Versatile synthetic methods for the engineering of thiophene-substituted Bodipy dyes

Article abstract of DOI:10.1016/j.tetlet.2009.09.163

Novel thienyl-borondiazadipyrromethene (Bodipy) dyes have been prepared using boronic acids or boronate reagents as cross-coupling mediators. A key dichloro/bromo-Bodipy starting material appears to be a useful starting material for such coupling reaction

Full text of DOI:10.1016/j.tetlet.2009.09.163

Tetrahedron Letters 50 (2009) 7008–7013
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Versatile synthetic methods for the engineering of thiophene-substituted
Bodipy dyes
a,
Sandra Rihn a, Pascal Retailleau b, Nicolas Bugsaliewicz a, Antoinette De Nicola a, , Raymond Ziessel
*
*
a Laboratoire de Chimie Organique et Spectroscopies Avancées (LCOSA), Ecole Chimie Polymères, Matériaux (ECPM), 25 rue Becquerel, 67087 Strasbourg Cedex, France
b Laboratoire de Cristallochimie, ICSN-CNRS, Bât 27, 1 Avenue de la Terrasse, 91198 Gif-sur-Yvette, Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2009
Revised 24 September 2009
Accepted 28 September 2009
Available online 3 October 2009
Novel thienyl-borondiazadipyrromethene (Bodipy) dyes have been prepared using boronic acids or bor-
onate reagents as cross-coupling mediators. A key dichloro/bromo-Bodipy starting material appears to be
a useful starting material for such coupling reactions, enabling the synthesis of various symmetrically and
unsymmetrically substituted thienyl-dyes. An alternative means of introducing thienyl substituents is
through Knoevenagel substitution in the 3 and 5 positions of Bodipy by reaction with a formyl-function-
alized thiophene derivative, resulting in systems of extended delocalization. All these Bodipy derivatives
are stable and exhibit pronounced fluorescence on irradiation in the S0?S1 transition.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Bodipys’
Thiophene
Suzuki coupling
X-ray structures
Fluorescence
Thiophene derivatives have been found to exhibit a wide range
of biological activities1 and many are active compounds as chemo-
therapeutics,2 while many have also been used in various materials
science applications.3 Polythiophenes obtained both chemically
and electrochemically display interesting optical and charge trans-
port properties. Variation of substituents, geometries, and shapes
has provided detailed insights into the structure–activity relation-
ships that have been scrutinized in several excellent reviews.4,5
The use of oligothiophenethynylene6 for the construction of cyclic
nanoarrays7,8 and linear scaffoldings capped by various stoppers
have also been investigated.9,10 Furthermore, thiophene-based
functional polymers have attracted significant attention due to
their application as conductive and active layers in OLEDs,11 and
their high conductivity provides them with excellent characteris-
tics as organic thin film transistors.12,13 Based on a thorough
understanding of structure–property relationships and the ingenu-
ity of synthetic chemists, ambipolarity, tunability, high quantum
efficiencies, and stability have all been achieved.13 Our findings
have revealed that thiophenes are among the most promising units
for use to connect chromophoric centers and to provide interesting
optical properties.14,15
a bathochromic shift of the absorption and emission wavelengths.
In some rare cases, near infra-red emission occurs. As a matter of
fact, only a few such dyes have been prepared up to now, mostly
due to the difficulty in preparation of the necessary thiophene
functionalized starting materials.18–20
Here, we disclose our attempts to selectively functionalize Bod-
ipy with various thiophene residues in order to control the optical
properties of the dyes. While various means of so-doing can be
envisaged, we chose to use tri-substituted reactants shown in
Chart 1, with the objective of selectively introducing thienyl units
at the 3 and 5 positions.
These reactants were prepared by a conventional multistep syn-
thesis from the corresponding substituted benzaldehydes and pyr-
role. Halogenation of the intermediate dipyrromethane was
performed by using NCS or NBS followed by DDQ oxidation and
stabilization of the dipyrromethene with BF3ÁEt2O in basic condi-
tions.21 The X-ray crystal structures of the three compounds have
been determined (Fig. 1). All three molecules have approximate
NO2
Br
NO2
In extending this work, we have recently found that the grafting
of thiophene subunits onto difluoro-boradiaza-s-indacenes, com-
monly termed Bodipy (for boron dipyrromethene),16,17 results in
8
1
7
2
6
N
F
N
B
N
F
N
F
N
F
N
B
* Corresponding authors. Fax: +33 (0)3 68 85 27 61.
