J. Phys. Chem. B 2006, 110, 23075-23080
23075
Conformational Polymorphism and Thermochemical Analysis of 5,5′′′-Bis[(2,2,5,5-tetramethyl-1-aza-2,5-disila-1-cyclopentyl)ethyl]-2,2′:5′,2′′:5′′,2′′′ -quaterthiophene Hitoshi Muguruma*,† and Shu Hotta‡ Department of Electronic Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan, and Department of Macromolecular Science and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ReceiVed: June 8, 2006; In Final Form: September 4, 2006
The titled compound exists as two polymorphic solid phases (denoted form-I and form-II). Form-I obtained by as-synthesized material is a more stable phase. Form-II is a less stable phase. Spontaneous solid-solid transformation from form-II to form-I is observed in the temperature range between room temperature and the melting point of form-I (Tm ) 156.5 °C), and its activation energy is estimated to be 96 kJ mol-1 by Arrhenius plot. The solid-solute-solid transformation (recrystallization from solution) from form-II to form-I is also observed. In contrast, form-II is obtained only by a solid-melt-solid transformation from form-I. Therefore, the system of two polymorphs is monotropic. The solid-state NMR measurement shows that form-I has the molecular conformation of complete S-syn-anti-syn in the oligothiophene backbone, whereas form-II has that of S-all-anti. With the solution NMR data, the polymorphism could not be observed. Therefore, the polymorphs originate from the different molecular packing involving the conformational change of the molecule. This unique property is attributed to the extra bulky terminal groups of the compounds. However, despite the extra bulky terminal groups, the mentioned polymorphism is not observed in the titled compound analogue which has S-all-anti conformation (like form-II).
1. Introduction Polymorphs are a solid phase in which the chemical composition is equal but the crystal structure differs. Different polymorphs are due to different arrangements and/or conformations of the molecules in the crystal lattice. Since the phenomenon of polymorphism is commonly encountered in the production of special chemicals such as dyestuffs, pharmaceuticals, pesticides, and molecular conductors, it is important to characterize and control the phenomena from the standpoint of the structureproperties relationship and crystal design. Oligothiophene and its derivative, in which the several thiophene rings are connected with each other, are a new class of organic semiconducting materials, and they are applied to device fabrication such as field effect transistors (FETs)1-3 and organic electroluminescence devices.4-6 In most of the oligothiophenes, the molecular chains assume a fully stretched S-anti conformation and co-planer adjacent thiophene rings because they are electronically most stable.7-9 One of the most attractive aspects of the materials offers all-printed or flexible electronic circuits, which is an easy and low-cost process compared with that for conventional silicon devices. Therefore, the modification of the R- and/or β-carbon of thiophene rings is often carried out and causes a strongly twisted S-syn conformation.10,11 Barballela et al. reported that β-methylated sexithiophenes12 and β-substituted quaterthiophene13 show conformational polymorphism. We have already reported that 5,5′′′-bis[(2,2,5,5-tetramethyl1-aza-2,5-disila-1-cyclopentyl)ethyl]-2,2′:5′,2′′:5′′,2′′′-quater* To whom correspondence should be addressed. E-mail: muguruma@ sic.shibaura-it.ac.jp. † Shibaura Institute of Technology. ‡ Kyoto Institute of Technology.
thiophene (2, Figure 1) has the complete S-syn conformation of oligothiphene and that the vacuum-evaporated “thin film” of 2 shows conformational polymorphism.14,15 In contrast, 5,5′′′-bis[(2,2,5,5-tetramethyl-1-aza-2,5-disila-1-cyclopentyl)methyl]-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (1, Figure 1), which is the analogue compound of 2, has the complete S-all-anti conformation and does not show polymorphism. Therefore, this polymorphism is remarkable because the previously reported S-syn was “incomplete” in the sense that this form is largely twisted.10-13 Moreover, the two conformers are definitively observed as stable forms according to the subtle change in the terminal groups; notice that the only difference between 1 and 2 is that the methylene group in 1 is replaced with the ethylene group in 2 (Figure 1). In this paper, we characterized the conformational polymorphism of 2 in “bulk” phase by differential calorimetry (DSC), X-ray powder diffraction (XRPD), and solid-state nuclear magnetic resonance (NMR). We also discuss the difference between 1 and 2. 2. Experimental Section Synthesis and purification of compounds 1 and 2 (form-I) were previously reported elsewhere.16 Form-II of 2 was obtained as follows: several grams of 2 (form-I) were put into flask where nitrogen is substituted. The flask was heated with an oil bath. After compound 2 has melted, the flask was cooled with an ice bath. The melted compound of 2 rapidly solidified and changed to form-II. The solution 13C NMR spectra were measured at 125.8 MHz using a JEOL JNM-A500 spectrometer equipped with standard variable-temperature devices. The measurements were carried out in deuteriochloroform at room temperature.
