Spectrophotometric studies of complexation of [60]fullerene with series of aromatic hydrocarbon molecules containing flexible phenyl substituents

Spectrochimica Acta Part A 65 (2006) 659–666

Spectrophotometric studies of complexation of [60]fullerene with series of aromatic hydrocarbon molecules containing flexible phenyl substituents Kalyan Ghosh a , Subrata Chattopadhyay b , Manas Banerjee a , Sumanta Bhattacharya a,∗ a

Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713104, West Bengal, India b Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 24 August 2005; accepted 16 December 2005

Abstract The absorption spectra of the electron donor–acceptor complexes of [60]fullerene with five different aromatic hydrocarbon (AH) molecules containing flexible phenyl substituents have been investigated in toluene medium. An absorption band due to charge transfer (CT) transition is observed in each case in the visible region. The experimental CT transition energies are well correlated with the vertical ionization potentials of the AHs studied (through Mulliken’s equation) from which we extract degrees of charge transfer, oscillator and transition dipole strengths of the CT complexes. The degrees of CT in the ground state of the complexes have been found to be very low (0.49–0.55%). The formation constants (K) for the complexes of [60]fullerene with the aromatic hydrocarbons have been determined by UV–vis spectroscopy. Both K values and PM3 calculations on [60]fullerene/AH complexes reveal that nature of substitution in the donor moiety as well as steric compatibility with the acceptor molecule govern the process of EDA complex formation. © 2005 Elsevier B.V. All rights reserved. Keywords: [60]Fullerene; Aromatic hydrocarbons; CT bands; Formation constants

1. Introduction Since the discovery [1–3], there has been a considerable amount of work done on the spectroscopic characterization of [60]fullerene adducts with various donor molecules showing charge transfer (CT) absorption bands in the solid state [4,5] or in solution [6–15]. Different fullerene based composites with conjugated polymer, dyad and triad molecules with covalently and non-covalently attached porphyrin, phthalocyanine and other molecules show efficient electron transfer from chromophores to fullerenes which makes fullerene based compounds promising materials for use in artificial photosynthesis [16] and in photovoltaic devices [17] with long lived charge separation states [18,19]. Our present investigations are directed towards the possible effects of EDA interaction of [60]fullerene with aromatic hydrocarbons (AH) containing flexible phenyl substituents. The importance of [60]fullerene/aromatic hydro-

∗

Corresponding author. Tel.: +91 3326327254; fax: +91 3422530452. E-mail address: sum [email protected] (S. Bhattacharya).

1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.12.025

carbon interaction is also important for interpretation of various spectroscopic data. For example, it has been suggested that the appearance of new features and frequency shifts in the [60]fullerene resonance Raman spectrum in aromatic solvents is due to the fullerene/solvent interaction [20]. Also, in addition reaction of [60]fullerene, 1,2-addition usually gives an absorption bands near 435 nm [21], while 1,4-addition produces band near 448 nm [22]. Various aspects of fullerene donor–acceptor interaction with aromatic hydrocarbons have been studied so far [23–26]. The CT interaction of [60]fullerene with substituted naphthalene was studied by Scurlock and Ogilby [23], who reported a very weakly bound ground-state CT complex with a small equilibrium constant of 0.08 M−1 for the [60]fullerene–1methyl naphthalene complex in toluene. Later, Sibley et al. [24] have studied the interaction of [60]fullerene with polycyclic aromatic hydrocarbons, reporting formation constant (K) = 0.1 M−1 for the [60]fullerene–naphthalene complex and increasing values by an order of magnitude as the number of the aromatic rings of the donor is increased. In some of our very recent work, we have reported the CT interaction of different polynuclear aromatic molecules, like 2,3-[1,8-naphtho]-6-

