Synthesis, spectroscopic characterization and dyeing performance of novel bis azo dyes derived from benzidine

Benzidine was coupled with ethyl cyanoacetate, and malononitrile, to give azo-hydrazo products which in turn were cyclized by using hydrazine and phenyl hydrazine to give 4,4'-([1,1'-biphenyl]-4,4'-diylbis(hydrazin-2-yl-1-ylidene))bis pyrazole derivatives 5–7. These compounds were identified by various spectral analysis. The examination of 0.1 M NaOH and 0.1 M HCl in DMF revealed that the λmax of the synthesized dyes are quite sensitive to pH variation and slightly affected by the coupler moieties. Utilizing the dispersion agent DYEWELL-002, polyester fabric (PE-F) was dyed in water. The color strength (K/S), its summation (K/Ssum), dye exhaustion (%E) and reflectance values were measured and discussed. The DFT method estimates the chemical descriptor parameters of the titled dyes, using B3LYP/6-31G(d,p) level to investigate the performance of dyes as well as to postulate a mechanism of dyeing process.

www.nature.com/scientificreports/ of hydrazine and phenylhydrazine on dye 3 forming dyes 5a and 5b, respectively. The IR spectra of dyes 3 and 4 showed an intense peak at υ 2210-2220 cm − 1 due to cyano groups vibrations. The disappearance of the -CN stretching vibration in IR spectra of compounds 3 and 4 and the presence of NH 2 stretching vibration at ʋ www.nature.com/scientificreports/ 3419-3366 cm − 1 in dyes 5-7 also confirmed the formation of pyrazole rings. The weak peak at ʋ 1552 cm − 1 for azo group (N = N) is assigned to dye 7. The observed medium peak at υ 1625-1539 cm − 1 of dyes 3-7 is assigned to imino (C = N), while dyes 3-7 showed a weak peak at υ 3064-3010 cm − 1 due (C-H sp 2 ) stretching vibration and weak peak at υ 2988 cm − 1 for (C-H sp 3

) of dye 3.
According to the IR measurement in the solid phase, dyes 3-5 lacked the -OH peak at 3368-3401 cm − 1 , and the appearance of the peak at ʋ 3414-3182 cm − 1 corresponding to NH groups of the hydrazo moiety indicate that the dyes 3 and 5 exist in the hydrazo form 3(I), and 5(I), respectively. The absence of NH peak and the appearance weak peaks at ʋ 1563 cm − 1 for the azo group (N = N) in dye 7, indicate its existence in the azo form 7.
An initial clue for assigning the tautomeric state was provided by the 1 H NMR spectra, which demonstrated conclusively that a range of newly dyes exist in solution exclusively in the hydrazo, azo or an equilibrium mixture of several tautomeric species and definitive assignments of all the signals in the spectra were made. At first, the 1 H NMR spectral data do not show any more high field signals around 5.5 ppm for CH of tautomeric forms 3(III), 4(II), 5(III) and 6(III) of the annulated similar compounds, respectively 24,25 . Therefore, these tautomers are hardly formed because of their low stabilities due to the less degree of conjugation.
The 1 H NMR spectra of bis azo dyes 3-6 in DMSO-d 6 (solution phase) showed highly deshielded protons in the range δ 13.13-10.79 ppm, attributable to two NH protons of hydrazo tautomers, except dye 7. It was reported that the hydrazo NH proton resonance often develops between 12.0 and 15.0 ppm 26 .
The appearance of exchangeable broad singlet signals at δ 12.99-10.55 ppm in the downfield region for dye 5a, are assigned to NH protons of the of pyrazole ring moieties. The 1 H NMR of dyes 3-7 exhibited two doublet signals in the range δ 7.97-7.54 ppm, assigned to the aromatic protons of the biphenyl moiety. The 1 H NMR spectra exhibited signals in the range δ 7.73-7.11 ppm, assigned to phenyl ring moieties for dyes 5b and 7. Compound 3 showed a highly shielded proton at δ 1.32 ppm, attributable to CH 3 protons and a quartet signal at δ 4.31 for the methylene group. The measurement of IR in the solid phase and the 1 HNMR in the solution phase showed that all dyes under investigation exist as hydrazo tautomers in both phases, while dye 7 exists only in the azo form in both solid and solution phases.
Theoretical analysis has been applied to calculate the tautomerization energy, Table 1, for the two tautomers of dyes 3-6 and the azo tautomer of dye 7, which lacks conjugation to an acidic proton. The geometric optimization of dyes 3-6 revealed that the hydrazo tautomer is more stable, with a lower relative energy (∆E = 24.43-35.72 kcal/mol) than the azo tautomer, which supports the experimental data, see the Supplementary information.
