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Low-temperature photoluminescence of 5CB liquid crystal
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1134–1141
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Low-temperature photoluminescence of 5CB liquid crystal T. Bezrodna n, V. Melnyk, V. Vorobjev, G. Puchkovska Institute of Physics, National Academy of Sciences Ukraine, 46 Nauki Prosp., Kyiv 03022 Ukraine
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 September 2008 Received in revised form 13 December 2009 Accepted 4 February 2010 Available online 10 February 2010
The photoluminescence spectra of the 4-n-pentyl-40 -cyanobiphenyl (5CB) liquid crystal have been investigated in detail at low temperatures 4.2–200 K, for the first time. The spectral data obtained are compared to the results of the luminescence study for the 5CB nematic phase at T= 300 K. The luminescence of the 5CB crystal state is characterized by emissions of both monomer and dimer structures. Besides, there are several energetically and conformationally non-equivalent types of monomers and dimers, and their amounts change with the temperature growth non-monotonously. The 5CB several crystal modifications, having different 5CB monomer and dimer conformers, have been found out below T= 160 K. The 5CB crystal–crystal transition at T= 80 K is characterized with a total loss of the fine structure in the 5CB photoluminescence spectrum and a disappearance of the spectral band at 343 nm. At T= 140 K, a new spectral band at 424 nm appears; it corresponds to the significantly overlapping 5CB dimers, its intensity grows under further heating. The present investigation gives a tool for the further characterization of the molecular alignments and changes in the 5CB molecular conformations, using the monomer and excimer fluorescence emissions as a probe. The conclusions made are confirmed by the IR-spectroscopy data, measured and analyzed for the 5CB in the same temperature region. & 2010 Elsevier B.V. All rights reserved.
Keywords: 5CB Photoluminescence Monomer and excimer emission IR-spectroscopy
1. Introduction 4-n-pentyl-40 -cyanobiphenyl (5CB) is one of the most studied liquid crystal materials, since it has an available nematic phase near room temperature and a simple molecular structure. These facts allow using this compound as a suitable model substance in the investigations of the physical behaviour of the simple nematics. 5CB belongs to an important class of thermotropic liquid crystals, nowadays widely used in the development of new functional materials for the modern devices in optoelectronics, information display, storage technologies, etc. [1,2]. These are mainly caused by some physico-chemical characteristics of cyanobiphenyls, such as weak absorption in the visible spectral region, chemical stability, high dielectric and optical anisotropy, and also by the presence of stable mesophases. Since the works of Shashidar and Venkatesh [3], Chielewski [4], Leadbetter et al. [5] and Dong [6], substantial efforts have been devoted to the investigations of structural transformation, intermolecular order and orientational molecular dynamics in the isotropic and nematic phases of 5CB liquid crystal. To date, little information is available from the low temperature phase transformations. In spite of the fact that the main interest on liquid crystals is based
n
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0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.02.009
on the particular properties of the fluid anisotropic phases that they are able to develop, in some cases the low-temperature properties of liquid crystals, i.e. the detail characteristics of their crystalline state, are important to be known. Nowadays, 5CB is widely used in composite materials as organic fillers in polymer or inorganic oxide matrices [7–10]. In such heterogeneous systems, even at room temperature 5CB often forms a state, close to the glass-like or crystalline ones, since the mobility of its molecules is significantly limited due to the confinement effects. Therefore, this detail spectroscopic investigation of the 5CB solidstate phases will assist the analysis of the structural and temperature changes, occurring in 5CB in the composites of the ‘‘guest-host’’ type. In particular, we have started the investigations of such complex materials based on organo-modified clay minerals. On the other hand, the low-temperature photoluminescence is known to be very sensitive even to small changes in the structure of the crystalline solid, its defects and dopants, so this experimental method is an efficient tool to study the composite materials of different nature. Moreover, 5CB is one of the model compounds whose structure can be theoretically described for different phase states. The results of our work can be useful for the calculations mentioned. Furthermore, cyanobiphenyls were found to form excimers, which could provide us with additional information on orientational ordering [11]. In fact, excimer formation in liquid crystal systems is a topic of current interest relating to how the morphology of the chromophores in the ground state affects excimer formation.
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First, Subramanian et al. [11] reported the fluorescent behaviour of a luminescent liquid crystal, dodecylcyanobiphenyl (12CB). It has been demonstrated that two kinds of fluorescence are emitted from 12CB in various phases. They interpreted the dual fluorescence emission in terms of singlet monomer and excimer formation. Since that, a number of papers concerning the fluorescence behaviour of cyanobiphenyls appeared [12–20]. In these works the main attention has been paid to the investigations for homogeneous solutions of cyanobiphenyls, as well as for CB’s pure materials in different mesogenic and isotropic liquid phases. It should be pointed out that the number of studies dealing with a solid phase of liquid crystals is surprisingly low in comparison with the number of papers dedicated to the mesophases themselves. Moreover, despite the presence of some literature data on the 5CB photoluminescence at low temperatures, the temperature evolution of the spectrum remains undiscovered, and also there are significant disagreements between different authors concerning the shape of the spectral contour, and its interpretation in terms of the formation of dimer and monomer structures. Excited-singlet 5CB (15CBn) strongly emits at 330 nm with a short lifetime ( 1 ns) at low concentrations (Eq. (1)) and competitively forms the excimer 1(5CB)n2 at higher concentrations, which efficiently emits at 400 nm with a longer lifetime ( 10 ns) (Eq. (2)) [15]. The shorter- and longer-wavelength emissions are hereafter abbreviated as ‘‘monomer emission’’ and ‘‘excimer emission’’, respectively. In the nematic phase of neat nCB, the excimer emission appears exclusively, because the molecular core packing is favourable for excimer formation [14– 16]. In the isotropic phase, on the other hand, both the monomer and excimer emissions comparably contribute to the net fluorescence, since randomly oriented nCB molecules should reorient and diffuse in the viscous fluid to form the excimer in competition with the fast decay of 1nCBn: 5CB þ hv-1 5CB -5CB þhvM 1
5CB þ 5CB-1 ð5CBÞ2 -5CB þ5CB þhvE
ð1Þ ð2Þ
In this work, we have carried out the detail analysis of the 5CB photoluminescence spectral contour at low temperatures and its changes at heating, and also we give the assignment of the spectral components. The spectral changes are connected with the formation and destruction of different monomer and dimer structures of the 5CB molecules, and also their conformational changes. The conclusions made have been proved by the results of the IR-spectroscopy experiments on the 5CB liquid crystal, measured in the same temperature region.