E-mail addresses: denicola@chimie.u-strasbg.fr (A.D. Nicola), ziessel@chimie.
u-strasbg.fr (R. Ziessel).
5
Cl
3
Cl
B
Br
Br
Cl
Cl
F
F
1
2
3
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.163
Chart 1.

S. Rihn et al. / Tetrahedron Letters 50 (2009) 7008–7013
7009
twofold rotational symmetry, though this is not crystallographi-
cally imposed. The Bodipy cores (12 atoms) are quasi-planar for
each structure, but the rms deviation from the least-squares mean
plane is significantly higher in compound 1 than those of the nitro-
phenyl compounds 2 and 3 (0.09 Å against 0.016 and 0.019 Å,
respectively), owing to an apparent bend at the extremities and a
distortion at the boron atom for compound 1. The value of the
dihedral angle of the substituted phenyl groups with respect to
the Bodipy platforms increases from 55.2(2)° in 1 to 59.6(1)° in 2,
and then to 60.2(1)° in 3. The average B–N and B–F bond lengths
are, respectively, around 1.560(6) Å and 1.372(9) Å and the average
N1–B1–N2, F1–B1–F2, and N–B–F, angles are 105.5(5)°, 111.0(3)°,
and 109.9(3)°, closely similar to the literature values. Another
recurrent observation is the pronounced double bond character
for the C4–N1 and C5–N2 bonds [1.350(6) Å] in contrast with the
longer C1–N1 and C8–N2 bond lengths [1.396(5) Å]. Molecules of
compound 1 are all packed similarly in the crystalline state with
the bromophenyl groups directed along the b-axis (with short
BrÁ Á ÁF and ClÁ Á ÁCl intermolecular distances of 3.3 Å) and the core
units lying in a plane quasi-parallel to ab, resulting in some in-
stances of short contacts between adjacent molecules consistent
It has previously been suggested that both chloro groups in dye
1 are potentially reactive toward nucleophiles in SNAr reactions.22
In our case, the reaction of thienyl magnesium bromide23 with 1 at
À40 °C provided an intractable mixture of compounds. After some
experimentation, it became obvious that a simpler strategy would
be to use cross-coupling reactions. The first attempt to functional-
ize derivative 1 was made with 3.6 equiv of tolylboronic acid under
conventional conditions [toluene, Pd(PPh3)4, with aqueous K2CO3
at 110 °C] providing in 15 min the trisubstituted compound 4 in
56% yield (Chart 2). The reaction with thienylboronic acid in the
same conditions led to a different scenario. The grafting of thio-
phene fragments was then relatively selective in the 3 and 5 posi-
tions, affording compound 5 in modest yield (29%).
Both experiments were considered quite promising in regard to
producing functionalized Bodipy dyes which emit in the near infra-
red. However, it was found that both thienylboronic acid and the
dichloro derivative 1 were not stable under the harsh Suzuki
cross-coupling reactions. Thus, to explore possible improvements,
changes were made in the nature of the thienylboronate derivative,
the nature of the base, and the temperature. The essential out-
comes are summarized in Table 1 and Scheme 1.
with
p
–
p
interactions.
As shown in Table 1, replacing the 2-thienylboronic acid by the
more stable 2-thienylboronic acid pinacol ester enabled the use of
a stoichiometric quantity of boronate to obtain 5 and 6 in a shorter
time. Additionally, the presence of water (possible nucleophilic
species for SNAr reaction) was avoided by using anhydrous Cs2CO3
instead of aqueous K2CO3 aq Despite these two modifications, the
yield of compound 5 remained approximatively the same, around
33%, at 110 °C. The solution for the problem of instability of com-
pound 1 was found by changing the temperature. The yield of 5
was increased to 62% by decreasing the temperature to 90 °C. A
balance between reaction time and progress of the reaction to ob-
tain the mono-substituted derivative 6 in acceptable yield was not
found, however. Like the dichloro compound 1, the monochloro
derivatives 6 and 7 are heat sensitive. However, the faster rate of
the reaction with tolylboronic acid at 110 °C allowed 7 to be iso-
lated in 56% yield (Scheme 2).
