Pyridine-substituted BODIPY as fluorescent probe for Hg2+

Article abstract of DOI:10.1002/ejoc.201101839

Dialkynylated BODIPYs bearing pyridine or dibutylaniline substituents have been synthesized, and their photophyisical properties in the presence of different metal ions have been studied. 4-Pyridyl substitution gives a highly selective fluorescence probe

Full text of DOI:10.1002/ejoc.201101839

FULL PAPER
DOI: 10.1002/ejoc.201101839
Pyridine-Substituted BODIPY as Fluorescent Probe for Hg2+
Sally Wagner,[a] Kerstin Brödner,[a] Benjamin A. Coombs,[a] and Uwe H. F. Bunz*[a]
Keywords: Alkynes / Chromophores / Sensors / Mercury
Dialkynylated BODIPYs bearing pyridine or dibutylaniline
substituents have been synthesized, and their photophyisical
properties in the presence of different metal ions have been
studied. 4-Pyridyl substitution gives a highly selective fluo-
rescence probe for HgII ions.
formed by condensation of 1 with 2 to give 3 in good to
excellent yields. The iodination of 3 with 2 equiv. of iodine
yielded the 2,6-diiodo compounds 4a and 4b, respectively.
A subsequent Sonogashira reaction of 4a with 3-ethynyl-
pyridine or with N,N-dibutyl-4-ethynylaniline furnished the
targets 5 and 6 in 30 and 25% yields, respectively.
Introduction
Herein we describe a simple pyridine-based derivative of
4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY), 7,
that displays a strong and selective fluorescence change in
the presence of mercury ions.
The design of novel fluorophores as chemosensors for
the detection of analytically important metal ions is a fasci-
nating field of research.[1] In the last decade, BODIPY[2]
dyes have captured a prominent place in the field of fluores-
cent probes.[3] BODIPYs have a wide range of applica-
tions[4] due to their outstanding photophysical properties
such as high extinction coefficients, high quantum yields,
and great photochemical stability.[5]
Reactive chromophores consist of a functional, inter-
acting part connected to a suitable π-system, which gives it
the necessary “color”, in this case a BODIPY unit. Func-
tionalization at the meso position of BODIPY is common,
but leaves the functional element electronically decoupled
and conformationally twisted with respect to the core.[6] As
an alternative, the attachment of styrene arms[7] in com-
pounds such as 8 have been described. There are, however,
fewer reports dealing with the 2,6-functionalization of the
BODIPY core.[8] In our search for new fluorogenic,
metallo-responsive materials, we have investigated the func-
tionalization of the 2,6-positions of the BODIPY skeleton
with aromatic acceptor and donor moieties containing pyr-
idines or dialkylanilines, such as compounds 5–7, which
should be able to bind to metal cations and would represent
simple operationally functional sensor cores.
Attempts to perform the Sonogashira reaction of 4a with
4-ethynylpyridine, however, furnished a material that could
not be purified; therefore, the meso-methoxy-substituted 4b
was prepared, which successfully underwent Sonogashira
coupling with 4-ethynylpyridine to give 7 in 28% yield. The
yields for this reaction were not great but gave the target in
sufficient amounts. Substitution of the meso-phenyl group
does not have any consequence for the HOMO and LUMO
positions, because they are located perpendicular to the
BODIPY–alkyne axis and do not interact with the π-orbit-
als of the meso-attached arene, which is twisted out of con-
jugation. The absorption spectra of compounds 6 and 7
show the characteristic properties of the BODIPY chromo-
phore, whereas 5 possesses a broad redshifted band at
613 nm, as described previously by Ziessel (Figure 1, top)
for a similar compound.[8]
The emission maximum of 6 (Figure 1, middle, Table 1)
is slightly bathochromically shifted relative to that of 7,
whereas 5 is nonfluorescent. Intramolecular charge transfer
from the dibutylaniline to the BODIPY core could explain
the fluorescence quenching of compound 5. Upon the ad-
dition of an excess of TFA to 5, the usual BODIPY fluores-
cence at 591 nm is recovered. We also investigated the pro-
tonation of 6 and 7 (in MeCN and TFA) but could find
only minute changes in the absorption and emission (Fig-
ure 1, dotted spectra), perhaps due to the strongly electron-
accepting character of the BODIPY core. This is quite dif-
ferent from compounds such as 9, which show a significant
redshift upon protonation of their pyridine units.[1b]
Results and Discussion
The targets 5–7 were synthesized according to Scheme 1
and their photophysics investigated. The BODIPY core was
The emissive lifetimes of 5–7 range between 2.7 and
4.0 ns and do not seem to be very dependent upon their
protonation state. An exception is 5. Here we cannot obtain
an emissive lifetime as the material is nonfluorescent in its
unprotonated state, but the anilinium salts are fluorescent
[a] Organisch-Chemisches Institut, Ruprecht-Karls-Universität,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: uwe.bunz@oci.uni-heidelberg.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101839.