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copper holder, and the whole system is inserted to UTREX helium cryostat with a precise temperature control within an accuracy of 70.11. All the IR absorption spectra of 5CB liquid crystal have been measured on a Bruker IFS-88 FTIR-spectrometer, spectral range 400–4000 cm 1 at 10–300 K temperatures. The spectral resolution was 1 and 4 cm 1, and the number of scans was 64. Lowtemperature measurements were carried out in a nitrogen cryostat with a vacuum equipment and a temperature control system with an accuracy of not worse than 0.1 K. For these experiments, 5CB is held between two polished KBr plates. A drop of 5CB is placed on one salt plate, covered with another and slightly pressed to remove any air pockets and to spread the sample evenly. The PEAKFIT program set has been used to analyze the spectral shape of the photoluminescence and IR absorption bands and to make a graphical separation of them on the spectral components. The use of Gaussian distribution allows to fit perfectly our experimental data, providing all the necessary characteristics for each spectral component (spectral intensity, peak position, halfwidth, etc.).
3. Results and discussions 3.1. Excitation spectra Excitation spectra of 5CB have been measured on the same spectrofluorimeter and have been performed in the following way. First, the general scanning by the continuous excitation spectrum shows that the 5CB liquid crystal emits at the spectral region 380 nm. So when the excitation spectra of 5CB are recorded, their registration was done exactly at this wavelength. The excitation spectra of 5CB at the helium and room temperatures are shown in Fig. 1. Here the x-axis is for the excitation wavelength. One can see that the temperature growth results in the increase of the 5CB emission intensity, some broadening of the spectral contour and a shift of the own absorption edge (this value is connected with the excitation spectrum) towards high wavelengths. The data obtained at room temperature are in good agreement with the 5CB absorption spectra presented in Ref. [6], where the change of the spectral pattern was also detected under the transition from the nematic phase to the isotropic liquid state. However, the origin of spectral differences mentioned with temperature is not clear at the present stage. Based on the
2
The 5CB (99%) was donated from Merk (Germany) and was used as received. The temperatures of crystal – LC and LC – isotropic liquid phase transitions in 5CB were 22.5 and 35 1C, respectively [21,22]. Fluorescence techniques have been used as a highly sensitive probe to explore molecular aggregation and microscopic environments surrounding chromophores. Photoluminescence spectra of 5CB liquid crystal have been recorded on MPF-4 spectrofluorimeter (Hitachi) in the spectral region 300–500 nm and the temperature range 4.2–200 K, and at T= 300 K for a comparison with a nematic state of 5CB liquid crystal. In this experiment, we have measured the samples of the bulk 5CB, since its films can significantly differ by structure. The 5CB compound is placed in the quartz tubes with an inner diameter of 5 mm. The 5CB sample column is 25 mm. The quartz tube is attached to the special
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spectral data received we chose 315 nm as an excitation wavelength for our photoluminescence experiments.
3.2. Photoluminescence spectra Fig. 2 shows the emission spectra of 5CB at helium (4.2 K) and room (300 K) temperatures, where 5CB has its crystal and nematic phases, respectively. The excitation wavelength is lex = 315 nm. Besides, it should be noted that photoluminescence spectra, measured at T=300 K for the initial 5CB liquid crystal and this substance after it has been cooled down to the helium temperature and then heated to the room temperature, are the same. This fact states the absence of the delayed crystal state—nematic liquid crystal phase transition in our case. The luminescence spectra measured at T=4.2 and 300 K are seen to be very different from each other by the spectral shape, and also by other spectral parameters (intensity, width, etc.). The luminescence spectrum of 5CB at room temperature is completely structureless and red-shifted with respect to that of 5CB at T= 4.2 K from 370 to 390 nm. Besides, the heating from low temperatures to room temperature results in the decrease of the emission intensity on 17% and some narrowing of the whole spectrum on several nanometers. Under the heating, the 5CB luminescence band peak position is seen to shift monotonously towards the long wavelengths, whereas the spectral half-width changes irregularly, having at least two ‘‘turning points’’ at temperatures 80 and 140 K. It should be noted that the total intensity of the 5CB luminescence spectrum changes nonmonotonously at heating, having its maximum value at T= 80 K. In the 5CB crystalline state the alkyl chains of 5CB molecules are in the trans-conformation, and it is oriented almost perpendicularly to the plane of the nearest benzene ring (the torsion angle is 90.31), and the torsion angle between the normals of the two phenyl rings is equal to 26.31 [23]. The polar structure of 5CB molecules (the dipole moment is 4.0 D) leads to strong dipole interactions, causing conformational changes in the 5CB molecules (the molecules become more flat) and the formation of the dimers. The results of quantum chemical calculations (CNDO/ S) on planar and twisted cyanobiphenyl model compounds in the gas phase showed that the dipole moment of the excited state (S1) of the 5CB molecule in the planar conformation (8.3 D) is higher than this value in the 5CB molecule in the twisted 901 conformation (4.9 D) [14]. This fact can explain the drop of the
1
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Fig. 2. Photoluminescence spectra of 5CB: 1—in the crystal (T= 4.2 K) and 2—nematic (T= 300 K) phases.
