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Non-covalent dendrimer-based liquid crystalline complexes: Synthesis and characterization
European Polymer Journal 46 (2010) 1923–1931
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Non-covalent dendrimer-based liquid crystalline complexes: Synthesis and characterization Khadijeh Didehban a, Hassan Namazi b,*, Ali Akbar Entezami c a
Metallurgy Group, Material School, NSTRI, Tehran, Iran Laboratory of Carbohydrates and Biopolymers, Faculty of Chemistry, University of Tabriz, Tabriz P.O. Box 5166616471, Iran c Laboratory of Polymer, Faculty of Chemistry, University of Tabriz, Tabriz, Iran b
a r t i c l e
i n f o
Article history: Received 27 November 2009 Received in revised form 14 June 2010 Accepted 6 July 2010 Available online 13 July 2010 Keywords: Dendrimer Hydrogen-bonded Liquid crystal DSC POM
a b s t r a c t The synthesis and structural characterization of dendritic macromolecules based on 3,5dihydroxybenzoic acid are described. The molecular structures and purity of all new compounds were confirmed by 1H NMR, 13C NMR spectroscopy and elemental analysis. The dendritic non-covalent liquid crystalline complexes were prepared through the formation of hydrogen bonds between different generation of dendritic acids (proton donor) and stilbazole derivative containing bipyridyl units (proton acceptor). We found that, the hydrogen-bonded dendritic liquid crystals supramolecules (G1–2py, G2–2py and G3–2py) exhibits nematic and semectic phase. The polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) were used for investigation of the liquid crystalline properties of the hydrogen-bonded dendritic supramolecular complexes. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Dendrimers represent a special class of intermediate compound between low molecular mass substances and polymers. Dendrimers attract the attention of researchers engaged in various fields of science and technology [1]. Some of famous dendritic architectures such as polyamines [2], polyphenylethers [3], triazine-containing dendrimers [4,5], polyamidoamines [6] and carbosilanes [7] have been reported. Based on these novel dendritic scaffolds, some liquid crystalline dendrimers have been synthesized [8,9]. It is an intriguing question whether a dendritic skeleton is able to impede liquid crystalline properties, or whether the high local concentration of mesogenic units in a dendritic molecule leads to preorganisation and facilitates the formation of a mesophase. Liquid crystals are unique materials that combine dynamic nature with anisotropic structures [10]. Liquid crystals have been successfully applied to display material. These materials have more potential in variety of fields, such as information, mass or charge transportation, transforma* Corresponding author. Tel.: +98 411 3393121; fax: +98 411 3340191. E-mail address: [email protected] (H. Namazi). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.07.001
tion, and molecular sensing. For a further functionalization of liquid crystals, the use of specific interactions, such as hydrogen bonding, is one of different versatile approaches [11]. Self-assembled materials formed by non-covalent bonding have attracted much attention because these materials are good candidates for the next generation of materials [12], for which dynamic function, environmental benignity, and low energy processing are required. The hydrogen bond acts as a linkage to hold the two rigid components together and thus form a mesogenic phase. The study of interaction between the proton donor and proton acceptor as the hydrogen bond and thermal properties of mesophase formation are determined by Fourier transformation infrared (FT-IR) and Differential scanning calorimetry (DSC) technique [13,14]. All the liquid crystal dendrimers synthesized through covalent bonding. There is powerful method to prepare liquid crystal dendrimers via hydrogen bonding. Kato and Fréchet had described stable liquid crystalline supramolecular via hydrogen bonding between a non-mesogenic polymer containing a pyridyl group [15]. Based on specific non-covalent interaction, some liquid crystal dendritic complexes and another’s liquid crystalline supramolecular complexes such as hydrogen bonding, halogen-bonded, charge
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transfer, ion binding or coordination complexation some liquid crystal dendritic complexes have been synthesized [16–23]. Recently, dendrimers as a macromolecular species instead of the conventional polymer have been explored to complex with various supramolecular systems with complex architectures [24–30]. Serrano and co-workers reported liquid crystal behavior of two series of dendrimer complexes with three aliphatic carboxylic acid [31]. Due to scientific values and potential technological applications of hydrogen-bonded supramolecules or polymers as optical storage device, information displays and optical switching elements have attracted much attention in past years [32,33]. A number of liquid crystal supramolecular assembled via H-bonds between pyridyl–acid functional groups have been prepared [34–37]. Recently, we reported the synthesis of liquid crystalline dendrimer via covalent bonding (containing mesogenic units) [38], and non-covalent bonding [39]. From this point of view, here we report a convergent synthesis and the structural analysis of three generation of dendritic macromolecules and preparation of the dendritic hydrogen-bonded supramolecular liquid crystalline complexes from different generation of dendritic acids and stilbazole derivative containing bipyridyl units. Then we studied liquid crystalline properties of dendritic complexes that are described in details.
2.4. G1–CO2H It was obtained by recrystallized twice in ethyl acetate. H NMR (400 MHz, DMSO-d6, ppm): d 8.35 (d, Jab = 8.93 Hz, 4H aromatic, ortho to NO2), 7.96 (d, Jba = 8.93 Hz, 4H aromatic, meta to NO2), 7.94 (d, Jcd = 8.83, 4H aromatic, meta to O), 7.17 (d, Jef = 2 Hz, 2H), 7.01 (d, Jdc = 8.87, 4H aromatic, ortho to O), 6.91 (t, Jfe = 2 Hz, 1H), 4.09 (t, Jgh = 5.91, 8H, CH2, a to –OPh), 1.90–1.84 (m, 8H, CH2), 1.54–1.62 (m, 8H, CH2). FT-IR (KBr, cm1): 3613–2625 (mCOOH), 2994 (mC–H), 1585 (mN@N), 1511, 1337 (mNO2 ), 1633 (mC@C), 1247 (mC–O), 1687 (mC@O). Elem. Anal. Calcd for C43N6O10H44: C, 64.18%, N, 10.45%, H, 5.47%. Found: C, 64.44%, N, 10.57%, H, 5.17%.
