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J. Phys. Chem. B 2004, 108, 1314-1320

Freezing of Crankshaft Motion of trans-Stilbene Molecule in Charge-Transfer Complexes, STB-TCNQ and STB-TCNQF4† Kazuya Saito,* Mizuho Okada, Hiroki Akutsu,‡ Akane Sato,§ and Michio Sorai Research Center for Molecular Thermodynamics, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan ReceiVed: September 3, 2003; In Final Form: NoVember 19, 2003

Heat capacities of crystalline charge-transfer (CT) complexes between trans-stilbene (STB) and tetracyanoquinodimethane (TCNQ) or (TCNQ)F4 were precisely measured below room temperature by adiabatic calorimetry. Although orientational disorder is seemingly absent, a glass transition due to freezing of the crankshaft motion is observed in STB-(TCNQ)F4 around 240 K as in the case of highly disordered STBTCNQ around 250 K. Assessment of the degree of CT by IR and structural methods indicates that STB(TCNQ)F4 is not a fully but weakly (only partially) ionic complex with a similar degree of CT of 0.1-0.2 to that in STB-TCNQ, despite the large difference in acceptor ability.

1. Introduction Coupling between motional and electronic degrees of freedom in solid states and resultant functionality is of current interest. Possible coupling between proton dynamics and conducting properties has been studied extensively.1-4 Large effects of ligand dynamics have also been identified on conducting and magnetic properties of halogen-bridged binuclear metal complexes, the so-called MMX chain complexes.5,6 Recently, a glass transition was found arising from conformational freezing of the terminal six-membered ring in organic conductors (DMET)2X (DMET ) dimethyl(ethylenedithio)diselenadithiafulvalene, X ) BF4 and ClO4) and superconductors κ-(ET)2Cu[N(CN)2]X [ET ) bis(ethylenedithio)tetrathiafulvalene, X ) Cl and Br].7-9 In the latter, the existence of the glass transition largely affects the conducting properties such as the superconducting transition temperature.8-10 It is widely accepted that the terminal ethylene moiety in the ET and DMET molecules bears only a little π electron density belonging to their HOMO. The following questions may arise: Is a reorientational motion possible for a molecule involved in the charge-transfer (CT) mechanism? What physical (electrical, optical, etc.) properties are caused from it? Crystals of trans-stilbene (STB) and its derivatives are known to show some structural peculiarities such as unusually short bond lengths and orientational disorder.11,12 Ogawa et al.11 performed systematic studies on the structures of a series of STB’s and suggested that the molecular dynamics will cause seeming shrinkage of the central CdC bond. For the crystal of neat STB, both peculiarities were reported and studied extensively.11-16 A lattice dynamical calculation12 showed that the vibration assumed by Ogawa et al.11 is certainly possible in the crystal. The structural disorder observed at room temperature is dynamical in nature as evidenced by variable-temperature † Contribution No. 82 from the Research Center for Molecular Thermodynamics. * Corresponding author. E-mail: [email protected]. ‡ Present address: Department of Material Science, Graduate School of Science, Himeji Institute of Technology, 3-2-1 Kouto, Kamigori-cho, Akogun, Hyogo 678-1297, Japan. § Present address: Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 1528552, Japan.

