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Photoluminescence and ElectronicInteraction of Anthracene DerivativesAdsorbed on Sidewalls of Single-Walled

Carbon NanotubesJian 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 coverageof 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 sincetheir introduction in the 1990s.1 Single-walled carbon nano-tubes (SWNTs) exhibit many interesting physical properties.A challenge is how to combine SWNTs with other chemicalentities in order to fabricate nanostructures with uniquefunctions and applications.2 The well-known3 adsorptive

interaction between polynuclear aromatic compounds andgraphitic surfaces can, for example, be exploited to attachbiological substances4 to the sidewalls of SWNTs, as detectedby imaging. Such interactions are also expected given thepersistent van der Waals interactions between graphenesheets in graphite and SWNTs in nanotube bundles.5 Becausepolynuclear aromatic compounds are often luminescent, theymay also be used as chromophore labels for detectingadsorption and immobilization of chemical or biologicalmolecules onto SWNTs. Because of the sidewall curvatureof SWNTs, it is also possible that the adsorption ofpolynuclear aromatic compounds there may differ fromanalogous adsorptions on flat graphitic surfaces by via

-stacking.6In this research, we detect the adsorption of anthracene

and several derivatives onto the sidewalls of cut SWNTs,using both absorbance and fluorescence spectra. The anthra-cene derivatives were substituted by different groups withvarious electrophilic capability and volume size including(1) anthracene; (2) anthraobin; (3) 9,10-dibromoanthracene;

(4) 9,10-anthracenedicarbonitrile; and (5) 9-anthracene-methanol (Scheme 1) in order to investigate the effect ofsubstituents on the electronic interaction of the adsorbate with

SWNT. SWNTs are labeled as SWNT-1, etc., according tothe adsorbed anthracene derivative.Experimental Section. SWNTs were obtained from

Carbon Nanotechnologies, Inc. (Rice University) and incu-bated in a 3:1 mixture of concentrated sulfuric and nitricacids for 8 h, by which the material is short-cut to segmentsas short as 3-4 m, but with many longer ones remaining(Figure 1). The end openings should bear oxidized carbonsites such as carboxylic acids.7 The cut SWNTs are readilydispersed in alcohol and THF but will settle out if leftovernight without stirring. The SWNT-anthracene absorp-

* Corresponding authors. Kenan Laboratories of Chemistry. Department of Physics and Astronomy.

Scheme 1. Chemical Structures of Anthracene Derivatives:(1) Anthracene; (2) Anthraobin; (3) 9,10-Dibromoanthracene;

(4) 9,10-Anthracenedicarbonitrile; and (5)9-Anthracene-methanol.

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10.1021/nl025952c CCC: $25.00 2003 American Chemical SocietyPublished on Web 02/12/2003


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tive adducts were prepared by stirring a solution of 2 mgcut SWNTs and 5 mg of anthracene derivatives in 10 mLTHF at room temperature for 72 h. This is a 1:2 mole ratio

of SWNT units to anthracene molecules, counting a SWNTunit as 14 carbon atoms (same as the anthracene structure).The SNWTs were isolated on a Millipore porous filter(FHLC04700, 0.45 m), with washing with excess amountof THF to remove any unattached anthracene, and dried inair. Transmission FTIR spectra of films cast on a KBr platewere taken with a Bio-RAD FTS 6000 FT-IR spectrometer,UV-vis absorption spectra with a Unicam UN4 spectro-photometer, and fluorescence spectra of THF dispersions in1 cm quartz cells with a Spex Fluorolog spectrofluorometer.Samples for transmission electron micrographs (TEM) werecast from THF solutions onto standard carbon-coated (200-300 ) Formvar films on copper grids (400 mesh) and taken

with a side-entry Phillips CM12 electron microscope operatedat 120 keV. Electrochemical measurements of the adsorbateswere carried out with a BAS Model 100B instrument, usingglassy carbon working, platinum wire counter, and silver wirequasi-reference electrodes (Ag QRE), in degassed 0.10 Macetonitrile solution of tetrabutylammonium bromide at apotential scan rate of 100 mV/s.

Results and Discussion. The FTIR spectrum of short-cutSWNTs (Figure 2, bottom) displays two weak C-H stretchpeaks at 2982 and 2875 cm-1 (see amplified spectrum from2700 to 3300 cm-1), which are probably from stretch modes

of aliphatic hydrogen on defects of the SWNT sidewall. Thepeak at 1701 cm-1 is assigned to the CdO stretch mode of

carboxylic acid, and the bands at 1597, 1543, 1499 cm-1probably represent the CdC aromatic stretch modes of theSWNT.8,9 The UV-vis spectrum of cut SWNTs dispersedin THF (0.3 mg /100 mL, 2.8 10-4 M based on carbon)displays a relatively featureless absorption gradually increas-ing from low to high energy.9,10 No detectable emission ofthe cut SWNTs was observed when excited at 300 nm.