1
2.5. G2–CO2H
Phenol, 3,5-dihydroxybenzoic acid, p-nitroaniline, 1,6dibromohexane, 4-hydroxybenzaldehyde, 4-methylpyridine, thionyl chloride were purchased from Merck and purified by common methods.
It was obtained by recrystallized twice in acetone. Further purification by chromatography on a silica gel eluted with eluted hexane/chloroform (2:1, v/v) Rf = 0.61. 1H NMR (400 MHz, DMSO-d6, ppm): d 8.35 (d, Jab = 8.93 Hz, 8H aromatic, ortho to NO2), 7.95 (d, Jba = 8.92 Hz, 8H aromatic, meta to NO2), 7.82 (d, Jef = 2 Hz, 2H) 7.88 (d, Jcd = 8.84, 8H aromatic, meta to O), 7.65 (t, Jfe = 2 Hz, 1H) 7.48(d, Jgh = 2 Hz, 4H) 7.04 (d, Jdc = 8.87, 8H aromatic, ortho to O), 6.84 (t, Jhg = 2 Hz, 2H) 4.11 (t, Jij = 5.91, 16H, CH2, a to –OPh), 1.86–1.80 (m, 16H, CH2), 1.49–1.60 (m, 16H, CH2). 13C NMR (100 MHz, DMSOd6, d): 171.34, 168.95, 165.48, 163.22, 163.78, 156.45, 152.33, 152.66, 142.34, 140.26, 127.11, 125.49, 131.23, 126.54, 123.72, 112.09, 29.47, 27.31, 26.52, 24.55, 20.48. FT-IR (KBr, cm1): 3617–2651 (mCOOH), 2988 (mC–H), 1582 (mN@N), 1511, 1334 (mNO2 ), 1631 (mC@C), 1241 (mC–O), 1688 (mC@O). Elem. Anal. Calcd. for C91N12O22H90: C, 64.16%, N, 9.87%, H, 5.47%. Found: C, 64.48%, N, 9.94%, H, 5.72%.
2.2. Instrumental measurements
2.6. Synthesis of G3–CO2H
2. Experimental 2.1. Materials
1
H NMR spectra were recorded on 400 MHz Brucker SP400 AVANC in CDCl3 and DMSO-d6. The elemental analyses were performed on a Vario EL III to characterization of synthetic compounds. FT-IR spectra were measured on a Shimadzu model FT-IR-8101 M spectrometer. A STA 625 DSC was used to determine phase transition temperatures at heating and cooling rates of 10 °C/min. Liquid crystal textures were observed using an optical Zeiss polarizing microscopy equipped with a TMS94 hot stage. 2.3. Synthetics procedure The synthesis of compounds Azo–OH, Azo–Br and G1– CO2Me were conducted by following the method described in the literature [35,36]. 2.3.1. General synthetic procedure for G1–CO2H, G2–CO2H and G3–CO2H Compound G1–CO2H was obtained by hydrolysis of esteric groups in mixture of KOH/EtOH. Compound G2–CO2H was obtained by chlorinated of G1–CO–Cl with refluxing in thionylchloride and additional of 3,5-dihydroxybenzoic acid according literature method [39]. Compounds G2–CO–Cl and G3–CO2H were obtained the same method.
It was obtained by chromatography on a silica gel. Rf = 0.58, (SiO2, hexane/CHCl3 3:2 v/v). 1H NMR (400 MHz, DMSO-d6, ppm): d 8.35 (d, Jab = 9.11 Hz, 16H aromatic, ortho to NO2), 8.06 (d, Jgh = 2 Hz, 4H) 7.93 (d, Jba = 9.11 Hz, 16H aromatic, meta to NO2), 7.91 (d, Jcd = 9.15, 16H aromatic, meta to O), 7.83 (d, Jef = 2 Hz, 2H), 7.77 (t, Jhg = 2 Hz, 3H), 7.37 (d, Jij = 2 Hz, 8H), 7.07 (t, Jji = 2 Hz, 4H), 6.88 (d, Jdc = 9.16, 16H aromatic, ortho to O), 4.17 (t, Jlk = 5.88, 32H, CH2, a to –OPh), 1.81–1.93 (m, 32H, CH2), 1.53–1.64 (m, 32H, CH2). 13C NMR (100 MHz, DMSO-d6, d): 170.67, 171.34, 169.27, 165.32, 164.12, 165.54, 164.55, 156.09, 152.77, 151.52, 144.32, 140.33, 132.21, 130.98, 129.30, 126.40, 125.12, 130.45, 129.91, 121.19, 112.32, 29.11, 28.06, 26.59, 24.95, 20.31. FT-IR (KBr, cm1): 3620–2653 (mCOOH), 2990 (mC–H), 1583 (mN@N), 1510, 1331 (mNO2 ), 1638 (mC@C), 1241 (mC–O), 1689 (mC@O). Elem. Anal. Calcd. for C187N24O46H182: C, 64.15%, N, 9.61%, H, 5.20%. Found: C, 64.48%, N, 9.93%, H, 5.51%. 2.7. Synthesis of proton acceptor compounds The synthesis of compounds py–Me and py–OH were conducted by following the method described in the literature [40,41]. Compounds 2py was obtained the similar method.
