Brominated boron dipyrrins: Synthesis, structure, spectral and electrochemical properties

Article abstract of DOI:10.1039/c2dt00019a

meso-Anisyl boron dipyrrins (BODIPYs) 1-6 containing one to six bromines at the pyrrole carbons have been synthesized by treating meso-anisyl dipyrromethane with 'n' equivalents of N-bromosuccinimide in THF at room temperature followed by oxidation with D

Full text of DOI:10.1039/c2dt00019a

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PAPER
Brominated boron dipyrrins: synthesis, structure, spectral and
electrochemical properties†
V. Lakshmi and M. Ravikanth*
Received 4th January 2012, Accepted 7th March 2012
DOI: 10.1039/c2dt00019a
meso-Anisyl boron dipyrrins (BODIPYs) 1–6 containing one to six bromines at the pyrrole carbons have
been synthesized by treating meso-anisyl dipyrromethane with ‘n’ equivalents of N-bromosuccinimide in
THF at room temperature followed by oxidation with DDQ, neutralization with triethylamine and further
complexation with BF3·OEt2. The brominated compounds were characterized by HR-MS mass, detailed
1H, 19F and 11B NMR and X-ray diffraction studies. The crystal structures solved for compounds 2–6
indicate that the boron dipyrrinato framework comprised two pyrrole rings and one six membered boron
containing ring in one plane like other reported BODIPYs. However, the dihedral angle between the
BODIPY core and the meso-anisyl group varied from 48° to 88° and the meso-anisyl ring has an almost
perpendicular orientation in penta 5 and hexabrominated 6 BODIPYs. The absorption and emission
studies showed a bathochromic shift and reached a maximum for tetrabrominated derivative 4, after which
there was no change in the peak maxima for penta 5 and hexabrominated 6 derivatives. However, the
quantum yields were reduced with the increasing number of bromines. The electrochemical studies
revealed that brominated BODIPY compounds 1–6 are easier to reduce compared to unsubstituted
meso-anisyl BODIPY 8 and the reduction potential is linearly related to the number of Br groups.
C-nucleophiles to synthesize a variety of 3,5-disubstituted
Introduction
BODIPYs.5 We recently reported the synthesis of sterically
crowded highly fluorescent polyarylated BODIPYs using
1,2,3,5,6,7-hexabromo meso-anisyl BODIPY as a key synthon.6
Thus, the halogenated BODIPYs are important precursors to
synthesize a variety of substituted BODIPYs. However, the
effect of halogen substitution at the pyrrole carbons of boron
dipyrrin core on spectral, structure, photophysical and electro-
chemical properties remained largely unexplored. It has been
shown with other chromophoric systems such as porphyrins,
how the systematic alteration of the electronic properties of the
porphyrins can be achieved as a function of the number of halo-
gens at the β-pyrrole carbons.7 Incidentally, while we were pre-
paring this manuscript, Jiao et al.8 reported regioselective
stepwise bromination of BODIPY by treating BODIPY with “n”
equivalents of Br2 in CHCl3. In this paper, we report the syn-
thesis, structure, spectral and electrochemical properties of
pyrrole brominated BODIPYs where the number of Br groups on
the BODIPY core has been varied from 0 to 6. However, our
approach for the synthesis of pyrrole brominated BODIPYs is
completely different from Jiao et al. We used meso-anisyl dipyr-
romethane 79 as key precursor and brominated it by treating with
“n” equivalents of N-bromosuccinimide in THF at room temp-
erature followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) and complexation with BF3·OEt2. The
monobromo and dibrominated BODIPYs prepared by this
approach are different from Jiao et al. method.8 Furthermore,
Jiao et al.8 failed to isolate the tribromo BODIPY which we
were successfully isolated and characterized. We also carried out
Among the fluorophores available, 4,4-difluoro-4-bora-3a,4a-
diaza-s-indacene (BODIPY) derivatives have become preferred
fluorophores which have applications in many different areas
because of their excellent properties.1 The valuable qualities of
BODIPY fluorophores are their high absorption coefficients,
high fluorescence quantum yields, narrow emission bandwidths
with high peak intensities, elevated photostability and chemical
stability, excitation/emission wavelengths above 500 nm. From
the synthetic viewpoint, it is relatively straightforward to syn-
thesize many different BODIPY analogues with their emission
maxima ranging from 500 to over 700 nm.2 Specifically, the
pyrrole substituted and pyrrole fused BODIPY systems possess
excellent photophysical properties and most of these systems
were prepared using the substituted pyrroles as key synthons.3
However, the synthesis of substituted pyrrole precursors is not
straightforward and has limited synthetic accessibility. Alter-
nately, the functionalized BODIPY dyes such as halogenated
BODIPYs can be used to synthesize substituted BODIPYs.
