New insights into the solubilization of Bodipy dyes

Article abstract of DOI:10.1016/j.tetlet.2009.04.017

Several Bodipy dyes were synthesized with various substituents designed to improve the water solubility. Among the different synthetic strategies protection of sulfonate groups by pyrrole of indopyrrole appears efficient but the deprotection step does not

Full text of DOI:10.1016/j.tetlet.2009.04.017

Tetrahedron Letters 50 (2009) 3840–3844
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
New insights into the solubilization of Bodipy dyes
b
c
a,
*
Song-Lin Niu a, Gilles Ulrich a, , Pascal Retailleau , Jack Harrowfield , Raymond Ziessel
*
a Laboratoire de Chimie Organique et Spectroscopies Avancées (LCOSA), 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
c ISIS 8 allée Gaspard Monge BP 70028 F-67083 Strasbourg Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Several Bodipy dyes were synthesized with various substituents designed to improve the water solubility.
Among the different synthetic strategies protection of sulfonate groups by pyrrole of indopyrrole appears
efficient but the deprotection step does not offer viable routes. Conversion of the bromopyrene or iodo
Bodipy compounds to sulfobetaine derivatives is feasible either by cross-coupling directly the ethynyl-
sulfobetaine or by first cross-coupling 1-(N,N-dimethylamino)-prop-2-yne followed by quaternization
of the dimethylamino residue with 1,3-propanesultone. Some of these dyes (Bodipy’s and pyrene) are
reasonably soluble in water and remain highly fluorescent in polar solvents and water.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 6 March 2009
Revised 6 April 2009
Accepted 8 April 2009
Available online 11 April 2009
Keywords:
Bodipy
Pyrene
Sulfobetaine
Water solubility
Fluorescence
Perhaps one of the most active areas of current research relating
to fluorescent dyes is that concerning the introduction of various
functionalities to improve their utility without significantly modify-
ing the intrinsic spectroscopic properties of the dyes. Cyanines, in
particular, are currently under intense investigation,1,2 the family
of difluoro-boradiaza-s-indacenes, which are commonly termed
Bodipy (for boron dipyrromethene) dyes, being especially promising
because of their exceptional spectroscopic properties.3,4 These in-
clude high extinction coefficients, sharp emission bands, high quan-
tum yields and behaviour as singlet emitters with little distortion of
the excited state. Bodipy dyes also exhibit exceptional stability,5
making them attractive for diverse applications such as donor-
acceptor systems,6 artificial light harvesters,7 fluorescent sensors,8
laserdyes,9 electron-transferreagents10 andlight-emittingdevices.11
To expand the utility of such dyes, we have developed recently an
arsenal of synthetic methods by which the pseudo-meso 8-posi-
tion,12 the 2,6-pyrrolic position,13 the 3,5-dimethyl fragments14
and the 4,40-boron positions can be routinely functionalized.15 Al-
kyne substitution tools have been extensively applied in these pro-
water-soluble Bodipys are all sulfonated species obtained either
by direct reaction with chlorosulfonic acid at the 2,6 positions,17
by Heck-type coupling18 to introduce hydrophilic (sulfonated) sub-
stituents at the 2,6 positions, by reaction, again at the 2,6 positions,
with chlorosulfonic acid after the introduction of other functional-
ity19 or by nucleophilic aromatic substitution of a 3-chloro substi-
tuent with 2-mercaptosulfonic acid.20
In this context, we report herein our attempts to prepare new
water-soluble Bodipy dyes appropriately functionalized in several
positions, together with description of the spectroscopic properties
of the new dyes in various polar solvents. Two synthetic pathways
were envisaged: (i) introduction of a protected sulfonate connected
via a ‘soft’ coupling reaction to the dye or (ii) a post-synthetic
introduction of a sulfobetaine unit. The critical aspect of this work
concerns the use of sulfobetaines as zwitterionic templates to pro-
mote water solubility. This is the first application of such a proce-
dure to Bodipy functionalization. Its initial step was the attachment
of the protected sulfonates 1 to 4 to various F-Bodipys.21
cedures. By virtue of their inherent linearity and p-orbitals, triple
bonds are not only an ideal conduit for electronic conductivity but also
provide a facile entry into the construction of C–C-linked appendages.16
Thus, Bodipys are fascinating dyes, although serious drawbacks
do exist which include the tendency to form aggregates and a very
low solubility in polar solvents.3 The few known examples of
O
S
O
S
N
S
N
S
O
O
O
O
O
O
O
O
* Corresponding authors. Tel.: +33 3 90 24 26 89; fax: +33 (0) 3 90 24 27 61.
