Synthesis of covalently linked boron-dipyrromethene-chromophore conjugates using 3-bromo boron-dipyrromethene as a key precursor

Article abstract of DOI:10.1016/j.tet.2011.05.089

3-Bromo boron dipyrromethene (3-bromo BODIPY) has been used as key synthon to prepare one ethynyl bridged and six ethynylphenyl bridged BODIPY-chromophore conjugates using mild Pd(0) coupling conditions. The chromophores possessing very distinct features,

Full text of DOI:10.1016/j.tet.2011.05.089

Tetrahedron 67 (2011) 5816e5824
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of covalently linked boronedipyrrometheneechromophore
conjugates using 3-bromo boronedipyrromethene as a key precursor
*
Tamanna K. Khan, M. Ravikanth
Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 April 2011
Received in revised form 18 May 2011
Accepted 20 May 2011
Available online 31 May 2011
3-Bromo boron dipyrromethene (3-bromo BODIPY) has been used as key synthon to prepare one ethynyl
bridged and six ethynylphenyl bridged BODIPYechromophore conjugates using mild Pd(0) coupling
conditions. The chromophores possessing very distinct features, such as anthracene, BODIPY, terpyridine,
porphyrin, Zn(II)porphyrin, 21,23-dithiaporphyrin and thiasapphyrin were connected at 3-position of
boronboronedipyrromethene dye by coupling of 3-bromo BODIPY with ethynyl or ethynylphenyl
chromophore in toluene/triethylamine in the presence of catalytic amount of AsPh3/Pd2(dba)3 at 40 ꢀC
followed by column chromatographic purification. The spectral studies indicated that the interaction is
stronger in ethynyl bridged BODIPYechromophore conjugate compared to ethynylphenyl bridged
BODIPYechromophore conjugates. The steady-state fluorescence indicated that in ethynyl bridged
BODIPYeanthracene conjugate, the BODIPY unit act as energy acceptor and showed a possibility of
energy transfer from donor anthracene unit to acceptor BODIPY unit on selective excitation of anthracene
unit. However, in ethynylphenyl bridged BODIPYeporphyrin conjugates, the BODIPY unit act as energy
donor and exhibited a possibility of singlet-singlet energy transfer from BODIPY unit to porphyrin unit.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Boronedipyrromethene
Chromophores
Anthracene
Porphyrin
Sapphyrin
Energy transfer
1. Introduction
porphyrin,6 pyrene,7 anthracene,8 BODIPY,9 Zn (II)/Ru(II) bis-ter-
pyridine,10 etc. complexes were connected covalently to BODIPY
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene dyes popularly
known as BODIPY dyes are known for their sharp bands in the
absorption spectra, large molar absorption coefficients, high fluo-
rescence quantum yields, reasonably long excited singlet state
lifetimes, excellent chemical and photochemical stability in both
solution and solid states and versatile charge-transfer properties.1
BODIPY dyes have good solubility in most common organic sol-
vents and are resistant towards aggregation in solution. BODIPY
dyes are amenable to structural modifications so that their spec-
troscopic properties can be fine tuned by introducing suitable
substituents at the right positions on BODIPY core.1 BODIPY dyes
have numerous applications that include light-harvesting arrays,2
biological probes,3 supramolecular fluorescent gels,4 fluorescent
switches and sensors,5 among others. In recent past, dual chro-
mophore systems containing BODIPY along with another chromo-
phore are receiving attention from several viewpoints.6ꢁ10 These
kinds of systems can be used to study photoinduced inter-
component electron and/or energy transfer processes, possibly
leading to valuable functions, such as charge separation and/or
energy migration. There are several BODIPYechromophore
conjugates reported in literature in which chromophores, such as
and explored their properties. However, in most of these systems,
the chromophores were connected via the meso-aryl group. Since
the meso-aryl group and the chromophore are perpendicular to
each other, electronic conjugation between two moieties is weak.
Recently, there are some reports on BODIPYechromophore conju-
gates in which the chromophores are connected directly at
b
a- or
-pyrrole positions of BODIPY ring.11 In these systems, BODIPY and
chromophore units interact very strongly and influence each others
properties. For example, Akkaya and co-workers synthesized
BODIPY tetrad in which two BODIPY units are covalently linked at
3,5-positions of another BODIPY via styryl linker, which in turn
linked to the fourth BODIPY through meso-phenyl.11b Very recently
Ziessel and co-workers reported panchromatic BODIPY dyes in
which five BODIPY units are covalently linked in an iterative fash-
ion.9c In both these novel cases, the singlet state energy levels of
BODIPY units are arranged in cascade manner for efficient intra-
molecular energy flow from BODIPY units that are at high energy to
the BODIPY units that are at lower in energy. Akkaya and
co-workers also reported BODIPYebipyridine/terpyridine conju-
gates in which bipyridine/terpyridine units are covalently linked at
b
-pyrrole positions and used for synthesis of metal complexes.11c
Although, some interesting reports on strongly coupled BODIPYe-
* Corresponding author. E-mail address: ravikanth@chem.iitb.ac.in (M. Ravikanth).
chromophore conjugates have been appeared, there is a need for
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.05.089

T.K. Khan, M. Ravikanth / Tetrahedron 67 (2011) 5816e5824
5817
more such systems for understanding electronic communication
between the BODIPYand the chromophore for their potential use in
photonic devices. Furthermore, bichromophoric systems contain-
ing two different chromophores possessing different absorption
properties and ground state/excited state redox potentials could
function in parallel in solar energy conversion schemes. We herein
report the synthesis of series of covalently linked BODIPYechro-
mophore conjugates 1e7 in which the chromophores, such as an-
thracene, terpyridine, BODIPY, porphyrin, thiaporphyrin and
ethyne bridge under mild Pd(0) coupling conditions. The absorp-
tion, electrochemical and fluorescence properties of some of these
compounds are also described.
