Absorption spectrometric study of molecular complex formation between [60]fullerene and a series of methylated pyridines

Spectrochimica Acta Part A 58 (2002) 2563– 2569 www.elsevier.com/locate/saa

Absorption spectrometric study of molecular complex formation between [60]fullerene and a series of methylated pyridines Sumanta Bhattacharya, Manas Banerjee, Asok K. Mukherjee * Department of Chemistry, The Uni6ersity of Burdwan, Golapbag, Burdwan 713104, India Received 1 November 2001; accepted 27 November 2001

Abstract [60]fullerene has been shown to form 1:1 molecular complexes with pyridine and some methylated pyridines such as 2-picoline, 3-picoline, 4-picoline, 2,6-lutidine and 2,4,6-collidine in CCl4 medium by absorption spectrometric method. Well defined charge transfer (CT) bands have been observed for complexes of C60 with all the pyridines studied except 4-picoline. From an analysis of the trends in the CT absorption bands the ionisation potentials of the methylpyridines have been determined. The electron affinity of C60 has also been determined from the spectral data. The formation constants of the complexes exhibit a very good linear free energy relationship from which the Hammett z parameter for the complexation process is found to be −2.96. © 2002 Elsevier Science B.V. All rights reserved. Keywords: [60]fullerene; Methylpyridine complexes; Electron affinity; Ionisation potentials; Hammett parameter

1. Introduction Since the discovery of fullerenes, in particular [60]fullerene [1], a great deal of experimental [2 – 4] and theoretical [5– 7] work has been done. The photophysics of C60 molecules in solution is quite well understood [8,9]. Recently the role of fullerenes in forming donor – acceptor complexes has received considerable attention from the viewpoint of both basic fullerene chemistry and fullerene based applications. A C60 molecule can act as an electron acceptor with a variety of electron donors, and optical and electronic prop* Corresponding author

erties of C60 with various donors have been reported by several groups [10 –17]. Charge transfer (CT) in C60-conducting polymer composites and other fullerene-based compounds is currently of great interest since these materials can be utilised in xerography, energy phototransdusers and molecular switches [18]. The treatment of C60 with tertiary amine tetrakis (dimethylamine) ethylene (TDAE) leads to the salt TDAE+C60 − which reveals an unusual molecular ferromagnetism [19]. Incorporation of C60 into polyacetylenes is of recent interest because the p–p interaction between the C60 cages and the polyacetylene chains may impart intriguing electronic and optical properties [20]. CT in fullerene compounds has been

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studied in solution and also in solid state [14– 17,21]. The CT transition energies (hwCT) have been determined by electronic absorption spectroscopy for several complexes of C60 [11 – 13], and the dependence of (hwCT) on the vertical ionisation potentials (ID) of the donors have been considered [12]. The present work is an UV– VIS absorption spectroscopic study which shows that C60 forms 1:1 electron donor– acceptor (EDA) complexes with pyridine, 2-picoline, 3-picoline, 4-picoline, 2,6-lutidine and 2,4,6-collidine in CCl4 medium. The importance of the pyridineC60 complexes is also reflected in a recent study [22], which reports the synthesis of a water soluble complex between [60]fullerene and a cholesteryl– group – bearing pullulan; the method of synthesis requires a solu-

tion of C60 in pyridine (10% v/v) indicating the possibility of a C60pyridine molecular complex. For this reason, it is felt necessary to study the interaction between C60 and pyridines (i.e., pyridine and methylpyridines) in solution phase.

2. Experimental [60]fullerene was collected from Sigma. Pyridine, 2-picoline, 3-picoline, 4-picoline, 2,6-lutidine and 2,4,6-collidine (commercial grade) were purified by repeated distillation with solid sodium hydroxide. HPLC grade CCl4 was used as solvent. Spectral measurements were carried out on a Shimadzu UV-2101 PC model spectrophotometer fitted with TB-85 thermo bath.

3. Results and discussions

3.1. Analysis of CT absorption spectra of the complexes and determination of 6ertical ionisation potentials (ID 6) of methyl pyridines

Fig. 1. Absorption spectra of C60 (5.4977× 10 − 5 mol dm − 3) and mixtures of (A) C60 (5.4977× 10 − 5 mol dm − 3)+ 3-picoline (0.0187 mol dm − 3), (B) C60 (5.4977× 10 − 5 mol dm − 3)+ 2,4,6-collidine (0.0178 mol dm − 3) and (C) C60 (5.4977× 10 − 5 mol dm − 3)+ 2,6-lutidine (0.025 mol dm − 3) in CCl4 medium against the solvent as reference.

