Synthesis of donor-substituted meso-phenyl and meso-ethynylphenyl BODIPYs with broad absorption

Article abstract of DOI:10.1039/c3nj00157a

We report the synthesis of meso-ethynylphenyl BODIPYs and compare their properties with the corresponding meso-phenyl derivatives. Both types of BODIPYs carry a 2-cyano-3-acrylic acid anchoring moiety and either methyl groups or 4,4′-dimethoxytriphenylamine (MeOTPA) donor groups at positions 3 and 5. All compounds were characterized by NMR, UV/vis and cyclic voltammetry. The MeOTPA-substituted BODIPYs show an excellent panchromatic absorption with high molar extinction coefficients over the whole UV/vis range up to the near-IR region. The most impressive absorption was exhibited by the MeOTPA-substituted meso-ethynylphenyl BODIPY which strongly absorbs up to 1030 nm. By cyclic voltammetry measurements, all compounds were identified to be electrochemically stable in solution. Further, it was observed that the value of the LUMO level can be tuned by the meso-substituent. The HOMO level is determined by the donor substituents (-5.41 ± 0.03 eV and -4.84 ± 0.01 eV for BODIPYs with methyl groups and MeOTPA donor groups, respectively). These findings were further supported by DFT calculations. To evaluate the potential of the BODIPYs as sensitizers, the incident photon-to-current conversion efficiencies of solid-state dye-sensitized solar cells were measured. The photoaction spectra clearly show that the BODIPYs contribute to the photocurrent generation over their entire absorption region.

Full text of DOI:10.1039/c3nj00157a

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Synthesis of donor-substituted meso-phenyl and
meso-ethynylphenyl BODIPYs with broad absorption†
Cite this: NewJ.Chem., 2013,
37, 1417
Katja Graf,a Thomas Korzdorfer,b Stephan Kummelb and Mukundan Thelakkat*a
¨
¨
¨
¨
We report the synthesis of meso-ethynylphenyl BODIPYs and compare their properties with the
corresponding meso-phenyl derivatives. Both types of BODIPYs carry a 2-cyano-3-acrylic acid anchoring
moiety and either methyl groups or 4,40-dimethoxytriphenylamine (MeOTPA) donor groups at positions
3 and 5. All compounds were characterized by NMR, UV/vis and cyclic voltammetry. The MeOTPA-
substituted BODIPYs show an excellent panchromatic absorption with high molar extinction coefficients
over the whole UV/vis range up to the near-IR region. The most impressive absorption was exhibited by
the MeOTPA-substituted meso-ethynylphenyl BODIPY which strongly absorbs up to 1030 nm. By cyclic
voltammetry measurements, all compounds were identified to be electrochemically stable in solution.
Further, it was observed that the value of the LUMO level can be tuned by the meso-substituent. The
HOMO level is determined by the donor substituents (ꢀ5.41 ꢁ 0.03 eV and ꢀ4.84 ꢁ 0.01 eV for
BODIPYs with methyl groups and MeOTPA donor groups, respectively). These findings were further
supported by DFT calculations. To evaluate the potential of the BODIPYs as sensitizers, the incident
photon-to-current conversion efficiencies of solid-state dye-sensitized solar cells were measured. The
photoaction spectra clearly show that the BODIPYs contribute to the photocurrent generation over
their entire absorption region.
Received (in Montpellier, France)
8th February 2013,
Accepted 16th February 2013
DOI: 10.1039/c3nj00157a
www.rsc.org/njc
materials in solar cells is in its infancy and the efficiencies are
still low. It has been shown that BODIPYs can be used as
Introduction
sensitizers in solid-state dye-sensitized solar cells,11 liquid
electrolyte dye-sensitized solar cells12–14 and also as active
material in organic bulk heterojunction solar cells.15
The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes, better known
as BODIPYs,1 have emerged as attractive compounds for many
research areas, including the application as laser dyes,2 bio-
chemical labeling agents,3 fluorescence sensors4/switches,5
electroluminescent materials6 or molecular photonics.7 The
large variety of applications of this class of dyes are based on
their outstanding properties.8–10 BODIPYs are thermally and
photochemically stable, chemically robust, redox active and
highly fluorescent. Most importantly, they are characterized
by an intensive absorption profile in the visible region that can
be easily tuned by chemical modification of the BODIPY core.9
This feature makes these dyes highly interesting for the appli-
cation as sensitizers in solar cells because it provides the
opportunity to improve the light harvesting by the extension
of the absorption. However, their use as sensitizers and donor
In the present work we focused on the structural modification
of the BODIPY core to accomplish excellent optical properties
with the goal of improving the light harvesting in solar cells.
For this, panchromatic absorption behaviour accompanied by high
extinction coefficients has to be realized. Therefore, we synthe-
sized a series of BODIPY dyes differing in the meso-substituent
and the groups at positions 3 and 5 (see 5, 8, 13 and 16,
Scheme 1). From the literature it is known that arylation at the
meso-position affects the optical and electrochemical properties
only marginally.16 The reason for this is the orthogonal orienta-
tion of the meso-phenyl moiety relative to the planar BODIPY
core. This configuration reduces the conjugation between the
two units. To improve the conjugation and thus the extent of
absorption, we introduced an ethynyl-bridge between the meso-
phenyl group and the BODIPY core creating meso-ethynylphenyl
BODIPYs (13, 16). The idea was to enable an efficient deloca-
lization due to the comparatively diffuse nature of the p-bonds
of the ethynyl-bridge creating a cylindrical electron cloud
around the s-bond. Porphyrin chemistry has already shown
a University of Bayreuth, Macromolecular Chemistry I, Applied Functional Polymers,
D-95447 Bayreuth, Germany. E-mail: Mukundan.Thelakkat@uni-bayreuth.de;
Fax: +49 921 55 3109; Tel: +49 921 55 3108
b University of Bayreuth, Theoretical Physics IV, D-95447 Bayreuth, Germany
† Electronic supplementary information (ESI) available: Mechanisms of the
Knoevenagel-type condensation, NMR-spectra, UV/vis-spectra, fluorescence-spectra,
cyclic voltammetry data. See DOI: 10.1039/c3nj00157a
c
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that ethynyl-type bridges can bring about strong electronic piperidine/p-toluenesulfonic acid catalyst system in non-polar
interactions between two connected moieties.17 For the synthesis solvents such as benzene or toluene. The proposed mechanism
of such meso-ethynylphenyl BODIPYs, we successfully used the of this reaction is presented in the ESI.† The basicity and
condensation of an alkynyl aldehyde with a pyrrole derivative. To nucleophilicity of the amine used generally determine the
further increase the extent of the absorption, we attached strong mechanism, either as the Hann–Lapworth (Fig. S1, ESI†) or
donor groups to the BODIPY core via vinylic bonds. To this end, the organocatalytic (Fig. S2, ESI†) mechanism; both being
4,40-dimethoxytriphenylamine (MeOTPA) was selected. MeOTPA well-studied for a variety of such condensations.20,21 For
is a strong donor due to the high electron density and the high instance, the Hann–Lapworth mechanism is the only valid
lying HOMO level. It is also stable against oxidative coupling. mechanism for reactions including tertiary amines because
Finally, a 2-cyano-3-acrylic acid anchoring moiety was attached they cannot perform a nucleophilic attack on the carbonyl
to the each BODIPY dye making them suitable for the applica- carbon of aldehydes as required in the organocatalytic mecha-
tion in hybrid solar cells. The structural, optical, electrochemical nism. For reactions using primary or secondary amines either
and photovoltaic properties of the meso-phenyl (5, 8) and meso- mechanism are conceivable and piperidine is a common
ethynylphenyl BODIPYs (13, 16) bearing methyl or MeOTPA secondary amine used for this reaction. But as we performed
groups at positions 3 and 5 were compared. Their applicability the reaction with 2,2,6,6-tetramethyl-piperidine (TMP) which is
as sensitizers for TiO2 for light harvesting and energy conversion also a secondary amine and as basic as piperidine but much more
was tested in hybrid solar cells.
bulky, we observed no reaction (even after one week under reflux).
