Charge transfer complexes of picolines with σ- and π-acceptors

Available online at www.sciencedirect.com

Spectrochimica Acta Part A 70 (2008) 1034–1040

Charge transfer complexes of picolines with ␴and ␲-acceptors Humaira Razzaq ∗ , Rumana Qureshi, Naveed Kausar Janjua, Farhat Saira, Samia Saleemi Chemistry Department, Quaid-i-Azam University, 45320 Islamabad, Pakistan Received 26 June 2007; received in revised form 2 October 2007; accepted 12 October 2007

Abstract In the present study CT complexes of 2-, 3- and 4-Picolines with (DDQ) 2, 3-dichloro-5, 6-dicyano parabenzoquinone (␲-acceptor) and (I2 ) Iodine (␴-acceptor) have been investigated spectrophotometrically in three different solvents (CCl4 , CHCl3 and CH2 Cl2 ) at six different temperatures. The formation constants of the CT complexes were determined by the Benesi-Hildebrand equation. The thermodynamic parameters were calculated  by Van t Hoff equation. The H◦ , G◦ and S◦ values are all negative implying that the formation of studied complexes is exothermic in nature. © 2007 Elsevier B.V. All rights reserved. Keywords: CT complex; ␴- and ␲-acceptors; Benesi-Hildebrand equation; Van’t Hoff equation; Thermodynamic parameters

1. Introduction The study of electron transfer process has occupied a central place in physical chemistry for more than 40 years. Molecular complexation and structural recognition are key processes in biological systems. For instance, enzyme catalysis, drug action, and ion transfer through lipophilic membranes all involve complexation between two or more distinct molecules [1]. The ubiquitous occurrence of N-heterocyclic compounds in living systems has prompted several investigations concerning the nature of their complexes with various compounds. Nitrogen bases are of special interest as electron donors because they can function as n- and ␲-donors. Moreover, the charge transfer involvement in biomolecular interactions can be better demonstrated by studying simple multisite donors which would serve as model compounds of biomolecules [2]. Accordingly, investigation of CT molecular complexes of such compounds is very important from the biological point of view. Pyridine is widely used by the pharmaceutical, textile and other industries as well as by the agricultural sector. In the agricultural sector, pyridine is used to exterminate weeds, fungi, insects, and other parasitic plants. In the pharmaceutical industry, pyridine

∗

Corresponding author. Tel.: +92 5190642142; fax: +92 5190642241. E-mail address: hunza [email protected] (H. Razzaq).

1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.10.030

is widely used as a material for making anti-allergic and antiTBC drugs, antiseptic, analgesic, and a treatment for vitamin B6 complex deficiency. In the textile industry, pyridine is used as a water-proofing material for making water-proof textile. The formation of charge transfer complexes of nitrogen heterocyclic compounds with iodine is known to involve the non-bonding electrons on the nitrogen atoms [3]. Charge transfer complexes are present in many synthetic and natural materials ranging from carbon black to metallo-proteins. The presence of charge transfer complexes in polymers and the interaction of oxygen and metals with radiation absorbing materials have far ranging consequences in applications that require environmental stability in space and aerospace applications. Charge transfer complex studies brought development in the field of chromatography, carbonless imaging and estimation of solvent polarity, ionization potential, electron affinity [4–8]. Recently charge transfer complexes are gaining importance as potential high efficiency non-linear optical materials [9]. Such complexes have been and are recently being reported to act as reaction intermediates [10–12]. CT complexes are also being regarded as important materials for use as organic superconductors, photo catalysts and dendimers. In the present work, 2-, 3- and 4-picolines were selected and their interaction with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as representative ␲-acceptor and with iodine as

