FT-IR Study on Liquid Crystal Phase Transitions of Thermotropic

Article pubs.acs.org/JPCB

FT-IR Study on Liquid Crystal Phase Transitions of Thermotropic Hydrogen-Bonded Cubic Mesogenes, 1,2Bis(4′‑n‑alkoxybenzoyl)hydrazines (BABH‑n) and 4′‑n‑Alkoxy-3′nitrobiphenyl-4-carboxlic acid (ANBC‑n): Spectroscopic Evidence for Quasibinary Picture Model Ryoji Ogawa, Yohei Miwa,* and Shoichi Kutsumizu

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 28, 2015 | http://pubs.acs.org Publication Date (Web): July 27, 2015 | doi: 10.1021/acs.jpcb.5b05498

Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan S Supporting Information *

ABSTRACT: Changes in intermolecular interactions and molecular geometry for two kinds of thermotropic cubic mesogenes, 4′-nalkoxy-3′-nitrobiphenyl-4-carboxlic acid (denoted as ANBC-n, where n represents the number of carbon atoms in the alkoxy group) and 1,2-bis(4′-n-alkoxybenzoyl)hydrazines (BABH-n), at liquid crystal (LC) phase transitions were revealed utilizing the frequency shifts in Fourier transform infrared (FT-IR) bands as a guide. The ANBC-n and BABH-n form two kinds of bicontinuous cubic (Cubbi), Ia3d and Im3m types, and smectic LC phases depending on the length of the alkyl chain and temperature. In the present work, two kinds of phase transitions, i.e., smectic C ↔ Ia3dCubbi phase transition for the ANBC-16 and BABH-9 and Ia3d-Cubbi ↔ Im3m-Cubbi phase transition for the BABH-13 and BABH-16, were examined, and the experimental result was compared to the entropy changes predicted by the quasibinary picture model. In this model, it is postulated that the basic units in the BABH-n and ANBC-n, i.e., the “chain” and “core”, would contribute to the phase transition entropy in different ways. A conclusion of the FT-IR result shows the adequacy of this model for the behavior of the alkyl chain. On the other hand, the FT-IR result suggested that entropy changes for the “core” predicted by this model are not directly related to changes in the intermolecular interactions between the aromatic cores of the LC molecules at the phase transitions.

1. INTRODUCTION Intense interests have been directed toward bicontinuous cubic (Cubbi) liquid crystals (LCs) in which two chemically incompatible components are both continuous in three dimensions and one forms interwoven networks in harmony with cubic symmetry.1−8 The resulting structures possess threedimensionally (3D) periodic lattices within which locally molecular diffusional motions are to some extent preserved. Although the Cubbi phases are more familiar in lyotropic LC9 and block copolymer (BC)10,11 systems than in thermotropic LC, the number of the thermotropic Cubbi LC compounds has been increasing as reviewed by one of the authors.7 In the thermotropic Cubbi LCs, 4′-n-alkoxy-3′-nitrobiphenyl4-carboxlic acid (denoted as ANBC-n, where n represents the number of carbon atoms in the alkoxy group)12−17 and 1,2bis(4′-n-alkoxybenzoyl)hydrazines (BABH-n)18−20 are representative compounds. They form two kinds of Cubbi, i.e., Ia3d and Im3m types, depending on the length of the alkyl chain. As shown in Figure 1, their molecular structures are centrosymmetric rod-shaped because the molecules of ANBC-n dimerize via hydrogen bonds between carboxylic acid moieties in crystalline and LC states.15 In contrast to the BC and lyotropic © 2015 American Chemical Society

Figure 1. Chemical structures of (A) ANBC-n and (B) BABH-n.

LC systems, the BABH-n and ANBC-n show the Cubbi LC phases for very wide ranges of temperature and the composition of the constituent molecules (Figure S1 in Supporting Information).17,20 This strongly suggests that the formation mechanisms for the thermotropic Cubbi LC phases Received: June 9, 2015 Revised: July 11, 2015 Published: July 13, 2015 10131

DOI: 10.1021/acs.jpcb.5b05498 J. Phys. Chem. B 2015, 119, 10131−10137

Article


The Journal of Physical Chemistry B

negative. In other words, at the Ia3d-Cubbi to SmC transition for the BABH-n (n = 8−10), the “core” gains entropy more than the amount of the entropy loss of the “chain” with an increase in temperature, as shown in Figure 2A. In the model, the contribution of the “chain” includes not only that which arises from the “static” arrangement but also those related to vibration, free volume, and unidentified specific excitation, etc. Namely, any entropy changes related to the alkyl chain are included in the “chain” contribution. On the other hand, in the “core” contribution, in addition to the above types of contributions but connected to the aromatic core part of constituent molecules, any other entropy changes in the system other than the “chain” contribution are included. For example, change in the translational degree of freedom of the constituent molecules induced by the phase structure change (e.g., from 1D layer SmC to 3D network Cubbi structure) is also included in the “core” contribution. The quasibinary picture model reasonably and systematically explains the complicated phase transition behaviors in the ANBC-n and BABH-n. However, there is no spectroscopic evidence to support the local entropy changes predicted by this model. In the present work, temperature-variable Fourier transform infrared (FT-IR) spectroscopic measurement is applied for this subject. The FT-IR spectroscopy is a powerful technique to determine changes in chemical interactions and molecular geometry in LC phases.15,27−30 The aim of this work is to compare the FT-IR results with the quasibinary picture model, and to reveal the adequacy of the model from the spectroscopic point of view.

