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A series of ortho fluoro-tolane liquid crystals was prepared via multi-step reactions based on 4-alkylcyclohexanecarboxylic acids. Their synthetic routes, thermotropic mesophases and UV stability properties are discussed by comparision with the non-fluorinated analogs. All of the measurements from DSC, UV-vis absorption and fluorescence emission spectra demonstrate that the presence of ortho fluorine can significantly improve the UV stability of tolane-liquid crystals. Meanwhile, the ultimate failure mechanism of tolane-LCs during UV exposure is deduced by a photochemical reaction. DFT calculations of molecular polarizabilities, triple bond lengths, electrostatic potential maps and molecular orbitals are used to correlate the experimental findings. The interaction mechanism for stabilization of the tolane structures by the ortho fluoro substitution is also explained. This work may provide an effective solution for the obstacle existed in tolane-liquid crystals and pave a way for their applications in liquid crystal photonics.
© 2015 Optical Society of America
1. Introduction
Liquid crystals (LCs), as a class of advanced opto-electronic materials, have been used in the area of flat-panel displays (liquid crystal displays, LCDs). Various information terminals and communication electronic devices have been realized from LCs [1, 2 ]. Recently, High-birefringence (Δn) LCs have attracted much attention because of their potential applications in liquid crystal photonics, such as beam steerers, tunable focus lens and electrically controlled phase shifter in GHz and THz region [3, 4 ]. Generally, high-Δn LCs can be obtained by extending π-electron conjugation, and the previous studies revealed that the introduction of tolane unit into a liquid crystal compound is an effective way to obtain high-Δn LCs with good solubility and low viscosity [5–7 ].
However, most of the high-Δn LCs are faced with a serious problem of photostability, which affects the lifetime of liquid-crystal devices. For the projection displays, although filters are used to cut off the light below 420 nm, residual UV light could still penetrate into the LC panels and cause performance degradation [8, 9 ]. For the direct-view LCDs, the backlight is either a white light emitting diode (LED) or quantum-dot LEDs or RGB LEDs for color sequential displays. In these displays, the shortest wavelength is 450 nm from the blue LED [10, 11 ]. Many photonic applications operate in the infrared region [12]. It seems that the photostability of high-Δn LCs for above applications should be forgiven. Nevertheless, the LC cells need to be hermetically sealed under UV radiation, which has a fatal effect on these tolane liquid crystals since the triple bond could be broken during UV exposure [13]. Anti-UV agents and antioxidants are usually added into the LCs to solve this issue. However, the addition of these reagents will lead to the performance deterioration of LCs [14]. Therefore, it is highly desirable to develop novel tolane-liquid crystal molecules with excellent UV stability for their applications in liquid-crystal devices.
It is well known that the physical properties of liquid crystal materials are highly dependent on the molecular structures. For example, the fluoro substituent has a remarkable effect on mesophase morphology, phase transition temperature, dielectric and optical anisotropy, resistivity and visco-elastic properties [15]. Additionally, the lateral fluoro substituents can be also beneficial for stabilizing liquid crystal blue phases [16]. Although the structure-property relationship focused on the fluoro-substituted LCs has been systematically investigated, the effect of ortho fluorine on UV stability of tolane-LCs has not yet been well explored.
In this work, fluorine atoms were introduced into the ortho position of triple bond of tolane, and three tolane compounds named as PTP, PFTP and PFTFP (Fig. 1 ) were synthesized to study the UV stability of these tolane compounds. It was found that the ortho fluorine can hinder the shift of the melting points after UV exposure. As thus, a series of fluoro-tolane liquid crystals (3Fn and 4Fn) was synthesized to insight into the effect of ortho fluoro substituents on UV stability of tolane-liquid crystals (Fig. 1). For comparison, the corresponding non-fluorinated analogs were also prepared [17]. These fluoro-tolane liquid crystals show the properties of broad nematic phase interval, high clearing point and high birefringence, making them potential applications in liquid crystal photonics [18]. Meanwhile, DFT calculations of molecular polarizabilities, triple bond lengths, and molecular orbitals were used to correlate the experimental results.
