Phenyliodine(III) bis(trifluoroacetate) (PIFA)-promoted synthesis of bodipy dimers displaying unusual redox properties

Article abstract of DOI:10.1021/ol200189y

Phenyliodine(III) bis(trifluoroacetate) (PIFA) in conjunctionwith a Lewis acid promotes C-C coupling of Bodipy monomers leading to mixtures of various oligomers. when a single position is blockedwith an iodo or phenyl group, formation of the dimer is favored. These dimers display two successive oxidation and two reductionWves separated on average by 260 and 130 mV, respectively, corresponding to each Bodipy subunit.

Full text of DOI:10.1021/ol200189y

ORGANIC
LETTERS
2011
Vol. 13, No. 8
1916–1919
Phenyliodine(III) Bis(trifluoroacetate)
(PIFA)-Promoted Synthesis of Bodipy
Dimers Displaying Unusual Redox
Properties
Sandra Rihn,† Mustafa Erdem,† Antoinette De Nicola,*,† Pascal Retailleau,‡ and
Raymond Ziessel*,†
ꢀ
Laboratoire de Chimie Organique et Spectroscopies Avancees (LCOSA), CNRS, Ecole
ꢀ
de Chimie, Polymeres, Materiaux de Strasbourg (ECPM), 25 rue Becquerel, 67087
ꢁ
ꢀ
^
Strasbourg, Cedex 02, France, and Laboratoire de Crystallochimie, ICSN-CNRS, Bat
27 - 1 avenue de la Terrasse, 91198 Gif-sur-Yvette, Cedex, France
denicola@unistra.fr; ziessel@unistra.fr
Received January 21, 2011
ABSTRACT
Phenyliodine(III) bis(trifluoroacetate) (PIFA) in conjunction with a Lewis acid promotes C-C coupling of Bodipy monomers leading to mixtures of
various oligomers. When a single position is blocked with an iodo or phenyl group, formation of the dimer is favored. These dimers display two
successive oxidation and two reduction waves separated on average by 260 and 130 mV, respectively, corresponding to each Bodipy subunit.
4,40-Difluoro-4-bora-3a,4a-diaza-s-indacene (Bodipy)
dyes have received renewed interest in recent years due to
their outstanding optical properties such as high absorption
coefficients and relatively sharp emission lines, high fluores-
cence quantum yields, excellent chemical and photochemical
stability, high solubility, and ease of functionalization at will.1
They have found widespread applications as chemiosensors,2
laser dyes,3 fluorescent labels,4 active layers in photovoltaic
cells,5 and two-photon absorbing dyes dedicated to cell
imaging,6 as well as recently in solar concentrators.7
In contrast, Bodipy dimers have received little attention
despite their interesting properties linked to charge de-
localization and exciton coupling, providing unusual fluo-
rescence and redox properties. Of this family, R,R0-linked
Bodipy dimers,8 boron-butadiene-boron linked dimers,9 and
cofacially arranged dibenzothiophene, dibenzofuran, and
9,9-dimethylxanthene bis-Bodipy10 dyes have been recently
prepared.
† ꢀ
ꢁ
ꢀ
Ecole de Chimie, Polymeres, Materiaux de Strasbourg (ECPM).
‡ ICSN-CNRS.
(1) (a) Ziessel, R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31,
496-501. (b) (b) Ziessel, R. Compt. Rendus Acad. Sciences Chimie 2007,
10, 622–629. (c) Loudet, A.; Burgess, K. Chem. Rev. 2007, 31, 496.
(d) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47,
1184–1201.
(2) (a) Kollmannsberger, M.; Heinl, S.; Werner, T.; Huber, C.; Boila-
€
Gockel, A.; Leiner, M. J. P. U.S. Patent 6001999, 1999. (b) Coskun, A.;
Akkaya, E. U. J. Am. Chem. Soc. 2006, 128, 14474–14475. (c) Coskun, A.;
Akkaya, E. U. J. Am. Chem. Soc. 2005, 127, 10464–10465.
(3) Shah, M.; Thangraj, K.; Soong, M. L.; Wolford, L.; Boyer, J. H.;
Politzer, I. R.; Pavlopoulos, T. G. Heteroat. Chem. 1990, 1, 389–399.
(4) (a) Haughland, R. P.; Kang, H. C. U.S. Patent US4774339, 1988.
