Synthesis and characterization of new π-conjugated polymers containing 1,8-naphthyridine in the main chain: Role of the 1,8-naphthyridine unit in π-conjugated polymers

Article abstract of DOI:10.1002/pola.24862

Alternating π-conjugated copolymers of 1,8-naphthyridine-2,6-diyl (1,8-Nap) with 9,9-dioctylfluorene-2,7-diyl (P(Flu-Ph-1,8-Nap)) and 2,5-didodecyloxy-1,4-phenylene (P(ROPh-Ph-1,8-Nap)) have been synthesized by Pd-catalyzed organometallic polycondensation

Full text of DOI:10.1002/pola.24862

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Synthesis and Characterization of New p-Conjugated Polymers
Containing 1,8-Naphthyridine in the Main Chain: Role of the
1,8-Naphthyridine Unit in p-Conjugated Polymers
Take-Aki Koizumi, Shinichirou Kato, Takakazu Yamamoto
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Correspondence to: T. Yamamoto (E-mail: tyamamot@res.titech.ac.jp) or T.-A. Koizumi (E-mail: tkoizumi@res.titch.ac.jp)
Received 20 April 2011; accepted 24 June 2011; published online 9 August 2011
DOI: 10.1002/pola.24862
ABSTRACT: Alternating p-conjugated copolymers of 1,8-naphthyri-
dine-2,6-diyl (1,8-Nap) with 9,9-dioctylfluorene-2,7-diyl (P(Flu-Ph-
1,8-Nap)) and 2,5-didodecyloxy-1,4-phenylene (P(ROPh-Ph-1,8-
Nap)) have been synthesized by Pd-catalyzed organometallic poly-
condensation. The copolymers showed UV-vis absorption peaks at
around 390 nm in o-dichlorobenzene. The polymers were photolu-
minescent both in o-dichlorobenzene and in the solid state. In o-
dichlorobenzene, the emission peaks of P(Flu-Ph-1,8-Nap) and
P(ROPh-Ph-1.,8-Nap) appeared at kEM ¼ 440 and 471 nm, with
quantum yields of 87% and 66%, respectively. Electrochemical
data revealed that 1,8-Nap behaved as a typical electron-accepting
unit. When P(Flu-Ph-1,8-Nap) was treated with 10-camphorsulfonic
C
V
acid, the emission peak shifted to kEM ¼ 598 nm. 2011 Wiley Peri-
odicals, Inc. J Polym Sci Part A: Polym Chem 49: 4204–4212, 2011
KEYWORDS: p-conjugated polymer; heteroatom-containing poly-
mers; luminescence; 1,8-naphthyridine; optical properties; poly-
aromatics; synthesis
INTRODUCTION Much attention has been paid to p-conjugated
polymers because of their interesting electronic and optical
functionalities.1–3 Nitrogen-containing six-membered heterocy-
clic units such as pyridine-2,5-diyl and pyrimidine-2,5-diyl units
shown in Chart 1 behave as electron acceptors when they have
electron-withdrawing imine AC¼¼N nitrogen functionalities.
1,8-Naphthyridine has two N atoms to bind protons and
metal ions,5 and it forms various adducts by hydrogen bond-
ing.6 Consequently, synthesis of p-conjugated polymers con-
sisting of 1,8-naphthyridine-based units is of interest. How-
ever, there have been only a few reports of the synthesis of
such polymers. Although Caluwe7 reported an alternating co-
polymer of 1,8-naphthrydine-2,7-diyl and pyridine-2,6-diyl
units, the chemical properties of the polymer were not
reported.
In this article, we present the synthesis and chemical prop-
erties of new alternating p-conjugated copolymers of 1,8-
Nap.
