NANO LETTERS
Photoluminescence and Electronic Interaction of Anthracene Derivatives Adsorbed on Sidewalls of Single-Walled Carbon Nanotubes
2003 Vol. 3, No. 3 403-407
Jian Zhang,† J.-K. Lee,‡ Yue Wu,*,‡ and Royce W. Murray*,† Kenan Laboratories of Chemistry and Department of Physics and Astronomy, UniVersity of North Carolina, Chapel Hill, North Carolina 27599 Received December 17, 2002; Revised Manuscript Received January 22, 2003
ABSTRACT Dye molecules (anthracene derivatives) are observed to strongly adsorb to single-walled carbon nanotubes (SWNTs). The adsorption coverage of anthracene molecules on SWNTs varied with the aromatic ring substituents. The observed red shifts of emission peaks of the absorptive adduct appear to depend on the energy level of the lowest unoccupied molecular orbital (LUMO) of the adsorbate, consistent with adsorption by a charge-transfer interaction, in which the SWNT is electron donor and anthracene is acceptor. The anthracene absorptive adducts can be displaced by adsorption of pyrene.
Carbon nanotubes have attracted considerable attention since their introduction in the 1990s.1 Single-walled carbon nanotubes (SWNTs) exhibit many interesting physical properties. A challenge is how to combine SWNTs with other chemical entities in order to fabricate nanostructures with unique functions and applications.2 The well-known3 adsorptive interaction between polynuclear aromatic compounds and graphitic surfaces can, for example, be exploited to attach biological substances4 to the sidewalls of SWNTs, as detected by imaging. Such interactions are also expected given the persistent van der Waals interactions between graphene sheets in graphite and SWNTs in nanotube bundles.5 Because polynuclear aromatic compounds are often luminescent, they may also be used as chromophore labels for detecting adsorption and immobilization of chemical or biological molecules onto SWNTs. Because of the sidewall curvature of SWNTs, it is also possible that the adsorption of polynuclear aromatic compounds there may differ from analogous adsorptions on flat graphitic surfaces by via π-stacking.6 In this research, we detect the adsorption of anthracene and several derivatives onto the sidewalls of cut SWNTs, using both absorbance and fluorescence spectra. The anthracene derivatives were substituted by different groups with various electrophilic capability and volume size including (1) anthracene; (2) anthraobin; (3) 9,10-dibromoanthracene; * Corresponding authors. † Kenan Laboratories of Chemistry. ‡ Department of Physics and Astronomy. 10.1021/nl025952c CCC: $25.00 Published on Web 02/12/2003
© 2003 American Chemical Society
Scheme 1. Chemical Structures of Anthracene Derivatives: (1) Anthracene; (2) Anthraobin; (3) 9,10-Dibromoanthracene; (4) 9,10-Anthracenedicarbonitrile; and (5) 9-Anthracene-methanol.
(4) 9,10-anthracenedicarbonitrile; and (5) 9-anthracenemethanol (Scheme 1) in order to investigate the effect of substituents on the electronic interaction of the adsorbate with SWNT. SWNTs are labeled as SWNT-1, etc., according to the adsorbed anthracene derivative. Experimental Section. SWNTs were obtained from Carbon Nanotechnologies, Inc. (Rice University) and incubated in a 3:1 mixture of concentrated sulfuric and nitric acids for 8 h, by which the material is short-cut to segments as short as 3-4 µm, but with many longer ones remaining (Figure 1). The end openings should bear oxidized carbon sites such as carboxylic acids.7 The cut SWNTs are readily dispersed in alcohol and THF but will settle out if left overnight without stirring. The SWNT-anthracene absorp-
Figure 1. Transmission electron micrographs (TEM) of (a) short-cut SWNT and (b) SWNT-1 absorptive adduct.
Figure 2. FT-IR spectra of SWNT, compound 1, and SWNT-1 absorptive adduct.
