Correlation between MetalâInsulator Transition and Hydrogen

Article pubs.acs.org/crystal

Correlation between Metal−Insulator Transition and HydrogenBonding Network in the Organic Metal δ‑(BEDT-TTF)4[2,6Anthracene-bis(sulfonate)]·(H2O)4 Franck Camerel,† Guillaume Le Helloco,† Thierry Guizouarn,† Olivier Jeannin,† Marc Fourmigué,*,† ,‡ ́ Arkadiusz Frąckowiak,‡ Iwona Olejniczak,‡ Roman Swietlik,* Andrea Marino,§ Eric Collet,§ § ∥ ,⊥ Loic Toupet, Pascale Auban-Senzier, and Enric Canadell* †

Institut des Sciences Chimiques de Rennes, Université Rennes 1, CNRS UMR 6226, Campus de Beaulieu, 35042 Rennes, France Institute of Molecular Physics, Polish Academy of Sciences, ul. Mariana Smoluchowskiego 17, 60-179 Poznań, Poland § Institut de Physique de Rennes, Université Rennes 1, CNRS UMR 6251, Campus de Beaulieu, 35042 Rennes, France ∥ Laboratoire de Physique des Solides, Université Paris-Sud, UMR CNRS 8502, Bât. 510, 91405 Orsay, France ⊥ Institut de Ciència de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra, Spain ‡

S Supporting Information *

ABSTRACT: The sensitivity of electronic properties of organic conductors to minute modifications of their solidstate structure is investigated here within BEDT-TTF (ET) salts with organic bis-sulfonate anions, where specific hydrogen bonds between water molecules and sulfonate moieties are shown to dynamically control the organic slabs’ electronic structure. While the mixed-valence, 2,6-naphthalene-bis(sulfonate) salt, (ET)4(NBS)·4H2O, exhibits a charge order state already at room temperature, the corresponding salt with the 2,6-anthracene-bis(sulfonate) dianion, formulated as (ET)4(ABS)·4H2O, is metallic at RT and exhibits a metal− insulator transition at TMI = 85 K. The origin of the MI transition is revealed from a combination of temperature-dependent spectroscopic (Raman) measurements, X-ray structure elucidations (from 300 to 15 K), and theoretical investigations, demonstrating that the charge disproportionation observed below TMI is associated here with the progressive switching of bifurcated OH···O hydrogen bonds between the sulfonate moieties of the anion and the trapped water molecules. These movements within the anion layer are transmitted through weaker C−H···O interactions to the two A and B donor molecules, modifying the details of the overlap interactions within AA and BB pairs and opening a gap in the band structure.

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all the more pertinent in nondimerized, 3/4 filled systems where 4kF instabilities are achieved by either the localization of each carrier within Mott dimers (MD) associated with a stack dimerization or a charge ordering (CO) with charge-rich molecules alternating with charge-poor molecules.4 In these situations, the structural intra- or intermolecular modifications are intimately linked to anion’s movements. However, detailed analyses of such correlated movements are very scarce as they most often involve weak C−H···X (X = O, N, Hal) hydrogen bonds, with, for example, subtle modifications of the methyl groups’ orientation in TMTTF or TMTSF salts, or of the ethylene bridge conformations in ET salts. In a very few instances only, CO was clearly identified as the result of the displacements of counterions, as, for example, in TMTTF or TMTSF salts with noncentrosymmetric anions thanks to anion

INTRODUCTION The electronic properties of organic conductors based on cation radical salts from tetrathiafulvalene derivatives, such as the prototypical TMTTF/TMTSF or BEDT-TTF salts, are known to be highly sensitive to minute modifications of their solid-state structure. This peculiarity makes such salts a magnificent playground for solid-state physics since a variety of different ground states with associated phase transitions can be revealed under application of external stimuli, such as temperature, pressure, or magnetic field. One point of importance is found in the details of the precise interactions between the organic stacks or slabs and the anion layers.1 Indeed, in many instances, the prototypical 2kF charge density wave (CDW) instability does not appear to be driven by a simple intrastack Peierls-type electron−phonon coupled mechanism, but seems to be more likely triggered by the anion layer, as recently outlined by Pouget, in Fabre and Bechgaard salts,2 or in α-(ET)2KHg(SCN)4.3 These effects are © 2013 American Chemical Society