CHART 1 Examples of electron-accepting heterocyclic units.1(c)
EXPERIMENTAL
Many reports have been published about p-conjugated homo-
and copolymers consisting of such N-containing heterocyclic
units.3 1,8-Naphthyridine has a pyridine-fused structure, and
its LUMO level (ꢀ2.43 eV) is positioned lower than those of
pyridine (ꢀ1.26 eV) and quinoline (ꢀ2.21 eV),4 suggesting that
a 1,8-naphthyridine-based unit (e.g., 1,8-naphthyridine-2,6-diyl,
1,8-Nap) behaves as a strong electron accepting unit.
Materials and Measurements
1H NMR and IR spectra were recorded on a JEOL EX-400
spectrometer and a JASCO IR 810 spectrophotometer, respec-
tively. UV-vis and photoluminescence (PL) spectra were
measured with a Shimadzu UV-3100 spectrophotometer and
a Hitachi F-4010 spectrophotometer, respectively. Thermal
analysis was performed with a Shimadzu TA-50 WS thermal
analyzer equipped with a Shimadzu DSC-50 differential scan-
ning calorimeter and a Shimadzu TGA-50 thermogravimetric
analyzer. Gel permeation chromatography (GPC) was carried
out with a Shimadzu liquid chromatography system with a
Shodex 80M column and a 6A refractive index detector; the
eluent was CHCl3. Electrochemical measurements were per-
formed with a Hokuto Denko HSV-100 automatic polarization
C
V
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system. A conventional three-electrode configuration was
used, with a glassy carbon working (BAS) electrode, a plati-
num wire auxiliary electrode (Tokuriki, special order), and a
0.10 M AgNO3/Ag reference (BAS RE-5). Cyclic voltammo-
grams were recorded at a scan rate of 50 mV sꢀ1. The fol-
lowing relations were established: Fcþ/Fc ¼ þ40 mV versus
0.1 M AgNO3/Ag and Fcþ/Fc ¼ þ529 mV versus SCE.
[Pd(PPh3)4]8 and monomer 39 were prepared in accordance
with the literature, and other reagents including monomer 2
were purchased from Tokyo Chemical Industry.
lected by filtration, washed with distilled water and MeOH,
and dried in vacuo.
Yield: 310 mg (96%). Anal. Calcd for (C43H48N2)n: C, 87.11%;
H, 8.16%; N, 4.23%. Found: C, 83.36%; H, 7.82%; Br, 1.17%;
N, 4.17% (cf. Calcd for Br(C43H48N2ꢂH2O)20C14H8N2Br: C,
83.44%; H, 8.08%; Br, 1.27%; N, 4.68%).
The discrepancy between the calculated and found values may
be partly due to the high thermal stability of the polymer.
IR (KBr, cmꢀ1): 2924, 2851, 1597, 1561, 1466, 1260, 1013,
808. 1H NMR (400 MHz, CDCl3): d 9.53 (br, 1H, Nap-H),
8.50–8.44 (br, 3H, PhAH and Nap-H), 8.15 (br, 1H, Nap-H),
7.90–7.28 (br, 9H, PhAH and Nap-H), 2.15 (br, 4H, CH2),
1.37–1.12 (br, 20H, CH2), 0.88–0.78 (br, 10H, bCH2 and
CH3).
Synthesis of Monomers
2.2.1. Synthesis of 2-amino-5-bromo-3-pyridinecarbalde-
hyde (preparation of the precursor of monomer 1)
To a dry THF (30 mL) solution of 2-amino-3-pyridinecarbal-
dehyde10 (2.40 g, 19.7 mmol) N-bromosuccinimide (3.50 g,
19.7 mmol) was added at 0 ꢁC in the dark. After 12 h, the
reaction was quenched with water (10 mL), and the product
was extracted with ethyl acetate (10 mL, 5 times). The or-
ganic layer was dried over Na2SO4, and the solvent was
removed by evaporation. Purification by column chromatog-
raphy on neutral Al2O3 (eluent: ethyl acetate) gave 2-amino-
5-bromo-3-pyridinecarbaldehyde as a yellow powder.
P(ROPh-Ph-1,8-Nap)
P(ROPh-Ph-1,8-Nap) was prepared analogously using 1,4-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,5-di(dodecylox-
y)benzene 3 (384 mg, 0.550 mmol) and 1 (200 mg, 0.550 mmol).
