Crystals of Charge-Transfer Complexes with Reorienting Polar

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Crystals of Charge-Transfer Complexes with Reorienting Polar Molecules: Dielectric Properties and Order–Disorder Phase Transitions Jun Harada, Naho Yoneyama, Shota Sato, Yukihiro Takahashi, and Tamotsu Inabe Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01418 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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Crystal Growth & Design

Crystals of Charge-Transfer Complexes with Reorienting Polar Molecules: Dielectric Properties and Order–Disorder Phase Transitions Jun Harada,*,†,‡ Naho Yoneyama,‡ Shota Sato,‡ Yukihiro Takahashi,†,‡ and Tamotsu Inabe†,‡ †

Department of Chemistry, Faculty of Science, and ‡Graduate School of Chemical Sciences and

Engineering, Hokkaido University, Sapporo 060-0810, Japan Abstract: Crystals of charge-transfer (CT) complexes were synthesized using the polar molecule tetrachlorophthalonitrile (TCPN) as the electron acceptor and nonpolar aromatic hydrocarbons, such as perylene, coronene, chrysene, and pyrene, as the electron donors. Variable-temperature X-ray crystal structure analyses revealed that the TCPN molecules in all the CT crystals exhibit orientational disorder at room temperature. Some of the CT crystals undergo an order–disordertype phase transition upon cooling, where the orientation of the TCPN molecules becomes ordered at low temperature. The CT crystals show large dielectric constants at room temperature arising from in-plane reorientation of the polar TCPN molecules. The order–disorder phase transition results in a drastic reduction of the dielectric constant of the CT crystals upon cooling. This study demonstrates that the formation of CT crystals from polar molecules represents a promising method for the development of dielectric crystalline materials, whose properties vary significantly in response to phase transitions.

Corresponding Author: Jun Harada Department of Chemistry, Faculty of Science Hokkaido University, Sapporo 060-0810, Japan Phone/Fax: +81 11 706 3563 E-mail: [email protected]

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Crystals of Charge-Transfer Complexes with Reorienting Polar Molecules: Dielectric Properties and Order–Disorder Phase Transitions Jun Harada,*,†,‡ Naho Yoneyama,‡ Shota Sato,‡ Yukihiro Takahashi,†,‡ and Tamotsu Inabe†,‡ †

Department of Chemistry, Faculty of Science, and ‡Graduate School of Chemical Sciences and

Engineering, Hokkaido University, Sapporo 060-0810, Japan

ABSTRACT: Crystals of charge-transfer (CT) complexes were synthesized using the polar molecule tetrachlorophthalonitrile (TCPN) as the electron acceptor and nonpolar aromatic hydrocarbons, such as perylene, coronene, chrysene, and pyrene, as the electron donors. Variable-temperature X-ray crystal structure analyses revealed that the TCPN molecules in all the CT crystals exhibit orientational disorder at room temperature. Some of the CT crystals undergo an order–disorder-type phase transition upon cooling, where the orientation of the TCPN molecules becomes ordered at low temperature. The CT crystals show large dielectric constants at room temperature arising from in-plane reorientation of the polar TCPN molecules. The order–disorder phase transition results in a drastic reduction of the dielectric constant of the CT crystals upon cooling. This study demonstrates that the formation of CT crystals from polar molecules represents a promising method for the development of dielectric crystalline materials, whose properties vary significantly in response to phase transitions.


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INTRODUCTION Molecular motion in crystals has been studied in various fields of chemistry.1–6 Recent topics of interest in this research field include the development of functional materials and establishing control over macroscopic properties by manipulation of the molecular motion in crystals. For example, orientational changes of aromatic molecules in semiconductor crystals influence the electric properties (e.g., the resistivity and transfer characteristics) of the semiconductors.7,8 Moreover, the rapid rotation of phenylene groups is related to the phosphorescent behavior of crystals,9 while the reorientation of polar molecules or polar moieties of molecules induces dielectric and ferroelectric properties in crystals.10–14 Changes in the intermolecular interactions in crystals induced by conformational changes on the molecules result in negative thermal expansion of the crystals.15 We have recently focused on the molecular-motion-based dielectric properties of crystals consisting of weakly bound charge-transfer (CT) complexes.16 Combining the polar molecule tetrabromophthalic anhydride (TBPA), which exhibits electron-accepting properties, with aromatic hydrocarbons that act as electron donors affords crystals of weak CT complexes, where the degree of charge transfer between the components is small and the molecules are electrically neutral in the ground state of the complexes. Despite the static nature of the single-component crystals, the polar TBPA molecule undergoes rapid in-plane reorientation, which has been attributed to additional free space available in the CT crystals. The orientational motion of the TBPA molecules induces orientational polarization, which results in a large dielectric constant of the CT crystals. In contrast to weak CT crystals, which have been relatively unexplored in the context of dielectric materials, considerable attention has been paid to the dielectric properties of strong CT


