Carbon nanotube-acridine nanohybrids: Spectroscopic characterization of photoinduced electron transfer

Article abstract of DOI:10.1002/chem.200801863

Single-walled carbon nanotubes (NT) were covalently functionalized with either 9-phenyl acridine (PhA) or 10-methyl-9-phenyl acridinium (PhMeA ⁺). Absorption and fluorescence properties of acridine derivatives tethered to the nanotubes were studied in homogeneous dispersions. Exciplex emission was observed for NT functionalized with 9-phenylacridine. This phenomenon was attributed to an " intramolecular" interaction between excited phenyl acridine and carbon nanotubes. Interestingly, reverse photoinduced electron transfer from the nanotube to 10-methyl-9-phenylacridinium was detected for the NTPhMeA⁺ nanohybrid. This electron transfer led to a strong quenching of the acridinium fluorescence and to the formation of a metastable acridine radical. Evidence for the formation of this radical was obtained by ESR studies.

Full text of DOI:10.1002/chem.200801863

DOI: 10.1002/chem.200801863
Carbon Nanotube–Acridine Nanohybrids: Spectroscopic Characterization of
Photoinduced Electron Transfer
Nicolas Mackiewicz,[a] Jacques A. Delaire,*[b] A. William Rutherford,[c]
Eric Doris,*[a] and Charles Mioskowski[a]
Abstract: Single-walled carbon nano-
tubes (NT) were covalently functional-
ized with either 9-phenyl acridine
(PhA) or 10-methyl-9-phenyl acridini-
um (PhMeA+). Absorption and fluo-
rescence properties of acridine deriva-
tives tethered to the nanotubes were
studied in homogeneous dispersions.
Exciplex emission was observed for NT
functionalized with 9-phenylacridine.
This phenomenon was attributed to an
“intramolecular” interaction between
excited phenyl acridine and carbon
nanotubes. Interestingly, reverse photo-
induced electron transfer from the
nanotube to 10-methyl-9-phenylacridi-
nium was detected for the NT-
PhMeA+ nanohybrid. This electron
transfer led to a strong quenching of
the acridinium fluorescence and to the
formation of a metastable acridine rad-
ical. Evidence for the formation of this
radical was obtained by ESR studies.
Keywords:
acridines
·
carbon
nanotubes
·
electron transfer ·
photochromism · radicals
Introduction
aging charge transfer. In addition to the fundamental inter-
est in photoinduced electron transfer, these studies also
open the door to new applications such as photovoltaics,
solar energy conversion, photochemical water splitting and
sensor design.[1–4] Two main strategies have been developed
for the linking of chromophoric species to the nanotube:
non-covalent assembly by adsorption or p-stacking and co-
valent functionalization by various chemical reactions.[5–7]
Both covalent and non-covalent approaches have been used
to build nanocomposites for photoelectron-transfer applica-
tions. Among the different chromophores, porphyrins and
metalloporphyrins have been extensively investigated either
adsorbed[8–12] or covalently linked to the nanotube sur-
face.[13–18] In most cases, a strong quenching of the porphyrin
fluorescence was observed. This effect was attributed to an
efficient electron transfer from the heterocycle to the nano-
tube. Transient absorption measurements provided further
evidence of the photogenerated radical ion pair by detecting
the porphyrin radical cation.[8–10,14–16,19] Although precise ki-
netics are difficult to measure because of the interference
from other phenomena (absorption of the porphyrin triplet
state and non-linear light scattering of carbon nanotubes),
the ion pair is long-lived due to the extended delocalization
of the electron through the nanotube lattice and lifetimes of
about 100 nanoseconds have been estimated.[9,10] Photoin-
duced electron transfer has also been studied using other
electron-donating groups such as pyrene,[20] tetrathiafulva-
lene,[5] 2,5-diarylpyrazoline,[21] or ferrocene.[22]
Single-walled carbon nanotubes (NT) and their surface-
modified derivatives have attracted considerable interest as
promising devices in molecular-scale electronics. In particu-
lar, the semiconducting or metallic properties of carbon
nanotubes can be adjusted by light excitation of a chromo-
phore bonded to their surface. The extended p-electron
system makes NT-based materials very useful for the man-
[a] Dr. N. Mackiewicz, Dr. E. Doris, Dr. C. Mioskowski+
CEA, iBiTecS
Service de Chimie Bioorganique et de Marquage
91191 Gif-sur-Yvette (France)
E-mail: eric.doris@cea.fr
[b] Prof. J. A. Delaire
Ecole Normale Supꢀrieure de Cachan
Laboratoire de Photophysique et Photochimie Supramolꢀculaires et
Macromolꢀculaires
61 avenue du Prꢀsident Wilson
94235 Cachan Cedex (France)
E-mail: jdelaire@ppsm.ens-cachan.fr
[c] Dr. A. W. Rutherford
CEA, iBiTecS
Service de Bioꢀnergꢀtique
Biologie Structurale et Mꢀcanismes
91191 Gif-sur-Yvette (France)
[+] Deceased June 2, 2007
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801863.