Recently Dehaen, Boens and co-workers used 3,5-dihalo BODI-
PYs4,5a to carry out nucleophilic substitution with O-, N-, S- and
Department of Chemistry, Indian Institute of Technology, Bombay,
Powai, Mumbai 400 076, India. E-mail: ravikanth@chem.iitb.ac.in
†Electronic supplementary information (ESI) available: Characterization
data for all new compounds. CCDC 2 (849331), 3 (849334), 4
(849332), 5 (849333) and 6 (823749). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2dt00019a
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 5903–5911 | 5903

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Scheme 1 Synthesis of brominated compounds 1–6.
systematic studies to increase the number of bromines at the
pyrrole carbons of BODIPY on the spectral as well as electroche-
mical properties. Thus, both these approaches are very useful in
synthesis of pyrrole brominated BODIPYs which can be used as
precursors for the synthesis of various new substituted BODIPY
derivatives.
followed by complexation with BF3·OEt2 at room temperature
for an additional 1 h. The progress of the reaction was followed
by TLC analysis and absorption spectroscopy. TLC analysis of
crude reaction mixture showed the major spot corresponding
to the desired brominated BODIPY along with two minor spots
corresponding to the unsubstituted BODIPY 8 and one higher
analogue of brominated BODIPY. The crude compounds were
subjected to silica gel column chromatographic purification
using petroleum ether–CH2Cl2 and isolated pure compounds 3
and 4 in ∼20% yield.
Results and discussion
Synthesis and characterization
The compounds 5 and 6 were prepared by treating 7 with five
and ten equivalents respectively with NBS in THF at room temp-
erature for 5–6 h. The resulting crude brominated dipyrro-
methanes were flash chromatographed on silica and then
subjected to oxidation with DDQ for 1 h followed by complexa-
tion with BF3·OEt2 for an additional 1 h at room temperature.
The crude reaction mixtures were subjected to silica gel column
chromatographic purification and afforded pure compounds 5
and 6 in ∼25% yield. All bromination reactions worked
smoothly and the Rf values of brominated BODIPYs are suffi-
ciently distinct from each other to isolate the pure compounds by
column chromatography. The compounds 1–6 are freely soluble
in common organic solvents and characterized by mass, NMR,
absorption, electrochemistry and fluorescence techniques.
The compounds 1–6 showed characteristic M+ and (M − F)+
ions in ES-MS mass spectra confirming the identity of the com-
The mono and dibromination at the 3-, 3,5-, 2- and 2,6-positions
of BODIPY has been reported previously by adopting different
synthetic routes. The bromination at 2- and 2,6-positions were
carried out by treating the corresponding meso-substituted
BODIPY with one and two equivalents of NBS respectively in
DMF at room temperature10 whereas the 3-bromo BODIPY 1
and 3,5-dibromo BODIPY 2 were prepared by treating the corre-
sponding meso-substituted dipyrromethane with one and two
equivalents of NBS in THF at −78 °C followed by oxidation
with DDQ and complexation with BF3·OEt2.4 Recently, the 1,7-
dibrominated BODIPY was prepared by treating the substituted
BODIPY with vacant 1,7-positions with two equivalents of Br2
in CH2Cl2 at 0 °C.11 We synthesized brominated BODIPYs 1–6
where the number of Br groups on the BODIPY core increased
from 0 to 6 using meso-anisyl dipyrromethane 79 as the key pre-
cursor (Scheme 1). The 3-bromo meso-anisyl BODIPY 1 and
3,5-dibromo meso-anisyl BODIPY 2 were synthesized by adopt-
ing the literature procedures.12 The other brominated BODIPYs
such as 2,3,5-tribromo BODIPY 3, 2,3,5,6-tetrabromo meso-
anisyl BODIPY 4, 1,2,3,5,6-pentabromo meso-anisyl BODIPY 5
and 1,2,3,5,6,7-hexabromo meso-anisyl BODIPY6 6 were pre-
pared by slightly modifying the reaction conditions. The tri-
bromo 3 and tetrabromo 4 BODIPYs were synthesized in two
steps. In the first step, the dipyrromethane 7 was treated with
three and four equivalents of NBS in THF at −78 °C for 30 min.
In the second step, the resulted brominated dipyrromethanes
without characterization were oxidized with DDQ for 1 h
1
pounds. The compounds 1–6 were characterized by H, 19F and
11B NMR studies. The comparison of 1H, 19F and 11B NMR
spectra of compounds 1–6 along with 8 is shown in Fig. 1 and
the relevant NMR data are presented in Table 1. As is clear from
Fig. 1 on introduction of bromines at the pyrrole carbons, the
corresponding pyrrole proton signals disappear and in the case
of compound 6, the pyrrole signals are completely absent confi-
rming the substitution of bromines at the pyrrole carbons of the
BODIPY core. In general, the pyrrole signals did not show sig-
nificant changes in their chemical shifts on increasing the
number of bromine groups from 0 to 4 but on introduction of
the 5th bromine group, the type ‘c’ proton adjacent to the
5904 | Dalton Trans., 2012, 41, 5903–5911
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Table 1 1H, 19F and 11B NMR data (δ in ppm) of compounds 1–6
1H NMR (δ in
ppm)
19F NMR (δ in
11B NMR (δ in
Compound
a
b
c
ppm)
ppm)
8
1
2
3
4
5
6
7.05 7.47 6.95 −145.03
7.05 7.50 6.84 −146.35
7.01 7.42 6.84 −146.64
7.05 7.46 6.90 −147.35
7.07 7.46 6.95 −148.11
7.03 7.25 6.76 −146.86
0.35
0.48
0.68
0.49
0.28
0.25
0.12
7.07 7.14
—
−145.73
dyes.13c The two fluoride atoms from boron are equidistant and
present above and below the plane of the pyrrole moieties. The
various bond lengths and bond angles observed in these com-
pounds are similar with the reported BODIPYs.13c The most
interesting feature of these structures is the variation of dihedral
angle between the meso-anisyl group and boron dipyrrin ring
(C4C5C10C15). This angle is in the range of 50°–60° in com-
pounds 2–4 which is commonly noted for other BODIPYs.
