FT-IR and Two-Dimensional Correlation Analysis of the Liquid

Article pubs.acs.org/JPCB

FT-IR and Two-Dimensional Correlation Analysis of the Liquid Crystalline Phase Transitions in the 4‑Bromobenzylidene-4′-alkyloxyanilines Natalia Osiecka,† Mirosław A. Czarnecki,‡ Zbigniew Galewski,‡ and Maria Massalska-Arodź*,† †

The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland ‡ Faculty of Chemistry, University of Wrocław, Joliot-Curie 14, 50-383 Wrocław, Poland S Supporting Information *

ABSTRACT: The FT-IR spectra of the 4-bromobenzylidene-4′-alkyloxyanilines (nBBAA, for n = 4−12) were studied as a function of temperature. The molten state of the alkyloxy chain in smectic B (SmB), smectic A (SmA), and isotropic phases was analyzed. Generalized two-dimensional (2D) correlation spectroscopy has been applied to study changes in the conformational structure and specific interactions of molecules at phase transition in homologous series of nBBAA. A windowed autocorrelation analysis enabled us to locate transition points basing on the spectroscopic data.

1. INTRODUCTION

about the specific relationships between various molecular fragments. The basic experimental principles of 2D correlation spectroscopy involve the exposition of an external perturbation that has selective influence on a given system. Nature of the perturbation variable can be optical, mechanical, thermal, chemical, magnetic, electrical, and so forth. Variations of the intensity, shift, and shape of bands in IR spectra are referred to a dynamic spectrum ỹ(ν,t), which can be expressed as

The properties of matter in the condensed states are determined by three factors: molecular structure, molecular interactions, and dynamics.1 The balance between these factors is delicate, and when it is lost, the phase transition occurs. Fourier-transformed infrared spectroscopy (FT-IR) is a powerful technique for investigation of the molecular interactions which are responsible for the type of molecular conformations in a given phase. It is known that the change in conformational degrees of freedom of the molecules may lead to the alternation in the sequence of the phase transitions.1 The IR study of the 4-bromobenzylidene-4′-alkyloxyanilines (nBBAA, for n = 4−12; BrC6H4CHNC6H4OCnH2n+1) was inspired by the data obtained from the X-ray diffraction (XRD) experiment.2 It was found by polarizing microscope method (POM) and DSC studies that the 4BBAA and 5BBAA exhibit only SmB phase, while in Schiff bases from 6BBAA to 12BBAA the SmA and SmB phases occur.3 The XRD results show that the smectic layer thickness in smectic A phase is smaller than in the smectic B phase, on the contrary to what is commonly observed.4,5 This unusual behavior may result from transition from trans to gauche conformation of the alkyloxy chains.6 We expect that IR study should verify this assumption. The excitation and relaxation processes influence the intensity, position, and shape of the bands in the IR spectra.7 Monitoring the changes of the complex spectra is not an easy task. 2D correlation spectroscopy offers new possibilities for the analysis and interpretation of vibrational spectra as it yields information © XXXX American Chemical Society

⎧ y(v , t ) − y ̃(v) for Tmin ≤ t ≤ Tmax y ̃ (v , t ) = ⎨ ⎩0 otherwise

where y(ν,t) - variation of a spectral intensity, t - external variable, ν - spectral index (wavenumber), and Tmin - Tmax minimum and maximum value of the external variable, ỹ(ν) - reference spectrum given by y ̃ (v ) =

Tmax

1 − Tmin

∫T

Tmax

y(v , t )dt

min

The 2D correlation spectrum can be described as X(v1 , v2) = ⟨y ̃(v1 , t ) ·y ̃(v2 , t ′)⟩

The 2D correlation spectrum X(ν1, ν2) is regarded as a complex number function. The real and imaginary components Received: April 29, 2013 Revised: August 5, 2013

A

dx.doi.org/10.1021/jp4042155 | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

