Dual Bodipy fluorophores linked by polyethyleneglycol spacers

Article abstract of DOI:10.1016/j.tetlet.2009.08.091

Several Bodipy dyes were synthesized with various substituents designed to potentially interact with adventurous cations including protons. Among the different synthetic strategies, protection of the amino group by a BOC appears efficient for the substitu

Full text of DOI:10.1016/j.tetlet.2009.08.091

Tetrahedron Letters 50 (2009) 6383–6388
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Dual Bodipy fluorophores linked by polyethyleneglycol spacers
Soumyaditya Mula a,b, Gilles Ulrich a, Raymond Ziessel a,
*
a Laboratoire de Chimie Organique et Spectroscopies Avancées (LCOSA), Ecole Chimie Polymères, Matériaux (ECPM), 25 rue Becquerel, 67087 Strasbourg Cedex, France
b Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 June 2009
Revised 31 July 2009
Accepted 26 August 2009
Available online 1 September 2009
Several Bodipy dyes were synthesized with various substituents designed to potentially interact with
adventurous cations including protons. Among the different synthetic strategies, protection of the amino
group by a BOC appears efficient for the substitution of the fluoro groups on the boron by Grignard
reagents. Single alkylation on the amino-Bodipy compounds by a polyethyleneglycol chain bearing an
alkyne end group is feasible using a biphasic strategy. Linkage of a blue dye on the alkyne fragment is
promoted by palladium(0) and it provides novel dual donor–acceptor dyes. Fluorescence of the mono-
alkylated amino dyes is heavily quenched but restored by protonation or interaction with Fe(II) salts.
Very efficient energy transfer from the donor (energy input at 20,000 cmꢀ1) to the acceptor (energy out-
put at 15,100 cmꢀ1) is quantitative and not distance dependent.
Keywords:
Bodipys’
Dual dyes
Amino groups
Polyethyleneglycol
Fluorescence
Sensors
Ó 2009 Elsevier Ltd. All rights reserved.
Linked fluorophores are interesting artificial models for mimick-
ing energy transfer processes, antenna effects, photon concentration
and charge separation occurring in photosynthetic organisms and
plants.1 There is also current interest in such molecules for use as
ratiometric dyes for sensing analytes,2 and in cascade energy trans-
fer events relatedto photonconcentrationand chemicaltransforma-
tions.3,4 The fundamental event involved in such systems is Förster
resonance energy transfer (FRET), which relies on judicious selection
of the absorption and emission properties of the dyes and the way
they are oriented and linked to the central structural platform.5 Re-
cently, several systems have been developed in which arrays of
sophisticated multi-chromophoric dyes ensure very fast multi-en-
ergy transfer processes, so that all absorbed photons are concen-
trated spatially to one single emitting dye.6–10
These systems are fascinating due to the synthetic challenges
associated with the need for selective cross-couplings, judicious
choice of the linking functions and careful purification of interme-
diates. We have found that the use of unsaturated linkages facili-
tates the synthesis of the building blocks as well as the
assemblage of the final molecules with shapes and functions re-
lated to molecular scale wires, dendrimers, tandem scaffoldings
and cassette edifices.11
Stokes shifts and (iii) the engendering of very efficient excitonic
energy transfer approaching the efficiency of natural systems.12,13
The concept exploited herein is to link two dyes, both deriva-
tives of difluoro-boradiaza-s-indacene, commonly termed Bodipy
(for boron dipyrromethene),12,13 having two different colours and
linked by a flexible bridge (polyethyleneglycol) in order to favour
FRET between the dyes, a process enhanced by folding the dyes
in the presence of cations. The system has been designed in such
a way that the dye emitting at higher energy (green emitter) is
fluorescent solely in the presence of proton or cations, due to the
fact that the fluorescence of amino-Bodipy is effectively quenched
by photoinduced electron transfer or more likely by a localized
charge transfer state. Our first choice was targeted on such deriv-
atives carrying suitable motifs for absorption and emission tun-
ings, with solubilizing groups attached on the boron to avoid
aggregation problems (Scheme 1).14
Compound 2 can be easily obtained from 1 by Pd-catalyzed
hydrogenation, as reported previously,15 but it requires 24 h to
complete the reaction (Scheme 1) which is disadvantageous when
the dye possesses a reducible group like a double bond, as was
found to be the case when starting with the distyryl derivative 7,
where we ended up with an inseparable mixture of compounds.
