# Effect of Structure of Monocyclic Aromatic Solvents on the Packing Density of FullereneC60 Solvation Shell

Authors: E. V. Chubarova; E. Yu. Melenevskaya
Journal: ISRN Spectroscopy
 (2012)

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## 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.


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## Body

## 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 numberN of excess (positive values) or deficient (negative values) molecules of the aromatic solvent per solvation shell of C60 molecule.
Solvent
Structural formula
N
Benzene
+650
Toluene
−110
o-xylene
−240
p-xylene
+940
Chlorbenzene
−190
o-dichlorobenzene
−120The 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 (cF) and of the AS used (cAS,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 cAS,sp calculated from the spectra of the (C60+AS) solution in THF must be equal to the concentration cAS,cal calculated for this solution from the weighed amounts of C60 solution in AS and of THF. To calculate cAS,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 cAS,sp concentrations of AS from the spectra of (C60+AS) solutions in THF.The graphic illustration of the algorithm proposed is presented in Figure1. Curve 1 in Figure 1 corresponds to the spectrum of (C60+AS) solution in THF with known concentration cAS,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 valuecAS,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, IAF=C·AF, of the integral absorbance IAF of C60 (the area under every individual spectral curve) in the wavelength range λ=240–365nm on the absorbance AF of C60 at λmax=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 (λmax=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 Figure1 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 IAF value to the C60 spectrum corresponding to the absorbance value at λmax=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 IAF=C·AF to the integral absorbance of the standard spectrum.From the UV spectra of AS solutions in THF recorded at specified concentrationscAS of AS (Figure 4) the linear calibration dependences IAAS=BAS·cAS (the insert in Figure 4) were obtained for all aromatic solvents used, where IAAS is the integral absorbance (the area under the spectral curve of AS at cAS) 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 atcchlorobenzene=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 calibrationIAAS=BAS·cAS, the standard spectrum of AS (the spectrum with a known value of IAAS) in THF was transformed to the AS spectrum corresponding to the respective concentration cAS,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 cAS,sp of AS in solution. The integral absorbance IAAS,sp of AS was obtained as the difference between the integral absorbance of (C60+AS) solution and the calculated integral absorbance IAF of C60. The concentration cAS,sp of AS was calculated from the value of IAAS,sp, obtained from the calibration IAAS=BAS·cAS.In accordance with thecAS,cal and cAS,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
(1)N=ΔcAScFMFMAS,
where ΔcAS=(cAS,sp-cAS,cal), MF, and MAS are the molecular masses of C60 and AS, respectively.


## 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 numberN of excess (positive values) or deficient (negative values) molecules of the aromatic solvent per solvation shell of C60 molecule.
Solvent
Structural formula
N
Benzene
+650
Toluene
−110
o-xylene
−240
p-xylene
+940
Chlorbenzene
−190
o-dichlorobenzene
−120The 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 (cF) and of the AS used (cAS,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 cAS,sp calculated from the spectra of the (C60+AS) solution in THF must be equal to the concentration cAS,cal calculated for this solution from the weighed amounts of C60 solution in AS and of THF. To calculate cAS,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 cAS,sp concentrations of AS from the spectra of (C60+AS) solutions in THF.The graphic illustration of the algorithm proposed is presented in Figure1. Curve 1 in Figure 1 corresponds to the spectrum of (C60+AS) solution in THF with known concentration cAS,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 valuecAS,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, IAF=C·AF, of the integral absorbance IAF of C60 (the area under every individual spectral curve) in the wavelength range λ=240–365nm on the absorbance AF of C60 at λmax=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 (λmax=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 Figure1 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 IAF value to the C60 spectrum corresponding to the absorbance value at λmax=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 IAF=C·AF to the integral absorbance of the standard spectrum.From the UV spectra of AS solutions in THF recorded at specified concentrationscAS of AS (Figure 4) the linear calibration dependences IAAS=BAS·cAS (the insert in Figure 4) were obtained for all aromatic solvents used, where IAAS is the integral absorbance (the area under the spectral curve of AS at cAS) 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 atcchlorobenzene=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 calibrationIAAS=BAS·cAS, the standard spectrum of AS (the spectrum with a known value of IAAS) in THF was transformed to the AS spectrum corresponding to the respective concentration cAS,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 cAS,sp of AS in solution. The integral absorbance IAAS,sp of AS was obtained as the difference between the integral absorbance of (C60+AS) solution and the calculated integral absorbance IAF of C60. The concentration cAS,sp of AS was calculated from the value of IAAS,sp, obtained from the calibration IAAS=BAS·cAS.In accordance with thecAS,cal and cAS,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
(1)N=ΔcAScFMFMAS,
where ΔcAS=(cAS,sp-cAS,cal), MF, and MAS are the molecular masses of C60 and AS, respectively.


## 3. Results and Discussion

For all (C60+AS) solutions in THF the AS concentration cAS,sp obtained with the proposed algorithm differed from the cAS,cal value. Thus, the comparison of the chlorbenzene spectrum corresponding to the calculated concentration cchlorobenzene,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 calculatedp-xylene concentration cp-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 thecAS,cal and cAS,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 theλ 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 cF and cAS,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.



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*Source: 906976-2012-11-22.xml*