Isolation of π-conjugated system through polyfluorene from electronic coupling with side-chain substituents by cardo structures

Article abstract of DOI:10.1002/pola.26249

The electronic coupling via the cardo structure in polyfluorene (PFs) was investigated. The series of fluorene units alternatively having alkoxyphenyl as an electron-donating group (EDG) and/or alkyl benzoate as an electron-withdrawing group (EWG) at the

Full text of DOI:10.1002/pola.26249

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Isolation of p-Conjugated System Through Polyfluorene from Electronic
Coupling with Side-Chain Substituents by Cardo Structures
Hyeonuk Yeo, Kazuo Tanaka, Yoshiki Chujo
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
Correspondence to: Y. Chujo (E-mail: chujo@chujo.synchem.kyoto-u.ac.jp)
Received 16 April 2012; accepted 25 June 2012; published online
DOI: 10.1002/pola.26249
ABSTRACT: The electronic coupling via the cardo structure in
polyfluorene (PFs) was investigated. The series of fluorene units
alternatively having alkoxyphenyl as an electron-donating group
(EDG) and/or alkyl benzoate as an electron-withdrawing group
(EWG) at the cardo carbon were synthesized. From the investiga-
tion of optical properties of the polymers containing these fluo-
rene units, it was found that the electronic states of the
substituents at the cardo carbons and the PF main chains should
be less influenced by the introduction of EDG and/or EWG at the
cardo structure. Furthermore, these preservation effects in the
cardo-PFs were observed in the film states even after the thermal
treatment. We conclude that the electronic structures of the PF
main chain are highly preserved from the correlations with the
substituents at the cardo carbons. This is the first example, to
the best of our knowledge, to survey the systematic information
on the electronic structures of the cardo-PFs and offer the preser-
vation effect of the optical properties from the introduction of
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EDGs and EWGs at the cardo carbon.
2012 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: cardo structure; fluorescence; photochemistry; poly-
fluorene; thermogravimetric analysis
INTRODUCTION The polyfluorene (PF) is one of the widely
known optical polymers having a bright blue emission.1–4
More recently, the robust p-conjugation system through the
PF main chain has been recognized as a scaffold for fabricat-
ing the advanced optoelectronic materials. For instance, the
PF films containing metalloporphyrin as a sensitizer can be
used as anti-Stokes fluorescent materials which show the
photon upconversion and the generation of higher energy
photons than those of the incident light.5–8 As the rigid
structure of the PFs can suppress the thermal deactivation of
the triplet-excited state produced by the sensitizing reaction
under the light irradiation, the singlet-excited state can be
efficiently generated via the triplet–triplet annihilation pro-
cess.9 These materials are promising to break through the
theoretical limitation on the transition efficiency in the pho-
tovoltaic cells.10 Although construction and elongation of the
conjugation systems are still main subjects, the disconnec-
tion and isolation of the electronic coupling are also of sig-
nificance to control a conjugation to receive superior proper-
ties from polymers.
processes is an introduciton of functional groups at the 9-
position. The phenyl groups bonded at the 9-position of a
fluorene are three dimensionally arranged to the fluorene
unit.14–17 The construction of such a tetrahedral bonding
carbon into the 9-position of the fluorene, called as a cardo
structure, is valid for improving thermal stability and dura-
bility against oxidation compared to those of dialkyl-substi-
tuted PFs.18–20 In the film states of the dialkyl-substituted
PFs, the annealing process should induce the fluorenone for-
mation by oxidation, resulting in a loss of the bright blue
emission of PFs.21–23 These unfavorable oxidation can be sig-
nificantly suppressed at the cardo structure. Moreover, owing
to the steric hindrance of the bulky phenyl groups, the inter-
chain and/or intrachain aggregations of the main chains are
efficiently disturbed.24–27 The isolation of the electronic
states of the PF main chains resulted in the strong blue
emission of the PFs. Thus, the cardo fluorenes are the attrac-
tive building block to construct the highly functional poly-
meric materials.28,29
Using polycardofluorenes as a template to build up func-
tional nanomaterials, the information on the electronic struc-
ture and coupling are essential. There are several reports on
the energy or charge transfer through the cardo structures
with the functional unit-tethered cardo-PFs.30,31 Although it
was presented that the weak electronic coupling through the
Optical properties observed from the solution state are often
compromised in the solid states. Particularly, in the case of
PFs, the excimer formation, the b-phase conformation, and
the interchain interactions are well known as a decay pro-
cess of an exciton.11–13 One of the solutions to avoide these
Additional Supporting Information may be found in the online version of this article.
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cardo structures could exist, most of the previous studies
mainly focused on the energy transferring between the func-
tional groups tethered to the 9-position of the fluorene and
the PF main chains.32 In particular, there is very few system-
atic information about the substituent effect on the elec-
tronic coupling and the extension of the conjugation through
the cardo structures in the PFs. For developing the applic-
ability of the PFs as a scaffold for the advanced all-organic
devices, the fundamental study on the conjugation through
the cardo structure should be necessary.
metric analyses (TGAs) were performed on a Seiko TG/DTA
6200 at a scan rate of 10ꢀC/min.
