Methoxy and Hydroxy Triphenylamine-Based Azo Dyes: Synthesis and Photophysical Properties on Polyester and Nylon Fabrics

Abstract

Four novel methoxy- and hydroxy-triphenylamine-based azo dyes were synthesized and applied on polyester and nylon fabrics. Photophysical properties of the dyes were studied in different organic solvents. Lightfastness and sublimation fastness properties were evaluated. Level dyeing with intense red and yellow colors was observed for these dyes on polyester fabric. Comparatively, brighter color and higher color strength (K/S) values were observed on polyester fabric than on nylon fabric dyed with these dyes. Good sublimation fastness and moderate lightfastness properties were achieved with all dyes on both fabrics. From net electrophilicity index and calculated energies, the azo tautomer was more stable than the respective hydrazone tautomer in a vacuum.

Keywords

Azo Dyes Dyeing Electrophilicity Index Exhaustion Fixation Lightfastness Sublimation Fastness Triphenylamine

Introduction

Azo dyes are the largest class of synthetic dyes—they cover the entire spectrum of colors.¹ Azo dye colors can be easily tuned by varying different auxochromic groups in the dye.² They have advanced applications³ and are used as food colors, sensors, and as nonlinear optical materials.^4–6 They are used for dyeing wool, nylon, cotton, and silk materials, as well as synthetic materials like polyester, acrylic, and cellulose acetates.^7–9

The photophysical properties of azo dyes are dependent on the donor and acceptor groups and strength of the substituents present on either side of the azo system.¹⁰ Various donor groups such as carbazole,¹¹ phenol,¹² and N,N-diethylaniline are used in azo dye synthesis. However, in spite of their wide application in organic light emitting devices, dye sensitized solar cell, non-linear optics, and metal sensing,^13–16few triphenylamine-based azo dyes have been reported as nonlinear optical materials.^17,18 Azo dyes that possess intramolecular hydrogen bonding improve their photostability.¹⁹

Azo disperse dyes are widely applied on polyester fabrics.^{20,21 This}class of dyes is mostly water-insoluble and is small in molecular weight. The small size has limitations for textile dyeing—these dyes can sublime, limiting their retention on fabric at higher temperatures.²² To avoid such problems, comparatively larger or more polar molecules are desired. There is a need for larger molecules with bulky substituents that can easily dye polyester.

Herein, we synthesized methoxy and hydroxy triphenylamine-based azo dyes. Different substituents were used to study changes in color shade and dyeing properties. Systematic changes in photophysical properties and color on dyed polyester and nylon were demonstrated for these new azo dyes.

Experimental

Materials and Methods

Triphenylamine and pyridine hydrochloride were procured from Sigma-Aldrich. 1,10-Phenanthroline and copper iodide (S. D. Fine Chemical Ltd.) and potassium tert-butoxide and sodium nitrite (Spectrochem Pvt. Ltd.) were of commercial grade and used without further purification. All other chemicals were of analytical grade and purchased from S. D. Fine Chemical Ltd. Hexane, ethylacetate, chloroform, dichloromethane, N,N-dimethylformamide, dimethylsulfoxide, and methanol solvents were purchased from S. D. Fine Chemical Ltd. and were purified by distillation on the rotary evaporator.

All reactions were monitored by TLC (thin-layer chromatography) with detection by UV light at 254 nm. Melting point was recorded in an open capillary on a Sunder Industrial Product and is uncorrected. The absorption spectra of the synthesized dyes were recorded on a Perkin-Elmer Lambda 950 spectro-photometer. Freshly prepared solutions in solvents of different polarities at a concentration of 1 × 10^–6 mol/L solution were used in 1-cm optical path length quartz cuvettes. ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were recorded on a Agilent 500 MHz instrument.

Ready for dyeing nylon (70 d/24 f, plain weave, 210 × 191/5 cm), and polyester substrate (weight 70 g/m², ends/in. = 105 and picks/in. = 94) were purchased from Suresh Kumar & Bros.

