Photoalignment dynamics of azo dyes series with different coordination metals Vadim S. Mikulich (SID Student Member) Alexander A. Muravsky (SID Member) Anatoli A. Murauski (SID Member) Iryna N. Kukhta Vladimir E. Agabekov (SID Member) Rashid Altamimi Abstract — There are many photoaligned azo dyes that can be used for orientation of liquid crystals in various display devices. However, the structure of these compounds needs to be optimized to increase the rate of the process of molecule photoalignment, as well as to spread the application of these compounds. The main coordination metal that presents in the molecules of azo dyes is sodium derivatives. The use of other alkali metals remains an open question. We used quantum-chemical computation methods and reversible intermolecular bonding model to determine the effect of metal coordination on the velocity of photoalignment. The theoretical predictions were experimentally verified using sodium, potassium, lithium, and cesium salts of the model azo dye synthesized by us. We conclude that potassium azo derivatives are the fastest, ceteris paribus. Keywords — photoalignment, intermolecular bonds, azo dye, orientation dynamics, liquid crystals. DOI # 10.1002/jsid.217 1 Introduction By now, a certain range of different liquid crystal (LC) photoalignment materials has been reported. Among it, photo cross-linkable polymers,1 azo polymers,2 and azo dyes 3 are the most developed and investigated. The azo dyes are attractive for LC photo orientation as it usually shows higher values of anchoring energy 4,5 and demonstrates significant thermal stability of the photoinduced anisotropy.6 According to the latest reports,7 the molecule design of successful azo dye photoalignment material meets the following requirements: azo group, rigid linear structure, and reversible intermolecular bonds. These requirements originate from the theory of photoalignment based on reversible intermolecular bonds.8 According to this theory, we can predict the probable structure of the molecule applicable as LC photoalignment material, based on the ability to form intermolecular bonds that fix anisotropic molecular orientational distribution, created through any applicable mechanism of photoselection with polarized light, i.e., cis–trans isomerization,9 non-linear phenomena 10, or statistical plane rotation.11 For a number of reasons related with organics synthesis, it turned out that most of the well-known azo dyes with verified photoalignment properties are sodium derivatives. Due to the fact that the role of the intermolecular bonds was not taken into account previously, its impact on the photoalignment process was not considered before. Recently, we have reported 12 that the type of an alkali metal impacts on the photoalignment dynamics. The latter was demonstrated in comparison of the same azo dye structure giving salt with different coordinating metals: sodium and potassium. It turned out that material with intermolecular coordination via potassium atom orients faster than sodium salts under the same conditions. This paper presents theoretical and experimental results of investigation of the role of the intermolecular coordination via metals of the first group in the periodic table and studies its impact on the photoalignment dynamics, concluding with metal salt giving the highest rate of the photoalignment process. 2 2.1 Experimental Equipment Spectra 1H and 13 NMR of the synthesized compounds were recorded in DMSO-d6 on Bruker AXS (Karlsruhe, Germany) Advance 500 with working frequency 500 MHz. IR spectra were recorded in KBr tablets on the FT-IR spectrometer Bruker Optics Tensor 27, Etlingen, Germany. The polarized absorption spectra were measured on Ocean Optics (Dunedin, FL) HR4000CG-UV-NIR with Moxtek’s (Orem, UT, USA) ProFlux Ultra broadband wire-grid polarizers UBB01. 