Sulfur-bridged BODIPY DYEmers

Article abstract of DOI:10.1002/chem.201300893

Reactions of BODIPY monomers with sulfur nucleophiles and electrophiles result in the formation of new BODIPY dimers. Mono- and disulfur bridges are established, and the new dyestuff molecules were studied with respect to their structural, optical, and el

Full text of DOI:10.1002/chem.201300893

FULL PAPER
The exciton coupling theory of the
Kasha model helps explaining the
changes in the optical spectra of sulfur-
bridged BODIPY DYEmers with
small subchromophore distances (see
figure). The preciseness of the predic-
tion depends on the degree of elec-
tronic communication and can be esti-
mated from a combination of optical
and electrochemical analyses with
DFT calculations.
Dyes/Pigments
J. Ahrens, B. Bçker, K. Brandhorst,
M. Funk, M. Brçring* ....... &&&& — &&&&
Sulfur-Bridged BODIPY DYEmers
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DOI: 10.1002/chem.201300893
Sulfur-Bridged BODIPY DYEmers
Johannes Ahrens, Birte Bçker, Kai Brandhorst, Markus Funk, and Martin Brçring*[a]
Dedicated to Professor Werner Uhl on the occasion of his 60th birthday
Abstract: Reactions of BODIPY mon-
omers with sulfur nucleophiles and
electrophiles result in the formation of
new BODIPY dimers. Mono- and di-
sulfur bridges are established, and the
new dyestuff molecules were studied
with respect to their structural, optical,
and electrochemical properties. X-ray
diffraction analyses reveal individual
angulated orientations of the BODIPY
subunits in all cases. DFT calculations
provide solution conformers of the
DYEmers which deviate in a specific
manner from the crystallographic re-
sults. Clear exciton-like splittings are
observed in the absorption spectra,
with maxima at up to 628 nm, in com-
bination with the expected weak fluo-
rescence in polar solvents. A strong
communication between the BODIPY
subunits was detected by cyclic voltam-
metry, where two separated one-elec-
tron oxidation and reduction waves
with peak-to-peak potential differences
of 120–400 mV are observed. The qual-
itative applicability of the exciton
model by Kasha for the interpretation
of the absorption spectral shape with
respect to the conformational state,
subunit orientation and distance, and
conjugation through the different
sulfur bridges, is discussed in detail for
the new BODIPY derivatives. This
work is part of our concept of DYEm-
ers, where the covalent oligomerisation
of BODIPY-type dye molecules with
close distances is leading to new func-
tional dyes with predictable properties.
Keywords: BODIPY dyes · boron ·
DYEmers · exciton coupling · tetra-
pyrroles
Introduction
take advantage of the potential of this luminescent dye for
many diverse optoelectronic applications.[7]
The boron dipyrrin complex (BODIPY, 4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene) was first described by Treibs and
Kreuzer in 1968.[1] Since the late 1990s this fluorescent dye
has become a topic of growing research, as the unique and
positive properties of BODIPYs show prospect for much
potential in manifold applications.[2] The high chemical and
physical stability, as well as well-defined absorption and flu-
orescence spectra with high quantum yields, are representa-
tive features of BODIPYs.[3] Many different architectures
and derivatizations on the BODIPY core are aiming at the
alteration of photophysical properties and/or the implemen-
tation of functionality, and have successfully been applied to
this class of dyes. Large red-shifts into the NIR region,[4]
large Stokes shifts in combination with high fluorescence
quantum yields,[5] and water solubility[6] have been some of
the key points in this erupting field of research. Further-
more, specific connectivities can be chosen synthetically to
One current aspect of the development of BODIPY
chemistry is the integration of the chromophore into poly-
mers and oligomers.[8,9] Depending on the position (a (3,5),
b (2,6), b’ (1,7), meso (8), and boron (4); see Figure 1) and
the linkage (distance, orientation, electronic coupling), more
or less interaction can be observed between the BODIPY
subunits by photophysical and/or electrochemical methods.[8]
Especially for dimers with close interchromophoric distances
significant property changes can be expected and have re-
cently been studied for the first examples.[10–12] Nevertheless,
the number of reports on well-defined BODIPY oligomers
is still limited.[13,14] Due to the well-developed synthetic ad-
dressability of the BODIPY framework, we have coined the
term “DYEmers” for this class
of BODIPY oligomers with sys-
tematically altered bridging pat-
terns. Recently, we have intro-
duced BODIPY DYEmers with
ꢀ
one direct C C connection and
reported about their new prop-
[a] J. Ahrens, B. Bçker, Dr. K. Brandhorst, Dr. M. Funk,
Prof. Dr. M. Brçring
Figure 1. BODIPY core struc-
ture with numbering scheme
and nomenclature.
erties like exceptional large
Institut fꢁr Anorganische und Analytische Chemie
Technische Universitꢂt Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
Fax : (+49)531-391-5387
Stokes shift with high fluores-
cence quantum yield.[11,15]
An intriguing possibility for bridging two BODIPY chro-
mophores at different positions is given by the use of sulfur-
based connecting units. Due to the possibility of an electron-
ic conjugation of the subchromophores through the sulfur
atom(s) it is in question, whether the absorption spectra of
E-mail: m.broering@tu-bs.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300893: Synthetic procedures
1
for 2, 3, and 4; H, 13C, 19F, 11B NMR spectra of all compounds; cyclic
voltammograms of 8 and 11.
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BODIPY DYEmers
FULL PAPER
such DYEmers could still be described, or even predicted,
by the simple exciton coupling model of Kasha.[16] In this
contribution we present preparative routes towards different
dimers derivatives, describe their solid state and solution
structures as well as their photophysical and electrochemical
characterization, and discuss the applicability of Kasha’s
model to each of the sulfur-bridged BODIPY DYEmers.
Scheme 3. Nucleophilic substitution to dimeric BODIPY disulfide 6: a)
Na2S2, MeOH, CH2Cl2, RT, 25 min.
Results and Discussion
sodium disulfide in MeOH. This latter reaction, however, re-
quires much less forcing conditions (CH2Cl2, RT) due to the
more pronounced nucleophilicity of the disulfide
(Scheme 3).
Synthesis: Two different synthetic strategies have been
tested in order to obtain sulfur-brigded symmetrical dimers
of BODIPYs linked at the a-positions, a nucleophilic
one[17,18] using sulfide anions as the sulfur source in SNAr re-
actions, and an electrophilic one employing sulfur chlor-
ides.[19,20] Therefore, the a-free BODIPY 3 and the 3-bromo
BODIPY 4 were chosen as unsymmetrical BODIPYs with
one reactive position (Scheme 1; for details see Supporting
Information). Note that the free meso-position of BODIPYs
is reactive only towards very strong nucleophiles such as
nBuLi.[21] Compounds 3 and 4 were obtained by BF2 com-
plexation of the corresponding dipyrrin hydrobromides 1[22]
and 2. Compound 2 was prepared from 1 by treatment with
elemental bromine.[23]
During the search for suitable reaction conditions towards
the disulfide 6, the formation of the dipyrrin-1-thione 7
from 4, sodium sulfide, and elemental sulfur was observed
(Scheme 4). Surprisingly, the rather tightly bound BF2 unit
of 4 is lost in the process of bromine substitution. To the
best of our knowledge, a similar result has only once be re-
ported for a 3-amino BODIPY, which also loses the BF2
group upon addition of acid.[24] Analytical and spectroscopic
data of 7 are in good agreement with those known from a
dipyrrin-1-thione study performed by Lightner.[25] In this
case, however, the sulfur atom has been introduced by the
action of Lawessonꢃs reagent onto the corresponding dipyr-
rinone. For comparison, the unsymmetrical BODIPY thio-
ether 8 has been produced by nucleophilic substitution of 4
with 4-tolylthiol (Scheme 4).
Scheme 4. Deboronation reaction to 7 and nucleophilic substitution to 8:
a) Na2S·9H2O, S8, EtOH, toluene, 908C; b) 4-methylthiophenole, NEt3,
MeCN, CH2Cl2, RT, 15 min.
Scheme 1. Syntheses of BODIPY starting materials 3 and 4: a) 1) Br2,
CH2Cl2, RT, 30 min, 2) cyclohexene, Et2O; b) 1) NEt3, CH2Cl2, 2)
BF3·OEt2, RT, 1 h.