H. Razzaq et al. / Spectrochimica Acta Part A 70 (2008) 1034–1040

a typical ␴-acceptor was studied in order to find out whether the picolines are n- and/or ␲-donors. 2. Experimental UV–vis spectrophotometry is an interesting technique and is employed to study the charge transfer phenomenon. 2.1. Materials and solutions Donors (2-Picoline, 3-Picoline and 4-Picoline), acceptors [iodine (␴-acceptor) and 2,3-dichloro-5,6-dicyano-1,4benzoquinone (␲-acceptor)] and solvents (carbon tetrachloride, dichloromethane and chloroform) used were all from Merck with percentage purity ranging from 98% to 99%. 2.2. Physical measurements The electronic absorption spectra were recorded on Shimadzu 1601 recording spectrophotometer equipped with a Julabo F-34 thermostat (±0.1 ◦ C) using 1.0 cm matched quartz cells. Freshly prepared solutions of donors (picolines) and acceptors (I2 and DDQ) were used in each case for the charge transfer complex formation. Stability constants and extinction coefficients of the charge transfer complexes were determined by making use of Benesi-Hildebrand equation. Thermodynamic  parameters were obtained by means of Van t Hoff plots. 3. Results and discussion 3.1. Spectral properties of donors and acceptors For the present study spectrophotometric investigations were carried out in three different solvents (CCl4 , CH2 Cl2 and CHCl3 ). The donors and acceptors were all soluble in the solvents used. DDQ solution in all the three solvents used in this study was light yellow in colour while iodine solution was of violet colour in all the three solvents. The solutions of donors were appeared to be colourless in three solvents.

1035

The wavelength for maximum absorbance λmax of acceptors and donors in all the three solvents was determined and then extinction coefficient max was calculated by Beer-Lambert’s law and the results are summarized in Table 1. Position of spectral bands may be affected by the solvent polarity. From the Table 1, it is clear that as the polarity of solvent is increased, λmax of pure iodine and DDQ is slightly shifted towards lower wavelength, i.e. blue shift is observed but for 2and 3-Picolines no effect of solvent polarity on λmax is observed. In case of 4-Picoline the slight shift in the λmax with increase of polarity of solvent is attributed to its higher dipole moment and charge density on N-atom as determined by semi empirical PM3 calculations using HYPER CHEM. The energies of HOMO and LUMO of donors and acceptors, respectively were determined by using by semi empirical PM3 calculations using HYPER CHEM and are reported in Table 1. Generally the energy gap of HOMO and LUMO is related to λobs by the following relation, ELUMO − EHOMO = hc/λobs . From this equation it is clear that E has inverse relation with λobs , i.e. lower wavelength corresponds to higher energy and vice versa. The expected inverse relationship of λmax with ELUMO − EHOMO is thus observed as seen in Table 1. For example, EHOMO−LUMO is relatively largest for 4-Picoline, spectral peak appears at shorter wavelength whereas for I2 HOMO − LUMO gap is smallest, spectral peak appears at larger wavelength. 3.2. Spectral properties of CT complexes When a solution of I2 or DDQ in any one of the solvents was mixed with a solution of donors in the same solvent, a slight colour change was observed. A new band appeared which is not the characteristic of either donor or acceptor alone. It clearly indicates the formation of charge transfer complex on mixing of donors and acceptors solution in the same solvent. The electronic absorption spectra in the wavelength range 320–600 nm were recorded for the CT complex solutions of the studied picolines with DDQ and I2 in three different solvents as mentioned above. In all the cases, the same concentration

Table 1 Values of λmax and εmax of donors & acceptors in three solvents Compound DDQ Iodine 2-Picoline 3-Picoline 4-Picoline DDQ Iodine 2-Picoline 3-Picoline 4-Picoline DDQ Iodine 2-Picoline 3-Picoline 4-Picoline a

Solvent

CCl4

CHCl3

CH2 Cl2

λmax (experimental) (nm)

λmax (Reported) (nm) [13]

εmax (L mol−1 cm−1 )

EHOMO − ELUMO (eV)a

288 516 263 264.5 260.5 287, 352 512 263 264 257 233, 276, 348 504 263 264 257

288

3973.00 893.00 28486.00 30871.00 13443.00 5899.00 1106.00 54245.00 51291.00 21930.00 10043.00 914.00 26186.00 27386.00 18486.00

DDQ = 7.52

292, 352

242, 288,349

Semi empirical PM3 calculations using HYPER CHEM for the energies of HOMO and LUMO of donors and acceptors.

I2 = 6.32 2-Picoline = 10.03

3-Picoline = 10.08 4-Picoline = 10.19

1036

H. Razzaq et al. / Spectrochimica Acta Part A 70 (2008) 1034–1040

Fig. 1. Spectra of Picolines–I2 C complexes in CHCl3 .

Fig. 4. Spectra of Picolines–DDQ CT complexes in CH2 Cl2 .