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(3 -methyl-4 -methoxyphenyl)-para-terphenyl, 2-phenyl-3-(3 methyl-4 -methoxyphenyl)-para-terphenyl and 2,3-benzo-1,4di-(4 -ethylphenyl)fluoren-9-one with [60]fullerene [25]. Very recently, Sarova and Berberan-Santos [26] have estimated the formation constants of the CT complexes of [60]fullerene with naphthalene, 1-methylnaphthalene and anthracene assuming the nature of the complexes as contact type. Molecular recognition is a major part of modern chemistry. Litvinov et al. [27] have synthesized the molecular complexes of [60]fullerene with aromatic hydrocarbons containing flexible phenyl substituents, namely tetraphenylethylene, 1,3,5-triphenylbenzene, 9-phenylanthracene and 9,10-diphenylanthracene. In their work special emphasis is given on the steric compatibility of the donor molecules with [60]fullerene during EDA interaction. Nonplanar hydrocarbons like dianthracene and triptycene also form complexes with fullerene due to steric conformity of their shapes to the spherical surface of the [60]fullerene molecule [28–30]. In view of the importance of the formation constant (K) for the communication of results in this field, it is usually essential that K be quantified. In order to get information on the dependence of complex stability upon the pattern of substitution in the aromatic moiety, we have determined the formation constants for the complexes of [60]fullerene with five different aromatic hydrocarbons (AH), namely l,4-di(4 -ethylphenyl)fluorine (B), 3,4-diphenyl2,5-n-propyl-m-terphenyl (D), l,4-di(4 -ethylphenyl)-6,7-(l,8naphtho)fluorine (G), 2,3-benzo-4,5-diphenyl-6(2 -naphthyl)para-terphenyl (H) and 1,4-diethyl-2-(2 -naphthyl)-3-phenyl5,6-(1,8-naphtho)benzene (K) (Fig. 1). Since formation constant

Fig. 1. Structures of B, D, G, H and K.

is an important parameter which measures the extent of binding, the object of the paper is an attempt along this line. A choice of these molecules for complexation with [60]fullerene is caused by their good steric compatibility to the fullerene cage. Quantum chemical investigations on the [60]fullerene/AH systems will be helpful in elucidating the electronic structures of the complexes. Finally, concept of CT interaction offers a platform for explaining interactions of the electronic subsystems of AH and [60]fullerene and hence widens the scope of the present investigation on molecular interactions of [60]fullerene with various other electron donors. 2. Materials and methods [60]Fullerene is obtained from Merck, Germany. AH compounds are collected from BDH, England. The solvent, toluene, is of UV spectroscopic grade. All UV–vis spectral measurements are recorded on a Shimadzu 1601 spectrophotometer fitted with TB 85 thermo bath. Third parametric method (PM3) theoretical calculations have been done using SPARTAN’02 software in silicon graphics workstation. 3. Results and discussions 3.1. Observation of CT bands Fig. 2(inset) shows the electronic absorption spectra of toluene solution of [60]fullerene with B, H and K. Spectra of the solutions of [60]fullerene with the above donors have been recorded against the pristine acceptor solution as reference to give new absorption peaks of the CT bands in the visible region. Since more concentrated solution of donor compared to acceptor is described to detect CT absorption bands, the absorption spectra are measured in the concentration range of 10−4 mol dm−3 for donor (i.e., AH) and 10−6 mol dm−3 for

Fig. 2. Absorption spectra of only (a) B (4.961 × 10−4 mol dm−3 ), (b) H (1.870 × 10−4 mol dm−3 ) and K (4.010 × 10−4 mol dm−3 ) recorded against the solvent as reference; CT absorption spectra of the mixtures of (d) [60]fullerene (2.564 × 10−5 mol dm−3 ) + B (1.654 × 10−4 mol dm−3 ), (e) [60]fullerene (5.648 × 10−5 mol dm−3 ) + K (9.115 × 10−4 mol dm−3 ) and (f) [60]fullerene (5.648 × 10−5 mol dm−3 ) + H (4.154 × 10−4 mol dm−3 ) recorded against the pristine acceptor solution as reference.

