A near-infrared fluoride sensor based on a substituted naphthalenediimide-anthraquinone conjugate

Article abstract of DOI:10.1016/j.tetlet.2015.06.050

The synthesis and characterization of a highly selective fluoride receptor based on a naphthalene diimide substituted with an anthraquinone (NDI-AQ) moiety is described. In CHCl₃, the receptor was shown to be highly selective for fluoride (K_a～10³ M^-1) over other anions (AcO^-, HSO₄^-, Br^-, Cl^-, I^-, ClO₄^-, H₂PO₄^-, and NO₃^-) with pronounced changes in absorption characteristics, that is, red-shift of the absorption band to 790 nm (NIR). The visual color change, NIR shift in absorption, emission spectroscopy, and electrochemistry thus support an intramolecular charge transfer (ICT) effect upon fluoride binding to the NDI-AQ receptor.

Full text of DOI:10.1016/j.tetlet.2015.06.050

Tetrahedron Letters 56 (2015) 4762–4766
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A near-infrared fluoride sensor based on a substituted
naphthalenediimide–anthraquinone conjugate
Sharad R. Bobe a, Sidhanath V. Bhosale a, , Lathe Jones b,d, Avinash L. Puyad c, Aaron M. Raynor b,
⇑
Sheshanath V. Bhosale b,
⇑
a Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, Telangana, India
b School of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne 3001, Victoria, Australia
c School of Chemical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 436 106, Maharashtra, India
d Centre for Advanced Materials and Industrial Chemistry (CAMIC), RMIT University, GPO Box 2476, Melbourne 3001, Victoria, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of a highly selective fluoride receptor based on a naphthalene diimide
substituted with an anthraquinone (NDI-AQ) moiety is described. In CHCl3, the receptor was shown to be
highly selective for fluoride (Kaꢀ103 MÀ1) over other anions (AcOÀ, HSOÀ4 , BrÀ, ClÀ, IÀ, ClO4À, H2POÀ4 , and
NOÀ3 ) with pronounced changes in absorption characteristics, that is, red-shift of the absorption band to
790 nm (NIR). The visual color change, NIR shift in absorption, emission spectroscopy, and electrochem-
istry thus support an intramolecular charge transfer (ICT) effect upon fluoride binding to the NDI-AQ
receptor.
Received 9 March 2015
Revised 19 May 2015
Accepted 17 June 2015
Available online 24 June 2015
Keywords:
Naphthalenediimide
Intramolecular charge transfer
Colorimetric and optical
Near-IR
Ó 2015 Elsevier Ltd. All rights reserved.
Fluoride sensing
The ability to sense fluoride ions is of importance to many areas
of chemistry due to its essential roles in medical, biological and
environmental processes.1 Consequently the development of fluo-
ride specific receptors is an area of continued focus and in recent
years the design of colorimetric and fluorescent receptors for the
sensing of fluoride anions has advanced steadily.2 The method of
covalently binding a receptor to a signaling subunit is the most
widely used approach, where pyrroles, urea, thiourea, amides, phe-
nols, imidazole, or calixarene have been used as coordination
sites.3 In such receptors, color or fluorescence emission changes
are observed due to interaction of the anion with the coordination
site of the sensor via hydrogen bonding and electrostatic interac-
tions, which induces changes in the electronic properties of the sig-
naling unit.4 These approaches are widely used by researchers due
to the low cost involved in the fabrication of the receptor and the
convenient detection of anions. The design and development of
anion sensors still remains a challenging task, as anions tend to
have a low stability constant, and some receptors also display a
complex pH-dependence.1d Chemosensors with a near-infrared
(NIR) optical response should be more useful, as they are more
likely to not show interference from other chromophores, and
can be run with an inexpensive compact laser, light emitting
diodes or NIR diodes.5,6
Among aromatic molecules that have found utility in the design
of chromophoric supramolecular materials, naphthalene diimide
(NDI) derivatives have attracted particular attention due to their
n-type properties.7,8 NDIs are compact, highly colored, and have
rich spectroscopic and electrochemical properties. Substituted
NDI derivatives, in particular, are ideally suited to sensor design.8
Recently, we and other researchers have shown that naph-
thalenediimide (NDI) derivatives are very effective colorimetric
sensors for a range of anions.7 In other reports, anthraquinone
(AQ) derivatives have also been employed as colorimetric sensors
for anions and cations.9 To the best of our knowledge, a donor–
acceptor system of this class of compound, exhibiting a solution
dependent intramolecular charge transfer (ICT) has not been
explored in either a receptor or sensor capacity.
Considering the above facts, and our interest in expanding the
development of chemosensors based on NDI probes. We herein
report the synthesis and characterization of a substituted NDI
derivative functionalized with an AQ subunit. The colorimetric
and spectroscopic behavior of the synthesized receptor toward
ion detection was studied via ICT effects.
⇑
Corresponding authors. Tel.: +91 4027191474 (Sidhanath V.B.), +61 399252680
(Sheshanath V.B.).
E-mail addresses: bhosale@iict.res.in (S.V. Bhosale, IICT), sheshanath.bhosale@
rmit.edu.au (S.V. Bhosale, RMIT).
http://dx.doi.org/10.1016/j.tetlet.2015.06.050
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