Similarly, by using the tertiary base NEt3 (which has also a
comparable basicity), we did not observe a condensation reaction.
Thus bases with the comparable basicity as piperidine that are not
able to activate the aldehyde did not promote the Knoevenagel
Results and discussion
Synthesis
The synthetic routes for the preparation of the target BODIPYs condensation at all. Moreover, we isolated the aminal (structure B,
5, 8, 13 and 16 are depicted in Scheme 1. The meso-phenyl Fig. S2 and S3, ESI†) expected in an organocatalytic route. From
derivatives 5 and 8 were prepared according to a conventional these results it can be deduced that the organocatalytic mecha-
coupling method using an aromatic aldehyde (Scheme 1, route a).14 nism is relevant for the condensation between BODIPYs and
Thus 5 and 8 were both obtained from 3, which was synthesized donor aldehydes. The proposed mechanism in Fig. S2 (ESI†) is
from 4-(1,3-dioxolan-2-yl)benzaldehyde 1 and kryptopyrrole 2. further supported by the exclusive formation of trans-substituted
Deprotection of 3 and subsequent Knoevenagel condensation BODIPYs (see 1H-NMR spectra of 8 and 16, Fig. S4 and S5, ESI,†
with 2-cyanoacetic acid resulted in the formation of 5. For respectively). In this way, 14 could be synthesised from 10 in a
obtaining 8, the MeOTPA-donor had to be attached first to 3. Knoevenagel-type condensation. After deprotection of 14, 15
Therefore, 4-(di(4-methoxyphenyl)amino)benzaldehyde was reacted was reacted with 12 in a Sonogashira coupling to afford 16. All
with the methyl groups of 3 at the positions 3 and 5 by a compounds were characterized using FT-IR, 1H-NMR, UV/vis
Knoevenagel-type condensation to yield 6. After the deprotection and cyclic voltammetry measurements.
of the aldehyde functionality, the resulting compound 7 was
condensed with 2-cyanoacetic acid to yield 8.
Characterization by NMR
For the synthesis of the meso-ethynylphenyl BODIPYs, we NMR analysis was used to investigate (1) the influence of the
developed a novel route (Scheme 1, route b). These BODIPYs meso-substituent on the electronic and magnetic environment
were synthesized starting from a non-aromatic aldehyde, viz. an of the BODIPY scaffold, (2) the stereochemistry of the vinyl
alkynyl aldehyde (9). Typically aromatic aldehydes, acid chlorides bonds and (3) the geometry of the 4-(2-carboxy-2-cyanovinyl)-
or anhydrides are used for the preparation of BODIPYs. To the phenyl anchoring unit.
best of our knowledge, no alkenyl or alkynyl aldehydes have
The influence of the meso-substituent was examined by
been used for this reaction. However, a comparable reaction comparing the 1H-NMR spectra of 18, 3 and 17 (Fig. 1, full
route is already known from porphyrin chemistry where 9 was spectra in Fig. S7, ESI†) differing only in the substituent at the
reacted with pyrrole to give the associated dipyrromethane and meso position: meso-proton, meso-phenyl and meso-ethynylphenyl.
finally the meso-ethynyl porphyrin.18,19 The meso-ethynylphenyl The main difference between meso-phenyl and meso-ethynyl-
BODIPYs (13 and 16) were synthesized starting from krypto- phenyl BODIPYs is the chemical shift of the resonance signal
pyrrole 2 and the alkynyl aldehyde 3-(trimethylsilyl)-2-propynal assigned to the 1,7-methyl groups of the BODIPY core. These
9 to give 10. After deprotection of 10, a Sonogashira coupling of occur at about 1.28 ppm for meso-phenyl derivatives (e.g. 3),
11 with the final 4-(2-carboxy-2-cyanovinyl)phenyl anchoring whereas they occur at about 2.53 ppm for the meso-ethynyl-
group 12 was performed to yield directly the meso-ethynyl- phenyl BODIPYs (e.g. 17). This gives clear evidence that the
phenyl BODIPY 13. To get the MeOTPA-substituted meso-ethynyl chemical and/or magnetic environment of these protons is
counterpart of 8, the TMS-protected meso-ethynyl BODIPY strongly influenced by the substituent at the meso-position.
10 was used for the Knoevenagel-type condensation with Relative to the corresponding meso-proton compound 18, the
4-(di(4-methoxyphenyl)amino)benzaldehyde. To this end, the orthogonal phenyl moiety in 3 seems to increase the electron
aromatic aldehyde was reacted with the 3,5-dimethyl BODIPY density and hence the shielding of the 1,7-methyl groups.
10 in the presence of a piperidine/glacial acetic acid or a The interesting resonance signal appears at a distinctly lower
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Scheme 1 Synthetic routes for the preparation of (a) the meso-phenyl BODIPYs (5 and 8) and (b) the meso-ethynylphenyl BODIPYs (13 and 16). (i) Trifluoroacetic
acid, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, NEt3, BF3ꢂOEt2, in CH2Cl2, RT. (ii) 5% HClaq, in THF, RT. (iii) 2-Cyanoacetic acid, piperidine, in acetonitrile, reflux.
(iv) 4-(Di(4-methoxyphenyl)amino)benzaldehyde, piperidine, glacial acetic acid, in benzene, reflux. (v) KF, in MeOH or MeOH : THF 1 : 1, RT. (vi) Pd(PPh3)4, CuI, NEt3, in THF, RT.
ppm value for 3. This is not due to the +M-effect of the phenyl Consequently, the generated magnetic field weakens the
group because it is not in conjugation with the BODIPY core. applied one at the 1,7-methyl groups, which necessitates a
One possible influence affecting the shift is the spatial proxi- higher external field. This equals a shift of the signal to lower
mity between the electron density of the meso-phenyl ring and ppm values. Therefore, it can be deduced that the meso-phenyl
that of the methyl groups. However, the main effect influencing group does not influence the electron density at the 1,7-methyl
the shift is the ring current of the meso-phenyl group. groups by conjugation, but rather by the spatial proximity.
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Fig. 1 Excerpts of the 1H-NMR spectra of (a) the meso-proton BODIPY 1,3,5,7-
tetramethyl-2,6-diethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (18), (b) the meso-
phenyl BODIPY (3) and (c) the corresponding meso-ethynylphenyl BODIPY (17). The
red boxes mark the resonance signal attributed to the 1,7-methyl protons. (The signals
at 1.56 and 1.44 ppm can be neglected because they arise from water and
cyclohexane, respectively. The whole spectra are depicted in Fig. S7, ESI†.)
