Effective light-harvesting antennae based on BODIPY-tethered cardo polyfluorenes via rapid energy transferring and low concentration quenching

Article abstract of DOI:10.1021/ma400015d

Light-harvesting antennae (LHA) were demonstrated using polyfluorenes (PFs) modified with borondipyrromethene (BODIPY) dyes tethered to the cardo structures. PFs work as a light absorber and an energy donor to the BODIPY units. The series of BODIPY-tether

Full text of DOI:10.1021/ma400015d

Article
pubs.acs.org/Macromolecules
Effective Light-Harvesting Antennae Based on BODIPY-Tethered
Cardo Polyfluorenes via Rapid Energy Transferring and Low
Concentration Quenching
Hyeonuk Yeo, Kazuo Tanaka, and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: Light-harvesting antennae (LHA) were dem-
onstrated using polyfluorenes (PFs) modified with borondi-
pyrromethene (BODIPY) dyes tethered to the cardo
structures. PFs work as a light absorber and an energy donor
to the BODIPY units. The series of BODIPY-tethering PFs via
the cardo carbon including homocardo PFs and alternative
polymers with fluorene and the cardo fluorene were
synthesized, and their optical properties were investigated.
Initially, highly efficient energy transferring was observed from
the PF main chains to the BODIPY unit (99%). It was found
that PFs can work as an efficient light absorber because of the large molar extinction coefficient and cause the rapid energy
transfer through the cardo structure. Next, from the comparison with the emission efficiency of the BODIPY units in the series of
the synthetic polymers, the favorable position of the BODIPY units was obtained to avoid the concentration quenching: The
alternative polymer with cardo fluorene and dialkyl-substituted fluorene showed the largest emission efficiency in this study.
Finally, we received the effective LHA with the 9 times larger amplification efficiency compared to that of the unimolar BODIPY
unit. The results from the computer modeling suggest that the positions of the BODIPY units via the cardo structure could play a
significant role in the inhibition of aggregation and electronic coupling with the BODIPY units, leading to the suppression of
concentration quenching. Here is presented the feasibility of the cardo structure in fluorene as a scaffold for designing advanced
optical materials.
been developed.9 The tetrahedral bonding carbon (the 9-
position of a fluorene) at the center of the cardo structures
holds the substituted aromatic rings arranged three-dimension-
INTRODUCTION
■
Light-harvesting antennae (LHA), which can enhance the
amount of light absorbing and transport the energy to a
ally to the fluorene unit. Because of the steric hindrance of the
chromophore, are feasible for improving the efficiencies of solar
cells or electroluminescence devices.1,2 Because of the
bulky rings, the interchain and/or intrachain aggregations of the
main chains are inhibited.10 Consequently, unexpected changes
extremely high efficiency of the photosynthesis, many
researchers have aimed to mimic the natural LHA systems or
in the emission properties such as red-shifting or self-quenching
are highly suppressed.10 Moreover, we have recently reported
build new nanoarchitectures and molecular assemblies to close
and overcome the efficiency of the natural ones.3−5 On the
construction of LHA systems, precise controls of assemblies
that the electronic coupling via the cardo structures hardly exist
for the electronic properties of the PF main chains having the
electron-donating and/or the electron-withdrawing groups at
and distributions between energy donors and acceptors to
the aromatic rings of the cardo structures.8 From these
realize electronic communications with incorporated molecules
are crucial.6 To achieve the construction of these nano-
advantages of the cardo PF, we expected that the cardo PFs
should be suitable scaffold to define the positions of donor and
assemblies with the functional molecules according to the
preprogrammed design, one feasible strategy is using the
acceptor units on the polymer main chains and mediate the
conjugated polymers as a template.7 By introducing the
transferring of the excitation energy for achieving highly
efficient LHA systems.
functional units into the polymer main chains and interacting
them via the conjugation systems, the gathering of the light
energy and the effective energy transferring can be expected.
There are several previous works on the energy transferring
involving the cardo PFs.11 Xu and co-workers reported a series
of the LHA involving fluorescence resonance energy transfer
We paid attention to the cardo polyfluorene (PFs) as a
using the PF-based conjugated polymers as an energy donor.12
scaffold for constructing the functional assembly.8 The PF is
one of conventional optoelectronic polymeric materials because
of the bright blue emission between 400 and 500 nm and the
high hole-transport abilities. Because of these characteristics,
PFs-based optical materials involving the LHA systems have
Received: January 3, 2013
Revised: March 3, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ma400015d | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
mL) were added. After stirring overnight, the solution was evaporated.
After flash chromatography on silica gel (10:1 = hexane:EtOAc, Rf =
0.45), recrystallization by hexane provided 1 (1.38 g, 1.52 mmol, 37%)
Their systems were composed of a dye molecule as an energy
acceptor in a mixture solution and could enhance the two-
photon excitation fluorescence. In another study with the dye-
tethered cardo PFs, effective energy transfer was observed
through the cardo structures.13 These results suggest the
feasibility of the cardo PFs to fabricate the practical
optoelectronic materials. On the other hand, although emission
properties of dyes are generally spoiled when the dyes are
gathered into local spaces, the influence of locations and
positional relationships with dyes was hardly examined. This
information should be necessary on the material design to avoid
undesired annihilation process, called concentration quench-
ing.14 In other words, the degree of electronic interaction
among dye molecules should be clarified schematically in the
dye-tethered cardo PFs.
