Study of charge transfer complexes of [70]fullerene with phenol and substituted phenols

Spectrochimica Acta Part A 61 (2005) 2065–2071

Study of charge transfer complexes of [70]fullerene with phenol and substituted phenols Sumanta Bhattacharyaa,∗ , Shrabanti Banerjeeb , Manas Banerjeea b

a Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713104, India Department of Chemistry, Raja Rammohan Roy Mohavidyalaya, Khanakul, Hooghly, West Bengal, India

Received 13 June 2004; accepted 20 August 2004

Abstract To improve the understanding of the charge transfer (CT) interaction of [70]fullerene with electron donors, interaction of [70]fullerene with a series of phenols, e.g., phenol, resorcinol and p-quinol were studied in 1,4-dioxan medium using absorption spectroscopy. An absorption band due to CT transition was observed in the visible region. The experimental CT transition energies (hνCT ) are well correlated (through Mulliken’s equation) with the vertical ionisation potentials (IDv ) of the series of phenols studied. From an analysis of this correlation degrees of charge transfer for the [70]fullerene-phenol complexes were estimated. The degrees of charge transfer in the ground state of the complexes have been found to be very low (<2%). The hνCT values change systematically as the number and position of the OH groups change on the aromatic ring of the phenol moiety. From the trends in the hνCT values, the H¨uckel parameters (hO¨ and kC O¨ ) for the OH group were obtained in a straightforward way and the values so obtained, viz., 1.91 and 1.0, respectively, are close to the ones (1.8 and 0.8) recommended by Streitwieser on the basis of other evidence. Oscillator strengths, transition dipole strengths and resonance energies of the [70]fullerene-phenol complexes were determined. Formation constants of the CT complexes were determined at four different temperatures from which enthalpies and entropies of formation of the complexes were estimated. © 2004 Elsevier B.V. All rights reserved. Keywords: [70]Fullerene; Phenols; CT bands; Enthalpies and entropies of formation

1. Introduction Since the discovery of [70]fullerene [1], a great deal of work has been done on this novel ␲-system [2–5]. In contrast to planar ␲-electron acceptors, [70]fullerene possess a number of characteristic features, namely, spherical shape, unique electronic structure, high symmetry, and polarizability [6]. That extraordinary properties of [70]fullerene have attracted the attention and provoked experimental studies in different laboratories. Because of high electron affinity (EA = 2.73 eV) [7,8], [70]fullerene can act as an acceptor in the charge transfer (CT) complexation with either electron rich aromatics (␲-donor) or interaction through an electron pair from a non-bonding orbital of heteroatoms like N, I, Br, Cl, ∗

Corresponding author. Tel.: +91 342 2558545; fax: +91 342 2530452. E-mail address: sum [email protected] (S. Bhattacharya).

1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.08.006

F and S (n-donors) or in combination. CT phenomenon of [70]fullerene with electron donors [9–16] is well documented and has been shown to exhibit interesting physical properties (e.g., superconductivity and ferromagnetism) [17,18] and may be of interest as promising photoactive materials [19]. Though interaction of [70]fullerene with amine has now been studied [20], less attention has been paid to its complexation with phenol and substituted phenols similar to those used in the optical spectroscopic investigations. This lack of study is quite surprising since the [70]fullerene absorbance spectrum, is known to be solvent dependent [21]. In addition, aromatic solvents are known to cause pronounced changes in vibronic spectral features of fullerene luminescence spectra [22,23]. Study of [70]fullerene/phenol complexation is also important for the interpretation of other spectroscopic data. Our rationale which is based on the molecular interactions of [70]fullerene with a series of phenols, addresses to this issue

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and the results described herein open the doors on a variety of fronts for future scientific endeavors with [70]fullerene.

2. Materials and methods [70]fullerene was collected from Lanchaster, UK. Phenol was distilled twice just before use. The other phenols, viz., resorcinol and p-quinol were purified by sublimation. The solvent 1,4-dioxan, was of UV spectroscopic grade. All spectral measurements were done on a UV-1601 PC model Shimadzu spectrophotometer fitted with a Peltier controlled thermo-bath.