S. R. Bobe et al. / Tetrahedron Letters 56 (2015) 4762–4766
4763
The synthesis of NDI-AQ (1) was achieved by reacting 1,4,5,8-
naphthalenetetracarboxylic dianhydride (NDA) with 5.3 equiv of
Br2 in a mixture of sulfuric acid and oleum (v:v, 4:1) at 140 °C for
4 weeks. 2,3,6,7-Tetrabromonaphthalene diimide (3) was prepared
over two steps by the reaction of 2 and 4 equiv of n-octylamine in
refluxing acetic acid for 30 min to give 2,3,6,7-tetrabromo-4,8-
bis(octylcarbamoyl) naphthalene-1,5-dicarboxylic acid, which
was further treated with excess PBr3 in refluxing toluene for 12 h
to afford 3 in 31% overall yield.10 The NDI-AQ conjugate was syn-
thesized via nucleophilic substitution of the substituted tetra-
bromonaphthalene diimide 3 with 1,2-diamino anthraquinone
(4) in dry DMF at 120 °C (Scheme 1). The receptor 1 was obtained
as a dark blue solid in 80% yield after purification by column chro-
matography (for details see ESI). It is important to note that the
addition of a second equivalent of anthraquinone 4 to compound
3, substituting the two remaining bromine atoms, may have
increased the sensitivity of the sensor toward fluoride. However,
this was not possible despite many attempts. The fact that
monoannulation resulted in high yield could be attributed to two
main factors. Firstly, aniline amines such as 4 are less nucleophilic
than the aliphatic amines where tetra-substitution takes place,11
and secondly, once the first addition is complete, the deactivating
feature of the two donating amino groups prevents further addi-
tion.12 This result is also important as it may allow variation to
the structure through the two remaining aryl bromide groups.
The colorimetric behavior of the NDIA-AQ (1) receptor toward
ion detection was studied by employing various anions such as
AcOÀ, HSOÀ4 , BrÀ, ClÀ, IÀ, ClO4À, H2PO4À, NOÀ3 , and FÀ (added as their
tert-butyl-ammonium salts). The result of adding 5 equiv of vari-
ous anions to 1 in CHCl3 is shown in Figure 1. It was observed that
Figure 1. Color changes upon addition of 5 equiv of anions (as their tert-butyl-
ammonium salts) to receptor 1 (1 Â 10À5 M in chloroform).
Figure 2. UV–vis absorption titration of NDI-AQ ([1] = 1 Â 10À5 M in CHCl3) upon
addition of fluoride ions (0–5 equiv of the solution 5 Â 10À4 M). Inset shows the Job
plot of fluoride to receptor 1 with 2:1 stoichiometry of the complex.
the NDI-AQ 1 receptor was selective for fluoride at lM concentra-
tions. Complete loss of the blue color was observed when FÀ was
introduced to a solution of receptor 1 in chloroform due to the
ICT effect.
The fluorescence emission spectra of NDI-AQ (Figure 3) consists
of a strong bands at 677 (excitation at 600 nm). As shown in
Figure 3, ꢀ70% quenching was observed for NDI-AQ upon addition
of 0-5 equiv. of FÀ (added as their tert-butyl-ammonium salts),
due to the fluorides high charge density.8b Smaller effects (<5%
quenching) were observed for all other anions requiring 20 equiv
to achieve this response (Fig. S4). These results clearly shows
that NDI-AQ has a selective colorimetric and emission response
to the presence of fluoride.
These results indicated that 1 had a selective colorimetric
response to the presence of fluoride ions and a 2:1 stoichiometry
of the complex was determined by a Job plot (inset Fig. 2). The
binding constant of NDI-AQ to FÀ was determined to be
4 Â 103 MÀ1 (Fig. S2). The quantification limit and detection limit
UV–vis absorption spectroscopy was employed to determine
the selectivity of receptor 1 and quantify the spectral changes upon
FÀ binding. Typically, receptor 1 (1 Â 10À5 M in CHCl3) exhibited
three absorption bands; strong bands at 600 and 651 nm, and a
weak band at 290 nm. Upon addition of up to 5 equiv of the
FÀ ion, the absorption intensity bands at 600 and 651 nm steadily
decreased along with the appearance of a new band at 790 nm
(near-infrared). In the presence of 5 equiv of FÀ, the band at
790 nm predominated with an isosbestic point at 685 nm (Fig. 2).
In contrast, the addition of other anions (AcOÀ, HSO4À, BrÀ, ClÀ, IÀ,
ClOÀ4 , H2POÀ4 or NO3À) to a CHCl3 solution of the receptor 1 did
not induce any spectral changes (Fig. S1, ESI), indicating that
anion–p interactions were not significant with these anions.
C8H17
O
O
N
O
O
O
O
O
O
O
O
O
O
(i) n-octylamine,
AcOH, reflux,
30 min
Br2, H2SO4/oleum
(v:v, 4:1)
Br
Br
Br
Br
Br
Br
Br
Br
140 oC, 4 weeks
ii) PBr3, toluene,
reflux, 12 h
100%
O
O
O
N
C8H17
O
31%
NDA
2
3
C8H17
O
O
N
O
O
NH2
O
DMF, 120 oC
30 min
H
N
H2N
Br
Br
O
3
+
80%
N
H
O
N
O
C8H17
4
NDI-AQ (1)
Scheme 1. Synthesis of the NDI-AQ (1) receptor.

