Solvatochromism and temperature effects on the electronic absorption spectra of some azo dyes

Spectrochimica Acta Part A 74 (2009) 691–694

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Solvatochromism and temperature effects on the electronic absorption spectra of some azo dyes Kamal Alizadeh a,∗ , Susan Seyyedi a , Mojtaba Shamsipur b , Shohreh Rouhani c , Kamaladin Haghbeen d a

Department of Chemistry, Lorestan University, Khorramabad, Iran Department of Chemistry, Razi University, Kermanshah, Iran Iran Color Research Center, Department of Colorant Manufacture, Tehran, Iran d National Research Center of Genetic Engineering and Biotechnology, Tehran, Iran b c

a r t i c l e

i n f o

Article history: Received 19 November 2008 Received in revised form 10 July 2009 Accepted 29 July 2009 Keywords: Azo dyes Electronic absorption spectra Solvatochromic behaviour Intermolecular charge-transfer Hydrogen bonding

a b s t r a c t The UV–visible electronic spectra of some azo dyes have been studied. The different bands observed in the electronic spectra of the compounds in various organic solvents have been assigned to the proper electronic transitions. The solvatochromic behaviour of these compounds was investigated by studying their visible spectra in several pure and mixed organic solvents. The longer wavelength band displayed by para-nitro azo dyes is assigned to an intermolecular charge-transfer transition. The solvated H-bonding complexes formed between N,N-dimethylformamide and the para-nitro azo dyes were investigated. G and formation constant, the values of Kf of the molecular complexes formed in solution have been determined. The effects of increase of temperature and concentration dependent spectroscopic changes on the longer wavelength visible band of para-nitro azo dyes were also investigated. © 2009 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

The structure of azo compounds, especially those containing phenol moieties, has invoked many investigations. This is due to their chromophoric nature and to the bidentate character of their ortho-phenolic hydroxyl groups. These properties have made them useful for metal complexation studies and for spectroscopic determination of cationic species [1–3]. Moreover, in biological systems, some of these compounds could be used as inhibitors for tumor-growth. Careful examination of the literature reveals that considerable work has been reported on the spectroscopic and acid–base properties of these compounds [4–6]. In continuation of our study on the acid–base properties of these molecules in aqueous solution [7], this article presents an investigations of the electronic spectra of some azo compounds in pure and in mixed organic solvents of different polarities. The structural features of the studied azo dye derivatives D1–D7 are given in Fig. 1. In addition to their solvatochromic properties, some of these azo compound derivatives (i.e., D3, D5 and D7) show a unique solvatochromism originating from hydrogen bonding interaction with the low ionization potential solvent molecules. Such a behaviour is much more pronounced in the case of p-NO2 azo derivatives, which means that the ionization is easier in view of the high electron accepting propriety of the NO2 group.

The azo dyes 4-[(4-sulfonamidophenyl)azo]phenol (D1), 4[(4-methylphenyl) azo]phenol (D2), 4-[(4-nitrophenyl)azo]phenol (D3), 2-methoxy-4-[(4-sulfonamidophenyl)azo]phenol (D4), 2methoxy-4-[(4-nitrophenyl)azo]phenol (D5), 4-(phenylazo)-l,2benzenediol (D6) and 4-[(4-nitrophenyl)azo]-l,2-benzenediol (D7) were synthesized and purified using a previously reported method [8]. Reagent grade hydrochloric acid and potassium nitrate were all from Merck Company and were used as received. The organic solvents used (MeOH, EtOH, CCl4 , CHCl3 , CH3 CN, THF, DMSO and DMF) were all of analytical grade from Merck or BDH companies. Stock solutions (10−3 mol dm−3 ) of the compounds D1–D7 were prepared by dissolving an accurately weighed amount of the target compound in the required amount of the appropriate solvent. Solutions of lower concentrations, used in the spectral measurements, were obtained by appropriate dilutions. A Shimadzu (Japan) model 1650PC double-beam spectrophotometer was used for running the electronic absorption spectra (controlled to ±0.1 ◦ C).

∗ Corresponding author. Tel.: +98 661 2200185; fax: +98 661 2200185. E-mail addresses: [email protected], [email protected] (K. Alizadeh). 1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.07.020

3. Results and discussion In this paper we have used seven azo dyes which have been reported previously by Haghbeen and Tan [8]. The UV–visible bands in the electronic spectra of compounds D1–D7 in eight organic solvents with different polarities, viz. tetrachloromethane, acetonitrile, chloroform, methanol, tetrahydrofuran, ethanol,

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Fig. 1. Structures of azo dyes D1–D7.

Fig. 3. The electronic absorption spectra of 3.8 × 10−5 mol L−1 D7 in various solvents: (1) CCl4 , (2) CH3 CN, (3) CHCl3 , (4) MeOH, (5) THF, (6) EtOH, (7) DMF, and (8) DMSO.

Fig. 2. The UV–visible absorption spectra of compounds D1–D7 in methanol solution.