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acceptor (i.e., [60]fullerene), respectively. In Fig. 1(a–c), the CT peaks are identified to be the peaks in the region of 380–440 nm, because they normally have the longest wavelength among the peaks different from those obtained from the spectra of the components. If we consider the spectra of pure B, H and K (Fig. 2), then we can found that, for B, there are bands near 330 and 390.5 nm, for H compound, bands are observed at 272, 307 and 397 nm and for K compound, bands are observed at 231, 253 and 400.5 nm. Thus, the absorption bands at 435, 385 and 419.3 nm for [60]fullerene + B, [60]fullerene + H and [60]fullerene + K may arise due to formation of charge transfer complexes as the donor B, H and K and the acceptor [60]fullerene do not possess such bands. The presence of CT absorption bands in the complex may be evidence of the ␲-interaction of aromatic hydrocarbon molecules with [60]fullerene. However, except [60]fullerene–D complex, the energies of charge transfer corresponding to the maximum of the charge transfer absorption bands are rather high for the complexes of B, G, H and K with [60]fullerene. This can be attributed to the weak hydrocarbon property of the aromatic hydrocarbons [33]. In this regard, it should be kept in mind that for [60]fullerene, the assignment of the spectral transitions has been carried out using the results of theoretical calculations [30–32]. The absorptions between 190 and 410 nm are due to allowed 1 T1u –1 Ag transitions, whereas those between 410 and 620 nm are due to forbidden singlet–singlet transitions. These later absorptions in the visible region are responsible for the purple colour of [60]fullerene. The CT absorption spectra in the present work have been analyzed √ by fitting to the Gaussian function of type y = y0 + [A/(w π/2] exp[−2(x − xc )2 /w2 ], where x and y denote wavenumber and molar extinction coefficient, respectively. The authors have tested other mathematical functions like Lorentzian and Sigmoidial functions, etc. But the Gaussian curve analysis of the spectral fittings for the complexes of [60]fullerene with B, H and K gives best result with least error compared to other fitting functions. When such a Gaussian fit (approximated by given approximation) is applied over

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Fig. 3. Gaussian curve analysis of the shoulder region of the CT band for [60]fullerene/K system at 303 K.

a wide range of (x, y) points, y0 obviously represents a lowest bound value above which y-values of all other points in the range will come up, because the remaining term in the expression is a definitely positive quantity. At the value of √ x = xc , the derivative dy/dx disappears giving yc = y0 + A/{w π/2} which has numerical significance in view of plot only. However, at x → ∞ then y → y0 , which may give a significance of hypothetical absorbance value in the limit of infinite wavenumber provided that it is included within the range. One such Gaussian plot for [60]fullerene/K system is shown in Fig. 3. The Gaussian fit of CT absorption of [60]fullerene/B, [60]fullerene/D, [60]fullerene/G and [60]fullerene/H complexes exhibits excellent result as obtained for [60]fullerene/K system. The results of the Gaussian analysis for all the [60]fullerene/AH systems under study are shown in Table 1. The wavelengths at these new absorption maxima (λmax = xc ) and the corresponding transition energies (hν) are summarized in Table 2. The Gaussian analysis fitting is done in accordance with the method developed by Gould et al. [31]. One important point to mention here is that Gaussian analysis of a curve generally gives a decent result near

Table 1 Gaussian curve analysis for the CT spectra of [60]fullerene with B, D, G, H and K System

Area of the curve (A) (×10−7 dm3 mol−1 cm−2 )

[60]Fullerene–B [60]Fullerene–D [60]Fullerene–G [60]Fullerene–H [60]Fullerene–K

3.12 1.77 0.43 4.3 7.62

± ± ± ± ±

0.25 0.14 0.054 0.44 0.8

Width of the curve (w) (cm−1 ) 1288 2023 1307 1025 717

± ± ± ± ±

64 92 91 55 43

Center of the curve (xc ) (cm−1 ) 22985.6 29318.5 21023.7 25966.7 23851.4

± ± ± ± ±

y0 (dm3 mol−1 cm−1 ) −950 −254 −450 29843 −1734

20 51 25 10 11

± ± ± ± ±

620 138 145 1712 3908

Table 2 CT absorption maxima and transition energies of the [60]fullerene/AH complexes; degrees of charge transfer (α), oscillator strengths (f), transition dipole strengths (μEN ) and resonance energies (RN ) of the complexes of [60]fullerene with B, D, G, H and K; PM3 determined vertical ionization potential (ILv ) of the donors; dipole moments of the [60]fullerene/AH complexes Complex

λmax (nm)

hνCT (eV)

α (×103 )

f

μEN

|RN | (eV)

ILv (eV)

Dipole moment (Debye)

