Synthesis of imidazoaryl-BODIPY derivatives for anion sensing applications

Article abstract of DOI:10.1016/j.jphotochem.2017.04.018

Imidazoaryl-BODIPY derivatives were synthesized and characterized for anion sensing applications. Absorption and emission methods were used to monitor the sensing of different anions. Our studies showed that these sensors interact with F^? selec

Full text of DOI:10.1016/j.jphotochem.2017.04.018

Accepted Manuscript
Title: Synthesis of imidazoaryl-BODIPY derivatives for anion
sensing applications
Authors: Swati Dixit, Neeraj Agarwal
PII:
S1010-6030(17)30272-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.04.018
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10-4-2017
17-4-2017
Please cite this article as: Swati Dixit, Neeraj Agarwal, Synthesis of imidazoaryl-
BODIPY derivatives for anion sensing applications, Journal of Photochemistry and
Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.04.018
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Synthesis of Imidazoaryl-BODIPY derivatives for anion
sensing applications
Swati Dixit and Neeraj Agarwal *
UM-DAE, Centre for Excellence in Basic Sciences, Health Center Building,
University of Mumbai, Santacruz (E), Mumbai 400098, India
Graphical Abstract
Highlights



New imidazoaryl-BODIPY derivatives were synthesised.
Selective and strong binding with fluoride among other halides is observed.
These sensors show marked colour change in presence of F-, OH- and CN- under visible light
also shows complete fluorescence quenching under 365 nm light.


Limit of detection for fluoride is found to be as low as 4 X 10-6 M.
Anion sensing results were further studied by 1H NMR and cyclic voltammetric titration.
Abstract: Imidazoaryl-BODIPY derivatives were synthesized and characterized for anion
sensing applications. Absorption and emission methods were used to monitor the sensing of
different anions. Our studies showed that these sensors interact with F- selectively and
strongly as compare to other halides and hence are capable of detecting F- easily, in very
1
small amount (4.3 X 10-6 M). To understand the interaction of anions with sensors H-NMR
titrations were carried out. It was found that anion interacts with N-H which results in the
change in the absorption and emission properties. Binding constant for F- was found to be
1

high, 4642900 M-1. For environmental applications and qualitative analysis detection of F-,
CN- and OH- by these sensors in doped polymer films is also shown.
Keywords: BODIPY; Absorption; Anion sensor
Introduction
Anions have a fundamental role in many chemical processes and hence carry a lot of
importance in biological and metabolic activities.1 It becomes very important to detect minute
amount of anions in environmental, food, biological and medicinal samples.2 Inexpensive real
time methods and new molecular probes are required to detect the small quantity of anions,
cations or neutral molecules. A considerable progress has been made to understand the anion
sensing properties of variety of host molecules. Absorption, fluorescence, electrochemical
properties etc. of host have been monitored quite often to detect a wide range of analytes.3
Colorimetric and fluorimetric based sensors offer convenient and easy detection of anions.4 In
these methods, change of color or emission of molecular probe is monitored on interaction
with anion. Ratiometric fluorescence analysis is found to be very useful as this method is
independent of the concentration of sensor and to some extent sensitivity and intensity of
light source.5 Fluorescent sensors have been designed based on the intramolecular charge
transfer (ICT), fluorescence energy transfer, photoinduced electron transfer etc.6 In recent
time, proton transfer and excited state charge transfer mechanism have been utilized to
develop the sensor probes.7 Hydrogen bonding between anions and NH of fluorescent probe
is also used to detect several anions.8 Anions, such as cyanide (CN-) and fluoride (F-), play
important roles in biological activities.9 For example, CN- interacts very strongly with heme
protein which causes inhibition of respiration and hence disrupts several other metabolic
activities.10 Although, fluoride is beneficial to human for its role in curing of dental and bone
2