are not always the same as other Cubbi systems such as the lyotropic LC and BC ones, and in the thermotropic Cubbi systems, hydrogen bonding and/or dipole interactions would play an important role in stabilizing the structures; from another viewpoint, in addition to the nanosegregation mechanism, the preferential parallel orientation of the long axes of the mesogenic cores is operative, as in typical low molecular mass rodlike LCs. In the packing structures of the Cubbi phases for the BABH-n and ANBC-n, the aromatic cores are placed on the 3D networks and the alkyl chains are in the remaining space.20,21 In this case the phase sequence of smectic (Sm) → Cubbi → columnar (Col) with increasing temperature is reasonably predicted by the analogy of BC and lyotropic LC systems. In fact, the Cubbi phases are observed at higher temperature than the smectic C (SmC) phase for the ANBC-n.17 On the other hand, the SmC phase is located at higher temperature than the Ia3d-Cubbi phase for the BABH-n in the case of n ranging 8− 10.20 This phase sequence contradicts the analogy of BC and lyotropic LC systems. That is, the phase behavior of the thermotropic cubic LCs is unable to be fully explained with the rule of the BC and lyotropic LC systems. In addition, even in a single system of the BABH-n, different transition orders between the Ia3d- and Im3m-Cubbi phases with temperature variation are observed: The Ia3d-Cubbi phase transforms into the Im3m-Cubbi phase with an increase in temperature for the BABH-13 whereas the inverted phase transition is observed for the BABH-16.20,22 Saito and Sorai applied the quasibinary picture model to explain the phase inversions of the thermotropic cubic LCs.6,23−26 In their model, it is postulated that the two “components” of the BABH-n and ANBC-n, i.e., the alkyl chain and aromatic core parts would contribute to the phase transition entropy in different ways, as described in Figure 2.

2. EXPERIMENTAL SECTION 2.1. Sample Preparations. The ANBC-n and BABH-n were basically prepared with the established method of Gray et al.12 and Schubert et al.,18 respectively, although the methods were slightly modified to improve the purity and/or to obtain longer alkyl chain homologues. The samples were identified by 1 H NMR and FT-IR spectroscopies and confirmed to be fully pure by DSC, thin layer chromatography, and elemental analysis. 2.2. Measurements. Fourier Transform Infrared Spectroscopy. Spectrum 400 FT-IR spectrometer manufactured by PerkinElmer was used for measurements. The temperature was controlled using a custom-designed horizontal sample cell holder. The sample was cast from the chloroform solution onto a KBr plate to make a thin film, and the film was dried overnight at 353 K under a vacuum. The sample film coated on the KBr plate was covered with another KBr plate. The sandwiched sample was heated above the melting point and cooled to room temperature. The measurements were performed on the transmittance mode with more than 32 scans at 2 cm−1 optical resolution. The data points were collected with 0.25 cm−1 digital resolution. Temperaturevariable FT-IR measurements were performed on the heating process. The baseline of each spectrum was corrected using the data analytical software provided by Nicolet (OMNIC version 5.1). To determine the frequency at the peak position of a band accurately, the curve-fitting was performed partially only around the peak maximum, typically ±5 cm−1, by using Igor software package (version 6, WaveMetrics, Inc.). Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) measurement was carried out using DSC7020 manufactured by Seiko Instruments Inc. and calibrated with indium, zinc, tin, and lead standards. For the

Figure 2. Description of entropy changes for “chain” and “core” parts of LC molecules at phase transitions based on quasibinary picture model. (A) Transition between SmC and Ia3d-Cubbi phases for BABH-9 and ANBC-16. (B) Transition between Ia3d- and Im3mCubbi phases for BABH-13 and BABH-16.