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crystals, some of which exhibit neutral-to-ionic phase transitions and concomitant emergence of ferroelectricity.17–20 However, this phenomenon has only been encountered in a limited number of CT crystals such as tetrathiafulvalene–chloranil (TTF–CA). This situation stands in sharp contrast to the dielectric phenomena of weak CT crystals with reorienting polar molecules, which can be found in a wide variety of CT crystals of donors and acceptors with weak and moderate electron-donating and -accepting abilities, respectively. By taking advantage of the wide range of available exchangeable constituent components, a systematical exploration of such weak CT crystals may lead to the development of dielectric crystalline materials with specifically designed properties. In this study, we have furthered the development of CT crystals with molecularmotion-induced

dielectric

properties

by

synthesizing

CT

crystals

composed

of

tetrachlorophthalonitrile (TCPN; 1) and aromatic hydrocarbons. TCPN is a polar planar molecule with a large dipole moment ( = 5.5 D, calculated at the MP2/6-31+G* level of theory), attributed to the electron-withdrawing cyano groups. These groups also endow the molecule with electron-accepting characteristics, whereby TCPN is able to form CT crystals with electron-donating molecules. TCPN exhibits an approximately circular disk-like structure. This polar molecule is therefore expected to undergo in-plane reorientation within crystals inducing a large orientational polarization. Although the molecular orientation and intermolecular interactions have been intensively studied for single-component crystals of TCPN and its CT crystals with hexamethylbenzene,21,22 the dynamic nature of TCPN molecules in crystals has not yet been reported. In this work, we synthesized CT crystals of TCPN and the nonpolar aromatic hydrocarbons perylene, coronene, chrysene, and pyrene, and examined their structures and dielectric properties together with their temperature dependence. The TCPN molecules in the CT crystals exhibit in-plane reorientation and orientational disorder with up to


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six different orientations at high temperatures, which result in large dielectric constants of the CT crystals. Some of the crystals show order–disorder-type phase transitions, which induce marked changes in the dielectric constants at the transition temperatures.

EXPERIMENTAL SECTION Materials. Crystals of CT complexes of perylene–TCPN (2), coronene–TCPN (3), chrysene– TCPN (4), and pyrene–TCPN (5) were grown by slow evaporation of solutions that contained equimolar amounts of 1 and the corresponding aromatic hydrocarbon (2: red crystals from acetonitrile/acetone; melting point (mp): 241 °C; 3: yellow crystals from benzene; no mp up to 300 °C; 4: yellow crystals from chloroform; mp: 199 °C; 5: yellow crystals from chloroform; mp: 222 °C). Single crystals of 2, 4, and 5 suitable for single-crystal X-ray diffraction analyses were obtained by the aforementioned method, while those of 3 were grown by co-sublimation of 1 and coronene under vacuum. Details of the formation of CT crystals can be found in the Supporting Information. Single-crystal X-ray Diffraction Analyses. Single-crystal X-ray diffraction analyses were conducted on a Bruker APEX II Ultra diffractometer (Mo K radiation,  = 0.71073 Å). The temperature of the samples was regulated using flowing nitrogen gas and calibrated using a thermocouple. The structures were solved by intrinsic phasing (SHELXT-2014)23 and refined by


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full-matrix least-squares on F2 using SHELXL–2014.24 All hydrogen atoms were refined using riding models. For fully ordered crystal structures, all non-hydrogen atoms were refined anisotropically. For disordered crystal structures (2 at 300 and 100 K, 3 at 300 and 100 K, 4 at 310 and 300 K, and 5 at 300 and 230 K), details of the refinement are provided in the Supporting Information (SHELXL-2014 res files). The crystal and experimental data are summarized in Table 1. Measurements. Differential scanning calorimetry (DSC) measurements were conducted on a Rigaku Thermo Plus DSC8230 apparatus using heating/cooling rates of 10 K min–1. Microcrystalline samples were weighed and sealed in aluminum pans. The DSC traces of 2–5 are shown in Figure 1. The powder X-ray diffraction patterns of the microcrystalline powder samples were recorded on a Bruker D8 ADVANCE diffractometer to confirm that the powders used for DSC measurement presented a crystal structure identical to that obtained by singlecrystal X-ray diffraction analysis. The dielectric constants were measured with an Agilent E4980A precision LCR meter (500 Hz to 1 MHz). A homemade cryostat was used to control the temperature of the samples (95–350 K) under helium atmosphere. The temperature was measured using a silicon-diode thermometer attached to the sample holder. Single crystals of 2, 3, and 5 were used as the samples with painted gold paste as the electrodes. General interpretations of the dielectric constants and their temperature dependence with respect to orientational changes of the polar molecules are documented in the literature.16


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Figure 1. DSC traces for (a) perylene–TCPN (2), (b) coronene–TCPN (3), (c) chrysene–TCPN (4), and (d) pyrene–TCPN (5).