(b) Monsma, F. J.; Barton, A. C.; Kang, H. C.; Brassard, D. L.; Haughland,
R. P.; Sibley, D. R. J. Neurochem. 1989, 52, 1641–1644.
(5) (a) Erten-Ela, S.; Yilmaz, D.; Icli, B.; Dede, Y.; Icli, S.; Akkaya,
U. E. Org. Lett. 2008, 10, 3299–3302. (b) Kolemen, S.; Cakmak, Y.;
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.
Chem.;Eur. J. 2009, 15, 6335–6338. (e) Rousseau, T.; Cravino, A.;
Ripaud, E.; Leriche, P.; Rihn, S.; De Nicola, A.; Ziessel, R.; Roncali, J.
Chem. Commun. 2010, 46, 5082–5084.
ꢀ
(6) Didier, P.; Ulrich, G.; Mely, Y.; Ziessel, R. Org. Biomol. Chem.
2009, 7, 3639–3642.
(7) Ziessel, R.; Harriman, A. Chem. Commun. 2011, 47, 611–631.
€
€
€
(8) (a) Broring, M; Kruger, R.; Link, S.; Kleeberg, C.; Kohler, S.;
Xie, X.; Ventura, B.; Flamigni, L. Chem.;Eur. J. 2008, 14, 2976–2983.
€
€
(b) Nepomnyashchii, A. B.; Broring, M.; Ahrens, J.; Kruger, R.; Bard,
A. J. J. Phys. Chem. C 2010, 114, 14453–14460.
(9) Goze, C.; Ulrich, G.; Ziessel, R. Org. Lett. 2006, 8, 4445–4448.
r
10.1021/ol200189y
Published on Web 03/11/2011
2011 American Chemical Society

Bodipy-based conjugated oligomers and polymers have
also been prepared using palladium-mediated cross-cou-
pling chemistry, in particular at the β,β0-positions using
triple bonds as linkers.11 While exciton coupling of the
subunits of β-β linked Bodipy oligomers is clearly possi-
ble, its occurrence has not yet been established.
Herein, we focus on the preparation of such Bodipy dimers
using a hypervalent iodine reagent.12 Phenyliodine(III) bis-
(trifluoroacetate) (PIFA) has previously been used to prepare
bisaryls in the presence of a Lewis acid or a fluoroalcohol
as a solvent,13 such as meso-meso-linked linear arrays of
porphyrins,14 and fused diporphyrin scaffoldings.15
NaHCO3 or Na2S2O3, aqueous ascorbic acid, H2 or NaBH4)
was unsuccessful.
The single-crystal molecular structure of dimer 2 reveals a
64.1(1)° dihedral angle between the mean planes of the two
indacene units (Figure 1). As expected, in each Bodipy unit the
central six-membered ring lies coplanar with the two adjacent
five-membered rings. The rms deviation from planarity in-
˚
cluding the methyl substituents is 0.041 and 0.064 A, and the
maximum deviation is observed for the boron atom which is
˚
0.111 (13) and -0.271 (11) A away from the mean plane,
respectively. The elongated dimers propagate along the b
direction forming chevron-like columns. Nonclassic H-bonds
involving either the methyl hydrogen and the fluorine atoms
˚
(with a bond distance of ca. 2.4 A) or methyl hydrogen atoms
contribute to connect columns to their inverted neighbors
generated by a half-unit translation along the caxis(Figure1).
Scheme 1. Synthesis of Bodipy Oligomers
Initially, we concentrated on the use of 4,4-difluoro-
1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene 1 as
the starting material plus PIFA (0.3 equiv) at low tempera-
ture. In the absence of a strong Lewis acid, no reaction occurs
but fast oligomerization even at -78 °C is found in the
presence of BF3 OEt2 (0.6 equiv) (Scheme 1). A series of
3
oligomers were isolated by column chromatography (dimer
2, 10%; trimer 3, 4%),16 and higher oligomers were char-
acterized in particular by 1H NMR (Figure S1). In each case,
the degree of oligomerization was readily deduced by inte-
gration of the signals in the 1H NMR spectra for the methyl
groups and the two β-pyrrolic protons. The major draw-
backs of this coupling reaction are the difficulty in stopping
the reaction and in controlling the oligomer distribution and
the separation of the oligomers (tetramer, pentamer,...). The
use of various treatments designed to quench PIFA or the
reaction intermediate after a given time (e.g., with aqueous
Figure 1. ORTEP view for 2 with thermal ellipsoids plotted at
the 30% level. H atoms are omitted for clarity. Molecular
packing along the b axis.
