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27 Oct 2010 - Bulavinb a Joint Institute for Nuclear Research, Dubna, Moscow, Russia b Kyiv Taras Shevchenko National. University, Kyiv, Ukraine c Moscow ...
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Fullerenes, Nanotubes and Carbon Nanostructures

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Solvatochromism and Fullerene Cluster Formation in C60/N-methyl-2pyrrolidone

O. A. Kyzymaab; M. V. Korobovc; M. V. Avdeeva; V. M. Garamusd; V. I. Petrenkoab; V. L. Aksenovae; L. A. Bulavinb a Joint Institute for Nuclear Research, Dubna, Moscow, Russia b Kyiv Taras Shevchenko National University, Kyiv, Ukraine c Moscow State University, Vorobjovy Gory, Moscow, Russia d GKSS Research Centre, Geesthacht, Germany e Russian Research Center Kurchatov Institute, Moscow, Russia Online publication date: 27 October 2010

To cite this Article Kyzyma, O. A. , Korobov, M. V. , Avdeev, M. V. , Garamus, V. M. , Petrenko, V. I. , Aksenov, V. L. and

Bulavin, L. A.(2010) 'Solvatochromism and Fullerene Cluster Formation in C60/N-methyl-2-pyrrolidone', Fullerenes, Nanotubes and Carbon Nanostructures, 18: 4, 458 — 461 To link to this Article: DOI: 10.1080/1536383X.2010.487778 URL: http://dx.doi.org/10.1080/1536383X.2010.487778

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Fullerenes, Nanotubes, and Carbon Nanostructures, 18: 458–461, 2010 Copyright © Taylor & Francis Group, LLC ISSN: 1536-383X print / 1536-4046 online DOI: 10.1080/1536383X.2010.487778

Solvatochromism and Fullerene Cluster Formation in C60/N-methyl-2-pyrrolidone O. A. KYZYMA1,2, M. V. KOROBOV3, M. V. AVDEEV1, V. M. GARAMUS4, V. I. PETRENKO1,2, V. L. AKSENOV1,5 AND L. A. BULAVIN2 1

Joint Institute for Nuclear Research, Dubna, Moscow, Russia Kyiv Taras Shevchenko National University, Kyiv, Ukraine 3 Moscow State University, Vorobjovy Gory, Moscow, Russia 4 GKSS Research Centre, Geesthacht, Germany 5 Russian Research Center Kurchatov Institute, Moscow, Russia

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UV-Vis spectroscopy and small-angle neutron scattering experiments are performed on the cluster solution of fullerene C60 in N-methyl-2-pyrrolidone before and after dilution of the system with pure solvent. Some changes in the UV-Vis spectra showing solvatochromism at dilution are observed, while the neutron scattering signal does not change. The effect is discussed with respect to the relation between solvatochromism and cluster formation for fullerene solutions in nitrogen-containing solvents. Keywords Fullerene clusters, Fullerene solutions, Small-angle neutron scattering, Solvatochromism, UV-Vis spectroscopy

Introduction The solutions of fullerene C60 in nitrogen-containing solvents (pyridine, N-methyl-2pyrrolidone (NMP), benzonitrile and acetonitrile) are characterized (1–5) by the time solvatochromism, which is the time evolution of photoluminescence, Raman, and UV-Vis spectra. Often, this effect is accompanied by the formation of fullerene clusters (up to 500 nm) in the solutions (1–3,6), so one can assume (3) that is the reason the solvatochromism is directly determined by the cluster growth. This implies that new bonds between fullerene molecules in the clusters appear, which changes the mentioned kinds of spectra. On the other hand, the solvatochromism can be related to the formation of some complexes between fullerene and solvent molecules, such as donor-acceptor complexes between fullerene and nitrogen in the solvent (NMP) molecules (7). In the given work, we consider these two possibilities from the viewpoint of the cluster reorganization after dissolution of the system. In particular, such reorganization is observed (8–11) for C60/NMP solutions (above 40% of water in the final mixture) as a comparatively sharp increase in the small-angle neutron scattering (SANS) over the interval of momentum transfer of 0.1–0.5 nm-1. This points out that the large clusters destroy at some extent, and clusters with characteristic sizes between 10 and 100 nm appear. It is interesting that some sharp solvatochromism takes place at the water dilution as well (9). It was shown (10) that water addition leads to the decomposition of Address correspondence to O. A. Kyzyma, Joint Institute for Nuclear Research, Joliot-Curie 6, Dubna, Moscow reg., 141980, Russia. E-mail: [email protected]

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fullerene clusters in the solution, which is a result of the detachment of monomers from the clusters. Here, we repeat the dissolution of the cluster solution C60/NMP using pure NMP. This also results in some changes of UV-Vis spectra, while the SANS signal, which is reflecting the formation of the new clusters of the previous case, does not increase.

