c QS, and R3c QS.

Figure 12. Pressure dependence of (a) room-temperature conductivity σRT and (b) the thermal activation energy of the nanocrystalline (nano) and crystalline (P21/c and R3c) phases of QS.

Molecular Electronic Structure Calculations. To gain insight into the similarities and differences between QT and QS, we performed a series of DFT calculations at the (U)B3LYP/6-311G(d) level, to compare the charge distributions of the ground and excited states of the molecules themselves (Tables S2 and S3) and to assess the energetics of association of the structurally relevant supramolecular dimers (Table S4) described in previous sections. To summarize the results, we illustrate in Figure 13 the frontier orbitals of QS

While a full analysis of the PXRD data for this nano-QS phase is beyond the scope of this work, we note that the dominant d spacing of 3.33 Å is only slightly larger than the interplanar separation found in Cmc21 QT (3.164 Å); this may reflect a similar lamellar or layered sheet structure. Further support for possible structural similarities can be gained by tracking the changes in this d spacing with pressure; this provides a measure of axial compressibility and hence insight into the nature of the packing pattern. As shown in Figure 11b, the compression profile of d/d0 for nano-QS differs from those found for the stacking axes in the P21/c and R3c phases, that is, a/a0 and c/c0, respectively. By contrast, the responses of d/d0 for nano-QS and a/a0 for Cmc21 QT are almost superimposable. On the basis of this close match, it is tempting to suggest similar stacking arrangements for the two materials; that is, the structure of nano-QS is based on a disordered or fragmented lamellar or brick-wall framework. Its relatively high conductivity (described below) is consistent with this possibility. Conductivity Measurements. To quantify the initial twoprobe conductivity measurements on the three phases of QS, we have explored the response of their room-temperature conductivity σRT and thermal activation energy Eact to applied pressure using multianvil press (MAP) techniques. Previous high-pressure measurements on the Cmc21 phase of QT established a value of σRT near 10 S cm−1 at 8 GPa, with Eact reaching zero at this pressure.23 The corresponding results for QS are presented in Figure 12, which illustrates plots of σRT and Eact for the nanocrystalline (nano) and crystalline (P21c and R3c) phases over the range of 0−10 GPa. As suggested by the initial ambient-pressure measurements, the performance of the monoclinic phase is the poorest, with σRT increasing from near 10−5 S cm−1 at 0 GPa to near 10−1 S cm−1 at 8 GPa. At this pressure, the conductivity still requires significant thermal activation, with an Eact of ∼0.2 eV. The nano phase shows a better response, with σRT increasing to near 10 S cm−1 and Eact dropping to near 0.04 eV at 8 GPa. Extrapolation of the data to higher pressures suggests a complete loss of thermal activation near 12 GPa. The performance of the R3c phase is the most impressive, with σRT increasing to 102 S cm−1 near 6 GPa, a value not far below the Mott-Ioffe Regel limit37 for metallic conductivity of 103 S cm−1. At the same pressure, Eact is reduced to zero, heralding formation of a metallic state.

Figure 13. Frontier Kohn−Sham π-orbital manifold (not to scale) of the zwitterionic 1A1 state of QS, showing excitations to the open-shell singlet 1B2 and triplet 3B2 states. For the zwitterionic state, only one of the two degenerate VB representations (see Scheme 1) is shown.

that, as expected from its pseudodiradical zwitterionic configuration, has a small HOMO−LUMO gap of 1.49 eV, similar to that found in QT (1.54 eV). These values of ΔE can be related to the respective optical gaps to the 1B2 state: QS (729 nm, 1.70 eV in DMSO) and QT (732 nm, 1.69 eV in DMSO). The associated 3B2 triplet states are calculated to lie just above the closed-shell zwitterionic 1A1 ground states, with a calculated promotion energy near 0.15 eV for both QS and QT. From a structural perspective, the internal metrics of QS and [QS]H+, gauged by both experiment (Figure 2) and calculation, parallel those previously found for QT and [QT]H+23 and are consistent with the presence of a quinoidal bridge between the two DTA rings. The calculated charge density distribution and low molecular dipole moment (Table S2) obtained for both molecules are also consistent with this description. G