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In this paper, we report the study of the photochemical
interaction of 9-phenylacridine and 10-methyl-9-phenylacri-
dinium with NTs. These chromophores are attractive for
several reasons: i) they are both fluorescent and can lead to
photoinduced charge/electron transfer reactions; ii) the ex-
cited singlet state of phenylacridine could play a dual role
that is, electron donor or -acceptor depending on the partner
involved and iii) the acridinium ion is expected to act as an
electron acceptor in the excited state.[23–26]
Scheme 2. Synthesis of reference compounds 6 and 7.
To the best of our knowledge, comparative studies of
carbon nanotube-based acridineACTHNUGTRNEUNG(ium) nanohybrids have not
been investigated thus far. The aim of the present study is
to describe their variable behaviour towards charge-transfer
processes.
Characterization: The nanotube samples were characterized
by standard analytical techniques such as Raman spectrosco-
py, UV/Vis-NIR spectroscopy, thermogravimetric analysis
(TGA) and transmission electron microscopy (TEM). The
Raman spectra of pristine NT and NT-PhA were recorded at
1064 nm (Figure 1a). The intensity of the signal at 1278 cmꢀ1
(D band) was used as a probe to detect covalent functionali-
zation.[27] The relative intensity of the D band relative to the
G band at 1592 cmꢀ1 (ID/IG) increased by a factor 4.5, com-
pared with pristine nanotubes. This indicated a significant
conversion of sp2 to sp3-hybridized carbon atoms induced by
the grafting of the acridine which disrupted the extended p-
network.
Results and Discussion
Synthesis of the functionalized nanotubes: For this study,
two different carbon nanotube-based nanohybrids 4 and 5
(Scheme 1) were synthesized. Both hybrids were prepared
starting from acridine salt 1, which was converted in the first
step to 9-(4-aminophenyl)acridine (2), by coupling with ani-
line in presence of silver chloride (Scheme 1). The amino
Figure 1. a) Raman spectra of functionalized (g) and pristine nano-
tubes (c) at 1064 nm. b) UV/Vis-NIR absorption spectra of functional-
ized and pristine nanotubes in DMF (inset: zoom of the 320-420 nm
region). c) TGA of functionalized and pristine nanotubes.
Scheme 1. Synthesis of functionnalized nanotubes 4 and 5.
group of aniline was then oxidized to the corresponding di-
azonium salt 3 using nitrosonium tetrafluoroborate. Subse-
quent treatment of HiPCO carbon nanotubes with 3 then af-
forded functionalized NTs 4 (NT-PhA). A portion of 4 was
further reacted in refluxing methyl iodide to give the corre-
sponding acridinium–SWNT sample 5 (NT-PhMeA+). The
functionalized nanotubes 4 and 5 were found to be soluble
in common organic solvents. Our approach also necessitated
the preparation of reference samples 6 and 7 (Scheme 2).
These references were used as mixture with pristine NTs for
spectroscopic comparison with the covalently functionalized
nanotubes 4 and 5. Reference compound 6 (PhA) was thus
synthesized by hydrodediazoniation of 3 by hypophosphorus
acid and the corresponding acridinium 7 (PhMeA+) was ob-
tained by refluxing 6 in pure methyl iodide (Scheme 2).
Covalent functionalization was further confirmed by UV/
Vis-NIR absorption spectroscopy. Figure 1b shows the char-
acteristic interband transitions between van Hove singulari-
ties of pristine nanotubes and the loss of these transitions in
the functionalized sample 4. It is worthwhile to note that the
main UV absorption band of PhA (360 nm) was detected.
