Synthesis and fluorescent properties of conjugated copolymers containing maleimide and fluorene units at the main chain

Article abstract of DOI:10.1002/pola.26924

Yamamoto or Suzuki-Miyaura coupling polymerizations of 2,3-diiodo-N- cyclohexylmaleimide with fluorene derivatives (2,7-dibromo-9,9′- dihexylfluorene and 9,9′-dihexylfluorene-2,7-diboronic acid) were carried out. The number-average molecular weights (M_n) of the resulting copolymers were 2600-3500 by gel permeation chromatography analysis. The fluorescence emission of the alternating copolymer showed the emission maxima at 551 nm in THF. On the other hand, the random copolymers showed the bimodal emission peaks at 418-420 and 555-557 nm region, respectively. The fluorescence peaks of the random copolymers on the long wavelength region (555-557 nm) were attributed to the conjugated neighboring N-cyclohexylmaleimide-9,9′- dihexylfluorene units in the polymer main chain. Furthermore, the copolymers exhibited the fluorescence solvatochromism by the difference of the polarity of solvents. The alternating and random copolymers showed the different fluorescence solvatochromism, and the emission colors are distinguishable by the naked eye, respectively. Copyright

Full text of DOI:10.1002/pola.26924

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Synthesis and Fluorescent Properties of Conjugated Copolymers
Containing Maleimide and Fluorene Units at the Main Chain
Munetoshi Nakamura, Kazuhiro Yamabuki, Tsutomu Oishi, Kenjiro Onimura
Department of Applied Chemistry, Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai,
Ube, Yamaguchi, 755-8611, Japan
Correspondence to: K. Onimura (E-mail: onimura@yamaguchi-u.ac.jp)
Received 23 May 2013; accepted 11 August 2013; published online 18 September 2013
DOI: 10.1002/pola.26924
ABSTRACT: Yamamoto or Suzuki–Miyaura coupling polymeri-
zations of 2,3-diiodo-N-cyclohexylmaleimide with fluorene
in the polymer main chain. Furthermore, the copolymers exhib-
ited the fluorescence solvatochromism by the difference of the
polarity of solvents. The alternating and random copolymers
showed the different fluorescence solvatochromism, and the
emission colors are distinguishable by the naked eye, respec-
0
0
derivatives (2,7-dibromo-9,9 -dihexylfluorene and 9,9 -dihexyl-
fluorene-2,7-diboronic acid) were carried out. The number-
n
average molecular weights (M ) of the resulting copolymers
were 2600–3500 by gel permeation chromatography analysis.
The fluorescence emission of the alternating copolymer
showed the emission maxima at 551 nm in THF. On the other
hand, the random copolymers showed the bimodal emission
peaks at 418–420 and 555–557 nm region, respectively. The flu-
orescence peaks of the random copolymers on the long wave-
length region (555–557 nm) were attributed to the conjugated
tively. V
2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
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Polym. Chem. 2013, 51, 4945–4956
KEYWORDS: conjugated polymers; dyes/pigments; fluorene; flu-
orescence property; luminescence; N-substituted maleimide;
solvatochromism; Suzuki–Miyaura coupling polymerization;
Yamamoto coupling polymerization
0
neighboring N-cyclohexylmaleimide-9,9 -dihexylfluorene units
INTRODUCTION Organic p-conjugated materials, especially
polymers, are able to control not only the low cost and com-
patibility but also intrinsic property such as electronic, opti-
cal, conductivity, and stability by the structure design of the
starting material. Thus, they find a variety of advanced tech-
nological applications in the fields of light-emitting diodes
Furthermore, fluorene copolymers are designed by the alter-
nating or random polymerization. The random copolymer
can induce various feed ratios of an acceptor unit over a
wide range in comparison with the alternating one. There-
fore, the design of the random copolymer is able to change
3
5,36
the band gap of fluorene copolymer.
Also, the result that
1
–10
11–15
16–18
the introduction of an acceptor unit can lead to improve the
electronic transport property of the random copolymer, and
the copolymer is suitable for applications in multilayer
(
PLEDs),
photovoltaics,
transistors,
and molecu-
1
9–21
lar electronics.
Among many promising conjugated poly-
mers, polyfluorene, and its derivatives have been widely
studied as emitting materials of organic electronics such as
PLEDs because they show blue emission around at 420 nm,
3
3
PLEDs, is also reported.
On the other hand, maleimide and its homopolymer are con-
sidered to be cis-olefin and consist of a repetition unit of the
cis configuration like polyacetylene known as materials of
the organic electronic device. Maleimide resembles n-type
organic semiconductor materials such as phthalimide, naph-
2
2–29
high PL, EL properties, and good thermal stability.
Recently, various polyfluorene derivatives containing the n-
type semiconductor units such as benzothiadiazole, pyridine,
naphthoselenadiazole, quinoxaline, and oxadiazole are
3
0–34
reported.