3. Results and discussions 3.1. Observation of CT bands For a series of related donors with a single acceptor, a plot of the peak maximum, hνCT , of a charge transfer band versus the vertical ionisation potential IDv of the donor should be roughly linear. This behaviour has been empirically observed for [70]fullerene with substituted phenols as that observed for [70]fullerene with substituted anilines [20]. To obtain the CT bands, the spectrum of each of the solutions (in dioxan) containing the [70]fullerene as acceptor and the donors (phenol, resorcinol and p-quinol) were recorded separately against the pristine acceptor solution as reference. It is a common practise that CT bands in solution are detected only by taking a high concentration of donor compared to that of the acceptor. In present case [phenol] ∼ = 10−2 to 10−3 mol dm−3 and concentration of the acceptor was ∼ =10−6 mol dm−3 . Two typical CT absorption bands are shown in Fig. 1. The solvent dioxan and the donors do not absorb in the visible range. The absorption spectra were analyzed by fitting them to the Gauss function, and the results are illustrated in Table 1. One such Gaussian analysis plot is shown in Fig. 2. The energies (hνCT ) corresponding to the maxima of the CT bands for the various [70]fullerene-phenol CT complexes are given in Table 2.

Fig. 1. CT absorption spectra of: (a) [70]fullerene (5.952 × 10−6 mol dm−3 ) + phenol (15.50 × 10−3 mol dm−3 ) and (b) [70]fullerene (4.761 × 10−6 mol dm−3 ) + resorcinol (9.85 × 10−3 mol dm−3 ) against pristine acceptor solution as reference.

3.2. Degree of charge transfer (α) To discuss the degrees of charge transfer (α) for the [70] fullerene-phenol CT complexes, we have used the Mulliken’s two-state model [24]. In this model, α is expressed by
 v α = (C2 /2)/ (IDv − EA + C1 )2 + (C2 /2) (1)

Fig. 2. Gaussian analysis curve of the shoulder region of Fig. 1(a).

Table 1 Gaussian curve analysis for the CT spectrum of [70]fullerene with phenol, resorcinol and p-quinol System

Area of the curve, A

Width of the curve, w

Center of the curve, xc

y0

[70]Fullerene-phenol [70]Fullerene-resorcinol [70]Fullerene-p-quinol

1.594 ± 0.080 0.28 ± 0.07 0.0092 ± 0.0011

103.58 ± 2.798 45.19 ± 5.58 9.70 ± 0.80

667 ± 0.43 746 ± 0.51 925 ± 0.26

0.066 ± 0.0003 0.105 ± 0.0007 0.129 ± 0.00004

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Table 2 CT absorption maxima and transition energies of [70]fullerene complexes; degrees of charge transfer (α), oscillator strength (f), transition dipole strengths (µEN ) and resonance energy (RN ) of the complexes of [70]fullerene with phenol, resorcinol and p-quinol Donor

λCT (nm)

hνCT (eV)

IDv (eV)

hνCT + ε0j (eV)

α (×102 )

f (×103 )

µEN (D)

|RN |

Phenol Resorcinol p-Quinol

667 746 925

1.859 1.662 1.341

9.115 9.126 8.712

1.8589 1.6623 2.3053

1.72 1.71 1.83

7.47 2.08 0.92

12.30 9.82 14.11

0.32 0.23 0.35

The values of α were calculated by using Eq. (1) and given in Table 2. The low value of α indicates that very little charge transfer takes place in the ground state. Dependence of α on IDv of the donors is shown in Fig. 3. It is found that α decreases with increasing ionisation potential of the donors, as expected. 3.3. Determination of H¨uckel parameters (hO¨ and kC O¨ ) The λCT values and the corresponding transition energies for the [70]fullerene-phenol CT complexes were found to change systematically as the number and position of the phenolic OH groups change on the phenol moiety. To explain this with the help of Mulliken’s theory [24], a simple perturbational calculation using the Coulson–Longuet–Higgins method [25] was carried out. According to this method, the energy (εj ) of the highest occupied molecular orbital (HOMO) of a phenol is given by   2 εj = ε0j + hO¨ β Crj + 2(kC O¨ − 1)β Crj Csj (2) r