The chemisorptive attachment of anthracene derivativesto the cut SWNT is made clearly evident by changes in boththe UV-vis (Figure 3) and the FTIR (Figure 2, top) spectraof the resulting SWNT-1 adsorptive adduct. The FTIRspectrum of SWNT-1 in Figure 2 is similar to that of free

anthracene 1 (Figure 2, middle), with C-H stretch bandsfrom the aromatic structure appearing at 2969 and 2881 cm-1

and weak bands at 1477 and 1390 cm-1 arising from aromaticring CdC stretching modes. There are some changes in theadsorbates spectrum; for example, the C-H wagging bandof compound 1 at 1268 cm-1 disappeared completely afteradsorption onto the SWNT, implying that this vibration hasbeen seriously broadened by the strong adsorptive interactionwith the SWNT wall. The weakly absorbing spectrum fromSWNT (note the 5 expansion of the lower curve) becameindistinct compared to that of the adsorbed anthracene. The

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-1absorptive adduct.

Figure 3. Absorption spectra of compound 1 (1.2 10-5 M) andSWNT-1 (absorptive adduct 0.3 mg in 100 mL THF) in THF. Insetrepresents the spectrum from adsorbed 1 obtained by taking thedifference of SWNT-1 from SWNT, and the concentration isestimated to be 3 10-7 M from its absorbance.

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other SWNT-anthracene adsorptive adducts displayed simi-lar 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 evidentin the UV-vis spectrum of SWNT-1. Figure 3 shows thespectrum of a solution of anthracene 1, which displays astrong absorbance between 300 and 400 nm, with pronouncedvibronic structure producing peaks at 329, 341, 357, and 375

nm, characteristic of anthracene and its derivatives.11 Thespectrum 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 theoriginal SWNT (at an identical mass concentration), exceptfor small shoulders in the 320-420 nm range. The inset toFigure 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 similarto that of 1 (see above). The other adsorptive adducts,SWNT-3, -4, -5, give analogous difference spectra andvibronic pattern, but the absorption intensity varies with theanthracene derivative. No bands could be detected foradsorption of2, 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 deriva-tives on the SWNT. Assuming that the anthracene absorbancecoefficient is unchanged by its chemisorption, the ratio ofabsorbance of1 in a solution of known concentration to thatseen in the difference spectrum of Figure 3 produces theconcentration of adsorbed 1 in a solution of SWNT-1. Theresults for anthracene and the other derivatives (3, 4, 5) areshown in Table 1. The estimated coverage is a few percentfor compounds 1, 3, 4, and 5. Coverage represents, roughly,

the fractional surface area of the SWNT occupied by thechemisorbed anthracene (see the method of calculatingcoverage in the table footnote).

Anthracenes are strongly luminescent, as illustrated by the

emission spectrum of1 in Figure 4 (excited at 350 nm). TheSWNT-anthracene adsorptive adducts also displayed strongfluorescence as seen from the emission spectrum of SWNT-1in Figure 4 (also excited at 350 nm; the SWNT yields noemission at this excitation wavelength). The emissionspectrum of SWNT-1 clearly displays the distinctive vibronicpattern of the parent aromatic hydrocarbon. Similar emissionresults are seen for the other chemisorbed anthracenederivatives. Further, the emission intensities from the chemi-sorbed anthracenes, when normalized to the same concentra-tions as standard solutions of free anthracenes, are nearlythe same when chemisorbed on the SWNT. This is aremarkable result, showing that excited states of the anthra-

cenes are not significantly quenched when chemisorbed onthe SWNT. This result is very different from that of covalentattachments to fullerene derivatives such as C60, which hasbeen reported to quench anthracene and pyrene luminescenceby factors of 103 to 104.12

As for the adsorptive adduct SWNT-2, while its absorptionspectrum could not be distinguished from that of the originalSWNT, weak emission peaks attributable to 2 could be seenin the luminescence of SWNT-2 (Figure 5). It seems thatthe concentration of adsorbed 2 is too small to be detectedby absorbance but can be seen by luminescence. Assumingthat the emission efficiency of chemisorbed 2 on SWNT-2is the same as free 2 (analogous to the results for the other

anthracenes), the concentration of chemisorbed 2 on the 0.3mg of SWNT-2 in Figure 2 is estimated to be about 5 10-9 M. This result and the calculated coverage are includedin Table 1.

The absorbance spectra of anthracene derivatives ofSWNTs-1, 3, 4, 5 did not display obvious shifts relative tothe corresponding unattached compounds. The fluorescencemaxima of chemisorbed anthracenes were, on the other hand,all red-shifted relative to the free compounds. The shift ofemission to lower energy varied with the anthracene sub-stituents, as shown in Table 1, in the order 2 > 1,4 > 3,5.

Table 1. Results for Chemisorption of Anthracene Derivativesonto Short-Cut SWNT in THF Solution.

