Efficient synthesis of panchromatic dyes for energy concentration

Article abstract of DOI:10.1002/anie.201003206

Channel your energy: Bodipy dyes were linked covalently by a method involving three key steps, including a carboformylation reaction. All multichromophoric dyes exhibited highly efficient photoinduced energy transfer to the terminal Bodipy modules, which fluoresced at 652 nm with 57 □ % quantum efficiency (see absorption (gray) and emission (orange) spectra of a dye containing five Bodipy units).

Full text of DOI:10.1002/anie.201003206

Angewandte
Chemie
DOI: 10.1002/anie.201003206
Panchromatic Dyes
Efficient Synthesis of Panchromatic Dyes for Energy Concentration
Thomas Bura, Pascal Retailleau, and Raymond Ziessel*
Energy collection and migration are fundamental aspects of
the functioning of both photosynthetic organisms and artifi-
cial solar concentrators.[1,2] In the past decade, many systems
based on mimicry of energy concentration in natural systems
have been scrutinized.[3,4] In most cases, their development
required tedious multistep synthetic procedures involving
masked functionalities, protecting groups, highly reactive
intermediates, and toxic residues, which hampered their
application.
The synthetic advantages of 4,4-difluoro-4-bora-3a,4a-
diaza-s-indacene (Bodipy) rigid cyanine dyes derive from the
possibility of controlling the reactivity at the central core
(dipyrromethene) and at the boron atom.[5] The upsurge of
interest in such dyes stems from their pronounced stability,
high absorption coefficients, narrow emission profiles, and
outstanding emission quantum yields, which reach unity in the
best cases.[6] The numerous applications of Bodipy dyes were
recently reviewed.[5,7]
Scheme 1. Iterative approach to the synthesis of multichromophoric
Herein, we report a novel strategy for the creation of
covalently linked dyes in an iterative fashion, whereby each
cycle provides an additional module capable of energy
transfer. The strategy involves (Scheme 1): 1) substitution at
the boron atom of a Bodipy dye with a Grignard reagent;
2) formylation of a phenyliodo residue; 3) a Knoevenagel
reaction between methyl groups of another Bodipy unit and
the formyl group to provide both divinyl and monovinyl
derivatives, which could be used in another sequence of
reactions to produce higher oligomers in a controlled fashion.
The essence of the strategy is that substitution at boron can be
used to control the accessibility of the two methyl groups in
the 3,5-positions nearest to the boron center (see compound 1
in Scheme 2).[8] This strategy avoids self-condensation of the
carbaldehyde in the 8-meso position and the methyl residues
in the 3,5-positions on the same molecule.
dyes.
production of aldehydes without side reactions, even in the
presence of vinyl groups (see below).
As anticipated, the reaction of dye 3 with dye 1 provided a
mixture of the monovinyl derivative 4a (magenta) and the
bisvinyl derivative 4b (blue) in good yields (Scheme 2).[10,11]
The ratio of mono- to bisvinyl products can be controlled
routinely by the amount of aldehyde 3 used.
In both cases, the observed proton–proton coupling
constant of 16.4 Hz for the vinyl group is in keeping with an
E conformation of the double bonds, as expected for this type
of condensation. Interestingly, there was no evidence for the
self-condensation of 3. The inertness of the methyl groups in
this compound for a Knoevenagel condensation is possibly
due to the steric crowding caused by the substituents on the
boron atom.
The pivotal formyl dye 3 was prepared by a carboformy-
lation reaction[9] catalyzed by Pd0 with sodium formate as the
reductant and
(Scheme 2). Under these conditions, no alkyne reduction
was observed. This approach appears to be very useful for the
a
flow of CO under mild conditions
Our next objective was the substitution of the iodo group
in compound 4a by a formyl group to repeat the synthesis of
vinyl derivatives for the next generation of multichromo-
phoric dyes. Compound 4a was transformed into compound 6
in a straightforward manner in two steps (Scheme 3).
Remarkably, formylation occurred without reduction of the
alkene functionality. We prepared compounds 7a and 7b, in
which three and five colored subunits, respectively, are linked
through conjugated bridges, by similar procedures
(Scheme 3). Use of an excess of the formyl derivative 6
drove the reaction toward the bisvinyl derivative 7b prefer-
entially. Under these conditions, 7b was obtained in up to
75% yield.
[*] T. Bura, Dr. R. Ziessel
Laboratoire de Chimie Molꢀculaire et
Spectroscopies Avancꢀes (LCOSA)
Ecole Europꢀenne de Chimie, Polymꢁres et Matꢀriaux
CNRS, 25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
E-mail: ziessel@unistra.fr
Homepage: http://www-lmspc.u-strasbg.fr/lcosa
www.lcosa.fr
Dr. P. Retailleau
The level of Bodipy substitution can be monitored readily
on the basis of the H NMR chemical shifts of the b-pyrrolic
hydrogen atoms, which resonate at d = 6.17 ppm in the
Laboratoire de Cristallochimie, ICSN—CNRS, Gif-sur-Yvette (France)
1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003206.