For example, in the case of the transition from the SmC to Ia3d-Cubbi phases for the ANBC-16, the “chain” gains large entropy because of the relatively long alkyl chain whereas the “core” slightly loses entropy when the system goes from the SmC to the Ia3d-Cubbi phases. In this case, the total amount of transition entropy is reduced but still positive, leading to the observation that the Ia3d-Cubbi phase is at the hightemperature side of the SmC phase as illustrated in Figure 2A. On the other hand, in the case of the BABH-n, at the transition from the SmC to Ia3d-Cubbi phases, the amount of entropy obtained by the “chain” is comparably small, less than the entropy loss of the “core”, because the alkyl chains of the BABH-n that exhibit both SmC and Ia3d-Cubbi phases are relatively short (n = 8−10). Thus, the total entropy at the transition from the SmC to Ia3d-Cubbi phases becomes 10132


Article


Table 1. Polymorphism, Phase Transition Temperaturesa in Kelvin, and their Entropy Changes in kJ mol−1 (in Parentheses) for the Compounds ANBC-16

Cr

BABH-9

Cr2b

BABH-13

Cr2b

BABH-16

Cr1b

400 (41) 400 (49) 400 (36) 403 (88)

SmC Cr1b Cr1b Im3mCubbi

Ia3dCubbi Ia3dCubbi Ia3dCubbi Ia3dCubbi

451 (0.6) 417 (23) 407 (27) 417 (0.6)

471 (ca. 1) 431 (1.0) 415 (0.2) 434 (11)

SmA SmC Im3mCubbi I

473 (ca. 1) 440 (8.7) 435 (10)

I1 c

478 (ca. 6)

I2c

I I

Determined from DSC peak temperature in the second heating. bSubscript of the crystalline phases for BABH-n was given only in order of temperature from the higher temperature side in each compound. cI1 and I2 are structured and normal liquid phases (ref 16).


a

FT-IR band for ANBC-n15,30 and BABH-n20 were described in our previous papers. The FT-IR bands selected to monitor changes in the degree of the alkyl chain order and the intermolecular interactions between the aromatic cores are listed in Table 2. Arrhenius plots of the FT-IR band frequencies for the ANBC-16 and BABH-9 in the LC region were shown in Figure 4A,B, respectively. As shown in our previous papers, the

cooling of samples a quench cooler accessory was used. The DSC cell was purged with dry nitrogen gas during the measurement at the flow rate of 40 mL min−1. Samples were heated from room temperature to Tc + 30 K (Tc: clearing temperature) at a rate of 5 K min−1, kept for 3 min, cooled to 323 K at a rate of 5 K min−1, and heated again at a rate of 5 K min−1. The data collection was carried out on the second heating process. The DSC thermograms for the samples studied in this work are shown in Supporting Information Figure S2. The phase sequences and their transition temperatures and enthalpies obtained during the heating scan are listed in Table 1, which are essentially the same as those reported previously.17,20

3. RESULTS 3.1. Transition between SmC and Ia3d-Cubbi LC Phases. The BABH-9 shows the SmC phase at higher temperature side of the Ia3d-Cubbi phase whereas for the ANBC-16 the SmC phase locates at lower temperature side of the Ia3d-Cubbi phase. In this section, the temperature dependences of the FT-IR bands for the ANBC-16 and BABH-9 are shown. In Figure 3, FT-IR spectra for the ANBC-16 and BABH-9 measured at 425 K are shown. Detailed assignments of each

Figure 4. Plots of natural logarithm of frequency of the indicated band versus the inverse of temperature for (A) ANBC-16 and (B) BABH-9 in the LC phase region. Bold arrows in graphs indicate the frequency shift at the transitions. Vertical broken lines indicate the transition temperatures determined by DSC.

Figure 3. FT-IR spectra for BABH-9 and ANBC-16 at 425 K where Ia3d-Cubbi and SmC phases are formed, respectively.

Table 2. Assignments of FT-IR Bands for ANBC-16 and BABH-9, -13, and -16 at 425 K frequency/cm−1 BABH-9

BABH-13

BABH-16

3293 2926 2856

3289 2924 2854

3292 2925 2854

1606

1606

1606

1250

1250

1250

ANBC-16

assignment

notation

2923 2853 1688 1607 1534 1278

stretching of hydrogen-bonded amide NH asymmetric stretching of CH2 symmetric stretching of CH2 stretching of hydrogen-bonded carbonyl CO ring CC stretching NO2 asymmetric stretching alkyl COaromatic C asymmetric stretching

ν(NH)H‑bond νas(CH)CH2 νs(CH)CH2 ν(CO)H‑bond ν(CC)ring ν(NO2) νas(COC)

