DOI: 10.1002/chem.200900372

Photoinduced Formation and Characterization of Electron–Hole Pairs inAzaxanthylium-Derivatized Short Single-Walled Carbon Nanotubes

Roberto Mart�n,[a] Liliana B. Jim�nez,[b] Mercedes �lvaro,[a] J. C. Scaiano,*[b] andHermenegildo Garc�a*[a]

Introduction

Single-wall carbon nanotubes (CNTs) hold promise forfuture applications in nanophotonics, where understandingof photoinduced charge separation will play a major role inthe development of novel nanodevices.[1] The developmentof reliable techniques to produce CNTs has enhanced op-portunities for the study of photochemical processes inthese materials and its derivatives.[2] We report here a CNTderivative that undergoes efficient generation of electron–hole pairs by UV excitation; further, we were able to scav-enge the electron to yield a moderately long lived (micro-seconds) CNT-supported hole that is of interest for nano-photonic applications.

The photochemistry of xanthone and its derivatives hasbeen extensively studied in a large variety of homogeneous

and heterogeneous media.[3–8] Xanthone triplet excitedstates, typically having lifetimes of a few microseconds andcharacterized by a strong absorption band around 600 nm,are commonly used as probes for calibration of laser flashphotolysis systems upon 355 nm excitation.[8] Compared toother aromatic ketones, xanthone and its derivatives, whichgenerally have triplet P–P* states, are significantly muchmore inert towards photochemical degradation.[3,8,10,11] Theexact position of the triplet maximum-intensity wavelengthhas been shown to depend on the polarity of the mediumexperienced by the electronic excited state.[3,6] In addition tothe triplet excited state, ketyl radicals (lmax =330 nm) andradical ions (lmax =750 nm) have also been reported as pos-sible transient species that can be generated upon photo-chemical excitation.[5,10, 12,13] Covalent functionalization ofsingle-walled CNTs to obtain photo-responsive nanomateri-als is a topic of much current interest.[14,15] In contrast to ful-lerenes, which act exclusively as electron acceptors,[15,16] thesemiconducting properties of some CNTs allow them to actas electron donors or acceptors depending on the partner in-troduced by functionalization.[15] Although the photochemis-try of CNTs in the presence of non-bonded molecules maybe similar to that of covalently functionalized analogues, ithas been found that linking the two subunits increases re-markably the interaction between the components.[17] Sincexanthone derivatives are among the best studied moleculesin photochemistry,[7,8] and considering potential applications

Abstract: 2-Azaxanthone, a nitrogenat-ed derivative of the well-studied organ-ic chromophore xanthone, has been co-valently bound through 2-(ethylthio)e-thylamido linkers to the carboxylic acidgroups of short, soluble single-walledcarbon nanotubes (CNTs) of 450 nmaverage length, and the resulting aza-xanthylium-functionalized CNTs (AZX-CNT, 8.5 wt % AZX content) charac-terized by solution 1H NMR, Raman

and IR spectroscopy and thermogravi-metric analysis. Comparison of thequenching of the triplet excited state ofAZX (steady-state and time-resolved)and of the transient optical spectra ofCNTs and AZX-CNT shows that the

covalent linkage boosts the interactionbetween the azaxanthylium moiety andthe short CNT units. The triplet excitedstate of the azaxanthylium derivative isquenched by CNT with and without co-valent bonding, but when it is covalent-ly bonded, the singular transient spec-trum is compatible with the photogen-eration of electron holes through elec-tron transfer from CNT to excitedazaxanthylium units.

Keywords: carbon nanotubes ·chromophores · electron transfer ·photophysics

[a] R. Mart�n, Dr. M. �lvaro, Prof. Dr. H. Garc�aInstituto de Tecnolog�a Qu�mica CSIC-UPVUniversidad Polit�cnica de ValenciaAv. De Los Naranjos s/n, 46022 Valencia (Spain)Fax: (+34) 963877809E-mail : hgarcia@quim.upv.es

[b] Dr. L. B. Jim�nez, Prof. Dr. J. C. ScaianoDepartment of Chemistry, University of Ottawa10 Marie Curie, Ottawa K1N 6N5 (Canada)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.200900372.

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in photo-responsive CNTs, it is of interest to study the pho-tophysical properties of xanthone derivatives covalentlyanchored to single-walled CNTs. Here we describe the syn-thesis, characterisation and photophysics of a 2-azaxanthonederivative linked through the nitrogen atom by a long alkylchain to soluble, short, single-walled CNT (AZX-CNT).

Results and Discussion

Synthesis and characterization of AZX-CNT: By carefulcontrol of the oxidative treatment of purified micrometricCNTs, it is possible to obtain samples of this carbonaceousmaterial of 200–600 nm length. These short CNTs are re-markably soluble in water and in organic solvents and aresusceptible to functionalization through the carboxyl groupspresent at their tips and wall defects.[17,18] The strategy usedhere to functionalize short CNT with 2-azaxanthone isshown in Scheme 1. The key steps in the synthesis are 1) thepreparation of thiol-modified short CNTs (SH-CNT)through a peptidic bond; 2) 2,2’-azobisisobutyronitrile-initi-ated radical-chain addition of the thiol groups of SH-CNTto a terminal C=C double bond of N-undecenyl-2-azaxanth-N-ylium compound 2, which was obtained by treating 2-azaxanthone (1) with 11-bromo-1-undecene.