Materials
Tetrahydrofuran (THF), diethyl ether (Et2O), and triethyl-
amine (Et3N) were purified using a two-column solid-state
purification system (Glasscontour System, Joerg Meyer,
Irvine, CA). 2,7-Dibromo-9-fluorenone, 2,7-dibromofluorene,
1-bromo-4-(diethoxymethyl)benzene, and 2,7-dibromo-9,9-
dioctyl-9H-fluorene were obtained commercially and used
without further purification. 4,40-(2,7-Dibromo-9H-fluorene-
9,9-diyl)diphenol (1) and 4-(2,7-dibromo-9-hydroxy-9H-fluo-
ren-9-yl)benzaldehyde (6) were prepared according to the
literatures.33,34 All reactions were performed under nitrogen
atmosphere.
Herein, to clarify the electronic interaction via the cardo
structures particularly the electronic substituent effect, we
synthesized and characterized the electronic properties of
the PFs with the electron-donating groups (EDGs) and/or
the electron-withdrawing groups (EWGs) at cardo structures.
The dibromo monomers containing dual alkyl benzoate
groups as an EWG, dual alkoxyphenyl groups as an EDG, and
single each group were, respectively, prepared, and the cor-
responding polymers were obtained via Yamamoto coupling.
The influence on the optical properties by introducing the
substituents was evaluated both in the solution and in the
film states. The computer calculation models and the data
from cyclic voltammetry (CV) support the discussions on the
electronic substituent effect of the PF main chains. This is
the first example, to the best of our knowledge, to offer the
systematic study on the electronic states of the cardo-PFs.
2,7-Dibromo-9,9-bis(4-(octyloxy)phenyl)-9H-fluorene
(2, M1)
Bromooctane (1.52 g, 7.88 mmol) was added to a methanol
solution (15 mL) of 133 (2.00 g, 3.94 mmol) and potassium
hydroxide (0.44 g, 7.88 mmol). After refluxing for 24 h, the
resulting mixture was poured into brine, extracted with
cyclopentyl methyl ether (CPME), and dried over anhydrous
Na2SO4. After evaporation to remove the solvent, the crude
product was purified by the silica gel column chromatogra-
phy with eluent ethyl acetate/n-hexane (1/16) to afford M1
(1.74 g, 61%) as a colorless oil.
1H NMR (CDCl3, ppm) 7.54 (d, J ¼ 7.9 Hz, 2H), 7.46 (d, J ¼
5.0 Hz, 2H), 7.43 (d, J ¼ 3.5 Hz, 2H), 7.04 (d, J ¼ 8.8 Hz,
4H), 6.75 (d, J ¼ 9.0 Hz, 4H), 3.88 (t, J ¼ 6.6 Hz, 4H), 1.73
(dt, J ¼ 7.0 Hz, 4H), 1.46–1.22 (m, 24H), 0.87 (t, J ¼ 7.1 Hz,
6H). 13C NMR (CDCl3, ppm) 158.23, 153.75, 137.83, 136.23,
130.70, 129.26, 128.97, 121.77, 121.48, 114.33, 67.92, 64.36,
31.78, 29.31, 29.24, 29.21, 26.03, 22.63, 14.07. HRMS (EI)
calcd for C41H48O2Br2: m/z ¼ 730.2021; Found: m/z ¼
730.2011.
EXPERIMENTAL
General
1H NMR and 13C NMR spectra were measured with a JEOL
EX-400 (400 MHz for H and 100 MHz for 13C) spectrometer.
