A new N-substituted heteroacene can detect CN- and F- anions via anion-π interaction

Article abstract of DOI:10.1039/c3ra40845k

A novel heteroacene with an aroyleneimidazophenazine moiety (1) has been successfully synthesized and characterized. Compound 1 can selectively detect CN^- and F^- over another ten anions (Cl^-, Br^-, I^-, Ac^-, H₂PO₄ ^-, NO₃^-, SO₄^2-, ClO ₄^- and BF₄^-, PF₆ ^-) in mixed solvents with multiple responses including fluorescent turn-off , colorimetric and near-infrared absorption signaling. The interactions between 1 and CN^-/F^- have been investigated through UV-vis and fluorescence experiments, and the results suggest that the anion-π interactions between 1 and CN^-/F^- might be reversible processes upon the addition of Cu²⁺ and Fe³⁺ cations.

Full text of DOI:10.1039/c3ra40845k

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A new N-substituted heteroacene can detect CN2 and
F2 anions via anion–p interaction3
Cite this: DOI: 10.1039/c3ra40845k
Jianfeng Zhao,ab Gang Li,a Chengyuan Wang,a Wangqiao Chen,ac Say Chye
Joachim Loo*ab and Qichun Zhang*abc
Received 20th February 2013,
Accepted 17th April 2013
DOI: 10.1039/c3ra40845k
www.rsc.org/advances
A novel heteroacene with an aroyleneimidazophenazine moiety
(1) has been successfully synthesized and characterized.
Compound 1 can selectively detect CN2 and F2 over another ten
chromophore12 to display a signaling response, while CN2 can be
recognized through ligand displacement13 or reaction-based
cyanide substitution.4,6,11,14 However, the application of these
sensors is limited by drawbacks such as low sensitivity and by not
being easily accessible. Therefore, developing new, simple and
highly sensitive CN2/F2 chemosensors is still a challenge.
Since naphthalene imides and azaacenes have been demon-
strated to be electron-deficient systems for anion recognition
through an anion–p interaction,4 the integration of a naphthalene
imide and azaacene into one molecule might show a synergistic
effect, which could be more sensitive and selective to anions.
Herein, we report a new aroyleneimidazophenazine derivative (a
hybrid molecule from naphthalene imide and an N-substituted
azaacene): 2-(2,6-diisopropylphenyl)benzo[19,109][3,8]phenanthro-
lino[39,29 : 1,2]imidazo[4,5-b]phenazine-1,3,6(2H)-trione (1), which
can selectively recognize CN2 and F2 with multiple responses
anions (Cl2, Br2, I2, Ac2, H2PO42, NO32, SO422, ClO4 and BF4
,
2
2
2
PF6
) in mixed solvents with multiple responses including
fluorescent ‘‘turn-off’’, colorimetric and near-infrared absorption
signaling. The interactions between 1 and CN2/F2 have been
investigated through UV-vis and fluorescence experiments, and
the results suggest that the anion–p interactions between 1 and
CN2/F2 might be reversible processes upon the addition of Cu2+
and Fe3+ cations.
N-Substituted heteroacenes are very attractive due to their
fundamental properties and technological applications.1–3
Especially, a new non-covalent binding motif between electron
deficient N-heteroarenes (or azaacenes) and anions, namely, an
anion–p interaction,4,5 has been receiving strong interest both in
theory and experiments, because anion–p or anion–proton
interactions play a crucial role in many chemical and biological
processes5,6 and have potential applications in biochemistry,
medicine and the environment.