Further, the meso-phenyl group mainly affects the magnetic
environment. In contrast, the meso-ethynyl bridge group is able
to reduce the electron density at these positions through the
conjugation. These observations are supported by the UV/vis
measurements showing that the meso-ethynylphenyl BODIPY
17 causes a strong red shift due to the electron withdrawing
ability of the meso-substituent whereas the corresponding meso-
phenyl BODIPY 3 shows hardly any difference in the absorption
relative to the meso-proton BODIPY 18 (Fig. S10, ESI†). However,
Fig. 2 (a) Electronic absorption spectra and (b) emission spectra of the BODIPYs
3 (in CH2Cl2), 8 (in CH2Cl2), 13 (in CH2Cl2 : THF 1 : 1) and 16 (in CH2Cl2) measured
at a concentration in the range of 1 ꢃ 10ꢀ5 M.
the 3,5-methyl groups are only marginally influenced by the shows the typical BODIPY absorption with the sharp S0–S1
substituent at the meso-position.
transition at 530 nm and a high extinction coefficient (e =
Further, the stereochemistry of the vinyl bond between the 4.21 ꢃ 104 Mꢀ1 cmꢀ1). Additionally, a shoulder appears at
BODIPY core and the donor group was investigated. On the 500 nm which is attributed to the 0–1 vibrational transition.
1
basis of the H-NMR spectra of 8 and 16 (Fig. S4 and S5, ESI†) In comparison to that the S0–S1 transition of its meso-
the configuration can be clearly identified to be trans because ethynylphenyl derivative (13) is red-shifted by 55 nm. The signal
3
the values for the vicinal JH/H couplings are 16.6 Hz and is broader and the extinction coefficient is reduced to 1.67 ꢃ
16.4 Hz for 8 and 16, respectively.
104 Mꢀ1 cmꢀ1. The 0–1 vibrational transition was detected at
Finally, only the geometry of the 4-(2-carboxy-2-cyanovinyl)- 542 nm. The reason for the reduced extinction coefficient of 13
phenyl anchoring unit of 5, 8, 13 and 16 required assignment. compared to 5 is the larger spatial separation of the molecular
Here, the carboxyl group can be oriented either cis or trans relative orbitals involved in the transition. This is supported by DFT
to the phenyl group. Since this group was attached in a compar- calculations (Fig. 3) which indicate an increased distance
able manner for all end-products and for 12, we used 13C-NMR- between HOMO and LUMO with the introduction of the
analysis of 12 for the investigation of the orientation (see Fig. S9, meso-ethynyl unit. The larger distance lowers the transition
ESI†). The double bond geometry of 12 could be unambiguously probability. Thus, also the extinction coefficient is reduced. A
assigned as trans. This is also supported by the results of further interesting feature of 13 is the absorption band at
Karas et al.22 who investigated the crystal structure of a very 397 nm. The extinction coefficient of this band is much higher
similar compound (2-cyano-3-(4-hydroxyphenyl)acrylic acid).
for 13 than for 5. We found that this band is associated with the
meso-phenyl/ethynylphenyl unit. The pronounced band of 13
indicates that the electronic interaction between the BODIPY
Optical properties
The electronic absorption spectra of 5, 8, 13 and 16 were core and the meso-substituent can be increased by the meso-
recorded in solution and are depicted in Fig. 2a. Compound 5 ethynyl group.
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Stokes shift of 13 is increased due to stronger structural
rearrangement. For the MeOTPA-substituted BODIPYs 8 and
16, the fluorescence was very weak or even below the resolution
of the spectrometer. This is attributed to a possible intra-
molecular charge transfer.
Electrochemical properties and DFT calculations
Cyclic voltammetry was used to examine the redox properties of
the BODIPY dyes in solution. The energy levels of the final
products are given in Table 1. Cyclic voltammograms of 5, 8, 13
and 16 and the deprotected counterparts 4, 7, 11, and 15 as well
as a table summarizing the redox values of all BODIPY deriva-
tives are included in the ESI† (Fig. S11, S12, and Table S1).
In general, the BODIPY core and the donor substituent show
a reversible behaviour, but on attaching the cyanocarboxylic
acid group, the anodic and cathodic peak currents decrease.
Therefore the peaks are less pronounced. In addition, a com-
parison of the calculated HOMO/LUMO energy levels of all
compounds reveals two general trends: first, for all BODIPY
compounds with methyl substituents, the only oxidation
(denoted EOX1 = HOMO = ꢀ5.41 ꢁ 0.03 eV) is unaffected by
the substituent at the meso-position (see Table 1 and Table S1,
ESI† for all compounds). This value of the EOX1 is thus assigned
to the BODIPY core. It is also known from the literature that the
HOMO of meso-phenyl BODIPY dyes without strong donor
groups is delocalized over the BODIPY framework.16,23
Secondly, with increasing electron withdrawing ability of the
meso-substituent, the energy level of the LUMO is shifted to
lower values. Consequently, the LUMO is dependent on the
meso-substituent.
Fig. 3 Orbital maps of the HOMO and LUMO of (a) 5 and (b) 13.
With the attachment of the MeOTPA donor groups, the
absorption spectra of 5 and 13 experience a strong bathochro-
mic shift. The absorption edge of 8 was detected at 840 nm and
that of 16 at 1030 nm. The absorption maxima appear at
725 nm (e = 4.09 ꢃ 104 Mꢀ1 cmꢀ1) and 835 nm (e = 2.83 ꢃ
104 Mꢀ1 cmꢀ1) for 8 and 16, respectively. These are excellent
absorption ranges and extinction coefficients providing the
possibility of improving the light harvesting of solar cells by
using these dyes as sensitizers. It is important to note that the
extinction coefficients of the MeOTPA-substituted BODIPYs are
higher than 1 ꢃ 104 Mꢀ1 cmꢀ1 over the whole visible spectrum
up to 787 nm and 940 nm for 8 and 16, respectively. This proves
the excellent optical behaviour of BODIPYs carrying strong
donor groups which is even more pronounced for meso-ethy-
nylphenyl BODIPYs. The low energy bands of 8 and 16 can be
attributed to intramolecular charge transfer.
These findings are supported by DFT calculations on 5
(meso-phenyl) and 13 (meso-ethynylphenyl) (see Fig. 3). The
LUMO of 5 is delocalized over the phenyl group and the
anchoring group. For 13, the LUMO is additionally distributed
over the ethynyl-bridge. The delocalization of the LUMO over
the meso-substituent is attributed to the strong electron with-
drawing para-2-carboxy-2-cyanovinyl group. This is also in
agreement with the literature.16
By the attachment of the MeOTPA-donor via a vinylic bond,
the conjugated system is further extended. This can not only be
seen in the absorption, but also in the cyclic voltammetry
experiments.
The cyclic voltammograms of all MeOTPA-substituted BODIPYs
show three reversible oxidation signals (Fig. S11 and S12, ESI†).