tive adducts were prepared by stirring a solution of 2 mg cut SWNTs and 5 mg of anthracene derivatives in 10 mL THF at room temperature for 72 h. This is a 1:2 mole ratio of SWNT units to anthracene molecules, counting a SWNT unit as 14 carbon atoms (same as the anthracene structure). The SNWTs were isolated on a Millipore porous filter (FHLC04700, 0.45 µm), with washing with excess amount of THF to remove any unattached anthracene, and dried in air. Transmission FTIR spectra of films cast on a KBr plate were taken with a Bio-RAD FTS 6000 FT-IR spectrometer, UV-vis absorption spectra with a Unicam UN4 spectrophotometer, and fluorescence spectra of THF dispersions in 1 cm quartz cells with a Spex Fluorolog spectrofluorometer. Samples for transmission electron micrographs (TEM) were cast from THF solutions onto standard carbon-coated (200300 Å) Formvar films on copper grids (400 mesh) and taken with a side-entry Phillips CM12 electron microscope operated at 120 keV. Electrochemical measurements of the adsorbates were carried out with a BAS Model 100B instrument, using glassy carbon working, platinum wire counter, and silver wire quasi-reference electrodes (Ag QRE), in degassed 0.10 M acetonitrile solution of tetrabutylammonium bromide at a potential scan rate of 100 mV/s. Results and Discussion. The FTIR spectrum of short-cut SWNTs (Figure 2, bottom) displays two weak C-H stretch peaks at 2982 and 2875 cm-1 (see amplified spectrum from 2700 to 3300 cm-1), which are probably from stretch modes 404
Figure 3. Absorption spectra of compound 1 (1.2 × 10-5 M) and SWNT-1 (absorptive adduct 0.3 mg in 100 mL THF) in THF. Inset represents the spectrum from adsorbed 1 obtained by taking the difference of SWNT-1 from SWNT, and the concentration is estimated to be 3 × 10-7 M from its absorbance.
of aliphatic hydrogen on defects of the SWNT sidewall. The peak at 1701 cm-1 is assigned to the CdO stretch mode of carboxylic acid, and the bands at 1597, 1543, 1499 cm-1 probably represent the CdC aromatic stretch modes of the SWNT.8,9 The UV-vis spectrum of cut SWNTs dispersed in THF (0.3 mg /100 mL, 2.8 × 10-4 M based on carbon) displays a relatively featureless absorption gradually increasing from low to high energy.9,10 No detectable emission of the cut SWNTs was observed when excited at 300 nm. The chemisorptive attachment of anthracene derivatives to the cut SWNT is made clearly evident by changes in both the UV-vis (Figure 3) and the FTIR (Figure 2, top) spectra of the resulting SWNT-1 adsorptive adduct. The FTIR spectrum of SWNT-1 in Figure 2 is similar to that of free anthracene 1 (Figure 2, middle), with C-H stretch bands from the aromatic structure appearing at 2969 and 2881 cm-1 and weak bands at 1477 and 1390 cm-1 arising from aromatic ring CdC stretching modes. There are some changes in the adsorbate’s spectrum; for example, the C-H wagging band of compound 1 at 1268 cm-1 disappeared completely after adsorption onto the SWNT, implying that this vibration has been seriously broadened by the strong adsorptive interaction with the SWNT wall. The weakly absorbing spectrum from SWNT (note the 5× expansion of the lower curve) became indistinct compared to that of the adsorbed anthracene. The Nano Lett., Vol. 3, No. 3, 2003
Table 1. Results for Chemisorption of Anthracene Derivatives onto Short-Cut SWNT in THF Solution.
concentration (M)a coverage (%)c emission red shift (nm)d relative intensitiese E1/2 (mV)f
SWNT-1
SWNT-2
3 × 10-7
5 × 10-9 b 9 × 10-7
1 × 10-6
1 × 10-6
2 11
0.03 17
5 8
6 12
6 7
1.3
1.3
1.3
-900
-415
-1525
1.4 -1550
-1780
SWNT-3 SWNT-4 SWNT-5
a The concentration of adsorbed anthracene and its derivatives in dispersions of 0.3 mg absorptive adduct in 100 mL THF. The concentration was estimated from the relative absorbance of the vibronic bands of free anthracene in a standard solution and in a solution of the SWNT adsorption adduct. b The concentration of 2 absorbed on SWNT was estimated from its fluorescence intensity, relative to that of free 2, in a dispersion of 0.3 mg SWNT-2 in 100 mL THF. c Adsorption coverage is the ratio of adsorbed anthracene concentration to the concentration of SWNT “units” (a unit is 14 carbons of the SWNT structure; the anthracene ring system contains 14 carbons so this coverage normalizes for the size of the anthracene adsorption footprint, assuming coplanar adsorption). d Luminescence peak red shift of anthracene absorptive adducts, relative to free anthracenes, in THF solution. e Intensity of free anthracene/intensity of adsorbed anthracene at corresponding concentrations in Row I, except 2. f First reduction formal potentials, from cyclic voltammetry at a glassy carbon working electrode, with platinum wire counter, and silver wire quasireference electrode (Ag QRE), in degassed acetonitrile/0.10 M Bu4NBr, at a potential scan rate of 100 mV/s.
