Synthesis and photophysical properties of borondipyrromethene dyes bearing aryl substituents at the boron center

Article abstract of DOI:10.1021/ja062405a

Several borondipyrromethene (Bodipy) dyes bearing an aryl nucleus linked directly to the boron center have been prepared under mild conditions. The choice of Grignard or lithio organo-metallic reagents allows the isolation of B(F)(aryl) or B(aryl)₂

Full text of DOI:10.1021/ja062405a

Published on Web 07/07/2006
Synthesis and Photophysical Properties of
Borondipyrromethene Dyes Bearing Aryl Substituents at the
Boron Center
†
†
‡
‡
Christine Goze, Gilles Ulrich, Laura J. Mallon, Ben D. Allen,
‡
,†
Anthony Harriman, and Raymond Ziessel*
Contribution from the Laboratoire de Chimie Mol e´ culaire, Centre National de la
Recherche Scientifique (CNRS), EÄ cole de Chimie, Polym e` res, Mat e´ riaux (ECPM),
5 rue Becquerel, 67087 Strasbourg Cedex 02, France, and Molecular Photonics Laboratory,
School of Natural Sciences, Bedson Building, UniVersity of Newcastle,
Newcastle upon Tyne, NE1 7RU, United Kingdom
2
Received April 7, 2006; E-mail: ziessel@chimie.u-strasbg.fr
Abstract: Several borondipyrromethene (Bodipy) dyes bearing an aryl nucleus linked directly to the boron
center have been prepared under mild conditions. The choice of Grignard or lithio organo-metallic reagents
allows the isolation of B(F)(aryl) or B(aryl) derivatives; where aryl refers to phenyl, anisyl, naphthyl, or
2
pyrenyl fragments. A single crystal, X-ray structure determination for the bis-anisyl compound shows that
3
the sp hybridized boron center remains pseudo-tetrahedral and that the B-C bond distances are 1.615
and 1.636 Å. All compounds are electrode active but replacement of the fluorine atoms by aryl fragments
renders the Bodipy unit more easily oxidized by 100 mV in the B(F)(aryl) and 180 mV in the B(aryl)
2
compounds whereas reduction is made more difficult by a comparable amount. Strong fluorescence is
observed from the Bodipy fluorophore present in each of the new dyes, with the radiative rate constant
being independent of the nature of the aryl substituent. The fluorescence quantum yields are solvent
dependent and, at least in some cases (aryl ) anisyl or pyrenyl), nonradiative decay from the first-excited
singlet state is strongly activated. There is no indication, however, for population of a charge-transfer state,
in which the aryl substituent acts as donor and the Bodipy fragment functions as acceptor, that is strongly
coupled to the ground state. Instead, it is conjectured that nonradiative decay involves a conformational
change driven by the solvophobic effect. Thus, the rate of nonradiative decay in any given solvent increases
with increasing surface accessibility (or molar volume) of the aryl substituent. Intramolecular energy transfer
from pyrene or naphthalene residues to Bodipy is quantitative.
can be tuned over a wide range.3,4,5 These properties have led
to numerous applications in the field of biological labeling.3
Introduction
,6
Modern biotechnological and electronic applications require
new fluorophores with predetermined properties, such as robust-
ness, high fluorescence quantum yield, large Stokes’ shift,
optimized absorption profile and suitable anchoring groups, so
as to meet the demands for more sensitive analytical protocols,
sensors and light-emitting devices.1,2 Among the many known
classes of fluorescent dyes, the family of difluoro-boradiaza-
indacenes (abbreviated herein as F-Bodipy), better known by
Furthermore, their electrochemical activity makes these dyes
interesting candidates for use as dopants in light-emitting
devices and as components in electroactive films.
7
However, in common with most other organic fluorophores,
F-Bodipy suffers from important inherent limitations. For
example, such dyes possess small Stokes’ shifts (around 600
-1
cm ), that require the use of precision optical filters and specific
excitation wavelengths for fluorescence microscopy and flow
cytometry. A large Stokes’ shift is of paramount importance in
3
the commercial name of BODIPY, has received particular
attention. The photophysical properties of this class of dyes are
characterized by high fluorescence quantum yields, low rates
of intersystem crossing, large molar absorption coefficients,
excellent photostability, and an emission spectral profile that
(
4) Kang, H. C.; Haugland, R. P. U.S. Patent 5,451,663; Sep 19 1995.
5) (a) Burghart, A.; Kim, H.; Welch, M. B.; Thoresen, L. H.; Reibenspies, J.;
Burgess, K. J. Org. Chem. 1999, 64, 7813-7819. (b) Chen, J.; Burghart,
A.; Derecskei-Kovacs, A.; Burgess, K. J. Org. Chem. 2000, 65, 2900-
(
2
906. (c) Turfan, B.; Akkaya, E. U. Org. Lett. 2002, 4, 2857-2859. (d)
Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2005, 127, 10464-10465.
e) Qin, W.; Rohand, T.; Baruah, M.; Stefan, A.; Van der Auweraer, M.;
Dehaen, W.; Boens, N. Chem. Phys. Lett. 2006, 420, 562-568.
†
Centre National de la Recherche Scientifique (CNRS).
University of Newcastle.
(
‡
(
(
(
1) Valeur, B. In Molecular Fluorescence: Principles and Applications; Wiley-
(6) Martin, H., Rudolf, H., Vlastimil, F., Eds.; Fluorescence Spectroscopy in
Biology: AdVanced Methods and their Applications to Membranes,
Proteins, DNA, and Cells; Springer-Verlag: Heidelberg, Germany, 2005.
(7) (a) Lai, R. Y.; Bard, A. J. J. Phys. Chem. B 2003, 107, 5036-5042. (b)
Brom, J. M. Jr.; Langer, J. L. J. Alloys Cmpds. 2002, 338, 112-115. (c)
Hepp, A.; Ulrich, G.; Schmechel, R.; von Seggern, H.; Ziessel R. Synth.
Met. 2004, 146, 11-15.
VCH: Weiheim, Germany, 2002.
2) Lakowicz, J. R. Probe Design and Chemical Sensing; Lakowicz, J. R.,
Ed.; Topics in Fluorescence Spectroscopy Vol. 4; Plenum: New York 1994.
3) Haugland, R. P. The Handbook A guide to Fluorescent Probes and Labeling
Technologies, 10th ed.; Spence, M. T. Z., Ed.; Molecular Probes: Eugene,
OR, 2005.
10.1021/ja062405a CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 10231-10239
9
10231

A R T I C L E S
Goze et al.
Scheme 1 a
multicolor labeling (i.e., chromosome mapping) where sophis-
ticated apparatus is used to simultaneously excite and analyze
numerous fluorophores present in the system. Several ap-
8
proaches are available by which to create an artificially large
Stokes’ shift. The most popular way involves the design of a
tandem molecular device in which fluorescence resonance
energy transfer occurs between discrete subunits; for example,
the development of fluorescence energy-transfer cassettes based
9
on modified oligo-deoxynucleotides has resulted in greatly
enhanced sensitivity for DNA recognition. In this way, an
anthracene-based donor has been covalently linked to various
F-Bodipy acceptors,10 leading to efficient intramolecular energy
transfer in a family of cassette dyes having a common excitation
wavelength and a tunable emission range. Further examples of
this approach have involved intramolecular energy transfer from
an appended pyrene-based chromophore to F-Bodipy, with the
aryl hydrocarbon being attached at the central pseudo-meso
position.11 Somewhat related systems have coupled fluorescent
proteins possessing strong absorption profiles to other organic
dyes, such as Alexa647.3
a
Keys. (i) Phenylmagnesium bromide (1 equiv), diethyl ether, 1 h,
0
°C; (ii) Phenylmagnesium bromide (2 equiv), diethyl ether, 1 h, rt.
