Synthesis and photophysical studies of heteroaryl substituted-BODIPy derivatives for biological applications

Article abstract of DOI:10.1016/j.tetlet.2014.11.004

Mono and di-heteroaryl-4,4′-difluoro-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (BODIPy) (1-5) were synthesized using Suzuki-Miyaura couplings. Hetero aryl substitution on 3- or 3,5-positions caused large bathochromic shifts (up to ～150 nm) in absorption (569

Full text of DOI:10.1016/j.tetlet.2014.11.004

Accepted Manuscript
Synthesis and photophysical studies of heteroaryl substituted-BODIPy deriva-
tives for biological applications
Dipti Lakhe, Karthika K. Jairaj, Madhura. Pradhan, Uma Ladiwala, Neeraj
Agarwal
PII:
S0040-4039(14)01877-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.11.004
TETL 45388
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 September 2014
2 November 2014
3 November 2014
Please cite this article as: Lakhe, D., Jairaj, K.K., Pradhan, Madhura., Ladiwala, U., Agarwal, N., Synthesis and
photophysical studies of heteroaryl substituted-BODIPy derivatives for biological applications, Tetrahedron
Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.11.004
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Synthesis and photophysical studies of
heteroaryl substituted-BODIPy derivatives
for biological applications
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D. Lakhe, K. K. Jairaj, M. Pradhan, U. Ladiwala and N. Agarwal
R1
N
N
B
F F
R2
R2
R
R
= Anisyl or 2-Thienyl
1
2
= 3-Thienyl or Benzothienyl

1
Tetrahedron Letters
journal homepage: www.els evier.com
Synthesis and photophysical studies of heteroaryl substituted-BODIPy derivatives for
biological applications
Dipti Lakhe, Karthika K. Jairaj, Madhura. Pradhan, Uma Ladiwala and Neeraj Agarwal *1
a UM-DAE, Centre for Excellence in Basic Sciences, University of Mumbai, Kalina campus, Santacruz (E), Mumbai 400098, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Mono and di-heteroaryl-4,4'-difluoro-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (BODIPy) (1-5)
were synthesized using Suzuki-Miyaura couplings. Hetero aryl substitution on 3- or 3,5-
positions caused large bathochromic shifts (up to ~150 nm) in absorption (569-652 nm) and
fluorescence maxima (586-679 nm) in comparison to classical BODIPy. Quantum yields were
found to be as high as 0.65. Singlet oxygen production activities of these compounds were
studied by monitoring the absorbance quenching of 1,3-diphenylisobenzofuran, on exposure to
light (> 600 nm). Cellular uptake of compound 4 was demonstrated using cervical cancer cells
and fibroblast cell line and was confirmed by the images obtained using confocal microscope.
Received in revised form
Accepted
Available online
Keywords:
Absorption
Fluorescence
2009 Elsevier Ltd. All rights reserved.
Boron
Suzuki-Miyaura coupling
Singlet-oxygen generation
Introduction
considerably and hence we believe that it will have direct effect
on singlet and triplet energy levels. Here, we aim to synthesize
4,4’-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPy) is one
of the most popular dye and has been studied extensively.1
BODIPy dyes are known for their high molar absorption
coefficient and sharp absorbance and fluorescence maxima.2,3
Photophysical properties of these dyes have been tuned by
changing the substituents of different electron density or by
manipulating the conjugation length.4-8 Properties like ease of
chemical modification, tunable absorbance and fluorescence
properties, high quantum yield and high photostability make
BODIPy dyes attractive for several applications such as
fluorescence imaging, electron transfer, chemiluminescence,
organic solar cells, chemo-sensors, energy transfer systems,
photodynamic therapy (PDT) etc.9-15 PDT is an effective and
approved way for the treatment of a variety of diseases such as
cancers, precancerous inductions, dermatological infections and
wound healing.16,17 Light, oxygen and photosensitizer (PS) are
major components for effective PDT. For application in PDT,
high molar absorption coefficient in therapeutic window and
ability to produce singlet oxygen on exposure to light are the pre-
requisites for PS. In PDT, it is required to populate the triplet
excited state on irradiation of PS by the inter-system crossing.
Triplet energy transfer from PS to surrounding molecular oxygen
(triplet), present in tissues, produces singlet oxygen which is the
key component in therapeutic process and can induce cell death
by apoptosis (type II process).18
new hetero-aryl substituted BODIPy derivatives having
absorption beyond 600 nm, large molar absorption coefficients
and high quantum yield. The Suzuki-Miyaura cross coupling (S-
M coupling) is one of the widely used methods to make aryl-aryl
bond and has been employed on the BODIPy core with aryl
(phenyl) boronic acids20-22 and recently with heteroaryl boronic
acid.23 Effect of 8-heteroaryls on the photophysical properties of
BODIPy have been studied.24 A few reports are available for the
synthesis of 3,5-diheteroaryl BODIPy in multiple steps, as shown
in Scheme 1. In these methods, as illustrated in scheme 1, 2-
heteroaryl substituted pyrroles were synthesized which were
further used in BODIPy synthesis.25,26 In one of these reported
method, 3-acetylbenzo[b]thiophene was reacted with NH2OH to
get oxime which on reaction with acylylene gas produced 2-
(Benzo[b]thiophene-3-yl)-1-vinylpyrrole. Devinylation of the
synthesized pyrrole led to the corresponding 2-
(benzo[b]thiophene-3-yl)pyrrole which was further used to get
BODIPy as usual.
O
NOH
N
NH
NH OH
2
Hg(OAc)
C
H
2 2
2
BODIPy
NaBH
4
S
S
S
S
NH2
SH
+
BODIPy
CN
N
H
S
N
H
HN
It has been shown that introduction of heavy metal or hetero-
aryl groups produces the triplet excited states efficiently.19
Similarly, hetero-aryls such as thiophene or benzothiophene
Scheme 1. Heteroaryl substituted pyrrole and their use in
BODIPy synthesis
g—ro—ups—on BODIPy core should alter photophysical properties
1 Corresponding author: Neeraj Agarwal, E-mail: na@cbs.ac.in; Tel: 91-22-26530227, Fax: 91-22-26524982