1
Coupling constants (J-value) are reported in hertz. The chem-
ical shifts are expressed in ppm downfield from tetramethyl-
silane using residual chloroform (d ¼ 7.24 in 1H NMR, d ¼
77.0 in 13C NMR) as an internal standard. The number-aver-
age molecular weight (Mn) and the molecular weight distri-
bution (weight-average molecular weight/number-average
molecular weight [Mw/Mn]) values of all polymers were esti-
mated by the size-exclusion chromatography (SEC) with a
TOSOH G3000HXI system equipped with three consecutive
polystyrene gel columns (TOSOH gels: a-4000, a-3000, and
a-2500) and ultraviolet detector at 40ꢀC. The system was
operated at a flow rate of 1.0 mL/min, with chloroform as
an eluent. Polystyrene standards were employed for the cali-
bration. UV–vis spectra were recorded on a Shimadzu UV-
3600 spectrophotometer. Emission spectra were recorded on
a HORIBA JOBIN YVON Fluoromax-4 spectrofluorometer, and
absolute quantum yields were determined by the integrating
sphere method on the HORIBA JOBIN YVON Fluoromax-4
spectrofluorometer in chloroform. Fluorescence lifetime anal-
ysis was carried out on a HORIBA FluoreCube spectrofluor-
ometer system with the excitation light at 375 nm from a UV
diode laser (NanoLED-375L). CV spectra were recorded on a
BAS ALS electrochemical analyzer model 600D. Fourier
transform infrared (FTIR) spectra were obtained using a
Shimadzu IRPrestige-21 infrared spectrometer. Thermogravi-
4,40-(2,7-Dibromo-9H-fluorene-9,9-diyl)dibenzonitrile (3)
A mixture of 2,7-dibromofluorene (10.00 g, 30.9 mmol), tolu-
ene (15 mL), DMF (50 mL), 18-crown-6-ether (4.08 g,
15.4 mmol), potassium carbonate (10.30 g, 75.1 mmol), and
4-fluorobenzonitrile (8.20 g, 67.7 mmol) was stirred at
140ꢀC for 15 h. After cooling, the solution was dropped into
500 mL of CPME. The product was filtered and washed with
water to give 3 5.76 g (35%) as a yellow solid.
1H NMR (CDCl3, ppm) 7.64 (d, J ¼ 8.0 Hz, 2H), 7.60 (d, J ¼
8.3 Hz, 4H), 7.57 (d, J ¼ 9.3 Hz, 1.7 Hz, 2H), 7.39 (d, J ¼ 1.5
Hz, 2H), 7.23 (d, J ¼ 8.3 Hz, 4H). 13C NMR (CDCl3, ppm)
150.42, 148.63, 138.10, 132.74, 132.11, 129.01, 128.53,
122.45, 122.17, 118.21, 111.84, 65.44. HRMS (FABþ) calcd
for C27H14Br2N2: m/z ¼ 523.9524; Found: m/z ¼ 523.9504.
4,40-(2,7-DIBROMO-9H-FLUORENE-9,9-DIYL)DIBENZOIC
ACID (4)
A mixture of potassium hydroxide (8.50 g, 151.5 mmol) and
3 (5.66 g, 10.8 mmol) in ethanol (25 mL) and distilled water
(30 mL) was stirred at 80ꢀC for 3 days. After cooling, the pH
value of the reaction mixture was adjusted to nearly 2 by 2N
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hydrochloric acid. The generated precipitate was collected by
filtration and washed with water to afford 6.03 g of the
product (99%) as a white powder.
22.45, 13.96. HRMS (MALDI) calcd for C34H32Br2O2: m/z ¼
632.07541; Found: m/z ¼ 632.07358.
4-(2,7-Dibromo-9-(4-(octyloxy)phenyl)-9H-fluoren-
9-yl)benzoic acid (8)
1H NMR (DMSO-d6, ppm) 12.92 (s, 2H), 7.97 (d, J ¼ 7.8 Hz,
2H), 7.87 (d, J ¼ 8.3 Hz, 4H), 7.67 (d, J ¼ 8.7 Hz, 2H), 7.63
(s, 2H), 7.24 (d, J ¼ 8.3 Hz, 4H). 13C NMR (DMSO-d6, ppm)
166.85, 151.46, 148.55, 137.88, 131.53, 129.85, 128.75,
127.83, 123.15, 121.56, 65.24. HRMS (ESI) calcd for
A mixture of NaClO2 (2.28 g, 25.2 mmol), NaH2PO4 (3.04 g,
25.3 mmol), and H2O (20 mL) was slowly added to an ace-
tone solution (20 mL) of 7 (2.00 g, 3.15 mmol) and 2-
methyl-2-butene (2 mL). After stirring at room temperature
for 1 h, the resulting mixture was diluted with 3 N HCl
(50 mL), and the product was extracted with CPME. The com-
bined organic extracts were washed with saturated aqueous
Na2S2O3 twice and dried over anhydrous Na2SO4. The product
8 was obtained as a white powder (1.85 g, 91%).
C
27H16Br2O4-Hþ: m/z ¼ 560.9343; Found: m/z ¼ 560.9330.
Dioctyl 4,40-(2,7-dibromo-9H-fluorene-9,
9-diyl)dibenzoate (5, M2)
Thionyl chloride (15 mL) was added to 4 (1.69 g, 3.0 mmol),
and then the solution was refluxed for 3 h. The residual thi-
onyl chloride was removed in vacuo. Dibenzoyl chloride was
obtained as a red solid and used for the next reaction with-
out further purification. A mixture of dibenzoyl chloride, pyr-
idine (15 mL), octanol (0.78 g, 6.0 mmol), and THF (30 mL)
was stirred at room temperature overnight. The resulting
mixture was diluted with 1N HCl, and the product was
extracted with CPME. The combined organic extracts were
washed with brine and dried over anhydrous Na2SO4. After
removing the solvent, the crude product was purified by
silica gel column chromatography with eluent ethyl acetate/
n-hexane (1/20) to afford M2 (1.58 g, 67%) as a colorless
oil.