related diseases,11 however, excess of it is harmful. Therefore, detection and quantification of
CN- and F- in very low quantity is essential. Strong hydrogen bond forming property of CN-
and F- has been used to design a variety of colorimetric and fluorimetric sensors for these
anions. Anthraimidazoledione derivatives have been shown as effective host for many anions
and cations.12 The N-H proton in anthraimidazoledione derivatives offers the possibility of
hydrogen bonding with CN- and F-. Here, in this article, we report the synthesis of
imidazoaryl-BODIPY derivatives 1-4. Compound 1 possesses anthraimidazoledione moiety
attached with BODIPY while 2-4 have different arylimidazole groups. The N-H of the fused
imidazole ring provides the opportunity to study the interaction of anions. Anion binding was
studied by photophysical (absorption & emission), NMR and electrochemical methods.
Experimental section
Materials and instruments
All the reagents were purchased from commercial sources and were used without any
further purification. Solvents used were of analytical grade and were dried according to
1
standard procedures whenever anhydrous solvents are required. H-NMR were recorded on
500 MHz (Bruker) or 600 MHz (Varian) with tetramethylsilane (TMS) as internal reference
and residual CHCl3 in CDCl3 as reference. Mass spectrometric measurements were recorded
using MALDI-TOF (Bruker). Cyclic voltammetry was carried out on a computer controlled
CH instruments. Glassy carbon as working electrode, Ag/AgCl as reference electrode and
platinum (Pt) wire as counter electrode were used. The measurements were carried in dry
acetonitrile with tetrabutylammonium hexafluorophosphate solution (0.1 M) as supporting
electrolyte with a scan rate of 100 mVs-1. UV-Visible spectra were recorded on SHIMADZU
UV‒1800 and fluorescence studies were done on Horiba Fluromax 4 at room temperature.
The neat solid films of compounds were prepared by using spin coater, Holmarc HO-TH-05.
3

Synthesis and characterization
1: Compound 1 was synthesized by using 1,2-diamminoanthraquione (0.045 g, 0.18 mmol)
and BODIPY-2-carboxaldehyde (0.04 g, 0.12 mmol) was heated in nitrobenzene (5 ml) at
160 οC for 18 h, completion of reaction was monitored by TLC. The reaction mixture was
cooled to room temperature, then for precipitation hexane was added. The solid obtained was
filtered off and washed several times with hexane. The crude product was purified by column
chromatography using hexane/ethyl acetate (70:30) as eluent. A pure red-brown solid was
obtained. Yield: (58 mg, 58 %); M.P: >220 οC; 1H-NMR (CDCl3, 500 MHz, δ ppm): 11.10
(s, 1H), 8.75 (s, 1H), 8.35 (d, 1H, J = 8.4 Hz), 8.27 (d, 1H , J = 7.5 Hz), 8.23 (d, 1H, J = 8.35
Hz), 8.09 (s, 1H), 8.05 (d, 1H, J = 8.35 Hz), 7.80-7.7 (m, 2H), 7.63 (d, 2H, J = 8.5 Hz), 7.50
(s, 1H), 7.14 (d, 3H, J = 8.5 Hz), 6.68 (d, 1H, J = 3.05 Hz), 3.97 (s, 3H); 13C-NMR (CDCl3,
500 MHz, δ ppm): 182.48, 174.58, 151.75, 146.85, 134.34, 133.65, 132.55, 127.48, 126.44,
122.06, 120.15, 117.69, 114.46, 29.55; MALDI-TOF: mass calcd. for C31H21BF2N4O3
546.33, found 545.57 (M)+, 525.63 (M-F)+.
2. Compound
2 was synthesized, similar to the 1, by using 5-Bromo-2,3-
diamminobenzene (0.043 g, 0.23 mmol) and BODIPY-2-carboxaldehyde (0.05 g, 0.15
mmol) was heated in nitrobenzene (5 ml) at 160 οC for 20 h. The crude compound was
purified by column chromatography using hexane/ethyl acetate (70:30) to afford solid
ο
1
2
. Yield: (0.04 g, 53 %); M.P.: above 220 C; H-NMR (CDCl3, 500 MHz, δ ppm):
8.35 (s, 1H), 8.05 (s, 1H), 7.51 (d, 3H, J = 8.35 Hz), 7.41 (s, 1H), 7.32 (t, 1H, J = 10.3,
8.45 Hz), 7.08 (d, 2H, J = 3.45 Hz), 7.04 (d, 3H, J = 8.4 Hz), 6.66 (d, 1H, J = 3.05
13
Hz), 3.92 (s, 3H); C-NMR (500 MHz, CDCl3, δ ppm): 161.06, 128.43, 122.86,
4

