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Triphenylide-Based Molecular SolidA New Candidate for a Quantum Spin-Liquid Compound Aleš Štefančič,†,‡ Gyöngyi Klupp,†,∥ Tilen Knaflič,# Dmitry S. Yufit,† Gašper Tavčar,# Anton Potočnik,#,⊥ Andrew Beeby,† and Denis Arčon*,#,§ †

Department of Chemistry, Durham University, Durham DH1 3LE, U.K. Jožef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia # Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia § Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia ‡

S Supporting Information *

ABSTRACT: The reduction of polycyclic aromatic hydrocarbons with alkali metals results in solids having intriguing magnetic properties the understanding of which has been hitherto severely hampered by the lack of single-phase samples. Here, we report on the successful reduction of triphenylene with stoichiometric amount of potassium in 1,2-dimethoxyethane (DME) solution. Comprehensive diffraction measurements of the obtained K2(C18H12)2(DME) solid demonstrate the importance of cation-π interactions as responsible for the characteristic stacking of the triphenylide molecular ions. Electron paramagnetic resonance and magnetization measurements reveal K2(C18H12)2(DME) is a Mott insulator with strikingly strong nearest neighbor antiferromagnetic interactions between S = 1/2 spins of (C18H12)•− radical anions. Low dimensionality hinders long-range magnetic ordering and establishes a spin state that resembles gapped quantum spin liquid state.

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INTRODUCTION Low-dimensional spin systems1,2 are an important class of emerging quantum materials in which strong quantum spin fluctuations can prevent classical long-range magnetic ordering even at temperature T = 0 K. Quantum spin liquid state, which is typically encountered in low-dimensional and geometrically frustrated lattices, with its unconventional properties (e.g., fractionalized spin excitations) serves as an ideal test bed for new concepts in quantum many-body physics. While search for new candidates for quantum spin-liquid states has traditionally focused on inorganic systems,1,3−5 several compounds based on organic components were also proposed to host such a state.6,7 These latter belong to a class of molecular solids, where the limited overlap of molecular orbitals lead to narrow electronic bandwidths, W. In the presence of correlation effects characterized by on-site Coulomb repulsion U, the Mottinsulating antiferromagnetic state is frequently established.6−10 For instance, in the case of organic Mott-insulating antiferromagnet, κ-(BEDT-TTF)2Cu2(CN)3 (here BEDTTTF stands for bis(ethylenedithio)tetrathiafulvalene),9,10 a triangular spin arrangement of BEDT-TTF dimers (each accommodating a spin of S = 1/2) creates a spin-liquid ground state where strong quantum fluctuations enhanced by competing exchange interactions suppress a magnetically ordered state down to T ≈ 30 mK. Polycyclic aromatic © 2017 American Chemical Society

hydrocarbons (PAHs), organic molecules made of fused conjugated aromatic rings, with their natural packing preference arising because of the π molecular-stacking11,12 are also expected to show similar low-dimensionality and geometrical frustration effects. They thus seem to be an ideal starting point to search for new compounds that host intricate quantum spin states. The progress in understanding the properties of PAH-based molecular solids has been largely impeded by difficulties involving controllable PAH reduction. Recent studies, where PAH salts were synthesized directly by diffusion controlled solid state reactions of PAHs and alkali metals or a halogen element, yielded materials with intriguing properties, including claimed superconductivity13−16 and low-dimensional magnetism.17−19 However, these samples either suffered from extremely poor crystallinity and likely multiphase composition or their structures were not determined. Therefore, it is imperative to develop new synthetic routes that would yield single phase reduced PAHs with a high degree of crystallinity. In this work, we utilize a synthetic route involving reactions in solution generally known to result in homogeneous and Received: March 23, 2017 Revised: May 10, 2017 Published: June 9, 2017 14864