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Experiment Fullerene (Fullerenovye Tekhnologii, purity > 99.5%) was dissolved in NMP (Merck, purity > 99.5%) to obtain the C60/NMP system. Then the solution was stirred for one hour at room temperature. UV-Vis absorption spectra were obtained using Helios UV-Visible Spectrophotometer (wavelength range of 200–1000 nm, quartz cells with 2 mm light pass). The spectra were obtained right after the solution preparation, several days after preparation and one month later. The new solution (several days after preparation) was dissolved with pure NMP. The volume fraction of added NMP was varied from 20 to 70vol.% with respect to the final solutions. SANS experiments were carried out on the SANS-1 instrument at the FRG-1 steady-state reactor of the GKSS Research Center, Geesthacht, Germany. The scattering intensity (differential cross-section per sample volume) was obtained according to the standard procedure (e.g., 12) on the solutions in quartz plane cells with 1 mm neutron flight pass. The small scattering signal above the background allowed us to estimate only the integral scattering intensity over the q-interval of 0.1-1 nm-1.

Results and Discussions The time smoothing of the characteristic peak of fullerene C60 at 330 nm wave length in the UV-Vis spectrum of C60/NMP (Figure 1) repeats the solvatochromic changes reported earlier (8,10). The level of the SANS intensity from the old solution is extremely low (about 0.01 cm-1). It is the same as was observed previously (8) and corresponds to quite large (size more than 150 nm) fullerene clusters. The dissolution of C60/NMP system with pure NMP results in solvatochromism (Figure 2), which is similar to that observed after the

Figure 1. Change in the UV-Vis spectrum of the C60/NMP solution with time: solid line corresponds to fresh C60/NMP and dashed line is the solution one month later after preparation.

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Figure 2. Absorption spectra of the C60/NMP system (several days after preparation) before (solid line) and after (dashed line) dissolution with pure solvent (NMP). The volume fraction of added NMP is 30vol.% with respect to the final solutions. Absorption spectra are normalized to the fullerene C60 concentration.

system is dissolved with water (9). One can see (Figure 2) that the characteristic peak is almost smoothed for C60/NMP solution for several days later after preparation. It is interesting that, as follows from Figure 2, even slight dissolution (about 30%) of the above-mentioned C60/NMP system leads to a solvatochromic effect (growth of the characteristic peak of fullerene C60 at 330 nm wave length in the UV-Vis spectrum). In contrast to the dissolution with water, in the given case the SANS intensity does not show any increase for all dissolution rates. It decreases in according with the dissolution rate, thus proving that the detected signal is the tale of the scattering curve from large fullerene clusters. This means that fullerene clusters do not change their structure after the dissolution of the system with NMP. Thus, we observe the solvatochromism without any reorganization of the clusters. This experiment proves that the reason of the solvatochromic effects in the system C60/NMP does not relate to the cluster formation but rather to formation of some complexes between fullerene and solvent molecules (e.g., donor-acceptor complexes). The addition of water and NMP changes the solvent organization at the interface with the cluster surface, which varied in time after the preparation of the solution. Despite the similarity of the effects for the two solvents, there are some significant differences. It seems that the effect of water is much stronger. Water molecules destroy the donor-acceptor bonds NMP-C60 and form their own complexes. It is confirmed by the two new bands in the UV-Vis spectra at 440 and 660 nm (3), which are specific for the interaction of water and fullerene in aqueous fullerene dispersions (13). Also, the detachment of single fullerene molecules from the clusters (10) after the addition of water suggests the formation of such complexes whose chemical potential is smaller in the solution as compared to pure fullerene molecules showing the tendency toward aggregation after they are placed in NMP.

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