CC BY
17
UDC 541.67:541.142
Yu. N. Biglova, V. V. Zagitov, M. S. Miftakhov, R. Z. Biglova, V. A. Kraikin, G. E. Zaikov
UV-SPECTROSCOPY STUDY OF 1,2-DIHYDRO-C60-FULLERENES IN POLAR SOLVENT
Keywords: fullerene, mono-substituted 1,2-dihydro-C60-fullerenes, [60]PCBM, ionization, UVspectroscopy, sulphuric acid.
We have carried out the spectroscopic study of C60 and its mono-substituted derivatives (methanofullerenes) of process dissolution in concentrated (98%) sulfuric acid. There has been found a number of absorption maxima, whose position does not depend on the type of substituent. Several maxima coincide with the absorption maxima of anion-and cation-radicals C60 whereas the distance between them (~80 nm) coincides with AX for ionized polyenes.
Ключевые слова: фуллерен, монозамещенный 1,2-дигидро-С60-фуллерен, [60] PCBM, ионизация, УФ-спектроскопия,
серная кислота.
Проведено спектроскопическое исследование С60 и его моно-замещенных производных (метанофуллеренов) в процессе растворения в концентрированной (98%) серной кислоте. Обнаружено несколько максимумов поглощения, положение которых не зависит от типа заместителя. Несколько максимумов совпадают с максимумами поглощения анионных и катионных радикалов С60, тогда как расстояние между ними (~ 80 нм) совпадает с AX для ионизированных полиенов.
Introduction
Fullerenes, i.e. specific carbon clusters with a closed system of double bonds with extremely low LUMO-energy are majorly of interest as n-type components of heterojunction phototransformators. Among functionalized fullerenes there were detected promising compounds for the practical use of renewable energy sources, molecular electronics and materials for medicine [1-3]. However the low solubility of Обо and its derivatives in polar and non-polar environments hinders their practical implementation. Water-soluble substances are required for medical use and film-forming properties, solubility in organic solvents, and stability in air are required for photovoltaic devices. Necessary and sometimes unique properties of Об0 derivatives are built, first of all, by the functionalization of the fullerene molecule. Extraordinary properties of fullerene are often manifested only in the ionized form.
It is of interest that fullerene and fullerene cations radical are present as reasonably common components in a series of planetary nebulae in the interstellar medium and around certain astrophysical-objects [4, 5]. With the use of IR-spectroscopy and the follow-up calculations Об0 (ionized form) was discovered in the cosmic space in the amount of 0.35% by weight of the total carbon.
There are several ways of generating Об0 cations. For example, the cation and cation-radical of Об0 were observed by spectroscope after irradiation of Об0 in frozen matrix of argon, freon, carbon tetrachloride [6-9]; and generated by the matrix photoionization method [10]. Об0 cation-radical were registered to be formed at dissolution of fullerene in oleum by way of EPR-spectroscopy [11] and UV spectroscopy [12]. The authors of the work [13] demonstrated that Об0 cation-radicals are easily formed at room temperature in solutions of superacids (trifluoromethanesulfonic acid with potassium persulphate or oleum). The results obtained are confirmed by using other synthetic and analytical
methods [14, 15]. There is spectroscopic evidence of such cations formation, based on the use of the model compounds spectra, as well as electronic spectra existing in scientific literature and theoretical calculations of C6o ion-radical [6, 10, 16, 17]. Cations are registered as well in organic solvents with the addition of Lewis acids (SbCl5 and SbF5) [18, 19]. Thus to generate superacid HSbF6 weak Bronsted acid HF is mixed with SbF5 [14].
Most carbocations are not sufficiently stable, which maintains their existence in the solution in such concentrations that are necessary to determine the electrical conductivity of the solution. UV-spectroscopy is most commonly used to register carbocations.
In this work, we carried out a spectroscopic study of a number of mono-substituted methanofullerenes and their initial C60 dissolved in 98% sulfuric acid. We discovered characteristic absorption bands whose maxima coincide with the absorption maxima of ionized fullerene C60 and polyenes.