1H NMR (DMSO-d6, ppm) 12.88 (s, 1H), 7.87 (m, 4H), 7.57
(m, 4H), 7.22 (d, J ¼ 7.6 Hz, 2H), 6.99 (d, J ¼ 7.5 Hz, 2H),
6.78 (d, J ¼ 7.8 Hz, 2H), 4.29 (t, J ¼ 6.7 Hz, 2H), 1.65–1.53
(m, 2H), 1.29–1.06 (m, 12H), 0.78 (t, J ¼ 6.6 Hz, 3H). 13C
NMR (DMSO-d6, ppm) 166.89, 157.86, 152.45, 149.27,
137.72, 135.12, 131.10, 129.68, 128.64, 127.67, 122.89,
121.36, 114.49, 67.32, 64.60, 31.18, 28.61, 25.46, 22.03,
13.86. HRMS (APCI) calcd for [C34H32Br2O3-H]ꢁ: m/z ¼
645.0616; Found: m/z ¼ 645.0645.
Octyl 4-(2,7-dibromo-9-(4-(octyloxy)phenyl)-9H-
fluoren-9-yl)benzoate (9, M3)
Thionyl chloride (8 mL) was added to 8 (1.80 g, 2.8 mmol),
and the solution was refluxed for 3 h. After the reaction, the
residual thionyl chloride was removed in vacuo. Benzoyl
chloride was obtained as a red solid and used for the next
reaction without further purification. A mixture of benzoyl
chloride, pyridine (10 mL), octanol (0.36 g, 2.8 mmol), and
THF (20 mL) was stirred at room temperature for 3 h. The
resulting mixture was diluted with 1N HCl, and the product
was extracted with CPME. The combined organic extracts
were washed with brine and dried over anhydrous Na2SO4.
After removing the solvent, the crude product was purified
by silica gel column chromatography with eluent ethyl
acetate/n-hexane (1/25) to afford M3 (1.92 g, 90%) as a
colorless oil.
1H NMR (CDCl3, ppm) 7.94 (d, J ¼ 8.3 Hz, 4H), 7.62 (d, J ¼
8.0 Hz, 2H), 7.53 (dd, J ¼ 8.1 Hz, 1.7 Hz, 2H), 7.45 (d, J ¼
1.7 Hz, 2H), 7.20 (d, J ¼ 8.4 Hz, 4H), 4.29 (t, J ¼ 6.6 Hz, 4H),
1.77–1.67 (m, 4H), 1.44–1.22 (m, 24H), 0.87 (t, J ¼ 7.1 Hz,
6H). 13C NMR (CDCl3, ppm) 166.11, 151.60, 148.81, 138.11,
131.52, 129.96, 129.24, 127.84, 125.49, 121.86, 65.18, 62.31,
31.75, 29.49, 29.20, 29.09, 26.00, 22.60, 14.06. HRMS (APCI)
calcd for [C43H48Br2O4þH]þ: m/z ¼ 787.1992; Found: m/z
¼ 787.1954.
4-(2,7-Dibromo-9-(4-(octyloxy)phenyl)-9H-fluoren-
9-yl)benzaldehyde (7)
Trifluoromethanesulfonic acid (5.99 g, 40.0 mmol) was
slowly added to a dioxane solution (100 mL) of 634 (13.8 g,
31.1 mmol) and n-octyl phenyl ether (7.80 g, 37.8 mmol). Af-
ter stirring at 70ꢀC for 4 h, the resulting mixture was diluted
with saturated aqueous NaHCO3, and the product was
extracted with CPME. The combined organic extracts were
washed with brine, and dried over anhydrous Na2SO4. After
removing the solvent, the crude product was purified by
silica gel column chromatography with eluent ethyl acetate/
n-hexane (1/12) to afford the product 7 (16.0 g, 82%) as a
white solid.
1H NMR (CDCl3, ppm) 7.91 (d, J ¼ 8.5 Hz, 2H), 7.59 (d, J ¼
8.1 Hz, 2H), 7.49 (d, J ¼ 8.3 Hz, 2H), 7.45 (s, 2H), 7.22 (d, J
¼ 8.5 Hz, 2H), 7.01 (d, J ¼ 8.8 Hz, 2H), 6.78 (d, J ¼ 8.8 Hz,
2H), 4.28 (t, J ¼ 6.6 Hz, 2H), 3.91 (t, J ¼ 6.5 Hz, 2H), 1.79–
1.65 (m, 4H), 1.45–1.22 (m, 24H), 0.90–0.84 (m, 6H). 13C
NMR (CDCl3, ppm) 166.26, 158.47, 152.71, 149.88, 138.00,
135.29, 131.12, 129.78, 129.45, 129.27, 128.96, 127.90,
121.95, 121.67, 114.57, 68.02, 65.11, 62.34, 31.78, 29.51,
29.31, 29.22, 29.17, 29.10, 28.72, 26.02, 25.75, 22.61, 14.06.