Scheme 2 a
From a synthetic viewpoint, the multiple functionalization
of F-Bodipy dyes is of particular interest but the scope is limited
by the tedious pyrrole chemistry and by the condensation step
that leads to the dipyrromethene intermediate. To circumvent
the linear multistep syntheses needed for preparation of new
4,5
Bodipy-based dyes we recently introduced the concept of
Cascatelle devices, in which a supplementary pendant arm
enables bio-labeling of proteins.12 Replacement of the fluorine
a Keys. (i) 2-Naphthylmagnesium bromide (1 equiv), diethyl ether, 2 h,
°C; (ii) 2-Naphthylmagnesium bromide (2 equiv), diethyl ether, 2 h, rt.
0
atoms ubiquitous to F-Bodipy dyes with ethynyl-aryl moieties,
thereby leading to E-Bodipy, is a further strategy for obtaining
enlarged (virtual) Stokes’ shifts.12 Two examples of the replace-
ment of the B-F units with B-phenyl fragments, and their basic
physical properties, were described in a patent by Murase et
directly to the boron center in place of the regular fluorine atoms
on a boradiazaindacene scaffold (C-Bodipy). It is shown that
these dyes are highly fluorescent in solution at ambient
temperature but some interesting photophysical properties
emerge as the size of the aryl substituent increases.
1
3
al. Aryl-Grignard reagents were found to be effective for
fluorine replacement. Similar reactions using organo-metallic
carbanions have been described with â-diketiminate boron
Results and Discussion
complexes, leading to insertion of a tetrahedral boron center
structurally related to difluoro-boradiazaindacene.14 Trigonal
To validate the fluorine substitution process, an aryl-Grignard
was reacted with the F-Bodipy 1 in anhydrous diethyl ether at
room temperature (Scheme 1), following the general conditions
boron salts have been converted to triaryl boron derivatives by
1
5
way of organo-metallic species, mostly aryl-Grignard or
13
organo-lithium reagents,16 and the resulting BR3 compounds are
described by Murase. When arylmagnesium bromide is added
at 0 °C, the major product is the monosubstituted derivative 2,
this being isolated in 40% yield. The disubstituted compound
3 cannot be observed by TLC, even with an excess of the
Grignard reagent. Under these latter conditions, an intractable
mixture of polar compounds is formed. However, the disubsti-
tuted derivative was obtained by adding the appropriate carb-
anion to a solution of the F-Bodipy at room temperature, giving
rise to 3 in 25% yield (Scheme 1).
stabilized with amino-donor groups. These synthetic procedures
have been used to build dendritic scaffolds with extended
17
π-conjugated systems including several boron centers in order
to obtain colorimetric fluoride ion sensors.18 Here, we introduce
a new family of fluorophores in which an aryl unit is connected
(
8) Ju, J.; Ruan, C.; Fuller, C. W.; Glazer, A. N.; Mathies, R. A. Proc. Natl.
Acad. Sci. U.S.A. 1995, 92, 4347-4351.
(9) (a) Glazer, A. N.; Mathies, R. A. Curr. Opin. Biotech. 1997, 8, 94-102.
(
b) Berti, L.; Xie, J.; Medintz, I. L.; Glazer, A. N.; Mathies, R. A. Anal.
This procedure was subsequently applied to the synthesis of
naphthalene-based analogues, using 2-naphthylmagnesium bro-
mide, generated from 2-bromonaphthalene and magnesium
turnings in diethyl ether. With this carbanion, the monosubsti-
tuted compound 4 was obtained in 30% yield by addition at
0 °C and the disubstituted compound 5 could be synthesized in
Biochem. 2001, 292, 188-197.
(
10) Wan, C.-W.; Burghart, A.; Chen, J.; Bergstr o¨ m, F.; Johanson, L. B.-A.;
Wolford, M. F.; T. G. Kim, M. R. Topp, R. M. Hochstrasser, K. Burgess
Chem.sEur. J. 2003, 9, 4430-4441.
(
(
(
(
11) Ziessel, R.; Goze, C.; Ulrich, G.; C e´ sario, M.; Retailleau, P.; Harriman,
A.; Rostron, J. P. Chem.sEur. J. 2005, 11, 7366-7378.
12) Ulrich, G.; Goze, C.; Guardigli, M.; Roda, A.; Ziessel, R. Angew. Chem.,
Int. Ed. 2005, 44, 3694-3698.
13) Murase, S.; Tominaga, T.; Kohama, A. Eur. Patent EP 1253151a1,
3
5% yield at 20 °C (Scheme 2). It was noted that additional
red-colored, polar compounds were also obtained under these
2
5.04.2002.
14) Qian, B.; Baek, S. W.; Smith, M. R. III Polyhedron, 1999, 18, 2405-
2
414.
(
(
15) Brown, H. C.; Racherla, U. S. J. Org. Chem. 1986, 51, 427-432.
16) Bayer M. J.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2002, 2069-
(18) (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001, 123,
11372-11375. (b) Kubo, Y.; Yamammoto, M.; Ikeda, M.; Takeuchi, M.;
Shinkai, S.; Yamaguchi, S.; Tamao, K. Angew. Chem., Int. Ed. 2003, 42,
2036-2040.
2
072.
(
17) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000, 122,
6
335-6336.
10232 J. AM. CHEM. SOC.
9
VOL. 128, NO. 31, 2006

Arylborondipyrromethene Dyes
A R T I C L E S
Scheme 3 a
a
Keys. (i) Phenyllithium (2 equiv) or 4-methoxyphenyllithium (2 equiv),
diethyl ether, rt, 5 min, 3 (40%), 6 (35%); (ii) 1-pyrenyllithium (2 equiv),
THF, rt, 5 min, 20%.
conditions but all attempts to isolate these latter materials were
unsuccessful.
To compare the reactivity of the two organo-metallic carb-
anions used traditionally in the synthesis of arylboron deriva-
tives, compound 3 was synthesized using phenylithium in diethyl
ether. This reaction was carried out at various temperatures
Figure 1. ORTEP views of compound 5 (Top) and 6 (bottom).
structure (Figure 1). Both structures are similar and only the
3
structure of 6 and is discussed. The sp hybridized boron center
(-78, 0, 20 °C), and was much faster than that with the Grignard
appears as a distorted tetrahedron with dihedral angles N1-
B-N2 of 104.54° and C1-B-C2 of 114.96°. The diazain-
dacene unit is almost planar, with internal bond lengths slightly
reagent. The reaction, however, has to be quenched rapidly to
avoid formation of secondary polar products. Surprisingly, only
formation of the disubstituted compound 3 was observed, even
when 1 equiv of anion was used. This method was extended to
other aromatic substituents, with the organo-lithium compound
being generated at low temperature (-78 °C) by reaction of
n-butylithium with an aryl bromide in THF. This latter solvent
is used to increase the solubility of the starting compound. The
aryllithium derivative is transferred to an F-Bodipy solution
maintained at room temperature, and the reaction is quenched
rapidly with water (5 min). The resultant products were stable
and purified by column chromatography, followed by recrys-
tallization. Compounds 6 and 7 were obtained in reasonable
yields (20-35%), starting respectively from 4-methoxyphenyl-
lithium and 1-pyrenyllithium (Scheme 3).
1
9
different to those of related F-bodipy analogues. The boron
atom is out of plane of the cyanine, being 0.50 Å from the mean
plane; this deformation is not observed in F-Bodipy or E-
1
2
Bodipy. The average B-N bond length is 1.583 Å, this being
slightly longer than that observed in F-bodipy (around 1.54 Å).