2. Results and discussion
To synthesize the BODIPYechromophore conjugates 1e7, we
need an access to a
-bromo boronedipyrromethene12 8, which was
prepared over sequence of steps. The meso-anisyl dipyrromethane
9 was prepared by treating 1 equiv of p-anisaldehyde with
expanded porphyrin are connected at
a-position of BODIPY unit by
OMe
OMe
OMe
THF, NBS
-78oC
NH
HN
NH
HN
9
Br
N
N
B
7
F
F
CH3
1) CH2Cl2,
DDQ
OMe
N
S
S
2) TEA,
BF3 OEt2
.
S
N
OMe
CH3
N
N
B
vi
v
F
F
OMe
i
H3C
1
N
N
B
F
F
6
CH3
OMe
ii
N
N
B
N
Br
iii
F
F
S
S
8
N
N
N
B
F
F
2
H3C
iv
CH3
OMe
OMe
N
B
OMe
N
F
F
N
N
B
N
N
B
F
F
3
F
F
5
N
N
B
CH3
F
F
N
N
CH3
N
4
N
N
Zn
N
N
N
NH
HN
N
CHCl3/MeOH
H3C
Zn(OAc)2
CH3
H3C
CH3
H
H
H
i;
iii;
ii;
N
N
B F
F
N
N
N
11
12
13
H
H
H
CH3
CH3
CH3
N
N
S
N
N
HN
N
S
S
S
iv;
v;
NH
vi;
S
N
CH3
H3C
H3C
CH3
CH3
15
14
16
H3C
Scheme 1. Synthetic scheme for the preparation of BODIPYechromophore conjugates 1e7. Reaction conditions used were Pd2(dba)3/AsPh3, toluene/TEA, 35e60 ꢀC.

5818
T.K. Khan, M. Ravikanth / Tetrahedron 67 (2011) 5816e5824
25 equiv of pyrrole under standard Lindsey’s dipyrromethane
forming conditions.13 The compound 8 was then prepared by
treating 9 with 1 equiv of N-bromosuccinimide at ꢁ78 ꢀC in THF
for 1 h and the resulted compound was oxidized with DDQ in
CH2Cl2 and complexed with BF3$OEt2. The compound 8 was con-
firmed by HR-MS mass spectrum and characterized by 1H, 19F and
11B NMR spectroscopic techniques. The absorption spectrum of
example, the BODIPY-conjugates 1, 2 and 7 were prepared by cou-
pling 8 with corresponding ethynyl chromophore 11, 12 and 16,
respectively, in the presence of catalytic amount of Pd2(dba)3/AsPh3
in toluene/triethylamine at 40 ꢀC for 2 h. Similarly, the BODIPY-
conjugates 3, 4 and 6 were prepared by coupling of 13, 14 and 15,
respectively, under similar reaction conditions using longer reaction
time (w6 h) and at slightly higher temperature (50e60 ꢀC).
The compound 5 was obtained by treating compound 4 with
Zn(OAc)2 in CHCl3/CH3OH at reflux temperature for 2 h. All com-
pounds were purified by column chromatography using either sil-
ica or alumina and confirmed by mass spectral analysis. The
compounds 1e7 are freely soluble in chlorinated solvents and
characterized by 1H, 13C, 19F and 11B NMR spectroscopic techniques.
In couple of cases, we recorded 1He1H COSY to identify the proton
signals of BODIPY core, such as Hd, the adjacent pyrrole proton of
BODIPY unit where the chromophore is connected.
compound
8 showed typical BODIPY absorption band and
exhibited slight red shift (8e10 nm) compared to meso-tolyl
boronedipyrromethene 10.14
The compound 8 was used to prepare a series of BODIPYechro-
mophore conjugates 1e7 as shown in Scheme 1. The other required
precursors, such as 9-ethynylanthracene 11,15 4,4-difluoro-8-(4-
ethynylphenyl)-4-bora-3a,4a-diaza-s-indacene 12,8a 40-(4-ethynyl
phenyl)-2,20:60,200-terpyridine 13,16 5-(4-ethynylphenyl)-10,15,20-
tri(p-tolyl)porphyrin 14,17 5-(4-ethynylphenyl)-10,15,20-tri(p-tolyl)
dithiaporphyrin 1518 and 5-(4-ethynylphenyl)-10,15,20-tri(p-tolyl)-
25,27,29-trithiasapphyrin 1619 were prepared by following the lit-
erature procedures. The compounds 1e7 except compound 5 were
prepared by coupling of 8 with appropriate ethynyl functionalized
chromophore in the presence of Pd2(dba)3/AsPh3 in toluene/trie-
thylamine20 (5:1) at 45e60 ꢀC for 2e6 h depending on the chro-
mophore followed by column chromatographic purification. For
The resonances of BODIPYechromophore conjugates were
assigned on the basis of the spectra observed for the two mono-
mers; the BODIPY monomer and chromophore monomer, taken
independently.
A comparison of 1H, 19F and 11B NMR spectra of compounds 1
and 2 are shown in Fig. 1aec, respectively; 1He1H COSY spectrum
for compound 2 in Fig. 2 and the relevant NMR data of compounds
Fig. 1. Comparison of (a) 1H, (b) 19F and (c) 11B NMR spectra of compounds 1 and 2 in selected region recorded in CDCl3.