HRMS (EIþ) calcd for C42H48Br2O3: m/z ¼ 758.1970; Found:
m/z ¼ 758.1941.
1H NMR (CDCl3, ppm) 9.88 (s, 1H), 7.70 (d, J ¼ 8.0 Hz, 2H),
7.56–7.50 (m, 4H), 7.44 (d, J ¼ 8.0 Hz, 2H), 7.33 (d, J ¼ 7.7
Hz, 2H), 7.05 (d, J ¼ 7.1 Hz, 2H), 6.77 (d, J ¼ 8.6 Hz, 2H),
3.84 (t, J ¼ 6.3 Hz, 2H), 1.75–1.66 (m, 2H), 1.45–1.19 (m,
12H), 0.86 (t, J ¼ 6.8 Hz, 3H). 13C NMR (CDCl3, ppm)
191.10, 158.33, 152.27, 151.52, 137.78, 135.07, 134.78,
131.04, 129.68, 129.03, 128.72, 128.30, 121.85, 121.60,
114.46, 67.62, 64.96, 31.58, 29.12, 29.01, 28.90, 25.83,
Synthesis of Polymer P1
A typical polymerization protocol via Yamamoto coupling16
is shown and discussed in this section. The solution contain-
ing 1,5-cyclooctadiene (COD) (0.16 mL, 1.30 mmol),
Ni(COD)2 (240 mg, 0.87 mmol), and 2,20-bipyridine (160 mg,
1.02 mmol) in 8 mL of anhydrous DMF was stirred at 60ꢀC
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FIGURE 1 Molecular orbital diagrams for the HOMO and LUMO of the models (B3LYP/6-31G(d)//B3LYP/6-31G(d)).
for 30 min under N2, and then the monomer M1 (303 mg,
0.414 mmol) in 4 mL of anhydrous DMF was added. The
reaction was stirred at 80ꢀC for 12 h in the dark. The prod-
ucts were reprecipitated several times with a small amount
of chloroform into 250 mL of methanol. After celite filtration
to remove Ni, the white solid (233 mg, 98%) was precipi-
tated by pouring the chloroform solution of the product into
50 mL of methanol.
22.98, 14.43. IR (KBr, m, cmꢁ1): 1718 (ArACOOAR), 1273
(ArACOOAR).
Synthesis of Polymer P3
The polymer P3 was prepared from the monomer M3 (338
mg, 0.44 mmol) in the presence of Ni(COD)2 (240 mg, 0.87
mmol), COD (0.16 mL, 1.30 mmol), and 2,20-bipyridine
(160 mg, 1.02 mmol) in 96% yield as a pale yellow solid
(256 mg).
1H NMR (CDCl3, ppm) 7.72 (2H), 7.53 (4H), 7.15 (4H), 6.74
(4H), 3.87 (4H), 1.71(4H), 1.26 (20H), 0.86 (6H). 13C NMR
(CDCl3, ppm) 157.89, 152.75, 140.71, 138.72, 137.87,
129.13, 126.60, 124.75, 120.18, 114.14, 67.97, 64.42, 31.85,
29.41, 29.29, 26.14, 22.70, 14.13. IR (KBr, m, cmꢁ1): 1246
(ArAOAR), 1032 (ArAOAR).
1H NMR (CDCl3, ppm) 7.90 (2H), 7.77 (2H), 7.51 (4H), 7.31
(2H), 7.11 (2H), 6.77 (2H), 4.26 (2H), 3.88 (2H), 1.72 (4H),
1.39 (4H), 1.25 (16H), 0.86 (6H). 13C NMR (CDCl3, ppm)
166.30, 159.97, 151.78, 151.32, 140.80, 138.88, 136.63,
129.52, 129.08, 128.03, 126.91, 124.56, 120.46, 114.33,
67.99, 65.10, 65.01, 31.84, 29.39, 29.25, 28.80, 26.12, 26.06,
22.67, 14.11. IR (KBr, m, cmꢁ1): 1719 (ArACOOAR), 1273
(ArACOOAR), 1250 (ArAOAR), 1020 (ArAOAR).
Synthesis of Polymer P2
Similarly to the preparation of P1, polymer P2 was prepared
from the monomer M2 (308 mg, 0.39 mmol) in the presence
of Ni(COD)2 (240 mg, 0.87 mmol), COD (0.16 ml, 1.30
mmol), and bipyridine (160 mg, 1.02 mmol) in 94% yield as
a pale green solid (230 mg).
Synthesis of Polymer PFO
Similarly to the preparation of P1, PFO was prepared from
2,7-dibromo-9,9-dioctylfluorene (226 mg, 0.41 mmol) in the
presence of Ni(COD)2 (240 mg, 0.87 mmol), COD (0.16 ml,
1.30 mmol), and 2,20-bipyridine (160 mg, 1.02 mmol) in
94% yield as a pale green solid (151 mg).