Herein, to gather the schematic information on the
interaction of the dye molecules in the PFs, we prepared a
series of the borondipyrromethene (BODIPY)-tethered PFs
involving the cardo structures at various positions. From the
evaluation of the efficiencies in each step of the LHA process
with the synthetic polymers, the optimal positions of the dyes
were explored. This is the first example, to the best of our
knowledge, to present the guideline for constructing highly
emissive materials based on the dye-tethered PFs.
1
as a red powder. H NMR (CDCl3, ppm): 7.60 (d, J = 8.0 Hz, 2H),
7.51 (d, J = 1.6 Hz, 2H), 7.48 (dd, J = 7.0 Hz, 1.6 Hz 2H), 7.26 (d, J =
8.6 Hz, 2H), 7.17 (d, J = 8.2 Hz, 2H), 7.06 (d, J = 8.8 Hz, 2H), 6.81
(d, J = 8.8 Hz, 2H), 3.92 (t, J = 6.5 Hz, 2H), 2.51 (s, 6H), 2.29 (q, J =
7.5 Hz, 4H), 1.80−1.71 (m, 2H), 1.48−1.39 (m, 2H), 1.38−1.21 (m,
14H), 0.97 (t, J = 7.4 Hz, 6H), 0.88 (t, J = 6.7 Hz, 3H). 13C NMR
(CDCl3, ppm): 158.48, 153.75, 153.26, 145.94, 139.63, 138.24, 138.01,
135.35, 134.77, 132.85, 131.05, 130.71, 129.18, 128.90, 128.56, 128.46,
121.87, 121.70, 114.64, 68.04, 64.96, 31.80, 29.32, 29.24, 29.22, 26.64,
22.64, 17.07, 14.58, 14.07, 12.47, 11.62. 11B NMR (CDCl3, ppm):
0.78. HRMS (p-ESI) calcd for C50H53BBr2F2N2O: m/z 906.2560;
found: m/z 906.2577.
Compound 2. The solution containing thionyl chloride (40 mL)
was added to 4,4′-(2,7-dibromo-9H-fluorene-9,9-diyl)dibenzoic acid
(4.18 g, 7.41 mmol) and was refluxed overnight, and then residual
thionyl chloride was removed under vacuum. Dibenzoyl chloride was
obtained as a white solid and used directly for the next reaction.
Degassed CH2Cl2 (250 mL) and 2,4-dimethyl-3-ethyl-1H-pyrrole
(3.65 g, 29.6 mmol) were mixed, and the resulting solution was
refluxed for 3 days. After cooling to ambient temperature, NEt3 (20
mL) and BF3·OEt2 (30 mL) were sequentially added. After refluxing
overnight, the reaction mixture was diluted with CH2Cl2 and extracted
with H2O and brine. After drying over MgSO4, the products were
concentrated. After flash chromatography on silica gel (8:1 =
hexane:EtOAc, Rf = 0.4), the compound 2 (0.90 g, 0.83 mmol,
11%) was obtained as an orange powder after recrystallization with
EXPERIMENTAL SECTION
■
1
General. 1H NMR, 13C NMR, and 11B NMR spectra were
hexane/EtOAc. H NMR (CDCl3, ppm): 7.64 (d, J = 8.0 Hz, 2H),
measured with a JEOL EX-400 (400 MHz for H, 100 MHz for 13C,
1
7.53 (dd, J = 8.0 Hz, 1.7 Hz, 2H), 7.50 (d, J = 1.7 Hz, 2H), 7.30 (d, J =
8.2 Hz, 4H), 7.22 (d, J = 8.2 Hz, 4H), 2.52 (s, 6H), 2.29 (q, J = 7.6 Hz,
4H), 1.28 (s, 6H), 0.97 (t, J = 7.7 Hz, 6H). 13C NMR (CDCl3, ppm):
153.82, 152.67, 144.98, 139.24, 138.06, 135.11, 132.89, 131.34, 130.62,
128.96, 128.82, 128.27, 121.92, 121.90, 65.47, 17.01, 14.54, 12.44,
11.61. 11B NMR (CDCl3, ppm): 0.78. HRMS (p-MALDI) calcd for
C59H58B2Br2F4N4: m/z 1080.315 10; found: m/z 1080.312 45.
Compound 3. Benzoyl chloride (1.14 g, 8.12 mmol), 2,4-dimethyl-
3-ethyl-1H-pyrrole (2.00 g, 16.2 mmol), and degassed CH2Cl2 (200
mL) were mixed, and the solution was refluxed for 2 days. After
cooling to ambient temperature, NEt3 (7 mL) and BF3·OEt2 (14 mL)
were sequentially added. The solution was concentrated by
evaporation, and the crude products were treated with short
chromatography on silica gel (3:1 = hexane:CH2Cl2, Rf = 0.7) for
removing BF3·OEt2. After flash chromatography on silica gel (9:1 =
hexane:EtOAc, Rf = 0.5), compound 3 was obtained as an orange
powder after recrystallization by hexane (0.40 g, 1.05 mmol, 13%). 1H
NMR (CDCl3, ppm): 7.48 (s, 1H), 7.47 (d, J = 2.0 Hz, 2H), 7.28 (m,
2H), 2.53 (s, 6H), 2.29 (q, J = 7.6 Hz, 4H), 1.28 (s, 6H), 0.98 (t, J =
9.2 Hz, 6H). 13C NMR (CDCl3, ppm): 153.67, 140.19, 138.40, 135.81,
132.75, 130.78, 128.98, 128.70, 128.27, 17.06, 14.58, 12.45, 11.59. 11B
NMR (CDCl3, ppm): 0.78. HRMS (p-ESI) calcd. for
C23H27BF2N2+H+: m/z 381.2308; found: m/z 381.2299.