The unsymmetrical 3-bromo BODIPY 4 was successfully
dimerized to the desired thioether 5 in 56% yield by treat-
ment with sodium sulfide as the nucleophile in toluene/
water at reflux and under phase-transfer conditions
(Scheme 2). Similarly, the symmetrical disulfide 6 forms
from 4 in 67% yield using a freshly prepared solution of
A second general preparative entry towards the DYEmers
5 and 6 is the use of electrophilic sulfur halides in terms of a
SEAr reaction. Following this approach the a-unsubstituted
pentamethyl BODIPY 3 reacts with sulfur dichloride (SCl2)
in dichloromethane to yield sulfide 5 in a diminished yield
of 24% (Scheme 5). If sulfur monochloride (S2Cl2) is used
instead, 3 is transformed into a mixture of symmetrical
BODIPY dimers bridged by polysulfides of different
lengths. Only the sulfide 5 and the disulfide 6 could be iso-
lated after column chromatography in yields of 21 and 33%,
respectively (Scheme 5). The trisulfide and higher polysul-
fides were detected by ESI mass spectrometry, but seem to
interconvert with time, yielding mainly 5 and 6. The occur-
rence of sulfur bridges of different length in the substitution
products of S2Cl2 and their thermodynamic conversion has
been reported earlier for related oligopyrrolic macrocy-
cles.[20]
Scheme 2. Nucleophilic substitution to dimeric BODIPY sulfide 5: a)
Na2S·9H2O, nBu4NBr, toluene, reflux, 3.5 h.
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M. Brçring et al.
is less pronounced, though still apparent for the dimer 10
which shows a violet-red color and no visible fluorescence in
CH2Cl2 solution.
Solid state X-ray structures of 5, 6, 8, 10, and 11: The three
new sulfur-bridged DYEmers 5, 6, and 10 as well as the thio-
ether monomers 8 and 11 were studied by single-crystal X-
ray diffraction (Figure 2–6). The sulfide 5 crystallizes in the
Scheme 5. Electrophilic substitution of 3 and formation of DYEmers 5
and 6: a) SCl2, CH2Cl2, 08C ! RT; b) S2Cl2, CH2Cl2, ꢀ658C ! RT.
¯
space group P1 with one dye molecule in the asymmetric
unit. For the disulfide 6 the same triclinic space group is
found with Z=2. However, one molecule of co-crystallized
solvent is found per formula unit. This solvent is severely
disordered and has been removed from the electron density
map using the SQUEEZE command in PLATON.[31] The
BODIPY subunit structures of 5 and 6 deviate very slightly
from each other. However, each of the BODIPY units is
characterized by a nearly flat C9BN2 framework, and by
angles at the boron atom close to 109.58. These structural
characteristics do not deviate significantly from those of
monomeric BODIPYs such as 8. The sulfur bridge in 5 is
For the introduction of a sulfur bridge at the b-position
(2,6-position) only the electrophilic route is feasible, as for
the nucleophilic route an electron poor (vinylogous) imini-
um halide function (3,5- and 1,7- and 8-positions) is re-
quired.[17,26–28] The known pentaalkylated BODIPY 9[29] was
reacted with sulfur monochloride in dichloromethane at low
temperature (Scheme 6). Interestingly, only the sulfide 10
could be isolated after column chromatography in low yield
as the main product, while a disulfide species could only be
detected in a different fraction by ESI mass spectrometry.
ꢀ
ꢀ
described by bond lengths C9 S1 of 1.755(2) ꢄ and C10 S1
of 1.752(2) ꢄ and a bond angle C9-S1-C10 of 100.5(1)8,
which are very similar to the values from a crystal structure
of 10-thiabilirubin[32] and similar to 8 (C14 S1 1.762(4) ꢄ,
ꢀ
C14-S1-C15 103.3(2)8). The disulfur bridge in 6 shows bond
ꢀ
ꢀ
ꢀ
lengths C9 S1 of 1.749(2) ꢄ, C10 S2 of 1.744(2) ꢄ, and S1
S2 of 2.088(1) ꢄ, bond angles C9-S1-S2 and C10-S2-S1 of
103.0(1) and 102.6(1)8, respectively, and a dihedral angle
C9-S1-S2-C10 of 94.5(1)8. These values are similar to those
of a bis(bipyrroledisulfide) macrocycle.[20] The b-bridged sul-
fide 10 crystallizes in the orthorhombic space group Pnc2
with two dye molecules in the asymmetric unit. The molecu-
lar geometries of the independent molecules of 10 differ
slightly, but insignificantly from each other, and show very
similar characteristics as the monomeric 11. For discussion,
one molecule was chosen. The BODIPY subunit structures
of 10 show crystallographic symmetry and are connected by
Scheme 6. Electrophilic substitution to DYEmer 10: a) S2Cl2, CH2Cl2,
ꢀ708C, 2 h.
As for the a-substituted examples, a monomeric b-tolylth-
io derivative 11 was prepared for comparison. Therefore,
the necessary 4-tolylsulfenyl chloride was prepared in situ
from N-chlorosuccinimide and 4-tolylthiol,[30] and reacted
with 9 to result in the desired thioether 11 in 96% yield
(Scheme 7).
ꢀ
a sulfur atom with bond lengths C2/C2a S1 of 1.756(3) ꢄ
and a bond angle C2-S1-C2a of 103.6(1)8. Table 1 summariz-
es selected molecular data for the new derivatives.
The intramolecular arrangement of the monomeric subu-
nits is of special interest for the photophysical behavior of 5,
6, and 10. For the case of 5, this arrangement is described by
a dihedral angle f of 71.0(1)8 between the C9BN2 mean
planes, a distance of 7.803(1) ꢄ between the centers of the
BODIPY moieties R, and by the closest distance between
fluorine atoms of different BF2 groups dFF of F2···F4 of
5.447(2) ꢄ (Figure 2). Apparently, the orientation of the
Scheme 7. Electrophilic substitution to thioether 11: a) 1) 4-Tolylthiol,
NCS, RT, 30 min; 2) 9, 08C, 15 min.
subchromophores of
6 deviates significantly, and the
BODIPY units are also further apart from each other
(Figure 3). This is quantified by a dihedral angle of f =
33.2(1)8 between the C9BN2 mean planes, an intercenter dis-
tance R of the BODIPY moieties of 8.541(1) ꢄ, and the
closest distance between fluorine atoms of different BF2
groups (F2···F3) of dFF =5.729(2) ꢄ. The angulated structure
of 10 finally leads to values for the dihedral angle of f=
79.7(1)8, for the distance between BODIPY centers of R=
In solution, the dimeric dyes 5, 6, and 10 are clearly dis-
tinct from the monomers 3, 4, 8, 9 and 11 in several aspects.
Most obvious is the color change from orange-yellow with
intense yellowish-green fluorescence for solutions of the
monomeric starting materials (3, 4, and 9) to blue and blue-
violet, respectively, without visible fluorescence by naked
eye for CH2Cl2 solutions of the dimers 5 and 6. This change
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BODIPY DYEmers
FULL PAPER
Table 1. Selected bond lengths [ꢄ], distances [ꢄ], and bond angles [8]
for 5, 6, 8, 10, and 11.
5
6
8
10[a]
11
ꢀ
B1 N1
1.555(3)
1.546(2)
1.561(2)
1.558(2)
1.393(2)
1.394(2)
1.385(2)
1.394(2)
2.491(2)
2.504(2)
106.9(1)
106.8(1)
109.7(1)
109.0(1)
1.567(3)
1.556(3)
1.550(3)
1.568(3)
1.392(3)
1.380(3)
1.382(3)
1.388(3)
2.493(2)
2.495(2)
106.0(2)
106.3(2)
109.8(2)
110.3(2)
1.536(5)
1.557(5)
–
–
1.380(5)
1.386(5)
–
–
2.484(4)
–
106.9(3)
–
109.8(3)
–
1.554(4)
1.552(2)
1.551(2)
–
–
1.391(2)
1.391(2)
–
–
2.489(2)
–
106.7(1)
–
109.0(1)
–
ꢀ
B1 N2
1.549(4)
[b]
ꢀ
B2 N3
[b]
ꢀ
B2 N4
ꢀ
B1 F1
1.386(4)
ꢀ
B1 F2
1.396(3)
[b]
ꢀ
B2 F3
[b]
ꢀ
B2 F4
Figure 3. Molecular structure of 6 (ellipsoids set at 50% probability; hy-
drogen atoms omitted for clarity). Left: View of the complete molecule.
Right: View along the intersection line of the C9BN2 mean squares
planes.
N1···N2
2.488(3)
[b]
N3···N4
N1-B1-N2
N3-B2-N4
F1-B1-F2
F3-B2-F4
106.6(2)
[b]
109.1(2)
[b]
[a] Data given only for one of the two crystallographically independent
molecules. [b] Due to a symmetry operation (two-fold rotational axis) the
halves of a molecule of 10 are crystallographically identical.
8.193(1) ꢄ, and for the closest distance between fluorine
atoms (F1···F1a) of dFF =7.518(2) ꢄ (Figure 5).
The intermolecular contacts between the BODIPY units
of the DYEmers are mainly based on p-stacking interac-
tions. For 5, 6, and 10, however, different types of arrange-
ments are evident from the crystal structures. The a-con-
Figure 4. Molecular structure of 8 (ellipsoids set at 50% probability; hy-
nected sulfide
5 shows pairs of antiparallel stacked
drogen atoms omitted for clarity).