T.K. Khan, M. Ravikanth / Tetrahedron 67 (2011) 5816e5824
5819
1e7 along with reference BODIPY 1014 is presented in Table 1. In
compound 1, the anthracene moiety is connected to BODIPY unit at
NMR, the Hd proton of BODIPY unit experiences 0.42 ppm down-
field shift, whereas compounds 2e7 experiences 0.20e0.32 ppm
shift compared to 10. Similarly, the downfield shifts were observed
in 19F and 11B NMR for all compounds 1e7 compared to reference
BODIPY 10 and maximum downfield shift was observed for com-
pound 1 in 11B NMR indicating the strong interaction between the
BODIPY and anthracenyl units. Some interesting observations were
a
-position by ethynyl bridge, whereas in compounds 2e7, the
chromophores are connected to BODIPY at -position by ethynyl-
a
phenyl bridge. Thus, in compound 1, the BODIPY and anthracenyl
moiety interacts more strongly compared to the other conjugates
2e7, which is reflected in their NMR studies. For example, in 1H
Fig. 2. 1He1H COSY NMR spectrum of compound 2 in selected region recorded in CDCl3.

5820
T.K. Khan, M. Ravikanth / Tetrahedron 67 (2011) 5816e5824
Table 1
The absorption spectrum of BODIPYeBODIPY conjugate 2
showed two strong bands at 509 and 562 nm with almost equal
intensity (Fig. 3a). The red shifted absorption band at 562 nm cor-
responds to BODIPY unit B to which ethynylphenyl group is con-
Comparison of 1H, 11B and 19F NMR chemical shift values (in ppm) of compounds
1e7 along with 10 recorded in CDCl3
Compound
1H NMR
Hd
19F NMR
11B NMR
nected at a-position and the absorption band at 506 nm is due to
the BODIPY unit A.14 This indicates that the two BODIPY units A and
B in conjugate 2 exhibit quite different absorption properties. In
BODIPYeterpyridine 3, a strong band at 563 nm corresponds to
BODIPY unit along with small bands of terpyridinyl unit in
300e400 nm region are present (Fig. S30).
10
1
2
6.55
6.97
6.79
ꢁ145.27
ꢁ145.34
ꢁ145.12
ꢁ146.08
ꢁ146.17
ꢁ145.85
ꢁ145.92
ꢁ146.07
ꢁ146.04
0.50
0.99
0.75
3
4
5
6
7
6.75
6.84
6.86
6.85
6.87
0.77
0.91
0.69
0.89
0.91
The BODIPY-N4 porphyrinyl conjugate 4 showed absorption
bands at 421, 564, 650 nm along with a shoulder band at 519 nm
(Fig. 3b). In general, porphyrin shows four well defined Q-bands in
700-450 nm region and one strong Soret band at w420 nm.17
However, the conjugate 4 did not exhibit typical Q-bands of por-
phyrin because of BODIPY unit whose strong absorption band at
564 nm was merged with the Q-bands of porphyrin unit. Thus, the
bands at 421 and 650 nm were exclusively due to porphyrin unit
and the band at 564 nm was mainly due to BODIPY unit.
In conjugate 5, the absorption bands at 425 and 597 nm are
mainly due to ZnTTP and a strong band at 563 nm is due to both
ZnTTP and BODIPY units (Fig. S31). The BODIPYeN2S2 porphyrin
conjugate 6 showed characteristic 21,23-dithiaporphyrin18 ab-
sorption bands at 438, 515, 633 and 699 nm (Fig. S32). The band at
567 nm is highly intense compared to other Q-bands due to the
absorption of BODIPY unit in the same region. In BODI-
PYesapphyrin conjugate, the BODIPY absorption was observed as
strong band at 560 nm and the bands at 515, 627, 689, 782 and
884 nm are due to sapphyrin19 unit (Fig. S33).
made for compound 2 in 19F and 11B NMR compared to the other
BODIPYechromophore conjugates.
In general, BODIPYechromophore conjugates 1 and 3e7 showed
a triplet in 11B NMR at 0.50 ppm and a single quartet in 19F NMR
spectrum because of coupling to 11B (I¼3/2, J¼32 Hz) and resonances
approximately at ꢁ146 ppm. However, in compound 2, due to the
presence of two BODIPY moieties, we observed two sets of quartets
at ꢁ145.12 and ꢁ146.08 ppm corresponding to BODIPY A and
BODIPY B, respectively. Similarly, in 11B NMR spectrum of compound
2, the two triplets of two BODIPY moieties A and B merged and
appeared as a quartet. This kind of observations were made earlier in
other systems containing two or more BODIPY units.9
The absorption properties of conjugates 1e7 were studied in
toluene at room temperature and the data are tabulated in Table 2.
The absorption spectra of conjugates 1e7 depend on the chromo-
phore, that is, connected to BODIPY unit. In all these conjugates, the
absorption bands corresponding to BODIPY unit as well as the
chromophore are present with alteration in peak maxima and in-
tensity. In compound 1, the strong band at 616 nm along with
a shoulder at 575 nm corresponding exclusively to BODIPY unit
along with the small ill-defined bands in 350e450 nm region,
which are due to both BODIPY and anthracene units are present
(Fig. S29). The large red shift of absorption band (w115 nm) of
BODIPY unit in 1 compared to the reference BODIPY 1014 indicates
that the BODIPY and anthracenyl moieties in 1 interact strongly.