Received: September 23, 2013 Published: October 1, 2013 5135

dx.doi.org/10.1021/cg401416h | Cryst. Growth Des. 2013, 13, 5135−5145

Crystal Growth & Design

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ordering (AO) transitions,2 or in (EDT-TTF-CONMe2)2Br,4 α-(ET)2KHg(SCN)4,3 or α-(ET)2I3.5 On these bases, one could anticipate that the modulation of stronger hydrogen bonds between donor and anion layers, or within the anion layer, could be associated with stronger structural modifications upon CO transitions. Such strategies have been indeed explored, with TTF derivatives functionalized with hydrogen bond donor substituents,6 such as alcohols,7 acids,8 amides,9 or imidazoles,10 and able to form hydrogen bonds with themselves and/or with the counterions.11 In some rare instances, a coupling between charge disproportionation and hydrogen-bonding motifs was identified, as the donor protonation increases indeed its oxidation potential.12 Another strategy uses hydrogen-bonded networks of anions, such as dicarboxylate monanion,13 dihydroxyquinone monoanion,14 or para-carboxybenzenesulfonate anion.15 To our knowledge, however, there is no example where a metal−insulator transition can be directly associated with a defined modification of the hydrogen bond pattern with or within the anion layers, as the hydrogen bond networks present in those salts are most often quite robust. We have found and described here one remarkable example where hydrogen-bonding interactions between water molecules and sulfonate anions exhibit a striking evolution upon cooling, associated with a concomitant metal− insulator transition. Organic anions, such as sulfonates, have been shown to act as very good hydrogen bond acceptors16 and have been investigated also in conducting cation radical salts with TTF derivatives.17 Besides aliphatic sulfonates, such as F3C− SO3−,18,19 or F5SCH2CF2SO3−,16,20 aromatic sulfonates were also investigated, for example, C6F5−SO3−, C6H5−CH2− SO3−,21 HO2C−p−C6H4−SO3−,22 N,N′-disulfo-1,4-benzoquinonediimine,23 or 1,1′-ferrocene-disulfonate.24 Their BEDTTTF salts are all characterized by the presence of C−H···O hydrogen bond interactions. Sulfonate anions substituted with aminoxyl radical species were also successfully used as counterions in purely organic paramagnetic semiconductors25,26 and metals.27 We have also recently investigated a series of bis-sulfonate anions that can act simultaneously as counterions and as halogen bond acceptors (rather than hydrogen bond acceptors) toward iodinated tetrathiafulvalenes, exhibiting strong CTTF−I···O3S−R halogen bonds.28,29 Among the centrosymmetric bis(sulfonate) anions, only 1,5-naphthalenedisulfonate had been engaged by Williams et al. with the prototypical BEDT-TTF donor molecule,20 while cation radical salts with other similar anions, such as 2,6-naphthalene- and 2,6-anthracene-bis(sulfonate), abbreviated NBS and ABS in the following (see the scheme below), were not described. We report here the electrocrystallization of their ET salts, and the structural and electronic properties of the two salts. It will be demonstrated that subtle modifications of the solid-state organization within 2,6-anthracene disulfonate anion layers, driven by switching of hydrogen-bonding interactions, prove to control in a striking way the transport properties of the whole salt.

Article

RESULTS AND DISCUSSION Syntheses. The tetraphenylphosphonium salts of naphthalene-2,6-bis(sulfonate) and anthracene-2,6-bis(sulfonate) used as electrolytes were prepared by metathesis of the known potassium salts and followed by careful recrystallization from MeOH/Et2O before electrocrystallization experiments.30 (PPh4)2(NBS) was found to crystallize in the monoclinic system, space group P21/n, with the PPh4+ cation in the general position and the NBS dianion on the inversion center (Figure 1a). (PPh4)2(ABS) crystallizes as MeOH solvate in the triclinic

Figure 1. ORTEP views of the dianionic species in (a) (PPh4)2(NBS) and (b) (PPh4)2(ABS)·(MeOH)2, at 150 K, with ellipsoids at the 50% probability level. Hydrogen bonds are shown with red dotted lines and exhibit the following structural characteristics: H1B···O2A 1.969(3) Å, O1B−H1B···O2A 170.6(1)°.

system, space group P1̅, with the PPh4+ cation and MeOH molecule in the general position and the ABS molecule on the inversion center. As shown in Figure 1b, the sulfonate anions are hydrogen-bonded to the MeOH molecules, demonstrating already here their hydrogen bond acceptor capability. In both structures, the PPh4+ cations adopt the so-called phenyl embrace dimeric structure.31 Electrocrystallization of BEDT-TTF (ET) in 1,1,2-trichloroethane in the presence of (PPh4)2(NBS) or (PPh4)2(ABS)· (MeOH)2 afforded, after several days, platelike crystals on the anode. With NBS, a salt formulated as (ET)4(NBS)·4H2O was isolated. Its stoichiometry indicates a mixed valence character, with four ET molecules for one dianion. It crystallizes in the monoclinic system, space group P21/a, with two crystallographically independent ET (noted mol. A and mol. B), and water molecules in the unit cell, and the dianion on the inversion center. Note that one of the two ethylenic moieties in mol. B is disordered on two positions, as well as the SO3− anionic substituent. The solid-state organization within the donor slab can be described as a δ-phase (Figure 2).32 Such phases are found, for example, in δ-(ET)2PF6,33 δ(ET)2[C(CN)3],34 δ-(ET)4[Ni(CN)4]·(H2O)4,35 or δ′-(ET)2FeCl4.36 Indeed, along the stacking axis a, we observe an alternation of molecules A and B, together with an alternation of overlap patterns, with the so-called ring-over-atom (RA) overlap mode (Figure 3a), and a twisted overlap mode (Figure 3b). The organic slabs are separated from each other by anionic layers incorporating the NBS dianion and water molecules, organized into centrosymmetric tetramers, with O···O dis5136