A yellow solid was obtained. Yield: 150 mg (95%). Anal. Calcd for
Br(C44H60N2O2)7-C14H8N2Br : C, 78.80%; H, 8.81%; Br, 3.26%; N,
4.57%; Mn (number average molecular weight) ¼ 4908. Found: C,
76.03%; H, 8.23%; Br, 3.45%; N, 4.37%; Mn ¼ 4800.
Yield: 3.11 g (78%). Anal. Calcd for C6H5BrN2O: C, 35.85%;
H, 2.51%; N, 13.94%; O, 7.96%. Found: C, 35.62%; H, 2.51%;
N, 13.88%, O, 8.22%. IR (KBr, cmꢀ1): 2924, 2851, 1597,
1561, 1466, 1260, 1013, 808. 1H NMR (300 MHz, CDCl3): d
9.80 (s, 1H, CHO), 8.29 (d, 1H, pyridine-H), 7.90 (d, 1H, pyri-
dine-H), 6.70 (br, 2H, NH2).
IR (KBr, cmꢀ1): 2922, 2853, 1599, 1474, 1419, 1207,
1
1016,810. H NMR (300 MHz, CDCl3): d 9.42 (br, 1H, Nap-H),
7.90–7.70 (br, 6H, PhAH and Nap-H), 7.54 (br, 1H, Nap-H),
7.06 (br, 2H, RO-PhAH), 4.05 (br, 4H, OCH2), 1.24 (br, 40H,
CH2), 0.87 (br, 6H, CH3).
RESULTS AND DISCUSSION
2.2.2. Synthesis of 2-(4-bromophenyl)-6-bromo-1,8-naph-
thyridine (1)
Monomer and Polymer Synthesis.
To a dry EtOH (15 mL) solution of 2-amino-5-bromo-3-pyri-
dinecarbaldehyde (300 mg, 1.49 mmol) and 4-bromoaceto-
phenone (297 mg, 1.49 mmol) KOH (46 mg, 0.82 mmol) was
added, and the solution was refluxed under N2. After 5 min,
the color turned orange and a yellow solid precipitated. After
5 h, the yellow solid was collected by filtration, washed with
EtOH and hexane, and dried in vacuo. 2-(4-Bromophenyl)-6-
Monomer 1 was prepared by the reaction of 2-amino-5-
bromo-3-pyridinecarbaldehyde with 4-bromoacetophenone
in the presence of KOH as shown in eq 1.
bromo-1,8-naphthyridine (1) was obtained as
a yellow
powder.
Yield: 288 mg (53%). Anal. Calcd for C14H8Br2N2: C, 46.19%;
H, 2.22%; N, 7.70%. Found: C, 46.01%; H, 2.04%; N, 7.62%.
IR (KBr, cmꢀ1): 1603, 1588, 1474, 1304, 1074, 1007, 897,
804. 1H NMR (400 MHz, CDCl3): d 9.13 (d, 1H, 1,8-Nap-H),
8.36 (d, 1H, 1,8-Nap-H), 8.22–8.17 (m, 3H, 1,8-Nap-H þ Ph-
H), 8.00 (d, 1H, 1,8-Nap-H), 7.67 (d, 2H, PhAH).
The following two comonomers were selected as polycon-
densation partners of 1.