r
Here the unperturbed system is taken to be the radical obtained by replacing the OH group of phenol by CH2 ; ε0j is the corresponding unperturbed HOMO energy. The perturbational H¨uckel parameters [25] (hO¨ and kC O¨ ) were defined as µO¨ = µ + hO¨ β

and

βC

¨ O

= kC

¨β O

where µ and β are, respectively, the Coulomb integral of a sp2 hybridised carbon atom and the resonance integral between

two adjacent sp2 carbon atoms in benzene. According to Mulliken’s theory [24], hνCT is related to the ionisation potential v of acceptor (A) by the equation (IDv ) of the donor (D) and EA v hνCT = IDv − EA +

(3)

where the energy term, , is composed of solvation, van der Waals interactions in ground state and Columbic interaction between D+ and A− in the excited state. For a fixed acceptor, and a series of structurally similar donors in a given solvent hνCT = IDv + constant

(4)

Since, IDv , in turn, is the negative of the HOMO energy of the phenol, we have from Eqs. (2) and (4)   2 hνCT + ε0j = − hO¨ β Crj + 2(kC O¨ − 1)β Crj Csj r

+ constant

r
(5) 

2 The plot of hνCT + ε0j against r Crj is linear with a correlation coefficient of 0.95. The linearity suggests that the second term in Eq. (5) has no effect which implies that kC O¨ ) = 1. From the slope of the line we find (hO¨ = 1.91 ± 0.85 (taking β = −3.1 eV as obtained from the first four singlet–singlet transitions in benzene). The values of (hO¨ and kC O¨ ) are in excellent agreement with Streitwieser’s [26] recommended values (1.8 and 0.8) respectively.

3.4. Determination of oscillator (f) and transition dipole strengths (µEN ) From the CT absorption spectra, we extracted an oscillator strength. The oscillator strength f is estimated using the formula:  −9 f = 4.32 × 10 εCT dν (6)  where εCT dν is the area under the curve of the extinction coefficient of the absorption band in question versus frequency. To a first approximation f = 4.32 × 10−9 εmax ν1/2

Fig. 3. Degrees of charge transfer (α) as a function of ionisation potential.

(7)

where εmax is the maximum extinction coefficient of the band and ν1/2 the half-width, i.e., the width of the band at half the maximum extinction. The observed oscillator strengths of the CT bands are summarized in Table 2. It is worth mentioning that we needed a proper calculation of oscillator strengths of [70]fullerene/phenol CT complexes; this is because oscillator strength is very sensitive to the molecular

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configuration and the electron charge distribution in the CT complex. For [70]fullerene/phenol CT complexes, we cannot using simple model assuming a charge localized at a certain cite of [70]fullerene molecule; this is due to the fact that [70]fullerene has ␲-bonds which are directed radially with a node on the molecular cage. The extinction coefficient is related to the transition dipole by 

µEN

εmax ν1/2 = 0.0952 ν

1/2 (8)

 where ν ≈ ν at εmax and µEN is defined as −e ψex  i ri ψg dτ. µEN values for the complexes of [70]fullerene with various phenols are given in Table 1.

3.5. Determination of resonance energy (RN ) Briegleb and Czekalla [27] theoretically derived the relation εmax =

7.7 × 104 hνCT /|RN | − 3.5

(9)

where εmax is the molar extinction coefficient of the complex at the maximum of the CT absorption, νCT the frequency of the CT peak and RN is the resonance energy of the complex in the ground state, which, obviously is a contributing factor to the stability constant of the complex (a ground state property).