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presence of disorder on the sulfonate moieties hinders a detailed analysis of hydrogen-bonding interactions between them and water molecules. Despite its mixed valence character, this salt behaves as a semiconductor, with a low roomtemperature conductivity, σRT = 10−2 S cm−1, and a large activation energy, Eact = 0.15 eV (Figure S1 in the Supporting Information). The origin of this behavior can be found in the charge distribution within the two crystallographically independent molecules A and B. Indeed, a side view of the two molecules (Figure 3a), in their RA overlap mode, clearly shows that molecule B is essentially planar while the dithiole rings in mol. A are strongly folded along the S−S hinges (by 16.1° and 19.8°), a characteristic of neutral, unoxidized molecules. Empirical calculations of the ET charges, based on the correlations developed by Day et al.37 between the intramolecular CC and C−S distances, and the actual charge, give calculated charges of ≈+0.23 and +0.95 for molecules A and B, respectively, indicating indeed that molecule A is essentially neutral and molecule B oxidized to the cation radical state. This extreme charge localization is associated with a segregation of the A and B molecules within the δ-type layers (Figure 3b) and explains the semiconducting behavior of this NBS salt. This charge distribution is further confirmed by Raman spectroscopic data (Figure S2 in the Supporting Information). Indeed, the ν2 ring stretching mode of the BEDT-TTF molecule is usually a very good indicator of the actual molecular charge as it is essentially unaffected by electron molecular vibration (emv) coupling.38 It is observed here at 1483 cm−1, corresponding to an estimated calculated charge of +0.70. As we will see below, the situation is different with the ABS salt. Formulated as (ET)4(ABS)·4H2O, it crystallizes in the monoclinic system, space group P21/c, with again two crystallographically independent ET molecules, one ABS dianion on an inversion center, and two water molecules. The solid-state organization within the organic slab can be also described as a δ-phase as above, but with notable differences (Figure 4). The two crystallographically independent ET

Figure 2. (a) Projection view along a of the unit cell of δ(ET)4(NBS)·4H2O. (b) View of the anionic layer showing the water tetramers.

Figure 4. Projection view along c of the unit cell of (ET)4(ABS)· 4H2O. Figure 3. Structure of δ-(ET)4(NBS)·4H2O: (a) Side view of two molecules interacting by ring-over-atom (RA) overlap mode, together with the central CC bond length (in Å). (b) Projection view of one δ-layer. (c, d) Overlap patterns between molecules A and B along the stacking axis in (c).

molecules organize into centrosymmetric dyads (AA) and (BB), which alternate both along the stacking c direction and lateral b direction into a chess-board pattern (Figure 5a). A ring-over-atom (RA) overlap mode is found within inversioncentered (AA) and (BB) dyads, and a twisted overlap mode between the two crystallographically independent molecules A and B (Figure 5b).

tances, 2.45(1) and 2.70(1) Å, indicating the occurrence of hydrogen bonds between water molecules. However, the 5137



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Figure 5. (a) Detail of the BEDT-TTF organization within the organic δ-slabs in (ET)4(ABS)·4H2O. The ethylene groups have been suppressed for clarity. (b) Detail of the overlap patterns.

The organic slabs are separated from each other by anionic layers incorporating the ABS dianion and water molecules. As shown in Figure 6, based on the shortest O···O distances, they

Figure 7. (a) Temperature dependence of the resistivity ρ (in mΩ cm) of (ET)4(ABS)·4H2O, at various pressures. In inset: idem, limited to ρmax = 25 mΩ cm. (b) Hysteretic behavior between warming and cooling modes at ambient pressure. In inset: evolution of d(ln(ρ))/ d(1/T) vs 1/T, giving an evaluation of the hysteresis width.