Electronic absorption spectra of azo dyes 3-7. The experimental and theoretical electronic absorptions data of bis azo dyes 3-7 are compared, Table 2. The B3LYP/6-31G(d,p) method was used to optimize the structures in both tautomeric forms and determine the computed λ max values, which lie in the range of (426-474 nm) for the hydrazo tautomer and in the range of (330-466 nm) for the azo tautomer. In contrast, the experimental absorption of the dyes 3-7 lies in the region (408-462 nm), Table 1 and Fig. 3. The estimated TD-DFT λ max of the hydrazo tautomer for dyes 3-6 agrees with the experimental absorption data.
The absorption spectra of dyes 3-7 were recorded in DMF. The value of λ max for all dyes depends on the coupler groups with the locality of -NH 2 (5-7) and C = O or enolic hydroxyl group (3, 5a,b) group present in the heterocyclic pyrazole moiety. The expected tendency of the higher λ max of dye 6-7 was observed because of the presence of four electron rich NH 2 groups at the 3,5-positions in the pyrazole rings compared with C = O of dyes 5a,b. While the higher λ max of dyes 5b and 7 than those 5a and 6, respectively is related to the presence of phenyl groups in the former dyes which increase unsaturation and conjugation.
The first absorption band appeared in the wavelength range 252-260 nm in DMF solution corresponding to the well-known low energy σ-σ* transition. The second band in the rang 408-462 nm is reported for n-π* transition of the hydrazone group 27 . Moreover, absorption band at 332-350 nm attributed to azo structures 28 is missing for dyes 3-6. Accordingly, the possibility of existence of azo structures of dyes 3-6, are rejected and provides support for the hydrazone structure as the most plausible tautomeric structure. While the dye 7 exhibit two absorption band at 462 and 350 nm assigned to the n-π* and π-π* transition corresponding to azo group 29 . www.nature.com/scientificreports/ Dyes 5b and 7 showed bathochromic shift is presumably due to the presence of extra phenyl rings, While dye 3 exhibited a hypsochromic shift. This suggests that the coupler moieties significantly influence the positions of the absorption bands.
Effect of acid and base. Azo dyes are considered as acid-base indicators because they affect the reaction color, which causes a change in UV/vis absorption due to a protonation equilibrium 30 . Due to the presence of four basic nitrogen atoms, the possibility of protonation and deprotonation of bis azo dyes 3-7 in DMF-H 2 O were investigated using 0.1 M sodium hydroxide and 0.1 M hydrochloric acid, Figs. 4 and 5, respectively. The impact of the acid and base on the absorption spectrum of the dye solutions was examined and the results revealed that the absorptions of the titled dyes are quite sensitive to pH medium, Table 2.
It was found that the addition of 0.1 M hydrochloric acid causes a hypsochromic shift in the UV/vis absorption maxima (max) of dyes 3-6 in DMF solution. This is because protonation of hydrazo groups can prevent resonance between biphenyl and nitrogen of hydrazo group. While the addition of acid to dyes 7 in DMF solution showed a small hypsochromic shift of maxima λ max , this is because resonance is slightly inhibited due to protonation of azo and/or amino groups.
The slight bathochromic shift of λ max for the bis azo dyes 3-6 upon addition of NaOH in DMF is presumably due to the hydrazo proton is deprotonated. As a result, the conjugation is increased either between anionic form of hydrazo group and pyrazole moiety of dye 4 and 6 or between anionic form of hydrazo group with cyano group as in dyes 3 and 4. On the other hand, dye 7 in DMF solution showed no significant bathochromic shift due to deprotonating process is not freely available and we suggest that dye 7 exists in the azo tautomer in neutral, acid and alkaline media.  www.nature.com/scientificreports/ Dyeing process and fastness properties. The disperse insoluble dyes in water that applied from aqueous dispersion rather than from solution was commercially and environmentally developed for coloration of synthetic fibers. The completely insoluble dyes in water were turned application-ready by being dispersed to microscopic-fine particles of the order of a few microns in the presence of dispersing agents. The more hydrophobic fiber could then be dyed using the resulting easily dispersible solid, which partitioned into the fiber from low concentrations of dye bath. The synthesized disperse dyes under study 3-7 were applied to polyester textiles (PE-F) using a high temperature (HT) dyeing process with a material to liquor ratio of 1:20 at 130 °C. In dyeing process, 2% dye based on the weight of the PE-F was utilized. To enhance the dye solubility in water, DYEWELL-002 was used as a dispersing agent in the dye bath. The dispersed dye was first adsorbed by the surface of PE-F followed by its diffusion through the fiber. The dyeing process was running using aqueous acetic acid at pH 4-5, and then temperature of the dye bath was raised to 130 °C in a dyeing machine under pressure at a rate of 3 °C/min. After being pierced for 60 min, the temperature was cooled to 50 °C. After the dyeing process was finished, the PE-F was rinsed and treated with aqueous solutions of sodium hydrosulphite (1 g/L) and NaOH (1 g/L) to achieve surface reduction, then the dyed PE-F washed with H 2 O and finally was air dried, Fig. 6.