10133


Article



derivative, ethyl 4-(4′-n-docosyloxyphenylazo)benzoate.35 On the other hand, the ν(CC)ring and νas(COC) bands of the BABH-9 show no step-like frequency change at the transition from the Ia3d-Cubbi to SmC phases. This indicates that the intermolecular interactions working on these bonds are almost unchanged at this transition. Hydrogen-Bonded Carbonyl CO and Amide NH Stretching Vibrations. Information on the hydrogen-bonding state is obtained from the hydrogen-bonded carbonyl CO (ν(CO)H‑bond)15,30 and amide NH (ν(NH)H‑bond)20,36 bands of the ANBC-16 and BABH-9, respectively. For the BABH-9, the conformation of amide group is trans because the ν(NH)H‑bond is observed around 3290 cm−1; therefore, multimer may be formed via hydrogen bonding. Moreover, formation of cis amide conformer (ca. 3160 cm−1) was not observed at any temperature. Thus, intramolecular hydrogen bonding such as a stabilized ring is not formed. Both ν(C O)H‑bond and ν(NH)H‑bond bands show a step-like frequency increase at the transitions from the SmC to Ia3d-Cubbi phases for the ANBC-16 and from the Ia3d-Cubbi to SmC phases for the BABH-9. The higher frequency shifts of these bands can be attributed to weakening of the intermolecular hydrogen bonding, which is necessary for the constituent molecules obtaining mobility. Some remarks will be mentioned in the Discussion section. 3.2. Transition between Ia3d- and Im3m-Cubbi LC Phases. The BABH-13 shows a phase transition from the Ia3dto Im3m-Cubbi phases with an increase in temperature while the inversion of the phase sequence is observed for the BABH16 (Table 1). In this section, temperature dependences of the FT-IR bands for BABH-13 and BABH-16 are compared, and changes in the molecular-level aggregation state at the transition between the Ia3d- and Im3m-Cubbi phases are shown. Alkyl CH2 Stretching Vibrations. The temperature dependences of the frequencies of the νs(CH)CH2 and νas(C H)CH2 bands for the BABH-13 and BABH-16 in the LC states are shown in Figure 5A,B, respectively. The BABH-16 shows clear step-like frequency increases in the νas(CH)CH2 and

most distinct frequency shift is observed at the melting point because the most remarkable changes in the degree of the alkyl chain order and intermolecular interactions occur at the melting transition.15,20,30 However, in the present work, the melting transition is out of scope, and small changes in the FTIR band frequencies at the transitions between LC phases are analyzed in detail. Alkyl CH2 Stretching Vibrations. Using the frequency shifts as a guide, we can determine the degree of the alkyl chain order. It is well-known that the higher frequency shifts of the symmetric and asymmetric stretching vibrations of CH2 in the alkyl chain (νs(C−H)CH2 and νas(C−H)CH2) reflect the disordering of chain packing and the introduction of gauche conformers on the alkyl chains.27−29,31−33 As shown in Figure 4A, the ANBC-16 shows clear step-like frequency increases in the νas(C−H)CH2 and νs(C−H)CH2 bands at the transitions from the SmC to Ia3d-Cubbi phase and from the SmA to isotropic liquid phase. At the Ia3d-Cubbi to SmA transition, on the other hand, no clear frequency shift is detected for the all FT-IR bands because of the narrow temperature range of the SmA phase. In the case of the BABH-9, the νas(C−H)CH2 and νs(C−H)CH2 bands show clear step-like frequency increases at the transition from the SmC to isotropic liquid phases; however, at the transition from the Ia3d-Cubbi to SmC phases, these bands show slight step-like frequency decreases. Namely, these results indicate that the alkyl chain in the ANBC-16 becomes more disordered at the SmC to Ia3d-Cubbi transition while the alkyl chain of the BABH-9 becomes slightly ordered at the Ia3d-Cubbi to SmC transition. The former result is consistent with the fact that the 1H NMR signal is sharper in the Ia3d-Cubbi phase than in the SmC phase of ANBC-16, indicating more mobile and disordered alkyl chain for Ia3dCubbi phase.34 Aromatic CC, COC, and NO2 Stretching Vibrations. When neighboring aromatic cores are more closely packed, stronger intermolecular π−π and polar interactions operate between them; therefore, frequencies of the stretching vibrations of CC bond in phenyl ring (ν(CC)ring), ether bond between alkyl chain and phenyl ring (νas(COC)), and nitro group (ν(NO2)) are utilized as a guide to monitor the molecular-level aggregation state of the aromatic cores in the LC phases.15,27−30 As shown in Figure 4A, the ν(CC)ring, νas(COC), and ν(NO2) of the ANBC-16 show clear steplike frequency increases at the transition from the SmA to isotropic liquid phases because of the disordering of the aggregation state. On the other hand, at the SmC to Ia3d-Cubbi transition in the ANBC-16, clear step-like frequency increases are observed for the νas(COC) and ν(NO2) while a steplike frequency decrease is observed for the ν(CC)ring. The ν(CC)ring band is in-plane deformation vibrations of inner phenyl rings, and its frequency change directly reflects the change of the π electron density.30 For example, enhancement of intermolecular π−π interaction would reduce the π electron density, leading to lower frequency shift of the ν(CC)ring band. Therefore, the observed frequency decrease is ascribed to some reduction in the intermolecular interactions. A similar argument would be valid for the νas(COC) and ν(NO2) bands. Hence, this result indicates weakening of the polar interactions working on the COC and NO2 groups, but intermolecular π−π interaction enhances at this phase transition. This complicated behavior was similarly observed for the transition from the SmC to Ia3d-Cubbi phases for a binary blend composed of ANBC-22 and its azobenzene