DOI: 10.1021/acs.jpcc.7b02763 J. Phys. Chem. C 2017, 121, 14864−14871

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The Journal of Physical Chemistry C crystalline products. The first reduction of PAHs with sodium was achieved by W. Schlenk already in 191420 and later a variety of reactions between PAHs and alkali metals have been successfully carried out in aprotic coordinating organic solvents.21−24 Nevertheless, affording these materials in crystalline form, which allows the determination of crystal structure, still poses a challenge19,25 and the unexplored electronic and magnetic response of these materials are serving with surprises. Within the family of PAHs the triphenylene (C18H12) is a remarkably stable molecule toward reduction26 and has one of the strongest electron−phonon coupling.27,28 In previous attempts to prepare Kx(C18H12) (with x = 1−3), electron paramagnetic resonance (EPR) and infrared spectroscopy confirmed a charge transfer from the alkali metal to triphenylene, and the dc conductivity with a low activation energy may suggest a semimetallic ground state.27 At this point, we stress that π-interactions between the potassium cations and the benzene rings of PAH anions, present when alkali metal coordinates to PAH,25,29−33 are expected to be strong25,34−36 and could critically affect the structural, magnetic and transport properties of the reduced solid. The charge transfer from the alkali metal to the PAH molecule is found theoretically and experimentally to be complete7,23,37 and as a result an ionic coordination bond is established by electrostatic forces that lead to interatomic contacts within the sum of the van der Waals radius of C and the ionic radius of the cation.25,38 However, the effects of these interactions on the stacking motif and the possible deformation of triphenylene aromatic system and thus the emerging electronic properties have not been previously discussed for Kx(C18H12) because of lack of single-phase samples and the structural data. Here we report the reduction of triphenylene with metal potassium in 1,2-dimethoxyethane (DME) solution yielding solid K2(C18H12)2(DME) (1). In the structure of 1, the K+ cations coordinate to terminal rings of triphenylene, which is then governing the packing of triphenylide anions. The molecules are magnetically coupled with strikingly strong antiferromagnetic exchange interactions within a Mott insulating state. Despite strong antiferromagnetic exchange interactions, no magnetic long-range order is established. The magnetic ground state is consistent with that of the gapped quantum spin liquid state, which emerges from the combination of structural low-dimensionality and geometrical frustration effects. Since cation-π interactions play an important role in the stacking of triphenylide anions, they may have a major effect also on the derived magnetism. We thus suggest exploiting these interactions for the design of new quantum spin materials based on PAH molecules.

two crystallographically independent triphenylide anions (here denoted as Tri1 and Tri2), one DME molecule and two potassium cations (denoted as K1 and K2, see Figure S1). Powder X-ray diffraction (Figure S2) together with elemental analysis verifies the phase-pure nature of the sample. The refinement of powder X-ray diffraction data fully corroborates the single crystal data. Moreover, no additional peaks that could be assigned to any impurity phase were observed. The anisotropic thermal parameters found by single crystal diffraction also did not show any anomaly, which would suggest the off-stoichiometry of 1. We also note that between 293 and 120 K there is no structural phase transition. The key structural building block of 1 consists of potassium ions coordinated to the nearest neighboring triphenylide anions and DME molecule (Figure 1): K1 is coordinated to both

Figure 1. Local coordination interactions of K1 and K2 cations (lilac color) with four neighboring triphenylide molecular ions and one DME molecule in the crystal structure of 1. The two independent triphenylene molecules are denoted with Tri1 (pale yellow planes) and Tri2 (pale blue planes). The oxygen atoms of the DME molecule are shown in red. Thermal ellipsoids are shown at 50% probability level and hydrogen atoms are omitted for clarity. The numbering of atoms refers to their labels in the CIF file of the Supporting Information.

oxygen atoms of DME and carbon atoms of two nearest neighboring Tri1 molecules. K2 coordinates to a single oxygen atom of DME and three triphenylide anions (two Tri1 and one Tri2). The K−O(DME) ion-dipole bonds are the strongest interactions in the crystal, but K−C interactions, which determine the position and orientation of the triphenylide ions directly, are also expected to be strong. The short K− C(triphenylene) contacts varying from 3.04−3.46 Å (Figure 1 and tabulated in Table S1) signal the importance of the cationπ interaction. These distances are in the expected range for potassium ions coordinated to PAH anions and echo the formation of multihapto-bonds.25,29−33 The K1 potassium cation is located in a slightly off-centered position with respect to the centroids of peripheral benzene ring planes I and III, interacting with them through η5 and η6 coordination modes, respectively (Table S1). The position of K2 is also somewhat off-centered from the middle of the benzene ring planes I, III, and IIIa, coordinated to them with η3, η6 and η5 hapticities (Figure 2 and Table S1). The structure of 1 is confirming the complete rearrangement of triphenylene moieties compared to the herringbone structure of pristine triphenylene39 to optimize the cation-π and DME ligand interactions analogous to rearrangements previously reported in a variety of other alkali metal reduced PAH structures.25