Bis azo dyes 3-7 have been subjected to the standard procedure 31 for color fastness, which includes washing, hot pressing (scorch) (cotton and polyester), perspiration (acidic and alkaline) and light fastness, Table 3. Gray scale ratings ranging from 1 (poor) to 5 (excellent) were used to evaluate all of the measured properties 32 .  www.nature.com/scientificreports/ Table 3 demonstrates that bis azo dyes 3-7 exhibited excellent fastness levels to washing, perspiration, and scorch on polyester fabric except dye 5b has moderate fastness levels. In terms of light fastness, dyes 4, 6, and 7 displayed good results.
Dye exhaustion, reflectance and color strength. The exhaustion percentage (%E) of the disperse dyes 3-7 by PE-F in H 2 O was optically calculated by measuring the concentration of dye bath before (C 1 ) and after (C 2 ) dyeing spectrophotometerically via UV/visible instrument (pg. T80 +) at λ max of the appropriate dye 33 by Eq. (1) 34 . Dyes 3 and 5b demonstrated comparatively excellent %E (above 80%), dyes 4 and 7 with good %E (above 70%) while dye 5a and 6 displayed relatively low %E for PE-F %E (above 60%), Table 4.
The quantity of visible light that a color of dyed fabric could reflect was represented by its reflectance value. As a result, white colored dyed fabric reflects all light components and has a 100% light reflectance, whereas black color has 0% reflectance since it absorbs all light components. Therefore, all other colors' reflectance ratings   www.nature.com/scientificreports/ fall between these two extremes. The measured reflectance values of dyed fabric by azo dyes 3-7, Table 4, were obtained using a UV/vis/NIR-spectrometer Jasco-V-570 (1), across the range of 190-2500 nm. The color strength (K/S ) of the dyed PE-F was determined at λ max by the use of Kubelka-Munk 34 Eq. (2) and given in Table 4.
where K the absorption coefficient, S the scattering coefficient and R is the decimal fraction of the reflection of the dyed fabric. Clearly, the current reflectance curves follow a similar pattern with a tiny band shift. As a result, all samples are yellowish orange bag-reddish orange and brown bag. The color strengths (K/S) of dyed PE-F, Table 4, ranged from 0.93 to 13.5, indicating that the coupler moiety mostly determines the color strength. This is obviously observed form the change in color from bright yellow to brown tones when the materials were dyed with dyes 3-7. While dye 5a displayed the lowest color strength value, the PE-F sample dyed with dyes 3 and 5b displayed a high K/S value. There are good agreements between the E% with color strength for dyeing of PE-F by dyes 3-7, Fig. 7.
The color strength summation (K/Ssum) values are calculated by Eq. (3) for dyes 3-7 samples in the visible spectrum ranging from 390 to 700 nm.
The K/Ssum metric is a widely used measure in the textile industry for evaluating color strength and is an important parameter for assessing the suitability of different dye-coupler combinations for various applications. As shown in Table 4, we observed a clear positive relationship between the K/Ssum value and the color yield for each dye-coupler combination. It was found that dye 5b produced the darkest brown hue when dyeing PE-F, as evidenced by its highest K/Ssum value among all the dyes tested. On the other hand, dye 5a produced the lightest color depth, with the lowest K/Ssum value.
Our results also demonstrate that even small differences in the structure of the dyes can have a significant impact on the K/Ssum value and ultimately the color yield. This underscores the importance of carefully selecting the appropriate dye-coupler combination for a given application, taking into account factors such as the fiber type, dyeing conditions, and desired color outcome. In addition to its value in laboratory-scale experiments, the K/Ssum metric has practical applications in industrial settings, where it can be used to assess the color strength and suitability of different dye formulations for large-scale production.
(2) www.nature.com/scientificreports/ DFT investigation and dyeing mechanism. Based on its theoretical basis, the density function theory (DFT) is the simplest method for investigating the molecular geometry 19 and it can be used to postulate the dyeing mechanism according to the dye class. Because of dye bath is acidic (pH 4-5 by acetic acid), theoretical investigation will perform on protonated dye species. The chemical descriptor parameters, Table 5, 35,36 of the protonated tautomer forms of the bis azo dyes 3-7 were calculated using the DFT method in order to study dyeing efficiency and to forecast the mechanism of the dyeing process.