However, the dihedral angle increases to 83° in compound 5 and
to 88° in compound 6. This is due to steric hindrance caused by
bromines present at 1 and 7-positions of BODIPY core which
restricts the rotation of meso-anisyl group resulting in a nearly
perpendicular orientation of the meso-anisyl group with the
BODIPY core. This kind of nearly perpendicular orientation of
the meso-aryl group was observed previously in other sterically
restricted BODIPY dyes such as meso-(o-tolyl) boron dipyrrin in
which the dihedral angle is 85°.13c Thus, X-ray studies of com-
pounds 2–6 indicate that only the bromine group(s) present at
the 1 and 7-positions causes steric strain and restricts the rotation
of the meso-aryl group.
Fig. 1 Comparison of 1H, 19F and 11B NMR spectra (δ in ppm) of
compounds 1–6 in a selected region.
meso-anisyl group experienced an ∼0.2 ppm upfield shift in
compound 5 compared to 8 (Fig. 1). This is attributed to the
near orthogonal orientation of the meso-anisyl group in com-
pound 5 because of steric hindrance between the bulky bromine
atom at the 7-position and the meso-anisyl ring which reduces
the π-conjugation in the BODIPY core. This is also clearly
evident in the chemical shift of meso-anisyl protons of type ‘a’
which experienced an upfield shift in compounds 5 and 6 com-
pared to other brominated BODIPYs. The compounds 1–6
showed a typical triplet in 11B and quartet in 19F NMR due to
B–F coupling. The 11B and 19F NMR also showed slight
changes in chemical shifts as the number of bromine groups
increased. For example, in 19F NMR, the signal shifts towards
upfield upto compounds 1–4 and then shifts towards downfield
in compounds 5 and 6 indicating that the π-delocalization is
altered in the boron-dipyrrin core upon introducing bromines at
the pyrrole carbons of BODIPY.
Photooptical and electrochemical studies
The absorption spectra of brominated BODIPYs 1–6 were
recorded in five different solvents of varying polarizability and
the data are presented in Table 4. The comparison of absorption
spectra of brominated BODIPYs 1–6 along with 8 recorded in
toluene using the same concentration is shown in Fig. 3. For all
brominated BODIPYs 1–6, the absorption spectra showed
typical BODIPY absorption features with a strong band in
500–550 nm region corresponding to the S0 → S1 transition with
a vibronic transition on the higher energy side as a shoulder and
an ill-defined, weak band corresponding to the S0 → S2 tran-
sition at ∼420 nm. However, each stepwise increase in the
number of bromine groups at the pyrrole carbons of BODIPY
core resulted in a bathochromic shift compared to 8 and the
magnitude of the red shift of the absorption band depends on the
number of bromines substituted at the BODIPY core.7,14 It is
noted that each bromine substitution at the pyrrole carbon con-
tributes an additional 10 nm red shift with respect to the absorp-
tion band observed for 8. However, this systematic red shift was
observed only up to the introduction of four bromines and com-
pounds 5 and 6 did not show any further red shift. This is also
evident in the non-linear nature of the plot of magnitude of red
shift versus the number of bromines substituted which indicates
Crystallographic studies
The brominated BODIPYs 2–6 were characterized by X-ray dif-
fraction analysis. The single crystals of compounds 2–6 were
obtained on slow evaporation of CH2Cl2–hexane mixture over a
period of one week. The compounds 2, 3 and 4 were crystallized
ˉ
in triclinic with a P1 space group whereas compounds 5 and 6
were crystallized in monoclinic with a P21/n space group. We
recently reported6 the crystal structure of compound 6 but it is
included here for comparison purposes. The crystal structures for
compounds 2–6 are shown in Fig. 2 and the selected bond
lengths, bond angles and dihedral angle are summarized in
Table 2. The crystallographic data for compounds 2–6 are listed
in Table 3. The common feature of all these structures is that the
two pyrrole rings and the central six-membered ring containing
boron atom are in one plane like any other reported BODIPY
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 5903–5911 | 5905

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Fig. 2 The X-ray crystal structures of compounds 2–6: (a) 2, (b) 3, (c) 4, (d) 5 and (e) 6.