The face-to-face distances range from 3.506(3) Å to 3.932(3) Å.
The two other structures adopt the same space group, P21/c. The
nitro groups interact with one of the X = Br, Cl atoms with O–X dis-
tances ranging from 3.101 to 3.251 Å, together with double F–X
interactions (with an F–Cl distance of 3.165 Å and F–Br 3.219 Å)
aligning the molecules along the [2 0 1] direction, but without
developing p–p stacking interactions.
ꢀ
Compounds 6 and 7 are interesting as precursors to unsymmet-
rically substituted Bodipy dyes such as compound 8 bearing one
thienyl and one tolyl residue. This dual substitution pattern allows
fine tuning of the emission channel of the dye (vide infra).
The X-ray structure of the bis-thiophene derivative 5 was also
determined and its features were compared to the previous struc-
tures. The dihedral angle between the Br-phenyl and the Bodipy
units is 50.3(3)°, which is less than that in the previous complexes.
The Bodipy core deviates from the planarity by a rms value of
0.059° with a slight twist observed at the remote rings, up and
down with respect to the least-squares mean plane. The thiophene
rings are tilted outwards from the Bodipy core by almost the same
angle, 25.2(4) and 21.6(4)°, leaving the sulfur atoms on the edges
Br
N
F
N
F
N
F
N
F
B
B
S
S
4
5
Figure 1. ORTEP view of 1 (top left), 2 (top right), and 3 (bottom). Displacement
ellipsoids are drawn at the 30% probability level.
Chart 2.

7010
S. Rihn et al. / Tetrahedron Letters 50 (2009) 7008–7013
Table 1
Reactivity of thienylboronate derivatives with dichloroBodipy 1
Reagent
Base
Equiv
Time (h)
T (°C)
Yielda (%)
5
6
K2CO3 aq
K2CO3 aq
(2 + 5.2b)
(1 + 9b)
60
35
110
110
29
9
12
10
B(OH)2
S
Cs2CO3 anhydrous
Cs2CO3 anhydrous
Cs2CO3 anhydrous
Cs2CO3 anhydrous
2.2
2.2
0.9
0.9
4
1.5
18
2
110
90
110
90
33
62
tr
tr
tr
tr
16
20
O
O
B
S
a
Isolated yield, tr for trace amounts.
Added in between three and five portions.
b
Br
Br
Br
OR
OR
B
S
+
N
F
N
F
N
F
N
F
N
F
N
F
[Pd(PPh3)4]
(5 mol %)
toluene
B
B
B
Cl
Cl
Cl
S
S
S
1
5
6
Scheme 1. Suzuki cross-coupling reactions of compound 1. For specific conditions
see Table 1.
Br
Br
Br
B(OH)2
O
O
B
S
N
F
N
F
N
F
N
F
N
F
N
F
B
B
B
(i)
(i)
Cl
Cl
Cl
S
1
7
8
Figure 2. ORTEP view of 5. Displacement ellipsoids are drawn at the 30%
probability level.
Scheme 2. Reagents and conditions: (i) [Pd(PPh3)4] (5 mol %), toluene, aqueous
K2CO3 5 equiv, 1 equiv of boronate derivative; 110 °C. Compound 7 obtained after
30 min in 56% and compound 8 after 6h in 34%.
lated in reasonably good yields (43% and 65% isolated yields,
respectively).
turned out. No
p
–
p
stacking interactions are observed between
The solubility of these novel dyes appeared to be good to excel-
lent for all compounds including alcohols and no degradations in
the solid state and the solution was observed within a given time.