Fig. 5. Spectra of Picolines–I2 C complexes in CH2 Cl2 . Fig. 2. Spectra of Picolines–DDQ CT complexes in CHCl3 .

od DDQ and I2 as in the test solution was used as a blank to eliminate any interference from the acceptor spectrum with that of the corresponding formed CT complex. The spectra for CT complexes are shown in Figs. 1–6. Figs. 1–6 represent CT complexes of Pic-I2 & Pic–DDQ in CH2 Cl2 , CHCl3 & CCl4 taking acceptor solution of same concentration as reference. Where conc. of pic = 6.667 × 10−2 M, conc.of I2 = 3.300 × 10−4 M and conc. of DDQ = 3.300 × 10−4 M in the complex solution. The λmax of CT complexes of DDQ and iodine with 2-, 3- and 4-picoline in all the three solvents was determined and then max

Fig. 3. Spectra of Picolines–I2 C complexes in CH2 Cl2 .

was calculated by using modified Benesi-Hildebrand equation [14], 1 1 1 [A]0 + AD = AD A KAD ελ [D]0 ελ

(1)

where A is the absorbance of the charge transfer band complex, [A0 ] the initial concentration of the electron acceptor, [D0 ] the initial concentration of the electron donor, KAD the association constant of charge transfer complex in solution, and AD is the molar extinction coefficient of charge transfer complex.

Fig. 6. Spectra of Picolines–DDQ CT complexes in CH2 Cl2 .

H. Razzaq et al. / Spectrochimica Acta Part A 70 (2008) 1034–1040 Table 2 Values of λmax , KAD and AD of CT complexes of DDQ & Iodine at 25 ◦ C in three solvents using Benesi-Hildebrand equation CT complexes DDQ-2Picoline DDQ-3-Picoline DDQ-4-Picoline Iodine-2-Picoline Iodine-3-Picoline Iodine-4-Picoline DDQ-2Picoline DDQ-3-Picoline DDQ-4-Picoline Iodine-2-Picoline Iodine-3-Picoline Iodine-4-Picoline DDQ-2Picoline DDQ-3-Picoline DDQ-4-Picoline Iodine-2-Picoline Iodine-3-Picoline Iodine-4-Picoline

Solvent

CCl4

CHCl3

CH2 Cl2

λmax (nm)

KAD (Lmol−1 )

AD

(Lmol−1 cm−1 )

– – 303 414 411 409

12.86 150.00 200.00 140.00

– – 11111.11 1428.57 1428.57 2000.00

389 379 383 397 398 378

160.00 166.67 285.00 100.00 115.00 75.00

2500.00 2000.00 2500.00 1666.67 1666.67 3333.33

350.00 1142.86 400.00 90.00 225.00 150.00

2000.00 1250.00 1666.67 2500.00 2000.00 3333.33

461 461.5 461 383 387 380

The results of λmax and max for CT complexes are summarized in Table 2. The stability constant and the extinction coefficient of CT complexes were determined by using modified BenesiHildebrand equation. The linearity of plots of [A0 ]/A vs.1/[D0 ] clearly indicates the formation of 1:1 complexes (Fig. 7). Eq. (1) is valid [15,16] under the condition [D0 ] > > [A0 ] for 1:1 donor–acceptor complexes. The intercept of the line with the ordinate is ( AD )−1 and the gradient is equal to ( AD KAD )−1 . So from the intercept slope of these plots extinction coefficient and the association constants were obtained. It is observed that picolines form more stable complexes with DDQ as is clear from the larger values of the stability constants for DDQ-Picolines CT complexes as compared to that with iodine. A decrease in stability constant with increase in temperature was observed. The observed decrease in stability constant values with rise in temperature indicates the exothermic nature of the interaction