1H NMR (CDCl3, ppm) 7.93 (4H), 7.79 (2H), 7.52 (2H), 7.49
(2H), 7.29 (4H), 4.27 (4H), 1.71(4H), 1.39 (4H), 1.24 (16H),
0.85 (6H). 13C NMR (CDCl3, ppm) 166.44, 151.14, 150.43,
141.23, 139.39, 130.06, 129.72, 128.29, 127.64, 124.83,
121.07, 66.10, 65.45, 32.13, 29.57, 29.52, 29.10, 26.37,
1H NMR (CDCl3, ppm) 7.85 (2H), 7.70 (4H), 2.14 (4H), 1.15
(24H), 0.83 (6H). 13C NMR (CDCl3, ppm) 151.43, 140.16,
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SCHEME 1 Synthetic scheme of the monomers containing cardo structure. Reagents and conditions: (a) MeSO3H, phenol, CCl4,
reflux, 48 h; (b) n-octyl bromide, KOH, MeOH, reflux, 24 h; (c) 1-cyano-4-fluorobenzene, 18-crown-6, K2CO3, DMF, toluene, 140ꢀC,
48 h; (d) KOH, ethanol, water, 80ꢀC, 4 d; (e) (i) SOCl2, reflux, 3 h, (ii) n-octanol, pyridine, THF, room temperature, 3 h; (f) (i) Mg, 4-
bromo-1-benzaldehyde diethyl acetal, THF, reflux, 16 h, (ii) 2 N HCl, room temperature, 1 h; (g) n-octyl phenyl ether, trifluoro-
methanesulfonic acid, 1,4-dioxane, 70ꢀC, 2 h; (h) NaClO2, NaH2PO4, 2-methyl-2-butene, H2O, acetone, room temperature, 1 h; (j) (i)
SOCl2, reflux, 3 h, (ii) n-octanol, pyridine, THF, room temperature, 3 h.
139.66, 125.81, 121.17, 119.59, 55.09, 40.13, 31.54, 29.79,
28.96, 23.72, 22.35, 13.80.
having the single repeat unit linked the fluorene moiety at
both sides. Figure 1 shows lowest unoccupied molecular
orbital (LUMO), highest occupied molecular orbital (HOMO),
and the resulting structures of the models. It was found that
both the HOMOs and the LUMOs in the models were located
on the PF main chain. It should be mentioned that the
HOMOs and LUMOs of models containing the cardo struc-
tures hardly localize on their phenyl groups. There are subtle
differences in the energy gaps (Eg) of the models. From these
data, we assumed that the EDGs and/or EWGs via the cardo
RESULTS AND DISCUSSION
Molecular Orbital Calculations
To estimate the electronic correlation via the cardo structure,
we employed the theoretical calculation for the models of
the polymers using density functional theory at the B3LYP/
6-31G(d)//B3LYP/6-31G(d). The models of the polymers
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SCHEME 2 Synthesis of the polymers. Reagents and conditions: (a) Ni(COD)2, COD, 2,20-bipyridine, DMF, 80ꢀC, 12 h.
carbon could less significantly affect the electronic states of
the PF main chain.
of the monomers. Thus, we concluded that the polymers
should possess the designed chemical structures, and un-
expected conformation or higher-ordered structures were
less obtained in the solutions.
Synthesis of the Monomers
The monomers containing the cardo structures with the dual
Thermal Properties of the Cardo Structure
EDGs,
2,7-dibromo-9,9-bis(4-(octyloxy)phenyl)-9H-fluorene
(M1), with the dual EWGs, dioctyl 4,40-(2,7-dibromo-9H-fluo-
rene-9,9-diyl)dibenzoate (M2), and with the coexisting single
EDG and EWG, octyl 4-(2,7-dibromo-9-(4-(octyloxy)phenyl)-
9H-fluoren-9-yl)benzoate (M3) were synthesized (Scheme 1).
The dibromo fluorene monomers M1 and M2 were prepared
according to the previous reports using 2,7-dibromofluore-
none and 2,7-dibromofluorene as a starting material, respec-
tively. To obtain the monomer M3, benzaldehyde and subse-
quently the 4-octyloxyphenyl group were introduced into
2,7-dibromofluorenone. All monomers were characterized by
1H and 13C NMR spectroscopies and mass measurements.
The dibromo monomers were obtained as colorless oils with
high viscosity without significant fluorescence emissions.
The thermal stabilities of the polymers were investigated
with TGA. The decomposition temperatures with 5% weight
loss are listed in Table 1. The chain scissions in the PF main
chain were observed at 403ꢀC in PFO. The decomposition
was observed at the similar temperature (405ꢀC) from the
polymer P1. These results indicate that the decomposition of
the cardo structures less occurred. The lower decomposition
temperatures of P2 and P3 would be derived from the deg-
radation at the ester groups at the side chains. From these
results, it is presented that the introduction of the cardo
structure can contribute to improve thermal stabilities of the
PF main chains.
