- Email: [email protected]
Solvent-induced optical properties of C60
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 787–791
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Solvent-induced optical properties of C60 Fang Zhang, Xiaoling Zhang n Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
a r t i c l e in fo
abstract
Article history: Received 30 September 2008 Received in revised form 7 October 2009 Accepted 30 November 2009 Available online 6 December 2009
Different C60 aggregates, i.e. nanoparticles, clusters of nanoparticles and microcrystals in roomtemperature solutions, are reported to account for the colors of fluorescence emissions centered at 440, 575 and 700 nm, respectively. And the configurations of C60 aggregation created in solutions are revealed to be closely associated with the characteristic interactions between C60 and solvent molecules. On this basis, aggregation behaviors and thus induced optical properties of C60 have been tentatively controlled through adopting solvent mixtures. & 2009 Elsevier B.V. All rights reserved.
Keywords: C60 Nanoparticles Microcrystals Fluorescence In-situ TEM Confocal fluorescence spectroscopy
1. Introduction Since the great application potential in areas such as optics, electrics, materials science, biomedicine, etc., photoluminescence properties of C60 have been extensively investigated. Although it is difficult to observe fluorescence from isolate C60 due to its high symmetry of Ih [1–3] the group degradation caused by certain ambient surroundings or lowing temperature has been proved to be efficient in exciting high-quality C60 fluorescence. Up to now, a great number of low-temperature-induced resolvable fluorescence spectra have been obtained from solid C60 [4–9]; however, room-temperature fluorescence spectra of C60 in common solvents or on various substrates are still structureless and limited in a wavelength region between 600 and 850 nm [10,11]. In contrast, a series of strong fluorescence bands of C60, centered at 440, 575 and 700 nm, respectively, were achieved from C60-pyridine solution by some of us [12]. Especially, different C60 aggregates produced in pyridine have been assumed to be related with the three distinctive fluorescence bands in our latest work [13]. This is of great interest for a variety of reasons, for example, the nature interaction between C60 and solvent, the inevitable correlation of C60 nano-aggregates with fluorescence emissions. In this work, steady-state absorption, fluorescence spectroscopy, in-situ transmission electron microscopy (TEM) and confocal fluorescence spectroscopy investigations on C60 in
n
Corresponding author. Tel.: + 86 010 88875298. E-mail address: [email protected] (X.L. Zhang).
0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.11.033
toluene and acetonitrile were carried out with the aim to ascertain the assignment of three distinctive fluorescence emission bands of C60 to different solvent-induced aggregates (i.e., nanoparticles, clusters of nanoparticles and microcrystals). Additionally, characteristic interactions of C60 with different solvent molecules are discussed, and optical properties of thus induced C60 aggregations are tentatively controlled through further experiments.
2. Experimental section C60 (99.9%) was ultrasonically dissolved in carbon disulfide, toluene, acetonitrile, and pyridine (HPLC grade). UV–visible absorption spectra were measured on UV-2401PC spectrophotometer (SHIMADZU). Steady-state fluorescence spectra were recorded by Fluorolog-3 spectrometer (JY), excited at 400 nm, with the excitation and emission slit widths being 5 nm. Corresponding C60 solutions were doped on ultrathin carbon films and dried before being taken as TEM samples. TEM images were obtained through H-600TEM (HiTachi), and the magnification was in the range from 5 103 to 8 104. Coating C60 solutions on glass substrates, the specimens were examined with a WITec CMR200 in the confocal Raman spectra mode. A picosecond pulse laser (405 nm, 135 mW, 20 MHz, PIL040F optical head, Advanced Laser Diode Systems, Germany) was used for excitation. By using an edge filter (from the CMR200 442 nm Raman filter slider assembly), only wavelength above 442 nm was detected. The confocal fluorescence spectra were measured with a SP2300i spectrograph (Acton Research Corp.) coupled with a
ARTICLE IN PRESS 788
F. Zhang, X.L. Zhang / Journal of Luminescence 130 (2010) 787–791
thermoelectrically cooled CCD Camera (DV401-BV-351, Andor Technology). Current–voltage curves were recorded by employing a CHI660A electrochemical working station. Three-electrode configuration was used, with a silver wire as reference electrode, platinum as counter and working electrodes. All measurements were taken at room temperature.
3. Results and discussion Fig. 1 shows UV–visible absorption spectra of C60 in different solvents. As is shown, C60 in carbon disulfide (Fig. 1a) and toluene (Fig. 1b) displays a similar sharp absorption peak at 407 nm, together with a broad absorption band in the range of 430– 650 nm attributed to the forbidden transition of t1u’hu commonly [14]. Comparatively, as for C60 in pyridine and acetonitrile, the characteristic peak at 407 nm decays relatively (see Fig. 1c) and finally turns to be unobservable (see Fig. 1d), with the absorbance in 420–590 nm region raising up and becoming a brand-new absorption band evidently (see Fig. 1d). Taking these absorption differences into account, diverse characteristic interaction patterns between C60 and solvents may be indicated to cause symmetry degradation of C60 and evoke various unique optical properties to some extent. And this presumption can be implied as well by the visual color variation of C60 solutions, given in the inset in Fig. 1. To analytically demonstrate the interactions of C60 with different organic solvents and thus induced distinctive optical properties, steady-state fluorescence, in-situ TEM and confocal fluorescence investigations on C60 in toluene and acetonitrile were carried out at room temperature. Fig. 2A (a) shows a fluorescence spectrum of 2.0 mg/ml C60 in toluene. Two strong fluorescence bands can be observed around 440 and 700 nm, besides, it is noteworthy that their relative intensities turn out to be insensitive to the concentration of C60 in toluene. After 0.5 mg/ ml C60/toluene solution was filtered through 0.2 mm hydrophobic PTFE filter (Nihon Millipore Ltd., Japan), a fluorescence spectrum of the filtrate was recorded (see Fig. 2A (b)). Surprisingly, only a strong purple-blue band centered at 440 nm can be detected, with a large amount of C60 particles dispersing in solution (see the in-
Fig. 1. UV–visible absorption spectra of C60 (2 10 5 M) in different solvents: (a) toluene, (b) carbon disulfide, (c) pyridine and (d) acetonitrile. The inset shows the corresponding visual images of C60 in different solvents: (A) toluene, (B) carbon disulfide, (C) pyridine and (D) acetonitrile.
