Solvated and Ferroelectric Phases of the Charge Transfer Co-crystal

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Solvated and Ferroelectric Phases of the Charge Transfer Co-crystal TMB-TCNQ Francesco Mezzadri, Nicola Castagnetti, Matteo Masino, and Alberto Girlando Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00905 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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Crystal Growth & Design

Solvated and Ferroelectric Phases of the Charge Transfer Co-crystal TMB-TCNQ

Francesco Mezzadri,* Nicola Castagnetti, Matteo Masino, Alberto Girlando

Dipartimento di Scienze Chimiche, della Vita e Sostenibilità Ambientale, Università di Parma, Parco Area delle Scienze 11A, I-43124 Parma, Italy

ABSTRACT: We have successfully determined the crystal structure of both the low temperature and solvated phases of 3,3’,5,5’-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The triclinic solvate phase was erroneously believed for years to be a polymorph of TMB-TCNQ, while thanks to X-rays diffraction, infrared spectroscopy and differential scanning calorimetry we have determined the presence of crystallization acetonitrile molecules within the structure and observed the conversion to the room temperature monoclinic phase of TMB-TCNQ upon desolvation promoted both by heating and grinding. The non-solvated phase undergoes inversion symmetry breaking below 160 K, giving rise to stack dimerization. A combination of the X-ray crystal structure determination, Hirschfeld surface analysis and density functional theory calculations suggest a rather complex scenario for the first order phase transition, which implies, besides stack dimerization, molecular inclination and small increase of the degree of charge transfer. The two donor-acceptor pairs within the unit cell are arranged in-phase, so that the low-temperature structure is potentially ferroelectric.

* Francesco Mezzadri, Dipartimento SCVSA, Università di Parma Parco Area delle Scienze 11A, I-43124 Parma, Italy tel. +39 0521 905548 [email protected]

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Solvated and Ferroelectric Phases of the Charge Transfer Co-crystal TMB-TCNQ

Francesco Mezzadri,* Nicola Castagnetti, Matteo Masino, Alberto Girlando

Dipartimento di Scienze Chimiche, della Vita e Sostenibilità Ambientale, Università di Parma, Parco Area delle Scienze 11A, I-43124 Parma, Italy

ABSTRACT: We have successfully determined the crystal structure of both the low temperature and solvated phases of 3,3’,5,5’-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The triclinic solvate phase was erroneously believed for years to be a polymorph of TMB-TCNQ, while thanks to X-rays diffraction, infrared spectroscopy and differential scanning calorimetry we have determined the presence of crystallization acetonitrile molecules within the structure and observed the conversion to the room temperature monoclinic phase of TMB-TCNQ upon desolvation promoted both by heating and grinding. The non-solvated phase undergoes inversion symmetry breaking below 160 K, giving rise to stack dimerization. A combination of the X-ray crystal structure determination, Hirschfeld surface analysis and density functional theory calculations suggest a rather complex scenario for the first order phase transition, which implies, besides stack dimerization, molecular inclination and small increase of the degree of charge transfer. The two donor-acceptor pairs within the unit cell are arranged in-phase, so that the low-temperature structure is potentially ferroelectric. INTRODUCTION Organic charge-transfer (CT) co-crystals are made up by planar π electron-donor (D) and electron acceptor (A) molecules that are arranged in stacks as a result of the strongly directional CT interaction. In principle, a proper choice of the D and A molecules allows fine tuning of the degree of charge-transfer (ρ) from D to A and of the electronic properties of the compounds, ranging from insulating to superconducting.1 In the case of mixed-stack (--DADAD--) CT crystals, D and A molecules are arranged alternatively along the stack, and the degree of charge-transfer, or ionicity, ranges from 0 to 1. A value of ρ = 0.5 is assumed as the conventional borderline between neutral (N) and ionic (I) ground states.2 Mixed stack CT crystals can undergo a very peculiar phase transition, the so-called neutral-ionic phase transition (NIT), a valence instability