Furthermore, Burgess and co-workers8a reported BODIPY-
eanthracene conjugates in which the anthracenyl moiety is con-
1.0
0.8
0.6
0.4
0.2
0.0
10
8
(a)
6
4
2
nected at
bridge exhibited BODIPY absorption band only at 540 nm. This in-
dicates that the anthracenyl moiety present at -position involves
in stronger interaction with BODIPY core compared to anthracenyl
b-pyrrole position of BODIPY core using the same ethynyl
0
a
350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
group present at the b-position of BODIPY unit.
(b)
1.0
Table 2
Absorption and fluorescence data of BODIPYechromophore conjugates 1e7 along
with 10 in toluene
0.8
0.6
0.4
0.2
0.0
Compound labs nm (log
3)
fDonor
Donor (%Q)a
10
1
502 (4.68)
0.05
d
354 (sh), 375 (sh), 401 (4.18), 424 (4.26), <0.0003b 98%
575 (sh), 616 (4.59)
2
3
423 (3.32), 509 (3.64), 562 (3.67)
<0.0001c 99%
d
299 (4.17), 328 (4.22), 422 (3.91), 529 (sh), 0.32c
563 (4.53)
4
5
6
421 (4.99), 519 (sh), 564 (4.19), 650 (3.29) <0.0001c 99%
425 (4.98), 563 (4.26), 597 (3.79)
438 (5.00), 515 (4.18), 567 (4.41),
633 (3.34), 699 (4.32)
0.003c
0.010c
83%
70%
7
515 (4.46), 560 (3.78), 627 (sh), 689 (3.75), d
d
400
500
600
700
800
782 (2.54), 884 (3.0)
Wavelength (nm)
a
(%Q) denotes percentage of quenching of fluorescence quantum yield.
b
c
Anthracene unit.
BODIPY unit.
Fig. 3. Normalized absorption (d) and emission spectra (————) of compounds (a) 2
and (b) 4 recorded in toluene.

T.K. Khan, M. Ravikanth / Tetrahedron 67 (2011) 5816e5824
5821
Table 3
Thus, the absorption study clearly indicated that in com-
pounds 2e7, the absorption band of BODIPY unit experienced
significant red shift compared to meso-tolyl BODIPY 10 due to
Electrochemical redox data (V) of compounds 1e7 along with 10 in dichloromethane
containing 0.1 M TBAP as supporting electrolyte
Compound BODIPY
Chromophore
the presence of ethynylphenyl group at a-position, whereas the
ox
red
E1re=d2Vð
D
Ep; mVÞ
E
ð
D
Ep; mVÞ
E
ð
D
Ep; mVÞ
chromophores connected to BODIPY unit which contains meso-
phenyl group did not show any shift in their respective ab-
sorption bands. However, in compound 1, due to the presence of
direct ethynyl bridge between the two chromophores, the
BODIPY unit showed more red shift compared to other
conjugates.
1=2
1=2
I
II
I
II
10
1
2
3
4
5
6
7
ꢁ1.15 (94)
ꢁ0.41, ꢁ0.46a
ꢁ0.49 (175)
ꢁ0.52 (145)
ꢁ0.59 (96)
ꢁ0.52 (85)
ꢁ0.64a
1.17 (92)
d
d
ꢁ1.24a
ꢁ1.16a
ꢁ1.36a
ꢁ1.58a
d
d
0.99a
d
d
1.57a
d
1.11a
ꢁ1.09 (86) ꢁ1.43 (100)
The redox potentials of conjugates 1e7 were measured in
0.91 (153) 1.21 (180) ꢁ1.18a
ꢁ1.57 (70)
CH2Cl2 by cyclic voltammetry at
a
variable scan rate
1.34a
d
ꢁ0.90a
ꢁ1.12a
ꢁ1.13a
(50e150 mV/s) using tetrabutylammonium perchlorate as the
supporting electrolyte. A comparison of reduction wave of BODIPY
unit in conjugates 1e5 is shown in Fig. 4 and the data are pre-
sented in Table 3. Bard and co-workers showed that the meso-aryl
substituted BODIPYs exhibit one reversible oxidation and one re-
versible reduction.21 In all conjugates, the BODIPY unit showed
one reversible or quasi-reversible reduction and did not show any
well defined oxidation wave. In compound 2, because of two
BODIPY units, the reduction of both BODIPY units were overlapped
and appeared as one broad reduction wave in cyclic voltammetry
(Fig. 4b).
ꢁ0.47 (84)
d
ꢁ1.37 (167)
a
Obtained from DPV.
and 3e7 showed their respective oxidation and reductions with
slight shifts in their potentials. The redox potentials of BODIPYe
chromophore conjugates 1e7 were assigned on the basis of the
redox potential data of their corresponding independent chro-
mophores. For example, the conjugate 7 showed a reduction at
ꢁ0.47 V, which corresponds exclusively to BODIPY unit and two
other reductions at ꢁ1.12 and ꢁ1.37 V were mainly due to sap-
phyrin unit (Fig. S34).
In all conjugates, the BODIPY unit showed one reversible re-
duction, which appeared at less negative potential (ꢁ0.40
to ꢁ0.65 V) compared to 10 (ꢁ1.15 V) indicating that the BODIPY
unit in conjugates 1e7 is easily reducible. The fluorescence prop-
erties of conjugates 1e7 were studied in toluene using different
excitation wavelengths.