122.24, 114.77, 107.73, 107.44, 29.38; MALDI-TOF: mass calcd. for
C23H16BBrF2N4O 493.11, found 493.12 (M)+, 473.28 (M-F)+.
3: Compound
3 was synthesized by using 1,2-diaminonapthalene (0.06 g, 0.22 mmol)
and BODIPY-2-carboxaldehyde (0.05 g, 0.15 mmol) was heated in nitrobenzene (5
ο
ml) at 160 C for 18 h. .The crude product was purified by silica gel chromatography
using hexane/ethyl acetate (70:30) to get pure compound 3. Yield: (0.037 g, 52 %);
ο
1
M.P.: above 220 C; H-NMR (CDCl3, 500 MHz, δ ppm): 8.49 (s, 1H), 8.10 (s, 1H),
7.96 (s, 3H), 7.60 (d, 2H, J = 7.8 Hz), 7.56(s, 1H), 7.4(t, 3H , J = 2.8, 2.8 Hz), 7.13(d,
13
1H, J = 3 Hz),7.09 (d, 3H, J = 8.2 Hz), 6.69 (d, 1H, J = 2.55 Hz), 3.96 (s, 3H); C-
NMR (CDCl3, 500 MHz, δ ppm): 162.73, 155.03, 150.86, 147.97, 147.46, 141.06,
137.78, 135.50, 134.57, 133.94, 133.35, 130.41, 129.13, 128.03, 127.30, 125.60,
123.77, 123.56, 121.06, 114.97, 114.84, 107.49, 29.55; MALDI-TOF: mass calcd. for
C27H19BF2N4O 464.27, found 464.41 (M)+, 445.50 (M-F)+.
4: Compound 4 was synthesized by using 1,2-diaminobenzene (o-phenylenediamine) (0.05 g,
0.22 mmol) and BODIPY-2-carboxaldehyde (0.05 g, 0.15 mmol) was heated in nitrobenzene
ο
(5 ml) at 160 C for 18 h. Crude reaction mixture was purified by silica gel chromatography
to get the pure product 4. Yield: (0.033 g, 52 %); M.P.: above 220.οC; 1H NMR (CDCl3, 500
MHz, δ ppm): 7.98 (d, 4H, J = 8.5 Hz), 7.60 (m, 3H), 7.24 (m, 3H), 6.98 (d, 4H, J = 8.55
13
Hz), 3.84 (s, 3H); C NMR (CDCl3, 500 MHz, δ ppm): 160.68, 151.28, 128.06, 122.49,
121.87, 114.40, 21.04; MALDI-TOF: mass calcd. for C23H17BF2N4O 414.22, found 414.67
(M)+, 395.39 (M-F)+.
Results and discussion
5

Synthesis of sensors
Imidazoaryl-BODIPY derivatives 1-4 were synthesized as shown in Scheme 1. BODIPY, 5
was synthesized according to literature method.13 Vilsmeyer-Hack reaction on 5 produced
BODIPY-2-carboxaldehyde, 6 in good yield. Compound 6 was reacted with diaminoarene in
nitrobenzene at 160 ◦C for 18-20 h. After completion of reaction solvent was evaporated and
crude product was purified by column chromatography to afford 1-4 in good yields.
Compounds 1-4 have moderate to good solubility in common organic solvents such as DCM,
ethyl acetate, chloroform, DMSO, DMF etc. Compounds 1-4 were characterized by NMR,
mass methods (see supporting information).
1
In H-NMR spectrum of 1 a broad singlet at 11.1 ppm for one proton is obtained
which is assigned for N-H proton. Similar feature for N-H is obtained in 2-3. The broad
singlet of N-H suggests the intermolecular hydrogen bonding between NH and carbonyl
groups in 1. The homonuclear correlation spectroscopy (COSY) was also recorded for
compound 1 to assign the signals obtained for 1 (Figure SI 34). Signals for protons at 1- and
3- positions of BODIPY were observed as two singlets at 8.10 and 8.57 ppm respectively.
Singlet nature of these protons in NMR confirms the substitution at 2- position. In MALDI-
TOF molecular ions corresponding to mass of 1-4 were observed which further support the
formation of desired 1-4. All the spectra related to NMR, MALDI, FTIR of 1-4 are given in
supplementary information.
Photophysical studies
6