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Figure 1. ORTEP views of the X-ray crystal structures of compounds
a) 3 and b) 4a. Disordered components with a higher occupancy factor
only are displayed. Ellipsoids are drawn at the 30% probability level,
H atoms as small spheres of arbitrary radius.
(ca. 0.3 ꢀ) of the boron atom from this plane found in F-
Bodipy systems.
In 3, which has approximate twofold rotational symmetry
consistent with the static disorder of the aldehyde group, the
indacene core is orthogonally substituted on both sides by the
benzaldehyde and the ethyne tether (C3’-C2’-C1’-B-C1’’-C2’’-
C3’’) moieties (the respective dihedral angles are 87.5(2) and
88.5(2)8). The slight tilt between the mean plane of the phenyl
substituent and that described by the ethynyl tethers is
10.1(2)8. For 4a, the orthogonality with the core is similar
(84.3(2) and 89.7(2)8, with 5.7(2)8 between the phenyl and the
ethynyl-tether planes). The additional iodophenyl-substituted
F-Bodipy group [despite the near orthogonality (dihedral
angle: 80.9(2)8) of its components and only a 9.6(4)8 tilt
between the iodophenyl and F-B-F planes] breaks the
apparent twofold symmetry of 3 and leads to a bending of
the overall elongated (ca. 30 ꢀ long) superstructure, as
characterized by a dihedral angle between the two indacene
cores of 64.1(1)8.
Scheme 2. a) EtMgBr, CH3OCH2CH2OCH2CꢀCH, THF, 608C, 2 h;
then 1, 608C overnight, 90%; b) [Pd(PPh3)2Cl2], DMF, HCOONa
(1.1 equiv), CO (1 atm), 908C, 3 h, 70%; c) 1 (1 equiv), 3 (1.5 equiv),
piperidine, toluene, p-TsOH (trace amount), 1408C; 4a: 33%, 4b:
66%. DMF=N,N-dimethylformamide.
tetramethyl-substituted dye and at d = 6.24 and 6.91 ppm in
the trimethyl-substituted dye (see compound 6 in Scheme 3),
and the integration of these signals. The most deshielded
hydrogen atom is assigned as that nearest to the vinyl bond.
These chemical shifts are similar for 7a and 7b. An additional
singlet appears at d = 6.99 ppm for the two b-pyrrolic hydro-
gen atoms of the dimethyl-substituted Bodipy unit in 7b.
The X-ray crystal structures of the two compounds 3 and
4a were determined (Figure 1). In both, the sterically
congested boron center adopts an almost tetrahedral geom-
etry, with an N-B-N angle of about 1068 and a C-B-C angle of
Intense bands in the UV/Vis region of the absorption
spectra reflect the presence of the different residues in the
final scaffolds: tetramethyl-substituted Bodipy, labs = 502 nm
(red);[12]
trimethyl-substituted
Bodipy,
labs = 569 nm
À
about 1128. The B C bond lengths range from 1.56 (for 4a) to
(magenta);[13] and dimethyl-substituted Bodipy, labs
=
1.59 ꢀ (for 3), and the ethynyl tethers retain bond lengths
typical of triple bonds (ca. 1.19 ꢀ). The bond lengths within
each indacene moiety remain characteristic of fully delocal-
ized cyanine units. Deviations from the mean ligand plane (17
atoms including methyl-substituent carbon atoms and the
aromatic carbon atom directly attached to the heterocyclic
core) range from 0.037 to 0.072 ꢀ, with the usual deviation
639 nm (blue)[14] (Figures 2 and 3 and Table 1). Prototypical
examples of the fluorescence behavior of these compounds
are provided by structures 4a and 4b, which exhibit intense
fluorescence at 578 and 650 nm, respectively, with the absence
of residual fluorescence at 513 nm due to the tetramethyl-
substituted Bodipy unit. These results demonstrate that the
extent of energy transfer from the tetramethyl Bodipy subunit
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Scheme 3. a) EtMgBr, CH3OCH2CH2OCH2CꢀCH, THF, 608C, 2 h; then 4a, 608C overnight, 52%; b) [Pd(PPh3)2Cl2], DMF, HCOONa (1.1 equiv),
1
CO (1 atm), 908C, 3 h, 70%; c) 1 (1 equiv), 6 (2 equiv), piperidine, toluene, p-TsOH (tr), 1408C; 7a: 10%, 7b: 75%. The H NMR chemical shifts
of the b-pyrrolic hydrogen atoms are given for compounds 6, 7a, and 7b in [D6]acetone.
to the tri- or dimethyl Bodipy is greater than 99%. Energy
transfer is promoted by a favorable spectral overlap between
the energy donor (tetramethyl-substituted Bodipy) and the
energy acceptor (tri- or dimethyl-substituted Bodipy).[15] The
rate of electronic energy transfer (EET) is expected to be very
fast: more than 2 ꢁ 1010 sÀ1, as estimated from our resolution
limits.
