Substituent Effects on Absorption Spectra of 7-Amino-4-Hydroxynaphthalene-2- Sulfonic Acid Based Dyes

The synthesis of dyes derived from coupling 7-amino-4-hydroxynaphthalene-2-sulfonic acid (a diazonium salt) with different coupling agents (phenol, p-nitrophenol, vanillin and salicylic acid) respectively yielded four different dyes namely; Dye A, 4-hydroxy-7-((2-hydroxyphenyl)diazenyl)naphthalene-2-sulfonicAcid, Dye B, 4-hydroxy-7-((2-hydroxy-5nitrophenyl)diazenyl)-4-hydroxynaphthalene-2-sulfonic Acid, Dye C, 7-((2-formyl-5-hydroxy-4-methoxyphenyl)diazenyl)-4hydroxy naphthalene-2-sulfonic Acid and Dye D, 2-hydroxy-6-((5-hydroxy-7-sulfonaphthalene-2-yl)diazenyl)benzoic Acid. The wavelengths of maximum absorption of the dyes were determined in different solvents; water, DMF, ethanol and acetone, their λmax was between 510-600nm. The synthesized dyes were characterized using IR and their melting points determined. The dyes were applied on wool and silk and the effect of pH, time, temperature on the %exhaustion for both fabrics was determined. The optimum pH for absorption of the dyes on both fabrics was 3 & 5, optimum time 50, 75 & 90 minutes and optimum temperature 75 and 90C depending on the dye and type of fabric. The Fastness (wash, rub and light) properties of the dyes were also assessed. DOI: https://dx.doi.org/10.4314/jasem.v24i7.17 Copyright: Copyright © 2020 Iyun et al. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 16 May 2020; Revised: 29 June 2020; Accepted: 07 July 2020

Dyes are colouring agents used in the manufacture of printing inks, paper, paint, pharmaceuticals and textile industries (Gouthaman et al., 2018). They are coloured compounds which show an affinity towards the substrate to which they are being applied. They are generally applied in an aqueous solution (Hassan, 2016;Agho et al., 2017). Considerable innovation has been witnessed in the field of azo dye chemistry based on heterocyclic systems and studies in the synthesis of such derivatives have been reported. Most of the recent research has focused on structural variations of existing types, for example, variations in substituent, especially on the side chains of the coupling components. Many different heterocyclic diazo components have been studied, especially derivatives of thiazole, imidazole etc. owing to the marked effect of such groups on the absorption spectra of dyes (Bashandy et al., 2016;Wadia and Patel 2008). Acid dyes are water soluble anionic dyes and are typically only applied to fibres with positive charges such as polyamide in an acidic bath. They are not used for cotton colouration owing to their low affinity for the fibre and small molecular size that makes it easy for the dye molecules to move out in water. (Vashi et al., 2014;Jabli et al., 2011). Acid dyes are primary organic acids, usually available to the dyer in the form of salts. They are generally applied to fibre from solutions containing sulphuric, formic or acetic acids. However, majority of them are sodium salts of aromatic sulphonic acids but there are a few containing carboxylic groups. Most acid dyestuffs acquire their acidity from the presence of sulphonic acid groups or nitro groups in the molecule (Musa et al., 2013). Modification of substituents e.g. electron donating or electron withdrawing leads to effects (such as bathochromic and hypochromic) in the absorption spectra of dyes (Jiang et al., 2018).

Synthesis of Dyes
General Procedure for Diazotization: 2.0ml of distilled water was added in a beaker with a drop wise (20 drops) addition of 10% H2SO4 (sulphuric acid), with continuous stirring while keeping the temperature constant (0-5 0 C) using an ice bath. 7-amino-4hydroxynaphthalene-2-sulfonic acid (2.39g, 0.01mol) in powdered form was added with constant stirring to the solution until properly dissolved. Sodium Nitrite NaNO3(0.8g) was added to another beaker and properly dissolved with 1ml H2O, this mixture was also kept at constant temperature (0-5 0 C) and then finally introduced drop wise to the earlier reaction mixture with constant stirring for about 20minutes.
General Procedure for Coupling: 0.01mole of the coupling agent (Phenol) was dissolved in 1ml 1:1 acetic acid/water while in an ice bath to maintain constant temperature (0-5 0 C). The previously prepared diazonium salt was then added dropwise to the solution over a period of 20minutes with vigorous stirring. 1M of NaOH was also added dropwise till the solution becomes neutral. Stirring was continued for another 20minutes and the pH of the solution measured and the colour noted. 0.5ml of the reaction mixture was then added to 2ml of 1M NaOH and the colour also noted. 0.5ml of the reaction mixture was also added to 2ml of 1M HCl and the colour noted. The reaction mixture was changed to the pH that gives the best colour. The dye mixture was cooled on ice to precipitate. Once the solid had formed, the precipitate was filtered and washed with water. The precipitate was then recrystallized by heating using 1:1 ethanol/water. This procedure was also followed in coupling the other coupling agents (p-nitrophenol, salicylic acid and vanillin) with the diazo component.