E-mail addresses: gulrich@chimie.u-strasb.fr (G. Ulrich), ziessel@chimie.u-
strasbg.fr (R. Ziessel).
1
2
3
4
H
H
H
H
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.017

S.-L. Niu et al. / Tetrahedron Letters 50 (2009) 3840–3844
3841
Cl
S
Nu
S
Nu
S
Nu
S
O
O
O
O
O
O
O
O
O
N
O
O
N
O
S
S
(i)
(ii)
(iii)
N
N
I
B
I
B
4
Br
Br
N
N
H
TMS
O
O
S
O
S
O
N
N
Scheme 1. Reagents and conditions: (i) isobutanol, 2,6-lutidine, 1,2-dichloroeth-
ane, rt, 70%; plus either pyrrole, NaH, THF, rt, 76%; or isoindole, NaH, THF, rt, 67%;
(ii) TMSacetylene, TEA, benzene, [Pd(PPh3)2Cl2] (0.06 mol %), CuI (0.1 mol %), 50 °C,
80% to 94%; (iii) KF, methanol, rt, 83–97%.
11
12
(i)
H
+
-
H
N
SO3
N
These derivatives were prepared in three steps from 4-bro-
mophenylsulfonyl chloride according to Scheme 1.
13 , 85%
(ii)
The cross-coupling reaction of 1–4 with dyes bearing halo-aro-
matic groups, such as 8-(4-iodophenyl)-Bodipy12 is very efficient in
the presence of TEA, benzene and [Pd(PPh3)4] (6 mol %) at 60 °C,
leading to the desired compounds 5 to 7 in 85–91% yields.
MeO
+
-
SO3
N
14 , 54%
X
Scheme 2. Reagents and conditions: (i) 1,3-propanesultone, toluene, rt; (ii) 4-
iodoanisole, [Pd(PPh3)4] (6 mol %), DMF, TEA, 80 °C.
OH
O
S
X =
O
S
O
O
O
This encouraging result led us to examine the cross-coupling
reaction of 13 with Bodipy dyes to see how greatly their water sol-
ubility might be enhanced. Dyes 15 and 16 were easily prepared
from the corresponding iodophenyl derivatives,14 and were ob-
tained in 74 and 30% yields, respectively, after purification by flash
chromatography on silica. The first two sulfobetaine-Bodipys are
not highly soluble in water, unfortunately, tending to form aggre-
gates as a result of their amphiphilic character. Thus, it appeared
necessary to introduce hydrophilic substituents onto the aromatic
core of the dyes.
5
6
N
S
X =
O
O
8
N
F
N
F
7
B
Note that during the reaction with the terminal alkyne 1 the
ethyl sulfoester is hydrolyzed. Following the same strategy,
efficient cross-coupling with the 2-iodo-, or 2,6-diiodo-Bodipy13
provided the substituted dyes 8–10 in excellent isolated yields
(64–83%).
-
SO3
-
SO3
N
N
O
S
N
S
N
S
2
X
O
O
O
O
O
O
X =
N
F
N
F
B
8
9
10
The pyrrole- or indole-protected sulfonates can be used to sub-
stitute F by acetylide at B by means of the corresponding Grignard
reagents.15 These were prepared from compounds 3 and 4 and
EtMgBr in anhydrous THF at 60 °C. The resulting boron-functional-
ized dyes 11 and 12 were obtained in satisfactory yields after con-
ventional purification procedures. Deprotection of the sulfonate,
however, appeared to be difficult under various acidic or basic con-
ditions. These are known to work well in other cases.22
As the basis for an alternative strategy, we examined the
reactivity of the ethynylsulfobetaine derivative 13 towards
Sonogashira cross-coupling reaction catalyzed by low valent
Pd(0) (Scheme 2). Compound 13 was conveniently prepared
from 1-(N,N-dimethylamino)-prop-2-yne and was found to re-
act readily with 4-iodoanisole (as model compound) under
classical cross-coupling reactions to afford the water-soluble
compound 14.