Article

predominantly before that at ν2, the sign of cross peak become positive. If the intensity change at ν1 occurs predominantly after that at ν2, the sign of the cross peak becomes negative.8−10 The 2D correlation analysis does not provide information on the temperature when the changes of spectral elements occur. To solve this problem, Thomas and Richardson proposed a movingwindow 2D correlation analysis.6 This method consists of the following steps: (1) partition of the whole spectral data matrix into smaller subsets (windows) containing the spectra obtained in several subsequent temperatures, (2) autocorrelation analysis of each window, (3) plotting the autopeak intensity spectrum as a function of the average temperature. This approach successfully shows the effect of temperature on the intensity changes at a particular wavenumber. The aim of this paper is to show in detail the spectral variations in the isotropic liquid−SmA, SmA−SmB, and isotropic liquid− SmB phase transitions in nBBAA (n = 4−12) by using 2D correlation spectroscopy and moving-window analysis.

correspond to the synchronous and asynchronous 2D correlation intensities, respectively. Autopeaks and cross peaks of the synchronous spectrum show the similarity between response of different functional groups to a common perturbation (here − the changes of temperature). Autopeaks appear on the diagonal of the 2D X(ν1, ν2) matrix, while the cross peaks occur at off-diagonal positions. Sign of the autopeaks is always positive, while the sign of the cross peaks can be negative or positive. Autopeak occurs when intensity of a given band is changed with respect to the reference spectrum. If the intensities for two spectral bands respond in a similar way to the perturbation, then the cross peaks appear. Positive cross peaks correspond to the spectral bands with intensities changing in the same direction (both decreasing or both increasing), while negative cross peak indicates that one intensity decreases while the other increases. The asynchronous spectrum yields information about variations in spectra that are not synchronously correlated. Only cross peaks appear on the asynchronous spectrum. Sign of the asynchronous cross peaks provides information on the relative rate of intensity changes or sequential order of events observed by the spectroscopic method along changes of temperature. If the intensity change at ν1 occurs

Figure 3. IR spectra in the CH stretching region for homologous series of nBBAA in the isotropic phase. Spectra are shifted along vertical axes. Figure 1. Comparison of experimental (top) and calculated (bottom) spectra of 4BBAA. Wavenumber positions of bars corresponding to vibrational normal modes are collected in Table 1.

Figure 2. Temperature dependence of IR spectra for 12BBAA. The blue, red, green, and brown lines represent isotropic liquid, SmA, SmB, and crystal phases, respectively. Phase transition temperatures are marked for: (a) SmA-isotropic liquid, (b) SmB-SmA, and (c) Cr-SmB.

Figure 4. Temperature dependence of the spectra of 11BBAA in isotropic, SmA, SmB, and crystal phase. Spectra are shifted along vertical axes. Main bands correspond to vibrations of alkyloxy chains. B



Article

Table 1. Band Assignment for 4BBAAa experimental frequencies

a

B3lyp/6-311G** (freq)

assignment B3lyp/6-311G**

Iso

SmB

719.9 739.8 750.6 799.6 815.4 829.0 832.1 846.3 850.1 856.6 894.9 916.6 954.0 961.1 961.8 967.0 992.1 1003.4 1020.2 1023.5 1023.9 1056.4 1070.2 1079.5 1126.5 1132.6 1142.8 1179.4 1188.6 1191.6 1216.2 1252.6 1264.1 1276.8 1299.4 1313.6 1318.7 1321.9

ω[CC] ArII + ω[CC] ArI ω[CC] ArII ρ [CH] δ [CO] +δ [CN] ω[CH] ArII ρ [CH] δout of plane [CH] NCH + ω[CH] ArI ω[CH] ArI + ArII ω[CH] ArI + ArII δs [CC] ArI +ArII δs [CN] δs [CC] τ[CH] ArII τ[CH] ArI ρ [CH] τ[CH] ArII ρ [CH] δout of plane [CH] NCH + τ[CH] ArI δout of plane [CH] NCH νas[CC] + δ [CC] ArII νas[CC] δ [CC] ArI ν[CO] + νas[CC] νas[CC] δ [CC] ArI δs [CH] ArI δs [CH] ArII νsym [CC] δs [CH] ArII ρ [CH] δs [CH] ArI ν [NC] τ[CH] δs [CH] ArI + ArII + δin plane [CH] NCH ν [C−O] ω [CH] ν [C−C] ArI + ArII + δin plane [CH] NCH δs [CH] ArI τ[CH]