2
Tetrahedron
In this article we report the use of heteroaryl-boronic acids in
mixture of THF : toluene: water (1:1:1) was added to the reaction
flask and stirred under argon gas. Pd(PPh3)4 was used as catalyst
and Na2CO3 as base and reaction was refluxed at 85 °C. The
progress of reaction was monitored by TLC, which clearly
indicated the formation of new spot corresponding to compounds
1-5 along with the reactant. For the completion of reaction, after
six hours, one or two equivalents of corresponding boronic acid
were further added. Reactions were continued till complete
disappearance of starting material. The reaction mixture was
cooled and washed with brine and water successively. The
organic layers were dried over anhydrous sodium sulphate and
solvent was evaporated. The crude product was purified by
chromatography on silica gel column to afford dark lustrous
solids of BODIPys 1-5. The yields of compounds 1-5 were low
(~14%) which is unlikely for the Suzuki-Miyaura coupling. Low
yields with hetero-aryl boronic acids, in particular, have been
observed earlier and attributed to the protodeboronation of
hetero-aryl boronic acids in aqueous basic conditions.29,30
the synthesis of BODIPy derivatives 1-5 using slightly modified
S-M coupling conditions. Photo-physical and singlet oxygen
production properties of these BODIPy derivatives are studied.
Cellular uptake of compound 4 is also demonstrated using
cervical cancer cells (HeLa cells) and fibroblast cell line (NIH-
3T3). Cellular uptake of compound 4 in the cytoplasm is
observed by fluorescence imaging.
Results and discussions
Compounds 1-5 were synthesized as outlined in Scheme 2.
Meso-substituted dipyrromethanes 6 and 7 were synthesized as
per the reported procedures.27 Purification by column
chromatography afforded 6-7 as white solids. These
dipyrromethanes 6, 7 were used in the synthesis of bromo
BODIPys 8a-b and 9a-b by using a sequence of reactions.28
Bromination of 6, 7 with one or two equivalents of N-
bromosuccinimide at low temperature (-78 °C) in THF afforded
the mono and dibromo dipyrromethanes, respectively which were
further reacted with DDQ (2,3-dichloro-5,6-dicyano-1,4-
benzoquinone) at room temperature to get the mono and
dibromo-dipyrromethenes. The obtained crude products were
quickly passed through the silica gel column to remove the
excess DDQ and purified product was reacted with BF3-etherate
to get the mono (8a-9a) and dibromo (8b-9b) BODIPy. Minor
amounts (less than 2%) of 8a, 9a were also observed along with
major amount (~65%) of 8b, 9b during the dibromination of 6,7.
i) NBS, THF, -78 0
C
R
R
ii) DDQ, THF, RT
InCl3
R
iii) TEA, BF3-etherate,
+
N
N
Toluene, 90 0
C
CHO
N
H
NHHN
B
X1
X2
F
F
R = Anisyl, X1 = H, X2 = Br, 8a
R = Anisyl, X1 = X2 = Br, 8b
R = Thienyl, X1 = H, X2 = Br, 9a