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Inorganic Chemistry

are reversed, an observation that augurs well for the future isolation of a Cmc21 phase for this material. For the all-selenium (Se−Se−N) structure, the binding of the six-center, head-tohead model returns to being favored over that of the five-center head-to-tail variant, although the energy difference is somewhat smaller than that found for QS (S−Se−N). Band Electronic Structure Calculations. In our earlier report on QT, we provided the results of DFT band structure calculations obtained using the Perdew−Burke−Ernzerhof (PBE) functional with a plane-wave basis set.23 However, the generalized gradient approximation to the density function in the PBE method tends to underestimate the HOMO−LUMO gap for most materials.39,40 The deficiency is mainly due to the neglect of self-exchange interactions. A remedy is to construct a “hybrid” functional that incorporates a portion of Hartree− Fock exchange with the rest of the exchange-correlation energy from the density functional. The most successful implementation of the hybrid functional is the screened hybrid functional (HSE) proposed by Heyd, Scuseria, and Ernzerhof.41 It has been shown that HSE06 is able to predict energy gaps consistently over a broad range of semiconductors.42 An improved hybrid functional for solid HSEsolb, based on PBEsol, a revised Perdew−Burke−Ernzerhof functional for solids,43 was used in all the band structure calculations reported here. Electronic band dispersion diagrams for the Cmc21 phase of QT and the P21/c and R3c phases of QS are presented in Figure 15. For each material, two plots are provided, based on ambient- and high-pressure crystal coordinates; for the Cmc21 and R3c phases, the high pressure that is selected corresponds nominally to where a metallic state (Eact = 0) is observed experimentally. In the case of the P21/c phase of QS, where metallization occurs at P ≫ 10 GPa, the pressure of 6 GPa was selected to allow a direct comparison with the R3c phase at the same pressure. The band electronic structures echo the electronic features of the isolated molecules, with three bands found near the Fermi level comprised of crystal orbitals (COs) derived from the molecular HOMO, LUMO, and LUMO+1 (Figure 13), although the proximity of the LUMO and LUMO+1 gives rise to considerable overlap and/or mixing of the resulting COs. Nonetheless, the overall dispersion of the valence bands (shaded blue) at 0 GPa provides a clear indication of the increased bandwidth found in the more 2D structures Cmc21 (0.77 eV) and R3c (0.64 eV) relative to the highly 1D P21/c phase (0.37 eV); the resulting band gaps, Eg (∼2Eact), follow suit. With the application of pressure, there is a significant increase in bandwidth of both the valence and conduction bands of the Cmc21 and R3c phases, with band gap closure near 8 and 6 GPa, respectively. By contrast, the effect of pressure on CO dispersion in the P21/c phase is minimal, so that at 6 GPa, there is still a substantial band gap Eg of ∼0.5 eV, in agreement with the measured Eact value (∼0.25 eV) at the same pressure. In summary, the qualitative and quantitative correspondence between theory and experiment across the range of structures (chemical pressure) and with applied pressure (physical pressure) is satisfying. Of particular note is the fact that band gap closure is both found and predicted for R3c QS at just 6 GPa. Equally, there is merit in the fact that while the 1D material P21c QS is found to be functionally less effective, the band calculations predict that this should be the case.

On the basis of the evidence provided above, the two building blocks, QT and QS, seem to be virtually identical. Because this is the case, we return to the question raised earlier regarding the difference in their crystal structures. Both contain molecular ribbon motifs; however, in Cmc21 QT, neighboring zwitterions along the ribbons are related by translation and linked by head-to-tail five-center S···N′/O′ contacts (Figure 9b), while in P21/c QS, they are related by inversion centers. The resulting dimers are linked by head-to-head six-center (Se···N′/O′)2 contacts (Figure 10). To explore the relative stability of these two patterns, that is, supramolecular dimers of QT and QS bound in head-to-tail and head-to-head fashions, we performed a series of dispersion-corrected DFT calculations at the B3LYP-D3/6-311g(d) level38 on these two geometries for QT and its three Se-containing variants; the resulting bond dissociation enthalpies (ΔHdis) are shown in Figure 14.