After some experimentation, we found that Pd-catalyzed hydroge-
nation in an ethanol/acetonitrile mixture at 70 °C was a good alter-
native as it took only 15 min to complete the reduction (Scheme 1).
In addition to its rapidity, in this method it is possible to synthesize
the N-ethylamino dye16 which is a very useful precursor for the
preparation of unsymmetrical tertiary amino-Bodipy dyes. With a
prolonged reaction time (24 h) under the same conditions,
The main results obtained so far and scrutinized in several re-
views concern: (i) the ratiometric sensing of cations and protons
in solution and cellular environments, (ii) the amplification of
* Corresponding author. Tel./fax: +33 3 68 85 2761.
E-mail address: ziessel@unistra.fr (R. Ziessel).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.091

6384
S. Mula et al. / Tetrahedron Letters 50 (2009) 6383–6388
Table 2
HN
NO2
NH2
Reaction conditions and synthetic outline
Reagent key (ii)
Time (h) Ratio 8 versus Ratio 9
H2, 10% Pd/C (10%), three drops of water,
EtOH/CH3CN (1:1), 70 °C
H2, 20% Pd/C (10%), three drops of water,
4
2
95:5
i)
N
F
N
F
N
F
N
B
F
N
F
N
B
F
0:100
B
EtOH/CH3CN (1:1), 70 °C
1
2
3
ii)
wavelength and improved solubility was obtained after N-Boc
cleavage by TFA (see Scheme 2).
95%
HN
O
O
HN
O
O
NH2
The nucleophilic character of the amino function is unfortu-
nately less reactive than related systems, such as p-nitrophenyl-
sulfonamide-protected amino groups.21 In our case, several
efforts to substitute the amino function in 6 and 12 were unsuc-
cessful. So we decided to keep the fluoride on the Boron centre
and to solve the solubility problem in a different fashion. Thus,
we tried substitution reactions at the amino centres of 2 and 8
using different, protected polyethyleneglycol-based substrates. Bi-
phasic reaction in water using NaHCO3 as the base was found to be
the best strategy (Scheme 3).22 Though the yield was moderate, the
reaction furnished exclusively the mono-N-substituted product 13
with a linker suitable for subsequent substitution. Unfortunately
this biphasic reaction of product 13 with the blue amino-deriva-
tives 8 or 12 was ineffective probably because of the weaker nucle-
ophilicity of the amino function in this delocalized dye.
In order to reach the target we chose to prepare an unsymmet-
rical linker bearing on one side a tosylate (activable with derivative
2) and on the other an ethynyl substituent able to be cross-linked
conveniently using routine palladium catalysis with the iodo-
substituted blue dye 15. The hybrid linkers were prepared from
the ditosylate of polyethylene glycol with propargylic alcohol un-
der basic conditions (Scheme 4).
iv)
iii)
58%
N
N
F
N
N
O
N
O
N
B
83%
B
B
F
4
O
O
O
O
O
O
6
5
Scheme 1. Reagents and conditions: (i) see Table 1; (ii) di-tert-butyldicarbonate,
dry THF, 60 °C, 2 days; (iii) CH3OCH2CH2OCH2C„CMgBr, dry THF, 60 °C, 12 h; (iv)
6% TFA, dry CH2Cl2, 2 h.
Table 1
Reaction conditions and synthetic outline
Reagents key (i)
Time
Ratio 2 versus
Ratio 3
Substitution of the TsO fragment by derivative 2 was then
straightforward, affording the green emitters 14a–c in acceptable
yields (Scheme 5). To explore the synthetic utility of compounds
14, we cross-coupled 4,4-difluoro-3,7-di-p-methoxystyryl-1,7-di-
methyl-8-p-iodophenyl-4-bora-3a,4a-diaza-s-indacene (15) via a
Sonogashira reaction promoted by Pd(0) to furnish 16a–c in satis-
factory yields (Scheme 5). Finally, we also succeeded in coupling
dye 14a with 1-bromopyrene 17 to get the linked derivative 18
absorbing at about 380 nm (Scheme 6).