Lightfastness of dyed fabric was measured on a Q-Sun Xenon Test Chamber (Q-Lab Corp.) and sublimation fastness tested on a sublimation fastness tester (RBE Electronics and Engineering Pvt. Ltd.) A computational study was performed using the Gaussian 09W software.²³ Ground state optimization modeling was done using a B3LYP functional²⁴ and 6-31G (d) basis set.²⁵

Synthesis

3-Methoxy-N,N-diphenylaniline (1) and 3-(diphenylamino)phenol (2) were synthesized according to literature procedures.^26,27 Aniline, p-anisidine, and p-nitroaniline were diazotized and coupled with 1 and 2 (Scheme 1). All dyes were characterized by ¹H NMR, ¹³C NMR, and CHN analyses. Structures of the synthesized dyes are given in Fig. 1.

Scheme 1

Synthetic scheme for dyes 3 and 4a-c.

Fig. 1

Structures of the triphenylamine-based azo dyes 3, 4a, 4b, 4c.

3-Methoxy-4-((4-nitrophenyl)diazenyl)-N,N-diphenylaniline (3)

p-Nitroaniline (0.27 g, 1.95 mmol) was dissolved in water (0.5 mL) by adding conc. HCl (2 mL). To this solution, sodium nitrite (0.2 g, 2.93 mmol) in 1 mL of distilled water was added at 0 °C. Urea (0.01 g) was added to consume excess nitrous acid. This solution was added dropwise to the solution of 1 (0.5 g, 1.817 mmol) in methanol (5 mL) and water (2 mL) at 0 °C. Neutral pH was maintained using aqueous NaOH solution. The mixture was stirred for 4 h at room temperature (RT). The reaction was monitored by TLC. The precipitate that formed was filtered and dried. The product was purified by column chromatography using petroleum ether as eluent to give the brick red solid 3 (40%), mp 166-170 °C. ¹H NMR (CDCl₃, δ): 8.32 (d, J = 7.8 Hz, 2H), 7.93 (d, J = 7.9 Hz, 2H), 7.71 (dd, J = 9.0, 1.2 Hz, 1H), 7.36 (t, J = 7.4 Hz, 4H), 7.22 (d, J = 8.6 Hz, 4H), 7.19 (t, J = 6.9 Hz, 2H), 6.62 (d, J =1.8 Hz, 1H), 6.59 (d, J = 9.0 Hz, 1H), 3.83 (s, 3H). ¹³C NMR (CDCl₃, δ): 159.47, 156.98, 153.85, 147.57, 146.12, 136.61, 129.65, 126.31, 125.12, 124.69, 122.92, 117.80, 113.07, 103.35, 56.18. Anal. Calcd for C₂₅H₂₀N₄O₃: C, 70.74; H, 4.75; N, 13.20. Found: C, 70.76; H, 4.75; N, 13.18.

Synthesis of Dyes 4a-c

p-Nitroaniline, p-anisidine, or aniline (2.10 mmol) was dissolved in water (0.5 mL) by adding conc. HCl (2 mL). To this solution, sodium nitrite (3.15 mmol) in 1 mL of distilled water was added at 0 °C. Urea (0.01 g) was added to consume excess nitrous acid. This solution was added dropwise to the solution of 2 (0.5 g, 1.91 mmol) in sodium hydroxide (0.076 g, 1.913 mmol), methanol (5 mL), and water (2 mL) at 0 °C. The pH was maintained between 8 and 9. The mixture was stirred for 4 h at RT. The reaction was monitored by TLC. The precipitate that formed was filtered and dried. Product was separated by column chromatography using petroleum ether as eluent to give solids 4a-c.