Absorption Spectra of the Polymers
The optical properties of the polymers were initially investi-
gated by UV–vis absorption in CHCl3 solution and solid thin
film state as compared to those of PFO (Fig. 2). The results
from the absorption and emission spectra are summarized in
Table 2. The absorption of PFO consists of a p–p* transition
Synthesis of the Polymers
The polymerization with the dibromo monomers via Yama-
moto coupling35 was executed as shown in Scheme 2. The
polymer properties determined from the GPC analysis are
summarized in Table 1. To remove the metal species origi-
nated from the catalyst thoroughly, the celite filtration and
the following reprecipitation from methanol were repeated
at least three times. The polymers P1, P2, P3, and PFO were
obtained as pale green or white solids in good yields (94–
98%). The solubility in conventional organic solvents seemed
to be improved by constructing the cardo structures on the
PF main chains. The number-average molecular weights (Mn)
and the molecular weight distributions (Mw/Mn) of the poly-
mers, measured by the SEC in chloroform toward polysty-
rene standards were from 10,500 to 46,600 with 2.8–8.1,
respectively. These data suggest that the influence of the
polymer ends should be negligible on the optical properties.
In addition, the data for structural analysis by 1H and 13C
NMR and FTIR measurements were corresponded to those
TABLE 1 Polymerization Results
b
b
d
Polymers
Yield (%)a
Mn
Mw/Mn
DPc
T5d
P1
P2
P3
PFO
a
98
94
96
94
46,600
10,500
14,900
18,500
8.1
2.8
3.0
4.2
81
17
25
48
405
364
365
403
Isolated yields after precipitation.
b
c
Estimated by SEC based on polystyrene standards in chloroform.
Degree of polymerization estimated by number-average molecular
weight.
d
TGA: heating rate 10 K/min under N2; values given for weight loss of
5%.
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FIGURE 2 UV–vis spectra of the polymers in CHCl3 (1.0 ꢂ 10ꢁ5
FIGURE 3 Photoluminescence spectra of the polymers in
CHCl3 (1.0 ꢂ 10ꢁ5 M).
M).
that peaks at about 388 nm both in the solution and in the
solid state. Then, the polymers P1, P2, and P3 exhibited the
strong absorption band in the region from 378 to 394 nm
nearby absorption peak of PFO. In addition, the absorption
onsets of the polymers, which are often used to define the
optical gap, are in the range from 414 nm (3.00 eV) to
416 nm (2.99 eV). In particular, the differences in the
HOMO–LUMO gaps of the polymers having EDGs, EWGs, or
both groups were within 0.01 eV. Moreover, similar results
were obtained from the film samples of the cardo-PFs. These
results significantly indicate that the electronic structures of
the PF main chains should be isolated from the influences
from the EDG and the EWG at the cardo sites, and the
energy levels of the PF main chains were maintained at the
ground states.
involving the PFO solution showed similar spectra with the
peaks at 418, 444, and 475 nm assigned as the 0–0, 0–1,
and 0–2 intrachain singlet transition.36 It should be men-
tioned that the red-shift of the peaks or annihilation was
hardly observed in the spectra. The absolute photolumines-
cence quantum efficiencies of the cardo-PFs are large and
the values are >80% (Table 2). These results clearly indicate
that the electronic structures at the excitation states were
less influenced by the introduction of the substituents at the
phenyl groups of the cardo structures. In particular, despite
the electron negativity of the PF main chains,37–39 the bi-sub-
stituted EWGs hardly cause the change in the intrinsic opti-
cal properties of the PF main chains. Moreover, electronic
interaction between the modified phenyl groups via the
cardo carbon should less exist.
Emission Properties of the Polymers
The emission properties of the cardo-PFs were investigated
in the solution states (Fig. 3). The polymers were dissolved
in chloroform, and the photoluminescence spectra were
obtained with the excitation light at 375 nm. All samples
TABLE 2 Photophysical Properties of the Polymers
Solutiona
Filmb
kabs,max kPL,max
kabs,max kPL,max
(nm)c
UPL
(nm)
(nm)c
UPL
d
d
Polymer (nm)
P1
P2
P3
PFO
a
392
378
383
388
418
417
416
416
0.82
0.87
0.90
0.72
394
379
385
388
422
420
422
439
0.22
0.25
0.21
0.24
Evaluated in chloroform (1.0 ꢂ 10ꢁ5 M).
b
c
Evaluated in the solid state; prepared from chloroform solutions.
Excited at 385 nm.
FIGURE 4 Photoluminescence spectra of the polymers in the
d
film states.
Absolute quantum yield.
