Synthesis, structure, spectral, electrochemical and sensing properties of 3-amino boron-dipyrromethene and its derivatives

Article abstract of DOI:10.1016/j.dyepig.2013.10.038

We report the synthesis of 3-amino boron-dipyrromethene (3-amino BODIPY) by treating 3-bromo boron-dipyrromethene (3-bromo BODIPY) initially with sodium azide in acetonitrile followed by triphenylphosphine(PPh₃)/H ₂O in tetrahydrofur

Full text of DOI:10.1016/j.dyepig.2013.10.038

Accepted Manuscript
Synthesis, Structure, Spectral, Electrochemical and Sensing Properties of 3-Amino
Boron-Dipyrromethene and Its Derivatives
Emandi Ganapathi, Sheri Madhu, Tamal Chatterjee, Rajesh Gonnade, Mangalampalli
Ravikanth
PII:
S0143-7208(13)00419-1
10.1016/j.dyepig.2013.10.038
DYPI 4159
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 12 September 2013
Revised Date: 21 October 2013
Accepted Date: 28 October 2013
Please cite this article as: Ganapathi E, Madhu S, Chatterjee T, Gonnade R, Ravikanth M, Synthesis,
Structure, Spectral, Electrochemical and Sensing Properties of 3-Amino Boron-Dipyrromethene and Its
Derivatives, Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.10.038.
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ACCEPTED MANUSCRIPT
Synthesis, Structure, Spectral, Electrochemical and Sensing Properties of 3-Amino
Boron-Dipyrromethene and Its Derivatives
a
a
a
b
Emandi Ganapathi , Sheri Madhu , Tamal Chatterjee , Rajesh Gonnade and Mangalampalli
a
Ravikanth *
a
Department of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai 400 076, India. E-mail: ravikanth@chem.iitb.ac.in
b
Center for Materials Characterization, National Chemical Laboratory, Pune-411008.
1

ACCEPTED MANUSCRIPT
Synthesis, Structure, Spectral, Electrochemical and Sensing Properties of 3-Amino
Boron-Dipyrromethene and Its Derivatives
a
a
a
b
Emandi Ganapathi , Sheri Madhu , Tamal Chatterjee , Rajesh Gonnade and Mangalampalli
a
Ravikanth *
a
Department of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai 400 076, India. E-mail: ravikanth@chem.iitb.ac.in
Center for Materials Characterization, National Chemical Laboratory, Pune-411008.
b
Abstract
We report the synthesis of 3-amino boron-dipyrromethene (3-amino BODIPY) by
treating 3-bromo boron-dipyrromethene (3-bromo BODIPY) initially with sodium azide in
acetonitrile followed by triphenylphosphine(PPh )/H O in tetrahydrofuran in three steps under
3
2
mild reaction conditions. In this reaction, 3-azido BODIPY which formed in the first step was
not isolated but the 3-iminophosphorane BODIPY which formed in the second step was isolated
and characterized crystallographically. The 3-amino BODIPY was characterized by various
spectroscopic and X-ray analytical techniques. To test the reactivity of amine functionality on
BODIPY core, we prepared 1-(meso-anisyl BODIPY)-3-phenyl urea /thiourea derivatives under
simple reaction conditions. Our studies indicated that 1-(meso-anisyl BODIPY)-3-pheny thiourea
2
+
can act as specific chemodosimetric sensor for Hg ion and 1-(meso-anisyl BODIPY)-3-phenyl
-
urea as colorimetric and ratiometric sensor for F ion.
Keywords: 3-amino boron dipyrromethene, iminophospharane BODIPY, fluorescent probes,
-
Mercury(II) sensor, chemodosimeter, F sensor.
2

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1
. Introduction
Boron-dipyrromethenes (BODIPYs) has gained tremendous recognition as being one of
the versatile fluorescent probes along with fluorescein, rhodamine and cyanine dyes in
biotechnology field owing to their advantageous spectroscopic properties such as intense
absorption and emission in desired regions with high absorption coefficients, fluorescence yields,
singlet state lifetimes and considerably high photostability under physiological conditions [1-3].
Besides their extensive applications in biology, these dyes are widely used as cation and anion
sensors, drug delivery agents, fluorescent switches, electroluminescent films, laser dyes, light
harvesters and sensitizers for solar cells [4-6]. The main reasons for the rapid growth of
BODIPYs are their ease of synthesis and potential for derivatization. Several new types of
BODIPY dyes have been synthesized recently by using functionalized BODIPYs since these
dyes are amenable for functionalization at all positions of core [7-10]. For example, BODIPYs
are subjected to halogenation at all positions and the halogenated BODIPYs have been used as
building blocks to synthesize different types of new BODIPY derivatives [11-14]. Similarly, the
BODIPY systems containing active methyl groups at 3,5-positions were subjected to Knoevengal
reaction to synthesize π-extended conjugated BODIPY systems [15]. Ziessel et al. developed a
route to introduce ethynyl functional groups in place of fluorides of BODIPY and used these
dyes for the synthesis of very interesting light-harvesting systems [16-19]. Recently, BODIPYs
containing one or two formyl groups have been synthesized and explored their use for the
synthesis of new derivatives and their applications [20-23]. In addition to halogens and formyl
groups, the most bio-compatible functional groups are amines and carboxylic acid groups on
BODIPY which can be activated and linked to proteins or DNA-derivatives [24-26]. In recent
literature, there are several examples in which BODIPY fluorophores were linked to proteins and
3

