Strategy for the increasing the solid-state fluorescence intensity of pyrromethene-BF2 complexes

Article abstract of DOI:10.1016/j.tetlet.2010.09.106

In this study, we attempted to increase the fluorescence quantum yield of pyrromethene-BF₂ complex (BODIPY) in the solid state by introducing bulky groups as substituents at the boron atom. Although the BF₂ complex did not exhibit an

Full text of DOI:10.1016/j.tetlet.2010.09.106

Tetrahedron Letters 51 (2010) 6195–6198
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Strategy for the increasing the solid-state fluorescence intensity
of pyrromethene–BF2 complexes
Yasuhiro Kubota a, , Jun Uehara a, Kazumasa Funabiki a, Masahiro Ebihara b, Masaki Matsui a,
⇑
⇑
a Department of Materials Science and Technology, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan
b Department of Chemistry, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, we attempted to increase the fluorescence quantum yield of pyrromethene–BF2 complex
(BODIPY) in the solid state by introducing bulky groups as substituents at the boron atom. Although
the BF2 complex did not exhibit any fluorescence in the solid state (Uf = 0.00), the B(OMe)2, B(OPh)2,
and BPh2 complexes did show solid-state fluorescence with quantum yields of 0.02, 0.04, and 0.22,
respectively.
Received 20 August 2010
Revised 17 September 2010
Accepted 22 September 2010
Available online 27 September 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
BODIPY
Solid-state fluorescence
Pyrromethene
Solid-state fluorescent dyes have attracted much attention be-
cause of their potential applications in optoelectronic materials
such as organic electroluminescence devices1 and solid-state dye
lasers.2 However, in many cases, fluorescent dyes that show in-
tense fluorescence in solution exhibit decreased fluorescence in
the solid state as a result of concentration quenching.3 Therefore,
the number of organic dyes that can show intense fluorescence
in the solid state is limited and active research is being conducted
to develop such dyes.4
cases, two fluorine atoms are substituted at the boron atom of a
BODIPY dye. These fluorine atoms are located above and below
the
p plane which consists of the pyrromethene skeleton
(Fig. S1). Therefore, the substitution of the fluorine atoms by bulky
groups is considered as a very effective technique for preventing
the p–p stacking of BODIPY dyes. In this Letter, we report a strat-
egy for increasing the solid-state fluorescence intensity of BODIPY
dyes by introducing bulky groups at the boron atom in the center.
We selected methoxy, phenyloxy, and phenyl groups as the
bulky substituents (Fig. 1). We synthesized methoxy derivative 2,
phenyloxy derivative 3, and phenyl derivative 4 using previously
reported methods.10 Compound 1 was allowed to react with meth-
anol and phenol in the presence of AlCl3 to yield the methoxy
derivative 2 and the phenoxy derivative 3, respectively. The reac-
tion of 1 with phenyl lithium yielded the phenyl derivative 4.11
The UV–vis absorption and the fluorescence spectra of 1–4 in hex-
ane are shown in Figure 2. The UV–vis absorption spectra of 1–4
show two absorption peaks at wavelengths of around 380 and
The pyrromethene–BF2 complex (BODIPY)5 is known as an
excellent type of fluorescent dye; it exhibits high fluorescence
quantum yield and sharp fluorescence spectra in solution. The fluo-
rescence wavelength of BODIPY dyes can be changed from blue6 to
near infrared7 in solution. Therefore, BODIPY dyes are considered
to be strong candidates for the realization of a wavelength-tunable
solid-state fluorescent dye. However, most BODIPY dyes hardly
fluoresce in the solid state. This has been attributed to their very
small Stokes shift (5–20 nm, in most cases) and high planarity.8 Re-
cently, some groups9 have reported solid-state emissive BODIPY
dyes with bulky substituents introduced at the meso-position
(i.e., 8 position) and/or 3 position. This suggests that the inhibition
of molecular aggregation which causes concentration quenching is
important, and that the introduction of sterically bulky substitu-
ent(s) in the BODIPY core is an effective strategy for improving
the solid-state fluorescence intensity of BODIPY dyes. In many
Me
Me
Me
8
1: R = F
1
7
2
: R = OMe
3: R = OPh
2
Et
Et
6
4
B
N
N
5
4
3
: R = Ph
⇑
Corresponding authors. Tel.: +81 59 293 2596; fax: +81 58 230 1893 (Y.K.); tel.:
Me
R
R
+81 59 293 2601; fax: +81 58 230 1893 (M.M.).
E-mail addresses: kubota@gifu-u.ac.jp (Y. Kubota), matsuim@gifu-u.ac.jp (M.
Matsui).
Me
Figure 1. Synthesized BODIPY dyes.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.106

