Molecules in Benzene Solutions Prepared by Various Methods

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Bulletin of National University of Uzbekistan: Mathematics and Natural Sciences

Volume 1 Issue 2 Article 4

9-25-2018

Analysis of clusterization of C70 molecules in benzene solutions prepared by various methods

U. Makhmanov Tashkent State Technical University

O. Ismailova Tashkent State Technical University

A. Kokhkharov Tashkent State Technical University

S. Bakhramov Tashkent State Technical University

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Recommended Citation Makhmanov, U.; Ismailova, O.; Kokhkharov, A.; and Bakhramov, S. (2018) "Analysis of clusterization of C70 molecules in benzene solutions prepared by various methods," Bulletin of National University of Uzbekistan: Mathematics and Natural Sciences: Vol. 1 : Iss. 2 , Article 4. Available at: https://uzjournals.edu.uz/mns_nuu/vol1/iss2/4

This Article is brought to you for free and open access by 2030 Uzbekistan Research Online. It has been accepted for inclusion in Bulletin of National University of Uzbekistan: Mathematics and Natural Sciences by an authorized editor of 2030 Uzbekistan Research Online. For more information, please contact [email protected]. Volume 1, Issue 2 (2018)

ANALYSIS OF CLUSTERIZATION OF C70 MOLECULES IN BENZENE SOLUTIONS PREPARED BY VARIOUS METHODS

Makhmanov U. K., Ismailova O. B., Kokhkharov A. M., Bakhramov S. A. Tashkent State Technical University, Tashkent, Uzbekistan

Abstract

Self-assembling features of molecules of C70 in benzene solution prepared in two various ways – equilibrium and non-equilibrium – has been investigated ex- perimentally by method of high-resolution transmission electron microscopy. It was demonstrated, that the formation of densely packed monomolecular fullerene aggregates with a diameter of not more than 30 nm in solutions prepared by the equilibrium method (without the use of external mechanical in- ﬂuences on solution). In the case of solutions C70, which were prepared by the non-equilibrium method (stirring of solution of C70 by a mechanical rotator), large quasispherical aggregates of nanoporous structure with fractal size D ≈ 2.19 were synthesized. Keywords: fullerene C70, benzene solution, electronic microscopy, clusterization, monomolecular, self-aggregation, morphology, fractal dimension, stabil- ity, application.

1 Introduction

Fullerenes C60 and C70 represent new allotropes of carbon [1] and exhibit unique properties that presently being pursued globally for a wide range of high performance applications. Since then, light C60 and C70 fullerenes have invited much attention and been extensively investigated. These fullerenes are soluble in many nonpolar organic solvents [2], are insoluble in polar and H-bonding solvents [3]. Light fullerenes are easily transferrable into water [4]. Fullerene C70 is extremely valuable for innovative applications in medicine [5–7], chemical synthesis and catalysis [8], biomedical testing [9], electronics [10], solar energy [11] and optics [12]. A number of studies have shown promising potential in using fullerene C70 as nonlinear optical laser-radiation limiters [13–16]. The limiters obtained on the base of such nonlinear optical media allow to decrease the laser radiation to an eye-safe level and protect high-sensitive light sensors from damage in the visible, near-, and mid- IR regions in a wide range of laser-pulse widths [17]. Fullerenes are excellent organic semiconductors, with some rather speciﬁc features, such as low LUMO level and high electron mobility [18]. C70 fullerene is a nontoxic substance and having powerful antioxidant properties due to its high activity as free radicals acceptor [19]. It can be used in photodynamic therapy for treatment of oncological diseases [20–21], in targeted delivery of various drugs in cells [22]. During the last years, new experimental data [23-26] reveal that light fullerenes molecules have tendency to self-assembling and formation of large negatively charged

66 Bulletin of National University of Uzbekistan: Mathematics and Natural Sciences aggregates of fullerenes of different shapes and sizes in a wide range of mono- and multi-component organic and inorganic solvents. Clusterization of fullerene molecules in solutions may cause significant changes in their physical properties and result in a dramatic change in their technical, medical and other applications. Most scien- tific articles regarding fullerene aggregation have been focused on the C60 behavior in different solvents, and only a few studies were reported on the C70 aggregation. In view of the above, development of novel selective methods of synthesis of (C70)m aggregates (where m is a number of fullerene molecules in aggregate) consisting both of intermediate macromolecular C70 formations (porous fractal aggregates) and individual C70 molecules (densely packed monomolecular aggregate) in solutions with predictable characteristics such as aggregate size and shape, density of packing. In this work, we exhibit for the first time, the possibility of directed synthesis of both close-packed monomolecular and porous fractal aggregates of C70 in benzene. We note that observation of the formation of stable C60 aggregate forms in toluene and their electronic absorption behaviors was reported in [27].

2 Experimental details

2.1 Materials

Fullerene C70 (99.7 % purity – SES Research, USA) and benzene (99.8 %, analytical grade – Sigma-Aldrich, USA) were used in measurements.

2.2 Preparation of C70 solutions To prepare equilibrium solutions of fullerene of different concentrations, powders of ◦ C70 were dissolved in benzene and equilibrium mixtures were kept at ∼ 25 C in test tubes for 15 days without external mechanical influences. Non-equilibrium C70 solution was prepared by mechanical mixing of C70 suspen- sion in benzene in a hermetically sealed flask mounted on the programmable rotator Multi RS-60 (BioSan) in a mechanical swinging mode at a room temperature (∼ 25◦C), 12 rpm for 14 days. Before adding to the solution, C70 was weighted on an analytical balance EP214C (Ohaus Explorer Pro, Switzerland) with an accuracy of ≤ 0.0001g.