Eur. J. Org. Chem. 2012, 2237–2242
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Wagner, K. Brödner, B. A. Coombs, U. H. F. Bunz
FULL PAPER
Scheme 1. Synthetic route to 5–7 and their photophysical properties.
and exhibit a monoexponential fluorescence lifetime of sponses of 5 towards increasing amounts of lead salts (Fig-
3.6 ns. ure 3). The absorption maximum at 613 nm decreases upon
Figure 2 shows the results of DFT calculations (SPAR- the addition of lead ions, and a new maximum appears at
TAN) performed on model compounds, 5Ј and 7Ј, of 5 and around 595 nm. The fluorescence intensity increases imme-
7, respectively. In both cases, the LUMO is either exclu- diately upon the addition of lead ions, and a slight redshift
sively (5Ј) or mostly (7Ј) localized on the BODIPY moiety, is also observed (Figure 3). With the data gleaned from Fig-
as a testament to the strong electron-accepting property of ure 2 and a simplified 1:1 binding model, one can extract
this core. The HOMO, however, is in both cases delocalized binding constants of around 5.5(Ϯ1.3)ϫ102 m–1 (613 nm)
over the whole molecule. The HOMO–LUMO gap is calcu- and 5.4(Ϯ1.3)ϫ102 m–1 (564 nm) after curve fitting with
lated to be 2.22 eV (560 nm, 17857 cm–1) for 5Ј and 2.67 eV the Origin software.[11] These numbers suggest that the di-
(466 nm, 21459 cm–1) for 7Ј. Thus, the calculations correctly butylamino groups in 5 bind to lead reasonably well con-
reproduce the trend, in which the presence of a dialk- sidering the fact that they only coordinate to an aniline.
ylamino group, as in 5, not unexpectedly lowers the optical
gap.
The absorption and emission spectra of BODIPY 7 (Fig-
ure 4) remain unchanged in the presence of Ca2+, Cd2+
,
Addition of Cd2+, Pb2+, Mn2+, or Zn2+ salts to 5 in Mg2+, Mn2+, and Zn2+, whereas the paramagnetic ions
acetonitrile solution leads to a strong fluorescence turn-on Co2+, Cu2+, and Fe2+ lead to fluorescence quenching with
as the charge-transfer character disappears. Addition of unchanged absorption spectra. We did not detect any
Ca2+ had little effect on the fluorescence response of 5, nei- changes when Pb2+ was added. Only Hg2+ leads to a hyp-
ther did Hg2+, but this ion did induce a hypsochromic shift sochromic shift in both the absorption and emission (from
of the absorption band of 5 (Figure 1).[9]
558 to 518 nm, absorption; from 580 to 530 nm, emission),
Lead is an interesting analyte, as it is present in a signifi- changing the emission color from orange to green. The
cant number of household goods and old paint.[10] We changes were recorded spectroscopically but are also easily
therefore investigated the emission and absorption re- visible with the naked eye (Figure 5). Silver ions also inter-
2238
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 2237–2242

Pyridine-Substituted BODIPY as Fluorescent Probe for Hg2+
Figure 2. Frontier molecular orbitals (DFT calculation, B3LYP 6-
31G**//B3LYP 6-31G** performed by using SPARTAN) of 5Ј
(model form of 5 with methyl groups instead of butyl groups and
phenyl groups instead of the more complex aryl groups attached
to the meso position, left) and 7Ј (right, H instead of methoxy
groups). The LUMOs are shown at the top (5Ј: –2.31 eV; 7Ј:
–2.92 eV) and the HOMOs (5Ј: –4.53 eV, gap 2.22 eV, 560 nm; 7Ј:
–5.59 eV, gap 2.67 eV, 466 nm) are depicted at the bottom of the
figure.