Experimental
The objects of study were selected to be fullerene C60 (CJSC "Innovations of Leningrad Institutes and Enterprises", 99.5% of basic substance) and mono-substituted 1,2-dihydro-C60-fullerenes (known as [60]PCBM) which we previously produced (I)20 and synthesized by modified Bingel-Hirsch method
(II - IV) (ESIf):
methyl ether of [6,6]-phenyl-C61-butyl acid (I)
{1-chloro-1-[2-
(methacryloyloxy)ethoxy carbonyl]-1,2-methane}-1,2-dihydro-C60-fullerene (II)
O
O O
O
{(1-methoxycarbonyl-1-[(acryloyloxy)ethoxycarb onyl]-1,2-methane]}-1,2-dihydro-Ceo-fullerene (III)
{(1-methoxycarbonyl-1-[2-
(methacryloyloxy)ethoxy
carbonyl]-1,2-methane]}-1,2-dihydro-Ceo-fullerene (IV)
A test portion of fullerene (methanofullerenes) was dissolved at room temperature in chloroform. The aliquot of yellowish-brown solution with a concentration of the compound 10-4 M was placed in a volumetric flask, the solvent was evaporated and the dry residue was filled with 98% sulfuric acid. The concentration of completely dissolved methanofullerenes amounted to 1.70*10-5 M.
The electronic spectra of fullerene and methanofullerenes sulfuric acid solutions absorption were registered within the grid coordinates: optical density (A) - wavelength (X) on UV-spectrophotometer UVmini 1240 produced by "Shimadzu" with UVProbe software. The spectra were recorded at room temperature in the wavelength range from 190 to 1100 nm (2.0 nm slit width, fast scanning speed) using 1 cm thick quartz cuvette. The start of the spectra capture was 5 min after the acid was added and it was repeated every 10 min.
Обо fullerene and its derivatives were weighed on Sartiorius M2P scales (sensitivity 10-6 g). The micropipettes Biohit Proline (Finland) with sample ranges of 10-100 and 100-1000 цЬ were used for aliquots intake. 98% sulfuric acid was distilled out of 94% H2SO4 in air at atmospheric pressure. Chloroform was purified by common methods [21].
UV spectra were researched by Zindo and TDDFT methods for fullerene and methanofullerenes cation structures. Geometrical parameters of claimed compounds studies were optimized by B3LYP/6-31g (d). In the case of Zindo research, 500 transitions were taken into account for the analysis of the spectrum and in the case of TD research there were 50 transitions. All calculations were done using the software package Gaussian09.
Results and Discussion
Due to its high hydrophobicity crystalline fullerene Обо is poorly soluble in 98% sulfuric acid. Apparently, only a small portion of the surface of the fullerene nuclei in the crystal contacts with the acid. This is insufficient for a comprehensive protonation and solvation, and, therefore, for the transition of fullerene molecules in the solution. There are 4 characteristic absorption bands with maxima at 317, 260, 218 and 196 nm in the short-wave range in the electronic spectra of solutions derived by dissolving suspended fullerene in sulphuric acid. In the long-wave range, the characteristic bands are unidentifiable due to
insufficient background indices (Fig. 1). As the fullerene dissolves, the optical density in the absorption maxima grows linearly with different and very low speeds (Fig. 2).
390 lull
Fig. 1 - The evolution of fullerene C6o absorption electron spectrum dissolved in 98% sulfuric acid
100
200
300 I- »nil
Fig. 2 - Change of optical density in maxima of absorption bands in the process of C6o fullerene dissolution in 98% sulfuric acid
Methanofullerenes dissolve in sulphuric acid much better than the original C60. For the functionalized fullerenes under analysis, the solubility increases sequentially as follows: {(1-methoxycarbonyl-1-[2-(methacryloyloxy)ethoxycarbonyl] -1,2-methane]}-1,2-dihydro-C60-fullerene (IV) > {(1-methoxycarbonyl-1-[(acryloyloxy)ethoxycarbonyl] -1,2-methane] }-1,2-dihydro-C60-fullerene (III) > methyl ether of [6,6]-phenyl-C6i-butyl acid (I) > {1-chloro-1-[2-(methacryloyloxy)ethoxycarbonyl] -1,2-methane}-1,2-dihydro-C60-fullerene (II) (Fig. 3).
The best solubility is demonstrated by methanofullerenes with the largest number of easily protonatable oxygen atoms in the substituent (simple etheric and carbonyl ones), the second best is the compound I with phenyl group a-positioned in cyclopropane ring, and the least soluble is adduct II containing chlorine atom in the same position.