On the basis of the idea of energy transfer
thylimide, and pyreneimide for an electron-deficient hetero-
from a electron donor to an acceptor molecule, copolymers
consist of fluorene and n-type units show green, yellow, and
even red emission compared with the blue emission poly-
37–41
cyclic ring.
For example, Cola and coworkers reported
fluorescence sensors of the zinc ion by the maleimide deri-
4
2
vative. Furthermore, Chen and coworkers reported alter-
nating copolymers obtained from N-substituted
,3-diphenylmaleimide and boronate derivative of fluorene
or bithiophene. The provided copolymer showed EL
3
2,35
fluorene.
In this way, the emission property of fluorene
copolymer changes from blue to red color by the introduc-
tion of n-type unit.
2
Additional Supporting Information may be found in the online version of this article.
2013 Wiley Periodicals, Inc.
V
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4
3
emissions of orange–red in the thin film. This was the first
report that maleimide copolymers showed PL and EL spectra
at long wavelength and that maleimide comonomers were an
important roles as an accepter molecule such as benzosele-
nadiazole and naphthoselenadiazole. Some articles relevant
to the polymer of N-substituted-2,3-diarylmaleimide are
reported, but there are few reports of copolymer that malei-
Materials
All chemicals were used without any further purification.
Fluorene, triisopropyl borate, and 1,5-cyclooctadiene (COD)
were purchased from TCI. Potassium iodide and sodium
iodide were available from Ishizu Seiyaku Lto. N-Bromosucci-
nimide was purchased from Wako Pure Chemical Industries.
Sodium thiosulfate 5-hydrate, potassium carbonate, propyle-
necarbonate, and 2,2 -bipyridyl (a,a -) were available from
Kishida Reagents Chemicals. Bromine, potassium hydroxide,
n-hexyl bromide, tetrakis(triphenylphosphine) palladium (0),
4
3–46
0
0
mide is connected directly.
In this article, the authors report Yamamoto and Suzuki–
Miyaura cross-coupling polymerization of 2,3-diiodo-N-cyclo-
hexylmaleimide (DICHMI) as an acceptor comonomer with
fluorene derivatives (2,7-dibromo-9,9-dihexylfluorene or 9,9-
dihexylfluorene-2,7-diboronic acid) as a donor comonomer,
and examine polymerization properties and fluorescence
properties of the copolymers obtained.
and anhydrous magnesium sulfate (MgSO ) were purchased
4
from Nakalai Tesque. Triethylamine, n-butyllithium (n-BuLi),
and bis(1,5-cyclooctadiene) nickel (0) were available from
Kanto Chemical. N-Cyclohexylmaleimide was refined and
used a thing offered by NOF Corporation. n-Hexane, ethyl
acetate, THF, N,N-dimethylformamide (DMF), dimethyl sulfox-
ide (DMSO), dichloromethane, and chloroform were dried
according to the standard procedure and distilled under
nitrogen. Analytical thin-layer chromatography was per-
formed on Merck silica gel plate 60F254. Column chromatog-
raphy was performed with silica gel 60 (0.063–0.200 mm,
Merck).
EXPERIMENTAL
Measurements
H (500 MHz) and C (125 MHz) nuclear magnetic reso-
1
13
nance (NMR) spectra were recorded on a JNM-LA500 (JEOL)
1
spectrometer using tetramethylsilane (TMS) ( H NMR, d
4
7,48
0
49,50
1
3
DICHMI,
9
2,7-dibromo-9,9 -dihexylfluorene (DBrDHF),
0
.00) or CDCl ( C NMR, d 77.0) as internal reference peaks
3
51
0
,9 -dihexylfluorene-2,7-diboronic acid (DHFDBa),
bis(9,9-dihexylfluorene-3-yl)-N-cyclohexylmaleimide,
each copolymers
ture method.
2,3-
and
at room temperature at the Collaborative Center for Engi-
neering Research Equipment, Faculty of Engineering in
Yamaguchi University. Splitting patterns are designated as s
5
2,53
3
5,54
were prepared by following the litera-
(
singlet), d (doublet), t (triplet), q (quartet), m (multiplet),
and br (broad). Number-average-molecular (M ) and molecu-
lar weight distributions (M /M ) of polymers were obtained
n
Synthesis of 2,3-Diiodo-N-cyclohexylmaleimide
w
n
A solution of DBrCHMI (5.06 g, 15.0 mmol) and sodium
iodide (6.75 g, 45.0 mmol) in acetic acid (55 mL) was heated
with gel permeation chromatography (GPC) on a CROMATO-
PAC C-R7Ae plus (LC-10AS, CTO-2A, SPD-10A) equipped
with polystyrene gel columns (HSG-40G, HSG-20G, HSG-15G,
and HSG-10G), using tetrahydrofuran (THF) as an eluent at a
48
at reflux for 2 h. After that, removal of the solvent gave
the crude product, which was purified by recrystallization
from ethyl acetate/n-hexane (1/10 5 v/v) to obtain a yellow
crystal (5.53 g, 12.8 mmol).