Figure 14. Calculated [B3LYP-D3/6-311g(d)] enthalpies of dissociation (ΔHdis) of six-center and five-center SBIs in QT (S−S−N) and its Se-containing variants (S−Se−N, Se−S−N, and Se−Se−N).

Taken collectively, it is immediately apparent that the fivecenter (head-to-tail) and six-center (head-to-head) interactions described here are significantly stronger than the more common four-center (S/Se···N′)2 SBIs found, for example, in 1,2,5-thia- and 1,2,5-selenodiazole dimers,30b,d although internal comparison of the five- and six-center options for QT (S−S− N) indicates little difference between the two. However, with the incorporation of selenium to form QS (S−Se−N), binding energies increase for both dimerization modes, but more so for the six-center head-to-head variant, resulting in a clear preference for this motif over the five-center option. Indeed, the binding energy of the six-center SBI of QS approaches that found for the four-center (Te···N′)2 SBIs in 1,2,5-tellurodiazoles.30b The computational results thus provide a satisfying correlation with experiment. Theory suggests a significantly greater stability for the centrosymmetric six-center SBI of QS, and the crystallographic evidence indicates a clear preference for this unit as the primary supramolecular building block in the R3c and P21/c phases. That being said, we note that the incorporation of selenium into alternate or additional sites of QT may lead to different outcomes. For example, in the putative isomer of QS generated by positional exchange of sulfur and selenium, that is, the −Se−S−N− variant, the relative stabilities of the five- and six-center association modes H


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Inorganic Chemistry

Figure 15. HSE calculated band structures of (a) QT (Cmc21 phase), (b) QS (P21/c phase), and (c) QS (R3c phase) at ambient pressure (above) and elevated pressure (below). The valence and conduction bands are highlighted in blue and green, respectively, and the Fermi level is shown with a dashed red line. Band gaps (Eg) are indicated for semiconducting states.

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SUMMARY AND CONCLUSIONS The results described here stem from the idea of taking a sulfurbased heterocyclic zwitterion with a small HOMO−LUMO gap and a perfectly 2D brick-wall packing pattern and applying “chemical pressure” to it by replacing sulfur with its heavier congener, selenium, to improve bandwidth and reduce the valence/conduction bandgap Eg. The initial strategy was that S/ Se exchange would be isomorphous and that the same solid state “brick-wall” packing pattern found in QT would also be present in QS. While this turned out not to be the case, the selenium-based zwitterion QS displays a rich structural phase diagram; we have been able to structurally characterize and explore the transport properties of two crystalline phases (space groups P21/c and R3c) and one nanocrystalline phase. All three phases are small band gap semiconductors, with σRT at ambient pressure ranging from 10−5 S cm−1 for the P21/c phase to 10−3 S cm−1 for the R3c phase. As expected, the bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy Eact. For the R3c phase, whose unusual 3-fold spiral packing pattern affords a high degree of twodimensionality, band gap closure to yield an organic molecular metal with a σRT of ∼102 S cm−1 occurs at just 6 GPa. In the absence of such features, as in P21/c QS, where a 1D slipped πstack architecture militates against band spreading, performance declines sharply. Understanding the factors that determine crystal packing is of vital importance for the design of new functional materials. For heterocyclic systems like QT and QS, control of packing by the standard approach of modification of ligands is impossible, as there are no ligands to modify. However, the exposed periphery of these all-heteroatom rings, coupled with the polar CO and −N−S/Se− framework bonds, sets up the opportunity to control supramolecular architecture by using intermolecular donor−acceptor SBIs as structure-making tools. As shown here, where several SBI options (five-center and six-