Table 2 Comparison of some selected bond lengths [Å], bond angles [°] and dihedral angles [°] obtained from X-ray crystal structure
BODIPY
2
3
4
5
6
C4C5C10C15
C5C6C7
C2C3C4
C6C5C10
F1BF2
B–N1
B–N2
C5–C10
C5–C6
B–F1
60(1)
53(3)
48(5)
52(5)
83(1)
88(1)
130.4(1)
107.3(1)
120.2(1)
110.3(1)
1.547(2)
1.547(2)
1.481(1)
1.396(2)
1.381(1)
1.381(1)
130.1(2)
108.3(2)
119.8(2)
110.9(2)
1.569(4)
1.557(3)
1.479(4)
1.405(4)
1.370(4)
1.389(4)
130.0(3)
108.0(3)
119.8(3)
110.8(3)
1.559(6)
1.545(6)
1.477(4)
1.412(5)
1.383(5)
1.376(4)
130.1(3)
107.0(3)
120.4(3)
111.4(3)
1.559(4)
1.568(6)
1.470(4)
1.401(5)
1.364(3)
1.377(4)
130.1(7)
106.7(7)
115.1(7)
113.7(8)
1.58(1)
1.58(1)
1.47(1)
1.39(1)
1.36(1)
1.37(1)
132.3(5)
107.9(5)
116.1(5)
110.8(5)
1.572(8)
1.538(9)
1.491(8)
1.416(8)
1.377(7)
1.383(7)
B–F2
BODIPY = 4,4-difluoro-8-phenyl-4-bora-3a,4a-diaza-s-indacene.13c
that the magnitude of red shift with each bromine substitution is
non-additive (inset in Fig. 3). The bathochromic shift of absorp-
tion band and non-linear nature of plot of absorption band shift
versus the number of bromines in brominated BODIPYs has
been ascribed to an antagonistic inductive effect by bromines
present at the pyrrole carbons of the BODIPY core. The elec-
tronic absorption spectra of brominated BODIPYs 1–6 in differ-
ent solvents show a blue shift of the S0 → S1 absorption band
with increasing solvent polarizability as was observed for apolar
BODIPY derivatives without an amine donor.13 Furthermore, the
absorption bandwidths and the full-width at half maximum
(FWHM) is independent of solvent polarizability for compounds
1–6.
fluorescence techniques and the data are tabulated in Table 4. A
comparison of steady state fluorescence spectra of compounds
1–6 along with 8 recorded using the same concentration is
shown in Fig. 4. The steady state fluorescence studies on com-
pounds 1–6 reveal the following: (1) The compounds 1–6
showed one single broad emission band which shifts gradually
to higher wavelength with the increase of number of bromine
groups (Fig. 4). (2) The maximum red shifts were observed for
compounds 5 and 6. (3) The quantum yields were decent for
compounds 1–4 but significantly low for compounds 5 and 6.
The presence of bromines on fluorophores generally decreases
the quantum yield due to the heavy halogen effect. However,
it is pronounced only in compounds 5 and 6. (4) The
emission maxima are slightly blue shifted and the fluorescence
quantum yield decreases significantly with increasing solvent
The fluorescence properties of compounds 1–6 were studied
in five different solvents by steady state and time-resolved
5906 | Dalton Trans., 2012, 41, 5903–5911
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Table 3 Crystal data and structure refinement parameters for compounds 2, 3, 4, 5 and 6
Parameters
2
3
4
5
6
Mol. formula
For. weight
Temp/K
Cryst sym
Space group
λ/Å
a/Å
b/Å
c/Å
α (°)
C16H11BBr2F2N2O
455.90
150(2)
C16H10BBr3F2N2O
534.80
293(2)
C16H9BBr4F2N2O
613.70
150(2)
C16H8BBr5F2N2O
692.60
150(2) K
Monoclinic
P21/n
C16H7BBr6F2N2O
771.51
150(2) K
Monoclinic
P21/n
Triclinic
Triclinic
Triclinic
ˉ
ˉ
ˉ
P1
P1
P1
0.71073
0.71073
0.71073
0.71073
0.71073
8.0532(4)
9.4760(4)
11.2338(5)
78.222(4)
83.573(4)
67.746(4)
776.15(6)
2
8.2231(10)
10.6391(10)
10.7273(6)
105.580(7)
92.262(7)
106.758(9)
858.63(14)
2
7.8922(4)
11.1893(6)
11.3508(5)
73.442(4)
74.044(4)
84.062(4)
923.44(8)
2
8.9535(17)
19.7020(4)
11.1720(3)
90
90.84(2)
90
1970.60(7)
4
10.224
11.7228(3)
8.8692(2)
19.6265(5)
90
92.39(2)
90
2038.83(9)
4
11.844
β (°)
γ (°)
Volume/Å3
Z
μ/mm−1
5.250
1.951
7.074
2.069
8.744
2.207
D
calcd/Mg m−3
2.335
2.513
F(000)
θ range (°)
e data/unique
Rint
444
512
580
1296
1432
3.14–25.00
5582/2727
0.0154
3.47–25.00
6019/3015
0.0279
3.69–32.80
10 011/6014
0.0255
3.56–30.51
21 069/5993
0.1355
3.48–25.00
13 921/3595
0.0509
Data/restraints/parameters
2727/0/218
1.075
3015/0/227
0.953
6014/0/236
1.058
5993/0/245
0.863
3595/0/254
1.082
GOF on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
Largest diff. peak/hole, e Å−3
0.0216, 0.0584
0.0269, 0.0593
0.593/−0.334
0.0298, 0.0751
0.0436, 0.0777
0.844/−0.631
0.0391, 0.0795
0.0593, 0.0893
1.497/−1.225
0.0686, 0.1528
0.1842, 0.1679
2.201/−1.301
0.0379, 0.0954
0.0483, 0.0968
1.430/−1.105
Table 4 Photophysical data of BODIPYs 1–6 in different solvents
Compd
Solvent
λabs [nm]
λemi [nm]