1037

between the studied acceptors and donor molecules and also that the formation of CT complexes is favorable at lower temperature. The experimental oscillator strength (f) and the transition dipole moment (μ) of the CT complex were calculated using the following appropriate formulae, f = 4.32 × 10−9 [εmax υ1/2 ] and μ = 0.0958[εmax υ1/2 /’␷max ]. The results for υ1/2 , f and μ are reported in Table 3. The values of calculated oscillator strength are rather relatively large indicating a strong interaction between the donor–acceptor pair with relative high probabilities of CT transitions. This is also supported by the relatively large heat of formation. The ionization potential of the donor, in different solvents, was determined from the CT-energies of the CT-band of its complex with DDQ and Iodine making use of the following relationship [17]: IP = 5.76 + 1.52 × 10−4 υ¯ (acceptor) (cm−1 ) where υ is the wave number corresponding to the maxima of CT-band of the complex. The values of ionization potentials thus determined are given in Table 2. It has been reported that the ionization potential of the electron donor may be correlated with the charge transfer transition energy of the complex [17]. The evidence for the nature of CT-interaction in the present systems is the calculation of dissociation energy (W) of the CT excited state of the complex in different solvents. Hence the dissociation energies of the complex were calculated from their CT transition energy, hυCT , the ionization potential of the donor, Ip and electron affinity, EA , of the acceptor using the empirical relation [17] given in the equation, hυCT = Ip − EA − W. The calculated values of W are given in Table 2. The plot of W versus Ip shown in Figs. 8–9 is linear, as was expected. Thus calculated values of dissociation energy of charge transfer excited states of the complex in different solvents are constant, which suggests that the investigated complexes are reasonably strong and stable under the studied conditions with higher resonance stabilization energy [17]. The strength of the CT complex can also be determined from the ratio b2 /a2 by using the proposed relation b2 /a2 = − H/hυ [3]. In this relation, hυ is the energy of absorption band of the complex, whereas according to Mullikens theory, a and b are the coefficients of the dative bond and the nonbonding wave functions (ΨD+ A− and ψD−A , respectively) of the CT complex. H is the change in enthalpy of reaction and it is thermodynamic parameter. Referring to many studied CT complexes [3], one can deduce that the obtained ratios (b2 /a2 ) are of the same magnitude as the results reported in literature (Table 3). 3.3. Composition of CT complexes

Fig. 7. Plot of 1/[D0 ] vs. [A0 ]/Abs for 4-Picoline–DDQ CT complex in CCl4 at 10 ◦ C, 15 ◦ C, 25 ◦ C, 30 ◦ C and 35 ◦ C at 303 nm using Benesi-Hildebrand equation.

Job’s method [18] of continuous variation was used to confirm the 1:1 stoichiometry of picolines to ␴- and ␲-acceptors as obtained by Benesi Hildebrand equation. The symmetrical curves with maximum at 0.5 mole fraction confirms the formation of 1:1 CT complex in all the solvents studied and a representative plot is shown in Fig. 10.

1038

H. Razzaq et al. / Spectrochimica Acta Part A 70 (2008) 1034–1040

Table 3 Spectral properties of CT complexes of DDQ and iodine in different solvents CT complex

Solvent

υ1/2 (cm−1 )

f

μ

b2 /a2

Ip (eV)

W (eV)

DDQ-2- Pic DDQ-3-Pic DDQ-4-Pic I2 -2-Pic I2 -3-Pic I2 -4-Pic

CCl4

– – 36210.89 137356.98 147640.70 124478.75

– – 0.26 0.85 0.79 1.08

– – 10.58 8.63 8.92 9.67

– – 0.08 0.06 0.08 0.08

– – 10.77 9.43 9.45 9.47

– 4.78 3.38 3.38 3.37

DDQ-2-Pic DDQ-3-Pic DDQ-4-Pic I2 -2-Pic I2 -3-Pic I2 -4-Pic

CHCl3

209648.00 265554.88 209823.96 107655.37 142261.67 104824.00

2.26 2.29 2.27 0.77 1.02 1.51

13.68 13.59 13.58 8.09 9.31 11.01

0.07 0.08 0.12 0.06 0.07 0.07

9.67 9.77 9.73 9.59 9.58 9.78

4.58 4.60 4.59 3.40 3.40 3.44

DDQ-2-Pic DDQ-3-Pic DDQ-4-Pic I2 -2-Pic I2 -3-Pic I2 -4-Pic

CH2 Cl2

336620.99 90530.51 92894.50 104825.10 128495.06 120707.35

2.12 0.49 0.80 1.13 1.11 2.61

16.88 6.92 8.09 9.59 9.55 11.84

0.07 0.09 0.11 0.05 0.07 0.07

9.06 9.06 9.06 9.73 9.69 9.76

4.46 4.46 4.46 3.43 3.42 3.43

3.4. Determination of thermodynamic parameters of the CT complexes The thermodynamic parameters (G, H, S) associated with the CT complex formation of the picolines with iodine and DDQ were evaluated from the spectral measurements at different temperatures. For this purpose, the KAD values of these complexes at six different temperatures (10, 15, 20, 25, 30 and 30 ◦ C) were determined in three different solvents. The enthalpy (H) was determined from the computed KAD values at different  temperatures making use of Van t Hoff equation given as ln KAD =

−H RT

The plot of ln KAD vs. 1/T is shown in Figs. 11 and 12. The Gibbs free energy change (G) was calculated by the equation, G = −RT ln KAD . The entropy change (S) was also Fig. 9. Plot of Ip vs. W for iodine–Picolines CT complexes.