N,N-dimethylformamide, and dimethylsulfoxide, were recorded. Sample spectra in methanol solution are shown in Fig. 2. Generally, the electronic spectra of these compounds in solution exhibit two bands [8–10], one band in the UV region of 260–290 nm and one at longer wavelength in the UV–visible region relative to the former one. For the sake of simplicity, in this work, we have limited ourselves to the study of solvent effects on the second band in spectra of the compounds D1–D7, based on the data given in Table 1. The shorter wavelength band in the UV region of 260–290 nm observed for all seven compounds in different solvent sysTable 1 Absorption maxima (max ) of azo dyes (D1–D7) in various eight organic solvents. Solvent

max (nm) D1

D2

D3

D4

D5

D6

D7

CCl4 CHCl3 THF MeOH EtOH CH3 CN DMF

364 372 361 360 362.6 335 364

320 316 321 314 314 313 318

360 374 379 378 381 375 386

367

321

386 390 402 398 401 392 409 605 415 614

368.5 375 387.5 380 386 378.5 391.5

DMSO

369 376 388 380.5 387 385 392.5 585 398 586

394 399 410 402 410 395 413 608 419 616

389

397

tems is ascribed to ␲–␲* transition of the benzenoid system present in their structure. This assignment is quite reasonable since max of this band is slightly altered from one derivative to another, a behaviour which is characteristic of the type of electronic transition corresponding to these bands. The second band (Fig. 2), observed in the region of 310–550 nm for azo compounds D1–D7, can be assigned to a ␲–␲* transition with a considerable charge-transfer character (CT transition). This is most possibly originated by the phenolic moiety pointing (hydroxyl phenyl azo) towards the other ring, which is characterized by a high electron accepting character. The charge-transfer nature of this band is deduced from its broadness as from the sensitivity of its max to the type of substituent attached to the azo coupler. This band acquires an appreciable shift towards lower energy (red shift) when Z is an electron acceptor (i.e., compounds D3, D5 and D7) as it compared with the cases where Z is an electron donor (i.e., compound D2). This shift can be considered as an evidence for the CT character of the band [10–15]. Fig. 3 is a representative example for the solvatochromic behaviour of the dyes under investigation. As it can be seen, the main band of compound D7, located in the spectral range of 310–550 nm, exhibits an apparent shift towards longer wavelengths in different solvents according to the sequence: CCl4 < CH3 CN < CHCl3 < MeOH < THF < EtOH < DMF < DMSO. This shift does not agree with the change in the polarity of the organic solvents and, therefore, it can be considered as a result of combination of several solvent characteristics such as polarity, basicity, and H-bond-accepting ability. Furthermore, the electronic spectra of compounds D3, D5 and D7 in DMF and DMSO comprise a new band appearing at much longer wavelength, as shown for D7 in Fig. 3 curves 7 and 8. This behaviour can presumably be due to the fact that the solute molecules are liable to form a solvated complex with DMF and DMSO molecules through an intermolecular H-bonding [13]. Since the charge-transfer forces play an important role in Hbonding, this additional band is a result of an intermolecular CT transition. This transition involves an electron transfer from the lone pair of electrons at the oxygen atom of the DMF or DMSO molecules to the antibonding orbital of the OH bond of the phenolic moiety. Good convincing evidence for the intermolecular CT nature of this band is attained from the nonlinear relationship between the absorbance of this band and the molar concentration

K. Alizadeh et al. / Spectrochimica Acta Part A 74 (2009) 691–694

Fig. 4. Effect of concentration on the electronic absorption spectra of D7 in DMF. Curves 1–15 represent: 11.6, 23.15, 34.73, 46.3, 57.87, 69.45, 81, 92.6, 104.2, 115.7, 127.3, 138.9, 150.5, 162, and 173.6 × 10−6 mol L−1 solutions, respectively. The inset shows the corresponding plot for more details in two dimension. The arrows show the directions of absorbance changes by increasing the concentration of D7.

of the azo compound; the resulting spectra for varying concentration of D7 in DMF as are depicted in Fig. 4, and the corresponding absorbance–concentration plots are shown in Fig. 5. As it is evident from Fig. 5, while the absorbance of 413 nm band increases linearly with concentration of the dye D7 from 11.6 × 10−6 to 17.36 × 10−5 mol L−1 with R2 of 0.99, the corresponding plot at 608 nm shows a nonlinear behaviour. It is interesting to note that the observed straight line at 413 nm is in confirmation of the assignment of this band to an intramolecular electronic transition [10,13]. It should be noted that the appearance of such a band only in the spectra of the para-nitro azo derivatives (i.e., D3, D5 and D7) is due to the high accepting propriety of the NO2 group, which reflects weakening of the O–H bond as a result of the decreased electron density on the oxygen atom. On plotting the absorbance of new band appearing at longer wavelengths against the molar concentration of compounds from 1.16 × 10−7 to 1.74 × 10−6 mol L−1 , a nonlinear relationship was obtained, which can be considered as

Fig. 5. Plot of absorption vs. concentration for D7 in DMF at room temperature at the position of the two absorption maxima according to Fig. 4.