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2.8. Synthesis of trans-4-hexyloxy-40 -stilbazole (2py) It was obtained by chromatography on a silica gel. Rf = 0.65, (SiO2, chloroform/methnol v/v, 3:1). 1H NMR (400 MHz, ppm, DMSO): d 8.58 (d, Jad = 4.80 Hz, 4H), 7.53 (d, Jbc = 6.83 Hz, 4H,), 7.42 (d, Jda = 4.83 Hz, 4H), 7.25 (d, Jfe = 16.31 Hz, 2H), 7.10 (d, Jcb = 6.80 Hz, 4H), 6.96 (d, Jef = 16.32 Hz, 2H), 4.71 (t, Jgh = 6.51 Hz, 4H,), 1.87–96 (m, 4H), 1.69–80 (m, 4H). 13C NMR (ppm, CDCl3, d): 159.5, 151.1, 145.1, 132.9, 129.5, 129.3, 123.8, 121.7115.1, 70.9,
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70.6, 69.7, 67.7. FT-IR (KBr, cm1): 3064, 2923(C–H), 1942, 1631, 1605(C@C), 1589(C@N), 1551, 1513, 1451, 1416, 1359, 1325. Elem. Anal. Calcd. For C32N2O2H32: C, 80.67%, N, 5.88%, H, 6.72%. Found: C, 80.84%, N, 6.02%, H, 6.95%. 2.9. Dendritic complex preparation All hydrogen-bonded complexes were prepared through melt method described by Namazi et al. in the literature [39].
Scheme 1. Synthetic route leading to dendritic macromolecules structures.
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Fig. 1. 1H NMR spectrum of G3–COOH.
Scheme 2. Synthetic procedure for the compound 2py and conditions: (i) (CH3CO)2O, 120 °C; (ii) NaHCO3, ethanol/H2O, reflux, N2; (iii) K2CO3, DMF, 65 °C, N2.
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Scheme 3. Synthetic procedure for the dendritic complexes.
Table 1 FT-IR absorption signals (cm1) for, Gn–COOH and its complexes. Assignment
O–H
C@C
C@O
C@N
G1–COOH G2–COOH G3–COOH 2py G1–2py G2–2py G3–2py
2630–3613 2651–3617 2653–3620 – 1930–2637 1968–2645 1975–2662
1633 1631 1638 1605 1606 1607 1605
1687 1688 1689 – 1697 1693 1690
– – – 1589 1597 1595 1593
3. Results and discussion 3.1. Synthesis and characterization In this work we report, the synthesis of dendritic macromolecules based on 3,5-dihydroxybenzoic acid with convergent growth approach. A combination of 1H NMR and 13 C NMR spectroscopy and elemental analysis were used for characterization of dendritic fragments. The synthetic route leading to dendritic polyester macromolecules struc-
tures is outlined in Scheme 1. The compound Azo–OH was synthesized from the diazotation reaction of p-nitroaniline and phenol in low temperature in yield 83% [35]. The synthesis of compound G1–COOMe is described elsewhere [36]. Compound G1–COOH was synthesized through convergent method using Azo–Br and 3,5-dihydroxybenzoic acid methyl ester in the presence of sodium bicarbonate in acetone as a solvent subsequently deprotection of G1– COOMe with KOH in ethanol then chlorination using thionylchloride. The 1H NMR spectrum of G1–COOH shows doublet signals at 8.35, (7.96, 7.94), 7.17 and 7.01 ppm for the aromatic ortho protons, to NO2 group, aromatic ortho to azo-group, aromatic ortho to acidic groups, and aromatic meta protons to –O– group, respectively. Other triplet signals at 6.91 and 4.09 ppm for the aromatic proton between esteric group and aliphatic protons alpha to –OPh group. G1–COOH was synthesized through chlorination of G1– COOH then addition of 3,5-dihydroxybenzoic acid in DMF as a solvent. The 1H NMR spectrum of G2–COOH shows doublet signals at 8.35, 7.95, 7.82, 7.88, 7.48 and 7.04 ppm for the aromatic ortho protons, to NO2 group,
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aromatic meta to NO2 group, aromatic ortho to acidic groups, aromatic meta protons to –O– group, aromatic protons between esteric–etheric group and aromatic ortho protons to –O– group, respectively. Other triplet signals at 7.65, 6.84 and 4.11 ppm for the aromatic proton between esteric-group, aromatic proton between etheric group and aliphatic protons alpha to –OPh group. The comparison of the proton numbers of aromatic ortho to NO2 group and the number of aromatic protons, ortho to acidic groups, which indicates the formation of G2–COOH. Fig. 1, the 1H NMR spectrum of G3–COOH in DMSO shows a doublet signal at 8.35 ppm for the aromatic ortho to NO2 group and other doublet signals at 8.06, (7.93, 7.91), 7.83, 7.37 and 6.88 ppm for the aromatic ortho to azo-group, aromatic proton between esteric-group, aromatic ortho to acidic groups, aromatic protons between esteric–etheric group and aromatic ortho protons to –OR group, respectively. Also in this spectrum two triplet signals appeared at 7.77, 7.07 and 4.17 ppm related to protons between esteric groups and etheric group, and aliphatic protons alpha to –OPh group, respectively. The comparison of the proton numbers of aromatic ortho to NO2 group and the number of aromatic protons, ortho to acidic groups, which indicates the formation of G3–COOH. Also integral ratio of aromatic protons of ortho to NO2 to aromatic protons, between esteric group or another signal is 3.94 (in comparison to four as a theoretical calculation) shows that the reaction was completed and the growth of dendrimer was confirmed G3–COOH. The growth of dendritic parts, confirmed through the 1H NMR data and CHN analysis. Therefore, this method could be applied for the determination of the growth of dendrimer generations. In the next step compound 2Py as a acceptor was prepared by condensation of 4-hydroxybenzaldehyde
with 4-picoline then reaction of trans-4-hydroxy-40 -stilbazole with dialkylbromide [40,41]. The synthetic route to pyridyl compound is shown in Scheme 2. The dendritic complexes were prepared by the melt method described in Section 2. The supramolecular dendritic complexes structures are shown in Scheme 3.