other SWNT-anthracene adsorptive adducts displayed similar vibrational results. Transmission electron microscopy of SWNT-1 (Figure 1) was not significantly changed relative to the original SWNT. The chemisorption of anthracene to SWNT is also evident in the UV-vis spectrum of SWNT-1. Figure 3 shows the spectrum of a solution of anthracene 1, which displays a strong absorbance between 300 and 400 nm, with pronounced vibronic structure producing peaks at 329, 341, 357, and 375 nm, characteristic of anthracene and its derivatives.11 The spectrum of SWNT-1, dispersed in THF at 3.0 mg /100 mL, 2.8 × 10-4 M in carbon), is nearly the same as that of the original SWNT (at an identical mass concentration), except for small shoulders in the 320-420 nm range. The inset to Figure 3 shows a difference spectrum of SWNT-1 vs SWNT; this spectrum displays vibronic fine structure at 327, 341, 358, 378 nm, wavelengths and with vibronic spacings similar to that of 1 (see above). The other adsorptive adducts, SWNT-3, -4, -5, give analogous difference spectra and vibronic pattern, but the absorption intensity varies with the anthracene derivative. No bands could be detected for adsorption of 2, which adsorbed more weakly. Its adsorption was detected, however, by its fluorescence (see below). Spectra like Figure 3 present an opportunity to estimate the surface coverage of the chemisorbed anthracene derivatives on the SWNT. Assuming that the anthracene absorbance coefficient is unchanged by its chemisorption, the ratio of absorbance of 1 in a solution of known concentration to that seen in the difference spectrum of Figure 3 produces the concentration of adsorbed 1 in a solution of SWNT-1. The results for anthracene and the other derivatives (3, 4, 5) are shown in Table 1. The estimated coverage is a few percent for compounds 1, 3, 4, and 5. Coverage represents, roughly, Nano Lett., Vol. 3, No. 3, 2003
Figure 4. Fluorescence spectra of uncapped anthracene (3 × 10-7 M) and SWNT-1 (0.3 mg in 100 mL THF) in THF contains the concentration of absorbed 1 (3 × 10-7 M) when photoexcited at 350 nm.
the fractional surface area of the SWNT occupied by the chemisorbed anthracene (see the method of calculating coverage in the table footnote). Anthracenes are strongly luminescent, as illustrated by the emission spectrum of 1 in Figure 4 (excited at 350 nm). The SWNT-anthracene adsorptive adducts also displayed strong fluorescence as seen from the emission spectrum of SWNT-1 in Figure 4 (also excited at 350 nm; the SWNT yields no emission at this excitation wavelength). The emission spectrum of SWNT-1 clearly displays the distinctive vibronic pattern of the parent aromatic hydrocarbon. Similar emission results are seen for the other chemisorbed anthracene derivatives. Further, the emission intensities from the chemisorbed anthracenes, when normalized to the same concentrations as standard solutions of free anthracenes, are nearly the same when chemisorbed on the SWNT. This is a remarkable result, showing that excited states of the anthracenes are not significantly quenched when chemisorbed on the SWNT. This result is very different from that of covalent attachments to fullerene derivatives such as C60, which has been reported to quench anthracene and pyrene luminescence by factors of 103 to 104.12 As for the adsorptive adduct SWNT-2, while its absorption spectrum could not be distinguished from that of the original SWNT, weak emission peaks attributable to 2 could be seen in the luminescence of SWNT-2 (Figure 5). It seems that the concentration of adsorbed 2 is too small to be detected by absorbance but can be seen by luminescence. Assuming that the emission efficiency of chemisorbed 2 on SWNT-2 is the same as free 2 (analogous to the results for the other anthracenes), the concentration of chemisorbed 2 on the 0.3 mg of SWNT-2 in Figure 2 is estimated to be about 5 × 10-9 M. This result and the calculated coverage are included in Table 1. The absorbance spectra of anthracene derivatives of SWNTs-1, 3, 4, 5 did not display obvious shifts relative to the corresponding unattached compounds. The fluorescence maxima of chemisorbed anthracenes were, on the other hand, all red-shifted relative to the free compounds. The shift of emission to lower energy varied with the anthracene substituents, as shown in Table 1, in the order 2 > 1,4 > 3,5. 405
Figure 5. Fluorescence spectra of uncapped 2 (7.7 × 10-6 M) and SWNT-2 (0.3 mg in 100 mL THF) in THF when photoexcited at 350 nm.