To further investigate the influence of the ethynyl spacer
and the donor substituents, steady-state emission measure-
ments were done. Compared to 5, the emission of 13 was
remarkably reduced by the introduction of the ethynyl spacer
(Fig. 2b). The fluorescence quantum yields in solution are 2.18
and 1.69% for 5 and 13, respectively. For 8 and 16, the
fluorescence was too weak to determine the fluorescence quan-
tum yield reliably. Additionally, the Stokes shift of 13 (DStokes
=
Table 1 Summary of the energy levels calculated from cyclic voltammetry
experiments measured at 50 mV sꢀ1 in CH2Cl2 with 0.1 M tetrabutylammonium
hexafluorophosphate using ferrocene as reference
42 nm) was more than thrice that of 5 (DStokes = 12 nm). The
explanation for both observations lies within the different
freedom of rotation of the phenyl rings.16 The increased
distance due to the ethynyl-bridge enables free rotation of the
phenyl group around the axis given by the meso-spacer. In
contrast, the meso-phenyl ring in 5 is restricted to an almost
orthogonal position by the 1,7-dimethyl groups. Consequently,
the emission probability of 13 is reduced via non-radiative
decay due to the motion of the phenyl group.16 Additionally, the
a
b
Compound
EOX1 [eV]
EOX2 [eV]
EOX3 [eV]
ELUMO [eV]
5
13
8
16
ꢀ5.40
ꢀ5.44
ꢀ4.84
ꢀ4.84
—
—
—
—
ꢀ3.18
ꢀ3.48
ꢀ3.28
ꢀ3.59
ꢀ4.97
ꢀ5.42
ꢀ4.98
ꢀ5.43
a
b
EOX1 is considered as the HOMO energy value. The LUMO levels
were calculated from the optical band gap.
c
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The one at ꢀ5.4 eV arising from the BODIPY core as discussed over their entire absorption region; 8 and 16 generating photo-
above and the two lower lying peaks (ꢀ4.84 ꢁ 0.01 eV, ꢀ4.97 ꢁ current up to 900 nm and 1000 nm, respectively. All the
0.01 eV) originate from the MeOTPA group. In fact, for non- sensitizers provide a high potential for improvement by further
conjugated systems, only one oxidation peak would be expected optimization, e.g. by selecting suitable hole transport materials,
for the MeOTPA unit. However, we detected two reversible tuning the thickness to match recombination and transport, etc.
oxidations for the MeOTPA substituent. This is due to the
conjugation of the MeOTPA-moieties through the BODIPY core.
Conclusions
The oxidation of one donor unit influences the electronic
surrounding of the second one due to the conjugation.24
We reported the tailor-made synthesis of novel BODIPY sensi-
A further characteristic of the BODIPY derivatives is their tizers capable of light harvesting and energy conversion in a
stability against repeated oxidation and reduction cycles. broad range. The synthesis included the variation of the meso-
Even, the MeOTPA-substituted BODIPYs were stable during substituent (phenyl vs. ethynylphenyl) and the substituents at
repeated electrochemical measurements because the para- positions 3 and 5 (methyl vs. MeOTPA).
methoxy groups of the donors inhibit radical coupling of the
triphenylamines.
Investigations of the optical properties of 5, 8, 13 and 16
showed that the introduction of the ethynyl-bridge between the
In a nutshell, the BODIPYs are redox stable, their LUMO BODIPY core and the phenyl ring causes a strong bathochromic
level can be tuned by the meso-substituent and the HOMO level shift of the absorption (5 vs. 13 and 8 vs. 16). Further, a more
can be adjusted by the donor groups.
than panchromatic behaviour resulted for both the meso-
phenyl and the meso-ethynylphenyl BODIPYs as a consequence
of the attachment of MeOTPA-donor groups (8 and 16). In
particular, 8 and 16 provide molar extinction coefficients higher
than 1 ꢃ 104 Mꢀ1 cmꢀ1 over the whole UV/vis region and up to
the near-IR part. 16 absorbs up to 1030 nm making this dye
particularly interesting as sensitizers for solar cells.
Furthermore, cyclic voltammetry experiments showed that
the HOMO level of the BODIPY depends solely on the donor
unit regardless of the substituents at the meso-position. The
measured HOMO levels were ꢀ5.41 ꢁ 0.03 eV and ꢀ4.84 ꢁ 0.01 eV
for BODIPYs with methyl and MeOTPA donor groups, respec-
tively. In contrast, the LUMO level depends on the meso-
substituent. Electron withdrawing substituents like the ethynyl
group shift the level to lower values. The HOMO/LUMO distri-
butions are further supported by DFT calculations.
Photocurrent generation
To elucidate the potential of BODIPYs 5, 8, 13 and 16 as light
harvesters, we prepared solid-state dye-sensitized solar cells
with these sensitizers and measured the incident photon-to-
current conversion efficiency (IPCE) for each sample (Fig. 4).
The final IPCE values depend on the optimization of device
structure and adaptation of HOMO/LUMO values of the hole
transport material used. Here we tested the applicability of the
dyes using a reference hole transport material, spiro-OMeTAD.
In general, both 8 and 16 show broad photoaction spectra. A
comparison between the performance of 5 and 13 shows that 13
carrying a meso-ethynyl substituent contributes to the current
generation over a much broader region. In accordance with the
absorption measurements, 5 generates current up to 600 nm,
whereas 13 contributes up to 700 nm. Due to a mismatch in the
energy levels of the MeOTPA-substituted BODIPYs 8 and 16
(EOX1 = ꢀ4.84 eV) relative to the hole transport material spiro-
OMeTAD (EOX1 = ꢀ4.91 eV),25 the absolute IPCE values for 8 and
16 are low. Nevertheless, the IPCE measurements clearly show
that BODIPYs 8 and 16 contribute to the current generation
IPCE measurements on solid-state dye-sensitized solar cells
sensitized with BODIPYs 5, 8, 13 and 16 showed that the
BODIPYs can contribute to the photocurrent generation over
the entire absorption region.
Experimental
General
1H-NMR spectra were recorded on a Bruker Avance 300 spectro-
1
meter at a transmitter frequency of 300 MHz. H–13C coupled
NMR spectra were recorded at a frequency of 125 MHz. The
spectra were calibrated relative to the chemical shift of the
respective solvent residual signal. The chemical shifts (d) are
given in ppm and the coupling constants (J) in Hz. UV/vis
spectra were recorded in CH2Cl2 (or in THF : CH2Cl2 1 : 1 for 13)
on a Hitachi U-3000 spectrophotometer and in a Bentham
DTR6 integrating sphere at a concentration in the range of
1 ꢃ 10ꢀ5 M. The extinction coefficients (e) were calculated
according to the Beer–Lambert law. Steady-state fluorescence
measurements were performed on a JASCO Spectrofluorometer
ST-8600. The slit width was 5 nm for excitation and emission.
The photomultiplier voltage was 800 V. For the determination
of the fluorescence quantum yields, the area for the corrected
Fig. 4 IPCE-curves of solid-state dye-sensitized solar cells sensitized with
BODIPYs 5, 8, 13 and 16.
c
1422 New J. Chem., 2013, 37, 1417--1426
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NJC
Paper
emission spectra was divided by the corrected area of the dissolved in CH2Cl2 washed with water (4 ꢃ 80 mL), dried over
absorbed light peak. The samples were excited at a wavelength Na2SO4 and is concentrated. The raw product is purified by
30 nm blue-shifted relative to the longest wavelength absorption column chromatography with toluene : methanol 5 : 1 as eluent.
maximum. Cyclic voltammetry was carried out under moisture- Yield after freeze-drying from 1,4-dioxane: 158 mg (red-orange
and oxygen-free conditions using a standard three-electrode solid, Rf (SiO2; toluene : methanol 5 : 1)
=
0.20), 90%.