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Table 1. Crystal Data and Structure Refinement Information for 2–5 compounds

perylene–TCPN (2)

coronene–TCPN (3)

chrysene–TCPN (4)

empirical formula

C28H12N2Cl4

C32H12N2Cl4

C26H12N2Cl4

formula weight

518.20

566.24

494.18

temperature/K

300

100

300

100

310

300

100

crystal system

monoclinic

monoclinic

monoclinic

monoclinic

monoclinic

monoclinic

monoclinic

space group

P21/c

P21/c

P21

P21

P21/n

P21/n

P21/n

a/Å

7.6882(5)

7.5507(5)

7.2815(6)

7.1358(8)

8.8112(5)

17.6211(13)

17.4861(10)

b/Å

8.8520(6)

8.8207(6)

9.5109(7)

9.4968(10)

7.7752(5)

7.7676(6)

7.6902(4)

c/Å

16.3080(11)

16.0840(11)

17.4214(13)

17.1695(19)

16.1554(10)

16.6296(12)

16.3972(9)

/°

90.4470(10)

91.0660(10)

96.0134(13)

94.594(2)

102.4590(10) 71.3218(12)

70.9950(10)

V/Å3

1109.82(13)

1071.05(13)

1199.86(16)

1159.8(2)

1080.72(11)

2156.3(3)

2084.8(2)

Z

2

2

2

2

2

4

4

reflections collected

15949

15175

17721

16786

15461

30671

29658

independent reflections

2588

2444

5460

5251

2513

4929

4791

Rint

0.0224

0.0164

0.0201

0.0229

0.0190

0.0223

0.0155

data/restraints/parameters 2588/166/203 2444/166/202 5460/370/448 5251/447/446 2513/128/172 4929/325/344 4791/0/289 goodness-of-fit on F2

1.049

1.068

1.042

1.132

1.059

1.027

1.066

R[F2 > 2(F2)]

0.0344

0.0260

0.0622

0.0424

0.0400

0.0487

0.0264

wR(F2) (all data)

0.1032

0.0700

0.1831

0.1004

0.1197

0.1572

0.0714

CCDC deposit number

1813534

1813535

1813536

1813537

1813538

1813539

1813540


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Table 1. Continued compounds

pyrene–TCPN (5)

chemical formula

C24H10N2Cl4

formula weight

468.14

temperature/K

300

230

220

115

crystal system

monoclinic

monoclinic

triclinic

triclinic

space group

P21/n

P21/n

P1̅

P1̅

a/Å

7.2509(5)

7.1751(5)

7.1779(10)

7.1106(13)

b/Å

9.1166(7)

9.1122(6)

17.415(2)

17.278(3)

c/Å

15.2982(12)

15.1404(11)

17.827(2)

17.748(3)

/°

90

90

62.427(2)

62.924(3)

/°

99.6590(10)

99.0425(11)

81.939(2)

82.368(3)

/°

90

90

82.145(2)

82.571(3)

V/Å3

996.93(13)

977.59(12)

1949.3(5)

1918.5(6)

Z

2

2

4

4

reflections collected

13693

18236

47320

32082

independent reflections

2282

2244

12294

10974

Rint

0.0494

0.0184

0.0609

0.0520

data/restraints/parameters 2282/210/257 2244/142/184 12294/0/542

10974/0/542

goodness-of-fit on F2

1.034

1.032

1.079

1.053

R[F2 > 2(F2)]

0.0480

0.0326

0.0581

0.0353

wR(F2) (all data)

0.1363

0.0912

0.1889

0.0949

CCDC deposit number

1813541

1813542

1813543

1813544

RESULTS AND DISCUSSION Crystal Structure of Perylene–TCPN (2). The acceptor molecule TCPN forms CT crystals with perylene as the donor molecule (2). The crystal structures at 300 and 100 K were determined by X-ray diffraction analysis. The CT crystals consist of a 1:1 mixture of the acceptor and donor molecules arranged in a mixed-stack architecture with acceptor and donor


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molecules alternately stacked along the crystallographic a-axis (Figure 2a). Both the acceptor (TCPN) and donor (perylene) molecules are located on crystallographic inversion centers, and the central benzene rings of both molecules overlap to form one-dimensional molecular columns oriented in parallel to the a-axis (Figure 2b). The TCPN molecules are orientationally disordered; each molecule can adopt six different orientations that share the central benzene ring of the molecule. The six orientations of the TCPN molecules are approximately related to each other by 60° rotations around the axis perpendicular to the molecular plane passing through the center of the benzene ring. Moreover, each orientation (A, B, and C) has a crystallographic equivalent (A’, B’, and C’, respectively) (Figure 2c). The site occupation factors of the A, B, and C orientations show significant temperature-dependent changes, which were refined as 0.096(4), 0.195(4), and 0.207(4) at 300 K and 0.046(3), 0.189(3), and 0.264(3) at 100 K, respectively. The temperature dependence of the orientation populations clearly indicates that the TCPN molecules in 2 undergo orientational interconversion through in-plane reorientation and that the six orientations equilibrate through multiple 60° orientational jumps. The crystal structure analysis revealed no phase transitions between 300 and 100 K, and the DSC measurements indicated that 2 exhibits no phase transitions below 400 K (Figure 1a).