2
1
flow rate of 1.0 mL min , calibrated by polystyrene stand-
ꢀ
ards at 50 C. The ultraviolet–visible (UV–vis) spectra were
ꢀ
C. Rf 5 0.17 (n-hexane/
recorded on a UV-1650PC (Shimadzu Corporation) spectrom-
eter. The photoluminescence (PL) spectra were recorded on
a FP-6300 (JASCO Corporation) spectrophotometer.
Yield 86%. m.p. 156–157
1
dichloromethane 5 1/2). H NMR (CDCl ) d (ppm from
3
TMS): 1.10–1.42 (3H, m, cyclohexyl), 1.60–1.74 (3H, m,
CHART 1 Yamamoto coupling polymerization of DICHMI with fluorene derivatives.
4
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cyclohexyl), 1.76–1.92 (2H, m, cyclohexyl), 1.94–2.14 (2H, m,
cyclohexyl), 3.90–4.10 (1H, m, @CHA).
product was dissolved in a small amount of THF, and pre-
cipitated into 125 mL of methanol. The product was col-
lected as a white powder and dried in vacuo.
Synthesis of 2,3-Bis(9,9-dihexylfluorene-3-yl)-N-
cyclohexylmaleimide (Model Compound)
A mixture of DICHMI (0.173 g, 0.400 mmol), 9,9-dihexyl-
Poly(CHMI-co-DHF) by Yamamoto Coupling Reaction
0
COD (0.132 g, 1.22 mmol), 2,2 -bipyridyl (0.189 g, 1.21
5
2
fluorene-2-boronic acid
Pd(PPh3)4 (0.0192 g, 0.0166 mmol) as catalyst was stirred
in dry toluene (6 mL) under N . Then, K CO (0.222 g, 1.61
The reaction mix-
(0.303 g, 0.800 mmol), and
mmol), Ni(COD) (0.332 g, 1.21 mmol), and dry DMF (3 mL)
2
were placed in a 50-mL schlenk flask. The mixture was
ꢀ
2
2
2,53
3
stirred at 60 C for 0.5 h under a nitrogen atmosphere, and
5
mmol) was added in water (0.8 mL).
then a mixture of DICHMI (0.174 g, 0.405 mmol), 2,7-
DBrDHF (0.197 g, 0.401 mmol) in dry DMF (3 mL) was
ꢀ
ture was heated to 80 C for 72 h. The reaction mixture was
poured into dichloromethane (50 mL), and the combined
organic layer was washed with water. The organic layer was
ꢀ
added. The reaction was maintained at 60 C for 24–48 h.
The resultant mixture was poured into the dichloromethane
(50 mL), and washed with 1N HCl and water. The organic
layer dried over MgSO4 and the solvent evaporated. The
crude product was dissolved in a small amount of THF, and
precipitated into 125 mL of methanol. The product was col-
lected as a white powder and dried in vacuo.
dried over MgSO and the solvent was evaporated. The crude
4
product was purified on a silica gel column using n-hexane/
dichloromethane (3/1 5 v/v). The solution was concentrated,
and dissolved in a small amount of THF, and precipitated
into 125 mL of methanol. An orange powder was collected
and dried in vacuo. An orange powder was obtained (0.209
g, 0.250 mmol).
Poly(CHMI-alt-DHF) by Suzuki–Miyaura Coupling Reaction
A mixture of DICHMI (0.173 g, 0.400 mmol), DHFDBa (0.170
ꢀ
Yield 62%. m.p. 118–120
C. Rf 5 0.12 (n-hexane/
) d (ppm from
TMS): 0.46–0.68 (8H, m, hexyl), 0.76 (12H, t, hexyl), 0.91–
.17 (27H, m, cyclohexyl and hexyl), 1.22–1.45 (3H, m, cyclo-
hexyl), 1.75–1.94 (12H, m cyclohexyl and hexyl), 2.16–2.29
2H, m, cyclohexyl), 4.02–4.16 (1H, m, @CHA), 7.31 (6H, d,
aromatic protons), 7.40–7.47 (4H, m, aromatic protons), 7.58
2H, d, aromatic protons), 7.62–7.68 (2H, m, aromatic pro-
g, 0.402 mmol), Pd(PPh ) catalyst (0.0185 g, 0.0160 mmol),
3 4
1
dichloromethane 5 3/1). H NMR (CDCl
3
and dry toluene (6 mL) was placed in a 50-mL schlenk flask.
Then, K CO3 (0.225 g, 1.63 mmol) in water (1.6 mL) was
2
ꢀ
1
added via syringe. The mixture was stirred at 90 C for 48 h
under N . The reaction mixture was poured into dichlorome-
2
(
thane (50 mL) and washed with water. The organic layer
was dried over MgSO and the solvent was evaporated. The
4
(
crude product was dissolved in a small amount of THF, and
precipitated into 125 mL of methanol. The product was col-
lected as a brown powder and dried in vacuo.