However, using differential pulse voltammetric (DPV) studies,
we identified the reduction corresponding to each BODIPY unit in
compound 2. Furthermore, the chromophores in compounds 1
The fluorescence properties of conjugates 1e7 are dependent on
BODIPY as well as chromophore units. In these conjugates, the
BODIPY unit is acting either as energy donor or as energy acceptor,
which depends on the chromophore that is attached to it. In con-
jugate 1, the anthracene unit acts as energy donor and BODIPY unit
acts as energy acceptor. On excitation at 350, 420 and 488 nm, the
emission was mainly observed at 650 nm corresponding to the
BODIPY unit with negligible emission from anthracene unit. The 1:1
mixture of the corresponding monomers (8 and 11) of compound 1,
on excitation at 350 nm showed mainly emission coming from
anthracene unit (Fig. S29). The excitation spectrum recorded at
680 nm matches with its absorption spectrum. These observations
indicate a possibility of singlet-singlet energy transfer from an-
thracene to BODIPY unit in compound 1. In BODIPYeBODIPY con-
jugate 2, the BODIPY unit B, which contains ethynylphenyl group at
a-position absorbs at lower energy, whereas the BODIPY unit A
absorbs at higher energy. This is clearly reflected in its absorption
spectrum (Fig. 3a). Thus, on excitation of BODIPY unit at 488 nm,
the emission was exclusively observed from the BODIPY unit, which
contains ethynylphenyl group at a-position and no emission was
noticed from the donor BODIPY unit due to the energy transfer from
BODIPY unit A to BODIPY unit B in conjugate 2. The BODI-
PYeterpyridine conjugate 3, on excitation at 488 nm, the emission
from BODIPY unit was observed with quantum yield of 0.32
(Fig. S30). The conjugate 4 containing N4 porphyrin and BODIPY
units, on excitation at BODIPY unit at 488 nm, the emission was
mainly observed from N4 porphyrin unit because of singletesinglet
energy transfer from BODIPY unit to N4 porphyrin (Fig. 3b). How-
ever, in conjugate 5 (Fig. S31) and 6 (Fig. S32), on excitation of
BODIPY unit, the emission was observed from both BODIPY and
porphyrin units. In these two cases, the energy transfer from
BODIPY unit to porphyrin unit was not very efficient hence emis-
sion was also observed from the donor BODIPY unit. The conjugate
7 is almost non-fluorescent indicating that the thiasapphyrin unit
Fig. 4. Comparison of reduction waves of cyclic voltammograms along with differen-
tial pulse voltammograms of (a) 1, (b) 2, (c) 3, (d) 4 and (e) 5 in dichloromethane
containing 0.1 M TBAP as supporting electrolyte recorded at 50 mV/s scan speed.
connected at a-position of BODIPY unit enhances the non-radiative

5822
T.K. Khan, M. Ravikanth / Tetrahedron 67 (2011) 5816e5824
decay channels. Although, the quenching of donor emission in
conjugates was mainly attributed to singlet-singlet energy transfer
from donor to acceptor unit, we did not rule out the possibility of
some contribution from electron transfer for quenching the donor
emission. A detailed time-resolved studies are required to un-
derstand the photophysical properties of these novel conjugates.
calculated manually by taking the average of the cathodic and an-
odic peak potentials. All potentials were calibrated versus saturated
calomel electrode by the addition of ferrocene as an internal
standard, taking E1/2 (Fc/Fcþ)¼0.42 V, versus SCE.23 The ES-MS mass
spectra were recorded with a Q-Tof micro mass spectrometer. High-
resolution mass spectrum was obtained from Q-TOF instrument by
electron spray ionization (ESI) technique.
3. Conclusions
4.3. 3-Bromo-4,4-difluoro-8-(4-methoxyphenyl)-4-bora-
3a,4a-diaza-s-indacene (8)
In conclusion, we used 3-bromo BODIPY as a synthon to prepare
one ethynyl bridged BODIPYeanthracene conjugate and six ethy-
nylphenyl bridged BODIPYechromophore conjugates, such as
BODIPY-BODIPY, BODIPYeterpyridine, BODIPYeporphyrin, BODI-
PYeZn(II)porphyrin, BODIPYe21,23-dithiaporphyrin and BODI-
PYethiasapphyrin by coupling with chromophores containing
ethynyl or ethynylphenyl functional group under mild Pd(0) cou-
pling conditions. The spectral studies indicated that the interaction
between the two chromophores is stronger in ethynyl bridged
BODIPYeanthracene conjugate compared to the ethynylphenyl
bridged BODIPYechromophore conjugates. We showed that, in
these conjugates, the BODIPY can act as either energy donor or
energy acceptor depending on the type of chromophore to which it
This compound was prepared in sequence of steps in one pot
reaction. meso-(p-Methoxyphenyl)-dipyrromethane 9 (500 mg,
2.12 mmol) was treated with 1 equiv of N-bromosuccinimide
(377 mg, 2.12 mmol) in dry THF (50 mL) at ꢁ78 ꢀC under nitrogen
for 1 h. The reaction mixture was brought to room temperature. The
reaction mixture was subjected to flash column chromatography
using CH2Cl2, concentrated on rotary evaporator, dissolved in
CH2Cl2 and DDQ (483 mg, 2.12 mmol) was added. The reaction
mixture was stirred for 1 h at room temperature and then neu-
tralized with triethylamine (10.49 mL, 75.4 mmol) and treated with
BF3$Et2O (13.44 mL, 107.0 mmol) at room temperature for addi-
tional 1 h. The reaction mixture was washed successively with
0.1 M NaOH solution and water. The organic layers were combined,
dried over Na2SO4, filtered and evaporated. The crude compound
was subjected to silica gel column chromatography and the re-
is linked. Furthermore, the chromophores connected at a-position
of BODIPY would exhibit different trends in properties compared to
the reported BODIPYechromophore conjugates in which the
chromophores are connected at b-position. We are presently ex-
ploring a detailed photodynamics of novel BODIPYechromophore
quired a-bromo derivative of BODIPY 8 was collected as second
conjugates in our laboratory.