This allowed quantification of the dye bonded to the nano-
tube by correlating absorbance to concentration. Contribu-
tion of dye to the nanohybrid absorption was deduced by
subtracting pristine nanotube absorbance from that of NT-
PhA (inset, Figure 1b). Using this technique, the concentra-
tion of PhA on the nanotubes was estimated to ca.
1.1 mgmLꢀ1 starting from a 13.6 mgmLꢀ1 solution of NT-
PhA. This value corresponds to 8% wt PhA on NT.
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E. Doris, J. A. Delaire et al.
The degree of dye coverage was confirmed by TGA of
the nanohybrid which was carried out under argon (100 to
8008C; 108Cminꢀ1). The latter indicated a weight loss of
8% compared to pristine NT (Figure 1c). This value is simi-
lar to the one determined by UV spectroscopy and corre-
sponds to one functional group in 245 carbon atoms.
Taken together, these data suggest effective covalent func-
tionalization of the nanotube by acridine. The nanohybrids 4
and 5 were then evaluated for their photoinduced charge/
electron transfer properties and the process was monitored
by fluorescence spectroscopy.
Figure 3. Fluorescence decay of of NT-PhA (g) and PhA (c) in
DMF at 470 nm (excitation at 336 nm). Thicker lines correspond to
curves calculated by the apparatus function.
Photophysical properties: The steady-state fluorescence
spectrum of NT-PhA was recorded in DMF. The emission
spectrum of the nanohybrid was compared to that of PhA
only (Figure 2) and a mixture of NT/PhA (see Supporting
Information).
Table 1. Fluorescence lifetimes ti (in nanoseconds), pre-exponential fac-
tors ai and c2; residual for PhA and NT-PhA in DMF. The irradiation
wavelength was 336 nm.
a1
t1
a2
t2
a3
t3
c2
PhA
NT-PhA
113.27
16.33
0.14
0.37
1.17
1.31
24.63
0.03
6.38
1.94
our single photon counting setup. The spectral changes ob-
served for NT-PhA could neither be attributed to the well-
known quenching of the first excited singlet state of aromat-
ics by the nanotube nor to the formation of excimers.[6,28]
Even though excimers of acridine have been observed at
77 K in organic glasses[29] and in solid-state copolymers at
room temperature,[30] chromophoric concentration on the
nanotube is very low. Excimer formation between two adja-
cent PhA is thus not likely. However, acridine is also known
to form exciplexes with other aromatic moieties.[31,32] As NT
can be seen as a polyaromatic species, we speculate that the
observed emission is due to the formation of an exciplex be-
tween bonded PhA and the nanotube. This phenomenon
could originate from the relative flexibility of PhA on the
anchoring sp3 carbon atom which could allow p-orbital to
overlap, thus giving rise to an exciplex with the nanotube.
Another likely explanation to exciplex formation could be
that the nanohybrids form aggregates (bundles) in solution.
Excited PhA moieties from one nanohybrid may then inter-
act with a neighbouring NT. This second hypothesis could
explain the different lifetimes observed for the decay of ex-
ciplex emission, fitted with a triexponential law as a result
of different environments (exciplexes inside or at the pe-
riphery of the bundles). Noteworthy, while classical aromatic
species interacting with carbon nanotubes usually lose their
luminescence, the NT–PhA nanohybrid remains fluorescent.
While maximum emission of NT–PhA is centred at 468 nm
in DMF, a peculiar solvent effect was observed for THF,
with a maximum emission at 520 nm, and for ethanol at
435 nm. Furthermore, the intensity of the fluorescence de-
creases in the order THF > DMF ꢁ ethanol (see Support-
ing Information for details). Usually, increasing solvent po-
larity shifts exciplexes fluorescence towards longer wave-
length because of the stabilizing effect of the solvent on
charge transfer. Surprisingly, the trend is reversed since the
Figure 2. Fluorescence spectra of PhA (c) and NT-PhA in DMF
(g). Excitation wavelength: 360 nm. Inset: Excitation spectrum of NT-
PhA (g, emission wavelength: 468 nm) and absorption spectrum of
PhA (c) in DMF.
The fluorescence spectrum of the NT/PhA mixture has
the same maximum (430 nm) and shape as that of PhA and
the excitation spectrum of the 430 nm band corresponds to
the absorption of PhA (inset, Figure 2).