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bio-molecules using amine and carboxylic acid functional groups [27]. However, the functional
groups in these cases were not present directly on the BODIPY core but present either on meso-
aryl group or on long chains attached to the different positions of BODIPY core. A perusal of
literature revealed that there are two reports on BODIPY having amino functional group at 3-
position. Liras et al. serendipitously synthesized meso-free
β-pyrrole substituted 3-amino
BODIPY along with 3-acetamido BODIPY in one pot reaction during their attempts to
synthesize 8-aza-BODIPY [28]. They carried out the reaction by treating 2, 4-dimethyl-3-
ethylpyrrole with aqueous sodium nitrite in the presence of acetic acid/acetic anhydride followed
by complexation with BF ·OEt . This reaction resulted in the formation of mixture of two
3
2
compounds which were separated by column chromatography. However, this method works only
for meso-free with pyrrole substituted 3-amino BODIPYs and may not be applicable for pyrrole
unsubstituted meso-aryl BODIPYs. Very recently Talukdar and co-workers prepared meso-aryl
3
-amino BODIPY by treating 3-bromo BODIPY with 15%NH in CH CN/H O at room
3 3 2
temperature [29]. Herein we report an alternate approach for the synthesis of 3-amino meso-
anisyl BODIPY 1 (Scheme 1) under mild reaction conditions. The 3-amino BODIPY 1 was used
to synthesize 1-(meso-anisyl BODIPY)-3-phenyl urea 3 and 1-(meso-anisyl BODIPY)-3-phenyl
thiourea 5 (Scheme 2) [30] and demonstrated their use for cation and anion sensing applications.
2
2
. Experimental Section
.1. General
THF and toluene were dried over sodium benzophenone ketyl and chloroform, ethyl-
acetate, methanol, acetonitrile were dried under calcium hydride. BF ·OEt and 2,3-dichloro-5,6-
3
2
dicyano-1,4-benzoquinone (DDQ) obtained from Sigma-Aldrich (USA) were used as obtained.
4

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All other chemicals used for the synthesis were reagent grade unless otherwise specified.
Column chromatography was performed on silica (60-120 mesh).
2
.2. Instrumentation
All the NMR spectra were recorded in CDCl on Bruker 400 MHz instrument using
3
tetramethylsilnae (Si(CH ) ) as internal standard. Absorption and steady state fluorescence
3
4
spectra were obtained with Perkin–Elmer Lambda-35 and PC1 photon counting
spectrofluorometer manufactured by ISS, USA instruments, respectively. The fluorimeter is
corrected for the wavelength dependence of the monochromator and the sensitivity of the
detector throughout the experiment. Square Wave Voltammetric (SWV) studies were carried out
with BAS electrochemical system utilizing the three electrode configuration consisting of a
Glassy carbon (working electrode), platinum wire (auxillary electrode) and saturated calomel
(reference electrode) electrodes. The experiments were done in dry dichloromethane using 0.1 M
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The HR mass spectra were
recorded with a Q-Tof micro mass spectrometer. The quantum yields were calculated using
Rhodamine 6G as reference (Ф = 0.88 in ethanol, λexc = 488 nm) [31]. All Ф are corrected for
changes in refractive index. The association constant of the anion complex formed in solution
has been estimated by using the standard Benesi–Hildebrand equation, viz.,
1
1
1
=
+
-
I - I0
I1 - I0
(I - I )K [A ]
1 0 a
Where I is the intensity of the compounds 3 and 5 before addition of anion, where I is the
0
-
-
intensity in the presence of A , I is intensity upon saturation with A , and K is the association
1
a
constant of the complex formed. The solutions of Bu N salts and metal perchlorates were
4
5

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-2
prepared (1×10 M) in CH CN. The solution containing compounds 3 and 5 were placed in
3
quartz cell (1 cm width), and Bu NF and Hg(ClO ) solutions were added in an incremental
4
4 2
1
fashion. Their corresponding UV-Vis and fluorescence spectra were recorded at 298 K. In H
NMR titration, the spectra were measured on 400 MHz NMR spectrometer. A solutions of 3 and
-3
5
in DMSO-d was prepared (5×10 M), and a 0.4 mL portion of this solution was transferred to
6
a 5-mm NMR tube. A small aliquots of Bu NF and Hg(ClO ) in DMSO-d were added in an
4
4 2
6
incremental fashion, and their corresponding spectra were recorded.
.3 X-ray crystallography: X-ray intensity data measurements of BODIPYs 1, 4 and 8 were
carried out on a SMART APEX II CCD diffractometer with graphite-monochromatized (MoKα=
.71073Å) radiation at 297 (2) K. Data were collected with ω scan width of 0.5° at different
settings of ϕ and 2θ with a frame time of 10 sec keeping the sample-to-detector distance fixed at
0 mm. The X-ray data collection was monitored by APEX2 program [32]. The data was
2
0
5
corrected for Lorentzian, polarization and absorption effects using SAINT and SADABS
programs. SHELX-97 was used for structure solution and full matrix least-squares refinement on
2
F [33]. All the H-atoms were placed in geometrically idealized position and constrained to ride
on their parent atoms. The asymmetric unit contained two molecules of acetonitrile along with
one molecule of BODIPY 2, thus the host to guest ratio is 1:2.
2
.4 General method for the synthesis of 4, 4-Difluoro-4-bora-3a, 4a-diaza-3-amino- 8-(4-
methoxyphenyl)-s-indacene (1-2)
Sample of 3-bromo BODIPY 6 (0.13 mmol) and sodium azide (0.13 mmol) were dissolved in
acetonitrile (10 ml) and reaction mixture was stirred at room temperature for 4 h. Progress of the
reaction was monitored by TLC. After the completion of the reaction, the solvent was removed
under reduced pressure and mixture was washed with CH Cl and water to obtain 3-azido
2
2
6