5CB total emission intensity under the temperature growth. Also, these authors showed that the energy gap between the lowest energy (ground state S0) conformation and the first excited one (S1) decreases at the transition from the twisted conformation to the planar one of the 5CB molecules. This explains the red shift of the 5CB luminescence spectra at the temperature growth. It is worth mentioning that in the solutions of 5CB at the increase of the solvent polarity the 5CB luminescence spectra also lose their structure and shift towards the long wavelengths [15,20]. As in the case of the spectral red shift at the temperature growth, this effect is explained by the increase of the dipole moment of the 5CB molecule excited state. In the solutions, excimer fluorescence is known to consist of a broad, unstructured emission in the range 380–390 nm, almost independent of solvent polarity since the excited dipole moment of excimers has a very low or null value [15]. In other words, the fine structure of the 5CB luminescence spectra in solutions and at low temperatures is mainly determined by the monomer emission; however, the nature of this spectral similarity between the 5CB luminescence at low temperatures and in the non-polar solvents is not absolutely clear to understand. Probably, the last fact mentioned is connected with very weak interactions between the 5CB molecules and their surroundings in both cases, resulting in the formation of similar configurations of emission centers, and finally, the similarity of the 5CB luminescence spectra structure. It is a common opinion for cyanobiphenyl compounds that their luminescence is characterized by the emission of monomers in the crystalline state and of both monomers and excimers in the mesogenic and isotropic liquid phases. Nevertheless, in the case of 12CB liquid crystal the excimer-like fluorescence appears even in the crystalline phase [11]. Moreover, some authors [17,18] made an assumption about the simultaneous monomer and excimer emission in the crystal of 5CB. The strong overlap of our luminescence spectra, obtained for 5CB at very low and room temperatures can confirm the last statement. However, up to now, there is no definite conclusion made on this matter. In order to analyze the 5CB luminescence spectra in detail, we made a graphical separation of the complex emission band into several components, as shown in Fig. 3 for temperatures (a) 4.2, (b) 100, (c) 200 and (d) 300 K. At T= 4.2 K (Fig. 3a) the 5CB luminescence band consists of at least four components: 343, 360, 383 and 407 nm. By analogy with the 5CB luminescence data presented for its nematic liquid crystal phase [16,18,24,25], and in the diluted and concentrated solutions of cyclohexane and ethanol [15], spectral components at 343 and 360 nm can be ascribed to the monomer emission, and 383 and 407 nm bands to the excimer fluorescence. The 5CB crystals have a crystal lattice of a monoclinic type with a centrosymmetrical space group (P21/a) and the following ˚ the monoclinic angle parameters, a=8.25, b=16.02 and c=10.94 A, is equal to 951 [26]. The number of molecules in the elementary unit is Z=4. The analysis of the experimental results obtained by the X-ray diffraction method, concerning a determination of the intermolecular distances between a cyano-group of one molecule layer and the nearest phenyl ring of another one showed the possibility of two types for the molecule alignments: parallel and anti-parallel ones. When the anti-parallel orientation is realized, the CN-group of one molecule is located near the nearest phenyl ring CH2-group of a pentyl chain of the neighbour molecule with an overlap of the biphenyl cores, and a distance between the molecules from the neighbouring layers is less, compared to a case of the molecule parallel orientation. Probably, these two non-equivalent states of the 5CB molecules in a crystal explain the presence of two spectral components in the 5CB luminescence spectra, 343 and 360 nm, corresponding to the monomer emission.
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Fig. 3. Graphical separations of the 5CB photoluminescence spectral contours: (a) T= 4.2 K; (b) T= 100 K; (c) T =200 K; and ( d) T= 300 K.
This can be supported by a potential model, proposed by Rapp and co-workers [27] to explain the appearance and loss of fine structure in the 9CB luminescence spectra. This model accounts for steric hindrance to planarity at certain temperatures and the intramolecular twisting relaxation towards planarity at other temperatures. In other words, the conformations with more or less range of the twist angles are able to emit at different temperatures, causing different luminescence spectral contours. For the formation of excimers in bulk phases, two monomer units must come close to one another to form a sandwich pair. It has been suggested from X-ray data that these pairs are preformed in the ground state, and that their overlap is such that the length of the dimer becomes about 1.6 times the monomer length [28,29]. Further, time-resolved luminescence measurements proved a model of planar preformed pairs of chromophores in the cyanobiphenyl liquid crystals [14]. These results can indirectly support our further conclusion that even at low temperatures some part of 5CB molecules exists in the dimer state and is characterized by excimer photoluminescence, seen as spectral components at 383 and 407 nm for the 5CB at T=4.2 K (Fig. 3a).
Each spectral component of the 5CB photoluminescence spectrum has its own temperature behaviour at heating up to 300 K. These dependencies are non-monotonous and allow one to conclude about some abrupt re-alignments of the 5CB crystal structure at T= 80 and 140 K. As is seen from Fig. 3b, the crystal phase above T= 80 K is characterized with a loss of the spectral fine structure, and a disappearance of the spectral band at 343 nm. Further heating leads to a new band at 424 nm (Fig. 3c). At T= 300 K the spectral contour of the 5CB luminescence can be also separated into four spectral components with their peak positions at 362, 383, 404 and 424 nm (Fig. 3d). Compared to the spectrum at T= 4.2 K (Fig. 3a), at room temperature the only component, which corresponds to the monomer emission and remains in the spectrum, is 362 nm. However, its intensity decreases significantly under heating (Fig. 3). This result could be expected, since the nematic liquid crystal phase is characterized mainly by dimer structures [30,31]. According to Klock et al. [14], the 5CB dimer sandwich-type structures can be more fully overlapping and more extended (Scheme 1). As a result, they are characterized with different
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Table 1 Peak positions of the photoluminescence spectral components and their assignments. Spectral components at T= 4.2 K (nm)
Spectral components at T= 300 K (nm)
344 361 383
363 383
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Assignment
Monomer emission Monomer emission Excimer emission (Scheme 1c) Excimer emission (Scheme 1a) Excimer emission (Scheme 1b)
dimers to the other dimer structures of the nematic phase. The spectral position of this component in relation to the other luminescence bands of the 5CB excimers corresponds to its position in the energy scale for the mentioned dimer structures. This is an additional confirmation of our assignment made for the spectral bands. Table 1 summarizes the data obtained here for the 5CB luminescence and the assignments of the spectral bands.
3.3. IR-spectroscopy data
Scheme 1. Different possible 5CB excimer structures.