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involving a collective CT from D to A, with the ionicity ρ crossing the N-I borderline.3,4 The NIT driving force is the increase of Madelung energy, that follows lattice contraction induced by lowering temperature or increasing pressure. On the other hand, mixed stack CT crystals, as quasi-one-dimensional systems, are also subject to the Peierls instability, yielding stack dimerization DA DA DA.5 In fact, on the ionic side a regular stack is intrinsically unstable towards dimerization, whereas in the proximity of the N-I borderline (ρ ≥ 0.3–0.4) the instability is conditional, i.e., depends on the strength of electron-lattice phonon coupling.6 After its discovery more than 30 years ago, NIT has been extensively studied in view of the many fascinating phenomena associated with it.5,7,8 In particular, attention has been recently addressed to the so-called “electronic ferroelectricity” exhibited by the prototypical tetrathiafulvalene-chloranil (TTF-CA) in the low-temperature ionic dimerized phase.9 However, the search for other ferroelectric mixed stack CT crystals proved to be very challenging,10,11 as the occurrence of quasi-ionic systems is relatively rare, and stack dimerization is not the only condition to be met. In the search of other potentially ferroelectric crystals, we have decided to reinvestigate an old system, 3,3’,5,5’-tetramethylbenzidinetetracyanoquinodimethane (TMB-TCNQ). This CT co-crystal was reported to occur into two forms, triclinic and monoclinic.12 The former, for which only the cell parameters have been published,13 did not exhibit phase transitions down to 24 K,12 whereas the latter was reported to exhibit temperature induced NIT around 200 K.14 According to our recent reanalysis, the phase transition, first order with strong hysteresis, can hardly be termed NIT, as the 0.5 borderline is not crossed, but unquestionable spectroscopic evidence of stack dimerization in the low-temperature (LT) phase has been found.15 However, cracking of the sample at the phase transition prevented the X-ray determination (XRD) of its structure. Here we extend the investigation to the triclinic form, obtaining its full crystal structure and showing that, contrary to what was thought initially, it is not a polymorph but a solvated crystal. The embedded acetonitrile sublimes away upon heating, and the usual monoclinic form is obtained. In addition, particular care in sample preparation and XRD collection, and the experience gained in several attempts, has allowed us to obtain also the TMB-TCNQ LT crystal structure, which is shown to have a ferroelecric arrangement of the DA pairs inside the unit cell, contrarily to what was initially tought.15 The LT structure is presented here, together with an analysis of temperature evolution of the cell parameters and of the Hirshfeld surfaces, aimed at obtaining clues about the complex phase transition mechanism.

EXPERIMENTAL SECTION TMB and TCNQ were commercial products, used without further purification. Single crystals of solvated TMB-TCNQ were grown by precipitation from solution, by mixing saturated solutions of the two components in hot acetonitrile. Slow



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cooling and solvent evaporation yielded crystals of the triclinic form.13,15 Physical vapour transport (PVT) in an open tube apparatus under controlled nitrogen ﬂux and a proper temperature profile was used to obtain the monoclinic TMB-TCNQ phase.16 An outer quartz tube, inserted in the furnace and connected to the inert gas line, contained an inner quartz tube (length = 45 cm, internal diameter = 1.8 cm). Reactants, put in glass bowls, were positioned in different points along the inner tube. Positions were chosen in order to have temperatures of ≈190°C and ≈155°C at the location of TCNQ and TMB, respectively, the latter being shifted by 6 cm in the direction of the gas ﬂux. The growth lasted 40 h yielding millimeter size needle-shaped crystals of the monoclinic form. This method gave crystals of better quality with the respect to the ones grown by sublimation of the two components in a closed evacuated ampoule.15 X-rays powder diffraction was performed by using a Thermo Electron X’Tra diffractometer making use of CuKα radiation and solid state Thermo Electron Si(Li) detector. Single crystal diffraction data for the samples obtained from solution were collected with a Bruker D8 Venture instrument equipped with a Photon CCD area detector. CuKα radiation was used in order to have information about the absolute structure of the compound. Low temperature single crystal diffraction data from the samples obtained by PVT required a different experimental setup due to the strong breaking tendency displayed by the crystal when the transition temperature is crossed, in particular in presence of thermal gradients. Noteworthy, enclosing the sample into viscous fluids inhibits the transition. As a consequence the crystal was introduced into a glass capillary and blocked by pressing a capillary of smaller diameter at its bottom, allowing the collection of the diffraction data despite minor cracking of the sample. Although copper radiation would be more appropriate for the study of an organic molecular crystal, in this case the higher energy of MoKα was needed in order to pass through the capillary walls. As a consequence, diffraction data were collected with a Bruker Smart diffractometer equipped with an APEX II CCD detector. Temperature control was achieved thanks to an Oxford Cryosystems cryostream, cooling/heating rate was set to 50°/h. Temperature dependent lattice parameters determination was carried out collecting 3.6° of three nonplanar sections of the reciprocal space, so that about 100 reflections were available for unit cell indexing and refinement. The single crystals data reduction was carried out by using the SADABS program.17 The software SIR201418 was used for structures solution while refinement was carried out full-matrix by using the Shelxl program.19 Thermal analyses were carried out using a Perkin Elmer DSC 6000 instrument under nitrogen atmosphere in aluminum crucibles by heating and cooling the polycrystalline samples at a rate of 5° C /min. The infrared (IR) spectra were recorded with a Bruker IFS66 Fourier transform IR (FT-IR) spectrometer coupled to the


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Hyperion 1000 IR microscope, and equipped with a wire-grid polarizer. A Linkam HFS 91 stage was used for collecting IR spectra as a function of temperature. Spectral resolution: 2 cm-1.

RESULTS AND DISCUSSION Powder X-Rays diffraction The samples obtained by PVT and from acetonitrile solution were first analyzed by powder X-rays diffraction. PVT ground samples and as grown ones showed good agreement with the pattern calculated on the basis of the previously published monoclinic crystal structure,14,15 hereinafter denoted as phase HT as in Ref. 15. On the other hand the samples obtained from solution showed significant differences. Initially small crystals were placed on a zero-background sample holder without grinding. The diffraction pattern is similar to the one corresponding to the samples obtained by the sublimation technique, however the principal peaks occur at lower 2θ angles, indicating a larger cell volume, while small amounts of phase HT are detected, as shown in Figure 1. We call this phase S. Noteworthy, grinding crystals of phase S yields the conversion to phase HT, with reduced crystallinity evidenced by a sizeable increase of the peaks width (Figure S1).