N
F
N
F
B
N
F
N
F
B
15
16
MeO
OMe
With this objective, we turned our attention to cross-coupling
reaction with Bodipys bearing a reactive iodo function at the pyrro-
lic ring (2 position). Surprisingly, the reaction did not produce the
target betaine but produced a rather complicated mixture of highly
coloured and non-fluorescent compounds that were difficult to
separate properly. To circumvent this problem, we designed a
two-step protocol based on (i) Pd(0) cross-coupling the dye with
1-dimethylamino-2-propyne and (ii) quaternization with1,3-pro-
panesultone under apolar conditions.23 We first tested the concept

3842
S.-L. Niu et al. / Tetrahedron Letters 50 (2009) 3840–3844
on a pyrene model compound. Both steps proved to be straightfor-
ward and compounds 17 and 18 were obtained in excellent yields,
providing awater-soluble pyrene. Thus, we cross-coupled 1-
dimethylamino-2-propyne with Bodipy 19,13 affording compound
20 in 82% yield, and subsequent alkylation with 1,3-propanesul-
tone readily provided the desired water-soluble dye 21. The use
of a non-polar solvent caused the desired dye to precipitate during
the course of the reaction and betaine 21 was isolated by centrifu-
gation without additional treatment.
+
C'2
N
C'1
N
-
SO3
17, 62%
18, 81%
A single-crystal structure determination24 for compound 18
established its expected structure (Fig. 1), with a C„C distance
of 1.185(4) Å and the puckered sulfobetaine chain folded back to-
wards the pyrene ring. The mean plane of the sulfobetaine chain
lies close to that of the ethynylpyrene unit, the quaternary-N and
sulfonate-S atoms lying approximately 1 Å to opposite sides of this
plane. Within the lattice, the molecules form columnar stacks par-
allel to a (Fig. 2), with the pyrene substituent orientation alternat-
ing down the column. The normal to the pyrene plane is inclined at
20.5° to axis-a and alternates in orientation from one column to
Figure 2. Top: Non-centrosymmetric
p-stacked pairs viewed down the axis-a. In
the next. While p-stacking is commonly observed in the solid state
structures of pyrene and its derivatives,25 and, in the present case,
the rings in the columns down to axis-a are close to parallel (mean
dihedral 2.5°) and separated by only 3.5 Å, consistent with such an
interaction, analysis of the structure using Hirshfeld surfaces gen-
grey, molecule at positions x ꢁ 1/2, y, 1/2 ꢁ z; hydrogen atoms are omitted for
clarity. Bottom: Enlarged view of the crystal packing down the axis-a. The water
molecules are omitted for clarity.
the same sulfonate-O has a contact to C5A of 3.502 Å. Note that
there is also a short contact of O1W to C16A of 3.266 Å, though
the (water) oxygen atom lies well out of the plane of the ring
erated with CrystalExplorer26 indicates that
p-stacking is less
important than interactions of the peripheral hydrogen atoms
probably involving CHꢀ ꢀ ꢀO bonding (note that there is some evi-
dence that the ethyne unit is involved in interaction with an adja-
cent pyrene ring). Thus, within a column, there are intermolecular
contacts such that C16A is 3.502 Å from (sulfonate) O3, with C6A
3.596 Å from the same oxygen, while between columns, again
and thus the interaction may be of
p
ꢀ ꢀ ꢀO character. This water mol-
ecule is clearly involved in H-bonding to sulfonate-O2 (Oꢀ ꢀ ꢀO
2.803 Å) and has another short contact of 3.319 Å to C2 in an adja-
cent column. In any case, the Hirshfeld surface properties indicate
that the strongest lattice interactions are those between the zwit-
terionic arms, which form head-to-tail arrays (i.e., antiparallel
alignment of the dipoles) in columns also parallel to a. Interactions
Figure 1. The molecular structure of 18, showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 30% probability level and H atoms are
shown as small spheres of arbitrary radii.