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Figure 2. Crystal and molecular structures of perylene–TCPN (2) at 300 K with thermal ellipsoids drawn at 50% probability. (a) Projection along the b-axis. (b) Projection along the aaxis. (c) (Top) Orientationally disordered TCPN molecule; (bottom) the six different orientations of the TCPN molecule. Dielectric Properties of Perylene–TCPN (2). Reorientation of the polar TCPN molecules in the CT crystals of 2 results in orientational polarization and large dielectric constants of the crystal. The dielectric constant was measured at several frequencies of an a.c. electric field as a function of the temperature using a single crystal of 2 (Figure 3a,b). Both the real (’) and imaginary (’’) parts of the complex dielectric constant experienced significant changes at temperatures above 200 K, which depended on the frequency of the measurement electric field. These features are


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characteristic of orientational polarization induced by the reorientation of polar molecules in crystals whose relaxation times are comparable to the inverse of the frequency of the a.c. electric field in the examined temperature range.25 The large dielectric constant reaching 50 near room temperature was ascribed to the large dipole moment of the TCPN molecules. Small ’ values (~6) and those close to zero ’’ below ~200 K indicate that the reorientation of the polar TCPN molecules is much slower than the electric fields (500 Hz – 1 MHz) of the measurements and does hence not contribute to the orientational polarization.

Figure 3. Dielectric properties of perylene–TCPN (2). Temperature dependence of the (a) real (’) and (b) imaginary (’’) parts of the complex dielectric constants. The electric field was applied through gold paste painted on two parallel {021} faces of a single crystal of 2. Frequency dependence of the (c) real (’) and (d) imaginary (’’) parts at several selected temperatures. (e)


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Relationship between ’ and ’’ (Cole–Cole diagram) at 290 K. The solid curves in (c)–(e) represent the fit of the experimental data to the Havriliak–Negami model. A quantitative analysis of the complex dielectric constant of 2 was performed based on the real and imaginary parts plotted against the frequency of the electric field at several selected temperatures (Figure 3c,d), and the Cole–Cole diagram (’’ vs. ’) at 290 K is shown in Figure 3e. The Cole–Cole plot deviates significantly from the semicircles and circular arcs expected from the Debye and Cole–Cole relaxation functions, respectively.26 The skewed circular arc is well represented by the phenomenological relaxation function known as the Havriliak–Negami equation: 𝜀 ∗ (𝜔) = 𝜀∞ +

𝜀0 − 𝜀∞ (1) [1 + (𝑖𝜔𝜏)1−𝛼 ]𝛽

where * is the complex dielectric constant and 0 as well as  are the low- and high-frequency limits of the real part of *, respectively (0 > );  represents the relaxation time;  is equal to 2 times the frequency; and the parameters  and  (0 ≤ , ≤ 1) are related to the distribution of relaxation times.27 The parameters for 2 were obtained by fitting the experimental data of the imaginary part (’’) using eq. 1 (Figure 3d). The real part and Cole–Cole diagram are also described well by eq. 1 using the obtained parameters. The relaxation time () at each temperature obeys the Arrhenius equation (eq. 2) with good accuracy, yielding an estimated activation energy (Ea) of 55 kJ mol–1 for the dielectric relaxation (Figure 4). The results indicate that the TCPN molecules undergo in-plane reorientation in 2 with a relatively low activation energy, comparable to the reported values of 56 kJ mol–1 for perylene– TBPA and 48 kJ mol–1 for coronene–TBPA, in which the polar TBPA molecules undergo inplane reorientation.16


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𝜏 = 𝐶exp (

Page 14 of 31

𝐸a ) (2) 𝑘𝑇

Figure 4. Arrhenius plot of the relaxation time () as a function of the inverse temperature for perylene–TCPN (2). The solid line represents the least-squares fit of the data. Crystal Structure of Coronene–TCPN (3). The acceptor molecule TCPN also presents orientational disorder in CT crystals with coronene. Coronene and TCPN molecules form a 1:1 mixed-stack architecture in crystals of 3, wherein the molecular columns are parallel to the aaxis and both the donor and acceptor molecules are orientationally disordered (Figure 5a). Each coronene molecule adopts one of three different orientations (A, B, and C) at 300 K (Figure 5b), whose occupation factors are 0.453(4), 0.350(4), and 0.197(4), respectively. The populations of the orientations depend on temperature, and only two orientations (A and B) are observed at 100 K with occupation factors of 0.815(4) and 0.185(4), respectively (Figure 5c). These results indicate that the coronene molecules in crystals of 3 undergo orientational interconversion through in-plane reorientation and that only the energetically favorable orientations (A and B) are present at low temperatures. Similar motions of coronene molecules have been previously reported for single-component crystals and CT crystals with several acceptor molecules.28–31


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Figure 5. Crystal and molecular structures of coronene–TCPN (3) with thermal ellipsoids drawn at 50% probability. (a) Projection along the a-axis at 300 K. Orientationally disordered coronene molecule at (b) 300 and (c) 100 K. The atoms with orientation A, B, and C are depicted in grey, blue, and green, respectively. (d) Orientationally disordered TCPN molecule at 300 K. The orientation of the TCPN molecules is also disordered, with each molecule taking one of two orientations that differ by 60° with respect to each other (Figure 5d). The ratio of the siteoccupation factors shows no significant changes with temperature (0.465(10):0.535(10) at 300 K