structural studies.13,14 On cooling, this disorder is frozen-in around 170 K in the normal observation time scale (≈103 s), resulting in the glass transition.15 The motion equilibrating two orientations of the molecule is not overall rotation but crankshaft motion that keeps the sense of the orientation of the benzene rings nearly constant, as revealed by NMR studies.15,16 STB is a weak donor having a large electron density of the HOMO on the central CdC bond. Several CT complexes have been reported. Even for crystals of the CT complexes, the orientational disorder of the STB molecule has been reported.17-20 The properties of neat STB crystal suggest that the central CdC moiety reorients itself by the crankshaft motion in the crystal lattice of CT complexes at rather high temperatures. In the crystal of the title complex between STB and tetracyanoquinodimethane (TCNQ), STB-TCNQ, STB and TCNQ molecules stack alternately to form columns.18,19 The complex is, accordingly, an insulator or a semiconductor having poor conductivity (10-10 S cm-1 at room temperature) and the large activation energy for electric conduction (Ea ) 0.66 eV).19 The STB molecules in STB-TCNQ were originally reported to be in an orientationally disordered state corresponding to the site symmetry of 2/m at room temperature. We first selected this complex in order to show that the reorientation of the central CdC moiety is active even in CT complexes. A positive result was already reported preliminarily.21 The complex between STB and tetrafluorotetracyanoquinodimethane ((TCNQ)F4), STB-(TCNQ)F4, was synthesized by us to examine whether similar reorientation is possible in an enhanced CT state, which was expected because of the strong acceptor ability of (TCNQ)F4. The complex has a different crystal structure compared with STB-TCNQ though having a mixed-stack structure.22 The STB molecules are seemingly ordered even at room temperature, while the central CdC bond length is slightly shorter than a normal CdC bond. Since the frozen-in disorder below the glass transition was not detected by diffractometry in neat STB13 and trans-azobenzene,23 the apparent completeness of orientational order in STB-(TCNQ)F4 does not mean at all that the order is perfect. It is rather reasonable, on considering the short CdC bond, to assume the presence of some disorder. To see if the crankshaft motion is

10.1021/jp036630f CCC: $27.50 © 2004 American Chemical Society Published on Web 01/03/2004

Freezing of Crankshaft Motion of trans-Stilbene

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active in STB-(TCNQ)F4, the search for a possible glass transition was performed by making use of precise calorimetry in this study. In this paper, the results of precise calorimetry on STB-TCNQ and STB-(TCNQ)F4 are described in full detail. The presence of the glass transition in STB-(TCNQ)F4 is shown around 240 K, which indicates that the STB molecules undergo the crankshaft motion above this temperature. 2. Experimental Section A. Materials. The starting materials (STB, TCNQ, and (TCNQ)F4) were purchased from Tokyo Chemical Industry and purified by fractional sublimation under vacuum. The sample of STB-TCNQ was synthesized by mixing hot acetonitrile solutions of purified STB and TCNQ. Microcrystalline needles of STB-TCNQ (dark purple) that readily precipitated were filtered, washed by acetonitrile, and dried under vacuum for 60 h. Elemental analysis gave the following values: C, 81.4% (calcd, 81.2%); H, 4.1% (4.2%); N, 14.6% (14.6%). The amount of acetonitrile occluded in the sample used for calorimetric measurement was determined as 0.05 wt % from the latent heat of fusion at 228 K. The sample of STB-(TCNQ)F4 was synthesized in a similar way using acetone as the solvent starting from purified STB and (TCNQ)F4. Microcrystalline needles of STB-(TCNQ)F4 (dark green) that precipitated were filtered, washed by acetone, and dried under vacuum for 20 h. Elemental analysis gave the following values: C, 68.5% (calcd, 68.4%); H, 2.6% (2.5%); N, 12.2% (12.3%). The amount of acetone occluded in the sample used for calorimetric measurement was determined as 0.02 wt % from the latent heat of fusion at 175 K. B. IR Spectroscopy. IR spectra were recorded for KBr pellets containing ca. 1 wt % of the powdered CT complex (STBTCNQ or STB-(TCNQ)F4) using JASCO FT/IR-8300. The wavenumbers were calibrated against those of polyethylene. C. Adiabatic Calorimetry. The sample was loaded in a goldplated copper calorimeter vessel with helium gas (105 Pa at room temperature) to improve thermal uniformity within the vessel. After the buoyancy correction, the mass of the sample loaded was 2.1175 g in the case of STB-TCNQ and 2.1956 g for STB(TCNQ)F4. They contributed more than 30% in the whole temperature range studied to the total heat capacity including that of the vessel. The vessel was set in a laboratory-made adiabatic calorimeter, of which details of the design, operation, and performance were described elsewhere.24 Working thermometers mounted on the calorimeter vessel are platinum (Minco Products, S1055) and germanium (LakeShore Cryotronics, GR-500-200B) resistance thermometers, which are used above 13.8 K and below 14 K, respectively. Their temperature scales are based upon the ITS-90. 3. Results and Discussion A. Degree of Charge Transfer. To get information on the degree of CT, IR spectra were recorded for both complexes, as shown in Figures 1 and 2. Wavenumbers of selected strong absorption bands are listed in Table 1. Taking into account the existing vibrational analyses for STB,25 TCNQ,26 and (TCNQ)F4,27 most bands listed in Table 1 can be assigned to intramolecular vibrations. The wavenumbers of IR absorption of STB-TCNQ were reported previously by Kao et al.28 Although all lines reported by them can be identified in the present spectrum, their wavenumbers are generally smaller by ca. 10-40 cm-1 than the present ones. Moreover, the reported ν(CtN) ) 2175 cm-1