FWHM (abs.) [cm−1
]
Δvst [cm−1
]
log εmax
Φf
τ [ns]
kf (109 s−1
)
knr (109 s−1
)
8
Toluene
CHCl3
THF
EtOAc
MeCN
Toluene
CHCl3
THF
EtOAc
MeCN
Toluene
CHCl3
THF
EtOAc
MeCN
Toluene
CHCl3
THF
EtOAc
MeCN
Toluene
CHCl3
THF
EtOAc
MeCN
Toluene
CHCl3
THF
EtOAc
MeCN
Toluene
CHCl3
THF
EtOAc
MeCN
501
499
497
495
494
510
509
506
504
502
522
521
517
515
514
537
536
530
529
526
554
553
546
545
541
554
552
547
545
543
554
551
547
545
543
517
515
515
511
511
526
523
521
519
518
535
532
530
528
526
551
550
545
544
544
569
568
563
562
559
569
565
562
560
558
564
565
560
559
558
1079
1173
1180
1184
1317
1145
1100
1199
1390
1209
975
1070
881
982
982
960
617
620
701
632
673
597
526
573
571
610
467
394
472
477
444
472
477
518
524
631
481
457
555
559
594
579
515
694
624
496
508
574
681
709
686
4.76
4.76
4.74
4.75
4.71
4.77
4.75
4.74
4.76
4.70
4.86
4.91
4.81
4.87
4.83
4.76
4.81
4.53
4.82
4.70
4.77
4.88
4.67
4.79
4.69
4.93
4.99
4.93
5.00
4.94
5.00
5.08
4.99
5.05
4.99
0.11
0.72
0.72
0.41
0.32
0.24
1.58
1.21
0.69
0.64
0.36
2.23
1.30
0.74
0.73
0.30
2.51
1.83
1.03
1.02
0.43
2.38
2.14
1.24
1.19
0.53
nd
0.16
0.14
0.13
0.18
0.13
0.16
0.16
0.165
0.17
0.17
0.20
0.23
0.21
0.24
0.23
0.11
0.19
0.12
0.17
0.21
0.13
0.15
0.09
0.13
0.17
nd
1.23
1.25
2.3
1.93
4.04
0.47
0.66
1.28
1.38
2.6
0.25
0.54
1.14
1.12
3.1
0.28
0.35
0.84
0.81
2.12
0.37
0.54
0.71
0.70
1.71
nd
0.097
0.052
0.035
0.0013
0.255
0.194
0.120
0.064
0.043
0.29
0.25
0.11
0.14
0.048
0.26
0.33
0.12
0.14
0.08
0.30
0.22
0.097
0.14
0.009
0.027
0.033
0.008
0.007
0.001
0.016
0.009
0.008
0.004
0.003
1
2
3
4
5
6
968
1117
959
955
1043
953
1283
960
1172
1232
1107
1361
1173
1216
1035
852
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
995
966
1341
nd
nd
nd
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 5903–5911 | 5907

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Fig. 5 Comparison of first reduction waves of compounds 1–6 along
with 8 in CH2Cl2, measured using n-Bu4N+P(ClO4)6 (0.1 M) as sup-
−
Fig. 3 Comparison of absorption spectra of compounds 1–6 recorded
in toluene. The concentration used was 1 × 10–6 M. The inset shows the
plot of magnitude of red shift in absorption band versus the number of
bromines.
porting electrolyte at a scan rate of 50 mV s−1. The inset shows the plot
between the no. of bromine atoms versus reduction potential.
Table 5 Electrochemical redox data of compounds 1–6 along with
compound 8 in CH2Cl2
Reduction potential (V)
Compound
I
II
8
1
2
3
4
5
6
−0.81
−0.71
−0.66
−0.56
−0.44
−0.39
−0.31
−1.91
−1.76
−1.70
−1.60
−1.43
−1.37
−1.29
increasing number of Br groups. Thus, the difference between
the first reduction potential ΔE1/2 of unbrominated BODIPY 8
and hexabrominated BODIPY 6 is 500 mV. The relationship
between E1/2 for the first reduction and the number of Br groups
on the BODIPY core is shown as an inset in Fig. 5. The positive
shift in E1/2 with increase in the number of Br groups is additive
and the slope of the straight-line plot is 80 mV/Br group. This
kind of linear relationship between reduction potential shift
and the number of bromine groups were observed earlier for
β-brominated porphyrins, metalloporphyrins and halogenated
metallacarboranes.7
Time-resolved fluorescence studies were carried out for com-
pounds 1–6 in different solvents and the representative fluor-
escence decay profile for compound 4 in CHCl3 collected at the
corresponding emission wavelength is shown in Fig. 6. We
failed to measure the singlet state lifetime τ for compounds 5
and 6 which are very weakly fluorescent. The fluorescence decay
profiles of compounds 1–4 were fitted to a single exponential
and the lifetimes are parallel to the fluorescence quantum yields
of the compounds. The fluorescence lifetimes of compounds 1–4
increase with increasing solvent polarizability which is in
line with the fluorescence quantum yields. Using the experimen-
tal Φf and τ values, we calculated the radiative kf and nonradia-
tive knr rate constants which are consistent with the quantum
yield and singlet state life time data. Thus, the steady state and
Fig. 4 Comparison of the fluorescence spectra of brominated com-
pounds 1–6 along with 8 recorded in toluene using the same concen-
tration (1 × 10–6 M).
polarizability. (5) The Stokes shift was small and independent of
solvent polarizability indicating that the excited state is not very
distorted from the ground state.