band using of petroleum ether/dichloromethane (90:10). The sol-
vent was removed on rotary evaporator under vacuo and afforded
pure 8 as orange powder (250 mg, 33% yield). Rf (25% pet. ether/
CH2Cl2) 0.70; IR (KBr, cmꢁ1) 737, 780, 837, 970, 1076, 1106, 1178,
1251, 1338, 1382, 1542, 1571, 1602, 2840, 2924; 1H NMR (400 MHz,
4. Experimental section
4.1. Chemicals
CDCl3,
d in ppm): 3.91 (3H, s, OMe), 6.53 (2H, m, Py), 6.57 (2H, m,
THF and toluene were dried over sodium benzophenone ketyl
and chlorofom, ethyl-acetate, methanol, acetonitrile dried over
calcium hydride prior to use. BF3$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 (60e120 mesh) or
alumina.
Py), 6.84 (2H, d, J¼4.28 Hz, py), 6.93 (2H, d, J¼3.70 Hz, py), 7.38 (2H,
d, J¼7.95 Hz, Ar), 7.44 (2H, d, J¼7.95 Hz, Ar). 19F NMR (282.2 MHz,
CDCl3,
d
in ppm): ꢁ146.34 (q, JBeF¼56.5 Hz). HR-MS: calcd for
(C16H13BBrF2N2) 377.0272 found 377.0271.
4.4. General procedure for the synthesis of
BODIPYechromophore conjugates 1e7
3-Bromo-4,4-difluoro-8-(4-methoxyphenyl)-4-bora-3a,4a-
4.2. Instrumentation
diaza-s-indacene 8 and appropriate chromophore containing
ethynyl or ethynylphenyl functional group 11e16 were dissolved in
dry toluene/Et3N (6 mL, 5:1) in a 25 mL, two-necked, round-bot-
tomed flask fitted with a reflux condenser, gas inlet and gas outlet
tubes for nitrogen purging. The reaction vessel was placed in an oil
bath preheated to 35 ꢀC. After purging the flask with nitrogen for
15 min, AsPh3 (3.5 equiv) and Pd2(dba)3 (0.44 equiv) were added,
and the reaction mixture was stirred at 35 ꢀC for 4 h. TLC analysis of
the reaction mixture indicated the appearance of a dark new spot
apart from the two minor spots corresponding to starting pre-
cursors. The solvent was removed under reduced pressure, and the
crude compound was purified by silica gel column chromatogra-
phy. The excess AsPh3 and the small amounts of unreacted starting
precursors were removed with petroleum ether and the required
pure BODIPYechromophore conjugate was then collected with
petroleum ether/CH2Cl2.
1H NMR spectra (
d in ppm) were recorded using Varian VXR 300,
400 MHz and Bruker 400 MHz spectrometer. 13C NMR spectra were
recorded on Bruker operating at 100.6 MHz 19F NMR spectra were
recorded on Varian spectrometer operating at 282.2 MHz 11B NMR
spectra were recorded on Varian spectrometer operating at
96.3 MHz. TMS was used as an internal reference for recording 1H
(of residual proton;
d d 77.0 signal) in CDCl3. Ab-
7.26) and 13C (
sorption and steady-state fluorescence spectra were obtained with
PerkineElmer Lambda-35 and PC1 Photon Counting Spectrofluo-
rometer manufactured by ISS, USA instruments, respectively.
Fluorescence spectra were recorded at 25 ꢀC in a 1 cm quartz
fluorescence cuvette. The fluorescence quantum yields (Ff) were
estimated from the emission and absorption spectra by compara-
tive method at the excitation wavelength of 488 nm using Rhoda-
mine 6G (Ff¼0.88).22 Cyclic voltammetric (CV) and differential
pulse voltammetric (DPV) studies were carried out with electro-
chemical system utilizing the three electrode configuration con-
sisting of a glassy carbon (working electrode), platinum wire
(auxillary electrode) and saturated calomel (reference electrode)
electrodes.22b The experiments were done in dry dichloromethane
using 0.1 M tetrabutylammonium perchlorate as supporting elec-
trolyte. Half wave potentials were measured using DPV and also
4.4.1. BODIPYeanthracene conjugate 1. Alumina column (petro-
leum ether/dichloromethane 95:5) Rf (5% pet. ether/CH2Cl2) 0.42;
42% yield; IR (KBr, cmꢁ1) 669, 768, 929, 1014, 1071, 1128, 1216, 1261,
1528, 1604, 2179, 2400, 2854, 2927, 3019, 3272; 1H NMR (400 MHz,
CDCl3): 3.92 (3H, s, OMe), 6.61 (1H, m, b-Py), 6.97 (1H, m, b-Py),
7.03e7.08 (2H, m, b-Py), 7.53e7.59 (6H, m), 7.67e7.71 (2H, m), 8.02
(1H, s,
a
-Py), 8.04 (2H, s), 8.50 (1H, s), 8.84 (2H, d, J¼8.2 Hz).13C NMR

T.K. Khan, M. Ravikanth / Tetrahedron 67 (2011) 5816e5824
5823
(100 MHz, CDCl3,
d
in ppm): 55.74, 77.3, 94.28, 99.93, 114.29, 116.31,
b
a
m,
-Py), 7.07 (2H, d, J¼8.7 Hz, BDP-Ar), 7.55e7.58 (8H, m), 8.03 (1H, s,
-Py), 8.05e8.11 (8H, m, Ar), 8.24 (2H, d, J¼8.2 Hz), 8.94e9.00 (8H,
118.74, 124.08, 126.15, 126.67, 127.17, 127.68, 128.87, 129.03, 129.90,
130.92, 131.09, 131.33, 132.64, 133.70, 136.46, 137.30, 144.01, 162.14.