Figure 1. Absorption and emission spectra of 5–7 (top, middle: 5
blue, 6 yellow, 7 green). The emission properties of 5–7 in an excess
of TFA are shown by dotted lines with the same color scheme.
Absorption spectra of BODIPY 5 in MeCN in the presence of
different metal ions (bottom).
Figure 3. Absorption (top) and emission response (bottom) of
compound 5 (20 μm solution in MeCN) towards an increasing
amount of Pb(ClO4)2.
Table 1. Absorption and emission maxima of 5–7.
The insensitivity of 7 to most metal cations is in line with
the material’s lack of spectral response when exposed to
acid. However, the selectivity towards Hg2+ is unusual and
will make 7 attractive as a core for the development of fur-
ther mercury-sensitive materials. Figure 5 shows the results
of a qualitative fluorescence titration of 7 with Hg2+. When
assuming a simple 1:1 complex, bound and unbound li-
gands are at equal concentration at a concentration of
around 8 mm Hg2+, which suggests a binding constant be-
Property
5
6
7
Absorbance
λmax [nm] (logε [m–1 cm–1])
λmax (+ H+) [nm]
Emission
613 (4.5)
561
558 (5.0) 554 (4.5)
550
557
λmax [nm]
quenched
591
quenched
591
570
533
580
572
533
λmax (+ H+) [nm]
λmax (+ Hg2+) [nm]
act with 7 and induce a redshift in the absorption (to tween metal and ligand in the range of 102. This is not very
609 nm), but the emission of 7 is quenched in the presence high, but still unusual, as there are no specific mercury-
of Ag+.
binding sites in 7, just pyridines.
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2239

S. Wagner, K. Brödner, B. A. Coombs, U. H. F. Bunz
FULL PAPER
is not clear, the observed selectivity is quite surprising, very
intriguing, and could not have been predicted. We will use
the BODIPY core in further experiments with substituents
tailored to bind more strongly to mercury salts.
Experimental Section
General: All chemicals and solvents were obtained from Sigma-
Aldrich, Fluka, Merck or ABCR. Air- and moisture-sensitive reac-
tions were carried out in heat-gun-dried glassware under nitrogen.
NMR spectra were recorded with a Bruker Avance 300 (300 MHz
for 1H and 75 MHz for 13C) or a Bruker Avance 500 (500 MHz
1
for H and 125 MHz for 13C) NMR spectrometer. IR spectra were
obained with a Jasco FT/IR-4100 spectrometer, while absorption
spectra were taken with a Jasco UV-Vis V660. Emision spectra and
quantum yields were obtained with a PTI Quantum Master 40.
Mass spectra were obtained with a Bruker ApexQe FT-ICR instru-
ment or a JEOL JMS-700.
4,4Ј-Difluoro-1,3,5,7-tetramethyl-8-(3,4,5-tris{2-[2-(2-methoxyeth-
oxy)ethoxy]ethoxy})-4-bora-3a,4a-diaza-s-indacene (3a): In a 1 L
round-bottomed flask under N2, aldehyde 2a (2.90 g, 4.90 mmol)
and 2,4-dimethylpyrrole (0.900 g, 9.80 mmol) were dissolved in dry
CH2Cl2 (300 mL), and then eight drops of trifluoroacetic acid were
added. The mixture was stirred at room temperature for around
18 h. After consumption of the aldehyde (monitored by TLC),
DDQ (1.11 g, 4.90 mmol) was added, and the mixture was further
stirred for 1.5 h. Then the mixture was cooled to 0 °C, and ethyldi-
isopropylamine (10 mL) was added followed by BF3·OEt2 (10 mL).
The mixture was warmed to room temperature, and half of the
solvent was removed in vacuo. After reduction of the volume, the
organic phase was washed with water and brine, and dried with
MgSO4. The crude product was purified by flash chromatography
on silica gel by using DCM/MeOH (95:5) as eluent. Compound 3a
was isolated as a red oil (2.20 g, 56 %). 1H NMR (300 MHz,
Figure 4. Absorption and emssion response of BODIPY 7 (25 μm;
λex = 554 nm; 518 nm Hg2+; 608 nm Ag+) in MeCN in the presence
of an excess of different metal ions.