1
3
tons). C (CDCl ) d (ppm from TMS): 171.0, 151.2, 150.7,
3
1
1
1
42,5, 140.3, 135.2, 129.0, 127.7, 126.9, 124.6, 122.9, 120.1,
19.6, 55.1, 51.2, 40.3, 31.6, 30.2, 29.7, 26.1, 25.2, 23.8, 22.6,
4.0.
Poly(CHMI-co-DHF) by Suzuki–Miyaura Coupling Reaction
A mixture of DICHMI (0.0874 g, 0.203 mmol), DBrDHF
Synthesis of Polymers
(0.0992 g, 0.201 mmol), DHFDBa (0.170 g, 0.402 mmol), and
Poly(CHMI)
Pd(PPh ) (0.0187 g, 0.0162 mmol) dry toluene (6 mL) was
placed in a 50-mL schlenk flask. Then, K CO (0.221 g, 1.60
2 3
3
4
0
COD (0.132 g, 1.22 mmol), 2,2 -bipyridyl (0.188 g, 1.20
mmol), Ni(COD) (0.331 g, 1.20 mmol), and dry DMF (3 mL)
were placed in a 50-mL schlenk flask.
stirred at 60 C for 0.5 h under a nitrogen atmosphere, and
then DICHMI (0.174 g, 0.803 mmol) in dry DMF (3 mL) was
added. The reaction was maintained at 60 C for 48 h. The
resultant mixture was poured into the dichloromethane (50
mL), and washed with 1N HCl and water. The organic layer
dried over MgSO4 and the solvent evaporated. The crude
product was dissolved in a small amount of THF, and pre-
cipitated into 125 mL of methanol. The product was col-
lected as a black powder and dried in vacuo.
2
mmol) in water (1.6 mL) was added via syringe. The mixture
3
5,54
ꢀ
The mixture was
was stirred at 90 C for 48 h under N . The resultant mix-
2
ꢀ
ture was poured into dichloromethane (50 mL), and washed
with water. The organic layer was dried over MgSO and the
4
ꢀ
solvent was evaporated. The crude product was dissolved in
a small amount of THF, and precipitated into 125 mL of
methanol. The product was collected as a brown powder
and dried in vacuo.
RESULT AND DISCUSSION
Synthesis and Characterization
Poly(DHF)
Scheme 1 illustrates the synthetic route for maleimide mono-
mers and fluorene derivatives. In the synthesis of DICHMI as
the key monomer, 2,3-dibromo-N-cyclohexylmaleimide
(DBrCHMI) was prepared by bromination of N-cyclohexylma-
0
COD (0.132 g, 1.22 mmol), 2,2 -bipyridyl (0.187 g, 1.20
mmol), Ni(COD) (0.331 g, 1.20 mmol), and dry DMF (3 mL)
2
were placed in 50-mL schlenk flask. The mixture was stirred
ꢀ
47
at 60 C for 0.5 h under a nitrogen atmosphere, and then
leimide (CHMI) according to the literature. The iodination
DBrDHF (0.397 g, 0.806 mmol) in dry DMF (3 mL) was
added. The reaction was maintained at 60 C for 48 h. The
resultant mixture was poured into the dichloromethane (50
mL), and washed with 1N HCl and water. The organic layer
dried over MgSO4 and the solvent evaporated. The crude
of DBrCHMI with sodium iodide in acetic acid afforded
DICHMI in good yield (86%). 2,7-Dibromo-9,9-dihexylfluor-
ene (DBrDHF) was synthesized from fluorene by bromina-
tion with bromine in CHCl3 and alkylation with KOH in
DMSO. 9,9-Dihexylfluorene-2,7-diboronic acid (DHFBa) was
ꢀ
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SCHEME 1 Synthetic route of monomers.
synthesized via lithiation of DBrDHF using n-BuLi, and then
boronation by triiropropyl borate in THF.
carried out in the presence of comonomers of DICHMI and
DHFDBa (monomer feed ratio was 0.40/0.40 [mmol/mmol]),
tetrakis(triphenylphosphine)palladium as
a catalyst, and
In order to reveal relationship between properties of fluores-
cence and structures of resulting copolymers, the authors
prepared 2,3-bis(9,9-dihexylfluorene-3-yl)-N-cyclohexylmalei-
mide as a model compound of conjugated copolymers con-
taining a CHMI and a DHF unit. The synthetic approach to a
model compound is presented in Scheme 2. The model com-
pound was synthesized in four steps from fluorene according
potassium carbonate as a base in a mixed solution of toluene
and water. In addition, in Suzuki–Miyaura coupling, the
copolymerization was also carried out using DICHMI,
DBrDHF, and DHFDBa (DICHMI/DBrDHF/DHFDBa 5 0.20/
0.20/0.40 [mmol/mmol/mmol]). In these polymerizations,
the random copolymers were obtained along with the result-
ing copolymer by Yamamoto coupling reaction. After the end
of polymerization, metal catalysts were removed by washing
the reaction mixture with 1N HCl. The organic phase was
evaporated in vacuo. The residue was purified by reprecipita-
tion method with excess MeOH. The results of the copoly-
merization of DICHMI with fluorene derivatives are
summarized in Table 1. Copolymers were white or brown
powders, which were quite soluble in common organic sol-
5
2,53
to the literature procedures.