BODIPY moieties in distances of 3.448 and 3.561 ꢄ. Both
BODIPY subunits of a given molecule show short contacts
to different neighbors and form supramolecular 1D poly-
meric strands. The interaction of the second side of each
BODIPY is inhibited sterically by a methyl group of the re-
spective other half of the molecule (at C8 and C11, see
Figure 2). Compound 6 forms two distinct and loosely
stacked columns in the crystal. These columns contain
BODIPY subunits in distances between 3.461 and 3.813 ꢄ
and progress in two orientations along the crystallographic a
direction. 10 finally contains tilted and slipped stacks in esti-
mated distances of 3.39 and 3.70 ꢄ. These stacks are point-
ing in two different directions, which organizes the DYEmer
molecules in a zig-zag orientation towards each other.
DFT structures and solution dynamics: For an analysis of
the influence of the different p-stacking interactions of the
sulfur-bridged DYEmers on the conformation and intrachro-
Figure 5. Molecular structure of 10 (ellipsoids set at 50% probability; hy-
drogen atoms omitted for clarity). Top: View of the complete molecule.
Bottom: View along the intersection line of the C9BN2 mean squares
planes.
Figure 2. Molecular structure of 5 (ellipsoids set at 50% probability; hy-
drogen atoms omitted for clarity). Left: View of the complete molecule.
Right: View along the intersection line of the C9BN2 mean squares
planes.
Figure 6. Molecular structure of 11 (ellipsoids set at 50% probability; hy-
drogen atoms omitted for clarity).
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M. Brçring et al.
mophore arrangement, DFT calculations of the solution
structures of 5, 6, and 10’[33] have been undertaken. A con-
formational analysis using the OPLS_2005 force field and a
Monte Carlo sampling was first carried out to find low-
energy conformations of the compounds in dichloromethane
solution, which were then further optimized using density
functional theory (B3LYP/6-31GACTHNUGTRNEUNG(d,p)). The results of these
calculations are given in Figures 7–9.
For 5, a stretched main conformer 5-I is found in solution,
for which a weak electronic conjugation of the BODIPY
units appears feasible via a free 3p orbital of the bridging
sulfur atom. The energetically closest conformer 5-II is
found 17.46 kJmolꢀ1 above the stretched form (Figure 7).
This high-energy conformer is characterized by a compact
overall structure with
a small distance between the
Figure 8. Calculated solution structures for the low-energy conformations
BODIPY unit centers R of 3.668 ꢄ, and an increased dihe-
dral angle f between the BODIPY planes of 83.198. As the
population of this conformer should be low, a rigid and
stretched out structure may be proposed for 5 in solution
(compare 19F NMR study below). Disulfide 6 resides in solu-
tion in three geometrically quite different conformers 6-I–6-
III (Figure 8) which are energetically very similar, so that a
high conformational flexibility should result. The b-bridged
sulfide 10’ also shows three energetically and structurally
similar conformers 10’-I–10’-III (Figure 9). These conformers
are distinct by different relative rotational states of the
BODIPY moieties, and therefore by the relative orientation
of the permanent dipol moments of the monomeric subunits.
Obviously, these permanent dipoles play only a minor role
for the stabilization of a certain conformer. Generally, it is
obvious that the energetical minimum conformations from
the solution calculations 5-I, 6-I, and 10’-I are structurally
equivalent to the conformations found in the crystal lattices
of the DYEmers 5, 6, and 10. Structural deviations by p
stacking and other packing effects are minor and mainly ef-
fective by small rotations of one (5, 6) or two (10) of the
BODIPY units along the C–S axis.
of 6 (B3LYP/6-31GACTHNUTRGNEU(NG d,p), CH2Cl2; H atoms removed for clarity), with rel-
ative energy levels and characteristic metrics (see text).
Figure 9. Calculated solution structures for the low-energy conformations
of 10’ (B3LYP/6-31GACTHNUTRGNEUNG(d,p), CH2Cl2; H atoms removed for clarity), with
relative energy levels and characteristic metrics (see text).
As another aspect of the solution structures of the
DYEmers 5, 6, and 10, the NMR characterisation of the
three new BODIPY dimers reveals a special feature. In the
19F NMR spectrum of 5, two broad and unspecific signals at
d ꢁ ꢀ139 and ꢀ148 ppm appear at ambient temperature.
These signals indicate two different fluorine positions and
dynamic behavior for the symmetric compound. In contrast,
the disulfide 6 and the sulfide 10 give a single quartet in the
19F NMR spectrum (d ꢁ ꢀ142 and ꢀ147 ppm, respectively)
under the same conditions, which is typical for BODIPYs
with equal positions of the fluorine atoms, like for example
for 8 or 11. Apparently, for 5 the rotation of the two
BODIPY units relative to each other is sterically hindered.
The effect is reminiscent to the behavior of BODIPY
ꢀ
dimers with direct C C connections. These DYEmers pos-
sess a rigid angulated structure and were studied in detail by
Figure 7. Calculated solution structures for the low-energy conformations
of 5 (B3LYP/6-31G
ACHTUNGTRENNUNG(d,p), CH2Cl2; H atoms removed for clarity), with rel-
ative energy levels and characteristic metrics (see text).
NMR spectroscopy at room temperature.[11,34]
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FULL PAPER
For sulfide 5 the two broad 19F signals are sharpening
upon lowering the temperature stepwise from 300 to 213 K
(Figure 10, right), and complex coupling patterns appear for
both signals. This reveals that the S-bridged BODIPY dimer
5 resides in a locked conformation and behaves similar to
directly coupled DYEmers. Upon heating the signals broad-
en, and coalescence is observed at ~344 K (Figure 10, left).
The broad signal for the fast exchange is observed at a
chemical shift of d ꢁ ꢀ143 ppm, typical for rotationally un-
hindered BODIPYs such as 8. From the coalescence tem-
perature of Tc =344(2) K and a peak separation of 3300 Hz,
the free energy of activation can be estimated to DG° =
59.2(4) kJmolꢀ1 for this rotational process.
is much larger for the b-substituted BODIPY than for 8,
and accompanied by a smaller quantum yield of 0.27 in
CH2Cl2. This differing optical behavior appears to depend
on the site of substitution alone and was found earlier for
regioisomeric BODIPYs with one alkynyl substituent.[27,36]
Table 2. Photophysical properties of the BODIPYs in CH2Cl2 at room
temperature.
Compound
Absorption
Fluorescence
lmax [nm] e [104 Lmolꢀ1 cmꢀ1
]
lmax [nm] Ff[a]
3
4
5
388.0
526.0
381.0
532.0
388.0
510.5
628.5
393.5
509.0
592.0
384.0
545.0
374.0
520.0
389.0
507.0
551.0
387.0
524.0
1.6
8.8
1.2
9.1
1.4
2.0
12.2
1.3
4.4
7.1
1.6
9.0
0.7
8.0
1.7
4.8
10.5
0.8
6.9
536
542
652
0.84
0.57
~0.007[b]
6
659
~0.006[b]
8
572
532
614
0.82
0.97
9
10
~0.005[b]
11
600
0.27
[a] Absolute fluorescence quantum yields in aerated solvent using an in-
tegrating sphere. [b] Note ref. [37].
Figure 10. 19F NMR spectra of 5 at different temperatures (376 Hz; left
side: high temperatures with coalescence, C2D2Cl4; right side: low tem-
peratures, CD2Cl2).
Photophysical properties: All BODIPY compounds were
examined by absorption and steady-state fluorescence spec-
troscopy in dichloromethane, and the results are summar-
ized in Table 2. Thioether 8 shows typical optical spectra for
a simple monomeric BODIPY (Figure 11). The shape of the
absorption spectrum is described by one main band at
!
545.0 nm, which is assigned to a strong S1 S0 transition, and
Figure 11. Optical spectra of 8 recorded in CH2Cl2.
!
one weaker, broad band at 384.0 nm attributed to the S2 S0
transition.[3] The excitation spectrum registered on the emis-
sion maximum is similar to the absorption spectrum. The in-
tense fluorescence of 8 occurs from the S1!S0 transition in
a mirror-shape band with a Stokes shift of 27 nm at 572 nm
(Ff =0.82 in CH2Cl2). The spectra of 8 are bathochromically
shifted relative to the a-free BODIPY 3 due to the elec-
tron-rich sulfur substituent.[35] The red-shift for the main ab-
sorption of the monomeric reference 11 relative to the start-
ing material 9 is much less pronounced (Table 2 and
Figure 12). Interestingly, however, the Stokes shift of 76 nm
In the absorption spectra of the two different dimers
bridged through the a-positions the expected exciton cou-
pling with splitting of the S1 level is observed. This typical
DYEmer behavior is likely due to the close spatial relation-
ship of the non-conjugated (or only weakly conjugated) sub-
chromophores, and to the observed conformational charac-
teristics.[16] The electronic transition dipole moment for the
!