Figure 5. Plots of natural logarithm of frequency of the indicated band versus the inverse of temperature for (A) BABH-13 and (B) BABH-16 in the LC region. Bold arrows in graphs indicate frequency shift at the transitions. Vertical broken lines indicate the transition temperatures determined by DSC. 10134


Article


The Journal of Physical Chemistry B νs(CH)CH2 bands at the transition from the Im3m- to Ia3dCubbi phases. On the other hand, the BABH-13 shows step-like frequency decreases in these bands at the transition from the Ia3d- to Im3m-Cubbi phases. Namely, this result indicates that the alkyl chain of the BABH-16 becomes more disordered because of an increase in the gauche conformer at the Im3m- to Ia3d-Cubbi transition whereas that of the BABH-13 becomes ordered at the Ia3d- to Im3m-Cubbi transition. Aromatic CC and COC Stretching Vibrations. Temperature dependences of the ν(CC)ring and νas(C OC) bands for the BABH-13 and BABH-16 are also given in Figure 5. For the BABH-13, the slopes of the plots slightly changed, but step-like frequency changes are not observed for these bands at the transition from the Ia3d- to Im3m-Cubbi phases. This result indicates that the intermolecular interactions working on the phenyl ring and the ether group are not discontinuously changed at the phase transition. For the BABH-16, although the νas(COC) band does not show a step-like frequency change, the ν(CC)ring band shows a steplike decrease at the Im3m- to Ia3d-Cubbi transition. Namely, the π−π interaction between the phenyl rings enhances at the phase transition. Hydrogen-Bonded Amide NH Stretching Vibration. A step-like frequency increase in the ν(NH)H‑bond band is observed for both transitions from the Ia3d- to Im3m-Cubbi phases for the BABH-13 and from the Im3m- to Ia3d-Cubbi phases for the BABH-16 (Figure 5). Namely, the hydrogen bonding is weakened at these phase transitions.

transitions from SmC to Im3m-Cubbi and from Im3m-Cubbi to Ia3d-Cubbi phases. However, the experimental result is opposite to this prediction. Therefore, explanation of the observed decrease in the Cp at the transitions needs another contribution, and the decrease would be attributed to restricted mobility of the “core”. From our FT-IR measurement, weakening of intermolecular polar interactions and hydrogen bonding at the transition from the SmC to Ia3d-Cubbi phases for the ANBC-16 was elucidated while change in the ν(CC)ring band observed suggests enhancement of the π−π interaction (Figure 4A). The weakening of the hydrogen bonding is consistent with our previous report where we evaluated the temperature variation of the molar fraction of the hydrogen-bonded carbonyl group ( f H‑bond) for the ANBC-16 (and its cyano analogue) from the FT-IR band intensities for the resolved hydrogen-bonded and free carbonyl components; lower f H‑bond in the Ia3d-Cubbi phase compared to the SmC phase was observed.15,30 For example, the f H‑bond’s at 440 K (SmC phase) and 460 K (Ia3dCubbi phase) are ca. 0.9 and 0.8, respectively. The BABH-16 also showed the weakening of the hydrogen bonding at the transition from the Im3m- to Ia3d-Cubbi phases whereas enhancement of the π−π interaction was observed (Figure 5B). The hydrogen bonding is formed between laterally neighboring molecules for the BABH-n while dimerization of the ANBC-n is formed between longitudinally aligned molecules via the hydrogen bonding as shown in Figure 1A. Probably, the hydrogen bonding would have more dynamic nature in LC phases, unlike in the crystalline state, changing its counterpart from time to time. Therefore, although the directions of the hydrogen bonding are different between the ANBC-n and BABH-n, weakening of the hydrogen bonding would lead to enhanced mobility of the aromatic cores for both types of molecules. The enhanced mobility of the aromatic cores is considered to be reasonable because the aromatic core is linked to the alkyl chains which become more mobile at the transitions. This result indicates that the mobility of the aromatic cores does not necessarily become restricted at these transitions although the enhancement of the π−π interaction is suggested. One may think this conclusion contradicts the prediction by the quasibinary picture model. In the model, however, any entropy changes in the system other than those related to the alkyl chain are included in the “core” contribution, while the contribution of “chain” is literally entropy changes related to the alkyl chain.6,23−26 Hence, for example, the translational degree of freedom of the constituent molecules should be decreased when going from “within a layer” in the SmC phase to “along a three dimensionally connected rod” in the Ia3d-Cubbi phase. This effect is included in the “core” contribution and is probably an origin for the decrease in the Cp at the transitions in the ANBC-22 as mentioned above. What we emphasize is the fact that mobility of the aromatic core part of constituent molecules does not necessarily become restricted at the phase transition never contradicts the entropy loss of the “core” predicted by the quasibinary picture model. 4.2. Cases of Ia3d-Cubbi → SmC and Ia3d-Cubbi → Im3m-Cubbi Transitions. In the cases of the transitions from the Ia3d-Cubbi to SmC phases for the BABH-9 and from the Ia3d- to Im3m-Cubbi phases for the BABH-13, the quasibinary picture model predicts the entropy loss and gain for the “chain” and “core”, respectively (Figure 2).6,23−26 From the temperature dependences of the νas(C−H)CH2 and νs(C−H)CH2

