Effect of Structure of Monocyclic Aromatic Solvents on the Packing Density of Fullerene C₆₀ Solvation Shell

Abstract

Fullerene C₆₀ solutions in various monocyclic aromatic solvents (AS) diluted by tetrahydrofurane (THF) were studied using UV spectroscopy. An algorithm was proposed which made it possible to obtain the data from the UV spectra of (C₆₀+AS) solutions in THF which pointed to the difference between the packing density of AS molecules in solvation shell of C₆₀ and that in the pure AS solvent. The presence of asymmetrical substituents in the AS molecule has been shown to result in loosening of solvent structure in the solvation shell, while the absence of the substituents or presence of symmetrical substituents leads to compacting of solvent structure.

1. Introduction

Donor-acceptor interactions of fullerene C₆₀ with solvent molecules are responsible for its good solubility in numerous organic solvents due to formation of strong solvation shells [1]. Fullerene C₆₀ manifests itself mainly as a π-acceptor [1–3], which predominates its high solubility in aromatic solvents due to formation of fullerene-solvent molecular complexes. The presence of substituents in the aromatic solvent (AS) molecule may be expected to result in the appearance of additional interactions between the AS and C₆₀ during solvation. For example, the spectroscopic study of fullerene solutions in various solvents [4, 5] pointed to the possibility of C₆₀ solvation by o-dichlorobenzene owing to both π-π interactions of the benzene ring of AS with the π-system of fullerene and partial charge transfer from the chlorine atom to fullerene. The presence of methyl group as a substituent in the AS molecule may open the way to formation of additional hydrogen-bonding complexes [6]. Interaction of some organic donors with C₆₀ has been shown to result in their coordination with fullerene C₆₀ [7]. The structure of solvation shell would be dependent on the forced coordination of solvent molecules by fullerene. The fullerene ability to structure a solvent was shown in [8]. It seems quite obvious that the coordination of monocyclic AS molecules by C₆₀ during solvation would depend on the presence of substituents in AS which may interact with fullerene. The forced coordination of solvent molecules by C₆₀ may be proposed to result in the difference between the packing density of AS molecules in the C₆₀ solvation shell and that in the pure AS. The examination of this idea was the aim of the work.

2. Experimental Part

2.1. Materials, Experimental Procedure, and Equipment

Fullerene C₆₀ (purity 99.5 wt%) (Fullerene Technologies, St. Petersburg) was used for preparation of its solutions with specified concentrations in several selected aromatic solvents with various substituents (Table 1) as well as in tetrahydrofurane (THF) and methylene chloride (CH₂Cl₂). All solvents were preliminary purified by distillation.

Table 1 

Aromatic solvents used and the number 𝑁 of excess (positive values) or deficient (negative values) molecules of the aromatic solvent per solvation shell of C₆₀ molecule.

The weighed amount of C₆₀ solution in AS with the known fullerene concentration was diluted by the weighed amount of THF, which allowed us to calculate the concentration of fullerene (𝑐_𝐹) and of the AS used (𝑐_AS,cal) in the (C₆₀+AS) solution in THF. No less than three solutions diluted by THF were prepared from the C₆₀ solution in each AS; for each (C₆₀+AS) solution in THF the UV spectrum was recorded.

The UV spectra of C₆₀ solutions were recorded using a UV VIS SF-2000 spectrophotometer (Russia); quartz cells of 2 mm thickness were used.

2.2. The Procedure of Calculation of the Number of Excess or Deficient Molecules of AS per Solvation Shell of C₆₀ Molecule

Tetrahydrofurane is a transparent in the UV-region solvent good for AS and poor for C₆₀. At addition of THF to C₆₀ solution in AS, fullerene solvation shell will be defined as the number of AS molecules which C₆₀ is able to retain in THF. If the packing density of AS molecules in solvation shell corresponds to that in the pure AS solvent, the concentration 𝑐_AS,sp calculated from the spectra of the (C₆₀+AS) solution in THF must be equal to the concentration 𝑐_AS,cal calculated for this solution from the weighed amounts of C₆₀ solution in AS and of THF. To calculate 𝑐_AS,sp concentration, it is necessary to isolate the C₆₀ spectrum from the (C₆₀+AS) spectrum in the region of the combined absorbance of AS and C₆₀ (at 𝜆<300nm). The following algorithm was developed to evaluate the 𝑐_AS,sp concentrations of AS from the spectra of (C₆₀+AS) solutions in THF.