Figure 1. IR spectra of crystalline STB-TCNQ at room temperature.

Figure 2. IR spectra of crystalline STB-(TCNQ)F4 at room temperature.

TABLE 1: Strong IR Absorption Bands of Crystalline STB-TCNQ and STB-(TCNQ)F4 STB-TCNQ

STB-TCNQF4

ν/cm-1

assignment25,26

ν/cm-1

assignment25,27

2219 1547 1498 1453 1353 1154 1072 988 972 913 841 764 689 528 465

ν19 TCNQ ν33 TCNQ ν57 STB ν58 STB ν35 TCNQ ν64 STB ν65 STB ν22 TCNQ ν27 STB ν29 STB ν50 TCNQ ? ν31 STB ν32 STB ν33 STB ν51 TCNQ

2227 1594 1546 1496 1485 1454 1391 1343 1190 1136 1074 805 776 696 531

ν18 (TCNQ)F4 ν33 (TCNQ)F4 ν19 (TCNQ)F4 ν57 STB ν58 STB ν34 (TCNQ)F4 ν20 (TCNQ)F4 ν35 (TCNQ)F4 ν21 (TCNQ)F4 ν65 STB ν22 (TCNQ)F4 ν31 STB ν32 STB ν33 STB

strangely corresponds to the degree of CT more than 1 (complete transfer of one electron). Considering the weak donor ability of STB, this is unphysical. We therefore shall proceed discussion without referring to their results. For TCNQ molecules, a good linear relation has been reported between the degree of CT and the wavenumver of ν(CtN) (ν19) for the range between 0 (no CT) and 1 (full transfer of one electron).29 By using this linear relation, the degree of CT is estimated as 0.2 for STB-TCNQ. On the other hand, Kistenmacher et al.30 proposed a method based on the molecular geometry. This method gives the degree of CT as 0.1 using the