721

722

795 818

797 818

834

835

969

969

1024

1024

1069

1069

1125

1125

1143

1143

1190 1217

1189 1216

1299

1299

experimental frequencies B3lyp/6-311G** (freq)

assignment B3lyp/6-311G**

1321.9 1326.9 1329.4 1331.8 1389.4 1399.2 1415.7 1430.9 1432.5 1453.5 1493.5 1499.6 1501.3 1511.4 1514.7 1522.9 1537.7 1599.7 1604.1 1623.8 1643.5 1684.8 2987.3 3005.2 3008.1 3022.4 3023.0 3029.7 3031.0 3070.2 3086.4 3090.5 3164.0 3174.1 3185.4 3192.9 3193.6 3204.1

τ[CH] δs [CH] ArII τ[CH] ν [C−C] ArII + ArI ω[CH] δin plane [CH] NCH ω[CH] ω[CH] δs [CH] ArI + δin plane [CH] NCH δs [CH] ArII + δin plane [CH] NCH δs [CH] δs [CH] ArII δs [CH] ArII δs [CH] δs [CH] ArI δs [CH] δs [CH] ν [C−C] ArI ν [C−C] ArII ν [C−C] ArI ν [C−C] ArI + ArII ν [NC] νsym [CH] ν [CH] NCH νsym [CH] νsym [CH] νas [CH] νsym [CH] νas [CH] νas [CH] νas [CH] νas [CH] νas [CH] ArI νas [CH] ArII νas [CH] ArII νas [CH] ArII νas [CH] ArII νsym [CH] ArI

Iso

SmB

1399

1400

1434

1432

1502

1502

1597

1596

1624

1624

1683 2961

1684 2957

3032 3068

3030 3069

ν − stretching mode, δs − scissoring mode, ρ − rocking mode, ω − wagging mode, τ − twisting mode.

to 400 cm−1. The synchronous and asynchronous 2D correlation spectra were calculated by using Scilab program.11 The spectra have been baseline-corrected by an offset at 2200 cm−1. Windowed data matrices were constructed by partitioning the entire data matrix into smaller subsets containing the three spectra measured in the subsequent temperatures. Each subset was mean-centered by subtracting its average spectrum. To guide the assignment of the IR bands, density functional theory (DFT) calculations at the B3LYP/6-31 level were conducted by means of Gaussian 09.12

2. EXPERIMENTAL SECTION 4-Bromobenzylidene-4′-alkyloxyanilines (nBBAA, for n = 4−12) were synthesized by Prof. Galewski’s group at the University of Wroclaw, Poland.3 All FT-IR spectra were recorded using FTIR Bruker Spectrometer 66/s for the sample sandwiched between two ZnSe disks. The disk surfaces were coated by 1% water solution of the polyvinyl alcohol and rubbed in one direction in order to force the sample orientation. The microscopic arrangement and macroscopic disorder of domains were observed using optical polarizing microscopy. The spectra of each sample were recorded each 1 °C during cooling from 120 to 5 °C below the crystallization temperature. A single data set consisted of 35−60 spectra depending on the temperature range. The temperature was stabilized by the Gresby controller. During the experiments, the spectrometer was purged with dry nitrogen. One hundred twenty-eight scans were collected at a 4 cm−1 resolution in the spectral region from 4000

3. RESULTS AND DISCUSSION Comparison of the experimental and calculated spectra of 4BBAA is presented in Figure 1. The band assignments for 4BBAA are collected in the Table 1. Figure 2 shows the exemplary FT-IR spectra of 12BBAA measured as a function of the temperature. One can see that around 2800−3000 cm−1 the intensity and half-width of the bands corresponding to the C



Article

Figure 5. Temperature dependence of the CH stretching bands in alkyloxy chain for isotropic liquid and SmB phase of 4BBAA.