4. DISCUSSION 4.1. Cases of SmC → Ia3d-Cubbi and Im3m-Cubbi → Ia3d-Cubbi Transitions. As shown in Figure 2, the quasibinary picture model predicts that the “chain” gains but the “core” loses entropy at the SmC to Ia3d-Cubbi transition for the ANBC-16 and the Im3m- to Ia3d-Cubbi transition for the BABH-16.6,23−26 In this section, changes in the molecular-level aggregation states detected by the FT-IR measurement at these transitions are discussed in comparison with the quasibinary picture model. Inspection of the νas(CH)CH2 and νs(CH)CH2 bands revealed that the alkyl chains become disordered at both transitions from the SmC to Ia3d-Cubbi phases for the ANBC16 (Figure 4A) and from the Im3m- to Ia3d-Cubbi phases for the BABH-16 (Figure 5B). Increased gauche conformations lead to a lateral expansion and a more liquid-like state of the alkyl chain. More laterally expanded conformation of the alkyl chain in the Ia3d-Cubbi phase compared to the SmC phase is consistent with our previous insight on the basis of the X-ray diffraction measurement.17,20 In conclusion, the behavior of the alkyl chain determined by the FT-IR measurement is in good agreement with the prediction by the quasibinary picture model. Previously, Saito et al. precisely measured the temperature variation of the heat capacity (Cp) for the ANBC-22 that shows the LC phase sequence of the SmC → Im3m-Cubbi → Ia3dCub bi with an increase in temperature. 25,37 The C p discontinuously decreased at the transitions from the SmC to Ia3d-Cubbi phases and from the Im3m- to Ia3d-Cubbi phases although it overall increased with temperature. They extended the quasibinary picture model and assumed that the alkyl chain becomes more disordered in the order SmC → Im3m-Cubbi → Ia3d-Cubbi stepwise at the transitions. If only the contribution of the alkyl chain is considered, the Cp should increase at the 10135



The Journal of Physical Chemistry B bands, it was revealed that the alkyl chains become ordered, and an increased packing density in the alkyl chain region is expected for both transitions of the BABH-9 (Figure 4B) and BABH-13 (Figure 5A). This result is reasonably consistent with the entropy loss of the “chain” predicted by the quasibinary picture model. Concerning the aromatic core, weakening of the intermolecular hydrogen bonding at these transitions was observed while no change was detected for the polar and π−π interactions on the ether and phenyl groups. As discussed in the previous section, ANBC-16 and BABH-16 also showed weakening of the hydrogen bonding, which is interpreted as an origin of the enhanced mobility of the constituent molecules. In the cases of BABH-9 and BABH-13, the decrease in the mobility of the alkyl chain was revealed at the transitions. Therefore, the weakening of the hydrogen bonding for the BABH-9 and BABH-13 may be attributed to a “static” effect, i.e., reduced packing density between the aromatic cores. This experimental result is consistent with the entropy gain of the “core” predicted by the quasibinary picture model. Nevertheless, we are considering that other factors than the decrease in the packing density in the aromatic core region may contribute to the entropy gain of the “core”.