The electronic absorption spectra of the new dyes are depicted
in Figure 3 and exhibit the characteristic S0?Sn transitions for
substituted Bodipy units. The weak absorptions in the 330–
430 nm window are typical of S0?S2 transitions, while in some
cases there are strong intramolecular charge-transfer (ICT) bands,
as well as a strong absorption between 510 and 630 nm which is
typical of a S0?S1 transition with a shoulder at the high-energy
side and which is attributed to the first vibrational band of the
same transition. All features are consistent with the absorption
spectroscopic behavior of standard Bodipy chromophores. As
would be expected from the design and previous measurements,28
the progressive substitution of the chloro groups in 1 by a phenyl
or thienyl group resulted in bathochromic shifts of 27 and 54 nm,
respectively. Substitution of the remaining chloro by the same sub-
stituent enhanced this shift to 45 and 106 nm, respectively, for
compounds 4 and 5. Interestingly, for the hybrid dye 8 bearing
one phenyl and one thienyl fragment an intermediate shift of
77 nm is found, allowing for control of the absorption and emission
channels (Table 2).
molecules, which are seemingly linked by C–HÁ Á ÁF hydrogen bonds
in the crystal (Fig. 2).
Finally, another useful way to fine-tune the optical properties of
such dyes is to extend the conjugation in the 3 and 5-position
using styryl-based modules.24 This scenario has previously been
used to promote solid state-gas sensors.25 The molecules 11 and
12 were designed such that polarity would be imparted by a short
polyethyleneglycol chain carried by the thiophene subunit. Com-
pound 10 was prepared by a two-step protocol from commercially
available 2-methanolthiophene (according to Scheme 3). The first
step consists of the alkylation of the alcohol using a polyethylene-
glycoltosylate and the second a Vilsmeier reaction using standard
reaction conditions.26 The mono- and distyryl compounds 11 and
12 were synthesized from a tetramethylBodipy derivative by a
Knoevenagel condensation.25 Both dyes were conveniently sepa-
rated by column chromatography, thanks to the presence of the
polyethyleneglycol side chains. As previously observed for similar
dyes, substitution of the fluoro groups on the boron is a convenient
way to improve the stability and processability of the dyes.27 The
use of alkynyl Grignard reagents is an efficient way to substitute
the fluoro groups on the boron center and dyes 13 and 14 were iso-

S. Rihn et al. / Tetrahedron Letters 50 (2009) 7008–7013
7011
H
S
S
(i)
O
O
(ii)
O
S
HO
O
O
9
10
I
O
O
I
I
N
N
F
B
F
(iii)
+
N
N
F
N
F
N
F
B
B
F
S
S
S
O
O
O
11
12
(iv)
(iv)
O
O
O
O
O
O
I
I
N
N
N
N
B
B
S
S
S
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
13
14
Scheme 3. Reagents and conditions: (i) NaH (1.4 equiv), THF, rt, MeO(CH2CH2O)2OTs (1 equiv), 12 h, 75%; (ii) POCl3 (1.6 equiv), DMF (2 equiv), dichloroethane, 80°C, 2h, 46%;
(iii) toluene, piperidine, Bodipy (1 equiv), aldehyde (1.5 equiv), 140°C, 10% for 11 and 13% for 12; (iv) THF, HC„CCH2OCH2CH2OMe (6.6 equiv), EtMgBr (5.6 equiv), 60°C, 2h,
43% for 13 and 65% for 14.
Excitation of these dyes in the lower energy transition produced
strong fluorescence within the 520–620 wavelength range (Fig. 4).
90000
The short lifetime (nanosecond range) and the high quantum yield
(Table 2) are in line with a singlet emitter. For each compound,
there is reasonably good mirror symmetry between the fluores-
cence and (lowest-energy) absorption spectral patterns while the
excitation spectra match well with the absorption spectra over
the entire wavelength range. Stokes’ shifts tend to be rather small
(Table 2), indicating that the geometry does not change signifi-
cantly upon excitation. Furthermore, the change in emission max-
imum tracks the shifts noted for the absorption band and kem
moves progressively toward lower energy on increasing the effec-
tive size of the ancillary substituent. In all cases, the interplay be-
tween short excited state lifetimes, the symmetry in a mirror
between the absorption and emission band and the relatively high
quantum yields determined at rt in fluid solution are in keeping
with a singlet excited state. No evidences for the population of a
low lying triplet excited state were obtained so far.
80000
70000
60000
50000
40000
30000
20000
10000
0
1
4
5
6
8
11
250
350
450
550
650
750
Wavelength (nm)
Our design opens up the possibility of engineering new sophis-
ticated dyes bearing thiophene residues at the periphery. The effi-
Figure 3. Absorption spectra for the substituted dyes in CH2Cl2 at rt. Inset
correspond to the color code and compound labeling.