3.6. Determination of formation constants The formation constants of the [70]fullerene/phenol complexes were determined at four different temperatures using the Benesi–Hildebrand (BH) [28] equation in the form [A]0 [D]0 [D]0 1 = + (10) d ε Kε Here [A]0 and [D]0 are the initial concentrations of the acceptor and donor, respectively, d is the absorbance of the donor–acceptor mixture at λCT against the pristine acceptor solution as reference, dA0 and dD0 are the absorbances of the acceptor and donor solutions with same molar concentrations as in the mixture at the same wavelength (i.e., λCT ). The quantity ε is the molar absorptivity of the complex at λCT . K is the formation constant of the complex. Eq. (10) is valid [28] under the condition [D]0  [A]0 for 1:1 donor–acceptor complexes. The intensity in the visible portion of the absorption band, measured against the acceptor solution of concentration [A]0 as reference, increases systematically with gradual addition of phenol. This indicates complex formation. Experimental data are given in Tables 3–5. In all the cases very good linear plots according to Eq. (10) are obtained, one typical case being shown in Fig. 4. The correlation coefficients of all such plots were above 0.95. The molar absorption coefficient ε is dependent on the donor species and increases with decreasing ionisation potential. The [70]fullerene/resorcinol complex exhibits highest value of formation constant. This result indicates that the distance between the [70]fullerene and donor molecules does not play significant role in forming donor–acceptor complexes in the present study. Rather,

Table 3 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities (ε) of the [70]fullerene-phenol complex Temperature (K)

Donor concentration (×103 mol dm−3 )

[A]0 (×106 mol dm−3 )

Absorbances at λCT

K (dm3 mol−1 )

ε (dm3 mol−1 cm−1 )

298

5.82 9.70 11.64 15.50 17.46

5.952

0.051 0.076 0.072 0.078 0.103

80 ± 7

33400 ± 1700

303

5.82 9.70 11.64 15.50 17.46

0.045 0.068 0.071 0.079 0.099

50 ± 5

308

5.82 9.70 11.64 15.50 17.46

0.040 0.064 0.066 0.075 0.095

40 ± 3

313

5.82 9.70 11.64 15.50 17.46

0.037 0.061 0.063 0.071 0.094

30 ± 3

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Table 4 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities (ε) of the [70]fullerene resorcinol complex Temperature (K)

Donor concentration (×103 mol dm−3 )

[A]0 (×106 mol dm−3 )

Absorbances at λCT

K (dm3 mol−1 )

ε (dm3 mol−1 cm−1 )

298

14.80 27.02 34.50 49.10 95.10 79.10 74.04

4.761

0.083 0.078 0.115 0.116 0.110 0.090 0.069

875 ± 120

21300 ± 1065

303

14.80 27.02 34.50 49.10 95.10 79.10

0.079 0.079 0.113 0.118 0.103 0.089

525 ± 75

308

14.80 27.02 34.50 49.10 95.10

0.072 0.074 0.109 0.114 0.096

265 ± 40

313

14.80 27.02 34.50 49.10 95.10

0.068 0.069 0.107 0.109 0.093

220 ± 30

we propose that the donor and acceptor molecules are apparently considerably less tilted out of a perpendicular orientation with respect to the line joining their centers. According to Mulliken’s theory [24], the perpendicular separation distances between the component rings diminishes with decreasing ionisation potentials of the donors. As resorcinol has higher value of ionisation potential (i.e., 9.126 eV) than that of phenol (i.e., 9.115 eV) and p-quinol (i.e., 8.712 eV), it forms strong complex with [70]fullerene. This explains the higher value of formation constant for the [70]fullereneresorcinol complex.