photoluminescence bands, explaining the appearance of several spectral components for excimer luminescence. It is reasonable to suggest, that the luminescence spectral component at 407 nm (T= 4.2 K) and 404 nm (T= 300 K) corresponds to the first type of the possible dimer structures (Scheme 1a), since this component significantly raises its intensity at heating. The last fact evidences the formation of the 5CB ‘‘classic’’ dimers, which are typical of the 5CB nematic liquid crystal phase [32]. At the same time, at higher temperatures, the denser packing of the nematic phase, compared to the crystal one, allows forming more overlapping dimers of 5CB molecules (Scheme 1b), which are characterized by the appearance of a new luminescence band at 424 nm. At least, two types of 5CB excimer structures were also predicted experimentally in Ref. [18]. In addition to the dimer structures mentioned (Scheme 1a, b), another excimer type can be proposed involving a self-solvated twisted intramolecular charge transfer state (Scheme 1c), i.e. a twisted intramolecular charge transfer state molecule interacting with a ground one of the same species [33]. The existence of these types of excimers was proved for 9CB liquid crystal [14], but there is no clear data about its spectral position in the luminescence contour. These structures are quite possible to be characterized with a spectral component at 383 nm, which is seen at both low and room temperatures. Its intensity does not practically change with temperature growth, decreasing only a little, which is probably an evidence of a partial transition of this type of 5CB
IR-spectroscopy allows studying the structure formation and conformation changes of 5CB in different phase states. The molecular vibrational analysis is well-known as a useful tool for directly detecting molecular conformations [34,35]. The results described above on the 5CB photoluminescence spectra have led to the conclusion about the simultaneous presence of the monomer and dimer structures in different crystalline states and nematic liquid crystal phase of this substance. This statement is well proved by the IR-spectroscopy data, analyzed in the region of the C N bond stretching vibrations of the 5CB molecules at different temperatures (Fig. 4). The graphical separation of this spectral band allows one to select two components at 2226 and 2229 cm 1, which according to Chandrasekar [36], are assigned to dimers and monomers of the 5CB molecules, respectively. It is seen that some amount of the 5CB dimers ( 11%) are presented even at T= 10 K, i.e. in the low-temperature crystalline state of 5CB (Fig. 4a). The temperature increase leads to the growth of the relative quantity of the 5CB dimer structures, reaching 36% at T= 100 K (Fig. 4b). The crystal–crystal phase transition at T= 140 K is characterized by the appearance of the domination of the 5CB dimer structures over the monomer ones ( 63% at T= 200 K, Fig. 4c), while the negligible amount of the monomers remains in the 5CB nematic liquid crystal phase ( 3%, Fig. 4d). The structure transformations mentioned above are reflected in the changes of the IR-spectral bands, corresponding to the CCH deformation vibrations of the 5CB phenyl rings (Fig. 5a, b). The formation of the dimer structures occurs due to the dipole–dipole interactions of the 5CB molecules, which dipole moment is mainly located on the CN-group. Thus, the most significant spectral changes concern the stretching vibrations of the C N bond (described above) and the deformation vibrations of the phenyl ring, close to this bond in the 5CB molecule. The vibrations of this ring in the IR-spectra are characterized by the spectral bands at 1186, 1286 and 1301 cm 1, thier intensities increase significantly at the sequential transitions from one crystalline phase to another and drop abruptly in the nematic liquid crystal state (Fig. 5a, b). Concerning other IR-spectral features of the 5CB liquid crystal, up to the crystal—nematic liquid crystal phase transition the spectral bands, corresponding to the deformation vibrations of the
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Fig. 4. The graphical separations of the 5CB IR-spectral band, corresponding to the C N bond stretching vibrations: (a) T= 4.2 K; (b)T= 100 K; (c) T=200 K; and (d) T= 300 K.
phenyl ring, close to the 5CB molecule alkyl chain (1180 cm 1, Fig. 5a), and of the CH2-groups of the alkyl chain (809, 828, 840 cm 1, Fig. 5c) do not practically change with the temperature growth. This proves the small changes of the alkyl chain conformation at heating, but according to Babkov et al. [34] and Tanaka et al. [35], at low temperatures the 5CB liquid crystal is already a mix of different conformers, where relative quantities do not change up to the nematic liquid-crystal state. Obviously, the simultaneous co-existence of the different conformers of the 5CB molecules in the low-temperature states explains the formation of different types of monomer and dimer structures, revealed by us during the analysis of the 5CB photoluminescence spectral properties. It is interesting to notice that the DSC and X-ray diffraction experiments allowed us to detect and describe several crystal modifications of 5CB, which were realized at different temperature histories [37]. So the suggestion about the formation of additional structure re-alignments in the crystal modification of 5CB is quite reasonable. Further investigations of the low-temperature states for the 5CB liquid crystal by other experimental techniques can give more detailed information about the structure transitions mentioned and explain their mechanisms.
4. Conclusions For the first time, the photoluminescence spectra of the 5CB liquid crystal have been investigated in detail at low temperatures 4.2–200 K. The spectral data obtained are compared with the results of the luminescence study for the 5CB nematic phase at T=300 K. The luminescence of the 5CB crystal state is characterized by emissions of both monomer and dimer structures. Besides, there are several energetically and conformationally non-equivalent types of monomers and dimers, and their amounts change with the temperature growth nonmonotonously. The 5CB several crystal modifications, having different 5CB monomer and dimer conformers, have been found out below T=200 K. The 5CB crystal–crystal transition at T= 80 K is characterized with a total loss of the fine structure in the 5CB photoluminescence spectrum and a disappearance of the spectral band at 343 nm. At T =140 K, a new spectral band at 424 nm appears, which corresponds to the significantly overlapping 5CB dimers, and its intensity grows under further heating. Unfortunately, the nature of these structure changes in the crystal state of 5CB cannot be unambiguously explained at the present time.
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Fig. 5. The IR-spectral fragments of the 5CB at different temperatures in the regions of the deformation vibrations of the CCH phenyl rings (a, b) and CH2-groups of the alkyl chain (c).
The simultaneous presence of the 5CB monomer and dimer structures in different crystalline and nematic liquid crystal states is proved by the IR-spectroscopy data in the region of the C N group stretching vibrations. The formation of the dipole–dipole bonded dimers affects the deformation vibrations of the 5CB phenyl ring, close to the CN-group, indicating some changes in the conformations of the 5CB molecules. The vibrations of other fragments in the 5CB molecules are slightly influenced up to the transition to the nematic liquid crystal phase. This investigation has demonstrated that the fluorescence methods using the monomer and excimer emissions of 5CB as the probe are potentially useful and sensitive for characterizing molecular alignments and changes in the molecular conformations.
Acknowledgements The authors would like to thank Prof. Jan Baran (Wroclaw, Poland) for his help in the IR-spectroscopic measurements. This work was partially funded by the NAS of Ukraine under the Program ‘‘Nanophysics and Nanoelectronics’’, project VC-138.
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[20] J.I. Kukielski, J. Mol. Struc. 478 (1999) 1. [21] S. Zhdanov, Liquid Crystals, Chemistry, Moscow, 1979 328 pp. (in Russian). [22] T. Mansare, R. Decressain, C. Gors, V. Dolganov, Mol. Cryst. Liq. Cryst. 382 (2002) 97. [23] S. Sinton, A. Pines, Chem. Phys. Lett. 76 (1980) 263. [24] T. Ikeda, S. Kurihara, S. Tazuke, J. Phys. Chem. 94 (1990) 6550. [25] T. Ikeda, S. Kurihara, S. Tazuke, Liq. Cryst. 7 (1990) 749. [26] T. Hanemann, W. Haase, I. Svoboda, H. Fuess, Liq. Cryst. 19 (1995) 699. [27] G. Swiatkowski, R. Menzel, W. Rapp, J. Lumin. 37 (1987) 183. [28] P. Cladis, D. Guillon, E. Bouchet, P.L. Finn, Phys. Rev. A 23 (1981) 2594. [29] L. Londa, W. de Jeu, Phys. Rev. A 26 (1982) 1632. [30] W. Haase, Z. Fan, H. Muller, J. Chem. Phys. 89 (1988) 3317.