2.2 Materials Benzidine (Vekton, St Petersburg, Russia), 4-methylsalicylic acid (Sigma-Aldrich, St. Louis, MO), and other reagents were used without further purification. Received 09/13/13; accepted 04/08/14. V. S. Mikulich, A. A. Muravsky, A. A. Murauski, I. N. Kukhta and V. E. Agabekov are with the Institute of Chemistry of New Materials, NAS of Belarus Minsk, Belarus; e-mail: alexander.muravsky@gmail.com. R. Altamimi is with Petrochemicals Research Institute, KACST Riyadh, Saudi Arabia. © Copyright 2014 Society for Information Display 1071-0922/14/2201-0217$1.00. Journal of the SID 22/1, 2014 29 2.3 2.5 Preparation of substrates and films Glass substrates are cleaned sequentially in an ultrasonic bath: an aqueous solution of surface-active substance, distilled water, and isopropyl alcohol. Additional purification and activation of the surface after drying by Photo Surface processor PL 16-110D (SEN Lights Corp., Toyonaka, Japan) are carried out . Dye solutions (1% in N ,N -dimethylformamide) were deposited on glass substrates activated using vacuum spin coater VTC-100 MTI Corp. (Richmond, CA) by spin coating in two steps: first, 30 s at 600 rpm and, second, 60 s at 2000 rpm. The films were dried at 100 °C for 5 min. 2.4 The study of the azo dyes molecules photoalignment dynamics in a thin amorphous film We used a computerized setup for investigation of the dynamics of photoalignment. The setup allows performing multiple polarized photoirradiation of the sample with consecutive measurement of its absorption spectra for the S- and P-polarization within the same area of the dye film. Such approach to photoalignment dynamics measurements provides a consistent set of data with small error spread. Details on the measurement setup can be found elsewhere.6 The photoinduced effect was analyzed at every time step by computation of the dichroic ratio after irradiation. The dichroic ratio was obtained according to Eq. (1) on the basis of experimental data: DR ¼ A? A jj (1) where DR is dichroic ratio, and A|| and A? are the absorption of linearly polarized light of the probing radiation in the parallel and perpendicular directions with respect to light exposure polarization. The dependence of dichroic ratio on time is the characteristic curve of the photoalignment process. Synthesis The synthesis of dyes and their structure are shown in Scheme 1. Azo dyes were obtained and purified by standard methods described elsewhere in literature.13 We used standard method of diazotization of benzidine 1 in acidic medium and the coupling of obtained diazo salt 2with 4-methylsalicylic acid 3 in a weakly alkaline medium. The precipitate (azo dye 4) was filtered and washed with brine and dried at room temperature. Compound 4 was dissolved in water and treated with hydrochloric acid solution to distinctly acidic medium. The precipitate 5 was filtered, thoroughly washed with hot water, and dried at room temperature. The salt form of the dye was prepared by treating the dye 5 with 2.5 equivalents of the appropriate alkali metal carbonate and purified by column chromatography (eluent water : ethanol, 1:4). Then azo dye dissolved in DMF, filtered of impurities, precipitated with diethyl ether, filtered, and dried under vacuum.14 Thus dyes, FbF-Li, FbF-Na, FbF-K, and FbF-Cs were obtained. We did not use rubidium derivative as it is highly hygroscopic. 2.5.1 Analytical data Lithium 4,4 -bis[1-(4-hydroxy-3-carboxylate-6-methyl)phenylazo] di-phenyl (FbF-Li). 1H NMR (500 MHz, DMSO-d6, δ): 2H 8.17 (s), 8H 7.94 (dd), 2H 6.64 (s), 6H 2.63 (s), (OH) 17.79 (s). 13C NMR (500 MHz, DMSO-d6, δ) δ: 17.45, 116.82, 117.89, 122.60, 127.52, 140.17, 140.90, 143.45, 152.17, 162.27, 168.34, 171.04. IR (KBr): ν, cm 1 = 3440.86, 2927.77, 1676.89, 1660.37, 1636.04, 1583.05, 1489.84, 1458.17, 1429.64, 1380.16, 1258.11, 1172.67, 1064.30. Sodium 4,4 -bis[1-(4-hydroxy-3-carboxylate-6-methyl)phenylazo] di-phenyl (FbF-Na). 1H NMR (500 MHz, DMSO-d6, δ): 2H 8.18 (s), 8H 7.94 (dd), 2H 6.65 (s). 6H 2.63 (s). (OH) 17.69. 