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BODIPY which without purification proceeded for 3-Amino BODIPY reaction. The crude
mixture of 3-azido BODIPY was reacted with triphenylphosphine (0.26 mmol) in THF solvent
we got corresponding phosphine intermediate of BODIPY compound 8, which intern hydrolysis
to give corresponding 3-Amino BODIPY 1. The crude product was subjected to silica gel
column chromatography using petether/ethylacetate (75:25) to afford pure compound 1 as stable
brown solid in 46% yield.
2
.4.1. BODIPY 8
1
3
Yield 54%. H NMR (400 MHz, CDCl , δ in ppm): 3.83 (s, 3H; -OCH ), 5.16-5.18 (d, J (H, H)
3
3
3
=
4.8 Hz, 1H; py), 6.29-6.30 (m, , 1H; py), 6.34-6.35 (d, J (H, H) = 3.2 Hz, 1H; py), 6.68-6.69
3
3
3
(
d, J (H, H) = 4.8 Hz, 1H; py), 6.90-6.92 (d, J (H, H) = 6.8 Hz, 2H; Ar), 7.38-7.39 (d, J (H, H)
1
1
=
6.8 Hz, 2H; Ar), 7.63-7.51 (m, 10H; Ar + py), 7.89-7.84 (m, 6H; Ar). B NMR (128.3 MHz,
1
19
CDCl , δ in ppm): 1.23 (t, J (B−F) = 33 MHz, 1B). F NMR (376.4 MHz, CDCl , δ in ppm):
3
3
1
31
13
−
147.5 (q, J (F−B) = 33 MHz, 2F). P NMR (400 MHz, CDCl , δ in ppm): 11.5. C NMR (100
3
MHz, CDCl3, δ in ppm) 55.5, 112.5, 113.5, 115.7, 117.7, 127.5, 128.1, 128.5, 129.3, 129.4,
+
1
5
2
30.6, 131.9, 132.9, 133.0, 133.1, 134.9, 160.0. HRMS calcd. [M+1] for C H BF N OP
3
4
28
2
3
74.2031, found 574.2037
.4.2. 4, 4-Difluoro-4-bora-3a, 4a-diaza-3-amino- 8-(4-methoxyphenyl)-s-indacene BODIPY
(1)
1
Yield 46%. H NMR (400 MHz, CDCl , δ in ppm): 3.88 (s, 3H; -OCH ), 5.78 (br s, 2H; NH ),
3
3
2
3
3
6
.07-6.08 (d, J (H, H) = 4.8 Hz, 1H; py), 6.35-6.34 (m, 1H; py), 6.49-6.50 (d, J (H, H) = 3.8
3
3
Hz, 1H; py), 6.93-6.94 (d, J (H, H) = 4.8 Hz, 1H; py), 6.98-6.99 (d, J (H, H) = 6.7 Hz, 2H; Ar),
3
11
7
.43-7.44 (d, J (H, H) = 6.6 Hz, 2H; Ar) 7.46 (br s, 1H; py). B NMR (128.3 MHz, CDCl , δ in
3
1
19
1
ppm): 0.94 (t, J (B−F) = 33 MHz, 1B). F NMR (376.4 MHz, CDCl , δ in ppm): −149.5 (q, J
3
7

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1
3
(
F−B) = 33 MHz, 2F). C NMR (100 MHz, CDCl3, δ in ppm) 55.7, 114.2, 118.4, 119.0, 121.9,
22.6, 126.4, 131.0, 131.5, 131.7, 132.4, 134.8, 135.7, 143.5, 145.5, 162.2; HRMS calcd.
M+Na] for C H BF N NaO 336.1093, found 336.1082
1
[
1
6
14
2
3
2
.4.3. 4, 4-Difluoro-4-bora-3a, 4a-diaza-3-amino- 8-(4-methylphenyl)-s-indacene BODIPY
(2)
1
Yield 48%. H NMR (400 MHz, CDCl , δ in ppm): 2.46 (s, 3H; -CH ), 5.77 (br s, 2H; NH ),
3
3
2
3
3
6
.06-6.07 (d, J (H, H) = 4.5 Hz, 1H; py), 6.33-6.34 (m, 1H; py), 6.47-6.48 (d, J (H, H) = 4.1
3
3
Hz, 1H; py), 6.91-6.93 (d, J (H, H) = 4.7 Hz, 1H; py), 6.97-6.99 (d, J (H, H) = 6.9 Hz, 2H; Ar),
3
11
7
.41-7.42 (d, J (H, H) = 6.9 Hz, 2H; Ar) 7.45 (br s, 1H; py). B NMR (128.3 MHz, CDCl , δ in
3
1
19
1
ppm): 0.92 (t, J (B−F) = 33 MHz, 1B). F NMR (376.4 MHz, CDCl , δ in ppm): −149.6 (q, J
3
1
3
(
F−B) = 33 MHz, 2F). C NMR (100 MHz, CDCl3, δ in ppm) 52.4, 114.3, 118.5, 119.0, 121.8,
22.2, 126.6, 131.0, 131.8, 132.1, 132.9, 133.9, 135.6, 143.2, 145.8, 160.3; HRMS calcd.
M+Na] for C H BF N Na 320.1144, found 320.1146
1
[
1
6
14
2
3
2
.4.4. General procedure for the synthesis of BODIPY 3, 4 and 5:
To a solution of BODIPY 1 (0.16 mmol) in CH CN was added isocyanate/isothiocyanate (0.16
3
mmol) and the reaction mixture was continued stirring at reflux for 12 h. After completion of the
reaction, solvent was removed under vacuum, and the residue was diluted with water and
extracted with dichloromethane. The organic phase was washed with brine, dried over Na SO ,
2
4
filtered, and evaporated to dryness. The crude compound was purified by flash column
chromatography on silica gel (9:1 petether/EtOAc) to afford pure compounds 3, 4 and 5 as tan
solids in 62-67% yield.
2
.4.5. 4, 4-Difluoro-4-bora-3a, 4a-diaza-3- (phenyl urea) - 8-(4-methoxyphenyl)-s-indacene
BODIPY 3
8