5-(Diphenylamino)-2-((4-nitrophenyl)diazenyl)phenol (4a)

Product 4a was isolated as a brick red solid (45%), mp 192-194 °C. ¹H NMR (DMSO-d₆, δ): 12.44 (s, 1H), 8.31 (d, J = 9.1 Hz, 2H), 7.98 (d, J = 9.0 Hz, 2H), 7.59 (d, J = 9.2 Hz, 1H), 7.44 (t, J = 7.9 Hz, 4H), 7.29–7.26 (m, 6H), 6.39 (dd, J = 9.2, 2.5 Hz, 1H), 6.10 (d, J = 2.5 Hz, 1H). ¹H NMR (DMSO-d₆, D₂O): δ 8.29 (d, J = 9.0 Hz, 2H), 7.95 (d, J = 9.0 Hz, 2H), 7.57 (d, J = 9.2 Hz, 1H), 7.43 (t, J = 7.8 Hz, 4H), 7.25 (dd, J = 9.5, 8.8 Hz, 6H), 6.37 (dd, J = 9.2, 2.5 Hz, 1H), 6.07 (d, J = 2.4 Hz, 1H). ¹³C NMR (DMSO-d₆, δ): 159.30, 154.90, 154.50, 147.06, 145.25, 133.94, 130.50, 128.34, 127.55, 126.75, 125.48, 122.35, 111.98, 104.55. Anal. Calcd for C₂₄H₁₈N₄O₃: C, 70.23; H, 4.42; N, 13.65. Found: C, 70.25; H, 4.42; N, 13.66.

5-(Diphenylamino)-2-((4-methoxyphenyl)diazenyl)phenol (4b)

Product 4b was isolated as an orange solid (40%), mp 122-126 °C. ¹H NMR (CDCl₃, δ): 13.64 (s, 1H), 7.77 (d, J = 8.7 Hz, 2H), 7.62 (d, J = 8.8 Hz, 1H), 7.34 (t, J = 7.7 Hz, 5H), 7.21 (d, J = 8.4 Hz, 4H), 7.18–7.14 (m, 2H), 6.99 (d, J = 8.7 Hz, 2H), 6.49 (d, J = 2.1 Hz, 1H), 3.88 (s, 3H). ¹³C NMR (CDCl₃, δ): 161.09, 155.05, 151.66, 146.28, 144.66, 133.45, 132.75, 129.55, 126.44, 124.85, 123.12, 114.45, 112.32, 107.39, 55.59. Anal. Calcd for C₂₅H₂₁N₃O₂: C, 75.93; H, 5.35; N, 10.63. Found: C, 75.95; H, 5.35; N, 10.64.

5-(Diphenylamino)-2-(phenyldiazenyl)phenol (4c)

Product 4c was isolated as a red solid (35%), mp 136-138 °C. ¹H NMR (DMSO-d₆, δ): 12.40 (s, 1H), 7.82 (d, J = 7.8 Hz, 2H), 7.63 (d, J = 9.0 Hz, 1H), 7.52 (t, J = 7.7 Hz, 2H), 7.43 (t, J = 7.8 Hz, 7H), 7.23 (d, J = 7.9 Hz, 4H), 6.43 (dd, J = 8.9, 2.3 Hz, 1H), 6.20 (d, J = 2.3 Hz, 1H).¹³C NMR (DMSO-d6, δ): 156.43, 152.76, 151.18, 145.84, 133.01, 130.41, 130.38, 129.84, 129.28, 127.15, 126.10, 122.01, 111.57, 105.68. Anal. Calcd for C₂₄H₁₉N₃O: C, 78.88; H, 5.24; N, 11.50. Found: C, 78.86; H, 5.24; N, 11.51.