The redox properties of brominated BODIPYs 1–6 were
probed through cyclic voltammetric and differential pulse
voltammetric studies using tetrabutylammonium perchlorate as
supporting electrolyte (0.1 M) in dichloromethane as solvent. A
comparison of first reduction waves of compounds 1–6 along
with 8 is shown in Fig. 5 and the data are presented in Table 5.
In general, the compounds 1–6 showed one reversible reduction
(ΔE = 60–80 mV) and one irreversible reduction. No oxidation
was noted between 0 to 2.0 V due to the electron deficient nature
of BODIPY dyes 1–6. It is clear from Fig. 5 and the data in
Table 5 that addition of each bromine resulted in a successive
anodic shift of reduction potential compared to 8 indicating that
the boron dipyrrin dye becomes easier to reduce with the
5908 | Dalton Trans., 2012, 41, 5903–5911
This journal is © The Royal Society of Chemistry 2012

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calcium hydride prior to use. BF3·OEt2 and 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) obtained from Spectrochem
(India) were used as obtained. All other chemicals used for the
synthesis were reagent grade unless otherwise specified. Column
chromatography was performed on silica (60–120 mesh and
100–120 mesh). 1H NMR spectra (δ in ppm) were recorded
using Varian VXR 300, 400 MHz and Bruker 400 MHz spec-
trometer. 13C NMR spectra were recorded on the Bruker operat-
ing at 100.6 MHz. 19F NMR spectra were recorded on the
Bruker operating at 376.4 MHz. 11B NMR spectra were recorded
on the Bruker operating at 128.4 MHz. TMS was used as an
1
internal reference for recording H (of residual proton; δ 7.26)
and 13C (δ 77.0 signal) in CDCl3. Absorption and steady-state
fluorescence spectra were obtained with Perkin-Elmer Lambda-
35 and PC1 Photon Counting Spectrofluorometer manufactured
by ISS, USA instruments respectively. Fluorescence spectra were
recorded at 25 °C in a 1 cm quartz fluorescence cuvette. The flu-
orescence quantum yields (Φf) were estimated from the emission
and absorption spectra by a comparative method at the excitation
wavelength of 488 nm in ethanol using Rhodamine 6G (Φf =
0.88)15 as standard. The time-resolved fluorescence decay
measurements were carried out at the magic angle using a
picosecond-diode-laser-based, time-correlated, single-photon-
counting (TCSPC) fluorescence spectrometer from IBH, UK. All
the decays were fitted to a single exponential.
Fig. 6
A representative fluorescence decay profile and weighted
residual distribution fit of 4 in CHCl3. The excitation wavelength used
was 440 nm and emission was detected at 570 nm.
time-resolved fluorescence studies indicates that compounds 1–4
are decently fluorescent and follow the same trend with the
change of polarizability of solvents as noted earlier for several
BODIPY dyes.13
Cyclic voltammetric (CV) and differential pulse voltammetric
(DPV) studies were carried out with an 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 tet-
rabutylammonium perchlorate as supporting electrolyte. Half
wave potentials were measured using DPV and also calculated
manually by taking the average of the cathodic and anodic peak
potentials. All potentials were calibrated versus saturated
calomel electrode by the addition of ferrocene as an internal
standard, taking E1/2 (Fc/Fc+) = 0.42 V, vs. SCE.16 The ES-MS
mass spectra were recorded with a Q-Tof micro mass spec-
trometer. A high-resolution mass spectrum was obtained from a
Q-TOF instrument by the electron spray ionization (ESI)
technique.
Conclusions
In summary, we systematically introduced one to six bromines
on the boron dipyrrin core by treating dipyrromethane with
different equivalents of N-bromosuccinimide in THF followed
by oxidation with DDQ and complexation with BF2 unit. The Rf
values of mono to hexabrominated BODIPYs are quite distinct
and so can be separated by column chromatography and the
compounds isolated in decent yields. 1H NMR spectroscopy
was used in identifying the location of the bromine group on
the BODIPY core. Absorption spectroscopy revealed that the
absorption band shift towards higher wavelength on addition of
each bromine group upto four bromines after which no further
shift was observed with the addition of five and six bromines.
The fluorescence band also experienced red shift on addition of
bromines. However, the quantum yields reduced significantly for
penta and hexabrominated BODIPYs. These low quantum yields
are due to the heavy halogen effect which because of spin–orbit
coupling enhances the non-radiative decay pathways. The first
reduction potential exhibits anodic shifts with the addition of
each bromine and follows a linear trend with increase in
the number of bromine groups. Thus, the BODIPYs with 1–6
bromines can be easily prepared which can be used as synthons
to prepare various BODIPY derivatives and such synthetic
efforts are currently underway in our laboratory.
