Oxidative coupling of 1,7,8-unsubstituted BODIPYs: Synthesis and electrochemical and spectroscopic properties

Article abstract of DOI:10.1021/jo301300b

We report the synthesis of BODIPYs with unsubstituted 1,7,8-positions and their dimerization by oxidative coupling with phenyliodine(III)- bis(trifluoroacetate) (PIFA). This dimerization was achieved for BODIPYs substituted in the 3,5-positions with either methyl or thienyl groups. The position and the type of the linkage in the resulting dimers depended on the nature of the substituent. The 3,5-dimethyl-BODIPY dyes were linked either via direct 1,1′-pyrrole-pyrrole coupling or via a 1,3′-methylene bridge. The 3,5-dithienyl-BODIPY dyes provided, in excellent yields, unique compounds linked exclusively via the α-thienyl positions. All dyes were unreactive in the 8-position. Electrochemical and spectroscopic measurements on the monomers and dimers provided evidence of interactions between the two halves of the dimers. Thus, oxidation and reduction potentials were split by up to 210 mV, and modest excitonic coupling and an internal charge transfer were observed in some cases.

Full text of DOI:10.1021/jo301300b

Article
pubs.acs.org/joc
Oxidative Coupling of 1,7,8-Unsubstituted BODIPYs: Synthesis and
Electrochemical and Spectroscopic Properties
Arnaud Poirel,† Antoinette De Nicola,†,* Pascal Retailleau,‡ and Raymond Ziessel*,†
†Laboratoire de Chimie Organique et Spectroscopies Avancee
́ ̀
s (LCOSA), UMR 7515 au CNRS, Ecole de Chimie, Polymeres et
́
Mater
́
iaux de Strasbourg (ECPM), 25 Rue Becquerel, 67087 Strasbourg, Cedex 02, France
27-1, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
‡Laboratoire de Crystallochimie, ICSN-CNRS, Bat
̂
S
* Supporting Information
ABSTRACT: We report the synthesis of BODIPYs with
unsubstituted 1,7,8-positions and their dimerization by
oxidative coupling with phenyliodine(III)-bis(trifluoroacetate)
(PIFA). This dimerization was achieved for BODIPYs
substituted in the 3,5-positions with either methyl or thienyl
groups. The position and the type of the linkage in the
resulting dimers depended on the nature of the substituent.
The 3,5-dimethyl-BODIPY dyes were linked either via direct
1,1′-pyrrole−pyrrole coupling or via a 1,3′-methylene bridge.
The 3,5-dithienyl-BODIPY dyes provided, in excellent yields,
unique compounds linked exclusively via the α-thienyl
positions. All dyes were unreactive in the 8-position.
Electrochemical and spectroscopic measurements on the
monomers and dimers provided evidence of interactions between the two halves of the dimers. Thus, oxidation and reduction
potentials were split by up to 210 mV, and modest excitonic coupling and an internal charge transfer were observed in some
cases.
of a hypervalent iodine reagent,17 such as PIFA, is one that
INTRODUCTION
■
enables direct production of BODIPY dimers without prior
functionalization of the BODIPY core. Few such dyes have
been prepared but some do display interesting properties linked
to charge delocalization and exciton coupling, providing
unusual fluorescence and redox properties. Recently studied
examples include α,α-linked BODIPY dimers,18 β,β-linked
BODIPY dimers,19 boron-butadiene-boron linked dimers20 and
cofacially arranged dibenzothiophene, dibenzofuran and 9,9-
dimethylxanthene bis-BODIPY21 dyes. The architecture of such
oligomers is of special importance for the electro-generated
chemiluminescence of thin films deposited on transparent
electrodes.19b
BODIPY oligomers are also of interest as models for
BODIPY-based conjugated polymers, in particular those linked
at the β,β-positions using triple bonds, to understand their
spectroscopic properties.22 While exciton coupling of the
subunits of β′,β′-linked BODIPY oligomers is clearly possible,
its occurrence has not yet been established.
There is increasing interest in the design, preparation and study
of task-specific fluorescent dyes that can be produced as very
pure materials using straightforward protocols.1,2 Many
research groups have sought pathways to a fluorescent probe
tailor-made in respect of its chemical and spectroscopic
properties. In this regard, 4,4′-difluoro-4-bora-3a,4a-diaza-s-
indacene (BODIPY) dyes have especially been of interest due
to their exceptional properties such as high absorption
coefficients and relatively sharp emission lines, high fluo-
rescence quantum yields, excellent chemical and photochemical
stability, high solubility and ease of functionalization.3,4 They
have found widespread applications as chemosensors,5 laser
dyes,6 fluorescent labels,7 active layers in photovoltaic cells,8
two-photon absorbing dyes dedicated to cell imaging,9 and
recently in solar concentrators.10 In some cases, substituents
have been introduced to render the dye sensitive to the
environment.11 Many synthetic protocols are available today to
introduce a reactive substituent at an appropriate position of
the indacene core to extend the conjugation and to optimize
the spectroscopic properties. Of the available protocols,
Knoevenagel condensation with tetramethyl-susbstituted BOD-
Increasing interest in fluorophore dimers has further been
due to their strong absorption and/or emission in the NIR
spectral region ranging from 700 to 1000 nm. For diverse
practical applications such as in solar cells, the materials should
have good light-harvesting capability not only in the visible
IPYs,12 Liebeskind-Srogl reactions,13 nucleophilic substitu-
̈
tions,14 palladium promoted cross-coupling reactions15 and
electrophilic attack16 on methyl-substituted BODIPYs are
particularly readily applied. Most rely on the direct introduction
of a specific group at the chosen substitution position. The use
Received: June 26, 2012
© XXXX American Chemical Society
A
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spectral range, but also in the NIR range, given that more than
50% of the energy in sunlight lies in the infrared region. In
contrast, biological samples have low background fluorescence
signals, and a concomitant high signal-to-noise ratio for
emission in this spectral region. Moreover, NIR light can
penetrate into sample matrices deeply due to low light
scattering. Many advanced technologies including high-contrast
bioimaging,23 optical recording,24 NIR laser filtering,25 NIR
photography,26 and solar cells27 could advantageously employ
NIR absorbing dyes. However, many commercially available
NIR dyes (e.g., cyanines and polyenes) suffer inherent
drawbacks of insufficient photostability and solubility.28 Thus,
soluble and stable organic NIR dyes are highly desirable
synthetic targets for organic chemists.
Here, we focus on the preparation of NIR-absorbing
BODIPY dimers using a hypervalent iodine reagent.
Phenyliodine(III)-bis(trifluoroacetate) (PIFA) has previously
been used to prepare bis-aryl compounds in the presence of a
Lewis acid or a fluoroalcohol as solvent,29 meso−meso-linked
linear arrays of porphyrins,30 and fused diporphyrin scaffold-
ings.31 Recently PIFA has been used to produce in a
straightforward manner biphenyl-fused BODIPYs32 through
an oxidative fusion reaction in the 3,5-positions. Similarly,
oxidation by FeCl3 has been used to produce perylene-fused
BODIPY33 and porphyrins34 with meso-substituents.
Scheme 1. Synthesis of Pyrroles 2a−c by the Trofimov
a
Reaction
a
Reagents and conditions: (i) BF3·Et2O (0.1 equiv), 90−115 °C, 30
min; (ii) (a) NH3OHCl, NaHCO3, DMSO, rt, overnight; (b)
acetylene, 100 °C, 30 min then KOH (1.5 equiv), 5 h.
Scheme 2. Synthesis of BODIPYs 3a−c with Triethyl
a
Orthoformate
a
Reagents and conditions: (i) HC(OEt)3 (15 equiv), TFA (6 equiv),
CH2Cl2, rt; (ii) Evaporation, Et3N (6 equiv), BF3·Et2O (8 equiv),
RESULTS AND DISCUSSION
■
CH2Cl2, rt, overnight.
Synthesis. Earlier work has shown that the oxidative
coupling reactions of 1,3,5,7,8-pentamethyl-substituted BODI-
PYs using either PIFA or Fe(III) salts proceed very rapidly at
the 2- and 6-substitution positions, leading to a variety of
oligomers displaying unusual redox properties.19a,b Presently, to
orientate the reaction in the 1-, 7- or 8-substitution positions of
BODIPY, all the other positions were blocked. To minimize
steric hindrance, methyl was chosen as the substituent for the
2,6-positions. Both methyl and thienyl derivatives were then
chosen as substituents for the 3,5-positions (Figure 1). Thienyl
substituents in those positions are known to give NIR BODIPY
emitters.15f
dipyrromethenes by condensation with triethyl orthoformate
followed a procedure established in porphyrin chemistry,40 only
one example being known for the preparation of BODIPY
dyes.41 This fast step was followed by in situ boron
complexation to afford the desired compounds. Dye 3a
(Scheme 2) was obtained in a typical yield of 41% but 3b in
an excellent yield of 79%. Compound 3c was very unstable on
silica and alumina, so that chromatographic purification resulted
in the very poor isolated yield of 8%.
To provide a comparison with oxidative coupling, at least
with that involving 1,7-linking, the use of 3a as a precursor of
substrates for Pd-catalyzed cross-coupling was investigated
(Scheme 3). Two pathways to dimerization were envisaged,
one involving the direct linkage of two BODIPYs and another
an acetylene bridge. While this work was in progress, others42
reported dibromination of the 1,7-positions, Pd-catalyzed cross-
coupling reactions and nucleophilic substitutions on chlori-
nated derivatives, while another study43 concerned polyiodi-
nated BODIPYs with some examples of 7-substitution.
