Results and Discussions

When a violet solution of C₆₀ in toluene is mixed with any colorless solution of podands 1-6, a pale yellow solution is developed. This change in color indicates the formation of a charge transfer complexes between these podands and C₆₀. The absorption spectra of a series of solutions of podand1 and C₆₀, as an example, are shown in Figure 1.

Figure 1: Absorption spectra of C₆₀ (1.3×10^-4 mol/L) in the presence of podand1, in toluene at 25°C, the concentration of 1 (from bottom to the top): 0.00 to 8.00 ×10^-3(mol/L).

Figure 1 reveals that the resulting complex has a new absorption band at λ =435 nm. None of the host or the guest shows any measurable absorption at 413-455 nm region. Thus the existence of this new band must be associated with the formation of donoracceptor molecular complex between podand1and C₆₀ in solution. Mukherjee et al. reported the formation of similar charge transfer absorption bands of the complexes of calyx [6] and calyx [8] arene derivatives with [60]fullerene [21]. Stoichiometries of all podands-C₆₀ complexes were determined to be 1:1 by mole ratio method and confirmed by using Job’s method. Figure 2 shows the mole ratio plot for the complex of podand 1 with C₆₀. The change in the slope near the 1:1 ratio is clearly shown, this indicates the formation of 1:1 complex. This stoichiometry is confirmed by Jobs plot which is shown in Figure 3 for the same system. The figure shows a maxima at 0.5 which confirms the formation of 1:1 complex. Complexes of C₆₀ with other podands show similar behavior.

Figure 2: Plot of absorbance change versus mole ratio [1]/[C₆₀] at λ = 435 nm in toluene at 25°C.

Figure 3: Job’s plot for the 1:C₆₀ complex in toluene at 25°C.

Supramolecular 1:1 complex formation between podand 1, as an example, with C₆₀ in solution can be represented by equation 1.

The corresponding formation constant Kc (stability constant) for1 is given by equation 2

Since both podand1 and [60 ]fullerene concentrations are very low, the stability constant Kc, can be calculated by using the following modified form of the Benesi-Hildebrand equation 3 [22]:

where [C₆₀]o is the initial concentration of C₆₀, Kc the formation constant, ΔA the absorbance change at λ = 435 nm of C₆₀ solution in toluene upon the successive addition of thiapodand1, Δε represents the difference in the molar extinction coefficient between the C₆₀.1 complex and that of uncomplexed C₆₀. When [C₆₀]o /ΔA is plotted against 1/ [1]o gives a straight line with a slope of 1/εKc and intercept of 1/ε is obtained as shown in Figure 4. Kc value in each case is calculated by dividing the intercept by the slope (determined bylinear regression analysis according to equation 3).

Figure 4: A typical Benesi-Hildebrand plot of C₆₀:1 complex in toluene.

In order to achieve a better understanding of the thermodynamicsof thiapodands-[60]fullerene complexation reactions, it would be useful to consider the enthalpic and entropic contributions to these complexation processes. These thermodynamic parameters (ΔG°, ΔH° and ΔS°, Table 1) were evaluated from the temperature-dependence of the stability constants (K_c) and applying the van’t Hoff equation.

Table 1: The thermodynamic parameters and formation constants for Thiapodands-C₆₀ fullerene complexes in toluene solution.

A plot of lnKc against 1/T gives a straight line with -ΔH°/ R as a slope, and ΔS°/R as an intercept as shown in Figure 5 for C₆₀ podand 1 complex. All other complexes of thiapodands with C₆₀ give similar linear plots.

Figure 5: A plot of lnkc against 1/T of C₆₀:1 complex in toluene.

Table 1 lists the stability constant values determined at 25°C and thermodynamic parameters ΔHo, ΔSo and ΔGo values for the chargetransfer complexes formed between C₆₀ and thiapodands 1-6.

