Supporting Information
© Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, 2009
Photoinduced Formation and Characterization of Electron-Hole Pairs in
Azaxanthylium Derivatized Short Single Wall Carbon Nanotubes
Roberto Martín,a Liliana B. Jiménez,b Mercedes Álvaro,a J. C. Scaianob and Hermenegildo
García.a
a
Instituto de Tecnología Química CSIC-UPV Universidad Politécnica de Valencia
Av. De los Naranjos s/n, 46022 Valencia, Spain.
Fax: +34963877809
b
Department of Chemistry, University of Ottawa
10 Marie Curie, Ottawa K1N 6N5, Canada.
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EXPERIMENTAL SECTION
Materials and Instrumentation
1
H NMR and 13C NMR spectra were recorded in CD3CN or [d 6]-DMSO as solvents on a Bruker
300 apparatus and, in the case of AZX-sSWNT, it was used a Varian Gemini 400 MHz
spectrometer. Chemical shifts are given in δ (ppm) using TMS as standard. UV-vis absorption
spectra were obtained in acetonitrile using quartz cuvettes on a Shimadzu spectrophotometer.
FT-IR spectra were recorded on a Nicolet Impact 410 spectrophotometer using KBr disks or selfsupported wafers compressed to 2 Ton x cm-2 for 2 min.
The starting samples of single walled carbon nanotubes used in this study were obtained from
Carbolex (www.carbolex.com) and have a carbonaceous purity of 80-90%. Azaxanthone was
purchased from Alfa Aesar, 11-bromoundecene, 10-chloro-1-decanol, oxalyl dichloride,
triethylamine, AIBN, thionyl chloride, aminoethanethiol and all the anhydrous solvents were
purchased from Sigma-Aldrich and used as received.
Functionalization of SWNTs
Purification of commercial SWNTs
1 g of commercial raw HiPCO (High pressure carbon monoxide) single-walled carbon nanotubes
(sSWNTs or CNTs) was refluxed in 500 ml 3 M HNO3 at 120ºC for 12 h; a reaction temperature
of 120°C was optimized in our experiment. Then, the mixture was diluted with distilled water
and cooled to room temperature. The excess of acid was removed performing five consecutive
centrifugation-redispersion cycles with Milli-Q water until the pH value of the filtrate was
neutral. Finally, they were submitted to overnight freeze-drying to obtain a dust-like material.
This procedure removes all catalyst particles contaminating the commercial sample. The
efficiency of the process was confirmed by TG analysis that shows complete combustion of the
material. The obtained material was used as the initial material for the following procedures.
CNT cutting
Purified HiPCO single-walled carbon nanotubes were cut by an ultrasonication / heating method
using a mixture of acids. An acid solution consisting in a 3:1 (vol/vol) mixture of 96% H2SO4
and 30% HNO3 was prepared immediately prior to use. Then, 150 mg of purified SWNTs were
sonicated at 60ºC in 8 ml of the acid mixture during 45-60 minutes. After this time, several
cycles of centrifugation-redispersion in Milli-Q water are performed until the pH of the solution
was neutral and finally the suspension was freeze-dried to obtain as residue the soluble short
SWNT. The average length of the cut sSWNT estimated by TEM microscopy is 500 nm.
Synthesis of mercaptoethylamido-functionalized CNTs (SH-CNT)
Chlorination of CNTs to transform carboxylic acid groups into acyl chlorides was carried out
after purification and cutting of commercial SWNT. Then, the resulting sample (500 mg) was
first sonicated in 60 mL of anhydrous DMF to give a homogeneous suspension and subsequently
oxalyl chloride (1.5 mL) was added dropwise to the sSWNT suspension at 0°C under N2. The
S2
mixture was stirred at 0°C for 2 h and then at room temperature for an additional 2 h. Finally, the
temperature was raised to 70 °C, and the mixture was stirred overnight. To remove the excess of
oxalyl chloride, the chlorinated CNT-COCl sample was collected by filtration through a PTFE
membrane (pore size 0.2 µm) and then re-dispersed in 60 mL of anhydrous DMF.
The mercaptoethylamide-functionalized CNTs were obtained by adding a DMF solution of
aminoethanethiol into the dispersion of CNT-COCl and heating overnight with magnetic
stirring at 90°C. The sample was purified by consecutive centrifugation/redispersion cycles and
final freeze-drying.
Synthesis of AZX-sSWNT
Anhydrous argon-purged acetonitrile (5 mL) was added to a mixture of azaxanthyl (2) (50 mg),
AIBN (20 mg) and mercaptoethylamido-functionalised CNTs (100 mg) and the mixture was
sonicated for 1 h to obtain a black dispersion. Then, the suspension was stirred at reflux under an
argon atmosphere for 48 h. After this time the reaction mixture was diluted with acetonitrile and
filtered through a 0.2 mm PTFE membrane. The isolated black material was washed
exhaustively several times with acetonitrile, redispersed in acetonitrile by sonication and finally
centrifuged at 15 000 rpm for 2 h. After this time, the solvent was decanted and the AXZ-CNT
were washed again with a mixture of toluene and diethyl ether (10:1). The samples were then
vacuum dried at 30º C for 1 day. The functionalised nanotubes were quite soluble in DMSO,
giving a dark solution.