Among the various anions, cyanide and fluoride are two of the
most toxic. It is well-known that excess fluoride will cause skeletal
fluorosis7 while a small amount of cyanide can cause a coma and
even death.8 Thus, the development of efficient and quickly
responding sensors for the recognition and monitoring of CN2
and F2 is very important. So far, most sensors for detecting CN2
and F2 focus on proton receptors9 or deprotonations.10 Beside
these, the F2 anion can also be detected through a desilylation
reaction11 relying on the electron delocalization adjustment of a
including fluorescent ‘‘turn-off’’, colorimetric and near-infrared
2
absorption signaling among a total of twelve anions (BF42, PF6
,
Cl2, F2, SO422, NO32, I2, H2PO42, ClO42, Ac2, Br2 and CN2,
which are balanced by the same cation, tetrabutylammonium
(TBA)). Interestingly, its responses to the CN2 and F2 anions are
reversible upon the addition of Cu2+ and Fe3+, which can trigger
the return of fluorescence due to the larger binding constant
between Cu2+/Fe3+ and CN2.13 A solution of 1 was prepared in
THF with a concentration of 1.0 6 1025 mol L21 while the
solutions of twelve anions, CuCl2, FeCl3, and Ca(NO3)2 were
prepared in DMF (1.0 6 1022 mol L21). High concentrations of
cations should reduce the volume effect during the detection.
As shown in Scheme 1, compound 1 was obtained as a red
colored powder in 32% yield through a condensation reaction
between 2,3-phenazinediamine and naphthalene tetracarboxylic
aSchool of Materials Science and Engineering, Nanyang Technological University,
Singapore 639798, Singapore. E-mail: qczhang@ntu.edu.sg; Fax: (+65)67909081;
Tel: (+65)67909081
monoimide monoanhydride (2) (Fig. S1, ESI ) according to a
3
similar report with a simple modification.15 Compound 1 was
bSingapore Centre on Environmental Life Sciences Engineering (SCELSE), Nanyang
Technological University, Singapore 637551, Singapore.
E-mail: joachimloo@ntu.edu.sg; Fax: 6790-9081
characterized by proton nuclear magnetic resonance spectroscopy
(1H NMR) (Fig. S2, ESI ) and high-resolution matrix-assisted laser
3
desorption/ionization time of flight (MALDI-TOF) (Fig. S3, ESI ).
3
cInstitute for Sports Research, Nanyang Technological University, 50 Nanyang
Due to its poor solubility, we could not obtain a reasonable 13C
Avenue, Singapore 639798
Electronic supplementary information (ESI) available: Experimental section,
NMR spectrum for compound 1.
3
MALDI-TOF, HR-TOF, 1H NMR, UV-vis and PL spectra. See DOI: 10.1039/c3ra40845k
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Scheme 1 Synthetic route of 1. Synthetic conditions (i): DMF, N2, reflux, 12 h. (ii)
Pyridine, N2, reflux, 4 h.
The UV-vis absorption spectrum of 1 in THF (1.0 6 1025 mol
L
21) solution exhibits a broad absorption band with peaks at 372,
407, and 431 nm (Fig. 1, hollow ball line), which can be assigned
to n–p* and p–p* transitions. The shoulder peaks at 485 and 525
nm probably result from intramolecular charge transfer (ICT) from
2,6-dialkylphenylene and two sp3-N atoms to electron deficient
zones (phenazine and imide-substituted naphthalene). The
emission peak in THF is at 593 nm (Fig. 1, hollow square line)
due to the large p-conjugated backbone.
Fig. 2a shows the color changes of 1 in a newly prepared THF
solution (1 eq., 2 mL, 1 6 1025 mol L21) with 30 eq. (the final
concentration of anion in solution after addition is 3 6 1024 mol
2
L
21) of twelve different anions: BF42, PF62, Cl2, F2, SO422, NO3
,
I2, H2PO42, ClO42, Ac2, Br2, and CN2 (using TBA as the cation).
Clearly, only CN2 and F2 have positive responses to compound 1,
displayed by the color changes from yellow to dark blue (for the
CN2 anion) and light blue (for the F2 anion). Besides that, the
fluorescence was quenched completely for CN2 and partially
quenched for F2 when they were excited at 365 nm. These results
show that compound 1 is more sensitive and selective for the
recognition of CN2 than for F2. It is noteworthy that compound 1
loses its response to F2 but still shows a positive response to CN2
Fig. 2 (a) A picture of color changes, (b) normalized UV-vis spectra and (c) PL
spectra of 1 (1 eq., 2 mL, 1.0 6 1025 mol L21) in THF solution upon addition of Cl2
(hollow square line), F2 (hollow ball line), and CN2 (hollow triangle line). The final
when the ratio of anions vs. 1 decreases to 15 : 1 (Fig. S4, ESI, the
3
concentration of anions is 3 6 1024 mol L21
.
concentration of anion in solution after addition is 3 6 1024 mol
21).