Our next subsequent synthesis concerned that of dimers with
one of the β-pyrrolic positions occupied by an iodo or phenyl
moiety in order to control the rate of the coupling reaction. For
this purpose, we first prepared the monoiodo derivative 4,17 the
monotolyl dye 5, and monoethynyltolyl dye 6,17 as shown in
Scheme 2.
Dimerization of monomers 4 and 5 was efficient in the
presence of PIFA (0.9 and 0.6 equiv, respectively) and
(10) Benniston, A. C.; Copley, G.; Harriman, A.; Howgego, D.;
Harrington, R. W.; Clegg, W. J. Org. Chem. 2010, 75, 2018–2027.
(11) (a) Nagai, A.; Miyake, J.; Kokado, K.; Nagata, Y.; Chujo, Y.
J. Am. Chem. Soc. 2008, 130, 15276–15278. (b) Cakmak, Y.; Akkaya,
E. U. Org. Lett. 2009, 11, 85–88. (c) Kim, B.; Ma, B.; Donuru, V. R.; Liu,
H.; Frechet, J. M. J. Chem. Commun. 2010, 46, 4148–4150. (d) Nagai, A.;
Chujo, Y Macromolecules 2010, 43, 193–200.
(12) (a) Zhdankin, V. V.; Stang, P. J. Chem. Rev. 2002, 102, 2523–
2584. (b) Moriarty, R. M. J. Org. Chem. 2005, 70, 2893–2903. (c) Wirth,
T. Angew. Chem., Int. Ed. 2005, 44, 3656–3665.
(13) (a) Dohi, T.; Morimoto, K.; Takenaga, N.; Goto, A.; Maruyama,
A.; Kiyono, Y.; Tohma, H.; Kita, Y. J. Org. Chem. 2007, 72, 109–116.
(b) Dohi, T.; Ito, M.; Yamaoka, N.; Morimoto, K.; Fujioka, H.; Kita, Y.
Tetrahedron 2009, 65, 10797–10815. (c) Dohi, T.; Yamaoka, N.; Kita, Y.
Tetrahedron 2010, 66, 5775–5785.
(14) Jin, L.-M.; Chen, L.; Yin, J.-J.; Guo, C.-C.; Chen, Q.-Y. Eur. J.
Org. Chem. 2005, 3994–4001.
BF3 OEt2 (0.6 equiv) at -78 °C (Scheme 2). Dimers 7 and
3
8 were isolated by column chromatography. After some
experimentation, we found that adding PIFA in portions
of 0.3 equiv up to a total of 0.9 equiv over a 10 min period
limited the degradation of the dyes at -78 °C. Under these
PIFA-mediated coupling conditions, compound 6 does not
give the desired dimer 9, due to fast degradation of the dye.
Fortunately, the three dimers 2, 7, and 8 showed essentially
the same stability and reaction pathways as those of mono-
meric Bodipy dyes. Extension of the πconjugation of dimer 7
by substitution of both iodo residues with ethynyltolyl
(15) Ouyang, Q.; Zhu, Y.-Z.; Zhang, C.-H.; Yan, K.-Q.; Li, Y.-C.;
Zheng, J.-Y. Org. Lett. 2009, 11, 5266–5269.
(16) Isolated as a diastereoisomeric mixture, and their separation is in
(17) Bonardi, L.; Ulrich, G.; Ziessel, R. Org. Lett. 2008, 10, 2183–2186.
(18) Olmsted, J. J. Phys. Chem. 1979, 83, 2581–2584.
progress.
Org. Lett., Vol. 13, No. 8, 2011
1917

Table 1. Optical Data Measured in Dichloromethane Solution
Scheme 2. Synthesis of Bodipy Monomers and Dimers
b
b
λabs
ε
λF
τF
kr
knr
dyes (nm) (M-1 cm-1
)
(nm) (ns) ΦF
108 s-1 108 s-1
a
3
1
496
509
509
526
526
554
544
546
569
520
360
94 000
90 000
509
529
544
575
562
588
579
581
601
562
-
7.6
<1
5.9
3.2
4.3
4.2
2.0
4.1
4.3
5
90
14
86
47
67
69
15
66
60
70
70
1.2
-
1.3
-
4
5
68 800
1.5
1.5
1.6
1.6
7.5
1.6
1.4
1.4
-
2.4
1.7
7.7
7.4
4.2
8.3
0.3
6.0
-
6
72 000
2
155 000
176 000
185 000
187 000
207 000
147 000
175 000
3
7
8
9
10
-
a Using Rhodamine 6G as reference, Φ = 0.78 in water,18 λex = 488 nm.