13 C NMR (500 MHz, DMSO-d6, δ): 17.45, 116.79, 117.9, 122.60, 127.51, 140.16, 140.89, 143.45, 152.15, 162.25, 168.32, 171.08. IR (KBr): ν, cm 1 = 3441.36, 2923.04, 1636.16, Scheme — Synthesis of azo dye FbF-Li, FbF-Na, FbF-K, and FbF-Cs. 30 Mikulich et al. / Photoalignment dynamics 1583.02, 1493.28, 1456.19, 1429.43, 1379.75, 1301.36, 1194.61, 1162.62, 1064.80. Potassium 4,4 -bis[1-(4-hydroxy-3-carboxylate-6-methyl) phenylazo]di-phenyl (FbF-K). 1H NMR (500 MHz, DMSO-d6, δ): 2H 8.17 (s), 8H 7.94 (dd), 2H 6.64 (s), 6H 2.63 (s), (OH) 17.80 (s). 13C NMR (500 MHz, DMSO-d6, δ): 17.45, 116.77, 117.9, 122.59, 127.50, 140.14, 140.82, 143.41, 152.16, 162.25, 168.51, 170.97. IR (KBr): ν, cm 1 = 3424.7, 2923.91, 1637.35, 1586.02, 1492.25, 1460.46, 1427.75, 1378.74, 1265.33, 1173.96, 1064.42. Cesium 4,4 -bis[1-(4-hydroxy-3-carboxylate-6-methyl)phenylazo] di-phenyl (FbF-Cs). 1H NMR (500 MHz, DMSO-d6, δ): 2H 8.17 (s), 8H 7.94 (dd), 2H 6.64 (s). 6H 2.63 (s), (OH) 17.72 (s). 13C NMR (500 MHz, DMSO-d6, δ): 17.43, 116.83, 117.98, 122.60, 127.50, 140.18, 140.99, 143.44, 152.14, 162.25, 168.03, 171.24. IR (KBr): ν, cm 1 = 3424.12, 2923.44, 1637.61, 1591.37, 1493.68, 1458.63, 1425.35, 1378.51, 1264.27, 1173.60, 1064.76. 2.6 Linearly polarized light exposure of films The polarized light exposure of dye films was performed with blue LED light source (λmax = 457 nm) through wire-grid polarizer. The output light power of LED was 8 mW/cm2, which provided 3 mW/cm2 of polarized light. The films were consequently exposed to photoinduce certain anisotropy characterized by measuring absorption spectra of S- and P-orthogonal polarizations and computing the dichroic ratio of the absorption peak wavelength. On having measured absorption spectra at each defined step of exposure, the induced dichroic ratio on time curve was obtained (Fig. 1). According to the obtained dependences, we conclude that there are two stages of photoinduction of the dichroic ratio observed during the exposure in the film: first, initial fast rise process, followed by, second, slow rise process. As we can see in Fig. 1, K derivative azo dye—FbF-K— possesses the highest velocity of the photoalignment. The photoinduced order parameter of the FbF-K film exposed to the dose of 30.7 J/cm2 is the next: s = (A|| 0.263. 2A?)/(A|| + 2A?) FIGURE 1 — Dichroic ratio dependence on time for various dyes. 3 Discussion Quantum-chemical computation methods were applied to give analysis and explanation of the experimental dependences on the molecular structures. Thus, inertia momenta were calculated for each corresponding dye structure. The theoretical computations have been carried out using the Gaussian’03 quantum-chemical package [15]. The computations have been performed using the density functional theory with the popular hybrid B3LYP functional at the basis set level of 6-31G. The B3LYP functional consists of Becke’s three parameter hybrid exchange functional combined with the Lee–Yang–Parr correlation functional. The gas phase ground state molecular geometries were fully optimized without imposing any molecular symmetry constraints. Mutual arrangement of molecules in the film may lead to the formation of hydrogen bonds between them, and if the hydrogen atoms are replaced by metal atoms—coordination bonds. In order to estimate the effect of the intermolecular bonding depending on the coordination metal, we studied a system of two molecules (Fig. 2). The binding radii and energies of the intermolecular bonds of dye molecular structures with different coordination metals were of particular interest. Remarkably, this case suggests one of the possible intermolecular bonding situations, as the molecules can form longer chains, and mutual arrangement of molecules is not always optimal. After full geometrical optimization, the molecules are arranged relatively to each other (Fig. 2). The energy of the system that consists of a pair of molecules, Epair, is lower (negative value) than the double energy of single free molecule, Esingl. Hence, such system is energetically more preferable, due to formation of additional intermolecular bonding. Let us define half of the energy difference between pairs of molecules and the double energy of single free molecule as