b
-py). 13C NMR (100 MHz, CDCl3,
d
in ppm): 21.69, 55.75, 65.73,
19F NMR (282.2 MHz, CDCl3,
d
in ppm): ꢁ145.49 (q, JBeF¼56.5 Hz).
102.00, 113.97, 114.27, 118.89, 120.52, 120.70,121.85, 123.49, 126.55,
127.32, 127.60, 128.90, 129.60, 130.71, 130.90, 131.09, 131.25, 131.70,
132.21, 132.50, 132.64, 134.21, 134.34, 134.67, 134.86, 135.72, 136.20,
137.09, 137.56, 139.35, 139.42, 141.91, 143.89, 144.32, 145.20, 162.18.
11B NMR (96.3 MHz, CDCl3,
d
in ppm): 0.991 (t, JBeF¼30.5 Hz). HR-MS
calcd for (C32H21BF2N2O): 499.1793 found: 499.1813.
4.4.2. BODIPYeBODIPY conjugate 2. Alumina column (petroleum
ether/dichloromethane 75:25) Rf (25% pet. ether/CH2Cl2) 0.30; 40%
yield; IR (KBr, cmꢁ1) 769, 754, 849, 928, 1021, 1081, 1119, 1218, 1527,
1604, 1672, 1965, 2400, 2855, 2927, 3020; 1H NMR (400 MHz,
19F NMR (282.2 MHz, CDCl3,
11B NMR (96.3 MHz, CDCl3,
d
in ppm): ꢁ145.92 (q, JBeF¼56.4 Hz).
d
in ppm): 0.69 (t, JBeF¼28.92 Hz). HR-
MS calcd for (C65H46BF2N6OZn) m/z 1041.3164 observed 1040.3192.
CDCl3): 3.94 (3H, s, OMe), 6.59 (2H, m,
(1H, m, -Py), 6.97 (2H, m,
-Py), 7.07 (2H, d, J¼8.2 Hz, Ar), 7.56 (2H,
d, J¼8.24 Hz, Ar), 7.61 (2H, d, J¼8.24 Hz, Ar), 7.72 (2H, m, -Py), 7.83
(2H, d, J¼7.9 Hz, Ar), 7.98 (2H, s, -Py), 8.02 (1H, s,
-Py). 13C NMR
(100 MHz, CDCl3, in ppm): 55.75, 65.73, 85.0, 100.0, 114.32, 18.98,
b
-Py), 6.63 (1H, s,
b
-Py), 6.79
4.4.6. BODIPYe21,23-dithiaporphyrin conjugate 6. Silica column
(petroleum ether/dichloromethane 50:50) Rf (50% pet. ether/
CH2Cl2) 0.30; 38% yield; IR (KBr, cmꢁ1) 669, 770, 849, 928, 1018,
1122, 1217, 1423, 1526, 1603, 1965, 2400, 2927, 3019, 3272; 1H NMR
(400 MHz, CDCl3): 2.71 (9H, s), 3.93 (3H, s, OMe), 6.59 (1H, s, BDP-
b
b
b
a
a
d
119.33, 123.32, 129.00, 130.74, 131.08, 131.57, 132.23, 132.46, 132.66,
b
-Py), 6.85 (1H, d, J¼3.3 Hz, BDP-
(2H, d, J¼8.2 Hz, BDP-Ar), 7.57 (2H, d, J¼8.5 Hz, BDP-Ar), 7.61e7.63
(8H, m), 8.04 (1H, s,
d, J¼8.2 Hz), 8.68e8.73 (4H, m,
b-Py), 7.01 (2H, m, BDP-b-Py), 7.07
134.88, 144.60, 144.96, 149.0, 162.0, 166.0, 167.89. 19F NMR
(282.2 MHz, CDCl3,
d
in ppm): ꢁ145.12 (q, JBeF¼56.4 Hz), ꢁ146.08
a
-py), 8.12e8.14 (6H, d, J¼7.02 Hz, Ar), 8.27 (2H,
(q, JBeF¼56.4 Hz). 11B NMR (96.3 MHz, CDCl3,
d
in ppm): 0.753 (m).
b
-py), 9.66e9.73 (4H, m,
b
-Th). 19
F
ES-MS calcd for (C33H22B2F4N4O): 588.19 found: 569.26 [Mꢁ19]þ.