The SH-CNT was prepared according to reported proce-dures,[2,17,19] and chemical analysis showed the expected 1:1nitrogen:sulfur atomic ratio and a thiol loading correspond-ing to 3.15 wt % of the solid. Synthetic intermediate 2 wascharacterized by analytical and spectroscopic techniques,

and the data agreed with the proposed structure (see theSupporting Information).

After functionalization the resulting AZX-CNT was puri-fied by consecutive cycles of filtration (Nylon, 0.2 mm) andredispersion in water and characterized by analytical andspectroscopic techniques. Combustion chemical analysis andparticularly the nitrogen-to-sulphur atomic ratio of 2.1 showthat, within experimental error, all thiol groups are now as-sociated with the azaxanthyl moiety. This exhaustive alkyla-tion of the thiol groups of SH-CNT is in accordance withthe twofold excess of undecenyl derivative of azaxanthylcompound 2 used in the synthesis of AZX-CNT with respectto thiol groups to achieve the maximum possible degree offunctionalization. The amount of azaxanthyl units in thesample is 8.5 wt %.

Thermogravimetric analyses of SH-CNT and AZX-CNTare shown in Figure 1. In contrast to the original commercialsample containing a significant percentage of non-carbona-

ceous residue (40 wt %), SH-CNT and AZX-CNT are almostfree of inorganic particles, and upon heating to increasingtemperatures in the presence of air undergo combustion of92 and 96 % of the total weight for SH-CNT and AZX-CNT,respectively. The thermogravimetric profiles of SH-CNT andAZX-CNT show, however, significant differences in the250–400 8C region with a variation in weight loss that can beeasily interpreted as the thermal decomposition/combustionof the 8.5 wt % of azaxanthyl units present in AZX-CNTbut not in SH-CNT.

Microscopy was used to study the average length andmorphology changes of the CNT samples upon purificationand functionalization. Transmission electron microscopy(TEM) images of AZX-CNT (Figure 2) show short tubes oflength 200–600 nm with a statistical average of 450 nm.

No morphological changes were observed by TEM micro-scopy (resolution about 3 nm) upon functionalization withazaxanthyl units. In contrast, atomic force microscopy(AFM) with subnanometre vertical resolution revealed sig-nificant changes on the CNT surface and an increase in theapparent diameter of the CNT samples after covalent func-

Scheme 1. Synthesis of short CNTs functionalized by 2-azaxanthyl units.

Figure 1. Thermogravimetric profiles recorded under air of raw commer-cial CNT (a), SH-CNT (b), and AZX-CNT (c).

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tionalization with the azaxanthyl derivative. Figure 3 showssome AFM images and cross-section profiles to illustratethese nanoscale morphological changes upon reaction of thethiol groups with 2. It is tempting to interpret this variationof CNT diameter upon functionalization as a reflection ofthe presence of azaxanthyl units homogeneously distributedover the CNT walls.

Raman spectroscopy is a powerful technique to prove thesingle-walled structure of CNT and to monitor the degree offunctionalization.[19] Figure 4 shows selected FT Ramanspectra of CNT, SH-CNT and AZX-CNT. Compared to pris-tine CNTs, purification, oxidative cutting and mercaptoethyl-amido functionalization to give SH-CNT decreases the in-tensity of the tangential C�C stretching band at 1574 cm�1

but increases the relative intensity of the peak associatedwith wall defects at about 1300 cm�1, while the characteristicradial breathing mode (RBM) at 189 cm�1 specific to single-walled CNTs is preserved throughout the modification pro-cedures.[21] The Raman spectrum of AZX-CNT is similar tothat of SH-CNT, and no new peaks derived from the azax-anthyl moiety could be observed, probably due to the low

degree of functionalization and the low Raman intensity ofthe AZX peaks. Importantly, the apparent size increase ob-served in AFM vertical profiles for AZX-CNT can not beattributed to variations in the diameter of the CNT moiety,since the average wavenumber of the radial breathing modeassociated with the diameter of the CNT has not changedand is maintained at 189 cm�1 in the process of covalentfunctionalization by azaxanthyl units of SH-CNT.

According to the equation that correlates diameter andwavenumber of the RBM,[26] a CNT with a diameter of3.0 nm, as observed in AFM, should give a peak at about150 cm�1, which is clearly not observed for any of the CNTsamples.

In contrast to Raman spectroscopy, evidence for the pres-ence of azaxanthyl groups on AZX-CNT can be obtained bythe appearance of a band at 1677 cm�1 in the FTIR spec-trum, characteristic of conjugated C=O stretching, which ispresent for AZX-CNT but absent in the FTIR spectrum ofSH-CNT or CNT (see Figure 5).

Also notable in the FT-IR spectrum of AZX-CNT is thepresence of a relatively intense peak at 2936 cm�1 attributa-ble to the CH2 vibration of the long alkyl chain linking aza-xanthyl and mercaptoethylamido groups. The aromatic region

Figure 2. TEM images of AZX-CNT at two different magnifications.