H2, 10% Pd/C (10%), three drops of water,
CH2Cl2/EtOH (1:1), rt
H2, 10% Pd/C (10%), three drops of water,
EtOH/CH3CN (1:1), 70 °C
H2, 10% Pd/C (10%), three drops of water,
EtOH/ CH3CN (1:1), 70 °C
24 h
100:0
95:5
15 min
24 h
15:85
compound 3 was obtained in very good yield at the expense of 2
(Table 1).
Preliminary electrochemical measurements for 3, 14a, 15 and
16a are given in Table 3. For all these dyes, a single reversible
cathodic wave was observed due to the (Bodipy/BodipyÅꢀ) couple.
In the case of 15, the reduction was facilitated by ꢁ300 mV as com-
pared to dyes 3 and 14a. Anodically, an irreversible wave was ob-
served along with a reversible wave for (Bodipy/BodipyÅ+). The
irreversible wave for 14a was due to the oxidation of the secondary
amine centre and was also seen for 3, but not for the 8-amino
derivatives. Alkyl substitution made the amine centre easily oxi-
dizable. As expected, the anodic behaviour of 15 was more compli-
cated than that of the other dyes. Two extra waves were observed
at higher potential and were tentatively assigned to oxidation of
the styryl entity and a second oxidation of the Bodipy core.
Interestingly, all the characteristic anodic and cathodic waves
for both the precursor dyes 14a and 15 were found in the final
dye 16a. In particular, two reversible cathodic waves separated
by 290 mV were found for the (Bodipy/BodipyÅꢀ) couples for both
the dye cores. As expected, the anodic part was more complicated
due to the overlap of the oxidation waves of the amine and styryl
units. These data are in keeping with a dual dye where both chro-
mophores/luminophores interact weakly.
One of the most critical aspects of Bodipy dye chemistry is con-
trol of their solubility, which is very important for their large scale
preparation. The boron centre provides a convenient site to attach
a functional group without affecting the optical properties of the
chromophore.17 Furthermore, the use of 2,5-dioxaoctyne is a con-
venient fragment which imports solubility, polarity and process-
ability to the dye. It has been found that 8-(N-Boc)-anilino Bodipy
4, synthesized from 2 via the reaction with BOC anhydride (Scheme
1), is stable enough to react with an alkynyl Grignard18 to substi-
tute the fluoro groups on boron centre, and 5 was isolated in rea-
sonably good yield (58%). Acid cleavage of the N-Boc with 6% TFA
then furnished the highly soluble dye 6 functionalized at the boron
centre. By syntheses analogous to those of the yellow amino deriv-
atives 2 and 6, we prepared the blue amino analogues 8 and 12.
The distyryl compound 7 was synthesized from 1 by a Knoevenagel
condensation in good yield.19,20 The reduction of 7 using the previ-
ous conditions did produce the desired amino derivative 8, with no
ethylamino derivative being isolated, but a non-conjugated yellow
dye 9 was also isolated in 5% yield (Table 2) as a result of over-
reduction of the double bonds.
The amino derivatives 2 and 8 gave similar fluorescence quan-
tum yields (QY = 32–38%), whereas substitution by a BOC-protect-
ing group in 4 and 10 increased the QYs to 84% and 49%,
After BOC-protection of the amino group of 8, reaction at the
boron centre with the alkynyl Grignard furnished the soluble com-
pound 11. The final amino-Bodipy 12 with a different emission

S. Mula et al. / Tetrahedron Letters 50 (2009) 6383–6388
6385
NO2
NH2
NH2
i)
ii)
1
N
F
N
B
F
N
F
N
F
N
F
N
B
F
95%
B
MeO
OMe MeO
OMe MeO
OMe
9
7
8
O
O
N
iii)
HN
O
HN
O
NH2
67%
v)
iv)
N
B
F
N
F
N
N
O
N
B
96%
55%
B
O
O
O
MeO
OMe MeO
OMe MeO
OMe
10
O
O
O
O
12
11
Scheme 2. Reagents and conditions: (i) p-methoxybenzaldehyde, piperidine, toluene, reflux; (ii) H2, Pd/C (10%), three drops of water, EtOH/CH3CN (1:1), 70 °C; (iii) BOC
anhydride, dry THF, 60 °C, 2 days; (iv) CH3OCH2CH2OCH2C„CMgBr, dry THF, 60 °C, 12 h; (v) 6% TFA, dry CH2Cl2, 2 h.