L
Fig. 2b and 2c display the UV-vis absorption and PL spectra of
compound 1 responding to Cl2, F2, and CN2 anions (all other
anions are provided and shown in Fig. S5, ESI ). Clearly, the Cl2
3
anion (other anions also, Fig. S5, ESI ) does not make any changes
3
in color, absorption and PL while the addition of CN2 causes the
original main absorption peaks (407 nm and 431 nm) of 1 to
disappear and several new peaks at 502 nm, 611 nm, 637 nm, 678
nm, and 729 nm to appear, which could be ascribed to the
intermolecular electron transfer (IET) between the anions and the
molecules of heteroacene 1.4 The difference in compound 1
responding to F2 and CN2 at the same concentration (3 6 1024
mol L21) is that the addition of F2 cannot completely depress the
Fig. 1 Normalized UV-vis (hollow ball line) and photoluminescence (PL) spectra
(hollow square line) of 1 in THF solution (1.0 6 1025 mol L21); Inset: the picture of 1
in THF solution.
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This phenomenon is also observed by the treatment of 1?CN2 with
original two main peaks (407 nm and 431 nm) of compound 1.
Meanwhile, the PL spectra (Fig. 2c) show that the CN2 anion can
completely quench the original emission peak (593 nm) while the
F2 anion only partially decreases the intensity of the original
fluorescence of compound 1. Even when the molar ratio of F2 vs. 1
is increased to 60, the fluorescence peak for compound 1 can still
Fe3+ (Fig. S7, ESI ), but Ca2+ shows no response due to the weak
3
binding constant with CN2 (Fig. S7, ESI ).
3
The mechanism of the intermolecular electron transfer (IET)
between the anions and azaacenes is further confirmed by the
titration of the aromatic proton NMR spectrum of 1 in 600 mL
CDCl3 (3.3 6 1023 mol L21) with different amounts of the CN2
anion (0, 1, 3, and 5 mL). As shown in Fig. 4, upon the addition of
the CN2 anion, Ha and Hg in the backbone obviously shift to low-
field, while Hh and Hi, belonging to the steric end capped
phenylene vertical to p-conjugated backbone display no field shift.
The addition of F2 showed a similar phenomenon (Fig. 2b). These
results further confirm that there is an anion–p interaction or
intermolecular electron transfer (IET) between the N-substituted
heteroacene and the anions.
be observed (Fig. S6, ESI ). These results clearly suggest that CN2
3
has the strongest affinity to the electron-deficient p-backbone
among the twelve anions due to the triple-bond nature of CN2 and
F2 is the second strongest one.
Interestingly, the color, UV-vis and PL responses of 1 in THF
solution can be recovered when Cu2+ is added into the solution of
1?CN2 or 1?F2. As shown in Fig. 3, when the THF solution of 1 is
treated with 20 eq. CN2, responses including the obvious color
changes (inset in Fig. 3a), the formation of new absorption peaks,
and the quenched fluorescence can be observed. Then, upon the
addition of 5 eq. of Cu2+ (5 6 1025 mol L21), the original features
of 1 can be recovered immediately (Fig. 3). These results suggest
that the anion–p interaction between 1 and CN2 is a reversible
process.16 Clearly, the anion–p interaction between 1 and CN2 is
much weaker than the binding constant between CN2 and Cu2+.
Conclusion
In summary, we have designed and synthesized a novel hybrid
molecule 1, which is a combination of naphthalene imide and
N-heteroarenes (or azaacenes). Compound 1 can selectively
recognize CN2 and F2 among twelve anions with multiple
responses such as color, UV-vis absorption, and photolumines-
cence. Interestingly, these responses are reversible upon the
addition of Cu2+ and Fe3+. Our strategy might be helpful to
develop new sensors for the detection of CN2 and F2.