Figure 3. AFM images of SH-CNT (a) and AZX-CNT (b). The plots onthe right show the vertical profile of the cross section indicated in theimage. The apparent diameter increases from 0.74 to 3.00 nm upon func-tionalization.

Figure 4. Raman spectra of raw CNT (a), SH-CNT (b), and AZX-CNT(c).

Figure 5. Room-temperature FTIR spectra of ambient-equilibrated rawCNT (a), SH-CNT (b), and AZX-CNT (c).

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of the FTIR spectrum of AZX-CNT shows peaks corre-sponding to the vibration of the aromatic rings present inazaxanthyl units. Compared to the FTIR spectrum of ambi-ent-equilibrated raw CNT, which shows peaks associatedwith co-adsorbed water at 3400 and 1620 cm�1, the FTIRspectrum of SH-CNT shows almost complete disappearanceof water, a weak vibration due to SH group at 2200 cm�1

and the amido C=O vibration at 1650 cm�1. Apparentlyfunctionalization has rendered CNT more hydrophobic withlower water affinity.

One of the major advantages of using soluble short CNTsfrom the characterization point of view is that it is possibleto record 1H NMR spectra of the samples in solution.Figure 6 shows the 1H NMR spectrum of AZX-CNT in[D6]DMSO. The good resolution of the aromatic region inthe 1H NMR spectrum allows detailed assignment of theseven aromatic protons in the azaxanthyl groups, whichappear between d=7.5 and 8.9 ppm.

Comparison of the chemical shifts of the characteristicprotons of azaxanthyl precursor 2 with the aromatic signalsrecorded for AZX-CNT reveals some significant changesthat are attributable to the anisotropy introduced by theCNT walls influencing the position of the aromatic protonsin AZX-CNT with respect to precursor 2. In contrast to thearomatic region, in which the seven protons of the azaxan-thone moiety can be identified, the zone corresponding tothe aliphatic protons is complicated by the intense peaksdue to [D6]DMSO solvent and accompanying water. Howev-er, even though the aliphatic protons in AZX-CNT can notbe completely assigned due to interference from the solvent,some important features are worth mentioning. Thus, no sig-nals corresponding to olefinic protons present in precursor 2are observed in the spectrum of AZX-CNT. Instead of ole-finic protons, two peaks at 3.15 and 3.03 ppm were recordedfor AZX-CNT, attributable to CH2 groups bonded to the ar-

omatic nitrogen atom and amide group, respectively. In anycase, considering the scarcity of 1H NMR data for function-alized CNTs,[2] the resolution of the spectrum of AZX-CNTis remarkable and provides strong support for the structureof the photo-responsive material. Particularly the covalentbinding of AZX and CNT can be inferred firmly from thepresence of the seven aromatic proton signals and the ab-sence of olefinic protons. All together the informationgained through the different analytical and spectroscopictechniques indicates that we have synthesized a sample ofshort (av length ca. 450 nm), single-walled CNTs containingabout 18 wt % weight of functional groups (azaxanthylgroups and linkers), covalently anchored mainly through thedefects at the walls (3 nm apparent diameter).

Photophysics of AZX-CNT: In agreement with literature re-ports,[9,11] 266 nm laser flash photolysis of azaxanthone 1 innitrogen-purged CH3CN solution gave a transient spectrumwith absorption bands at 330 and 620 nm exhibiting differ-ent decays. The 620 nm band is quenched by oxygen and isassigned to the AZX triplet excited state. The 330 nm peakgrows in the first 4 ms and then decays and is assigned to theketyl radical. Both transients and their respective maximumwavelengths are well documented.[6,10] The growth of thesignal monitored at 330 nm should correspond to some ketylformed from the triplet excited state. Upon laser excitationazaxanthyl compound 2 exhibits very similar behavior toazaxanthone with a transient spectrum containing two peaksat 640 and 330 nm with different temporal profiles that canbe attributed to the corresponding triplet excited state andketyl radical respectively (see Figure 7). The temporal pro-

file of the signal monitored at 640 nm can be adequatelyfitted to two first-order kinetics with lifetimes of 2.68 (82 %)and 13 ms (18 %) respectively (Figure 8).

These two populations of triplet excited states probablycorrespond to azaxanthylium molecules with different asso-

Figure 6. Solution 1H NMR spectrum of AZX-CNT in [D6]DMSO. Inset:expansion of the aromatic proton region for a) azaxanthylium derivative2 and b) AZX-CNT.

Figure 7. Transient spectra recorded 0.80, 2.54, 5.80, and 13.0 ms (top tobottom) after 266 nm laser pulse on N2-purged acetonitrile solution ofcompound 2.