situ TEM image 1 in Fig. 2A). This evidence provides a visual proof for the analysis on the solubility of C60 that C60 molecule in solution is not singly isolated but forms a finite cluster [15], and also signifies that the purple-blue fluorescence band at 440 nm may probably arise from C60 nanoparticles in solution. As for the C60 nanoparticle, we conjecture that its formation in solution may associate more strongly with the characters of C60 molecules. Since the molecular volume of C60 is up to five times that of solvent, the dissolution of such a large molecule may severely disrupt the local solvent structure and thus offer more opportunities for C60 to interact with each other. On the other hand, the electronegative nature of C60, acting via the abnormally large exterior p lobe of each carbon [16], may also stimulate the assembly of C60 molecules through the overlap of p orbit, since sharing electrons with congeneric elements is much easier than with solvent. Hence, in response to a lower cohesive energy (1.6 eV) per C60 in face-center-cubic solid [17] than charge transfer band (1.98 eV) for solid complex [18], the generation of C60 nanoparticles in toluene would be more reasonably ascribed to its own catachrestic geometry and electronic structure. Subsequently, to explicate the origin of fluorescence emission band centered at 700 nm, a fluorescence spectrum with considerable difference was obtained by increasing the concentration of C60/toluene to 3.0 mg/ml (see Fig. 2A (c)). Compared with Fig. 2A (b), a dramatic shift of the main fluorescence emission from 440 to 700 nm occurs, with four resolvable fluorescence peaks emerging at 692, 712, 730, and 764 nm particularly. To our surprise, these separated lines agree with those obtained from crystalline C60 at 1.2 K [9], especially, the fluorescence peak at 730 nm corresponds very well with the line interpreted as fluorescence from C60 molecules located in a perfect crystal environment [19]. Therefore, bulk C60 in supersaturate solution would be estimated to arouse the fluorescence emission centered at 700 nm. To establish this assumption, an in-situ TEM observation was taken on this solution, and amounts of C60 microcrystals can be discovered as the dominant component (shown in the TEM image 2 in Fig. 2A). As is well reported, the interaction between C60 molecules in room-temperature solid film is not so strong to induce structured fluorescence emission [17]. While weak electron–electron repulsion in delocalized system, viz., the distortion of molecule in solid matrix, would create C60 in microcrystals as a well-resolved fluorescence emission center [20]. As far as the main fluorescence emission around 700 nm emitted from this solution is concerned, it is not difficult to draw a conclusion that the broad salmon pink fluorescence band may probably originate from C60 microcrystals precipitated from solution, as is to be discussed further afterwards. Additionally, to give more evidence, corresponding C60/toluene solutions with different concentrations were doped on glass substrates and dried at room temperature. Confocal fluorescence spectroscopy investigations were taken on these samples. When the detection light (405 nm) is focused on the C60 nanoparticles (see Fig. 2B for the bright-field image of nanoparticles), only a purple-blue light around 440 nm is observed (see the inset in Fig. 2B); however, when the focus surface is played down to microcrystals (see Fig. 2C for the bright-field image of microcrystals), the relative intensity of fluorescence emission at 700 nm is enhanced rapidly (see the inset in Fig. 2C). This result ascertains the assignments of C60 fluorescence emission bands around 440 and 700 nm, respectively, to C60 nanoparticles and microcrystals in dispersion. Also, some identical conclusions can be obtained from the C60/CS2 system, which is consistent with the similar ground state absorptions of C60 in toluene and carbon disulfide (see Figs. 1a and b). According to Marcus et al. [21], electron-pair donation ability of solvent may considerably influence the solubility of C60 in
ARTICLE IN PRESS F. Zhang, X.L. Zhang / Journal of Luminescence 130 (2010) 787–791
789
Fig. 2. (A) Fluorescence spectrum of 2.0 mg/ml C60 in toluene at room temperature, excited at 400 nm. The inset in (A) shows fluorescence spectra and TEM images of C60 in toluene with different concentrations: (b) (1) 0.5 mg/ml, filtered through 0.2 mm hydrophobic PTFE filter; (c) (2) 3.0 mg/ml. Confocal fluorescence spectra of different C60 aggregates: (B) C60 nanoparticles; (C) C60 microcrystals, excited at 405 nm.