ACKNOWLEDGMENTS

■

REFERENCES

(1) Tschierske, C. Non-conventional Soft Matter. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 0, 97, 191−267. (2) Diele, S. On Thermotropic Cubic Mesophases. Curr. Opin. Colloid Interface Sci. 2002, 7, 333−342. (3) Kutsumizu, S. The Thermotropic Cubic Phase: A Curious Mesophase. Curr. Opin. Solid State Mater. Sci. 2002, 6, 537−543. (4) Impéror-Clerc, M. Thermotropic Cubic Mesophases. Curr. Opin. Colloid Interface Sci. 2005, 9, 370−376. (5) Holyst, R. Some Features of Soft Matter Systems. Soft Matter 2005, 1, 329−333. (6) Saito, K. Microphase-Separated Multicontinuous Phase in LowMolecular-Mass Thermotropic Liquid Crystals. Pure Appl. Chem. 2009, 81, 1783−1798. (7) Kutsumizu, S. Recent Progress in the Synthesis and Structural Clarification of Thermotropic Cubic Phases. Isr. J. Chem. 2012, 52, 844−853. (8) Tschierske, C. Development of Structural Complexity by LiquidCrystal Self-assembly. Angew. Chem., Int. Ed. 2013, 52, 8828−8878. (9) Mariani, P.; Luzzati, V.; Delacroix, H. Cubic Phases of LipidContaining Systems: Structure Analysis and Biological Implications. J. Mol. Biol. 1988, 204, 165−189. (10) Bates, F. S.; Fredrickson, G. H. Block Copolymer Thermodynamics: Theory and Experiment. Annu. Rev. Phys. Chem. 1990, 41, 525−557. (11) Matsen, M. W.; Bates, F. S. Unifying Weak- and StrongSegregation Block Copolymer Theories. Macromolecules 1996, 29, 1091−1098. (12) Gray, G. W.; Jones, B.; Marson, F. Mesomorphism and Chemical Constitution. Part VIII. The Effect of 3′-Substituents on the Mesomorphism of the 4-́ n-Alkoxydiphenyl-4-carboxylic Acids and Their Alkyl Esters. J. Chem. Soc. 1957, 393−401. (13) Demus, D.; Kunicke, G.; Neelsen, J.; Sackmann, H. Die Polymorphie der kristallin-flüssigen Modifikationen in der homologen Reihe der 4′-n-Alkoxy-3′-nitrodiphenyl-4-carbonsäuren. Z. Naturforsch., Teil A 1968, 23, 84−90. (14) Kutsumizu, S.; Yamada, M.; Yano, S. Mesomorphic Phase Transitions of a Series of D-Phase Compounds. Liq. Cryst. 1994, 16, 1109−1113. (15) Kutsumizu, S.; Kato, R.; Yamada, M.; Yano, S. Structural Studies of 4′-n-Alkoxy-3′-nitrobiphenyl-4-carboxylic Acids by Infrared Spectroscopic Analysis. J. Phys. Chem. B 1997, 101, 10666−10673. (16) Kutsumizu, S.; Ichikawa, T.; Yamada, M.; Nojima, S.; Yano, S. Phase Transitions of 4′-n-Hexacosyloxy-3′-nitrobiphenyl-4-carboxylic Acid (ANBC-26): Two Types of Thermotropic Cubic Phases. J. Phys. Chem. B 2000, 104, 10196−10205. (17) Kutsumizu, S.; Morita, K.; Ichikawa, T.; Yano, S.; Nojima, S.; Yamaguchi, T. Cubic Phases of 4′-n-Alkoxy-3′-nitrobiphenyl-4carboxylic Acids (ANBC-n). Liq. Cryst. 2002, 29, 1447−1458. (18) Schubert, H.; Hauschild, J.; Demus, D.; Hoffmann, S. Diacylhydrazin-Mesogene;) Symmetrische 1,2-Dibenzoylhydrazinderivate. Z. Chem. 1978, 18, 256−256. (19) Demus, D.; Gloza, A.; Hartung, H.; Hauser, A.; Rapthel, I.; Wiegeleben, A. New Thermotropic Cubic Mesophases. Krist. Tech. 1981, 16, 1445−1451. (20) Kutsumizu, S.; Mori, H.; Fukatami, M.; Naito, S.; Sakajiri, K.; Saito, K. Cubic Phase Formation and Interplay between Alkyl Chains

ASSOCIATED CONTENT

S Supporting Information *

Phase diagrams for ANBC-n and BABH-n on the heating process and DSC thermograms for the samples studied in this work. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcb.5b05498.

■

■

The authors thank Dr. Hiroyuki Mori, Mr. Eiichi Teraura, and Mr. Yoshitaka Yamada at Gifu University for their experimental aids, Professors Kazuya Saito and Yasuhisa Yamamura at University of Tsukuba for their valuable discussions. This work was partially supported by Grant-in-Aid for Scientific Research (C) [Grant 25410091] (for S.K.) and Grant-in-Aid for Young Scientists (B) [Grant 25810072] (for Y.M.) from Japan Society for the Promotion of Science (JSPS).

5. CONCLUSION Changes in the molecular-level aggregation states for the ANBC-n and BABH-n at the LC phase transitions were revealed from the frequency shifts of the FT-IR bands; the experimental results were compared with the prediction of the quasibinary picture model. The FT-IR result gave clear evidence that changes in the degree of the alkyl chain order at the transitions are in good agreement with the entropy changes of the “chain” predicted by the model. On the other hand, changes in the interactions between the aromatic cores of constituent molecules determined by our FT-IR measurement did not necessarily reflect the entropy change of the “core” expected by the model. However, this result never denies the quasibinary picture model, but is reasonably reconciled by considering that in this model any entropy changes in the system other than those related to the alkyl chain are included in the “core” contribution while the contribution of the “chain” is literally entropy changes related to the alkyl chain; e.g., the change in the translational degree of freedom of the constituent molecules induced by the phase structure change is included in the “core” contribution.