3
CDCl3): δ = 6.54 (s, 2 H), 5.98 (s, 2 H), 4.21 (t, JH,H = 5.1 Hz, 2
3
3
H), 4.11 (t, JH,H = 4.8 Hz, 4 H), 3.83 (t, JH,H = 4.8 Hz, 6 H),
3.77–3.58 (m, 18 H), 3.57–3.50 (m, 6 H), 3.37 (s, 3 H), 3.35 (s, 6
H), 2.54 (s, 6 H), 1.51 (s, 6 H) ppm. 13C{1H} NMR (75 MHz,
CDCl3): δ = 155.70, 153.83, 143.14, 141.34, 139.20, 131.45, 130.07,
121.2, 107.85, 72.85, 72.10, 72.05, 71.00, 70.84, 70.78, 70.70, 69.88,
69.36, 59.15, 14.71, 14.43 ppm. 11B NMR (96 MHz): δ = 0.79 (t,
1JB,F = 32.9 Hz) ppm. IR (KBr): ν = 2870, 2362, 1541, 1509, 1466,
˜
1411, 1325, 1302, 1191, 1103, 1082, 1019, 973, 839, 755 cm–1
.
HRMS (ESI): calcd. for [M + Na]+ 833.4184; found 833.4168;
calcd. for [M + K]+ 849.3917; found 849.3884. C40H61BF2N2O12
(810.73): calcd. C 59.20, H 7.58, N 3.46; found C 59.10, H 7.63, N
3.54.
Figure 5. Fluorescence titration of BODIPY 7 with mercury triflate
in MeCN (20 μm). The inset shows a photograph of a more concen-
trated solution of 7 in acetonitrile before (left) and after the ad-
dition of mercury triflate (right).
4,4Ј-Difluoro-2,6-diiodo-1,3,5,7-tetramethyl-8-(3,4,5-tris{2-[2-(2-meth-
oxyethoxy)ethoxy]ethoxy})-4-bora-3a,4a-diaza-s-indacene (4a):
Compound 3a (1.60 g, 2.06 mmol) was dissolved in ethanol
(25 mL). Iodine (1.20 g, 4.77 mmol) was suspended, and a solution
of iodic acid (725 mg, 4.12 mmol) in water (5 mL) was added drop-
wise. After stirring at room temperature for 2 h, the reaction was
quenched by the addition of an aqueous Na2S2O3 solution. The
mixture was extracted with DCM. The organic layer was washed
with water and dried with MgSO4. Removal of the solvent afforded
the product as a red oil (2.11 g, 96 %). 1H NMR (300 MHz,
Conclusions
BODIPY derivatives 5–7 have been prepared, and their
photophysical properties as well as their response to metal
cations were investigated. BODIPY 7 displays significant
selectivity towards mercury ions but is inert to the presence
of most other ions. Interestingly, Hg2+ has no effect on the
optical properties of the aniline-substituted BODIPY 5,
which, however, binds strongly but not very selectively to
3
CDCl3): δ = 6.50 (s, 2 H), 4.23 (t, JH,H = 5.1 Hz, 2 H), 4.10 (t,
3
3JH,H = 4.8 Hz, 4 H), 3.83 (t, JH,H = 4.8 Hz, 6 H), 3.77–3.57 (m,
18 H), 3.56–3.48 (m, 6 H), 3.37 (s, 3 H), 3.35 (s, 6 H), 2.62 (s, 6
H), 1.51 (s, 6 H) ppm. 13C{1H} NMR (75 MHz, CDCl3): δ =
lead. Although the sensing mechanism, particularly for 7, 156.98, 154.15, 145.37, 141.04, 139.63, 131.34, 129.62, 107.60,
2240
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Pyridine-Substituted BODIPY as Fluorescent Probe for Hg2+
85.74, 72.92, 72.11, 72.06, 71.01, 70.85, 70.9, 70.71, 69.87, 69.46,
were used. After the addition of [(PPh3)2PdCl2] (3 mg, 4 mol-%)
and CuI (1 mg, 5 mol-%), the mixture was stirred at 60 °C for 24 h.
Purification of the crude product by flash chromatography on silica
1
59.16, 17.07, 16.14 ppm. 11B NMR (96 MHz): δ = 0.58 (t, JB,F
=
32.3 Hz) ppm. IR (KBr): ν = 2872, 1527, 1453, 1419, 1397, 1344,
˜
1326, 1304, 1239, 1175, 1090, 1033, 992, 948, 918, 849, (MeOH/DCM, 3:97) gave a purple solid (16.0 mg, 28%). 1H NMR
756 cm–1. HRMS (ESI): calcd. for [M + Na]+ 1085.2118; found
1085.2098; calcd. for [M + K]+ 1101.1857; found 1101.1865.