and 128 MHz for 11B) spectrometer. Coupling constants (J value) are
1
reported in hertz. H and 13C NMR spectra used tetramethylsilane
(TMS) as an internal standard. 11B NMR spectra were referenced
externally to BF3·OEt2 (sealed capillary) in CDCl3. The number-
average molecular weight (Mn) and the molecular weight distribution
[weight-average molecular weight/number-average molecular weight
(Mw/Mn)] values of all polymers were estimated by size-exclusion
chromatography (SEC) with a TOSOH G3000HXI system equipped
with three consecutive polystyrene gel columns [TOSOH gels: α-
4000, α-3000, and α-2500] and ultraviolet detector at 40 °C. The
system was operated at a flow rate of 1.0 mL/min with THF as an
eluent. Polystyrene standards were employed for the calibration. UV−
vis spectra were recorded on a Shimadzu UV-3600 spectrophotometer.
Fluorescence emission spectra were recorded on a HORIBA Jobin
Yvon Fluoromax-4 spectrofluorometer, and the absolute quantum
yield was calculated with the integrating sphere on the HORIBA Jobin
Yvon Fluoromax-4 spectrofluorometer in chloroform. Fluorescence
lifetime analysis was carried out on a HORIBA FluoreCube
spectrofluorometer system; excitation at 375 nm was carried out
using a UV diode laser (NanoLED-375L). Cyclic voltammetry (CV)
spectra were recorded on a BAS ALS electrochemical analyzer model
600D.
Synthesis of Polymer P1. The solution containing 1,5-cyclo-
octadiene (COD) (26.8 mg, 0.25 mmol), Ni(COD)2 (68.0 mg, 0.25
mmol), and 2,2′-bipyridine (39.0 mg, 0.25 mmol) in 1 mL of
anhydrous DMF was stirred at 60 °C for 30 min under an argon
atmosphere, and then the monomer 1 (150 mg, 0.165 mmol) in 1 mL
of anhydrous DMF was added. The reaction was stirred at 80 °C for
12 h in the dark, and then the products were reprecipitated several
times with a small amount of chloroform into 150 mL of methanol.
After Celite filtration to remove Ni, the red solid (134 mg, quant) was
precipitated by pouring the chloroform solution of the product into 50
mL of methanol. 1H NMR (CDCl3, ppm): 7.80 (2H), 7.51 (4H), 7.39
(2H), 7.16 (4H), 6.77 (2H), 3.86 (2H), 2.50 (6H), 2.22 (4H), 1.71
(2H), 1.25 (16H), 0.89 (9H). 13C NMR (CDCl3, ppm): 158.21,
153.72, 152.39, 151.97, 147.26, 140.93, 139.73, 138.98, 137.97, 136.84,
134.34, 132.77, 130.71, 128.99, 128.72, 128.31, 126.89, 124.52, 120.59,
114.33, 67.94, 64.92, 31.77, 29.18, 26.03, 22.60, 17.02, 14.57, 14.04,
12.46, 11.58. 11B NMR (CDCl3, ppm): 0.78.
Materials. Tetrahydrofuran (THF), diethyl ether (Et2O), and
triethylamine (Et3N) were purified using a two-column solid-state
purification system (Glasscontour System, Joerg Meyer, Irvine, CA).
9,9-Didodecylfluorene-2,7-diboronic acid and 2,4-dimethyl-3-ethyl-
1H-pyrrole were obtained commercially and used without further
purification. The monomers 4-(2,7-dibromo-9-(4-(octyloxy)phenyl)-
9H-fluoren-9-yl)benzaldehyde and 4,4′-(2,7-dibromo-9H-fluorene-9,9-
diyl)dibenzoic acid were prepared according to the literature.8 All
reaction was performed under an argon atmosphere.
Compound 1. A flask charged with 100 mL of CH2Cl2 was
degassed with Ar for 1 h, and 4-(2,7-dibromo-9-(4-(octyloxy)phenyl)-
9H-fluoren-9-yl)benzaldehyde (2.57 g, 4.06 mmol), 2,4-dimethyl-3-
ethyl-1H-pyrrole (1.00 g, 8.12 mmol), and three drops of trifluoro-
acetic acid (TFA) were placed. After the strirring for 2.5 h at room
temperature, to the solution was added 2,3-dichloro-5,6-dicyano-p-
benzoquinone (DDQ) (1.00 g, 4.41 mmol). After stirring for an
additional 2 h at room temperature, NEt3 (8 mL) and BF3·OEt2 (8
B
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a
Scheme 1. Synthesis of the Monomers
a
Reagents and conditions: (a) Mg, 4-bromobenzaldehyde diethyl acetal, in THF, rt to reflux overnight and 2 N HCl(aq) 3 h; (b) n-octyl phenyl
ether, CF3SO3H, in dioxane, 70 °C, 5 h; (c) 2,4-dimethyl-3-ethyl-1H-pyrrole, TFA, in CH2Cl2, rt, 3 h; (d) DDQ, 3 h; (e) NEt3, BF3·OEt2, overnight;
(f) 4-fluorobenzonitrile, 18-crown-6, K2CO3, in DMF and toluene, 140 °C, overnight; (g) KOH, EtOH, H2O, reflux, 5 days; (h) SOCl2, reflux, 8
days; (i) 2,4-dimethyl-3-ethyl-1H-pyrrole, in CH2Cl2, reflux, 2 days; (j) NEt3, BF3·OEt2, reflux, overnight.