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 10136


Article


The Journal of Physical Chemistry B and Hydrogen Bonds in 1,2-Bis(4′-n-alkoxybenzoyl)hydrazines (BABH-n). Chem. Mater. 2008, 20, 3675−3687. (21) Kutsumizu, S.; Morita, K.; Yano, S.; Nojima, S. Cubic Phases of Binary Systems of 4′-n-Tetradecyloxy-3′-nitrobiphenyl-4-carboxylic Acid (ANBC-14)−n-Alkane. Liq. Cryst. 2002, 29, 1459−1468. (22) Mori, H.; Kutsumizu, S.; Saito, K.; Yamamoto, K.; Sakurai, K.; Sakajiri, K. Temperature-Jump Time-Resolved X-ray Diffraction Study of Cubic−Cubic Phase-Transition Kinetics in Thermotropic Cubic Mesogen 1,2-Bis(4′-n-alkoxybenzoyl)hydrazines (BABH-n). Langmuir 2010, 26, 11605−11608. (23) Sato, A.; Yamamura, Y.; Saito, K.; Sorai, M. Possibility of Isostructural Condensed States of Matter in the D Phase of ANBC and the Cubic Mesophase of BABH: Heat Capacity of 4′-n-Octadecyloxy3′-nitrobiphenyl-4-carboxlic Acid, ANBC(18). Liq. Cryst. 1999, 26, 1185−1195. (24) Saito, K.; Sorai, M. Quasi-Binary Picture of Thermotropic Liquid Crystals and its Application to Cubic Mesophases. Chem. Phys. Lett. 2002, 366, 56−61. (25) Saito, K.; Shinhara, T.; Nakamoto, T.; Kutsumizu, S.; Yano, S.; Sorai, M. Degree of Disorder in Cubic Mesophases in Thermotropics: Thermodynamic Study of a Liquid Crystal Showing Two Cubic Mesophases. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2002, 65, 031719-1−031719-7. (26) Sorai, M.; Saito, K. Alkyl Chains Acting as Entropy Reservoir in Liquid Crystalline Materials. Chem. Rec. 2003, 3, 29−39. (27) Galbiati, E.; Zerbi, G. Molecular Mobility and Phase Transitions in Thermotropic Liquid Crystals: A Spectroscopic Study of Dodecylcyano Biphenyl. J. Chem. Phys. 1986, 84, 3509−3518. (28) Galbiati, E.; Zerbi, G. Molecular Structural Changes at the Phase Transitions in Thermotropic Liquid Crystals: A Spectroscopic Study of 4-Dodecyloxy-4′-cyano Biphenyl. J. Chem. Phys. 1987, 87, 3653− 3659. (29) Lee, W. K.; Heiney, P. A.; Ohba, M.; Haseltine, J. N.; Smith, A. B., III Fourier Transform Infrared Study of Two Truxene-based Discotic Liquid Crystals. Liq. Cryst. 1990, 8, 839−861. (30) Ogawa, R.; Miwa, Y.; Sumita, M.; Saito, K.; Kutsumizu, S. Effects of the Lateral Substituent on the Cubic Phase Formation of Two Analogous Compounds, 4′-n-Hexadecyloxy-3′-cyanobiphenyl-4carboxylic Acid (ACBC-16) and Its 3′-Nitro Compound (ANBC-16). Liq. Cryst. 2015, 42, 143−157. (31) Snyder, R. G. Vibrational Study of the Chain Conformation of the Liquid n - Paraffins and Molten Polyethylene. J. Chem. Phys. 1967, 47, 1316−1360. (32) Kawai, T.; Umemura, J.; Takenaka, T.; Kodama, M.; Seki, S. Fourier Transform Infrared Study on The Phase Transitions of an Octadecyltrimethylammonium Chloride-water System. J. Colloid Interface Sci. 1985, 103, 56−61. (33) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Interlayer Structure and Molecular Environment of Alkylammonium Layered Silicates. Chem. Mater. 1994, 6, 1017−1022. (34) Ukleja, P.; Siatkowski, R. E.; Neubert, M. E. NMR Studies of a D-phase compound. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 4815− 4821. (35) Hori, R.; Miwa, Y.; Yamamoto, K.; Kutsumizu, S. Phase Structure and Phase Transition Mechanism for Light-Induced Ia3d Cubic Phase in 4′-n-Docosyloxy-3′-nitrobiphenyl-4-carboxlic acid/ Ethyl 4-(4′-n-Docosyloxyphenylazo)benzoate Binary Mixture. J. Phys. Chem. B 2014, 118, 3743−3749. (36) Wang, F. C.; Feve, M.; lam, T. M.; Pascault, J.-P. FTIR Analysis of Hydrogen Bonding in Amorphous Linear Aromatic Polyurethanes. I. Influence of Temperature. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 1305−1313. (37) Saito, K.; Nakamoto, T.; Sorai, M.; Yao, H.; Ema, K.; Takekoshi, K.; Kutsumizu, S. Thermodynamic Symptom of Coexistence of Two Aggregation Modes in the Im3m Cubic Phase Formed in Thermotropic Mesogen, ANBC(n). Chem. Phys. Lett. 2009, 469, 157−160.

10137


FT-IR Study on Liquid Crystal Phase Transitions of Thermotropic

Recommend Documents