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[31] M. Cook, M. Wilson, Liq. Cryst. 27 (2000) 1573. [32] C. Amovilli, I. Cacelli, S. Campanile, G. Prampolini, J. Chem. Phys. 117 (2002) 3003. [33] Z. Grabowski, K. Rotkiewicz, H. Leismann, W. Rettig, J. Photochem. Photobiol. A Chem. 49 (1989) 347. [34] L. Babkov, I. Gnatyuk, G. Puchkovska, S. Truhachev, Zh. Struk. Khim. 43 (2002) 1098 in Russian. [35] S. Tanaka, Ch. Kato, K. Horie, J. Mol. Struc. 735–736 (2005) 27. [36] S. Chandrasekar, in: Liquid Crystals, Cambridge University Press, Cambridge, 1977, p. 135–143. [37] T. Mansare, R. Decressain, C. Gors, V. Dolganov, Mol. Cryst. Liq. Cryst. 382 (2002) 97.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Low-temperature photoluminescence of 5CB liquid crystal T. Bezrodna n, V. Melnyk, V. Vorobjev, G. Puchkovska Institute of Physics, National Academy of Sciences Ukraine, 46 Nauki Prosp., Kyiv 03022 Ukraine
a r t i c l e in f o
a b s t r a c t
Article history: Received 21 September 2008 Received in revised form 13 December 2009 Accepted 4 February 2010 Available online 10 February 2010
The photoluminescence spectra of the 4-n-pentyl-40 -cyanobiphenyl (5CB) liquid crystal have been investigated in detail at low temperatures 4.2–200 K, for the first time. The spectral data obtained are compared to the results of the luminescence study for the 5CB nematic phase at T= 300 K. The luminescence of the 5CB crystal state is characterized by emissions of both monomer and dimer structures. Besides, there are several energetically and conformationally non-equivalent types of monomers and dimers, and their amounts change with the temperature growth non-monotonously. The 5CB several crystal modifications, having different 5CB monomer and dimer conformers, have been found out below T= 160 K. The 5CB crystal–crystal transition at T= 80 K is characterized with a total loss of the fine structure in the 5CB photoluminescence spectrum and a disappearance of the spectral band at 343 nm. At T= 140 K, a new spectral band at 424 nm appears; it corresponds to the significantly overlapping 5CB dimers, its intensity grows under further heating. The present investigation gives a tool for the further characterization of the molecular alignments and changes in the 5CB molecular conformations, using the monomer and excimer fluorescence emissions as a probe. The conclusions made are confirmed by the IR-spectroscopy data, measured and analyzed for the 5CB in the same temperature region. & 2010 Elsevier B.V. All rights reserved.
Keywords: 5CB Photoluminescence Monomer and excimer emission IR-spectroscopy
1. Introduction 4-n-pentyl-40 -cyanobiphenyl (5CB) is one of the most studied liquid crystal materials, since it has an available nematic phase near room temperature and a simple molecular structure. These facts allow using this compound as a suitable model substance in the investigations of the physical behaviour of the simple nematics. 5CB belongs to an important class of thermotropic liquid crystals, nowadays widely used in the development of new functional materials for the modern devices in optoelectronics, information display, storage technologies, etc. [1,2]. These are mainly caused by some physico-chemical characteristics of cyanobiphenyls, such as weak absorption in the visible spectral region, chemical stability, high dielectric and optical anisotropy, and also by the presence of stable mesophases. Since the works of Shashidar and Venkatesh [3], Chielewski [4], Leadbetter et al. [5] and Dong [6], substantial efforts have been devoted to the investigations of structural transformation, intermolecular order and orientational molecular dynamics in the isotropic and nematic phases of 5CB liquid crystal. To date, little information is available from the low temperature phase transformations. In spite of the fact that the main interest on liquid crystals is based
n
Corresponding author. Tel.: + 38 044 525 1552; fax: + 38 044 525 1589. E-mail address: [email protected] (T. Bezrodna).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.02.009
on the particular properties of the fluid anisotropic phases that they are able to develop, in some cases the low-temperature properties of liquid crystals, i.e. the detail characteristics of their crystalline state, are important to be known. Nowadays, 5CB is widely used in composite materials as organic fillers in polymer or inorganic oxide matrices [7–10]. In such heterogeneous systems, even at room temperature 5CB often forms a state, close to the glass-like or crystalline ones, since the mobility of its molecules is significantly limited due to the confinement effects. Therefore, this detail spectroscopic investigation of the 5CB solidstate phases will assist the analysis of the structural and temperature changes, occurring in 5CB in the composites of the ‘‘guest-host’’ type. In particular, we have started the investigations of such complex materials based on organo-modified clay minerals. On the other hand, the low-temperature photoluminescence is known to be very sensitive even to small changes in the structure of the crystalline solid, its defects and dopants, so this experimental method is an efficient tool to study the composite materials of different nature. Moreover, 5CB is one of the model compounds whose structure can be theoretically described for different phase states. The results of our work can be useful for the calculations mentioned. Furthermore, cyanobiphenyls were found to form excimers, which could provide us with additional information on orientational ordering [11]. In fact, excimer formation in liquid crystal systems is a topic of current interest relating to how the morphology of the chromophores in the ground state affects excimer formation.
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First, Subramanian et al. [11] reported the fluorescent behaviour of a luminescent liquid crystal, dodecylcyanobiphenyl (12CB). It has been demonstrated that two kinds of fluorescence are emitted from 12CB in various phases. They interpreted the dual fluorescence emission in terms of singlet monomer and excimer formation. Since that, a number of papers concerning the fluorescence behaviour of cyanobiphenyls appeared [12–20]. In these works the main attention has been paid to the investigations for homogeneous solutions of cyanobiphenyls, as well as for CB’s pure materials in different mesogenic and isotropic liquid phases. It should be pointed out that the number of studies dealing with a solid phase of liquid crystals is surprisingly low in comparison with the number of papers dedicated to the mesophases themselves. Moreover, despite the presence of some literature data on the 5CB photoluminescence at low temperatures, the temperature evolution of the spectrum remains undiscovered, and also there are significant disagreements between different authors concerning the shape of the spectral contour, and its interpretation in terms of the formation of dimer and monomer structures. Excited-singlet 5CB (15CBn) strongly emits at 330 nm with a short lifetime ( 1 ns) at low concentrations (Eq. (1)) and competitively forms the excimer 1(5CB)n2 at higher concentrations, which efficiently emits at 400 nm with a longer lifetime ( 10 ns) (Eq. (2)) [15]. The shorter- and longer-wavelength emissions are hereafter abbreviated as ‘‘monomer emission’’ and ‘‘excimer emission’’, respectively. In the nematic phase of neat nCB, the excimer emission appears exclusively, because the molecular core packing is favourable for excimer formation [14– 16]. In the isotropic phase, on the other hand, both the monomer and excimer emissions comparably contribute to the net fluorescence, since randomly oriented nCB molecules should reorient and diffuse in the viscous fluid to form the excimer in competition with the fast decay of 1nCBn: 5CB þ hv-1 5CB -5CB þhvM 1
5CB þ 5CB-1 ð5CBÞ2 -5CB þ5CB þhvE
ð1Þ ð2Þ
In this work, we have carried out the detail analysis of the 5CB photoluminescence spectral contour at low temperatures and its changes at heating, and also we give the assignment of the spectral components. The spectral changes are connected with the formation and destruction of different monomer and dimer structures of the 5CB molecules, and also their conformational changes. The conclusions made have been proved by the results of the IR-spectroscopy experiments on the 5CB liquid crystal, measured in the same temperature region.