Synthesis of Polymer P2. The solution containing tris-
RESULTS AND DISCUSSION
■
(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) (4 mg, 0.004
Synthesis of the Monomers. Because of strong emission
mmol), 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (S-Phos)
intensity, low environmental dependency of optical properties,
and high stability to photodegradation, BODIPY can be used as
a conventional fluorophore for emissive materials and
biomarkers.15,16 The monomers containing the cardo structures
with the single BODIPY and octyloxyphenyl group, 1, and with
the dual BODIPYs, 2, were synthesized (Scheme 1). The
dibromofluorene monomers 1 and 2 were obtained according
to the previous reports using 2,7-dibromofluorenone and 2,7-
dibromofluorene as a starting material, respectively.8 We also
prepared the compound 3 as a free BODIPY dye. All
monomers were characterized by 1H, 13C, and 11B NMR
spectroscopies and mass measurements. In 11B NMR spectra
using BF3·OEt2 as a standard, the products showed the peaks at
0.78 ppm assigned as a tetracoordinate boron. From these data,
we concluded that the BODIPY units were constructed.
Synthesis of the Polymers. The polymerizations with the
dibromo monomers 1 via Yamamoto coupling and with the
dibromo monomers 1, 2 and 9,9-didodecylfluorene diboronic
acid via Suzuki−Miyaura coupling were executed as shown in
Scheme 2. P1 and P3 have same number of the BODIPY unit
per a fluorene unit but different positions. P2 has one BODIPY
unit per two fluorene units. To remove the metal species
originated from the catalyst thoroughly, the Celite filtration for
P1 and the following reprecipitation from methanol were
repeated twice. The solubility in conventional organic solvents
seemed to be improved by introducing the cardo structures to
the polyfluorene main chains. The number-average molecular
(7 mg, 0.017 mmol), cesium carbonate (Cs2CO3) (0.40 g, 1.23
mmol), 1 (150 mg, 0.165 mmol), and 9,9-didodecylfluorene-2,7-
diboronic acid (97.7 mg, 0.165 mmol) in 1.5 mL of toluene and 1 mL
of H2O was stirred at 90 °C for 2 days under an argon atmosphere.
The orange solid (188 mg, 91%) was precipitated by pouring the
chloroform solution of the product into 50 mL of methanol twice. 1H
NMR (CDCl3, ppm): 7.90 (2H), 7.68 (6H), 7.51 (6H), 7.25 (4H),
6.82 (2H), 3.90 (2H), 2.52 (6H), 2.27 (4H), 2.00 (2H), 1.74 (2H),
1.18 (m, 58H), 0.97 (6H), 0.83 (9H). 13C NMR (CDCl3, ppm):
158.19, 153.70, 152.56, 151.73, 147.47, 141.86, 141.42, 140.02, 138.92,
138.12, 134.24, 132.76, 130.81, 129.18, 128.92, 128.30, 127.02, 126.19,
124.68, 121.45, 120.54, 119.95, 114.40, 67.97, 64.96, 55.25, 40.37,
31.89, 31.79, 30.06, 29.60, 29.31, 29.21, 26.06, 23.91, 22.65, 22.62,
17.09, 14.64, 14.09, 14.06, 12.47, 11.70. 11B NMR (CDCl3, ppm):
0.78.
Synthesis of Polymer P3. Similarly to the preparation of P2, the
polymer P3 was prepared with the monomer 2 (76 mg, 0.07 mmol)
and 9,9-didodecylfluorene-2,7-diboronic acid (41.5 mg, 0.07 mmol) in
the presence of Pd2(dba)3 (1 mg, 0.001 mmol), S-Phos (3 mg, 0.007
mmol), and Cs2CO3 (0.40 g, 1.23 mmol) as a red solid (105 mg,
1
quant). H NMR (CDCl3, ppm): 7.95 (2H), 7.76 (4H), 7.69 (4H),
7.54 (2H), 7.48 (6H), 7.26 (2H), 2.52 (12H), 2.26 (8H), 2.01 (4H),
1.34 (12H), 1.13 (36H), 0.95 (16H), 0.82 (6H). 13C NMR (CDCl3,
ppm): 153.86, 152.00, 151.81, 146.60, 141.58, 140.35, 140.05, 139.61,
139.04, 137.98, 136.79, 134.65, 132.84, 130.74, 128.68, 128.63, 127.41,
126.26, 124.62, 121.53, 121.22, 120.72, 119.99, 65.54, 55.25, 55.12,
40.32, 40.17, 31.87, 30.49, 30.04, 29.65, 29.59, 29.30, 28.94, 23.96,
22.63, 17.07, 14.62, 14.07, 12.48, 11.76. 11B NMR (CDCl3, ppm):
0.78.
C
dx.doi.org/10.1021/ma400015d | Macromolecules XXXX, XXX, XXX−XXX

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a
Scheme 2. Synthesis of the Polymers
a
Reagents and conditions: (a) Ni(COD)2, COD, 2,2-bpy, in DMF, 12 h; (b) 9,9-didodecylfluorene-2,7-diboronic acid, Pd2(dba)3, S-Phos, Cs2CO3,
toluene, H2O, 2 days.
weights (Mn) and the molecular weight distributions (Mw/Mn)
of the polymers, measured by the size-exclusion chromatog-
raphy (SEC) in tetrahydrofuran (THF) toward polystyrene
standards were from 5500 to 7000 with 2.1 to 3.7, respectively
(Table 1). The synthesized polymers have longer chain lengths
Table 1. Polymerization Results
a
b
b
polymer
yield (%)
Mn
Mw/Mn
DP
P1
P2
P3
quant
91
6200
7000
5500
3.95
2.30
2.10
8.3
11.2
7.7
quant
a
b
Isolated yields after precipitation. Estimated by size-exclusion
chromatography (SEC) based on polystyrene standards in chloroform.
c
Degree of polymerization estimated by number-average molecular
weight.