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and 0.486(8):0.514(8) at 100 K), indicating that the two orientations have almost identical potential energies in the crystal. Consequently, their populations show no significant temperature dependence irrespective of the orientational interconversion. The polar point group of the crystal structure (point group 2, space group P21) implies that the crystal belongs to pyroelectric crystals and can present spontaneous electric polarization along the b-axis, thus meeting a prerequisite for ferroelectricity. However, unlike most ferroelectric crystals, 3 exhibits no phase transitions up to 480 K (Figure 1b). Dielectric Properties of Coronene–TCPN (3). Dielectric measurements revealed that the polar TCPN molecules experience molecular reorientation in CT crystals of 3, although the contribution of the orientational polarization is smaller than that for crystals of 2. Figure 6 shows the dielectric constants of 3 obtained from a single crystal. The dielectric constants above 200 K show dependence on the temperature and frequency, which is characteristic of orientational polarization, as in the case of crystals of 2. Although the coronene molecules undergo in-plane reorientation in the crystal, the dynamic reorientation of the nonpolar molecules does not contribute to the observed orientational polarization, which was instead attributed to the reorientation of the polar TCPN molecules. In contrast to the six different orientations that TCPN molecules can take in crystals of 2, which allow 360° in-plane reorientation of the dipolar molecules, the TCPN molecules in crystals of 3 can assume only two orientations that differ by 60° with respect to each other. The restrictions on the orientational changes of the dipole moment reduce the contribution of the orientational polarization and the dielectric constants of crystals of 3 relative to those of 2 at high temperatures. The frequency dependence of the complex dielectric constants at several selected temperatures and the Cole–Cole diagram (290 K) are described well by the Havriliak–Negami equation (eq. 1) (Figure S1). After fitting, the obtained relaxation time,


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, obeys the Arrhenius equation (eq. 2), which provides an estimated activation energy Ea of 43 kJ mol–1 for the dielectric relaxation (Figure S2a).

Figure 6. Dielectric constants of coronene–TCPN (3). Temperature dependence of the (a) real (’) and (b) imaginary (’’) parts of the complex dielectric constants. The electric field was applied through gold paste painted on the two parallel {001} faces of a single crystal of 3. Due to its polar crystal structure, crystals of 3 are pyroelectric crystals and could potentially show ferroelectricity despite the absence of ferroelectric phase transitions. We have not yet been able to successfully prepare single crystals of 3 with sizes and shapes that are suitable for pyroelectric current measurements and polarization–electric field hysteresis measurements. The crystals of 3 can be spontaneously polarized along the b-axis, and pair of electrodes for the pyroelectric and ferroelectric measurements should preferably be formed on two parallel crystal faces perpendicular to the b-axis. However, only thin plates of single crystals of 3 with welldeveloped {001} crystal faces have been obtained so far.


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Crystal Structure and Phase Transition of Chrysene–TCPN (4). The CT crystals formed from TCPN and chrysene exhibits an order–disorder phase transition related to the orientational order of the TCPN molecules. An equimolecular mixture of chrysene and TCPN molecules affords molecular columns of alternating donor and acceptor molecules. Crystals of 4 belong to the monoclinic P21/n space group at 310 K (high-temperature phase), and both the donor and acceptor molecules reside on crystallographic inversion centers (Figure 7a). The TCPN molecules

exhibit

orientational

disorder;

each

molecule

can

adopt

one

of

two

crystallographically equivalent orientations that differ by 180° with respect to each other (Figure 7c). The DSC measurements for crystals of 4 revealed a phase transition at 305 K (Figure 1c). The transition enthalpy (H = 0.17 kJ mol-1) and entropy (S = 0.57 J K–1 mol–1) were calculated from the small DSC peak corresponding to the phase transition. The small value of the transition entropy (S = R ln(1.07)) where R is the gas constant, was attributed to small changes in the populations of the two orientations of TCPN during the phase transition (vide infra). The overall molecular arrangement in the crystal structure at 100 K (low-temperature phase) does not significantly differ from that in the high-temperature phase, except that neither chrysene nor TCPN are orientationally disordered (Figure 7b). The lattice in the low-temperature phase is approximately related to that in the high-temperature phase by the transformations: aLT = 2 aHT, bLT = bHT, and cLT = aHT + cHT. Although the crystal in the low-temperature phase belongs to the centrosymmetric space group P21/n, both molecules are not located on crystallographic inversion centers.


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Figure 7. Crystal and molecular structures of chrysene–TCPN (4) with thermal ellipsoids drawn at 50% probability. (a) Projection along the b-axis at 310 K. (b) Projection along the b-axis at 100 K. (c) Orientationally disordered and ordered TCPN molecules at several temperatures. The atoms of the chloro and cyano groups of the minor orientation at 300 K are depicted in white. Although each TCPN molecule takes only one orientation at 100 K, TCPN is still disordered over the two orientations at 300 K, which is just below the transition temperature and falls within the low temperature phase (Figure 7c). The ratio of the populations of the two orientations was