S1 S0 transition of a BODIPY unit is perpendicular (i.e.,
along the 2,6-positions) to the C2v symmetry axis of the core,
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M. Brçring et al.
Figure 13. Optical spectra of 5 recorded in CH2Cl2.
Figure 12. Optical spectra of 11 recorded in CH2Cl2.
which is in direction of the permanent electric dipole
moment from the boron atom to the meso-position.[38] Thus,
the absorption spectrum of the sulfide 5 in CH2Cl2 shows
three bands (Figure 13), one weak, broad band at 388.0 nm
band in the spectrum of 5. This difference in relative intensi-
ties suggests a more tilted alignment of the transition dipole
moments of the subchromophores of 6 in solution. Further-
more, the smaller exciton splitting can be ascribed to the in-
creased distance of the BODIPY subunits in the disulfide
!
for the S2 S0 transition and two bands for exciton split
!
!
S1 S0 transitions according to the model of Kasha. One of
compared to the sulfide.[16] The S2 S0 transitions in the
these latter bands at 510.5 nm has a small higher-energetic
shoulder and is hypsochromically shifted relative to the
monomeric 8, while the other band at 628.5 nm is far batho-
chromically shifted, which results in a large exciton splitting
of 118 nm. The split band to lower energy is much more in-
tense than the high-energy band, which accounts for a
stretched and only moderately angular alignment of the
transition dipole moments of the BODIPY subunits in solu-
tion.[16] The intramolecular nature of this phenomenon was
proven by absorption spectra from a dilution series. No con-
centration effects on band position and molar absorption co-
efficient could be observed from 10ꢀ4 to 10ꢀ6 molLꢀ1 in
CH2Cl2. The fluorescence emission of 5 occurs in one band
with a small Stokes shift at ~650 nm and is quenched in
polar solvents (Ff = ~0.007 in CH2Cl2). In non-polar sol-
vents with only poor solubility of 5 this dimer shows a visi-
ble red fluorescence, and significantly higher fluorescence
quantum yields were determined (Ff =0.44 in n-hexane;
Ff =0.11 in toluene). The excitation spectrum shows three
bands at positions similar to the absorption spectrum. Varia-
tion of the solvent exhibits a dependence of the absorption
and emission energies of 5 on the solvent polarity, resulting
in a negative solvatochromism. For MeCN, MeOH, EtOAc,
CH2Cl2, Et2O, n-hexane, and toluene, a shift of lmax,abs from
616 to 639 nm and of lmax,em from 639 to 655 nm is observed.
Such a behavior is generally typical for monomeric BODI-
PYs,[39] but appears to be much stronger for DYEmers.
spectra of both dimers show one broad, weak band, which
implies little exciton coupling for the S2 level. The fluores-
cence emission of 6 occurs at 659 nm (lexc =592 nm) and is
quenched in CH2Cl2 (Ff = ~0.006) as well as in toluene.
Quite unexpectedly, only two bands are present in the exci-
tation spectrum, a larger one at the S2 S0 transition region,
and a smaller one in the center of the split main absorption
bands at 532 nm.
!
Figure 14. Optical spectra of 6 recorded in CH2Cl2.
The absorption spectrum of disulfide 6 also shows a split
low-energy absorption in CH2Cl2 (Figure 14). In this case,
however, the exciton splitting of 83 nm is smaller than for 5.
The low-energy band at 592.0 nm is less red-shifted as in the
spectrum of 5 and similar to the signal of monomeric 8. The
second band at 509.0 nm is more intense than the respective
The b-bridged sulfide 10 produces yet different optical
spectra (Figure 15). The exciton coupling is not as pro-
nounced as for the a-bridged DYEmers 5 and 6 and results
for the S1 S0 transition in an intense band at 551.0 nm with
a distinct shoulder at 507.0 nm. As for the other dimers, the
!
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BODIPY DYEmers
FULL PAPER
exciton coupling on the S2 level of 10 is only of minor im-
portance. The small exciton splitting of 44 nm can be ex-
plained by assuming an increased distance of the subunits in
10 compared to 5 or 6, and the lower intensity of the high-
energy band accounts for a more linear alignment of the
transition dipole moments of the subchromophores of 10 in
solution. The relative excitonic coupling strengths of 5 and
tionless deactivation process through rotation along the S–S
axis of 6, these points have to remain highly speculative sug-
gestions until detailed lifetime analyses are available.
Analyses of excitonic couplings: The model of exciton cou-
pling[16,41–43] considers a dimer of two identical (or similar)
chromophores A and B, which are connected covalently
without conjugation (or are aggregated). The coupling is de-
scribed as an electrostatic interaction between the transition
dipole moments meg of the two subchromophores leading to
an energy splitting of the excited state. Only the strong cou-
pling case is assumed by this model elaborated for molecu-
lar dimers by Kasha.[16] In this model, VAB is the energy of
the exciton splitting, also called Davydov splitting,[44] which
is twice the energy of the exciton interaction. This energy
can be written after a point-dipole point-dipole approxima-
tion of the transition dipole moments as Equation (1):
ꢀ
10 coincidence with those of the directly C C coupled
BODIPY dimers. Here, the a-coupled homodimers show a
large split of the main absorption,[11,15,40] whereas for the b-
coupled homodimers only a bathochromic shift is found.[14]
The influence on the absorption spectra thus appears to be
depending on the linking position. In the fluorescence spec-
trum 10 emits with a Stokes shift of 63 nm at 614 nm, and
the fluorescence is largely quenched in CH2Cl2 (Ff = ~
0.005) as for 5 and 6. In non-polar toluene, however, a much
higher fluorescence quantum yield of 0.42 is observed. The
excitation spectrum of 10 follows the absorption spectrum
!
ꢀ
ꢁ
2
and supports, that the S1 S0 transition is slightly split.
monomer
eg
m
ð1Þ
VAB ¼ 2
where m
k
4pe0n2R3
monomer
eg
is the value of the transition dipole moment
of the monomer, R the intradimeric distance between the
centers of the transition dipole moments and k the orienta-
tion factor. The orientation factor describes the angle rela-
tion between the subchromophoric transition dipole mo-
ments relative to the center-to-center distance vector
(Figure 16) as described in Equation (2):
k ¼ cos qAB ꢀ 3 cos qA cos qB
ð2Þ
where qA and qB are the angles between the transition
dipole moment of the corresponding subchromophore and
the distance vector and qAB the angle between both dipole
Figure 15. Optical spectra of 10 recorded in CH2Cl2.
For both sulfides 5 and 10, the fluorescence quantum
yield is reduced in polar dichloromethane and strongly en-
hanced in unpolar solvents such as toluene, while no signifi-
cant changes are observed in absorption (meaning that the
ground state conformation is almost independent of the sol-
vent), despite from the negative solvatochromism. This be-
havior suggests an intramolecular charge transfer (ICT)
process in the excited state under solvent-induced symmetry
breaking. For other BODIPY DYEmers, but without a
ꢀ
bridging heteroatom (C C and methylene bridge), the same
conclusion was drawn and discussed in more detail by
Thompson[10d] and Ziessel[10f]. In the case of the disulfide 6,
the singlet quantum yield is low regardless of the solvent po-
larity. The unusual excitation spectrum of 6 in combination
with the accessibility of several low-lying conformers may
be interpreted in terms of an intramolecular excimer forma-
tion independent of the solvent. However, due to the uncer-
tain influence of the sulfur atoms, and to a probable radia-
Figure 16. Top: Angular relationship between the transition dipole mo-
ments of A and B connected by the distance vector R to determine the
orientation factor k. Bottom: Example for determination of the angles
qA, qB, and qAB (not shown), and of the center-to-center distance R for
the molecular structure of 5.
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M. Brçring et al.
moments. The medium is considered by the square of the re-
fractive index n multiplied with the vacuum permittivity e0.
The transition dipole moment of a monomeric BODIPY
unit is polarized along the b-positions, known from thin-film
examinations and quantum-chemical calculations.[38,45]
Therefore, the subchromophoric dipole vectors have been
marked as lines between these positions into the molecular
structure of the dimer. The centroids of the six-membered
C3BN2 rings served as centers of point dipoles. Figure 16
(bottom) illustrates this procedure for DYEmer 5. With k
and R (in ꢄ) from the molecular structure (either from
monomer
eg
XRD or DFT studies) and with m
(in Cm) from the
integration of the main absorption band of the monomeric
reference 8 or 11, the exciton splitting energy VAB (in J) of
the dimer can be calculated. The intensity ratio of the two
dimer
egꢂ
exciton splitted absorption bands D
is given by the ratio
Figure 17. Comparison of measured and calculated absorption bands for
the crystallographic conformer and for two calculated low-energy con-
formers of 5 (CH2Cl2). The calculated curves of the main absorption as a
function of the wavelength are given by the sum of the two exciton bands
assuming Lorentz band shape with a half width of 15 nm.
of the dipole strengths relative to the monomer, which is
only depending on the angle qAB between the subchromo-
phores given by Equation (3):
ꢂ
ꢂ
ꢂ
ꢂ
2
dimer
egꢂ
D
¼ mdimer ¼ Dmonomer ꢃ ð1 ꢂ cos qAB
Þ
ð3Þ
ꢂ
ꢂ
egꢂ
eg
Table 3 and Figures 17–19 summarize the results from the
calculations using Kashaꢃs model.