Recently, regioselective bromination on boron dipyrrins has
also been explored.44
Our key compound was the monoiodinated derivative 4
(Scheme 3) obtained in good yield by using 1 equivalent of ICl
(no trace of 1,7-diiodinated product was detected). The
Sonogashira coupling reaction with TMS-acetylene was very
efficient and led to 5 in 84% yield but failed completely when
applied to the reaction between the deprotected compound 6
and the iodide 4. The directly linked dimer could be prepared
by Suzuki coupling between 4 and the pinacolboronate 7 in an
acceptable yield. But it has been very difficult to obtain 7 as it is
shown by the unsatisfactory yield of 16%. In comparison, the
oxidative coupling reaction of 3a using PIFA at room
temperature in presence of BF3·Et2O (Scheme 4) gave better
results. Dimer 8 was isolated by column chromatography in
Figure 1. General formula of BODIPY with its numbering scheme.
The Trofimov reaction35 allowed us to synthesize the
pyrroles needed for the preparation of the BODIPY dyes. It
can lead to various 2,3-disubstituted pyrroles depending upon
the structure of the starting ketones. Pyrroles 2a−c36 were
prepared according to this reaction using a one-pot literature
procedure (Scheme 1).37
Either the ketones 1a,38 commercially available, or 1b−c,
obtained by acylation of thiophene with propanoic anhydride,39
were condensed with hydroxylamine, and acetylene was fed
into the reaction mixture under atmospheric pressure. The
pyrroles were purified by column chromatography and isolated
in quite acceptable yields (Scheme 1).
The synthesis of BODIPYs 3a−c (Scheme 2) was conducted
in two steps. Conversion of pyrroles 2a−c into the intermediate
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a
Scheme 3. Synthesis of the Dimer 8 by a Pd-catalyzed Cross-coupling Reaction
a
Reagents and conditions: (i) ICl (1 equiv), MeOH/DMF (1:1), rt, 45 min; (ii) TMS-acetylene, [PdCl2(PPh3)2] (6 mol %), CuI (6 mol %),
iPrNH2, THF, rt, 1 h; (iii) K2CO3 (5 equiv), THF/H2O/MeOH (10:5:2), rt, 20 min; (iv) Bis(pinacolato)diboron (2 equiv), CH3CO2K (1.5 equiv),
[PdCl2(dppf)] (10%), dioxane, 90 °C, 24 h; (v) 6, [Pd(PPh3)4] (5%), Et3N, THF, 60 °C, overnight; (vi) 7, CH3CO2K (1.5 equiv), [PdCl2(dppf)]
(10%), dioxane, 90 °C, 24 h.
coupling reaction, possibly by detaching by cleavage of one
trifluoroacetoxy ligand of PIFA to generate a reactive cationic
iodine(III) intermediate.46 Oxidation of 3a could lead to two
electrophilic intermediates: a cation radical on the BODIPY
core and a more stable benzylic-like cation.47 Nucleophilic
attack of a second molecule of 3a could then produce both 8
and 9, 8 only appearing when the more reactive reagent was
used to create the higher energy intermediate. Where a methyl
group was not present in the 3-position, as in compound 3b, no
methylene-bridged product could be detected.
Scheme 4. Synthesis of the Dimers 8 and 9 by Oxidative
Coupling
a
When 3b was reacted with PIFA/BF3·Et2O, a precipitate
appeared and consumption of the starting BODIPY was
complete in 30 min. The poorly soluble solid product could
not be purified by column chromatography but only by
successive washings and recrystallization to give a shiny dark
blue powder in 78% yield.
a
Reagents and conditions: (i) PIFA, BF3·EtO, CH2Cl2, −78 °C to rt,
overnight.
32% yield but was accompanied by the less polar compound 9
in 13% yield. This was characterized as a different dimer of 3a
in which the 1-position of one BODIPY was linked through a
methylene bridge to the 3-position of another. In the absence
of BF3·Et2O, only 9 was obtained, then in 27% isolated yield, a
result of interest in regard to the reaction mechanism.45
Thus, it is known that Lewis acids such as BF3·Et2O can be
used to increase the efficiency of PIFA-mediated oxidative
This compound exhibited spectral data [IR, NMR (1H, 11B,
13C, COSY, HSQC, HMBC, MS] and elemental analysis data
1
consistent with the structure 12 (Scheme 5). The H and 11B
NMR spectra were consistent with a symmetrical dimer
although the aromatic proton signals were poorly resolved
(Figure S41, Supporting Information). The COSY (Figure 2)
a
Scheme 5. Synthesis of the Dimers 12 and 13 by Oxidative Coupling
a
Reagents and conditions: (i) NBS (1 equiv), CH2Cl2/DMF (1:1), rt, 40 min; (ii) 3,4,5-tridodecyloxyphenylboronic acid pinacol ester, [Pd(PPh3)4]
(10%), Cs2CO3 (3 equiv), toluene, 60 °C, 18 h; (iii) PIFA, BF3·Et2O, CH2Cl2, rt, 30 min.
C
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Figure 2. COSY (solid red line) and HMBC (dotted red line)
1
correlations in compound 12, and H NMR chemical shifts in ppm.
correlations brought to light two major facts: no substituents in
the 1,7 and 8-positions and two different thienyl groups in the
molecule, one monosubstituted and the second one disub-
stituted. The COSY spectrum showed couplings between the
protons at 7.88, 7.71, and 7.25 ppm, at 7.70 and 7.57 ppm, at
7.22 and 2.26 ppm and at 7.24 and 2.20 ppm. The HMBC
correlations (Figure 2) between protons removed various
ambiguities, namely the couplings between the proton at 2.20
ppm and the carbon at 149.4 ppm, the proton at 7.22 ppm and
the carbon at 13.3 ppm, the proton at 7.24 ppm and the carbon
at 13.2 ppm, the proton at 7.25 ppm and the carbon at 130.6
ppm, the proton at 7.63 ppm and the carbon at 130.4 ppm and
the proton at 7.71 ppm and the carbon at 149.4 ppm. These are
consistent with a dimer structure involving linkage through the
thienyl units and, except for protons a and b (7.70/7.57 ppm),
all of the signals have been assigned (Figure 2).
Figure 3. ORTEP view of dye 3a. Displacements are drawn at the 30%
probability level.
again observed, as for the two other crystal structures.
Molecules 3a are organized in layers stacking along the [0 0
1] direction (Figure 4). Within each layer, all the molecules are
To increase the solubility of this dimer, BODIPYs substituted
by butyl groups (3c, Scheme 2) and pyrogallate (11, Scheme 5)
groups were synthesized. Bromination (Scheme 5) of 3b was
used to introduce the pyrogallate moiety by a Pd-catalyzed
cross-coupling reaction. A single monobrominated compound
was obtained with 1 equivalent of NBS. This electrophilic
aromatic substitution took place in the α-position of the
thiophene, not the 1-position of the BODIPY, in a good yield
of 66%, to afford 10. The Suzuki-Miyaura reaction of 10 with 1
equiv of 3,4,5-tridodecyloxyphenylboronic acid pinacol ester48
led to the highly soluble dye 11 in good yield. Attempts to
oxidatively couple 3c and of 11 were carried out under our
standard conditions (PIFA, BF3·Et2O, rt) but no pure
compound could be isolated from the product mixture, despite
the fact that the dimer of 3b was obtained (Scheme 5) in a yield
of 89% under the same conditions. As for 12, the poorly soluble
product was purified by successive washings and recrystalliza-
tion. This compound exhibited spectral data [IR, NMR (1H,
11B, 13C, COSY, HSQC, HMBC, MS] and elemental analysis
data consistent with the molecular structure 13, which was
unequivocally established by a single-crystal X-ray structure
determination (vide infra).
X-Ray Molecular Structures. The X-ray crystal structures
of the three compounds 3a, 3b and 13 were determined. Dye
3a, which consists of a F-BODIPY core tetra-substituted by
methyl groups, crystallizes in the orthorhombic space group,
Pccn (no 56), and lies on a crystallographic 2-fold rotational axis
that bisects the six-membered central ring along the direction
given by the C7, B1 atoms (Figure 3). The BODIPY core (12
atoms) is almost perfectly planar with a root-mean-square
(rms) deviation less than 0.01 Å. The B−N and B−F bond
lengths are respectively 1.554(3) and 1.388(2) Å and the
average N−B−N, F−B−F, and N−B−F angles are 107.4(2),
108.5(2), and 110.2(8)°, respectively. The pronounced double
bond character for the C4−N1 bond [1.358(3) Å] in contrast
with the longer C1−N1 bond lengths [1.390(3) Å] is once
Figure 4. View perpendicular to the [1 3 0] plane of the 3a crystal
packing.
aligned parallel either to [1 −3 0] or [1 3 0], with a pyrrole
group facing the second one in adjacent position along [0 1 0]
with a centroid-centroid distance of 3.79 Å.
Substitution of methyl groups attached to C4 and C5 atoms
by thienyl groups gave compound 3b, which crystallized in the
noncentrosymmetric orthorhombic space group, Pna21 (no
33). The molecules present a thienyl-ring flip disorder, with a
major-minor orientation ratio refined to values of 0.604(7):
0.396(7) and 0.630(7): 0.370(7), respectively. The dihedral
angles between the thienyl rings and the BODIPY core are
respectively 48 and 38° (Figure 5). The thienyl group disorder
likely contributes to the slight distortion of the BODIPY core
despite the overall low rms deviation of 0.06 Å. The average B−
N and B−F bond lengths are respectively 1.553(4) and
1.382(4) Å and the average N−B−N, F−B−F angles are
108.8(2), 109.9(3)°, whereas N−B−F angle values range from
107.8(3) to 111.3 (3)°.