Fig. 8. Plot of Ip vs. W for DDQ–Picolines CT complexes.

Fig. 10. Continuous variation method for the CT molecular complex of DDQ–Picoline (a, b, c) and I2 –Picoline (c, d, e) in CHCl3 , CH2 Cl2 and CCl4 .

H. Razzaq et al. / Spectrochimica Acta Part A 70 (2008) 1034–1040

1039

exothermic in nature. Thus the results of KAD and H are in good agreement with each other. The values of H and S generally become more negative as the stability constant for molecular complexes increases. As the bond between the components becomes stronger and thus the components are subjected to more physical strain or loss of freedom, the values of H and S should be more negative. In the present work a linear-correlation between H and S is found for both Picoline–DDQ and Picoline-Iodine CT-complex systems in different solvents indicating the stability of the CTcomplex. The H and S are more negative for Picoline–DDQ CT complex as compared to that for Picoline-Iodine CT complex indicating that Picolines–DDQ CT complexes are relatively more stable. The obtained values of G, H and S are most generally in accordance with those reported for middle strength n-␴ complexes [3] for iodine. It is observed that S has most negative values for DDQ-4-Picoline implying an ordered arrangement in the complex. This is expected from 4-Picoline due to its symmetric nature.



Fig. 11. Van t Hoff plot for Picolines–DDQ.

3.5. Effect of solvent on the studied CT complexes



Fig. 12. Van t Hoff plot for Picolines–I2 CT complex in CT complex in CHCl3 .

determined by making use of the equation, G = H − TS. The values of thermodynamic parameters listed in Table 4 show that complexation is thermodynamically favored. The enthalpy change of the complexation also reveals that the CT complex formation between the used donor and the acceptors is of Table 4 Thermodynamic parameters of CT complexes Donor

Solvent

Acceptor

−G (kJ/mol)

−H (kJ/mol)

2-Picoline 3-Picoline 4-Picoline 2-Picoline 3-Picoline 4-Picoline 2-Picoline 3-Picoline 4-Picoline 2-Picoline 3-Picoline 4-Picoline 2-Picoline 3-Picoline 4-Picoline 2-Picoline 3-Picoline 4-Picoline

CCl4

DDQ

– – 11.04 12.57 12.68 13.13 14.53 12.12 14.23 12.51 13.46 12.29 11.41 11.75 10.69 11.17 12.30 12.30

– – 36.02 22.32 26.31 38.68 20.13 25.40 30.09 19.40 25.36 25.36 17.50 23.24 24.78 18.34 23.49 23.02

CHCl3

CH2 Cl2

CCl4

CHCl3

CH2 Cl2

Iodine

−S (kJ/mol) – 0.08 0.03 0.04 0.09 0.02 0.05 0.05 0.02 0.04 0.04 0.02 0.04 0.04 0.02 0.04 0.03

The increase in the polarity of solvent may produce a red or a blue shift in λmax. A blue shift appears for a n–␴* or n–␲* transition whereas a red shift is present for a ␲–␲* transition [19]. In the present work it is observed that as the solvent polarity is increased, λmax of DDQ–Picolines CT complexes is shifted to the higher wavelength, i.e. a red shift is observed, thus implying a ␲–␲* instead of the expected n–␲* transition as observed in amino substituted pyrimidines–DDQ complexes [3]. This blue shift implies an n–␲* transition which may be due to the more basic nature of amino substituted pyrimidines. It may be inferred from this observation that a decreased basicity of the donor may lead to a ␲–␲* transition in methyl substituted pyrimidines instead of the expected n–␲* transition in amino substituted pyrimidines. For the iodine–Picoline CT complexes, a blue shift in λmax was observed implying an n–␴ transition. This is expected result as iodine is known to form complexes with lone pair donors via n–␴ transitions [3]. In case of iodine–picolines CT complexes the difference in KAD values in the three solvents are generally in the order as CCl4 > CH2 Cl2 > CHCl3 which indicate that the formation of the complexes in CCl4 is most favorable, most probably because the complexes formed are less polar than their components (donor and acceptor individually) [19]. In a complex of a non-polar donor with a non-polar acceptor, the dipole moment of the no-bond structure will usually be very small and may be equal to zero. However there will be a dipole moment associated with a dative bond structure and it will be directed from donor to acceptor. Since the actual ground state wave function of the complex is an admixture of both the nobond and the dative bond wave functions, this implies there will be a dipole moment of the complex. In the present study the donors have high dipole moments so we do expect that the formed complex would be less polar. This argument explains the higher stability constants of the complexes in the less polar