∆ Absorbance / a.u.
0.25
0.2
0.15
0.1
0.05
0
250
300
350
400
450
500
550
600
Wavelength, nm
Figure S1: UV-vis spectrum of AZX-CNT, 46 mg/mL in ACN
Spectroscopic details
Azaxanthyl (2).
11-bromoundecene (355 mg, 1.52 mmol) was added to a solution of 5H-chromen-5-one[2,3c]pyridine (azaxanthone 300 mg, 1.52 mmol) in dry CH3CN (5 ml). After stirring at reflux
S3
temperature for 24 h, the solvent was removed in vacuum and diethyl ether was added. AZXt
was obtained by precipitation (525 mg, 82%) as a white solid.
H NMR (300 MHz, CDCl3): δ (ppm) = 8.79 (dd, J1 =6.0 Hz, J2 = 2.25 Hz, 1H; arom. H), 8.69
(dd, J1 =6.0 Hz, J2 = 2.25 Hz, 1H; arom. H), 8.28 (dd, J1 =7.8 Hz, J2 = 1.8 Hz, 1H; arom. H),
7.91 (ddd, J1 = 6.6 Hz, J2 = 4.5 Hz, J3 = 1.8 Hz, 1H; arom H), 7.70 (d, J1 =7.8 Hz, 1H; arom. H),
7.55 (dd, J1 = 7.8 Hz, J2 = 4.5 Hz, 1H; arom H), 7.51 (t, J =6.6 Hz, 1H; arom H), 3.51 (t, J =6.9
Hz, 2H; N-CH2), 1.33 (bs,16H; alkyl. H); 13C NMR (300 MHz, CDCl3): δ (ppm) = 177.87,
160.82, 156.20, 154.81, 139.69, 137.22, 136.26, 126.56, 125.14, 121.94, 121.78, 118.86, 117.15,
114.19, 34.84, 33.91, 33.06, 29.55, 29.52, 29.25, 29.13, 28.86, 28.25; IR (KBr): ν (cm-1) = 3076,
2934, 2856, 1744, 1673, 1595, 1556, 1472, 1414, 1348, 1200, 1103, 935, 897, 832, 677, 625,
535, 444.
1
∆ Absorbance / a.u.
1.5
1
0.5
0
200
300
400
500
600
700
800
Wavelength, nm
Figure S2: UV-vis spectrum of Azaxanthyl (2) 9 mg/mL in ACN
bis-(10-chlorodecyl) oxalate (4)
10-Chlorodecanol (2 g, 9.34 mmol) was slowly dropped to a solution of oxalyl chloride (0.65 g,
5.1 mmol) and triethylamine (0.72 ml,5.1 mmol) in dry CH2Cl2 (15 ml) at 0ºC under an argon
atmosphere. After 1 hour it was allowed to reach room temperature and then heated at reflux
overnight. Removal of the solvent after filtration of the triethyl ammonium chloride gave the
product as a white solid (2.01 g, 92%)
H NMR (300 MHz, CD3CN): δ (ppm) = 4.29 (t, J1 =6.6 Hz, 4H), 3.63 (t, J1 =6.6 Hz, 4H), 1.78
(m, 4H), 1.36 (bs, 28H); 13C NMR (300 MHz, CD3CN): δ (ppm) = 158.04, 67.13, 45.16, 32.60,
29.45, 29.31, 29.06, 28.81, 28.23, 26.83, 25.65; IR (KBr): ν (cm-1) = 2930, 2858, 1754, 1654,
1468, 1303, 1222, 1175, 1030, 937, 784, 717, 651; GC-MS calcd. for C22H40Cl2O4 + (M+) m/z:
438,23 found 438 (M+)
1
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Azaxanthyl dimer (5)
An anhydrous CH3CN (10 ml) solution of bis-(10-chlorodecyl) oxalate(175.2 mg, 0.4 mmol) was
mixed with azaxanthone (157.3 mg, 0.79 mmol) and heated at reflux temperature for 48 h until
appearance of a white solid (291.5 mg, 87.5%) which was isolated by filtration washing with
diethyl ether.
H NMR (300 MHz, CD3CN): δ (ppm) = 8.76 (dd, J1 = 4.8 Hz, J2 = 2.1 Hz, 2H; arom. H), 8.65
(dd, J1 = 7.8 Hz, J2 = 1.8 Hz, 2H; arom. H), 8.23 (dd, J1 = 7.8 Hz, J2 = 1.6 Hz, 2H; arom. H),
7.87 (ddd, J1 = 7.2 Hz, J2 = 4.5 Hz, J3 = 1.8 Hz, 2H; arom. H), 7.66 (d, J1 = 8.5 Hz, 2H; arom.