center) are possible, the energetically preferred outcome depends on the nature of the chalcogen. Replacement of sulfur with selenium can reverse the relative stability of possible supramolecular building blocks (Figure 14) that, in turn, drives changes in crystal packing; hence, the centric P21/c space group for QS is preferred over the noncentric Cmc21-based option found for QT. The lessons learned from this work open the door to targeted explorations in crystal engineering, that is, the prediction and design of specific crystalline phases for related and/or entirely new heterocycles. For example, the incorporation of selenium into alternate or additional sites of the QT framework may be fruitful. More generally, the application of these design principles to other electron-rich zwitterions in which strong, multicenter, quasi-dipolar SBIs control packing should allow the development of new closed-shell p-block molecular conductors based on both closed- and open-shell44 building blocks. Given that strong secondary bonding interactions involving tellurium are known to afford impressive supramolecular architectures,45 incorporation of Te46 into the framework of QT and related heterocycles may also be rewarding.

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EXPERIMENTAL SECTION

General Methods and Procedures. Selenium dioxide, trimethylsilyl nonaflate (TMSONf), Proton Sponge [1,8-bis(dimethylamino)naphthalene], 3-cyanopyridine, absolute 1,3-dimethyl-2-imidazolidinone (DMI, 250 °C. ESI-MS (MeCN, m/z): 356.92 (M+). Anal. Calcd for C10HF9N2O5S3Se2: C, 18.36; H, 0.15; N, 4.28. Found: C, 18.07; H, 0.46; N, 4.40. IR (Nujol mull, KBr, cm−1): 1683 (m), 1499 (m), 1245 (s), 1224 (s), 1137 (m), 1091 (w), 1064 (m), 1017 (w), 804 (w), 770 (s), 738 (w), 660 (w), 630 (vs), 523 (m). UV−vis (MeCN): λmax = 648 nm (log ε = 4.4). Preparation of Nano-QS. A solution of Proton Sponge (120 mg, 0.561 mmol) in 5 mL of MeCN was added dropwise to a slurry of [QS][HONf] (303 mg, 0.463 mmol) in 20 mL of MeCN. This led to a rapid loss of the red color of the salt and formation of a fine black precipitate in a nearly colorless solution. After 3 h, the black powder of nano-QS was filtered off, washed with 3 × 15 mL of MeCN, and dried in vacuo; the yield was 164 mg (0.464 mmol, 100%). Mp: >200 °C dec. Anal. Calcd for C6N2O2S2Se2: C, 20.35; N, 7.91. Found: C, 20.60; N, 8.07. IR (Nujol mull, KBr, cm−1): 1660 (s), 1423 (s), 1319 (sh, m), 1297 (m), 1088 (w), 964 (vw), 777 (m), 723 (w), 687 (w), 621 (vs), 487 (m). UV−vis (DMSO): λmax = 729 nm. Preparation of R3c QS. Method 1. A solution of Proton Sponge (120 mg, 0.561 mmol) in 5 mL of EtCN was added dropwise over 10 min to a slurry of [QS][HONf] (300 mg, 0.459 mmol) in 20 mL of boiling EtCN, and the mixture was stirred and heated at reflux for 90 min. The mixture was cooled to room temperature, and the fine black microcrystals of R3c-QS were filtered off, washed with 3 × 15 mL of MeCN, and dried in vacuo. The yield was 163 mg (0.460 mmol, 100%). J