The small energy shifts may be attributed to how the solventrelated excited-state relaxations that lower excited/groundstate dipole moment differences13 have been changed when the anthracene derivatives are in the adsorbed state. The order of energy shifts among the absorptive adducts indicates that the changes depend on the particular anthracene substituents. We next consider the general nature of the chemisorptive interaction. The emission energy shift suggests some structuredependent aspect of the interaction, which might be correlated with electron donor or acceptor properties. The absorptive adducts are qualitatively expected to occur by a face-to-face interaction, common for π-π bond interactions. Might this interaction be accompanied by an electron donoracceptor charge-transfer interaction between the aromatic adsorbate and the aromatic SWNT sidewall? If this occurred, one might expect that the SWNT adsorption process would favor the anthracene derivatives in order of their electron affinity, which for the present substituents is -CN > -Br > -CH2OH > -H > -OH.14 This is, except for 5, in accordance with the chemisorption coverages (Table 1), where 4,5 > 3 > 1 > 2. The affinity of 5 for forming SWNT-5 may include the interaction of hydroxyl groups with the SWNT.14 The correlation of absorption coverage with anthracene substituents does not, however, extend to any understanding of the red shift of luminescence recorded in Table 1, although the red shift must logically have something to do with the electronic interaction between the anthracene derivatives and the SWNT surface. Electrochemical measurements of the relative electron affinity of the anthracene derivatives offer a clearer correlation and support a charge-transfer interaction in the chemisorption. Electrochemical potentials are well-known to track electron affinity energetics. Voltammetry of, for example, 1 and 4 (Figure 6) reveals very different potentials for the first one-electron transfer reductions. A less negative reduction potential corresponds to a lower LUMO state; such a state would be expected to electronically interact more strongly as an electron acceptor with the SWNT. To the extent that the strength of the charge-transfer interaction controls the relative extent of adsorption coverage, anthracene derivatives with more positive reduction potentials should exhibit the largest chemisorption coverages. Formal potentials for the first one-electron reduction of the anthracene derivatives are 406
Figure 6. Electrochemical measurements of 1 and 4 in 0.10 M CH3CN/Bu4NBr under N2 at 100 mV/s at glassy carbon working electrode, at potential scan rate of 100 mV/s.
listed in Table 1, and fall in an order of 4 > 3 > 5∼1 > 2. This order is indeed approximately the same as that of the coverage of the anthracene derivatives on SWNTs. As a final exploration step, we examined the reversibility of the chemisorption process. In one experiment, another strong adsorber, with a capacity for fluorescent detection, was used to displace the adsorbed anthracene. A mixture of SWNT-1 and a 100-fold molar excess of pyrene in THF was stirred at room temperature for 72 h; then the SWNT product was purified as done for the SWNT-anthracene absorptive adducts, to remove unattached fluorophore. The product’s emission and absorption spectra were characteristic solely of those of pyrene, indicating that anthracene had been quantitatively displaced by pyrene adsorption. This experiment shows that the chemisorptive attachment of anthracene to SWNT is a chemically reversible process. SWNT-5 was subjected to coupling by dansyl chloride and 1-pyrene butylic acid (with condensation reagent 1,3dicyclohexylcarbodiimide, DCC). Instead of the coupling reaction, only the replacement of attached anthracene by excess amount of pyrene in solution was observed, indicating that the physical interaction of polynuclear aromatic compounds with SWNT was not as strong as of ordinary chemical bonds. Finally, the absorbance spectra of SWNT-anthracene absorptive adducts were measured after immersion in THF for 24 h, followed by washing with THF. This experiment showed a loss of 10% of the adsorbed anthracenes. This loss also shows that the adsorption onto the nanotube is strong but reversible. Acknowledgment. This research was supported by the Office of Naval Research (MURI), the National Science Foundation (R.W.M.), and PRF 37310-ACS (Y.W.). References (1) Ijima, S. Nature (London) 1991, 354, 56-58. (2) (a) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes, Academic Press: New York, 1996. (b) Ebbesen, T. W. Carbon Nanotubes: Preparation and Properties; CRC Press: Boca Raton, FL, 1997. Nano Lett., Vol. 3, No. 3, 2003
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