Figure 1. UV−vis spectra of the polymers and the model compounds
in chloroform (1.0 × 10−5 M).
(P3: DP = 7.7) than the effective conjugation length at the
excited states (ca. 5) in polyfluorene.17 These results suggest
that the influence of the polymer ends should be negligible on
the optical properties as LHA. In addition, the data for
M1 containing 9,9-dioctylpolyfluorene (PFO, Mn = 18 500,
PDI = 4.2, fluorene unit =1.0 × 10−5 M) and 3 (1.0 × 10−5 M)
as a comparison to P1 and P3. M2 containing PFO (fluorene
unit = 2.0 × 10−5 M) and 3 (1.0 × 10−5 M) was also prepared
for the comparison to P2. All measurements were executed
with the samples containing 1.0 × 10−5 M of the BODIPY unit.
The results from the absorption and emission spectra are
summarized in Table 2. The unimolecular BODIPY 3 exhibited
the strong and sharp absorption peak at 527 nm. Correspond-
1
structural analysis by H, 13C, and 11B NMR corresponded to
those of the monomers. Thus, we concluded that the polymers
should possess the expected chemical structures as we designed.
Absorption Spectra of the Polymers. The optical
properties of the polymers were initially investigated by UV−
vis absorption in chloroform (Figure 1). The influence of the
BODIPY units on the electronic properties of the main chains
was examined. We prepared the mixture solutions named as
D
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Table 2. Photophysical Properties of the Polymers in Chloroform Solution
a
b
c
d
e
f
g
h
h
i
compd
λabs,max/nm
λPL,max/nm
ΦPL
IPL,max
ΦDA
Eeff,Φ
Eeff,I
τ/ns
χ
ELHA
3
377 (8100), 527 (76500)
388 (40400), 527 (76700)
388 (78700), 527 (76200)
377 (32000), 528 (60300)
387 (75300), 527 (63500)
383 (38900), 529 (63000)
538
0.68
1.29
1.31
1.30
0.46
1.00
4.99
1.013
1.0
M1
M2
P1
P2
P3
416, 539
416, 538
539
0.46
0.80
0.72
0.001
0.002
0.014
0.999
0.997
0.981
0.995
0.997
0.998
4.21
4.20
3.96
1.123
1.035
1.013
1.7
8.9
3.9
539
539
0.74
a
b
c
Evaluated in chloroform (1.0 × 10−5 M) (molar extinction coefficient). Excited at 380 nm in chloroform (1.0 × 10−7 M). Absolute quantum
d
e
f
yield. Fluorescence intensity of emission maxima excited at 525 nm. Absolute quantum yield of the PF main chain. Energy transfer efficiency
g
h
calculated by quantum yield. Energy transfer efficiency calculated by emission intensity at 420 nm. Photoluminescence lifetimes of the polymers on
i
emission wavelength 540 nm in chloroform (1.0 × 10−7 M). LHA efficiency compared to 3.
ingly, the polymers P1−P3 showed the strong absorption peak
at 527 nm. In absorption regions from 330 to 420 nm, P1 and
P3 possessing a single fluorene unit per a single BODIPY unit
showed similar levels of the molar extinction coefficient to
PFO. Moreover, P2 possessing two fluorene units per a single
BODIPY unit exhibited the twice-larger molar extinction
coefficient than that of PFO. These results mean that the
electronic properties of the BODIPY units tethered to the
polymers should be similar to that of a free BODIPY dye in the
solution. In particular, it should be mentioned that the
BODIPY units should be electronically independent of the
polymer main chains as well as adjacent BODIPY units as
representatively observed from P3. Accordingly, it is suggested
that the intrinsic optical properties of BODIPYs and PF main
chains can be observed from the synthesized polymers. These
results can be explained by our previous findings that the cardo
Figure 2. Photoluminescence spectra of the polymers in chloroform
(1.0 × 10−7 M, excitation wavelength 380 nm).
structure plays a role in inhibiting electronic couplings between
the main chains and the side chains.8 In contrast, P1 provided
red-shifted optical edge. It is assumed that the BODIPY units
could form the stacking structure. The electronic structure of
P1 is discussed in the emission spectra.
The absorption spectra of the polymers were measured with
the polymer films (Figure S1 and Table S1). The thin films
were cast on the quartz plate by the spin-coating method with
the chloroform solutions (1.0 mg/mL) of the polymers. Slightly
red-shifted peaks were obtained from the spectra comparing
with those from the solutions. These data imply that the
isolation effect owing to the cardo structure could be received
even in the condensed state such as in the film.
Emission Properties of the Polymers. The emission
properties and the LHA efficiencies of P1, P2, and P3 were
evaluated in the chloroform solutions (Figure 2). The LHA
efficiency was calculated from the ratios of the emission
intensities toward that from 3 with the excitation light at 380
nm. The results are listed in Table 2. Significantly, it was found
that P2 and P3 can work as a LHA with 8.9 and 3.9 times larger
efficiencies than that of 3. Even from the polymer P1 in which
the BODIPY units could form the aggregation, 1.7 times
stronger emission than that of 3 was observed with the peak at
540 nm. The samples were prepared with the same
concentrations of the BODIPY unit. It means that in the
sample of P2 the concentration of the PF main chains, light
absorber, is 2-fold larger than that of P3. Therefore, the actual
LHA value of P3 calculated from the PF standard concentration
should be similarly large value with that of P2. In summary, it
can be claimed that the synthesized polymers should be
efficient LHA.