The graphic illustration of the algorithm proposed is presented in Figure 1. Curve 1 in Figure 1 corresponds to the spectrum of (C₆₀+AS) solution in THF with known concentration 𝑐_AS,cal and, supposedly, is a superposition of C₆₀ and AS spectra.

Figure 1 

Graphic illustration of the procedure of obtaining the AS spectra corresponding to the true value 𝑐AS,sp of AS concentration from the spectrum of (1) (C60+AS) solution in THF. (2) is the transformed standard spectrum of C60; (3) is the spectrum of AS corresponding to 𝑐AS,cal; (4) is the spectrum of AS obtained as the difference (1) and (2) spectra and corresponding to the required concentration 𝑐AS,sp (AS is chlorobenzene; 𝑐𝐹=0.0058mg/mL, 𝑐chlorbenzene,cal=1.974mg/mL, and 𝑐chlorobenzene,sp=1.806mg/mL).

To recover the individual C₆₀ spectrum, the following calibration procedure was developed. From the UV spectra recorded for C₆₀ solution in CH₂Cl₂ under gradual addition of THF (Figure 2), the linear dependence, IA_𝐹=𝐶⋅A_𝐹, of the integral absorbance IA_𝐹 of C₆₀ (the area under every individual spectral curve) in the wavelength range 𝜆=240−365nm on the absorbance A_𝐹 of C₆₀ at 𝜆_max=329.3nm was obtained (the insert in Figure 2).

Figure 2 

The UV spectra of (C60+CH2Cl2) solution in THF obtained by serial dilution. The insert shows the calibration dependence of the integral absorbance IA𝐹 of (C60+CH2Cl2) solution in THF on the absorbance value A𝐹 at 𝜆=329.3nm.

The spectrum of (C₆₀+CH₂Cl₂) solution in THF was taken in the calculations for the standard spectrum of C₆₀, since (C₆₀+CH₂Cl₂) is transparent at 𝜆>230nm (curve 2 in Figure 3), and fullerene does not aggregate in CH₂Cl₂ in contrast to C₆₀ in THF (curve 1 in Figure 3) [1, 5]. Besides, the position of the absorption maximum (𝜆_max=329.3nm) of C₆₀ band and the width of this band for (C₆₀+CH₂Cl₂) solution in THF coincides with that of the visible C₆₀ band for all (C₆₀+AS) solutions in THF (the insert in Figure 3).

Figure 3 

The UV spectra of C60  (1) in THF and (2) in CH2Cl2. The insert shows the normalized UV spectra of (1) C60 in THF, (2) (C60+CH2Cl2) in THF, and (C60+AS) solutions in THF in the region transparent for C60: (3) AS is toluene, (4) AS is p-xylene, and (5) AS is chlorobenzene.

Curve 2 in Figure 1 corresponds to the C₆₀ spectrum obtained by transformation of the spectrum of (C₆₀+CH₂Cl₂) solution in THF with the known value of integral absorbance (for any spectral curve in Figure 2). The transformation of the selected standard C₆₀ spectrum with the known IA_𝐹 value to the C₆₀ spectrum corresponding to the absorbance value at 𝜆_max=329.3nm in the (C₆₀+AS) spectrum was carried out by multiplication of absorbance values of the standard C₆₀ spectrum by the ratio of integral absorbance calculated from the calibration dependence IA_𝐹=𝐶⋅A_𝐹 to the integral absorbance of the standard spectrum.

From the UV spectra of AS solutions in THF recorded at specified concentrations 𝑐_AS of AS (Figure 4) the linear calibration dependences IA_AS=𝐵_AS⋅𝑐_AS (the insert in Figure 4) were obtained for all aromatic solvents used, where IA_AS is the integral absorbance (the area under the spectral curve of AS at 𝑐_AS) within the absorption range of AS (from 𝜆₁ to 𝜆₂, Figure 4).