Synthesis of Polymers
P(Flu-Ph-1,8-Nap)
To a dry toluene (6 mL) solution containing 1 (200 mg,
0.550 mmol) and 9,9-dioctylfluorene-2,7-bis(trimethylenebo-
rate) 2 (307 mg, 0.550 mmol), a N2-bubbled aqueous solu-
tion of K2CO3 (2 M, 2 mL), several drops of Aliquat 336, and
[Pd(PPh3)4] (0.020 g, 0.017 mmol) were added. After the
mixture was stirred at 80 ꢁC for 48 h under N2, it was
poured into excess MeOH. The yellow precipitate was col-
Organometallic polycondensation of 1 with 2 and 3 under
Suzuki-Miyaura cross-coupling conditions gave the 1,8-Nap-
containing p-conjugated polymers. P(Flu-Ph-1,8-Nap) was
obtained in 96% yield as a yellow solid. Poly(fluorene)s and
fluorene-copolymers are the subject of recent interest
because of their excellent light emitting properties.11
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FIGURE 1 IR spectra of 1, 2, 3, P(Flu-Ph-1,8-Nap) and P(ROPh-
Ph-1,8-Nap).
P(ROPh-Ph-1,8-Nap) was obtained in 95% yield as a yellow
powder (eq 3). These polymers were partly soluble in or-
ganic solvents such as CHCl3, THF, toluene, and trifluoroace-
tic acid at room temperature, and P(ROPh-Ph-1,8-Nap) was
soluble in hot o-dichlorobenzene and 1,2,4-trichlorobenzene.
FIGURE 2 1H NMR spectra of (a) P(Flu-Ph-1,8-Nap) and (b)
P(ROPh-Ph-1,8-Nap) in CDCl3.Peaks with * are due to solvent
impurities (CHCl3 and H2O).
elemental analysis suggested that this polymer was partly
hydrated due to the affinity of 1,8-Nap for water.
Although the degree of polymerization of the polymers was
not large, chemical properties of the polymers revealed the
role of 1,8-Nap in the p-conjugated polymers. Figure 1
shows IR spectra of P(Flu-Ph-1,8-Nap), P(ROPh-Ph-1,8-
Nap), and their starting monomers. The IR spectra of the
Solutions of the polymer displayed strong blue emission
when irradiated with UV light.
GPC analysis of P(ROPh-Ph-1,8-Nap) using 1,2,4-trichloro-
benzene at 130 ꢁC showed Mn of 4800 (vs. polystyrene
standards) with Mw/Mn (Mw ¼ weight average molecular
weight) of 2.60. The Mn corresponded to a degree of poly-
merization of about 7, and the observed Br content of
P(ROPh-Ph-1,8-Nap) suggested that both polymer ends con-
tained a ACABr group (cf. Experimental section). Because
P(Flu-Ph-1,8-Nap) was only partly soluble in organic sol-
vents, GPC data for its THF-soluble part were obtained. The
Mn of the THF soluble part was 3300 with an Mw/Mn value
of 2.58. If P(Flu-Ph-1,8-Nap) has Br at both ends of the
polymer similar to P(ROPh-Ph-1,8-Nap), the Br content
(1.17%; cf. Experimental section) of the original polymer
corresponds to a degree of polymerization of 20. Data from
SCHEME 1 Possible microstructures of P(Ar-Ph-1,8-Nap).
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FIGURE 3 Top: UV-vis spectra of (a) monomer 1, (b) P(Flu-Ph-
1,8-Nap), and (c) P(ROPh-Ph-1,8-Nap) in o-dichlorobenzene at
room temperature. Once the polymers were dissolved in hot
o-dichlorobenzene, cooling the solution to room temperature
did not cause precipitation of the polymer for 24 h. Bottom:
UV-vis spectra of cast films.
FIGURE 4 Top: PL spectra of (a) P(Flu-Ph-1,8-Nap) and (b)
P(ROPh-Ph-1,8-Nap) in o-dichlorobenzene at room temperature.
Bottom: PL spectra of cast films.