H), 7.51 (dd, J1 = 7.5 Hz, J2 = 4.5 Hz, 2H; arom. H), 7.47 (t, J = 7.5 Hz, 2H; arom H), 4.25 (t, J
= 6.6 Hz, 2 x 2H; N-CH2-CH2), 3.59 (t, J = 6.6 Hz, 2 x 2H; O-CH2-CH2), 1.73 (m, 2 x 4H; CH2CH2-CH2), 1.31 (bs, 2 x 12H; CH2-CH2-CH2); 13C NMR (300 MHz, CD3CN): δ (ppm) = 193.44,
186.64, 159.94, 157.45, 153.92, 136.31, 135.36, 125.65, 124.23, 121.01, 120.87, 117.96, 66.24,
44.78, 31.91, 29.77, 28.58, 28.30, 28.04, 27.48, 26.05, 24.94, 24.39; IR (KBr): ν (cm-1) = 3062,
2920, 2856, 1937, 1756, 1672, 1595, 1465, 1413, 1336, 1226, 1103, 935, 889, 754.
1
LASER FLASH PHOTOLYSIS STUDY
Azaxanthyl (2) in acetonitrile under Argon.
Figure S3 shows the decay of AZXt under argon in acetonitrile. The main lifetime component of
2.7 µs is consistent with similar “microsecond” lifetimes for the unsubstituted azaxanthone.
Figure S3: Decay of triplet Azaxanthyl (2) monitored at 640 nm following 266 nm laser
excitation in acetonitrile under argon.
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Azaxanthyl dimer in acetonitrile under Argon
Control molecule azaxanthyl dimer was prepared in order to test for possible triplet selfquenching participation induced by the short distances that result from tethering the azaxanthone
moiety to supramolecular structures, such as the CNTs. In fact as shown in Figure S4, the
lifetimes are quite long, and when approximated as first order processes are 3.8 µs and 5.7 µs for
the high (23 µM) and low (8 µM) concentrations, respectively in acetonitrile under nitrogen. The
difference between these two lifetimes most likely reflects contributions from second order
kinetics due to triplet-triplet annihilation, and are common under these conditions, and are
expected to contribute more when the signals are stronger.
Figure S4: Decay of triplet azaxanthyl dimer monitored at 640 nm following 266 nm laser
excitation in acetonitrile under nitrogen, for two different substrate concentrations.
Detection of NT-supported hole in long time scales
Figure 10 (main paper) shows the formation and early decay of the NT –supported hole under
nitrogen and under oxygen. For the latter, the long time decay is illustrated in Figure S5, leading
to a lifetime of approximately 12 µs.
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Figure S5: Transient signals monitored at 600 nm after laser excitation of AZX-CNT in
acetonitrile, following 266 nm laser excitation under oxygen.
Intermolecular quenching of triplet Azaxanthyl (2) by CNTs.
These experiments were designed to address the question as to whether there was a real
advantage in tethering the azaxanthone moiety to the CNT, or if simple non-covalent association
was strong enough to yield a supramolecular structure with essentially the same properties. To
this effect, we monitored the quenching of the triplet state of azaxanthyl (2) approximately 26
µM by CNT in acetonitrile under a nitrogen atmosphere. All experiments were carried out using
266 nm laser excitation under conditions comparable to those employed with AZX-CNT. Figure
S6 illustrates the triplet decay monitored at 600 nm in short (top) and long (bottom) time scales
in the presence of various CNT concentrations. At concentrations of sSWNT above 3 µg/ml the
decays show readily observable two-component decay, with a fast decay which in the case of
Figure S6 (top) correspond to lifetimes of 240 ns and 160 ns for CNT concentrations of 4.6
µg/ml and 7.4 µg/ml, respectively. This fast component is attributed to azaxanthyl (2)
physisorbed on the nanotubes. Variable lifetimes may reflect the population of different sites at
different concentrations. Note that the small decrease in maximum absorbance is simply due to
competitive absorbance (at 266 nm) by the CNT added as quenchers.
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Figure S6: Decay of the triplet state of azaxanthyl (2) monitored at 600 nm in the presence
of various concentrations of CNT. Note that the decay in the presence of 0.9 µg/ml CNT is
essentially identical to that in the absence of CNT. The top panel shows a magnification of
short time scales from the same traces in the lower panel.
Figure S6 shows the effect of the addition of CNT on the long component of the triplet xanthone
decay. That is, they correspond to the exponential fit of the long time data as shown in Figure S6
(bottom); we note that this figure only shows representative traces, while Figure S5 includes the
complete data set. The vertical scale in Figure S6 was selected to emphasize the relatively
modest change in decay kinetics.
Typical quenching plots in simple systems usually give linear plots, but in this case the curvature
is quite evident. Most likely as some of the more reactive CNT are committed to non-covalent
association (as described above) the remaining sites show decreased reactivity and account for
the negative curvature.
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Figure S7: Effect of the addition of sSWNT on the long component of the triplet xanthone
decay. The data have been derived from the traces of Figure S6 and from similar traces
(not shown).
The results of Figures S6 and S7 indicate that while Azaxanthyl (2) and CNT undergo some noncovalent association, the effect cannot be compared with that achieved upon covalent binding;
further, the association seems relatively weak and thus strongly dependent on the concentrations
used.
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