1316 J. Phys. Chem. B, Vol. 108, No. 4, 2004 length in ref 19. Since the latter method relies on a small change in the bond length resulting in only one significant digit, discrepancy between two estimates is acceptable considering the difficulties arising from the presence of disorder in STBTCNQ.18,19 In any way, it is true that weak (partial) CT occurs in crystalline STB-TCNQ. In contrast to the case of TCNQ complexes, ν(CtN) (ν18) in (TCNQ)F4 is reported to be sensitive to the environmental effects such as intermolecular interaction, and consequently this vibrational mode is not appropriate for determination of the degree of CT.27 Instead, two absorption bands have been suggested to be suitable for this purpose. Those are ν19 (1551 cm-1 for (TCNQ)F40 and 1500 cm-1 for TCNQF4-) and ν33 (1598 and 1536 cm-1). Assuming that linear relations hold between the wavenumbers and the degree of CT, the two bands in the present spectrum consistently give the estimate of the degree of CT as 0.1 for STB-(TCNQ)F4. The same structural method as that in TCNQ was applied to (TCNQ)F4 and gave a reasonable result in comparison with the independent estimate from the oscillator strength of the CT absorption for partially charge transferred (BTT)2((TCNQ)F4) (BTT ) benzo[1,2-c:3,4-c′:5,6-c′′]trithiophene).31 For STB(TCNQ)F4, the molecular geometry gives the estimate of the degree of CT as 0.2. A discrepancy between the two methods is thus again encountered. Although the degree from the geometries has a larger uncertainty as described above, additional estimates are necessary to fix finally the degree of CT for the present complex. It is sure, however, that STB-(TCNQ)F4 is a rare example of (TCNQ)F4 complexes where partial CT occurs despite the strong acceptor ability of (TCNQ)F4. Indeed the degree of CT is 1 (full transfer of one electron to a (TCNQ)F4 molecule) in most CT complexes of (TCNQ)F4. As far as the authors know, the above-mentioned (BTT)2((TCNQ)F4)31 and HMTTeF-(TCNQ)F432 (HMTTeF ) hexamethylenetetratellurafulvalene) are only examples of this kind. The strong acceptor ability of (TCNQ)F4 suggests the presence of potential instability to an ionic phase with an enhanced degree of CT in STB-(TCNQ)F4. Enhancement of the intermolecular interaction by pressure will be interesting. It is noteworthy that the structural features that the molecules are located on inversion centers to form alternate stacks are the same as those in TTF-p-chloranil33 (TTF ) tetrathiafulvalene), the most famous compound undergoing a neutral-to-ionic phase transition. B. Heat Capacity and Thermodynamic Functions. The heat capacity of STB-TCNQ was measured below room temperature. Typical heat capacity data are shown in Figure 3. A spike at 228 K is attributed to fusion of acetonitrile occluded in the sample. From the magnitude of the latent heat34 and the integrated area under the spike, the mass of acetonitrile is estimated as 1.1 mg (0.05 wt % of the total). With the exception of the temperature regions near the solvent fusion and the glass transition, the sample-filled calorimeter achieved thermal equilibrium in a manner that is normal for the apparatus after the supply of heater current was stopped. The anomalous thermal equilibration in the glass-transition region will be discussed further later. Primary data were smoothed out using a least-squares spline fit procedure. Standard thermodynamic functions, enthalpy increment (H - H0), entropy increment (S - S0), and Gibbs energy (G - H0), were obtained from appropriate integration of the resulting fits as tabulated in Table 2. In the vicinity of the glass-transition region to be described later, the data obtained after annealing treatment were used to generate the thermodynamic functions.

Saito et al.

Figure 3. Heat capacities of STB-TCNQ (experimental data, open squares), STB35 (dotted curve), TCNQ36 (dotted curve), and the sum of STB and TCNQ (solid curve).

Also shown in Figure 3 is a comparison of the heat capacities of STB-TCNQ and the sum of those of neat STB35 and TCNQ.36 Roughly speaking, two curves are remarkably similar to each other except for the anomalous regions. Similar good coincidence was observed in TTF-TCNQ and TSF-TCNQ (TSF ) tetraselenafulvalene).37 The fact that the heat capacities of the STB-TCNQ complex are systematically smaller, though only slightly, than the sum of those of neat crystals of STB and TCNQ over a wide temperature region may indicate that the intermolecular interactions are fastened by the CT mechanisms. The heat capacity measurement was made on STB-(TCNQ)F4 up to 320 K as shown in Figure 4. Thermal equilibration inside the calorimeter vessel was normal except in the glasstransition region (centered around 240 K) discussed later. A small anomaly can be recognized at 175 K due to fusion of acetone occluded in the sample. From the magnitude of the latent heat,38 its mass is estimated as 0.4 mg (0.02 wt %). Thermodynamic functions were calculated in a way similar to those of STB-TCNQ as given in Table 2. Since no appreciable dependence was observed on thermal history even in the glasstransition region, no selection was made of the primary data in the calculation. Due to the lack of reliable heat capacity data for neat (TCNQ)F4, a comparison similar to that made in Figure 3 is impossible for STB-(TCNQ)F4. The difference in heat capacity between STB-(TCNQ)F4 and STB-TCNQ shown in Figure 5 can, however, be assumed as the difference between the contributions of four C-F and C-H bonds. Indeed, the difference is reasonably close to two-thirds of the heat capacity difference between hexafluorobenzene39 and benzene,40 although reasonable correspondence is hardly recognized between their normal modes reported.41,42 The good coincidence shown in Figure 5 therefore implies the insensitiveness of heat capacity, due to its “integrated” nature (integration over the frequency distribution). In turn, the coincidence enhances the reliability of the following estimate for TCNQF4 from the present results: Cp/(J K-1 mol-1), 130 ( 5 (at 100 K), 220 ( 5 (at 200 K), and 290 ( 5 (at 300 K). These estimates would be useful to scale results of relative (nonabsolute) calorimetry such as optical heating one on small amounts of samples. C. Glass Transition in STB-(TCNQ)F4. Although there seemed to exist no orientational disorder in STB-(TCNQ)F4 crystal at room temperature,22 anomalous behavior was detected around 240 K. This is clearly seen by plotting enclaty (heat

