Highly luminescent BODIPY-based organoboron polymer exhibiting supramolecular self-assemble structure

Article abstract of DOI:10.1021/ja806939w

A novel class of rod-coil type-organoboron polymers with p-phenylene-ethynylene as the rod segment and long alkyl chain (decyl group) as coil segment has been prepared from a Sonogashira-Hagihara coupling reaction of a BODIPY-based monomer having bis-iodophenyl and decyl groups with diyne monomers, 1,4-diethynylbenzene (a), 1,4-diethynyl-2,5-bis(trifluoromethyl)benzene (b), and 2,7-diethynyl-9,9-dihexyl-9H-fluorene (c). The characterization by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the strong tendency of the obtained polymers, 2a, 2b, and 2c, to self-assemble into particles in solution and as-casted films on a μm-nm scale. Especially, 2a showed the presence of nm-sized particles and μm-sized fiber-like structures formed by aggregation of each particle. Further, in CHCl₃, the gelation of 2a by three-dimensional aggregation of each fiber was observed at room temperature after 24 h. Their luminescent properties showed high energy transfer efficiency from π-conjugated polymer linkers to BODIPY moieties (ΦF > 71%). Copyright

Full text of DOI:10.1021/ja806939w

Published on Web 10/29/2008
Highly Luminescent BODIPY-Based Organoboron Polymer Exhibiting
Supramolecular Self-Assemble Structure
Atsushi Nagai, Junpei Miyake, Kenta Kokado, Yuuya Nagata, and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
Received September 2, 2008; E-mail: chujo@chujo.synchem.kyoto-u.ac.jp
BODIPYs (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes) and
their derivatives are an important class of highly luminescent
materials that have found their widespread applications in
fluorescent dyes for biochemical labeling, photonic molecular
systems, laser dyes, organogelators, and light-emitting devices1
due to high quantum yields, low rates of intersystem crossing,
large molar absorption coefficients, and excellent photostability.2
The recent discovery by Ziessel et al. that several BODIPY
derivatives replaced fluoride with aryl, ethynylaryl, ethynylth-
ienyl, and so on show high stability to polar solvents and large
Stokes’ shifts in their emission spectra revitalized research into
boron dipyrromethanes.3 Recently, the incorporation of these
boron-chelating dyes except for BODIPYs as electroluminescent
chromophores into a polymer main chain4 or side chain5 is more
attractive for applications as electroluminescent devices, organic
field-effect transistors, photovoltaics, etc.6 However, the fluo-
rescent quantum yields (ΦF) of their polymers were commonly
low (<30%). Herein, a candidate approach is the introduction
Figure 1. [A] Polymerization of BODIPY-based monomer 1 with diyne
monomers and [B] SEC profiles of 2a (a), 2b (b), and 2c (c).
of a BODIPY unit into the polymer chain, because the ΦF of its
dye is exceedingly high as compared to those of other orga-
noboron dyes such as boron quinolate and boron diketonate.
We designed a highly luminescent BODIPY-based monomer
(1) (absolute quantum yield: ΦF ) 92%) with two kinds of
substituents: the polymerizable bis-iodophenyl groups and the decyl
group, which can be soluble in most organic solvents. Monomer 1
could be prepared from F-BODIPY having a decyl group and
Grignard derivative, which was synthesized by the reaction of
1-ethynyl-4-iodobenzene and ethylmagnesium bromide, under a
similar manner.3c The Sonogashira-Hagihara coupling reaction of
1 with diyne compounds proceeds via p-phenylene-ethynylene for-
mation under mild conditions. The BODIPY-based poly(p-phe-
nylene-ethynylene)s awaken our interest in not only the high
luminescent properties but also supramolecular self-assembly of
soluble polymers due to the rod-coil type structure; i.e., the rod
and coil segments are a rigid π-conjugated polymer main chain
and decyl group, respectively. Their polymers are expected to be
arranged to form intriguing nanostructures by virtue of intermo-
lecular π-π stacking.7
Initially, the polymerization of 1 with 1,4-diethynylbenzene
(a) was conducted in the presence of Pd(PPh3)4 (5.0 mol%) and
CuI (5.0 mol%) in the mixed solvents (66.7 mM: THF/NEt3 )
2/1(v/v)) at room temperature for 24 h (Figure 1A). The soluble
polymer was obtained as a deep red solid after precipitation into
methanol and hexane, respectively, and the polymer yield was
87%. The 1H NMR and IR spectra agreed well with the expected
structure, and the tetracoordination state of the boron atom was
confirmed by the 11B NMR spectroscopy (δB ) -13.8 ppm),
indicating that the polymerization proceeded without any damage
to the tetrahedral structure in the BODIPY moiety. The SEC (a
size-exclusion chromatography) trace of 2a (eluent: THF)
showed two peaks in the high molecular region as a shoulder as
well as the main peak corresponding to 2a (Figure 1B(a)). The
observed shoulder peak (Mn ) ∼16 million) implies that this
polymer was arranged in supramolecular self-assembly in THF.