while the activation energy is decreased in the low-temperature regime at higher pressures. A hysteresis (Figure 7b) is observed between heating and cooling modes, indicating a first-order character for this metal−insulator transition. The temperature of the transition and the width of the hysteresis can be deduced from the evolution of d(ln(ρ))/d(1/T) vs 1/T (Figure 7b, inset). A hysteresis of 32 K is found at ambient pressure, decreasing under pressure to 8 K at 0.2 or 0.4 GPa (Figure S4 in the Supporting Information). The temperature dependence of the magnetic susceptibility (Figure S5 in the Supporting Information) shows a weak, essentially temperature-independent contribution at high temperature, with little variation down to 10 K, where a Curie tail becomes apparent. To understand the origin of this sharp metal−insulator transition and the nature of the low-temperature electronic ground state, we have particularly investigated the temperature dependence, (i) of the Raman spectroscopic properties, (ii) of the structural characteristics of the salts, and (iii) of the calculated band structures, as detailed below. Raman Spectroscopy. Room-temperature Raman spectra (Figure S6 in the Supporting Information) show numerous bands that can be assigned to the ABS dianion: 399, 645, 761, 775, 1003, 1032 (SO), 1188, 1256, 1408, 1556, and 1628 cm−1. Additionally, many bands are related to the ET donor: ν11(ag) = 316, ν10(ag) = 486, ν9(ag) = 499 and 506, ν8(ag) = 681, ν60(b3g) = 889, ν6(ag) = 974, ν3(ag) = 1467 cm−1. The assignment of the bands has been done on the basis of the Raman spectra of anthracene39 and of the ET donor40,41 from earlier work. The most interesting bands are related to the C C stretching of ET, ν2(ag) and ν3(ag), since they are especially sensitive to the actual charge residing on the molecule. A good

Figure 6. Detail of the hydrogen bond pattern within the anionic layer in (ET)4(ABS)·4H2O at room temperature. Short O···O contacts (2.78−2.93 Å) are in red dotted lines; longer contacts (3.22−3.23 Å) are in black dotted lines.

organize into hydrogen-bonded tetramers, with O···O contacts (b, c) at 2.78 and 2.93 Å, respectively, at room temperature (300 K), further anchored to the oxygen atoms of the sulfonate moieties. At room temperature, only one of the oxygen atoms of the sulfonate moiety is engaged in a short hydrogen bond (a: O···O: 2.80 Å), while the two other oxygen atoms exhibit only rather long and equivalent (3.23 Å) O···O contacts (d, e). Thus, the short O···O contacts ((a, b, c), in red in Figure 6) define here an inversion-centered isolated supramolecular motif, only weakly connected to neighboring tetramers through the longer O···O (d, e) contacts (in black in Figure 6). At variance with the previous NBS salt, the temperature dependence of the resistivity (Figure 7a) shows now a metallic behavior at room temperature, with a much higher (×104) room-temperature conductivity (σRT = 100 S cm−1). A sharp metal−insulator transition is observed around TMI = 90 K when cooling. Application of hydrostatic pressure does not modify significantly this behavior: the room-temperature conductivity increases to 250 S cm−1 at 1.1 GPa (Figure S3 in the Supporting Information); TMI remains essentially unchanged, 5138



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Figure 8. Temperature dependence of the Raman spectra of (ET)4(ABS)·4H2O in the selected energy ranges.

charge sensitive mode, namely, the ν9(ag) ring breathing mode of ET.40 Above TMI, the broad ν9(ag) spectral feature suggests that charge density fluctuates and is distributed rather uniformly. The existence of charge fluctuations in the metallic phase is also supported by a weak ν2(ag) feature and relatively broad ν3(ag) peak. Structural Evolutions with Temperature. To complement the room-temperature (300 K) crystal structure described above, X-ray data collections were performed at 250, 200, 150, 120, 100, 80, and 15 K for (ET)4(ABS)·4H2O. Temperature dependence of the unit cell parameters shows a uniform cell contraction down to 100 K, followed by a plateau (for a, b, and c parameters, and unit cell volume) and a sharp increase of the β angle at TMI (Figure S7 in the Supporting Information). The Raman data suggested that a charge disproportionation is at the origin of the metal−insulator transition in this salt. The temperature dependence of the calculated charges in each of the two crystallographically independent BEDT-TTF molecules, applying the Day’s formulas37 as for the NBS salt described above, is reported in Table 1. The absolute value of these calculated charges should be considered with great care. Accordingly, they have been also normalized to a total +1 charge for the two BEDT-TTF molecules. Despite large variations, it appears that the charge seems evenly distributed between molecules A and B in the high-temperature regime, as already suggested from the Raman data, with a possible charge disproportionation below the metal−insulator transition, but only clearly seen at 15 K. Considering now the anion layer, a careful inspection shows that the aforementioned cell contraction observed upon cooling (Figure S7, Supporting Information) also affects the hydrogen bond pattern originally shown in Figure 6 for the RT structure. Indeed, as shown in Figure 9 and Figure S8 (Supporting Information), the length of the three O···O contacts noted a−c are not modified upon cooling, while the d and e contacts,