Freezing of Crankshaft Motion of trans-Stilbene

J. Phys. Chem. B, Vol. 108, No. 4, 2004 1317

TABLE 2: Standard Thermodynamic Quantities of Crystalline STB-TCNQ and STB-(TCNQ)F4 STB- TCNQ T/K 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 298.15 a

STB-(TCNQ)F4

[H°(T) - H°(0)]/ S°(T) - S°(0)a/ -[G°(T) - H°(0)]/ Cp°/ [H°(T) - H°(0)]/ S°(T) - S°(0)a/ -[G°(T) - H°(0)]/ Cp°/ (J K-1 mol-1) T/(J K-1 mol-1) (J K-1 mol-1) T/(J K-1 mol-1) (J K-1 mol-1) T/(J K-1 mol-1) (J K-1 mol-1) T/(J K-1 mol-1) 5.34 27.62 56.70 85.29 110.78 132.76 151.83 168.94 184.69 199.60 214.1 228.2 241.9 255.4 269.0 282.6 296.3 309.8 323.2 336.6 350.3 364.2 377.8 391.1 409.9 428.7 444.2 449.5 459.2 471.0 468.5

1.30 8.38 19.60 32.49 45.66 58.39 70.41 81.67 92.25 102.24 111.76 120.88 129.66 138.16 146.42 154.51 162.45 170.26 177.96 185.55 193.07 200.5 208.0 215.3 222.7 230.3 237.9 245.4 252.6 259.6 258.3

1.69 11.52 28.16 48.44 70.27 92.46 114.39 135.80 156.62 176.85 196.56 215.8 234.6 253.0 271.1 288.9 306.4 323.8 340.9 357.8 374.5 391.2 407.7 424.0 440.3 456.8 473.3 489.6 505.5 521.2 518.3

0.39 3.14 8.56 15.94 24.61 34.07 43.98 54.13 64.37 74.61 84.80 94.9 104.9 114.9 124.7 134.4 144.0 153.5 162.9 172.2 181.5 190.6 199.7 208.7 217.6 226.5 235.4 244.2 252.9 261.6 260.0

5.90 29.22 59.08 89.25 116.73 141.65 163.51 184.42 203.6 221.7 239.2 256.0 272.3 288.3 304.2 319.7 334.9 349.8 364.4 378.9 393.3 407.5 421.6 436.3 452.2 468.2 483.2 497.4 511.2 524.3 521.9

1.59 9.08 20.75 34.12 47.94 61.53 74.56 86.99 98.90 110.28 121.21 131.75 141.94 151.82 161.45 170.86 180.06 189.07 197.92 206.6 215.2 223.6 231.9 240.1 248.2 256.4 264.5 272.6 280.6 288.5 287.0

2.14 12.68 30.15 51.30 74.22 97.75 121.25 144.46 167.31 189.70 211.7 233.2 254.3 275.1 295.5 315.7 335.5 355.1 374.4 393.4 412.3 430.9 449.3 467.6 485.7 503.7 521.7 539.5 557.2 574.8 571.5

0.55 3.61 9.40 17.18 26.27 36.22 46.69 57.47 68.41 79.42 90.5 101.4 112.4 123.3 134.1 144.8 155.4 166.0 176.5 186.8 197.1 207.3 217.4 227.5 237.4 247.3 257.2 266.9 276.6 286.3 284.5

S(0) * 0 because of the presence of glass transitions.