Fig. 3. (A) Fluorescence spectrum of 1.0 mg/ml C60 in acetonitrile at room temperature, excited at 400 nm. The inset in (A) shows fluorescence spectra and TEM images of C60 in solution settled for 7 days: (b) (1) the up layer solution; (c) (2) the low layer solution. Confocal fluorescence spectra of C60 aggregates: (B) clusters of C60 nanoparticles; (C) C60 microblocks, excited at 405 nm.
solution, thus characteristic interaction between C60 and electronpair donor would provoke some unique optical properties. Based on this supposition, acetonitrile was selected as solvent, and a fluorescence spectrum of 1.0 mg/ml C60/acetonitrile solution settled for 7 h was obtained (see Fig. 3A (a)). Differing markedly from the situation given in Fig. 2A (a), the former blue fluorescence band centered at 440 nm decays to be almost unobservable; the wide salmon pink fluorescence band in
700 nm region appears to be more informative, splitting into six peaks at 668, 685, 692, 712, 730, 764 nm respectively; in particular, an extra yellow-green fluorescence band emerges around 575 nm distinctly. In order to interpret these solvent effects on the optical properties of C60, 1.0 mg/ml C60/acetonitrile was settled for 7 days and finally separated into two layers (i.e., the upper brownish-yellow solution and the lower brownish-black opaque solution), then corresponding
ARTICLE IN PRESS 790
F. Zhang, X.L. Zhang / Journal of Luminescence 130 (2010) 787–791
fluorescence spectra were recorded, respectively. As is shown (see Fig. 3A (b)), the upper brownish-yellow solution only gives a emission band centered at 575 nm at the presence of a large number of C60 lace-like clusters which are factually composed of C60 nanoparticles within a size range of 20–80 nm (see the in-situ TEM image 1 in Fig. 3A). Considering the similar aggregation cannot be observed through time-dependent studies on C60/toluene and C60/CS2 solutions, thus it is not difficult to assume that the unique yellow-green fluorescence band centered at 575 nm may derive from these lacelike clusters of C60 nanoparticles produced in acetonitrile. Taking Fig. 3A (b) as reference, a dramatic increase of fluorescence band at 700 nm is noted for the lower brownish-black opaque solution (see
Fig. 4. Cyclic voltammetry of C60 in acetonitrile containing 0.1 M (TBA) ClO4 at room temperature.
Fig. 3A (c)) with the existence of more unslovated micron C60 blocks (see the in-situ TEM image 2 in Fig. 3A). This offers a supplemental confirmation for the corresponding conclusion achieved from C60/ toluene system that the fluorescence emission centered at 700 nm would be attributed to either precipitated C60 microcrystals or unslovated C60 microblocks in supersaturated solutions. Furthermore, to gain more confidence in our assignments, the upper and lower layer dispersions were subsequently doped on glass substrates, dried at room temperature, and confocal fluorescence spectroscopy investigations were taken on these samples. As is shown, a yellow-green emission around 575 nm is relatively enhanced (see the inset in Fig. 3B) when the detection light is focused on lace-like C60 nanoparticle clusters (see Fig. 3B); and the fluorescence emission at 700 nm increases suddenly (see the inset in Fig. 3C) when the focus surface of detection light is played down to C60 microblocks (see Fig. 3C). These results establish the foregoing assignments for fluorescence emission bands of C60 in acetonitrile. In view of the C60 lace-like cluster which seems similar to that created in pyridine [13], we suppose, besides the contribution of solvent polarity [22,23], its generation would be more related to the electron-pair donation ability of acetonitrile, with which little charge transfer would occur between acetonitrile and C60. To exactly confirm this strong interaction, cyclic voltammograms of C60 in acetonitrile were recorded at room temperature. Fig. 4 depicts a complete set of two waves obtained at a scan rate of 100 mV/s. The peak in the negative-direction scan in each step and the corresponding one in the positive-direction scan are nearly separated by about 0.3 V. According to Tetsuo Saji et al. C60 dissolved in DMF–toluene (2:3) could be reduced in six successive 88 1C. one-electron transfer steps to generate Cn60 (n=1–6) at [24] Thus, some strong interaction with little charge transfer (CT) may indeed occur between C60 and acetonitrile. Concerning the corresponding cyclic voltammograms of C60 in toluene cannot be acquired under the same conditions, it can be further ascertained that the formation of lace-like clusters of C60 nanoparticles and thus induced fluorescence emission around 575 nm could be both attributed to this characteristic interaction of C60 with acetonitrile.
Fig. 5. (A) Fluorescence spectra of 0.5 mg/ml C60 in room-temperature toluene/acetonitrile (volume ratios: (a) 10:1; (b) 5:1; (c) 1:1; and (d) 1:5), excited at 400 nm. The right inset in (A) shows the corresponding ground state absorption spectra of C60 in toluene/acetonitrile ( volume ratios: (a) 10:1; (b) 5:1; (c) 1:1; and (d) 1:5), and the left inset in (A) shows the TEM image of C60 in toluene/acetonitrile (volume ratio = 5:1). (B), (C) Confocal fluorescence image and spectrum of C60 aggregates formed in acetonitrile/(2.0 mg/ml C60/toluene), volume ratio = 1:5, excited at 405 nm.
ARTICLE IN PRESS F. Zhang, X.L. Zhang / Journal of Luminescence 130 (2010) 787–791
As demonstrated above, it can be clarified that nanoaggregates in dispersion definitely contribute the colors of C60 fluorescence emissions from blue to red, and the characteristic interaction between C60 and solvent would account for the aggregation behaviors of C60 in certain solution. To give further evidence directly for such solvent-induced optical properties of C60, acetonitrile was ultrasonically mixed into toluene, then fluorescence and absorption spectra of 0.5 mg/ml C60 were recorded in toluene/acetonitrile mixtures with different volume ratios at room temperature. As we can see in Fig. 5A, with the increasing volume ratio of acetonitrile, a yellow-green band at 570 nm comes into sight and rises gradually at expense of the strong purple-blue band at 440 nm; this corresponds well to that a broad absorption band turns out in 440–480 nm range with a decrease and final extinguishment of the narrow absorption peak at 407 nm (see the right inset in Fig. 5A), which has been also observed by other works [25–30]; simultaneously, compared with the C60/toluene dispersion (displayed in Fig. 2B), well-dispersed C60 nanoparticles can be found assembling with each other into clusters during our mixing toluene with acetonitrile (see the TEM image in Fig. 5A). These investigations give a further confirmation to the significantly solvent-influenced aggregations and thus induced optical red shift properties of C60 in solution. Moreover, on controlling this aggregating process of C60 in solution, confocal fluorescence experiments were taken, and three distinctive emission bands from blue to red (see Fig. 5C) have been obtained from C60 aggregates (see Fig. 5B) produced by adding acetonitrile into C60/toluene (2.0 mg/ml) dispersion with a volume ratio of 1:5. This may predict an important application potential for fullerene as unique luminescent nanomaterials with the help of solvents.