R = Thienyl, X1 = X2 = Br, 9b
R = Anisyl, 6
R = Thienyl, 7
OCH3
OCH3
Figure 1. Representative (a) 1H-NMR and (b) 13C-NMR spectra (selected
region) of compound 4 recorded in CDCl3
OH
Na2CO3, Pd(PPh3
)
4
HO
B
+
All compounds were found to be fairly soluble in common
organic solvents e.g. chloroform, dichloromethane, toluene and
dimethyl formamide. The identities of the compounds were
confirmed by several spectroscopic techniques. The 1H-NMR
spectra of compounds 1-5 showed the characteristic NMR signals
corresponding to BODIPy core with additional hetero aryl proton
signals of substituted groups. A representative 1H-NMR spectrum
for compound 4 recorded in CDCl3 is shown in Figure 1. A
singlet was observed for beta protons of pyrrole at 6.83 ppm.
Anisyl protons appeared as doublets at 6.96 and 7.42 ppm. Four
different sets of signals were obtained for benzothienyl at 7.31
(m), 7.76 (d), 7.86 (d) and 8.40 (s) ppm. Characteristic singlet for
N
N
B
Toluene, THF, H2
85 0
O
S
N
N
Br
Br
C
B
F
F
F
F
8b
S
S
1
R
R
Na2CO3, Pd(PPh3
)
4
N
N
+
B
N
N
HO
Toluene, THF, H2
O
B
S
B
F
F
85 0
C
Br
OH
S
F
F
R = Anisyl 2
R = Thienyl, 3
8a or 9a
R
1
R
Na2CO3, Pd(PPh3
)
4
methoxy protons was found at 3.83 ppm. The H-NMR spectra
N
N
for other compounds were also characterized and have been
summarized in experimental section. 13C-NMRs of compounds 1-
5 (except 3) were recorded in CDCl3.
N
N
HO
B
Toluene, THF, H2
85 0
O
B
S
B
F
F
Br
C
Br
OH
S
S
F
F
+
8b or 9b
R = Anisyl 4
R = Thienyl, 5
Photo-physical properties of compounds 1-5 were studied by
using absorption and emission spectroscopy in dichloromethane.
Absorption spectra of 1-5 in dichloromethane are shown in
Figure 2. Compounds 1-5 showed strong S0-S1 (π−π*) absorption
maxima in the range of 569-652 nm. This band is assigned to
diisoindolomethene core of compounds 1-5. The absorption
maxima of this band are directly dependent on the extent of de-
localization of π electrons of thiophene and benzothiophene
groups on BODIPy core. The broad band observed in the high
energy region 400-470 nm of spectrum corresponds to the S0-S2
transition of the boradiazaindacene. Absorptions for thiophene or
benzothiophene were also observed in the short wavelength
Scheme 2. Synthesis of heteroaryl-substituted BODIPys, 1-5.
Compounds 8a-b, 9a-b were subsequently used for Suzuki-
Miyaura coupling with heteroaryl boronic acids to synthesize
BODIPy derivatives 1-5 (Scheme 1). Under the well known S-M
coupling conditions i.e. using bromo BODIPy derivatives, hetero
aryl boronic acids and Pd catalyst with K2CO3 and DME/water
produced very small amount (less than 2-3%) of BODIPy 1-5.
The Suzuki-Miyaura coupling conditions were slightly modified,
where compounds 8a-b, 9a-b and the respective hetero aryl
boronic acid were degassed with argon gas for 15 minutes. The