X-ray diffraction studies
A single crystal X-ray structural study was performed on a CCD
Oxford Diffraction XCALIBUR-S diffractometer equipped with
an Oxford Instrument with a low-temperature attachment. Data
were collected at 293(2) K (in the case of compound 3) and 150
(2) K (in the case of 2, 4 and 5) using graphite-monochromated
Mo-Kα radiation (λα = 0.71073 Å). The strategy for the data col-
lection was evaluated by using the CrysAlisPro CCD software.
The data were collected by the standard ‘phi-omega scan’ tech-
niques and were scaled and reduced using CrysAlisPro RED
software. Structure solutions for the compounds 2–6 were
obtained using direct methods (SHELXS-97)17 and refined using
full-matrix least-squares methods on F2 using SHELXL-97.18
The positions of all the atoms were obtained by direct methods.
All non-hydrogen atoms were refined anisotropically. The
Experimental section
General
THF and toluene were dried over sodium benzophenone ketyl
and chloroform, ethyl-acetate, methanol, acetonitrile dried over
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 5903–5911 | 5909

View Online
hydrogen atoms were placed in geometrically constrained
positions and refined with isotropic temperature factors, gener-
ally 1.2 Ueq of their parent atoms.
= 28.2Hz); HRMS calcd for C16H11BBr2F2N2O 434.9315
[M − 19]+ found 434.9309 [M − 19]+.
The single crystals of compounds 2–6 were obtained on slow
evaporation of hexane–CH2Cl2 over a period of one week.
2,3,5-Tribromo-4,4-difluoro-8-(4-methoxyphenyl)-4-bora-3a,4a-
diaza-s-indacene (3)
1
(202 mg, 19% yield). H NMR (400 MHz, CDCl3, δ in ppm)
General method for the synthesis of compounds 1–4
3
3.91 (s, 3H; OCH3), 6.59 (d, J(H,H) = 4.28 Hz, 1H; Py), 6.90
3
3
The compounds 1–4 were prepared by a sequence of steps in a
one pot reaction. meso-(p-Methoxyphenyl)-dipyrromethane 7
(500 mg, 1.98 mmol) was treated with appropriate equivalents of
N-bromosuccinimide in dry THF (50 mL) at −78 °C under nitro-
gen for 1 h. The reaction mixture was warmed to room tempera-
ture and the solvent was evaporated by a rotary evaporator. The
crude compound was subjected to flash column chromatography
using CH2Cl2. The resultant compound was dissolved in CH2Cl2
and DDQ (483 mg, 2.12 mmol) was added to oxidize the com-
pound. The reaction mixture was stirred for 1 h at room tempera-
ture. Triethylamine (75.4 mmol) followed by BF3·Et2O
(89.97 mmol) were added and stirring continued at room temp-
erature for an additional 1 h. The reaction mixture was washed
successively with 0.1 M NaOH solution and water. The organic
layers were combined, dried over Na2SO4, filtered, and evapor-
ated. The TLC analysis of crude compound showed three spots;
the major spot corresponding to the required bromo derivative of
BODIPY and the two minor spots corresponding to the other
substituted bromo BODIPY and unsubstituted BODIPY. The
crude compound was subjected to silica gel column chromato-
graphy and the required bromo derivative of BODIPY was col-
lected as a second band using petroleum ether–dichloromethane
(90 : 10). The solvent was removed on a rotary evaporator under
vacuum and afforded pure bromo BODIPY in 20–40% yield.
(s, 2H; Py), 7.05 (d, J(H,H) = 4.28 Hz, 2H; Ar), 7.46 (d, J(H,
H) = 4.28 Hz, 2H; Ar); 13C NMR (400 MHz, CDCl3, δ in ppm)
55.7, 110.6, 114.5, 123.5, 124.5, 130.4, 131.8, 132.4, 132.9,
134.3, 134.5, 135.8, 143.3, 162.5; 19F NMR (376.5 MHz,
CDCl3, δ in ppm) −147.3; 11B NMR (96.3 MHz, CDCl3, δ
in ppm) 0.49 (t, 3J(B,F)
= 28.2 Hz); HRMS calcd
for (C16H10BBr3F2N2O) 512.8435 [M − F]+ found 512.8420
[M − F]+.
2,3,5,6-Tetrabromo-4,4-difluoro-8-(4-methoxyphenyl)-4-bora-
3a,4a-diaza-s-indacene (4)
1
(244 mg, 20% yield). H NMR (400 MHz, CDCl3, δ in ppm)
3.91 (s, 3H; OCH3), 6.95 (s, 2H; py), 7.07 (d, 3J(H,H) = 7.9 Hz,
2H; Ar), 7.46 (d, 3J(H,H) = 7.9 Hz, 2H; Ar); 13C NMR
(400 MHz, CDCl3, δ in ppm) 55.8, 111.8, 114.7, 124.3, 131.5,
132.5, 134.3, 134.8, 143.1, 162.8; 19F NMR (376.5 MHz,
CDCl3, δ in ppm) −148.1; 11B NMR (376.5 MHz, CDCl3, δ
in ppm) 0.28 (t, 3J(B,F)
= 28.2 Hz); HRMS calcd
for (C16H9BBr4F2N2O) 590.7525 [M − F]+ found 590.7523
[M − F]+.