assembly connected to a potentiostat (model 263A, EG&G
l
max(DCM)/nm = 530 (e/Mꢀ1 cmꢀ1 = 42 137), 500sh (18 610),
Princeton Applied Research) at a scanning rate of 50 mV sꢀ1. A 381 (6951). FT-IR (ATR): nmax/cmꢀ1 = 3464 (br), 2964 (w), 2223 (w)
Pt milli-electrode (model G0228, AMETEK Advanced Measure- 1629 (w), 1539 (s), 1629 (m), 1470 (m), 1393 (m), 1187 (s), 1057 (m),
ment Technology) was used as a working electrode. A platinum 977 (s), 708 (m). 1H-NMR (300 MHz, DMSO-d6): d/ppm = 0.94
3
wire in the respective solvent plus conducting salt (tetrabutylam- (t, J1H/1H = 7.02 Hz, 6H, 2/6-CH2CH3), 1.28 (s, 6H, 1/7-CH3), 2.27
monium hexafluorophosphate, 0.1 M) was used as a counter (q, 3J1H/1H = 6.99 Hz, 4H, 2/6-CH2CH3), 2.44 (s, 6H, 3/5-CH3), 7.57
3
3
electrode. The quasi-reference electrode consisted of an Ag-wire (d, J1H/1H = 8.16 Hz, 2H, 8-Har), 8.14 (d, J1H/1H = 8.07 Hz, 2H,
in an AgNO3/acetonitrile solution (0.1 M). Each measurement 8-Har), 8.27 (s, 1H, Ph–CHQC(COOH)(CN)).
was calibrated with the internal standard ferrocene/ferrocenium.
8-[4-(1,3-Dioxolan-2-yl)phenyl]-1,7-dimethyl-3,5-di[4-(di-
The energy levels were determined using the empirical relation (p-methoxyphenyl)-amino)styryl]-2,6-diethyl-4,4-difluoro-4-bora-
EHOMO or LUMO = [ꢀe(E1/2
ꢀ E1/2(Fc/Fc+vs.Ag/AgNO ))] ꢀ 3a,4a-diaza-s-indacene (6). To a solution of 3 (100.0 mg,
(x vs.Ag/AgNO3)
3
4.80 eV. Molecular geometries, energy levels and orbitals were 0.221 mmol, 1 eq.) and 4-(di(4-methoxyphenyl)amino)-benzaldehyde
calculated from density functional theory at the B3LYP/cc-pVTZ (221.1 mg, 0.663 mmol, 3 eq.) in benzene (20 mL), piperidine
level using the TURBOMOLE program package.26 Solid-state (0.3 mL) and glacial acetic acid (0.25 mL) were added. After
dye-sensitized solar cells were prepared according to a method heating the solution to reflux over a glass frit containing
reported elsewhere.27 IPCE spectra were recorded using a PVE300 molecular sieve (4 Å) for 11 h, the solvent is removed and the
photovoltaic device characterization system from Bentham.
raw product is purified by column chromatography using
cyclohexane : ethyl acetate 2 : 1 as eluent. Yield: 77 mg (green-
black solid, Rf (SiO2; cyclohexane : ethyl acetate 2 : 1) = 0.30),
Synthesis
4-(1,3-Dioxolan-2-yl)benzaldehyde (1),28 8-[4-(1,3-dioxolan-2-yl)- 32%. lmax(DCM)/nm = 715 (e/Mꢀ1 cmꢀ1 = 56 444), 534 (20 179),
phenyl]-1,3,5,7-tetramethyl-2,6-diethyl-4,4-difluoro-4-bora-3a,4a- 437 (21 739), 358 (50 498). FT-IR (ATR): nmax/cmꢀ1 = 2962 (w),
diaza-s-indacene (3),14 2-(4-iodophenyl)-1,3-dioxolane29 and 1587 (m), 1503 (s), 1441 (m), 1242 (s), 1174 (s), 1028 (m), 819 (m).
4-(di(4-methoxyphenyl)amino)benzaldehyde30 were prepared 1H-NMR (300 MHz, CDCl3): d/ppm = 1.15 (t, J1H/1H = 7.50 Hz,
3
3
according to standard procedures. 1,3,5,7-Tetramethyl-2,6- 6H, 2/6-CH2CH3), 1.32 (s, 6H, 1/7-CH3); 2.61 (q, J1H/1H
=
diethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (18) was isolated 7.47 Hz, 4H, 2/6-CH2CH3), 3.83 (s, 12H, OMe), 4.09–4.26
3
during the synthesis of 10. Compounds 10 and 11 can also be (m, 4H, O–CH2CH2–O), 5.90 (s, 1H, Ph–CH), 6.86 (d, J1H/1H
=
3
synthesised according to ref. 31. 3-Ethyl-2,4-dimethyl-1H-pyrrole 8.97 Hz, 8H, 3/5-Har), 6.93 (d, J1H/1H = 8.70 Hz, 4H, 3/5-Har),
3
3
and 3-trimethylsilylpropanal are commercially available. The 7.10 (d, J1H/1H = 8.94 Hz, 8H, 3/5-Har), 7.19 (d, J1H/1H
=
3
syntheses of all other compounds are described below.
16.71 Hz, 2H; 3/5CHQCH), 7.36 (d, J1H/1H = 8.07 Hz, 2H,
8-(4-Formylphenyl)-1,3,5,7-tetramethyl-2,6-diethyl-4,4-difluoro- 8-Har), 7.42 (d, J1H/1H = 8.73 Hz, 4H, 3/5-Har), 7.64 (d + d,
4-bora-3a,4a-diaza-s-indacene (4). 3 (200.0 mg, 0.463 mmol) is 3J1H/1H = 8.22 Hz, 2H + 2H, 3/5CHQCH + 8-Har).
3
dissolved in THF (50 mL) and 5% HClaq (10 mL) is added. The
8-(4-Formylphenyl)-1,7-dimethyl-3,5-di[4-(di(p-methoxy-phenyl)-
solution is stirred for 3.5 h. Then CH2Cl2 (100 mL) is added and amino)-styryl]-2,6-diethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
the solution is washed with diluted NaHCO3aq (50 mL) and (7). 6 (100.0 mg, 0.092 mmol) is dissolved in THF (40 mL) and
water (4 ꢃ 80 mL) until neutrality. The organic phase is dried 5% HClaq (5 mL) is added. The solution is stirred overnight.
over Na2SO4, filtered and the solvent is removed under vacuum Then CH2Cl2 (100 mL) is added and the solution is washed with
to yield the pure product. Yield: 189 mg (red solid), 100%. diluted NaHCO3aq (50 mL) and water (4 ꢃ 100 mL) until
l
max(DCM)/nm = 528 (e/Mꢀ1 cmꢀ1 = 58 300), 500sh (20 481). neutrality. The organic phase is dried over Na2SO4, filtered
FT-IR (ATR): nmax/cmꢀ1 = 3361 (w), 2965 (m), 1701 (m), 1536 (s), and the solvent is removed under vacuum to yield the pure
1470 (m), 1405 (m), 1317 (s), 1262 (m), 1184 (s), 1052 (m), product. Yield: 95 mg (green-black solid), 99%. lmax(DCM)/nm =
974 (s), 798 (m). 1H-NMR (300 MHz, CDCl3): d/ppm = 1.00 723 (e/Mꢀ1 cmꢀ1 = 55 388), 545 (21 318), 440 (21 318), 360 (51 403).