3
region. Substitution of heteroaryl groups produced large
bathochromic shift of around 65-150 nm in absorption maxima in
comparison to un-substituted BODIPy.31 As a result of this,
compounds 4, 5 observed the absorbance well within the
therapeutic window (600-850 nm). Molar absorption coefficients
for 1-5 were high and are summarized in Table 1.
single exponential decay in toluene. Fluorescence life time, for
compounds 1-5 are summarized in Table 1.
Figure 2. Normalized absorption spectra for compounds 1-5 recorded in
dichloromethane
Figure 3. Normalized emission spectra for compounds 1-5 recorded (top)
and fluorescence decay for 4 (bottom).
Table 1. Photophysical properties of BODIPy derivatives 1-5
The electrochemical properties of compounds 1-5 were
investigated by cyclic voltammetry in CH3CN at a scan rate of
100 mV/s using tetrabutylammoniumhexafluorophosphate
(TBAHFP) as supporting electrolyte. Oxidation waves were
observed for compounds 1-5 in these experimental conditions.
All compounds showed irreversible oxidations. A comparison of
oxidation waves of compounds 1-5 are shown in Figure 4 and
electrochemical data for compounds 1-5 are summarized in Table
2. Monosubstituted compounds 2, 3 showed oxidation at higher
potential as compare to disubstituted 1, 4 & 5 showing their
robustness towards oxidation. EHOMO of compounds 1-5 were
calculated using difference in oxidation potentials and using the
Fc/Fc+ value at 4.8 eV versus vacuum. Optical band gaps for 1-5
were obtained from the offset of absorption spectra and thus
ELUMO were calculated using the EHOMO and optical band gaps for
compounds 1-5. EHOMO and ELUMO of organic compounds are
representation of ionization potential and electron affinity or in
other words they reflects electron donating and accepting
tendency, respectively.
λ
abs & ε
τb
Stoke shift
Comp
λ
em [nm] Φa
-1
[ 103cm ]
[nm] (log value)
503 (4.73)
[ns]
-
Std.31
521
-
-
5.20
1
2
3
4
5
412 br (3.88), 582 (4.60) 605
419 br (4.13), 569 (4.52) 586
420 br (4.00), 582 (4.66) 608
450 br (4.13), 636 (4.65) 660
464 br (3.49), 652 (4.03) 679
0.45
0.65
0.03
0.49
0.26
434.7
588.2
384.6
416.6
370.3
2.59
0.43
5.67
3.94
a in dichloromethane
b in toluene
Emission spectra of compounds 1-5 recorded in
dichloromethane are shown in Figure 3. Compounds 1-5 showed
emission peak at 586 -679 nm with a low intensity shoulder in
further low energy region. A large bathochromic shift of around
~ 65 - 160 nm was observed in comparison to un-substituted
BODIPy. A very small Stoke shift was observed for 1-5 which
provides the information on little change occurred in the ground
and excited state geometry of compounds 1-5. Fluorescence
quantum yields of compound 1-5 were calculated with respect to
Rhodamine B (φ = 0.68 in ethanol)32 (Table 1). Fluorescence
quantum yields were found to be as high as 0.65. The brightness,
defined as the product of the molar absorption coefficient at the
excitation wavelength and the fluorescence quantum yield [ε(λ) x
φf], of these compounds were calculated. The maximum value of
brightness was found to be as high as 29539 L mol-1 cm-1 for
compound 2. Similarly for 1 and 4 it was found to be about
15562 L mol-1 cm-1. For Compound 3 and 4 it was about 761 L
mol-1 cm-1 and 1584 L mol-1 cm-1 respectively.
Fluorescence life time studies of 1-5 were carried out in
toluene and are presented in Table 1. Fluorescence decay profile
for compound 4 is shown in Figure 3. Compounds 1–5 showed a
Figure 4. Cyclic voltammograms showing oxidation waves of 1- 5 in
acetonitrile