The absorption spectral behavior of arylimidazo-BODIPY derivatives 1-4 were
studied in dichloromethane and their normalized UV-Visible absorption spectra are shown in
Figure 1. Electronic properties were slightly altered by changing the fused ring in these
derivatives. The absorption peaks in high energy region were assigned to n transitions
and peaks around 310-340 nm are originating from  transition from the imidazole ring.
A broad absorption peak at ~ 410 nm is assigned to S0-S2 transition of boradiazaindacene.
Compounds 1-4 showed an intense S0-S1 (*) absorption at about 535 nm which we
assigned to the diisoindolomethane core. Emission spectra of 1-4 recorded in
dichloromethane are shown in Figure 1. Compounds 1-4 showed an emission maxima in the
range of 576-601 nm (Table 1). These compounds showed a bathochromic shift of ~70-100
nm as compare to standard BODIPY. Fluorescence quantum yields of 1-4 were calculated
with respect to Rhodamine B ( = 0.68 in ethanol) and are found to be low to moderate. It can
be noticed that the Stoke shift, which generally gives an idea of change in geometry of
excited state as compare to ground state, for 1-2 is small as compare to 3,4. Absorption and
emission data of 1-4 are summarized in Table 1.
Anion sensing studies
Absorption and emission studies of compounds 1,3 with tetrabutylammonium salts of
-
-
-
several anions such as F-, CN-, OH-, Br-, I-, HSO4 , NO2 and ClO4 were carried out to study
the interaction behavior with anions. Addition of F-, OH- and CN- in acetonitrile resulted in
the formation of colorless solutions of 1,3. For spectrophotometric titration concentration of
compounds 1,3 were kept at ~3.2 X 10-5 M. For 1, a decrease in the absorbance at 538 nm
was observed on addition of increasing concentration of F-, CN- and OH- anions as shown in
Figure 2. Quenching was observed at very low concentration in the case of fluoride as
7

compare to hydroxide and cyanide ions for sensor 1. Complete quenching of absorption peak
at 538 nm for 1 was observed with F- (~8.0 X 10-6 M), CN- (~5.7 X 10-5 M) and OH- (~8.7 X
10-5 M). Anion sensing properties of compounds 1,3 were also studied by fluorimetric
titrations. Successive addition of F-, CN- and OH- to the solution of sensors causes decrease
in emission intensity which leads to complete fluorescence quenching. No considerable
-
absorption or fluorescence response was observed in for 1,3 in the presence of Br-, I-, HSO4 ,
-
-
NO2
and
ClO4
anions
(Figure
SI:
15-30).
8

For compound 1, binding constants calculated using Stern-Volmer equation were
found to be 4642900 M-1 for F-, 72478 M-1 for CN- and 27660 M-1 for OH-. (See supporting
information). Limit of detection (LOD) for these anions for 1 was calculated using the
formula LOD = 3 Sd/s. where ‘Sd’ stands for standard deviation of regression and ‘s’ stands
for slope (Table SI 1). LOD for F- was found to be 4.39 X 10-6 M.
The 1H-NMR titration experiments were performed to understand the binding sites for
compound 1. Compound 1 was dissolved in CD3CN (4.0 X 10-3 M) and a solution of NBut4F
1
in CD3CN was added in a known but increasing quantity. H-NMR spectrum were recorded
after each addition and are shown in Figure 3. The N-H signal of 1 disappeared completely
on addition of 0.1eq of F- which indicates that the F- ion is interacting with N-H proton very
strongly at a low concentration (4.0 X 10-4 M) of fluoride. The  hydrogen of BODIPY
which is closer to imidazole ring observes downfield shift of 0.2 ppm. Changes observed in
1
the chemical shifts of other protons in aromatic region are shown in Figure 3. H-NMR
signals related to protons ‘j’, ‘i’, ‘b’ & ‘c’ observed marked change in their positions as we
increase the fluoride concentration. As explained by Peng et al.,12c these up-field shifts of
NMR signals were attributed to the increase in electron density, through bond, at
anthraquinone ring adjacent to imidazole as well as distant pyrrole ring of BODIPY.
Downfield shift in NMR signal is ascribed to low electron density, through space effects of
H-bonding, on proximal C-H protons. During the NMR titration, we observed the increase in
the solubility of 1 on increasing the concentration of F- ion, interpreted from increased
intensity of signals in Figure 3. This increase in solubility indicates the dissociation of
1
intermolecular interaction between NH and carbonyl groups. Similar H-NMR titration was
1
carried out with Br-, however no change in the H-NMR spectra is observed on addition of
9