6196
Y. Kubota et al. / Tetrahedron Letters 51 (2010) 6195–6198
The fluorescence and excitation spectra of 1–4 in the solid state
1
0.8
0.6
0.4
0.2
0
1500
1000
500
0
are shown in Figure 3. The solid-state fluorescence intensity shows
significant fluctuations depending on the kind of substituent at the
boron atom. Compound 1 did not exhibit any fluorescence in the
solid state, similar to the behavior of other BODIPY dyes that have
been reported so far. On the other hand, 2–4 showed fluorescence
in the solid state. Compounds 2, 3, and 4 gave the fluorescence
maxima at 595, 580, and 640 nm, respectively. The solid-state fluo-
rescence quantum yields for 1, 2, 3, and 4 were 0.00, 0.02, 0.04, and
0.22 (Table 1).
X-ray crystallographic studies were carried out to investigate
the difference between 1 (which exhibited no fluorescence) and
4 (which exhibited the most intense fluorescence). The results for
1 and 4 are shown in Figures 4 and 5, respectively.
1
2
3
4
300
400
500
600
700
Wavelength / nm
Two crystallographically independent conformers 4A and 4B
were identified in the crystals of 4 (Fig. 5). The dihedral angles be-
tween the pyrromethene ring and the meso-tolyl group of the con-
formers 4A and 4B were almost orthogonal (4A: 89.1° and 4B:
88.5°), whereas the dihedral angle of 1 was 72.5°. Unlike the boron
atom of 1, the boron atom of 4 was not situated in the plane of the
pyrromethene ring. The dihedral angles 4A and 4B between the
plane containing atoms N1, C4, C5, C6, and, N2 and that containing
N1, B1, and N2 were 23.1° and 23.4°, respectively (Fig. S2). One of
the phenyl rings on the boron atom was oriented almost perpen-
dicular to the plane of the pyrromethene ring. In the crystal pack-
Figure 2. UV–vis and fluorescence spectra of BODIPY dyes 1–4 in hexane at the
concentration of 1 Â 10À5 mol dmÀ3 at 25 °C. Solid and dotted lines represent UV–
vis absorption and fluorescence spectra, respectively.
Table 1
Optical properties of BODIPY dyes 1–4
Compd
R
Hexanea
Fmax (nm)
Solid statec
kmax
(e) (nm)
Ufb
Fmax (nm)
Ufb
d
1
2
3
4
F
524 (79,000)
522 (70,000)
525 (82,000)
517 (79,000)
538
538
538
532
0.56
0.48
0.36
0.03
—
0.00
0.02
0.04
0.22
ing of 1, intermolecular p-p interactions were observed; hence, it
OMe
OPh
Ph
595
580, 617
601, 640
can be inferred that 1 did not exhibit any fluorescence.13 On the
other hand, in the crystal packing of 4, 4A and 4B formed indepen-
dent stacking columns, and the tilt angle between the two columns
a
Measured at a concentration of 1.0 Â 10À5 mol dmÀ3 at 25 °C.
b
Determined by a Hamamatsu Photonics Absolute PL Quantum Yield Measure-
was 30° (Fig. 6). Although the intermolecular CH/
tween the pyrromethene rings and the tolyl groups were detected,
the stacking between adjacent pyrromethene rings was not
p interactions be-
ment System C9920-02.
c
Excitation wavelength (kex) was obtained by measuring the diffuse reflectance
p–p
spectra given in Kubelka–Munk units.
d
observed in the crystal packing of 4. Because the
p–p interaction
Fluorescence was not observed.
was avoided, 4 exhibited intense fluorescence.
Two peaks were observed on the fluorescence spectra of 4 at
around 600 and 640 nm (Fig. 3). It was important to determine
whether the observed peaks were attributable to monomer-like
fluorescence or excimer-like fluorescence. To achieve this, we
investigated the concentration dependence of the excitation and
fluorescence spectra of 4. In sufficiently dilute conditions, 4 is con-
sidered to generate the monomer form. We selected potassium
bromide (KBr) as the inert material14 in which to dilute 4. Mixtures
of 4 and KBr with varying dye concentrations (0.01, 0.1, 1, 10, and
100 wt %) were prepared. The fluorescence and excitation spectra
of these mixtures were measured (Fig. 7). The spectral shapes of
excitation and fluorescence spectra of 4 were not extensively af-
fected by the changes in concentration, suggesting that the ob-
served peaks are not attributable to excimer-like fluorescence.
Therefore, the two observed peaks may be attributed to the mono-
mer-like S1?S0 and the vibrational transitions.
500
4: R = Ph
400
300
3: R = OPh
200
2: R = OMe
100
1: R = F
0
500
600
700
800
In conclusion, solid-state fluorescence properties of BODIPY
dyes with fluorine atoms 1, methoxy groups 2, phenoxy groups
3, and phenyl groups 4 substituted at their boron atom were inves-
tigated to clarify the effect of the substituent groups on the solid-
state fluorescence quantum yield. The fluorescence quantum yields
in the solid state are as follows: 1 = 0.00, 2 = 0.02, 3 = 0.04, and
4 = 0.22. Substitution of the fluorine atoms by more sterically
bulky substituents resulted in fluorescence in the solid state. In
particular, the BODIPY dye substituted with phenyl groups (i.e.,
4) exhibited the most intense fluorescence in the solid state. X-
ray crystallographic analysis of 4 showed that one of the phenyl
groups on the boron atom was oriented almost perpendicular to
the plane of the pyrromethene ring and that this phenyl group con-
Wavelength / nm
Figure 3. Excitation (dotted line) and fluorescence (solid line) spectra of BODIPY
dyes 1–4 in the solid state. The excitation spectra were monitored at 620 nm for 1,
595 nm for 2, 617 nm for 3, and 640 nm for 4.
520 nm. The weak absorption peaks at around 380 nm are attribut-
able to S0?S2 transitions. The strong absorption peaks at around
520 nm are typical of S0?S1 transitions and the shoulder peaks
at around 490 nm correspond to the vibrational band of the
S0?S1 transitions.12 BODIPY dyes 1–4 showed fluorescence maxi-
mum at around 540 nm with vibrational transitions. The fluores-
cence quantum yields in hexane are as follows: 1 = 0.56, 2 = 0.48,
3 = 0.36, and 4 = 0.03 (Table 1). The fluorescence quantum yield
of 4 was considerably lower than those of other compounds.
tributed to the prevention of the p–p interaction between the pyr-
romethene rings. Therefore, the introduction of bulky substituents
at the boron atom can be considered as an effective strategy for