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ciation with chloride as charge-balancing cation. The signalat 330 nm attributable to the corresponding azaxanthyl ketylradical was significantly longer lived than the triplet excitedstate. The signal monitored at 330 nm shows growth in thefirst 8 ms and complex decays that again fit to two first-orderkinetics with lifetimes of 38 (51 %) and 273 ms (49 %), re-spectively. These two lifetimes can be related to two popula-tions of ketyl radicals in which the chloride anion is in-cageor solvent-separarated. In contrast to the band at about640 nm present in the transient spectra of azaxanthone 1and azaxanthyl 2, the transient spectra recorded for N2-purged CH3CN solutions of AZX-CNT upon 266 nm laserexcitation gave a continuum absorption covering the wholewavelength range available to our detector (Figure 9).

Blank controls with SH-CNT under the same conditionsdid not show any detectable transient. The transient spec-trum recorded for AZX-CNT (Figure 9) presents, at shorttime delays, a broad band from 490 to 720 nm that could be

assigned to the residual triplet exciton located on the aza-xanthyl unit being quenched by the walls of the CNTmoiety.[2] The temporal profile monitored at 640 nm, that is,the region where azaxanthyl triplets absorb, shows two dif-ferent components, one with a very short lifetime (52 ns)and another, much longer lived residual (Figure 10). In con-

trast with azaxanthone 1 and azaxanthyl derivative 2, AZX-CNT shows a very short AZX triplet lifetime of about 50 ns,which clearly indicates that the interaction with the nano-tube leads to efficient triplet quenching; kinetic traces aresomewhat more complex than the simple monoexponentialdecays that characterize molecular models such as azaxanth-yl 2. Triplet decay of AZX-CNT leads to the formation of arather featureless intermediate with a lifetime on the micro-second timescale (Figure 10). The presence of oxygen led toan increase in the lifetime of this intermediate. Scheme 2shows the proposed interpretation of the data in schematicform. Electron transfer (A) from the nanotube to the AZXmoiety leads to a hole–electron pair, whereby the latter islocated at AZX (effectively the radical anion of AZX) andthe hole is supported by the nanotube p system. In the pres-ence of oxygen (or presumably many other electron accept-

Figure 8. Temporal profile of the signals monitored at 330 and 640 nm,after 266 nm laser pulse on N2-purged acetonitrile solution of 2.

Figure 9. Transient spectra recorded 0.062, 0.099, 0.166, and 0.385 ms (topto bottom) after 266 nm laser excitation of a N2-purged acetonitrile solu-tion of AZX-CNT.

Figure 10. Transient signals monitored at 600 nm after laser excitation ofAZX-CNT at 266 nm in acetonitrile. Use of a double y scale allows datanormalization without modifying the recorded absorbances.

Scheme 2. Proposed rationalization of the photochemistry of AZX-CNT;bet=back electron transfer.

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ors) the electron is scavenged and the hole is left without itsgeminate electronic partner, which enhances substantiallythe hole lifetime (C). In the presence of oxygen there is aclear minimum in the trace (B); the growth that follows istentatively assigned to hole relocation to a lower energy sitewith a slightly higher transient absorbance (for long time-scales, see the Supporting Information). The transient decayof Figure 10 C is assigned to the CNT-supported hole; whiledetailed analytical fits proved difficult, the half-life can bereadily measured, and for measurements starting at 1 ms it is1.9 ms under nitrogen and up to 6.2 ms under oxygen. Thisdecay probably incorporates reaction with O2C

� generated inthe quenching process. It is interesting that the growth (C)is not observed under nitrogen, although a rather flat regionis clear; we suggest that a weaker signal combined withfaster hole decay masks the growth in this case.

We interpret the fast decay as the residual exciton locatedon azaxanthyl moieties being quenched by the CNT walls,while the long component of the signal is due to the speciesarising from quenching localized mainly on the CNT moiety.Thus, the spectra at long delay times when the signal ofazaxanthyl triplets has disappeared originate mainly fromthe CNT moiety and do not show any features due to azax-anthyl units. This lack of spectral features due to azanthylunits in AZX-CNT is not totally unexpected considering thelow azaxanthyl fraction in the material. In agreement withour rationalisation the temporal profiles on the microsecondtimescale at different wavelengths in the range between 320and 800 nm were all coincident and can be attributed to asingle species. Considering precedents in the photochemistryof functionalized CNTs in where the walls of the nanotubesact as electron donors towards electron-acceptor moiet-ies,[2,22] we propose that the featureless continuous absorp-tion shown in Figure 9 for the laser flash photolysis ofAZX-CNT at delay times longer than 0.1 ms corresponds toan electron hole localized in the CNT moiety that originatesfrom photoinduced electron transfer from the CNT as donorto azaxanthyl as electron-acceptor terminus (Scheme 2). Toprovide support to our proposal that azaxanthyl triplets arequenched by the CNT walls, yielding a very short-lived tran-sient (52 ns) as compared to the triplet lifetime of azaxanth-yl 2 in acetonitrile (2.68 ms), we first ruled out the possibilityof azaxanthyl self-quenching. Since azaxanthyl units anch-ored to the nanotubes could be in close proximity with re-duced mobility, interaction between azaxanthyl neighbourscould be responsible for the shortening of the triplet life-time. To rule out self-quenching we synthesized an azaxan-thylium derivative containing two azaxanthyl units with sim-ilar structure as in AZX-CNT, but connected through anoxalic acid bridge (Scheme 3).