O
O
HN
O
NH2
OTs
i) 30%
N
N
N
F
N
B
TsO
O
OTs
O
O
B
F
F
F
2
13
Scheme 3. Reagents and conditions: (i) NaHCO3, dodecyl sulfate, C2H4Cl2, H2O, 80 °C, 24 h.
i) 22-36%
TsO
TsO
O
O
O
O
OTs
O
n
n
n = 0, 1 and 2
n = 0, 1 and 2
Scheme 4. Reagents and conditions: (i) propargylic alcohol (1.2 equiv, t-BuOK, THF).
respectively. Alkylation of the amino function by alkyl fragments
as in 3 (with ethyl) and 14a–c (with polyethyleneglycol) resulted
in a dramatic decrease of the QY to 2–3%, while the excited state
lifetimes remained similar (Table 4). It is likely that the electron-
donating fragment (NHR) and the electron-accepting Bodipy favour
the emergence of a localized charge transfer state responsible for
the marked quenching of the fluorescence. This non-emissive state
might however favour fast energy transfer from the green emitter
to the blue acceptor. Interestingly, in the dual dyes 16a–c where
the absorption spectra are simple sums of the separate dye spectra
( Fig. 1), the electronic energy transfer is almost quantitative,
despite the weak fluorescence of the model derivatives 14a–c,
and is not dependent on the distance between the two dyes.
For the pyrene-linked dye 18, excitation in the pyrene residue at
362 nm resulted in very weak pyrene fluorescence at 393 nm
(<0.2%) but higher fluorescence of S1–S0 transition of the Bodipy
unit at 516 nm (Fig. 2). A large weak broad emission band centred
at 650 nm which could be attributed to a low lying charge transfer
emissive state is also observed.24 Protonation of the amino moiety
in 18 using gaseous HCl resulted in a QY increase from 2% to 40%,
whereas spicing the solution with Fe2+ salts resulted in an increase
to 33% (Fig. 3). Iron(II) has a strong affinity for amino groups and is
relatively oxophile and might consequently strongly interact with
the amino-polyethyleneglycol spacer.
From a mechanistic viewpoint, this is an interesting situation
where the fluorescence of the donor can be switched on by protons
or Fe2+ (selectively versus other metallic salts) and a classical FRET
is then effective, favoured by the spectral overlap between the
emission of the green emitter and the absorption of the blue dye.
In the absence of cations, it is possible that a charge transfer state
effectively favours EET without the need to channel through the
emission of the donor.

6386
S. Mula et al. / Tetrahedron Letters 50 (2009) 6383–6388
O
O
NH2
HN
O
n
i) 28-48%
14a X = 0
14b X = 1
14c X = 2
N
N
N
N
B
B
F
F
F
F
I
2
ii)
22-33%
OMe
N
F
N
B
F
15
MeO
OMe
F
N
B
F
N
OMe
O
O
HN
O
x
16a X = 0
16b X = 1
16c X = 2
N
N
B
F
F
Scheme 5. Reagents and conditions: (i) NaHCO3, dodecyl sulfate, corresponding tosylate, C2H4Cl2, H2O, 80 °C, 24 h; (ii) [Pd(PPh3)4], benzene, Et3N, 50 °C.
O
O
HN
O
HN
O
Br
i)
N
N
F
N
N
F
23%
B
B
F
F
18
17
14a
Scheme 6. Reagents and conditions: (i) [Pd(PPh3)4], benzene, (iPr)2NH, 70 °C.
In short, new borondipyrromethene dyes carrying amino-linked
derivatives have been developed and dual-Bodipy species have
been engineered displaying both an orange-red colour (with emis-
sion at 540 nm) and a blue colour (emission at 650 nm). The meth-
od established for the synthesis of new dyads in which orange and
blue dyes are linked via various polyethyleneglycol chains has pro-
vided families of compounds where, in one case, the blue dye has
been replaced by a pyrene residue. Fluorescence measurements re-
vealed quenching processes inherent to the presence of the amino
fragments and probably due to the presence of a charge transfer
state which might be responsible for the efficient energy transfer
between the emitting states of the orange and blue species. Inter-
actions with protons and cations have also been evaluated and
more detailed mechanistic studies are currently in progress.