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copper holder, and the whole system is inserted to UTREX helium cryostat with a precise temperature control within an accuracy of 70.11. All the IR absorption spectra of 5CB liquid crystal have been measured on a Bruker IFS-88 FTIR-spectrometer, spectral range 400–4000 cm 1 at 10–300 K temperatures. The spectral resolution was 1 and 4 cm 1, and the number of scans was 64. Lowtemperature measurements were carried out in a nitrogen cryostat with a vacuum equipment and a temperature control system with an accuracy of not worse than 0.1 K. For these experiments, 5CB is held between two polished KBr plates. A drop of 5CB is placed on one salt plate, covered with another and slightly pressed to remove any air pockets and to spread the sample evenly. The PEAKFIT program set has been used to analyze the spectral shape of the photoluminescence and IR absorption bands and to make a graphical separation of them on the spectral components. The use of Gaussian distribution allows to fit perfectly our experimental data, providing all the necessary characteristics for each spectral component (spectral intensity, peak position, halfwidth, etc.).
3. Results and discussions 3.1. Excitation spectra Excitation spectra of 5CB have been measured on the same spectrofluorimeter and have been performed in the following way. First, the general scanning by the continuous excitation spectrum shows that the 5CB liquid crystal emits at the spectral region 380 nm. So when the excitation spectra of 5CB are recorded, their registration was done exactly at this wavelength. The excitation spectra of 5CB at the helium and room temperatures are shown in Fig. 1. Here the x-axis is for the excitation wavelength. One can see that the temperature growth results in the increase of the 5CB emission intensity, some broadening of the spectral contour and a shift of the own absorption edge (this value is connected with the excitation spectrum) towards high wavelengths. The data obtained at room temperature are in good agreement with the 5CB absorption spectra presented in Ref. [6], where the change of the spectral pattern was also detected under the transition from the nematic phase to the isotropic liquid state. However, the origin of spectral differences mentioned with temperature is not clear at the present stage. Based on the
2
The 5CB (99%) was donated from Merk (Germany) and was used as received. The temperatures of crystal – LC and LC – isotropic liquid phase transitions in 5CB were 22.5 and 35 1C, respectively [21,22]. Fluorescence techniques have been used as a highly sensitive probe to explore molecular aggregation and microscopic environments surrounding chromophores. Photoluminescence spectra of 5CB liquid crystal have been recorded on MPF-4 spectrofluorimeter (Hitachi) in the spectral region 300–500 nm and the temperature range 4.2–200 K, and at T= 300 K for a comparison with a nematic state of 5CB liquid crystal. In this experiment, we have measured the samples of the bulk 5CB, since its films can significantly differ by structure. The 5CB compound is placed in the quartz tubes with an inner diameter of 5 mm. The 5CB sample column is 25 mm. The quartz tube is attached to the special
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spectral data received we chose 315 nm as an excitation wavelength for our photoluminescence experiments.
3.2. Photoluminescence spectra Fig. 2 shows the emission spectra of 5CB at helium (4.2 K) and room (300 K) temperatures, where 5CB has its crystal and nematic phases, respectively. The excitation wavelength is lex = 315 nm. Besides, it should be noted that photoluminescence spectra, measured at T=300 K for the initial 5CB liquid crystal and this substance after it has been cooled down to the helium temperature and then heated to the room temperature, are the same. This fact states the absence of the delayed crystal state—nematic liquid crystal phase transition in our case. The luminescence spectra measured at T=4.2 and 300 K are seen to be very different from each other by the spectral shape, and also by other spectral parameters (intensity, width, etc.). The luminescence spectrum of 5CB at room temperature is completely structureless and red-shifted with respect to that of 5CB at T= 4.2 K from 370 to 390 nm. Besides, the heating from low temperatures to room temperature results in the decrease of the emission intensity on 17% and some narrowing of the whole spectrum on several nanometers. Under the heating, the 5CB luminescence band peak position is seen to shift monotonously towards the long wavelengths, whereas the spectral half-width changes irregularly, having at least two ‘‘turning points’’ at temperatures 80 and 140 K. It should be noted that the total intensity of the 5CB luminescence spectrum changes nonmonotonously at heating, having its maximum value at T= 80 K. In the 5CB crystalline state the alkyl chains of 5CB molecules are in the trans-conformation, and it is oriented almost perpendicularly to the plane of the nearest benzene ring (the torsion angle is 90.31), and the torsion angle between the normals of the two phenyl rings is equal to 26.31 [23]. The polar structure of 5CB molecules (the dipole moment is 4.0 D) leads to strong dipole interactions, causing conformational changes in the 5CB molecules (the molecules become more flat) and the formation of the dimers. The results of quantum chemical calculations (CNDO/ S) on planar and twisted cyanobiphenyl model compounds in the gas phase showed that the dipole moment of the excited state (S1) of the 5CB molecule in the planar conformation (8.3 D) is higher than this value in the 5CB molecule in the twisted 901 conformation (4.9 D) [14]. This fact can explain the drop of the
1
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Fig. 2. Photoluminescence spectra of 5CB: 1—in the crystal (T= 4.2 K) and 2—nematic (T= 300 K) phases.
