[2.2]paracyclophane-based through-space conjugated polymers with fluorescence quenchers

Article abstract of DOI:10.1002/pola.26382

New [2.2]paracyclophane-based through-space conjugated polymers containing fluorescence quenchers such as anthraquinone and ferrocene units at the polymer termini were designed and synthesized. Their optical properties were investigated in detail. Fluorescence emission from the stacked π-electron systems was effectively quenched by the stacked π-electron systems at the polymer termini due to the energy and electron transfer through a single polymer chain; thus, the polymers acted as the molecular wire.

Full text of DOI:10.1002/pola.26382

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[2.2]Paracyclophane-Based Through-Space Conjugated Polymers
with Fluorescence Quenchers
Yasuhiro Morisaki, Shizue Ueno, Yoshiki Chujo
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
Correspondence to: Y. Morisaki (E-mail: ymo@chujo.synchem.kyoto-u.ac.jp) or Y. Chujo (E-mail: chujo@chujo.synchem.kyoto-u.ac.jp)
Received 31 July 2012; accepted 8 September 2012; published online 9 October 2012
DOI: 10.1002/pola.26382
ABSTRACT: New
[2.2]paracyclophane-based
through-space
and electron transfer through a single polymer chain; thus,
C
V
conjugated polymers containing fluorescence quenchers such
as anthraquinone and ferrocene units at the polymer termini
were designed and synthesized. Their optical properties were
investigated in detail. Fluorescence emission from the stacked
p-electron systems was effectively quenched by the stacked
p-electron systems at the polymer termini due to the energy
the polymers acted as the molecular wire.
2012 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 51: 334–339,
2013
KEYWORDS: conjugated polymers; copolymerization; fluores-
cence; molecular wire; [2.2]paracyclophane
INTRODUCTION Since the discovery of conductive polyacety-
lenes three decades ago,1–3 there has been considerable
interest in the design and synthesis of new conjugated
polymers. Further, various p-electron systems such as
poly(p-arylene-ethynylene),4,5 poly(p-arylenevinylene),6,7 and
poly(p-arylene)8,9 have been synthesized.10–12 Although most
of such polymers comprise a through-bond conjugation sys-
tem composed of sp or sp2 carbon frameworks, there are
very few studies on through-space conjugated polymers with
stacked p-electron systems.
tems and the absorption of the end-capping anthracene moi-
ety.24 In this article, we report the synthesis and properties
of [2.2]paracyclophane-based through-space conjugated poly-
mers with fluorescence quenchers at the polymer termini;
they are responsible for photo-excited energy transfer from
the staked p-electron systems to the stacked terminal units.
These polymers give rise to a new class of single molecular
wires comprising stacked p-electron systems, in which
energy and/or charge transfer occurs via through-space
interactions among the stacked p-electron systems.
Introduction of a p-stacked structure into the conjugated
polymer backbone would allow for effective energy and
charge transfer in a single polymer chain. As a representative
example, Nakano et al. reported the synthesis of poly(diben-
zofulvene)s consisting of p-stacked fluorene rings13–15 that
showed higher hole mobility than the through-bond conju-
gated polymers.15 Recently, we have focused on the
p-stacked structure of [2.2]paracyclophane16–18 and synthe-
sized through-space conjugated polymers by incorporating
[2.2]paracyclophane, which has a p-stacked structure, into
the conjugated polymer backbone.19–29 Because of through-
space conjugation, the conjugation length of the resulting
polymers increased with an increase in the number of
stacked p-electron systems.24 In addition, end-capping of the
through-space conjugated polymers by anthracene-containing
p-electron systems allowed for fluorescence resonance
energy transfer from the stacked p-electron systems to the
end-capping p-electron systems, because of the efficient
overlap between the emission of the layered p-electron sys-
EXPERIMENTAL
General
1H and 13C NMR spectra were recorded on a JEOL JNM-