Laser flash photolysis of dimeric azaxanthyl 5 was per-formed at different concentrations in the range 6.4 to19 mgmL�1. As in the case of azaxanthone 1 and azaxanthy-lium compound 2, two transients corresponding to the trip-let excited state at 620 nm and the ketyl species at 330 nmwere observed (see the Supporting Information). The tem-poral profile of the signal monitored at 640 nm was fitted to

two first-order kinetics with lifetimes of 2.1 and 8.2 ms re-spectively. We interpret these two lifetimes as indicating twodifferent populations of triplet excited states, probably dueto in-cage and out-of-cage association with the charge-bal-ancing anion. Importantly, the data of the azaxanthyl dimergive a triplet lifetime very similar to that of the monomericazaxanthyl 2, that is, no inter- or intramolecular self-quench-ing seems to be responsible for the shortening of the tripletlifetime observed for AZX-CNT. Thus, it is reasonable topropose that quenching of the exciton localized on AZX isdue to the CNT moiety.

To support the idea that CNT can quench the triplet excit-ed state of azaxanthone, we studied the behavior of the trip-let of azaxanthyl compound 2 in the presence of increasingconcentrations of short, soluble CNT. With increasingamount of CNT two effects on the triplet excited state of 2occur: the top absorbance decreases and the lifetime of thesignal decreases. The diminution of the initial triplet absorb-ance by addition of CNT could be due to an internal filtereffect, since CNT absorbs in the whole UV/Vis range andalso at the excitation wavelength (266 nm). The possibilitythat static quenching arising from ground-state complexa-tion between compound 2 and CNT is responsible, at leastin part, for the observed decrease in top absorbance can notbe ruled out, particularly considering the chemical shifts ob-served in the 1H NMR spectrum of AZX-CNT compared tocompound 2. However, the fact that no changes are ob-served in the UV/Vis spectrum of azaxanthyl 2 upon addi-tion of CNT did not provide spectroscopic support to com-plexation between 2 and CNT in the ground state. Thesecond effect observed in the behavior of azaxanthyl tripletsin the presence of the CNT was shortening of the lifetime.Figure 11 shows some selected decays monitored at 640 nmto illustrate the changes occurring in the kinetics due to thepresence of CNT. A quantitative determination of thequenching constant is, however, complicated by the fact thatthe decay does not fit to single mono-exponential kinetics.

Independent plots of the short- and long-lifetime compo-nents as a function of the amount of CNT (Figure 12) clear-ly show lifetime shortening of both components, which indi-cates dynamic quenching of the azaxanthyl triplet by CNT.The slope of the plot of t2 versus the amount of CNT issmaller than for t1. In contrast to quenching of the triplet

Scheme 3. Synthetic route to and structure of azaxanthylium dimer 5used to refute self-quenching.

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excited state by CNT, the signal corresponding to the ketylradical at 330 nm does not experience quenching by CNT.(A full set of transient decays is provided in the SupportingInformation.) Therefore, based on the absence of inter- andintramolecular self-quenching for the azaxanthyl dimer andthe observation of CNT triplet quenching, we propose thatin the case of AZX-CNT the short-lived component ob-served in the transient signal monitored at 640 nm corre-sponds to residual triplet excitons located on the AZXmoiety undergoing quenching by the CNT moiety ratherthan AZX self-quenching. As mentioned above, except forvery short delay times, the transient spectrum recorded forAZX-CNT is due to the absorption localized on the CNTthat we have attributed to an electron hole (h+) on CNT. Inprevious studies with functionalized CNTs that undergophotoinduced electron transfer, it was observed that methylviologen (MV2+) can quench electrons delocalized on CNTs

to give methyl viologen radical cation (MV·+), which iseasily detected by its characteristic optical spectrum.[22]

In our case, addition of MV2+ to AZX-CNT did not allowthe detection of any difference in the transient spectrum, inline with our proposal in Scheme 2 that the spectrum shownin Figure 9 is that of a hole (h+) and not an electron on thenanotube. The fact that no signal attributable to the electronlocated on the AZX moiety is observed (probably due tothe low AZX fraction) is consistent with the fact that MV·+

is not detected.

Singlet-oxygen generation : To explore in some detail the in-teraction with O2, we performed near-infrared (NIR) lumi-nescence studies (Figure 13). Azaxanthone triplet excited

state is quenched by oxygen through an energy-transfermechanism whereby singlet oxygen is generated.[27] Consid-ering that the transient absorption spectrum of AZX-CNTdoes not show clear features corresponding to the triplet ex-cited state of the AZX moiety, an indirect way to investigatethe dynamics and concentration of AZX triplet excited stateis through its ability to generate singlet oxygen. Singletoxygen can be selectively monitored by its phosphorescenceemission appearing as a narrow band at 1270 nm in the NIRregion.[23] On the other hand, the NIR region is also of inter-est due to the emission of CNTs that occurs in this part ofthe electromagnetic spectrum.[24]

For azaxanthone 1 in acetonitrile, the typical spectrumcorresponding to 1O2 phosphorescence emission was ob-served. Likewise 1O2 is also observed for the azaxanthyl pre-cursor 2. Using methylene blue (MB) as standard (FACHTUNGTRENNUNG(1O2)=