Figure 4 

The UV spectra of AS solution in THF obtained at different concentrations of AS (AS is chlorbenzene; the UV spectra were obtained at 𝑐chlorobenzene=0.648,1.342,1.799,2.239, and 2.692 mg/mL). The insert shows the calibration dependence of the integral absorbance IAAS of AS solution in THF on AS concentration (AS is chlorobenzene; IAchlorobenzene=6.853⋅𝑐chlorbenzene; the 𝜆 range is 240–280 nm).

Using the calibration IA_AS=𝐵_AS⋅𝑐_AS, the standard spectrum of AS (the spectrum with a known value of IA_AS) in THF was transformed to the AS spectrum corresponding to the respective concentration 𝑐_AS,cal of AS in the (C₆₀+AS) solution in THF (curve 3 in Figure 1). The spectrum (curve 4 in Figure 1) obtained by subtraction of the absorbance values of C₆₀ (curve 2) from the absorbance values of (C₆₀+AS) (curve 1) is the spectrum of AS which corresponds to the true concentration 𝑐_AS,sp of AS in solution. The integral absorbance IA_AS,sp of AS was obtained as the difference between the integral absorbance of (C₆₀+AS) solution and the calculated integral absorbance IA_𝐹 of C₆₀. The concentration 𝑐_AS,sp of AS was calculated from the value of IA_AS,sp, obtained from the calibration IA_AS=𝐵_AS⋅𝑐_AS.

In accordance with the 𝑐_AS,cal and 𝑐_AS,sp values obtained for each (C₆₀+AS) solution in THF, the number 𝑁 of excess (positive values) or deficient (negative values) molecules of AS per solvation shell of C₆₀ molecule was calculated from the relation 𝑁=Δ𝑐_AS𝑐_𝐹𝑀_𝐹𝑀_AS, (1) where Δ𝑐_AS=(𝑐_AS,sp−𝑐_AS,cal), 𝑀_𝐹, and 𝑀_AS are the molecular masses of C₆₀ and AS, respectively.

3. Results and Discussion

For all (C₆₀+AS) solutions in THF the AS concentration 𝑐_AS,sp obtained with the proposed algorithm differed from the 𝑐_AS,cal value. Thus, the comparison of the chlorbenzene spectrum corresponding to the calculated concentration 𝑐_{chlorobenzene,cal} in THF (curve 3 in Figure 5) with the spectrum corresponding to the true concentration of chlorobenzene (curve 4 in Figure 5) shows that the true concentration is lower than the calculated one.

Figure 5 

The UV spectra of (1) (C60+o-dichlorobenzene) solution in THF (𝑐𝑜-dichlorobenzene,cal=2.113mg/mL and 𝑐𝐹=0.0095mg/mL), (2) the transformed standard spectrum of C60, (3) o-dichlorobenzene corresponding to 𝑐𝑜-dichlorobenzene,cal=2.113mg/mL (IA𝑜-dichlorobenzene=9.598⋅𝑐𝑜-dichlorobenzene in the λ range from 245 to 285 nm), and (4) o-dichlorobenzene obtained as a difference of (1) and (2) spectra (the calculated value 𝑐𝑜-dichlorobenzene,sp for the spectrum (4) is 1.851 mg/mL). The insert shows the normalized spectrum of  o-dichlorobenzene in THF (solid line) and the normalized spectra of o-dichlorobenzene obtained by subtraction of the C60 spectrum from the spectra of (C60+o-dichlorobenzene) solutions in THF with various concentrations of o-dichlorobenzene and C60: 𝑐𝑜-dichlorobenzene,cal=2.113mg/mL, 𝑐𝐹=0.0095mg/mL (dash line); 𝑐𝑜-dichlorobenzene,cal=2.190mg/mL, 𝑐𝐹=0.0069mg/mL (short dash line); 𝑐𝑜-dichlorobenzene,cal=1.758mg/mL, 𝑐𝐹=0.0081mg/mL (dot line).

