CiteScore: 1.8     h-index: 21

Document Type : Original Research Article

Authors

1 Tikrit University, College of Engineering, Department of Chemical Engineering, Tikrit, Iraq

2 Tikrit University, College of Science, Department of Chemistry, Tikrit, Iraq

Abstract

Many novel mesogenic azo-Schiff base compounds have been produced successfully. An azo compound was obtained by a reaction of p-nitroaniline and hydrochloric acid with NaNO2 to prepare diazonium salt, this was followed by a coupling reaction with aniline to produce the final azo dye (II). While alkoxybenzaldehyde was prepared by reacting benzaldehyde with an alkyl halide (III). Finally, azo-Schiff bases were prepared by reacting azo dye with alkoxybenzaldehyde. The prepared compounds were diagnosed using FT-IR and 1H NMR spectrometers. A polarizing optical microscope (POM) and a differential scanning calorimeter (DSC) were used to study liquid crystalline properties. It was observed that the thermal stability of the liquid crystalline phases of the prepared compounds increased with the length of the aliphatic chain. In addition, most of the compounds gave liquid crystalline properties and more than one transition, and the reason for this is the molecular structure of the compounds. Likewise, increasing the length of the terminal groups decreased the melting points of the products.

Graphical Abstract

Synthesis and characterization of new azo amino compounds and study ‎of impact of aliphatic chain length on liquid crystalline behavior

Keywords

Main Subjects

Introduction

Several organic compounds, as are widely known, can exhibit not only gas, liquid, and solid phases but also intermediate phases between liquid and solid, known as mesophases [1]. So, liquid crystals are intermediate states between solid crystalline and isotropic phases [2, 3].

Due to its unique properties, it has been used in many medical and industrial fields. Therefore, lately, LC has been employed in self-assembly transistors and organic photovoltaic cells [4, 5], and optical sensors for imaging trypsin activity [6], light polarization [7], and Elastomers were generated by weakly cross-linking thermotropic liquid crystal phase with reactive mesogens to create artificial muscles. [8]. Aromatic azo groups are highly colored and frequently used as dyes [9].

Azo compounds contain the R1–N=N–R2 group, When the R groups in aromatic azo compounds are Arene rings, the structures are more stable than when the R groups are alkyl groups [10, 11].

A coupling reaction between a diazonium salt and a coupling agent produces aromatic azo compounds [12]. When a diazonium salt reacts with aryl amines, a yellow color is frequently produced. So, many different azo compounds can be formed by coupling different diazonium salts with aryl amines [13, 14].

Because their availability, good photo-fatigue resistance, and the significant shift of their molecular structure during E-to-Z photoisomerization, azo-dyes liquid crystals have lately gained a lot of attention for optical storage applications [15, 16]. Azo-dye have a good dichroic ratio, making them ideal for the efficient liquid crystal devices with low power consumption [17]. The main objective of the research is to prepare new amino dyes and study their liquid crystal behavior, as well as study the effect of the length of aliphatic chains on the thermal stability of liquid crystal phases.

Experimental

Materials and methods

4-Nitro aniline (Merck), Sodium nitrite (Fluka), Aniline (Merck), 1-Bromobutane (Fluka), 1-Bromopentane (Fluka), 1-Bromohexane (Fluka), 1-Bromohepane (Fluka), 1-Bromooctane (Fluka), Sodium carbonate (Aldrich), 4-Hydroxy benzaldehyde (Aldrich), Sodium acetate (Merck), Hydrochloric acid (Fluka), and Ethanol Absolute (Merck). The IR spectrum was recorded using a Shimadzu FT-IR 8400S (KBr) scale Fourier transform infrared spectrometer (4000-400). An Ultra Shield 400 MHz Bruker 2003 was used to record a 1HNMR spectra. A Differential Scanning Calorimeter (DSC-60) Shimadzu, polarizing optical microscope (POM) type (Optike) was used to analyze the liquid crystalline characteristics.

General procedure

All compounds were prepared according to the Scheme 1.