Figure 4. Heat capacities of STB-(TCNQ)F4 (experimental data, open circles) and STB-TCNQ (solid curve).

Figure 5. Difference of heat capacities between STB-(TCNQ)F4 and STB-TCNQ (solid curve). Two-thirds of the heat capacity difference between hexafluorobenzene39 and benzene40 (crosses).

capacity divided by temperature), as shown in Figure 6. There exists a stepped increase in heat capacity around 240 K. In this region, the equilibration behavior after energy input was also anomalous. In a measurement run after normal cooling (ca. -2 K min-1 at 240 K) to liquid nitrogen temperature, the temperature drift in equilibration period showed a characteristic temperature dependence indicated by open circles in Figure 7. The drift is stationary below 200 K. This stationary drift is due to a small heat leak uncontrollable by the adiabatic control and depends weakly on temperature. The drift rapidly increases (exothermic) to a maximum at 230 K, turns into a decrease, changes its sign at 240 K (endothermic), shows a minimum around 250 K, and then finally recovers to the stationary level

above 270 K. This characteristic temperature dependence and the stepped increase in heat capacity are typical of enthalpy relaxation around glass transitions43 and can be explained as follows: On cooling, a molecular degree of freedom relevant to the glass transition is frozen-in around a glass-transition temperature because the relaxation time exceeds the time scale of observation. The enthalpy involved in the degree of freedom remains high on further cooling. In this frozen-in state, the degree of freedom does not contribute to the enthalpy of the system under consideration. Although the system behaves as in equilibrium state, the heat capacity is smaller by the contribution of the frozen-in degree of freedom. On heating this frozen-in system, the relaxation time decreases. The exothermic

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Saito et al.

Figure 6. Heat capacities of STB-(TCNQ)F4 around the glass transition shown in terms of enclaty: open circles, data obtained after normal cooling to liquid nitrogen temperature; filled circles, data after annealing at 230 K for 22 h. Solid curves are smooth extrapolation from the highand low-temperature sides. Figure 8. Heat capacities of STB-TCNQ around the phase transition and the glass transitions: open squares, data obtained after normal cooling to liquid nitrogen temperature; filled squares, data after annealing at 250 K for 50 h; plus signs, data after annealing at 250 K for 11 h. Assumed normal heat capacity (baseline) is drawn by a solid curve.

Figure 7. Temperature drift in the equilibration period after heat input around the glass transition in STB-(TCNQ)F4. Symbols are the same as those in Figure 6.

relaxation becomes detectable around but still below the temperature where the enthalpy of the system matches to that of the (virtual) equilibrated system. As the heating rate is larger than that of cooling, before the relaxation completes, the relaxing system is brought to temperatures where the enthalpy of the system is smaller than that of the system in equilibrium, resulting in an endothermic relaxation. On further heating, the relaxation time becomes shorter than the time scale of observation and the system remains in (nearly) equilibrium even while heating. The drift returns to stationary, and the heat capacity becomes larger by the contribution of the frozen-in degree of freedom than that at lower temperatures. If the above explanation is correct, a measurement run after annealing below the glass transition should suppress the endothermic relaxation. This was really observed in the run after the anneal at 230 K for 22 h, as shown by solid circles in Figure 7. From these observations, therefore, the stepped anomaly observed around 240 K is attributed to a glass transition. The glass-transition temperature where the relaxation time becomes ca. 103 s is determined as 240 K after the empirical criterion43 that a temperature where the drift changes its sign is taken. Since only peculiarity in the crystal structure at room temperature is a slightly short length of the central CdC bond,22 the glass transition is plausibly related to this. Besides, glass transitions due to freezing of crankshaft motion are observed in neat STB and trans-azobenzene crystals.15,16 It is therefore reasonable to regard the glass transition in STB-(TCNQ)F4 to