7012
S. Rihn et al. / Tetrahedron Letters 50 (2009) 7008–7013
Table 2
Selected spectroscopic data for the new dyesa
Dyes
kabs (nm)
e
(MÀ1 cmÀ1
)
kexc (nm)
kem (nm)
Ub
Stokes’ shifts (cmÀ1
)
1
4
5
6
7
516
561
622
570
543
593
580
657
576
658
79,200
59,600
69,700
71,500
60,200
73,000
98,400
118,000
108,300
83,400
356
525
338
535
513
554
560
648
562
640
527
591
643
587
569
622
593
672
587
670
0.36
0.15
0.82
0.40
0.08
0.68
0.82
0.37
0.81
0.33
400
900
525
508
842
786
380
230
240
270
8
11
12
13
14
a
Measured in dichloromethane except for compounds 12, 13, and 14 in ethanol at rt.
b
Quantum yields are determined using cresyl violet (Uref = 0.51, kexc = 623 nm) as the standard.29
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0.8
0.6
0.4
0.2
0
480
580
680
780
Wavelenght (nm)
Figure 4. Emission spectra for the substituted dyes in CH2Cl2 at rt, in each case the
excitation wavelength correspond to the maximum of the S0?S2 transition. Inset
correspond to the color code and compound labeling.
ciency of the substitution reactions depends on the reaction time,
temperature, and nature of the mineral base. Hybrid dyes bearing
thienyl and phenyl were also prepared along with 2-thienylvinyl-
based molecules. The mono- substituted and di-substituted thio-
phene dyes were easily separated thanks to the presence of short
polyethyleneglycol chains which could be incorporated either as
side chain on the thiophene or as ethynyl modules on the boron.
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substituents.
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21. Typical
preparation
for
compound
2:
A
solution
of
5-(4-
nitrophenyl)dipyrromethane: Rohand, T.; Dolusic, E.; Ngo, T. H.; Maes, W.;
Dehaen, W. Arkivoc, 2007, 10, 307, (100 mg, 0.374 mmol) in 6 mL of THF was
cooled to À78 °C under argon. NBS (133 mg, 0.748 mmol) was added in two
portions over 30 min period. Once all NBS had dissolved, DDQ (85 mg,
0.374 mmol) in 1.5 mL THF was added over 10 min. The reaction mixture
was warmed to rt. Triethylamine (8 mL, 4.61 mmol) and boron trifluoride
diethyletherate (1.3 mL, 10.28 mmol) were added dropwise successively. The
mixture was stirred at room temperature until the complete consumption of
the starting material and water was added. The organic extracts were washed
with water and brine. The solvent was removed by rotary evaporation. The
residue was purified by chromatography on silica gel eluting with petroleum
ether/dichloromethane (60:40) to give 67 mg (38%) of 2: 1H NMR (CDCl3) d
Supplementary data
CCDC 735530–735533 contain the supplementary crystallo-
graphic data for the four crystal structures of this paper. These data
can be obtained free of charge at www.ccdc.cam.an.uk/uk/conts/
retrieving.html or from the Cambridge Crystallographic Data Cen-
tre (CCDC), 12 Union Road, Cambridge CB2 1EZ, United Kingdom;
Fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk.
6.64 (AB, 4H, ABJ = 4.2 Hz,
m
m
dAB = 23.1 Hz); 8.05 (AB, 4H, ABJ = 8.0 Hz,
dAB = 141.2 Hz). 1H NMR for 3; (CDCl3) 6.62 (AB, 4H, ABJ = 4.3 Hz,
dAB = 54.4 Hz); 8.04 (AB, 4H, ABJ = 8.9 Hz, dAB = 141.9 Hz). 1H NMR for 1;
dAB = 109.3 Hz); 7.50 (AB, 4H, ABJ = 8.4 Hz,
d
m
m
(CDCl3) d 6.63 (AB, 4H, ABJ = 4.4 Hz,
m
Acknowledgments
m
dAB = 92.9 Hz).
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This work was partially supported by the CNRS providing re-
search facilities. We acknowledge Dr. Gilles Ulrich for helpful dis-
cussions and skill expertise in the spectroscopic measurements.
We also warmly thank Professor Jack Harrowfield (ISIS in Stras-
bourg) for commenting on the manuscript before publication.
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