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1
Yield 67%. H NMR (400 MHz, CDCl , δ in ppm): 3.91 (s, 3H; -OCH ), 6.40-6.41 (m, 1H; py),
3
3
3
3
6
1
7
1
1
3
1
.66-6.67 (d, J (H, H) = 3.7 Hz, 1H; py), 7.02-7.03 (d, J (H, H) = 4.8 Hz, 1H; py), 7.12-7.15 (t,
3
3
H; Ar), 7.27-7.29 (d, J (H, H) = 4.3 Hz, 1H; py), 7.32-7.34 (d, J (H, H) = 6.6 Hz, 2H; Ar),
3
3
.35-7.37 (d, J (H, H) = 7.3 Hz, 2H; Ar), 7.39-7.41 (d, J (H, H) = 8.1 Hz, 4H; Ar), 7.56 (br s,
1
1
H; py), 9.50 (br s, 1H; NH), 10.45 (br s, 1H; NH). B NMR (128.3 MHz, CDCl , δ in ppm):
3
1
19
1
.37 (t, J (B−F) = 33 MHz, 1B). F NMR (376.4 MHz, CDCl , δ in ppm): −146.9 (q, J (F−B) =
3
1
3
3 MHz, 2F). C NMR (100 MHz, CDCl , δ in ppm) 55.5, 113.2, 114.0, 116.2, 126.2, 126.9,
3
27.5, 127.6, 129.2, 131.1, 132.1, 132.4, 133.2, 133.6, 134.9, 137.3, 142.4, 152.3, 161.6, 163.9.
HRMS calcd. [M + Na] for C H BF N NaO 455.1467, found 455.1464
2
3
19
2
4
2
2
.4.6. 4, 4-Difluoro-4-bora-3a, 4a-diaza-3- (phenyl urea) - 8-(4-methylphenyl)-s-indacene
BODIPY 4
1
Yield 64%. H NMR (400 MHz, CDCl , δ in ppm): 2.49 (s, 3H; -CH ), 6.39-6.40 (m, 1H; py),
3
3
3
3
6
1
7
1
1
3
1
.67-6.68 (d, J (H, H) = 3.5 Hz, 1H; py), 7.02-7.03 (d, J (H, H) = 4.7 Hz, 1H; py), 7.11-7.13 (t,
3
3
H; Ar), 7.27-7.28 (d, J (H, H) = 4.6 Hz, 1H; py), 7.31-7.32 (d, J (H, H) = 6.2 Hz, 2H; Ar),
3
3
.34-7.35 (d, J (H, H) = 6.9 Hz, 2H; Ar), 7.39-7.41 (d, J (H, H) = 7.5 Hz, 4H; Ar), 7.55 (br s,
1
1
H; py), 9.51 (br s, 1H; NH), 10.46 (br s, 1H; NH). B NMR (128.3 MHz, CDCl , δ in ppm):
3
1
19
1
.30 (t, J (B−F) = 33 MHz, 1B). F NMR (376.4 MHz, CDCl , δ in ppm): −146.7 (q, J (F−B) =
3
1
3
3 MHz, 2F). C NMR (100 MHz, CDCl , δ in ppm) 52.7, 113.4, 114.0, 116.8, 126.5, 126.8,
3
27.3, 127.9, 129.5, 131.1, 132.5, 132.8, 133.6, 133.9, 134.4, 137.3, 142.5, 152.7, 161.6. HRMS
calcd. [M + Na] for C H BF N NaO 439.1516, found 439.1516
2
3
19
2
4
2
.4.7. 4, 4-Difluoro-4-bora-3a, 4a-diaza-3- (phenyl thiourea) - 8-(4-methylphenyl)-s-
indacene BODIPY 5
9

ACCEPTED MANUSCRIPT
1
Yield 62%. H NMR (400 MHz, CDCl , δ in ppm): 3.90 (s, 3H; -OCH ), 6.40-6.41 (m, 1H; py),
3
3
3
3
6
.73-6.74 (d, J (H, H) = 3.7 Hz, 1H; py), 6.97-6.99 (d, J (H, H) = 4.2 Hz, 1H; py), 7.01-7.03 (d,
3
3
3
J (H, H) = 8.2 Hz, 2H; Ar), 7.38-7.39 (d, J (H, H) = 7.5 Hz, 2H; Ar), 7.45-7.46 (d, J (H, H) =
3
3
4
.5 Hz, 1H; py), 7.47-7.49 (d, J (H, H) = 6.7 Hz, 2H; Ar), 7.53-7.55 (d, J (H, H) = 7.7 Hz, 2H;
3
Ar), 7.57 (br s, 1H; py), 7.88-7.89 (d, J (H, H) = 6.9 Hz, 1H; Ar), 8.25 (br s, 1H; NH), 9.28 (br
1
1
1
19
s, 1H; NH). B NMR (128.3 MHz, CDCl , δ in ppm): 0.99 (t, J (B−F) = 33 MHz, 1B).
F
3
1
13
NMR (376.4 MHz, CDCl , δ in ppm): −146.1 (q, J (F−B) = 33 MHz, 2F). C NMR (100 MHz,
3
CDCl , δ in ppm) 55.5, 111.0, 113.92, 113.99, 114.20, 120.79, 121.73, 122.63, 122.66, 124.65,
3
1
1
3
26.09, 126.96, 129.16, 129.88, 129.96, 130.01, 130.65, 131.89, 132.61, 135.77, 136.68, 137.36,
37.75, 153.60. HRMS calcd. [M + Na] for C H BF N NaOS 471.1237, found 471.1233
2
3
19
2
4
. Results and Discussion:
The synthetic route adopted here to prepare 3-amino BODIPY 1 is outlined in Scheme 1.
We also used the readily available 3-bromo BODIPY 6 as key precursor to synthesize 3-amino
BODIPY. In the first step, the 3-bromo BODIPY 6 was treated with one equivalent of NaN in
3
CH CN at room temperature for 4 h to obtain 3-azido BODIPY 7. The progress of the reaction
3
was followed by TLC, absorption and IR spectroscopy. In TLC analysis, we noted the complete
disappearance of spot corresponding to 6 and appearance of one new single spot corresponding
to the desired 3-azido BODIPY compound 7. In absorption spectroscopy, the absorption band of
compound 6 at 508 nm was bathochromically shifted and appeared at 524 nm in 7. The presence
-1
of azide group at 3-position of BODIPY was confirmed by strong peak at 2131 cm in IR
spectrum. Furthermore, the compound 7 was confirmed by molecular ion peak at 362.1008 in
HR-MS (Figure S1, Supporting Information). Our attempts to isolate pure stable 7 by subjecting
the reaction mixture to column chromatographic purification were failed due to decomposition of
1
0