4764
S. R. Bobe et al. / Tetrahedron Letters 56 (2015) 4762–4766
of the FÀ anion were estimated to be approximately 3.87 Â 10À5
M
and 1.16 Â 10À5 M, respectively (Fig. S3).
1H NMR titration experiments confirmed that F- complexation
with the receptor 1 occurred via hydrogen bonding (Fig. 4). Peaks
of the NDI-AQ receptor in CDCl3 that were observed at d 13.20
and 14.75, were shifted downfield (d 13.82 and 15.49) upon the
addition of 2 equiv of FÀ. When four equivalents of FÀ were used,
the peaks shifted further downfield (d 14.10 and 15.85) with a
decrease in intensity. Finally, upon the addition of 5 equivalents
of FÀ, complete disappearance of these peaks was observed. At
the same time, signals for protons on the aryl rings were shifted
upfield.
The voltammogram of NDI-AQ in the absence of fluoride
showed three prominent cathodic processes in the region of
À1.09 to À1.43 V (vs Fc/Fc+), two of which were chemically rever-
sible under the conditions examined, and an overlapping chemi-
cally irreversible peak. The anodic scan also showed a series of
overlapping peaks in the range +0.44 to +0.88 V.13 The voltammo-
grams were a series of overlapping peaks with no contributions
from both the NDI core and AQ group.14 In compound NDI-AQ,
the redox processes at the NDI and anthraquinone center overlap,
to give the complicated voltammogram seen in curve (a1), which
was shown by the DPV trace (a2) to be a series of overlapping
peaks on both the anodic and cathodic scan (Fig. 5).
Figure 3. Fluorescence spectra (kex = 600 nm) of NDI-AQ (1 Â 10À5 M in CHCl3) in
the presence of 0–5 equiv of FÀ (5 Â 10À4 M).
Upon the addition of 2 equiv of FÀ, the change in the voltammo-
gram and DPV was profound. The cathodic processes shifted to a
more negative potential, in one large overlapping peak in the
voltammogram (b1), which was revealed by DPV (b2) to be a series
of redox processes. The greatest change was seen in the oxidation
processes on the anodic scan. A large peak, revealed by DPV (b2) to
be at least two overlapping redox processes, was observed, which
was shifted to a far more cathodic potential (À0.2 to 0 V vs Fc/Fc+)
than the experiment conducted in the absence of F-. This indicated
that the binding of fluoride led to a change in the electronic struc-
ture of NDI-AQ resulting in a compound that was far more easily
oxidized than when the NDI-AQ was not bound to fluoride.
The visual observation of color change, NIR shift in absorption,
emission spectroscopy, and electrochemistry thus supported an
ICT effect upon fluoride binding to receptor 1.7 DFT studies were
carried out to gain additional understanding of the ICT mechanism.
DFT computational studies were performed using the Gaussian 09
Figure 4. 1H NMR (300 MHz) spectra of chemosensor NDI-AQ in CDCl3 in the
presence of 0–5 equiv of fluoride ions.
Figure 5. Voltammogram (a1) and differential pulse voltammogram (DPV) (a2) of 0.3 mM NDI-AQ. Voltammogram (b1) and DPV (b2) of 3 mM NDI-AQ in the presence of
0.6 mM TBA+FÀ4 . Conditions: supporting electrolyte 0.1 M TBAPF6/DCM, glassy carbon working electrode, voltammogram scan rate 50 mV/s. See ESI for all conditions.