Table 3. Angle relation and center-to-center distance between the sub-
chromophores, orientation factor
k and ratio of the relative dipole
strengths for the molecular structures from X-ray diffraction (5, 6, 10)
and from quantum-chemical calculations for a CH2Cl2 solution (5-I/II, 6-
I/II/III, 10’-I/II/III).
dimer
dimer
Cpd.[a,b] qA [8]
qB [8]
qAB [8]
R [ꢄ]
k
D
/D
egþ egꢀ
5
26.61(2) 40.81(2) 49.67(3) 7.803(1) ꢀ1.38 1.65:0.35
5-I
5-II
6
6-I
6-II
6-III
31.89
59.63
31.88
59.63
40.16
71.48
7.901
6.041
ꢀ1.40 1.76:0.24
ꢀ0.45 1.32:0.68
34.33(2) 41.89(2) 57.19(3) 8.541(1) ꢀ1.30 1.54:0.46
40.86
38.27
57.33
41.73
71.83
57.34
62.69
84.85
68.32
8.710
7.601
6.455
ꢀ1.23 1.46:0.54
ꢀ0.64 1.09:0.91
ꢀ0.50 1.37:0.63
37.19(2) 37.19(2) 74.36(3) 8.193(1) ꢀ1.63 1.27:0.73
Figure 18. Comparison of measured and calculated absorption bands for
the crystallographic conformer and for three calculated low-energy con-
formers of 6 (CH2Cl2). The calculated curves of the main absorption as a
function of the wavelength are given by the sum of the two exciton bands
assuming Lorentz band shape with a half width of 15 nm.
10
37.11(2) 37.11(2) 74.09(4) 8.199(1) ꢀ1.63 1.27:0.73
10’-I
10’-II
10’-III
35.41
35.39
35.06
35.56
35.40
35.98
70.93
70.75
71.04
8.362
8.262
8.306
ꢀ1.66 1.33:0.67
ꢀ1.66 1.33:0.67
ꢀ1.66 1.32:0.68
[a] For 8: m=3.43ꢅ10ꢀ29 Cm (at lmax =545 nm in CH2Cl2). [b] For 11: m=
2.53ꢅ10ꢀ29 Cm (at lmax =524 nm in CH2Cl2).
transition dipole moment as a point dipole, while the ob-
served red shift of the absorption bands indicates significant
conjugation between the two BODIPY subchromophores
through a free 3p electron pair of the sulfur bridge. The
same explanations can be used to also discuss the differen-
ces between the calculated and the measured spectra of 10/
10’. Other than above, however, only a slight bathochromic
shift is observed in the measurement. This indicates a small-
er degree of conjugation in 10 as expected for the special
conformations of this DYEmer in the solid and in solution.
For the three lowest-energy conformers of the disulfide 6,
significantly different optical absorption spectra are pro-
posed (Figure 18). The measured intensity ratio of the exci-
In the case of 5 the measured intensity ratio of the exci-
tonic absorptions is approximated equally well by the solu-
tion minimum conformation 5-I and by the conformation in
the crystalline state. Due to the rather high energy level, the
conformational state 5-II appears to play only a negligible
role. However, the observed absorption spectrum deviates
markedly from the calculated data by the strength of the ex-
citonic splitting, and by the center of gravity of the excitonic
absorption. This former finding is presumably being caused
by the oversimplifying description of the solvation by a ho-
mogeneous dielectric field and the approximation of the
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BODIPY DYEmers
FULL PAPER
step at ꢀ1.47 V (8) and ꢀ1.55 V (11) is irreversible with an
associated wave on the oxidation side (Eopx =ꢀ0.53 V for 8
and Eopx =ꢀ0.60 V for 11). This irreversible reduction proc-
ess is typical for meso-free BODIPYs.[46] There is no sign of
electrochemical activity associated with the tolyl thioether
moiety in either derivative. The major difference between
the monomeric compounds is that the electrochemical
bandgap of 11 is significantly larger (220 mV) than that of 8.
This compares well to the differences in the optical spectra
of these BODIPYs.
The cyclic voltammogram of the sulfide 5 indicates the di-
meric nature of the compound by electronic communication
between the subunits (Figure 20). The above-mentioned oxi-
dation and reduction processes for a single meso-free
BODIPY unit are now split into two monoelectronic redox
processes each, assuming an EE mechanism. The reversible
oxidation to the radical monocation is slightly easier than
for the monomer 8. The quasi-reversible oxidation to the di-
cation occurs at 0.88 V, which means that both oxidation
steps are well separated with a split of ~400 mV. The first
reduction step of 5 to the radical monoanion at ꢀ1.40 V
shows the same irreversible feature as for 8. The second re-
duction step to the dianion occurs clearly separated from
the first step at ꢀ1.77 V and is quasi-reversible. Again, no
sign of electrochemical activity can be associated with the
sulfide bridge. The high degree of electronic interaction be-
tween the subunits of dimer 5, which is apparent from the
clear separation of the redox waves, reflects also the strong
exciton coupling in the absorption spectrum. A similar cor-
relation has been seen for other DYEmers before.[12,14,15,47]
Similar to 5, a doubling of the number of waves for oxida-
tion and reduction is observed in the cyclic voltammogram
of the disulfide 6 (Figure 20). The potentials of the reduc-
tion steps to the monoanion and the dianion by consecutive
electron transfers onto the BODIPY moieties are similar to
those of the sulfide 5 and show a separation of ~350 mV.
The first and second oxidation of 6 are also split with
Figure 19. Comparison of measured and calculated absorption bands for
the crystallographic conformer and for three calculated low-energy con-
formers of 10/10’ (CH2Cl2). The calculated curves of the main absorption
as a function of the wavelength are given by the sum of the two exciton
bands assuming Lorentz band shape with a half width of 25 nm.
tonic splitting is best reproduced by a weighted mixture of
the three most favorable conformers 6-I, 6-II, and 6-III, al-
though again the strengths of the splitting is much smaller
than in the real spectrum.
Electrochemical investigations: Cyclic voltammetry of the
three S-bridged DYEmers 5, 6, and 10, and the two refer-
ence compounds 8 and 11 was carried out in dry CH2Cl2
(0.5 molLꢀ1 nBu4NPF6), using ferrocene as the internal
standard. The obtained potentials are listed in Table 4. The
Table 4. Electrochemical potentials of the sulfur-containing BODIPYs in
CH2Cl2 (0.5 molLꢀ1 nBu4NPF6) versus ferrocene as internal standard at
room temperature.
red
1=2
ox
1=2
Compound
E
[V]
E
[V]
Erped
Eopx
A
E
[V]
[V]
Aꢀ/A
2ꢀ/Aꢀ
A+/A
A2+/A+
2RSꢀ/(RS)2
5
ꢀ1.40
(irr.)
ꢀ1.77
(90)
ꢀ1.79
(70)
–
0.48
(80)
0.55
(210)
0.54
(100)
0.59
(100)
0.68
(160)
0.88
(150)
0.91
(150)
–
–
–
6
ꢀ1.44
ꢀ1.22 ꢀ0.33
(irr.)
8
ꢀ1.47
(irr.)
–
–
–
–
–
–
10
11
ꢀ1.55
ꢀ1.67
(120)
–
0.91
(130)
–
(irr.)
ꢀ1.55
(irr.)
cyclic voltammograms for the BODIPYs 8 and 11 (see Sup-
porting Information) are similar and characteristic for mon-
omeric BODIPYs. In either case the voltammogram displays
one one-electron oxidation and one one-electron reduction.
The oxidation step at 0.54 V (8) and 0.68 V (11) is electro-
chemically quasi-reversible (ipa/ipc ꢁ 1), while the reduction
Figure 20. Cyclic voltammograms of 5 (5.8 mmolLꢀ1), 6 (4.4 mmolLꢀ1),
and 10 (3.1 mmolLꢀ1) in CH2Cl2 containing nBu4NPF6 (0.5 mol/L) at a
scan rate of 50 mVsꢀ1 (5) resp. 100 mVsꢀ1 (6, 10) at room temperature.
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M. Brçring et al.
~360 mV, and the potential of the first oxidation is very sim-
ilar to that for the monomeric 8. In addition to these typical
characteristics of such a DYEmer, 6 shows an additional
redox process with a reduction at ꢀ1.22 V and a significantly
displaced back oxidation at ꢀ0.33 V. Each process repre-
sents a two-electron transfer and can be assigned to a disul-
fide/twofold thiolate transition. Related diaryl disulfides
(RS)2 are electrochemically reduced by an ECE mechanism
to two thiolate anions RSꢀ in an (electro)chemically reversi-
ble process.[48] The large peak separation of disulfide cleav-
age and formation of ~890 mV for 6 indicates significant ki-
netic hindrance and therefore bistability of the chemically
reversible disulfide/thiolate couple over this potential range.