10.1021/jp063560e CCC: $33.50 © 2006 American Chemical Society Published on Web 10/18/2006
23076 J. Phys. Chem. B, Vol. 110, No. 46, 2006
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Figure 2. Microphotograph of (a) form-I and (b) form-II of 2. Inset (b): X-ray diffraction pattern.
Figure 3. The path for the change between form-I and form-II of 2.
DSC measurements were performed with a TA DSC2920 instrument utilizing 30 mL min-1 flow of nitrogen as the purge gas. A sample of about 5 mg was heated in a sealed aluminum pan. The samples were heated and cooled at a rate of 1, 10, or 45 °C min-1. The instrument was calibrated using indium and tin. 3. Results
Figure 1. (a) Chemical structure and conformation of 1 and 2 (form-I and form-II). The backbone conformation is either S-all-anti (1 and 2 form-II) or S-syn-anti-syn (2 form-I). (b) A 3D model of 2 form-I is constructed by the crystallographic data in ref 14. A 3D model of 2 form-II is extrapolated from the structure of form-I.
The solid-state magic angle spinning (MAS) NMR spectra were recorded on a Chemagnetics CMX300 spectrometer equipped with a 7.5-mm Chemagnetics high-speed-spinning PENCIL probe at a 13C frequency of 75.497791 MHz. All spectra were acquired with cross-polarization (CP), 30.03-kHz spectral width, and a 4.5-µs 13C 90° pulse width, and they were referenced to the methyl peak of hexamethylbenzene at 17.35 ppm. The X-ray powder diffraction (XRPD) measurements were carried out at room temperature on a Rigaku X-ray powder diffractometer equipped with a fine-focus X-ray tube using Cu KR radiation (1.5406 Å). The tube voltage and amperage were set at 40 kV and 20 mA, respectively. The receiving slit was set at 1 mm. Diffracted radiation was detected by a NaI scintillation detector. θ-2θ continuous scans at 1° min-1 with a step size of 0.02° from 3 to 20° in 2θ were used. The XRPD measurements were done after the samples were annealed at various temperatures for a duration of varying time. The study for X-ray diffraction pattern was done in the reflection mode at 50 kV and 100 mA.
Figure 2 shows photographs of form-I and form-II of 2. The grain of form-I is plate-shaped and grows to a large size (ca. 1 mm); the crystal structure of form-I has already been reported.12 Since form-II is a polycrystalline powder (its diffraction pattern is shown in the inset in Figure 2b), its identification could be unsuccessful. The fact that the crystal size and quality of form-I are much larger and better than those of form-II is related to the stability of two phases. Figure 3 shows the phase diagrams of 2. The stable form-I can be obtained easily. Spontaneous transformation of form-II f form-I was observed in the temperature range between room temperature and the melting point of 2 (form-I). The solidsolute-solid transformation (recrystallization in solvent) of form-II f form-I is also observed. On the other hand, the less stable form-II can be obtained only when the melt of 2 was quickly cooled (a solid-melt-solid transformation from formI). 3.1. NMR. The experimental solid-state NMR data are presented in Table 1, and the spectra are shown in Figure 4. Since the rotational conformer of adjacent thiophene rings is addressed, we mark the peaks at 150-120 ppm due to the aromatic carbons. The nonaromatic region is less important to consider; the aromatic region is strongly related to conformation, whereas the aliphatic region at terminal groups of the molecule is not related to that. Since the molecules of 1 and 2 have the inversion center, the resonances are associated with the eight nonequivalent aromatic carbons in the unit cell and correspond to one-half of aromatic carbons of 1 and 2 molecules. In fact, eight independent peaks of aromatic carbon were observed although there were 16 aromatic carbons in those compounds. This symmetry persists under the polymorphic transformation. The peak assignment is according to previously reported NMR data of quaterthiophene derivatives.17 Since conformational polymorphism is due to the rotation of adjacent thiophene rings in this case, the chemical shift of β-carbons (C3, C4, C3′, and
Polymorphism and Analysis of an Oligothiophene TABLE 1:
13C
J. Phys. Chem. B, Vol. 110, No. 46, 2006 23077
Solid-State and Solution NMR Chemical Shifts (ppm) of Thiophene Rings of Compounds 1 and 2 chemic al shift of ring carbon [ppm]
compound solid 1 2, form-I 2, form-II solution 1 2, form-I 2, form-II a,b,c
C5
C4
C3
C2
C2′
C3′
C4′
C5′
149.87 141.52c 142.39
126.18 125.16a 126.95
121.76 125.16a 121.85
139.58 141.52c 137.93
137.14 136.67 136.68
123.80 125.16a 123.89
124.48 125.16a 124.91
137.88 135.74 136.23b
148.88 142.78 142.79
124.94 125.41 125.37
123.16 123.49 123.47
137.14 136.67 136.68
135.76 134.80 134.79
124.01 123.71 123.81
124.34 124.09 124.06
135.87 135.47 135.47
Those peaks coalesced at the position of chemical shift.