3.2. FT-IR investigation of dendritic complex The FT-IR data for the pure proton donor and pure proton acceptor, 2py and the hydrogen-bonded complexes G1–2py, G2–2py and G3–2py are shown in Table 1. The free proton donor exhibits a broad O–H band from 3600 to 2600 cm1 and the C@O band at about 1687 cm1. Complexes compounds show a similar IR profile, but in comparison with G1–COOH, G2–COOH and G3–COOH the stretching bands of hydroxyl group shifted to the lower wave numbers which are assigned to be the hydrogenbonding interaction (O–H N). These absorption bands must have resulted from the hydrogen-bonding interaction between the carboxylic acid and the pyridine, which is (O– H N). The absorption band at 1690 cm1 is attributed to the free carbonyl group of carboxylic acid, in which the oxygen in the carbonyl was not involved in the formation of the hydrogen band. The absorption of pyridine shifted from 1589 to 1597 cm1, suggesting the formation of the hydrogen bonding N H–O. The shift is due to the change in electron density in nitrogen. In contrast, the peak at 1606 cm1 in donors and in acceptors did not show any shift when compared with related peaks in the complexes because the electron density in phenyl group was not considerably disturbed by the formation of the hydrogen bond.
Fig. 2. DSC thermograms of Gn–2py complexes, with a heating rate 10 °C/min, (1) cooling and (2) heating.
K. Didehban et al. / European Polymer Journal 46 (2010) 1923–1931 Table 2 Phase transition temperatures of the dendritic complexes, determined by DSC at scan rate 10 °C/min on the first heating and cooling scan and microscope observation. Sample
First heating (°C)
First cooling(°C)
G1–2py G2–2py G3–2py
K149 N 167 I K 138 K0 150 I K 141 K0 159 I
I 161 N 127 K I 162 S 143 K I 143 S 126 K113 K0
*From microscope observation.
3.3. Liquid crystal properties of the synthesized dendrimers Dendritic compounds G1–COOH, G2–COOH, G3–COOH as a donor and 2py as a acceptor in our study are non-mesogenic. The structures of obtained complexes compounds used in this paper are shown in Scheme 3. A polarizing optical microscopy (POM) with a heating stage and differential scanning calorimetry (DSC) were used for measuring the melting temperature (Tm) and isotropization temperature (Ti) of the compounds. The DSC curves of supramolecular complexes were shown as Fig. 2 and phase transition data of the compound were listed in Table 1. The effective association between donor and acceptor lead to stable structural organization. The hydrogen bonds link between the rigid parts of the donor and acceptor caused length of mesogenic groups
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increased and subsequently liquid crystallinity of the resulting complexes. All the compounds used in this study do not exhibit any liquid crystallinity by themselves; but them mixing together in melt condition, liquid crystallinity was observed in all the cases (Table 2). The DSC thermogram of G1–2py, on heating exhibits two endothermic peaks at 149 and 167 °C. Correspond to the crystal–mesophase transition and the mesophase–isotropic transition. On cooling run, exhibits two exothermic peaks at 161 and 127 °C, correspond to the isotropic–mesophase transitions and mesophase–crystal transition. The phase transition was also studied using polarizing optical microscopy. The battonet textures of a smectic phase were observed on cooling. Polarizing photomicrographs of compound G1–2py are given in Fig. 3. From the DSC graphs of G2–2py, we observed two main exothermic peaks at 162 and 143 °C on the cooling, indicating the presence of phase transition. On heating run, exhibits two endothermic peaks at 138 and 150 °C, correspond to the crystal–crystal, and crystal–isotropic transitions. The polarizing optical micrographs of dendritic complex, G2–2py, showed the focal conic texture formation of the smectic phase from isotropic phase on cooling process (Fig. 3). The DSC thermogram of G3–2py, on heating exhibits two endothermic peaks at 141 and 159 °C. Correspond to the crystal–crystal and crystal–isotropic transitions. On
Fig. 3. Optical polarized micrographs of (a) G1–2py on cooling at 146 °C (b) G2–2py on cooling at 138 °C (c) G3–2py on cooling at 167 °C (d) G3–2py on cooling at 123 °C.
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cooling run, there are three exothermic peaks at 143, 126 and 113 °C, which indicate the transitions from isotropic phase to mesophase, mesophase to crystal and crystal to crystal. The observations during cooling process of G3–2py under POM the focal conic texture of the smectic phase from isotropic phase in the high temperature and Fan-shape texture formation of smectic phase in low temperature were formed. Fig. 3 illustrates the textures of G3–2py.
[14] [15]
[16] [17] [18]
4. Conclusion [19]
In this work, we synthesized three generations of dendritic supramolecular proton donor (G1–COOH, G2–COOH, G3–COOH) and proton acceptor (2py). The dendritic structure obtained was determined with common spectroscopic methods and elemental analysis. All of dendritic compound as a donor and pyridyl derivative as a acceptor were non-mesogenic. Hydrogen-bonded dendritic liquid crystals supramolecules were prepared via hydrogen bonding between pyridyl–acid groups. Hydrogen bonds formation studied by FT-IR spectroscopy. We found that, the hydrogen-bonded dendritic liquid crystals supramolecules (G1–2py, G2–2py and G3–2py) exhibits nematic and semectic phase. These properties of hydrogen-bonded liquid crystalline supramolecules are interest in the field of switching devices, optical data storage and optoelectronics.