However, a decrease in intensity was observed for the
NT/PhA mixture. As PhA does not p-stack on the nanotube
surface (see Supporting Information for details), this de-
crease was attributed to an internal filter effect of the nano-
tube whose absorbance and light scattering properties con-
tribute to the drop of fluorescence. Interestingly, the fluores-
cence spectrum of the covalently functionalized NT-PhA
nanohybrid exhibited the lowest intensity and a bathochro-
mic shift of 38 nm. The excitation spectrum of the nanohy-
brid fluorescence (inset, Figure 2) is superimposed with ab-
sorption of PhA which demonstrates that PhA excitation is
responsible for the observed emission.
While time-resolved fluorescence decay for PhA in DMF
was correctly described by a mono-exponential with a decay
time of 140 picoseconds, the fluorescence decay of NT-PhA
could only be fitted using a three-exponential function
(Figure 3 and Table 1). The fastest component, which has
the largest pre-exponential factor, has a decay time as fast
as 30 picoseconds, which is nearby the resolution limit of
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maximum emission of the exciplex shifts to shorter wave-
lengths when the polarity and/or hydrogen-donating effect
of the solvent increases (see Supporting Information). This
result may be linked to the following observation from the
literature: when ethanol is added to a toluene solution of
9,10-di-n-propylanthracene and acridine, exciplex fluores-
cence drops sharply.[32] This phenomenon was interpreted by
the authors as a consequence of hydrogen bonding between
the nitrogen lone pair of acridine and ethanol. This hydro-
gen bonding and/or the increase of polarity, is at the origin
of the drastic changes in fluorescence quantum yield and
lifetime of acridine derivatives.[33–38] The lowest singlet excit-
Figure 4. Fluorescence spectra of PhMeA+ (c) and NT-PhMeA+ in
DMF (g). Excitation wavelength: 360 nm; Inset: Excitation spectrum
of NT-PhMeA+ (g, emission wavelength: 470 nm) and absorption
spectrum of PhMeA+ (c) in DMF.
ed state is now supposed to be a
non-polar solvents, the
1(n,p*) state is coupled with the
1(p,p*) state. However, in
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
latter and leads to an increase of non-radiative processes
like intersystem crossing. In the case of our NT–PhA nano-
hybrid, we propose that exciplex formation, with phenylacri-
dine acting as the electron donor, induces a decrease of the
electron density on the nitrogen atom and a destabilization
of the (n, p*) state. This leads to a strong fluorescence emis-
sion of the exciplex in nonpolar solvents. In protic medium
(e.g. ethanol) hydrogen bonding hampers the electron-do-
nating ability of phenylacridine, thus leading to a weak sta-
bilization of the exciplex. Although more polar than etha-
nol, DMF is non-protic and leads to an intermediate posi-
tion of the exciplex spectrum.
Contrarily to NT–PhA, the spectral changes observed for
the NT–PhMeA+ nanohybrid could not be attributed to the
formation of an exciplex even though exciplexes of acridini-
um cations with aromatics have already been described at
very low temperature.[39] Indeed, an exciplex always emits at
longer wavelength compared with the corresponding mono-
mers. As acridinium ions are already known to behave as
electron acceptors in catalytic reactions and in photoinduced
DNA cleavage via electron transfer,[40–42] we suggest here
that the strong fluorescence quenching of acridinium is the
result of
a fast electron transfer from the nanotube
Interestingly, dissimilar behaviour was observed for the
parent NT–PhMeA+ nanohybrid. While the UV/Vis absorp-
tion spectrum of PhA exhibited a single absorption band
centred at 360 nm, that of PhMeA+ (inset, Figure 4) showed
two main bands at 360 and 420 nm. By analogy with the ab-
sorption spectrum of acridinium ion,[35] the 360 nm band was
attributed to the second p ! p* transition and the 420 nm
band to the shift of the first p ! p* transition after methyl-
ation of the acridine tethered to the nanotube (5 from 4).
Fluorescence studies of 5 were then undertaken using the
same procedure as above. Fluorescence of NT–PhMeA+
was compared to that of PhMeA+ and a mixture of NT/
PhMeA+ (see Supporting Information). The steady-state
fluorescence spectra are shown in Figure 4, the fluorescence
emission of PhMeA+ being centered at 510 nm. When pris-
tine nanotubes were added to the acridinium solution, a
quenching of about 60% of the fluorescence was observed
with no shift. As previously discussed, the decrease in inten-
sity was attributed to an internal filter effect of the nano-
tubes since no p-stacking of chromophoric PhMeA+ on the
nanotube was observed.