The bond length in the diazaindacene core is similar to that
1
9
found for F-bodipy derivatives, with average bonds values
for N1-C15/N2-C29 being 1.353(4) Å; these data tend to
confirm the pronounced double-bond character. The C22-N1/
C25N2 bond is longer, being 1.404 (5) Å. The B-C bonds in
6, 1.615(4) for B-C1 and 1.636(4) for B-C2, are slightly longer
than those found for somewhat similar 5-(2-pyridyl)pyrazolate
2
0
diphenyl boron complexes. Interestingly, in both structures
the boron atom is located above the mean plane of the
dipyrromethene residue by 0.50 Å for 6 and 0.57 Å for 5, a
situation which has never been observed in related F-Bodipy
dyes where the boron center fits perfectly in the mean plane of
1
13
All new compounds were fully characterized by H, C, and
B NMR, mass spectroscopy and elemental analysis. In H
1
1
1
NMR spectra, typical peaks due to the Bodipy core can be
observed; a characteristic feature being shielding of the singlet
corresponding to the methyl group R to the nitrogen, this signal
being shifted from 2.49 ppm for 1 to 2.16 ppm for 2. Stronger
shielding, induced by the phenyl ring, is observed for 3, with a
singlet appearing at 1.70 ppm. Similar behavior is observed for
each of the other compounds, with the chemical shift of this
methyl group moving from 2.39 ppm in the mononaphthyl
derivative 4 to 1.73 ppm in the binaphthyl compound 5. The
strongest shielding was observed with the bis-pyrene derivative
1
1,12
the indacene core.
Electrochemical Properties. An evaluation of the extent of
electronic interactions between the various subunits in these new
dyes was made by cyclic voltammetry in dichloromethane with
tetra-n-butylammonium hexafluorophosphate as supporting elec-
trolyte. The electrochemical data collected for the B-aryl
molecules are gathered in Table 1. The prototypic starting
material 1 displays a one-electron, reversible oxidation wave
with a half-wave potential of +0.95 V vs SCE. There is a
corresponding one-electron, reversible reduction wave with a
half-wave potential of -1.43 V vs SCE. In comparison,
compounds 2 and 3, which lack one or both fluorine atoms
respectively, are easier to oxidize, with half-wave potentials
being lowered by 100 and 180 mV, respectively. Reduction is
7
, where the singlet appears at 1.32 ppm.
In 11B NMR spectra, the triplet seen at 3.8 ppm can be
attributed to coupling with the two equivalent fluorine atoms
in 1. This signal is replaced by a unique singlet for the other
compounds, even for the mono-fluoro derivatives. The signal
shifts downfield by ca. 2 ppm for the monosubstituted C-Bodipy
whereas for the dipyrenyl compound the signal is shielded
upfield 1.2 ppm with respect to the BF2-starting material.
X-ray Crystallography. Single crystals were obtained for
compounds 5 and 6 used to elucidate the X-ray molecular
(19) Shen, Z.; R o¨ hr, H.; Rurack, K.; Uno, H.; Spieles, M.; Schulz, B.; Reck,
G.; Ono, N. Chem.sEur. J. 2004, 10, 4853-4871.
(
20) Cheng, C.-C.; Yu, W.-S.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Wu, P.-
C.; Song, Y.-W.; Chi, Y. Chem. Commun. 2003, 2628-2629.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 31, 2006 10233

A R T I C L E S
Goze et al.
Table 1. Electrochemical Properties of the C-Bodipy Dyes and
is apparent that the presence of the directly attached aryl moieties
renders the indacene unit more easy to oxidize but more difficult
to reduce. This observation can be explained in terms of
increased electron density at the indacene core. It is also
noticeable that in the anisyl, naphthyl and pyrenyl cases an
additional, irreversible oxidation wave is present in the cyclic
voltammograms. This latter wave can be assigned to one-
electron oxidation of the aryl nucleus. As expected in light of
a
Reference Compound
E
1/
2
, V (∆E, mV)
.
aryl/aryl-
.
cmpd
aryl+./aryl
bodipy+./bodipy bodipy/bodipy-
1
2
3
4
5
6
7
+0.95(70)
+0.85(70)
+0.77(60)
+0.86(70)
-1.43(60)
-1.61(60)
-1.79(70)
-1.62(70)
-1.74(70)
-1.77(70)
+1.60(irrev, Ic/Ia ≈ 0.1) +0.78(60)
+1.64(irrev, Ic/Ia ≈ 0.1) +0.74(60)
+1.48(irrev, Ic/Ia ≈ 0.2) +0.79(60)
1
1
previous observations the pyrenyl residue is easier to oxidize
than either the anisyl or naphthyl groups. For the pyrene
compound 7, an additional electrode process could be detected
at about -2.5 V vs SCE in ortho-dichlorobenzene using a
-1.76(70) -2.46(irrev)b
a
Potentials determined by cyclic voltammetry in deoxygenated CH2Cl2
solution containing 0.1 M TBAPF6, at a solute concentration of ca. 1 mM
1
1
and at 20 °C. Potentials were standardized versus ferrocene (Fc) as internal
mercury hanging drop electrode. This step is assigned to
+
reference and converted to SCE assuming E1/2 (Fc/Fc ) ) +0.38 V
reduction of the pyrene unit.
(
∆Ep ) 70 mV) vs SCE. Error in half-wave potentials is ((10 mV. Where
It is interesting to note that the energy gap between LUMO
and HOMO localized on the Bodipy dye is increased signifi-
cantly for the B-aryl derivatives relative to the parent dye, 1.
Thus, the energy gap increases from 2.38 to 2.56 eV on
replacing both fluorine atoms with phenyl rings. Replacing only
a single fluorine atom results in an energy gap of 2.46 eV,
midway between the disubstituted cases. The same behavior is
seen for the corresponding naphthyl derivatives, where the
energy gaps are 2.48 and 2.52 eV respectively for mono- and
disubstitution. The nature of the aryl substituent has little real
effect on this energy gap, being 2.55 eV for pyrenyl and 2.51
eV for anisyl substituents. The overall difference, however,
between 1 and 3 is surprisingly pronounced.
the redox process is irreversible, the peak potential (Eap) is quoted and the
relative peak heights are given as Ic/Ia. All reversible redox steps result
b
from one-electron transfer. In 1,2-dichlorobenzene using a Hg hanging
drop electrode. Ic/Ia ≈ 0.
Optical Properties. Spectroscopic data relevant to the present
discussion are collected in Table 2. In solution, the absorption
spectra of all compounds show a strong S0 f S1 (π-π*)
transition with a clear maximum (λMAX) between 510 and 525
nm. In most cases, the molar absorption coefficient at the peak
-
1
-1
(
ꢀMAX) has a value of about 75 000 M cm . This transition
5
,22
is unambiguously assigned to the Bodipy chromophore and
it is noteworthy that, except for 7, the B-aryl dyes show
somewhat larger ꢀMAX values than the parent dye 1. This is not
the case for the pyrene-based dye 7 where the lowest-energy
absorption band is noticeably broader and weaker. In all cases,
a broad absorption band, that can be assigned to the S0 f S2
2
3
(
π-π*) transition of the boradiazaindacene unit, is located
between 370 and 410 nm (Figure 3). Additional π-π* transi-
tions are found around 250 and 280 nm, respectively, for the
phenyl and the naphthyl residues, although these overlap higher-
energy bands due to the dye. For the pyrenyl fragments, π-π*
transitions can be seen across much of the near-UV region with
two clear sets of bands around 320-370 nm and 230-310 nm
n
Figure 2. Cyclic voltammetry in CH2Cl2 at 20 °C using 0.1 M Bu4PF6 as
supporting electrolyte at a scan rate of 200 mV/s. (a) Compounds 1 (black),
(blue), and 3 (red). (b) Compounds 4 (black) and 5 (red). Ferrocene is
used as internal standard (half-wave potential ) 0.38V vs SCE).