The thienyl substitution in 3b is associated with the absence
of π−π stacking interactions between molecules, which are
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approximately 60° angle, leaving the sulfur atoms on opposite
sides with respect to the molecular plane (Figure 7).
Figure 5. ORTEP view of dye 3b. Displacements are drawn at the 30%
probability level.
Figure 7. ORTEP view of dye 13. Displacements are drawn at the 30%
probability level.
seemingly linked by weak C−H···Cg (π−ring) hydrogen bonds
The molecules 13 lie within infinite sheets parallel to the (2 0
1) plane, with an interplanar distance of ca. 4 Å indicating π−π
interactions (Figure 8). Within the sheets, the molecules are
regularly aligned with the crystallographic 2-fold rotational axis,
the concavity of the molecule delineated by the thienyl
substituents leaving room for cylindrical solvent channels
running along the c axis (Figure 9).
and also C−H···F bonds (2.356 Å for the shortest one) (Figure
6).
Optical Properties. Spectroscopic data relevant to the
present discussion are collected in Table 1 and typical spectra
are given in Figures S53 to S59 (Supporting Information) for
dyes 3a−c, 4−7. All these monomeric dyes exhibit absorption
and emission patterns typical for BODIPY fluorophores,49 with
profiles and excited state lifetimes characteristic of singlet
emitters. In particular the absorption spectrum shows a strong
S0→S1 transition (of π−π* nature), located at 537 nm for dye
3a (Figure 10) which is shifted to lower energy for dye 3b (λabs
= 597 nm, Figure S54), highlighting the increase of conjugation
by increasing the electronic density in the 3,5-substition
positions. Such beneficial effects have previously been seen in
thiophene-grafted BODIPY scaffoldings.15f The quantum yield
remained very high for dye 3b (78%) despite the notable
decrease in energy of the main transition. Furthermore, for all
dyes the absorption spectrum shows a broader absorption at
higher energy with a lower extinction coefficient (around 10000
M−1 cm−1), safely assigned in light of previous work to the S0→
S2 transition (also of π−π* nature).49 In the case of the thienyl
substituted derivatives, additional transitions at 319 and 301 nm
are assigned to the π−π* transition of the thienyl subunits.
Both the fluorescence quantum yields and the excited-state
lifetimes (nanosecond regime) recorded for all monomeric dyes
are independent of excitation wavelength. In addition, the time-
resolved fluorescence decay profile remains monoexponential
under all conditions. The Stokes shifts (ΔSS = 240 cm−1) are
very slight for the compound 3a but greater for the thienyl
based BODIPYs 3b, 3c, 10 and 11 (ΔSS = 1229−1498 cm−1,
Table 1).
Figure 6. View down the b axis of the 3b crystal packing. Dotted lines
denote nonconventional H-bonds. Color code: molecules in general
position (gray), related by a 21 axis (green), by a glide mirror c
(magenta).
Finally, dye 13 is a dimer of the previous molecule 3b by
introducing a 1.45 Å C−C bond between two thienyl groups.
These two groups lie basically in the same plane made by the
two BODIPY cores and the S atoms face one another, 3.09 Å
apart. The dimer crystallized in the monoclinic space group,
C2/c (no 15) with 2-fold rotational crystallographic axis that
bisects the C−C bond. The second pair of (bromo)-thienyl
rings is tilted outward from the molecular platform by an
On substitution of the proton in 3a by ethynyl groups in 5
and 6 or boronate in 7, the absorption and emission profiles,
quantum yields and excited state lifetimes remained essentially
the same (Table 1). In the case of dye 4, the presence of an
iodo unit directly linked to the BODIPY core markedly
decreases the quantum yield and the excited state lifetime due
E
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Figure 8. View down the b axis of the dye 13 crystal packing.
Figure 9. View of a 2-D sheet of dye 13 crystal packing. Columnar voids filled by diffuse solvent are running along the c axis.
to the heavy atom effect which favors intersystem crossing to
the triplet state.50 This provides an additional deactivation
channel which causes a decrease of photoluminescence
efficiency. This effect was much weaker in case of dye 10
with the bromo group attached to the thienyl residue. The
quantum yield for dye 10 was 70% with respect to dye 3b
(78%) and the excited state lifetimes remained similar within
experimental error. The substitution of the bromo groups by an
electron rich pyrogallate platform in dye 11 shifted the
emission wavelength to 692 nm (compared to 654 nm for
dye 10) whereas the quantum yield and excited state lifetime
decreased by a factor of about 2 (see Table 1). Another point of
interest is that for all monomeric dyes the absorption and
emission profiles were not sensitive to the solvent polarity and
that excitation spectra perfectly matched the absorption spectra,
in keeping with the absence of aggregates (Figure 10). Finally,
for the tetramethyl-BODIPY dyes and derivatives the
fluorescence spectra showed good mirror symmetry with the
lowest energy absorption transition, confirming that these
transitions are due to the same weakly polarized excited state.
This is not true for spectra of the thienyl-substituted dyes,
which did not show mirror symmetry and had large Stokes
shifts. We cannot at the present stage exclude some charge
transfer from the electron rich thienyl subunits to the BODIPY
core in its excited state.
For the dimers, the situation is not as would be expected for
two close but independent chromophores. In the first series
constructed from the tetramethyl-BODIPY core, two different
situations were found (Figure 10). For dimer 8, the absorption
and emission profiles resembled that of the monomer 3a with a
set of vibronic bands extending to higher energies (spacing
1154 cm−1), indicating minimal ground-state interaction or
excitonic coupling between the two individual chromophores.51
The emission spectrum was also similar to that of the
F
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Table 1. Selected Spectroscopic Data for the Monomers and the Dimers
a
a
dye
λabs (nm)
ε (M−1cm−1
)
λem (nm)
ΦF
τF (ns)
kr (108 s−1
)
knr (108 s−1
)
solvent
3a
538
537
606
597
619
539
553
550
552
560
557
559
555
553
550
611
603
627
687
678
672
701
683
682
66000
76000
58000
58000
61000
93000
97000
96000
89000
69000
80000
70000
144000
139000
136000
57000
59000
58000
41000
54000
36000
58000
60000
52000
545
544
649
647
670
546
559
557
560
593
598
0.93
0.87
0.81
0.78
0.66
0.10
0.80
0.83
0.85
0.67
0.05
11.0
11.8
9.7
0.8
0.7
0.8
0.8
0.7
0.1
0.8
0.8
0.7
0.1
0.3
0.06
0.1
0.2
0.2
0.3
1.0
0.2
0.2
0.1
0.6
4.9
Toluene
CH2Cl2
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
CH2Cl2
DMSO
Toluene
CH2Cl2
DMSO
Toluene
CH2Cl2
CH2Cl2
Toluene
CH2Cl2
DMSO
Toluene
CH2Cl2
DMSO
3b
9.2
3c
4
9.5
0.9
5
10.1
11.1
12.1
5.3
6
7
8
1.9
9
563
562
561
657
654
692
796
790
0.36
0.01
4.9
0.3
0.1
9.2
8.6
4.2
1.1
0.7
0.3
1.3
0.8
0.8
1.0
0.1
1.3
33.0
124.0
0.3
<0.01
0.73
10
0.70
0.3
11
12
0.41
1.4
0.01
8.9
<0.01
13
805
801
0.06
0.03
1.1
0.7
0.6
0.4
8.9
14.6
a
Calculated using the following equations: kr = ΦF/τF, knr = (1 − ΦF)/τF, assuming that the emitting state is produced with unit quantum efficiency.
monomer. However, both the quantum yield and excited state
lifetime were significantly reduced compared to those of the
monomer 3a (Table 1). Furthermore, the photoluminescence
quantum efficiencies varied dramatically with the polarity of the
solvent being 67% in toluene, 5% in dichloromethane and 0%
in DMSO, although the shape of the absorption and emission
spectra did not significantly vary with the solvent polarity
(Figure S60−61, Supporting Information). Such behavior has
been previously observed in meso-linked BODIPY dimers and it
was hypothesized that a nonemissive charge-transfer state
entailing some degree of symmetry breaking is responsible for
the emission loss in polar solvents.52 The possibility that dimer
8 could undergo symmetry-breaking internal charge transfer
(ICT) was further assessed by cyclic voltammetry (vide infra).
The dimer 9 showed significantly different behavior
compared to 8 (Figure 10c). The absorption spectrum
indicated some excitonic coupling between the two BODIPY
subunits and the absorption was split into two well-defined
bands at 555 and 509 nm. The model compound 3a showed a
single absorption at 538 nm (in toluene). Dimer 9 exhibited a
fluorescence maximum at 563 nm, independent of the
excitation wavelength, with a quantum yield of 36% and a
single decay lifetime of 4.94 ns (Table 1). The shape of the
absorption spectrum but not that of the fluorescence band
depended on the solvent polarity (Figure S62−63, Supporting
Information). However, the photoluminescence efficiency
dramatically decreased with increasing solvent polarity,
indicating that a symmetry-breaking ICT was also effective in
this case. The presence of a flexible methylene bridge in dimer
9 probably favors a modest excitonic coupling, as indicated by
the splitting of the main absorption band into two overlapping
bands rather than one which would be expected for an
uncoupled dimer in which the low lying ICT is the main
channel for deactivation of the excited state. This analysis is
supported by the observations on the series of dimers based on
bis-thiophene linkers. Dimers 12 and 13 also exhibited a
splitting of the absorption band due to a modest excitonic
coupling between both BODIPY moieties, whereas a single
emission was found at 796 and 805 nm, respectively, in toluene
(Figure 11). In both cases the photoluminescence efficiency
remained low at 1 and 6%, respectively, for dimers 12 and 13,
while the excited state lifetimes were much shorter than those
of the related monomer analogues: 1 ns versus about 9 ns for
dyes 3b and 10. BODIPY dimers directly linked at the α- or β-
positions do not exhibit such a nonradiative symmetry-breaking
ICT.18a,19a,b However, dimers with an α,α-coupling exhibit very
efficient excitonic coupling.18
Electrochemical Properties. Cyclic voltammograms were
recorded in dichloromethane using tetrabutylammonium
hexafluorophosphate as supporting electrolyte (electrochemical
window from +1.9 to −2.2 V) and the ferricinium/ferrocene
couple as internal reference and the results are given in Table 2.