4
Tetrahedron
Table 2. Electrochemical data for compounds 1-5 in CH3CN
aEox bEg
Compound
Compound 4 was used as representative dye to study
cEHOMO dELUMO
cellular uptake. Good quantum yield in red region for compound
4 helped in microscopic fluorescence imaging. Incubation of
HeLa cells and NIH-3T3 fibroblasts with ~1 X 10-5 M aqueous 4
for 1 h at 37 °C showed efficient cellular uptake of
photosensitizer as shown in Figure 6 A & B. The cellular uptake
of 4 appeared to be localized to the cytoplasm (red color), with
the nuclei remaining free of 4 (dark area) in confocal microscopic
images (Figure 6A) of NIH-3T3 fibroblast cells. Images of HeLa
cells and NIH-3T3 fibroblasts taken using an epifluorescence
microscope also showed dye accumulated in the cytoplasm (red
area) with nuclei stained with Hoechst dye (blue area) (Figure 6
B). Singlet oxygen generation shares some dynamic processes
with fluorescence emission.34 The PS activation mechanisms
depends on solvent properties such as pH, hydrophobicity,
viscosity etc., most of which control photo-induced properties
and hence may be useful for selectivity of BODIPy towards the
cancer cells. More recently, heterocyclic photosensitizers have
been used in PDT with improved selectivity towards cancer
cells.35
[V] [eV] [eV]
[eV]
1
2
3
4
0.70 2.00 -5.50
0.84 2.07 -5.64
1.01 2.00 -5.81
0.70 1.81 -5.50
0.71 1.78 -5.51
-3.50
-3.57
-3.81
-3.69
-3.72
5
a with respect to Fc/Fc+
b optical band gap calculated from the offset of absorption spectrum,
c EHOMO = –(Eox (vs ferrocene) + 4.8 eV),
d ELUMO = EHOMO + Eg.
Ability of singlet oxygen production is one of the essential
requirements by any potential type II photonic therapeutic agent.
Taking the optical therapeutic window into consideration, only
the compounds with absorption maxima above 600 nm (4 and 5)
were studied for its singlet oxygen production ability. The singlet
oxygen production experiments were done by using the widely
used singlet oxygen acceptor 1,3-diphenylisobenzofuran (DPBF)
with the molecule(s) under study. Decrease in the absorbance of
DPBF was monitored on irradiating the solution with red light
source (>600 nm) at regular time intervals (2 min). Absorbance
of DPBF with time is plotted and is shown in Figure 5.
Hematoporphyrin (HP) was used as the standard photo-sensitizer
for comparison. Decrease in the absorbance of DPBF due to its
oxygenation was observed in case of 4 and 5 and were compared
with that of HP. Singlet oxygen generation quantum yield33,
related to HP, were calculated and found to be about 0.25 for 4
and 0.1 for 5. Compounds 4 and 5 showed effective absorbance
quenching which suggest that compound 4 and 5 could be
potential photosensitizers in PDT. Over HP, compounds 4 and 5
have the advantage that they can absorb efficiently in the
therapeutic window and has high molar extinction coefficient in
this region, much higher than that of HP. Compounds 1-5 are
photo-stable even after exposure to ambient light for several
hours as observed from their absorption spectra. No considerable
change was observed in absorbance maxima of BODIPy
derivatives in singlet oxygen generation titration which further
show the photostability of 4, 5 (see supporting information).
Figure 6. (A) Confocal images of NIH-3T3 cells: a) brightfield and b-d)
after incubation with Compound 4 showing uptake of dye in the cytoplasm
(red). Nuclear regions in the centre appear dark. (B) Fluorescence merged
images of HeLa cells (a,b) and NIH-3T3 fibroblasts (c,d) with 20x and 100x
oil immersion objectives, respectively, showing cellular uptake of 4 seen in
the cytoplasm (red) and Hoechst-stained nuclei (blue).
Conclusions
To summarize, we have synthesized five derivatives of
BODIPy having hetero-aryl substituents on -3 and/or -5 positions
using Suzuki-Miyaura coupling. Photo-physical studies showed
the large (upto ~150 nm) bathochromic shifts in absorption and
emission maxima as compare to the un-substituted BODIPy.
Absorption in therapeutic window, large molar absorption
coefficients and high fluorescence quantum yield of compounds
4 and 5 allowed us to examine them for singlet oxygen
production experiment and were found to be quite effective in
DPBF absorption quenching. Compounds 4 and 5 have
absorption maxima at 636 and 652 nm with higher extinction
coefficient than hematoporphyrin (4350 M-1cm-1 at 623 nm) and
hence we believe that they could be effective in PDT. Compound
4 showed sufficient cellular uptake with NIH-3T3 fibroblasts as
evident from confocal microscopy.
Acknowledgements
We thank, Centre for Excellence in Basic Sciences, Mumbai
for providing research facilities. Partial funding was provided by
Department of Science and Technology, India (SR/FT/CS-
87/2010). We thank Tata Institute of Fundamental Research,
Mumbai for providing the NMR, MALDI-TOF and TCSPC
facility. We also thank Indian Institute of Technology, Bombay,
for helping us in recording the HRMS spectra.
Figure 5. Comparison of singlet oxygen production using 4, 5 and HP
using DPBF as singlet oxygen acceptor

5
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Supplementary Material
Details of instrumentations and synthetic procedures for 1-5
1
along with H NMR, 13C NMR, mass spectra etc. are given in
Supplementary material.
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V.; Boens, N.; Dehaen, W. Chem. Commun. 2009, 45, 4515–4517.