The sulfide 10 exhibits a similar cyclic voltammogram
(Figure 20) as the sulfide 5 with a first irreversible and a
second quasi-reversible reduction step, and with two quasi-
reversible one-electron oxidation processes. The electronic
communication between the subunits is obviously effective
through the b-positions, too. Compared to the a-bridged
dimer 5, however, the splitting of the redox processes of
320 mV for the oxidation and 120 mV for the reduction side
is smaller for 10, and in particular the two consecutive re-
duction peaks are not as well separated. The smaller poten-
tial splitting corresponds with a less pronounced exciton
coupling in the absorption spectrum of 10. Similar differen-
served if an electronic conjugation between the BODIPY
subunits is conformationally possible and effective through
the sulfur bridge. With these deviations in mind, the Kasha
model of excitonic coupling in combination with a confor-
mational analysis provides a simple and acceptable method
for a qualitative prediction of optical absorption characteris-
tics for the class of BODIPY-based DYEmers.
Experimental Section
General: Solvents were dried according to standard procedures under
inert gas atmosphere of argon or nitrogen. All reagents were purchased
from commercial sources in reagent grade and used as received, unless
stated otherwise. NMR spectra were obtained by using a Bruker DRX
400 and a Bruker Avance 300 spectrometer with room temperature as
measuring temperature, if not otherwise indicated. Chemical shifts (d)
are given in ppm relative to residual protio solvent resonances (1H,
13C NMR spectra) or to external standards (BF3·Et2O for 11B and CFCl3
for 19F NMR spectra). High-resolution ESI mass spectra were recorded
with a Finnigan LTQ FT. MALDI-TOF spectra were recorded on a
Bruker Biflex IV with sinapinic acid in MeCN as matrix. A Shimadzu
UV-1601 PC spectrophotometer and a Varian Cary Eclipse spectrofluor-
ometer were used to acquire absorption, emission, and excitation spectra.
Absolute fluorescence quantum yields were determined in aerated sol-
vents by a PTI QuantaMaster 40 UV VIS spectrofluorometer equipped
with an integrating sphere. The provided corrections for excitation and
emission were applied. Cyclic voltammetry was performed with a Prince-
ton Applied Research VersaSTAT 3 potentiostat under inert conditions
at room temperature in absolute CH2Cl2 containing nBu4NPF6. A self-
made 3-electrode set-up with two platinum wires as working and counter
electrodes and a silver wire as quasi-reference electrode was used and
ferrocene as internal standard had been added. Single-crystal X-ray dif-
fraction studies were performed by using a Stoe IPDS-I X-ray diffrac-
tometer (8) or an Oxford Diffraction Xcalibur diffractometer with an
Atlas (Nova) detector (5, 6, 10, 11). All structures were solved and re-
fined by using the SHELXS programs for crystal structure determina-
tion[49] and refinement.[50] The gas phase global minima of 5, 6, and 10’
were obtained by first applying an extended conformational analysis
using the OPLS_2005 force field[51] together with a Monte Carlo torsional
sampling as implemented in the Macromodel (BatchMin 9.9) program.[52]
The lowest-energy conformations of 5, 6, and 10’ have then been further
optimized by applying density functional theory. For all optimizations
and subsequent calculations the B3LYP hybrid density functional[53] as
implemented in the Gaussian09 set of programs[54] was employed. All
atoms were described by standard double zeta all electron basis set aug-
ces between a,a- and b,b-coupled DYEmers have been
[14,15,47]
ꢀ
found earlier for the directly C C coupled derivatives.
Furthermore, the potential of the first oxidation of 10 is
slightly more positive than those of the two other DYEmers
5 and 6, and the first reduction wave occurs at a more nega-
tive potential of ꢀ1.55 V, which results in a larger electro-
chemical band gap for 10.
Conclusion
In summary, we have shown that different sulfur-bridged
DYEmers are available from simple BODIPY precursors by
nucleophilic and electrophilic aromatic substitution reac-
tions, and that the photophysical and electrochemical prop-
erties of these DYEmers are clearly distinct from those of
the monomers due to intense communication between the
BODIPY subunits. The structures of the DYEmers in the
solid state and in solution have been analyzed in some
detail and show far reaching similarities with respect to the
lowest-energy conformers, despite the fact that packing ef-
fects are clearly present in the crystalline states.
If the conformations and relative orientations of the
BODIPY subunits in a DYEmer chromophore are known,
an application of the Kasha model provides well adapted in-
formation concerning the intensity ratio of the exciton split-
ting in all cases. The strength of the excitonic splitting, how-
ever, is generally underestimated by this method. We ac-
count an oversimplified description of the solvation and the
point-dipole approximation used by the Kasha model for
this deviation, although other aspects certainly also contrib-
ute to this effect. Additional bathochromic shifts are ob-
mented with one set of polarization functions (6-31GACTHNUGRTENUNG(d,p)). First a geom-
etry optimization was carried out out for a single molecule in an implicit
solvent (dichloromethane). Solvent effects were incorporated by employ-
ing the polarizable continuum model (PCM).[55] After the relevant sta-
tionary points were localized on the energy surface, they were further
characterized as minima states by normal mode analysis based on the an-
alytical energy second derivatives. Enthalpic and entropic contributions
were calculated by statistical thermodynamics as implemented in the
Gaussian09 set of programs.[54]
Bis-(1,2,5,6,7-pentamethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-3-
yl)-sulfide (5): To a suspension of 4 (199 mg, 0.584 mmol) in toluene
(11 mL) a solution of Na2S·9H2O (77 mg, 0.321 mmol) in H2O (174 mL)
and nBu4NBr (catalytic amount) were added. The mixture was heated to
reflux for 3.5 h with a color change to dark violet. After cooling the sol-
vent was removed on a rotary evaporator. By column chromatography
(silica, CH2Cl2) the product was separated in the fourth colorful, deep
blue fraction. After evaporation of the solvent and drying in vacuo the
product was obtained as dark violet plates with intense green shine
(91 mg, 56%). 1H NMR (400 MHz, CD2Cl2): d=7.04 (s, 2H, 8/8’-CH),
2.51 (s, 6H, 5/5’-CCH3), 2.19 (s, 6H, 7/7’-CCH3), 2.13 (s, 6H, 1/1’-CCH3),
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BODIPY DYEmers
FULL PAPER
1.96 (s, 6H, 6/6’-CCH3), 1.60 ppm (s, 6H, 2/2’-CCH3); 13C NMR
(100 MHz, CD2Cl2): d=160.4 (2C, 5/5’-CCH3), 142.2 (2C, 3/3’-CS), 139.9
(2C, 7/7’-CCH3), 136.5 (2C, 1/1’-CCH3), 134.9 (2C, 7a/7’a-C), 133.3 (2C,
8a/8’a-C), 130.2 (2C, 2/2’-CCH3), 128.1 (2C, 6/6’-CCH3), 118.8 (2C, 8/8’-
CH), 13.5 (2C, 5/5’-CCH3), 10.0 (4C, 1/1’-CCH3, 7/7’-CCH3), 9.3 (2C, 2/2’-
CCH3), 9.1 ppm (2C, 6/6’-CCH3); 19F NMR (376 MHz, CD2Cl2): d=
ꢀ138.8 (brs, 2F, 2ꢅBFF), ꢀ147.7 ppm (br s, 2F, 2ꢅBFF); 19F NMR
(282 MHz, [D6]DMSO): d=ꢀ136.7 (br s, 2F, 2ꢅBFF), ꢀ146.1 ppm (br s,
13C NMR (100 MHz, CDCl3): d=186.2, 139.9, 136.1, 132.9, 130.1, 123.6,
118.7, 104.1, 12.1 (CH3), 10.7 (CH3), 10.5 (CH3), 10.1 (CH3), 9.0 ppm
(CH3); MS (ESI): m/z: 269 [M+Na]+, 247 [M+H]+; HRMS (ESI): m/z:
calcd for C14H19N2S [M+H]+: 247.1263; found: 247.1270.