2
(Figure 3). The lowest-energy absorption transition association
more difficult, with the corresponding half-wave potentials being
decreased by 180 and 360 mV, respectively, relative to 1 (Figure
with the pyrene fragment is seen as a weak, sharp band at about
375 nm in dichloromethane. No evidence was obtained for self-
association of these compounds in dichloromethane solution,
at least at concentrations below 2 mM.
An interesting, if fortuitous, feature of the optical absorption
spectrum recorded for 1 is that the energy of the absorption
maximum (λMAX ) 2.40 eV) is in excellent agreement with the
energy gap between LUMO and HOMO (∆ ) 2.38 eV) obtained
from cyclic voltammetry. It is also clear that whereas λMAX for
1 is essentially insensitive to changes in solvent polarity, those
2
a). Both processes remain electrochemically reversible. There
is no indication for oxidation to the Bodipy dication or reduction
to the Bodipy dianion within the given electrochemical window,
2
1
as observed previously for pyridine-linked B-F derivatives.
There is a small lowering (i.e., 30 mV) of the half-wave potential
for one-electron oxidation on replacing the phenyl rings with
anisole units, compound 6.
On replacing the phenyl rings in compound 3 with naphthyl
groups, in compound 5, or pyrenyl units, in compound 7, the
various half-wave potentials remain essentially unaffected. It
(
22) Kollmannsberger, M.; Rurack, K.; Resch-Genger, U.; Daub, J. J. Phys.
Chem. A 1998, 102, 10211-10220.
(23) Karolin, J.; Johansson, L. B.-A.; Strandberg, L.; Ny, T. J. Am. Chem. Soc.
1994, 116, 7801-7806.
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1
0234 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006
9

Arylborondipyrromethene Dyes
A R T I C L E S
Table 2. Spectroscopic Data for the Various Dyes in Solution at 298 K
a
a
MAX (M-1cm-1)
a
a
a
b
RAD (108 s-1)
FLU (nm)c
c
c
d
d
cmpd
λ
MAX (nm)
ꢀ
λ
FLU (nm)
Φ
FLU
τ
FLU (ns)
k
λ
Φ
FLU
τ
FLU (ns)
Φ
FLU
τ
FLU (ns)
1
2
3
4
5
6
7
517
524
513
513
516
513
526
64 500
75 200
73 300
77 300
74 500
77 400
46 000
538
543
548
543
552
546
562
0.83
0.90
0.91
0.92
0.80
0.78
0.19
6.2
6.4
6.1
6.1
5.7
6.0
2.0
1.35
1.40
1.50
1.50
1.40
1.30
0.95
536
-
539
-
541
540
558
0.90
-
0.77
-
0.65
0.64
0.12
6.7
-
5.0
-
4.9
4.7
1.7
0.88
-
0.76
-
0.60
0.57
0.14
6.6
-
5.0
-
5.0
4.8
1.6
a
Determined in deoxygenated dichloromethane solution. b Calculated from the Strickler-Berg expression. c Ethanol solution. d Acetonitrile solution.
Figure 4. Fluorescence spectrum (black curve) observed for 7 in ethanol
Figure 3. Absorption spectra, emission spectra (arbitrary units) for
compounds 1, 4, and 5. Absorption of 7 is shown as an inset. All spectra
were measured in CH2Cl2 at room temperature.
solution. The blue curves represent the deconvoluted Gaussian components
whose maxima are separated by the low-frequency vibrational mode hωL.
The sum of the Gaussian bands, red curve, is compared to the total spectrum.
for the B-aryl derivatives tend to undergo a small red shift with
increasing polarizability of the solvent. For example, λMAX for
excitation wavelength. Relative to 1, the Stokes’ shift is also
-
1
larger, having a value of 1230 cm for 7, and sensitive to the
size of the aryl substituent. With the exception of 7, φFLU and
τFLU measured for the new compounds in deoxygenated dichlo-
romethane remain comparable to those recorded for 1. In each
case, fluorescence is clearly characteristic of the Bodipy
6
is found at 516, 513, 512, 512, and 509 nm, respectively, in
cyclohexane, dichloromethane, ethanol, dimethyl sulfoxide, and
acetonitrile. A similar observation has been made for certain
e
3
,5-disubstituted F-bodipy dyes.5
5
,22,23
The parent dye 1 fluoresces with high efficiency in solution
fluorophore
and decay kinetics are monoexponential. For
8
-1
at ambient temperature. The emission maximum (λFLU) is located
at 538 nm in dichloromethane and is insensitive to changes in
solvent polarity; for example, λFLU shifts to 536 nm in
acetonitrile solution. The fluorescence quantum yield (φFLU) in
deoxygenated dichloromethane is 0.83, where the fluorescence
lifetime (τFLU) is 6.2 ns. Again, changes in solvent polarity have
minimal effect on these values (Table 2). The corrected fluo-
rescence excitation spectrum agrees well with the absorption
spectrum over the visible and near-UV regions and photons
absorbed by the S2 state are transferred quantitatively to S1.
these dyes, kRAD is found to be 1.3 ( 0.2 × 10 s in dichloro-
methane. There is, however, a modest decrease in both φFLU
and τFLU in ethanol (Table 2), where the Stokes’ shifts are
decreased by ca. 20%. Similar values are found in acetonitrile
2
5
but kRAD remains solvent independent. It is clear that there is
an increase in the rate of nonradiative decay for the B-aryl
compounds in these latter solvents, even for the phenyl-based
dyes where intramolecular charge-transfer is most unlikely.
The disubstituted pyrene-based dye 7 exhibits a relatively low
ꢀMAX in the region where Bodipy absorbs and both φLUM and
τLUM are decreased relative to the other B-aryl dyes (Table 2).
The absorption profile of the pyrene component is red shifted
-
1
The Stokes’ shift is only 650 cm in dichloromethane and is
-
1
increased to 700 cm in acetonitrile. The fluorescence decay
profiles were strictly monoexponential in all solvents and the
derived τFLU is independent of excitation or monitoring wave-
length. The radiative rate constant calculated from the Strickler-
2
6
and broadened with respect to pure pyrene and the S1-S0
fluorescence spectrum observed for 7 is the most red shifted of
the B-aryl dyes studied. Both φLUM and τLUM decrease in ethanol
and acetonitrile, where the Stokes’ shift remains at ca. 1200
Berg expression24 (kRAD ) 1.30 × 10 s ) is in excellent
8
-1
-
1
7 -1
agreement with that estimated directly by experiment (kRAD )
cm . For 7, the radiative rate constant is about 9 × 10 s in
8
-1
φFLU/τFLU ) 1.35 × 10 s ). All available indicators point to
minimal disruption of the geometry on excitation to the singlet
manifold. It is also clear that nonradiative deactivation of the
excited singlet state competes poorly with the radiative process.
Fluorescence is readily observed from the various B-aryl dyes
following excitation into the first-excited singlet state in
dichloromethane solution (Table 2). In each case, λFLU is red
shifted with respect to 1 (Figure 4) but remains independent of
all solvents; the decrease relative to the other B-aryl dyes
2
7
reflecting the drop in oscillator strength. It is clear that, in all
solvents, nonradiative decay plays an important role for this
system. The same fluorescence spectral profile is observed
(
25) (a) In fact, the small variation in kRAD found in different solvents can be
well explained in terms of changes in refractive index. (b) Meech, S. R.;
Phillips, D. J. Photochem. 1983, 23, 193-217.
(26) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New
York, 1970.