Continuously, the synthesis of 2b and 2c was also performed
by the polymerization of 1 with 1,4-diethynyl-2,5-bis(trifluo-
romethyl)benzene (b) and 2,7-diethynyl-9,9-dihexyl-9H-
fluorene (c) under similar conditions. Figure 1B presents the
SEC traces 2b and 2c, respectively. Analogously to 2a, 2b
showed the shoulder peak in the high molecular weight region
(Figure 1B(b)). However, the shoulder peak of 2b decreased in
comparison with that of 2a. In contrast, the absence of an
observable shoulder peak in 2c was displayed as shown in Figure
1B(c), probably suggesting that the higher steric hindrance of
the substituent in diyne monomer interferes with the construction
of self-organization of 2c. Scanning electron microscopy (SEM)
was used to visualize the assemblies of the obtained polymers
along with monomer after drying the THF solution on a glass
plate. Surprisingly, the SEM image of 1 revealed a wide range
of particle structures from µm to nm (Figure 2A), and the image
of 2a showed the presence of nm-sized particles and µm-sized
fiber-like structures formed by aggregation of each particle,
which was roughly 1.2 µm in diameter, as confirmed by
transmission electron microscopy (TEM) (Figure 2B and E).
Further, Figure 2C and D also illustrate the SEM images of 2b
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C O M M U N I C A T I O N S
Figure 2. SEM micrographs of 1 [A], 2a [B], 2b [C], and 2c [D] dried at rt for 3.0 h on a glass plate, TEM micrograph of 2a [E] dried at rt for 5.0 h on
a carbon-coated copper grid, and SEM micrograph of a dried gel of 2a [F] from CHCl3. The concentration of all samples except for that for sample F is 1.0
g/L of THF.
and 2c. The presence of particles of 2b was more predominant
than that of fibers of 2b. On the contrary, the fibers of 2c were
almost not observed, possibly suggesting that the fiber-like
architectures result from the steric factor of the substituents in
comonomer units similarly to SEC observations. The size
distributions of these aggregates in THF solution (c ) 1.0 g/L)
were in the order 1 (643.8 ( 144.3 nm) > 2a (11.5 ( 2.0 nm)
> 2b (10.9 ( 1.5 nm) > 2c (9.0 ( 1.9 nm) as measured from
dynamic light scattering (DLS) measurement. In particular, 1
with least molecule weight constructs larger aggregates as is
the SEM measurement as well (Figure S1). More surprisingly,
only in CHCl3 (2.0 × 10-2 mol/L) among various solvents such
as THF, benzene, and toluene, the gelation of 2a was observed
at room temperature after 24 h (Figure S2).8 The concentration
of gelation of 2a was checked in CHCl3, and the concentration
at more than 2.0 × 10-2 mol/L led to the efficient gelation of
2a in CHCl3. Therefore, we investigated the correlation between
specific viscosity (cP) and time (h) in CHCl3 (Figure S3). The
viscosity increased gradually with time, and the final viscosity
of gelation was very high (2.52 × 104 cP) after 24 h. The SEM
image of a dried gel of 2a represented the formation of an
entangled network structure with roughly a 1.2-1.9 µm width
(Figure 2F). X-ray diffraction (XRD) of a dried gel 2a showed
a d-spacing of 3.48 Å, which is characteristic of π-stacked
packing and a signal at the small angle region corresponding to
an interlayer spacing of 12.63 Å (Figure S4). The dried gel can
redissolve in benzene at more than 50 °C or with sonication.