Structure of phase S Spectroscopic and conductivity data have been published for TMB-TCNQ samples obtained by acetonitrile saturated solution, indicating triclinic symmetry, but the full crystal structure was not determined, and only the cell parameters have been reported.12 After several attempts we succeeded in solving the structure of a single crystal grown from acetonitrile. The reflections were indexed using a triclinic cell with a=6.6945(3) Å, b=8.1067(4) Å, c=12.3772(6) Å, α=91.4531(18)°, β=99.9242(17)°, γ= 101.9283(19)°. The centrosymmetric space group P-1 was used to solve and refine the structure since P1 gave similar results without a relevant improvement of the refinement. Besides the expected TMB and TCNQ moieties, it is apparent the presence of crystallization acetonitrile molecules within the cell, characterized by disorder. Refinement of the crystal structure was carried out full matrix, setting anisotropic thermal parameters for all atoms except hydrogens and the whole acetonitrile molecule. Data collection conditions and refinement parameters are reported in Table S1. Simulation of the powder diffraction pattern based on this structure agreed very well with that of phase S, as shown in figure S2. Similar cell parameters (a=8.105 Å, b=12.373 Å, c=6.702 Å, α=100.14°, β=101.84°, γ=91.39°) have been reported for what was believed to be a different TMB-TCNQ polymorph.12 On the basis of our data, we conclude that what we have called



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phase S indeed coincides with the triclinic phase reported by Iwasa et al. and is not a new polymorph but a solvated crystal. Constraints were used in order to model the disorder involving the acetonitrile molecules, whose longitudinal axis is parallel to the [100], with 50% probability of having the methyl group in the a or –a direction. The TMB and TCNQ molecules, laying in the bc plane, are stacked along a with a scheme of interactions similar to that observed in phase HT, with the intermolecular distance being 3.347(2) Å, compared to 3.354(3) Å for the HT phase. The fact phase S is a solvated phase also explains why by grinding it transforms in phase HT, as we have seen by the previous analysis. Clearly, acetonitrile evaporates away when more surface is exposed. In order to have a better insight into the thermodynamics of the system, we have performed DSC measurements on 0.490 mg of crystalline samples from room temperature to 200° C (300-473 K). As shown in Figure 3, two peaks are detected, at 329 and 365 K, associated with endothermic transformations characterized by enthalpy values ∆H=2.02 J/g and ∆H=34.09 J/g, respectively. No transitions are detected by cooling back the samples down 300 K, pointing out the irreversible nature of the observed phenomenon. It was not possible to unequivocally identify the nature of the transition located at 329 K by performing powder diffraction on a sample previously heated to 343 K, which does not show relevant modification from the as grown sample. On the other hand, samples heated above 365 K transform to phase HT upon evaporation of the solvent, as confirmed by the XRD measurements carried out on the samples unloaded from the DSC crucible. It is instructive at this point to compare the structures and physical properties of phase S and phase HT. As shown by the data reported in the first and second columns of Table 1, the lattice metrics of the two structures are quite similar, except for the long axis, which is 20.647(14) Å in phase HT and 12.3772(6) Å in phase S. Also the intermolecular interactions scheme is similar in the S and HT phases, with the noticeable difference constituted by the acetonitrile molecules acting as spacers between the stacks in the bc plane. Consequently the longitudinal axis of the TMB and TCNQ moieties forms zigzag chains in the bc plane in the first case, requiring more or less the doubling of the b cell parameter, while they are oriented parallel to each other along the [01-2] direction in the second one. On the other hand the relative orientation of the molecules stacked along the a axis is almost equivalent in both phases with the DA interplanar distances being practically indistinguishable within 2σ: 3.347(2) Å vs. 3.354(3) Å for the S and HT phases respectively. We have verified that this similarity also applies to the HOMO-LUMO overlap integral or to the DA CT integral (see the Supporting Information for details). The latter is calculated as 0.269 eV vs. 0.268 eV of phase HT. Therefore the strength of the CT interaction is practically equal in the two phases, and the acetonitrile molecules act simply as spacers between the stacks, with a slight increase of the volume per DA pair. At this point it is worth noting that, as shown in the following, no low-temperature phase transitions are observed in the S phase, likely due to the shielding effect of the acetonitrile molecule which hinders lattice contraction on cooling, therefore


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preventing an increase of the Madelung energy.3 Table 1. Basic structural parameters of the three phases of TMB-TCNQ.

Temperature/K Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z R1

Phase S 293(2) Triclinic P-1 6.6945(3) 8.1067(4) 12.3772(6) 91.4531(18) 99.9242(17) 101.9283(19) 646.08(5) 1 4.58

Phase HT* 230(2) Monoclinic P21/n 6.708(3) 21.797(7) 8.074(3) 90 100.39(5) 90 1161.3(8) 2 4.03

Phase LT 130(2) Monoclinic Pn 6.886(5) 20.647(14) 8.232(6) 90 97.438(11) 90 1160.5(14) 2 3.21

* data from Ref. 15.