Regardless of the nature and length of the substituent in the C60 nucleus, the spectral curves of mono-substituted 1,2-dihydro-C60 fullerenes are identical: there are three absorption maxima (at 218, 251 and 317 nm) recorded in the near ultraviolet range, their intensity increases over time and after some time it
reaches a constant value (Fig. 4(a)). Moreover, the higher the solubility, the earlier the curve of the optical density dependent on time plateaus. After that, the optical density does not change, which may indicate the completion of the dissolution process of 1,2-dihydro-Обо fullerenes (the maximum is missing in the 196 nm range, presumably due to the shift to the unregistable short-wave range). In the long-wave range of the spectrum, the opposite pattern is observed: the intensity of appeared absorption bands decreases over time. The changes in the range of 450-850 nm appear to be the most significant (Fig. 4(b)).
240 т. mill
Fig. 3 - Optical density dependent on the duration of dissolution at a fixed wavelength of 317 nm and unchanging final concentration of compounds 1.7X10"5 M
If the dissolution of the original Об0 in sulfuric acid is accompanied only by the increase in optical density of the solution, then, in case of methanofullerenes, there is as well a hypsochromic shift of the maxima for all of the compounds under study (Fig. 5). The authors registered similar process in the long-wavelength range of the electron when fullerene is dissolved in oleum [12, 13]. In our case, the amount of the peaks displacement in the short wavelength range of the spectrum is a fraction of that in the long-wavelength region. The phenomena observed can be caused by processes similar to the processes occurring during the dissolution of polyenes in high-ionizing environments: protonation and cationic polymerization.
Indeed, the ionizations of fullerene and of polyene have a lot in common. This is a close coincidence of the series of absorption maxima and the distances at which they occur (Fig. 6, Table 1). And in homologous series of polyenes, and in the series of the studied methanofullerenes, lengthening the chain of conjugation by 1 component leads to the shift of the absorption maximum by approximately 80 nm (Fig. 7). For all methanofullerenes the value of til calculated by regression equations (Table 2) match well with calculated values for polyenes [23].
A 0,6
0,4
0,2
Л/
step 10 rain
190
290
390 К nm b
400
600
800
i ooo У., nm
Fig. 4 - The evolution of the electron absorption spectrum in the short-wave (a) and long wave (b) ranges at the dissolution of mono-substituted adduct I in 98% sulfuric acid (similar character of spectrum changes was observed for all of the compounds under study)
Fig. 5 - Shifts in time of short-wave (314 nm) (a) and long-wave (797 nm) (b) maxima of the absorption bands for methanofullerene I
second derivative
-0,550
695,00
Fig. 6 - Electronic absorption spectrum (a) and its second derivative (b) of methanofullerene IV in 98% sulfuric acid
This match allows us to conclude that in the case of the fullerene core of methanofullerenes non-localized electron cloud splits into several oscillating linear vibrators (similar to polyene ones) of different lengths, each of which is characterized by its absorption band in the electronic spectrum. It can mean that, by analogy with polyenes, non-localized electron cloud does not split into separate vibrators, but oscillates as a whole. Thus, apparently, there is a set of fullerene nuclei with various lengths of adjacent sequences, whose specific content differs. Besides, the more atoms are harnessed by n-molecular orbital, i.e. the longer the system of conjugated sequences is, the lower the energy difference between the ground and upper states is and, therefore, the more visible is the shift of the absorption maximum toward longer wavelengths, and the higher the intensity of absorption is.