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the new emission band at 520 nm possessed the longer life
time than that of the singlet transition of the PF main chains
detected at 420 nm (Table 3). These facts indicate that the
thermally induced oxidative degradation in PFO should
occur.13 These results including the TGA experiments can be
summarized that the cardo is responsible for various prefer-
able properties of the PFs as optical materials: The thermal
stabilities against the production of the keto defect and py-
rolysis were enhanced, and the electronic structures, result-
ing in the optical properties, were efficiently preserved.
Electrochemical Properties
CV (Fig. 6) was employed to investigate the redox behaviors
of the polymers and to estimate their HOMO and LUMO
energy levels. The measurements were executed with the
films of the polymers on a glass substrate-coated indium tin
oxide in acetonitrile containing 0.1M lithium perchlorate
(LiClO4) as an electrolyte. Ferrocene was used in this experi-
ment as a reference to calculate the Eox or Ered. The HOMO
and LUMO levels were calculated using Eox (onset) from the
measurements. The values were obtained from the empirical
FIGURE 5 Photoluminescence spectra of the polymers in the
solid state after the thermal treatment at 200ꢀC for 2 h.
relation ELUMO¼ [(Ered ꢁ E1/2(ferrocene)) þ 4.8] eV or EHOMO
¼
[(Eox ꢁ E1/2(ferrocene)) þ 4.8] eV. Table 4 summarizes the
results of the measurements with the polymer films. The
cardo-PFs approximately showed EHOMO values in the range
from ꢁ5.85 to ꢁ5.88 eV, Egap from 3.20 to 3.24 eV, and
ELUMO from ꢁ2.64 to ꢁ2.65 eV. PFO has similar values
(EHOMO ¼ –5.68 eV, Egap ¼ 3.05 eV, and ELUMO ¼ ꢁ2.63 eV,
respectively). These results also indicate that the cardo-PFs
and PFO have almost the HOMO and LUMO at almost same
energy levels. From the optical and electrochemical data, it
can be concluded that the electronic energy levels of the PF
main chains can be highly preserved from the EWG and EDG
of the side chains on the cardo structures.
The preservation effect of the electronic structures of the PF
main chains by the cardo structures was observed in the
film states (Fig. 4). The thin films for the measurements
were prepared by the spin-coating method with the chloro-
form solutions of the polymers on the quartz plate. The pho-
toluminescence spectra were measured with the excitation
light at the same wavelength as the solutions (375 nm). The
shapes of the emission spectra from the cardo-PFs were very
similar not only to those of one another but also to the solu-
tion states. In particular, the significant red-shift and the
broadening of the peaks originated from the excimer forma-
tion between the fluorene units was observed only in the
PFO film.11 These data indicate that the cardo structure
should contribute to the preservation of the electronic states
of the PF main chains in the condensed states from the
interchain interaction as well as from the electronic correla-
tion via the cardo carbons. In addition, we investigated the
optical properties of the films after heating at 200ꢀC for 2 h
to induce the oxidation at the fluorene units (Fig. 5).21–23
Less significant changes from the films of the cardo-PFs
were observed. Only the PFO film after the thermal treat-
ment showed new broad peaks around 520 nm. From the
measurements of the emission decay rates, it was found that
CONCLUSIONS
Three significant issues were obtained from this study. The
synthesis of the cardo-PFs introduced by the EDG and/or the
EWG at the phenyl groups of the cardo structures was estab-
lished. The stepwise syntheses to integrate the heterogene-
ous EDG and EWG pair at the cardo structure were demon-
strated. These synthetic protocols can be applied for
preparing the dual-functional fluorene-based polymers. The
second issue concerns about the influence on the electronic
structures of the cardo-PFs. We clearly indicate that the
TABLE 3 Photoluminescence Lifetimes of the Polymers on Emission Wavelength at 520 nm
Thin Film Statea (% Composition)
v2
Thin Film Stateb (% Composition)
Polymer s1/ns
s2/ns
s3/ns
s1/ns
s2/ns
s3/ns
v2
P1
P2
P3
PFO
a
5.72 (14) 1.68 (56) 0.42 (30) 3.469 7.83 (49) 2.28 (34) 0.46 (17) 1.625
9.79 (41) 2.20 (20) 0.41 (39) 3.011 9.15 (53) 2.55 (32) 0.43 (15) 1.435
7.30 (13) 1.75 (36) 0.42 (51) 3.223 8.26 (29) 1.72 (45) 0.37 (26) 1.267
8.36 (19) 2.11 (33) 0.48 (48) 5.505 5.23 (73) 1.29 (19) 0.21 (8)
1.040
1.0 ꢂ 10ꢁ3 M Chloroform solution spin coated at 1,000 rpm for 10 s on quartz glasses.
b
After annealing at 200ꢀC for 2 h.
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ꢀ
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ꢁ5.85
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ꢁ2.64
ꢁ2.64
ꢁ2.65
ꢁ2.63
3.24
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