5CB total emission intensity under the temperature growth. Also, these authors showed that the energy gap between the lowest energy (ground state S0) conformation and the first excited one (S1) decreases at the transition from the twisted conformation to the planar one of the 5CB molecules. This explains the red shift of the 5CB luminescence spectra at the temperature growth. It is worth mentioning that in the solutions of 5CB at the increase of the solvent polarity the 5CB luminescence spectra also lose their structure and shift towards the long wavelengths [15,20]. As in the case of the spectral red shift at the temperature growth, this effect is explained by the increase of the dipole moment of the 5CB molecule excited state. In the solutions, excimer fluorescence is known to consist of a broad, unstructured emission in the range 380–390 nm, almost independent of solvent polarity since the excited dipole moment of excimers has a very low or null value [15]. In other words, the fine structure of the 5CB luminescence spectra in solutions and at low temperatures is mainly determined by the monomer emission; however, the nature of this spectral similarity between the 5CB luminescence at low temperatures and in the non-polar solvents is not absolutely clear to understand. Probably, the last fact mentioned is connected with very weak interactions between the 5CB molecules and their surroundings in both cases, resulting in the formation of similar configurations of emission centers, and finally, the similarity of the 5CB luminescence spectra structure. It is a common opinion for cyanobiphenyl compounds that their luminescence is characterized by the emission of monomers in the crystalline state and of both monomers and excimers in the mesogenic and isotropic liquid phases. Nevertheless, in the case of 12CB liquid crystal the excimer-like fluorescence appears even in the crystalline phase [11]. Moreover, some authors [17,18] made an assumption about the simultaneous monomer and excimer emission in the crystal of 5CB. The strong overlap of our luminescence spectra, obtained for 5CB at very low and room temperatures can confirm the last statement. However, up to now, there is no definite conclusion made on this matter. In order to analyze the 5CB luminescence spectra in detail, we made a graphical separation of the complex emission band into several components, as shown in Fig. 3 for temperatures (a) 4.2, (b) 100, (c) 200 and (d) 300 K. At T= 4.2 K (Fig. 3a) the 5CB luminescence band consists of at least four components: 343, 360, 383 and 407 nm. By analogy with the 5CB luminescence data presented for its nematic liquid crystal phase [16,18,24,25], and in the diluted and concentrated solutions of cyclohexane and ethanol [15], spectral components at 343 and 360 nm can be ascribed to the monomer emission, and 383 and 407 nm bands to the excimer fluorescence. The 5CB crystals have a crystal lattice of a monoclinic type with a centrosymmetrical space group (P21/a) and the following ˚ the monoclinic angle parameters, a=8.25, b=16.02 and c=10.94 A, is equal to 951 [26]. The number of molecules in the elementary unit is Z=4. The analysis of the experimental results obtained by the X-ray diffraction method, concerning a determination of the intermolecular distances between a cyano-group of one molecule layer and the nearest phenyl ring of another one showed the possibility of two types for the molecule alignments: parallel and anti-parallel ones. When the anti-parallel orientation is realized, the CN-group of one molecule is located near the nearest phenyl ring CH2-group of a pentyl chain of the neighbour molecule with an overlap of the biphenyl cores, and a distance between the molecules from the neighbouring layers is less, compared to a case of the molecule parallel orientation. Probably, these two non-equivalent states of the 5CB molecules in a crystal explain the presence of two spectral components in the 5CB luminescence spectra, 343 and 360 nm, corresponding to the monomer emission.
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Fig. 3. Graphical separations of the 5CB photoluminescence spectral contours: (a) T= 4.2 K; (b) T= 100 K; (c) T =200 K; and ( d) T= 300 K.
This can be supported by a potential model, proposed by Rapp and co-workers [27] to explain the appearance and loss of fine structure in the 9CB luminescence spectra. This model accounts for steric hindrance to planarity at certain temperatures and the intramolecular twisting relaxation towards planarity at other temperatures. In other words, the conformations with more or less range of the twist angles are able to emit at different temperatures, causing different luminescence spectral contours. For the formation of excimers in bulk phases, two monomer units must come close to one another to form a sandwich pair. It has been suggested from X-ray data that these pairs are preformed in the ground state, and that their overlap is such that the length of the dimer becomes about 1.6 times the monomer length [28,29]. Further, time-resolved luminescence measurements proved a model of planar preformed pairs of chromophores in the cyanobiphenyl liquid crystals [14]. These results can indirectly support our further conclusion that even at low temperatures some part of 5CB molecules exists in the dimer state and is characterized by excimer photoluminescence, seen as spectral components at 383 and 407 nm for the 5CB at T=4.2 K (Fig. 3a).
Each spectral component of the 5CB photoluminescence spectrum has its own temperature behaviour at heating up to 300 K. These dependencies are non-monotonous and allow one to conclude about some abrupt re-alignments of the 5CB crystal structure at T= 80 and 140 K. As is seen from Fig. 3b, the crystal phase above T= 80 K is characterized with a loss of the spectral fine structure, and a disappearance of the spectral band at 343 nm. Further heating leads to a new band at 424 nm (Fig. 3c). At T= 300 K the spectral contour of the 5CB luminescence can be also separated into four spectral components with their peak positions at 362, 383, 404 and 424 nm (Fig. 3d). Compared to the spectrum at T= 4.2 K (Fig. 3a), at room temperature the only component, which corresponds to the monomer emission and remains in the spectrum, is 362 nm. However, its intensity decreases significantly under heating (Fig. 3). This result could be expected, since the nematic liquid crystal phase is characterized mainly by dimer structures [30,31]. According to Klock et al. [14], the 5CB dimer sandwich-type structures can be more fully overlapping and more extended (Scheme 1). As a result, they are characterized with different
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Table 1 Peak positions of the photoluminescence spectral components and their assignments. Spectral components at T= 4.2 K (nm)
Spectral components at T= 300 K (nm)
344 361 383
363 383
407
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Assignment
Monomer emission Monomer emission Excimer emission (Scheme 1c) Excimer emission (Scheme 1a) Excimer emission (Scheme 1b)
dimers to the other dimer structures of the nematic phase. The spectral position of this component in relation to the other luminescence bands of the 5CB excimers corresponds to its position in the energy scale for the mentioned dimer structures. This is an additional confirmation of our assignment made for the spectral bands. Table 1 summarizes the data obtained here for the 5CB luminescence and the assignments of the spectral bands.