Application of the dye to Fabric (silk and wool):
1% stock solution was prepared and 2ml solution measured so as to obtain 2% shade. The dye bath prepared for each dye was varied at different pH values (3,5,7,9,12), at different time intervals (10,30 ,50 ,70, 90 minutes) and different temperature (20 0 , 45 0 , 60 0 , 75 0 , 90 0 C). 1g of the fabric was weighed and wet before dyeing at the above conditions. Fastness to Washing Test: ISO wash test NO.3 was used. The prepared dyed sample was placed in a conical flask with a solution of Soda Ash 2g/L, Soap 5g/L, Liquor Ratio (L.R) 50:1 for 30 minutes at 60±2 0 C. The washed samples were rinsed in cold water and then dried at room temperature. The grey scale was used to assess the change in colour between the washed and unwashed sample.
Fastness to Light Test: The dyed wool and silk samples (about 3cmx2cm) was placed under direct sunlight for about 6-8hrs daily. It was left for about 2 weeks and adequate ventilation of the sample during exposure was ensured. As exposure proceeds, the samples under test and the standard dyed material were examined at frequent intervals and the change in color of the sample compared visually with the changes that have occurred in the standard. The exposure of the test samples was terminated after about 80hrs of exposure was ascertained and change in color was examined using the grey scale.
Fastness to Rubbing: Dry rub fastness method was used here. A White felt (1cmx1cm) was put into the grove of the rubbing hand and staged. The dyed sample fastened on the tester and run automatically for 1000counts and then rubbing fastness of the sample cloth and degree of staining is accessed by measuring the stained fret using the grey scale.

RESULTS AND METHODS
Four azo acid dyes were synthesized by coupling with a diazo component (J-acid) and four different coupling agents. The structures and physical characteristics of the dyes obtained are shown in Table 1 3  3  3  3  3  4  4  Dye C  3  3  4  3  4  4  3  3  3  3  3  3  3  3  Dye D  4  4  3  3  5  3  4  3  4  3  3  3  3 3 Where 1= minimum 5= maximum. Cs-Change of shade of dyed fabric, Sc-Staining of felt Substituent effects on Absorption Spectra: Substituents attached to a chromophore (the part of the molecule responsible for color) most times have a role to play in the change of spectral band position in the absorption of Electromagnetic Radiation by the dye molecule. These substituents are known as auxochromes. All dyes contain at least one chromophore i.e. a structure with alternating double and single bonds (Broadbent, 2001;Zollinger, 2003;Mather and Wardman, 2011). The four dyes synthesized in this research project share the same chromophores but have different auxochromes hence the difference in color and wavelength. Dye A has the presence of three major auxochromes (two OH groups and one SO3 group) with a wavelength of 541nm and a pink color in solution but Dye B has an additional auxochrome NO2 which is an electron accepting group, caused a bathochromic shift which increased its wavelength to 576nm and hence enhancing its colour to blue black in aqueous solution. Whereas Dye C has addition of two major auxochromes (OCH3, CHO) which are both electron accepting groups. This caused Dye C to have the longest wavelength (583nm) in water and the deepest color (greenish blue). Dye D had only the addition of a COOH group which is electron donating hence causing a hypsochromic shift and the shortest wavelength (539nm) in water.
Percentage Absorption: Figures 1-9 shows the percentage absorption of the dye by silk and wool fabrics, the wool showed mostly a higher percentage of uptake compared to the silk and this can be attributed to the structure of the fabric being dyed. Silk is more crystalline than wool hence the particles involved in the crystal of silk are more tightly packed thereby limiting the influx of the dye molecules within its packed space but wool on the other hand is not so crystalline which denotes that its structure is less packed and therefore the dye molecules can move more freely through it, enhancing its dye uptake . Silk possesses only one site for easy attachment of the dye i.e. the -NH site, for bonding to occur on the right site, the silk has to be treated first which was not carried out primarily in the application process of this project work, whereas, wool contains double sites for attachment on the right and left. Although polymeric materials like silk and wool use up their lone pairs and site of attraction to form the polymeric bonds, wool still showed greater absorption.
Wash fastness: The four dyes synthesized in this research project are all acid dyes which are known for their great solubility in aqueous solution, Therefore, wash fastness depends majorly upon the dyes (Zollinger, 2003). Due to the polar nature of water, the ionic bonds between the fabric and the dye will easily dissociate in water hence causing the colour fastness to be poor in aqueous solution. The high solubility of these acid dyes caused a high percentage exhaustion because the dye molecules from the dye bath easily bonded to the fabric due to high polarity but when placed in the wash liquid the dye which had been attached also easily to the fabric can dissociate.

Conclusion:
The study showed that a new set of azo acid dyes were synthesized in fairly moderate and good yields with very good to excellent uniform dyeing. The diazo component and nature of the substituents in the coupling components have effects on the visible absorption and the shade of dyeing. Parameters such as pH, time and temperature also have effects on the percentage absorption and fastness properties of said dyes. The synthesized dyes were all soluble in water as this can reduce the cost for purchasing of solvent for dilution, testing and application.