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Inorganic Chemistry

based on k-point grids available on the Bilbao Crystallographic Server.64

and isotropic thermal parameters taken from the initial DASH refinement. Final Rietveld indices Rp and Rwp are listed in Table S1. Atomic positions were not further refined, and as a result, standard deviations for atomic coordinates are not available. For the highpressure PXRD measurements, powdered samples were mounted in a diamond anvil cell, with helium used as the hydrostatic-pressure transmitting medium. The diffraction data were collected at ambient temperature (293 K) and as a function of increasing pressure, estimated from the photoinduced luminescence of a microsphere of ruby. For both crystalline phases, a series of data sets up to 15 GPa was indexed and cell parameters were refined as described above; unit cell data as a function of pressure are shown in Figures 5 and 8. Structural solutions (Figure S6 and Table S1) for selected data sets were refined in DASH and GSAS, as described above. Multianvil Press Conductivity Measurements. High-pressure conductivity experiments on the nanocrystalline and crystalline phases of QS were performed in a 3000 t multianvil press using a Cr2O3doped MgO octahedron as the pressure-transmitting medium.56 The pressure was generated by three electric oil pumps and transmitted through a split-cylinder module to six steel anvils, then to eight tungsten carbide (WC) cubes with a 32 mm edge length, and finally through the eight truncated corners of these cubes to the octahedral pressure medium. The force−pressure relationships for the 18/11 [octahedral edge length (mm)/truncated edge length (mm)] cell configurations adopted in these experiments were determined from prior calibrations of the applied hydraulic load against pressures of structure transformations in standards at room temperature (Bi I ↔ II at 2.55 GPa, Bi III ↔ V at 7.7 GPa, Sn I ↔ II at 9.4 GPa, and Pb I ↔ II at 13.4 GPa). The pressure cell was modified to include a cylindrical heater made from a 0.05 mm thick rhenium (Re) foil and a type C thermocouple with its junction placed in contact with the outside wall of the Re heater. Powder samples were densely packed in a boron nitride (σBN = 10−11 S cm−1) cup with Pt disk electrodes in direct contact with the samples at both ends. Four-wire AC (Solartron 1260 Impedance Analyzer) resistance measurements were taken at a frequency of 1 kHz. A series of resistance measurements were performed at pressures of ≤10 GPa and temperatures ranging from 298 to 400 K. In each series, the pressure was first increased to the target value and then resistance measurements were taken at fixed temperature intervals of 10 K on heating or cooling at a constant pressure (Figure S8). The contiguous cylinder-shaped sample was extracted from the recovered pressure cell, and the sample geometry was measured to convert resistance to conductivity. The effects of lead resistance have been applied to the data. The conductivity and activation energy plots shown in Figure 12 refer to data collected during the cooling mode. For the sake of convenience, the term σRT refers to the conductivity measured at 298 K. Electronic Structure Calculations. DFT calculations of singlet and triplet state energies and charge densities for the QS molecule were performed with the Gaussian 09 suite of programs57 using the (U)B3LYP functional and a polarized, split valence basis set with triple-ζ [6-311G(d)] functions. All calculations on single molecules refer to fully optimized gas phase structures. Dissociation enthalpies ΔHdis (at 0 K) of SBI-bound dimers (Figure 14) were derived with the B3LYP-D3 method, that is, the B3LYP functional with the D3 empirical dispersion correction developed by Grimme,58 using differences in the total electronic energies of the native zwitterions and their five- and six-center dimers optimized in Cs and Ci symmetry, respectively. Final coordinates are provided in archive files (Table S4). Band electronic structure calculations were performed with CRYSTAL1459 based on atom-centered Gaussian basis sets on all atoms. A valence triple-ξ basis set was used on all atoms: Se,60 S,61 O,62 N, and C.63 Atomic coordinates were taken from the experimentally determined structures. The HSEsolb functional was used for all band calculations. For the calculations on the R3c phase of QS, atomic coordinates were transformed to the trigonal setting, where a = b = c = 14.7547 Å and α = β = γ = 117.509°; when Z = 6, there are six crystal orbitals per band, rather than 18 for the full hexagonal setting. The band dispersion diagrams in Figure 14 are

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00485. Details of UV−vis and FTIR spectra, crystal and PXRD data, and DFT calculations (PDF) Accession Codes

CCDC 1825118−1825123 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

John S. Tse: 0000-0001-8389-7615 Richard T. Oakley: 0000-0002-7185-2580 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERCC) and the Canada Foundation for Innovation (CFI) for financial support. HPXRD data were collected at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, the NSERCC, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council of Canada, and the Canadian Institutes of Health Research. The authors also thank the NSERCC for a Vanier Graduate Scholarship to J.D. and the Government of Canada for a Tier I Canada Research Chair to J.S.T.

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REFERENCES

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Inorganic Chemistry

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