UV-vis and PL Data
Figures 3 and 4 show UV-vis and PL spectra of P(Flu-Ph-
1,8-Nap) and P(ROPh-Ph-1,8-Nap), respectively, and optical
data for the polymers are summarized in Table 1. The UV-vis
peaks of P(Flu-Ph-1,8-Nap) and P(ROPh-Ph-1,8-Nap) in
o-dichlorobenzene appear at kmax ¼ 399 nm and 392 nm,
respectively. The UV-vis peak is shifted by about 60 nm to a
longer wavelength from that of monomer 1 because of
expansion of the p-conjugated system, and the kmax values
polymers show the m(C¼¼N) peak of the 1,8-Nap ring at
about 1600 cmꢀ1. Figure 2 shows 1H NMR spectra of the
polymers, and the 1H NMR peaks of P(Flu-Ph-1,8-Nap) and
P(ROPh-Ph-1,8-Nap) appear at expected positions with
appropriate areas. The characteristic peak of 7-H of 1,8-
Nap is observed at about d 9.5 for both polymers. In the
case of P(Flu-Ph-1,8-Nap), the 1H NMR signal of the
bACH2 group of the octyl side chain in the fluorene unit
appears at a high magnetic field of about d 0.8, and the
peak overlaps with the signal of the terminal CH3 group of
the octyl chain, similar to reported examples.12 Because
monomer 1 has an asymmetric structure, the polymers are
considered to have both head-to-tail and head-to-head
microstructures, as shown in Scheme 1. However, the 1,8-
Nap signals of the polymers appear as slightly broadened
peaks, and determining the microstructure from 1H NMR
spectra was difficult.
TABLE 1 UV-vis and PL Data of P(Flu-Ph-1,8-Nap) and
P(ROPh-Ph-1,8-Nap) in o-Dichlorobenzene and Cast Film
a
k
max/nm
konset/nm
kEM/nm
UF
P(Flu-Ph-1,8-Nap)
Solution
Film
399
407
445
444
440
479
0.87
P(ROPh-Ph-1,8-Nap)
Solution
392
393
472
478
0.66
Film
P(Flu-Ph-1,8-Nap) and P(ROPh-Ph-1,8-Nap) had high ther-
mal stability, and gave 5% weight loss temperatures (T5%) at
430 and 340 C, respectively.
a
The photoluminescence quantum efficiency UF of the polymer in an
o-dichlorobenzene solution was measured using quinine sulfate (ca. 1ꢃ
10ꢀ5 M solution in 0.1 M H2SO4 with UF of 0.55) as a standard.
ꢁ
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tion of aggregates in the solid phase where intermolecular
electronic interaction takes place.
P(Flu-Ph-1,8-Nap) and P(ROPh-Ph-1,8-Nap) show PL
peaks at kEM ¼ 440 nm and 472 nm, respectively, in
o-dichlorobenzene, and the kEM positions agree with the
onset positions of the UV-vis absorption bands, as is usually
observed with p-conjugated compounds and polymers. The
PL spectrum of P(Flu-Ph-1,8-Nap) has a shoulder at about
475 nm, which is thought to arise from vibronic coupling.
P(Flu-Ph-1,8-Nap) and P(ROPh-Ph-1,8-Nap) gave high
quantum yields of 87% and 66%, respectively, in o-dichloro-
benzene; the quantum yield of P(Flu-Ph-1,8-Nap) is compa-
rable to those of reported fluorene-based p-conjugated
polymers.13
A cast film of P(Flu-Ph-1,8-Nap) shows a shift of the PL
peak by about 40 nm to a longer wavelength from that in
the solution. The shift is not large, and the emission is con-
sidered to occur from a single molecule in the solid state.
Formation of an excimer or an excimer-like adduct by inter-
molecular electronic interaction usually gives a larger shift,14
and the small shift agrees with a strong retardation effect on
the intermolecular electronic interaction caused by the two
octyl groups in the fluorene unit. The cast film of P(ROPh-
Ph-1,8-Nap) also shows a minor shift in kEM as shown in
Figure 4, although the kEM values of arylene-dialkoxypheny-
lene copolymers in the solid state often show a large shift
(e.g., by ca. 100 nm) to a larger wavelength from that in the
solution due to the formation of an excimer-like adduct.15
FIGURE 5 Cyclic voltammograms of (a) P(Flu-Ph-1,8-Nap) film
and (b) P(ROPh-Ph-1,8-Nap) film under N2.