[60]Fullerene–B [60]Fullerene–D [60]Fullerene–G [60]Fullerene–H [60]Fullerene–K

435 341 475.6 385 419.3

2.851 3.637 2.608 3.221 2.958

5.20 4.98 5.47 5.24 5.36

0.164 0.060 0.012 0.822 0.328

3.87 2.10 1.08 8.15 5.40

0.377 0.251 0.0686 0.685 0.671

8.7863 9.0935 8.4407 8.7409 8.5857

0.350 0.511 0.205 0.321 0.308

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the maxima of the curve spread over a very small region. For this reason, although the errors in the center of the CT spectra for the complexes of [60]fullerene with various electron donors are very small, there are appreciable errors in the y0 value. 3.2. Degrees of charge transfer (α) The present work explores the possible method for estimating degrees of charge transfer for the complexes of [60]fullerene with aromatic hydrocarbon molecules. The ionicity which is determined by the degrees of charge transfer is one of the most important parameter which determines electronic and optical properties in CT complexes. According to Mulliken’s theory [32] the ground state of the CT complex is a resonance hybrid of a ‘no-bond’ state (L···A) and a dative state (L+ A− ) with the former predominating; the excited state is a resonance hybrid of the same two structures with the dative one predominating. CT transition energies of these complexes are related to the vertical ionization potentials (ILv ) of the donors by the non-linear equation of type,   C2 (1) hνCT = ILv − C1 + ILv − C1 Here C1 =

v EA

+ G0 + G1

(2)

v is the vertical electron affinity of the acceptor; G the where EA 0 sum of several energy terms (like dipole–dipole, van der Waals interactions, etc.) in the ‘no-bond’ state; G1 is the sum of a number of energy terms in the ‘dative’ state. In most cases G0 is very small and can be neglected while G1 is largely the electrostatic energy of attraction between L+ and A− . The term C2 in Eq. (2) is related to the resonance energy of interaction between the ‘no-bond’ and ‘dative’ forms in the ground and excited states and for a given acceptor it may be supposed constant [32]. A rearrangement of Eq. (2) yields:

2ILv − hνCT = (1/C1 )ILv (ILv − hνCT ) + {C1 + (C2 /C1 )}

(3)

with the observed transition energies and presently determined experimental ILv values, we have obtained the following correlation: 2ILv − hνCT = (0.1289 ± 0.0308)ILv (ILv − hνCT ) + (8.0185 ± 1.5255)

(4)

with a correlation coefficient of 0.99. This also conforms the CT nature of the transition observed and yields C1 = 7.758 eV and C2 = 2.021 (eV)2 . Neglecting G0 and applying assuming the CT complexes studied in the present investigation are of ␲-type, the major part of G1 is estimated to be e2 /4πε0 r = 4.13 eV. According to Mulliken’s two state model, the ground state (ψg ) and excited (ψex ) state wave functions of the CT complexes are described by a linear combination of dative ψ(L0 , A0 ) and ionic ψ(L+ , A− ) states,  √ (5) ψg = { (1 − α)}ψ(L0 , A0 ) + ( α)ψ(L+ , A− )

Fig. 4. Plot of degrees of charge transfer (α) vs. vertical ionization potential of the donor (ILv ).

√ √ ψex = { 1 − α}ψ(L+ , A− ) − ( α)ψ(L0 , A0 )

(6)

where α is the degree of charge transfer. The function ψ(L+ , A− ) differs from ψ(L0 , A0 ) by the promotion of an electron from the donor to the acceptor. α [33] is given by α=

C2 /2 v + C )2 + (C /2) (ILv − EA 1 2

(7)

The values of α (calculated by using Eq. (7) and given in Table 2) are small and indicate that very little charge transfer occurs in the ground state (about 0.5%). The dependence of α on ILv of the donors is shown in Fig. 4. It is also observed that α decreases with the increasing ionization potentials of the donors, as expected. The following correlation is obtained with the present data: α = (−7.526 × 10−4 ± 1.056 × 10−5 )ILv + (11.8 × 10−3 ± 9.22 × 10−5 )

(8)

3.3. Determination of oscillator (f) and transition dipole strengths (μEN ) From the CT absorption spectra, we can extract the oscillator strength. The oscillator strength f is estimated using the formula  −9 f = 4.32 × 10 εCT dν (9)  Here 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

(10)

where εmax is the maximum molar extinction coefficient of the CT band and ν1/2 is the half-width, i.e., the width of the CT band at half the maximum extinction. The observed oscillator strengths of the CT bands are summarized in Table 2. These values are larger in magnitude than that of [60]fullerene itself (∼0.014 for the S0 –S1 transition). This phenomenon indicates that the AHs studied in the present work are very much capable to form CT complexes when they are subjected to