(
27) From the Strickler-Berg expression, kRAD for the pyrene-based dye 7 is
7
-1
(24) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814-817.
calculated to be 9 × 10 s in dichloromethane solution.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 31, 2006 10235

A R T I C L E S
Goze et al.
Table 3. Derived Rate Constants for Nonradiative Decay of the
First-Excited Singlet State of the Various Dyes in Different
Solvents
h
ω
L
V
MOL
k
NR/CH
2
Cl
2
k
NR/C
2
H
5
OH
3
kNR/CH CN
(107 s-1)
cmpd
(cm-1)
(cm3 mol-1)
(107 s-1)
(107 s-1)
3
5
6
7
1060
950
950
263.5
409.5
337.5
619.5
1.5
3.5
3.7
4.6
7.1
7.7
4.8
9.0
8.0
1010
37.5
51.8
56.2
following excitation into the pyrene unit. Furthermore, the
corrected excitation spectrum matches with the absorption
spectrum over the range 260 to 530 nm in all solvents studied.
Thus, photons collected by the pyrene unit are transferred
quantitatively to the Bodipy chromophore. The same is true for
the naphthyl derivative 5.
Figure 5. Effect of temperature on the rate constants for nonradiative decay
as measured for 6 (O) and 7 (b) in ethanol. The solid line drawn through
each set of data points is a nonlinear, least-squares fit to eq 1 with the
parameters given in the text.
Detailed studies carried out with the anisyl derivative 6
showed that the Stokes’ shift does not vary systematically with
solvent polarity and, in particular, does not follow Lippert-
reasonably independent of temperature until around 230 K.
Above this temperature, τFLU decreases smoothly with increasing
temperature. Expressing the kinetic data obtained in fluid
solution in the form of eq 1, where kNR is the rate constant for
nonradiative decay of the excited state, allows estimation of
the activation energy for the activated decay route as being 17.7
2
8
Mataga behavior. This suggests that there is not a large
difference in dipole moment between ground and excited states.
2
9
The same situation was found for 7. Deconvolution of the
emission spectral profile (Figure 4) leads to the conclusion that
nonradiative decay of the lowest-energy singlet excited state is
coupled to a low-frequency vibrational mode (hωL) of ca.
-
1
kJ mol . The derived activated preexponential factor (kACT)
1
1
-1
has a value of 1.1 × 10
s
while the activationless rate
-
1
9
90 ( 70 cm (Table 3), although the same analysis carried
constant (k ) describing nonradiative decay at low temperature
IND
-
1
6
-1
out for 1 showed a vibrational mode of ca. 650 cm . There
was no systematic effect of solvent polarity on the size of the
coupled vibrational mode,30 whereas similar analysis applied
to the absorption spectral profiles for the S0-S1 transition gave
comparable but consistently smaller hωL values. Furthermore,
the reorganization energies32 (λ) accompanying formation of
the S1 state are larger for the B-aryl dyes than for the parent.
Indeed, λ for the phenyl (400 cm ), anisyl (500 cm ) and
naphthyl (520 cm ) substituted Bodipy dyes significantly
exceed that for 1 (280 cm ) under identical conditions. The
reorganization energy (λ ) 630 cm ) is somewhat larger for 7
is calculated to be 2.9 × 10 s . The same behavior is found
-
1
for 7 (Figure 5), where the activation energy is 19.4 kJ mol .
7
-1
For this latter system, k
IND
-1
) 7.1 × 10 s , whereas k
ACT
has
1
2
a value of 1.3 × 10 s . Nonradiative decay is much more
31
pronounced for the bulkier pyrene substituent.
(1 - φFLU
)
(EA/RT)
kNR
)
) kIND + kACTe-
(1)
-
1
-1
τFLU
-1
-
1
The additional nonradiative decay process characterized by
-
1
kACT is solvent dependent and does not result in enhanced triplet
34
than for the other dyes.
state formation, which remains extremely inefficient under all
conditions. A possible explanation for the temperature effect
could involve population of a high-lying, charge-transfer state
(CTS) in which the aryl substituent acts as donor and the Bodipy
residue functions as acceptor. The cyclic voltammetry results
indicate irreversible oxidation of the anisole unit at about 1.64
V vs SCE but this value can be lowered to about 1.5 V vs SCE
The fluorescence lifetime recorded for 1 in ethanol solution
was found to be insensitive to changes in temperature over the
3
3
range 100-300 K. This was not the case for 6 where there is
clear evidence for the onset of an activated decay route at higher
temperature (Figure 5). Thus, in a frozen ethanol glass, τFLU
remains constant at 10.2 ns, which is remarkably close to the
radiative lifetime calculated under these conditions. Clearly,
nonradiative decay is unimportant in the glassy matrix. On
melting the glass, τFLU decreases to ca. 7 ns but remains
3
5
by allowing for electrostatic effects. This would place the
energy of any such CTS at about 3.1 eV in ethanol solution,
but this is still 0.7 eV above the π,π* excited singlet state
associated with the Bodipy unit. Population of the CTS via a
thermal process from S1, therefore, is inconsistent with the
observed activation energy. A similar conclusion is reached for
7, although the energy of the CTS (ECTS ≈ 2.9 eV) is projected
to be closer to that of the S1 state (ES ) 2.37 eV). The
(
28) (a) Lippert, E. Z. Naturforsch. Teil A 1955, 10, 541-545. (b) Mataga, N.;
Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465-470.
(29) (a) Smedarchina, Z.; Fernandez-Ramos, A.; Siebrand, W. J. Comput. Chem.
2
001, 22, 787. (b) Chang, R.; Hsu, J. H.; Fann, W. S.; Liang, K. K.; Chang,
C. H.; Hayashi, M.; Yu, J.; Lin, S. H.; Chang, E. C.; Chuang, K. R.; Chen,
S. A. Chem. Phys. Lett. 2000, 317, 142-152.
(
30) For example, for 6 hω was determined from emission spectral profiles to
L
-1
be 923, 980, 948, 925, and 976 cm , respectively, in dichloromethane,
dimethyl sulfoxide, ethanol, hexane, and acetonitrile.
(34) The triplet excited state was hardly detectable by laser flash photolysis
studies carried out in deoxygenated solution following excitation with a
4-ns pulse. Recovery of the ground state, measured by following the
bleaching signal at the absorption maximum, was essentially complete
within the laser pulse. Low-temperature phosphorescence in an ethanol glass
could not be detected, in any case.
(35) The electrostatic correction term was estimated by considering point charges
separated by the distance between the midpoints of the aryl substituent
and Bodipy framework. The distances were obtained from energy-
minimized structures computed at the AM1 level. It was assumed that the
point charges were embedded in a continuum solvent of known dielectric
constant. It is recognized that such calculations are necessarily crude.
(
31) For example, for 6 hω was determined from absorption spectral profiles
L
-1
to be 871, 809, 911, 820, and 858 cm , respectively, in dichloromethane,
dimethyl sulfoxide, ethanol, hexane, and acetonitrile. Closely comparable
values were obtained by analysis of the corrected excitation spectra.
32) The total reorganization energy (λ) was calculated from the fluorescence
spectrum by examining the temperature dependence for the half-width of
the low-frequency vibrational modes and correcting for instrumental
broadening.
(
(
33) Harriman, A.; Rostron, J. P.; Cesario, M.; Ulrich, G.; Ziessel, R. J. Phys.
Chem. A 2006, ASAP.
10236 J. AM. CHEM. SOC.
9
VOL. 128, NO. 31, 2006

Arylborondipyrromethene Dyes
A R T I C L E S
Figure 7. (a) Left-hand panel shows the computed geometry for the ground
state of 3. (b) Right-hand panel shows the computed lowest-energy geometry
for the corresponding first-excited singlet state of 3.
glass. In this medium, the fluorescence quantum yield was found
to be essentially independent of temperature over the range 295
to 80 K. This finding is consistent with geometry changes
playing a major role in promoting nonradiative decay for the
dyes bearing large aryl substituents in fluid solution. It should,
Figure 6. Simplified potential energy level diagram summarizing the
proposed conformational change available to the excited singlet state.