Fig. 1 shows the electronic absorption spectra of C60 and mixtures of C60 with 2-picoline, 3-picoline and 2,6-lutidine in CCl4 medium against the solvent as reference. At 440 nm, where C60 has negligible absorption (owing to the small concentration used), the mixture of C60 and 2,6-lutidine shows an absorption band. Similar spectral features were obtained with C602-picoline, C603-picoline, C60pyridine and C602,4,6-collidine systems but with different umax. The wavelengths at these new absorption maxima and the corresponding transition energies (hw) are summarised in Table 1. A plot of the observed hw values against the donor ionisation potentials (calculated by the AM1 method [23]) is approximately linear, the least square correlation coefficient being 0.87. This confirms that the observed new absorption bands of the C60pyridine mixtures are of CT nature and henceforth we shall write umax as uCT. It is a common experience that theoretically calculated ionisation potentials differ appreciably from experimental values. Because theoretical calculation considers only the isolated molecule and ig-

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Table 1 CT absorption maxima and transition energies of C60pyridine complexes; AM1 and experimental ionisation potentials of the donors Donor

uCT (nm)

hwCT (eV)

AM1 IDv (eV)

Experimental IDv (eV)

Pyridine 2-Picoline 3-Picoline 2,6-Lutidine 2,4,6-Collidine

421 423 430 440 451

2.945 2.932 2.884 2.818 2.750

9.9322 9.6327 9.6331 9.4041 9.3716

9.23 9.22 9.17 9.10 9.03

nores bulk (mainly, solvation) effects. A survey of literature shows that for the present series of donors experimental IDv is known only for pyridine [24]. Hence in the present work the observed CT transition energies (hwCT) have been utilised to obtain the experimental IDv values of the other pyridines as follows: The energy of a CT transition (hwCT) can be expressed according to Mulliken’s theory [25] as: hwCT = I −M + 2 i /(I v D

2

v − M) D

(1)

where IDv is the vertical ionization potential of the donor, i is related to the overlap of donating and accepting orbitals, and M is comprised of the affinity of the acceptor, electrostatic interactions, and other terms. When, in a CT complex, i is small, (Eq. (1)) reduces to its more frequently used form (Eq. (2)) which is valid when 2i 2/(I vD −M) is sufficiently small: hwCT = I vD −M

(2)

M is taken to be a constant for a series of EDA complexes with the same acceptor. Experimental IDv of pyridine was collected from ref [24]. Putting this IDv value in (Eq. (2)) we get the value of M. As M is constant we then can easily obtain the experimental I vD value of all the other methylpyridines using (Eq. (2)) and the observed values of hwCT. Results are given in Table 1. The goodness of these newly determined I vD values of the pyridines were tested by plotting them against the observed hwCT energies. The following correlation was obtained: hwCT = (0.99890.0003)I vD −(6.283 90.00273), correlation coefficient =0.99

The nearly unit slope is an accordance with Mulliken’s theory [25].

3.2. Determination of 6ertical electron affinity (EA 6) of C60 According to Mulliken’s theory [25] the quantity M of (Eq. (1)) is given by M= E vA + G0 + G1

(3)

where EAv is the vertical electron affinity of the acceptor, G0 is the sum of several energy terms (like dipole–dipole, van der Waals interaction, etc.) in the ‘no-bond’ state and G1 is the sum of number of energy terms in the dative state. In most cases G0 is small and can be neglected while G1 is largely the electrostatic energy of attraction between D+ and A−. A rearrangement of (Eq. (1)) yields 2I vD − hwCT = (1/M)I vD (I vD − hwCT) +M+ (2i 2/M)

(4)

Using the observed transition energies and the experimentally determined vertical ionisation potentials (shown in Table 1) we have obtained the correlation 2I vD − hwCT = (0.1589790.000838) I vD (I vD − hwCT) + (6.29379 0.04822)