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undergo interaction with [60]fullerene in any non-polar solvent. It is worth mentioning that we need a proper calculation of oscillator strengths of [60]fullerene/AH CT complexes. This is because oscillator strength is very much sensitive to the molecular configuration and the electron charge distribution in the CT complex. In the [60]fullerenes/AH complexes, we cannot using simple model assuming a charge localized at a certain cite of fullerene sphere, since the fullerenes have ␲-bonds which are directed radially with a node on the molecular cage. The extinction coefficient is related to the transition dipole by   εmax ν1/2 1/2 μEN = 0.0952 (11) ν  where ν ≈ υ at εmax and μEN is defined as −e ψex i ri ψg dτ. μEN for the complexes of [60]fullerene with various AHs are given in Table 2. It has been observed that transition dipole strength (μEN ) for the [60]fullerene/H complex is somewhat higher than that of other AH/[60]fullerene complexes. Quantum mechanical calculations reveal that μEN is given by (a∗ bμ1 ) − (ab∗ μ0 ) + (aa∗ − bb∗ )μ0 = a∗ b(μ1 − μ0 ) + (aa∗ − bb∗ )(μ01 − Sμ0 )

(12)

where a, a* , b and b* can be determined from Mulliken’s equation [32] of following type: ΨN = aΨ(A,L) + bΨ(A− L+ ) ∗

∗

where a b

ΨE = b Ψ(A− ,L+ ) − a Ψ(A,L)

∗

where b a

(13) ∗

(14)

where ψN and ψE denote ground and excited state wave functions, respectively. The dipole moments μ01 is equal to the moment generated when an amount of charge Se is transferred from the donor to somewhere between the donor and acceptor. Hence, μ01 ≈ (l/2)μ1 S. μ1 is approximately equal to erAL /E, where rAL is the separation distance between L and A in the complex AL and E is the dielectric constant. For the case μ0 = 0 μEN ≈ (a∗ b)μ1 + (aa∗ − bb∗ )μ01 . As μ01 = (1/2)μ1 S we can write that   1 μEN = a∗ b + S(aa∗ − bb∗ ) μ1 2

3.4. Determination of resonance energy (RN ) Briegleb and Czekalla [34] theoretically derived the relation 7.7 × 104 hνCT /|RN | − 3.5

3.5. Theoretical model in favour of electric dipole—dipole interaction between the [60]fullerene molecule and donors Consider the interaction of [60]fullerene and D. The interaction between the dipole–dipole transitions of [60]fullerene and D can be represented in the form H=

N max

D )(1 − 3 cos2 θ ) (d[60]fullerence σx[60]fullerence dD σx,i i

i=1

ε∞ ri3

(16)

(i)

(17) dDi

are dipole moments of the correwhere d[60]fullerene and sponding transitions in [60]fullerene and the ith D molecule; D the corresponding Pauly matrices; r the σx[60]fullerence and σx,i i distance between [60]fullerene and D; ε∞ in Eq. (17) is the high-frequency dielectric constant. The reconstruction of the resulting spectrum taking into account Eq. (17) is determined by mixing of the states of the [60]fullerene molecule and the surrounding D molecules. For one [60]fullerene/D pair, Eq. (17) gives (i)

E± =

E[60]fullerence + ED 2 1/2   E[60]fullerene − ED 2 + |Vi |2 ± 2

(18)

where E[60]fullerene and ED are the energies of dipole transitions (i) for [60]fullerene and D, respectively; Vi = [d[60]fullerene dD (1 − −3 2 −1 3 cos θi )]ε∞ ri is the matrix element of the state mixing. The final expression has the form |Vi |N 1/2 (19) 2 where V is the amplitude of the non-diagonal flip–flop dipole–dipole matrix element for [60]fullerene and of the dipole transitions in neighboring D molecules and N is the number of neighboring D molecules. In Eq. (19) the difference between ε− (0) and −ε− may be termed as the shift of absorption band edge. Such a dependence of the absorption band edge is valid only under the condition N < Nthr , where Nthr is the maximum number of D molecules that can take part in the dipole–dipole flip–flop interaction with [60]fullerene. A further increase in the concentration of D does not increase the number of these molecules in the nearest environment of [60]fullerene. Fig. 5 shows that the dependences are saturated when the concentration of D exceeds 4.0 × 10−4 mol dm−3 concentration in agreement with the theory. It is noteworthy to mention here that no such saturation of absorbance takes place for other [60]fullerene/AH systems. This mechanism also allows us to explain the formation of the (0)

The dipole moments of the complexes of B, D, G, H and K with [60]fullerene have been calculated by PM3 method and they are given in Table 2. It is observed that the trend in transition dipole strengths of the [60]fullerene complexes of B, D, G, H and K do not corroborates very well with the order of dipole moment values of such systems.