2.3 Investigation of structural features of C70 solutions We have used the transmission electron microscope (TEM) LEO-912 AB (ZEISS, Germany) with accelerating voltage of 120 kV and spatial resolution of ∼ 0.34 nm to determine morphological features and dimensional characteristics of aggregates of fullerents synthesized in equilibrium and non-equilibrium solutions. For transmission electron microscopy, copper grids (diameter 3.05 mm, 300 mesh, Ted Pella Inc., USA) without formvar coating were used. Digital micrographs of samples were analysed using UTHSCSA Image Tool software (UTHSCSA, USA).

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3 Results and discussions

Figure 1 represents electron microscopic images of (C70)m aggregates synthesized in benzene solutions of C70 at fullerene concentration of ∼ 1.0 mg/ml prepared by equilibrium method.

Figure 1. Transmission electronic microscope image show the formation of (C70)m aggregates of fullerenes synthesized in C70 benzene solution prepared by equilibrium method (without using of external mechanical inﬂuences on the solution). The initial concentration of C70 in solution is ∼ 1.0 mg/ml.

It has been shown that processes of self-organization of C70 molecules in solutions prepared by equilibrium method is synthesized close-packed aggregate with a diameter of d ∼ 25 nm. Additional morphological investigations of a peripheral part of an individual C70 aggregate using high resolution TEM have allowed to ﬁnd out that (C70)m aggregate has a close-packed monomolecular dispersion (see Fig-1). C70 molecules have unique electronic properties that make them attractive candidates for diagnostic and therapeutic applications.

Figure 2 shows a TEM image of (C70)m aggregate, which has been synthesized in the benzene solvent prepared by other "non-equilibrium method" of preparation of C70 solution. In a solution prepared by non-equilibrium method, self-aggregation of fullerene molecules and synthesis of large porous fractal aggregates of quasispherical shape with a diameter of d ∼ 220 ± 10 nm, consisting of smaller intermediate "discrete" aggregates of C70 with a diameter of d0 ∼ 20 ÷ 35 nm were observed during 12 ÷ 15 days (see Fig-2). It has been established [28] that the minimum of free energy (E) of the system "fullerene cluster + benzene solution" (normalised to one molecule of fullerene) at the room temperature can only be achieved when a fractal aggregate (C70)55 with a radius R = 2.3 nm is formed (symmetry group D5h). According to the fractal concept [29], the relationship between number of fullerene molecules in the cluster (m), its radius (R) and the fractal dimension (D) can be expressed as

−1 D = ln m · (lnR − lnro) (1)

68 Bulletin of National University of Uzbekistan: Mathematics and Natural Sciences

where r0 is average radius of C70 molecule. Fractal dimension of the aggregate synthesized in the solution does not depend on the initial concentration of fullerene, however, geometrical dimension of the aggregate increases proportionally with the concentration of C70 in the solution. Using Eq. 1 and the numbers m = 55, R = 2.3 nm and r0 = 0.37 nm, we found that the fractal dimension of the synthesized (C70)m aggregate in benzene is D ≈ 2.19 (see Fig-2).

Figure 1. Transmission electronic microscope image show the formation of large (C70)m aggregates of fullerenes synthesized in C70 benzene solution prepared by non-equilibrium method (mechanical mixing of C70 solution using rotator).

Precise measurements of the inner structure of fractal (C70)m aggregate performed using high-resolution TEM and analysis using UTHSCSA Image Tool software have shown that the shortest distance (i.e. spatial gap) between neighbouring discrete structural units of "small dimension" located inside fullerene aggregate does not exceed ∆L ∼ 1.8 nm (Fig-2).

4 Conclusions

TEM microscopic investigations of C70 solutions in benzene revealed the possibility of synthesis of both fractal and closed-packed (C70)m aggregates. It was observed that in solutions prepared by the non-equilibrium method at the room temperature, mainly large quasispherical aggregates of fullerene with a diameter of ∼ 220 ± 10 nm nm having porous structure with fractal dimension D ≈ 2.19 are synthesized. In equilibrium solutions of C70 of the same initial concentration, the formation of densely packed monomolecular fullerene aggregates with D ≈ 3 was observed. In both cases, the ﬁnite geometrical dimensions of the synthesized nano-porous (C70)m aggregates are determined by the initial concentration of fullerene. Higher initial concentration of C70 in the solution requires a greater number of iterations of self-assembly of fullerene molecules which leads to an increase in the geometrical dimension of the synthesized (C70)m aggregates.

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Fractal (C70)m aggregates synthesized in benzene are stable to external mechanical (for instance, intensive mixing of solution, repeated transfusion from flask to flask) and thermal influences (heating up to ≤ 50◦C in the water bath followed by slow cooling to a room temperature). Experimental results obtained in this study may be used for a wide range of practical applications of C70 fullerene dispersions in nanotechnologies, chemistry and biomedicine.

Acknowledgements

This study has been supported by the Grants No F 2 − FA − F 143 and No F 2 − FA−F 146 for the fundamental investigations of the Academy Sciences of Uzbekistan and by a Grant No CA − 4 − 002 Committee for coordination science and technology development under the Cabinet of Ministers of Uzbekistan.

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