3.3. IR-spectroscopy data
Scheme 1. Different possible 5CB excimer structures.
photoluminescence bands, explaining the appearance of several spectral components for excimer luminescence. It is reasonable to suggest, that the luminescence spectral component at 407 nm (T= 4.2 K) and 404 nm (T= 300 K) corresponds to the first type of the possible dimer structures (Scheme 1a), since this component significantly raises its intensity at heating. The last fact evidences the formation of the 5CB ‘‘classic’’ dimers, which are typical of the 5CB nematic liquid crystal phase [32]. At the same time, at higher temperatures, the denser packing of the nematic phase, compared to the crystal one, allows forming more overlapping dimers of 5CB molecules (Scheme 1b), which are characterized by the appearance of a new luminescence band at 424 nm. At least, two types of 5CB excimer structures were also predicted experimentally in Ref. [18]. In addition to the dimer structures mentioned (Scheme 1a, b), another excimer type can be proposed involving a self-solvated twisted intramolecular charge transfer state (Scheme 1c), i.e. a twisted intramolecular charge transfer state molecule interacting with a ground one of the same species [33]. The existence of these types of excimers was proved for 9CB liquid crystal [14], but there is no clear data about its spectral position in the luminescence contour. These structures are quite possible to be characterized with a spectral component at 383 nm, which is seen at both low and room temperatures. Its intensity does not practically change with temperature growth, decreasing only a little, which is probably an evidence of a partial transition of this type of 5CB
IR-spectroscopy allows studying the structure formation and conformation changes of 5CB in different phase states. The molecular vibrational analysis is well-known as a useful tool for directly detecting molecular conformations [34,35]. The results described above on the 5CB photoluminescence spectra have led to the conclusion about the simultaneous presence of the monomer and dimer structures in different crystalline states and nematic liquid crystal phase of this substance. This statement is well proved by the IR-spectroscopy data, analyzed in the region of the C N bond stretching vibrations of the 5CB molecules at different temperatures (Fig. 4). The graphical separation of this spectral band allows one to select two components at 2226 and 2229 cm 1, which according to Chandrasekar [36], are assigned to dimers and monomers of the 5CB molecules, respectively. It is seen that some amount of the 5CB dimers ( 11%) are presented even at T= 10 K, i.e. in the low-temperature crystalline state of 5CB (Fig. 4a). The temperature increase leads to the growth of the relative quantity of the 5CB dimer structures, reaching 36% at T= 100 K (Fig. 4b). The crystal–crystal phase transition at T= 140 K is characterized by the appearance of the domination of the 5CB dimer structures over the monomer ones ( 63% at T= 200 K, Fig. 4c), while the negligible amount of the monomers remains in the 5CB nematic liquid crystal phase ( 3%, Fig. 4d). The structure transformations mentioned above are reflected in the changes of the IR-spectral bands, corresponding to the CCH deformation vibrations of the 5CB phenyl rings (Fig. 5a, b). The formation of the dimer structures occurs due to the dipole–dipole interactions of the 5CB molecules, which dipole moment is mainly located on the CN-group. Thus, the most significant spectral changes concern the stretching vibrations of the C N bond (described above) and the deformation vibrations of the phenyl ring, close to this bond in the 5CB molecule. The vibrations of this ring in the IR-spectra are characterized by the spectral bands at 1186, 1286 and 1301 cm 1, thier intensities increase significantly at the sequential transitions from one crystalline phase to another and drop abruptly in the nematic liquid crystal state (Fig. 5a, b). Concerning other IR-spectral features of the 5CB liquid crystal, up to the crystal—nematic liquid crystal phase transition the spectral bands, corresponding to the deformation vibrations of the
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Fig. 4. The graphical separations of the 5CB IR-spectral band, corresponding to the C N bond stretching vibrations: (a) T= 4.2 K; (b)T= 100 K; (c) T=200 K; and (d) T= 300 K.
phenyl ring, close to the 5CB molecule alkyl chain (1180 cm 1, Fig. 5a), and of the CH2-groups of the alkyl chain (809, 828, 840 cm 1, Fig. 5c) do not practically change with the temperature growth. This proves the small changes of the alkyl chain conformation at heating, but according to Babkov et al. [34] and Tanaka et al. [35], at low temperatures the 5CB liquid crystal is already a mix of different conformers, where relative quantities do not change up to the nematic liquid-crystal state. Obviously, the simultaneous co-existence of the different conformers of the 5CB molecules in the low-temperature states explains the formation of different types of monomer and dimer structures, revealed by us during the analysis of the 5CB photoluminescence spectral properties. It is interesting to notice that the DSC and X-ray diffraction experiments allowed us to detect and describe several crystal modifications of 5CB, which were realized at different temperature histories [37]. So the suggestion about the formation of additional structure re-alignments in the crystal modification of 5CB is quite reasonable. Further investigations of the low-temperature states for the 5CB liquid crystal by other experimental techniques can give more detailed information about the structure transitions mentioned and explain their mechanisms.
4. Conclusions For the first time, the photoluminescence spectra of the 5CB liquid crystal have been investigated in detail at low temperatures 4.2–200 K. The spectral data obtained are compared with the results of the luminescence study for the 5CB nematic phase at T=300 K. The luminescence of the 5CB crystal state is characterized by emissions of both monomer and dimer structures. Besides, there are several energetically and conformationally non-equivalent types of monomers and dimers, and their amounts change with the temperature growth nonmonotonously. The 5CB several crystal modifications, having different 5CB monomer and dimer conformers, have been found out below T=200 K. The 5CB crystal–crystal transition at T= 80 K is characterized with a total loss of the fine structure in the 5CB photoluminescence spectrum and a disappearance of the spectral band at 343 nm. At T =140 K, a new spectral band at 424 nm appears, which corresponds to the significantly overlapping 5CB dimers, and its intensity grows under further heating. Unfortunately, the nature of these structure changes in the crystal state of 5CB cannot be unambiguously explained at the present time.
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780
795
810
825
840
wavenumbers, cm
855
870
-1
Fig. 5. The IR-spectral fragments of the 5CB at different temperatures in the regions of the deformation vibrations of the CCH phenyl rings (a, b) and CH2-groups of the alkyl chain (c).
The simultaneous presence of the 5CB monomer and dimer structures in different crystalline and nematic liquid crystal states is proved by the IR-spectroscopy data in the region of the C N group stretching vibrations. The formation of the dipole–dipole bonded dimers affects the deformation vibrations of the 5CB phenyl ring, close to the CN-group, indicating some changes in the conformations of the 5CB molecules. The vibrations of other fragments in the 5CB molecules are slightly influenced up to the transition to the nematic liquid crystal phase. This investigation has demonstrated that the fluorescence methods using the monomer and excimer emissions of 5CB as the probe are potentially useful and sensitive for characterizing molecular alignments and changes in the molecular conformations.
Acknowledgements The authors would like to thank Prof. Jan Baran (Wroclaw, Poland) for his help in the IR-spectroscopic measurements. This work was partially funded by the NAS of Ukraine under the Program ‘‘Nanophysics and Nanoelectronics’’, project VC-138.
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