(Figure 3A). The maxima, corresponding to a πfπ* transition, of
the obtained polymers (2a: λmax ) 348 nm, 2b: λmax ) 370 nm,
and 2c: λmax ) 372 nm) are red-shifted as compared to those of 1
(λmax ) 265 nm), and the molar absorption coefficients of the poly-
mers (2a: ꢀ ) 18.9 × 104 M-1 cm-1, 2b: ꢀ ) 12.5 × 104 M-1
cm-1, and 2c: ꢀ ) 14.8 × 104 M-1 cm-1) are also higher than
that of 1 (ꢀ ) 11.5 × 104 M-1 cm-1), indicating the extension of
π-conjugation along the polymer main chain. Notably, the absorp-
tion coefficient of 2a was slightly higher than those of other
polymers, resulting from the formation of a regular aggregate such
as fiber and network structures in CHCl3 as considering the above
morphologic studies. In the fluorescence spectra in CHCl3 (Figure
3B), all compounds showed almost the same emission maxima at
∼532 nm, which was excited at absorption maxima of both 519
nm corresponding to BODIPY ligand moiety and the λmax corre-
sponding to the πfπ* transition moiety of each compound. This
result means that their emissive sources consist of the BODIPY
ligand moieties.9 The absolute fluorescence quantum yields of the
obtained polymers, which excited at λmax corresponding to πfπ*
transition moiety of each compound, were barely smaller than
that of 1 (ΦF ) 92%) (excited at 519 nm), actually, 2a: ΦF )
80%, 2b: ΦF ) 71%, and 2c: ΦF ) 85%. The high ΦF of their
polymers were in the order 2c > 2a > 2b, probably originating
from the energy transfer efficiency of each comonomer.10 Their
values were high enough as compared with general poly(p-
phenylene-ethynylene)s11 and could clearly indicate high energy
transfer efficiency from π-conjugated linkers to BODIPY moie-
ties. The ΦF of a wet gel 2a, which was prepared in CHCl3 at 2.0
× 10-2 mol/L for 24 h, was 37% (excited at 348 nm), indicating
that decrease in the ΦF of the gel originates from collisional
quenching of the excited state by π-π stacking.12 Further, the
emission intensity and ΦF of 2a have been observed in THF/
benzene mixed solvents. The values in 100% benzene solution were
higher than those in 50% THF/benzene and 100% THF solutions
with decreasing π-stacking, implying that 2a self-assembles into
aggregates (Figure S7).13 In the film state, the emission spectra of
the polymers were red-shifted compared with those for the solution
state; especially, a dramatic bathochromic shift of 2a was observed
due to a higher degree of ordering in the polymer than those of
1
The H NMR studies of 1 and 2a were recorded with benzene-
d6 and CDCl3 at room temperature. Their signals assignable to
aromatic protons and methyl and methylene protons in BODIPY
ligand in CDCl3 were almost shifted upfield in benzene-d6,
indicating the inhibition of the π-stacked self-assembly by
interaction between 1 or 2a and benzene (Figures S5 and S6).
These data support that the construction of supramolecular self-
assembly consists of π-stacking interaction.
The UV-vis absorption and photoluminescence experi-
ments of monomer and the obtained polymers were carried out in
CHCl3 (1.0 × 10-5 mol/L). Commonly, all compounds showed
the absorption maxima at 519 nm assignable to BODIPY ligand
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lengths of monomers in controlling self-assembly to research
potential device uses in the development of semiconductors and
organic light emitting diodes.
Acknowledgment. We thank Dr. Y. Morisaki and Dr. K.
Tanaka for helpful discussions at Kyoto University.
Supporting Information Available: Text giving typical experi-
mental procedures, data for all new compounds, DLS of all com-
pounds, photograph of gelation of 2a, the relationship between specific
1
viscosity and time of 2a, H NMR spectra of 1 and 2a in CDCl3 and
benzene-d6, XRD profile of a dried gel 2a, and UV and fluorescence
spectra of 2a in THF/benzene mixed solvent. This material is available
free of charge via the Internet at http://pubs.acs.org.
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