A monobromination of fluo-
rene was carried out by NBS in propylenecarbonate (PC) to
give 2-bromofluorene. 2-Bromo-9,9-dihexylfluorene and 9,9-
dihexylfluorene-2-boronic acid were synthesized from 2-
bromofluorene described above. Finally, Suzuki–Miyaura cou-
pling reaction of DICHMI and 9,9-dihexylfluorene-2-boronic
acid catalyzed by Pd(PPh
model compound.
3 4
) in toluene produced a desired
3
vents such as THF, CHCl , and DMF. Interestingly, these
The copolymers were prepared from DICHMI, DBrDHF, and
DHFDBa by Yamamoto or Suzuki–Miyaura coupling polymer-
ization. In Yamamoto coupling reaction, the copolymerization
was carried out in the presence of each comonomer of
DICHMI and DBrDHF (monomer feed ratio was 0.40/0.40
copolymers showed solubility in n-hexane. This solubility in
n-hexane was due to the presence of the long alkyl chains of
fluorene units. The ratios of a CHMI unit in copolymers
1
(MIcont.) were determined by H NMR and were shown in
1
Table 1. H NMR spectra of poly(CHMI-co-DHF) (Table 1,
[
2
mmol/mmol]), bis(cyclooctadiene)nickel as a catalyst, and
,2 -bipyridyl and 1,5-COD as a ligand in DMF. On the other
Run 4) and poly(CHMI-alt-DHF) were shown in Figure 1. A
broad peak in the aromatic region at 7.3–8.0 ppm was attrib-
uted to protons on the fluorene ring. A signal at 3.9–4.2 ppm
0
hand, in Suzuki–Miyaura coupling, the copolymerization was
4
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SCHEME 2 Synthesis of model compound.
was assigned to a proton at the 1-position of the cyclohexyl
substituent. Peaks at 0.5–2.3 ppm were due to methylene
protons of the cyclohexyl ring and n-hexyl on the side chain
in a fluorene unit. In the same manner, the contents (mol %)
of CHMI units in other poly(CHMI-co-DHF)s were also deter-
son with ones by Yamamoto coupling reaction (Table 1, Runs
5 and 6). The authors were reported that the low polymeri-
zablites of 2,3-dihalido-N-substituted maleimide were
responsible for side reactions such as a dehalogenation at
4
7(b)
the terminal end.
The homopolymerizations and copoly-
1
merizations of DICHMI proceeded by side reactions such as
a dehalogenation in this article.
mined in Figure 1(a). H NMR spectrum of poly(CHMI-alt-
1
3
DHF) was shown in Figure 1(b). In addition,
C NMR
spectra of copolymers (Table 1, Runs 3 and 5) were shown
in Figures S1 and S2.
Optical Properties of Copolymers
The normalized absorption (UV–vis) and PL spectra of
copolymers and a model compound were measured in a THF
solution (concentration based on monomeric unit: 2.5 3
The homopolymer of DICHMI was obtained in low yield and
number average molecular weight compared with poly(DHF)
25
21
10
mol L ), and illustrated in Figure 2. The photophysical
(
Table 1, Runs 1, 2). In the copolymerization of DICHMI with
properties of copolymers in a dilute solution are summarized
in Table 2.
DBrDHF, the number average molecular weights (M s)
n
increased with increasing polymerization times (Table 1,
Runs 3 and 4). But MIcont. and yield slightly decreased with
increasing polymerization times. The copolymers by Suzuki–
Miyaura coupling reaction were obtained in good yield (70–
The homopolymer of DICHMI showed absorption peaks at
*
299 nm and 496 nm, which were attributed to the p–p tran-
*
sition of enone groups and the p–p transition of the conju-
7
8%) and confirmed a small improvement of M in compari-
gated main chain, respectively [Fig. 2(a), Run 2]. The
n
TABLE 1 Copolymerization of DICHMI with Fluorene Derivatives
a
b
c
23
c
Run
Time (h)
Yield (%)
MIcont. (mol %)
M
n
3 10
w n
M /M
d
1
2
3
4
5
6
Poly(CHMI)
48
48
24
48
48
48
13
87
60
52
78
70
100
0
1.0
1.1
e
Poly(DHF)
17.0, 4.1
2.6
1.3, 1.1
1.6
f
Poly(CHMI-co-DHF)
Poly(CHMI-co-DHF)
44
40
50
24
f
3.0
1.4
g
Poly(CHMI-alt-DHF)
3.5
1.4
h
Poly(CHMI-co-DHF)
2.6
1.4
a
f
MeOH insoluble part.
Yamamoto coupling polymerization of DICHMI and DBrDHF.
b
c
1
g
h
By H NMR.
By GPC with poly(styrene) standard.
Suzuki–Miyaura coupling polymerization of DICHMI and DHFDBa.