Figure 3. Photoluminescence spectra of the polymers in the film states
(excitation wavelength 380 nm).
aggregation ability of BODIPY dyes, the emission drastically
decreased in the solid states because of concentration
quenching.14 Indeed, the unimolecular BODIPY 3 showed no
emission. On the other hand, the polymers P2 and P3 gave
strong fluorescence emissions. These data indicate that the
cardo PFs-based LHA systems should be valid for the practical
usages as a device. The weak emission from P1 can be
explained by the aggregation formation similarly as observed in
the solution state. It should be mentioned that the synthetic
polymers have good processability similarly as PFs. These
advantages as a material involving optical properties in the film
states could be feasible for the application to practical devices.
The LHA efficiencies of the polymers were also measured in
the film states (Figure 3). Generally, because of the strong
E
dx.doi.org/10.1021/ma400015d | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Chart 1. Schematic Diagram for Energy Transferring
a
b
Determined from cyclic voltammograms. Calculated from the onsets of the polymers in the absorption spectra.
To investigate the photochemical process on the LHA, the
the excitation light at 380 nm. The alternative polymer P2
showed the highest QY value of the samples (ΦPL = 0.80).
Interestingly, the polymer P3 that has the same backbone
structure, but the intermolecular distances between the
BODIPY units are much closer than those in P2 presented
similar degree of the QY value (ΦPL = 0.72). In particular, the
BODIPY units in these polymers showed higher QY values
than that of 3. Thermal quenching path could be suppressed by
tethering the BODIPY unit to polymer chains via the rigid
cardo structures. The smaller value obtained from the polymer
P1 than that of 3 implies the aggregation-induced quenching of
the emission. Next, to evaluate the favorable position of the
BODIPY unit for improving the emission efficiency, the
photoluminescence spectra were obtained with the excitation
light at 525 nm (Figure 4). From the emission intensities at the
spectra were analyzed in further detail. Initially, the polymers
and models were dissolved in chloroform, and the photo-
luminescence spectra were obtained with the excitation light at
380 nm corresponding to absorption maxima of the PFs
(Figure 2). In the emission region from 400 to 500 nm of the
PFs, M1 and M2 showed similar spectra with the peaks at 420,
445, and 470 nm assigned as the 0−0, 0−1, and 0−2 intrachain
singlet transition originated from PFO.18 In contrast, the
emission peaks originated from the PFs main chain were not
observed in all polymers, but strong emission was obtained with
the peak at 540 nm assigned as the fluorescence of BODIPY.
Furthermore, the excitation spectra of the polymers were
measured with the detection wavelength at 540 nm (Figure
S2). The spectra were normalized by the peak top of BODIPY
emissions. The peak positions showed good agreements with
those in UV spectra. These data represent that efficient energy
transfer from the PF main chains to the BODIPY units should
occur through the cardo structures in the polymers. The energy
transfer efficiencies were calculated according to the eq 1,19 and
the values are listed in Table 2:
Eeff = (1 − IDA/ID) = (1 − τDA/τD) = (1 − ΦDA/ΦD)
(1)
where IDA is a fluorescence intensity of donor in the presence of
acceptor and ID in the absence of acceptor, τDA is a fluorescence
lifetime of donor in the presence of acceptor and τD in the
absence of acceptor, and ΦDA is a quantum yield of donor in
the presence of acceptor and ΦD in the absence of acceptor. All
polymers indicated energy transfer efficiencies over 0.99 from
the PF main chain to the BODIPY units. According to the
energy levels of the PF and the BODIPY unit determined from
optical and electrochemical measurements, the proposed
mechanism on the energy transfer is illustrated in Chart 1.
Surprisingly, although the electronic coupling via the cardo
structure is very few, the energy transfer can proceed with high
efficiency in the synthetic polymers. To gather the kinetics of
the energy transfer in the polymers, the fluorescence lifetimes
were measured. Because of too rapid process, the energy
transfer rates from the PFs to the BODIPY units were not
determined (<100 ps). These results imply the efficient energy
transfer could occur.
In general, to receive highly emissive polymers, it is necessary
to avoid the concentration quenching induced by the
accumulation of chromophores.10 To evaluate the degree of
the emission properties of the BODIPY units in the polymers,
we compared the photoluminescence quantum yields (QY) of
these polymers. Initially, the quantum yields through the total
process from the light absorption at the PF main chains to the
emission from the BODIPY unit were calculated. By using the
integration sphere, the absolute values were determined with
Figure 4. Photoluminescence spectra of the polymers in chloroform
(1.0 × 10−7 M, excitation wavelength 525 nm).
peak tops, the degree of the concentration quenching was
evaluated as a relative value (Table 2). The BODIPY units in
the polymer P2 showed the largest value of the polymers.
Notably, similarly as the QY values, the concentration
quenching of the BODIPY units in the polymer P3 was
efficiently suppressed. These data clearly indicate that the cardo
structure should play an important role in the preservation of
the intrinsic properties of dyes.
