In this study, the synthesized coupling component 1-(2-benzothiazolyl)-3-methylpyrazol-5-one reacted with diazotised heterocyclic amines to afford six novel hetarylazopyrazolone dyes. These azo dyes based on benzothiazole and benzimidazole ring systems were characterized by spectral methods and elemental analyses. The solvatochromic behaviors of these dyes in various solvents were evaluated. The ground state geometries of the dyes were optimized using density functional theory (DFT). Solvent, acid-base, and substituent influences on the wavelength of the maximum absorption were examined in detail. Time-dependent density functional theory (TD-DFT) calculations were performed to obtain the absorption spectra of the dyes in various solvents and the results compared with experimental values. Besides, frontier molecular orbitals (FMO) analysis for the dyes is also described from the computational process.

1. Introduction

The ease of availability of raw materials and ease of preparation [1], wide range of uses as dyestuff [2, 3], advanced applications such as liquid crystal displays (LCDs) [4], nonlinear optical (NLO) devices [5], dye-sensitized solar cells (DSSCs) [6], optoelectronic systems [7, 8], and wide application areas in biological and medical studies [9] are all desirable characteristics of azo dyes.

Various experimental and theoretical studies on the structure, tautomeric behavior, dyeing properties, and vibrational analysis of azo dyes [1019] and their metal complexes [2022] have been done. Due to their chemical significance and spectroscopic properties, hetarylazo dyes were studied extensively by spectroscopic and theoretical methods. Their higher tinctorial strength and brighter dyeing make azo dyes based on heterocyclic amines superior than aniline based azo dyes [2328]. Some hetarylazopyrazolone dyes have not only potential use as dye stuff, but also other potential nontextile applications such as optical materials [29].

Many studies via theoretical calculations have been performed for determination of the structural, spectroscopic properties, and tautomeric behaviors of azo dyes [30, 31]. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been one of the most widely used methods in computational chemistry [3237].

We have previously reported the synthesis, characterization, tautomeric behavior, and solvent effects on their absorption spectra of some hetarylazopyrazolone dyes with thiazole moiety together with their theoretical studies [38]. In continuation of this study, we report here the synthesis and characterization of a series of new monoazo dyes based on the benzothiazole and benzimidazole ring systems. Optimization calculations of the dyes in the ground state were done at the DFT B3LYP/6-31G(d,p) level. Absorption spectra based on the DFT-optimized structures were obtained by the time-dependent density functional theory (TD-DFT/B3LYP) level with the 6-31G(d,p) basis set.

2. Experimental

2.1. General

The reagents of analytical grade were purchased from commercial sources and used without any further purification. 1H NMR spectra were measured with a Bruker DPX-400 MHz spectrometer at room temperature in DMSO-d6 with tetramethylsilane as the internal reference. UV-visible absorption spectra were recorded on Analytik Jena Specord 200 double-beam spectrophotometer, using dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetonitrile, methanol, acetic acid, and chloroform as solvent. Infrared spectra were taken in KBr on a Mattson 1000 spectrophotometer. Mass spectra were measured with a Micromass UK Platform II LC-MS spectrometer. Elemental analyses were performed with a LECO-CHNS-9320 instrument. Melting points were obtained with a Gallenkamp capillary melting apparatus and they are uncorrected.

2.2. Synthesis

The methods used for the syntheses of 2-hydrazinobenzothiazole (1) and coupling component 1-(2-benzothiazolyl)-3-methylpyrazol-5-one (2) were described in the previous part of our study [38].

2.2.1. Synthesis of Hetarylazopyrazolone Dyes (3a–3f)

Diazotisation of various amines was affected with nitrosylsulfuric acid. General procedure was described below for 1-(2-benzothiazolyl)-3-methyl-4-(2-benzothiazolylazo)pyrazol-5-one and the other dyes were synthesized in a similar way. The characteristic properties and elemental analysis of the dyes are shown in Table 1. The synthesized dyes (3a–3f) are shown in Figure 1.

2.2.2. Synthesis of 1-(2-Benzothiazolyl)-3-methyl-4-(2-benzothiazolylazo)pyrazol-5-one (1)

2-Aminobenzothiazole (0.002 mol) was dissolved in icy acetic acid-propionic acid mixture (3 mL/2 mL) and was cooled to −5°C in ice-salt bath. Nitrosylsulfuric acid, prepared with dissolving sodium nitrite (0.002 mol, 0.14 g) in sulfuric acid (4 mL), was added to the heterocyclic amine dropwise in 30 min at −5°C. The mixture was stirred in cold for additional 2 hours. Urea was added to the mixture in order to decompose the unreacted nitrous acid. 1-(2-Benzothiazolyl)-3-methylpyrazol-5-one (0.002 mol, 0.46 g) was dissolved in KOH solution (10 mL 0.2 M) and cooled. The prepared coupling compound solution was added to the diazonium solution in half an hour. The mixture was stirred for additional 2 hours at 0–5°C and Na2CO3 solution was added till the mixture had a value of pH 5-6. The mixture was stirred for 30 min. Water was added to precipitate the product. The product was filtered, washed with water, and air-dried. Recrystallization was performed in ethanol to obtain the pure orange compound (Yield: 87% (m.p. 255–257°C)).