(t, 3J1H/1H = 7.53 Hz, 6H, 2/6-CH2CH3), 1.27 (s, 6H, 1/7-CH3), 2.32 FT-IR (ATR): nmax/cmꢀ1 = 2961 (w), 1705 (w), 1590 (m), 1502 (s),
3
(q, J1H/1H = 7.59 Hz, 4H, 2/6-CH2CH3), 2.56 (s, 6H, 3/5-CH3), 1440 (m), 1240 (m), 1171 (m), 1035 (m), 822 (m). 1H-NMR (300 MHz,
3
3
7.53 (d, J1H/1H = 8.04 Hz, 2H, 8-Har), 8.04 (d, J1H/1H = 8.25 Hz, CDCl3): d/ppm = 1.16 (t, 3J1H/1H = 7.32 Hz, 6H, 2/6-CH2CH3), 1.30
2H, 8-Har), 10.14 (s, 1H, COH).
(s, 6H, 1/7-CH3), 2.61 (q, 3J1H/1H = 7.35 Hz, 4H; 2/6-CH2CH3), 3.83
3
8-[4-(2-Carboxy-2-cyanovinyl)phenyl]-1,3,5,7-tetramethyl-2,6- (s, 12H, OMe), 6.87 (d, J1H/1H = 8.94 Hz, 8H, 3/5-Har), 6.93 (d,
diethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (5). 2-Cyano- 3J1H/1H = 8.67 Hz, 4H, 3/5-Har), 7.10 (d, J1H/1H = 8.94 Hz, 8H,
3
3
acetic acid (218.7 mg, 2.572 mmol, 7 eq.) and 4 (150.0 mg, 3/5-Har), 7.20 (d, J1H/1H = 16.66 Hz, 2H, 3/5CHQCH), 7.45 (d,
0.367 mmol, 1 eq.) are dissolved in dry acetonitrile (35 mL) and 3J1H/1H = 8.67 Hz, 4H, 3/5-Har), 7.55 (d, 3J1H/1H = 8.01 Hz, 2H, 8-Har),
3
3
piperidine (50 mL) is added. Then, the solution is heated to 7.64 (d, J1H/1H = 16.54 Hz, 2H, 3/5CHQCH), 8.04 (d, J1H/1H
reflux for 3 h. The solvent is removed and the residue is 8.07 Hz, 2H, 8-Har), 10.15 (s, 1H, COH).
=
c
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8-[4-(2-Carboxy-2-cyanovinyl)phenyl]-1,7-dimethyl-3,5-di-
NJC
l
max(DCM)/nm = 572 (e/Mꢀ1 cmꢀ1 = 59 850), 534sh (26 194),
[4-(di(p-methoxy-phenyl)amino)styryl]-2,6-diethyl-4,4-difluoro- 352 (8205), 265 (21 775) FT-IR (ATR): nmax/cmꢀ1 = 3263 (m),
4-bora-3a,4a-diaza-s-indacene (8). 2-Cyanoacetic acid (65.6 mg, 2960 (m), 2107 (m), 1535 (m), 1456 (m), 1394 (m), 1311 (m),
0.772 mmol, 7 eq.) and 7 (114.3 mg, 0.110 mmol, 1 eq.) are 1264 (m), 1187 (m), 1041 (s), 970 (s), 754 (m). 1H-NMR
3
dissolved in dry acetonitrile (35 mL) and three drops of piper- (300 MHz, CDCl3): d/ppm = 1.07 (t, J1H/1H = 7.53 Hz, 6H,
idine are added. Then, the solution is heated to reflux for 5 h. 2/6-CH2CH3), 2.41 (s, 6H, 1/7-CH3), 2.41 (q, J1H/1H = 7.56 Hz,
3
The solvent is removed and the residue is dissolved in CH2Cl2, 4H, 2/6-CH2CH3), 2.52 (s, 6H, 3/5-CH3), 3.89 (s, 1H, RH).
washed with water (4 ꢃ 80 mL), dried over Na2SO4 and the
2-Cyano-3-(4-iodophenyl)acrylic acid (12). Under dry condi-
solvent is removed under vacuum. The raw product is purified tions, 4-iodobenzaldehyde (7.400 g, 31.894 mmol, 1 eq.) is
by column chromatography with CH2Cl2 : methanol 15 : 1 as dissolved in benzene (90 mL). Piperidine (0.90 mL), glacial
eluent. Yield after freeze-drying from 1,4-dioxane: 72 mg acetic acid (0.75 mL) and 2-cyanoacetic acid (18.990 g, 0.223 mol,
(brown-black solid, Rf (SiO2; CH2Cl2 : methanol 15 : 1) = 0.45), 7 eq.) are added. The solution is heated to reflux over a glass frit
59%. lmax(DCM)/nm = 725 (e/Mꢀ1 cmꢀ1 = 40 889), 543 (17 258), containing molecular sieve (4 Å) for 40 h. Then, the solvent is
439 (17 846), 358 (48 342). FT-IR (ATR): nmax/cmꢀ1 = 3405 (br), removed and the raw product is purified by repetitive recrys-
2963 (w), 2222 (w), 1593 (m), 1503 (s), 1440 (m), 1332 (m), 1241 tallisation from CH2Cl2. Yield: 5.03 g (white solid), 53%. FT-IR
1
(s), 1171 (s), 1034 (m), 823 (m). H-NMR (300 MHz, DMSO-d6): (ATR): nmax/cmꢀ1 = 3312 (br), 2222 (m), 1623 (s), 1578 (s), 1481
d/ppm = 1.07 (t, 3J1H/1H = 7.26 Hz, 6H, 2/6-CH2CH3), 1.31 (s, 6H; (m), 1387 (s), 1187 (w), 1058 (m), 1005 (s), 817 (s), 781 (m).
3
3
1/7-CH3), 2.58 (q, J1H/1H = 6.75 Hz, 4H; 2/6-CH2CH3), 3.76 1H-NMR (300 MHz, DMSO-d6): d/ppm = 7.79 (d, J1H/1H
=
3
3
(s, 12H, OMe), 6.78 (d, J1H/1H = 8.73 Hz, 4H, 3/5-Har), 6.93 8.52 Hz, 2H, 2-Har), 7.98 (d, J1H/1H = 8.46 Hz, 2H, 3-Har), 8.30
(d, 3J1H/1H = 9.00 Hz, 8H, 3/5-Har), 7.07 (d, 3J1H/1H = 8.94 Hz, 8H, (s, 1H, Ph–CHQC(COOH)(CN)), 14.04 (s (broad), 1H, COOH).
3/5-Har), 7.21 (d, J1H/1H = 16.59 Hz, 2H, 3/5CHQCH), 7.41 13C-NMR (125 MHz, DMSO-d6): d = 163.19 (d, J1H/13C = 6.7 Hz,
3
3
(d, 3J1H/1H = 16.65 Hz, 2H, + d, 4H, 3J1H/1H = 8.85 Hz, 3/5CHQCH COOH), 153.46 (d, J1H/13C = 162.2 Hz, CHQC(COOH)(CN)),
1
+ 3/5-Har), 7.56 (d, 3J1H/1H = 8.22 Hz, 2H, 8-Har), 8.10 (d, 3J1H/1H
8.37 Hz, 2H, 8-Har), 8.16 (s, 1H, Ph–CHQC(COOH)(CN)).
=
138.29 (d, 1J1H/13C = 167.6 Hz, 3/5-Car), 132.06 (d, 1J1H/13C = 162.8 Hz,
2/6-Car), 130.91 (s, 1-Car), 115.97 (d, 3J1H/13C = 13.9 Hz, CN), 104.43
8-[(Trimethylsilyl)ethynyl]-1,3,5,7-tetramethyl-2,6-diethyl-4,4- (s, CHQC(COOH)(CN)), 101.50 (s, 4-Car).