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Fig. 6. The electronic absorption spectra of 8.1 × 10−5 mol L−1 D7 in DMF–CHCl3 mixtures: (1) 0.87 M DMF; (2) 2.73 M DMF; (3) 5.33 M DMF; (4) 7.93 M DMF; (5) 10.53 M DMF; (6) 13 M DMF. The arrows show the directions of absorbance changes with increasing DMF concentration.

an evidence for the intermolecular CT nature of this band, as shown for D7 in Fig. 5 at 608 nm. Generally, the spectra recorded in binary mixed solvents DMF–CCl4 , DMF–CHCl3 , DMF–EtOH and DMF–CH3 CN exhibited an isosbestic point indicating the establishment of equilibrium between the free and H-bond solvated species of the compound. Representative spectra for compound D7 in DMF–CHCl3 mixture are shown in Fig. 6. The observed behaviour in different solvent mixtures indicated that DMF molecules have a greater tendency to form solvated complexes with the solute molecules than acetonitrile, ethanol, chloroform and tetrachloromethane. This is due to the low ionization potential of DMF as well as its high H-bondaccepting character. The formation constants, Kf , of different solvated molecular complexes of compounds D3, D5 and D7 with DMF were determined from the variation of the absorbance at a given wavelength of their intermolecular CT band with increasing DMF concentration. The equation used [10,16] is log CDMF = log Kf + log

A − A  0

A1 − A

where A0 and A1 are absorbances of the compounds D3, D5 and D7 in the low dielectric constant solvent and DMF, as a high dielectric constant solvent, and A is the absorbance in the mixed solvent. A plot of log CDMF versus log(A − A0 /A1 − A) gives a straight line, from the intercept of which Kf can be evaluated. The Gibbs free energy (G) of the solvated complexes can be obtained from G = −RT ln Kf . The Kf and G values for various molecular complexes with D3, D5 and D7 are given in Table 2. As it is obvious from Table 2, the resulting Kf and G values in different mixed solvents decrease in the order DMF–CCl4 > DMF–CHCl3 > DMF–EtOH > DMF–CH3 CN. It is clearly observed that this sequence is very sensitive to the difference in the dielectric constants of binary mixture components, i.e., the Kf and G values decrease with increasing dielectric constant of the solvent mixed with DMF [10,13]. An interesting behaviour is obtained by studying the effect of temperature on the low energy band of compounds D3, D5 and D7 in DMF as solvent [13,17]. Sample spectra for compound D7 in DMF at different temperatures are shown in Fig. 7. In all cases, raising the solution temperature from 19 ◦ C to 72 ◦ C resulted in significant decrease in the band intensity. Based on the above results, it can be concluded that an increase in temperature of a 4.6 × 10−5 mol L−1

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Table 2 Kf values and G for the hydrogen bonding solvated complexes of some azo dyes (D3, D5 and D7). System

Kf

G (KJ mol−1 )

D3 DMF–CCl4 DMF–CHCl3 DMF–EtOH DMF–CH3 CN

37.15 22.39 21.88 7.00

−2.14 −1.84 −1.83 −1.15

D5 DMF–CCl4 DMF–CHCl3 DMF–EtOH DMF–CH3 CN

23.99 16.98 10.71 6.59

−1.88 −1.68 −1.41 −1.12

D7 DMF–CCL4 DMF–CHCl3 DMF–EtOH DMF–CH3 CN

80.90 63.10 57.54 8.17

−2.61 −2.46 −2.40 −1.25

4. Conclusions All azo compounds D1–D7 displayed two or three bands in their electronic absorption spectra in various organic solvents. The first and second bands attributed to ␲–␲* transition in benzenoid system involving the entire electronic system of the compounds. Whereas the third band at longer wavelengths for some azo compounds (i.e., D3, D5 and D7) are sensitive to small amounts of DMF or DMSO (Figs. 3 and 4), leading to new bands appearing at larger wavelengths (Table 1). In the case of dyes D1, D2, D4, and D6, no intermolecular hydrogen bonding occurs, while the dyes D3, D5 and D7 dissolved in DMF reveal an additional band. The observation of a special behaviour for compounds D3, D5 and D7 in the presence of DMF is examined and which indicates that DMF has a greater tendency to form a solvated complex with the solute molecules. This is due to the low ionization potential and high hydrogen bondaccepting character of DMF. The G and Kf values for the resulting these complexes have been determined. Acknowledgement The authors wish to thank the Lorestan University Research Council for supporting this work. References

Fig. 7. Effect of increase of temperature on the electronic absorption spectra of 4.6 × 10−5 mol L−1 solution of D7 in DMF. Curves 1–13: 19, 21, 24, 28, 32, 37, 42, 47, 52, 57, 62, 67, and 72 ◦ C.

solution of dyes in DMF increases the molecule energy, charge delocalization and weakens the intermolecular hydrogen bonding, and consequently assists the solute–solvent dissociation. This can be considered as a further evidence that this band corresponds to an association process through solute–solvent interactions.

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