• [70]fullerene-phenol complex: ln K =

5867 ± 515 − (15 ± 1.7), T correlation coefficient = 0.99

(12a)

• [70]fullerene-resorcinol complex: ln K =

9066 ± 1263 − (23.7 ± 4.1), T correlation coefficient = 0.98

(12b)

3.7. Enthalpies (∆Hf0 ) and entropies of formation (∆Sf0 ) of the complexes of [70]fullerene with phenol and substituted phenols Evaluation of K for the [70]fullerene-phenol complexes at four different temperatures allows for determination of enthalpies (Hf0 ) and entropies of formation (Sf0 ) by a van’t Hoff plot of ln K versus 1/T (Eq. (11)). As measured, these terms will represent the net change in enthalpy and entropy for the solvated species: ln K = −

Hf0 + constant RT

(11)

The following linear regression equations have been obtained with the present data:

Fig. 4. Benesi–Hildebrand plot for the complex of [70]fullerene with pquinol at 298 K.

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Table 5 Data for spectrophotometric determination of stoichiometry, formation constants (K) and molar absorptivities (ε) of the [70]fullerene p-quinol complex Temperature (K)

Donor concentration (×103 mol dm−3 )

[A]0 (×106 mol dm−3 )

Absorbances at λCT

K (dm3 mol−1 )

ε (dm3 mol−1 cm−1 )

298

21.50 16.70 18.80 34.04 31.50 41.50 54.50

5.952

0.101 0.090 0.100 0.131 0.130 0.133 0.170

32 ± 3

43960 ± 2200

303

21.50 16.70 18.80 34.04 31.50 41.50 54.50

0.094 0.089 0.097 0.132 0.125 0.133 0.163

29 ± 3

308

21.50 16.70 18.80 34.04 31.50 41.50 54.50

0.091 0.083 0.092 0.129 0.121 0.130 0.160

27 ± 2

313

21.50 16.70 18.80 34.04 31.50 41.50 54.50

0.087 0.079 0.090 0.125 0.121 0.125 0.159

24.6 ± 2.5

• [70]fullerene-p-quinol complex: ln K =

1611 ± 65 − (1.9 ± 0.2), T coefficient = 0.99

p-quinol 8.712 eV) were plotted against Gibbs energies of the [70]fullerene-phenol complexes. The excellent linear plot with a correlation coefficient of 0.91 supports the above view.

correlation (12c)

The positive slope in each case indicates that the complexation process is exothermic and thus enthalpy favoured. The Hf0 and Sf0 values of the complexes are listed in Table 6. One typical plot of ln K against 1/T is shown in Fig. 5. The large enthalpy for the [70]fullerene/resorcinol complex is consistent with the larger value of K in the [70]fullerene/resorcinol system. Finally, it can be concluded that, since the guest molecule (i.e., [70]fullerene) is a weak electron acceptor, CT interaction plays very important role in complexation. To examine this idea, the HOMO energies of the host (phenol 9.115 eV, resorcinol 9.126 eV and Table 6 Enthalpies and entropies of formation of the [70]fullerene-phenol complexes Complex

Hf0 (kJ mol−1 )

Sf0 (J K−1 mol−1 )

[70]Fullerene-phenol [70]Fullerene-resorcinol [70]Fullerene-p-quinol

−48.8 ± 4.3 −75.4 ± 10.5 −13.4 ± 10.0

−124.7± 14.1 −197.0 ± 34.3 −16.1 ± 1.7

Fig. 5. Plot for determination of enthalpy of formation of the complex of p-quinol with [70]fullerene.

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4. Conclusions In the present study ground state complex formation between [70]fullerene and a series of phenols have been established. CT bands could be located in all the cases and from an analysis of these bands a good estimate of perturbational H¨uckel parameters of the OH group have been obtained. Using the full form of Mulliken’s equation, the degrees of charge transfer have been determined; the estimated values have the right trend. The very low values of degrees of charge transfer indicate that the CT complexes studied here have almost neutral character in their ground states. At any temperature, the formation constants are in the order resorcinol > phenol > p-quinol. The trend in the formation constants suggests that the number and position of the OH groups determine the magnitude of the formation constants.

Acknowledgements S. Bhattacharya thanks the Council of Scientific and Industrial Research (CSIR), India, for a Senior Research Fellowship. Financial assistance by the UGC, New Delhi, extended through the DSA project in Chemistry, is also gratefully acknowledged. The authors also thank the learned referee for making valuable comments.

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