ACCEPTED MANUSCRIPT
compound 7 on silica or alumina column. Without purification, the compound 7 was subjected to
Staudinger reaction conditions by treating it with PPh in THF at refluxing temperature for 4 h
3
[
34, 35]. Column chromatographic purification on silica afforded stable pure compound 3-
iminophosphorane BODIPY 8 in 54% yield. The compound 8 was confirmed by molecular ion
1
1
1
31
13
19
11
peak at 574.2037 in HR-MS. H, H- H COSY, P, C, F and B NMR techniques as well as
1
X-ray crystallography were used to characterize the compound 8. In H NMR of compound 8
(Figure S3, Supporting Information), the five pyrrole protons of BODIPY unit (type a to type e)
appeared as five sets of signals which were identified based on the correlations observed in the
1
1
H- H COSY spectrum (Figure S4, Supporting Information). The pyrrole signals of compound 8
were slightly upfield shifted compared to compound 6 and the maximum upfield shift was noted
for pyrrole proton of type e which was adjacent to the iminophosphorane group. The compound 8
3
1
showed a single resonance at 11.57 ppm in P NMR (Figure S8, Supporting Information); a
11
characteristic triplet at 1.2 ppm in B NMR and slightly upfield shifted quartet at -147 ppm in
1
9
F NMR.
The single crystals of compound 8 (CCDC No. 900428) were obtained from n-
hexane/chloroform on slow evaporation and crystallized in a triclinic P2 unit cell. The crystal
1
structure of compound 8 is shown in Figure 1. The dihedral angle between the meso-aryl ring
and the BODIPY core is 58°. The compound posseses a planar indacene plane and the three
phenyl groups of iminotriphenylphospharane were perpendicular to the BODIPY core connected
through imino bridge. The bond length between “N” atom of iminophospharane and carbon in 3-
position of BODIPY is 1.335 Å which clearly indicates the presence of single bond (1.330-1.390
Å) between nitrogen and carbon atoms. The angle between the phenyl group and imino group
1
1

ACCEPTED MANUSCRIPT
was in the range of 118-122°. The torsion angle between pyrrole and phenyl group of
iminotriphenylphospharane was 10.1°.
In the next step, the compound 8 was subjected to hydrolysis in THF/H O mixture and
2
the progress of the reaction was monitored by TLC and absorption spectroscopy. After standard
work-up followed by column chromatographic purification on silica afforded the desired stable
BODIPY 1 in 46% yield. Alternately, the compound 1 was also obtained in one pot reaction by
treating compound 6 first with NaN followed by PPh /H O without isolating compound 8. The
3
3
2
1
compound 1 was confirmed by molecular ion peak at 336.1082 in HR-MS. In H NMR, the five
pyrrole protons (type a to type e) appeared as five sets of signals (Figure 2a) which were upfield
shifted compared to 6 but downfield shifted compared to compound 8. Furthermore, the NH2
protons of 1 were appeared as slightly broad peak at 5.78 ppm. The compound 1 also showed
19
11
typical quartet at -149 ppm in F and triplet at 0.9 ppm in B NMR like compounds 6 and 8.
Crystals of BODIPY 1 (CCDC No. 900427) suitable for X-ay analysis were obtained via
slow evaporation of CH Cl solution. The crystal structure of compound 1 was shown in Figure 3
2
2
and the relevant crystallographic data was included in Table 1. Due to symmetry, there was
.85/0.15 disorder present in the amino and H-atom positions. The amino group (NH ) was
0
2
statistically averaged over two positions with major occupancies at C16 (0.85) and minor one at
C13 (0.15), and vice-versa for H-atom (H13 and H16) thus summing the site-occupancy factors
for the disordered atoms equal to unity. The compound 1 was planar with the central six-
membered ring with two adjacent five-membered rings were in the same plane. The plane
defined by F-B-F atoms was perpendicular to that of BODIPY core. The bond length between
“N” atom of amine and carbon in 3-position of BODIPY is 1.358 Å. The dihedral angle between
1
2

ACCEPTED MANUSCRIPT
meso-aryl group and the BODIPY core was 51° indicating that the meso-aryl group was not in
the same plane.
To show the reactivity of amino group of compound 1, we prepared BODIPYs 3, 4 and 5
as shown in Scheme 2. The compounds 3, 4 and 5 were prepared by treating BODIPY 1 with
phenyl isocyanate and phenyl thioisocyanate respectively in CH CN at reflux for 12 h. The
3
progress of the reaction was monitored by TLC analysis which showed the complete
disappearance of the spot corresponding to BODIPY 1 and appearance of less polar spot
corresponding to the desired compounds 3-5. The crude mixtures were subjected to silica gel
column chromatographic purification and afforded pure compound 3-5 in 62-67% yields. The
molecular ion peak at 455.1464 for compound 3, at 439.1516 for 4 and at 471.1233 for
compound 5 in HR-MS spectra confirmed their identities. The compounds 3-5 were freely
soluble in common organic solvents and characterized by various techniques including X-ray
1
crystallography. In H NMR of compound 3 (Figure 2a), the two amide NH protons appeared as
two broad singlets at 9.50 and 10.45 ppm whereas in compound 5, the two amide NH protons
appeared as two broad singlets at 8.25 and 9.38 ppm. Compared to compound 1, the pyrrole
protons (type a to type e) in compounds 3 and 5 experienced downfield shift of ~0.2 ppm (Figure
1
9
11
2
a). In F (Figure 2b) and B NMR (Figure 2c) spectra of compounds 3 and 5 showed typical
quartet at ~-146 ppm and triplet at ~-1.25 ppm respectively.
The single crystals of compound 4 (CCDC 951085) were obtained from n-
hexane/chloroform on slow evaporation and crystallized in a triclinic P2 unit cell. The crystal
1
structure of compound 4 was shown in Figure 4 and the relevant crystallographic data was
included in Table 1. The compound 4 has a planar BODIPY framework which was comprised of
two pyrrole rings and the central six membered ring containing boron. The dihedral angle
1
3