Scheme 1. Synthesis pathway of the prepared compounds

Synthesis of 1-(4-nitrophenyl)-3-phenyltriaz-1-ene (I) [18]

0.03 mol (3.09 mL) of HCl was dissolved in 20 mL of water and 0.02 mol (2.76 g) of p-nitro aniline was added to it (A). Stir the mixture until the amine dissolves (The solution was cooled by an ice bath (0-5 °C)). After that, 0.02 mol (1.38 g) of NaNO2 dissolved in the least amount of water is added. Then, the reaction mixture was stirred for half an hour. In another beaker (B), 0.02 mol (1.86 g) of aniline was dissolved in 20 mL of ethanol. Solution (B) was progressively added to the solution (A) and stirring continued for an hour. The suction filtration is used to filter the mixture with a small amount of cold water and wash the solid product off the Buchner funnel, and recrystallized from 60-80 °C petroleum ether. Yield=90%, orange powder, m.p=139-141 °C, IR (KBr): 3402 cm-1 (NH), 3032 cm-1 (C–H aromatic); (1493/1577) cm-1 (C=C aromatic); 1448 cm-1 (N=N). 1308 cm-1, 1647 cm-1 (NO2) [19].

Synthesis of 4-(4-nitrophenyl)diazenyl)aniline (II)

After dissolving 0.02 mol (4.84 g) of azo dye in the least amount of aniline, 0.01 mol (1.295 g) of anilinium chloride was added to it. The reaction mixture was refluxed for one hour in a water bath at 40-45 °C. Then, drop by drop, added the mixture to a solution of glacial acetic acid diluted with an equal amount of distilling water. The resulting product was filtered, washed in cold water, and recrystallized from CCl4. Yield=88%, orange powder, m.p= 236-238 °C, IR (KBr): (Asym. 3364, sym. 3402) cm-1 (NH2), 3009 cm-1 (C–H aromatic); (1507/1586) cm-1 (C=C aromatic); 1422 cm-1 (N=N). 1319 cm-1, 1651 cm-1 (NO2).

Synthesis of 4-alkoxybenzaldehyde (III) [20]

0.02 mol (2.76 g) of 4-hydroxy benzaldehyde was dissolved in 30 mL of DMF in a round flask and 0.02 mol (0.8 g) of sodium hydroxide was added to it. After 20 minutes of refluxing, 0.025 moles of alkyl halide was added to the mixture. The reflux was kept at 130 °C for 6 hours in an oil bath. The mixture is cooled and added drop by drop to ice water, then the product was extracted. Table 1 shows some properties of the prepared compounds.

All the FT-IR spectra of the prepared compounds indicated the disappearance of the peak at 3276 cm-1 for the OH bond of the hydroxyl group, while the carbonyl group of aldehydes remained within the range 1687-1694 cm-1. Also, the infrared spectrum of the prepared compounds showed other spectroscopic, as show Table 2.

Table 1. Shows some properties of 4-alkoxybenzaldehyde (C4-C8) (III)

Density g/mL

Yield%

Color

bp. °C

R

Comp. NO.

1.035

79

yellow

283-285

C4

1

1.022

73

Light yellow

144-146

C5

2

0.993

75

yellow

150-152

C6

3

0.98

72

yellow

152-154

C7

4

0.979

70

yellow

138-140

C8

5

Table 2. Shows some infrared spectra of the prepared compounds (C4–C8) (III)

Synthesis of N-(4-((4-nitrophenyl)diazenyl)phenyl)-1-(4-alkoxyphenyl)methanimine (IV)

0.01 mol of 4-alkoxybenzaldehyde (III) was dissolved in 20 mL of absolute ethanol and drops of glacial acetic acid were added. The mixture was then treated with 0.01 mol of 4-((4-nitrophenyl) diazenyl) aniline (II) under reflux for 3–4 hours. The product was cooled, filtered, and recrystallized from the ethanol. Table 3, shows some properties of prepared compounds (IV).

The FT-IR spectra of the Schiff bases were studied, and it was observed that the peak of the symmetric and asymmetric stretching of the amino group at (Asym. 3364, sym. 3402) cm-1 had disappeared, as well as the disappearance of the peak of the carbonyl group within the range 1687-1694 cm-1, as the disappearance of these peak is an indication of the occurrence of the interaction. Also, A new peak to azomethene group (C=N) in FT-IR spectrum was appeared at range (1629-1659) cm-1, Table 4: shows the FT-IR absorption values for the prepared series compounds.

The structural formulas of the prepared compounds were confirmed using 1H-NMR and using the solvent [DMSO-d6], which gave further evidence of the correctness of the product composition.

1H NMR (300 MHz, DMSO-d6): δ to the (C6) Compound: 0.89 (t, 3H, 3JHH=8.0 Hz, CH3), 1.24-1.44 (m, 6H, 3CH2), 1.74 (m, 2H, CH2), 4.04 (t, 2H, CH2), 6.88-8.50 (m, 12H, aromatic benzene), 8.64 (s, 1H, CH=N), as displayed in Figure 1 [21].