2. Experimental details
2.1 Synthesis of compounds
Unless otherwise noted, all chemicals were commercially available and used as received. The liquid crystal mixture (NR-5336LA) was purchased from Chisso Corporation. The target compounds were obtained by the similar procedures reported [17, 19 ].
2.2 Measurement techniques
The phase transition temperatures were measured by Shimatsu differential scanning calorimetry (DSC-60) under nitrogen atmosphere with the heating and cooling rate of 10 °C min−1. Polarized optical microscopy (POM) was performed by a LEICA DM2500P with a Linkam THMS600 hot-stage and control unit with a heating and cooling rate of 0.5 °C min−1. Phase identification was carried out by comparing the observed textures. The phase transition temperatures were the peak values of the transition on DSC curves. The UV absorption spectra and emission spectra were measured by using HITACHI U–3900 UV/VIS spectrophotometer and Hitachi F-7000 spectrometer, respectively. Thermogravimetric analysis (TGA) was carried out on a TA Q50 in nitrogen (flow rate: 100 cm3 min−1) with the heating rate of 10 °C min−1. Birefringence was measured on an Abbe refractometer (NAR-4T) at 18 °C, using a 589 nm wavelength light source in a liquid crystal mixture (NR-5336LA). Quantum chemical calculations were performed based on the DFT model by employing the combination of B3LYP functional and 6-31g (d, p) basis set [20].
2.3 UV-irradiation process
The UV-irradiation of the tolane compounds was tested in the n-heptane solution (sample concentration: 6 × 10−2 mol/L) using a portable UV lamp (power: 8 W; wavelength: 365 nm). The distance between the lamp and the sample was about 2 cm. After removing the solvent, the dried solid sample was used for DSC and UV stability measurements.
2.4 UV-degradation process
The sample (23 mg) was dissolved in ethanol (30 ml), the solution was put into a quartz tube. Then the quartz tube was irradiated within a photo-reactor (XPA-7, Xujiang Electromechanic-al Plant, Nanjing, China) under a UV light (power: 500 W; wavelength: 365 nm) for 4h in argon atmosphere. After the irritation, the solid was collected and dissolved in CDCl3 to measure NMR spectrum.
3. Results and discussion
3.1 Synthesis
The tolane-based LCs are commonly synthesized by cross coulping reaction from terminal alkynes and aryl halides, which leads to some by-products and difficulty for purification [21]. One pot Sonogashira cross coulping reaction was employed in this work to synthesize the targeted compounds. All of the compounds show overall yield of 45-55% with purity higher than 99% (HPLC), indicating that this synthetic pathway is facile and efficient, which is beneficial to avoid contamination of chemical impurities [20].
3.2 Liquid crystalline properties
The phase transition temperatures and mesophase textures of the target compounds are summarized in Fig. 2 . All compounds show enantiotropic nematic phases with relatively broad temperature ranges of 95.1-118.2 °C at the heating process, and 98.1-120.3 °C at the cooling process. Meanwhile, all compounds are thermotropic liquid crystals with high clearing points of 222.1-239.7 °C. As shown in Fig. 2, the presence of ortho fluoro substituent decreased the melting points, while enhanced the nematic phase intervals and limited the smectic phases. For example, the melting point of 3F2 is lower than that of 3-2 without fluoro substitution, while the mesophase interval of 3F2 is wider than that of 3-2. Compared to the non-fluorinated analogue of 4-5, 4F5 shows a limited smectic phase. In addition, the birefringence of a nematic liquid crystal mostly depends on the polarisability anisotropy Δα, which can explain why compound 3-2 exhibits a slightly larger Δn value than that of 3F2 (Table 1 ). Even though, compound 3F2 still reveals a relatively high birefringence (Δn>0.3).
Fig. 2 The mesophase range of target compounds during heating. Cr: crystal; S: smectic A phase interval; N: nematic phase interval. See Data File 1 for underlying values.
Table 1. The birefringencesa and DFT calculated principal components (αXX, αYY, αZZ), isotropic component αb, anisotropy Δαc, triple bond length (l) of molecules 3-2 and 3F2.d
3.3 UV stability
As an efficient method, DSC was used to monitor UV stability of the compounds by observation of the melting and clearing point temperatures (T c) before and after UV illumination [22]. As an example, two representative samples 3-2 and 3F2 were used to study the effect of ortho fluoro substituent on UV stability of tolane-liquid crystals.