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RESULTS The reduction of triphenylene with a stoichiometric amount of potassium metal in DME solution immediately leads to the lavender-purple coloration, which is a hallmark of the triphenylide radical anion, (C18H12)•−, formation.27 The growth of highly air-sensitive needle-like, dark purple/black crystals with blue luster was observed after layering the solution with npentane. X-ray structural analysis of isolated crystals performed at T = 120 K revealed that the product crystallizes in the orthorhombic space group P212121 with a unit cell of a = 10.2589(3) Å, b = 15.7908(5) Å, c = 18.7420(6) Å, and the unit cell volume of V = 3036.13(16) Å 3 (see Supporting Information). The asymmetric part of the unit cell comprises 14865


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The Journal of Physical Chemistry C

Figure 2. Facial projection of Tri1 (a) and Tri2 (b) molecular anions. The elongation (shortening) of C−C bonds, compared to that of pristine triphenylene39 larger than 0.01 Å, are given in units of Ångstroms and are represented with blue (red circle-headed) arrows. Potassium ions coordinating to triphenylide anions are depicted in lilac color and hydrogen atoms were omitted for clarity. Coordinates of the C atoms of Tri1 (c) and Tri2 (d) relative to the plane defined by the principal and secondary inertial axes of the molecule (shown with yellow and cyan planes for Tri1 and Tri2, respectively). The orientation of the molecules is the same as that of their counterparts in panels a and b. The out-of-plane distortion of the molecular ions are visualized by having a stretched z axis displaying the deviation in these coordinates from the ideal planar configuration. The full list of the bond length changes and the out-of-plane displacements of the atoms are collected in Table S2.

Figure 3. Room-temperature infrared (top panel) and Raman (bottom panel) spectra of K2(C18H12)2(DME) (colored lines) and pristine triphenylene (black lines). The manifold of peaks in the infrared (red line) and in the Raman spectrum obtained with 532 nm excitation (green line) evidence the large distortion of the triphenylene molecule. The asterisks in the infrared spectrum mark the peaks of a small amount of pristine triphenylene produced during pellet preparation, which was absent in the control attenuated total reflection (ATR) measurement (Figure S4). The Raman spectrum of K2(C18H12)2(DME) with 785 nm excitation (brown) and the spectrum of its DME solution with the DME peaks subtracted (violet) is also depicted proving the presence of sole triphenylide monoanion in the solid.

A further important structural feature of 1 is the statistically clearly present distortion of the molecules both in the in-plane and the out-of-plane directions (Figure 2, Table S2). The bond lengths of the molecule change upon reduction in an asymmetric manner with the largest change as 0.050(6) Å for Tri1. The out-of-plane distortion has a considerable size with the largest displacement of 0.100(2) Å at 120 K. The change in molecular geometry involves a twist of the outer benzene rings in an asymmetric fashion leading to the lowering of the anticipated D3h symmetry of triphenylide ions to C1 symmetry. Although the precise shape of the out-of-plane distortion is slightly different for the Tri1 and Tri2 units, the pattern of the distortions is still alike to that in pristine triphenylene (Figure S3), where the distortion is governed by the intramolecular crowding of bay hydrogens.40−42 The less pronounced differences in the magnitude of the out-of-plane movement for specific atoms between the two anions and the neutral molecule (Figure S3) arise mainly from the K+−π coordination resulting in attractive forces that pull the nearby C atoms in the direction of K+ ions. Similar molecular distortions originating from the cation-π interactions have been found across the whole family of alkali−metal−PAH compounds.25 The electronic properties were first probed by infrared spectroscopy. The spectrum shows neither an enhanced background nor pronounced Fano line shapes43 (Figure 3). We therefore conclude that 1 is not a conventional metal but rather an insulator. Since charge disproportionation is a common phenomenon in organic semiconductors,44,45 we next address the reduction state of the triphenylene molecules in 1. In Figure 4a we compare the visible spectra of solid 1 and

Figure 4. (a) Room-temperature visible and near-infrared spectrum of K2(C18H12)2(DME) (black) and its solution in DME (purple). The inset visualizes the singly occupied molecular orbital (SOMO) of Tri1, which is occupied by the extra electron of the molecule, with the SOMO of Tri2 having a similar shape. (b) Schematic energy diagram of the SOMO and the lower unoccupied molecular orbitals illustrating the effect of symmetry reduction caused by the distortion of the molecule. The excitations appearing in the visible range are indicated with black arrows.