General Procedure for Dyeing

Dyeing of polyester and nylon was carried out in a Rossari Labtech Flexi Dyer Machine using a material to liquor ratio (LR) of 1:30. The depth of the shade was 2% owf.²⁸ All synthesized azo dyes were solubilized in 5 mL of N,N-dimeth-ylformamide (DMF) and further diluted with a buffered solution of acetic acid pH 4-5 in water. Metamol was used as a dispersing agent. Dyeing was started at 80 °C for 15 min, the temperature was raised to 130 °C over 30 min, maintained at 130 °C for 1 h, and then rapidly cooled to 50 °C (Fig. 2). The dyed fabrics were rinsed with warm and cold water. For polyester, reduction clearing treatment using 2 g/L of sodium hydrosulfite, 2 g/L of sodium hydroxide, and 1 g/L soap solution in a 1:50 LR at 70 °C for 30 min. The treated fabrics were then rinsed with cold water and air dried.

Fig. 2

Dyeing curve.

Results and Discussion

Photophysical Properties

A photophysical study of the synthesized dyes (3 and 4a-c) was carried out in different organic solvents of varying polarity. The high-energy absorption peak in the UV spectrum at nearly 300 nm is due to π-π* transitions. A second, low-energy peak is due to the π-π* charge-transfer in chromophore. Due to the strong electron withdrawing nitro group, 4a shows red-shifted absorption at 505 nm in toluene. As the electron withdrawing strength of the substituents decreases (NO₂ > OCH₃ > H), the wavelength of maximum absorption (λ_max) also decreases for the corresponding dyes (4a > 4b > 4c). Dyes 4a, 4b, and 4c show λ_max values of 505, 464, and 459 nm in toluene, respectively (Fig. 3).

Fig. 3

Normalized absorption spectra of dyes 3 and 4a-c in toluene.

Absorption peak characteristics were evaluated using full width half maxima (FWHM). FWHM is the wavelength difference between the two points on either side of the maximum at which the intensity is at half of the peak's maximum value. FWHM of dyes 4a-c ranged from 80 nm (3946 cm^–1) to 103 nm (4712 cm^–1), and dye 3 showed a FWHM of 107 nm (4595 cm^–1) to 121 nm (4962 cm^–1). Dye 3 exhibited a broader peak than dyes 4a-c. Dye 4a gave high molar absorptivity values compared to 3, 4b, and 4c. Dyes 4a, 4b, 4c, and 3 gave molar absorptivity values of 67,400, 64,100, 52,650, and 58,200 dm³/mol∙cm respectively in toluene (Table I).

Table I

Absorbance Maxima, FWHM, and Molar Absorptivity of Dyes in Various Solvents

Solvents^a	Dye 3			Dye 4a			Dye 4b			Dye 4c
Solvents^a	λ_abs(nm)	FWHM (nm)	ε^b	λ_abs(nm)	FWHM (nm)	ε	λ_abs(nm)	FWHM (nm)	ε	λ_abs(nm)	FWHM (nm)	ε
Toluene	484	112	58,200	505	86	67,400	464	80	64,100	459	81	52,650
CHCl₃	499	113	58,713	507	93	70,200	463	82	61,650	459	83	45,800
DCM	505	118	55,350	505	95	72,700	465	85	74,500	455	86	55,550
THF	488	107	50,300	502	91	69,250	462	82	66,850	457	81	50,300
Acetone	488	114	52,300	498	91	68,290	458	84	64,750	453	85	49,900
Dioxane	481	108	46,700	499	93	70,000	458	84	63,350	455	85	50,550
EA	482	110	51,500	497	94	71,600	455	83	59,850	453	85	50,100
DMF	498	118	49,650	502	97	61,550	461	88	62,600	456	88	47,950
DMSO	504	121	51,492	504	103	60,100	461	93	60,027	456	94	45,021
CH₃CN	489	114	61,010	496	94	66,538	455	84	66,816	450	89	57,944
CH₃OH	486	117	55,750	487	97	67,231	457	85	63,850	453	88	64,463