Figure 3. A view of the Hirshfeld surface calculated for the stoichiometric unit of
crystalline compound 18. By convention, red regions indicate points of unusually
close contact to adjacent atoms.

S.-L. Niu et al. / Tetrahedron Letters 50 (2009) 3840–3844
3843
(i)
+
N
I
-
N
N
N
N
SO3
B
B
F
F
F
F
21
19
O
S
O
(ii)
(iii)
O
N
N
N
B
20
F
F
Scheme 3. Reagents and conditions: (i) Compound 13, [Pd(PPh3)4] (6 mol %), DMF, TEA, 80 °C; (ii) 1-dimethylamino-2-propyne, [Pd(PPh3)2Cl2] (6 mol %), CuI (10 mol %),
benzene, TEA, 60 °C, 82%; (iii) 1,3-propanesultone, toluene, 58 °C, 78%.
i)
ii)
N
F
N
F
N
N
N
N
B
B
B
-
+
-
+
N
N
O3S
N
N
SO3
22
23
Scheme 4. Reagents and conditions: (i) 1-(N,N-dimethylamino)-propyne magnesium bromide, THF, 60 °C, 55%; (ii) propanesultone, toluene, 60 °C, 78%.
analogue of 1-(N,N-dimethylamino)-prop-2-yne. The alkyne-
substituted dye 22 was prepared in 55% isolated yield, then alkyl-
ated in toluene with 1,3-propanesultone as the reagent to give
compound 23 in 78% yield after collection by centrifugation and
washing with common apolar solvents (Schemes 3 and 4).
The electronic absorption spectrum of the water-soluble pyrene
35000
30000
25000
20000
15000
10000
5000
0
ab
ex
em
18 exhibits characteristic p,p* transitions for the various S3, S2 and
S1 excited states. Excitation spectra match the absorption spectra
allowing to conclude that all the transitions are involved in the sin-
gle and structured emission centred at 368 nm (Fig. 4). This emis-
sion mirrors the absorption and its single exponential decay in the
nanosecond regime is typical of a singlet excited state. The water-
soluble Bodipy 23 has a weak absorption at 368 nm typical of a
S0?S2 transition and a strong absorption at 510 nm typical of a
S0?S1 transition (Fig. 5). The short lifetime (9.2 ns) and the high
quantum yield (81%) are in line with a singlet emitter. Finally,
the blue dye 16 exhibits the characteristic structured absorption
with a maximum at 643 nm and a pronounced intramolecular
charge transfer (ICT) absorption at 369 nm (Fig. 6). The intense
red emission at 660 nm has a short lifetime typical of a singlet
230
280
330
380
Wavelength
430
480
Figure 4. Absorption, emission and excitation spectra of 18 in water.
between the formally oppositely charged ammonium and sulfo-
nate centres appear again to be mediated through CHꢀ ꢀ ꢀO interac-
tions (Fig. 3).
Finally, this interesting strategy was applied towards the
grafting of such betaine residues onto boron using the Grignard
70000
50000
ab
em
ex
ab
ex
em
45000
60000
40000
35000
30000
25000
20000
15000
10000
5000
50000
40000
30000
20000
10000
0
0
250
250
350
450
550
650
350
450
550
650
750
Wavelength (nm)
Wavelength (nm)
Figure 5. Absorption, emission and excitation spectra of 23 in water.
Figure 6. Absorption, emission and excitation spectra of 16 in EtOH.

3844
S.-L. Niu et al. / Tetrahedron Letters 50 (2009) 3840–3844
Table 1
References and notes
Optical properties of selected compounds at rta
1. Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera, G. B. Chem. Rev.
2000, 100, 1973–2012.
Compds
kabs
(nm)
emax
kem
(nm)
sem
(ns)
UF
(%)
kr
knr
(Mꢁ1 cmꢁ1
)
(108 sꢁ1
)
(106 sꢁ1
)
2. Ziessel, R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31, 496–501.
3. Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932.