Those of monomer 3a and dimer 8 are shown in Figure 12a. A
reversible cathodic wave was observed at +1.02 V (ΔEp = 70
mV), corresponding to a monoelectronic exchange and is
attributed to the oxidation of the BODIPY core to its radical
cation (BOD/BOD•+).53 A second monoelectronic, irreversible
reduction wave at −1.21 V is attributed to the formation of the
BODIPY radical anion (BODIPY/BODIPY•‑). For the dimer 8,
the oxidation waves appeared as two overlapping signals
separated by 80 mV. Furthermore, two successive reduction
waves separated by 210 mV were also clearly observed (Figure
12a). The first reduction is facilitated by 170 mV compared to
the monomer 3a. These results clearly reflect the breaking of
the symmetry during the oxidation and the reduction of the
dimer 8. The fact that the splitting of the wave is more effective
in reduction might reflect of better delocalization of the charge
over the BODIPY cores. Within the same family of dyes the
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Figure 11. (a) Absorption (plain trace), emission (dashed trace) and
excitation (dotted trace) spectra for the monomer 3b (blue trace) and
dimer 12 (red trace). (b) Absorption (plain trace), emission (dashed
trace) and excitation (dotted trace) spectra for the monomer 10 (blue
trace) and dimer 13 (red trace). All spectra in toluene at rt with c = 2.1
to 12 × 10−6 M for the absorption, c = 9.6 × 10−7 to 2.1 × 10−5 M for
the emission and c = 4.8 × 10−7 to 2.1 × 10−6 M for the excitation.
Table 2. Solution Electrochemical Properties of the
a
Monomeric and Dimeric Dyes
compounds
Eox, V (ΔE, mV)
+1.02 (70)
Ered, V (ΔE, mV)
−1.21 (irr.)
−0.90 (irr.)
−1.04 (60); −1.25 (60)
−1.17 (irr.); −1.35 (70)
−0.88 (irr.)
Figure 10. Absorption (blue trace), emission (red trace) and
excitation (blue dashed trace) spectra for dyes at rt in toluene at c =
8 to 18 × 10−6 M for the absorption, c = 0.8 to 1.8 × 10−6 M for the
emission and c = 4.4 × 10−8 to 5.3 × 10−6 M for the excitation.
3a
3b
8
+0.93 (70)
+1.06 (60); +1.14 (60)
+1.02 (70); +1.22 (60)
+0.95 (75)
9
10
11
12
methylene bridged dimer 9 also displayed a pronounced
splitting of both cathodic and anodic waves (Figure 12b). In
this case, the cathodic waves were separated by 200 mV and the
anodic waves by 180 mV. This is a remarkable result which
highlights the electronic interaction between the two parts of
the molecules and the breaking of symmetry as hypothesized
above. Worth noting is the fact that in the second family of
dimers based on thienyl modules exhibited the same splitting of
the cathodic and anodic waves (Figure 12c). However, the
splitting was less pronounced than in the previous cases, with
differences of 90 mV in oxidation and 80 mV in reduction. The
nature and size of a dithienyl bridge versus that of a direct
linkage or a methylene bridge makes the electronic interaction
weaker.
+0.82 (70); +1.07 (70)
−0.93 (irr.)
−0.58 (irr.); −0.83 (irr.)
+0.62 (irr.); +0.80 (40);
+0.95 (70)
13
+0.82 (50); +0.91 (60)
−0.78 (irr.); −0.86 (irr.)
a
Potentials determined by cyclic voltammetry in deoxygenated
CH2Cl2 solutions, containing 0.1 M TBAPF6, at a solute concentration
range of 1 mM and at rt. Potentials are given versus the saturated
calomel electrode (SCE) and standardized vs ferrocene (Fc) as
internal reference assuming that E1/2 (Fc/Fc+) = +0.38 V (ΔEp = 70
mV) vs SCE. The error in half-wave potentials is 15 mV. Where the
redox processes are irreversible, the peak potentials (Eap or Ecp) are
quoted. All reversible redox steps result from one-electron processes
unless otherwise specified.
obtained, one directly linked through the BODIPY 1-positions
and the second one with a methylene bridge. In the latter case,
one methyl of one partner and the pyrrole of the second
partner are used to form the linkage. In the thiophene cases,
thienyl−thienyl coupling is preferred and allows the prepara-
tion of dimers in excellent yields. The dimers exhibit a marked
splitting of the oxidation and reduction waves as great as 210
CONCLUSION
■
In conclusion, we have designed and synthesized a series of
novel monomeric and dimeric BODIPY dyes constructed from
2,3,5,6-substituted cores. The nature of the substituent drives
the dimerization process when PIFA is the oxidant. With
methyl groups in 3,5-positions, two types of dimers are
H
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spectrometer using BF3·Et2O as reference. FT-IR spectra were
recorded using a spectrometer equipped with an ATR “diamond”
apparatus. Chromatographic purification was conducted using 40−63
μm silica gel or aluminum oxide 90 standardized. Thin layer
chromatography (TLC) was performed on silica gel or aluminum
oxide plates coated with fluorescent indicator. All mixtures of solvents
are given in v/v ratio. All anhydrous reactions were carried out under
dry argon by using Schlenk tube techniques.
Spectroscopic Measurements. UV−visible spectra were re-
corded using a dual-beam grating spectrophotometer with a 1 cm
quartz cell. All fluorescence spectra were corrected. The fluorescence
quantum yield (Φcmp) was calculated from eq 1:
n2
OD
OD
I
ref
Φ
= Φ
cmp
ref
nr2ef
I
(1)
ref
Here, I denotes the integral of the corrected emission spectrum, OD
is the optical density at the excitation wavelength and n is the refractive
index of the medium. The reference systems used were Rhodamine 6G
(Φem = 0.78 in H2O) and Cresyl violet (Φem = 0.53 in CH3OH).54
Luminescence lifetimes were measured on a spectrofluorimeter
using software with Time-Correlated Single Photon Mode coupled to
a stroboscopic system. The excitation source was a laser diode (λ =
310 nm). No filter was used for the excitation. The instrument
response function was determined by using a light-scattering solution
(LUDOX).
Electrochemistry. Electrochemical studies employed cyclic
voltammetry with a conventional 3-electrode system using a
voltammetric analyzer equipped with a platinum micro disk (2
mm2) working electrode and a silver wire counter electrode.
Ferrocene was used as an internal standard and was calibrated against
a saturated calomel reference electrode solution (SSCE) separated
from the electrolysis cell by a glass frit presoaked with electrolyte
solution. Solutions contained the electroactive substrate in deoxy-
genated and anhydrous CH2Cl2 containing doubly crystallized tetra-n-
butylammonium hexafluorophosphate (0.1 M) as supporting electro-
lyte. The electrochemical cell was flushed with anhydrous nitrogen gas.
Materials. All chemicals were used as received from commercial
sources without further purification. CH2Cl2 was distilled over P2O5
and THF was distilled over sodium and benzophenone under an argon
atmosphere. [Pd(PPh3)4],55 [PdCl2(PPh3)2],56 [PdCl2(dppf)],57 1-
(thien-2-yl)propan-1-on,58 2,3-dimethyl-1H-pyrrole,59 3-methyl-2-
(thien-2-yl)-1H-pyrrole60 and 4,4,5,5-tetramethyl-2-[3,4,5-tris-
(dodecyloxy)phenyl]-1,3,2-dioxaborolane61 were synthesized accord-
ing to the literature procedures. N-Bromosuccinimide was recrystal-
lized from hot water.
Figure 12. Cyclic voltammograms of (a) monomer 3a (red trace) and
dimer 8 (blue trace), (b) 3a (red trace) and dimer 9 (blue trace), (c)
monomer 10 (red trace) and dimer 13 (blue trace). Concentration of
dyes 1.5 × 10−3 M in CH2Cl2 (0.1 M, nBu4NPF6), Fc+/Fc refers to the
ferricinium/ferrocene couple used as internal reference E1/2 (Fc+/Fc)
= +0.38 V (ΔEp = 60 mV).
Representative Procedure for the Synthesis of Ketones 1b−
c. 1-(Thien-2-yl)propan-1-on (1b). In a two-necked flask equipped
with a reflux condenser to a mixture of propionic anhydride (37.2 mL,
290 mmol, 1.16 equiv) and thiophene (20.0 mL, 250 mmol, 1 equiv)
was added BF3·Et2O (3.1 mL, 25 mmol, 0.1 equiv) at rt. The
temperature rose rapidly to 90−115 °C upon the addition of the
catalyst. After 30 min the mixture had cooled to rt and water was
added. The reaction mixture was extracted with Et2O. The combined
extracts were washed with a saturated solution of NaHCO3 and dried
over MgSO4. The residue was distilled under vacuum (110 °C at 22
mmHg). The product 1b was obtained as a colorless oil in 66% yield
and 180 mV in the best cases, highlighting a pronounced
electronic interaction and symmetry breaking during electron
exchange processes. For the dimers, modest excitonic coupling
is observed in apolar solvents and the photoluminescence
efficiency is dramatically sensitive to the solvent polarity. The
absence of emission in DMSO is likely due to an internal
charge transfer state likely formed by symmetry breaking in the
excited state. These results should be extremely useful for the
further design of relevant fluorescent compounds absorbing in
the red and NIR with tunable emission properties. Current
work is focused on enhancing the fluorescence properties by
restricting the rotation of the thienyl rings.