The ionisation potential IP and electron affinities EA values of any chemical species have been related to their lowest E LUMO and highest occupied molecular orbitals E HOMO , respectively 35,36 , Eqs. (4)(5).
The energy lowering caused by the maximum electron flow between the donor and acceptor was measured by the electrophilicity index parameter ω 39 , Eq. (11).
The inclination of a molecule to transfer electrons to acceptor molecules with lower energy MO is reflected by the high values of the E HOMO . The energy E LUMO represents a molecule's capacity to take electrons 40 . In general, as the HOMO and LUMO energy levels of a molecule change, so does its capacity to bind. In other words, independent of lowering HOMO and increasing LUMO energy levels, a molecule's capacity to bind is improved when ∆E value is lowered. i.e. Accordingly, the dyeing strength increases when the dye molecule has a low ∆E value. While the softness parameter σ will reflect strong dyeing strength, the greater values of chemical hardness η will provide low dyeing for fibre.
The electrophilicity index ω 41 is a description that might depict the dye power of chemical species. The molecule's global electrophilicity index enables quantitative assessment of its reactivity [42][43][44] . The electrophilicity index displays the ability of the electron-accepting ability 45 . It should be emphasised that strong colour strength should have low electronegativity values 46 . As a result, the increase in chemical potential causes increasing of dyeing strength. www.nature.com/scientificreports/ Table 5 points out that, dye 3 has the highest dyeing strength based on parameters E HOMO , E LUMO , ∆E, η and S while dye 4 is the highest depending on parameters χ and µ eV. The correlation results of each calculated parameter with %E and K/S revealed that color strength (K/S) and %E depend mainly on the effect of dipole moment. The higher in dipole moment in dyes 3 and 4 (5.3932 and 4.2716, respectively), the higher in both K/S and %E. The cyclized pyrazole dyes 5a,b and 6-7 showed lower in dipole moment than their parent dyes 3 and 4 which correlate with their lower value of K/S and %E, respectively.
Dyeing mechanism. The proposed method for dyeing using protonated tautomers of dyes 3-7 involves the initial adsorption of the dye onto the PE surface via hydrogen bonding and/or electrostatic attraction. The polyester's carbonyl oxygen and/or etheric linkages can create a hydrogen bond with the protonated hydrazo moiety from the dye (NH….O = C) with length = 1.60681 Å for dye 3 and length = 1.6337 Å for dye 5a. Finally, the diffusion process is completed by heating, Figs. 8 and 9.

Conclusion
The bis azo dyes 3-6 were found to exist in the hydrazone tautomer in both the solid and solution states, while dye 7 existed in the azo form due to the lack of a conjugation to an acidic proton. The estimated TD-DFT λ max of the titled dyes agreed with the experimental absorption data. Dyes 5b and 7 showed bathochromic shift, whereas dye 3 exhibited a hypsochromic shift, suggesting that the coupler moieties significantly influence the positions The correlation analysis between the calculated chemical descriptor and the dyeing data (%E and K/S) indicated that K/S and %E mainly dependent on the dipole moment.

Experimental
General method for synthesis of dyes 3, 4.
Step 1: Preparation of diazonium 2. Benzidine 1 (5.52 g, 0.03 mol) was placed in a 250 mL conical flask, a mixture of 15 mL conc. HC1 with 10 mL water was added and stirring till clear solution, the mixture kept at 0 °C in an ice bath. Mixture of sodium nitrite (4.14 g, 0.06 mol) in 10 mL cold water added to the benzidine hydrochloride mixture slowly with constant stirring.
Step 2: Reaction of diazonium salt with coupling reagent. The former prepared diazonium salt solution 2 was then added dropwise to the coupler solution, namely, ethyl cyanoacetate and malononitrile, (0.06 mol) in 17 mL ethanol and 30 g sodium acetate in 15 mL of water (keep the mixture alkaline) at 0 °C. The progress of the reaction was monitored by TLC and then crude dyes were filtered, washed with hot water for several times.
General method for synthesis of compounds 5a, and 6. A mixture of compound 3 or 4 (0.004 mol) and hydrazine hydrate 98% (0.008 mol) was refluxed in 15 mL ethanol for 4 h. The progress of the reaction was monitored by TLC. The formed precipitate was filtered, dried and recrystallized from methylene chloride.
General method for synthesis of compounds 5b, and 7. A mixture of compound 3 or 4 (0.004 mol) and phenyl hydrazine (0.008 mol) was refluxed in 15 mL ethanol for 4 h. The progress of the reaction was monitored by TLC. The formed precipitate was filtered, dried and recrystallized from methylene chloride.  Figure 9. The optimized structure of dye 5a and methyl terephthalate monomer and the bond lengths between the hydrazo hydrogen atom with the carbonyl oxygen of the methyl terephthalate monomer.

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.