We now integrate the information coming from X-ray with that from infrared (IR) spectroscopy. Figure 4 shows the temperature evolution of the significant frequency ranges of the polarized IR spectra. The performed temperature cycle has been as follows (from top to bottom in the figure): we started from phase S at room temperature, heated it first at 348 K, above the first peak in the DSC analysis, then we went up to 383 K, above the second DSC peak. Subsequently we went back to room temperature, and finally to 180 K, below the phase transition of phase HT.15 The right panel shows the spectra polarized parallel to the stack axis in the spectral region of the CN stretching modes. The CN vibrations of TCNQ are not expected in this polarization, since their oscillating dipole moment is perpendicular to the stack. Instead, the CN stretching of the acetonitrile, that is located between the stacks and directed along them, can be clearly identified in the top spectrum of Figure 4. The band is found at 2251 cm-1 (it is located at 2252 cm-1 in liquid acetonitrile20) and is marked by an asterisk in the figure. It is seen that by increasing the temperature above the first DSC peak nothing changes. However, the acetonitrile band disappears when the temperature is brought above the second DSC peak (green line spectrum in the figure), confirming the X-ray datum. Of course, the acetonitrile band does not come back when we return to room temperature (blue line spectrum). If with the same sample, that now has become phase HT, we go below 200 K (magenta spectrum in the figure), we indeed observe the well-known valence instability transition, which is accompanied by stack dimerization marked by the vibronic activation of the totallysymmetric CN stretching bands polarized along the stack direction.15 Another important parameter which can be derived from the IR measurements is the degree of CT (ρ). As in previous publications, to such aim we shall use the charge sensitive C=C stretching mode b1u ν20 located at 1545 in neutral TCNQ, exhibiting a ionization frequency shift of 41 cm-1.21 The left panel of Figure 4 shows the temperature evolution of this mode:



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it is located at 1534 cm-1 in phase S (black and red curves), at 1532 cm-1 in phase HT (green and blue curves) and at 1528 cm-1 in the LT phase. The corresponding ionicity is 0.27, 0.31 and 0.41±0.02.22 This datum shows that despite the comparable CT interaction in phase S as compared to phase HT, as evidenced by DA interplanar distance and CT integral, the corresponding degree of CT is slightly less in the former. This finding clearly shows that it is inappropriate to directly correlate the strength of DA CT interaction with the degree of CT,23 as the latter depends also from the Madelung energy of the crystal.24 And we may well suppose that the Madelung energy of phase S crystal is less than that of phase HT, in view of the larger cell volume per DA pair and of the screening effect of acetonitrile molecules. As a matter of fact, phase S does not undergo the Madelung energy driven valence instability of phase HT, not even at lower temperatures since its volume contraction is hindered by the presence of interstack acetonitrile molecules.12

Structure of phase LT In the previous Section we have shown how by heating phase S above 343 K phase HT is obtained, which undergoes precisely the same valence instability of the crystals directly grown by PVT,15 accompanied, also in this case, by severe cracking of the crystals. We now analyze the crystal structure of the LT phase. The reflections collected at 130 K from a sample obtained by PVT were indexed on the basis of a monoclinic cell with a=6.886(5) Å, b=20.647(14) Å, c=8.232(6) Å, β=97.438(11)°. Extinction rules involving the h0l reflections with h+l=2n are detected, however differently from the room temperature structure with space group symmetry P21/n (s.g. 14),15 no systematic absences are observed involving the 0k0 sets. As a consequence the structure solution and refinement was carried out in the noncentrosymmetric space group Pn (s.g. 7). This observation is in agreement with the recent observations of Castagnetti et al. suggesting a symmetry breaking at low temperature related to the loss of the inversion center.15 The crystal structure was refined making use of anisotropic thermal parameters for all the atoms except hydrogens, located in the difference Fourier map and refined by using constrains. The full data collection and refinement parameters are collected in Table S2. The comparison with the HT phase (Figure 5) points out that no relevant variation of the molecular bond distances is detected, in agreement with the small change in ionicity (about 0.1). A subtle distortion of the molecules is present, with the benzene rings of TMB passing from a planar geometry to a twisted configuration with torsion angle 8.1°, while the TCNQ moiety appears to be somewhat bent, with the nitrogen atoms lying below the mean plane of the molecule and the central carbon atoms of the benzene ring being slightly shifted above (Figure S3). Nonetheless, the major changes observed in the structure are ascribed to the crystal packing, as shown in the comparison of the cell parameters in Table 1 and in Figures 6 and 7. Indeed, the relative position of the two molecules slightly changes along the stack direction with a