Table 3
Properties of selected dyesa
Compd
E0ox V (
D
E, mV)
E0red, V (
DE, mV)
3
14a
15
+1.04 (irrev); +1.21 (90)
+0.98 (irrev); +1.12 (90)
+0.74 (60); +1.08 (70); +1.58 (irrev)
+0.72 (60); +0.98 (irrev); +1.11 (irrev); +1.22 (irrev); +1.57 (irrev)
ꢀ1.30 (70)
ꢀ1.27 (70)
ꢀ0.97 (60); ꢀ1.90 (irrev)
16a
ꢀ1.01 (70); ꢀ1.29 (74); ꢀ1.88 (irrev)
a
Potential determined by cyclic voltammetry in deoxygenated CH2Cl2 solutions, containing 0.1 M TBAPF6, at a solute concentration range 1–5 ꢂ 10ꢀ3 M, at 23 °C. Potentials
were standardized with ferrocene (Fc) as internal reference and converted to SSCE assuming that E1/2 (Fc=Fþc ) = 0.38 V (
DEp = 70 mV) SSCE. The error in half-wave potentials
was 10 mV, where the redox process was irreversible and the peak potential (Eap/cp) was quoted. All waves were monoelectronic unless otherwise specified.

S. Mula et al. / Tetrahedron Letters 50 (2009) 6383–6388
6387
Table 4
Optical properties of selected compounds at rta
c
c
Compd
kabs (nm)
emax (Mꢀ1 cmꢀ1
)
kem (nm)
UF (%)b
sem (ns) 0.01
kr (108 sꢀ1
)
knr (106 sꢀ1
)
2
3
4
5
6
8
500
499
501
499
500
641
368
504
642
369
642
369
640
367
500
500
500
646
500
372
646
501
372
646
500
371
500
362
80,000
86,000
86,000
81,000
79,000
102,000
69,000
99,000
99,000
62,000
90,000
52,000
68,000
41,000
103,000
98,000
74,000
120,000
86,000
80,000
112,000
77,000
74,000
107,000
73,000
71,000
77,000
42,000
510
513
510
507
509
656
38
2
84
85
34
32
3.14
3.00
3.32
3.27
2.98
5.75
1.21
0.07
2.53
2.60
0.80
0.56
197.4
326.7
48.2
15.3
221.5
118.3
9
10
514
658
38
49
3.04
5.67
1.25
0.86
203.9
89.9
11
12
656
659
54
40
5.49
5.79
0.98
0.69
83.8
103.6
14a
14b
14c
16a
515
515
514
668
514
3
3
3
39
1
3.12
2.95
3.00
7.01
0.01
0.10
0.10
0.56
310.9
328.8
323.3
87.0
16b
16c
18
668
514
38
0.7
5.97
5.81
4.46
0.64
0.72
0.04
103.8
99.8
662
512
42
0.8
516
393
2
0.2
220.6
a
b
c
Determined in anhydrous CH2Cl2 solution.
Determined using rhodamine 6G, as reference
Calculated using the following equations: kr = UF
U
= 0.78 in water, kexc = 488 nm [Ref. 23], all UF are corrected for changes in refractive index.
sF and knr = (1 ꢀ UF)/sF
/
.
Figure 3. Fluorescence spectra of 18 (blue), 18 + H+ (green) and 18 + Fe2+ (pink),
kexc = 480 nm in dichloromethane at rt. Acid was introduced as vapours and the iron
salt in a methanol solution.
Figure 1. Absorption (blue) and fluorescence (pink) spectra of 16a in dichloro-
methane at rt.
Acknowledgements
This work was partially supported by the CNRS providing re-
search facilities. Soumyaditya Mula is very grateful to the French
Embassy in India for the award of a Sandwich-Ph.D. Scholarship
and also to the Bhabha Atomic Research Centre for permission to
conduct research in France. We also warmly thank Professor Jack
Harrowfield (ISIS in Strasbourg) for commenting on the manuscript
before publication.
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