1 dissolved in DME. In solution synproportionation between molecules with different charge states is facilitated. Therefore, 14866


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The Journal of Physical Chemistry C the solution of 1 can only contain monoanions of triphenylene. The positions of the absorption peaks of solid 1 coincide nicely with those of the solution, which are in perfect agreement with the spectrum published for triphenylide monoanions.21,24 Furthermore, an excellent matching of the Raman spectra for 1 in DME solution and solid 1 (Figure 3) provides an additional evidence for the sole presence of monoanions also in the solid. This proves the complete charge transfer between K and triphenylene and shows the expected ionic character of the K+−π coordination bonds. We conclude that the observed insulating state is not a consequence of charge disproportionation. To address the effect of triphenylide molecular anion deformations, we next performed density functional theory (DFT) calculations for the triphenylide monoanion in the geometries of Tri1 and Tri2. As expected, the C1 distortion of both molecular ions removes the degeneracy of the pair of lowest unoccupied molecular orbitals (LUMOs) that would be adopted in a symmetric molecule with D3h point group (Figure 4b) and lowers orbital energy of both molecules. The splitting of the E″ LUMO of a symmetric triphenylene, ΔE, is 0.13 eV for both Tri1 and Tri2. The lower of the two E″ LUMOs, whose shape is depicted in the inset of Figure 4a, is thus singly occupied with the electron donated by the potassium metal. Its bonding−antibonding nature is reflected in the C−C bondlength changes determined by the single crystal X-ray diffraction (Figure 1). The E″ LUMOs on neighboring triphenylide monoanions in the absence of electron correlations thereby form a half-filled metallic band. To explain the insulating state of 1 other interactions, such as electron correlations or electron-phonon coupling, have to be taken into account. The vibrational excitations of the triphenylide monoanion probed by infrared and Raman spectroscopy (Figure 3) are next compared to those of neutral triphenylene (Table S3). The Raman spectra obtained with the two different laser excitations are not the same because of resonance effects. The 532 nm and the 785 nm laser light excite transitions to the former A1″ and E″ orbitals, respectively, which couple resonantly and thus enhance different vibrations. The peak-assignment (Table S3) implies charge induced softening due to the average decrease of bond strengths, in accordance with calculations on a flat triphenylide monoanion.46 The peaks are also noticeably broader than in pristine triphenylene. The broadening cannot be attributed to unresolved splittings only. Instead, the magnitude of the broadening provides a firm evidence of a significant electron−phonon coupling. Since the peak widths of the Raman peaks of the solid bear a remarkable resemblance to that of the solution, we conclude that the intramolecular vibrations couple to the molecular electronic excitations. Moreover, the observation of a sizable downshift of vibrational frequencies also signifies strong electron−phonon coupling in agreement with theoretical predictions.28 The magnetic response of insulating 1 depends on the packing of the PAH building blocks in the network of antiferromagnetically coupled S = 1/2 triphenylide anions. The characteristic structural motif of 1 is the chain of Tri1 units running along the shortest crystallographic axis a (Figure 5). The nearest neighboring Tri1 molecular ions of the chains form an angle of 62.60(3)o and are rotated (because of the 2-fold screw axis) by 180° with respect to each other. The neighboring chains of Tri1 units are separated by Tri2 molecular ions that are each connected to the main chain via

Figure 5. (a) Projection of structure 1 along [100] crystallographic direction showing a two-dimensional arrangement of chains. (b) A chain with a backbone of Tri1 units running along [100] (the DME molecules are omitted for clarity). The red triangle indicates a geometrical frustration in antiferromagnetic exchange interactions introduced by the triangular arrangement of Tri1 and Tri2 units. The color code is the same as in Figure 1.