be of the same kind. The magnitude of the stepped increase is roughly estimated as 3-4 J K-1 mol-1 as in Figure 6, which roughly corresponds to the maximum magnitude if a simple two-level system is assumed to model the orientational disorder. Within this naive model, the population of the misorientated STB molecules is less than 10% at room temperature. The estimated population is compatible with the apparent absence of orientational disorder. D. Phase Transition and Glass Transition in STB-TCNQ. Figure 3 shows the presence of a trapezoidal anomaly between 250 and 280 K. This region is plotted in an enlarged scale in Figure 8. In the measurement run after normal cooling rate to liquid nitrogen temperature, the data shown by open squares were obtained. Stepped increase and decrease are recognized around 260 and 285 K, respectively. Since the orientations of the STB molecules are highly disordered at room temperature,18,19 this excess heat capacity is attributed to a phase transition related to the symptom of orientational order. The shape of the excess heat capacity is, however, curious if this anomaly is due only to a phase transition. As described in detail in the previous section, a stepped increase in heat capacity is typical for a glass transition.43 Around the stepped anomaly in STB-TCNQ, anomalous behavior was also detected in the thermal equilibration period as shown in Figure 9. Large exothermic relaxation was observed. This suggests that the step originates in a glass transition. An anneal around 250 K indeed suppressed the exothermic relaxation and enhanced the endothermic relaxation as shown by closed squares (50 h around 250 K) and plus signs (11 h around 250 K) in Figure 9. Besides, the annealing makes the step shift downward as seen in Figure 8. These are the trends expected for a glass transition. It is therefore concluded that the stepped anomaly arises from a glass transition. Since the glass transition occurs in the course of the low-temperature tail of the ordering transition concerning the orientation of the STB molecule, the presence of this glass transition directly proves that the correlation time of the crankshaft motion of the STB molecule in this system is about 103 s around 250 K. To estimate what fraction of disorder is removed at the phase transition, a smooth interpolating curve is drawn as a baseline

Freezing of Crankshaft Motion of trans-Stilbene

Figure 9. Temperature drift in the equilibration period after heat input around the glass transition in STB-TCNQ. Symbols are the same as those in Figure 8.

between the high- and low-temperature sides as shown in Figure 8. By subtracting this normal heat capacity, excess heat capacities were separated. The excess shows the maximum at 273 K, which is regarded as the temperature of phase transition. Estimates of the excess enthalpy and entropy due to this phase transition are obtained as about 2.9 × 102 J mol-1 and 1.1 J K-1 mol-1, respectively, at most based on the result in the run after the longest anneal. This magnitude of the excess entropy is smaller than R ln 2 (≈5.8 J K-1 mol-1), which is an expected entropy of transition for the complete removal of the orientational disorder at room temperature.18,19 The excess entropy experimentally observed corresponds to the situation in which one-third of the STB molecules are ordered while the rest remain (frozen-in as) disordered at low temperatures; i.e., the order parameter in the simple mean-field treatment is ca. 1/2 at low temperature. The temperature dependence of the drift in the run after normal cooling (open squares in Figure 9) is different from that of STB-(TCNQ)F4 shown in Figure 7. That is, there is no endothermic relaxation. Also different from the case of STB(TCNQ)F4 is the downward shift of the location of the stepped increase in heat capacity. While the former difference is probably related to the fact that the glass transition is on the course of the ordering phase transition, the latter is not, because there exist some examples with detectable difference in heat capacity without a phase transition closely related to the glass transition.43 E. Crankshaft Motion of STB Molecule in Solid States. While in the crystal of neat STB the glass transition due to freezing of the crankshaft motion takes place around 170 K,15 those occur about 240 and 250 K in STB-TCNQ and STB(TCNQ)F4, respectively. It is interesting if the correlation can be found between the glass-transition temperatures and the degree of CT. This is, however, impossible because of insufficient determination as described in the previous section. It is emphasized that, nevertheless, the crankshaft motion is really active above the glass-transition temperatures. The crankshaft motion changes the orientation of the central CdC moiety while keeping the orientations of the benzene rings nearly unchanged. Since the HOMO density is the highest on the central CdC moiety, the motion causes the reorientation of the CT interacting part. It is noted that this conclusion does not assume that the CT interaction is realized through the CdC moiety while undergoing the crankshaft motion. Since electron transfer is much faster than molecular motion, optical studies are necessary to clarify the point.