indicator of charge density is the ν2(ag) mode related to the stretching of outer CC ring bonds,38 but in the roomtemperature Raman spectra (Figure S6 in the Supporting Information), this band is not well-seen (it is hidden in the wing of the strong ν3(ag) band). The mode ν3(ag) = 1467 cm−1 is attributed to the stretching of the central CC bond. Usually, this band is not very useful for evaluation of charge residing on the ET molecule, since its position can be disturbed by strong coupling with electrons. Nevertheless, in the present case, neglecting the coupling, we can estimate from the position of ν3(ag) an averaged ET charge of about +0.42e, close to the +0.5e value expected if both crystallographically independent molecules bear the same charge.40 The Raman spectra within this CC stretching region are strongly modified below the metal−insulator transition TMI = 90 K, providing an unambiguous evidence of a charge disproportionation phenomenon (Figure 8b). Indeed, the ν3(ag) band reduces its intensity and a new peak at about 1484 cm−1 grows, which was assigned to the ν2(ag) mode; its position corresponds to a charge of +0.7e.38 Moreover, below TMI, the band ν3(ag) is split into two components at 1467 and 1475 cm−1. It results from these Raman data that the metal− insulator phase transition is coupled to a charge ordering, most probably from a relatively uniform charge distribution with ≈+0.5e on each molecule to a tentative +0.3,42 +0.5, and +0.7e charge distribution on the molecules below TMI. Moreover, the crystal structure is probably slightly reorganized. Indeed, the two ABS bands visible at 1409 and 1412 cm−1 above TMI merged into a single peak at 1408 cm−1 below the transition (Figure 8b). The low-frequency part of the Raman spectrum characterizes lattice modes and thus is the most sensitive to the crystalline packing; this part of the spectrum undergoes also significant changes, especially in the region of 240−300 cm−1 (Figure 8a). Below TMI, four well-resolved peaks at 488, 494, 504, and 509 cm−1 appeared and are attributed to another 5139



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calculated band structure at 300 K is shown in Figure 10a. There are eight bands mostly based on the BEDT-TTF’s

Table 1. Calculated Charge in the Two Crystallographically Independent BEDT-TTF Molecules A and B in (ET)4(ABS)· 4H2Oa T (K) 300 250 200 150 120 100 80 15

ρMol. A 0.71 0.69 0.68 0.57 0.39 0.45 0.53 0.32

(+0.52) (+0.52) (+0.56) (+0.53) (+0.47) (+0.53) (+0.47) (+0.40)

.ρMol. B 0.66 0.63 0.53 0.50 0.43 0.40 0.60 0.47

(+0.48) (+0.48) (+0.44) (+0.47) (+0.53) (+0.47) (+0.53) (+0.60)

a

Values in parentheses are normalized values, calculated on the basis of a total +1 charge for the two BEDT-TTF molecules.

Figure 9. Evolution of the O···O contacts (see Figure 6 for a−e definition) within the hydrogen-bonded anionic layer in (ET)4(ABS)· 4H2O as a function of the temperature. The solid lines in the temperature dependence of the O···O distances are only a guide to the eyes. Estimated standard deviations (esd’s) on the O···O distances are less than 0.01 Å.

notably longer at room temperature, actually evolve toward a much shorter hydrogen bond (contact d) and a much longer one (contact e), with no further evolution below TMI. In other words, a weaker, probably bifurcated, hydrogen bond toward two oxygen atoms of the sulfonate moiety actually localizes toward one specific oxygen atom of the sulfonate moiety, while, at the same time, a concomitant charge differentiation is observed within the donor slab. Note also that the shortening of this O···O contact (contact d) allows now for an extended, interconnected hydrogen-bonded system that develops within the whole anion layer, at variance with the room-temperature structure where the hydrogen-bonded motif was localized. The formation of this polymeric 2D network in the anion layer might be connected to the first-order character of the metal− insulator transition. Band Structure Calculation. We have shown above that the observed evolutions of the charge of the BEDT-TTF molecules are intimately correlated with minute structural modifications of the anionic layers, with the striking evolution of a symmetric, bifurcated hydrogen bond at RT toward an oxygen atom differentiation below TMI. To correlate these structural and charge distribution evolutions with the occurrence of the sharp metal−insulator transition, the electronic structure of (BEDT-TTF)4(ABS)·4H2O was calculated at different temperatures, using the X-ray crystal structures determined at 300, 200, 100, 80, and 15 K. The

Figure 10. Band structure calculated for the donor layers of (ET)4(ABS)·4H2O at 300 K (a) and 80 K (c). The dashed line refers to the Fermi level. Shown in (b) is the Fermi surface calculated at 300 K. Γ = (0, 0), Y = (b*/2, 0), Z = (0, c*/2), and M = (b*/2, c*/2).