the K2 cation coordinating to the triphenylide terminal ring plane IIIa. Although the complex arrangement of Tri1 and Tri2 units implies an elaborate network of exchange interactions between neighboring S = 1/2 triphenylide ions, the structural anisotropy is nevertheless suggestive of low-dimensional quantum magnetism. Magnetization measurements (Figure 6a) combined with complementary EPR spectroscopy (Figure 6b and c) is an established approach to study magnetism in low-dimensional molecular solids.47−53 The X-band EPR spectrum measured between room temperature and 4 K on a single crystal of K2(C18H12)2(DME) for a magnetic field B ⊥ a comprises an extremely strong and very narrow signal at g = 2.0028 (Figure 6b). The room-temperature EPR spectrum can be fitted to a Lorentzian line shape with the line width of 0.158(1) mT. The calculation of an EPR second moment M2 for dipolar interactions between S = 1/2 triphenylide ions in the lattice of 1 yields ΔBdip = M 2 = 18.4 mT, which is by more than 2 orders of magnitude larger than the measured EPR line width. The extreme narrowness of the EPR signal is therefore consistent with the exchange narrowing regime driven by the strong exchange interactions between nearest neighboring triphenylide spins. On cooling, the line shape remains Lorentzian, while its line width is progressively narrowed (not shown) reaching 0.042(1) mT at 60 K. Below ∼50 K deviations from a single Lorentzian line shape can be observed and another slightly anisotropic signal gradually increases in intensity. The latter EPR signal is very weak even at the lowest temperatures below 10 K. However, as the high-temperature EPR signal completely vanishes at temperatures below ∼20 K, the anisotropic signal dominates the EPR spectrum at 4 K (Figure 6b). Its g-factor and the line width are temperature independent (g = 2.0032(3) and ΔB = 0.33(2) mT) whereas its intensity follows a Curie-like dependence (Figure 6c) and corresponds to 0.6% of all triphenylide units. Additional control measurements of calibrated L-band EPR spectra on powder sample (Figure S5), show slightly stronger Curie tail (Cimp = 4.2 × 10−3 emu/mol K) corresponding to 0.7% of all triphenylide molecular anions (Table S4) and are in agreement with the magnetic susceptibility data (Figure 6a). The anisotropic signal is thus attributed to structural defects or other unidentified paramagnetic impurities and not to some 14867


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measurements (Figure S5). On further cooling, the intensity of the Lorentzian EPR signal completely vanishes below ∼20 K. The rapid suppression of χEPR (and likewise χSQUID) below Tmax implies the existence of a spin-gap Δ in the spin excitation spectrum (at least for temperatures down to T ≈ 20 K, that is, to temperature where the high-temperature EPR signal can still be traced). A logarithmic plot of χEPR T1/2 versus 1/T (inset to Figure 6c) demonstrates that the low-temperature spin susceptibility of 1 follows χ ∝

1/2

( ΔT )

exp( −Δ/T ) which is

applicable to gapped low-dimensional antiferromagnets,54 yielding Δ = 120 ± 1 K. The insulating 1 emerges as an intriguing candidate for lowdimensional quantum molecular magnetism. The main hallmark, the temperature dependence of magnetic spin susceptibility exhibiting a maximum at Tmax, may be found for a number of spin models, including alternating spin chains (spin chains with alternating antiferromagnetic interactions J and αJ between two nearest neighbors; here α is the alternating parameter),55 spin ladders (spin ladders are formed when two parallel spin chains with exchange interactions Jrail between nearest neighboring spins along the chains are exchange coupled via interaction Jrung) or perhaps isolated dimers.56 An excellent fit of the data can be found for an alternating S = 1/2 spin chain model and equally good for a spin ladder with strong rung exchange (Figure 6a and c). In the former case, we extract from EPR data the exchange strength J = 268 ± 4 K and the alternating parameter α = 0.59 ± 0.02, whereas in the latter case Jrail = 152 ± 6 K and Jrung = 226 ± 2 K (Table S4). We stress that the present data does not allow us to unambiguously distinguish between these two (or any other similar) quantum spin models. However, the data clearly demonstrates that magnetically the main building block comprises a pair of antiferromagnetically coupled S = 1/2 triphenylide anions. These dimers are coupled through a network of non-negligible interdimer exchange interactions giving rise to a quantum magnetism characterized by a spin-gap in the ground state.