0.52) and measuring the intensity of the 1O2 emission at1270 nm for different azaxanthone concentrations, we deter-mined the quantum yield for 1O2 formation for azaxanthone1 and azaxanthyl 2 as 0.39 and 0.32, respectively. In sharpcontrast to the behavior of compound 2, when covalentlybonded to CNT, the quantum yield for 1O2 formation, al-though detectable, was dramatically reduced to 0.015, thatis, 34 times lower than MB.[25] Interestingly the observed 1O2

emission intensity is of comparable magnitude to that for

Figure 11. Temporal profile of the triplet excited state of azaxanthyl de-rivative 2 measured at 640 nm upon 266 nm laser excitation for N2-purged acetonitrile solutions in the presence of increasing amounts ofCNT.

Figure 12. Plot of the short (t1 ~) and long (t2 *) lifetimes as a functionof the concentration of CNT.

Figure 13. NIR spectra recorded about 2 ms after laser excitation of ace-tonitrile solutions of AZX-CNT under different atmospheres.

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CNT, whereas in reality unmodified CNT emits with a quan-tum yield about 400 times higher than that of 1O2. This im-plies that the NIR emission from CNT is largely suppressedby AZX derivatization, and further that quenching of AZXtriplet does not yield the nanotube NIR emissive state, con-sistent with our proposal that CT is the dominant interac-tion. The very small 1O2 yield is also consistent with the factthat the decay (region A in Figure 10) is essentially unaffect-ed by oxygen. In fact, it is remarkable that this emission canbe readily detected in spite of its small yield.

Conclusions

A derivative of the xanthone chromophore, one of the fa-vourite probes in photochemistry, has been covalently at-tached to short single-walled carbon nanotubes through aC11 chain connected to mercaptoethylamido groups. The re-sulting entity containing 8.5 wt % of azaxanthyl groups issoluble in water and some organic solvents, and this allowsfirm 1H NMR spectroscopic evidence of covalent binding tobe obtained. The triplet excited state of azaxanthone isquenched by CNT with and without covalent bonding, butwhen it is covalently bound, the transient spectra are com-patible with the photogeneration of electron holes throughelectron transfer from CNT to excited azaxanthyl units. Thisindicates that the covalent bond enhances the interaction ofthe two subunits. We have devised a simple way to injectelectron–hole pairs into CNT and characterized their dy-namic behavior as geminate pairs, as well as the behavior ofthe hole once the electron has been scavenged. This ap-proach may prove useful in studies leading to nanophotonicapplications, as well in the understanding of the way inwhich different modifications influence nanotube photonicproperties.

Experimental Section

Materials : CNT was a commercial sample (www.carbolex.com) and waspurified by oxidative acid treatment with 3 m HNO3 at 120 8C for 12 h.Purified CNTs were subjected to oxidative shortening by ultrasonication/heating with a 3/:1 (v/v) mixture of 96% H2SO4 and 30% HNO3 as previ-ously reported.[16] Mercaptoethylamide-functionalized CNT (SH-CNT)was then prepared by treating purified, short CNTs with thionyl chloridein DMF for 2 h at room temperature followed by addition of an excess of2-mercaptoethanamine, as reported in the literature.[15]

Preparation of AZX-CNT: Anhydrous argon-purged acetonitrile (5 mL)was added to a mixture of azaxanthylium derivative 2 (50 mg), AIBN(20 mg), and mercaptoethylamido-functionalized CNTs (100 mg), and themixture was sonicated for 1 h to obtain a black dispersion. Then the sus-pension was stirred at reflux under an argon atmosphere for 48 h. Thenthe reaction mixture was diluted with acetonitrile and filtered through a0.2 mm PTFE membrane. The isolated black material was washed exhaus-tively several times with acetonitrile, redispersed in acetonitrile by soni-cation and finally centrifuged at 15 000 rpm for 2 h. Then the solvent wasdecanted and AXZ-CNT washed again with toluene/diethyl ether (10/1).The product was then vacuum-dried at 30 8C for 1 d. The functionalizednanotubes were quite soluble in DMSO, giving a dark solution. The

1H NMR spectrum of AZX-CNT (Figure 6) was recorded in [D6]DMSOwith TMS as internal standard on Varian Gemini 400 MHz spectrometer.

Photophysical measurements : Laser flash photolysis were carried outwith a LuzChem LFP-111 System and the fourth harmonic of a Nd:YAGlaser (Minilite-II 266 nm, 7 ns fwhp, 5 mJ/pulse) as excitation source. Thesamples were diluted to an absorbance of around 0.15–0.20 units andthen introduced into 1� 1 cm2 cuvettes capped with septa, which werepurged with N2 or O2 for at least 15 min before measurements. NIRemission studies were carried out with a Peltier-cooled (�62 8C) Hama-matsu NIR detector operating at 900 V coupled with a computer-con-trolled grating monochromator. For excitation, the fourth harmonic(266 nm, 7 ns fwhh, 50 mJ/pulse) of the primary beam of a Nd:YAG laseror an Nd:YAG-pumped optical parametric oscillator operating at 540 nm(7 ns fwhh, 40 mJ/pulse) was used. The solutions were placed in 1� 1 cm2

Suprasil quartz cuvettes capped with septa. The solutions were purgedwith N2 or O2 for at least 15 min before measurement. The system wascontrolled with a PC running LuzChem LFPv3 software.