the potential energy of the newly formed bonds in the system of two molecules, Eq. (2): ΔE ¼ Epair 2E singl 2 (2) The dependence of the potential energy of the intermolecular interaction (i.e., a pair of intermolecular coordination bonds formation) on the intermolecular distance between two molecules of the same structure, but different salt giving atoms, was completed in the next way. First, the system of two molecules with bonding atoms H, Li, Na, K, and Cs was fully optimized. Next, the distance between the molecules was varied by setting it closer or further in the direction parallel to the bonds. Then the system of two molecules was optimized. On performing the optimization procedure, the coordinates of the part of atoms bounded by an oval (Fig. 2) were free. The other coordinates of the remaining atoms of two molecules were fixed in space (“frozen”). The optimization results into the system energy for different distances between the molecules, which was Journal of the SID 22/1, 2014 31 FIGURE 2 — Formation of the intermolecular bonds’ pair by two model dye molecules. compared with the double energy of single free molecule to compute the bonding potential, according to Eq. (2). Figure 3 shows the model computation results for molecules bonded through H, Li, Na, K, and Cs atoms. The obtained data on characteristic values for each metal are summarized in Table 1. The sequence in the table corresponds to the increase of the photoalignment velocity: from dyes that are oriented the fastest to dyes that are oriented the slowest. Thus, we determined that the fastest dye possesses potassium coordinating atoms. Let us analyze the experimental results and try to exclude the reasonable factors impacting the rate of the photoalignment process. In general, the rate of photoalignment is influenced by a large number of various factors, while in our suggested experiments, the dynamics of photoalignment is limited exclusively by the influence of metal coordination only, by application of automated measurement setup with the same exposure conditions within single probing point, and utilization of the same core molecule with the same absorption spectra. In our case, photoalignment of dye molecule includes several repetitive steps:11 (1) photo dissociation intermolecular coordination bonds, (2) rotation of dye structure, and (3) FIGURE 3 — Intermolecular interaction potential dependence on the intermolecular distance, where ΔΕ is the potential energy and m is distance between two molecules. 32 Mikulich et al. / Photoalignment dynamics TABLE 1 — Dyes’ characteristics obtained by quantum-chemical computations. Dye FbFK FbFCs FbFLi FbFNa Energy Molecular Inertia momentum Coordination Bonding 2 number radius (Å) (kJ/mol) weight (g/mol) (g/mol Å ) 41,767 6–8 10.9 85 586,68 73,994 12 11.4 70 774,29 31,586 4 10.5 120 522,36 36,358 4–6 10.7 98 554,46 formation of new bonds. In this process, the important point is the coordination number of the atom, responsible for formation of the intermolecular bonds. Thus, bigger coordination numbers increase the probability of new bonds formation. We conclude that it accelerates the induction of the anisotropy in the film. The fastest are the azo dyes molecules with K and Cs, which show high values of coordination numbers (6–8 and 12, respectively), while Li and Na derivatives do not show high values of coordination numbers (4 and 4–6, respectively) and demonstrate slower photoalignment rates. It is known that the intermolecular interaction energy must be higher than the thermal motion (about 2.5 kJ/mol at room temperature of 25 °C), but lower than the energy of a photon with a wavelength of 450 nm (about 300 kJ/mol). The first condition is required in order to form the anisotropic molecular distribution and preserve it in time, while the second condition allows photo dissociation of intermolecular bonds. Thus, the larger the potential energy value, the more difficult it is to break the intermolecular bonds and, hence, we conclude, the lower the rate of azo dyes