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small rotation around the [100] axis. Noteworthy is the fact that also in the solvated phase the same relative orientation of the molecules along the stack is detected (Figure S4), pointing out that the interactions among the two facing molecules is strongly maximized in this particular reciprocal orientation. On the other hand, due to the loss of the 21 screw axis following the HT-LT transition, also the molecular ordering pattern observed in the ab plane is modified, with the mean planes of the molecules forming puckered sheets at an angle of 34.26°. This is the most evident change with respect to phase HT, where all the molecules lie on planes perpendicular to the [100] direction (see Figure 6), i.e. the stack axis. This structural variation is responsible for the large contraction of the b axis and likely for the cracking phenomena observed at the transition. In the solvated phase (Figure S5) the molecules lie in a plane somewhat parallel to the (011), similarly to what observed in the HT structure. However, in the latter all the molecules are stacked along x approximately at the same 0 and 0.5 fractionary coordinates, while in the S phase adjacent stacks are shifted one respect to the other giving rise to a ladder-like arrangement as the effect of the acetonitrile moieties, whose effect is to shield the interstack interactions. Finally, the projection on the bc plane does not display substantial differences between the non-solvated phases being both the structures characterized by a herringbone-like pattern of the TMB and TCNQ moieties whose bisecting lines form an angle of 61.1° and 55.0° in phase HT and LT respectively. On the contrary, in phase S all the molecules are parallel (Figure S6), with the longitudinal axes oriented approximately along the [0-13] direction. Most important, the transition from the P21/n to the Pn space group removes the inversion center, yielding stack dimerization of the crystal structure. At 130 K indeed the average distance between the molecular planes stacked along a is almost equivalent but gives rise to an alternate pattern in which each molecule is 3.207(4) Å and 3.614(5) Å apart from its first neighborhoods (an uniform distance of 3.354(3) Å is observed in the HT structure in both directions). The low temperature dimerization amplitude δ = (d1 - d2)/( d1 + d2) = 0.06 appears rather large if compared to the 0.02 value of the low temperature phase in TTF-CA,25 but in both cases the electric dipoles P1 and P2 of the two DA pairs in the unit cell present the same ferroelectric arrangement, as pictorially represented in Figure 7. In the image we have shown the dipoles resulting from the displacements of the ions, which give rise to the so-called ionic ferroelectricity.8 However, since the degree of CT of the LT phase of TMB-TCNQ is close to the N-I borderline like that of TTF-CA (ρ = 0.41 vs. 0.51 of TTF-CA),5 we may well expect that TMB-TCNQ, as TTF-CA, will possess an electronic polarization, of larger magnitude and opposite direction with respect to the ionic polarization.8 Unfortunately, the damage of the TMB-TCNQ crystal at the phase transition prevented so far the delicate ferroelectricity measurements.

HT-LT phase transition Temperature dependent single crystal XRD was used to examine the lattice parameters variation in the 130-310 K range.



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The transition inducing the P21/n - Pn symmetry change is observed in cooling at about 160 K, however variability in the transition temperature is detected when different crystals are considered. This phenomenon may be related to the peculiar measurement setup, preventing the accurate determination of the temperature on the sample, or to the hysteretic nature of the process, so that the temperature transition likely depends on the initial state and microstructure of the system. The lattice parameters refinement carried out at different temperatures clearly shows a discontinuous variations affecting a, b, c and β (Figures S7-S9) being however characterized by strong anisotropy. As shown in Figure 8, despite the stack dimerization, the a stack axis increases by about 2% at the transition, as the c axis does. On the other hand, the b axis decreases by about 8%. The volume jump is quite low: 1161.3(8) and 1160.5(14) Å3 at 230 and 130 K respectively, corresponding to a 0.07% variation. The crystal breaking at the transition is likely due to the expansion of the a and c axes, and indeed the breaking occurs precisely along these axes.15 Moreover, at variance with TTFCA26 the thermal expansion coefficients above and below the transition do not differ noticeably, displaying the same slope of the thermal parameters vs T. On heating the transition is observed at about 230 K, pointing out a 70 K thermal hysteresis of the process (Figures S7-S9). In this case severe cracking of the samples is observed, which is likely to be accompanied to microstructural damaging so that the data taken above the transition temperature are characterized by increased error bars. By the structural point of view however it was observed that the initial structure is completely recovered. In order to gather information regarding the molecular interactions driving the HT-LT phase transition of TMB-TCNQ, Hirshfeld surfaces were computed by using the Crystal Explorer package.27 The crystallographic structure used to perform the calculations on phase HT is the one reported in Ref. 15. The dnorm surface computed for the HT phase is shown in Figure 9, clearly pointing out that the intermolecular interactions within the bc plane are mainly ascribed to weak N···H hydrogen bonds (2.186(19) Å), corresponding to the red regions of the surface. At low temperature this picture does not change significantly, and the same hydrogen bonding pattern is detected, involving the same atoms, however characterized by increased N···H distances: 2.232(3) Å and 2.246(3) Å. The scheme of intermolecular interactions (short contacts) can be better appraised in figure S6, where also the S structure is represented, evidencing a complex network of short contacts involving the acetonitrile molecule. Despite weak, these interaction are well developed tridimensionally so that the effect of the solvent molecules in docking the TMB and TCNQ moieties is rather evident, reducing the possibility of a temperatureinduced transition. Along the perpendicular [100] direction a more complex picture is detected in the non-solvated phases. At RT just the weak N2···H14C (2.598(2) Å) and N2···H14B (2.592(2) Å) hydrogen bonds are present, while at 130 K an increased interaction, evidenced by the red regions of Figure 9, is observed involving the central parts of both the molecules, likely due to an increase of the frontier orbitals overlap (Figure 10).