Excitation of the ground-state system produces a metastable geometry, after
thermal equilibration, that undergoes fluorescence and activationless non-
radiative decay. A more distorted geometry can be accessed by passage
over a barrier, denoted by EA. This “dark” state decays rapidly to re-form
the ground state.
5
e,39
however, be noted that earlier work
has also reported that
for certain F-bodipy dyes kNR increases with increasing polarity
of the solvent. In most cases, this polarity effect can be traced
to intramolecular charge-transfer effects associated with an aryl
donor substituted at the meso position. Charge-transfer interac-
tions are less likely in our systems, although such effects might
make a minor contribution toward the observed kNR values found
in polar solvents.
impression, therefore, is that charge-transfer effects are not
responsible for the enhanced rates of nonradiative decay seen
with these B-aryl dyes.
At present, the nature of the proposed conformational change
is unknown, but it might involve concerted distortion of the
indacene backbone, resulting in a shift from planar to bent
geometries. Thus, the computed geometry of each of the new
derivatives was found to be pseudotetrahedral around the boron
center but with the indacene fragment being strictly planar
The rate constants for nonradiative decay at room temperature
kNR ) (1 - φFLU)/τFLU) increase with increasing size of the
(
aryl substituent and also depend on the nature of the solvent
5
e
(
Table S1). This finding can be used to suggest that kNR is in
some way connected to a steric effect. There is a crude
3
6
correlation between kNR and the molar volume (VMOL) of the
aryl substituent but this does not take into account the noted
solvent effects. A more reasonable way to examine the data
involves consideration of the solvent accessible surface area of
the substituent and the solvophobicity of the aryl group. Thus,
following an earlier study of somewhat related Bodipy-based
(Figure 7a). The computed excited-state geometries show some
measure of geometric distortion in the planarity of the indacene
4
0
unit (Figure 7b). The magnitude of this buckling effect
increases with increasing size of the aryl substituent, corre-
sponding to a distortion of about 22° for the pyrene-derivative
7. The computed excitation energy for 3 (∆ ) 2.54 eV) is in
excellent agreement with the HOMO-LUMO energy gap
3
7
dyes, it is considered that the ground state conformation has
a global minimum corresponding to a geometry related to the
X-ray structure. Excitation into the excited singlet state manifold
gives rise to a relaxed geometry that is not too dissimilar to
that of the ground state (Figure 6). This relaxed geometry is
responsible for the fluorescence observed in fluid solution. The
excited state surface, however, is considered to possess several
minima and, in competition to fluorescence, an activated
geometry change is possible. Passage over a modest barrier,
characterized experimentally by EA in eq 1, provides access to
a distorted geometry that has no ground-state counterpart (Figure
(∆ ) 2.56 eV) obtained by cyclic voltammetry, For none of
the compounds, however, was the structural distortion ac-
companying excitation sufficient to account for the enhanced
rates of nonradiative decay found in polar solvents. As such,
we conclude that the driving force for conformational change
is provided, in part, by the solvophobic effect. Here, the
substituent seeks to minimize surface exposure to polar solvents,
such as ethanol and acetonitrile. The solvophobic effect would
drive the aryl substituent toward a configuration that minimizes
its solvent exposed area.
6
). Nonradiative deactivation from this new geometry is fast
An additional feature of a major conformational change in
the excited state might involve transfer of electron density from
the substituent to the highest-energy, semi-occupied molecular
orbital. This latter situation would be favored by a geometry in
which one of the aryl substituents lies coplanar with the Bodipy
nucleus and would result in increased resonance stabilization
of the excited state. Presumably, the barrier for such confor-
because of the decreased energy gap and an increased Franck-
Condon overlap factor.38
It is interesting to note that fluorescence from 7 is restored
in the solid state at ambient temperature. Thus, τFLU is 5.9 ns
when the sample is dispersed in a sucrose octa-acetate glass at
2
5 °C. Conformational changes are less likely to occur in the
(
36) (a) Molar volumes were calculated from energy-minimized structures for
TM
each substituent using SPARTAN. (b) SPARTAN, Wavefunction, Inc.,
(39) (a) Qin, W.; Baruah, M.; Van der Auweraer, M.; De Schryver, F. C.; Boens,
N. J. Phys. Chem. A 2005, 109, 7371-7384. (b) Qin, W.; Baruah, M.;
Stefan, A.; Van der Auweraer, M.; Boens, N. ChemPhysChem 2005, 6,
2343-2351.
(40) Figure 7 shows the computed structures for 3 but similar conclusions hold
for the 5, 6, and 7. Where comparison is possible, the computed ground-
state geometries were in quite good agreement (bond lengths and angles
around the B center) with the X-ray structures.
Irvine, CA. (c) Hehre, W. J.; Shusterman, A. J.; Huang, W. W. A Laboratory
Book of Computational Organic Chemistry; Wavefunction, Inc.: Irvine,
CA. 1998.
(
37) Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H.; III; Singh, D. L.;
Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am.
Chem. Soc. 1998, 120, 10001-10009.
(
38) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145-164.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 31, 2006 10237

A R T I C L E S
Goze et al.
Figure 8. Computed highest-occupied molecular orbitals for the phenyl derivative 3.
mational exchange is too high for the ground state but becomes
possible by way of the increased potential energy available to
the excited state. Preliminary computational studies have
confirmed that the LUMO is localized on the Bodipy fragment
in each case and that the first-allowed transition is from the
Bodipy-localized HOMO to the LUMO. As the size of the aryl
groups increases, there is an increased contribution to the S1
state from lower-lying Bodipy-based orbitals. Furthermore, for
to a solution of magnesium (0.076 g, 3.1 mmol) in anhydrous diethyl
ether (20 mL) maintained under argon. The mixture was stirred at room
temperature for 1 h. The resulting solution was cooled to 0 °C and
transferred via cannula to a solution of 1 (0.2 g, 0.63 mmol) in
deoxygenated, freshly distilled diethyl ether (20 mL). The solution was
stirred at 0 °C for 30 min, during which complete consumption of the
starting material was observed by TLC. Water was added (5 mL), and
the solution was extracted with CH
the organic layer was purified by column chromatography on alumina
CH Cl /cyclohexane, 20:80), followed by recrystallization from CH Cl
hexane, yielding the desired compound (0.095 g, 40%).
2 2
Cl (20 mL). After evaporation,
3, the CI computational studies indicate the availability of
(
2
2
2
2
/
occupied orbitals with considerable involvement of the phenyl
rings (Figure 8). Mixing of these latter orbitals into the S1
manifold might become more pronounced as the geometry is
perturbed by solvent effects.