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Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184–1201.
5. Haughland, R. P. Handbook of Fluorescent Probes and Research Products, 9th ed.;
Molecular Probes: Eugene, OR, 2002.
6. Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006, 128, 14474–14475.
7. Harriman, A.; Izzet, G.; Ziessel, R. J. Am. Chem. Soc. 2006, 128, 10868–10875.
8. Bricks, J. L.; Kovalchuk, A.; Trieflinger, C.; Nofz, M.; Büschel, M.; Tolmachev, A.
I.; Daub, J.; Rurack, K. J. Am. Chem. Soc. 2005, 127, 13522–13529.
9. Lopez-Arbeloa, F.; Banuelos Prieto, J.; Martinez Martinez, V.; Arbeloa Lopez, T.;
Lopez Arbeloa, I. Chem. Phys. Chem. 2004, 5, 1762–1771.
10. Sartin, M. M.; Camerel, F.; Ziessel, R.; Bard, A. J. J. Phys. Chem. C 2008, 112,
10833–10841.
11. Lai, R. Y.; Bard, A. J. J. Phys. Chem. B 2003, 107, 5036–5042; Hepp, A.; Ulrich, G.;
Schmechel, R.; von Seggern, H.; Ziessel, R. Synth. Metals 2004, 146, 11–15.
12. Ulrich, G.; Ziessel, R. J. Org. Chem. 2004, 69, 2070.
5 (EtOH)
524
527
527
523
523
523
522
523
501
496
643
359
54,100
65,000
92,600
67,000
48,000
71,500
103,000
54,000
77,600
75,500
47,000
30,600
537
542
541
548
548
548
534
534
512
511
660
358/
405
387/
407
529
528
4.3
5.1
5.5
4.5
3.9
5.1
6.9
6.4
2.55
3.18
4.66
11.6
66
48
64
78
79
74
50
82
43
20
60
32
1.53
0.94
1.16
1.73
2.04
1.47
0.72
1.61
1.68
0.63
1.29
0.28
79
102
65
49
54
49
72
35
6 (CH2Cl2)
7 (CH2Cl2)
8 (CH2Cl2)
9 (CH2Cl2)
10 (CH2Cl2)
11 (CH2Cl2)
12 (CH2Cl2)
15 (EtOH)
15 (H2O)
223
252
86
16 (EtOH)
18 (H2O)
59
18 (DMSO)
365
39,000
17.4
38
0.22
36
13. Bonardi, L.; Ulrich, G.; Ziessel, R. Org. Lett. 2008, 10, 2183–2186.
14. Ziessel, R.; Ulrich, G.; Harriman, A.; Alamiry, M. A. H.; Stewart, B.; Retailleau, P.
Chem. Eur. J. 2009, 15, 1359–1369.
15. Goze, C.; Ulrich, G.; Ziessel, R. J. Org. Chem. 2007, 72, 313–322; Goze, C.; Ulrich,
G.; Ziessel, R. Org. Lett. 2006, 8, 4445–4448; Ziessel, R.; Goze, C.; Ulrich, G.
Synthesis 2007, 936–949.
21 (DMSO)
23 (H2O)
506
510
44,700
63,300
7.2
9.2
80
81
1.11
0.88
28
21
a
Quantum yields determined in dilute solution (c ꢃ 1.10–6 M) using Rhodamine
6G as reference (UF = 0.78 in water, kexc = 488 nm) or quinine sulfate (UF = 0.55 in
1 M H2SO4, kexc = 366 nm).27 All UF are corrected for changes in refractive index. kr
16. Harriman, A.; Ziessel, R. In Carbon-Rich Compounds; Haley, M. M., Tykwinski, R.
R., Eds.; Wiley-VCH: Weiheim, 2006; pp 26–89. and references cited therein.
17. Boyer, J. H.; Haag, A. M.; Sathyamoorthi, G.; Soong, M. L.; Thangaraj, K.
Heteroatom Chem. 1993, 4, 39–49.
and knr were calculated using the following equations: kr = UF
assuming that the emitting state is produced with unit quantum efficiency.