1,2,5,6,7-Pentamethyl-3-(4-methylphenylthio)-4,4-difluoro-4-bora-3a,4a-
diaza-s-indacene (8): To an orange-yellow solution of
4 (48 mg,
0.141 mmol) in a mixture of MeCN (40 mL) and CH2Cl2 (10 mL) NEt3
(29 mL, 0.211 mmol) was added at room temperature. Then addition of 4-
methylthiophenol (28 mg, 0.225 mmol) followed, whereupon the solution
rapidly turned red-orange. After 15 min the reaction mixture was poured
into CH2Cl2 (100 mL), washed with saturated aqueous NaCl solution
(20 mL) and H2O (20 mL), dried over Na2SO4, and the solvent was re-
moved on a rotary evaporator. Purification by column chromatography
(silica, CH2Cl2/n-pentane 1:1) gave the product as red-orange fraction of
orange fluorescence, which was obtained after drying in vacuo as red, mi-
crocrystalline solid with golden shine (39 mg, 72%). 1H NMR (400 MHz,
2F, 2ꢅBFF); 11B NMR (128 MHz, CD2Cl2): d=0.54 ppm (pseudo t, JBF
=
33 Hz, 2B, 2ꢅBF2); MS (MALDI-TOF): m/z: 554 [M]+; MS (ESI): m/z:
577 [M+Na]+, 535 [MꢀF]+; HRMS (ESI): m/z: calcd for
C28H32B2F4N4SNa [M+Na]+: 577.2362; found: 577.2362.
To a solution of 3 (32 mg, 0.122 mmol) in dry CH2Cl2 (13 mL) every hour
SCl2 (4ꢅ5 mL, 4ꢅ0.067 mmol) was added at 08C. After each addition the
ice bath was removed and meanwhile the mixture was stirred at room
temperature. The initially orange solution turned violet after complete
addition. After 5 h the solvent was removed on a rotary evaporator and
the residue was purified by column chromatography (silica, CH2Cl2). The
deep blue fraction gave after evaporation and drying in vacuo product 5
(8 mg, 24%).
3
CDCl3): d=7.20 (m, J=8.2 Hz, 2H, 2ꢅmeta-tolyl-CH), 7.04 (m, 2H, 2ꢅ
ortho-tolyl-CH), 6.99 (s, 1H, 8-CH), 2.53 (s, 3H, 5-CCH3), 2.28 (s, 3H,
tolyl-CH3), 2.16 (s, 3H, 7-CCH3), 2.13 (s, 3H, 1-CCH3), 1.94 (s, 3H, 6-
CCH3), 1.77 ppm (s, 3H, 2-CCH3); 13C NMR (75 MHz, CDCl3): d=161.1
(1C, 5-CCH3), 143.0 (1C, 3-CS), 139.7 (1C, 7-CCH3), 136.3 (1C, tolyl-
CCH3), 135.5 (1C, 1-CCH3), 134.9 (1C, 7a-C), 132.8 (1C, 8a-C), 132.2
(1C, tolyl-CS), 130.0 (1C, 2-CCH3), 129.8 (2C, 2ꢅortho-tolyl-CH), 129.5
(2C, 2ꢅmeta-tolyl-CH), 127.7 (1C, 6-CCH3), 118.8 (1C, 8-CH), 21.2 (1C,
tolyl-CH3), 13.3 (br s, 1C, 5-CCH3), 9.9 (1C, 1-CCH3), 9.9 (1C, 2-CCH3),
9.8 (1C, 7-CCH3), 9.1 ppm (1C, 6-CCH3); 19F NMR (188 MHz, CDCl3):
d=ꢀ143.4 ppm (q, JBF =32 Hz, 2F, BF2); 11B NMR (96 MHz, CDCl3): d=
0.99 ppm (t, JBF =32 Hz, 1B, BF2); MS (ESI): m/z: 407 [M+Na]+; HRMS
(ESI): m/z: calcd for C21H23BF2N2SNa [M+Na]+: 407.1535; found:
407.1539.
Bis-(1,2,5,6,7-pentamethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-3-
yl)-disulfide (6): To an orange solution of 4 (50 mg, 0.147 mmol) in
CH2Cl2 (12 mL) a freshly prepared yellow solution of Na2S·9H2O (20 mg,
0.083 mmol) and S (2.7 mg, 0.083 mmol) in MeOH (1 mL) was added and
stirred at room temperature. After 25 min the reaction was quenched by
filtering the now deep red reaction mixture through a short plug of silica
with CH2Cl2/Et2O 2:1. After evaporation the residue was purified by
column chromatography (silica, CH2Cl2/Et2O 49:1). First, residual start-
ing material was eluted (2 mg, 4%), then the product as a blue-violet
fraction without visible fluorescence. After evaporation and drying in
vacuo the product was obtained as a dark violet, microcrystalline solid
Bis-(6-ethyl-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacen-
2-yl)-sulfide (10): To an orange solution of 9 (60 mg, 0.219 mmol) in dry
CH2Cl2 (6 mL) a solution of S2Cl2 (11 mL, 0.138 mmol) in dry CH2Cl2
(2 mL) was added at ꢀ708C slowly within 2 h. After complete addition
the now red reaction mixture was quenched with H2O (20 mL) and
warmed up to room temperature. After phase separation the organic
phase was dried over Na2SO4 and the solvent was removed on a rotary
evaporator. The residue was purified by column chromatography (silica,
CH2Cl2/n-pentane 1:1). First, some orange to red-orange fractions with-
out visible fluorescence were eluted (presumably disulfides according to
mass spectrometry), then the violet-red main fraction without visible flu-
orescence of the sulfide 10 was eluted. The isolable product was freed
from the solvent in vacuo and gave a red, microcrystalline solid (16 mg,
1
with intense green shine (29 mg, 67%). H NMR (400 MHz, CD2Cl2): d=
7.08 (s, 2H, 8/8’-CH), 2.54 (s, 6H, 5/5’-CCH3), 2.20 (s, 6H, 7/7’-CCH3),
2.14 (s, 6H, 1/1’-CCH3), 1.97 (s, 6H, 6/6’-CCH3), 1.79 ppm (s, 6H, 2/2’-
CCH3); 13C NMR (100 MHz, CD2Cl2): d=164.3 (2C, 5/5’-CCH3), 142.4
(2C, 3/3’-CS), 141.1 (2C, 7/7’-CCH3), 136.7 (2C, 7a/7’a-C), 135.0 (2C, 8a/
8’a-C), 133.9 (2C), 132.6 (2C), 129.5 (2C, 6/6’-CCH3), 119.7 (2C, 8/8’-CH),
13.9 (2C, 5/5’-CCH3), 10.1 (2C, 1/1’-CCH3), 10.0 (2C, 7/7’-CCH3), 9.8 (2C,
2/2’-CCH3), 9.4 ppm (2C, 6/6’-CCH3); 19F NMR (376 MHz, CD2Cl2): d=
ꢀ141.9 ppm (q, JBF =31 Hz, 4F, 2ꢅBF2); 11B NMR (128 MHz, CD2Cl2):
d=0.89 ppm (t, JBF =31 Hz, 2B, 2ꢅBF2); MS (ESI): m/z: 609 [M+Na]+;
HRMS (ESI): m/z: calcd for C28H32B2F4N4S2Na [M+Na]+: 609.2083;
found: 609.2083.
1
25%). H NMR (400 MHz, CDCl3): d=6.97 (s, 2H, 8/8’-CH), 2.57 (s, 6H,
To a solution of 3 (84 mg, 0.320 mmol) in dry CH2Cl2 (6 mL) at ꢀ658C a
solution of S2Cl2 (15 mL, 0.184 mmol) in dry CH2Cl2 (1 mL) was added
dropwise within 25 min. After complete addition the reaction mixture
was stirred until warmed up to room temperature. The organic phase was
washed with H2O (10 mL) and then the solvent was removed on rotary
evaporator. The residue was separated by column chromatography
(silica, CH2Cl2). First, a violet fraction was eluted (presumably the trisul-
fide (MS (ESI): m/z: 641 [M+Na]+; HRMS (ESI): m/z: calcd for
C28H32B2F4N4S3Na [M+Na]+: 641.1804; found: 641.1805; also tetrasulfide
detected: MS (ESI): m/z: 673 [M+Na]+; HRMS (ESI): m/z calcd for
C28H32B2F4N4S4Na [M+Na]+: 673.1524; found: 673.1533), but instable
with decomposition to sulfide 5 and disulfide 6), then the blue fraction of
the sulfide 5, finally the blue-violet fraction of the disulfide 6. Both isola-
ble products were freed from the solvent in vacuo (5: 19 mg, 21%; 6:
31 mg, 33%).