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refined to 0.684(3):0.316(3) at 300 K, close to the 0.5:0.5 value for the high-temperature phase above 305 K. This small variation in the populations just above (310 K) and below (300 K) the phase-transition temperature (305 K) is attributable to the observed small changes in the entropy and enthalpy at this order–disorder phase transition. The observed temperature dependence of the occupation factors of the TCPN orientations clearly indicates that the disorder is dynamic and that the TCPN molecules undergo orientational changes in crystals of 4. The small population variations also imply that the difference in energy of the two orientations is quite small in the low-temperature phase, although not zero, and that the phase transition involves small changes in the crystal structure that induce a small energy difference between the two TCPN orientations. Although we have not yet obtained good single crystals with well-developed crystal faces suitable for dielectric measurements, we can still conclude that the TCPN molecules undergo inplane reorientation in crystals of 4. Crystal Structure and Phase Transition of Pyrene–TCPN (5). Crystals of 5 also exhibit an order–disorder-type phase transition at low temperature and the structural changes associated with the transition were elucidated by X-ray diffraction analysis. The DSC measurements for crystals of 5 revealed that they undergo a phase transition at 223 K during cooling (Figure 1d). The transition enthalpy (H = 1.15 kJ mol–1) and entropy (S = 5.13 J K–1 mol–1) were calculated from the sharp peak corresponding to the phase transition. The transition entropy (S = R ln(1.85)), which is much larger than that for 4, was ascribed to drastic changes in the orientational disorder at the transition temperature (vide infra). Pyrene and TCPN molecules form mixed-stack one-dimensional molecular columns in crystals of 5. The crystals belong to the monoclinic P21/n space group at 300 K (high-temperature phase), and both the donor and acceptor molecules are located on crystallographic inversion centers


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(Figure 8a). Both the pyrene and TCPN molecules are orientationally disordered at 300 K (Figure 8b,c). Each pyrene molecule adopts one of two possible orientations, whose occupation factors are 88.0(4) and 12.0(4). The six-fold orientational disorder of TCPN in crystals of 5 is similar to that in crystals of 2, and the populations of the three crystallographically independent orientations were refined as 14.0(5), 7.3(4), and 28.7(5).

Figure 8. Crystal and molecular structures of pyrene–TCPN (5) with thermal ellipsoids drawn at 50% probability. (a) Projection along the a-axis at 300 K. (b) Orientationally disordered pyrene molecule at 300 K. The atoms of the minor orientation are depicted in white. (c) Orientationally disordered TCPN molecule at 300 K. (d) Projection along the a-axis at 230 K. (e) Projection along the a-axis at 220 K.


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Crystals of 5 at 230 K remain in the high-temperature phase, although the disorder of the pyrene molecules disappears at this temperature (Figure 8d). The observed temperature dependence indicates that the pyrene molecules undergo orientational changes in the hightemperature phase. However, the disorder of the TCPN molecules remains at 230 K, and the populations of the three orientations are 14.9(4), 4.8(3), and 30.3(4). As shown below, the dielectric measurements indicate that the disorder of the TCPN molecules is also dynamic; thus, the polar molecules undergo in-plane reorientation in crystals of 5. Crystals of 5 exhibit a low-temperature phase at 220 K. The lattice changes associated with the phase transition are approximately represented by the following transformations: aLT = aHT, bLT = bHT + cHT, and cLT = cHT – bHT. The crystals belong to the triclinic crystal system and space group P1̅ . The overall arrangement of the molecules is almost identical to that in the high-temperature phase, while both the pyrene and TCPN molecules are orientationally ordered (Figure 8e). The drastic changes in the degree of orientational order of the TCPN molecules at these two temperatures (230 and 220 K), both of which are near the transition temperature (223 K), are consistent with a large transition entropy, which stands in sharp contrast to the transition in crystals of 4. The order–disorder phase transition cannot be a ferroelectric one as the crystals in the low-temperature phase belong to the nonpolar point group 1̅ . The orientationally ordered crystals did not exhibit phase transitions upon further cooling to 115 K, where the structure was essentially identical to that at 220 K. Dielectric Properties of Pyrene–TCPN (5). Dielectric measurements of crystals of 5 showed that TCPN molecules undergo in-plane reorientation and that the dielectric constant of the crystal changes abruptly at the order–disorder phase transition. The dielectric constant was measured for a single crystal of 5 (Figure 9). The real and imaginary parts of the dielectric constant showed


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drastic reductions at the phase-transition temperature upon cooling. These results are consistent with the drastic changes observed for the degree of orientational order of TCPN molecules at the phase transition.

Figure 9. Dielectric constant of pyrene–TCPN (5). Temperature dependence of the (a) real (’) and (b) the imaginary (’’) parts of the complex dielectric constants. The electric field was applied through gold paste painted on two parallel {011} faces (high-temperature phase) of a single crystal of 5. The phase-transition temperature is indicated by arrows. Above the transition temperature, both the real and imaginary parts of the complex dielectric constant show temperature and frequency dependence, which is characteristic of orientational polarization, similar to the case of crystals of 2 and 3. This was attributed to the in-plane reorientation of the polar TCPN molecules. The frequency dependence of the dielectric constant and the Cole–Cole diagram are satisfactorily described by the Havriliak–Negami equation (eq. 1) (Figure S3). The Arrhenius plot of the relaxation time  provided an estimated activation energy Ea of 47 kJ mol–1 for the dynamic molecular process (Figure S2b). Overall, the activation


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energies for the reorientation of TCPN in crystals of 2, 3, and 5 fall within a small range (55, 43, and 47 kJ mol–1) regardless of the number of possible TCPN molecular orientations (2 or 6).