C40H59BF2I2N2O12 (1062.52): calcd. C 45.22, H 5.60, N 2.64; found
C 45.48, H 5.83, N 2.78.
(300 MHz, CDCl3): δ = 8.57 (d, JH,H = 4.8 Hz, 4 H), 7.30 (d,
3
3
3
3JH,H = 6.0 Hz, 4 H), 7.19 (d, JH,H = 8.7 Hz, 2 H), 7.07 (d, JH,H
= 8.7 Hz, 2 H), 3.91 (s, 3 H), 2.73 (s, 6 H), 1.60 (s, 6 H) ppm.
13C{1H} NMR (75 MHz, CDCl3): δ = 160.8, 158.9, 149.8, 145.4,
140.3, 131.7, 129.2, 128.6, 126.2, 125.3, 115.1, 94.2, 86.8, 55.6,
4,4Ј-Difluoro-1,3,5,7-tetramethyl-8-(3,4,5-tris{2-[2-(2-methoxyeth-
oxy)ethoxy]ethoxy})-2,6-bis(4-di-tert-butylaminophenylethynyl)-4-
bora-3a,4a-diaza-s-indacene (5): Under nitrogen, 4a (200 mg,
0.190 mmol) and N,N-dibutyl-4-ethynylaniline (114 mg,
0.470 mmol) were dissolved in a mixture of dry THF (4 mL) and
dry diisopropylamine (2 mL). The mixture was purged with nitro-
gen for 10 min before [Pd(PPh3)2Cl2] (5.00 mg, 4 mol-%) and CuI
(2.00 mg, 5 mol-%) were added. After stirring at ambient tempera-
ture for 3 d, the solvents were evaporated. The residue was taken
up in DCM and washed with water and brine. The organic layers
were dried with MgSO4, and the solvent was removed. Flash
chromatography on silica (PE/EA/DCM/EtOH, 6:1:3:0.5) gave
(69.0 mg, 30%) of a blue oil. 1H NMR (300 MHz, CDCl3): δ =
13.9 ppm. 11B NMR (96 MHz): δ = 0.59 (t, JB,F = 31.9 Hz) ppm.
1
IR (KBr): ν = 2922, 2210, 1595, 1527, 1321, 1264, 1245, 1186, 1071,
˜
1005, 814 cm–1. UV/Vis (MeCN): λmax (logε) = 554 (4.50 m–1 cm–1)
nm. Emission (MeCN): λmax = 580 nm; Φ = 97%.
Fluorescence Quantum Yields: The fluorescence quantum yields
were determined by the comparative method. Fluorescein, excited
at 480 nm in 1 n NaOH (Φref = 85%), was used as the fluorescence
standard. The absorption and emission of each substance were
measured at four concentrations. The area under the emission curve
was plotted against the absorption at the excitation wavelength.
Comparison of the gradients for the fluorescence standard and
sample afforded the quantum yield according to Equation (1) in
which gradx is the gradient of the sample, ηx is the refraction index
of the sample solvent, gradref is the gradient of the reference, and
ηref is the refraction index of the reference solvent. The quantum
yields were measured twice and have an approximate error of 10%.
3
3
7.29 (d, JH,H = 8.92 Hz, 4 H), 6.56 (m, 6 H), 4.24 (t, JH,H
=
=
3
3
5.04 Hz, 2 H), 4.13 (t, JH,H = 4.86 Hz, 4 H), 3.84 (t, JH,H
4.72 Hz, 6 H), 3.78–3.59 (m, 18 H), 3.59–3.49 (m, 6 H), 3.38 (s, 6
3
H), 3.35 (s, 6 H), 3.27 (t, JH,H = 7.68 Hz, 8 H), 2.69 (s, 6 H), 1.63
3
(s, 6 H), 1.62–1.50 (m, 8 H), 1.42–1.28 (m, 8 H), 0.95 (t, JH,H
=
7.20 Hz, 12 H) ppm. 13C{1H} NMR (75 MHz, CDCl3): δ = 158.2,
153.9, 148.0, 142.6, 139.3, 131.1, 129.7, 117.2, 111.4, 108.9, 107.8,
97.9, 79.0, 72.8, 72.0, 71.0, 70.8, 70.7, 69.9, 69.4, 59.2, 59.1, 50.8,
29.5. 20.4, 14.1, 13.8, 13.4 ppm. 11B NMR (96 MHz): δ = 0.60 (t,
(1)
1JB,F = 31.6 Hz) ppm. IR (KBr): ν = 2954, 2927, 2867, 2369, 2323,
˜
Supporting Information (see footnote on the first page of this arti-
cle): Synthesis of the precursor alkynes and NMR spectra of the
target BODIPYs 3a, 4a, and 5–7.