Figure 7. Moving-window analysis of 4BBAA (a) and power spectrum at the Iso−SmB (120−115 °C) phase transition from 700 to 1700 cm−1 (b) and from 2800 to 3100 cm−1 (c).

alkyloxy chain, the maxima of bands are red-shifted, suggesting changes in the ratio of gauche to trans conformers. The longer the alkyloxy chain, the more preferable the trans conformation of the alkyloxy chain is.13,14 It is known that in isotropic liquid phase the alkyloxy chain of nBBAA is molten, which means a rotational freedom between gauche and trans conformers.15 To observe the rotational freedom of alkyloxy chains in SmA and SmB phases, the spectral features due to the molecular chains were analyzed. Comparison of the spectra in the 2700−3100 cm−1 region as a function of temperature is presented in Figure 4 for 11BBAA and in Figure 5 for 4BBAA. It was found that from 6BBAA to 12BBAA the intensity and band positions remain the same at the Is-SmA and the SmA-SmB phase transitions (Figure 4). This observation shows that the alkyloxy chain is already molten in the SmA and SmB phases to the same extent as that in the isotropic liquid.16 Y. Yamamura et al.16 have shown spectroscopic evidence that the alkyl chains of nTCB (n = 2−10) are already molten in the SmE phase.17−21 One can see that in the highly ordered smectic phases of different arrangements of molecules (hexagonal in nBBAA or orthorhombic in nTCB) the chain rotational freedom is similar. For 4BBAA and 5BBAA the band shift was observed (Figure 5) at the transition from isotropic liquid to SmB phase. This observation suggests that the alkyloxy chain is fully molten in isotropic liquid phase only. Below Is−SmB phase transition the

Figure 6. Moving-window analysis of 11BBAA (a) and power spectra at the Iso−SmA (120−118 °C - blue dashed line) and SmA−SmB (117− 112 °C - red solid line) phase transitions from 700 to 1700 cm−1 (b) and from 2700 to 3100 cm−1 (c).

stretching vibrations of CH bond in the alkyloxy chain undergo significant changes in the course of the phase transition. Spectra in this region for nBBAA (n = 4−12) in the isotropic phase (Is) are collected in Figure 3. As can be seen, with elongation of the D



Article

Figure 10. Synchronous spectrum constructed from the data taken around the SmA to SmB phase transition (117−112 °C) for 11BBAA from 800 to 1700 cm−1 (a) and heterocorrelation between 800 and 1700 cm−1 and 2800−3100 cm−1 regions (b). The filled-red contours represent positive peaks while the empty-blue contours correspond to negative peaks.

Figure 8. Synchronous spectrum constructed from the data taken around the isotropic liquid to SmA phase transition (120−118 °C) for 11BBAA from 800 to 1700 cm−1 (a) and heterocorrelation between 800 and 1700 cm−1 and 2700−3100 cm−1 regions (b). The filled-red contours represent positive peaks while the empty-blue contours correspond to negative peaks.

spectral elements occurs.7,23 For that reason the moving-window analysis was carried out (see Figures 6 and 7). Peaks appear on the moving-window plot if the temperature variation induces spectral changes at particular wavenumbers. As one can see, in the case of 11BBAA the peaks appear in temperatures regions between Is−SmA, SmA−SmB, and SmB−crystal phase transition. It is worth noting that in the spectral region of the CH stretching mode (Figure 6a) no peaks appear. This confirms the suggestion of a fully molten state of the alkyloxy chain in the temperatures down to crystallization. However, for 4BBAA at the phase transition between isotropic liquid and SmB peaks appear at 815 cm−1, 828 cm−1, 842 cm−1, 1009 cm−1, 1068 cm−1, 1245 cm−1, 1502 cm−1, and 1625 cm−1 related to the alkyloxy chain and molecular rigid core vibrations (Figure 7a). This suggests that in the SmB phase the alkyloxy chain is no longer fully molten. In Figures 6b,c and 7b the changes of vibrational dynamics caused by phase transitions are illustrated in the form of the power spectra. The data were partitioned around a singular transition point and then the generalized 2D correlation analysis was performed. We selected 11BBAA and 4BBAA to show the exemplary results for the two groups of nBBAA (compare Figures 8−11 and Figures 12−13). For molecules with n from 6 to 12 the correlation analysis for isotropic liquid−SmA and SmA−SmB phase transitions yield similar synchronous and asynchronous contour plots. The synchronous spectrum constructed from the data taken around the isotropic liquid to the SmA phase transition 120−118 °C for 11BBAA shows the autopeaks at 1244 and 1502 cm−1 (Figure 8). The contour plot of the asynchronous spectrum is shown in Figure 9. All positions of the peaks are