Figure 4. 13C solid-state NMR spectra (low-field region only) of (a) 1, (b) 2 form-I, (c) mixture of form-I and form-II, and (d) 2 form-II. (e) Solution NMR spectra in CDCl3. The peak assignment is according to the numbering system in Figure 1. NMR spectra for all regions are presented in Supporting Information.
C4′) at 120-130 ppm is more sensitive to local environment than that of R-carbons (C2, C5, C2′, and C5′) at 130-135 ppm. In a word, the change of chemical shift assigned to β-carbons is much larger than that to R-carbons. In form-I of 2 (Figure 4b), only one peak where some peaks coalesced is observed due to β-carbons around 125 ppm. In contrast, in form-II of 2 (Figure 4d), the distinct four peaks are observed around 125 ppm. The feature is similar to that of 1 in which the molecular form shows all S-anti conformation. The peaks assigned to R-carbons are also similar in situation to that of β-carbons. The peaks due to three kinds of R-carbons (C2, C5′, and C2′) coalesced in form-I, whereas the peak due to C2 is separated in form-II. The peaks in the coexistence of two phases (Figure 4c) show a spectrum shape mixed with both phases. This result implies that 2 shows the conformational polymorphism of thiophene ring rotation. The reason C5 is deshielded on structure 1 is considered as follows: the chemical environment is much different from that of the other carbons because the C5 carbon is located at the edge of the oligothiophene backbone where this environment is scarcely varied. The solution-state NMR data of form-I and form-II of 2 exactly correspond (Table 1). This indicated that the conformational polymorphism of 2 is not attributed to characteristics
of the isolated molecules but to the difference of molecular packing. Thus, the packing crystal forces supply the energy required to stabilize the conformation of form-I. This issue is discussed in more detail in section 4. 3.2. XRPD. The structure of the films was determined using XRPD in a symmetric reflection. The tube anode was Cu (λ ) 1.5406 Å). The reported single-crystal structure of 2 (form-I) is monoclinic, space group P21/c, with a ) 19.809 Å, b ) 6.158 Å, c ) 15.749 Å, β ) 97.20°, and Z ) 2.14 The molecule of 2 form-I has the inversion center, complete S-syn-anti-syn conformation, and coplanar oligothiophene backbone. Because of bulky terminal groups, molecular packing is much slacker than that of conventional oligothiophene derivatives; the molecules in the 2 crystal are packed in such a way that only “two” thiophene rings are staggered and arranged “face-to-face”, whereas for alkyl-substituted quaterthiophenes “four” thiophene rings are overlapped in such a way that the molecules are turned aside from the face-to-face arrangement and their molecular planes meet at an angle of about 60°.22 This molecular arrangement in such conventional oligothiophenes is well-known as the herringbone structure in which the long axis of molecules is stacked parallel to the long axis of the unit cell.7 Figure 5a,e shows the XRPD patterns of form-I and form-II, respectively. The peaks in Figure 5b,c have a d spacing of 19.62 and 18.17 Å, respectively. The former spacing closely matches the (100) spacing defined as a sin β ()19.65 Å), where angle β is one of the lattice constants of the monoclinic crystals. The higher-order reflections (300) and (400) at 6.54 and 4.91 Å, respectively, are also observed; however, the (200) peak was annihilated. Therefore, the (100) spacing is due to the crystal structure of form-I. The latter peaks comprising a primary diffraction of 18.17 Å (defined as (h′00)) and its higher-order reflections (6.09 Å (3′00) and 4.56 Å (4′00)) do not correspond directly to the crystallographic data. It is determined as the one of the crystal unit cell of form-II. The spacing due to form-II is smaller than that of form-I. There is no difference of molecular length of oligothiophene backbone comprised with conformational polymorphs. Therefore, this means that the molecules in the crystal lattice of form-II are slightly more inclined against one axis of the unit cell than that of form-I because of the difference of molecular packing. Figure 5a-d also shows the XRPD of form-II as a function of aging time at room temperature (25 °C). The solid-solid transformation of form-II f form-I is clearly noted by comparing their characteristics peaks (see Figure 5). We have determined the ratio of the two forms by that of the relevant peak intensities. However, the peak intensity of complete changing to form-I (Figure 5d) was one-eighths smaller than that of the original form-II (Figure 5a), suggesting that the crystallinity of the transformed sample was poorer than that of single-crystal 2.