S. R. Bobe et al. / Tetrahedron Letters 56 (2015) 4762–4766
4765
mostly located on the complete NDI-AQ molecular system
(Fig. 6). It was observed that the AQ subunit contributed little to
the HOMO, but its contribution to the LUMO was greatly increased.
This indicated that the electron flow takes place from NDI to AQ in
both states, that is, HOMO and LUMO. The HOMO and LUMO pic-
tures of NDI-AQ bonded to F- and deprotonated NDI-AQ species
are shown in Figure 6 and Figures S5–S8. It was also found that
the energy gap between the HOMO and LUMO of the FÀ bonded
and deprotonated probes of NDI-AQ decreases, and became smal-
ler than that of the NDI-AQ receptor, which was clearly observed in
the voltammograms in Figure 5, where the first cathodic, and first
anodic processes was clearly compressed when F- was added to the
solution.
These results clearly support hydrogen bonding interaction
between the N–H of the receptor 1, and the FÀ anions, followed
by deprotonation of N–H. With excess FÀ, fluoride ions interact
with both the core N–Hs and the induced negative charge is
delocalized over the molecule (Fig. 7).
In conclusion, we have reported a near-infrared (NIR) donor–
acceptor based sensor, that is, a fluorescent, neutral and selective
colorimetric sensor for the fluoride ion. Among the anions tested,
NDI-AQ showed excellent selectivity for fluoride anion over other
anions (AcOÀ, HSOÀ4 , BrÀ, ClÀ, IÀ, ClOÀ4 , H2PO4À and NOÀ3 ). Sensing
of ions via an ICT effect was confirmed via UV–vis, fluorescence,
electrochemistry, and DFT calculations. Due to the visible color
changes, as well as the appearance of a NIR signal at 790 nm, it
is possible to conceive the use of this receptor in various sensing
applications, including situations such as anion transport and
purification, where the availability of cheap and easy-to-make
anion receptors would be advantageous.
Figure 6. HOMO and LUMO orbitals for NDI-AQ (left) and deprotonated species
(right).
X no
-
I
C8H17
C
C8H17
T
X
O
O
N
O
O
O
H
N
O
F
H
N
H
N
Br
Br
Br
Br
F-
O
Acknowledgments
O
N
H
N
H
n-Bu4NF
N
O
O
N
O
O
S.V.B. (IICT) is grateful for the financial support from the
DAE-BRNS (Project Code: 37(2)/14/08/2014-BRNS), Mumbai, and
Intelcoat project CSC0114, Council of Scientific and Industrial
Research (CSIR), New Delhi, India. S.R.B. acknowledges CSIR, New
Delhi, for SRF support. S.V.B. (RMIT) acknowledges the financial
support from the Australian Research Council under a Future
Fellowship Scheme (FT110100152). A.L.P. acknowledges DST
(Project. No. FS/FST/PSI-018/2009) for financial assistance. L.A.J.
acknowledges RMIT University for receipt of a Vice Chancellors
(Industry) Fellowship.
C8H17
C8H17
ICT
C8H17
O
N
O
F
Br
N
Excess n-Bu4NF
O
Br
O
N
H
F
N
O
O
C8H17
2 tBuN
Supplementary data
Figure 7. Anion sensor structure illustrating the recognition of fluoride via an ICT
effect.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.06.
050.
ab initio/DFT quantum chemical simulation package to determine
the HOMO and LUMO energy differences of the receptor NDI-AQ
and the corresponding deprotonated species (Fig. 6).15 The geome-
try optimization of NDI-AQ, NDI-AQ+F, NDI-AQ+2F, the NDI-AQ
anion and of NDI-AQ dianion molecules with truncated alkyl
chains were carried out at the B3LYP/6-311++G⁄⁄ level of theory.
The polarizable continuum model (PCM) was employed to investi-
gate the effect of the solvent (CHCl3) on geometries. In order to
confirm that the optimization for NDI-AQ and its deprotonated
species were real structures on the potential energy surface,
frequency calculations were also carried out at the same level of
theory, and frequencies for all species were found to be positive.
Based upon the molecular geometries obtained, the frontier molec-
ular orbitals of all five species were calculated at the same level of
theory and pictures generated using Avogadro.16 For the receptor
NDI-AQ, the HOMO orbitals were found to be mainly distributed
at the NDI along with substituted six membered ring systems
and half of the anthraquinone subunit, whereas the LUMO was
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