Figure 9. Asynchronous spectrum constructed from the data taken around the isotropic liquid to SmA phase transition (120−118 °C) for 11BBAA. The asynchronous spectrum was multiplied by a sign of the corresponding synchronous spectrum and the filled-red contours represent positive peaks.

conformational arrangement of molecular chains is no longer random. The results corroborate with dielectric measurements data.11 The conformational motion of alkyloxy chains in SmB phase was clearly seen for 5BBAA. In contrast, this process was not observed in the case of 6BBAA.22 The two-dimensional correlation analysis gives no direct information on the condition in which a change of various E



Article

Figure 11. Asynchronous spectrum constructed from the data taken around the SmA to SmB phase transition (117−112 °C) for 11BBAA from 1100 to 1600 cm−1 (a) and heterocorrelation between 1100 and 1600 cm−1 and 2800−3100 cm−1 regions (b). The asynchronous spectrum was multiplied by a sign of the corresponding synchronous spectrum and the filled-red contours represent positive peaks, while the empty-blue contours correspond to negative peaks.

Figure 12. Synchronous spectrum constructed from the data taken around the Iso to SmB phase transition (120−115 °C) for 4BBAA from 800 to 1700 cm−1 (a) and heterocorrelation between 1200 and 1600 cm−1 and 2800−3100 cm−1 regions (b). The filled-red contours represent positive peaks while the empty-blue contours correspond to negative peaks.

pointed out in the Table 1S in the Supporting Information. The peaks located at 1244, 2856, and 2925 cm−1 correspond to vibrations of the alkyloxy chain, while the peak near 1502 cm−1 corresponds to vibration of the rigid core. The positive sign of the asynchronous peaks shows that the vibrations due to alkyloxy chain have intensity modulations at a higher temperature than the vibration related with the rigid core.4 This observation suggests that the phase transition isotropic to SmA starts rather by ordering the alkyloxy chain than by the biphenyl ring alignment. The synchronous and asynchronous contour plots for the SmA−SmB phase transition are presented in Figures 10 and 11. The autopeaks are located at 1244 and 1502 cm−1. The peaks at 2925 and 2964 cm−1 are related to alkyloxy chain, while the peak at 1502 is related to the stretching mode of biphenyl. The organization of molecules in the SmA−SmB phase transition in 6BBAA to 12BBAA starts by ordering of the alkyloxy chain. The synchronous and asynchronous spectra for Is−SmB phase transition in 4BBAA are shown in Figures 12 and 13. The peaks at 1502 and 1244 cm−1 correspond to the mode of biphenyl ring. The peaks at 1485, 1301, 1162, 1011, and 877 correspond to the deformation mode of the alkyloxy chain. The sign of the peaks at the asynchronous spectrum shows that the vibrations related to the rigid core have intensity modulations at a higher temperature than the vibrations of the alkyloxy chains. This observation suggests that the isotropic to the SmB phase transition in 4BBAA and 5BBAA starts by ordering of the biphenyl rings rather than of the alkyloxy chains.

Figure 13. Asynchronous spectrum constructed from the data taken around the Iso to SmB phase transition (120−115 °C) for 4BBAA. The asynchronous spectrum was multiplied by a sign of the corresponding synchronous spectrum and the filled-red contours represent positive peaks, while the empty-blue contours correspond to negative peaks.