(Scheme 3). Similarly with the discussion on exciplex forma-
Scheme 3. Photoinduced electron transfer between a nanotube and 10-
phenyl-9-methylacridinium iodide.
tion between NT and PhA, the electron transfer could also
occur “intermolecularly” between one nanotube and one ex-
cited PhMeA+ linked to another nanotube in a bundle.
Fluorescence quenching of 9-phenylacridinium cation by ar-
omatics has been extensively studied, as it gives a long-lived
radical of 9-phenylacridine resulting from fast electron
transfer.[43] Photoinduced intramolecular processes has also
been observed for donor–acceptor dyads such as methylacri-
The fluorescence of NT–PhMeA+ was very different from
that of PhMeA+ as the 510 nm fluorescence band disap-
peared and was replaced by a weak band peaking at 430 nm.
The excitation spectrum of this new band (inset, Figure 4)
was different from that of PhMeA+, so we attributed it to
an unidentified photoproduct. Hence a strong quenching of
fluorescence was observed for the nanohybrid. This quench-
ing leads to the appearance of fast decay components in the
time resolved fluorescence analysis of NT–PhMeA+ (see
Supporting Information).
dinium linked to a mesityl group (Acr+–Mes). This led the
+
C
C
formation of an electron transfer state (Acr –Mes ) with an
exceptionally long lifetime and a quenching of the fluores-
cence. In our case, the carbon nanotube acts as the electron
donor. At room temperature, the electron transfer is so fast
that no fluorescence from some excited precursor (exciplex)
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E. Doris, J. A. Delaire et al.
could be detected. However, such a precursor cannot be ex-
cluded at very short interval after excitation.
Conclusions
To validate the above hypothesis and find further evi-
dence for the emergence of an acridine radical, ESR studies
of an irradiated solution of NT–PhMeA+ were performed.
A solution of the NT–PhMeA+ nanohybrid in DMF was il-
luminated for 60 min with a 100 W mercury lamp (365 nm)
at room temperature and the ESR spectrum was immediate-
ly recorded at 4 K. A control experiment was run in parallel
by irradiating a suspension of HiPCO single-walled nano-
tubes that were mixed with PhMeA+. The two spectra are
depicted in Figure 5.
Single-walled carbon nanotubes were functionalized with
either 9-phenylacridine or 10-methyl-9-phenylacridinium
and their spectroscopic profiles were compared. Fluores-
cence spectrum of NT–PhA shows a large bathochromic
shift compared to PhA and the new fluorescence band was
attributed to the formation of an exciplex between grafted
PhA and the nanotube. This is a rare example where cova-
lent association of an aromatic molecule with a NT does not
lead to a drastic quenching of the fluorescence. To the best
of our knowledge, this is the first example of exciplex for-
mation between nanotubes and aromatic molecules. In con-
trast, fluorescence of the NT–PhMeA+ nanohybrid is effi-
ciently quenched. This quenching has been interpreted as a
consequence of a photoinduced electron transfer from the
nanotube to 10-phenyl-9-methylacridinium ion. The result-
ing 10-phenyl-9-methylacridinyl radical was detected by
ESR at 4 K. In the two nanohybrids, the nanotube plays a
dual role of electron-acceptor/-donor, depending on the
redox property of the excited partner (acridine/acridinium).
This new nanohybrid could be of interest in molecular elec-
tronics and photovoltaic applications.
Figure 5. ESR spectra at 4 K (9 GHz) of NT-PhMeA+ (g) and pristine
nanotubes mixed with PhMeA+ (c) in DMF after illumination at
365 nm.
Experimental Section
General information: Chemicals were purchased from Aldrich and used
as received. NTs (90% purity) were synthesized by the HiPCO process
and were purchased from CNI, Houston TX (USA). Solvents were dried
prior to use. Centrifugation was performed using an Eppendorf 5804 ap-
paratus. Sonication was done using either an ultrasonicating probe Bran-
son Sonifier 450 or an ultrasonicating Fisher Bioblock Scientific bath.