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Noteworthy, the low dnorm values are observed just on one side of both the surfaces, thus giving rise to an alternate pattern, obvious consequence of the dimerization process. The fingerprint plots of the Hirshfeld surface of the hydrogen bond interactions do not change significantly by crossing the phase transition (Figures S11-S12), except for a slight increase of the (already weak) C-H bonding. This process is indeed allowed by the noncentrosymmetric nature of the LT phase, and the observed increased π-π interactions at low temperature are at the origin of the onset of a net dipole moment in the compound and consequently to potential pyro- or ferro-electric properties. The Hirschfeld analysis of Figures 9 and 10 points out that phase LT is characterized by short contacts between the central carbon atoms of TMB (C13, C20, C21 and C22) and of TCNQ (C1, C3, C4, and C6), suggesting an increased overlap (i.e., CT interaction) between the frontier orbitals. Such phenomenon is testified by the fingerprint plot of Figure 11, clearly displaying a shortening of the (de, di) pair relative to the C-C interaction which passes from 1.7 Å to 1.55 Å as temperature is lowered. In the LT phase the stack dimerizes, but the a stack axis increases by about 2% at the transition due to the inclination and distortion of the molecules (Figure 6). Therefore the increased overlap is not associated only with the stack dimerization. To give a quantitative basis to the above idea, we have calculated the DA overlap and CT integrals (t). Details of the computation are provided as Supporting Information. In the HT phase, where the DA distance is 3.354(3) Å, the CT integral t is calculated as 0.268 eV, and at low temperature, where the DA distances alternate (3.207(4) and 3.614(5) Å), the intradimer t increases to 0.368 eV. However, this increase would be significantly lower if the molecules had retained the same orientation as at room temperature. Indeed, a calculation of a DA pair with the same reciprocal orientation as in the HT phase (left side of Figure 5), but at the same 3.207(4) Å distance as the LT phase, gives t = 0.334 eV, about 9% lower than the actual low-temperature value. Therefore the LT packing is a compromise between the energy gain due to the increase of the CT interaction following the molecular inclination and distortion, and the cost of expanding the crystal along the a stack axis.

CONCLUSION The present accurate structural study of the phase transition of TMB-TCNQ, coupled to the full characterization of the triclinic (solvated) phase, gives us important indications about the subtle interplay among the different intermolecular interactions at stake in this peculiar CT crystal. We have seen that the basic stack structure, determined by prevailing CT interaction, is affected by inter-stack N—H interactions, both in the solvated and in the pristine HT monoclinic phase. The degree of CT, on the other hand, is also affected by the long-range, three dimensional electrostatic Madelung interaction,



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whereas the Peierls electron-phonon interaction underlies the dimerization of the stack in the LT phase. The S phase does not undergo valence instability phase transitions, due to the shielding effect of the acetonitrile molecule, but once the solvent is removed, it undergoes the transition with exactly the same features of strong hysteresis and crystal damage as the transition of the as-grown monoclinic crystal.15 This finding suggests that the Madelung energy rather obviously plays a fundamental role, but at the same time that N—H interactions albeit weak, are important in triggering the transition and determining the strongly dimerized LT phase. We may envision a mechanism by which changes in the weak N—H interactions in a system at the borderline of a valence instability trigger the transition towards a dimerized stack structure bound to the Peierls mechanism. However, the consequent increase of the CT interaction is associated to an inclination and distortion of the molecules with respect to the stack axis, again connected with weak inter-stack forces. We then have anisotropic changes in cell volume (lengthening of the a and c axes, shortening of b), that cause tension and crystal cracking, and at the same time yield to a very small decrease of the overall cell volume. As a consequence, the Madelung energy increase is likely small, so that the increase of the degree of CT is limited, and the N-I borderline is not crossed.15 Such a kind of complex scenario explains why true temperature induced NITs are so rare, and why the actual type of phase transition occurring in mixed stack CT crystals is difficult to predict.5 Last but not least, the arrangements of the two DA pairs in the LT cell turns out to be polar, contrarily to what it was initially hypothesized on the basis of semiempirical calculations.15 As we have seen (Figure 6), the two DA pairs in the unit cell are indeed oriented in the same direction. If we compare the valence instability of TMB-TCNQ with the NIT transition of TTF-CA we see strong analogies. The change of space group at the transition is the same, from P21/n to Pn, and in both cases the ionicity is close to the N-I interface. Therefore also TMB-TCNQ might exhibit electronic ferroelectricity like TTF-CA, but at higher temperature (below 200 K rather than below 80 K). The damage of the TMB-TCNQ crystal at the valence instability transition might be overcome by subtle chemical changes, like for instance selective deuteration,28 or one could see if transient ferroelectricity arises upon a possible photoinduced transition.29

ASSOCIATED CONTENT

Supporting Information. Additional XRD patterns, tables gathering the data collection and refinement parameters for phases LT and S, lattice parameters thermal evolution, Hirshfeld surface computed for TCNQ, C-H and N-H fingerprint plots, computational details.