4. Conclusion In summary, solvent-induced optical properties of C60 were investigated at room temperature. Three distinctive fluorescence emission bands have been revealed to derive from C60 nanoaggregates produced by the characteristic interactions between C60 and solvents, i.e., C60 nanoparticles can arouse a strong purple-blue fluorescence emission around 440 nm; lace-like C60 clusters would be responsible for a distinctive yellow-green fluorescence band at 575 nm, both precipitated C60 microcrystals and unslovated microblocks in supersaturated solutions are able to provide a series of salmon pink fluorescence peaks centered at 700 nm. Based on these results, aggregations and thus induced optical properties of C60 in solution have been tentatively controlled by adopting solvent mixtures. This work might not
791
only supply an optical detection method for identifying the nanostructure configurations of C60 in solution, but also predict a nano-technology-derived application of the fullerene as luminescent materials.
Acknowledgments The authors are grateful for the support of this research by the National Natural Science Foundation of China and the Natural Science Foundation of Beijing. References ¨ [1] W. Kratschmer, L.D. Lamb, K. Fostiropoulos, D.R. Huffman, Nature 347 (1990) 354. [2] D.S. Bethune, G. Meijer, W.C. Tang, H.J. Rosen, Chem. Phys. Lett. 174 (1990) 219. [3] R.D. Johnson, G. Meijer, D.S. Bethune, J. Am. Chem. Soc. 112 (1990) 8983. [4] J.L. Sauvajol, Z. Hricha, N. Coustel, A. Zahab, J. AznarJ. Phys.: Condens. Matter 5 (1993) 2045. ¨ ¨ [5] J. Feldmann, W. Guss, U. Lemmer, E.O. Goebl, C. Taliani, H. Mohn, W. Muller, P. ¨ Haussler, H.U. Ter meer, Mol. Cryst. Liq. Cryst. 256 (1994) 757. [6] C. Wen, N. Minami, Synth. Met. 86 (1997) 2301. [7] Y. Wang, J. Phys. Chem. 96 (1992) 764. [8] Y. Zeng, L. Biczok, H. Linschitz, J. Phys. Chem. 96 (1992) 5237. [9] D.J. Yan Den Heuvel, G.J.B. V.an Den Berg, E.J.J. G.roenen, J. Schmidt, I. Holleman, G. Meijer, J. Phys. Chem. 99 (1995) 11644. [10] X. Andong, P. Haibin, Zh. Xinyi, H. Yilin, F. Shaojun, S. Junyan, Z. Jian, X. Cunyi, J. Lumin. 63 (1995) 301. [11] Dongho Kim, Minyung Lee, Yung Doug Suh, Seong Keun Kim, J. Am. Chem. Soc. 114 (1992) 4429. [12] Y. Zhao, Y. Fang, J. Phys. Chem. B 108 (2004) 13586. [13] F. Zhang, Y. Fang, J. Phys. Chem. B 110 (2006) 9022. [14] O.H. Kwon, K. Park, D.J. Jang, Chem. Phys. Lett. 346 (2001) 195. [15] K. Kaneto, T. Abe, W. Takashima, Solid State Commun. 96 (1995) 259. [16] R.C. Haddon, Acc. Chem. Res. 25 (1992) 127. [17] S. Saito, A. Oshiyama, Phys. Rev. Lett. 66 (1991) 2637. [18] D.V. Konarev, R.N. Lyubovskaya, N.V. Drichko, V.N. Semkin, A. Graja, Synth. Met. 103 (1999) 2466. ¨ ¨ [19] J. Feldmann, W. Guss, U. Lemmer, E.O. Goebl, C. Taliani, H. Mohn, W. Muller, ¨ P. Haussler, H.U. Ter Meer, Mol. Cryst. Liq. Cryst. 256 (1994) 757. [20] J.W. Arbogast, A.P. Darmanyan, C.S. Foote, Y. Rubin, F.N. Diederich, M.M. Alvarez, S.J. Anz, R.L. Whetten, J. Phys. Chem. 95 (1991) 11. [21] Y. Marcus, A. Smith, M.V. Korobov, A.L. Mirakyan, N.V. Avramenko, E.B. Stukalin, J. Phys. Chem. B 105 (2001) 2499. [22] S. Nath, H. Pal, A.V. Sapre, Chem. Phys. Lett. 327 (2000) 143. [23] S. Nath, H. Pal, A.V. Sapre, Chem. Phys. Lett. 360 (2002) 422. [24] Y. Ohsawa, T. Saji, J. Chem. Soc. Chem. Commun. 10 (1992) 781. [25] Y.-P. Sun, C.E. Bunker, Nature 365 (1993) 398. [26] R.V. Bensasson, E. Bienvenue, M. Dellinger, S. Leach, P. Seta, J. Phys. Chem. 98 (1994) 3492. [27] Q. Ying, J. Marecek, B. Chu, Chem. Phys. Lett. 219 (1994) 214. [28] I. Renge, J. Phys. Chem. 99 (1995) 15955. [29] H.N. Ghosh, A.V. Sapre, J.P. Mittal, J. Phys. Chem. 100 (1996) 9439. [30] S. Nath, H. Pal, D.K. Palit, A.V. Sapre, J.P. Mittal, J. Phys. Chem. B 102 (1998) 10158.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Solvent-induced optical properties of C60 Fang Zhang, Xiaoling Zhang n Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
a r t i c l e in fo
abstract
Article history: Received 30 September 2008 Received in revised form 7 October 2009 Accepted 30 November 2009 Available online 6 December 2009
Different C60 aggregates, i.e. nanoparticles, clusters of nanoparticles and microcrystals in roomtemperature solutions, are reported to account for the colors of fluorescence emissions centered at 440, 575 and 700 nm, respectively. And the configurations of C60 aggregation created in solutions are revealed to be closely associated with the characteristic interactions between C60 and solvent molecules. On this basis, aggregation behaviors and thus induced optical properties of C60 have been tentatively controlled through adopting solvent mixtures. & 2009 Elsevier B.V. All rights reserved.