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Non-covalent dendrimer-based liquid crystalline complexes: Synthesis and characterization Khadijeh Didehban a, Hassan Namazi b,*, Ali Akbar Entezami c a
Metallurgy Group, Material School, NSTRI, Tehran, Iran Laboratory of Carbohydrates and Biopolymers, Faculty of Chemistry, University of Tabriz, Tabriz P.O. Box 5166616471, Iran c Laboratory of Polymer, Faculty of Chemistry, University of Tabriz, Tabriz, Iran b
a r t i c l e
i n f o
Article history: Received 27 November 2009 Received in revised form 14 June 2010 Accepted 6 July 2010 Available online 13 July 2010 Keywords: Dendrimer Hydrogen-bonded Liquid crystal DSC POM
a b s t r a c t The synthesis and structural characterization of dendritic macromolecules based on 3,5dihydroxybenzoic acid are described. The molecular structures and purity of all new compounds were confirmed by 1H NMR, 13C NMR spectroscopy and elemental analysis. The dendritic non-covalent liquid crystalline complexes were prepared through the formation of hydrogen bonds between different generation of dendritic acids (proton donor) and stilbazole derivative containing bipyridyl units (proton acceptor). We found that, the hydrogen-bonded dendritic liquid crystals supramolecules (G1–2py, G2–2py and G3–2py) exhibits nematic and semectic phase. The polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) were used for investigation of the liquid crystalline properties of the hydrogen-bonded dendritic supramolecular complexes. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Dendrimers represent a special class of intermediate compound between low molecular mass substances and polymers. Dendrimers attract the attention of researchers engaged in various fields of science and technology [1]. Some of famous dendritic architectures such as polyamines [2], polyphenylethers [3], triazine-containing dendrimers [4,5], polyamidoamines [6] and carbosilanes [7] have been reported. Based on these novel dendritic scaffolds, some liquid crystalline dendrimers have been synthesized [8,9]. It is an intriguing question whether a dendritic skeleton is able to impede liquid crystalline properties, or whether the high local concentration of mesogenic units in a dendritic molecule leads to preorganisation and facilitates the formation of a mesophase. Liquid crystals are unique materials that combine dynamic nature with anisotropic structures [10]. Liquid crystals have been successfully applied to display material. These materials have more potential in variety of fields, such as information, mass or charge transportation, transforma* Corresponding author. Tel.: +98 411 3393121; fax: +98 411 3340191. E-mail address: [email protected] (H. Namazi). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.07.001
tion, and molecular sensing. For a further functionalization of liquid crystals, the use of specific interactions, such as hydrogen bonding, is one of different versatile approaches [11]. Self-assembled materials formed by non-covalent bonding have attracted much attention because these materials are good candidates for the next generation of materials [12], for which dynamic function, environmental benignity, and low energy processing are required. The hydrogen bond acts as a linkage to hold the two rigid components together and thus form a mesogenic phase. The study of interaction between the proton donor and proton acceptor as the hydrogen bond and thermal properties of mesophase formation are determined by Fourier transformation infrared (FT-IR) and Differential scanning calorimetry (DSC) technique [13,14]. All the liquid crystal dendrimers synthesized through covalent bonding. There is powerful method to prepare liquid crystal dendrimers via hydrogen bonding. Kato and Fréchet had described stable liquid crystalline supramolecular via hydrogen bonding between a non-mesogenic polymer containing a pyridyl group [15]. Based on specific non-covalent interaction, some liquid crystal dendritic complexes and another’s liquid crystalline supramolecular complexes such as hydrogen bonding, halogen-bonded, charge
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transfer, ion binding or coordination complexation some liquid crystal dendritic complexes have been synthesized [16–23]. Recently, dendrimers as a macromolecular species instead of the conventional polymer have been explored to complex with various supramolecular systems with complex architectures [24–30]. Serrano and co-workers reported liquid crystal behavior of two series of dendrimer complexes with three aliphatic carboxylic acid [31]. Due to scientific values and potential technological applications of hydrogen-bonded supramolecules or polymers as optical storage device, information displays and optical switching elements have attracted much attention in past years [32,33]. A number of liquid crystal supramolecular assembled via H-bonds between pyridyl–acid functional groups have been prepared [34–37]. Recently, we reported the synthesis of liquid crystalline dendrimer via covalent bonding (containing mesogenic units) [38], and non-covalent bonding [39]. From this point of view, here we report a convergent synthesis and the structural analysis of three generation of dendritic macromolecules and preparation of the dendritic hydrogen-bonded supramolecular liquid crystalline complexes from different generation of dendritic acids and stilbazole derivative containing bipyridyl units. Then we studied liquid crystalline properties of dendritic complexes that are described in details.
2.4. G1–CO2H It was obtained by recrystallized twice in ethyl acetate. H NMR (400 MHz, DMSO-d6, ppm): d 8.35 (d, Jab = 8.93 Hz, 4H aromatic, ortho to NO2), 7.96 (d, Jba = 8.93 Hz, 4H aromatic, meta to NO2), 7.94 (d, Jcd = 8.83, 4H aromatic, meta to O), 7.17 (d, Jef = 2 Hz, 2H), 7.01 (d, Jdc = 8.87, 4H aromatic, ortho to O), 6.91 (t, Jfe = 2 Hz, 1H), 4.09 (t, Jgh = 5.91, 8H, CH2, a to –OPh), 1.90–1.84 (m, 8H, CH2), 1.54–1.62 (m, 8H, CH2). FT-IR (KBr, cm1): 3613–2625 (mCOOH), 2994 (mC–H), 1585 (mN@N), 1511, 1337 (mNO2 ), 1633 (mC@C), 1247 (mC–O), 1687 (mC@O). Elem. Anal. Calcd for C43N6O10H44: C, 64.18%, N, 10.45%, H, 5.47%. Found: C, 64.44%, N, 10.57%, H, 5.17%.