ACCEPTED MANUSCRIPT
between the meso-aryl ring and the BODIPY core was 55° which indicate the presence of free
rotation of meso-aryl group in compound 4. The indacene ring and the amide functional group
were in same plane in compound 4. The plane defined by F-B-F atoms was perpendicular to that
of BODIPY core. The bond length between “N” atom of amide in urea and carbon in 3-position
of BODIPY is 1.369 Å. However, the phenyl group at 3-position of urea moiety was slightly
deviated from the indacene plane as well as urea functionality by 34°. Importantly, the two N-H
protons of urea moiety were facing towards fluoride atoms and the type- f amide NH proton was
involved in weak intramolecular hydrogen bonding with boron bound fluoride atoms.
3
.1.
Absorption, electrochemical and fluorescence properties of compounds 1-6 and 8:
Absorption, electrochemical and fluorescence properties were investigated for
compounds 1-5 along with compounds 6 and 8. The compounds 1-6 and 8 showed typical
BODIPY absorption features with a strong band in 480-520 nm region corresponding to the
S →S transition with a vibronic transition on the higher energy side as a shoulder (Figure 5).
0
1
The BODIPY 8 exhibited a hypsochromic shift of ~20 nm compared to BODIPY 6 whereas
BODIPY 1 showed ~10 nm hypsochromic shift compared to BODIPY 6. However, the
compound 3 showed a bathochromic shift of ~20 nm compared to compound 1 but in case of
BODIPY 5, the absorption band was hypsochromically shifted by ~15 nm compared to BODIPY
1
. Furthermore, it is clear from the Figure 5a, compounds 1 and 5 showed broader absorption
spectra compared to compounds 3 and 6. This is attributed to the presence of electron donor
substituent at 3-position of BODIPY in compounds 1 and 5 which induces a weak charge
transfer character for the lowest electronic excitation in the ground state geometry [36]. The
fluorescence properties of BODIPYs 1-5 are very sensitive to the solvent. Upon increasing the
solvent polarizability from hexane to DMSO, the emission maxima showed 8-10 nm
1
4

ACCEPTED MANUSCRIPT
hypsochromic shift with decrease in the the fluorescence quantum yields and singlet-state life
times which was in consistent with the general behaviour of BODIPY chromophores (Table S3)
[
37]. The electrochemical properties of compounds 1-6 and 8 were investigated by cyclic
voltammetry and square-wave voltammetry in CH Cl using tetrabutylammonum perchlorate
2
2
(TBAP) as the supporting electrolyte. The compounds 1, 3, 5 and 8 showed an ill-defined
oxidation and one or two reductions. The compound 8 containing electron donating
iminotriphenylphosphorane group showed one reversible reduction at -1.40 V which was shifted
towards more negative by ~635 mV compared to compound 6 indicating the electron rich nature
of BODIPY core in compound 8. Similarly, the presence of electron donating amine
functionality in compound 1 also resulted in the shift of reduction potential towards more
negative by ~480 mV compared to compound 6 (Table S2, Supporting Information). However,
the BODIPYs 3 and 5 were difficult to oxidize but easier to reduce compared to BODIPY 1. The
fluorescence properties of compounds 1-6 and 8 were studied in different solvents using steady
state and time-resolved fluorescence techniques. In acetonitrile solution, the compound 8 showed
one fluorescence band at 539 nm with quantum yield of ~0.03 whereas the compound 1 showed
blue shifted fluorescence band at 524 nm with decreased quantum yield of 0.007. However, in
case of compounds 3 and 5, the fluorescence band was bathochromically shifted by ~20 nm
compared to compound 1 with increased quantum yield (~ Φ = 0.02) and singlet state life times
(~ τ = 0.32 ns) as shown in Table 2.
3
.2.
Hg(II) ion Sensing:
Since the compound 5 was synthesized with an assumption that it would sense Hg(II) ion
specifically and convert to compound 3, we carried out systematic studies of compound 5 with
various metal ions and followed the changes by different spectroscopic techniques. Our
1
5

ACCEPTED MANUSCRIPT
screening studies with various metal ions clearly indicated that compound 5 binds specifically to
2
+
Hg ion with clear colour change from yellow fluorescent to green fluorescent but it did not
show any changes in the presence of other metal ions (Figure S36-S37, Supporting Information).
Thus, BODIPY 5 can be used as an exclusive chemodosimeter for Hg(II) ions which we
1
confirmed by using absorption, emission and H NMR studies as described below.
The systematic changes in absorption spectra of compound 5 upon addition of increasing
equivalents of Hg(II) ions were shown in figure 6a. Upon addition of increasing equivalents of
Hg(II) ion to compound 5 in CH CN resulted in the decrease in the absorption band at 480 nm
3
with an appearance and concomitant increase in new absorption band at 520 nm corresponding to
compound 3 with two clear isosbestic points at 500 nm and 412 nm. This was also clearly
evident in the plot of ratio of absorbances at 513 and 480 nm versus the concentration of Hg(II)
ion added (inset figure 6a). Thus, the study clearly showed that in the presence of Hg(II) ion, the
BODIPY 5 was converted to BODIPY 3 which was confirmed by comparing the spectroscopic
data with the synthesized BODIPY 3.
We also monitored the chemidosimetric sensing of Hg(II) by BODIPY 5 using
fluorescence studies. Upon addition of Hg(II), a gradual decrease in the intensity of fluorescence
band at 543 nm with a shift of ~ 5 nm in emission peak maxima was observed (Figure 6b). The
fluorescence titration profile of BODIPY 5 versus concentration of Hg(II) ions revealed that the
gradual decrease of the emission was observed with the increasing addition of Hg(II) ions up to
6
0 ꢀM (inset in figure 6b) and the detection limit was found to 3 ppm. From the Benesi-
6
-1
Hildebrand equation, the association constant calculated was 1.6x10 M . Furthermore, the
decrease in fluorescence intensity of BODIPY 5 upon addition of Hg(II) ion was attributed to the
desulfurization of compound 5 to form compound 3. Thus, compound 5 can act as
1
6