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Accession Codes (for the CIF files deposited at CCDC)

AUTHOR INFORMATION Corresponding Author *[email protected]

ORCHID Alberto Girlando: 0000-0003-1887-709X Francesco Mezzadri: 0000-0001-9505-1457 Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS Roberta Magnani is acknowledged for DSC measurements. Chiesi Farmaceutici SpA is acknowledged for the support for the D8 Venture X-ray equipment.

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Castagnetti, N.; Masino, M.; Rizzoli, C.; Girlando, A.; Rovira, C. Mixed stack charge transfer crystals: Crossing

the neutral-ionic borderline by chemical substitution. Phys. Rev. Mat. 2018, 2, 024602. (3)

Torrance, J. B.; Vazquez, J. E.; Mayerle, J. J.; Lee, V. Y. Discovery of a Neutral-to-Ionic Phase Transition in

Organic Materials. Phys. Rev. Lett. 1981, 46, 253–257. (4)

Torrance, J. B.; Girlando, A.; Mayerle, J. J.; Crowley, J. I.; Lee, V. Y.; Batail, P.; LaPlaca, S. J. Anomalous Nature

of Neutral-to-Ionic Phase Transition in Tetrathiafulvalene-Chloranil. Phys. Rev. Lett. 1981, 47, 1747–1750. (5)

Masino, M.; Castagnetti, N.; Girlando, A. Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack



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Charge-Transfer Crystals. Crystals 2017, 7, 108. (6)

Girlando, A.; Painelli, A. Regular-Dimerized Stack and Neutral-Ionic Interfaces in Mixed-Stack Organic Charge-

Transfer Crystals. Phys. Rev. B 1986, 34, 2131–2139. (7)

D’Avino, G.; Painelli, A.; Soos, Z. G. Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to

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Horiuchi, S.; Kobayashi, K.; Kumai, R.; Ishibashi, S. Ionic versus Electronic Ferroelectricity in Donor-Acceptor

Molecular Sequences. Chem. Lett. 2014, 43, 26–35. (9)

Kobayashi, K.; Horiuchi, S.; Kumai, R.; Kagawa, F.; Murakami, Y.; Tokura, Y. Electronic Ferroelectricity in a

Molecular Crystal with Large Polarization Directing Antiparallel to Ionic Displacement. Phys. Rev. Lett. 2012, 108, 237601. (10) Tayi, A. S.; Shveyd, A. K.; Sue, A. C.-H.; Szarko, J. M.; Rolczynski, B. S.; Cao, D.; Kennedy, T. J.; Sarjeant, A. A.; Stern, C. L.; Paxton, W. F.; et al. Room-Temperature Ferroelectricity in Supramolecular Networks of Charge-Transfer Complexes. Nature 2012, 488, 485–489. (11) D’Avino, G.; Souto, M.; Masino, M.; Fischer, J. K. H.; Ratera, I.; Fontrodona, X.; Giovannetti, G.; Verstraete, M. J.; Painelli, A.; Lunkenheimer, P.; et al. Conflicting Evidence for Ferroelectricity. Nature 2017, 547, E9–E10. (12) Iwasa, Y.; Koda, T.; Koshlhara, S.; Tokura, T.; Saito, G. Spectroscopic Study on (Anti)Ferroelectric Molecular Systems. Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. Mol. Cryst. Liq. Cryst. 1992, 216, 195–200. (13) Imaeda, K.; Enoki, T.; Inokuchi, H.; Saito, G. Electronic Properties of 3,3′,5,5′-Tetramethyl-Benzidine Complexes with TCNQ Derivatives. Mol. Cryst. Liq. Cryst. 1986, 141, 131–140. (14) Iwasa, Y.; Koda, T.; Tokura, Y.; Kobayashi, A.; Iwasawa, N.; Saito, G. Temperature-Induced Neutral-Ionic Transition in Tetramethylbenzidine-Tetracyanoquinodimethane (TMB-TCNQ). Phys. Rev. B 1990, 42, 2374–2377. (15) Castagnetti, N.; Kociok-Köhn, G.; Da Como, E.; Girlando, A. Temperature-Induced Valence Instability in the Charge-Transfer Crystal TMB-TCNQ. Phys. Rev. B 2017, 95, 024101. (16) Laudise, R.A.; Kloc, Ch.; Simpkins, P.G.; Siegrist, T. Physical Vapor Growth of Organic Semiconductors. J. Cryst. Growth 1998, 187, 449–454. (17) Bruker (2008). APEX2, SAINT and SADABS, Bruker AXS Inc., Madison, Wisconsin, USA. (18) Burla, M.C.; Caliandro, R.; Carrozzini, B.; Cascarano, G.L.; Cuocci, C.; Giacovazzo, C.; Mallamo, M.; Mazzone A.; Polidori G. Crystal structure determination and refinement via SIR2014. J. Appl. Cryst. 2015, 48, 306–309. (19) Sheldrick, G.; Crystal structure refinement with SHELXL, Acta Cryst C, 2015, 71, 3-8. (20) Kosower, E. M.; Marcovich, G.; Borz, G. Thin‐Film Infrared Spectroscopy of Acetonitrile. Chem. Phys. Chem. 2007,