b Calculated using the following equations: kr = ΦF/τF, knr = (1 - ΦF)/τF,
assuming that the emitting state is produced with unit quantum efficiency.
properties, as would be expected by the absence of empty
orbitals on the boron center.1d The structured absorption
bands localized around 285 and 369 nm are attributed to
the π-π* transitions of the pyrene moieties. Irradiation in
these bands resulted in the absence of pyrene emission at
370-380 nm20 but intense emission of the Bodipy dimer at
562 nm. This indicates that almost quantitative (>99%)
modules through palladium-catalyzed cross-coupling with p-
tolylacetylene proved facile, affording dimer 9 in 88% yield.
Dimer 2 was also appealing as a substrate for the introduction
of arylethynyl units on the boron atom as possible absorbing
units for excitonic energy transfer (vide infra). The use of excess
ethynyl-Grignard reagent led to rapid substitution of the
fluorine atom and allowed the preparation of compound 10
in good yield (Scheme 2).
Photophysical parameters are gathered in Table 1. Linked
Bodipy dimers and trimers all show high absorption coeffi-
cients (>150 000 M-1 cm-1) and relatively high fluorescence
quantum yields, except those having a iodo atom on the
β-position which show reduced fluorescence lifetimes and
quantum yields (Table 1). This is largely due to an efficient
intersystem crossing leading to a low-lying triplet state.19
Interestingly, the electronic absorption profiles are shifted by
30 nm in dimer 2 and by 58 nm in trimer 3 with respect to the
monomer 1 and by 43 nm in dimer 9 with respect to the
monomer 6 (Figure 2). This observation indicates that some
electronic communication between the subunits is effective.
In the emission spectra a progressive shift of 51 nm for dimer
2 and 79 nm for trimer 3 was also observed.
The Stokes shifts progressively increase from 515 cm-1 for
1to 1200 cm-1 for the dimer 2. In all cases, the emission band
shows good mirror symmetry with the lowest-energy band
and the radiative deactivation rate is higher by 1 order of
magnitude than the nonradiative deactivation rate (Table 1).
Furthermore, the nanosecond deactivation regime of the
excited state is in keeping with a singlet emitter and confirms
that little rearrangement occurs in the excited state.
Figure 2. Absorption, emission, and excitation spectra mea-
sured in dichloromethane: (a) green lines 1, red lines 2, blue lines
3; (b) red lines 5 and blue lines 10. For the excitation spectra the
emission is scaled at 600 nm for 1, 620 nm for 2, 640 nm for 3, 580
nm for 5, and 610 nm for 10.
Interestingly, the substitution of the boron center by
1-ethynylpyrene fragments has no effect on the emission
(19) (a) Yogo, T.; Urano, Y; Ishitsuka, Y.; Maniwa, F.; Nagano, T.
J. Am. Chem. Soc. 2005, 127, 12162–12163.
1918
Org. Lett., Vol. 13, No. 8, 2011

excitonic energy transfer is occurring from the pyrene to
the dimeric Bodipy framework. The efficiency of this
process is likely due to a good spectral overlap between
the emission of the pyrene and the second excited state of
the Bodipy subunits. As shown in Figure 2, the excitation
spectra match the absorption spectra fairly well over the
entire absorption window (250 to 560 nm).
Table 2. Electrochemical Data for the Bodipy Monomers and
Dimersa
dyes
E0°oxy, V (ΔE, mV)
E0°red, V (ΔE, mV)
-1.33 (60)
-1.27 (70) ; -1.40 (70)
-1.29 (100)b; -1.49 (70)
-1.23 (70)
-1.16 (60); -1.23 (70)
-1.30 (60)
-1.17 (70); -1.30 (70)
-1.32 (70)
1
þ 1.11 (70)
Figure 3. Cyclic voltammograms in dichloromethane at rt for
the monomer 1 (mauve trace), the dimer 2 (blue trace), and
trimer 3 (green trace). The redox couple at þ0.55 V vs Pt is due to
added ferrocene (Fc) in all cases.