Figure 6. (a) The evolution of the static spin susceptibility of K2(C18H12)2(DME) with temperature measured by the superconducting quantum interference device (SQUID) magnetometer on randomly oriented crystals at a magnetic field of 1 T (open circles). The solid red line is a fit to strong rung ladder model yielding Jrail = 138 ± 8 K and Jrung = 269 ± 2 K. The low temperature Curie-like contribution attributed to 1.6% impurities has been also considered in the fitting. (b) X-band EPR spectrum of K2(C18H12)2(DME) single crystal measured at 100 K (solid black circles) is fitted to a Lorentzian line shape (solid red line). The EPR spectrum at 4 K is composed of a very weak impurity line fitted to an uniaxially anisotropic g-factor line shape (solid red line). (c) Temperature dependence of spin-only susceptibility of the main spectral component as measured by X-band EPR on a K2(C18H12)2(DME) single crystal (open circles). The solid black circles present the EPR intensity of the weak impurity line that could be fitted to a Curie−Weiss law with a negligibly small Curie− Weiss temperature (Table S4) and the Curie constant corresponding to 0.6% impurities. The solid red line shows a fit to the strong rung ladder model56 (data are fitted only down to 50 K where the hightemperature Lorentzian line dominates the spectrum and its intensity is unambiguously determined without the disruption caused by the presence of the impurity line). The inset shows a fit to the natural logarithm of spin-only EPR susceptibility versus inverse temperature. Data highlighted within the gray area are used to extract the spin gap Δ = 120 ± 1 K.

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DISCUSSION The applied method of triphenylene reduction with an alkali metal in DME solution enables us to isolate single phase and perfectly crystalline samples of 1, where the structure can be determined with high precision and related to the electronic and magnetic properties. K+ ions exhibit slightly off-centered coordination to singly charged triphenylide ions to achieve advantageous cation-π interactions. For the electronic structure of the K+−triphenylide ion pair the unpaired electron has been calculated to reside on the triphenylide ion,23 which is in agreement with the mononegative charge state supported by the Raman and visible spectroscopy and nearly free electron value of EPR g-factor. The K+−π coordination bond is thus ionic and its attractive force is also partially responsible for the out-of-plane distortion of triphenylene units. This effect together with intramolecular crowding is sufficient to remove the degeneracy of E″ LUMO. As a result, the electronic structure of 1 is built up from an isolated singly occupied molecular orbital. The packing motif of triphenylide ions in the structure implies low-dimensional electronic properties. The intermolecular contacts where the closest C−C distance is 3.45 Å between Tri1 and Tri2 molecules would naively suggest a metallic state with a very narrow bandwidth W and a half-filled conduction band. However, in sharp contrast to claimed metallicity and superconductivity in certain alkali-metal-PAH

low-temperature magnetically ordered state. Finally, we stress that the disappearance of the high-temperature signal at low temperatures is not accompanied by the emergence of another signal that could be attributed to the antiferromagnetic resonance. We thus conclude that no long-range antiferromagnetic ordering takes place down to 4 K. The EPR signal intensity of the exchange-narrowed Lorentzian line first increases with decreasing temperature, but then reaches a maximum at TEPR max = 160 K. This maximum differs from the maximum found in measurements of the magnetic susceptibility by 15 K (i.e., TSQUID = 175 K). The max difference in chemical stoichiometry (e.g., a minuscule difference in the DME content invisible by diffraction) between single crystal and powder samples is most likely the reason for this discrepancy. The maximum found from the complementary L-band EPR measurements of the powder sample, indeed coincides nicely with that of the magnetic susceptibility 14868