difluoro-4-bora-3a,4a-diaza-s-indacene (10). Under dry condi-
8-[(4-(2-Carboxy-2-cyanovinyl)phenyl)ethynyl]-1,3,5,7-tetramethyl-
tions, 3-ethyl-2,4-dimethyl-1H-pyrrole (13.664 g, 110.909 mmol, 2,6-diethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (13). Under
2 eq.) is dissolved in dry CH2Cl2 (300 mL), cooled to ꢀ5 1C and dry and oxygen-free conditions, 12 (820.0 mg, 2.742 mmol,
degassed with argon for 30 min. Then 3-trimethylsilylpropanal 3 eq.), Pd(PPh3)4 (63.4 mg, 0.055 mmol, 0.06 eq.), CuI (6.9 mg,
(7.000 g, 55.454 mmol, 1 eq.) and one drop of trifluoroacetic 0.036 mmol, 0.04 eq.) and NEt3 (0.60 mL, 4.296 mmol, 4.7 eq.)
acid are added. The solution instantly becomes orange, and are dissolved in dry THF (13 mL). Then, 11 (300.0 mg,
changes the colour firstly to red and then to deep violet. 0.914 mmol, 1 eq.) is dissolved in dry THF (20 mL) and added
After 1 h 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (12.588 g, dropwise over 3 h at RT to the reaction solution. After further
55.454 mmol, 1 eq.) is added at RT and stirring is continued 60 min stirring at RT, the solvent is removed. The residue is
overnight. NEt3 (46.4 mL, 332.726 mmol, 6 eq.) and 30 min later dissolved in CHCl3 and washed with water (4 ꢃ 100 mL). The
BF3ꢂOEt2 (55.7 mL, 443.635 mmol, 8 eq.) are added slowly. organic fraction is dried over Na2SO4 and filtered. After removal
Stirring is continued for further 3 h. Then the solvent is of the solvent, the raw product is purified by column chromato-
removed under reduced pressure and the raw product is graphy with cyclohexane : ethyl acetate 2 : 1 + 1% of glacial
purified by column chromatography using cyclohexane : ethyl acetic acid, then the solvent is changed to CH2Cl2 followed by
acetate 13 : 1 as eluent. Yield: 9.66 g (pink-black solid, Rf (SiO2; a change to CH2Cl2 : methanol 20 : 1 with a gradient to 10 : 1.
cyclohexane : ethyl acetate 13 : 1) = 0.49), 44%. lmax(DCM)/nm = Yield: 218 mg (purple-black solid, Rf (SiO2; CH2Cl2 : methanol
574 (e/Mꢀ1 cmꢀ1 = 49 206), 537sh (22 374), 360 (12 028), 274 10 : 1) = 0.50), 48%. lmax(DCM : THF 1 : 1)/nm = 585 (e/Mꢀ1 cmꢀ1
=
(23 552). FT-IR (ATR): nmax/cmꢀ1 = 2963 (w), 1533 (m), 1455 (m), 16 708), 542sh (9628), 397 (16 047), 279 (21 144). FT-IR (ATR):
1311 (m), 1181 (m), 1072 (m), 971 (m), 843 (m), 751 (m).
n
max/cmꢀ1 = 3403 (br), 2963 (w), 2214 (w), 1631 (w), 1539 (m),
1H-NMR (300 MHz, CDCl3): d/ppm = 0.31 (s, 9H, Si(CH)3), 1.06 1472 (m), 1392 (m), 1321 (m), 1194 (s), 1043 (m), 977 (s),
(t, 3J1H/1H = 7.53 Hz, 6H, 2/6-CH2CH3), 2.41 (s, 6H, 1/7-CH3), 2.41 801 (m), 754 (m). 1H-NMR (300 MHz, DMF-d7): d/ppm = 1.07
3
(q, J1H/1H = 7.56 Hz, 4H, 2/6-CH2CH3), 2.51 (s, 6H; 3/5-CH3).
(t, 3J1H/1H = 7.38 Hz, 6H, 2/6-CH2CH3), 2.46 (q, 3J1H/1H = 7.53 Hz,
8-Ethynyl-1,3,5,7-tetramethyl-2,6-diethyl-4,4-difluoro-4-bora- 4H, 2/6-CH2CH3), 2.53 (s, 6H, 3/5-CH3), 2.58 (s, 6H, 1/7-CH3),
3
3
3a,4a-diaza-s-indacene (11). 10 (400.0 mg, 0.999 mmol, 1 eq.) is 7.87 (d, J1H/1H = 8.34 Hz, 2H, 8-Har), 8.09 (d, J1H/1H = 8.28 Hz,
dissolved in methanol (80 mL), KF (290.2 mg, 4.995 mmol, 2H, 8-Har), 8.29 (s, 1H, Ph–CHQC(COOH)(CN)).
5 eq.) is added and the reaction solution is stirred for 1.5 h at
8-[(Trimethylsilyl)ethynyl]-1,7-dimethyl-3,5-di[4-(di(p-methoxy-
RT. After full consumption of the starting material, 1% phenyl)-amino)styryl]-2,6-diethyl-4,4-difluoro-4-bora-3a,4a-diaza-
CH3COOHaq (50 mL) is added. Then, CH2Cl2 (150 mL) is added s-indacene (14). Under dry conditions, 4-(di(4-methoxyphenyl)-
and the organic layer is washed with water (4 ꢃ 80 mL) until amino)benzaldehyde (1.873 g, 5.620 mmol, 3 eq.) is dissolved in
neutrality. It is dried over Na2SO4 and the solvent is removed dry benzene (40 mL). Piperidine (0.30 mL), glacial acetic acid
under vacuum. Yield: 327 mg (pink-black solid), 100%. (0.25 mL) and finally 10 (750.0 mg, 1.873 mmol, 1 eq.) are added.
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1424 New J. Chem., 2013, 37, 1417--1426
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The reaction solution is heated to reflux over a glass frit 1171 (s), 1031 (m), 821 (m). 1H-NMR (300 MHz, DMSO-d6):
containing molecular sieve (4 Å) to remove the formed water. d/ppm = 1.13 (t, 3J1H/1H = 7.14 Hz, 6H, 2/6-CH2CH3), 2.49 (s, 6H,
3
After complete consumption of the staring material (21 h), the 1/7-CH3), 2.67 (q, J1H/1H = 6.75 Hz, 4H, 2/6-CH2CH3), 3.75
3
solvent is removed and the raw product is purified by column (s, 12H, OMe), 6.76 (d, J1H/1H = 8.67 Hz, 4H, 3/5-Har), 6.92
chromatography using CH2Cl2 : hexane 9 : 1 as eluent. Yield: (d, 3J1H/1H = 9.03 Hz, 8H, 3/5-Har), 7.05 (d, 3J1H/1H = 8.88 Hz, 8H,
3
800 mg (blue-black solid, Rf (SiO2; CH2Cl2 : hexane 9 : 1) = 0.23), 3/5-Har), 7.20 (d, J1H/1H = 16.39 Hz, 2H, 3/5CHQCH), 7.38 (d,
41%. lmax(DCM)/nm = 784 (e/Mꢀ1 cmꢀ1 = 62 815), 582 (28 346), 3J1H/1H = 16.36 Hz, 2H, + d, 4H, 3J1H/1H = 8.22 Hz, 3/5CHQCH +
3
3
465 (17 482), 381 (60 544). FT-IR (ATR): nmax/cmꢀ1 = 2959 (w), 3/5-Har), 7.72 (d, J1H/1H = 8.22 Hz, 2H, 8-Har), 7.95 (d, J1H/1H
=
2148 (w), 1587 (m), 1497 (s), 1440 (m), 1237 (m), 1161 (m), 1029 8.34 Hz, 2H, 8-Har), 8.00 (s, 1H, Ph–CHQC(COOH)(CN)) ppm.