NMR (282.2 MHz, CDCl3,
NMR (96.3 MHz, CDCl3,
calcd for (C65H45BF2N4OS2): calculated 1011.2994 found: 1011.3018.
d
in ppm): ꢁ146.07 (q, JBeF¼56.4 Hz). 11
B
HR-MS calcd for (C33H22B2F3N4O): 569.1932 found 569.1945.
d
in ppm):0.896 (t, JBeF¼27.4 Hz). HR-MS
4.4.3. BODIPYeterpyridine conjugate 3. Alumina column (petro-
leum ether/dichloromethane 40:60) Rf (60% pet. ether/CH2Cl2)
0.45; 50% yield; IR (KBr, cmꢁ1) 710, 758, 887, 983, 1023, 1081, 1121,
1216, 1527, 1604, 1965, 2201, 2855, 2926, 3018; 1H NMR (400 MHz,
4.4.7. BODIPYethiasapphyrin conjugate 7. Alumina column (pe-
troleum ether/dichloromethane 40:60) Rf (60% pet. ether/CH2Cl2)
0.60; 53% yield; IR (KBr, cmꢁ1) 710, 799, 856, 945, 1076, 1081, 1261,
1553, 2123, 2921, 2852, 3322; 1H NMR (400 MHz, CDCl3): ꢁ0.84
CDCl3): 3.94 (3H, s, OMe), 6.59 (1H, m,
-Py), 6.95 (2H, m,
-Py), 7.33e7.39 (4H, m, pyridylþBDP-Ar), 7.47
(2H,d, J¼7.9 Hz, BDP-Ar), 7.69 (2H, d, J¼8.2 Hz, Tpy-Ar), 7.80 (2H, d,
b-Py), 6.75 (1H, d, J¼3.97 Hz,
b
b
(2H, s,
(1H, s, BDP-
m, BDP[
-PyþAr]), 7.60 (10H, m, Ar[BDPþsap]), 8.06 (1H, s, BDP-
Py), 8.12e8.21 (4H, m, Ar), 8.27 (2H, d, J¼7.3 Hz, Ar), 8.35 (2H, d,
J¼8.2 Hz, Ar), 8.59e8.71 (4H, m, -Py sap), 9.8 (2H, d, J¼8.54 Hz,
b-Th), 2.64 (6H, s, tol), 2.74 (3H, s, tol), 3.93 (3H, s, OMe), 6.62
b
-Py), 6.87 (1H, d, J¼4.27 Hz, BDP-
b-Py), 7.02e7.18 (4H,
J¼7.6 Hz, Tpy-Ar), 7.88e7.96 (2H, m, pyridyl), 8.0 (1H, s,
a
-Py), 8.67
b
a-
(2H, d, J¼7.63 Hz, pyridyl), 8.75 (4H, d, J¼3.97 Hz, pyridyl). 13C NMR
(100 MHz, CDCl3,
d
in ppm): 55.56, 114.10, 118.76, 121.42, 123.40,
b
123.95, 127.36, 130.63, 132.47, 132.79, 136.94, 145.00, 149.18, 156.11.
19F NMR (282.2 MHz, CDCl3,
in ppm): ꢁ146.17 (q, JBeF¼56.4 Hz).
11B NMR (96.3 MHz, CDCl3,
b-Th-bth), 10.25e10.28 (2H, m, b
-Th-bth). 19F NMR (282.2 MHz,
d
CDCl3,
CDCl3,
d
in ppm): ꢁ146.04 (q, JBeF¼56.0 Hz). 11B NMR (96.3 MHz,
d
in ppm): 0.769 (t, JBeF¼25.9 Hz). HR-
d
in ppm): 0.912 (t, JBeF¼28.9 Hz). HR-MS calcd for
MS calcd for (C39H27BF2N5O): 631.2355 found: 631.2367.
(C69H47BF2N4OS3): 1093.3047 found: 1093.3051.
4.4.4. BODIPYeporphyrin conjugate 4. Silica column (petroleum
ether/dichloromethane 60:40) Rf (40% pet. ether/CH2Cl2) 0.55; 40%
yield; IR (KBr, cmꢁ1) 710, 758, 888, 983, 1023, 1081, 1121, 1216, 1286,
1507, 1604, 1965, 2201, 2855, 2926, 3018, 3320; 1H NMR (400 MHz,
Acknowledgements
M.R. thanks Department of Atomic Energy (DAE-BRNS) and
Department of Science and Technology for financial support. T.K.K.
thanks Indian Institute of Technology, Bombay for fellowship.
CDCl3): ꢁ2.7 (1H, s), 2.70 (9H, s), 3.9 (3H, s), 6.59 (1H, s, BDP-
6.84 (1H, d, J¼3.9 Hz, BDP- -Py), 6.99e7.02 (2H, m, BDP- -Py), 7.04
(2H, d, J¼8.7 Hz, BDP-Ar), 7.54e7.58 (8H, m), 8.03 (1H, s, -Py),
8.05e8.10 (8H, m, Ar), 8.24 (2H, d, J¼8.2 Hz), 8.86e8.89 (8H, m,
-py). 13C NMR (100 MHz, CDCl3,
in ppm): 21.69, 55.69, 65.76,
b-Py),
b
b
a
Supplementary material
Complete characterization data of compounds, 1H and 13C NMR
and 19F spectra. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.tet.2011.05.089.
b
d
70.74, 84.09, 102.00, 113.97, 114.28, 118.89, 120.55, 120.70, 121.85,
123.49, 126.55, 127.32, 127.62, 128.90, 129.60, 130.73, 130.90, 131.09,
131.25, 131.70, 132.21, 132.50, 132.64, 134.21, 134.40, 134.68, 134.87,
135.72, 136.20, 137.09, 137.57, 139.37, 139.42, 141.91, 143.89, 144.32,
References and notes
144.32, 145.20, 162.19. 19F NMR (282.2 MHz, CDCl3,
d
in ppm):
ꢁ145.85 (q, JBeF¼56.4 Hz). 11B NMR (96.3 MHz, CDCl3,
d in ppm):
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