It was found that the phase transition temperatures of 3-2 and 3F2 did not change when the samples were illuminated under UV light in the solid state. However, big differences in the phase transition temperatures were observed if the samples were illuminated by UV light in n-heptane solution under argon atmosphere. As shown in Figs. 3(a) and 3(b) , the T c onset point temperature of 3-2 was decreased by 5.2 °C, while the T c onset point temperature of 3F2 almost was retained after 30 minutes illumination. The peak symmetry of the nematic to isotropic phase transition indicates that the purity of 3F2 is higher than that of 3-2. This demonstrated that the presence of ortho fluorine increased the UV stability.
Fig. 3 DSC traces of compound 3-2 (A) and 3F2 (B) in the heating process before and after the 30 minutes of UV illumination. DSC traces of compound 3-2 (C) and 3F2 (D) in the heating process before and after the 240 minutes of UV illumination.
Two factors may contribute to the UV stability: absorption and molecular structure [23]. As shown in Fig. 4 , 3F2 shows blue-shifted absorption edge of 325 nm compared with 3-2 (345 nm), which could be attributed to the higher HOMO energy of 3-2 (−5.43 eV) than 3F2 (−5.49 eV) [24]. Compared with other reported tolane compounds [25], these compounds exhibit strong absorption bands, which can be explained by their flat tolane structures and intensive π-electron conjugations. The absorption bands for 3-2 and 3F2 are below 300 nm, which are similar with that of 5CB. Compound 3-2 exhibits stronger photoluminescence emission than 3F2 with the similar emission peaks at around 315 and 333 nm. These results demonstrated that the presence of ortho fluorine can improve the UV stability, which are consistent with the DSC measurements.
Fig. 4 UV-vis absorption spectra (2 × 10−4 mol/L) and FL spectra (8 × 10−7 mol/L) of 3-2 (A, C) and 3F2 (B, D) in cyclohexane solution before and after the 240 minutes of UV illumination.
Further exposure these compounds under UV light for longer time, further decreased T c was observed (Figs. 3C and 3D). And some other physical properties, such as absorption and photoluminescence emission spectra changed as well (Fig. 4). For example, the T c were decreased by 4.8 °C for 3-2 and 2.6 °C for 3F2, respectively, after 240 minutes UV illumination. Similar results were also observed by Wu et al. [7]. They reported that there were several changes of physical properties after UV exposure, such as birefringence and viscosity. In addition, the mesophases of compounds 3-2 and 3F2 were also observed with POM before and after 240 minutes of UV illumination (Fig. 5 ). Both 3-2 and 3F2 show typical thread-like textures of the nematic mesophase in the initial stage (Figs. 5A and 5C), then they changed to highly fluid nematic schlieren textures (Figs. 5B and 5D) after UV illumination for 240 minutes. No any obvious change for the mesophases from the nematic phase could be observed before and after UV illumination.
Fig. 5 POM images ( × 200): Thread-like textures of nematic mesophases of 3-2 (A) and 3F2 (C); Schlieren textures of nematic mesophases of 3-2 (B) and 3F2 (D) after the 240 minutes of UV illumination.
3.4 UV-degradation mechanism
Wu et al. [13] reported that the degradation mechanism of diphenyl-diacetylenes is believed to originate from the UV-induced free radicals of the diacetylene group, which is deduced by relatively physical methods. From the chemistry point of view, the alkyne bond contains one π bond (268 kJ/mol), another π bond (202 kJ/mol) and one δ bond (369 kJ/mol). According to the photon energy equation, 365 nm wavelength is equal to the energy of 327.67 kJ/mol, thereby it has a chance to break π bond of the alkyne to occur photopolymerization reactions. The possible products of PTP after UV exposure are shown in Fig. 6(a) . In addition, the alkyne bond could be aggregated to form big ring structures, but it is difficult to happen.