Suzuki–Miyaura coupling polymerization of DICHMI, DBrDHF, and
d
e
Yamamoto coupling polymerization of DICHMI.
Yamamoto coupling polymerization of DBrDHF.
DHFDBa.
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1
3
FIGURE 1 H NMR spectra of (a) poly(CHMI-co-DHF) (Table 1, Run 4) and (b) poly(CHMI-alt-DHF) (Table 1, Run 5) in CDCl .
*
homopolymer of DBrDHF showed absorption peaks at 339
and 397 nm attributed to the p–p transition of aromatic
rings of fluorene and conjugated chain [Fig. 2(a), Run 3]. The
model compound showed broad peaks at 317 and 411 nm
transition of carbonyl groups and the p–p transition of
*
aromatic rings of fluorene at around 303 nm [Fig. 2(a), Runs
4 and 5]. Further, the copolymers showed the maximum and
shoulder peaks at around 364 and 440 nm, respectively. On
the other hand, in the copolymers by Suzuki–Miyaura cou-
pling reaction, the alternating copolymer showed a spectrum
[
Fig. 2(a), Run 1]. The copolymers by Yamamoto coupling
*
reaction exhibited shoulder peaks attributed to the n–p
FIGURE 2 Normalized absorption (a) and emission (b) spectra of poly(CHMI) (Run 2), poly(DHF) (Run 3), and copolymers (Runs
25
21
4
–7) and model compound (Run 1 in Table 2) in THF (concentration; 2.5 3 10 mol L base on monomeric units). [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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a
TABLE 2 UV–Vis Absorption and PL Emission Spectral Data for the Copolymers and Model Compound in THF
b
c
d
e
f
Run
Time (h)
MIcont. (mol %)
k
abs. (nm)
E.W. (nm)
k
Em. (nm)
1
2
3
4
5
6
7
Model compound
–
–
317, 411
299, 496
339, 397
302, 365
304, 363
321, 440
338, 398, 433
411
299
397
365
363
440
398
548
g
h
Poly(CHMI)
48
48
24
48
48
48
100
0
nd
i
Poly(DHF)
419
j
Poly(CHMI-co-DHF)
Poly(CHMI-co-DHF)
44
40
50
24
418, 556
418, 557
551
j
k
Poly(CHMI-alt-DHF)
l
Poly(CHMI-co-DHF)
420, 555
a
25
21
h
2
.5 3 10 mol L based on monomeric units in THF.
nd, not determined.
Yamamoto coupling polymerization of DBrDHF.
b
c
d
e
f
i
Time 5 polymerization time.
By determined by H NMR.
1
j
Yamamoto coupling polymerization of DICHMI and DBrDHF.
k
l
k
abs. 5 absorption peaks.
Suzuki–Miyaura coupling polymerization of DICHMI and DHFDBa.
Suzuki–Miyaura coupling polymerization of DICHMI, DBrDHF, and
E.W. 5 excitation wavelength.
Em. 5 emission peaks.
Yamamoto coupling polymerization of DICHMI.
k
DHFDBa.
g
pattern similar to the model compound [Fig. 2(a), Runs 1
and 6]. But, the absorption peak on the part of the long
wavelength showed a red shift about 30 nm in comparison
with the model compound. This red shift was thought to be
due to the extension of conjugated chains by the polymeriza-
tion. The random copolymer by Suzuki–Miyaura coupling
reaction showed absorption peaks at 338 and 398 nm, and a
shoulder peak was observed at 433nm [Fig. 2(a), Run 7].
From the above UV–vis spectra, the alternating copolymer
showed the recognizable absorption peak attributed to the
conjugated main chain on the long wavelength region. Mean-
while, the random copolymers showed a shoulder peak on
the long wavelength region. A coplanarity of the conjugated
main chain was collapsed by a twist with the presence of a
continuous DHF unit in the random copolymers. Thus, the
effective conjugation length of random copolymers was
shorter than alternating one, and the absorption peak on the
long wavelength region was shoulder.
mer of DBrDHF is considered to be attributed an intramolec-
ular donor/acceptor influence in the presence of an
N-cyclohexylmaleimide-9,9 -dihexylfluorene (CHMI-DHF) unit.
0
On the other hand, the random copolymers by Yamamoto cou-
pling reaction showed the bimodal emission peaks at the posi-
tion each similar to the homopolymer of DBrDHF and the
model compound. The change of emission spectra was not
seen by difference of polymerization time [Fig. 2(b), Runs 5
and 6]. In addition, the random copolymer by Suzuki–Miyaura
coupling reaction showed the modest reduction of the emis-
sion peak on the part of the short wavelength in comparison
with one by Yamamoto coupling reaction. Judging from the
above results, it was thought that emission peaks of the ran-
dom copolymers on the short wavelength region were attrib-
uted to the fluorene unit and the others were attributed to
CHMI-DHF unit in the main chain.