orientation. The data listed in Table 1 show that FbF-K and FbF-Cs are in the upper rows with, correspondingly, 85 and 70 kJ/mol, while Li and Na derivatives possess energy of 120 and 98 kJ/mol, correspondingly. Thus, we conclude that molecules with lowenergy intermolecular bonds orient faster. Dyes based on K and Cs possess 10.9 and 11.4 Å values of intermolecular bonding radius, while for Li and Na case, it is 10.5 and 10.7 Å, correspondingly. It indicates that the intermolecular distances for the first pair of metals are higher, which confirms the presence of larger free volume, probably, required for effective reorientation of molecules. The molecular weight of azo dyes and the distribution of mass within the molecule in terms of inertia momentum influence the rate of photoalignment. The increase of the molecular weight of azo dye slows photoalignment, as seen by comparing a pair of Li and Na. At practically the same radius and the binding energies of intermolecular bonds, Li-FbF oriented faster because this dye molecule is lightweight compared with Na-derived molecules. At the same time, the opposite effect is observed in a pair of metal K and Cs. The bonding radius, the value of binding energy of intermolecular interactions, and the coordination number altogether suggest Cs to be faster than K in velocity of photoalignment, but due to the larger molecular weight and almost twice value of inertia momentum, FbF-Cs demonstrates slower photoalignment dynamics than FbF-K. All of the aforementioned points explain why potassium derivatives orient faster than other alkali metals in the azo dye molecule, and it gives ground for molecular design of materials with target rate of photoinduction of anisotropic properties, due to the high velocity of photoalignment based on reversible intermolecular bonds. 4 Conclusion The paper considered derivatives of alkali metals of the first group of one of the dye in the dynamics of photoalignment. It was found and confirmed in the experiment that potassium derivatives orient faster than other alkali metal salts of the same molecular structure. By the correlation of the basic characteristics of intermolecular coordination bonds, such as metal coordination number, bonding radius, and potential energy, inertia momentum obtained by quantum-chemical computations allow the explanation of the observed photoalignment dynamics behaviors. The explanation in terms of the theory of reversible intermolecular bonds photoalignment is in full agreement with results of experiments on photoinduction of the dichroic ratio in the azo dye films, bringing to light the molecular design of the chemical structure of efficient photoalignment materials. References 1 2 3 4 5 6 7 8 9 10 11 M. Schadt et al., J. Jap. Appl. Phys. 31, 2155–2164 (1992). K. Ichimura, et al., Langmuir 4, 1214 (1988). W. M. Gibbons et al., Nature 351, 49–50 (1991). A. Murauski et al., Physical Review E. 71, 061707 (2005). V. Chigrinov et al., Phys. Rev E. 68, 061702 (2003). V. G. Chigrinov et al., Photoalignment of liquid crystalline materials: physics and applications, Wiley: Chichester (2008). A. Muravsky et al., SID Int. Symp. Dig. Tech. Pap. 40, 1623–1626 (2009). A. Muravsky, Next generation of photoalignment, VDM Verlag: Dr. Müller, Saarbrücken (2009). C. Jones and S. Day, Nature 351, 15 (1991). C. Manzo et al., Phys. Rev. E. 73, 051707 (2006). V. Chigrinov et al., Phys. Rev. E. 69, 061713 (2004). 