CONCLUSIONS We have shown that the polar acceptor TCPN easily undergoes in-plane reorientation in its CT crystals with nonpolar aromatic hydrocarbons. The TCPN molecules are orientationally disordered at room temperature, and each molecule can take up to six different orientations. The dynamic transitions between the different orientations induce orientational polarization of the CT crystals, which thus exhibit large dielectric constants. Together with our previous work,16 this study demonstrates that CT crystal preparation with polar molecules is a versatile method for the development of crystalline materials with large dielectric constants and also a promising strategy for the introduction of phase transitions involving drastic changes in the dielectric properties. Although the occurrence of the phase transitions and their nature, e.g. ferroelectric or antiferroelectric, are still difficult to control or to predict based on the structures of the constituent donor and acceptor molecules, we have found that the combination of pseudocentrosymmetric polar acceptors and centrosymmetric donors yields with high probability CT crystals that exhibit order-disorder phase transitions, the introduction of which is necessary for the design of ferroelectric or antiferroelectric crystals. Moreover, the present study based on TCPN molecules has made significant progress in the development of such dielectric CT crystals by demonstrating that one of the CT crystals exhibits a pyroelectric crystal structure that meets a prerequisite for ferroelectric crystals, which has not been achieved in the previous study. Further exploration of such CT crystals composed of polar planar molecules and donor aromatics with different characteristics, e.g., molecular symmetry and dipole moments, will promote the


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development of dielectric materials with desired properties, leading to novel functional materials including ferroelectric and antiferroelectric molecular crystals. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1012/asc.cgd.xxxxxxx. Preparation of CT crystals, dielectric properties of 3 and 5, Arrhenius plots for 3 and 5 (PDF), and SHELXL-2014 res files of disordered crystal structures. Accession Codes CCDC 1813534–1813544 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by a JSPS KAKENHI grant JP16H04126, as well as a Grant-in-Aid for Scientific Research on Innovative Areas “-System Figuration: Control of Electron and


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Structural Dynamism for Innovative Functions” (JP17H05135). The authors would like to thank Dr. A. Kobayashi (Hokkaido University) for access to a Bruker D8 ADVANCE powder X-ray diffractometer. REFERENCES (1) Gavezzotti, A.; Simonetta, M. Crystal Chemistry in Organic Solids. Chem. Rev. 1982, 82, 1-13. (2) Dunitz, J. D.; Maverick, E. F.; Trueblood, K. N. Atomic Motions in Molecular Crystals from Diffraction Measurements. Angew. Chem., Int. Ed. 1988, 27, 880-895. (3) Khuong, T.-A. V.; Nunez, J. E.; Godinez, C. E.; Garcia-Garibay, M. A. Crystalline Molecular Machines: A Quest Toward Solid-State Dynamics and Function. Acc. Chem. Res. 2006, 39, 413-422. (4) Akutagawa, T.; Nakamura, T. Supramolecular approach for solid state Brownian rotators. Dalton Trans. 2008, 6335-6345. (5) Harada, J.; Ogawa, K. Pedal motion in crystals. Chem. Soc. Rev. 2009, 38, 2244-2252. (6) Vogelsberg, C. S.; Garcia-Garibay, M. A. Crystalline molecular machines: function, phase order, dimensionality, and composition. Chem. Soc. Rev. 2012, 41, 1892-1910. (7) Goetz, K. P.; Fonari, A.; Vermeulen, D.; Hu, P.; Jiang, H.; Diemer, P. J.; Ward, J. W.; Payne, M. E.; Day, C. S.; Kloc, C.; Coropceanu, V.; McNeil, L. E.; Jurchescu, O. D. Freezing-in orientational disorder induces crossover from thermally-activated to temperature-independent transport in organic semiconductors. Nat. Commun. 2014, 5, 5642.


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Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Yokokura, S.; Takahashi, Y.; Nonaka, H.; Hasegawa, H.; Harada, J.; Inabe, T.; Kumai, R.; Okamoto, H.; Matsushita, M. M.; Awaga, K. Switching of Transfer Characteristics of an Organic Field-Effect Transistor by Phase Transitions: Sensitive Response to Molecular Dynamics and Charge Fluctuation. Chem. Mater. 2015, 27, 4441-4449. (9) Jin, M.; Chung, T. S.; Seki, T.; Ito, H.; Garcia-Garibay, M. A. Phosphorescence Control Mediated by Molecular Rotation and Aurophilic Interactions in Amphidynamic Crystals of 1,4Bis[tri-(p-fluorophenyl)phosphane-gold(I)-ethynyl]benzene. J. Am. Chem. Soc. 2017, 139, 18115-18121. (10) Horansky, R. D.; Clarke, L. I.; Price, J. C.; Khuong, T.-A. V.; Jarowski, P. D.; GarciaGaribay, M. A. Dielectric response of a dipolar molecular rotor crystal. Phys. Rev. B 2005, 72, 014302. (11) Horansky, R. D.; Clarke, L. I.; Winston, E. B.; Price, J. C.; Karlen, S. D.; Jarowski, P. D.; Santillan, R.; Garcia-Garibay, M. A. Dipolar rotor-rotor interactions in a difluorobenzene molecular rotor crystal. Phys. Rev. B 2006, 74, 054306. (12) Akutagawa, T.; Koshinaka, H.; Sato, D.; Takeda, S.; Noro, S.-I.; Takahashi, H.; Kumai, R.; Tokura, Y.; Nakamura, T. Ferroelectricity and polarity control in solid-state flip-flop supramolecular rotators. Nat. Mater. 2009, 8, 342-347. (13) Ichikawa, J.; Hoshino, N.; Takeda, T.; Akutagawa, T. Collective In-Plane Molecular Rotator Based on Dibromoiodomesitylene -Stacks. J. Am. Chem. Soc. 2015, 137, 13155-13160.