Y. Kubota et al. / Tetrahedron Letters 51 (2010) 6195–6198
6197
Figure 4. Crystal structure of 1 (CCDC 785006). (a) ORTEP drawing of 1. (b) Global crystal packing of 1. (c) Side view of crystal packing of 1. (d) Top view of crystal packing of
1. Dotted lines show intermolecular interactions. Hydrogen atoms have been omitted for clarity.
p–p
Figure 5. Crystal structure of 4 (CCDC 784322). ORTEP views of (a) 4A and (b) 4B. Hydrogen atoms have been omitted for clarity.
Figure 6. (a) Global crystal packing of 4. (b) Intermolecular interactions of 4. Dotted lines show intermolecular CH/p interactions. Hydrogen atoms have been omitted for
clarity. Blue and red molecules represent the conformers 4A and 4B, respectively.

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Y. Kubota et al. / Tetrahedron Letters 51 (2010) 6195–6198
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10 wt%
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Figure 7. Excitation (dotted line) and fluorescence (solid line) spectra of BODIPY
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increasing the solid-state fluorescence quantum yield of BODIPY
dyes. Further, the applicability of this strategy to other BF2 com-
plexes is being investigated.
Supplementary data
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Met. 2004, 146, 11.
Supplementary data (synthetic procedures and crystallographic
information file) associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2010.09.106.
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11. BODIPY dyes 1–4 were able to be stored at room temperature for 5 months
without decomposition. Although BODIPY dyes 1–4 were stored in CDCl3
solution at room temperature for 5 days, any spectral changes were not
observed in the 1H NMR spectra. Therefore, BODIPY dyes 1–4 are considered to
be comparatively stable.
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