Generally, the stability constant value is an indication about thestability of the complex between donor and acceptor, i.e the strength of acceptor-donor interaction. The larger the value, the higher is the stability of the complex. Many factors influence the stability constants, among them; the size of the guest, number and type of the donor atoms and a macrocyclic effect which involves the dimensional compatibility between the size of the macrocycle and the size of the guest. Results in Table 1, show the order of stability constants of C₆₀ and ester group pearing thiapodands 1-3 complexes is in the order of 1>2>3. This behavior of the podands can be explained by considering the cavity sizes of the podands and the ligating heteroatoms, oxygen and sulfur atoms at the periphery of the podands. The higher stability of the complex of 1 over the complexes of 2 and 3 could be due to the cavity size ratio of podand to [60]fullerene that gives more proper spatial orientation for the interaction, despite the larger number of heteroatoms in each of 2 and 3 as compared to 1. The replacement of one of the sulfur atoms in podand 2 with oxygen atom gives podand 3 causes a decrease in the stability constant. This is because sulfur atom is better donating base toward neutral acceptors than oxygen atom [23-25]. For C₆₀ and amide group pearing thiapodands 4-6 complexes; the order of stability constants is in the order of 6 >5 >4. The situation here is completely reversing the order of stability constants in C₆₀- thiapodands 1-3 complexes. This dramatical change could be due to changing the nature of the ending groups; from ester ending groups to amide ending groups. Previous studies on the complexation of podands 1-6 with some transition metals showed clear evidence that the nature of the ending groups takes part in determining the value of stability constants [19,20].

Table 1 also shows that podands 1 and 2 complexes are enthalpy stabilized and entropy destabilized while podands 3, 4, 5 and 6 are enthalpy destabilized, but are highly entropy stabilized.

The less negative ΔH° value of 3:[60]fullerene complex as compared to 1 and 2:[60]fullerene complexes indicates weaker interaction between [60]fullerene and podand 3 compared to that between [60]fullerene and podands 1 or 2. This probably due to the donation power of the softer sulfur atoms over the harder oxygen atoms.

The extent of flexibility is expected to vary with the size of the podand due to the number and the type of atoms at the periphery of the podands. The interaction of a C₆₀ molecule with a flexible podand results in the formation of a less flexible charge-transfer complex. This will lead to a negative ΔS° values for the complexation process. It is known that the most flexible starting host will form a complex with the most negative ΔS° value. This explains the most negative ΔS° value for the 2:[60]fullerene complex as compared to podands 1 and 3.

For comparison purposes, the stability constants of some macrocyclic compounds 8-14 (Scheme 2) with C₆₀ were incorporated (Table 2). It is clear that the stability constants of podands 1-6 are larger than the stability constants of compounds 8-14 macrocylic-C₆₀ complexes.

Scheme 2: Some macrocyclic compounds can complex with [60]fullerene.

Table 2: log K values for selected macrocyclic-C₆₀ complexes.

Experimental

Thiapodands 1-6 were prepared according to the published procedures [20]. Toluene (HPLC grade, GCC, assay 99.8%) and [60]fullerene (99.5%, Aldrich) were used without any further purification. Spectrophotometric measurements were carried out using a UVspectrophotometer (SHIMADZU), Model UV-2450, equipped with a thermostat cell holder. The temperature controller (SHIMADZU) was provided with a cooling facility and used to control the cell temperature in the range (0.00-80.0°C) with an accuracy of ± 0.1°C.

The concentration of [60]fullerene in each solution was kept constant at 1.30 × 10-⁴ M, and podands 1-6 concentrations were increased from 0.00 up to 8.00 ×10-3 M. Three runs were made for the calculation of certain K value. Mole ratio method was used to determine the stoichiometry for the complexes. Job’s method was applied to confirm the 1:1 stoichiometry of the complexes [15]. The stability constants were evaluated at various temperatures (15-35°C)using Bensesi-Hildebrand equation and van’t-Hoff equation was applied to determine the thermodynamic parameters [22].