Keywords: C60 Nanoparticles Microcrystals Fluorescence In-situ TEM Confocal fluorescence spectroscopy
1. Introduction Since the great application potential in areas such as optics, electrics, materials science, biomedicine, etc., photoluminescence properties of C60 have been extensively investigated. Although it is difficult to observe fluorescence from isolate C60 due to its high symmetry of Ih [1–3] the group degradation caused by certain ambient surroundings or lowing temperature has been proved to be efficient in exciting high-quality C60 fluorescence. Up to now, a great number of low-temperature-induced resolvable fluorescence spectra have been obtained from solid C60 [4–9]; however, room-temperature fluorescence spectra of C60 in common solvents or on various substrates are still structureless and limited in a wavelength region between 600 and 850 nm [10,11]. In contrast, a series of strong fluorescence bands of C60, centered at 440, 575 and 700 nm, respectively, were achieved from C60-pyridine solution by some of us [12]. Especially, different C60 aggregates produced in pyridine have been assumed to be related with the three distinctive fluorescence bands in our latest work [13]. This is of great interest for a variety of reasons, for example, the nature interaction between C60 and solvent, the inevitable correlation of C60 nano-aggregates with fluorescence emissions. In this work, steady-state absorption, fluorescence spectroscopy, in-situ transmission electron microscopy (TEM) and confocal fluorescence spectroscopy investigations on C60 in
n
Corresponding author. Tel.: + 86 010 88875298. E-mail address: [email protected] (X.L. Zhang).
0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.11.033
toluene and acetonitrile were carried out with the aim to ascertain the assignment of three distinctive fluorescence emission bands of C60 to different solvent-induced aggregates (i.e., nanoparticles, clusters of nanoparticles and microcrystals). Additionally, characteristic interactions of C60 with different solvent molecules are discussed, and optical properties of thus induced C60 aggregations are tentatively controlled through further experiments.
2. Experimental section C60 (99.9%) was ultrasonically dissolved in carbon disulfide, toluene, acetonitrile, and pyridine (HPLC grade). UV–visible absorption spectra were measured on UV-2401PC spectrophotometer (SHIMADZU). Steady-state fluorescence spectra were recorded by Fluorolog-3 spectrometer (JY), excited at 400 nm, with the excitation and emission slit widths being 5 nm. Corresponding C60 solutions were doped on ultrathin carbon films and dried before being taken as TEM samples. TEM images were obtained through H-600TEM (HiTachi), and the magnification was in the range from 5 103 to 8 104. Coating C60 solutions on glass substrates, the specimens were examined with a WITec CMR200 in the confocal Raman spectra mode. A picosecond pulse laser (405 nm, 135 mW, 20 MHz, PIL040F optical head, Advanced Laser Diode Systems, Germany) was used for excitation. By using an edge filter (from the CMR200 442 nm Raman filter slider assembly), only wavelength above 442 nm was detected. The confocal fluorescence spectra were measured with a SP2300i spectrograph (Acton Research Corp.) coupled with a
ARTICLE IN PRESS 788
F. Zhang, X.L. Zhang / Journal of Luminescence 130 (2010) 787–791
thermoelectrically cooled CCD Camera (DV401-BV-351, Andor Technology). Current–voltage curves were recorded by employing a CHI660A electrochemical working station. Three-electrode configuration was used, with a silver wire as reference electrode, platinum as counter and working electrodes. All measurements were taken at room temperature.
3. Results and discussion Fig. 1 shows UV–visible absorption spectra of C60 in different solvents. As is shown, C60 in carbon disulfide (Fig. 1a) and toluene (Fig. 1b) displays a similar sharp absorption peak at 407 nm, together with a broad absorption band in the range of 430– 650 nm attributed to the forbidden transition of t1u’hu commonly [14]. Comparatively, as for C60 in pyridine and acetonitrile, the characteristic peak at 407 nm decays relatively (see Fig. 1c) and finally turns to be unobservable (see Fig. 1d), with the absorbance in 420–590 nm region raising up and becoming a brand-new absorption band evidently (see Fig. 1d). Taking these absorption differences into account, diverse characteristic interaction patterns between C60 and solvents may be indicated to cause symmetry degradation of C60 and evoke various unique optical properties to some extent. And this presumption can be implied as well by the visual color variation of C60 solutions, given in the inset in Fig. 1. To analytically demonstrate the interactions of C60 with different organic solvents and thus induced distinctive optical properties, steady-state fluorescence, in-situ TEM and confocal fluorescence investigations on C60 in toluene and acetonitrile were carried out at room temperature. Fig. 2A (a) shows a fluorescence spectrum of 2.0 mg/ml C60 in toluene. Two strong fluorescence bands can be observed around 440 and 700 nm, besides, it is noteworthy that their relative intensities turn out to be insensitive to the concentration of C60 in toluene. After 0.5 mg/ ml C60/toluene solution was filtered through 0.2 mm hydrophobic PTFE filter (Nihon Millipore Ltd., Japan), a fluorescence spectrum of the filtrate was recorded (see Fig. 2A (b)). Surprisingly, only a strong purple-blue band centered at 440 nm can be detected, with a large amount of C60 particles dispersing in solution (see the in-
Fig. 1. UV–visible absorption spectra of C60 (2 10 5 M) in different solvents: (a) toluene, (b) carbon disulfide, (c) pyridine and (d) acetonitrile. The inset shows the corresponding visual images of C60 in different solvents: (A) toluene, (B) carbon disulfide, (C) pyridine and (D) acetonitrile.