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8, 2513-2519. (21) Bozio, R.; Zanon, I.; Girlando, A.; Pecile, C. Influence of the intermolecular charge transfer interaction on the solution and solid state infrared spectra of 7,7,8,8-tetracyanoquinodimethane (TCNQ) alkaline salts. J. Chem. Soc., Faraday Trans. 2, 1978,74, 235-248. (22) There is a misprint in Ref. 13, as the ρ of phase HT is erroneously quoted as 0.29. The erroneous value is in any case within the error estimate. (23) Hu, P.; Wang, S.; Chaturvedi, A.; Wei, F.; Zhu, X.; Zhang, X.; Li, R.; Li, Y.; Jiang, H.; Long, Y.; Kloc, C. Impact of C–H···X (X = F, N) and π–π Interactions on Tuning the Degree of Charge Transfer in F6TNAP-Based Organic Binary Compound Single Crystals. Cryst. Growth Des. 2018, 18, 1776-1785. (24) Delchiaro, F.; Girlando, A.; Painelli, A.; Bandyopadhyay, A.; Pati S.K.; D'Avino, G. Towards first-principles prediction of valence instabilities in mixed stack charge-transfer crystals. Phys. Rev. B 2017, 95, 155125. (25) Le Cointe, M.; Lemée-Cailleau, M. H.; Cailleau, H.; Toudic, B.; Toupet, L.; Heger, G.; Moussa, F.; Schweiss, P.; Kraft, K.H.; Karl, N. Symmetry breaking and structural changes at the neutral-to-ionic transition in tetrathiafulvalene-pchloranil. Phys. Rev. B 1995, 51, 3374. (26) Batail, P.; LaPlaca, S.J.; Mayerle, J.J.; Torrance J.B. Structural Characterization of the Neutral-Ionic PhaseTransition in Tetrathiafulvalene-Chloranil: Evidence for C-H-0 Hydrogen Bonding. J. Am. Chem. Soc. 1981, 103, 951-953. (27) McKinnon, J.J.; Jayatilaka, D.; Spackman, M.A. Towards quantitative analysis of intermolecular interactions with Hirshfeld surfaces. Chem Commun., 2007, 0, 3814-3816. (28) Ueda, A. Development of Novel Organic Functional crystals by Utilizing Proton and π-Electron Donating/Accepting Abilities, Bull. Chem. Soc. Japan, 2017, 90, 1181. (29) Morimoto, T.; Miyamoto, T.; Okamoto, H. Ultrafast Electron and Molecular Dynamics in Photoinduced and ElectricField-Induced Neutral–Ionic Transitions. Crystals 2017, 7, 132.



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For Table of Contents Use Only

Solvated and Ferroelectric Phases of the Charge Transfer Co-crystal TMB-TCNQ Francesco Mezzadri, Nicola Castagnetti, Matteo Masino, and Alberto Girlando

SYNOPSIS: TMB-TCNQ undergoes inversion symmetry breaking by cooling below 160 K, giving rise to stack dimerization ascribed to increase of the π-π overlap. The occurrence of charge transfer joined to dimerization indicates TMB-TCNQ as a potential ferroelectric molecular crystal. Crystallization from acetonitrile solution yields a solvated phase, not showing low temperature transitions, that converts to TMB-TCNQ both on heating and grinding.


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Powder X-ray patterns of as-grown microcrystals of TMB-TCNQ obtained from acetonitrile (top trace), by PVT (middle trace) and simulated from phase HT structure of Ref. 13 (bottom trace). 84x60mm (300 x 300 DPI)



Asymmetric unit of the solvated TMB-TCNQ phase projected along the [100] direction. Just one of the possible orientations of the disordered acetonitrile molecules is drawn for clarity. 76x69mm (300 x 300 DPI)


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Temperature evolution (from top to bottom) of the IR spectra of TMB-TCNQ solvated phase. Left panel: spectra polarized perpendicular to the stack axis. The position of the charge sensitive b1u ν20 mode of TCNQ in the three phases is evidenced by a dashed line. Right panel: Spectra polarized parallel to the stack axis. The asterisk indicates the band corresponding to the acetonitrile CN stretching mode. 85x60mm (300 x 300 DPI)



TMB-TCNQ moieties in the HT (left) and LT (right) phases, viewed in [001] projection. 85x27mm (300 x 300 DPI)


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Representation of the ferroelectric arrangements of the dipoles P in the LT phase of TMB-TCNQ. The direction of the dipoles corresponds to the so-called ionic ferroelectricity. 85x56mm (300 x 300 DPI)



Relative lattice parameters variation vs. temperature measured in heating. 84x61mm (300 x 300 DPI)


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Hirshfeld dnorm surface computed for the TMB molecule. The analogue image relative to TCNQ is reported as Supporting Information. Top: phase HT, bottom: LT. 85x93mm (300 x 300 DPI)



Hirshfeld dnorm surface computed for the TMB-TCNQ system at 130 K. 85x63mm (300 x 300 DPI)


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Fingerprint plot of to the C-C interactions. Left: 230 K, right: 130 K. 85x42mm (300 x 300 DPI)


Solvated and Ferroelectric Phases of the Charge Transfer Co-crystal

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