1
3
(23.10 g): H NMR (300 MHz, CDCl3, ppm): δ = 7.70 (dd, J = 3.8
Hz, 4J = 1.1 Hz, 1H), 7.61 (dd, 3J = 5.0 Hz, 4J = 1.1 Hz, 1H), 7.12 (dd,
3J = 5.0 Hz, J = 3.8 Hz, 1H), 2.94 (q, J = 7.4 Hz, 2H), 1.23 (t, J =
7.4 Hz, 3H); 13C NMR (75 MHz, CDCl3, ppm): δ = 193.8, 144.2,
133.1, 131.5, 128.0, 32.6, 8.5.
3
3
3
1-(5-Butylthien-2-yl)propan-1-one (1c). Colorless oil. Yield 82%.
1H NMR (300 MHz, CDCl3, ppm): δ = 7.53 (d, 3J = 3.8 Hz, 1H), 6.79
EXPERIMENTAL SECTION
■
3
3
3
General Methods. 1H and 13C spectra were recorded at rt on 200,
(d, J = 3.8 Hz, 1H), 2.86 (q, J = 7.3 Hz, 2H), 2.84 (q, J = 7.6 Hz,
2H), 1.67 (m, 2H), 1.38 (m, 2H), 1.21 (t, 3J = 7.4 Hz, 3H), 0.93 (t, 3J
= 7.3 Hz, 3H); 13C NMR (75 MHz, CDCl3, ppm): δ = 193.6, 155.3,
141.5, 131.9, 125.4, 33.4, 32.1, 30.3, 22.1, 13.7, 8.7; IR (ν, cm−1): 2958,
2934, 2873, 1659, 1533, 1455, 1417; EI-MS m/z (nature of the peak,
300, and 400 MHz spectrometers using perdeuterated solvents as
internal standards. Chemical shifts of H and 13C spectra are given in
1
ppm relative to residual protiated solvent and relative to the solvent
respectively. 11B spectra were recorded at rt on a 400 MHz
I
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relative intensity): 196.1 ([M]+, 100); Anal. Calcd for C11H16OS: C,
67.30; H, 8.22. Found: C, 67.54; H, 8.42.
127.6, 124.8, 13.5; 11B NMR (128 MHz, CDCl3, ppm): δ = 1.24 (t, 3J
= 32.9 Hz); UV−vis (CH2Cl2) λ nm (ε, M−1cm−1): 597 (60000), 383
(9000), 319 (17000), 301 (17000), 232 (19000); IR (ν, cm−1): 3099,
3068, 2916, 2856, 1601, 1515, 1451, 1435, 1397, 1345, 1278; EI-MS
m/z (nature of the peak, relative intensity): 384.0 ([M]+, 100); Anal.
Calcd for C19H15BF2N2S2: C, 59.39; H, 3.93; N, 7.29. Found: C,
59.21; H, 3.77; N, 7.02.
Representative Procedure for the Synthesis of Pyrroles 2a−
c. 2,3-Dimethyl-1H-pyrrole (2a). To a solution of hydroxylamine
hydrochloride (1.92 g, 27.7 mmol, 1 equiv) in DMSO (55 mL) were
added NaHCO3 (2.33 g, 27.7 mmol, 1 equiv) and the ketone 1a (2.48
mL, 27.7 mmol, 1 equiv). The mixture was allowed to stand for a
night. Then the mixture was heated to 100−110 °C and acetylene was
fed upon stirring for 30 min. After addition of potassium hydroxide
(2.33 g, 41.6 mmol, 1.5 equiv), acetylene feeding was continued at the
same temperature for 5 h at a rate of ∼15 cm3min−1. Then the mixture
was cooled down, diluted with water and extracted with Et2O. The
extracts were washed with water and dried over MgSO4. The residue
was purified by column chromatography (Al2O3, petroleum ether/
Et2O 75:25). The product 2a was obtained as a brown oil in 59% yield
(1.57 g): 1H NMR (200 MHz, CDCl3, ppm): δ = 7.72 (brs, 1H), 6.59
(t, JH1H4H5 = 2.7 Hz, 1H), 6.00 (t, JH1H4H5 = 2.7 Hz, 1H), 2.19 (s, 3H),
2.04 (s, 3H); 13C NMR (75 MHz, CDCl3, ppm): δ = 123.6, 114.8,
114.1, 110.0, 10.9, 10.8.
3,5-[Di-(5-butylthien-2-yl)]-4,4′-difluoro-2,6-dimethyl-4-bora-
3a,4a-diaza-s-indacene (3c). Purification by column chromatography
(SiO2, petroleum ether/CH2Cl2 70:30) and recrystallization. Pink
1
metallic needles. Yield 8%. mp 260−263 °C; H NMR (300 MHz,
CDCl3, ppm): δ = 7.66 (d, 3J = 3.8 Hz, 2H), 6.90 (s, 1H), 6.83 (d, 3J =
3.8 Hz, 2H), 6.79 (s, 2H), 2.86 (t, 3J = 7.6 Hz, 4H), 2.24 (s, 6H), 1.71
(m, 4H), 1.45 (m, 4H), 0.95 (t, 3J = 7.3 Hz, 6H); 13C NMR (75 MHz,
CDCl3, ppm): δ = 150.1, 149.8, 134.8, 132.0 (t, JC−F = 6.3 Hz), 130.00,
129.95, 129.1, 125.1, 123.4, 33.5, 29.9, 22.4, 13.84, 13.81; 11B NMR
(128 MHz, CDCl3, ppm): δ = 1.35 (t, 1J = 32.9 Hz); UV−vis
(CH2Cl2) λ nm (ε, M−1cm−1): 619 (61000), 396 (10000), 336
(19000), 307 (17000), 232 (20000); IR (ν, cm−1): 3082, 2954, 2926,
2871, 2855, 1594, 1552, 1493, 1465, 1453, 1427, 1399, 1337, 1317; EI-
MS m/z (nature of the peak, relative intensity): 496.2 ([M]+, 100);
Anal. Calcd for C27H31BF2N2S2: C, 65.32; H, 6.29; N, 5.64. Found: C,
65.07; H, 5.89; N, 5.38.
3-Methyl-2-(thien-2-yl)-1H-pyrrole (2b). Purification by column
chromatography (Al2O3, petroleum ether/Et2O 80:20). Brown oil.
Yield 52% (3.63 g). 1H NMR (200 MHz, CDCl3, ppm): δ = 8.14 (brs,
1H), 7.21 (dd, 3J = 5.1 Hz, 4J = 1.3 Hz, 1H), 7.06 (dd, 3J = 5.1 Hz, 3J =
3.5 Hz, 1H), 7.00 (dd, 3J = 3.5 Hz, 4J = 1.3 Hz, 1H), 6.73 (t, JH1H4H5
=
4,4′-Difluoro-1-iodo-2,3,5,6-tetramethyl-4-bora-3a,4a-diaza-s-
indacene (4). To a solution of 3a (100 mg, 0.40 mmol, 1 equiv) in
MeOH/DMF (20 mL, 1:1) was added dropwise a solution of ICl (71
mg, 0.44 mmol, 1 equiv) in MeOH (5 mL) at rt. The reaction was
monitored by TLC inspection. After 45 min the reaction mixture was
diluted with CH2Cl2, washed with a saturated solution of Na2S2O3,
abundantly with water and extracted with CH2Cl2. The organic layer
was dried over MgSO4. The solvent was removed under vacuum. The
residue was purified by column chromatography (SiO2, petroleum
ether/CH2Cl2 60:40) and recrystallized from CH2Cl2/EtOH. The
product 4 was obtained as green metallic crystals in 66% yield (99
2.7 Hz, 1H), 6.13 (t, JH1H4H5 = 2.7 Hz, 1H), 2.28 (s, 3H); 13C NMR
(50 MHz, CDCl3, ppm): δ = 141.6, 135.9, 127.4, 122.8, 121.8, 117.4,
116.9, 112.0, 12.4.
2-(5-Butylthien-2-yl)-3-methyl-1H-pyrrole (2c). Purification by
column chromatography (Al2O3, petroleum ether). Yellow oil. Yield
30% (2.21 g). 1H NMR (300 MHz, CDCl3, ppm): δ = 8.04 (brs, 1H),
6.79 (d, 3J = 3.5 Hz, 1H), 6.71 (d, 3J = 3.5 Hz, 1H), 6.69 (t, JH1H4H5
=
2.8 Hz, 1H), 6.10 (t, JH1H4H5 = 2.8 Hz, 1H), 2.81 (t, 3J = 7.9 Hz, 2H),
3
2.26 (s, 3H), 1.68 (m, 2H), 1.42 (m, 2H), 0.95 (t, J = 7.4 Hz, 3H);
13C NMR (75 MHz, CDCl3, ppm): δ = 143.6, 133.3, 124.3, 123.3,
1
mg): mp 196−198 °C; H NMR (300 MHz, CDCl3, ppm): δ = 6.90
121.6, 116.9, 116.3, 111.9, 33.8, 29.7, 22.2, 13.8, 12.4. This compound
was quite unstable even in the freezer and has been used right after.