1
2.5. G2–CO2H
Phenol, 3,5-dihydroxybenzoic acid, p-nitroaniline, 1,6dibromohexane, 4-hydroxybenzaldehyde, 4-methylpyridine, thionyl chloride were purchased from Merck and purified by common methods.
It was obtained by recrystallized twice in acetone. Further purification by chromatography on a silica gel eluted with eluted hexane/chloroform (2:1, v/v) Rf = 0.61. 1H NMR (400 MHz, DMSO-d6, ppm): d 8.35 (d, Jab = 8.93 Hz, 8H aromatic, ortho to NO2), 7.95 (d, Jba = 8.92 Hz, 8H aromatic, meta to NO2), 7.82 (d, Jef = 2 Hz, 2H) 7.88 (d, Jcd = 8.84, 8H aromatic, meta to O), 7.65 (t, Jfe = 2 Hz, 1H) 7.48(d, Jgh = 2 Hz, 4H) 7.04 (d, Jdc = 8.87, 8H aromatic, ortho to O), 6.84 (t, Jhg = 2 Hz, 2H) 4.11 (t, Jij = 5.91, 16H, CH2, a to –OPh), 1.86–1.80 (m, 16H, CH2), 1.49–1.60 (m, 16H, CH2). 13C NMR (100 MHz, DMSOd6, d): 171.34, 168.95, 165.48, 163.22, 163.78, 156.45, 152.33, 152.66, 142.34, 140.26, 127.11, 125.49, 131.23, 126.54, 123.72, 112.09, 29.47, 27.31, 26.52, 24.55, 20.48. FT-IR (KBr, cm1): 3617–2651 (mCOOH), 2988 (mC–H), 1582 (mN@N), 1511, 1334 (mNO2 ), 1631 (mC@C), 1241 (mC–O), 1688 (mC@O). Elem. Anal. Calcd. for C91N12O22H90: C, 64.16%, N, 9.87%, H, 5.47%. Found: C, 64.48%, N, 9.94%, H, 5.72%.
2.2. Instrumental measurements
2.6. Synthesis of G3–CO2H
2. Experimental 2.1. Materials
1
H NMR spectra were recorded on 400 MHz Brucker SP400 AVANC in CDCl3 and DMSO-d6. The elemental analyses were performed on a Vario EL III to characterization of synthetic compounds. FT-IR spectra were measured on a Shimadzu model FT-IR-8101 M spectrometer. A STA 625 DSC was used to determine phase transition temperatures at heating and cooling rates of 10 °C/min. Liquid crystal textures were observed using an optical Zeiss polarizing microscopy equipped with a TMS94 hot stage. 2.3. Synthetics procedure The synthesis of compounds Azo–OH, Azo–Br and G1– CO2Me were conducted by following the method described in the literature [35,36]. 2.3.1. General synthetic procedure for G1–CO2H, G2–CO2H and G3–CO2H Compound G1–CO2H was obtained by hydrolysis of esteric groups in mixture of KOH/EtOH. Compound G2–CO2H was obtained by chlorinated of G1–CO–Cl with refluxing in thionylchloride and additional of 3,5-dihydroxybenzoic acid according literature method [39]. Compounds G2–CO–Cl and G3–CO2H were obtained the same method.
It was obtained by chromatography on a silica gel. Rf = 0.58, (SiO2, hexane/CHCl3 3:2 v/v). 1H NMR (400 MHz, DMSO-d6, ppm): d 8.35 (d, Jab = 9.11 Hz, 16H aromatic, ortho to NO2), 8.06 (d, Jgh = 2 Hz, 4H) 7.93 (d, Jba = 9.11 Hz, 16H aromatic, meta to NO2), 7.91 (d, Jcd = 9.15, 16H aromatic, meta to O), 7.83 (d, Jef = 2 Hz, 2H), 7.77 (t, Jhg = 2 Hz, 3H), 7.37 (d, Jij = 2 Hz, 8H), 7.07 (t, Jji = 2 Hz, 4H), 6.88 (d, Jdc = 9.16, 16H aromatic, ortho to O), 4.17 (t, Jlk = 5.88, 32H, CH2, a to –OPh), 1.81–1.93 (m, 32H, CH2), 1.53–1.64 (m, 32H, CH2). 13C NMR (100 MHz, DMSO-d6, d): 170.67, 171.34, 169.27, 165.32, 164.12, 165.54, 164.55, 156.09, 152.77, 151.52, 144.32, 140.33, 132.21, 130.98, 129.30, 126.40, 125.12, 130.45, 129.91, 121.19, 112.32, 29.11, 28.06, 26.59, 24.95, 20.31. FT-IR (KBr, cm1): 3620–2653 (mCOOH), 2990 (mC–H), 1583 (mN@N), 1510, 1331 (mNO2 ), 1638 (mC@C), 1241 (mC–O), 1689 (mC@O). Elem. Anal. Calcd. for C187N24O46H182: C, 64.15%, N, 9.61%, H, 5.20%. Found: C, 64.48%, N, 9.93%, H, 5.51%. 2.7. Synthesis of proton acceptor compounds The synthesis of compounds py–Me and py–OH were conducted by following the method described in the literature [40,41]. Compounds 2py was obtained the similar method.