S WN T -1 S WN T -2 S WN T -3 S WN T -4 S WN T -5

concentration

(M)a

3 10 -7 5 10 -9 b 9 10 -7 1 10 -6 1 10 -6

coverage (%)c 2 0.03 5 6 6

emission r ed

shift (nm)d

11 17 8 12 7

relative

intensities e

1.4 1.3 1 .3 1 .3

E1/2 (mV)f

-1550 -1780 -900 -415 -1525

a The concentration of adsorbed anthracene and its derivatives indispersions of 0.3 mg absorptive adduct in 100 mL THF. The concentrationwas estimated from the relative absorbance of the vibronic bands of freeanthracene in a standard solution and in a solution of the SWNT adsorptionadduct. b The concentration of 2 absorbed on SWNT was estimated fromits fluorescence intensity, relative to that of free 2, in a dispersion of 0.3mg SWNT-2 in 100 mL THF. c Adsorption coverage is the ratio of adsorbedanthracene concentration to the concentration of SWNT units (a unit is14 carbons of the SWNT structure; the anthracene ring system contains 14carbons so this coverage normalizes for the size of the anthracene adsorptionfootprint, assuming coplanar adsorption). d Luminescence peak red shift ofanthracene absorptive adducts, relative to free anthracenes, in THF solution.e Intensity of free anthracene/intensity of adsorbed anthracene at corre-sponding 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 (AgQRE), in degassed acetonitrile/0.10 M Bu4NBr, at a potential scan rate of100 mV/s.

Figure 4. Fluorescence spectra of uncapped anthracene (3 10-7

M) and SWNT-1 (0.3 mg in 100 mL THF) in THF contains theconcentration of absorbed 1 (3 10-7 M) when photoexcited at350 nm.

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The small energy shifts may be attributed to how the solvent-related excited-state relaxations that lower excited/ground-state dipole moment differences13 have been changed whenthe anthracene derivatives are in the adsorbed state. The orderof energy shifts among the absorptive adducts indicates thatthe changes depend on the particular anthracene substituents.

We next consider the general nature of the chemisorptiveinteraction. The emission energy shift suggests some structure-dependent aspect of the interaction, which might be cor-related with electron donor or acceptor properties. Theabsorptive adducts are qualitatively expected to occur by aface-to-face interaction, common for -bond interactions.Might this interaction be accompanied by an electron donor-acceptor charge-transfer interaction between the aromaticadsorbate and the aromatic SWNT sidewall? If this occurred,one might expect that the SWNT adsorption process wouldfavor the anthracene derivatives in order of their electronaffinity, 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 formingSWNT-5 may include the interaction of hydroxyl groups withthe SWNT.14 The correlation of absorption coverage withanthracene substituents does not, however, extend to anyunderstanding of the red shift of luminescence recorded inTable 1, although the red shift must logically have somethingto do with the electronic interaction between the anthracenederivatives and the SWNT surface.

Electrochemical measurements of the relative electronaffinity of the anthracene derivatives offer a clearer correla-tion and support a charge-transfer interaction in the chemi-sorption. Electrochemical potentials are well-known to track

electron affinity energetics. Voltammetry of, for example, 1and 4 (Figure 6) reveals very different potentials for the firstone-electron transfer reductions. A less negative reductionpotential corresponds to a lower LUMO state; such a statewould be expected to electronically interact more stronglyas an electron acceptor with the SWNT. To the extent thatthe strength of the charge-transfer interaction controls therelative extent of adsorption coverage, anthracene derivativeswith more positive reduction potentials should exhibit thelargest chemisorption coverages. Formal potentials for thefirst one-electron reduction of the anthracene derivatives are

listed in Table 1, and fall in an order of 4 > 3 > 51 > 2.This order is indeed approximately the same as that of thecoverage of the anthracene derivatives on SWNTs.

As a final exploration step, we examined the reversibilityof the chemisorption process. In one experiment, anotherstrong adsorber, with a capacity for fluorescent detection,was used to displace the adsorbed anthracene. A mixture ofSWNT-1 and a 100-fold molar excess of pyrene in THF wasstirred at room temperature for 72 h; then the SWNT productwas purified as done for the SWNT-anthracene absorptiveadducts, to remove unattached fluorophore. The productsemission and absorption spectra were characteristic solelyof those of pyrene, indicating that anthracene had beenquantitatively displaced by pyrene adsorption. This experi-ment shows that the chemisorptive attachment of anthraceneto SWNT is a chemically reversible process.

SWNT-5 was subjected to coupling by dansyl chloride

and 1-pyrene butylic acid (with condensation reagent 1,3-dicyclohexylcarbodiimide, DCC). Instead of the couplingreaction, only the replacement of attached anthracene byexcess amount of pyrene in solution was observed, indicatingthat the physical interaction of polynuclear aromatic com-pounds with SWNT was not as strong as of ordinarychemical bonds.

Finally, the absorbance spectra of SWNT-anthraceneabsorptive adducts were measured after immersion in THFfor 24 h, followed by washing with THF. This experimentshowed a loss of 10% of the adsorbed anthracenes. This lossalso shows that the adsorption onto the nanotube is strongbut reversible.

Acknowledgment. This research was supported by theOffice of Naval Research (MURI), the National ScienceFoundation (R.W.M.), and PRF 37310-ACS (Y.W.).

References

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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 photoexcitedat 350 nm.

Figure 6. Electrochemical measurements of 1 and 4 in 0.10 MCH3CN/Bu4NBr under N2 at 100 mV/s at glassy carbon workingelectrode, at potential scan rate of 100 mV/s.

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Der Vermögensberater, May 2011