Representative Procedure for the Synthesis of BODIPY 3a−
c. 4,4′-Difluoro-2,3,5,6-tetramethyl-4-bora-3a,4a-diaza-s-indacene
(3a). To a solution of pyrrole 2a (800 mg, 8.4 mmol, 2 equiv) in dry
CH2Cl2 (100 mL) were successively added triethyl orthoformate (10.5
mL, 63.1 mmol, 15 equiv) and trifluoroacetic acid (1.9 mL, 25.2 mmol,
6 equiv). The mixture was stirred at rt for 45 min, concentrated and
the excess of triethyl orthoformate was removed under vacuum. The
residue was resolubilized in dry CH2Cl2 (40 mL) and triethylamine
(3.5 mL, 25.2 mmol, 6 equiv) was added. After 10 min of stirring,
BF3·Et2O (4.2 mL, 33.6 mmol, 8 equiv) was added. The solution was
stirred at rt for a night, then poured into a saturated solution of
NaHCO3 and stirred further for 1 h (2 × 50 mL). The reaction
mixture was washed with water, with brine, dried over MgSO4, filtered
and evaporated. The residue was purified by column chromatography
(SiO2, petroleum ether/CH2Cl2 60:40) and the product 3a was
recrystallized from CH2Cl2/EtOH to afford green metallic crystals in
(s, 1H), 6.75 (s, 1H), 2.55 (s, 3H), 2.50 (s, 3H), 2.04 (s, 3H), 2.02 (s,
3H); 13C NMR (100 MHz, CDCl3, ppm): δ = 158.9, 153.4, 134.2,
134.0, 131.1, 129.5, 129.1, 124.4, 92.4, 13.2, 12.9, 12.7, 11.2; 11B NMR
(128 MHz, CDCl3, ppm): δ = 0.79 (t, 1J = 32.9 Hz); UV−vis
(CH2Cl2) λ nm (ε, M−1cm−1): 539 (93000), 512 (36000), 376
(8000), 231 (19000); IR (ν, cm−1): 2918, 2851, 1591, 1464, 1406,
1371, 1340; EI-MS m/z (nature of the peak, relative intensity): 374.0
([M]+, 100), 247.0 ([M − I]+, 30); Anal. Calcd for C13H14BF2IN2: C,
41.75; H, 3.77; N, 7.49. Found: C, 41.62; H, 3.37; N, 7.31.
4,4′-Difluoro-2,3,5,6-tetramethyl-1-(trimethylsilylethynyl)-4-
bora-3a,4a-diaza-s-indacene (5). To a solution of 4 (100 mg, 0.27
mmol, 1 equiv) in a mixture of THF (15 mL) and diisopropylamine
(2.4 mL) were added PdCl2(PPh3)2 (11 mg, 0.016 mmol, 0.06 equiv)
and CuI (3 mg, 0.016 mmol, 0.06 equiv). The solution was degassed
for 30 min. Ethynyltrimethylsilane (53 mg, 0.53 mmol, 2 equiv) was
then added. The solution was stirred at rt and monitored by TLC.
After 1 h the solvent was removed under vacuum. The residue was
treated with water, extracted with CH2Cl2 and dried over MgSO4. The
solvent was removed under vacuum. The residue was purified by
column chromatography (SiO2, petroleum ether/CH2Cl2 60:40) and
recrystallized from CH2Cl2/EtOH. The product 5 was obtained as
1
41% yield (429 mg): mp 230−232 °C; H NMR (200 MHz, CDCl3,
ppm): δ = 6.83 (s, 1H), 6.65 (s, 2H), 2.51 (s, 6H), 2.02 (s, 6H); 13C
NMR (50 MHz, CDCl3, ppm): δ = 156.3, 133.0, 127.9, 124.4, 30.9,
12.7 (t, JC−F = 2.3 Hz), 11.0; 11B NMR (128 MHz, CDCl3, ppm): δ =
1
1
0.85 (t, J = 33.3 Hz); UV−vis (CH2Cl2) λ nm (ε, M−1cm−1): 537
green metallic crystals in 94% yield (87 mg): mp 202−205 °C; H
(77000), 510 (32000), 359 (8000), 232 (15000); IR (ν, cm−1): 3076,
2926, 2864, 1612, 1596, 1462, 1409, 1367; EI-MS m/z (nature of the
peak, relative intensity): 248.0 ([M]+, 100), 229.0 ([M − F]+, 30);
Anal. Calcd for C13H15BF2N2: C, 62.94; H, 6.09; N, 11.29. Found: C,
62.64; H, 5.85; N, 10.96.
NMR (300 MHz, CDCl3, ppm): δ = 7.01 (s, 1H), 6.75 (s, 1H), 2.53
(s, 3H), 2.47 (s, 3H), 2.05 (s, 3H), 2.04 (s, 3H), 0.28 (s, 9H); 13C
NMR (75 MHz, CDCl3, ppm): δ = 158.8, 153.3, 134.6, 133.9, 129.6,
129.5, 129.4, 129.0, 122.0, 104.9, 97.1, 12.9, 12.5, 11.2, 10.0, 0.1; 11B
1
NMR (128 MHz, CDCl3, ppm): δ = 0.74 (t, J = 33.9 Hz); UV−vis
4,4′-Difluoro-2,6-dimethyl-3,5-(dithien-2-yl)-4-bora-3a,4a-diaza-
s-indacene (3b). Purification by column chromatography (SiO2,
petroleum ether/CH2Cl2 60:40) and recrystallization. Blue metallic
(CH2Cl2) λ nm (ε, M−1cm−1): 553 (97000), 523 (35000), 404
(8000), 223 (19000); IR (ν, cm−1): 2953, 2919, 2856, 2148, 1597,
1465, 1414, 1392; EI-MS m/z (nature of the peak, relative intensity):
344.1 ([M]+, 100), 325.1 ([M − F]+, 20); Anal. Calcd for
C18H23BF2N2Si: C, 62.80; H, 6.73; N, 8.14. Found: C, 62.72; H,
6.59; N, 8.09.
1
crystals. Yield 79%. mp 203−206 °C; H NMR (300 MHz, CDCl3,
3
4
3
ppm): δ = 7.76 (dd, J = 3.8 Hz, J = 1.0 Hz, 2H), 7.51 (dd, J = 5.2
Hz, 4J = 1.0 Hz, 2H), 7.15 (dd, 3J = 3.8 Hz, 3J = 5.2 Hz, 2H), 7.00 (s,
1H), 6.85 (s, 2H), 2.22 (s, 6H); 13C NMR (75 MHz, CDCl3, ppm): δ
= 150.0, 134.9, 132.3, 131.8 (t, JC−F = 5.8 Hz), 130.4, 129.4, 128.8,
1-Ethynyl-4,4′-difluoro-2,3,5,6-tetramethyl-4-bora-3a,4a-diaza-
s-indacene (6). To a solution of 5 (50 mg, 0.15 mmol, 1 equiv) in
J
dx.doi.org/10.1021/jo301300b | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
THF (10 mL) was added a solution of K2CO3 (104 mg, 0.75 mmol, 5
equiv) in water (5 mL) and MeOH (2 mL). The mixture was stirred
for 20 min at rt. The solvent was removed under vacuum. The residue
was treated with water and extracted with CH2Cl2. The organic layer
was washed with water, with brine and dried over MgSO4. The solvent
was removed under vacuum. The residue was purified by column
chromatography (SiO2, petroleum ether/CH2Cl2 60:40) and recrystal-
lized from CH2Cl2/EtOH. The product 6 was obtained as green
− 2F]+, 10); Anal. Calcd for C26H28B2F4N4: C, 63.20; H, 5.71; N,
11.34. Found: C, 62.84; H, 5.54; N, 11.09.
4,4′-Difluoro-5-(4,4′-difluoro-2,3,5,6-tetramethyl-4-bora-3a,4a-
diaza-s-indacen-1-yl)methyl-2,3,6-trimethyl-4-bora-3a,4a-diaza-s-
indacene (9). 3a (200 mg, 0.80 mmol, 1 equiv) was dissolved in dry
CH2Cl2 (20 mL) in a Schlenk flask. PIFA (172 mg, 0.40 mmol, 0.5
equiv) was added and the mixture was stirred at rt. The reaction was
monitored by TLC. When the consumption of 3a was stabilized (1 h),
NaBH4 (14 mg, 0.40 mmol, 0.5 equiv) was added. After 15 min the
reaction mixture was poured into water and extracted with CH2Cl2.
The organic layer was dried over MgSO4. The solvent was removed
under vacuum. The residue was purified by column chromatography
(SiO2, petroleum ether/CH2Cl2 60:40 to 40:60) and recrystallized
from CH2Cl2/EtOH. The product 9 was obtained as a red powder in
1
metallic crystals in 71% yield (29 mg): mp 192−193 °C; H NMR
(200 MHz, CDCl3, ppm): δ = 7.04 (s, 1H), 6.74 (s, 1H), 3.51 (s, 1H),
2.53 (s, 3H), 2.48 (s, 3H), 2.07 (s, 3H), 2.04 (s, 3H); 13C NMR (75
MHz, CDCl3, ppm): δ = 159.5, 152.8, 134.6, 134.0, 129.8, 129.7,
129.3, 122.1, 120.4, 86.2, 76.2, 13.0, 12.5, 11.2, 9.9; 11B NMR (128
1
MHz, CDCl3, ppm): δ = 0.74 (t, J = 31.8 Hz); UV−vis (CH2Cl2) λ
1
nm (ε, M−1cm−1): 550 (96000), 522 (37000), 395 (8000), 223
(21000); IR (ν, cm−1): 3266, 3058, 2925, 2857, 1608, 1560, 1502,
1465, 1416, 1387; EI-MS m/z (nature of the peak, relative intensity):
272.1 ([M]+, 100); Anal. Calcd for C15H15BF2N2: C, 66.21; H, 5.56;
N, 10.30. Found: C, 66.04; H, 5.29; N, 10.19.