1H NMR (CDCl
3
, 300 MHz): δ ) 7.36-7.33 (m, 2H), 7.19-7.08
3
(
m, 3H), 2.70 (s, 3H), 2.36 (s, 6H), 2.31 (q, 2H, J ) 7.7 Hz), 2.30 (q,
3
3
13
1
2H, J ) 7.7 Hz), 2.16 (s, 6H), 0.96 (t, 6H, J ) 7.7 Hz); C { H}
NMR (CDCl , 75 MHz): 151.9, 139.8, 134.95, 134.93, 132.4, 132.0,
31.9, 131.4, 126.9, 125.9, 31.0, 17.4, 17.2, 15.0, 14.6, 13.05, 13.00;
3
Concluding Remarks
1
11
1
B { H} NMR (CDCl
3
, 128 MHz): 5.73 (s); UV-vis (CH
ꢀ, M cm ) ) 519 (175000), 370 (5200), 233 (17000); IR (KBr):
2 2
Cl ) λ nm
A new class of Bodipy dyes has been prepared by replacing
the fluorine substituents present in the more conventional
chromophores with aryl substituents. Synthesis makes use of
aryl-magnesium or aryl-lithium reagents. Using Grignard re-
agents, the monosubstituted derivatives are isolated but the use
of lithium reagents leads to exclusive formation of the disub-
stituted derivatives. These synthetic protocols are generic and
a wide range of substituted dyes is possible; the scope can be
further extended by attaching chromophores to the meso position
of the Bodipy unit. The optical and photophysical properties of
the resultant B-aryl dyes depend on the size of the aryl group
and on the nature of the surrounding solvent. The controlling
feature appears to be a solvophobic effect in which the aryl
substituent seeks to minimize surface contact with certain types
of solvent. This geometry change might also involve buckling
of the Bodipy framework. An interesting feature of the B-aryl
dyes bearing pyrene substituents concerns the fact that intramo-
lecular energy transfer from pyrene to Bodipy is quantitative.
This effect serves to provide a series of additional excitation
wavelengths that increase the virtual Stokes’ shift; a highly
desirable feature for flow cytometry and fluorescence micros-
copy.
-
1
-1
(
ν ) 3368 (m), 2961 (s), 1553 (s), 1447 (s), 1320 (s), 1190 (s), 978 (s),
+
7
62 (s); FAB m/z (nature of peak, relative intensity): 377.2 ([M +
+
+
H] , 100), 357.2 ([M-F] , 50); Anal. Calcd for C24
H, 8.04; N, 7.44. Found: C, 76.72; H, 8.25; N, 7.56.
-Fluoro-4-(2-naphthyl)-1,3,5,7,8-pentamethyl-2,4-diethyl-4-bora-
a,4a-diaza-s-indacene (4). 2-Bromonaphthalene (52 mg, 0.25 mmol)
2
H30BFN : C, 76.60;
4
3
was added to a solution of magnesium (6 mg, 0.25 mmol) in anhydrous
THF (5 mL) maintained under argon. The mixture was stirred at room
temperature for 1 h. The resulting solution was cooled to 0 °C and
transferred via cannula to a degassed solution of 1 (80 mg, 0.25 mmol)
in anhydrous THF (20 mL). The solution was stirred at 0 °C for 2 h,
until complete consumption of the starting material was apparent by
TLC. A solution of 3 M HCl in water was added (10 mL), and the
solution was extracted with CH
organic layer was purified by column chromatography on silica (CH
Cl /cyclohexane, gradient from 20:80 to 30:70), followed by recrys-
2 2
Cl (20 mL). After evaporation, the
2
-
2
tallization from CH
2
Cl
2
/hexane, to yield pure 4 (30 mg, 30%).
1
H NMR (CDCl
3
, 300 MHz): δ ) 7.97 (s, 1H), 7.81-7.72 (m, 2H),
3
7
6
.62 (d, 1H, J ) 8.3 Hz), 7.39-7.26 (m, 3H), 2.75 (s, 3H), 2.39 (s,
H), 2.29 (q, 4H, J ) 7.5 Hz), 2.17 (s, 6H), 0.94 (t, 6H, J ) 7.7 Hz);
C{ H}NMR (CDCl
3
3
13
1
3
, 75 MHz): 152.1, 139.9, 135.1, 133.1, 132.7,
1
1
5
32.5, 131.5, 131.1, 130.44, 130.38, 128.1, 127.6, 126.1, 125.0, 124.8,
7.4, 17.3, 15.0, 14.7, 13.2, 13.1; 11B { H} NMR (CDCl
1
, 128 MHz):
3
Experimental Section
.75 (s); UV-vis (CH Cl
2 2
) λ nm (ꢀ, M- cm ) ) 516 (57000), 371
1
-1
General methods are provide as Supporting Information.
(5000), 230 (44500); IR (KBr): ν ) 3047 (m), 2962 (s), 2928 (m),
4
-Fluoro-4-phenyl-1,3,5,7,8-pentamethyl-2,4-diethyl-4-bora-3a, 4a-
2869 (m), 1554 (s), 1478 (s), 1403 (m), 1360 (m), 1320 (s), 1186 (s),
1110 (s), 977 (s), 915 (m), 819 (m), 716 (m); FAB m/z (nature of
+
diaza-s-indacene (2). Bromobenzene (126 µL, 1.3 mmol) was added
1
0238 J. AM. CHEM. SOC. VOL. 128, NO. 31, 2006
9

Arylborondipyrromethene Dyes
A R T I C L E S
+
+
+
peak, relative intensity): 427.2 ([M + H] , 100); Anal. Calcd for
: C, 78.87; H, 7.56; N, 6.57. Found: C, 78.95; H, 7.82;
FAB m/z (nature of peak, relative intensity): 435.2 ([M + H] , 100),
+
C
28
H
32BFN
2
357.2 ([M-C
H
6 5
] , 30); Anal. Calcd. for C30
H35BN
2
: C, 82.94; H,
N, 6.63.
,4-Bis(2-naphthyl)-1,3,5,7,8-pentamethyl-2,4-diethyl-4-bora-3a,-
a-diaza-s-indacene (5). 2-Bromonaphthalene (104 mg, 0.5 mmol) was
added to a solution of magnesium (12 mg, 0.5 mmol) in anhydrous
THF (5 mL) maintained under argon. The mixture was stirred at room
temperature for 1 h. The resulting solution, held at room temperature,
was transferred via cannula to a degassed solution of 1 (80 mg, 0.25
mmol) in freshly distilled THF (20 mL). The solution was stirred for
8.12; N, 6.45. Found: C, 82.70; H, 7.87; N, 6.28.
4
4,4-Bis(4-methoxyphenyl)-1,3,5,7,8-pentamethyl-2,4-diethyl-4-
bora-3a,4a-diaza-s-indacene (6). This compound was prepared ac-
cording to the general procedure with 4-bromoanisole (79 µL, 0.63
mmol) in 10 mL of THF, 0.44 mL of n-butyllithium (1.55 M in
n-hexane), and 1 (0.1 g, 0.31 mmol) in 20 mL of THF. Complete
consumption of the starting material was observed after 5 min. Column
4
2 2
chromatography was performed on alumina (CH Cl /cyclohexane, 20:
2
h, until complete consumption of the starting material was observed
by TLC. A solution of 3M HCl in water was added (10 mL), and the
solution was extracted with CH Cl (20 mL). After evaporation, the
organic layer was purified by column chromatography on silica (CH
Cl /cyclohexane, gradient from 20:80 to 30:70), followed by recrys-
tallization from CH Cl /hexane, yielding pure 5 (53 mg, 30%).
, 300 MHz): δ ) 7.81-7.72 (m, 2H), 7.73-7.70
80), and recrystallization gave 0.05 g of 6 (40% yield).