HOMOs, because the repeat unit of the layer contains eight donor molecules. Since there is one dianion per four donor molecules, these bands must accommodate four holes. Although the band structure of Figure 10a may suggest that there is a small band gap between the second and third bands from the top, a careful exploration of the full Brillouin zone showed that this is not the case. The two bands slightly overlap, leading to the Fermi surface of Figure 10b, so that the conductivity should be nonactivated at RT, in agreement with the transport measurements (see Figure 7). This Fermi surface has obviously no nesting properties; hence, the metal-toinsulator transition does not originate here from a Fermi surface instability. The electronic structure changes very slowly from 300 K until 100 K, but at 80 K a band gap of 28 meV definitely opens and increases up to 50 meV at 15 K. The band structure calculated at 80 K is shown in Figure 10c. Although the band gap is now clearly visible, the shape of the different bands remains practically the same as in the 300 K structure. Thus, the abrupt change in the conductivity regime must be 5140



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Table 2. |βHOMO−HOMO| Interaction Energies (eV) at Different Temperatures for the Donor···Donor Interactions in (ET)4(ABS)·4H2O

associated with a very subtle geometry change that does not substantially change the nature of the donor···donor interactions in the layer. This is understandable because the semimetallic overlap between the valence band and the conduction band is quite small, and consequently, minor displacements of the donors can destroy this overlap. A possible key to understanding the nature of this transition is provided by the evolution of the HOMO levels of the two different BEDT-TTF molecules A and B. Whereas between 300 and 100 K, the energy difference is only 15−28 meV, at 80 K, it is almost twice larger, 49 meV and, at 15 K, becomes 87 meV. Because of the well-known inter-relation between the energy of the HOMO and the charge of BEDT-TTF, this evolution also suggests some kind of charge differentiation at the transition. Globally, the situation is clearly reminiscent of the classical charge ordering transition in α-(ET)2I3. Although the origin of the transition has long been debated, recent first-principles calculations43 have shown that this salt also exhibits a very small semimetallic overlap that disappears, leading to a small band gap, of the same order as that found here, and a charge redistribution. These changes are associated with subtle anion movements. This scenario most likely applies also to the present salt. To get some more insight into the nature of the transition, we have also evaluated the strength of the different HOMO··· HOMO interactions in the donor layers (see Figure 11). As

interaction I II III IV V VI VII VIII

(A−B) (A−B) (A−A) (A−A) (B−B) (B−B) (A−B) (A−B)

300 K

200 K

80 K

15 K

0.2694 0.2830 0.1684 0.0579 0.0590 0.1582 0.1642 0.1649

0.2796 0.2941 0.1786 0.0585 0.0625 0.1783 0.1687 0.1682

0.2784 0.3108 0.2285 0.0605 0.0605 0.1970 0.1756 0.1674

0.2806 0.3120 0.2388 0.0614 0.0639 0.1967 0.1756 0.1658

twisted overlap; see Figure 5b, where one of the donors is displaced to the adjacent site in the layer), interactions IV and V are considerably weaker. When looking at the results in Table 2, it is clear that most of the interactions change very slightly with temperature. However, two of them, III and, to a lesser extent, VI, abruptly increase around the transition temperature. This means that the coupling between the zigzag chains of dimers increases when the transition occurs. The slightly avoided crossing near the Fermi level along the Γ to Y direction (i.e., the direction of the interchain interactions) becomes more strongly avoided when the coupling between the chains increases. Thus, it is understandable that, between 100 and 80 K, when this coupling suddenly increases, the band crossing is more strongly avoided, a band gap is generated, and the conductivity becomes activated. What is the origin of the increased interactions III and VI? The oxygen atoms of the sulfonate groups also make C−H···O hydrogen-bonding interactions between the hydrogen atoms of the ethylenic moieties in either donor A or B, and the sulfonate moiety (Figure 12a). As illustrated in Figure 12b, these C−H··· O contacts also evolve with temperature and the shortest ones actually establish, below the transition, with the most oxidized

Figure 11. Schematic representation of a donor layer of (ET)4(ABS)· 4H2O where the different donors and intermolecular interactions are labeled (see also Figure 5a).

mentioned, there are two different types of BEDT-TTF donors (labeled A and B in Figure 11) and eight different intermolecular interactions (labeled I−VIII in Figure 11). The absolute values of the so-called |βHOMO−HOMO| interaction energies44 at different temperatures are reported in Table 2. Two of the interactions, I and II, are clearly stronger than all others so that, as far as the HOMO···HOMO interactions are concerned (i.e., those determining the shape of the HOMO bands and thus the transport properties of the salt), the ET donor layers may be described as a series of zigzag chains of A− B ET dimers. These chains interact through π-type interactions, VII and VIII, or mostly σ-type interactions, III−VI. However, because of the opposite rotation of successive dimers (i.e., the

Figure 12. (a) Detail of the C−H···O hydrogen bonds (thick dotted black lines) between the ET molecules and the sulfonate moiety at 300 K in (ET)4(ABS)·4H2O. Crystallographically independent ET molecules A are in red, molecules B in blue. (b) Temperature evolution of these H···O contacts. The solid lines are only a guide to the eyes. 5141