Molecular Orbital Calculations. To understand a
quenching phenomenon observed from P1, we employed the
theoretical calculation for the models of the polymers using
density-functional theory (DFT) at the B3LYP/6-31G(d)//
B3LYP/6-31G(d) (Figure S4). The models of the polymers
have the two or three repeat units, substituting alkyl chains to
methyl groups for simplifying calculation. It was found that
intramolecular dimer of the nearby BODIPY units could be
F
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S.; Kuramochi, Y.; Hirota, S.; Kobuke, Y. Chem.Eur. J. 2011, 17,
855−865. (e) Fujita, S.; Inagaki, S. Chem. Mater. 2008, 20, 891−908.
(5) (a) Patra, A.; Koenen, J.-M.; Scherf, U. Chem. Commun. 2011, 47,
9612−9614. (b) Kameta, N.; Ishikawa, K.; Masuda, M.; Asakawa, M.;
Shimizu, T. Chem. Mater. 2012, 24, 209−228.
formed on the ground states in the P1 model. On the other
hand, each BODIPY unit hardly shows the interaction in the P2
and P3 models. It is known that the fluorescence emission from
BODIPY dyes can be drastically quenched by the aggregation.14
Thereby, the decrease of the emission intensity could be
observed from P1. In contrast, in the P3 model, plane angles of
two BODIPY units are ∼120°, meaning little interaction
between two BODIPYs on a cardo structure. These data
suggest that the rigid and the orthogonal structure of the cardo
should be favorable not only to avoid the concentration
quenching of the dyes but also to accumulate the emissive dyes
without unexpected changes in the electronic properties.
(6) Scholes, G. D.; Fleming, G. R.; Olaya-Castro, A.; van Grondelle,
R. Nat. Chem. 2011, 3, 763−774.
(7) (a) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C.; George, S. J.
Angew. Chem., Int. Ed. 2007, 46, 6260−6265. (b) Abbel, R.; Grenier,
C.; Pouderoijen, M. J.; Stouwdam, J. W.; Leclere, P. E. L. G.; Sijbesma,
R. P.; Meijer, E. W.; Schenning, A. P. H. J. J. Am. Chem. Soc. 2009, 131,
833−843.
(8) Yeo, H.; Tanaka, K.; Chujo, Y. J. Polym. Sci., Part A: Polym. Chem.
2012, 50, 4433−4442.
(9) (a) Liu, S.-J.; Xu, W.-J.; Ma, T.-C.; Zhao, Q.; Fan, Q.-L.; Ling, Q.-
D.; Huang, W. Macromol. Rapid Commun. 2010, 31, 629−6333.
(b) Evans, N. R.; Devi, L. S.; Mak, C. S. K.; Watkins, S. E.; Pascu, S. I.;
CONCLUSION
■
We present the first example to offer the schematic information
on the changes of optical properties of the dyes depending on
the location at the PF main chains. Considering the
experimental results described here, we summarize that three
significant advantages on the usages of the cardo structure
involving PFs as a scaffold: (i) Large light absorption and
effective energy transfer can be obtained. (ii) The intrinsic
property of the dye can be observed after conjugation with the
cardo PFs. (iii) The desired optical properties can be received
in the film state. Finally, highly effective LHA systems were
realized based on the BODIPY-tethered cardo PFs. Advanced
LHA systems based on our design concept for higher efficiency
or diverse light wavelength of absorption and emission are
promised to be realized by modulating the location, the
numbers, and the type of chromophores. In addition, further
applications are in progress to utilize the cardo fluorene as a
scaffold for constructing highly functional optical materials.
Kohler, A.; Friend, R. H.; Williams, C. K.; Holmes, A. B. J. Am. Chem.
̈
Soc. 2006, 128, 6647−6656. (c) Ego, C.; Marsitzky, D.; Becker, S.;
Zhang, J.; Grimsdale, A. C.; Mullen, K.; MacKenzie, J. D.; Silva, C.;
̈
Friend, R. H. J. Am. Chem. Soc. 2003, 125, 437−443. (d) Chen, X.;
Liao, J.-L.; Liang, Y.; Ahmed, M. O.; Tseng, H.-E.; Chen, S.-A. J. Am.
Chem. Soc. 2003, 125, 636−637. (e) Wang, L.; Puodziukynaite, E.;
Vary, R. P.; Grumstrup, E. M.; Walczak, R. M.; Zolotarskaya, O. Y.;
Schanze, K. S.; Reynolds, J. R.; Papanikolas, J. M. J. Phys. Chem. Lett.
2012, 3, 2453−2457. (f) Buckley, A. R.; Rahn, M. D.; Hill, J.;
Cabanillas-Gonzalez, J.; Fox, A. M.; Bradley, D. D. C. Chem. Phys. Lett.
2001, 339, 331−336. (g) Thivierge, C.; Loudet, A.; Burgess, K.
Macromolecules 2011, 44, 4012−4015. (h) Wu, C.; Zheng, Y.;
Szymanski, C.; McNeill, J. J. Phys. Chem. C 2008, 112, 1772−1781.
(i) Becker, K.; Lupton, J. M. J. Am. Chem. Soc. 2006, 128, 6466−6479.
(j) Russell, D. M.; Arias, A. C.; Friend, R. H.; Silva, C.; Ego, C.;
Grimsdale, A. C.; Mullen, K. Appl. Phys. Lett. 2002, 80, 2204−2206.
̈
(k) Troshin, P. A.; Koeppe, R.; Susarova, D. K.; Polyakova, N. V.;
Peregudov, A. S.; Razumov, V. F.; Sariciftci, N. S.; Lyubovskaya, R. N.