DCM = dichloromethane, THF = tetrahydrofuran, EA = ethyl acetate, DMF = dimethylformamide, DMSO = dimethylsulfoxide

dm³/mol∙cm

Intramolecular Hydrogen Bonding

To study the effect of intramolecular hydrogen bonding on the photophysical properties of these dyes, we have selected dye 4a, where intramolecular hydrogen bonding was possible, and dye 3, where this possibility was ruled out. Dye 4a absorbed at a longer wavelength than 3 and the nature of the absorption bands were different (Fig. 4), with those for 3 being broader. The FWHM was 112 nm for 3 and 86 nm for 4a. Due to the intramolecular hydrogen bonding exhibited by –OH and –N=N–, the molar absorptivity of 4a should be greater than that of 3.¹⁹ Dye 4a had a molar absorptivity of 67,400 dm³/ mol∙cm and that for dye 3 was 58,200 dm³/mol∙cm.

Fig. 4

Comparison of properties of dyes 3 and 4a in toluene.

pH Effect

The presence of hydroxyl group in these dyes suggested that pH studies would be of value. Studies were carried out from pH 2 to 12. Aqueous DMF solutions were prepared and pH 2, 4, 6, 8, 10, and 12 solutions were created using 5% hydrochloric acid and 5% sodium hydroxide solutions. Fig. 5 shows the effect of various pHs on the new synthetic dyes. Dyes containing hydroxyl groups (4a-c) showed considerable changes in spectra as the pH changed. For 4a at acidic pH, 2 to 6 new peaks were observed at 317 and 365 nm. At basic pH (10 and 12), the peak at 502 nm diminished and a new peak at 632 nm was observed. This bathochromic shift of 130 nm was due to the formation of an enolate ion, which was stabilized by charge transfer from enolate to the nitro group. For 4b and 4c at basic pH, a new peak appeared at 514 and 521 nm, respectively. Any change in absorption of dye 3 at basic pH was not observed. For 3 at acidic pH, a peak at 370 nm appeared.

Fig. 5

Effect of different pH on the absorption of dyes 3 and 4a-c.

Dye Optical Properties

When we compared the synthesized dyes 3 and 4a with alkyl analogs 5 and 6 (Fig. 6),¹⁹ the alkyl analogs showed red-shifted absorption. The corresponding absorptions were at 488 nm (3), 502 nm (5), 498 nm (4a), and 516 nm(6) in acetone. The hypsochromic shift for the synthesized compounds as compared to the alkyl analogs was due to the delocalization of the nitrogen electron pair into the phenyl rings of triphenylamine.

Fig. 6

Absorption of synthesized dyes 3 and 4a versus alkyl analog dyes 5 and 6 in acetone.

Polyester and Nylon Dyeing

Color Properties

The colorimetric properties of the fabric dyeings were expressed in terms of the CIELab system. The K/S and L*, a*, and b* values of the dyed samples were determined from Spectrascan 5100+ (D₆₅ illuminant and 10° observer) instrument data²⁸and the results are presented in Tables II and III. The Kubelka-Munk equation (Eq. 1) was used.

Table II

Color Coordinates (CIELab) for Polyester Dyeing

Dye	L*	a*	b*	c*	h°	K/S
4a	31.13	49.828	20.341	53.82	22.198	38.283
4b	52.07	38.797	81.328	90.108	64.471	11.3507
4c	51.65	38.537	78.754	87.67	63.9	16.814
3	30.84	46.082	24.009	51.96	27.509	26.91

Table III

Color Coordinates (CIELab) for Nylon Dyeing

Dye	L*	a*	b*	c*	h°	K/S
4a	66.951	20.436	11.689	23.543	29.757	1.112
4b	72.121	9.955	35.03	36.417	74.106	1.2889
4c	72.03	9.966	34.871	36.267	74.02	1.2664
3	40.555	–34.736	–32.288	47.425	222.891	1.0269

\frac{K}{S} = \frac{{(1 - R)}^{2}}{2 R}

(Eq. 1)

K is the absorbance coefficient, S is the scattering coefficient, and R is the reflectance ratio.