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Table 3. Crystallographic Data (PPh4)2(NBS) formula fw (g·mol−1) crystal system space group T (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g·cm−3) μ (mm−1) total reflns. θmin, θmax abs. corr. unique reflns. Rint unique reflns. (I > 2σ(I)) R1 (I > 2σ(I)) wR2 (all data) goodness-of-fit res. dens. (e− Å−3) formula fw (g·mol−1) crystal system space group T (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g·cm−3) μ (mm−1) total reflns. θmin, θmax abs. corr. unique reflns. Rint unique reflns. (I > 2σ(I)) R1 (I > 2σ(I)) wR2 (all data) goodness-of-fit res. dens. (e− Å−3)

formula fw (g·mol−1) crystal system space group T (K) a (Å) b (Å)

C29H23O3PS 482.51 monoclinic P21/n 150(2) 12.2340(3) 7.0165(2) 26.8821(6) 98.3500(10) 2283.09(10) 4 1.404 0.243 18 505 1.53, 27.49 multiscan 5222 0.0297 4542 0.0409 0.1110 1.053 0.562, −0.453 (ET)4(ABS)·4H2O

(ET)4(NBS)·4H2O C50H38O10S34 1889.18 monoclinic P21/a 150(2) 14.8206(14) 13.3595(12) 18.0157(18) 90.00 101.297(3) 90.00 3497.9(6) 2 1.793 1.087 33 453 1.15, 27.53 multiscan 8023 0.0275 6804 0.0652 0.2110 1.061 2.94, −1.02 (ET)4(ABS)·4H2O (ET)4(ABS)·4H2O

(PPh4)2(ABS)·2MeOH C32H28O4PS 539.58 triclinic P1̅ 150(2) 9.7713(3) 10.3882(3) 13.5829(4) 77.7220(10) 80.7490(10) 84.3990(10) 1326.81(7) 2 1.351 0.220 21 940 1.55, 27.45 multiscan 6022 0.0267 5417 0.0352 0.0974 1.048 0.48, −0.41 (ET)4(ABS)·4H2O

C54H48O10S34 1943.27 monoclinic P21/c 15(2) 16.9980(4) 13.1085(2) 16.7578(4) 90.00 106.666(2) 90.00 3577.09(13) 2 1.801 1.066 27 510 2.91, 27.00 none 7772 0.0469 4032 0.0626 0.1846 0.967 3.58, −1.02




(ET)4(ABS)·4H2O

(ET)4(ABS)·4H2O

(ET)4(ABS)·4H2O

(ET)4(ABS)·4H2O

C54H48O10S34 1943.27 monoclinic P21/c 150(2) 17.0782(4) 13.1485(2)




5142



Article

Table 3. continued c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g·cm−3) μ (mm−1) total reflns. θmin, θmax abs. corr. unique reflns. Rint unique reflns. (I > 2σ(I)) R1 (I > 2σ(I)) wR2 (all data) goodness-of-fit res. dens. (e− Å−3)

(ET)4(ABS)·4H2O

(ET)4(ABS)·4H2O

16.8083(4) 90.00 106.501(2) 90.00 3618.90(13) 2 1.780 1.053 24 851 2.90, 27.00 none 7839 0.0282 4621 0.0809 0.2666 1.099 2.33, −1.12

16.8588(5) 90.00 106.732(3) 90.00 3644.08(17) 2 1.767 1.046 24 985 2.89, 27.00 none 7905 0.0381 4159 0.0767 0.2532 0.974 2.07, −0.740

16.9054(6) 90.00 106.968(4) 90.00 3672.2(2) 2 1.754 1.038 25 132 2.88, 27.00 none 7958 0.0415 3837 0.0782 0.2506 0.918 1.91, −0.63

(ET)4(ABS)·4H2O 16.9505(6) 90.00 107.315(3) 90.00 3705.64(19) 2 1.738 1.029 27 023 2.86, 27.00 none 8017 0.0325 3858 0.0809 0.2733 0.873 1.96, −0.47