situ TEM image 1 in Fig. 2A). This evidence provides a visual proof for the analysis on the solubility of C60 that C60 molecule in solution is not singly isolated but forms a finite cluster [15], and also signifies that the purple-blue fluorescence band at 440 nm may probably arise from C60 nanoparticles in solution. As for the C60 nanoparticle, we conjecture that its formation in solution may associate more strongly with the characters of C60 molecules. Since the molecular volume of C60 is up to five times that of solvent, the dissolution of such a large molecule may severely disrupt the local solvent structure and thus offer more opportunities for C60 to interact with each other. On the other hand, the electronegative nature of C60, acting via the abnormally large exterior p lobe of each carbon [16], may also stimulate the assembly of C60 molecules through the overlap of p orbit, since sharing electrons with congeneric elements is much easier than with solvent. Hence, in response to a lower cohesive energy (1.6 eV) per C60 in face-center-cubic solid [17] than charge transfer band (1.98 eV) for solid complex [18], the generation of C60 nanoparticles in toluene would be more reasonably ascribed to its own catachrestic geometry and electronic structure. Subsequently, to explicate the origin of fluorescence emission band centered at 700 nm, a fluorescence spectrum with considerable difference was obtained by increasing the concentration of C60/toluene to 3.0 mg/ml (see Fig. 2A (c)). Compared with Fig. 2A (b), a dramatic shift of the main fluorescence emission from 440 to 700 nm occurs, with four resolvable fluorescence peaks emerging at 692, 712, 730, and 764 nm particularly. To our surprise, these separated lines agree with those obtained from crystalline C60 at 1.2 K [9], especially, the fluorescence peak at 730 nm corresponds very well with the line interpreted as fluorescence from C60 molecules located in a perfect crystal environment [19]. Therefore, bulk C60 in supersaturate solution would be estimated to arouse the fluorescence emission centered at 700 nm. To establish this assumption, an in-situ TEM observation was taken on this solution, and amounts of C60 microcrystals can be discovered as the dominant component (shown in the TEM image 2 in Fig. 2A). As is well reported, the interaction between C60 molecules in room-temperature solid film is not so strong to induce structured fluorescence emission [17]. While weak electron–electron repulsion in delocalized system, viz., the distortion of molecule in solid matrix, would create C60 in microcrystals as a well-resolved fluorescence emission center [20]. As far as the main fluorescence emission around 700 nm emitted from this solution is concerned, it is not difficult to draw a conclusion that the broad salmon pink fluorescence band may probably originate from C60 microcrystals precipitated from solution, as is to be discussed further afterwards. Additionally, to give more evidence, corresponding C60/toluene solutions with different concentrations were doped on glass substrates and dried at room temperature. Confocal fluorescence spectroscopy investigations were taken on these samples. When the detection light (405 nm) is focused on the C60 nanoparticles (see Fig. 2B for the bright-field image of nanoparticles), only a purple-blue light around 440 nm is observed (see the inset in Fig. 2B); however, when the focus surface is played down to microcrystals (see Fig. 2C for the bright-field image of microcrystals), the relative intensity of fluorescence emission at 700 nm is enhanced rapidly (see the inset in Fig. 2C). This result ascertains the assignments of C60 fluorescence emission bands around 440 and 700 nm, respectively, to C60 nanoparticles and microcrystals in dispersion. Also, some identical conclusions can be obtained from the C60/CS2 system, which is consistent with the similar ground state absorptions of C60 in toluene and carbon disulfide (see Figs. 1a and b). According to Marcus et al. [21], electron-pair donation ability of solvent may considerably influence the solubility of C60 in
ARTICLE IN PRESS F. Zhang, X.L. Zhang / Journal of Luminescence 130 (2010) 787–791
789
Fig. 2. (A) Fluorescence spectrum of 2.0 mg/ml C60 in toluene at room temperature, excited at 400 nm. The inset in (A) shows fluorescence spectra and TEM images of C60 in toluene with different concentrations: (b) (1) 0.5 mg/ml, filtered through 0.2 mm hydrophobic PTFE filter; (c) (2) 3.0 mg/ml. Confocal fluorescence spectra of different C60 aggregates: (B) C60 nanoparticles; (C) C60 microcrystals, excited at 405 nm.
Fig. 3. (A) Fluorescence spectrum of 1.0 mg/ml C60 in acetonitrile at room temperature, excited at 400 nm. The inset in (A) shows fluorescence spectra and TEM images of C60 in solution settled for 7 days: (b) (1) the up layer solution; (c) (2) the low layer solution. Confocal fluorescence spectra of C60 aggregates: (B) clusters of C60 nanoparticles; (C) C60 microblocks, excited at 405 nm.