Experimental
Synthesis of 1
1,4,5,8-Naphthalene tetracarboxylic dianhydride (2.68 g, 10 mmol)
was added to DMF (10 mL) and refluxed for 30 min at 180 uC, then
2,6-diisopropylaniline (1.98 mL, 10 mmol) in 10 mL DMF was
added dropwise into the refluxing mixture over 30 min, under N2
with strong stirring. The resulting solution was heated at 180 uC
for 12 h. It was cooled to r.t. and the light yellow precipitate was
collected by filtration, washed with methanol and dried in a
vacuum oven. The solid was confirmed to be naphthalene
monoimide monoanhydride (2) as the main product (Fig. S1,
ESI ). MALDI-TOF mass of 2: C H NO calcd. 427.14 [M]+; found
3
26 21
5
428.19[M + H]+. The solid (2) was used directly for the next
synthetic procedure: 2 (0.25 g, y0.5 mmol) and phenazine-2,3-
diamine (0.11 g, 0.5 mmol) were added to pyridine (10 ml) and
Fig. 3 (a) The normalized UV–vis spectra and (b) PL spectra of 1 (2 mL, 1 6 1025
mol L21) in THF solution upon the addition of 20 eq. CN2 (40 mL, 1 6 1025 mol L21
)
Fig. 4 Quantitative titration of the aromatic proton NMR spectrum of 1 in 600 mL
CDCl3 (3.3 6 1023 mol L21), by CN2 with 0, 1, 3, and 5 mL added (0, 0.05, 0.15, and
0.25 eq., CHCl3, 1022 mol L21, respectively).
and 5 eq. Cu2+ (10 mL, 1 6 1022 mol L21); hollow ball line: 1 in THF; hollow square:
1 + CN2; hollow triangle: 1 + CN2 + Cu2+. Inset: a picture of the color changes.
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refluxed for 4 h. The solution was cooled to r.t. and diluted with
methanol (100 ml 6 2) under strong agitation for 1 h. The solid
was collected by vacuum filtration and purified using silica gel
column chromatography with chloroform–methanol (40 : 1) to
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afford
2-(2,6-diisopropylphenyl)benzo[19,109][3,8]phenanthro-
lino[39,29 : 1,2]imidazo[4,5-b]phenazine-1,3,6(2H)-trione (1) (96
1
mg, 32%) as a red powder. H NMR (CDCl3, 400 MHz) d 9.52 (s,
1H), 9.23 (d, J = 7.71 Hz, 1H), 9.09 (d, J = 7.62 Hz, 1H), 8.99 (d, J =
7.70 Hz, 2H), 8.87 (s, 1H), 8.37 (m, 2H), 7.96 (m, 2H), 7.56 (t, J =
7.44 Hz, 1H), 7.71 (d, J = 8.00 Hz, 2H), 2.77 (t, J = 6.78 Hz, 2H), 1.22
(s, 6H), 1.20 (s, 6H). HiRes MALDI-TOF mass: C38H27N5O3 calcd.
601.21 [M]+; found 602.2188 [M + H]+.
Methods
1
Solution H NMR spectra were measured on a Bruker ARX 400
spectrometer. The UV–vis absorption and fluorescence spectra of 1
and 1?anions were recorded on Shimadzu UV-2501 and RF-5301
spectrophotometers, respectively. MS were collected on MALDI
TOF2 AXIMA mass spectrometer. HiRes MALDI-TOF spectra were
recorded on a Waters Q-Tof premier TM mass spectrometer.
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Acknowledgements
Q.Z. acknowledges financial support from AcRF Tier 1 (RG 18/
09) and Tier 2 (MOE2012-t2-1-019, ARC 20/12) from MOE, the
CREATE program (Nanomaterials for Energy and Water
Management) from NRF, and the New Initiative Fund from
NTU, Singapore. J.Loo acknowledges a NMRC-EDG grant
(EDG09may011) and the SCELSE project (M020070110).
´
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