Acknowledgements

Financial support by the Spanish DGT (CTQ2006-05869) and CanadianNSERC is gratefully acknowledged. R.M. thanks the Spanish Ministry ofEducation for a postgraduate Scholarship.

[1] P. K. Mohseni, G. Lawson, C. Couteau, G. Weihs, A. Adronov, R. R.LaPierre, Nano. Lett. 2008, 8, 4075 –4080.

[2] M. Alvaro, C. Aprile, B. Ferrer, H. Garcia, J. Am. Chem. Soc. 2007,129, 5647 –5655.

[3] J. C. Scaiano, J. Am. Chem. Soc. 1980, 102, 7747 –7753.[4] N. Mohtat, F. L. Cozens, J. C. Scaiano, J. Phys. Chem. B 1998, 102,

7557 – 7562; G. C. Ferreira, C. C. Schmitt, M. G. Neumann, J. Braz.Chem. Soc. 2006, 17, 905 – 909; L. T. Okano, T. C. Barros, D. T. H.Chou, A. J. Bennet, C. Bohne, J. Phys. Chem. B 2001, 105, 2122 –2128; C. Ley, F. Morlet-Savary, J. P. Fouassier, P. Jacques, J. Photo-chem. Photobiol. A 2000, 137, 87 –92; M. Barra, K. A. Agha, Supra-mol. Chem. 1998, 10, 91 –95; M. Barra, J. C. Scaiano, Photochem.Photobiol. 1995, 62, 60 –64; Y. Liao, J. Frank, J. F. Holzwarth, C.Bohne, J. Chem. Soc. Chem. Commun. 1995, 199 –200; C. Bohne, M.Barra, R. Boch, E. B. Abuin, J. C. Scaiano, J. Photochem. Photobiol.A 1992, 65, 249 – 265; M. Barra, C. Bohne, J. C. Scaiano, J. Photo-chem. Photobiol. A 1991, 54, 1 –5; M. Barra, C. Bohne, J. C. Scaiano,J. Am. Chem. Soc. 1990, 112, 8075 – 8079.

[5] M. Goez, B. Hussein, M. Hussein, Phys. Chem. Chem. Phys. 2004, 6,5490 – 5497; C. Coenjarts, J. C. Scaiano, J. Am. Chem. Soc. 2000, 122,3635 – 3641.

[6] J. C. Scaiano, D. Weldon, C. N. Pliva, L. J. Martinez, J. Phys. Chem.A 1998, 102, 6898 – 6903.

[7] J. C. Scaiano, J. Am. Chem. Soc. 1980, 102, 7747 –7753; A. Gilbert, J.Baggott, Essentials of Molecular Photochemistry, Blackwell, Oxford,1990.

[8] N. J. Turro, Modern Molecular Photochemistry, Benjamin Cummings,Menlo Park, 1978.

[9] C. Bohne, R. W. Redmond, J. C. Scaiano in Photochemistry in Or-ganized and Constrained Media (Ed.: V. Ramamurthy), VCH, Wein-heim, 1991, Chap. 3, pp. 79–132; F. Wilkinson, G. Kelly in Hand-book of Organic Photochemistry, Vol. 1 (Ed.: J. C. Scaiano), CRCPress, Boca Raton, 1989, p. 293; F. Wilkinson, D. R. Worrall, Proc.Indian Acad. Sci. Chem. Sci. 1992, 104, 287 –298; L. M. Hadel inHandbook of Organic Photochemistry (Ed.: J. C. Scaiano), CRCPress, Boca Raton, 1989.

[10] L. J. Martinez, J. C. Scaiano, J. Phys. Chem. A 1999, 103, 203 – 208.[11] I. Carmichael, G. L. Hug in Handbook of Organic Photochemistry,

Vol. I (Ed.: J. C. Scaiano), CRC Press, Boca Raton, 1989, pp. 369 –403.

www.chemeurj.org � 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2009, 15, 8751 – 87598758

H. Garc�a et al.

[12] A. Garner, F. Wilkinson, J. Chem. Soc. Faraday Trans. 2 1976, 72,1010 – 1020.

[13] K. Maruyama, M. Yoshida, I. Tanimoto, J. Osugi, Rev. Phys. Chem.Jpn. 1969, 39, 117; K. Maruyama, M. Yoshida, Osugi, Rev. Phys.Chem. Jap. 1969, 39, 123 – 130; M. K. Kalinowski, Z. R. Grabowski,Trans. Faraday Soc. 1966, 62, 926 – 935; M. Sakamoto, X. C. Cai, M.Hara, M. Fujitsuka, T. Majima, J. Phys. Chem. A 2005, 109, 2452 –2458; E. C. Lathioor, W. Leigh, J. Photochem. Photobiol. A 2006, 82,291 – 300.