Fig. 6 (a) The possible products of PTP after UV exposure. (b) TGA curve of PTP in the n-heptane solution before and after 12h UV exposure.
For in-depth studying the change of tolane after UV exposure, the sample PTP in n-heptane solution (sample concentration: 6 × 10−2 mol/L) was illuminated under a UV light for 12h in argon atmosphere. The IR spectra showed that the possible products preclude the existance of ethylenic bond (1680~1620 cm−1), while the TGA results (Fig. 6(b)) revealed that there exist some changes of PTP after UV exposure. Meanwhile, 1H NMR spectra showed negligible changes for the sample PTP after UV exposure, 13C NMR spectra showed that changes were only observed for the relative peak heights in aromatic carbon region. The above experimental results did not show convinced evidences for the changes of tolane after UV exposure. Therefore, 500 W mercury lamp was employed to illuminate the sample PTP dissolved in ethanol solution (sample concentration: 4 × 10−3 mol/L) under argon atmosphere for 4h. Based on the GC/EI-MS spectra, one could conclude that PTP occured a photochemical reaction with the alcohol molecule, which confirms that the degradation mechanism is originated from the UV-induced free radicals of the alkyne bond. These free radicals could transfer charges to the neighboring ethanol molecules, resulting in the molecular structure change of tolane. When the ethanol solution was changed into n-heptane (GW) solution, only some new small peaks in aromatic region on 600MHz NMR spectra were observed (Figs. 7(a) and 7(c) ). The simulated 13C NMR spectrum (Fig. 7(b)) by the ChemBioOffice software reveals that the most possible product of PTP after UV exposure is structure 4.
Fig. 7 The selected region of the 1H NMR (a) and whole 13C NMR (c) spectra of PTP before and after 4h UV exposure; The simulated 13C NMR (b) spectrum of structure 7; The selected region of the 13C NMR (d) spectrum of PTP after 4h UV exposure.
3.5 Interaction mechanism for stabilization of the tolane structures
To understand the effect of ortho fluoro substitution on the UV stability of tolane-liquid crystals, DFT calculations of molecular properties from compound 3-2 and 3F2 were performed to gain futher insight into the influence of the geometry, molecular orbital and triple bond length (l) on the nature of UV stability. The energy gap (E g) is an important parameter, which affects molecular electrical transport proprieties and photostability [26]. The substitution of fluorine atom leads to a slightly decrease in HOMO and LUMO levels, and E g value [27]. Hence, the lower E g of 3F2 is beneficial for the eventual charge transfer interactions taking place within the molecules [26].
It is known that the shorter of the bond length is, the more stable of a molecular structure is. Actually, the introduction of ortho fluorine atom into the tolane structures can shorten the triple bond length (l) (Table 1), from the chemistry point of view, this explains why the presence of ortho fluorine can increase the UV stability of the tolane-liquid crystals. Moreover, there exists a strong interaction between a lone electron pair from the ortho fluorine atom and the π electrons of the triple bond because of the resonance structures [5] as shown in Fig. 8 . For PTP, PFTP and PFTFP, their melting points are 59.6 °C, 44.6 °C and 52.5 °C, respectively. After 60 minutes illumination, their melting points are changed to 58.4 °C, 43.9 °C and 52.3 °C, respectively, which indicates that their melting point temperatures decrease slowly along with the increasing ortho fluorine atoms. Obviously, two fluorine atoms near to the triple bond can be more beneficial for the strong interaction. In addition, introduction of the highly electronegative fluorine also facilitates the formation of a weak hydrogen bond C–H…F [28], which is also beneficial for the stability of tolane structures. Furthermore, the molecular conjugation and polarization are also enhanced because the lone electron pair of ortho fluorine atom increases the electron cloud density around the triple bond, which can as far as possible protect the triple bond from being broken by UV illumination. Overall, it is more difficult to destroy these interactions than that of a single triple bond. Therefore, the fluoro-tolane liquid crystals exhibit excellent UV resistance.