For CIELab determinations, L* is lightness, a* represents the degree of redness (positive) and greenness (negative), and b* represents the degree of yellowness (positive) and blueness (negative).

All dyes gave uniform colors on the test fabrics. Dyes 4a and 3 were red and 4b and 4c were yellow. Brighter colors and greater K/S values (Table II and III) were obtained for the polyester fabrics when compared to nylon.

Lightfastness

Lightfastness is the degree to which a dye resists fading due to light exposure. The lightfastness of the dyed samples was measured by AATCC Test Method (TM) 16-2004²⁸ in which half of the sample was exposed to a xenon lamp for 17 h while the other half was covered. The samples were then compared with the Blue Wool standard scale and fastness ratings were given. Lightfastness of dyes on polyester was better than on nylon fabrics. All dyes showed moderate lightfastness on both polyester and nylon (Table IV).

Sublimation Fastness

Sublimation fastness is the most significant requirement of dyed fabrics. It is determined by standard method ISO 105-F04.²⁸ The dyed samples were sandwiched between two undyed cloth pieces (polyester and nylon depending on which fabric was tested) and were subjected to 150 °C, 180 °C, and 210 °C for 30 s. Two parameters, color change (cc) and color staining (cs, on undyed cloth) were rated.

Sublimation fastness ranged from low to moderate. It was a half scale better for dyed polyester than for the dyed nylon samples (Table IV).

Table IV

Lightfastness and Sublimation Fastness Rating for Dyed Polyester and Nylon Fabrics

Dye	Polyester				Nylon
	Light Fastness Rating^a	Sublimation Fastness Rating^b			Light Fastness Rating	Sublimation Fastness Rating
	Light Fastness Rating^a	150 °C	180 °C	210 °C	Light Fastness Rating	150 °C	180 °C	210 °C
4a	5	4	3–4	2	4	3	3	1–2
4b	5	3–4	4	1–2	3	3–4	3–4	1
4c	4–5	4	3–4	1–2	3	3	3	1–2
3	5–6	4	3	2	3–4	3	2–3	2

Ratings for lightfastness: 1-poor to 8-excellent

Sublimation fastness: 1-poor to 5-excellent

Percent Dye Exhaustion and Fixation

The percent exhaustion of the dyebath plays an important role in the kinetics of the dyeing process. The amount of dye exhausted from dye bath during dyeing is calculated using Eq. 2.²⁹

% E x h a u s t i o n = \frac{I n i t i a l O . D . - F i n a l O . D .}{I n i t i a l O . D .} \times 100

(Eq. 2)

Initial O.D. is the optical density of dyebath before dyeing and final O.D. is the optical density of dyebath after dyeing.

Percent fixation gives the amount of dye fixed on the fabric. It is calculated by taking the ratio of fabric weight before and after dyeing (Eq. 3).

% F i x a t i o n = \frac{C 2}{C 1} \times 100

(Eq. 3)

C2 is the weight of fabric after dyeing and C1 is the weight of fabric before dyeing.

Fixation values were comparable for both fabrics. The color and exhaustion values were greater for polyester than for nylon (Table V). This was probably because polyester allowed better diffusion of the dye into the fibers than nylon.

Table V

Percent Exhaustion and Fixation on Polyester and Nylon Fabrics

Dye	Polyester		Nylon
Dye	E (%)	F (%)	E (%)	F (%)
4a	78	69	68	70
4b	71	71	61	69
4c	82	79	60	72
3	83	65	71	68

Comparative Study

Spectroscopic and dyeing properties of the newly synthesized dyes 3 and 4a were compared with the reported dyes 7 and 8 (Table VI).^30,31 The molar absorptivities of 3 and 4a were greater than those for dyes 7 and 8. Dye 4a gave the greatest molar absorptivity value of 71,600 dm³/mol∙cm. Color values (K/S) were also greater for the newly synthesized dyes on polyester fabric. Lightfastness values were 4, 3–4, 5–6, and 5 for dyes 7, 8, 3, and 4a, respectively. Sublimation fastness values were nearly the same for dyes 7, 8, 3, and 4a.