J. Phys. Chem. B, Vol. 108, No. 4, 2004 1319 An STB molecule seems to be the smallest one that can undergo the crankshaft motion. The crankshaft motion can be regarded as an elementary process in conformational changes of molecules having long chain(s) and polymers. In this respect, it is important to clarify factors deteremining the molecular mobility and, consequently, a glass-transition temperature. The steric hindrance is certainly the primary factor. Indeed, the crystal of trans-azobenzene, which has a similar molecular structure without hydrogen atoms on the central moiety, undergoes a glass transition due to freezing of the crankshaft motion at a lower temperature, 110 K.15,16 As for STB and its complexes, the volume available for an STB molecule, which is estimated by subtracting the molar volume of TCNQ44 or (TCNQ)F4,45 is the largest in neat STB among neat STB (257.5 Å3 molecule-1), STB-TCNQ (243.7 Å3), and (TCNQ)F4 (231.5 Å3). A large difference in a glass-transition temperature between neat STB and the CT complexes might also be ascribed to the effect of the CT interaction in part, though their degrees are small in these complexes (0.1-0.2). It is noted that the crankshaft dynamics treated in this study takes place in a welldefined environment in crystalline states. Systematic studies on the crankshaft motion in these compounds would, therefore, open the possibility to “decompose” complex dynamics in polymers into elementary dynamics on an experimental basis. 4. Summary and Conclusion The degree of charge-transfer is studied for crystalline STBTCNQ and STB-(TCNQ)F4 by using IR spectrum and molecular geometries. It was found that the latter shows, similarly to the former, partial CT (0.1-0.2), despite the strong acceptor ability of (TCNQ)F4. Possible occurrence under pressure is pointed out for a neutral-to-ionic phase transition in STB-(TCNQ)F4. The thermal behavior of the two complexes was examined by precise calorimetry, and standard thermodynamic functions were determined. For STB-TCNQ, a phase transition due to the orientational order of STB molecules and a glass transition due to freezing of the crankshaft motion were detected at 273 K and around 250 K, respectively. The degree of frozen-in disorder is estimated as ca. 5 J K-1 mol-1 in terms of residual entropy. A glass transition of the same kind was found around 240 K for STB-(TCNQ)F4, in which no apparent disorder in molecular orientation was reported to exist at room temperature. A discussion based on the magnitude of the stepped increase in heat capacity and the glass-transition temperature implies the consistency between the presence of the glass-transition and the previous structural result. Since the HOMO density is maximum on the central CdC moiety in STB molecules, the presence of glass transition directly proves the reorientation of the CT interacting part in crystalline lattices. The glass-transition temperature was found to depend strongly on the volume available for an STB molecule. References and Notes (1) Endres, H. Angew. Chem., Intl. Ed. Engl. 1982, 21, 524. (2) Mitani, T.; Saito, G.; Urayama, H. Phys. ReV. Lett. 1988, 60, 2299. (3) Nakasuji, K.; Sugiura, K.; Kitagawa, T.; Toyoda, J.; Okamoto, H.; Okaniwa, K.; Mitani, T.; Yamamoto, H.; Miura. I.; Kawamoto, A.; Tanaka, J. J. Am. Chem. Soc. 1991, 113, 1862. (4) Saito, K.; Yamamura, Y.; Kitagawa, H.; Yoshida, D.; Mitani, T.; Sorai, M. J. Phys. Soc. Jpn. 1999, 68, 3592. (5) Ikeuchi, S.; Saito, K.; Nakazawa, Y.; Sato, A.; Mitsumi, M.; Toriumi, K.; Sorai, M. Phys. ReV. B 2002, 66, 115110. (6) Ikeuchi, S.; Saito, K.; Nakazawa, Y.; Mitsumi, M.; Toriumi, K.; Sorai, M. J. Phys. Chem. B 2004, 108, 387. (7) Akutsu, H.; Saito, K.; Yamamura, Y.; Kikuchi, K.; Nishikawa, H.; Ikemoto, I.; Sorai, M. J. Phys. Soc. Jpn. 1999, 68, 1968.

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