UV/Vis-NIR spectra were recorded in 1 cm quartz cuvettes on a Perkin–
Elmer Lambda 900 spectrophotometer. Fluorescence spectra were re-
corded on a Horiba Jobin Yvon Fluoromax-3 spectrofluorimeter. Ther-
mogravimetric analyses were performed with a TGA 92 Setaram at
108Cminꢀ1 until 8008C under ultrapure argon. Raman spectra were run
on a RFS100 Bruker spectrophotometer. The samples were excited at
1064 nm using the fundamental line of a Nd/YAG laser; spectral resolu-
tion was ca. 4 cmꢀ1. NMR spectra were recorded on a Bruker Advanced
While the ESR spectrum of the carbon nanotube/acridini-
um mixture showed no significant signal, a well defined iso-
tropic signal centred at g=2.0017 was detected for the NT–
PhMeA+ nanohybrid. The observed value is slightly shifted
compared to that of the free electron (g=2.0023). This dis-
crepancy was attributed to the presence of contaminating
iron that was used in the synthesis of HiPCO carbon nano-
tubes. Indeed, when the same type of experiments were con-
ducted with functionalized multiwalled carbon nanotubes
that contained no iron (from n-Tec), a signal centred at g=
2.0023 was observed (see Figure S9 in the Supporting Infor-
mation). These signals were attributed to the phenylacridin-
yl radical formed after photoinduced electron transfer from
400 with a resonance frequency of 400 MHz and 100 MHz for H and 13C
1
respectively. Time-resolved fluorescence intensity decays were obtained
by the single photon timing method, with picosecond laser excitation
with a Spectra-Physics set-up composed of a Titanium Sapphire Tsunami
laser pumped by an argon ion laser, a pulse selector and a doubling
(LBO) crystal. Light pulses were selected by acousto-optic crystals at a
repetition rate of 4 MHz. Fluorescence photons were detected by means
of a Hamamatsu MCP R3809U photomultiplier connected to a constant
fraction discriminator. The time-to-amplitude converter was purchased
from Tennelec. In this investigation, the FWHM (full width at half maxi-
mum height) instrument response was 30 ps. ESR spectra were recorded
on an ESR Bruker ES-300 with a standard cavity TE102 equipped with a
liquid helium cryostat (Oxford Instrument).
the nanotube. Taken together the data corroborate
a
charge-transfer process originating from NT to the acridini-
+
C
C
um that yields a long-lived NT –PhMeA species. The re-
markable stability of the ion pair results from two factors: i)
the phenylacridinyl radical has been recognized as extreme-
ly stable in water and has been used in homogeneous cataly-
sis,[44] ii) the hole can be easily delocalized on the nanotube
lattice, thus preventing fast back recombination and if the
electron transfer is intermolecular, delocalization would in-
crease the stability of the radical pair. In the litera-
ture,[5,8,10,20–22] there are some examples of photoinduced rad-
ical–ion pairs formation between carbon nanotubes and aro-
matic compounds, but nanotubes always behaved as electron
acceptor and lifetime of the separated ion pair never ex-
ceeded a few microseconds.
9-(4-Aminophenyl)acridine (2): Anhydrous HCl was passed through a so-
lution of acridine (1.60 g, 8.9 mmol, 1 equiv) diluted in EtOH (25 mL).
After 15 min, a precipitate was obtained and the solvent was removed
under vacuum. To the pale brown crystals, corresponding to acridine hy-
drochloride, were added aniline (1.63 mL, 2 equiv) and silver chloride
(3.84 g, 3 equiv). The mixture was heated at 1308C for 6 h with vigorous
stirring. After cooling to RT, the black solid was washed with Et2O (3ꢂ
20 mL) and dissolved in 10% HCl (50 mL). The aqueous acidic solution
was washed with Et2O (4ꢂ50 mL) and basified to pH 8.5 using 28%
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Carbon Nanotube–Acridine Nanohybrids
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NH3. A yellow precipitate was obtained and collected by filtration. Re-
crystallization from EtOH provided 2 as brownish crystals (1.7 g, 70%).
1H NMR (CDCl3): d=8.26 (d, 2H, J=8.8 Hz), 7.84 (d, 2H, J=8.8 Hz),
7.76 (m, 2H), 7.42 (m, 2H), 7.24 (d, 2H, J=6.4 Hz), 6.91 (d, 2H, J=
6.4 Hz), 3.93 ppm (brs, 2H); 13C NMR (CDCl3): d=148.8, 147.9, 146.6,
131.7, 129.9, 129.5, 127.2, 125.6, 125.5, 125.3, 114.8 ppm; IR (NaCl): n˜ =
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3463, 3316, 3208, 3037, 1630, 1604, 1515, 1412, 1288, 821, 760 cmꢀ1
.