3/3’-CCH3), 2.51 (s, 6H, 5/5’-CCH3), 2.39 (q, 3J=7.6 Hz, 4H, 6/6’-CCH2),
2.18 (s, 6H, 1/1’-CCH3), 2.17 (s, 6H, 7/7’-CCH3), 1.06 ppm (t, 3J=7.6 Hz,
6H, 2ꢅCH2CH3); 13C NMR (100 MHz, CDCl3): d=158.8 (2C, 5/5’-
CCH3), 155.8 (2C, 3/3’-CCH3), 140.5 (2C, 1/1’-CCH3), 138.8 (2C, 7/7’-
CCH3), 134.0 (2C, 7a/7’a-C), 133.5 (2C, 6/6’-CCH2), 131.4 (2C, 8a/8’a-C),
119.4 (4C, 8/8’-CH and 2/2’-CS), 17.4 (2C, 6/6’-CCH2), 14.5 (2C, 2ꢅ
CH2CH3), 13.2 (br s, 2C, 3/3’-CCH3), 13.0 (br s, 2C, 5/5’-CCH3), 10.7 (2C,
1/1’-CCH3), 9.6 ppm (2C, 7/7’-CCH3); 19F NMR (376 MHz, CDCl3): d=
ꢀ146.7 ppm (q, JBF =33 Hz, 4F, 2ꢅBF2); 11B NMR (128 MHz, CDCl3):
d=0.78 ppm (t, JBF =33 Hz, 2B, 2ꢅBF2); MS (ESI): m/z: 605 [M+Na]+,
582 [M]+, 563 [MꢀF]+; HRMS (ESI): m/z: calcd for C30H36B2F4N4S
[M]+: 582.2777; found: 582.2787.
6-Ethyl-1,3,5,7-tetramethyl-2-(4-methylphenylthio)-4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene (11): To an orange-yellow solution of 9 (107 mg,
0.387 mmol) in dry CH2Cl2 (26 mL) at 08C a solution of 4-methylphenyl-
sulfenyl chloride (167 mmolLꢀ1 in CH2Cl2) (2.8 mL, 0.470 mmol) was
added dropwise within 15 min. After complete addition H2O (20 mL)
was added to the deep red reaction mixture. After phase separation the
organic phase was washed with H2O (20 mL), dried over Na2SO4, and the
solvent was removed on a rotary evaporator. Purification by column
chromatography (silica, CH2Cl2/n-pentane 1:1) gave the product as red
fraction of orange fluorescence, which was obtained after drying in vacuo
as red, microcrystalline solid (146 mg, 96%). 1H NMR (400 MHz,
CDCl3): d=7.08 (s, 1H; 8-CH), 7.03 (m, 3J=8.2 Hz, 2H; 2ꢅtolyl-meta-
2,3,7,8,9-Pentamethyldipyrrin-1-thione (7): A suspension of 4 (42 mg,
0.123 mmol) in toluene (4 mL) was heated to 908C and a solution of
Na2S·9H2O (21 mg, 0.088 mmol) and S (3 mg, 0.088 mmol) in EtOH/H2O
(5 mL, 4:1) was added. The reaction mixture was kept at 908C for 2 h.
After cooling the solvent was reduced on a rotary evaporator and the
dark red residue was purified by column chromatography (silica, CH2Cl2,
then CH2Cl2/EtOAc 9:1). The red product fraction gave a red-black solid
(24 mg, 78%). 1H NMR (300 MHz, CDCl3): d=11.16 (br s, 1H, NH),
9.75 (br s, 1H, NH), 6.24 (s, 1H, CH), 2.37 (s, 3H, CH3), 2.11 (s, 3H,
CH3), 2.09 (s, 3H, CH3), 2.07 (s, 3H, CH3), 1.91 ppm (s, 3H, CH3);
Chem. Eur. J. 2013, 00, 0 – 0
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M. Brçring et al.
CH), 6.95 (m, 3J=8.2 Hz, 2H; 2ꢅtolyl-ortho-CH), 2.55 (s, 6H; 3/5-
CCH3), 2.42 (q, J=7.6 Hz, 2H; 6-CCH2), 2.28 (s, 3H; tolyl-CH3), 2.24 (s,
A. Romieu, Org. Lett. 2009, 11, 2049–2052; c) T. Bura, R. Ziessel,
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Adv. Funct. Mater. 2008, 18, 2017–2025; b) J. BaÇuelos, V. Martꢆn,
C. F. A. Gꢇmes-Durꢈn, I. J. A. Cꢇrdoba, E. PeÇa-Cabrera, I. Garcꢆa-
Moreno, A. Costela, M. E. Pꢉrez-Ojeda, T. Arbeloa, I. L. Arbeloa,
Chem. Eur. J. 2011, 17, 7261–7270.
3
3H; 1-CCH3), 2.20 (s, 3H; 7-CCH3), 1.09 ppm (t, 3J=7.6 Hz, 3H;
CH2CH3); 13C NMR (100 MHz, CDCl3): d=159.6 (1C; 5-CCH3), 157.3
(1C; 3-CCH3), 143.1 (1C; 1-CCH3), 139.3 (1C; 7-CCH3), 134.9 (1C; tolyl-
CCH3), 134.5 (1C; tolyl-CS), 134.2 (1C; 8a-C), 133.8 (1C; 6-CCH2), 131.4
(1C; 7a-C), 129.8 (2C; 2ꢅtolyl-meta-CH), 126.1 (2C; 2ꢅtolyl-ortho-CH),
119.8 (1C; 2-CS), 117.0 (1C; 8-CH), 21.0 (1C; tolyl-CH3), 17.4 (1C; 6-
CCH2), 14.5 (1C; CH2CH3), 13.1 (br s, 1C; 3/5-CCH3), 12.9 (br s, 1C; 3/5-
CCH3), 10.6 (1C; 1-CCH3), 9.6 ppm (1C; 7-CCH3); 19F NMR (376 MHz,
CDCl3): d=ꢀ146.6 ppm (q, JBF =33 Hz, 2F; BF2); 11B NMR (128 MHz,
CDCl3): d=1.04 ppm (t, JBF =33 Hz, 1B; BF2); MS (ESI): m/z: 421
[M+Na]+, 398 [M]+, 383 [MꢀCH3]+; HRMS (ESI): m/z: calcd for
C22H25BF2N2SNa [M+Na]+: 421.1692; found: 421.1695.
[8] M. Benstead, G. H. Mehl, R. W. Boyle, Tetrahedron 2011, 67, 3573–
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The solution of 4-methylphenylsulfenyl chloride was prepared by drop-
wise addition of a solution of 4-methylthiophenol (248 mg, 2 mmol) in
dry CH2Cl2 (6 mL) to a suspension of N-chlorosuccinimide (267 mg,
2 mmol) in CH2Cl2 (6 mL) at room temperature. After stirring for 30 min
the orange-yellow solution of the sulfenyl chloride was used in situ as-
suming a complete conversion (167 mmolLꢀ1).
¯
Crystal data for 5: C28H32B2F4N4S, M=554.26, triclinic, space group P1,
a=8.632(1), b=11.111(1), c=15.069(2) ꢄ, a=107.78(1), b=96.81(1), g=
99.02(1)8, V=1337.6(3) ꢄ3, Z=2, 1calcd =1.376 gcmꢀ3
1.536 mmꢀ1, R1 [I>2s(I)]=0.0374, wR2 (all data)=0.1026.
,
mACHTNGUTRENNU(G CuKa)=
¯
Crystal data for 6: C28H32B2F4N4S2, M=586.32, triclinic, space group P1,
a=8.2876(8), b=10.186(1), c=18.139(2) ꢄ, a=83.293(8), b=82.031(8),
g=76.182(8)8, V=1467.1(3) ꢄ3, Z=2, 1calcd =1.327 gcmꢀ3
2.078 mmꢀ1, R1 [I>2s(I)]=0.0421, wR2 (all data)=0.1083.
, mACHTNUGTRNUEGN(CuKa)=
Crystal data for 8: C21H23BF2N2S, M=384.28, monoclinic, space group
P21/c, a=8.124(1), b=22.745(3), c=10.977(1) ꢄ, b=107.00(2)8, V=
ACTHNUTRGNEUNG
1939.7(4) ꢄ3, Z=4, 1calcd =1.316 gcmꢀ3, m(MoKa)=0.193 mmꢀ1, R1 [I>
2s(I)]=0.0505, wR2 (all data)=0.1207.
Crystal data for 10: C30H36B2F4N4S, M=582.31, orthorhombic, space
group Pnc2, a=35.229(3), b=8.7558(6), c=9.2389(8) ꢄ, V=
2849.8(4) ꢄ3, Z=4, 1calcd =1.357 gcmꢀ3
2s(I)]=0.0434, wR2 (all data)=0.1124.
(CuKa)=1.468 mmꢀ1
, mACHTUNREGTNNUGN , R1 [I>
Crystal data for 11: C22H25BF2N2S, M=398.31, orthorhombic, space
group Pbca, a=17.5637(5), b=9.8783(3), c=23.4254(8) ꢄ, V=
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ACTHNUTRGNEUNG
4064.3(2) ꢄ3, Z=8, 1calcd =1.302 gcmꢀ3, m(MoKa)=0.187 mmꢀ1, R1 [I>
2s(I)]=0.0408, wR2 (all data)=0.1092.
Crystallographic data: CCDC-926196 (5), -926197 (6)-926198 (8)-926199
(10) and -926200 (11) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
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Acknowledgements
Support for this work by the Deutsche Forschungsgemeinschaft (DFG) is
gratefully acknowledged.
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Received: March 8, 2013
Revised: May 1, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
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www.chemeurj.org
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