2.3. Computational Details

All calculations were carried out with Gaussian09 program [39]. The geometries of the dyes were fully optimized and calculated by B3LYP method and 6-31+G(d) basis set without any symmetry constraints. Vibrational frequency analysis confirms that all the optimized geometries correspond to minima on the potential energy surface. Time-dependent density functional theory/integral equation formalism polarisable continuum model (TD-DFT/IEFPCM) method has been used to obtain electronic absorption spectra of the dyes [40]. The (default) nonequilibrium procedure was selected for the TD-DFT calculations [41].

3. Results and Discussion

3.1. Molecular Structure

Azo dyes can exist in four possible tautomeric forms, namely, the keto-azo forms A and D, keto-hydrazo form B, and enol-azo form C as shown in Figure 2. The deprotonation of these four tautomers lead to common anion form E.

1H NMR spectrum of dye 3a exhibits a broad peak at 14.94 ppm, which was attributed to NH of the hydrazo tautomer. This peak was observed at 15.00 ppm for dye 3b. These results suggest that dyes 3a and 3b are present as keto-hydrazo form in DMSO-d6. The absence of hydroxyl (OH), imino (NH), and hydrazo (NH) peaks in the 1H NMR spectra suggests that dyes 3c, 3d, 3e, and 3f exist in keto-azo (A) form or in anionic form (E). The 1H NMR spectra are measured in DMSO-d6 for the dyes except dye 3c, which is recorded in CDCl3. Dye 3b showed a singlet peak at 2.30 ppm for methyl group at pyrazolone ring. The 1H NMR spectra for the dyes 3a–3f showed a singlet peak from 2.30 to 2.42 ppm for pyrazolone CH3. Aromatic protons of the dyes 3a–3f appear at 7.00–8.14 ppm. Dye 3c showed a singlet peak at 3.90 ppm for OCH3 proton at benzothiazole ring and a peak at 2.40 ppm for methyl group at pyrazolone ring. Dye 3d showed a singlet peak at 2.31 ppm for methyl groups at benzothiazole ring and two doubled peak at 7.79 ppm and 7.99 ppm for aromatic protons at benzothiazole ring. 1H NMR spectra of dyes 3e and 3f exhibit a broad peak at 13.30 and 13.24 ppm, respectively, which were due to NH benzimidazole proton.

FT-IR spectra of dyes 3a–3f showed a band at 1638–1684 cm−1 for carbonyl group and a band at 3390–3445 cm−1 for NH group. These results suggest that these dyes are predominantly in keto-hydrazo (B) form in solid state. The imidazole NH peaks of the dyes 3e and 3f were recorded at 3194 cm−1 and 3147 cm−1. The other values of 3021–3070 cm−1 (aromatic C–H) and 2915–2994 cm−1 (aliphatic C–H) were also recorded.

The molecular ion [M]+ peaks of the dyes are observed at the expected values as the main peak. Dyes give [C11H8N5OS]+ ion peak at except dye 3c with relative intensities 10–33%. The ion peak [C4N2H3]+ is observed at 79 (10%) for dye 3c. 1H NMR, FT-IR, and mass spectra results of the dyes are shown in Table 2.

A detailed potential energy scan was performed on dihedral angles (S24–C7–N22–C13) (Figure 3(a)) and (N26–C16–N19–N27) (Figure 3(b)) in the range of 0°–360° at HF/6-31G(d) level by 10° intervals to reveal all possible conformations of the dye 3a.

Then, final geometry optimizations were performed within the framework of DFT/B3LYP at the level of 6-31+G(d) without any geometrical restriction. The most stable conformation of the keto-hydrazo tautomer is shown in Figure 4 with atomic numbering.

3.2. Effect of Solvent

The UV-Vis absorption spectra of the dyes 3a–3f were recorded over the range of λ between 300 and 700 nm and the results are given in Table 3.

values of dye 3a, dye 3b, and dye 3c exhibit bathochromic shift in methanol, DMF, and DMSO. These dyes show only one maximum and also only one tautomeric form in all solvents. The isosbestic points are around 442 nm, 435 nm, and 465 nm, respectively. Absorption spectra of dye 3b in various solvents are given in Figure 5. It was determined that the absorption maxima of the dye 3d in methanol, DMF, and DMSO show no significant change, despite blue shift in chloroform and acetic acid. The difference between the experimental absorption maxima of dyes in the most apolar solvent chloroform and in the most polar solvent DMSO we used lies between 5 and 50 nm while the calculated absorption maxima of each dye do not change significantly with solvent polarity (max. 4 nm). The difference between the experimental and calculated absorption maxima lies between 0 and 36 nm. The absorption maxima of dye 3e and dye 3f vary less with the solvent. All of the absorption bands of the dye 3e and dye 3f show shoulders in a short wavelength, and their symmetries are distorted. The shoulders can be attributed either to the interference of a tautomer over the dominant one or to the transitions π-, n-, or charge transfer transitions.