9-Phenylacridinediazonium tetrafluoroborate (3): To a solution of nitro-
sonium tetrafluoroborate (526 mg, 1.4 equiv) in dry CH3CN (5 mL) was
added, at ꢀ308C, a solution of 2 (870 mg, 3.2 mmol, 1equiv) dissolved in
anhydrous CH3CN/CH2Cl2 5:1 (6 mL). The mixture was stirred for 1.5 h
at ꢀ308C and further stirred at RT for 1 h. Et2O (50 mL) was added and
the precipitate was collected by filtration. The resulting solid was washed
with Et2O and concentrated under vacuum to give a yellow powder cor-
responding to 3 in quantitative yield (1.2 g). 1H RMN (CD3CN): d=8.80
(d, 2H, J=8.4 Hz), 8.41 (d, 2H, J=8.8 Hz), 8.23 (t, 2H, J=7.8 Hz), 8.08
(d, 2H, J=8.4 Hz), 7.82 (m, 2H), 7.71 ppm (d, 2H, J=8.8 Hz); IR
(KBr): n˜ =3122, 3092, 3058, 2295, 1641, 1585, 1050, 758 cmꢀ1
.
NT–9-Phenylacridine (NT-PhA), 4: NTs (10 mg) were dispersed in anhy-
drous o-dichlorobenzene (6 mL) using a sonicating probe for 15 min. To
this suspension was added diazonium 3 (100 mg) dissolved in anhydrous
CH3CN (3 mL). The mixture was flushed with nitrogen for 10 min and
heated to 608C for 15 h. After cooling to RT, the functionalized NTs
(NT-PhA 4) were collected by centrifugation at 15000 rpm and extensive-
ly washed with THF, CH2Cl2 and finally Et2O.
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NT–10-Methyl 9-phenylacridinium iodide (NT-PhMeA+), 5: NT-PhA
(3 mg) was dispersed in CH3I (5 mL) using a sonicating probe. The sus-
pension was heated to reflux for 3 d. NT-PhMeA+ 5 was collected by
centrifugation at 15000 rpm and extensively washed with THF, CH2Cl2
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9-Phenylacridine (PhA), 6: To a solution of 3 (1.1 g) in anhydrous
CH3CN/CH2Cl2 10:5 (15 mL) were added cold (08C) hypophosphorous
acid (50% in water) (5 mL) and a catalytic amount of Cu2O. The reaction
was stirred for 1 h at RT and saturated NaHCO3 (20 mL) was added. The
aqueous phase was extracted with CH2Cl2 (3ꢂ10 mL) and the combined
organic phases were dried over Na2SO4, filtered and evaporated under
vacuum. Pale yellow crystals of PhA 6 were obtained in quantitative
yield without further purification (760 mg). 1H NMR (CDCl3): d=8.29
(d, 2H, J=8.8 Hz), 7.78 (t, 2H, J=7.6 Hz), 7.71 (d, 2H, J=8.8 Hz), 7.58–
7.64 (m, 3H), 7.41–7.46 ppm (m, 4H); 13C NMR (CDCl3): d=148.5,
135.7, 130.2, 129.8, 129.3, 128.3, 128.2, 126.7, 125.4, 124.9 ppm; IR (KBr):
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n˜ =3043, 1539, 1512, 1436, 1412, 756, 703 cmꢀ1
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10-Methyl-9-phenylacridinium iodide (PhMeA+), 7: 9-Phenylacridine
(8.7 mg) was dissolved in CH3I (2 mL) and the solution was heated to
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2H, J=9.2 Hz), 8.41 (t, 2H, J=7.6 Hz), 7.96 (d, 2H, J=8.8 Hz), 7.78 (t,
2H, J=7.6 Hz), 7.70 (m, 3H), 7.45 (d, 2H, J=7.6 Hz), 5.15 ppm (s, 3H);
13C NMR (CDCl3): d=161.1, 141.5, 139.2, 132.9, 130.4, 129.9–128.9, 127.9,
125.9, 119.4, 54.2 ppm; IR (NaCl): n˜ =3484, 1609, 1578, 1550, 1454, 1380,
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Received: September 10, 2008
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Chem. Eur. J. 2009, 15, 3882 – 3888