The situation is even more interesting in the next
generation of multichromophoric dyes, 7a and 7b. In both
cases, the absorption corresponds to a linear combination of
the three and five modules in 7a and 7b, respectively. For 7a
and 7b, one and two intense intramolecular-charge-transfer
bands (ICTB), respectively, were observed around 340 nm
(Figure 3). One ICTB band, at 336 nm, corresponds to the
two individual styryl moieties of compound 7a (Figure 3a),
the less energetic ICTB band, at 359 nm, to the presence of
the bisstyryl fragment in compound 7b (Figure 3b). Three
well-defined absorption bands corresponding to the singlet
absorption of each type of Bodipy moiety were deciphered.
Upon excitation at 480 nm, no residual emission of the
tetramethyl-substituted Bodipy moiety was detected (also in
the case of 4a, 4b, 5, and 6), whereas a weak emission, which
could be assigned unambiguously to residual fluorescence of
the trimethyl-substituted Bodipy moiety, was observed at
583 nm.
An estimated rate constant of 1.6 ꢁ 108 sÀ1 was calculated
on the basis of the quantum yields for energy transfer from
the internal trimethyl-substituted Bodipy moieties in 7b to
the blue dimethyl-substituted Bodipy moieties at the termini
with an efficiency of 82% (kEET = [(f0/f)À1]/t2; f0 is the
quantum yield of the unquenched compound 5, f is the
quantum yield of the fragment equivalent to 5 (quenched) in
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Communications
Figure 3. a) Absorption (solid line), emission (dotted line), and excita-
tion spectra (triangles) of a) 7a and b) 7b. N Emission wavelength.
Figure 2. a) Absorption (solid line), emission (dotted line), and excita-
tion spectra (triangles) of a) 4a and b) 4b.
Table 1: Selected spectroscopic data.
[a]
[b]
[b]
[b]
Dye
labs
[nm]
e
lF
FF
(lexc [nm])
t1
t2
t3
[mcmÀ1
]
[nm]
[ns]
[ns]
[ns]
molecule 7b, and t2 is the lifetime of the unquenched
compound 5). A similar rate constant of 1.0 ꢁ 108 sÀ1 was
calculated on the basis of lifetimes (kEET = 1/t2*À1/t2; t2* is the
lifetime of the fragment equivalent to 5 (quenched) in
molecule 7b, and t2 is the lifetime of the unquenched
compound 5). As confirmed by the excitation spectra for
emission at 652 nm, all the modules participated in the
energy-transfer process (Figure 3b). Furthermore, virtual
Stokes shifts of about 5500 cmÀ1 were calculated. Thus,
multicascade energy transfer from the tetramethyl Bodipy
to the trimethyl Bodipy, and finally to the dimethyl Bodipy is
very efficient within these linked dyes, and the input energy is
efficiently concentrated on a single emitting dye, as observed
in bacteria and higher plants.[17]
In short, we have described a facile and straightforward
route to panchromatic fluorescent materials. The key step is
the introduction of an aromatic aldehyde group by a smooth
and selective carbopalladation reaction with CO and
NaHCOO. This carbaldehyde functionality is essential for
linking the modules and creating a gradient of energy levels
along each lateral arm. The presence of a phenyl fragment in
the middle of the starting Bodipy moiety ensures that each
linked Bodipy subunit remains electronically isolated. By the
excitation of each module, a cascade energy transfer is
effective with photons channeled to the dyes localized at the
termini. Further chemical modifications at the terminal
iodophenyl group can be foreseen. For example, anionic
2
3
4a
502
504
503
569
74000
80500
90000
115000
513
518
–
0.55 (480)
0.42 (480)
–
0.66 (480)
0.68 (550)
–
0.52 (480)
0.53 (605)
–
0.73 (480)
0.73 (550)
–
0.72 (480)
0.73 (550)
0.03 (480)
0.36 (480)
0.37 (560)
–
0.10 (480)
0.08 (550)
0.47 (480)
0.47 (550)
0.57 (625)
5.28
3.11
–
–
–
–
6.43
–
–
–
-
578
–
4b
5
502
639
148000
137500
–
650
–
–
–
–
–
5.70
502
569
84000
108000
–
579
–
–
–
–
5.54
–
3.00
–
–
–
–
6.20
–
–
–
–
–
3.66
–
–
–
–
–
–
–
–
–
–
–
–
5.21
–
–
6
503
570
80000
104000
–
581
7a
7b
503
571
84000
157000
517
581
–
–
503
568
193000
221000
–
583
–
–
–
–
3.83[c]
–
–
–
–
641
102000
652
–
[a] The fluorescence quantum yield was determined in toluene
(cꢁ1.10À6 m) at 298 K with Rhodamine 6G as a reference (FF =0.78 in
water, lexc =488 nm).[16] All FF values are corrected for changes in the
refractive index. [b] Lifetimes are denoted t1 for the tetramethyl-
substituted subunit, t2 for the trimethyl-substituted subunit, and t3 for
the dimethyl-substituted subunit. [c] This value corresponds to t2* in the
kEET equation.
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Received: May 27, 2010
Published online: August 2, 2010
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Keywords: dyes/pigments · energy transfer · fluorescence ·
formylation · iterative synthesis
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