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(14) Harada, J.; Shimojo, T.; Oyamaguchi, H.; Hasegawa, H.; Takahashi, Y.; Satomi, K.; Suzuki, Y.; Kawamata, J.; Inabe, T. Directionally tunable and mechanically deformable ferroelectric crystals from rotating polar globular ionic molecules. Nat. Chem. 2016, 8, 946-952. (15) Hutchins, K. M.; Groeneman, R. H.; Reinheimer, E. W.; Swenson, D. C.; MacGillivray, L. R. Achieving dynamic behaviour and thermal expansion in the organic solid state via cocrystallization. Chem. Sci. 2015, 6, 4717-4722. (16) Harada, J.; Ohtani, M.; Takahashi, Y.; Inabe, T. Molecular Motion, Dielectric Response, and Phase Transition of Charge-Transfer Crystals: Acquired Dynamic and Dielectric Properties of Polar Molecules in Crystals. J. Am. Chem. Soc. 2015, 137, 4477-4486. (17) 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. (18) 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. (19) 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. (20) Horiuchi, S.; Kobayashi, K.; Kumai, R.; Ishibashi, S. Ionic versus Electronic Ferroelectricity in Donor–Acceptor Molecular Sequences. Chem. Lett. 2014, 43, 26-35. (21) Britton, D. 3,4,5,6-Tetrachloro-1,2-dicyanobenzene, Cl4C6(CN). Cryst. Struct. Commun. 1981, 10, 1509-1512.


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(22) Britton, D. Planar packing of tetrachlorodicyanobenzene isomers. Acta Crystallogr. Sect. B 2002, 58, 553-563. (23) Sheldrick, G. M. SHELXT - Integrated space-group and crystal-structure determination. Acta Crystallogr. Sect. A 2015, 71, 3-8. (24) Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr. Sect. C 2015, 71, 3-8. (25) Kao, K. C. Dielectric Phenomena in Solids: With Emphasis on Physical Concepts of Electronic Processes, Elsevier Academic press, San Diego, USA, 2004. (26) Cole, K. S.; Cole, R. H. Dispersion and Absorption in Dielectrics I. Alternating Current Characteristics. J. Chem. Phys. 1941, 9, 341-351. (27) Havriliak, S.; Negami, S. A Complex Plane Representation of Dielectric and Mechanical Relaxation Processes in Some Polymers. Polymer 1967, 8, 161-210. (28) Fyfe, C. A.; Dunell, B. A.; Ripmeester, J. Nuclear Magnetic Resonance Investigation of Possible Molecular Motion in Coronene, Perlene, and Triphenlene in the Solid State. Can. J. Chem. 1971, 49, 3332-3335. (29) Yoshida, Y.; Shimizu, Y.; Yajima, T.; Maruta, G.; Takeda, S.; Nakano, Y.; Hiramatsu, T.; Kageyama, H.; Yamochi, H.; Saito, G. Molecular Rotors of Coronene in Charge-Transfer Solids. Chem.-Eur. J. 2013, 19, 12313-12324. (30) Yoshida, Y.; Kumagai, Y.; Mizuno, M.; Isomura, K.; Nakamura, Y.; Kishida, H.; Saito, G. Improved Dynamic Properties of Charge-Transfer-Type Supramolecular Rotor Composed of Coronene and F4TCNQ. Cryst. Growth Des. 2015, 15, 5513-5518.


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(31) Islamov, D. R.; Shtyrlin, V. G.; Serov, N. Y.; Fedyanin, I. V.; Lyssenko, K. A. Symmetry Influence on the Rotation of Molecules in Crystals. Cryst. Growth Des. 2017, 17, 4703-4709.


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For Table of Contents Use Only

Crystals of Charge-Transfer Complexes with Reorienting Polar Molecules: Dielectric Properties and Order–Disorder Phase Transitions Jun Harada,* Naho Yoneyama, Shota Sato, Yukihiro Takahashi, and Tamotsu Inabe

SYNOPSIS. Crystals of charge-transfer complexes of disk-shaped polar molecules exhibit large dielectric constants due to orientational polarization at room temperature and order–disorder phase transitions that cause abrupt changes in the dielectric properties.


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Crystals of Charge-Transfer Complexes with Reorienting Polar

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