Synthesis and fluorescence properties of a pyridomethene-BF2 complex

Article abstract of DOI:10.1021/ol101612z

A fluorescent dye, the pyridomethene-BF₂ complex, has been synthesized. Although pyridomethenes did not exhibit fluorescence, pyridomethene-BF₂ complexes exhibited fluorescence both in solution and in the solid state. The trifluoromethyl-substituted BF₂ complex formed a J-aggregate and showed the highest fluorescence quantum yield in the solid state among all pyridomethene-BF₂ complexes.

Full text of DOI:10.1021/ol101612z

ORGANIC
LETTERS
2010
Vol. 12, No. 18
4010-4013
Synthesis and Fluorescence Properties
of a Pyridomethene-BF2 Complex
Yasuhiro Kubota,*,† Toshihiro Tsuzuki,† Kazumasa Funabiki,† Masahiro Ebihara,‡
and Masaki Matsui*,†
Department of Materials Science and Technology, Faculty of Engineering, Gifu
UniVersity, Yanagido, Gifu 501-1193, Japan, and Department of Chemistry, Faculty of
Engineering, Gifu UniVersity, Yanagido, Gifu 501-1193, Japan
kubota@gifu-u.ac.jp; matsuim@gifu-u.ac.jp
Received July 13, 2010
ABSTRACT
A fluorescent dye, the pyridomethene-BF2 complex, has been synthesized. Although pyridomethenes did not exhibit fluorescence,
pyridomethene-BF2 complexes exhibited fluorescence both in solution and in the solid state. The trifluoromethyl-substituted BF2 complex
formed a J-aggregate and showed the highest fluorescence quantum yield in the solid state among all pyridomethene-BF2 com-
plexes.
Borondipyrromethene (BODIPY) dyes1,2 are well-known
fluorescent dyes with high fluorescence quantum yields, high
extinction coefficients, sharp absorption and fluorescence
emission spectra, and high photo- and chemical stability.
Owing to these excellent characteristics, BODIPY dyes have
a wide range of applications. They can be used in chemosen-
sors,3 in biolabels,4 as sensitizers for dye-sensitized solar cells,5
and as donor materials for bulk heterojunction solar cells,6 as
well as in photodynamic therapy.7 However, most BODIPY
dyes have defects such as very small Stokes shifts (5-20 nm,
in most cases) and high planarity.8 These defects cause the self-
quenching of BODIPY dyes in the solid state. Therefore, most
BODIPY dyes hardly fluoresce in the solid state.9
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† Department of Materials Science and Technology.
‡ Department of Chemistry.
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G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184
.
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Pigm. 2009, 82, 392. (b) Arroyo, I. J.; Hu, R.; Merino, G.; Tang, B. Z.;
Pen˜a-Cabrera, E. J. Org. Chem. 2009, 74, 5719. (c) Schmitt, A.; Hinkeldey,
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E. U. Org. Lett. 2008, 10, 3299. (b) Hattori, S.; Ohkubo, K.; Urano, Y.;
Sunahara, H.; Nagano, T.; Wada, Y.; Tkachenko, N. V.; Lemmetyinen,
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10.1021/ol101612z  2010 American Chemical Society
Published on Web 08/20/2010