/
s
F, knr = (1 ꢁ UF)/sF
,
18. Thivierge, C.; Bandichhor, R.; Burgess, K. Org. Lett. 2007, 9, 2135–2138.
19. Li, L.; Han, J.; Nguyen, B.; Burgess, K. J. Org. Chem. 2008, 73, 1963–1970.
20. Li, L.; Nguyen, B.; Burgess, K. Bioorg. Med. Chem. Lett. 2008, 18, 3112–3116.
21. Roberts, J. C.; Gao, H.; Gopalsamy, A.; Kongsjahju, A.; Patch, R. J. Tetrahedron
Lett. 1997, 38, 355–358.
22. Jolicoeur, B.; Chapman, E. E.; Thompson, A.; Lubell, W. D. Tetrahedron 2006, 62,
11531–11563.
23. Kong, X.; Migneault, D.; Valade, I.; Wu, X.; Gervais, F. US Patent 2007/0010573
A1, 2007.
emission and a 60% quantum yield in ethanol. In fact, most of the
radiative rate constants of around 108 sꢁ1 are the same within
experimental error (Table 1). However, the radiative rate constants
are significantly weaker for the pyrene compounds 18 compared to
those of the others, mostly because of the weaker quantum yield,
which may be attributed to the presence of aggregates.
In short, we have established new synthetic routes for function-
alization of Bodipy dyes with sulfonate residues. The use of unusual
betaine building blocks carrying a terminal alkyne allows cross-
coupling reactions to be effective under mild operational condi-
tions. In some cases, a two-step synthesis is required to first graft
the 1-dimethylamino-2-propyne fragment either on the dipyrro-
methene core or on the boron and to then quaternize the tertiary
amine centre with 1,3-propanesultone. Interestingly, in all cases
the optical properties are retained with respect to the non-substi-
tuted Bodipys. This work opens the door for the engineering of fluo-
rescent dyes in aqueous medium and a vast range of new
molecules, some being highly hydrophilic, can now be envisaged.
By extension of these protocols, the engineering of amphoteric
dyes bearing large functionalities and which are potentially able
to be properly grafted on surfaces should also be possible. Work
along these lines is currently in progress.
24. The crystal data of 18: C24H23NO3, H2O, M = 423.51, orthorhombic, space group
Pbca, a = 7.517(2), b = 19.331(3), c = 28.791(4) Å, V = 4183.7(14) Å3, Z = 8,
Dc = 1.345 g cmꢁ3
yellow,
[Rint = 0.047], 3834 unique, wR2 = 0.141 for all data, conventional R = 0.051
[( max = 0.001] on F-values of 2404 reflections with I > 2 (I), S = 1.018 for all
,
l(Mo-K
a
) = 0.186 mmꢁ1, F(0 0 0) = 1792, prismatic crystal,
size = 0.60 ꢂ 0.30 ꢂ 0.10 mm,
21,734
reflections
measured
D
r)
r
data and 273 parameters. Unit cell determination and intensity data collection
(2h = 50.7°) were performed on a Enraf-Nonius KAPPA CCD diffractometer at
293(2) K. Structure solution by direct methods and refinement by full-matrix
least-squares on F2. Programs: COLLECT [Nonius, B. V. (1999)], HKL2000
[Otwinowski, Z.; Minor, W. 1997], SHELX97 [Sheldrick, G. M. 2008]. CCDC
[deposit No: 716693] contains the supplementary crystallographic data for this
Letter. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
25. (a) Robertson, J. M.; White, J. G. J. Chem. Soc. 1947, 358–368; (b) Ziessel, R.;
Goze, C.; Ulrich, G.; Césario, M.; Retailleau, P.; Harriman, A.; Rostron, J. P. Chem.
Eur. J. 2005, 11, 7366–7378.
26. Spackman, M. A.; Jayatilaka, D. CrystEngCommun 2009, 11, 19–32.
27. Olmsted, J., III J. Phys. Chem. 1979, 83, 2581–2584.