Electrochemical Response
Electrochemical properties of the polymers were investigated
by cyclic voltammetry (CV) using cast films on a Pt plate
electrode. Figure 5 shows the CV curves of cast films of
P(Flu-Ph-1,8-Nap) and P(ROPh-Ph-1,8-Nap), and electro-
chemical data are summarized in Table 2. The P(Flu-Ph-1,8-
Nap) film displays one irreversible oxidation peak at þ1.64 V
versus Agþ/Ag and one quasi-reversible redox couple at
E1/2 ¼ ꢀ1.98 V versus Agþ/Ag. During the electrochemical
process, the color of the film changes as shown in Figure 5.
Yellow P(Flu-Ph-1,8-Nap) film turns red in the reduced
state (ca. –2.2 V) and oxidized state (ca. þ1.2 V). The color
of the P(ROPh-Ph-1,8-Nap) film turns from yellow to or-
ange in both the oxidized and reduced states.
are comparable with that of poly(p-phenylene) (PPP; kmax
¼
ca. 380 nm).1 Cast films of P(Flu-Ph-1,8-Nap) and P(ROPh-
Ph-1,8-Nap) show kmax at longer wavelengths, 407 nm and
393 nm, respectively. However, the shift from solutions to
films is not large, suggesting the absence of a strong elec-
tronic interaction between the polymer molecules even in
the solid phase. Powder X-ray diffraction (XRD) patterns of
the polymers indicated that they were amorphous in the
solid state. However, tailing of the UV-vis absorption bands
of P(Flu-Ph-1,8-Nap) and P(ROPh-Ph-1,8-Nap) films in the
longer wavelength region (cf. Fig. 3) implies partial forma-
TABLE 2 Electrochemical Data for P(Flu-Ph-1,8-Nap) and P(ROPh-Ph-1,8-Nap)
E/V (vs. Agþ/Ag)
n-doping/
p-doping/
onset
pc
onset
pa
Polymer
n-dedoping
p-dedoping
E
E
E
HOMO/eV
ELUMO/eV
Eg,e/eV
P(Flu-Ph-1,8-Nap)a
P(ROPh-Ph-1,8-Nap)a
ꢀ2.07/ꢀ1.88
Nd
þ1.64/–
þ1.18/–
ꢀ1.80
ꢀ1.90
þ1.00
þ0.81
ꢀ5.76
ꢀ5.57
ꢀ2.96
ꢀ2.86
2.80
2.71
a
The cast film was prepared from an o-dichlorobenzene solution of the polymer on an Pt plate. Conditions: Electrolyte: [n-Bu4N]BF4 0.10 M; Working
1
electrode: Pt plate; Counter electrode: Pt wire. Sweep rate: 50 mV sꢀ .
onset
E
E
E
: Onset potential of the oxidation peak.
: Onset potential of the reduction peak.
pa
onset
pc
g,e: Band gap determined from the cyclic voltammogram of the polymer film.
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TABLE 3 Electrochemical Data (vs. Ag1/Ag) of Fluorene Copolymers -(Ar-Flu)n-(Flu 5 9,9-dialkylfluorene-2,7-diyl)
onset
onset
onset onset
No.
1
AArA
Epa
Epc
ꢀ2.07
E
pa-Epc
Ref.
E
E
E
pa
-Epc
Ref.
pa
pc
1.64
3.71
This work
1.00
ꢀ1.80
2.80
This work
2
3
1.73
0.93
ꢀ2.77
ꢀ2.73
4.20
3.66
[16a]
[16a]
1.07
0.76
ꢀ2.12
ꢀ1.89
3.19
2.65
[16e]
[16e]
4
5
0.88
0.83
ꢀ2.39
ꢀ2.01
3.27
2.84
[16c]
[16c]
0.78
0.76
ꢀ2.08
ꢀ1.59
2.86
2.35
[16c]
[16c]
6
7
1.47
1.31
about ꢀ2.5
ꢀ2.36
about 4
3.67
[15b]
[15a]
0.82
ꢀ2.29
3.11
[15b]
8
1.19
ꢀ2.16
3.35
[16d]
Table 3 shows a comparison of electrochemical data of
P(Flu-Ph-1,8-Nap) with other alternating fluorene-based
pꢀconjugated copolymers containing electron withdrawing
units in the main chain.15,16 For application of fluorene-
based pꢀconjugated copolymers in electronic and optical
devices such as polymer light emitting diodes (PLED), effects
of the comonomer unit on the electronic structure of the co-
polymer are important.