2
þ 1.07 (60); þ 1.33 (70)
þ 1.06 (80)b; þ 1.18 (70)
þ 1.18 (70)
3
4
7
þ 1.16 (60); þ 1.36 (70)
þ 1.16 (60)
6
The cyclic voltammogram of trimer 3 shows that both
first reduction and oxidation potentials are bielectronic
and resulted from an overlapping between two successive
but very close single redox processes which can just be
deconvoluted. These first redox processes are close to that
observed for dimer 2 (Table 2). However, both of these
processes are followed by monoelectronic ones less catho-
dic by 150 mV in oxidation and more anodic by 90 mV in
reduction with respect to dimer 2. These results are in line
with the simultaneous oxidation and reduction of the two
Bodipy subunits at the periphery of the trimer resulting in a
dielectronic process very close to that observed for the
dimer. The second redox process is assigned to the oxida-
tion and reduction of the central Bodipy residue.
In summary, we have developed an unprecedented oxida-
tive coupling reaction of Bodipy dyes using PIFA with a Lewis
acid providing Bodipy oligomers linked by C-C tethers at the
β pyrrolic positions. This combination of reagents provides an
electrophilic intermediate (likely a radical cation)13c which
rapidly reacts with electron-rich Bodipy’s to provide fluor-
escent oligomers or preferentially dimers when one of the
pyrrolic sites is blocked. The absorption and emission
properties of the dimers are shifted to lower energies, with a
maximum shift of 2640 cm-1 and Stokes shift of ∼1200 cm-1
with respect to the monomers. All dimers display two reversible
monoelectronic reduction and oxidation waves separated by
130 and 260 mV, respectively. In the trimers, both Bodipy units
at the periphery are oxidized and reduced at similar but
distinguishable potentials from those of the central unit. Future
work will include the preparation of mixed valence dyes to
determine the rate of charge transfer between the Bodipy
subunits and the influence on the optical properties.
9
þ 1.06 (70); þ 1.19 (60)
þ 1.07 (70)
5
10
þ 1.23 (irrev.)c
-1.37 (60); -1.52 (70)
a Potentials determined by cyclic voltammetry in deoxygenated CH2Cl2
solutions, containing 0.1 M TBAPF6, at a solute concentration range of 1.5
ꢀ 10-3 M, at rt. Potentials were standardized using added ferrocene (Fc) as
an internal reference and converted to SCE assuming that E1/2 (Fc/Fcþ) =
þ0.38 V (ΔEp = 60 mV) vs SCE. Error in half-wave potentials is (15 mV.
b Two-electron process. c The peak potential is quoted.
The electrochemical activity of the new dyes was deter-
mined in dichloromethane using tetrabutylammonium
hexafluorophosphate as a supporting electrolyte (electro-
chemical window from þ1.7 to -2.1 V), and the results are
gathered in Table 2. All four monomers exhibit a reversible
one-electron oxidation around þ1.1 V and a reversible
one-electron reduction around -1.2 V, demonstrating that
the β-substitution position has little electronic effect on the
redox activity of the central Bodipy core. These redox
processes are reversible (ipa/ipc ≈ 1), with a shape that is
characteristic of Nernstian one-electron processes (ΔEp =
60 mV). A typical example is given in Figure 3, the
characteristics being closely similar to those obtained with
simpler Bodipy dyes.21
Interestingly, all dimers display two reversible one-electron
waves on both reduction and oxidation (Figure 3 and
Table 2), and addition to the dimer solution of the corre-
sponding monomer clearly increases the first reduction and
oxidation potentials, demonstrating only a slight influence of
the second unit on the redox properties of the first. The degree
of separation between the first and second oxidation and
reduction potentials and the gap between the first and second
oxidation do not depend on the substituents of the dimers
(Table 2). Remarkably, in the case of dimer 10 the substitu-
tion of the fluoro groups by ethynylpyrene shifted the reduc-
tion potentials anodically by 100 mV while maintaining the
degree of separation.
Acknowledgment. We also acknowledge the CNRS for
providing research facilities and financial support and
Professor Jack Harrowfield (ISIS in Strasbourg) for com-
menting on the manuscript before publication.
Supporting Information Available. Synthetic proce-
dures and analytical data reported herein (PDF). Centre
under the number 796243. This material is available free
of charge via the Internet at http://pubs.acs.org.
(20) Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R. Chem.;Eur.
J. 1999, 5, 3366–3378.
(21) (a) Ziessel, R.; Bonardi, L.; Retailleau, P.; Ulrich, G. J. Org.
Chem. 2006, 71, 3093–3102. (b) Bura, T.; Ziessel, R. Tetrahedron Lett.
2010, 51, 2875–2879.
Org. Lett., Vol. 13, No. 8, 2011
1919