12 13 14 15 A. Muravsky et al., SID Int. Symp. Dig. Tech. Pap. 41, 1724–1726 (2010). H. Rudyk et al., Eur. J. Med. Chem. 38, 567–579 (2003). Y. D. Kim et al., Dyes and Pigments 89, 1–8 (2011). M. J. Frisch et al., Gaussian 03, Revision C 02, Gaussian, Inc., Wallingford CT (2004). Vadim Mikulich graduated from the Chemistry Department of Belarusian State University (BSU), Belarus, in 2010. Currently, he is a PhD student at the Institute of Chemistry of New Materials (IChNM) NAS of Belarus, Belarus. His main research interests are organic synthesis, polymer science, aligning materials, surface alignment of liquid crystals, and photoalignment. Alexander Muravsky graduated from the Physics Department of Belarusian State University (BSU), Belarus, in 2005. He received his PhD (2008) degree in Electrical and Electronic Engineering from Hong Kong University of Science and Technology (HKUST), Hong Kong. Since 2009 till present, he is the Head of “Materials and Technologies of LC Devices” laboratory at the Institute of Chemistry of New Materials (IChNM) NAS of Belarus, Belarus. His main research interests are liquid crystal technologies, patterned alignment, anisotropic materials for photonic, and display devices. He is a member of SID. Anatoli Murauski graduated from the Physics Department of Belarusian State University (BSU), Belarus, in 1981. He received his PhD (2007) degree in Electrical and Electronic Engineering from Hong Kong University of Science and Technology (HKUST), Hong Kong. From 1981 till 2005, he worked at the Institute of Applied Physical Problems, Belarus. From 2005 till 2010, he was the visiting scholar in Center for Display Research, HKUST. Currently, he is the leader researcher of laboratory at the Institute of Chemistry of New Materials (IChNM) NAS of Belarus, Belarus. His research area includes the investigation electro-optical effects in LC and the R&D of LCD devices. He develops different methods for investigating the alignment properties of LCD materials. He is a member of SID. Iryna Kukhta graduated from the Physics Department of Belarusian State University (BSU), Belarus, in 1985. She was junior researcher (1985–1996) at the Institute of Physics, NAS of Belarus, technical scientific editor (1997–2005) of the Journal of Applied Spectroscopy (Minsk), and researcher (2005–2008) at the Institute of Molecular and Atomic Physics, Institute of Physics, NAS of Belarus. Since 2008 till present, she is the Researcher at the Institute of Chemistry of New Materials (IChNM), NAS of Belarus, Belarus. Her main research interests are theoretical study, quantum-chemical calculations, and modeling of the structure, spectra, and electronic properties of electroactive and liquid crystal molecules. She has authored more than 30 scientific papers. Journal of the SID 22/1, 2014 33 Vladimir Agabekov has a PhD (1969) degree in Chemistry. He became the Doctor of Chemical Sciences (1981) and Professor (1987); Corresponding Member of the National Academy of Sciences (NAS) of Belarus (1996); Academician of the NAS of Belarus (2003, Physical Chemistry); Honored Scientist of Republic Belarus (2008); and Foreign Member of the NAS of Armenia (2009). Since 1998 till present, he is the Director of the Institute of Chemistry of New Materials (IChNM), NAS of Belarus, Belarus. His research areas include physical chemistry of organic substances in different structural phase states; development of principles of chemical reactions control in thin organic films of submicron thickness, in monolayer and multilayer Langmuir–Blodgett; finding of interconnection between chemical structures of organic compounds and formation mechanism of corresponding thin film materials, their properties, and transformations; and homogeneous and heterogeneous kinetics and catalysis of organic substances. He is a member of Academician of the International Academy of Engineering (Russia), member of the Scientific Council in Colloid Chemistry and Physico&chemical Mechanics (Russian Academy of Sciences), and Foreign Member of the Scientific Council in Chemistry of fossil and renewable carbon-containing raw stuff (Russian Academy of Sciences). Also, he is a member of SID and IEEE. 34 Mikulich et al. / Photoalignment dynamics Rashid Altamimi received his PhD (2010) degree in Chemistry from the University of Akron (UA), USA. Currently, he is the Assistant Director for Academic Affairs at the Petrochemicals Research Institute, King Abdulaziz City for Science and Technology (KACST), Kingdom of Saudi Arabia. His research areas include multipurpose chemistry, material science, and new materials for various applications.