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2.8. Synthesis of trans-4-hexyloxy-40 -stilbazole (2py) It was obtained by chromatography on a silica gel. Rf = 0.65, (SiO2, chloroform/methnol v/v, 3:1). 1H NMR (400 MHz, ppm, DMSO): d 8.58 (d, Jad = 4.80 Hz, 4H), 7.53 (d, Jbc = 6.83 Hz, 4H,), 7.42 (d, Jda = 4.83 Hz, 4H), 7.25 (d, Jfe = 16.31 Hz, 2H), 7.10 (d, Jcb = 6.80 Hz, 4H), 6.96 (d, Jef = 16.32 Hz, 2H), 4.71 (t, Jgh = 6.51 Hz, 4H,), 1.87–96 (m, 4H), 1.69–80 (m, 4H). 13C NMR (ppm, CDCl3, d): 159.5, 151.1, 145.1, 132.9, 129.5, 129.3, 123.8, 121.7115.1, 70.9,
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70.6, 69.7, 67.7. FT-IR (KBr, cm1): 3064, 2923(C–H), 1942, 1631, 1605(C@C), 1589(C@N), 1551, 1513, 1451, 1416, 1359, 1325. Elem. Anal. Calcd. For C32N2O2H32: C, 80.67%, N, 5.88%, H, 6.72%. Found: C, 80.84%, N, 6.02%, H, 6.95%. 2.9. Dendritic complex preparation All hydrogen-bonded complexes were prepared through melt method described by Namazi et al. in the literature [39].
Scheme 1. Synthetic route leading to dendritic macromolecules structures.
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Fig. 1. 1H NMR spectrum of G3–COOH.
Scheme 2. Synthetic procedure for the compound 2py and conditions: (i) (CH3CO)2O, 120 °C; (ii) NaHCO3, ethanol/H2O, reflux, N2; (iii) K2CO3, DMF, 65 °C, N2.
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Scheme 3. Synthetic procedure for the dendritic complexes.
Table 1 FT-IR absorption signals (cm1) for, Gn–COOH and its complexes. Assignment
O–H
C@C
C@O
C@N
G1–COOH G2–COOH G3–COOH 2py G1–2py G2–2py G3–2py
2630–3613 2651–3617 2653–3620 – 1930–2637 1968–2645 1975–2662
1633 1631 1638 1605 1606 1607 1605
1687 1688 1689 – 1697 1693 1690
– – – 1589 1597 1595 1593
3. Results and discussion 3.1. Synthesis and characterization In this work we report, the synthesis of dendritic macromolecules based on 3,5-dihydroxybenzoic acid with convergent growth approach. A combination of 1H NMR and 13 C NMR spectroscopy and elemental analysis were used for characterization of dendritic fragments. The synthetic route leading to dendritic polyester macromolecules struc-
tures is outlined in Scheme 1. The compound Azo–OH was synthesized from the diazotation reaction of p-nitroaniline and phenol in low temperature in yield 83% [35]. The synthesis of compound G1–COOMe is described elsewhere [36]. Compound G1–COOH was synthesized through convergent method using Azo–Br and 3,5-dihydroxybenzoic acid methyl ester in the presence of sodium bicarbonate in acetone as a solvent subsequently deprotection of G1– COOMe with KOH in ethanol then chlorination using thionylchloride. The 1H NMR spectrum of G1–COOH shows doublet signals at 8.35, (7.96, 7.94), 7.17 and 7.01 ppm for the aromatic ortho protons, to NO2 group, aromatic ortho to azo-group, aromatic ortho to acidic groups, and aromatic meta protons to –O– group, respectively. Other triplet signals at 6.91 and 4.09 ppm for the aromatic proton between esteric group and aliphatic protons alpha to –OPh group. G1–COOH was synthesized through chlorination of G1– COOH then addition of 3,5-dihydroxybenzoic acid in DMF as a solvent. The 1H NMR spectrum of G2–COOH shows doublet signals at 8.35, 7.95, 7.82, 7.88, 7.48 and 7.04 ppm for the aromatic ortho protons, to NO2 group,
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aromatic meta to NO2 group, aromatic ortho to acidic groups, aromatic meta protons to –O– group, aromatic protons between esteric–etheric group and aromatic ortho protons to –O– group, respectively. Other triplet signals at 7.65, 6.84 and 4.11 ppm for the aromatic proton between esteric-group, aromatic proton between etheric group and aliphatic protons alpha to –OPh group. The comparison of the proton numbers of aromatic ortho to NO2 group and the number of aromatic protons, ortho to acidic groups, which indicates the formation of G2–COOH. Fig. 1, the 1H NMR spectrum of G3–COOH in DMSO shows a doublet signal at 8.35 ppm for the aromatic ortho to NO2 group and other doublet signals at 8.06, (7.93, 7.91), 7.83, 7.37 and 6.88 ppm for the aromatic ortho to azo-group, aromatic proton between esteric-group, aromatic ortho to acidic groups, aromatic protons between esteric–etheric group and aromatic ortho protons to –OR group, respectively. Also in this spectrum two triplet signals appeared at 7.77, 7.07 and 4.17 ppm related to protons between esteric groups and etheric group, and aliphatic protons alpha to –OPh group, respectively. The comparison of the proton numbers of aromatic ortho to NO2 group and the number of aromatic protons, ortho to acidic groups, which indicates the formation of G3–COOH. Also integral ratio of aromatic protons of ortho to NO2 to aromatic protons, between esteric group or another signal is 3.94 (in comparison to four as a theoretical calculation) shows that the reaction was completed and the growth of dendrimer was confirmed G3–COOH. The growth of dendritic parts, confirmed through the 1H NMR data and CHN analysis. Therefore, this method could be applied for the determination of the growth of dendrimer generations. In the next step compound 2Py as a acceptor was prepared by condensation of 4-hydroxybenzaldehyde
with 4-picoline then reaction of trans-4-hydroxy-40 -stilbazole with dialkylbromide [40,41]. The synthetic route to pyridyl compound is shown in Scheme 2. The dendritic complexes were prepared by the melt method described in Section 2. The supramolecular dendritic complexes structures are shown in Scheme 3.