Table 1 - Wavelengths in the absorption band maxima (resulted from experimental and calculation methods) and number of conjugated bonds for cations formed by dissolving adducts and polyenes in 98% sulfuric acid
Scheme*/ Number of conjugated bonds, n / Polyenes [22] Experimental method, Xmax* (nm) Calculation method by Zindo, Xmax (nm)
I II III IV Сбо IV
1/1 /305.0 325.5 324.0 326.0 326.0 317.0 328.2
2/2/397.0 400.5 398.0 402.0 401.5 388.7 400.3
3/3/473.0 480.5 474.0 475.5 474.5 470.0 485.5
4/4/550.0 559.5 559.5 563.0 562.0 552.2 553.6
5/5/630.0*** 628.0 628.0 626.5** 628.0** 620.9 636.1
6/6/710.0*** 692.5** 694.0** - - 681.5 694.2
7/7/790.0*** 806.0 804.5 803.5 805.0 - 825.8
8/8/870.0*** 886.5 885.5 886.5 886.0 853.6 -
9/9/950.0*** 956.5 953.0 954.5 955.0 964.1 965.2
10/10/1030.0*** 1029.5 1033.0 1031.0 1030.5 - -
* - 1- у
2 - +
3 - У в
End table 1
* for methanofullerenes defined by the second derivative 5 min after sulfuric acid was introduced ** arm on the absorption spectra *** calculated continuable series [22]
X. mil
2(Ш -I-г-1-1-1-г-
2 4 fi 8 10
number of double bonds, п
Fig. 7 - Dependence of wavelength in the absorption maxima on the number of conjugated bonds in polyenes and methanofullerenes
Table 2 - Regression equation and correlation coefficients for methanofullerenes and polyenes
Compound Equation R2
I y=79.1x+236.6 0.9918
II y=79.0x+240.7 0.9960
III y=78.8x+232.6 0.9954
IV y=78.5x+228.0 0.9916
Ref.[22] y=81.1x+228.5 0.9979
A significant part of the absorption bands in the electronic spectra registered in methanofullerenes sulfuric acid solutions electon stpectra is present in the spectra of fullerene Обо+ core of these compounds, derived by calculation (Fig. 8, Table 1).
As for the comparative analysis of the data obtained in the present work and in the scientific literature per se, namely in the overview [15], the spectroscopically measured values of the maximum absorption of C60+ in various publications are very different. The authors as a rule confine themselves to
haphazard statement of the absorption maxima of emerging C6o+. B Research [13] determined the peaks of absorbtion maxima at 477.4, 633.3 nm, and research [12] identified the peaks corresponding to wavelengths of 320.9 and 478.2 nm, which agrees with the values (Table 1) determined in the present paper. Bearing in mind that the fullerene is a linear structure, we identified the patterns for C6o+ correlating with those for carbocation structures with long system of conjugated double bonds.
a b
Fig. 8 - Visualizations of the C60+ (a) and carbocation of methanofullerene IV (b)
Some of the absorption bands that we have identified before were attributed to both anion-radicals and cation-radicals [23].
Thus, the modification of fullerene by functionalization of its core by easily protonating double bonds and oxygen atoms can significantly increase the local concentration of fullerene cores in concentrated (98%) sulfuric acid. The latter, in its turn, allows to reliably identify a series of the absorption bands in the visible spectrum, many of the bands are identical to the absorption bands of fullerene cation-radicals derived by dissolving it in oleum and in superacids.
Acknowledgements
The study was carried out under a financial support of the Russian Foundation for Basic Research (grants № 14-03-31610 mo^, 14-02-97008).
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© Yu. N. Biglova - Ph.D., Assistant Professor, Bashkir State University, Ufa, Russia, e-mail: bn.yulya@mail.ru, V. V. Zagitov -Student, Bashkir State University, Ufa, Russia, M. S. Miftakhov - Doctor of Chemistry, Full Professor, Head of Laboratory, Institute of Organic Chemistry, Ufa Scientific Centre of RAS, Ufa, Russia, R. Z. Biglova - Doctor of Chemistry, Full Professor, Bashkir State University, Ufa, Russia, V. A. Kraikin - Doctor of Chemistry, Head of Laboratory, Institute of Organic Chemistry, Ufa Scientific Centre of RAS, Ufa, Russia, G E. Zaikov - Doctor of Chemistry, Full Professor, Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia.
© Ю. Н. Биглова - кандидат химических наук, ассистент, Башкирский государственный университет, Уфа, Россия, bn.yulya@mail.ru, В. В. Загитов - студент, Башкирский государственный университет, Уфа, Россия, М. С. Мифтахов - доктор химических наук, профессор, заведующий лабораторией, Институт органической химии, Уфимский научный центр РАН, Уфа, Россия, Р. З. Биглова - доктор химических наук, профессор, Башкирский государственный университет, Уфа, Россия, В. А. Крайкин - доктор химических наук, заведующий лабораторией, Институт органической химии, Уфимский научный центр РАН, Уфа, Россия, Г. Е. Заиков - доктор химических наук, профессор, кафедра Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия.