4. CONCLUSION In the present paper, 2D correlation analysis of IR spectra of the homologous series of nBBAA (n = 4−12) in various phases was performed. From 6BBAA to 12BBAA the absorbance and the position of bands remain the same at the isotropic liquid to the SmA and SmA−SmB phase transitions. This observation shows that the alkyloxy chain in SmA and SmB phases is already molten by the same degree as that in the isotropic liquid. The 2D F



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(9) Noda, I. Generalized Two-Dimensional Correlation Method Applicable to Infrared, Raman, and Other Types of Spectroscopy. Appl. Spectrosc. 1993, 47, 1329−1336. (10) Noda, I. Determination of Two-Dimensional Correlation Spectra Using the Hilbert Transform. Appl. Spectrosc. 2000, 54, 994−999. (11) Campbell, S. L.; Hancelier, J.-P.; Nikoukhah, R. Modeling and Simulation in Scilab/Scicos, Springer: New York, USA, 2006. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09; Gaussian Inc.: Wallingford CT, USA, 2009. (13) Nabet, A.; Auger, M.; Pezolet, M. Investigation of the Temperature Behaviour of the Band Due to the Methylene Vibrations of Acyl Chains by Two-Dimensional Infrared Correlation Spectroscopy. Appl. Spectrosc. 2000, 54, 948−955. (14) Senak, L.; Moore, D.; Mendelsohn, R. CH2 Wagging Progressions as IR Probes of Slightly Disordered Phospholipid Acyl Chain States. J. Phys. Chem. 1992, 96, 2749−2754. (15) Saito, K. Microphase-Separated Multicontinuous Phase in LowMolecular-Mass Thermotropic Liquid Crystals. Pure Appl. Chem. 2009, 81, 1783−1798. (16) Yamamura, Y.; Adachi, T.; Horiuchi, K.; Sumita, M.; Yasuzuka, S.; Massalska-Arodź, M.; Urban, S.; Saito, K. Calorimetric and Spectroscopic Evidence of Chain-Melting in Smectic E and Smectic A Phases of 4-Alkyl-4′-isothiocyanatobiphenyl (nTCB). J. Phys. Chem. B 2012, 116, 9255−9260. (17) Urban, S.; Czupryński, K.; Dąbrowski, R.; Gestblom, B.; Janik, J.; Kresse, H.; Schmalfuss, H. Dielectric Studies of the nBT Homologous Series in the Isotropic and Smectic E Phases. Liq. Cryst. 2001, 28, 691− 696. (18) Urban, S.; Czub, J.; Dąbrowski, R.; Wüflinger, A. PressureTemperature Phase Diagrams for Four Higher Members of nBT Homologous Series. Phase Trans. 2006, 79, 331−342. (19) Jasiurkowska, M.; Budziak, A.; Czub, J.; Urban, S. Dielectric and X-ray Studies of Eleventh and Twelfth Members of Two Isothiocyanato Mesogenic Compounds. Acta Phys. Pol., A 2006, 110, 795−805. (20) Jasiurkowska, M.; Budziak, A.; Czub, J.; Massalska-Arodź, M.; Urban, S. X-Ray Studies on the Crystalline E Phase of the 4-n-Alkyl-4′isothiocyanatobiphenyl Homologous Series (nBT, n=2−10). Liq. Cryst. 2008, 35, 513−518. (21) Massalska-Arodź, M.; Schmalfuss, H.; Witko, W.; Kresse, H.; Wuerflinger, A. Dielectric Relaxation of the 4-n-Butyl-4′-thiocyanatobiphenyl (4TCB). Mol. Cryst. Liq. Cryst. 2001, 366, 221−227. (22) Osiecka, N.; Massalska-Arodź, M.; Galewski, Z.; Chłędowska, K.; Wróbel, S.; Morito, T.; Yamamura, Y.; Saito, K. Dynamics and Phase Transitions of 4-Bromobenzylidene-4′-pentyloxyaniline and 4-Bromobenzylidene-4′-hexyloxyanline as Studied by Dielectric Spectroscopy. Acta Phys. Pol., A 2013, in press. (23) Czarnecki, M. A. Two-Dimensional Correlation Analysis of Hydrogen Bonded System: Basic Molecules. Appl. Spectrosc. Rev. 2011, 46, 67−103. (24) Conboy, C. C.; Messmer, M. C.; Richmond, G. L. Effect of Alkyl Chain length of Simple Ionic Surfactants Adsorbed at the D2O/CCl4 Interface as Studied by Sum-Frequency Vibrational Spectroscopy. Langmuir 1998, 14, 6722−6727.