27% yield (52 mg): mp >260 °C dec.; H NMR (200 MHz, CDCl3,
ppm): δ = 6.91 (s, 1H), 6.85 (s, 1H), 6.75 (s, 1H), 6.63 (s, 1H), 6.58
(s, 1H), 4.28 (s, 2H), 2.54 (s, 3H), 2.51 (s, 3H), 2.48 (s, 3H), 2.06 (s,
3H), 1.98 (s, 3H), 1.91 (s, 3H), 1.70 (s, 3H); 13C NMR (75 MHz,
CDCl3, ppm): δ = 158.3, 155.9, 155.2, 154.5, 136.6, 133.6, 132.8,
132.7, 132.3, 129.2, 128.8, 128.7, 128.2, 128.0, 127.4, 126.2, 125.1,
122.6, 24.1, 12.88, 12.86, 12.6, 11.4, 11.1, 11.0, 9.0; 11B NMR (128
MHz, CDCl3, ppm): δ = 0.97 (t, 1J = 33.9 Hz), 0.85 (t, 1J = 33.9 Hz);
UV−vis (CH2Cl2) λ nm (ε, M−1cm−1): 553 (143000), 507 (82000),
362 (16000); IR (ν, cm−1): 2924, 2856, 1602, 1464, 1410, 1377; EI-
MS m/z (nature of the peak, relative intensity): 494.1 ([M]+, 100),
456.1 ([M − 2F]+, 10); Anal. Calcd for C26H28B2F4N4: C, 63.20; H,
5.71; N, 11.34. Found: C, 62.82; H, 5.54; N, 11.24.
4,4′-Difluoro-2,3,5,6-tetramethyl-1-(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolan-2-yl)-4-bora-3a,4a-diaza-s-indacene (7). 4 (50 mg, 0.13
mmol, 1 equiv), bis(pinacolato)diboron (66 mg, 0.26 mmol, 2 equiv),
potassium acetate (20 mg, 0.20 mmol, 1.5 equiv) and PdCl2(dppf) (10
mg, 0.013 mmol, 0.1 equiv) were dissolved in dioxane (5 mL) and
heated at 90 °C for 24 h. The reaction mixture was diluted with
CH2Cl2, washed with water, and extracted with CH2Cl2. The organic
layer was dried over MgSO4. The solvent was removed under vacuum.
The residue was purified by column chromatography (SiO2, petroleum
ether/CH2Cl2 60:40) and recrystallized from CH2Cl2/EtOH. The
product 7 was obtained as green metallic crystals in 16% yield (8 mg):
3-(5-Bromothien-2-yl)-4,4′-difluoro-2,6-dimethyl-5-(thien-2-yl)-4-
bora-3a,4a-diaza-s-indacene (10). To a solution of 3b (500 mg, 1.30
mmol, 1 equiv) in CH2Cl2/DMF (100 mL, 1:1) was added N-
bromosuccinimide (231 mg, 1.30 mmol, 1 equiv) at rt. The reaction
was monitored by TLC. After 40 min the reaction mixture was washed
with water (5 × 150 mL), extracted with CH2Cl2 and dried over
MgSO4. The solvent was removed under vacuum. The residue was
purified by column chromatography (SiO2, petroleum ether/CH2Cl2
70:30 to 60:40) and recrystallized from CH2Cl2/EtOH. The product
10 was obtained as blue metallic crystals in 66% yield (399 mg): mp
1
mp 231−234 °C; H NMR (300 MHz, CDCl3, ppm): δ = 7.46 (s,
1H), 6.75 (s, 1H), 2.52 (s, 3H), 2.50 (s, 3H), 2.19 (s, 3H), 2.03 (s,
3H), 1.33 (s, 12H); 13C NMR (75 MHz, CDCl3, ppm): δ = 157.3,
154.8, 138.3, 136.0, 134.0, 128.8, 128.43, 128.41, 126.0, 83.2, 24.9,
12.8, 12.3, 11.2, 11.1; 11B NMR (128 MHz, CDCl3, ppm): δ = 0.87 (t,
1J = 31.8 Hz); UV−vis (CH2Cl2) λ nm (ε, M−1cm−1): 552 (89000),
365 (9000), 233 (12000); IR (ν, cm−1): 2980, 2922, 2856, 1600, 1521,
1461, 1406; EI-MS m/z (nature of the peak, relative intensity): 374.1
([M]+, 100); Anal. Calcd for C19H26B2F2N2O2: C, 61.01; H, 7.01; N,
7.49. Found: C, 60.77; H, 6.89; N, 7.22.
1
3
186−187 °C; H NMR (200 MHz, CDCl3, ppm): δ = 7.80 (dd, J =
3.8 Hz, 4J = 1.1 Hz, 1H), 7.54 (dd, 3J = 5.1 Hz, 4J = 1.1 Hz, 1H), 7.45
(d, 3J = 4.0 Hz, 1H), 7.18 (dd, 3J = 5.1 Hz, 3J = 3.8 Hz, 1H), 7.10 (d, 3J
= 4.0 Hz, 1H), 7.00 (s, 1H), 6.86 (s, 1H), 6.81 (s, 1H), 2.23 (s, 3H),
2.19 (s, 3H); 13C NMR (75 MHz, CDCl3, ppm): δ = 150.9, 147.6,
135.2, 134.7, 134.0, 132.0 (t, JC−F = 6.3 Hz), 131.7 (t, JC−F = 5.5 Hz),
130.9, 130.8, 130.5, 130.0, 129.9, 129.2, 129.0, 127.7, 125.0, 116.1,
1,1′-Bi-(4,4′-difluoro-2,3,5,6-tetramethyl-4-bora-3a,4a-diaza-s-
indacene) (8). Suzuki coupling procedure: 4 (25 mg, 0.07 mmol, 1
equiv), potassium acetate (10 mg, 0.10 mmol, 1.5 equiv), 7 (25 mg,
0.07 mmol, 1 equiv) and PdCl2(dppf) (5 mg, 0.007 mmol, 0.1 equiv)
were dissolved in dioxane (5 mL) in a Schlenk flask and heated at 90
°C for 24 h. The reaction mixture was diluted with CH2Cl2, washed
with water, and extracted with CH2Cl2. The organic layer was dried
over MgSO4. The solvent was removed under vacuum. The residue
was purified by column chromatography (SiO2, petroleum ether/
CH2Cl2 60:40), then recrystallized from CH2Cl2/cyclohexane. The
product 8 was obtained as a red powder in 43% yield (15 mg).
Oxidative coupling procedure: 3a (200 mg, 0.80 mmol, 1 equiv) was
dissolved in dry CH2Cl2 (20 mL) in a Schlenk flask. The solution was
cooled to −78 °C and PIFA (172 mg, 0.40 mmol, 0.5 equiv), then
BF3·Et2O (0.1 mL, 0.80 mmol, 1 equiv) were added. The solution was
allowed to rise at rt during the night. After 18 h the mixture was
poured into water and extracted with CH2Cl2. The organic layer was
dried over MgSO4. The solvent was removed under vacuum. The
residue was purified by column chromatography (SiO2, petroleum
ether/CH2Cl2 60:40 to 40:60) and recrystallized from CH2Cl2/EtOH.
The product 8 was obtained as a red powder in 32% yield (63 mg):
1
13.6, 13.3; 11B NMR (128 MHz, CDCl3, ppm): δ = 1.18 (t, J = 32.9
Hz); UV−vis (CH2Cl2) λ nm (ε, M−1cm−1): 603 (59000), 395
(10000), 327 (20000), 304 (17000), 233 (21000); IR (ν, cm−1): 3094,
2957, 2920, 2851, 1599, 1515, 1452, 1432, 1410, 1394, 1339; EI-MS
m/z (nature of the peak, relative intensity): 463.9 ([M, 81Br]+, 95),
461.9 ([M, 79Br]+, 100), 383.1 ([M − Br]+, 35); Anal. Calcd for
C19H14BBrF2N2S2: C, 49.27; H, 3.05; N, 6.05. Found: C, 48.88; H,
2.72; N, 5.82.
4,4′-Difluoro-2,6-dimethyl-5-(thien-2-yl)-3-{[5-(3,4,5-
tridodecyloxy)phen-1-yl]thien-2-yl}-4-bora-3a,4a-diaza-s-indacene
(11). 10 (31 mg, 0.07 mmol, 1 equiv), Cs2CO3 (65 mg, 0.20 mmol, 3
equiv) and 4,4,5,5-tetramethyl-2-[3,4,5-tris(dodecyloxy)phenyl]-1,3,2-
dioxaborolane (50 mg, 0.07 mmol, 1 equiv) were dissolved in toluene
(4 mL) in a Schlenk flask and degassed for 30 min. Then Pd(PPh3)4
(8 mg, 0.0066 mmol, 0.1 equiv) was added and the solution was
heated to 60 °C. The reaction was monitored by TLC inspection.
After 18 h the reaction was stopped by addition of water (30 mL),
extracted with CH2Cl2 and dried over MgSO4. The solvent was
removed under vacuum. The residue was purified by column
chromatography (SiO2, petroleum ether/CH2Cl2 70:30 to 60:40).