ACCEPTED MANUSCRIPT
chemodosimeter for Hg(II) ion. This hypothesis was further supported by ESI-MS analysis. In
the absence of Hg(II), a peak was observed at m/z 471.1233 corresponding to the BODIPY 5.
After the addition of Hg(II) to the solution of BODIPY 5, a new peak appeared at m/z 455.1641
confirming the formation of BODIPY 3 (Figure S38, Supporting Information).
1
Furthermore, we carried out H NMR titration experiments for sensing of Hg(II) by
1
BODIPY 5 in DMSO-d . The systematic changes in H NMR signals of BODIPY 5 upon
6
addition of increasing amounts of Hg(II) ion was shown in Figure 7. As clear from figure 7, upon
addition of increasing amounts of Hg(II) ion to BODIPY 5, the signals corresponding to the
amide N-H protons (type f and type g) at 8.25 and 9.38 ppm experienced a continuous down filed
shift of ~1.2 ppm and appeared at 9.51 ppm and 10.46 ppm respectively. This significant down
field shifts of amide N-H signals (type f and type g) was attributed to the conversion of BODIPY
5
to BODIPY 3 in the presence of Hg(II) ion. As expected, we did not observe any significant
changes for signals corresponding to BODIPY core as well as meso-aryl protons of BODIPY 5
2
+
upon addition of Hg ion. Thus, all our studies clearly showed that BODIPY 5 can act as
specific chemodosimetric sensor for the detection of Hg(II) ion.
3
.3.
Fluoride ion sensing:
It is known in the literature that amide containing fluorophores can be good anion sensors
[
38, 39], we investigated anion sensing property of compound 3 with different anions. Initially,
we checked the colour change of compound 3 by adding different anions. The compound 3 was
-
fluorescent under Uv light. Upon addition of F ion, the fluorescence of the compound 3 was
enhanced significantly under Uv lamp and the colour change was also significant to the naked
-
-
-
-
-
-
-
eye. Whereas the addition of various other anions such as Cl , Br , I , H PO , HSO , ClO , OH
2
4
4
4
did not show any change in the fluorescence of the compound 3 (Figure S39-S40, Supporting
1
7

ACCEPTED MANUSCRIPT
-
Information). Thus, we realized that compound 3 can act as specific colorimetric sensor for F
-
ion. Hence, we investigated the F ion sensing behaviour of compound 3 by using absorption,
1
fluorescence, electrochemical and H NMR studies.
The changes in the absorption spectra of BODIPY 3 upon addition of increasing amounts
-
of F ions in CH CN were shown in figure 8a. Upon systematic addition increasing equivalents
3
-
of F ion, the intensity of the absorption band at 512 nm was decreased with an appearance and
concomitant increase of absorption band at 468 nm with two clear isosbestic points at 490 and
3
86 nm indicating the binding of fluoride ion to compound 3. This was also clearly evident in the
ratiometric plot (inset of figure 8a) which shows the changes in the absorption bands at 512 nm
-
-
and 468 nm versus the concentration of F ion added. We also monitored the F ion sensing
phenomenon of BODIPY 3 by fluorescence studies (Figure 8b). Upon systematic addition of
-
increasing equivalents of F ion, the fluorescence intensity of BODIPY 3 at 540 nm experienced
slight bathochromic shift of ~8 nm with significant enhancement in the fluorescence intensity
-
reflected in 5-fold increase in the fluorescence quantum yield. The Job’s plot analysis of the F
ion titration with BODIPY 3 was also corroborated to be 1:1 binding stoichiometry (Figure S43
in Supporting Information). From Benesi−Hildebrand equation, the binding constant of
3
−1
-
4
.033x10 M was evaluated for BODIPY 3–F anion interaction. Furthermore, we did not
notice any changes in the absorption and fluorescence bands of BODIPY 3 upon addition of
various other anions (Figure S38, Supporting Information). Thus, absorption and fluorescence
studies clearly showed that BODIPY 3 can act as a specific colorimetric and ratiometric
-
chemosensor for detection of F ion.
-
We further tested specific binding of BODIPY 3 with F ion by following the changes in
-
the first reduction of 3 upon addition of increasing amounts of F ion by square wave
1
8

ACCEPTED MANUSCRIPT
voltammetric technique. The systematic changes in the first reduction wave of BODIPY 3 upon
-
addition of increasing amounts of F ion were presented in the Figure 9. As it was clear from
-
figure 9 that the increasing addition of F ions to BODIPY 3 resulted in the significant decrease
in current intensity of reduction wave at -0.89 V and at the same time, the appearance of a new
reduction wave with a gradual increase in current intensity at -1.34 V was noted. This significant
-
shift in the reduction potential towards more negative by ~450 mV indicating that F ions were
binding to amide moiety of BODIPY 3 and makes the dye 3 more electron rich. Furthermore, no
changes in the reduction wave of BODIPY 3 were observed upon addition of various other
anions. Thus, the square wave voltammetric studies also supported that the BODIPY 3 can be
-
used as specific electrochemical sensor for F ion.
1
The fluoride anion binding to BODIPY 3 was also monitored by H NMR titration
1
studies in DMSO-d . The systematic changes in H NMR signals of BODIPY 3 upon addition of
6
increasing amounts of fluoride ion were shown in Figure 10. As clear from figure 10 that upon
-
addition of only 0.5 equivalents of F ion, the signal corresponding to the amide N-H proton of
type f experienced downfield shift and became broadened indicating the hydrogen-bonding
-
interaction between fluoride ions with type f N-H proton. Upon addition of 1.5 equiv. of F ion,
the type-f N-H proton of 3 was disappeared completely. However, upon addition of more
-
-
equivalents of F ion, we noticed one triplet at ~16.5 ppm suggesting the formation of HF ion
2
which confirmed the deprotonation of the type f amide N-H proton. The deprotonation of type f
amide NH proton of BODIPY was also evident in the upfiled shifts of various BODIPY core
protons (types a-e and type g).
4
. Conclusion
1
9