23078 J. Phys. Chem. B, Vol. 110, No. 46, 2006
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Figure 7. DSC heating, cooling, and reheating curves of (a) 1 and (b) 2. The heating and cooling rates were both 10 °C min-1.
Figure 5. θ-2θ X-ray powder diffraction patterns from 2 form-II as a function of aging time at room temperature: (a) 0; (b) 3 days; (c) 10 days; (d) 21 days. (e) XRPD of 2 form-I. The primary spacing (100) and (1′00) were 19.62 and 18.17 Å, respectively.
Figure 6. Arrhenius plot of transformation of form-II f form-I of 2.
Spontaneous polymorphic transformations can only be noticed from the less stable form (form-II) to the stable one (form-I). The raised annealing temperature shortens the time of this transformation. The rate constants (k) of the transformation (form-II f form-I) can be determined as a function of temperature. The rate constants increase with increasing annealing temperatures. This relationship can be properly expressed by assuming the following Arrhenius behavior:
k ) A exp(-Ea/RT) where Ea is activation energy, R is the Boltzmann constant, and T is absolute temperature. Figure 6 shows the linear least-squares plots of the logarithm of the rate constant versus the reciprocal of the absolute temperature. A straight line indicates that the polymorphic transformation between the two polymorphs follows the law of the first-order reaction, for which the activation
energy is 96 kJ mol-1. To the best of our knowledge, this high number for polymorphism in oligothiophene compounds is the first report. 3.3. DSC. Melting behavior is investigated. DSC heating, cooling, and reheating curves of 1 and 2 (form-I) are shown in Figure 7. The endothermic and exothermic peaks of virgin samples of 1 (2) were observed at 176.4 (156.2) °C and 122.4 (122.4) °C, respectively. The enthalpies of melting (∆Hm) and solidification (∆Hs) transition of 1 (2) were 56.1 (62.9) kJ mol-1 and 47.3 (39.4) kJ mol-1, respectively. The melting point was determined by the onset temperature in which the baseline intersected the extrapolated tangents at the midpoints of the fusion peak. The melting points of 1 and 2 were determined as 174.4 and 156.5 °C, respectively. The values are much smaller than that of R-sexithiophene18 (220 °C) and R,β-end-capped quaterthiophene19 (225-226 °C); the melting point is similar to that of R-didodecylquaterthiophene,20 indicating that the packing energy of 1 and 2 was reduced by the bulky terminal groups. This is consistent with the fact that there is a relationship between the melting point and the packing coefficient21 defined as the ratio of volume occupied by the molecule to available volume. In fact, the density of 1 and 2 (1.2 g cm-3)14 is much smaller than that of conventional oligothiophenes (1.4-1.5 g cm-3).22,23 The difference between endo- and exothermic peaks of 1 and 2 (i.e., those compounds that have a wide range of supercooling) is much larger than that of R-alkyl-substituted quaterthiophenes20 and R-sexithiophene.18 This probably means that the rate for crystallization is much slower than that of R-quaterthiophene derivatives and R-sexithiophene because the π-π stacking force with thiophene rings is reduced with the steric hindrance of extra bulky terminal groups. This is supported by the fact that two thiophene rings are staggered and only two rings meet each other in the crystal structure of 2. In the reheating curves of 1, only one fusion peak is observed, indicating that the polymorph or mesophase (e.g., nematic phase) did not exist. In contrast, the reheating curve of 2 is more complicated with the three small endothermic peaks at 68, 96, and 120 °C. There is also the shoulder in the fusion peak. The enthalpy of remelting (∆Hrm) of 1 (total enthalpy of the transitions) and 2 were 49.3 and 49.4 kJ mol-1, respectively. The ∆Hrm of 1 is near ∆Hm, whereas ∆Hrm of 2 is far different from ∆Hm. This suggests that 2 has a polymorph and the reheating sample contains the polymorphic phase.
Polymorphism and Analysis of an Oligothiophene
J. Phys. Chem. B, Vol. 110, No. 46, 2006 23079
Figure 9. Gibbs free energy curves for a monotropic polymorophic system where form-I (higher melting polymorph) is more stable than form-II over the entire temperature range. TmI and TmII denote the melting point of form-I and form-II, respectively.
Figure 8. DSC heating curves of 2 form-II as a function of heating rate: (a) 1, (b) 10, (c) 45 °C min-1. Under insets are zoom for fusion peak involving that of 2 form-I (red line).
The heating rate is a decisive parameter, which can determine the polymorph observed and the rate of polymorphic transformation. Figure 8 shows the heating curve of the virgin sample of form-II as a function of heating rate. The fusion peak at the heating rate 10 °C min-1 is similar in shape to that of the reheating of 2. However, the three small endothermic peaks observed in the reheating sample (Figure 7b) is not observed in that of the virgin sample of form-II. This indicates that the endothermic events at