εmax =

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). The values of RN for the complexes under study have been given in Table 2.

ε− = ε− − (15)

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Fig. 5. Absorbance data of [60]fullerene/D system against molar concentration of D.

strong donor–acceptor complex between [60]fullerene and D under study [35] as revealed by very high formation constant value in case of [60]fullerene/D system (to be discussed in Section 3.6). 3.6. Spectrophotometric study of formation equilibria of the complexes of AH with [60]fullerene The formation constants of the [60]fullerene/AH complexes have been determined at four different temperatures using the Benesi–Hildebrand (BH) [36] equation in the form [A]0 [L]0 [L]0 1 =  + d ε Kε

(20)

Here [A]0 and [L]0 are the initial concentrations of the fullerene and AH (i.e., B, D, G, H and K), respectively, d (= d − dA0 − dL0 ) the corrected absorbance of the donor–acceptor mixture at 550 nm against the solvent as reference and dA0 and dL0 are the absorbance of the fullerene and AH solutions with same molar concentrations as in the mixture at the same wavelength (i.e., 550 nm). The corrected molar extinction coefficient, ε , is not quite that of the complex. The Benesi–Hildebrand [36] method is an approximation that we have used many times and it gives decent answers. But the extinction coefficient is really a different one between the complex and free species that absorbs at the same wavelength. K is the formation constant of the complex, Eq. (20) [36] is valid under the condition [L]0 [A]0 for 1:1 donor–acceptor complexes. The intensity in the visible portion of the absorption band, measured against the solvent as reference, increases systematically with gradual addition of AH. Thus, it is definitely established in this work that the systematic increase in intensity of the broad 400–700 nm absorption band (resulting from a forbidden singlet–singlet transition in [60]fullerene [37,38]) is due to 1:1 molecular complex formation between [60]fullerene and AH. This indicates complex formation. The equilibrium constant values calculated using the BH model is only an approximation. Sibley et al. [39] have also estimated the formation constants for the complexes of [60]fullerene with aniline and substituted anilines using BH equation. Typical absorbance data for [60]fullerene/AH systems at room temperature in toluene

medium are given in Table 3. In all the cases very good linear plots according to Eq. (20) are obtained, one typical case is being shown in Fig. 6. Formation constants for the complexes of AH with [60]fullerene (determined from the BH plots) at four different temperatures are summarized in Table 3. It is also observed that D binds more tightly with [60]fullerene rather than B, G, H and K. One of the reasons is that a specific nearly spherical shape of fullerene molecule imposes steric demands upon the shape of D molecule. We surmise that there is a difference in the rotational freedom among the multinuclear aromatic molecules during complexation with [60]fullerene. [60]Fullerene loses scarcely its rotational freedom in the complexation of D, whereas the rotation of [60]fullerene is restricted in greater extent during its complexation with B, G, H and K. This restricted rotation of [60]fullerene molecule also supports the higher magnitude of formation constant in [60]fullerene/D complex. PM3 calculation also reveals that the complex formation between aromatic hydrocarbons and [60]fullerene is attained due to the flexibility of the phenyl substituents of the aromatic hydrocarbon molecules. In D, the phenyl substituents insert into the cavitand formed in the layers by the spherical [60]fullerene molecule. Such packing allows van der Waals contacts with the spherical [60]fullerene molecule with relatively short C(aromatic hydrocarbon)···C([60]fullerene) dis˚ These contacts are comtances in the range of 3.59–3.65 A. parable with the similar C···C distances in the [60]fullerene complexes with concave aromatic hydrocarbons. For example, it is found that the [60]fullerene complexes with triptycene and dianthracene have the shortest C···C distances, in the ˚ range [40,41]. However, the interaction between 3.27–3.35 A [60]fullerene and other aromatic hydrocarbons like B, G, H and K is rather weak as revealed from lower value of formation constants. The C(aromatic hydrocarbon)···C([60]fullerene) distances in the [60]fullerene–B, [60]fullerene–G, [60]fullerene–H ˚ and [60]fullerene–K complexes are 4.50, 4.20, 4.37 and 4.10 A, respectively. These results also show in the spatial disorder of [60]fullerene molecule during complexation with such aromatic hydrocarbons. The stereoscopic structures of the [60]fullerene complexes with B, D, G, H and K (optimized by PM3 calculation) are shown in Fig. 7.