ACCEPTED MANUSCRIPT
In conclusion, we developed a simple facile three step synthetic route for the preparation
of 3-amino BODIPY 1 from 3-bromo BODIPY 6 under mild reaction conditions. One of the
intermediate compounds such as iminophospharane BODIPY 8 was also isolated and well
characterized by various techniques including X-ray analysis. The BODIPY 1 was characterized
in detail by HR-MS, NMR, absorption, fluorescence, electrochemical and X-ray crystallographic
techniques. The spectral and electrochemical properties of 3-amino BODIPY indicated that the
presence of amino group on the BODIPY core alter the electronic properties of the dye. We
tested the reactivity of amine functional group on BODIPY core of compound 1 by synthesizing
1
-(meso-anisyl BODIPY)-3-phenyl urea 3/thiourea 5 derivatives under simple reaction
conditions. The spectral and electrochemical properties of BODIPY compounds 3 and 5 clearly
showed that the absorption and emission maxima were bathochromically shifted with increasing
quantum yields and singlet state life times compared to BODIPY 1. We tested the compounds 3
and 5 for sensing various anions and cations. Our studies clearly indicated that BODIPY 5 can
2
+
act as specific chemodosimetric sensor for the detection Hg ions over other metal ions as
1
verified by absorption, fluorescence and H NMR techniques. Furthermore, the studies also
-
indicated that BODIPY 3 can act as specific colorimetric and ratiometric chemosensor for F ion.
Acknowledgments M.R. thanks Council of Scientific & Industrial Research (CSIR), Govt. of
India for financial support; E. G. thanks UGC for a fellowship, S. M. thanks IIT-Bombay and
T.C thanks CSIR for a fellowship.
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Table 1: Crystal data and structure refinement parameters for compounds 1, 4 and 8
Parameters
BODIPY 1
BODIPY 4
BODIPY 8
CCDC No.
Formula
Mw
Temperature [K]
Wavelength [Å]
Crystal system
Space group
a [Å]
900427
951085
900428
C H BF N O
C H BF N O
C H BCl F N OP
16
14
2
3
23 19
2
4
35 28
3
2
3
313.11
297
0.71073
monoclinic
P 21/c
19.9215(12)
10.2446(6)
7.1570(4)
90
416.23
100(2)
0.71075
monoclinic
P 21/c
6.130(3)
29.249(14)
10.712(5)
90
692.73
298
0.71073
monoclinic
P 21
9.1765(15)
15.631(3)
11.6493(19)
90
b [Å]
c [Å]
α (°)
β (°)
99.218(3)
90
1441.79(15)
4
0.109
1.442
91.917(8)
90
1919.5(16)
4
0.104
1.440
91.847(3)
90
1670.1(5)
2
0.367
1.378
γ (°) 3
V [Å ]
Z
-1
µ [mm ]
ρ calcd [gcm ]
-3
F(000)
648.0
864.0
712.0
2
θ range [°]
2.07-27.73
12431/3389
0.0230
3.33 - 25.50
15248 / 3533
0.1273
1.75 - 25.00
16074/ 5858
0.0226
e data
Rint
R , wR [1>2
σ
(I)]
0.0472, 0.1235
0.0662, 0.1347
1.034
0.0797, 0.1957
0.0942, 0.2133
1.238
0.0501, 0.1364
0.0530, 0.1394
1.027
1
2
R , wR (all data)
1
2
GOF
-3
Largest diff. peak/hole, [eÅ ] 0.671/-0.275
0.443 / -0.375
0.570/ -0.445
2
6

ACCEPTED MANUSCRIPT
Table 2: Photophysical and electrochemical data of compounds 1-6 and 8 recorded in CH CN
3
2
7

ACCEPTED MANUSCRIPT
Figure Captions
Chart 1: Structures of BODIPYs 1-8
Scheme 1: Synthesis of 3-amino BODIPY 1
Scheme 2: Synthesis of BODIPYs 3, 4 and 5
Figure 1: ORTEP diagram (50% probability) of compound 8
1
11
19
Figure 2: Comparision of a) H, b) B, c) F NMR spectra of compounds 1, 3, and 5 recorded
in CDCl3
Figure 3: ORTEP diagram (50% probability) of compound 1
Figure 4: ORTEP diagram (50% probability) of compound 4
Figure 5: Comparision of normalized (a) absorption and (b) emission spectra of BODIPYs 1-5
(
5 ꢀM) recorded in CH CN solution.
3
-6
Figure 6: a) Absorption and b) emission spectra of BODIPY 5 (5x10 M) upon titration with
+
2
different concentrations of Hg [Hg(ClO ) ] solution (0-6 equiv.) in CH CN. Inset: a) shows the
4
2
3
2
+
plot of ratio of absorbance at 478 nm and 518 nm vs concentration of Hg (ꢀM) ions added, b)
2
+
plot of fluorescence intensity at 546 nm vs concentration of Hg (ꢀM) ions added.
1
-2
Figure 7: Partial H NMR titration spectra of BODIPY 5 (1.67 x 10 M) upon systematic
+
2
addition of Hg (0-1.5 equivalents) ions in DMSO-d solvent.
6
-6
Figure 8: a) Absorption and b) emission spectra of BODIPY 3 (5x10 M) upon titration with
-
different concentrations of F [TBAF] ion solution (0-10 equiv.) in CH CN. Inset: a) shows the
3
-
plot of ratio of absorbance at 468 nm and 512 nm vs concentration of F ion (ꢀM) added, b) plot
-
of fluorescence intensity at 553 nm vs [F ] ion added.
-3
Figure 9: Square Wave Voltammogram of compound 3 (1.92 x 10 M) upon addition of
-
increasing equivalents of F (TBAF) ions (0-1.2 equiv.) in CH CN solution.
3
2
8

ACCEPTED MANUSCRIPT
1
-2
Figure 10: Partial H NMR titration spectra of BODIPY 3 (1.80 x 10 M) upon systematic
-
addition of F (0-5 equivalents) in DMSO-d solvent.
6
R
B
R
B
OCH3
OCH3
OCH3
N
N
N
N
N
N
B
F
N
N
B
F
N
N
B
F
X
NH2
F
F
HN
Br
F
N3
F
N
P
F
F
F
HN
R = -OCH 1
X = O, R = -OCH 3
3
3
6
7
8
X = O, R = -CH3
4
R = -CH3
2
X = S, R = -OCH 5
3
Chart 1
2
9