J. Mater. Chem. 2009, 19, 7738−7744. (l) He, F.; Feng, F.; Wang, S.;
Li, Y.; Zhu, D. J. Mater. Chem. 2007, 17, 3702−3707.
ASSOCIATED CONTENT
■
S
* Supporting Information
(10) Khan, A. L. T.; Sreearunothai, P.; Herz, L. M.; Banach, M. J.;
Figures S1−S23 and Table S1. This material is available free of
Kohler, A. Phys. Rev. B 2004, 69, 085201.
̈
(11) Hsu, F.-M.; Chien, C.-H.; Shu, C.-F.; Lai, C.-H.; Hsieh, C.-C.;
Wang, K.-W.; Chou, P.-T. Adv. Funct. Mater. 2009, 19, 2834−2843.
(12) He, F.; Ren, X.; Shen, X.; Xu, Q.-H. Macromolecules 2011, 44,
5373−5380.
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail chujo@chujo.synchem.kyoto-u.ac.jp; Fax +81-75-383-
2605; Ph +81-75-383-2604.
■
(13) (a) Jin, J.-K.; Kwon, S.-K.; Kim, Y.-H.; Shin, D.-C.; You, H.;
Jung, H.-T. Macromolecules 2009, 42, 6339−6347. (b) Wu, F.-I.; Shih,
P.-I.; Shu, C.-F. Macromolecules 2005, 38, 9028−9036.
(14) (a) Kubota, Y.; Uehara, J.; Funabiki, K.; Ebihara, M.; Matsui, M.
Tetrahedron Lett. 2010, 51, 6195−6198. (b) Zhang, D.; Wen, Y.; Xiao,
Y.; Yu, G.; Liu, Y.; Qian, X. Chem. Commun. 2008, 4777−4779.
(c) Hepp, A.; Ulrich, G.; Schmechel, R.; Von Seggern, H.; Ziessel, R.
Synth. Met. 2004, 146, 11−15.
(15) (a) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed.
2008, 47, 1184−1201. (b) Ziessel, R.; Ulrich, G.; Harriman, A. New J.
Chem. 2007, 31, 496−501. (c) Bones, N.; Leen, V.; Dehaen, W. Chem.
Soc. Rev. 2012, 41, 1130−1172.
(16) (a) Ziessel, R.; Harrimanb, A. Chem. Commun. 2011, 47, 611−
631. (b) El-Khouly, M. E.; Amin, A. N.; Zandler, M. E.; Fukuzumi, S.;
D’Souza, F. Chem.Eur. J. 2012, 18, 5239−5247. (c) Gabe, Y.; Urano,
Y.; Kikuchi, K.; Kojima, H.; Nagano, T. J. Am. Chem. Soc. 2004, 126,
3357−3367. (d) Mula, S.; Ray, A. K.; Banerjee, M.; Chaudhuri, T.;
Dasgupta, K.; Chattopadhyay, S. J. Org. Chem. 2008, 73, 2146−2154.
(17) Klaerner, G.; Miller, R. D. Macromolecules 1998, 31, 2007−2009.
(18) Dieter, N. Macromol. Rapid Commun. 2001, 22, 1365−1385.
(19) Gu, Z.-Y.; Guo, D.-S.; Sun, M.; Liu, Y. J. Org. Chem. 2010, 75,
3600−3607.
Notes
The authors declare no competing financial interest.
REFERENCES
■
(1) (a) Chen, C.-Y.; Wu, S.-J.; Li, J.-Y.; Wu, C.-G.; Chen, J.-G.; Ho,
K.-C. Adv. Mater. 2007, 19, 3888−3891. (b) Hasobe, T.; Hattori, S.;
Kamat, P. V.; Wada, Y.; Fukuzumi, S. J. Mater. Chem. 2005, 15, 372−
380. (c) Warnan, J.; Pellegrin, Y.; Blart, E.; Odobel, F. Chem. Commun.
2012, 48, 675−677. (d) Li, Y.; Bian, Y.; Yan, M.; Thapaliya, P. S.;
Johns, D.; Yan, X.; Galipeau, D.; Jiang, J. J. Mater. Chem. 2011, 21,
11131−11141.
(2) (a) Brovelli, S.; Meinardi, F.; Winroth, G.; Fenwick, O.;
Sforazzini, G.; Frampton, M. J.; Zalewski, L.; Levitt, J. A.; Marinello,
F.; Schiavuta, P.; Suhling, K.; Anderson, H. L.; Cacialli, F. Adv. Funct.
Mater. 2010, 20, 272−280. (b) Chen, Z.; Jiang, C.; Niu, Q.; Peng, J.;
Cao, Y. Org. Electron. 2008, 9, 1002−1009.
(3) Collini1, E.; Wong, C. Y.; Wilk, K. E.; Curmi, P. M. G.; Brumer,
P.; Scholes, G. D. Nature 2010, 463, 644−647.
(4) (a) Taranekar, P.; Huang, C.; Fulghum, T. M.; Baba, A.; Jiang, G.;
Park, J.-Y.; Advincula, R. C. Adv. Funct. Mater. 2008, 18, 347−354.
(b) Wu, C.; Zheng, Y.; Szymanski, C.; McNeill, J. J. Phys. Chem. C
2008, 112, 1772−1781. (c) Wu, C.; Bull, B.; Christensen, K.; McNeill,
J. Angew. Chem., Int. Ed. 2009, 48, 2741−2745. (d) Satake, A.; Azuma,
G
dx.doi.org/10.1021/ma400015d | Macromolecules XXXX, XXX, XXX−XXX