Synthesis, characterization and solvatochromic behaviour of new water-soluble 8-hydroxy-3,6-disulphonaphthyl azohydroxynaphthalenes

Figures & data

Table 1. Optimized procedures for the synthesis of 8-hydroxy-3,6-disulphonaphthyl azohydroxynaphthalenes (3a–e).

Dye	Substituted naphthol	Optimized coupling solutions	Final recrystallization	Yield^a (%)
3a	1-naphthol	50 mL 0.5M NaOH	Reflux in ethanol	65.3
3b	2-naphthol	50 mL 0.5M NaOH	Reflux in ethanol	69.4
3c	1-amino-8-hydroxynaphthalene-3,6-disulphonic acid	60 mL aqueous solution of Na2CO3 (15 mmol,) and NaHCO3 (9.57 mmol)	Reflux in DMF then precipitation with acetone	74.9
3d	7-amino-4-hydroxyl-2-naphthalenesulphonic acid	60 mL aqueous solution of Na2CO3 (15 mmol,) and NaHCO3 (9.57 mmol)	Reflux in methanol then precipitation with diethyl ether	48.5
3e	1-amino-8-naphthol-4-sulphonic acid	60 mL aqueous solution of Na2CO3 (15 mmol,) and NaHCO3 (9.57 mmol)	Methanol/ethyl acetate (45:55, v/v) binary solvent at room temperature	47.1

^a Yield refers to isolated pure compound.

Figure 1. ¹³C NMR peaks of 3a–e in DMSO-d₆.

Figure 2. Peak splits in the aromatic protons of 3b.

Figure 3. ¹H NMR of 3c and 3d showing highly deshielded protons of the hydrazone tautomers.

Figure 4. Effect of pH on azo-hydrazone tautomers of 3a–e at $1.48-2.11\times {10}^{-5}$ concentrations.

Figure 5. Electronic absorption spectra of the five dyes in (a) ethanol, (b) acetonitrile, (c) acetone, (d) ethyl acetate, (e) DMF at $1.44-2.01\times {10}^{-5}\mathit{M}$ concentrations.

Table 2. Colours of the dyes in various solvents.

Compound	Methanol	1,4-dioxane	Acetonitrile	DMF	Ethyl acetate	Methanol + 1M HCl	Methanol + 1M NaOH
3a	Pink	Pink	Red	Blue	Brown	Orange	Purple
3b	Pink	Pink	Pink	Pink	Brown	Pink	Pink
3c	Purple	Magenta	Magenta	Purple	Brown	Magenta	Magenta
3d	Orange	Orange	Peach	Pink	Brown	Orange	Purple
3e	Purple	Purple	Purple	Violet	Yellow	Purple	Indigo

Table 3. Variation of the molar transition energies of the dyes with Kamlet–Taft parameters of solvents.

	$E_T\left(\log ϵ\right)$					KAT parameters
Solvent	3a	3b	3c	3d	3e	α	β	${\pi }^{\ast }$
Acetone	55.30(3.94)	54.36(3.61)	53.44(4.19)	60.57(4.25)	51.89(4.20)	0.08	0.48	0.62
Acetonitrile	56.95(4.01)	54.56(3.72)	53.05(4.23)	60.83(4.12)	51.24(4.17)	0.19	0.40	0.66
1-Propanol	54.67(4.05)	54.15(3.73)	59.56(3.93)	59.69(4.19)	51.89(4.28)	0.84	0.9	0.52
2-Propanol	55.52(4.05)	54.36(3.75)	53.14(4.23)	59.19(4.16)	51.89(4.26)	0.76	0.81	0.48
Ethanol	56.07(4.07)	54.46(3.76)	52.65(4.35)	59.31(4.20)	51.89(4.24)	0.86	0.75	0.54
Methanol	56.62(4.05)	55.19(3.67)	52.46(4.34)	57.88(4.19)	51.89(4.24)	0.98	0.66	0.60
1,4-dioxane	56.40(3.97)	54.56(3.66)	52.37(4.20)	59.93(4.19)	51.33(4.16)	0.00	0.37	0.49
DMF	48.79(4.31)	53.44(3.69)	49.38(4.32)	56.17(4.20)	49.90(4.26)	0.00	0.69	0.88
DMSO	48.30(4.36)	53.84(3.73)	51.33(4.30)	56.06(4.11)	49.90(4.26)	0.00	0.76	1.00
Toluene	56.28(4.07)	54.67(3.75)	53.04(4.17)	60.57(3.95)	52.08(4.28)	0.00	0.11	0.49
Chloroform	59.57(3.82)	58.83(3.57)	58.11(3.61)	61.89(3.92)	53.34(3.72)	0.20	0.10	0.69
Ethyl acetate	59.07(4.16)	56.28(3.88)	60.70(4.10)	63.54(4.38)	54.88(4.28)	0.00	0.45	0.45
Dichloromethane	59.94(3.91)	58.35(3.47)	57.18(3.68)	62.15(4.06)	53.24(3.98)	0.13	0.10	0.73
Water	50.78(3.81)	53.44(3.54)	51.15(4.25)	55.41(4.09)	51.42(3.65)	1.17	0.47	1.09

E_T = molar transition energies of dyes; ϵ = molar absorption coefficients.

Figure 6. Tauc’s plots of 3a–e in (a) acetone and (b) ethyl acetate.

Scheme 1. Synthesis of 8-hydroxy-3,6-disulphonaphthyl azohydroxynaphthalenes (3a–e) with yields.

Scheme 2. Azo-hydrazone tautomerism of 3a–e.

Table 4. Multiple linear regression analysis of the KAT equations for molar transition energies of the dye probes.

Compound	$A_{\circ }$	A	b	p	F	$r^2$
3a	63.43			−13.60	31.49	0.76
3b	56.05			−2.47	9.55	0.49
3c	59.16			−8.65	4.85	0.33
3d	64.97			−9.01	22.66	0.70
3e	54.06			−3.65	6.66	0.39