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The Journal of Physical Chemistry C materials containing open shell molecular ions13−16 we find 1 to be insulating. We stress that the metallic/superconducting state has been experimentally ruled out in potassium intercalated picene and coronene films57,58 as well as in cesium doped phenanthrene7 whereas the importance of electron correlations in such systems became evident from the ab initio calculations.59,60 Although previous theoretical investigation of La-phenanthrene suggested charge disproportionation of two inequivalent polyaromatic molecular ions,45 the present observation of triphenylide monoanions in 1 clearly rules out such possibility as the driving force for charge localization. On the other hand, the interplay of electron−electron interactions and the electron−phonon coupling is known to lead to three different competing states: metallic, bipolaronic, and Mott insulating phases.61,62 The metallic state is ruled out by infrared spectroscopy data (Figure 3) and the Lorentzian EPR line shape (Figure 6b), whereas the bipolaronic state is eliminated by the large spin susceptibility. If the original triphenylene double degeneracy of LUMO is retained, then a single electron transferred from the potassium would lead to a 1/4 filled band, where the effect of electron correlations is diminished and 1 should be a metal. However, because of the loss of the 2-fold LUMO degeneracy 1 is actually a correlated half-filled metal and is therefore easily driven to the Mott insulating state,63,64 as experimentally observed. Within such Mott insulating state, we found exceptionally strong intermolecular antiferromagnetic exchange interactions significantly exceeding the expected range of organic molecular materials.65 For instance, in recently reported Cs(C14H10) (ref 7), the antiferromagnetic exchange between nearest neighboring (C14H10)•− radical anions is for about 3−4 times weaker compared to 1. In this respect we recall that important interactions in the crystal are the K+−π-interactions providing intermolecular distances well within the van der Waals contacts of adjacent atoms. These interactions are important for the stacking of triphenylide units and tune both the direct exchange as well as superexchange pathways through the p orbitals of K+ ions in close analogy to other low-dimensional p-electron antiferromagnets.3−5 The resulting network of exchange interactions between neighboring triphenylide units becomes complicated, as reflected in our difficulties in identifying a single antiferromagnetic quantum spin model to describe the temperature evolution of χ(T). Nevertheless, we find a remarkable absence of long-range magnetic ordering which is a hallmark of a robust gapped spin-liquid state over broad temperature range. This state is likely stabilized by a combination of frustration introduced by the triangular arrangement of Tri1 and Tri2 units (Figure 5b) and lowdimensionality, which are inherent to PAH-derived molecular solids. Such concept leading to spin liquid states seems to be more general than believed so far.7,66

the surprisingly strong antiferromagnetic exchange interactions controlled by the alkali-cation-π interactions. This feature may open a new avenue for the synthesis of reduced PAH solids where the quantum spin state of novel materials based on all carbon π-orbitals can be tailored and in the future expand the family of organic molecular antiferromagnets beyond BEDTTTF type compounds.

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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.jpcc.7b02763. Crystal data for 1 (CIF) Experimental details and additional results of structural, spectroscopic, and magnetic studies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gyöngyi Klupp: 0000-0001-8062-1760 Denis Arčon: 0000-0002-1207-8337 Present Addresses ∥

G.K.: Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary. ⊥ A.P.: Department of Physics, ETH Zürich, CH-8093 Zürich, Switzerland. Author Contributions

D.A., A.Š., and A.P. conceived the project. D.A. and A.B. directed and coordinated the research. A.Š. synthesized the material. A.Š. and D.S.Y. carried out structural measurements. A.Š., D.S.Y., and G.K. discussed the structure. A.Š. conducted magnetization measurements, and T.K. and D.A. interpreted magnetization data. G.K. carried out optical spectroscopy measurements and DFT calculations, and G.K. and A.B. discussed its results. T.K., A.P., and A.D. carried out and discussed EPR work. A.Š., G.T., A.P., and D.A. conducted early synthesis and magnetic work. The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.K. acknowledges the Newton International Fellowship of the Royal Society. D.A. acknowledges the financial support of the Slovenian Research Agency through grant No. N1-0052. G.T. acknowledges the financial support of the Slovenian Research Agency through grant No. P1-0045. We thank the help of Katalin Kamarás with the infrared measurements.

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CONCLUSIONS In this study, we demonstrated that the reduction of triphenylene with a stoichiometric amount of potassium metal in DME solution allows the synthesis of a highly crystalline compound 1 whose crystal structure has been precisely determined. The structure is stabilized by the optimization of alkali-cation to PAH π-interactions generating the deformation of PAH units and the characteristic stacking of triphenylide anions. These materials are Mott insulators showing exciting quantum magnetism originating from the special packing of electronically active triphenylide anions and

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REFERENCES

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