solutions were mixed under stirring. A white precipitate readily formed, and the suspension was left overnight under stirring. The precipitate was filtered and washed with distilled water. After drying, the precipitate was dissolved in methanol, and crystals were obtained by slow diffusion of diethyl ether (2.27 g, 78%) into the methanolic solution. 1H NMR (MeOD, 300 MHz): 7.6−7.8 (m, 32H, phosphonium), 7.85−7.95 (m, 12H, phosphonium + naphthalene), 8.30 (s, 2H). Anal. Calcd for C58H46O6P2S2: C, 72.18; H, 4.80; S, 6.65. Found: C, 72.13; H, 4.75; S, 6.49. (PPh4)2(ABS)·2MeOH. 2,6-Anthracenedisulfonic acid, dipotassium salt, K2(ABS) (400 mg, 0.95 mmol), was dissolved in 100 mL of hot water. Ph4PCl (724 mg, 1.9 mmol) was added at room temperature to the solution. After stirring overnight, the precipitate was filtered and washed with distilled water. The dried precipitate was dissolved in a minimum of methanol and crystallized by slow diffusion of diethyl ether in a closed flask (580 mg, 59%). 1H NMR (MeOD, 300 MHz): 7.6−7.8 (m, 32H, phosphonium), 7.84 (dd, 3J = 9.0 Hz, 4J = 1.8 Hz, 2H), 7.9−8.0 (m, 8H, phosphonium), 8.04 (d, 3J = 9.0 Hz, 2H), 8.48 (d, 3J = 1.5 Hz, 2H), 8.50 (s, 2H). Anal. Calcd for C62H48O6P2S2, CH3OH: C, 72.26; H, 5.01; S, 6.12. Anal. Calcd for C62H48O6P2S2, 2(CH3OH): C, 71.23; H, 5.23; S, 5.94. Found: C, 72.60; H, 4.82; S, 5.96. Elemental analysis shows some MeOH loss. Crystal Growth. (BEDT-TTF)4(NBS)·4H2O. Two-compartment electrocrystallization cells equipped with Pt wires (diameter, 1 mm; length, 2 cm) were filled with ET (10 mg) in the anodic compartment and a solution of (PPh4)2(NBS) (50 mg) in 1,1,2-trichloroethane (5 mL) as the electrolyte in both compartments. Electrocrystallizations were performed at 40 °C with a constant current of 0.5 μA during 10 days. (BEDT-TTF)4(ABS)·4H2O. Similar conditions as above were used, with (PPh4)2(ABS)·2MeOH as the electrolyte. Addition of water (100 μL) allowed increasing the crystal size and quality. Crystallography. Data collections for (BEDT-TTF)4(ABS)·4H2O at various temperatures were performed on an Xcalibur Saphir 3 diffractometer. Other data were obtained on a Bruker model AXS X8APEX II Oxford Diffraction system at 150 K. Both diffractometers operate with graphite monochromatized Mo Kα radiation. Structures were solved by direct methods (SHELXS-97, SIR97)48 and refined (SHELXL-97) by full-matrix least-squares methods,49 as implemented in the WinGX software package.50 C−H hydrogen atoms were introduced at calculated positions (riding model), included in structure factor calculations, and not refined, with thermal parameters fixed as 1.2 times Ueq of the attached carbon atom. Hydrogen atoms on water molecules could not be properly identified. Crystallographic data on Xray data collections and structure refinements are given Table 3.

B molecule. This direct correlation between the charge distribution in molecules A and B and the C−H···O hydrogen bonding is also demonstrated by the fact that they exhibit indeed an abrupt change around the phase transition, by contrast with the much smoother O···O evolution.45 Consequently, it is the change in the hydrogen-bonding network of the water molecules and the sulfonate anions that, via weaker C−H···O hydrogen bonding with the terminal CH2 groups of the ET donors, is transmitted separately to the two types of donors, increasing the A−A and B−B interchain interactions. This leads to the splitting of the bands and, therefore, a band gap opening and a charge ordering on the two donors below 90 K. In conclusion, the ET anthracene-bis(sulfonate) salt described here shows a clear correlation between the change in O−H···O hydrogen-bonding pattern within the anion layer, the induced subtle variations of the donor layers, and the opening of a band gap associated with a charge differentiation seen in the resistivity as well as Raman measurements. It demonstrates how structural modifications within the anion layers, as also anion ordering phenomena (AO), are directly transferred to the organic stacks or slabs through weaker C− H···O hydrogen bonds, inducing a polarization of the organic donors. The question may then arise if the original trigger is the electronic instability or the structural instability. While 2kF transitions are most often driving the associated structural transitions, it seems that 4kF instabilities toward Mott dimer (MD) or charge ordering (CO) states, because they involve charge distribution modifications, are recurrently triggered by an underlying structural modification in the anion layer, whose evolution (with temperature or pressure) leads at some point to the observed electronic transitions.

■

(ET)4(ABS)·4H2O

EXPERIMENTAL SECTION

Syntheses. The ABS potassium salt, K2(ABS), was prepared according to a published procedure,46 whereas Na2(NBS) (97%) was purchased from Aldrich and used as received. BEDT-TTF was prepared as previously described according to the Larsen−Lenoir synthesis.47 (PPh4)2(NBS). 2,6-Naphthalenedisulfonic acid, disodium salt, Na2(NBS) (1.00 g, 3 mmol), was dissolved in 20 mL of water. Ph4PCl (2.25 g, 6 mmol) was also dissolved in 20 mL of water. The two 5143



Article

Transport Measurements. Electrical resistivity was measured on platelet-shaped single crystals using a four-point method. Four gold contacts were evaporated on both faces of the crystals, and gold wires were glued with silver paste on those contacts. A low frequency (

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