1H NMR (300 MHz, DMSO-d6) δ to the (C8) Compound: 0.90 (t, 3H, 3JHH = 8.2 Hz, CH3), 1.22-1.46 (m, 10H, 5CH2), 1.74 (m, 2H, CH2), 4.03 (t, 2H, CH2) 7.04-8.0 (m, 12H, aromatic benzene), 8.66 (s, 1H, CH=N), as displayed in Figure 2.

Table 3. Shows some infrared spectra of the prepared compounds (C4 – C8) (IV)

Table 4. shows the FT-IR absorption values for the Schiff bases compounds

Figure 1. The 1H NMR spectrum of the compound C6

Figure 2. The 1HNMR spectrum of the compound C8

Results and Discussion

Discussion of liquid crystalline behavior (General concepts)

As shown in the above scheme, a new series of azo-Schiff bases was produced by coupling the reaction of the diazonium salt with aniline, then the output was reacted with an aldehyde to obtain the final product.

The liquid crystalline behavior was determined for most of the prepared compounds by using polarizing the optical microscope POM and Differential Scanning Calorimetry (DSC) techniques. It has been taken roughly (2–3) mg of dry matter and heated it in an inert environment of N2 [22-25].

DSC was used to identify thermal transitions in liquid crystal compounds, as show in Figure 3 and 4, also determined the nature and type of thermal liquid crystalline behavior with the use of POM. Figures 5-12 shows the results of texture of liquid crystalline phases.

Most of the compounds are mesomorphic, with a variety of SmA, SmC, and N phases, based on the findings. Most of the prepared compounds indicated liquid crystal phases with a high-temperature range, and the reason for this is the presence of three aromatic rings linked by bonds that increase the electronic conjugated along the axis of the molecule, as well as the presence of the aliphatic terminal groups.

The nematic phase of some of the prepared compounds revealed the schlieren texture [26, 27], as well as, the marble texture [28].

As shown in Table 5, the reason for the appearance of liquid crystalline phases in most of the prepared compounds was the construction of a mesogenic unit consisting of three aromatic rings linked by bond that increase the electronic conjugated, in addition to the presence of aliphatic terminal groups that increase the flexibility of the molecule at the ends.

All the above effects were the main reason for the liquid crystal behavior of the prepared compounds.

POM was used to determine the types of liquid crystal phases [29, 30]. All the compounds were monotropic and it was only in heating.

This study showed that the thermal range of the liquid crystalline phases exhibited by compounds was affected by the length of the aliphatic chain.

Figure 3. DSC analysis of the compound C5

Figure 4. DSC analysis of the compound C6

Figure 5. SA comp. C4

Figure 6. SA comp. C6

Figure 7. N comp. C6

Figure 8. SC comp. C6

Figure 9. SC comp. C8

Figure 10. N comp. C5

Figure 11. N comp. C4

Figure 12. N comp. C8

Table 5. Liquid crystal phase transitions in a differential scanning calorimeter (DSC) and a polarizing optical microscope (POM) for prepared substances

Conclusions

From the practical results which were obtained and discussed, the conclusions can be summarized as follow:

  • The prepared compounds were of high purity, and this was confirmed by various spectroscopic tests.
  • Raising the number of aromatic rings led to an increase in the hardness of the molecules, which in turn caused an increase in their melting points.
  • There is no odd-even effect on the liquid crystalline properties of the series compounds.
  • Most of the prepared compounds revealed liquid crystal phases with a high-temperature range due to the presence of three aromatic rings in the center of the molecule linked by bonds increase the electronic conjugated, as well as the presence of aliphatic terminal groups that increase the flexibility of the molecule.

Acknowledgements

The authors appreciate everyone who assisted them get the chemicals material. The authors would like to express their gratitude to everyone who assisted them in calculating, analyzing, and interpreting the data.

Disclosure Statement

No potential conflict of interest was reported by the authors.

How to cite this manuscript: Mohammed Mezher Aftan*, Fadhil Dawood Khalid, Hanaa Kaain Salih. Synthesis and characterization of new azo amino compounds and study of impact of aliphatic chain length on liquid crystalline behavior. Asian Journal of Green Chemistry, 6(2) 2022, 155-165.  DOI: 10.22034/ajgc.2022.2.5

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