Maximum absorption values and oscillator strengths of the dyes were calculated by using the TD-DFT/IEFPCM method with 6-31G(d,p) basis set in different solvents and the results are shown in Table 4.

3.3. Effect of Acid and Base

Acid-base effects on the absorption spectra of the dyes were investigated. Changes in the maximum absorption wavelength of the dyes with different solvents obtained by addition of the acid-base solution are also provided in Table 5.

When acid was added to the methanol solutions of the dyes 3c, 3d, and 3e, the absorption maxima shift to the bathochromic area while the absorption maxima of the dyes 3a and 3b shift to the hypsochromic area. By the addition of KOH to methanol solutions of the dyes hypsochromic shifts were observed for dye 3f and the other dyes did not represent a significant change by their absorption maxima.

The absorption spectra of the dyes in chloroform are sensitive to the addition of piperidine while piperidine addition to DMF and DMSO solutions led to no notable change in the absorption spectra. When piperidine was added to chloroform solution the absorption peaks obtained are similar to the ones measured in DMF and DMSO solutions. There is no significant change in the absorption spectra of the dyes by the addition of trifluoroacetic acid to the chloroform solutions. Absorption spectra of dye 3a in acid and base are given in Figure 6.

Absorption spectra of the dyes 3a–3f were taken in pure chloroform and chloroform/DMF mixtures of 80/20, 60/40, 40/60, 20/80 (v/v), and pure DMF to understand whether the bathochromic shift results from tautomerization or ionization in basic chloroform solution (Table 6). Even a slight increase in the percentage of DMF in chloroform solution shifted the absorption maxima of the dyes 3a–3d to a sudden longer wavelength. These results suggest that these dyes absorbed at long wavelength exist in anionic form. But such a shift was not observed for the dyes 3e and 3f.

3.4. Effect of Concentration and Temperature

The effect of molarity and temperature on the absorption spectra of the dyes 3a–3f was studied and the results are listed in Table 7. Therefore, chloroform and DMF solutions were diluted 50% and the absorption spectra were recorded at 25°C versus 40°C. The difference between the values of the neutral molecule and the anion is in the range 1–48 nm. The dyes can easily ionize even in neutral polar solvents and the equilibrium resulting from the solvent change is an ionic equilibrium rather than a tautomeric equilibrium. As seen in Table 7, the absorption spectra are not affected by the concentration and temperature. The reason for this can be ionic equilibria.

3.5. Effect of Substituent

As seen in Table 4, the substituent effect is mostly observed in the most apolar solvent chloroform among all the solvents. The substituent affected values less in DMF and DMSO when compared to other solvents. The substituent effect on values in chloroform is evaluated and the results can be summarized as follows:(i)The introduction of chlorine group into the sixth position of the benzothiazole ring did not have a notable effect on the absorption maxima. On the other hand, substitution of the methoxy group on the sixth position in dye 3c and methyl groups on the fifth and sixth position in dye 3d had the equal effect on values resulting in 31 nm bathochromic shift.(ii)The substituted methyl groups on the fifth and sixth positions on the benzimidazole ring in dye 3f caused 12 nm increase in bathochromic shift when it is compared with dye 3e.(iii)As the electron donating ability of the substituents on the benzothiazole and benzimidazole in diazo compound increases, more electron donation occurs and in parallel to this bathochromic shift also increases.

3.6. Frontier Molecular Orbitals (FMOs)

DFT computational results given in Table 7 reveal that the energy gaps () for benzothiazole moiety bearing azo dyes 3a–3d are 3.08, 3.04, 2.85, and 3.01 eV, respectively, while those for the benzimidazole moiety bearing azo dyes 3e and 3f are 3.04 and 2.87 eV, respectively. The frontier orbitals LUMO+1, LUMO, HOMO, and HOMO+1 are represented in Figure 7. HOMOs and LUMOs of the dyes 3a–3d are mainly localized on pyrazole and benzothiazolyl diazo moiety. The main peaks in UV-Vis spectra are attributed to the HOMO-LUMO electronic transitions. As seen from Table 8, the increase in the wavelength values is in agreement with bathochromic shifts and decreasing. The electron densities in the HOMOs of the dyes 3e and 3f are mainly localized on pyrazole and benzothiazole moiety while electron densities in their LUMOs are localized on pyrazole and benzimidazolyl diazo moiety.

4. Conclusions

A series of azo dyes based on benzothiazolyl pyrazole and simultaneously benzothiazolyl and benzimidazolyl diazo moieties were synthesized and characterized by elemental analyses, FT-IR, 1H NMR spectra, and DFT calculations. Absorption spectra of the dyes were reported both experimentally and theoretically by TD-DFT calculations. The absorption maxima of the hetarylazopyrazolone dyes bearing benzothiazolyl diazo moiety showed bathochromic shifts because these dyes give easily acidic protons to the solvent like DMF and DMSO. The maximum absorption values obtained with TD-DFT/B3LYP method using nonequilibrium procedure are in a good agreement with the experimental results.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.


This work has been supported by the Gazi University Research Fund through Grant no. 05/2002-08.