(m), 818 (m). 1H-NMR (300 MHz, benzene-d6): d/ppm = 0.22
8-[(4-(1,3-Dioxolanyl)phenyl)ethynyl]-1,3,5,7-tetramethyl-2,6-
(s, 9H, Si(CH)3), 1.07 (t, 3J1H/1H = 7.29 Hz, 6H, 2/6-CH2CH3), 2.36 diethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (17). Under
3
(s, 6H, 1/7-CH3), 2.55 (q, J1H/1H = 7.38 Hz, 4H, 2/6-CH2CH3), dry and oxygen-free conditions, 2-(4-iodophenyl)-1,3-dioxolane
3.37 (s, 12H, OMe), 6.75 (d, 3J1H/1H = 9.00 Hz, 8H, 3/5-Har), 6.84 (4.54 g, 16.453 mmol, 3 eq.), Pd(PPh3)4 (0.38 g, 0.329 mmol,
(d, 3J1H/1H = 8.70 Hz, 4H, 3/5-Har), 7.04 (d, 3J1H/1H = 8.94 Hz, 8H, 0.06 eq.), CuI (4.2 mg, 0.219 mmol, 0.04 eq.) and NEt3 (3.5 mL,
3
3/5-Har), 7.38 (d, J1H/1H = 16.66 Hz, 2H, 3/5CHQCH), 7.48 25.250 mmol, 4.7 eq.) are dissolved in dry THF (80 mL). Then,
3
3
(d, J1H/1H = 8.73 Hz, 4H, 3/5-Har), 8.42 (d, J1H/1H = 16.51 Hz, 11 (1.80 mg, 5.484 mmol, 1 eq.) is dissolved in dry THF (20 mL)
2H, 3/5CHQCH). and added dropwise over 3.5 h at RT to the reaction solution.
8-Ethynyl-1,7-dimethyl-3,5-di[4-(di(p-methoxyphenyl)-amino)- After further 4 h stirring at RT, the solvent is removed. The
styryl]-2,6-diethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (15). residue is dissolved in CHCl3 and washed with water (4 ꢃ 100 mL).
14 (790.0 mg, 0.766 mmol, 1 eq.) is dissolved in THF : methanol The organic fraction is dried over Na2SO4 and filtered. After
1 : 1 (80 mL of each), KF (222.6 mg, 3.831 mmol, 5 eq.) is added removal of the solvent, the raw product is purified by column
and the reaction solution is stirred for 30 min at RT. Then 1% chromatography with cyclohexane : ethyl acetate 6 : 1. Yield:
CH3COOHaq (50 mL) and CH2Cl2 (150 mL) are added. The 2.22 g (dark purple solid, Rf (SiO2; cyclohexane : ethyl acetate
organic layer is washed with water (4 ꢃ 100 mL) until neutrality, 6 : 1) = 0.47), 85%. lmax(DCM)/nm = 579 (e/Mꢀ1 cmꢀ1 = 30 887),
dried over Na2SO4, filtered and the solvent is removed under 542 (15 910), 395 (16 874). FT-IR (ATR): nmax/cmꢀ1
=
1H-NMR
3
vacuum. Yield: 710 mg (blue-black solid), 97%. lmax(DCM)/nm = (300 MHz, DMF-d7): d/ppm = 1.08 (t, J1H/1H = 7.59 Hz, 6H,
3
781 (e/Mꢀ1 cmꢀ1 = 49 415), 580 (22 659), 461 (16 994), 372 2/6-CH2CH3), 2.44 (q, J1H/1H = 7.59 Hz, 4H, 2/6-CH2CH3), 2.51
(49 970). FT-IR (ATR): nmax/cmꢀ1 = 3274 (w), 2961 (w), 1587 (s, 6H, 3/5-CH3), 2.54 (s, 6H, 1/7-CH3), 4.00–4.20 (m, 4H,
1
3
(m), 1500 (s), 1240 (m), 1171 (m), 1033 (m), 821 (m). H-NMR CH2–CH2), 7.54 (d, J1H/1H = 8.37 Hz, 2H, 8-Har), 7.59 (d,
(300 MHz, THF-d8): d/ppm = 1.24 (t, J1H/1H = 7.17 Hz, 6H, 3J1H/1H = 8.43 Hz, 2H, 8-Har).
2/6-CH2CH3), 2.53 (s, 6H, 1/7-CH3), 2.78 (q, J1H/1H = 7.44 Hz,
3
3
4H, 2/6-CH2CH3), 3.80 (s, 12H, OMe), 4.69 (s, 1H; RH), 6.88
Acknowledgements
3
3
(d + d, J1H/1H = 7.02 Hz, 4H + 8H, 3/5-Har), 7.08 (d, J1H/1H
=
3
We acknowledge financial support from the Graduiertenkolleg
GRAKO 1640 (DFG).
8.94 Hz, 8H, 3/5-Har), 7.28 (d, J1H/1H = 16.74 Hz, 2H,
3
3/5CHQCH), 7.41 (d, J1H/1H = 8.73 Hz, 4H, 3/5-Har), 7.63
3
(d, J1H/1H = 16.60 Hz, 2H, 3/5CHQCH).
8-[(4-(2-Carboxy-2-cyanovinyl)phenyl)ethynyl]-1,7-dimethyl-
3,5-di[4-(di(p-methoxyphenyl)amino)styryl]-2,6-diethyl-4,4-difluoro-
4-bora-3a,4a-diaza-s-indacene (16). Under dry and oxygen-free
conditions, 12 (280.1 mg, 0.939 mmol, 3 eq.), Pd(PPh3)4
(21.7 mg, 0.019 mmol, 0.06 eq.), CuI (2.4 mg, 0.013 mmol,
0.04 eq.) and NEt3 (0.20 mL, 1.408 mmol, 4.5 eq.) are dissolved
in dry THF (15 mL). Then, 15 (300.0 mg, 0.313 mmol, 1 eq.) is
likewise dissolved in dry THF (20 mL) and added dropwise over
3 h at RT to the reaction solution. After further 60 min stirring
at RT, the solvent is removed. The raw product is purified by
column chromatography starting with cyclohexane : ethyl acet-
ate 2 : 1 + 1% of glacial acetic acid, then the solvent is changed
to CH2Cl2 followed by a change to CH2Cl2 : methanol 20 : 1 with
a gradient to 10 : 1. The product fractions are collected and
concentrated. The pure product is precipitated from ethanol,
washed with ethanol, methanol, hexane and water. Yield:
170 mg (green-black solid, Rf (SiO2; CH2Cl2 : methanol 10 : 1) =
0.41), 48%. lmax(DCM)/nm = 835 (e/Mꢀ1 cmꢀ1 = 28 272), 611
(27 287), 403 (52 941). FT-IR (ATR): nmax/cmꢀ1 = 3476 (br), 2930
(w), 2210 (w), 1581 (m), 1500 (s), 1439 (w), 1323 (w), 1234 (m),
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