3.2. FT-IR investigation of dendritic complex The FT-IR data for the pure proton donor and pure proton acceptor, 2py and the hydrogen-bonded complexes G1–2py, G2–2py and G3–2py are shown in Table 1. The free proton donor exhibits a broad O–H band from 3600 to 2600 cm1 and the C@O band at about 1687 cm1. Complexes compounds show a similar IR profile, but in comparison with G1–COOH, G2–COOH and G3–COOH the stretching bands of hydroxyl group shifted to the lower wave numbers which are assigned to be the hydrogenbonding interaction (O–H N). These absorption bands must have resulted from the hydrogen-bonding interaction between the carboxylic acid and the pyridine, which is (O– H N). The absorption band at 1690 cm1 is attributed to the free carbonyl group of carboxylic acid, in which the oxygen in the carbonyl was not involved in the formation of the hydrogen band. The absorption of pyridine shifted from 1589 to 1597 cm1, suggesting the formation of the hydrogen bonding N H–O. The shift is due to the change in electron density in nitrogen. In contrast, the peak at 1606 cm1 in donors and in acceptors did not show any shift when compared with related peaks in the complexes because the electron density in phenyl group was not considerably disturbed by the formation of the hydrogen bond.
Fig. 2. DSC thermograms of Gn–2py complexes, with a heating rate 10 °C/min, (1) cooling and (2) heating.
K. Didehban et al. / European Polymer Journal 46 (2010) 1923–1931 Table 2 Phase transition temperatures of the dendritic complexes, determined by DSC at scan rate 10 °C/min on the first heating and cooling scan and microscope observation. Sample
First heating (°C)
First cooling(°C)
G1–2py G2–2py G3–2py
K149 N 167 I K 138 K0 150 I K 141 K0 159 I
I 161 N 127 K I 162 S 143 K I 143 S 126 K113 K0
*From microscope observation.
3.3. Liquid crystal properties of the synthesized dendrimers Dendritic compounds G1–COOH, G2–COOH, G3–COOH as a donor and 2py as a acceptor in our study are non-mesogenic. The structures of obtained complexes compounds used in this paper are shown in Scheme 3. A polarizing optical microscopy (POM) with a heating stage and differential scanning calorimetry (DSC) were used for measuring the melting temperature (Tm) and isotropization temperature (Ti) of the compounds. The DSC curves of supramolecular complexes were shown as Fig. 2 and phase transition data of the compound were listed in Table 1. The effective association between donor and acceptor lead to stable structural organization. The hydrogen bonds link between the rigid parts of the donor and acceptor caused length of mesogenic groups
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increased and subsequently liquid crystallinity of the resulting complexes. All the compounds used in this study do not exhibit any liquid crystallinity by themselves; but them mixing together in melt condition, liquid crystallinity was observed in all the cases (Table 2). The DSC thermogram of G1–2py, on heating exhibits two endothermic peaks at 149 and 167 °C. Correspond to the crystal–mesophase transition and the mesophase–isotropic transition. On cooling run, exhibits two exothermic peaks at 161 and 127 °C, correspond to the isotropic–mesophase transitions and mesophase–crystal transition. The phase transition was also studied using polarizing optical microscopy. The battonet textures of a smectic phase were observed on cooling. Polarizing photomicrographs of compound G1–2py are given in Fig. 3. From the DSC graphs of G2–2py, we observed two main exothermic peaks at 162 and 143 °C on the cooling, indicating the presence of phase transition. On heating run, exhibits two endothermic peaks at 138 and 150 °C, correspond to the crystal–crystal, and crystal–isotropic transitions. The polarizing optical micrographs of dendritic complex, G2–2py, showed the focal conic texture formation of the smectic phase from isotropic phase on cooling process (Fig. 3). The DSC thermogram of G3–2py, on heating exhibits two endothermic peaks at 141 and 159 °C. Correspond to the crystal–crystal and crystal–isotropic transitions. On
Fig. 3. Optical polarized micrographs of (a) G1–2py on cooling at 146 °C (b) G2–2py on cooling at 138 °C (c) G3–2py on cooling at 167 °C (d) G3–2py on cooling at 123 °C.
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cooling run, there are three exothermic peaks at 143, 126 and 113 °C, which indicate the transitions from isotropic phase to mesophase, mesophase to crystal and crystal to crystal. The observations during cooling process of G3–2py under POM the focal conic texture of the smectic phase from isotropic phase in the high temperature and Fan-shape texture formation of smectic phase in low temperature were formed. Fig. 3 illustrates the textures of G3–2py.
[14] [15]
[16] [17] [18]
4. Conclusion [19]
In this work, we synthesized three generations of dendritic supramolecular proton donor (G1–COOH, G2–COOH, G3–COOH) and proton acceptor (2py). The dendritic structure obtained was determined with common spectroscopic methods and elemental analysis. All of dendritic compound as a donor and pyridyl derivative as a acceptor were non-mesogenic. Hydrogen-bonded dendritic liquid crystals supramolecules were prepared via hydrogen bonding between pyridyl–acid groups. Hydrogen bonds formation studied by FT-IR spectroscopy. We found that, the hydrogen-bonded dendritic liquid crystals supramolecules (G1–2py, G2–2py and G3–2py) exhibits nematic and semectic phase. These properties of hydrogen-bonded liquid crystalline supramolecules are interest in the field of switching devices, optical data storage and optoelectronics.
[20]
[21]
[22]
[23]
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