Table VI

Comparison of the Spectroscopic and Dyeing Properties of the Dyes

Molecular Modeling Studies

Optimized Dye Geometry

Ground state geometries of 4a-c were optimized by using B3LYP functional and 6-31(G)d basis set in the gas phase and solvents. From the optimized geometry, O-H, N-H, and N=N bond lengths were determined (Fig. 7). The O-H bond length for the azo and hydrazone tauto-mers increased from 1.000 Å to 1.703 Å in 4a, 0.999 Å to 1.691 Å in 4b, and 1.000 Å to 1.703 Å in 4c respectively. A similar trend was observed for N=N bond length. It increased from 1.283 Å to 1.318 Å in 4a, 1.277 Å to 1.306 Å in 4b, and 1.277 Å to 1.308 Å in 4c for the azo and hydrazone tautomers respectively. The N-H bond length decreased from the azo to hydrazone tautomer from 1.708 Å to 1.039 Å for 4a, 1.714 Å to 1.041 Å in 4b, and 1.711 Å to 1.039 Å in 4c. The dihedral angles in 4a, 4b, and 4c were very small (i.e., from 0.147210 to 1.063370) and therefore favorable for intramolecular hydrogen-bonding.

Fig. 7

Optimized geometry of dye 4a (a) azo and (b) hydrazone tautomer in chloroform.

The computed energies of the tautomers were compared. Dyes 4a, 4b, and 4c gave values of –1369.946063, –1279.967017, and –1165.443588 hartree for the azo tautomer and –1369.945731, –1279.964543, and-1165.442509 hartree for the hydrazone tautomer, respectively. The azo tautomer was predicted to be more stable than the hydrazone tautomer for 4a-c. Existing in azo form only as it contains an ortho methoxy group to the azo linkage, dye 3 showed a total energy of -1409.230314 hartree.

Electrophilicity Index

To understand the chemical stability of these dyes, global electrophilicity index (ω)³² values were calculated according to Eq. 4.

ω = \frac{μ^{2}}{2 η}

(Eq. 4)

μ is the negative of the electronegativity obtained using Eq. 5.

μ = - \frac{1 + A}{2}

(Eq. 5)

η is chemical hardness as defined by Eq. 6.

η = I - A

(Eq. 6)

I is the electron potential (IP) and A is the electron affinity (EA). By Koopmans theorem,³³ IP is related to the negative energy of the Highest Occupied Molecular Orbital (HOMO) and EA is related to the Lowest Unoccupied Molecular Orbital (LUMO).

Energies of the HOMO and LUMO of the dyes were obtained from optimized geometries of the dyes by the density functional theory (DFT) method,³⁴ and then substituted into Eqs. 5 and 6 to give Eqs. 7 and 8.

μ = \frac{E_{H O M O} + E_{L U M O}}{2}

(Eq. 7)

η = E_{L U M O} - E_{H O M O}

(Eq. 8)

Therefore, to propose dye stabilities, the net electrophilicity index (ω^±) was calculated using Eq. 9.

ω^{\pm} = ω^{+} + ω^{-}

(Eq. 9)

ω⁺ is electron accepting power and or is electron donating power and were obtained from Eqs 10 and 11.

ω^{\pm} = \frac{{(E_{L U M O})}^{2}}{2 (E_{L U M O} - E_{H O M O})}

(Eq. 10)

ω^{-} = \frac{{(E_{H O M O})}^{2}}{2 (E_{L U M O} - E_{H O M O})}

(Eq. 11)

From Table VII, the azo tautomer gave greater η, ω, and ω^±values than the hydrazone tautomer for dyes 4a-c, indicating that azo tautomer of 4a-4c was the more stable tautomer in a vacuum.