strong electron-withdrawing ability, and P(Flu-Ph-1,8-Nap)
accepts electron easily.
onset
pa
onset
pc
onset
pa
It is reported that E
the anodic peak; E
–E
(E
¼ onset potential for
onset
¼ onset potential for the cathodic
pc
peak) corresponds to the band gap of p-conjugated poly-
mers.17 An E
–E
onset
pa
onset
value of P(Flu-Ph-1,8-Nap) was esti-
pc
mated at 2.80 eV (or 22,600 cmꢀ1 and 442 nm), which
roughly agrees with the band gap of P(Flu-Ph-1,8-Nap)
estimated from the onset position of the UV-vis absorption
band of P(Flu-Ph-1,8-Nap) in film (18,100 cmꢀ1 and
2.53 eV).
As shown in Table 3, the peak current cathodic potential,
Epc, of P(Flu-Ph-1,8-Nap) film (No. 1 in Table 3) is located
at a more positive potential (ꢀ2.07 V vs. Agþ/Ag) than those
of fluorene-quinoxaline (ꢀ2.39 V, No. 4) and fluorene-phe-
nanthroline (ꢀ2.36 V, No. 7) copolymers, in which quinoxa-
line and phenanthroline units are typical electron-accepting
units. These data indicate that the Ph-1,8-Nap unit has a
onset
From the onset reduction potential (E
¼ ꢀ1.80 V), the
red
LUMO level of P(Flu-Ph-1,8-Nap) was calculated as –2.96
eV using the following empirical relationships:18,19
TABLE 4 UV-vis and PL Data of P(Flu-Ph-1,8-Nap) in Acidic
Media
UV-vis
max/nm
543
PL
Acid (pKa)a
k
k
EM/nm
k
EX/nm UF
b
Methanesulfonic
acid (1.6)
–
–
Quenched
0.31
10-camphorsulfonic 470
acidc
598
–
460
–
Trifluoroacetic
acid (3.45)
472
Quenched
a
In DMSO.
b
The UF of the polymer in a chloroform solution was measured using
quinine sulfate (ca. 1 ꢃ 10ꢀ5 M solution in 0.1 M H2SO4, assuming UF of
0.55) as a standard.
FIGURE 6 UV-vis absorption spectra of P(Flu-Ph-1,8-Nap) in (a)
CHCl3, (b) methanesulfonic acid (MSA), (c) CSA (10-camphorsul-
fonic acid)-saturated CHCl3, and (d) trifluoroacetic acid (TFA).
c
Solvent ¼ CSA-saturated CHCl3.
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SCHEME 2 Intramolecular donor-acceptor charge transfer.
FIGURE 9 PL spectra of P(Flu-Ph-1,8-Nap) (a) in CHCl3, (b) after
addition of 30 equiv of CSA, and (c) after addition of 30 equiv
of DBU to the acidic solution.
SCHEME 3 Plausible protonation forms of P(Flu-Ph-1,8-Nap) in
onset
the presence of CSA, TFA and MSA.