Fluorescence Solvatocromism of the Copolymers in
Various Solvent
In PL spectra, the homopolymer of DICHMI showed no emis-
sion property. The homopolymer of DBrDHF showed an
emission peak at 419 nm [Fig. 2(b), Run 3]. The model com-
pound and the alternating copolymer exhibited emission
peaks at around 548–551 nm [Fig. 2(b), Runs 1 and 7]. This
red shift of about 120 nm in comparison with the homopoly-
As described above, the resulting copolymers showed high
solubility in n-hexane along with THF and DMF. Furthermore,
emission colors of these copolymers were different in vari-
ous organic solvent. The UV–vis and PL spectra of the alter-
nating copolymer and the model compound in n-hexane,
a
TABLE 3 UV–Vis Absorption and PL Emission Spectral Data for Poly(CHMI-alt-DHF) and Model Compound in Various Solvent
b
c
d
Run
Solv.
k
abs. (nm)
E.W. (nm)
kEm. (nm)
1
2
3
4
5
6
Model compound
Model compound
Model compound
Poly(CHMI-alt-DHF)
Poly(CHMI-alt-DHF)
Poly(CHMI-alt-DHF)
n-Hexane
THF
315, 416
317, 411
313, 414
308, 434
321, 452
308, 433
416
411
414
434
452
433
540
548
553
545
551
559
DMF
n-Hexane
THF
DMF
a
25
21
c
2
.5 3 10 mol L based on monomeric units in various solvent.
E.W. 5 excitation wavelength.
b
d
k
abs. 5 absorption peaks.
kEm. 5 emission peaks.
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emission spectra, it seemed that emission properties of the
alternating copolymer and model compound were dependent
on the polarity of solvents.
The UV–vis and emission spectra of the random copolymer
and model compound in n-hexane, THF, and DMF are showed
in Figures 4 and 5, and the measurement results are sum-
marized in Table 4. The photographs of the solution of the
random copolymers in various solvents under a UV light are
shown in Figure 6.
Spectral patterns of the absorbance peaks of the random
copolymers by Yamamoto coupling reaction did not change
by changing the measurement solvent [Fig. 4(a), Runs 4–9].
However, the PL spectra of random copolymers showed the
significant change. The random copolymers showed bimodal
emission peaks in THF, but the peaks on the long wavelength
region remarkably increased in n-hexane and decreased in
DMF [Fig. 4(b), Runs 4–9]. Along with a change of this spec-
tral pattern, the emission colors observed by the naked eye
were very different, and the copolymers showed yellow in n-
hexane, orange in THF and blue in DMF, respectively (Fig. 6,
left). In this bimodal emission peaks, one on the short wave-
length region was attributed to fluorene units and the other
on the long wavelength region was attributed to CHMI-DHF
units in the main chain from the above. In emission peaks,
kinds of solvent gave the strongest influence to the peak on
the long wavelength region. Thus, the authors clarified that
CHMI-DHF units in the main chain affected some sorts of sol-
vent effect.
FIGURE 3 Photographs of poly(CHMI-alt-DHF) (top) and model
compound (bottom) under the visible light (left) and the UV
25
light (right) in various solvent (concentration; 2.5 3 10
mol
2
1
L
base on monomeric units).
THF, and DMF are measured, and the measurement results
are summarized in Table 3.
In UV–vis spectra, the model compound exhibited similar spec-
tra in various kinds of solvent (Table 3, Runs 1–3). On the
other hand, the alternating copolymer showed the change of
the absorption peak attributed to the conjugated main chain
on the long wavelength region (Table 3, Runs 4–6). The peaks
were observed to the red shift of about 20 nm and increasing
of peak intensity in comparison with n-hexane and DMF when
the measurement solvent was THF (Table 3, Run 5). In PL
spectra, the emission peaks of the alternating copolymer and
the model compound showed a red shift with increasing the
polarity of solvents such as THF and DMF. The photographs of
the solution of the alternating copolymer and model com-
pound in various solvents under a visible light and a UV light
are shown in Figure 3. The emission colors of the alternating
copolymer and model compound observed by the naked eye
showed the bathochromic shift. From the photographs and the
The random copolymers by Suzuki–Miyaura coupling reac-
tion were affected by kinds of solvent on UV–vis spectra. The
absorption peaks attributed to the conjugated main chain at
around 422–433 nm decreased with increasing the polarity
of solvent [Fig. 5(a), Runs 10–12]. Meanwhile, an emission
peak on the long wavelength region increased in nonpolar
solvent and decreased in polar solvent along with the ran-
dom copolymers by Yamamoto coupling reaction [Fig. 5(b),
Runs 10–12]. However, the peak intensity on the short
FIGURE 4 Normalized absorption (a) and emission (b) spectra of poly(CHMI-co-DHF) by Yamamoto coupling reaction (Runs 4–9)
25
21
and model compound (Runs 1–3 in Table 4) in various solvent (concentration; 2.5 3 10
Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
mol L
base on monomeric units).