Table VII

Electrophilicity Index of Dyes 3 and 4a-c^a

Dye #	E_HOMO(au)	E_LUMO (au)	μ (eV)	η (eV)	ω (eV)	ω⁺(eV)	ω⁻(eV)	ω^±(eV)
3	–0.3002	–0.2261	–7.16	2.017	12.713	9.385	16.545	25.93
4a Azo	–0.2999	–0.2278	–7.179	1.962	13.133	9.789	16.968	26.757
4a Hydrazone	–0.3018	–0.2316	–7.257	1.91	13.785	10.395	17.652	28.047
4b Azo	–0.2968	–0.218	–7.003	2.144	11.439	8.205	15.209	23.414
4b Hydrazone	–0.2961	–0.2247	–7.085	1.943	12.916	9.617	16.702	26.318
4c Azo	–0.3005	–0.2237	–7.132	2.092	12.158	8.854	15.985	24.839
4c Hydrazone	–0.3022	–0.229	–7.227	1.993	13.1	9.736	16.963	26.699

E_HOMO = Energy of the HOMO, E_L_UMO = Energy of the LUMO, μ = chemical potential, η = hardness, ω = electrophilicity index, ω⁺ =electron accepting ability, ω^– = electron donating ability, and ω^± = net electrophilicity index

Frontier Molecular Orbital (FMO) Approach

The frontier molecular orbitals diagram (Fig. 8) of the dyes shows that, for dyes 3 and 4a, the electron density was located on the azo linkage and donor part in the HOMO, which is shifted towards the acceptor nitro group in the LUMO. In the HOMO of 4b and 4c, the electron density was spread over all of the molecule, including the azo linkage and donor part. In the LUMO of 4b and 4c, the electron density of the two twisted phenyl rings of triphe-nylamine shifted to the molecule. The HOMO-LUMO band gap trend was 4a < 3 < 4c < 4b. As 4a and 3 contain electron withdrawing nitro groups, the charge transfer was greater, lessening the HOMO-LUMO band gap.

Fig. 8

FMO diagram for dyes 3 and 4a-c (azo tautomer).

Molecular Electrostatic Potential (MEP) Plots

To determine the reactive sites of the dyes, MEP analysis was performed at the B3LYP/6-31G(d) level of theory. Fig. 9 shows the MEP surface plots for dyes 3 and 4a-c. The negative electron potential of the -NO₂ group pulled electrons from the triphenylamine core in the azo tautomer of dyes 3 and 4a. In dyes 4b and 4c, the reactive sites are the methoxy group and benzene ring, respectively, indicated in red.

Fig. 9

MEP plots of dyes 3 and 4a-c (top-azo tautomer and bottom-hydrazone tautomer).

For the hydrazone form, the C=O group has a negative electron potential. MEP values observed for the azo tautomer of dyes 3 and 4a-c were ±0.00609, ±0.00567, ±0.0.00567, and ±0.00589, and for hydrazone tautomer of dyes 4a-c were ±0.00838, ±0.00979, and ±0.00907, respectively.

Conclusion

Four methoxy- and hydroxy-based triphenylamine azo dyes (3 and 4a-c) were successfully synthesized and characterized using ¹H and ¹³C NMR. Hydroxy-based triphenylamine azo dyes (4a-c) provided additional dye photostability through intramolecular hydrogen bonding to exhibit bathochromic shifts in absorption spectra. Deep and homogeneous red and yellow shades were observed when these dyes were applied on polyester and nylon fabric. Sublimation fastness of these dyes was enhanced due to the bulky triphenylamine group and was achieved up to 180 °C. Color strength, lightfastness and sublimation fastness were greater for polyester fabric than for nylon. Calculated energies and the net electrophilicity index showed the azo tautomer to be more stable for dyes 4a-c.

Supporting Information

NMR spectra and absorption plots of the dyes 3 and 4a-c in various solvents, and calculated bond lengths, dihedral angles, and energy tables from the molecular modeling studies are available from the author upon request.

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