4. Conclusion
A series of ortho fluoro-tolane liquid crystals was synthesized and the effect of ortho fluorine on UV stability of tolane-liquid crystals was investigated. These fluoro-tolane liquid crystals show the properties of broad nematic phase interval, high clearing point and high birefringence, making them potential applications in liquid crystal photonics. The results demonstrated that the ortho fluorine can improve the UV stability of tolane-liquid crystals, which is in line with the DFT calculation results. Meanwhile, the experimental results confirm that the ultimate failure mechanism of tolane-LCs is originated from the UV-induced free radicals of the alkynes by a photochemical reaction. A strong interaction between a lone electron pair from the ortho fluorine atom and the π electrons of the triple bond is thought to be the main reason for the improvement of UV stability. This study may provide an effective solution for the obstacle existed in tolane liquid crystals and pave a way for their applications in liquid crystal photonics.
Acknowledgments
The authors would like to thank the Defense Industrial Technology Development Program of China (B0520132007, B1120132028), Shaanxi National Science Foundation (2014JM7270), Key Technologies R&D Program of Xi'an (CXY1430(2)), the Key Technologies R&D Program of Shaanxi Province (2014K10-06), Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33) and the Fundamental Research Funds for the Central Universities (GK201504008) for financial support of this work.
References and links
1. T. Geelhaar, K. Griesar, and B. Reckmann, “125 years of liquid crystals--a scientific revolution in the home,” Angew. Chem. Int. Ed. Engl. 52(34), 8798–8809 (2013). [CrossRef] [PubMed]
2. Z. Luo, F. Peng, H. Chen, M. Hu, J. Li, Z. An, and S.-T. Wu, “Fast-response liquid crystals for high image quality wearable displays,” Opt. Mater. Express 5(3), 603–610 (2015). [CrossRef]
3. L. Wang, X. Lin, X. Liang, J. Wu, W. Hu, Z. Zheng, B. Jin, Y. Qin, and Y. Lu, “Large birefringence liquid crystal material in terahertz range,” Opt. Mater. Express 2(10), 1314–1319 (2012). [CrossRef]
4. G. Y. Si, Y. H. Zhao, E. S. P. Leong, and Y. J. Liu, “Liquid-crystal-enabled active plasmonics: a review,” Materials (Basel) 7(2), 1296–1317 (2014). [CrossRef]
5. R. Dąbrowski, P. Kula, and J. Herman, “High birefringence liquid crystals,” Crystals 3(3), 443–482 (2013). [CrossRef]
6. Y. M. Liao, H. L. Chen, C. S. Hsu, S. Gauza, and S. T. Wu, “Synthesis and mesomorphic properties of super high birefringence isothiocyanato bistolane liquid crystals,” Liq. Cryst. 34(4), 507–517 (2007). [CrossRef]
7. P. T. Lin, S. T. Wu, C. Y. Chang, and C. S. Hsu, “UV stability of high birefringence liquid crystals,” Mol. Cryst. Liq. Cryst. (Phila. Pa.) 411(1), 1285–1295 (2004). [CrossRef]
8. S. Gauza, C. H. Wen, B. J. Tan, and S. T. Wu, “UV stable high birefringence liquid crystals,” Jpn. J. Appl. Phys. 43(10), 7176–7180 (2004). [CrossRef]
9. S. Gauza, C. H. Wen, B. Wu, S. T. Wu, A. Spadlo, and R. Dąbrowski, “Fast-response nematic liquid-crystal mixtures,” Journal of the SID 14(3), 241–246 (2006).
10. J. Lim, W. K. Bae, J. Kwak, S. Lee, C. Lee, and K. Char, “Perspective on synthesis, device structures, and printing processes for quantum dot displays,” Opt. Mater. Express 2(5), 594–628 (2012). [CrossRef]
11. S. Gauza, X. Zhu, K. Piecek, R. Dąbrowski, and S. T. Wu, “Fast switching liquid crystals for color-sequential LCDs,” J. Disp. Technol. 3(3), 250–252 (2007). [CrossRef]
12. W. Lee and S.-T. Wu, “Focus issue introduction: liquid crystal materials for photonic applications,” Opt. Mater. Express 1(8), 1585–1587 (2011). [CrossRef]
13. S. T. Wu, “Molecular design strategies for high birefringence liquid crystals,” MRS. Symposium Proceedings 709, 219–228 (2002).