HOMO ¼ ꢀðE
þ 4:76Þ eV
þ 4:76Þ eV
pa
onset
pc
LUMO ¼ ꢀðE
where E
and E
are the potentials versus Agþ/Ag.
onset
pa
onset
pc
HOMO levels of P(Flu-Ph-1,8-Nap) and P(ROPh-Ph-1,8-
Nap) were estimated to be ꢀ5.76 and ꢀ5.57 eV, and
LUMO levels of the polymers were estimated as ꢀ2.96
and ꢀ2.86 eV, respectively. The band gap estimated by the
electrochemical measurement of P(Flu-Ph-1,8-Nap) is
2.80 eV. On the other hand, the band gaps estimated by
the optical measurement of P(Flu-Ph-1,8-Nap) and P(ROPh-
Ph-1,8-Nap) are 2.24 and 2.50 eV, respectively. They are
smaller than the electrochemically measured band gaps. This
may be due to the interfacial energy barrier for charge injec-
tion to the polymer film in the electrochemical process.18
FIGURE 7 PL spectra of P(Flu-Ph-1,8-Nap) in (a) CHCl3, (b)
MSA, (c) CSA-saturated CHCl3, and (d) TFA.
Response to Protic Acids
N-Containing heterocycles in p-conjugated polymers are of-
ten protonated, and the protonated polymers show optical
and electrochemical properties different from those of the
original polymers.15(a),20–22 Recently, Hancock and Jenekhe
have reported tuning the luminescent color of a quinoline-
containing compound by treatment with camphorsulfonic
acid (CSA).20
When a protic acid (HX) was added to an o-dichlorobenzene
solution of P(Flu-Ph-1,8-Nap), the solution color changed,
suggesting the formation of a naphthyridinium unit in the
polymer, as shown in eq 4.
FIGURE 8 UV-vis absorption spectra of P(Flu-Ph-1,8-Nap) (a) in
CHCl3, (b) after addition of 30 equiv of CSA, and (c) after addi-
tion of 30 equiv of DBU to the acidic solution.
SCHEME 4 Protonation of naphthyridine.
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CONCLUSION
Two new 1,8-naphthyridine unit-containing p-conjugated
polymers have been synthesized using the Suzuki cross-cou-
pling reaction and characterized. Monomer 1 serves as a
good starting material to introduce the 1,8-Nap unit into p-
conjugated polymers. The polymers obtained were soluble in
organic solvents and showed high thermal stability. UV-vis
and XRD data indicate that the polymers are amorphous in
the solid state. The polymers show strong PL; in particular,
P(Flu-Ph-1,8-Nap) gives a quantum yield of 87%, which is
comparable to or larger than those of other fluorene poly-
mers. The polymers are electrochemically active both in the
oxidation and reduction regions and exhibit electrochromism.
The 1,8-Nap unit in the polymers is protonated by acids. Af-
ter the addition of acids, the values of kmax in UV-vis spectra
are shifted to longer wavelengths by 60–160 nm. P(Flu-Ph-
1,8-Nap) showed a new emission peak at kEM ¼ 598 nm
with a shift of 158 nm to a longer wavelength in a camphor-
sulfonic acid-saturated CHCl3 solution, and the shift was re-
versible upon addition of a base.
Figure 6 shows electronic spectra of P(Flu-Ph-1,8-Nap) in
CHCl3, methanesulfonic acid (MSA), 10-camphorsulfonic acid
(CSA)-saturated CHCl3, and trifluoroacetic acid (TFA). Optical
data of the polymers in protic acid media are summarized in
Table 4.
In the CSA-CHCl3 solution, the color of P(Flu-Ph-1,8-Nap)
turned from yellow to orange. On the other hand, the MSA
and TFA solutions were purple and red, respectively. The
kmax was shifted to longer wavelengths by about 140 nm (in
MSA) or by about 70 nm (in CSA-CHCl3 and TFA) from that
of P(Flu-Ph-1,8-Nap). The protonation of 1,8-Nap (eq 4)
increases the electron accepting ability of the unit, resulting
in easier intramolecular donor-to-acceptor charge transfer
(CT) (Scheme 2).
Such a phenomenon is observed in pyridine and quinoline-
containing oligomers and polymers.15(a),20–23 The larger shift
to longer wavelengths observed in MSA may be due to multi-
site protonation of 1,8-Nap caused by strong acidity and
high concentration of MSA, as shown in Scheme 3.
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