bromide ions. (Figure: SI 32). This result further supports the selected binding with fluoride
ion as also observed from photophysical methods.
Compounds 1-4 were also studied by cyclic voltammetry to get the insight on their
oxidation and reduction behavior. Compounds 1-4 showed one reversible oxidation peak and
two reduction peaks (Table SI 2, see supporting information). The redox behavior of 1 was
also studied in presence of F-. Cyclic voltammogram of compound 1 with increasing amount
of F- is shown in Figure 4. On addition of F-, no prominent difference was seen in oxidation
peak potential, however, it resulted in the increase of oxidation peak current. This may be due
to the increase of solubility on interaction of F- which is also seen in NMR studies.
Interestingly, second reduction peak observed the anodic shift and a decrease in the peak
current which is attributed to the interaction of 1 with F- and forming a different species.
To demonstrate the application of these sensors in thin films, we doped compound 1
with polyvinyl pyrrolidone (PVP) in 1:100 (w/w) ratio and films were casted on glass slides
using spin coating method. Thin film showed the orange fluorescence on exposure to UV
light (365 nm) (See Figure SI: 35). On this film, a drop of solution of anions in acetonitrile
was added and solvent was evaporated. Under UV light exposure, the area exposed with the
fluoride and cyanide anions did not show any emission which proves the interaction of
fluoride and cyanide with sensors in the solid thin films. No change in the emission properties
of thin film was observed with Br-. Anion binding is also demonstrated on paper strip (See
Figure SI: 35). In a similar way, the naked eye colorimetric detection of anions by
10

compounds 1,3 was observed in solution (Figure 2 and SI 36). A solution of 1,3 in
acetonitrile showed marked change in the color on addition of F-, CN- & OH-.
Conclusions
We have designed and synthesized imidazoaryl-BODIPY derivatives for anion sensing
purpose. Anion sensing was studied by absorption, emission and NMR methods. Our
observations with 1,3 suggest (i) the stronger binding with fluoride ion which is evident from
the high binding constants at a very low concentration of F- and (ii) F- interacts selectively
among other halides and hence are capable of detecting F-. This behavior of sensors with F-
can be attributed to the high electronegativity, small size of fluoride ion and possible stronger
H-bonding interaction between sensors and F-. For environmental applications and qualitative
analysis detection of F-, CN- by these sensors in doped polymer films is shown.
Supporting Information
Synthesis, characterization data, NMR, mass and anion binding data of selected compounds
are given.
Acknowledgements
Centre for Excellence in Basic Sciences, Mumbai is thanked for providing research facilities.
We also thank Tata Institute of Fundamental Research, Mumbai for providing the NMR and
MALDI-TOF facilities.
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15

Fig. 1: Normalized absorption (above) and emission (below) spectra of compound 1-4 in
dichloromethane
16

Fig. 2: Absorption studies of anion sensing properties of 1 (3.2 X 10-5 M) with varying
amount of (a) fluoride; [F-]stock = 1 X 10-3 M, (b) cyanide;- [CN-]stock = 1 X 10-3 M in
acetonitrile.
Fig. 3: 1H-NMR titration of 1 with increasing amount of F- in CD3CN
17

Fig. 4: Cyclic voltammograms of 1 and with increasing amount of F- ion in acetonitrile.
Scheme 1: Synthesis of Imidazoaryl-BODIPY derivatives, 1-4.
18

Table 1: Photophysical and redox data of 1-4.
Comp
bEox (V) bERed
(V)
a휆abs nm
a휆em

nm
(log )
410 (4.4), 538 (4.9)
576
601
576
590
0.48 1.92
0.12 1.84
1.04, 1.75
0.94, 1.53
1.10, 1.70
1.08, 1.65
1
2
3
4
302 (4.1), 313 (4.1), 412 (3.9), 534 (4.1)
339 (4.0), 408 (3.7), 532 (4.0)
308 (4.7), 323 (4.5), 536 (3.8)
0.1
0.1
1.89
1.96
a: in dichloromethane; b: in acetonitrile
19