correlation analysis shows that at the phase transition from the isotropic liquid to the SmA and from SmA to the SmB phase the molecules of 6BBAA−12BBAA reorient faster as a result of organization of the alkyloxy chains. In contrast, for 4BBAA and 5BBAA at isotropic liquid to SmB phase transitions, shifts in the band positions were observed. The 2D correlation analysis reveals that for 4BBAA and 5BBAA the isotropic to SmB phase transition starts by ordering of the biphenyl rings rather than the alkyloxy chains. The CH bands are red-shifted with increasing chain length. This means a stronger tendency of the alkyloxy chains to take alltrans conformation in longer chains, as the characteristic wavenumbers of all-trans conformations are smaller than those of the gauche ones.24 The present study does not reveal change in the conformation order of the alkyloxy chain for 6BBAA−12BBAA in phase transitions. The increase in the smectic layer thickness in the SmA−SmB phase transition originates not from the conformation change of the alkyloxy chains but is due to the mutual shift of the molecules in the smectic layer, as was suggested by our XRD and DFT studies.2

■

ASSOCIATED CONTENT

S Supporting Information *

Peak positions at the synchronous and asynchronous spectra at phase transitions for 4BBAA and 11BBAA. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +48 12 662 8439. Fax: +48 12 662 8458. E-mail: Maria. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partly supported by the EU Human Capital Operation Program, Polish Project No. PKOL.04.0101-00-434/ 08-00. The DFT calculations have been performed at the ACK CYFRONET AGH in Cracow.

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REFERENCES

(1) Sorai, M.; Saito, K. Alkyl Chains Acting as Entropy Reservoir in Liquid Crystalline Materials. Chem. Rec. 2003, 3, 29−39. (2) Osiecka, N.; Budziak, A.; Galewski, Z.; Massalska-Arodź, M. X-ray Studies of the Smectic B Phase of the 4-Bromobenzylidene-4′alkoxyanilines. Phase Trans. 2012, 85, 314−321. (3) Galewski, Z. Liquid Crystalline Properties of 4-Halogenobenzylidene-4′-alkoxyanilines. Mol. Cryst. Liq. Cryst. 1994, 249, 43−49. (4) Gushiken, T.; Ujiie, S.; Ubukata, T.; Yokoyama, Y. AlkoxyphenylSubstituted Symmetric Liquid Crystalline Diamantane Derivatives. Bull. Chem. Soc. Jpn. 2011, 84, 269−282. (5) Kang, Y.-S.; Zin, W.-C. Liquid Crystalline Behaviour of HydrogenBond Complexes of Alkoxycinnemic Acids with Octyloxystilbazole. Liq. Cryst. 2002, 29, 369−375. (6) Kirov, N.; Simova, P. Vibrational Spectroscopy of Liquid Crystals; Publishing House of the Bulgarian Academy of Sciences: Sofia, Bulgaria, 1984. (7) Thomas, M.; Richardson, H. Two-Dimensional FT-IR Correlation Analysis of the Phase Transitions in a Liquid Crystal, 4′-n-Octyl-4cyanobiphenyl (8CB). Vib. Spectrosc. 2000, 24, 137−146. (8) Noda, I.; Ozaki, Y. Two-Dimensional Correlation Spectroscopy − Applications in Vibrational and Optical Spectroscopy; Wiley: Chichester, England, 2004. G


FT-IR and Two-Dimensional Correlation Analysis of the Liquid

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