Synthesis of 1,2,3,5,6-pentabromo-4,4-difluoro-8-(4-
methoxyphenyl)-4-bora-3a,4a-diaza-s-indacene (5)
To a solution of 8-(4-methoxyphenyl)dipyrromethane 7 (0.5 g,
1.98 mmol) in dry THF, five equivalents of N-bromosuccinimide
(1.76 g, 9.9 mmol) were added under N2 atmosphere and
the reaction mixture was allowed to stir at room temperature for
4 h. The TLC analysis indicated the disappearance of the spot
corresponding to 7 and the appearance of two new spots corre-
sponding to compound 5 along with a faint spot which is corre-
sponding to compound 6. The solvent was removed on a rotary
evaporator under vacuum and the crude compound was passed
through a flash silica gel column chromatograph using dichloro-
methane. The resultant compound was dissolved in freshly dis-
tilled dichloromethane and oxidized with DDQ (2.38 mmol) for
30 min at room temperature. Triethylamine (75.4 mmol) fol-
lowed by BF3·OEt2 (89.97 mmol) were added to the reaction
mixture and stirring was continued at room temperature for an
additional 30 min. The solvent was removed and the crude com-
pounds were purified using silica gel column chromatography
using petroleum ether–ethyl acetate (98 : 2) which afforded the
desired pentabromo-BODIPYs 5 as green colour solid.
3-Bromo-4,4-difluoro-8-(4-methoxyphenyl)-4-bora-3a,4a-diaza-s-
indacene (1)
1
(250 mg, 40% yield). H NMR (400 MHz, CDCl3, δ in ppm)
3.91 (s, 3H; OCH3), 6.53 (m, 1H; py), 6.57 (m, 1H; py), 6.84
3
3
(d, J = 4.28 Hz, 1H; py), 6.96 (d, J = 3.70 Hz, 1H; py), 7.05
3
3
(d, J = 7.95 Hz, 2H; Ar), 7.51 (d, J = 7.95 Hz, 2H; Ar), 7.94
(s, 1H, py); 13C NMR (400 MHz, CDCl3, δ in ppm) 55.7, 114.2,
114.3, 118.4, 119.0, 122, 126.4, 132.4, 132.5, 132.6, 134.8,
143.5, 144.2, 145.6, 162.3; 19F NMR (282.2 MHz, CDCl3, δ in
ppm) −146.34 (q, JB–F = 56.5 Hz); 11B NMR (96.3 MHz,
CDCl3, δ in ppm) 0.48 (t, JB,F = 28.2 Hz); HRMS calcd
for (C16H13BBrF2N2) 357.0205 [M − F]+ found 357.0210
[M − F]+.
3,5-Dibromo-4,4-difluoro-8-(4-methoxyphenyl)-4-bora-3a,4a-
diaza-s-indacene (2)
1
(208 mg, 25% yield); H NMR (400 MHz, CDCl3, δ in ppm):
3.9 (s, 3H; –OCH3), 6.52 (d, 3J = 4.28 Hz, 2H; py), 6.82 (d, 3J =
(0.25 g, 16% yield). H NMR (400 MHz, CDCl3, δ in ppm):
1
3
3
3
4.28 Hz, 2H; py), 7.01 (d, J = 7.9 Hz, 2H; Ar), 7.42 (d, J =
7.9 Hz, 2H; Ar); 13C NMR (100 MHz, CDCl3, δ in ppm) 55.6,
114.2, 122.5, 124.8, 131.6, 131.8, 132.3, 135.4, 148.4, 162.2;
3.91 (s, 3H; –OCH3), 6.76 (s, 1H; py), 7.03 (d, 2H, J(H,H) =
9.0 Hz; Ar), 7.25 (d, 2H, 3J(H,H) = 8.7 Hz; Ar); 13C NMR
(400 MHz, CDCl3, δ in ppm): 55.6, 114.3, 114.3, 122.4, 122.5,
123.2, 131.4, 132.3, 136.4, 136.6, 143.2, 144.8, 145.5, 162.0;
19F NMR (376.5 MHz, CDCl3, δ in ppm):−145.76 (q, 3J(B,F) =
19F NMR (282.2 MHz, CDCl3, δ in ppm)−146.64 (q, JB,F
=
57.8 Hz); 11B NMR (96.3 MHz, CDCl3, δ in ppm) 0.68 (t, JB,F
5910 | Dalton Trans., 2012, 41, 5903–5911
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57.8 Hz); 11B NMR (96.3 MHz, CDCl3, δ in ppm) 0.88 (t, 3J(B,
F) = 28.2 Hz); HRMS calcd for C16H7BBr5F2N2O 688.6631
(M − F)+ found 668.6657 (M − F)+.
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Acknowledgements
We thank the Board of Research in Nuclear Sciences, Depart-
ment of Atomic Energy, Government of India for funding
the project and VL thank CSIR for fellowship. We thank the
DST-funded National Single Crystal X-ray Diffraction facility
for diffraction data and Dr S. M. Mobin and Professor Ray
Butcher for their help in crystal structure diffraction.
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