Fig. 6. Benesi–Hildebrand plot of [60]fullerene/D system at 298 K.

K. Ghosh et al. / Spectrochimica Acta Part A 65 (2006) 659–666

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Table 3 Absorbance data for spectrophotometric determination of stoichiometry and formation constants (K) of the [60]fullerene/AH complexes in toluene at 298 K Donor concentration (×104 mol dm−3 )

System

[A]0 (×105 mol dm−3 )

Absorbance

K (dm3 mol−1 )

[60]Fullerene–B

1.654 2.205 2.756 3.307 4.410 4.961

2.564

0.188 0.220 0.250 0.274 0.350 0.368

1885 ± 80

[60]Fullerene–D

1.060 1.588 2.117 2.647 3.176 3.706 4.235 4.764

2.564

0.548 0.542 0.548 0.530 0.551 0.576 0.580 0.583

59800 ± 2000

[60]Fullerene–G

3.466 5.777 6.932 8.010 9.243 10.40

2.564

0.069 0.110 0.132 0.152 0.174 0.196

80 ± 15

[60]Fullerene–H

4.154 6.231 8.320 10.385 12.461 14.540 16.615 18.692

5.468

0.070 0.090 0.090 0.103 0.114 0.126 0.138 0.152

1070 ± 20

[60]Fullerene–K

9.115 13.673 18.230 22.800 27.346 31.904 36.461

5.468

0.203 0.273 0.333 0.397 0.080 0.500 0.550

200 ± 7.5

Fig. 7. Stereoscopic view for the PM3 optimized geometry of the complexes of [60]fullerene with various aromatic hydrocarbons.

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4. Conclusions [60]Fullerene is shown to form 1:1 electron donor–acceptor complex with various aromatic hydrocarbon molecules containing flexible phenyl substituents. Well defined charge transfer absorption bands have been located in each of the [60]fullerene/AH systems studied. The hνCT –ILv dependences have been utilized to determine the degrees of charge transfer for [60]fullerene/AH complexes. The very low values of degrees of charge transfer and oscillator strengths indicate that the CT complexes studied here have almost neutral character in their ground states. Formation constants of the [60]fullerene/AH CT complexes have been determined by UV–vis absorption spectrophotometric method. The formation constants are in the order: K[60]fullerene–D > K[60]fullerene–B > K[60]fullerene–H > K[60]fullerene–K > K[60]fullerene-G . This trend in the formation constants values suggests that the arrangement of aromatic ring and position of different functional groups in the aromatic hydrocarbon molecules governs the EDA interaction processes with [60]fullerene. Finally we can say that the fullerene complexes with aromatic hydrocarbons have various structures and neutral ground states; therefore, these complexes can be promising in intercalation by alkali metals. Acknowledgements S.B. thanks CSIR, New Delhi, for providing Research Associateship to him. The authors acknowledge UGC, India, for providing financial assistance in this work through the DSA project in Chemistry, Department of Chemistry, The University of Burdwan, India. References [1] W. Kratschmer, L.D. Lamb, K. Forstiropoulos, D.R. Huffman, Nature 347 (1990) 354. [2] M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996. [3] C.A. Reed, R.D. Bolskar, Chem. Rev. 100 (2000) 1075. [4] D.V. Konarev, A.V. Kovalevsky, D.V. Lopatin, A.V. Umrikhin, E.I. Eudanova, P. Coppens, R.N. Lyubovskaya, G. Saito, Dalton Trans. (2005) 1821. [5] D.V. Konarev, A.Yu. Kovalevsky, A.L. Litvinov, N.V. Drichko, B.P. Tarasov, P. Coppens, R.N. Lyubovskaya, J. Solid State Chem. 168 (2002) 474. [6] D.V. Lopatin, V.V. Rodaev, A.V. Umrikhin, D.V. Konarev, A.L. Litvinov, R.N. Lyubovskaya, J. Mater. Chem. 15 (2005) 657. [7] D.M. Guldi, Chem. Soc. Rev. 31 (2002) 22. [8] D.V. Konarev, R.N. Lyubovskaya, N.V. Drichko, V.N. Semkin, A. Graja, Chem. Phys. Lett. 314 (1999) 570. [9] M.V. Korobov, A.L. Mirakyan, N.V. Avramenko, G. Olofsson, A.L. Smith, R.S. Ruoff, J. Phys. Chem. B 103 (1999) 1339.

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