[14] D. M. Guldi, G. M. A. Rahman, M. Prato, N. Jux, S. H. Qin, W.Ford, Angew. Chem. 2005, 117, 2051 – 2054; Angew. Chem. Int. Ed.2005, 44, 2015 –2018; D. M. Guldi, Nature 2007, 447, 50– 51.

[15] D. M. Guldi, A. Rahman, V. Sgobba, C. Ehli, Chem. Soc. Rev. 2006,35, 471 –487.

[16] J. W. Arbogast, A. P. Darmanyan, C. S. Foote, Y. Rubin, F. N. Die-derich, M. M. Alvarez, S. J. Anz, R. L. Whetten, J. Phys. Chem.1991, 95, 11– 12; Di Valentin, M. A. Bisol, G. Agostini, P. A. Liddell,G. Kodis, A. L. Moore, T. A. Moore, D. Gust, D. Carbonera, J. Phys.Chem. B 2005, 109, 14 401 – 14409; T. W. Ebbesen, K. Tanigaki, S.Kuroshima, Chem. Phys. Lett. 1991, 181, 501 –504; M. Gevaert, P. V.Kamat, J. Phys. Chem. 1992, 96, 9883 – 9888; V. A. Nadtochenko,N. N. Denisov, I. V. Rubtsov, A. S. Lobach, A. P. Moravskii, Chem.Phys. Lett. 1993, 208, 431 –435; J. L. Segura, N. Martin, D. M. Guldi,Chem. Soc. Rev. 2005, 34, 31 –47.

[17] C. Aprile, R. Martin, M. Alvaro, J. C. Scaiano, H. Garcia, Chem.Eur. J. 2008, 14, 5030 –5038.

[18] H. Murakami, N. Nakashima, J. Nanosci. Nanotechnol. 2006, 6, 16 –27; N. Nakashima, T. Fujigaya, Chem. Lett. 2007, 36, 692 – 697; D.Tasis, N. Tagmatarchis, V. Georgakilas, M. Prato, Chem. Eur. J. 2003,9, 4000 –4008; N. Tagmatarchis, M. Prato, D. M. Guldi, Physica E2005, 29, 546 –550.

[19] J. L. Hudson, M. J. Casavant, J. M. Tour, J. Am. Chem. Soc. 2004,126, 11158 –11159; J. L. Bahr, J. M. Tour, J. Mater. Chem. 2002, 12,1952 – 1958.

[20] K. Balasubramanian, M. Burghard, Small 2005, 1, 180 –192; R.Saito, H. Kataura, Carbon Nanotubes 2001, 80, 213 –246.

[21] A. Kukovecz, C. Kramberger, V. Georgakilas, M. Prato, H. Kuzma-ny, Eur. Phys. J. B 2002, 28, 223 –230; C. D. Scott, A. Povitsky, C.Dateo, T. Gokcen, P. A. Willis, R. E. Smalley, J. Nanosci. Nanotech-nol. 2003, 3, 63 –73; E. Menna, F. Della Negra, M. Dalla Fontana,M. Meneghetti, Phys. Rev. B 2003, 68, 193412; A. C. Dillon, M. Yu-dasaka, M. S. Dresselhaus, J. Nanosci. Nanotechnol. 2004, 4, 691 –703; Z. T. F. Luo, R. Li, S. N. Kim, F. Papadimitrakopoulos, Phys.Rev. B 2004, 70, 245429; L. Lafi, D. Cossement, R. Chahine, Carbon2005, 43, 1347 – 1357; M. Selfi, D. K. Ross, A. Giannasi, Carbon2007, 45, 1871 –1879.

[22] M. Alvaro, C. Aprile, P. Atienzar, H. Garcia, J. Phys. Chem. B 2005,109, 7692 –7697.

[23] B. Cojocaru, M. Laferriere, E. Carbonell, V. Parvulescu, H. Garcia,J. C. Scaiano, Langmuir 2008, 24, 4478 – 4481.

[24] M. J. O�Connell, S. M. Bachilo, C. B. Huffman, V. C. Moore, M. S.Strano, E. H. Haroz, K. L. Rialon, P. J. Boul, W. H. Noon, C. Kit-trell, J. Ma, R. H. Hauge, R. B. Weisman, R. E. Smalley, Science2002, 297, 593 – 596; Y. Oyama, R. Saito, K. Sato, J. Jiang, G. G.Samsonidze, A. Gruneis, Y. Miyauchi, S. Maruyama, A. Jorio, G.Dresselhaus, M. S. Dresselhaus, Carbon 2006, 44, 873 – 879.

[25] R. Schmidt, Chem. Phys. Lett. 1988, 151, 369.[26] B. Gao, Y. Zhang, J. Zhang, J. Kong, Z. Liu, J. Phys. Chem. C 2008,

112, 8319 –8323.[27] J. C. Scaiano, T. J. Connolly, N. Mohtat, C. N. Pliva, Can. J. Chem.

1997, 75, 92–97.Received: February 10, 2009

Published online: July 14, 2009

Chem. Eur. J. 2009, 15, 8751 – 8759 � 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 8759

FULL PAPERElectron–Hole Pairs in Carbon Nanotubes