(5)

with a correlation coefficient of 0.99 (Fig. 2). This also confirms the CT nature of the transition observed and the slope of (Eq. (4)) yields M= 6.29 eV. Neglecting G0 and taking the typical D–A distance in p-type EDA complexes to be 3.5 A, , the major part of G1 is estimated to be e 2/ 4pm0r= 4.13 eV. Now using (Eq. (3)), EAv of C60

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in solution is found to be 2.16 eV which is in fair agreement with the value of 2.31 eV obtained in some earlier works [21,26]. The present EAv value is somewhat less than that of 2.65 eV obtained in gas phase by Smalley et al. [27] (and later by Boltalina et al. [28] and Chen et al. [29]). The difference (0.49 eV) between the electron affinity values in gas phase and in solution is due to solvation effect, which has significant contribution to G1. Owing to lack of suitable data this contribution can not be estimated correctly.

3.3. Determination of formation constants (K) Stoichiometry and formation constants of the complexes were determined by using Benesi– Hildebrand (BH) [30] equation for cells with 1 cm optical path length: [A]0 [B]0/d%= [B]0/m% +1/Km% 0 A

d%= d−d −d

0 B

(6) (7)

Here [A]0 and [B]0 are the initial concentra-

tions of the acceptor and donor, respectively, d% is the absorbance of the donor–acceptor mixture measured against the solvent as reference, dA0 and dB0 are the absorbances of the acceptor and donor solutions with same molar concentrations as in the mixture and at the same wavelength. The quantity m% means mC − mA − mB where mC is the molar absorptivity of the complex, mA and mB are those of the acceptor and the donor respectively at the wavelength of measurement. K is the formation constant of the complex. (Eq. (6)) is valid under the condition [B]0  [A]0 for 1:1 donor –acceptor complex. Experimental data are shown in Tables 2 and 3. In all the cases very good linear plots according to (Eq. (6)) were obtained, one such plot being shown in Fig. 3. Values of K and m% of the complexes obtained from such plots are shown in Tables 2 and 3. The values of K thus obtained exhibit a good linear free energy relationship (Fig. 4) (Hammett) [31]: logK = (− 2.9690.24)| + (1.749 0.025)

Fig. 2. Plot of 2IDv − hnCT against IDv (IDv − hwCT) using the presently determined values of IDv.

(8)

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Table 2 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C60pyridine, C603-picoline and C604-picoline complexes (Temperature =303 K) Donor

102 Donor concentration (mol dm−3)

Pyridine

0.625 1.250 1.875 2.500 3.125 3.725

3-Picoline

4-Picoline

0.625 1.250 1.875 2.500 3.725 1.875 2.500 3.125 3.725 4.375

K (dm3 mol−1)

m% (dm3 mol−1 cm−1)

57

6369

1.8904

0.006 0.008 0.007 0.007 0.012

84

714

1.7361

0.224 0.222 0.231 0.232 0.254

179

15954

105 [A]0 (mol dm−3)

2.0399

Absorbances at 406 nm 0.032 0.057 0.067 0.082 0.084 0.086

Table 3 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities of the C602-picoline, C602,6-lutidine and C602,4,6-collidine complexes (Temperature = 303 K) Donor

102 Donor concentration (mol dm−3)

2-Picoline

0.625 1.875 2.500 3.125 4.375 5.000

2,6-Lutidine

2,4,6-Collidine

0.625 1.250 1.875 2.500 3.125 4.400 5.000 0.712 1.068 1.425 1.780 2.134

105 [A]0 (mol dm−3)

1.7882

3.5764

1.7361

Absorbances at 411 nm 0.021 0.020 0.028 0.030 0.029 0.028 0.061 0.057 0.067 0.063 0.065 0.062 0.073 0.322 0.349 0.352 0.374 0.379

K (dm3 mol−1)

m% (dm3 mol−1 cm−1)

186

1821

474

1972

468

23 697

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Fig. 3. Benesi – Hildebrand plot for C604-picoline system at 303 K.

Fig. 4. Hammett plot for the complexes of C60 with pyridine, 3-picoline and 4-picoline.

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Acknowledgements S. Bhattacharya thanks the Council of Scientific and Industrial Research (C.S.I.R.), India for a Senior Research Fellowship. Financial assistance by the U.G.C., India extended through the DSA project in Chemistry, is also gratefully acknowledged. We also acknowledge Miss Shrabanti Banerjee for providing 2,6-lutidine and 2,4,6collidine.

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