1040

H. Razzaq et al. / Spectrochimica Acta Part A 70 (2008) 1034–1040

solvent, carbon tetrachloride. While in case of DDQ–Picoline CT complexes the decreasing order of KAD values in the three solvents is in the order as CH2 Cl2 > CHCl3 > CCl4 . On the basis of discussion for Iodine–Picolines CT complexes it might be concluded that the dipole moment of the resulting CT complexes in case of DDQ–Picoline CT complexes is greater than that of the donors or acceptors alone and this is the reason for smaller KAD value of 4-Picoline–DDQ CT complex which is much lower than that for the observed value of KAD for 4-Picoline–Iodine CT complex. Thus the values of KAD increase with increase in solvent polarity in case of ␲-acceptor while reverse order is observed for ␴-acceptor for most of the cases. It may also be concluded that for the same class of donors (e.g., picolines), the CT complex formed, the transition may be ␲–␲* or n–␲* depending on the substituent. 4. Conclusions From the discussion given above, it may be concluded that the charge transfer interaction between the picolines with iodine as a ␴-acceptor and with DDQ as a ␲-acceptor have indicated the capability of picolines to form CT complexes of n-␴ and ␲–␲ types. The obtained formation constants and ionization potentials reflect that the picolines are strong donors and the formed complexes are of 1:1 stoichiometry. The data obtained for thermodynamic parameters suggest the exothermic nature of CT complexes. From the trends in CT-absorption bands, the vertical ionization potential of picolines has been estimated in different solvents. The oscillator and transition

dipole moment of the complexes being formed have also been determined. References [1] M.M.A. Hamed, M.I. Abdel-Hamid, M.R. Mahmoud, Monatsh. Chem. 129 (1998) 121–127. [2] E.H. EI-Mossalamy, A.S. Amin, A.A. Khalil, Spectrochim. Acta A 58 (2002) 67–72. [3] H.M. Salmana, U.M. Rabiea, E.M. Abd-Allab, Can. J. Anal. Sci. Spectrosc. (2004) 49. [4] M.S. Newman, J. Blum, J. Am. Chem. Soc 86 (1964) 5600. [5] K.G. Gunningham, W. Dawsen, F.S. Spring, J. Chem. Soc. (1951) 2305. [6] R. Foster, C.A. Fyfe, J. Chem. Soc. (B) (1966) 926. [7] R.S. Mulliken, W.B. Person, Annual Rev. Phys. Chem. 13 (1962) 107. [8] S.M. Sydnye, R.D. Shackle, TAPPI Fall Coat Graphic Arts Din. Week Prep. (1975) 14–50. [9] S.D. Bella, I.L. Fragala, M.A. Ratner, T.J. Marks, J. Am. Chem. Soc. 115 (1993) 682. [10] F.P. Pla, J. Palou, R. Valero, C.D. Hall, P. Speers, J.C.S Perkin Trans. 2 (1991) 1925. [11] K. Datta, A.K. Mukberjee, M. Banerjee, B.K. Seal, Spectrochim. Acta Part A. 53 (1997) 2587. [12] T. Roy, K. Datta, M.K. Nayek, A.K. Mukherjee, M. Banerjee, B.K. Seal, J.C.S. Perkin Trans. 2 (1999) 2219. [13] M.S. Subhani, N.K. Bhatti, M. Mohammad, A.Y. Khan, Turk. J. Chem. 24 (2000) 223–230. [14] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2103. [15] S.M. Teleb, M.S. Refat, Spectrochim. Acta A 60 (2004) 1579–1586. [16] S. Bhattacharya, M. Banerjee, A.K. Mukherjee, Spectrochim. Acta A 57 (2001) 1463–1470. [17] M. Pandeeswaran, K.P. Elango, Spectrochim. Acta A 65 (2006) 1148–1153. [18] P. Job, Ann. Chim. Phys. 9 (1928) 113. [19] M. Tarek, M. El Gogary, A. Mostafa, Diaib, F. Shreen, E.L. Tantawz, Monatsh. Chem. 137 (2006) 1027–1042.