[
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FIGURE 5 Normalized absorption (a) and emission (b) spectra of poly(CHMI-co-DHF) by Suzuki–Miyaura coupling reaction (Runs
2
5
21
10–12) and model compound (Runs 1–3 in Table 4) in various solvent (concentration; 2.5 3 10
mol L
base on monomeric
units, inset, expanded emission spectra of Runs 10, 11). [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
5
5,56
wavelength for the other on the long wavelength region was
smaller than the random copolymers by Yamamoto coupling
reaction. By the slight difference, the emission colors
observed by the naked eye significantly differed. The random
copolymer showed white in n-hexane and magenta in THF in
contrast to that one by Yamamoto coupling reaction exhib-
ited yellow in n-hexane and orange in THF (Fig. 6, right).
enhancement molecular.
To demonstrate this hypothesis, a
PL spectrum in regard to the concentration change was meas-
ured in various solvents. The PL spectra of random copoly-
mers by Yamamoto reaction (Table 2, Run 4) are shown in
Figure 7. The measurements were carried out by the concen-
25
27
tration of the polymer change from 2.5 3 10 to 2.5 3 10
2
1
mol L . An emission peak intensity on the long wavelength
region decreased with decreasing of the polymer concentra-
tion in n-hexane [Fig. 7(a)]. On the other hand, a peak intensity
on the short wavelength region significantly increased in THF
While the alternating copolymer showed the bathochromic
shift with increasing the polarity of solvent, an emission peak
intensity of the random copolymers on the long wavelength
region changed. The authors proposed the mechanism of this
fluorescence solvatochromism of the random copolymers. At
the first setout, the emissions of CHMI-DHF units in the polar
solvent were probably quenched by the occurring rotation of
the conjugated chains like an aggregation-induced emission
[
Fig. 7(b)]. The peak intensity on the short wavelength region
increased even in DMF [Fig. 7(c)]. These increases of the peak
intensity on the part of the short wavelength with the concen-
tration reduction were suggested to the moderation of concen-
tration quenching. In addition, the peak intensity on the short
wavelength region increased when the polymer concentration
a
TABLE 4 UV–Vis Absorption and PL Emission Spectral Data for Poly(CHMI-co-DHF) and Model Compound in Various Solvent
b
c
d
Run
Reaction Name
Time (h)
Solv.
k
abs. (nm)
E.W. (nm)
kEm. (nm)
1
2
3
4
5
6
7
8
9
Model compound
Model compound
Model compound
Yamamoto coupling
–
n-Hexane
THF
348, 416
416
411
414
363
365
365
363
363
364
398
398
396
540
–
317, 411
548
–
DMF
313, 414
553
24
24
24
48
48
48
48
48
48
n-Hexane
THF
303, 363
412, 546
418, 556
421, 549
411, 546
418, 557
421, 552
545
302, 365
DMF
302, 365
n-Hexane
THF
303, 363
304, 363
DMF
309, 364
1
1
1
0
1
2
Suzuki–Miyaura coupling
n-Hexane
THF
337, 398, 422
338, 398, 433
337, 396, 431
420, 555
441, 555
DMF
a
25
21
c
2
k
.5 3 10 mol L based on monomeric units in various solvent.
E.W. 5 excitation wavelength.
kEm. 5 emission peaks.
b
d
abs. 5 absorption peaks.
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FIGURE 6 Photographs of poly(CHMI-co-DHF) by Yamamoto
coupling reaction (left) and Suzuki–Miyaura coupling (right)
under the UV light in various solvent (concentration; 2.5 3
25
21
10
mol L base on monomeric units).
FIGURE 8 Emission spectra of poly(CHMI-co-DHF) by Suzuki–
2
7
was 2.5 3 10 M, but the intensity in regard to the long
wavelength region decreased. The PL spectra of the random
copolymer by Suzuki–Miyaura reaction (Table 2, Run 7) are
shown in Figure 8. The copolymer showed the decreasing of
the peak intensity on the long wavelength region and the
increasing of the other on the short wavelength region with
concentration reduction.
Miyaura coupling reaction (Run 7 in Table 2) by concentration
25
change in n-hexane (concentration; from 2.5 3 10
to 2.5 3
27
21
1
0
mol L base on monomeric units). [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
intensity on the long wavelength region attributed to CHMI-
DHF would increase by the intermolecular aggregation. But
the peak intensity was decreased and shown red shifts with
increasing of MeOH. From these measurement results, it was
indicated that the fluorescence solvatochromism was strongly
attributed to not only the intermolecular aggregation but also
To additionally investigate the effect of Aggregation-Induced
Emission Enhancement, PL spectra of a THF solution of
copolymers adding MeOH as poor solvent (Table 2, Run 5)
were measured (in Fig. 9). The authors expected that the peak
FIGURE 7 Emission spectra of poly(CHMI-co-DHF) by Yamamoto coupling reaction (Run 4 in Table 2) by concentration change in
25
27
21
n-hexane (a), THF (b), and DMF (c) (concentration; from 2.5 3 10 to 2.5 3 10 mol L base on monomeric units). [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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excited state. This is why the authors could obtain the
copolymers which showed various fluorescences by change
of the polarity of the solvent.
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