2198, 1604, 1526, 1505, 1320, 1180, 1104, 1056, 998, 810 cm–1
.
HRMS (ESI): calcd. for [M + Na]+ 1287.7537; found 1287.7544;
calcd. for 1303.7265 [M + K]+; found 1303.7293. C72H103BF2N4O12
(1265.42): calcd. C 68.34, H 8.20, N 4.43; found C 68.24, H 8.36,
N 4.31. UV/Vis (MeCN): λmax (logε) = 613 (4.49 m–1 cm–1), 319 nm.
Acknowledgments
4,4Ј-Difluoro-1,3,5,7-tetramethyl-2,6-bis(3-pyridylethynyl)-8-(3,4,5-
tris{2-[2-(2-methoxyethoxy)ethoxy]ethoxy})-4-bora-3a,4a-diaza-s-
indacene (6): The Sonogashira reaction was carried out as described
for compound 5 with 4a (250 mg, 0.240 mmol), 3-ethynylpyridine
We thank the Struktur- und Innovationfonds of the State of Baden-
Württemberg for generous support.
(100 mg, 1.00 mmol), [(PPh3)2PdCl2] (10.0 mg, 6 mol-%), and CuI [1] a) A. J. Zucchero, P. L. McGrier, U. H. F. Bunz, Acc. Chem.
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chromatography on silica by using DCM/MeOH (98:2) as eluent
afforded (59.0 mg, 25%) of a coppery solid. H NMR (300 MHz,
1
CDCl3): δ = 8.68 (br. s, 2 H), 8.55–8.48 (m, 2 H), 7.75 (dt, 3JH,H
=
3
7.9, JH,H = 1.87 Hz), 7.30–7.22 (m, 2 H), 6.56 (s, 2 H), 4.25 (t,
3
3
3JH,H = 5.0 Hz, 2 H), 4.13 (t, JH,H = 4.9 Hz, 4 H), 3.84 (t, JH,H
= 4.8 Hz, 6 H), 3.78–3.58 (m, 18 H), 3.58–3.46 (m, 6 H), 3.37 (s, 3
H), 3.33 (s, 6 H), 2.71 (s, 6 H), 1.67 (s, 6 H) ppm. 13C{1H} NMR
(75 MHz, CDCl3): δ = 158.80, 154.11, 152.02, 148.56, 144.59,
142.63, 139.63, 138.22, 131.32, 129.09, 123.16, 120.65, 115.66,
107.57, 93.25, 84.99, 72.89, 72.06, 72.01, 70.97, 70.81, 70.75, 70.67,
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69.83, 69.45, 59.10, 13.85, 13.59 ppm. 11B NMR (96 MHz): δ =
1
0.57 (t, JB,F = 32.1 Hz) ppm. IR (KBr): ν = 2920, 2870, 2367,
˜
2325, 2208, 1720, 1526, 1480, 1402, 1317, 1263, 1181, 1110, 1079,
1005, 759, 584 cm–1. HRMS (ESI): calcd. for [M + Na]+ 1035.4717;
found 1035.4712; calcd. for [M + K]+ 1051.4448; found 1051.4455.
UV/Vis (MeCN): λmax (logε) = 558 (4.96 m–1 cm–1) nm. Emission
(MeCN): λmax = 591 nm; Φ = 52%.
4,4Ј-Difluoro-8-(4-methoxyphenyl)-1,3,5,7-tetramethyl-2,6-bis(4-pyr-
idinylethynyl)-4-bora-3a,4a-diaza-s-indacene (7): Compound 4b
(70.0 mg, 0.120 mmol) and 4-ethynylpyridine (70.0 mg, 0.68 mmol)
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Eur. J. Org. Chem. 2012, 2237–2242
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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S. Wagner, K. Brödner, B. A. Coombs, U. H. F. Bunz
FULL PAPER
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Published Online: March 13, 2012
2242
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Eur. J. Org. Chem. 2012, 2237–2242