1
H NMR (CDCl , 300 MHz): δ ) 6.94 (ABsys, 8H, J ) 8.6 Hz,
3
AB
2
2
ν δ ) 114.1 Hz), 3.77 (s, 6H), 2.67 (s, 3H), 2.36 (s, 6H), 2.33 (q, 4H,
0
3
3
13
1
2
-
J ) 7.5 Hz), 1.72 (s, 6H), 0.98 (t, 6H, J ) 7.5 Hz); C { H} NMR
2
(CDCl , 75 MHz): 157.8, 151.3, 140.0, 134.8, 133.4, 132.4, 132.3,
3
1
2
2
112.7, 55.0, 18.1, 17.6, 15.2, 14.8, 14.7; 11B { H} NMR (CDCl , 128
3
1
-1
-1
H NMR (CDCl
m, 6H), 7.47 (dd, 2H, J ) 8.3 Hz, J ) 1.3 Hz), 7.41-7.38 (m, 4H),
3
MHz): 2.56 (s); UV-vis (CH Cl ) λ nm (ꢀ, M cm ) ) 514 (93100),
2
2
3
4
(
380 (5400), 276 (28700), 233 (48000); IR (KBr): ν ) 2960 (s), 1556
3
+
2
.72 (s, 3H), 2.42 (s, 6H), 2.36 (q, 4H, J ) 7.5 Hz), 1.73 (s, 6H), 0.99
(s), 1176 (s), 974 (s), 819 (s); FAB m/z (nature of peak, relative
3
13
1
+
(t, 6H, J ) 7.5 Hz); C{ H}NMR (CDCl
3
, 75 MHz):152.8, 140.1,
intensity): 495.2 ([M + H] , 100); Anal. Calcd. for C H BN O : C,
3
2
39
2
2
1
1
33.7, 133.3, 132.7, 132.5, 132.4, 132.3, 132.1, 128.1, 127.4, 126.3,
77.73; H, 7.95; N, 5.67. Found: C, 77.98; H, 8.04; N, 5.62.
1
1
1
24.8, 124.7, 18.0, 17.4, 15.0, 14.8; B { H} NMR (CDCl
3
, 128
4,4-Bis(1-pyrenyl)-1,3,5,7,8-pentamethyl-2,4-diethyl-4-bora-3a, 4a-
diaza-s-indacene (7). This compound was prepared according to the
general procedure with 1-bromopyrene (0.176 g, 0.63 mmol) in 10 mL
of THF, 0.51 mL of n-butyllithium (1.55 M in n-hexane) (formation
of a dark brown anion), and 1 (0.1 g, 0.31 mmol) in 20 mL of THF.
Complete consumption of the starting material was observed after 30
min. Column chromatography was performed on alumina (CH Cl /
) λ nm (ꢀ, M- cm ) ) 516 (74500),
1
-1
MHz): 2.87 (s); UV-vis (CH
3
Cl
2 2
72 (10300), 234 (142000); IR (KBr): ν ) 3360 (m), 3051 (m), 2958
(
(
(
s), 2925 (s), 2854 (s), 1728 (s), 1631 (m), 1594 (m), 1557 (s), 1457
s), 1262 (s), 1179 (m), 1119 (s), 1072 (m), 976 (m), 811 (m), 738
m); FAB m/z (nature of peak, relative intensity): 535.2 ([M + H] ,
00); Anal. Calcd. for C38 : C, 85.38; H, 7.35; N, 5.24. Found:
C, 85.08; H, 7.52; N, 5.06.
+
+
1
H39BN
2
2
2
cyclohexane, 20:80), and recrystallization gave 0.043 g of 7 (20%).
General Procedure for the Preparation of Disubstituted C-
Bodipy Compounds. In a Schlenk flask maintained at -78 °C,
n-butyllithium (2.2 equiv) was added to a stirred, degassed solution of
the relevant aryl bromide (2.2 equiv) in anhydrous THF or diethyl ether.
The mixture was stirred at -78 °C for 1 h before warming to room
temperature for a further 30 min. The resulting solution was transferred
via cannula to a degassed solution of the precursor F-bodipy (1 equiv)
in anhydrous THF (or diethyl ether). The solution was stirred at room
temperature until complete consumption of the starting material was
observed by TLC. Water was added, and the solution was extracted
with CH
column chromatography and recrystallized from CH
,4-Diphenyl-1,3,5,7,8-pentamethyl-2,4-diethyl-4-bora-3a, 4a-
diaza-s-indacene (3). This compound was prepared according to the
general procedure with bromobenzene (63 µL, 0.63 mmol) in 10 mL
of diethyl ether, 0.51 mL of n-butyllithium (1.55 M in n-hexane)
1
3
H NMR (CDCl , 400 MHz): δ ) 8.25 (d, 2H, J ) 9.0 Hz), 8.12
3
3
4
3
4
(
dd, 2H, J ) 8.0 Hz, J ) 1.5 Hz), 8.08 (dd, 2H, J ) 8.0 Hz, J )
.0 Hz), 8.01 (s, 4H), 7.94 (m, 4H), 7.79 (d, 2H, J ) 9.6 Hz), 7.38 (d,
H, J ) 8.0 Hz), 2.48 (s, 3H), 2.32 (m, 10H), 1.38 (s, 6H), 0.96 (t,
3
H, J ) 7.5 Hz), 0.91 (t, 3H, J ) 7.1 Hz); C { H} NMR (CDCl ,
00 MHz): 140.8, 135.7, 135.2, 133.1, 132.7, 131.8, 131.1, 130.4,
28.5, 127.9, 126.5, 125.6, 125.3, 124.9, 124.7, 124.16, 124.12, 18.6,
3
7.8, 15.4, 15.1, 14.8; B { H} NMR (CDCl , 128 MHz): 4.97 (s);
UV-vis (CH Cl
3
1
2
3
1
1
1
3
3
3
13
1
11
1
) λ nm (ꢀ, M-1 cm-1) ) 524 (46300), 351 (80000),
2 2
3
2
35 (50300), 279 (67000), 269 (43500), 247 (92000); IR (KBr): ν )
2
Cl
2
. After evaporation, the organic layer was purified by
+
960 (s), 1551 (s), 1432 (s), 1176 (s), 976 (s), 842 (s); FAB m/z (nature
Cl /hexane.
2
+
+
2
of peak, relative intensity): 683.2 ([M + H] , 100), 481.1 ([M-pyrene] ,
0); Anal. Calcd. for C50 : C, 87.96; H, 6.35; N, 4.10. Found:
C, 87.75; H, 6.17; N, 3.84.
4
1
H43BN
2
Acknowledgment. This work was supported by EPSRC (EP/
D001994/1), CNRS, Universit e´ Louis Pasteur de Strasbourg and
the University of Newcastle. C.G. thanks IFET-ILO Contract
(formation of a pale yellow anion), and 1 (0.1 g, 0.31 mmol) in 20 mL
of diethyl ether. Complete consumption of the starting material was
observed after 30 min. Column chromatography was performed on
2
001-33057 for financial support.
alumina (CH
g of 3 (34%).
1H NMR (CDCl
H), 2.37 (s, 6H), 2.33 (q, 4H, J ) 7.5 Hz), 1.7 (s, 6H), 0.96 (t, 6H,
2 2
Cl /cyclohexane, 20:80), and recrystallization gave 0.04
Supporting Information Available: General experimental
procedure, reagents, materials physical methods, detailed X-ray
crystal structure determination and geometrical parameters
3
, 400 MHz): δ ) 7.24-7.11 (m, 10H), 2.67 (s,
3
3
3
13
1
J ) 7.5 Hz); C { H} NMR (CDCl
3
, 100 MHz): 151.1, 139.9, 133.6,
(Table S1) for compounds 4 and 6 (6 pages, print/PDF). This
1
33.4, 133.3, 132.3, 132.2, 127.1, 125.4, 17.9, 17.4, 15.2, 14.7, 14.6;
material is available free of charge via the Internet at
http://pubs.acs.org.
11
1
B { H} NMR (CDCl
3
, 128 MHz): 2.70 (s); UV-vis (CH
2 2
Cl ) λ nm
-
1
-1
(ꢀ, M cm ) ) 514 (64500), 376 (7800), 229 (28000); IR (KBr):
ν ) 3435 (m), 2961 (s), 1556 (s), 1451 (s), 1178 (s), 974 (s), 765 (s);
JA062405A
J. AM. CHEM. SOC.
9
VOL. 128, NO. 31, 2006 10239