On the other hand, fluorescent boron complexes excluding
BODIPY dyes have attracted considerable attention in recent
times.10 A pyridomethene-BF2 complex is an analogue of
a BODIPY dye. To the best of our knowledge, some
papers11 and some patents12 have been published on the
pyridomethene-BF2 complex. However, the fluorescence
properties of the complex have not been investigated in detail.
In the present paper, we not only describe a convenient
method for synthesizing the pyridomethene-BF2 complex
but also present its fluorescence properties.
BODIPY dyes are synthesized by the reaction of py-
rromethene (dipyrrin)13 with trifluoroborane. Therefore, it
was supposed that pyridomethene would be a good precursor
to the pyridomethene-BF2 complex. Pyridomethene is an
enamine tautomer of bis(2-pyridyl)methane (Figure S1,
Supporting Information (SI)). Because of the aromatic
stability of the pyridine ring, pyridomethene mostly exists
in the imine form, i.e., bis(2-pyridyl)methane.14 However,
several reports have described the synthesis of meso-cyano-
substituted pyridomethene derivertives.15 The findings of
these reports indicate that the introduction of a cyano group
at the meso position is an effective way to stabilize the
pyridomethene structure. Our DFT calculations support these
experimental results (Figure S2, SI). We chose meso-cyano-
substituted pyridomethene derivatives as the precursor
because they are easy to obtain and they enable the retention
of the pyridomethene structure.
meso-Cyano-substituted pyridomethene 1 was obtained by
the reaction of 2-(cyanomethyl)pyridine with 2-bromopyri-
dine (Scheme S1, SI).15b The pyridomethene structure of 1
1
was confirmed by the H NMR signal at δ 16.3 (brs, 1H,
D2O exchangeable, NH) and 13C NMR signal at δ 68.2
(s, meso-carbon atom) (Figures S3, S4, SI). The observed
1
symmetrical H and 13C NMR spectra indicate that the rate
of interconversion between 1 and 1′ is faster than the NMR
time scale (Figure S5, SI). The rapid interconversion between
1 and 1′ can be attributed to an imine-enamine tautomerism
and/or a direct hydrogen shift between two nitrogen atoms
in the vinamidine conjugate system. A similar symmetric
property in the NMR spectrum was reported for another
pentacyclic amidine (vinamidine).16
The reaction of 1 with boron trifluoride diethyl ether
complex in the presence of triethylamine yielded the
pyridomethene-BF2 complex 5 (Scheme 1). The structure
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(8) Small Stokes shift causes the reabsorption of its own fluorescence,
so the fluorescence intensity was decreased. High planarity leads to
increasing intermolecular interactions which cause concentration quench-
ing.
Scheme 1. Synthesis of Pyridomethene-BF2 Complexes
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of 517 was confirmed by NMR spectra and X-ray crystal-
lographic analysis (Figure S6, SI). The UV-vis absorption
and fluorescence spectra of 1 and 5 in hexane are shown in
Figure 1. The UV-vis absorption spectra of 5 showed two
absorption peaks at 319 and 450 nm along with vibrational
peaks at 306 and 425 nm, respectively. The weak absorption
peak at 319 nm and the strong absorption peak at 450 nm
can be attributed to an S0fS2 transition and an S0fS1
transition, respectively.
Although 1 did not show any fluorescence, 5 exhibited
fluorescence at 456 nm in hexane. The difference between
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(17) Although compound 5 was reacted with HCl aq or NaOH aq in
THF at room temperature for 3 days, decomposition of 5 was not observed.
Therefore, pyridomethene-BF2 complexes are considered to be compara-
tively stable against acid and base.
Org. Lett., Vol. 12, No. 18, 2010
4011

Table 1. Optical Properties of Pyridomethene-BF2 Complexes
Figure 1. UV-vis absorption and fluorescence spectra of 1 and 5
in hexane. Measured at a concentration of 1 × 10-5 mol dm-3 of
substrate at 25 °C. Solid and dotted lines represent UV-vis
absorption and fluorescence spectra, respectively.
the fluorescence properties of 1 and 5 can be attributed to
the increase in rigidity of the pyridomethene skeleton of 5
caused by the chelation by the boron atom. This reduced
the loss of energy via radiationless thermal vibrations.1b,18
Compound 5 showed a very small Stokes shift (6 nm). The
absorption and fluorescence properties of 5 in various
solvents were investigated (Figures S7, S8 and Table S1,
SI). The absorption and fluorescence maxima of 5 were only
slightly affected by solvent polarity. The fluorescence of 5
a Measured at a concentration of 1.0 × 10-5 mol dm-3 at 25 °C.
b Determined using a Hamamatsu Photonics Absolute PL Quantum Yield
Measurement System C9920-02. c Excitation wavelength (λex) was obtained
by measuring the diffuse reflectance spectra given in Kubelka-Munk units.
can be observed in any solvent (φtoluene ) 0.30, φCH2Cl2
0.29, φTHF ) 0.29, φCHCl3 ) 0.27, φEtOH ) 0.27, φMeCN
0.26, φMeOH ) 0.26, φhexane ) 0.19).
)
)
fluorescence in the solid state.9 This is a significant difference
between the pyridomethene-BF2 complexes and the BO-
DIPY dyes. The solid-state fluorescence spectra of
pyridomethene-BF2 complexes are shown in Figure S11
(SI). In the solid state, these compounds exhibited fluores-
cence maxima and fluorescence quantum yields in the range
of 496-524 nm and 0.02-0.17, respectively (Table 1). In
aryl-substituted compounds 9-12, fluorescence quantum
yield increased as the substituent group was hindered. The
trifluoromethyl-substituted derivative 8 showed the highest
fluorescence quantum yield (0.17). The fluorescence quantum
yield of 8 was identical in both the hexane solution and the
solid state, whereas the fluorescence quantum yields of other
compounds in the solid state were lower than in solution.
To investigate the difference in the solid-state fluorescence
properties of each compound, X-ray crystallographic analysis
was performed for compounds 5 (φf ) 0.03) and 8 (φf )
0.17). In the crystal structure of 5, the boron atom (B1) is in
the same plane containing atoms N1, N2, C5, C6, and C7,
and the molecule has an almost planar structure (Figure S6,
SI). The crystal packing is shown in Figure S12 (SI) and
Figure 2. As shown in Figure 2, the blue molecules and red
molecules are arranged in a columnar fashion. Significant
intermolecular π-π interactions were observed between the
blue molecules and the red molecules. The intermolecular
π-π interactions link the neighboring molecules and result
in the formation of network π-π interactions. The π-π
interactions are known to cause fluorescence quenching in
A similar procedure for 2-(cyanomethyl)pyridine with 2,5-
dibromopyridine, 2-bromo-5-methylpyridine, and 2-bromo-
5-trifluoromethylpyridine gave meso-cyano-substituted py-
ridomethenes 2, 3, and 4, respectively (Scheme S1, SI).
Compounds 6-8 were synthesized by the reaction of the
corresponding pyridomethene with the boron trifluoride
diethyl ether complex (Scheme 1). Aryl-substituted
pyridomethene-BF2 complexes 9-12 were also synthesized
by the Suzuki-Miyaura cross-coupling reaction (Scheme S2,
SI). The absorption and fluorescence properties of 5-12 in
hexane are shown in Table 1 and Figures S9 and S10 (SI).
It was observed that the absorption and fluorescence maxima
of pyridomethene-BF2 complexes were only slightly af-
fected by the substitution of the aryl group. Although all
pyridomethene-BF2 complexes exhibited fluorescence in
hexane, the bromo derivative 6 exhibited a relatively low
fluorescence quantum yield (0.03) when compared with the
other compounds (0.17-0.25). This difference in quantum
yield may be attributed to the heavy-atom effect, which
causes an intersystem crossing to a nonradiative triplet state.19
It is interesting to note that all the synthesized
pyridomethene-BF2 complexes exhibited fluorescence in the
solid state, whereas most BODIPY dyes did not exhibit
(18) Ikeda, C.; Ueda, S.; Nabeshima, T. Chem. Commun. 2009, 2544
(19) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W. M.;
O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619.
.
4012
Org. Lett., Vol. 12, No. 18, 2010