14. S. T. Wu and D. K. Yang, Reflective Liquid Crystal Displays (Wiley-SID, 2001).
15. M. Hird, “Fluorinated liquid crystals--properties and applications,” Chem. Soc. Rev. 36(12), 2070–2095 (2007). [CrossRef] [PubMed]
16. B. Li, W. He, L. Wang, X. Xiao, and H. Yang, “Effect of lateral fluoro substituents of rodlike tolane cyano mesogens on blue phase temperature ranges,” Soft Matter 9(4), 1172–1177 (2013). [CrossRef]
17. R. Chen, Z. W. An, X. B. Chen, and P. Chen, “Syntheses and properties of four-ring liquid crystals with ethylene and internal alkyne bridge,” Chem. J. Chinese Universities 35(7), 1433–1438 (2014).
18. N. Vieweg, C. Jansen, and M. Koch, “Liquid Crystals and their Applications in the THz Frequency Range,” in Terahertz Spectroscopy and Imaging, K.-E. Peiponen et al, eds. (Berlin, 2013), pp. 301—326.
19. H. F. Chow, C. W. Wan, K. H. Low, and Y. Y. Yeung, “A highly selective synthesis of diarylethynes and their oligomers by a palladium-catalyzed Sonogashira coupling reaction under phase transfer conditions,” J. Org. Chem. 66(5), 1910–1913 (2001). [CrossRef] [PubMed]
20. R. Chen, Y. Jiang, J. Li, Z. An, X. Chen, and P. Chen, “Dielectric and optical anisotropy enhanced by 1,3-dioxolane terminal substitution on tolane-liquid crystals,” J. Mater. Chem. C Mater. Opt. Electron. Devices 3(33), 8706–8711 (2015). [CrossRef]
21. I. C. Khoo, S. Webster, S. Kubo, W. J. Youngblood, J. D. Liou, T. E. Mallouk, P. Lin, D. J. Hagan, and S. E. W. Van, “Synthesis and characterization of the multi-photon absorption and excited-state properties of a neat liquid 4-propyl 4′-butyl diphenyl acetylene,” J. Mater. Chem. 19(40), 7525–7531 (2009). [CrossRef]
22. C. H. Wen, S. Gauza, and S. T. Wu, “Ultraviolet stability of liquid crystals containing cyano and isothiocyanato terminal groups,” Liq. Cryst. 31(11), 1479–1485 (2004). [CrossRef]
23. S. Gauza, C. H. Wen, S. T. Wu, R. Dąbrowski, C. S. Hsu, C. O. Catanescu, and L. C. Chien, “High birefringence liquid crystals for photonic applications,” Proc. SPIE 5947, 594706 (2005). [CrossRef]
24. A. Yuki, K. Sungmin, N. Shunpei, S. Koichi, K. Susumu, W. Junji, and K. Genichi, “Synthesis of new wide nematic diaryl-diacetylenes containing thiophene-based heteromonocyclic and heterobicyclic structures, and their birefringence properties,” Liq. Cryst. 41(5), 642–651 (2014). [CrossRef]
25. S. T. Wu, E. Ramos, and U. Finkenzeller, “Polarized UV spectroscopy of conjugated liquid crystals,” J. Appl. Phys. 68(1), 78–85 (1990). [CrossRef]
26. P. L. Praveen and P. O. Durga, “Effect of substituent on UV-visible absorption and photostability of liquid crystals: DFT study,” Phase Transit. 87(5), 515–525 (2014). [CrossRef]
27. M. Zhang, X. Guo, S. Zhang, and J. Hou, “Synergistic effect of fluorination on molecular energy level modulation in highly efficient photovoltaic polymers,” Adv. Mater. 26(7), 1118–1123 (2014). [CrossRef] [PubMed]
28. L. Hunter, A. M. Z. Slawin, P. Kirsch, and D. O’Hagan, “Synthesis and conformation of multi-vicinal fluoroalkane diastereoisomers,” Angew. Chem. Int. Ed. Engl. 46(41), 7887–7890 (2007). [CrossRef] [PubMed]