solution, thus characteristic interaction between C60 and electronpair donor would provoke some unique optical properties. Based on this supposition, acetonitrile was selected as solvent, and a fluorescence spectrum of 1.0 mg/ml C60/acetonitrile solution settled for 7 h was obtained (see Fig. 3A (a)). Differing markedly from the situation given in Fig. 2A (a), the former blue fluorescence band centered at 440 nm decays to be almost unobservable; the wide salmon pink fluorescence band in
700 nm region appears to be more informative, splitting into six peaks at 668, 685, 692, 712, 730, 764 nm respectively; in particular, an extra yellow-green fluorescence band emerges around 575 nm distinctly. In order to interpret these solvent effects on the optical properties of C60, 1.0 mg/ml C60/acetonitrile was settled for 7 days and finally separated into two layers (i.e., the upper brownish-yellow solution and the lower brownish-black opaque solution), then corresponding
ARTICLE IN PRESS 790
F. Zhang, X.L. Zhang / Journal of Luminescence 130 (2010) 787–791
fluorescence spectra were recorded, respectively. As is shown (see Fig. 3A (b)), the upper brownish-yellow solution only gives a emission band centered at 575 nm at the presence of a large number of C60 lace-like clusters which are factually composed of C60 nanoparticles within a size range of 20–80 nm (see the in-situ TEM image 1 in Fig. 3A). Considering the similar aggregation cannot be observed through time-dependent studies on C60/toluene and C60/CS2 solutions, thus it is not difficult to assume that the unique yellow-green fluorescence band centered at 575 nm may derive from these lacelike clusters of C60 nanoparticles produced in acetonitrile. Taking Fig. 3A (b) as reference, a dramatic increase of fluorescence band at 700 nm is noted for the lower brownish-black opaque solution (see
Fig. 4. Cyclic voltammetry of C60 in acetonitrile containing 0.1 M (TBA) ClO4 at room temperature.
Fig. 3A (c)) with the existence of more unslovated micron C60 blocks (see the in-situ TEM image 2 in Fig. 3A). This offers a supplemental confirmation for the corresponding conclusion achieved from C60/ toluene system that the fluorescence emission centered at 700 nm would be attributed to either precipitated C60 microcrystals or unslovated C60 microblocks in supersaturated solutions. Furthermore, to gain more confidence in our assignments, the upper and lower layer dispersions were subsequently doped on glass substrates, dried at room temperature, and confocal fluorescence spectroscopy investigations were taken on these samples. As is shown, a yellow-green emission around 575 nm is relatively enhanced (see the inset in Fig. 3B) when the detection light is focused on lace-like C60 nanoparticle clusters (see Fig. 3B); and the fluorescence emission at 700 nm increases suddenly (see the inset in Fig. 3C) when the focus surface of detection light is played down to C60 microblocks (see Fig. 3C). These results establish the foregoing assignments for fluorescence emission bands of C60 in acetonitrile. In view of the C60 lace-like cluster which seems similar to that created in pyridine [13], we suppose, besides the contribution of solvent polarity [22,23], its generation would be more related to the electron-pair donation ability of acetonitrile, with which little charge transfer would occur between acetonitrile and C60. To exactly confirm this strong interaction, cyclic voltammograms of C60 in acetonitrile were recorded at room temperature. Fig. 4 depicts a complete set of two waves obtained at a scan rate of 100 mV/s. The peak in the negative-direction scan in each step and the corresponding one in the positive-direction scan are nearly separated by about 0.3 V. According to Tetsuo Saji et al. C60 dissolved in DMF–toluene (2:3) could be reduced in six successive 88 1C. one-electron transfer steps to generate Cn60 (n=1–6) at [24] Thus, some strong interaction with little charge transfer (CT) may indeed occur between C60 and acetonitrile. Concerning the corresponding cyclic voltammograms of C60 in toluene cannot be acquired under the same conditions, it can be further ascertained that the formation of lace-like clusters of C60 nanoparticles and thus induced fluorescence emission around 575 nm could be both attributed to this characteristic interaction of C60 with acetonitrile.
Fig. 5. (A) Fluorescence spectra of 0.5 mg/ml C60 in room-temperature toluene/acetonitrile (volume ratios: (a) 10:1; (b) 5:1; (c) 1:1; and (d) 1:5), excited at 400 nm. The right inset in (A) shows the corresponding ground state absorption spectra of C60 in toluene/acetonitrile ( volume ratios: (a) 10:1; (b) 5:1; (c) 1:1; and (d) 1:5), and the left inset in (A) shows the TEM image of C60 in toluene/acetonitrile (volume ratio = 5:1). (B), (C) Confocal fluorescence image and spectrum of C60 aggregates formed in acetonitrile/(2.0 mg/ml C60/toluene), volume ratio = 1:5, excited at 405 nm.
ARTICLE IN PRESS F. Zhang, X.L. Zhang / Journal of Luminescence 130 (2010) 787–791
As demonstrated above, it can be clarified that nanoaggregates in dispersion definitely contribute the colors of C60 fluorescence emissions from blue to red, and the characteristic interaction between C60 and solvent would account for the aggregation behaviors of C60 in certain solution. To give further evidence directly for such solvent-induced optical properties of C60, acetonitrile was ultrasonically mixed into toluene, then fluorescence and absorption spectra of 0.5 mg/ml C60 were recorded in toluene/acetonitrile mixtures with different volume ratios at room temperature. As we can see in Fig. 5A, with the increasing volume ratio of acetonitrile, a yellow-green band at 570 nm comes into sight and rises gradually at expense of the strong purple-blue band at 440 nm; this corresponds well to that a broad absorption band turns out in 440–480 nm range with a decrease and final extinguishment of the narrow absorption peak at 407 nm (see the right inset in Fig. 5A), which has been also observed by other works [25–30]; simultaneously, compared with the C60/toluene dispersion (displayed in Fig. 2B), well-dispersed C60 nanoparticles can be found assembling with each other into clusters during our mixing toluene with acetonitrile (see the TEM image in Fig. 5A). These investigations give a further confirmation to the significantly solvent-influenced aggregations and thus induced optical red shift properties of C60 in solution. Moreover, on controlling this aggregating process of C60 in solution, confocal fluorescence experiments were taken, and three distinctive emission bands from blue to red (see Fig. 5C) have been obtained from C60 aggregates (see Fig. 5B) produced by adding acetonitrile into C60/toluene (2.0 mg/ml) dispersion with a volume ratio of 1:5. This may predict an important application potential for fullerene as unique luminescent nanomaterials with the help of solvents.