The comparison of the spectrum corresponding to the calculated p-xylene concentration 𝑐_{𝑝-xylene,cal} (curve 3 in Figure 6) with the spectrum corresponding to the true concentration of p-xylene (curve 4 in Figure 6) shows that the true concentration exceeds considerably the calculated concentration.

Figure 6 

The UV spectra of (1) (C60+p-xylene) solution in THF (𝑐𝑝-xylene,cal=1.548mg/mL and 𝑐𝐹=0.0061mg/mL), (2) the transformed standard spectrum of C60, (3) p-xylene in THF corresponding to 𝑐𝑝-xylene=1.548mg/mL (IA𝑝-xylene=9.013⋅𝑐𝑝-xylene in the λ range from 240 to 280 nm), and (4) p-xylene obtained as a difference of (1) and (2) spectra (with the calculated value of 𝑐𝑝-xylene,sp=2.495mg/mL). The insert shows the normalized spectrum of p-xylene in THF (solid line) and normalized spectra of p-xylene obtained by subtraction of the C60 spectrum from the spectra of (C60+p-xylene) solutions in THF with various concentrations of p-xylene and C60: 𝑐𝑝-xylene,cal=1.548mg/mL, 𝑐𝐹=0.0061mg/mL (dash line); 𝑐𝑝-xylene,cal=1.166mg/mL, 𝑐𝐹=0.0063mg/mL (short dash line); 𝑐𝑝-xylene,cal=1.378mg/mL, 𝑐𝐹=0.0070mg/mL (dot line).

The difference between the 𝑐_AS,cal and 𝑐_AS,sp values found for each (C₆₀+AS) solution in THF points to the change of solvation shell density with respect to the density of pure solvent. The calculated 𝑁 values are given in Table 1. The data show that the presence of asymmetrical substituents (chlorine atoms or the methyl group) in the AS molecule results in the loosening of solvent structure in the solvation shell of C₆₀, while the absence of substituents (benzene) or the presence of symmetrical substituents (p-xylene) leads to compacting of solvent structure in the C₆₀ solvation shell. The insignificant spread in the calculated 𝑁 values (below 10%) supports the validity of the proposed algorithm for calculation of the true AS concentration from the UV spectroscopic data. The validity of the proposed algorithm is supported as well by the practical identity of the normalized real spectra of AS in THF and of the AS spectra obtained as a difference of spectra of (C₆₀+AS) and C₆₀ (see inserts in Figures 5 and 6). The concentration dependences of the integral absorbance of C₆₀ in various aromatic solvents (Figure 7) correlate with the data listed in Table 1. Absorption decreases with the increase of packing density of solvent molecules in the solvation shell of C₆₀.

Figure 7 

Concentration dependences of the integral absorbance (in the 𝜆 range from 300 to 365.1 nm) of C60 in (1) o-xylene, (2) toluene, and (3) p-xylene.

4. Conclusions

The idea has been examined that the forced coordination of monocyclic molecules with various substituents of aromatic solvents by fullerene C₆₀ may result in the difference between the packing density of AS molecules in the C₆₀ solvation shell and that in the pure AS. In this work, the fullerene solvation shell was defined as the number of AS molecules which the molecule of fullerene is able to retain at addition of THF which is a good solvent for AS and poor for C₆₀, to C₆₀ solution in AS. The analytical techniques applied for preparation of C₆₀ solutions in aromatic solvents and for determination of accurate concentrations 𝑐_𝐹 and 𝑐_AS,cal after addition of THF, as well as the developed calibration procedures, made it possible to obtain the data from the UV spectra of (C₆₀+AS) solutions in THF which is the supporting evidence of the difference between the packing density of AS molecules in the solvation shell of C₆₀ and that in the pure AS solvent. The data obtained show that the presence of asymmetrical substituents (chlorine atoms or methyl group) in the AS molecule results in the loosening of solvent structure in the solvation shell of C₆₀, while the absence of substituents (benzene) or the presence of symmetrical substituents (p-xylene) leads to compacting of solvent structure in the C₆₀ solvation shell.