The product 11 was obtained as blue-green amorphous solid in 68%
1
mp >260 °C dec.; H NMR (300 MHz, CDCl3, ppm): δ = 6.66 (s,
2H), 6.62 (s, 2H), 2.58 (s, 6H), 2.55 (s, 6H), 2.03 (s, 6H), 1.92 (s,
6H); 13C NMR (75 MHz, CDCl3, ppm): δ = 157.6, 155.0, 133.7,
132.9, 132.5, 128.8, 128.6, 126.5, 124.0, 12.83, 12.82, 11.1, 10.1; 11B
1
3
yield (48 mg): H NMR (300 MHz, CDCl3, ppm): δ = 7.85 (d, J =
1
3.6 Hz, 1H), 7.77 (d, 3J = 3.6 Hz, 1H), 7.51 (d, 3J = 5.0 Hz, 1H), 7.26
(d, J = 3.8 Hz, 1H), 7.16 (dd, J = 5.0 Hz, J = 3.8 Hz, 1H), 6.95 (s,
1H), 6.83 (s, 2H), 6.81 (s, 2H), 4.01 (m, 6H), 2.30 (s, 3H), 2.22 (s,
3H), 1.77 (m, 6H), 1.50 (m, 6H), 1.28 (m, 48H), 0.89 (t, 3J = 6.8 Hz,
NMR (128 MHz, CDCl3, ppm): δ = 0.97 (t, J = 33.9 Hz); UV−vis
(CH2Cl2) λ nm (ε, M−1cm−1): 557 (80000), 523 (57000), 371
(12000); IR (ν, cm−1): 2926, 2863, 1590, 1555, 1463, 1406; EI-MS m/
z (nature of the peak, relative intensity): 494.1 ([M]+, 100), 456.1 ([M
3
3
3
K
dx.doi.org/10.1021/jo301300b | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
9H); 13C NMR (75 MHz, CDCl3, ppm): δ = 153.4, 149.7, 149.3,
148.4, 138.7, 135.2, 134.8, 133.2, 132.4, 131.6, 131.2, 130.4, 130.1,
129.7, 129.1, 129.0, 128.7, 127.5, 124.1, 123.8, 105.5, 73.5, 69.3, 31.92,
31.91, 30.3, 29.74, 29.73, 29.69, 29.64, 29.61, 29.42, 29.38, 29.36, 26.1,
22.7, 14.1, 13.9, 13.4; 11B NMR (128 MHz, CDCl3, ppm): δ = 1.33 (t,
1J = 31.8 Hz); UV−vis (CH2Cl2) λ nm (ε, M−1cm−1): 627 (59000),
ACKNOWLEDGMENTS
■
We acknowledge the CNRS providing research facilities and
financial support, the Ministere de l’Enseignement Superieur et
̀
́
de la Recherche for a MENRT fellowship for AP. We also thank
Dr G. Ulrich for helpful and fruitful discussions and Professor
Jack Harrowfield from ISIS in Strasbourg for reading the
manuscript before publication.
349 (20000), 308 (22000), 232 (30000); IR (ν, cm−1): 2918, 2851,
1603, 1548, 1524, 1455, 1395; EI-MS m/z (nature of the peak, relative
intensity): 1012.4 ([M]+, 100); Anal. Calcd for C61H91BF2N2O3S2: C,
72.30; H, 9.05; N, 2.76. Found: C, 72.12; H, 8.84; N, 2.42.
REFERENCES
■
Representative Procedure for the Synthesis of Dimers 12
and 13 of BODIPY. 5,5′-Di-[4,4′-difluoro-2,6-dimethyl-3-(thien-2-
yl)-4-bora-3a,4a-diaza-s-indacen-5-yl]-2,2′-bithiophene (12). To a
solution of BODIPY 3b (50 mg, 0.14 mmol, 1 equiv) in dry CH2Cl2
(10 mL) in a Schlenk flask, PIFA (36 mg, 0.08 mmol, 0.6 equiv) and
BF3·Et2O (24 mg, 0.17 mmol, 1.2 equiv) were added. The reaction
mixture was stirred at rt for 30 min, then diluted with petroleum ether
(10 mL) and centrifugated. The solid was washed with petroleum
ether (10 mL), water (10 mL), EtOH (10 mL), Et2O (5 mL), pentane
(5 mL), then recrystallized from THF/EtOH. The product 12 was
obtained as a blue metallic powder in 78% yield (42 mg): mp >260 °C
dec.; 1H NMR (300 MHz, DMSO-d6, ppm): δ = 7.88 (dd, 3J = 5.0 Hz,
4J = 1.0 Hz, 2H), 7.71 (dd, 3J = 3.6 Hz, 4J = 1.0 Hz, 2H), 7.70 (d, 3J =
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Erten-Ela, S.; Altay, Y.; Brendel, J.; Thelakkat, M.; Akkaya, E. U. Org.
Lett. 2010, 12, 3812−3815. (c) Rousseau, T.; Cravino, A.; Roncali, J.;
Bura, T.; Ulrich, G.; Ziessel, R. Chem. Commun. 2009, 1673−1675.
(d) Kumaresan, D.; Thummel, R. P.; Bura, T.; Ulrich, G.; Ziessel, R.
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Ripaud, E.; Leriche, P.; Rihn, S.; De Nicola, A.; Ziessel, R.; Roncali, J.
Chem. Commun. 2010, 46, 5082−5084.
4.1 Hz, 2H), 7.63 (s, 2H), 7.56 (d, 3J = 4.1 Hz, 2H), 7.25 (dd, 3J = 5.0
Hz, 3J = 3.6 Hz, 2H), 7.23 (s, 2H), 7.21 (s, 2H), 2.25 (s, 6H), 2.20 (s,
6H); 13C NMR (500 MHz, DMSO-d6, ppm): δ = 149.5, 147.0, 139.1,
134.9, 134.7, 133.0 (t, JC−F = 4.9 Hz), 131.8 (t, JC−F = 4.9 Hz), 131.6,
131.5, 130.7, 130.6, 130.4, 130.3, 130.0, 127.7, 126.2, 125.6, 13.3, 13.2;
̈
1
11B NMR (128 MHz,, ppm): δ = 1.12 (t, J = 33.9 Hz); UV−vis
(CH2Cl2) λ nm (ε, M−1cm−1): 679 (55000), 597 (53000), 458
(10000), 368 (24000), 305 (20000); IR (ν, cm−1): 3112, 3053, 2962,
2922, 2865, 1591, 1521, 1501, 1435, 1394; EI-MS m/z (nature of the
peak, relative intensity): 766.1 ([M]+, 100); Anal. Calcd for
C38H28B2F4N4S4: C, 59.54; H, 3.68; N, 7.31. Found: C, 59.37; H,
3.44; N, 7.08.
5,5′-Di-[4,4′-difluoro-2,6-dimethyl-3-(5-bromothien-2-yl)-4-bora-
3a,4a-diaza-s-indacen-5-yl]-2,2′-bithiophene (13). Blue metallic
1
crystals. Yield 89%. Mp 283−285 °C; H NMR (300 MHz, DMSO-
d6, ppm): δ = 7.76 (d, 3J = 4.1 Hz, 2H), 7.67 (s, 2H), 7.63 (d, 3J = 4.1
Hz, 2H), 7.46 (d, 3J = 4.0 Hz, 2H), 7.38 (d, 3J = 4.0 Hz, 2H), 7.27 (s,
2H), 7.22 (s, 2H), 2.28 (s, 6H), 2.17 (s, 6H); 13C NMR (500 MHz,
DMSO-d6, ppm): δ = 148.2, 146.8, 139.5, 135.2, 134.7, 133.4 (t, JC−F
=
(9) Didier, P.; Ulrich, G.; Mel
2009, 7, 3639−3642.
́
y, Y.; Ziessel, R. Org. Biomol. Chem.
5.1 Hz), 133.3, 132.3 (t, JC−F = 3.5 Hz), 131.5, 131.2, 131.0, 130.6,
130.2, 130.1, 126.5, 126.0, 115.8, 13.4, 12.9; 11B NMR (128 MHz,
DMSO-d6, ppm): δ = 1.05 (t, 1J = 33.9 Hz); UV−vis (CH2Cl2) λ nm
(ε, M−1cm−1): 683 (59000), 600 (53000), 459 (14000), 369 (33000);
IR (ν, cm−1): 3075, 2966, 2922, 2856, 1594, 1529, 1502, 1453, 1435,
1393; EI-MS m/z (nature of the peak, relative intensity): 923.1 ([M],
100), 921.1 ([M], 50), 845.0 ([M − Br], 40), 843.0 ([M − Br], 40);
Anal. Calcd for C38H26B2Br2F4N4S4: C, 49.38; H, 2.84; N, 6.06.
Found: C, 49.28; H, 2.54; N, 5.73.
(10) Harriman, A.; J. Mallon, L. J.; Kristopher J. Elliott, K. J.;
Haeffele, A.; Ulrich, G.; Ziessel, R. J. Am. Chem. Soc. 2009, 131,
13375−13386.
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G. A.; Overkleft, H. S. Org. Lett. 2011, 13, 5656−5659 and references
cited therein.
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(b) Kumaresan, D.; Thummel, R. P.; Bura, T.; Ulrich, G.; Ziessel, R.
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Akkaya, E. U. Tetrahedron Lett. 2009, 50, 1738−1740. (d) Buyukcakir,
O.; Bozdemir, O. A.; Kolemen, S.; Erbas, S.; Akkaya, E. U. Org. Lett.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Optimized experimental conditions for use of PIFA; proton,
carbon and boron NMR traces for all compounds; COSY,
HSQC and HMBC spectra; spectroscopic data and crystallo-
graphic data of compounds 3a, 3b and 13. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*denicola@unistra.fr; ziessel@unistra.fr
Nazeeruddin, M. K.; Gratzel, M.; Akkaya, E. U. Chem. Sci. 2011, 2,
̈
Notes
949−954. (j) Bura, T.; Retailleau, P.; Ulrich, G.; Ziessel, R. J. Org.
The authors declare no competing financial interest.
Chem. 2011, 76, 1109−1117.
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