4. Conclusion In summary, solvent-induced optical properties of C60 were investigated at room temperature. Three distinctive fluorescence emission bands have been revealed to derive from C60 nanoaggregates produced by the characteristic interactions between C60 and solvents, i.e., C60 nanoparticles can arouse a strong purple-blue fluorescence emission around 440 nm; lace-like C60 clusters would be responsible for a distinctive yellow-green fluorescence band at 575 nm, both precipitated C60 microcrystals and unslovated microblocks in supersaturated solutions are able to provide a series of salmon pink fluorescence peaks centered at 700 nm. Based on these results, aggregations and thus induced optical properties of C60 in solution have been tentatively controlled by adopting solvent mixtures. This work might not
791
only supply an optical detection method for identifying the nanostructure configurations of C60 in solution, but also predict a nano-technology-derived application of the fullerene as luminescent materials.
Acknowledgments The authors are grateful for the support of this research by the National Natural Science Foundation of China and the Natural Science Foundation of Beijing. References ¨ [1] W. Kratschmer, L.D. Lamb, K. Fostiropoulos, D.R. Huffman, Nature 347 (1990) 354. [2] D.S. Bethune, G. Meijer, W.C. Tang, H.J. Rosen, Chem. Phys. Lett. 174 (1990) 219. [3] R.D. Johnson, G. Meijer, D.S. Bethune, J. Am. Chem. Soc. 112 (1990) 8983. [4] J.L. Sauvajol, Z. Hricha, N. Coustel, A. Zahab, J. AznarJ. Phys.: Condens. Matter 5 (1993) 2045. ¨ ¨ [5] J. Feldmann, W. Guss, U. Lemmer, E.O. Goebl, C. Taliani, H. Mohn, W. Muller, P. ¨ Haussler, H.U. Ter meer, Mol. Cryst. Liq. Cryst. 256 (1994) 757. [6] C. Wen, N. Minami, Synth. Met. 86 (1997) 2301. [7] Y. Wang, J. Phys. Chem. 96 (1992) 764. [8] Y. Zeng, L. Biczok, H. Linschitz, J. Phys. Chem. 96 (1992) 5237. [9] D.J. Yan Den Heuvel, G.J.B. V.an Den Berg, E.J.J. G.roenen, J. Schmidt, I. Holleman, G. Meijer, J. Phys. Chem. 99 (1995) 11644. [10] X. Andong, P. Haibin, Zh. Xinyi, H. Yilin, F. Shaojun, S. Junyan, Z. Jian, X. Cunyi, J. Lumin. 63 (1995) 301. [11] Dongho Kim, Minyung Lee, Yung Doug Suh, Seong Keun Kim, J. Am. Chem. Soc. 114 (1992) 4429. [12] Y. Zhao, Y. Fang, J. Phys. Chem. B 108 (2004) 13586. [13] F. Zhang, Y. Fang, J. Phys. Chem. B 110 (2006) 9022. [14] O.H. Kwon, K. Park, D.J. Jang, Chem. Phys. Lett. 346 (2001) 195. [15] K. Kaneto, T. Abe, W. Takashima, Solid State Commun. 96 (1995) 259. [16] R.C. Haddon, Acc. Chem. Res. 25 (1992) 127. [17] S. Saito, A. Oshiyama, Phys. Rev. Lett. 66 (1991) 2637. [18] D.V. Konarev, R.N. Lyubovskaya, N.V. Drichko, V.N. Semkin, A. Graja, Synth. Met. 103 (1999) 2466. ¨ ¨ [19] J. Feldmann, W. Guss, U. Lemmer, E.O. Goebl, C. Taliani, H. Mohn, W. Muller, ¨ P. Haussler, H.U. Ter Meer, Mol. Cryst. Liq. Cryst. 256 (1994) 757. [20] J.W. Arbogast, A.P. Darmanyan, C.S. Foote, Y. Rubin, F.N. Diederich, M.M. Alvarez, S.J. Anz, R.L. Whetten, J. Phys. Chem. 95 (1991) 11. [21] Y. Marcus, A. Smith, M.V. Korobov, A.L. Mirakyan, N.V. Avramenko, E.B. Stukalin, J. Phys. Chem. B 105 (2001) 2499. [22] S. Nath, H. Pal, A.V. Sapre, Chem. Phys. Lett. 327 (2000) 143. [23] S. Nath, H. Pal, A.V. Sapre, Chem. Phys. Lett. 360 (2002) 422. [24] Y. Ohsawa, T. Saji, J. Chem. Soc. Chem. Commun. 10 (1992) 781. [25] Y.-P. Sun, C.E. Bunker, Nature 365 (1993) 398. [26] R.V. Bensasson, E. Bienvenue, M. Dellinger, S. Leach, P. Seta, J. Phys. Chem. 98 (1994) 3492. [27] Q. Ying, J. Marecek, B. Chu, Chem. Phys. Lett. 219 (1994) 214. [28] I. Renge, J. Phys. Chem. 99 (1995) 15955. [29] H.N. Ghosh, A.V. Sapre, J.P. Mittal, J. Phys. Chem. 100 (1996) 9439. [30] S. Nath, H. Pal, D.K. Palit, A.V. Sapre, J.P. Mittal, J. Phys. Chem. B 102 (1998) 10158.