8 formed a dimer in the crystal (Figure 3). The transition
dipole moment of the dimer was determined by a TDDFT
calculation at the B3LYP/6-31G(d,p) level (Figure S15, SI).
The dimers are arranged head to tail along the transition
dipole moment (Figure 3). The corresponding angle between
two transition dipole moments is 48°. According to the
excitation interaction model,21 these results indicate that
compound 8 forms a typical J-aggregate.
It is known that the formation of a J-aggregate causes
bathochromic shifts of the absorption and fluorescence
spectra.21 To confirm the formation of the J-aggregate of 8,
the effect of the concentration of the substrate in the range
of 10-6-10-2 M on the fluorescence and excitation spectra
was examined in dichloromethane. Because the concentra-
tions were too high to measure the absorption spectra, the
excitation spectra were measured instead (Figures S16, S17,
SI). As the concentration increased from 10-6 to 10-2 M,
red shifts were observed in both the fluorescence and the
excitation spectra. These results suggest that compound 8
forms a J-aggregate in the concentrated state. It has been
reported that the formation of a J-aggregate leads to an
increase in the fluorescence intensity.22 Therefore, because
of the formation of the J-aggregate, it is likely that 8 may
show the highest fluorescence quantum yield in the solid
state.
Figure 2. Crystal packing of 5: (a) top view and (b) side view of
5. Hydrogen atoms have been omitted for clarity. The dotted lines
represent intermolecular C···C and C···N interactions.
the solid state.20 Hence, it can be supposed that compound
5 shows a lower fluorescence quantum yield.
The ORTEP drawing of 8 is shown in Figure S13 (SI),
and its crystal packing is shown in Figure S14 (SI) and Figure
3. In the crystal structure of 8, the boron atom (B1) is slightly
In conclusion, we have synthesized a pyridomethene-BF2
complex by the reaction of pyridomethene with boron
trifluoride and investigated its fluorescence properties both
in solution and in the solid state. All the synthesized
pyridomethene-BF2 complexes exhibited fluorescence in
solution. The fluorescence quantum yields of the complexes
were in the range of 0.03-0.25 in hexane solution. Further-
more, the complexes exhibited solid-state fluorescence. The
fluorescence maximum and fluorescence quantum yield were
in the range of 496-524 nm and 0.02-0.17, respectively.
It was observed that the fluorescence quantum yield of these
complexes tended to increase in the solid state as the
substituent group became more sterically hindered. In
addition, a trifluoromethyl-substituted derivative formed a
J-aggregate and showed an exceptionally high fluorescence
quantum yield in the solid state.
Figure 3. Crystal packing of 8: (a) top view, (b) side view, and (c)
stacking of 8. Hydrogen atoms have been omitted for clarity. The
dotted lines represent intermolecular C···C and C···N interactions.
The green arrows indicate the transition dipole moment.
out of the plane that contains atoms N1, N2, C5, C6, and
C7, and as a result, the molecule appears slightly bent. As
in the case of compound 5, networks of π-π interactions
were observed in the crystal of 8. Surprisingly, despite the
existence of π-π networks, compound 8 exhibited strong
fluorescence in the solid state. The packing patterns of
compounds 5 and 8 are considerably different from each
other. X-ray crystallographic results revealed that compound
Supporting Information Available: Details of experi-
1
mental procedures and copies of H, 13C NMR, and 2D
spectra for new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL101612Z
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