EX400 instrument at 400 and 100 MHz, respectively. The
chemical shift values were expressed relative to Me4Si as an
internal standard. High-resolution mass spectra (HRMS)
were obtained on a JEOL JMS-SX102A spectrometer. Analyti-
cal thin-layer chromatography was performed with silica gel
60 Merck F254 plates. Column chromatography was per-
formed with Wakogel C-300 silica gel. Gel permeation chro-
matography (GPC) was carried out on a TOSOH 8020
(TSKgel G3000HXL column) instrument using CHCl3 as an
eluent after calibration with standard polystyrene samples.
Recyclable preparative high-performance liquid chromatogra-
phy was performed on a Japan Analytical Industry LC-9204
(JAIGEL-2.5H and 3H columns) using CHCl3 as an eluent.
UV–vis absorption spectra were obtained on a SHIMADZU
UV3600 spectrophotometer. Photoluminescence spectra were
Additional Supporting information may be found in the online version of this article.
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SCHEME 1 Synthesis of polymers P1–P3.
obtained on a Horiba FluoroMax-4 luminescence spectrome-
ter. Elemental analyses were performed at the Microanalyti-
cal Center of Kyoto University.
23.1, 26.6, 30 (m), 32.3, 34.4 (m), 69.8, 89.9, 95.5, 114.3,
116.6, 125.2, 133.5, 137.5, 139.9, 142.5, 153.9 ppm.
Polymer P3: 1H NMR (CDCl3, 400 MHz): d 0.88 (t, J ¼ 6.4
Hz), 1.26 (br), 1.59 (br), 1.87 (m), 1.96 (br), 2.33 (s), 3.05
(s), 2.94 (br), 3.05 (br), 3.35 (br), 3.82 (br), 4.14 (m), 6.53
(m), 6.65 (br), 7.05–7.15 (m), 7.36 (br) ppm; 13C NMR
(CDCl3, 100 MHz): d 14.1, 20.3, 20.8, 22.7, 26.2, 26.3, 29.5
(m), 32.0, 33.8, 34.1, 69.4, 69.5, 89.6, 91.3, 95.2, 114.0,
116.2, 116.6, 123.1, 125.0, 129.3, 130.6, 132.3, 133.3, 135.0,
137.2, 139.6, 142.3, 153.5, 153.6 ppm.
Materials
Tetrahydrofuran (THF) and Et3N were purchased and puri-
fied by passage through purification column under Ar pres-
sure.30 Pd(PPh3)4, CuI, ethynylferrocene (3), and 2,5-dime-
thylphenylacetylene (5) were obtained commercially, and
used without further purification. Pseudo-p-diethynyl[2.2]-
paracyclophane (1),31 1,4-diiodo-2,5-didodecyloxybenzene
(2),32,33 1-ethynylanthraquinone (4),34 and 1,4-bis(2,5-dime-
thylphenylethynyl)-2,5-didodecyloxybenzene (7)35 were pre-
pared as described in the literature. All reactions were per-
formed under Ar atmosphere.
Synthesis of Compound 6
1-Bromo-4-(2,5-dimethylphenylethynyl)-2,5-didodecyloxyben-
zene (0.14 g, 0.20 mmol),24 ethynylferrocene (3) (0.063 g,
0.30 mmol), PdCl2(PPh3)2 (4.2 mg, 0.006 mmol), and CuI
(2.2 mg, 0.012 mmol) were placed in a 50-mL Pyrex flask
equipped with a magnetic stirrer bar and a reflux condenser.
The equipment was purged with Ar, followed by adding THF
(30 mL) and NEt3 (15 mL). The reaction was carried out at
50ꢀC for 24 h. After cooling, the reaction mixture was con-
centrated in vacuo to afford the crude product, which was
purified by silica gel column chromatography (hexane/CHCl3,
vol/vol ¼ 4/1 as an eluent) to afford 6 as an orange solid
(19 mg, 0.024 mmol, 12%). Rf ¼ 0.15 (hexane/CHCl3, vol/
Polymerization
Compounds 1 (0.026 g, 0.10 mmol), 2 (0.077 g, 0.11 mmol),
Pd(PPh3)4 (0.012 g, 0.010 mmol), and CuI (2.3 mg, 0.020
mmol) were placed in a Schlenk flask equipped with a mag-
netic stirrer bar and a reflux condenser. The equipment was
purged with Ar, and THF (4.0 mL) and NEt3 (2.0 mL) were
added. The polymerization was carried out at reflux temper-
ature. After 11 h, mono-acetylenes 3–5 (0.10 mmol) were
added to the reaction mixture, and it was stirred overnight.
After cooling, the reaction mixture was diluted with CHCl3,
and washed with 0.1 N HCl, 28% aqueous NH3, water, and
brine. The organic layer was dried over MgSO4. The solvent
was concentrated in vacuo, and the residue was reprecipi-
tated with MeOH to obtain polymers P1–P3.
1
vol ¼ 3/1). H NMR (CDCl3, 400 MHz): d 0.88 (t, J ¼ 6.4 Hz,
6H), 1.2–1.6 (br m, 36H), 1.85 (m, J ¼ 7.2 Hz, 4H), 2.31 (s,
3H), 2.50 (s, 3H), 4.01 (q, J ¼ 7.2 Hz, 4H), 4.24 (d, J ¼ 2.0
Hz, 2H), 4.26 (s, 5H), 4.51 (d, J ¼ 2.0 Hz, 2H), 6.95 (s, 1H),
6.98 (s, 1H), 7.03 (d, J ¼ 7.6 Hz, 1H), 7.11 (d, J ¼ 7.6 Hz,
1H), 7.33 (s, 1H); 13C NMR (CDCl3, 100 MHz): d 14.1, 20.2,
20.7, 22.7, 26.10, 26.15, 29.5 (m), 31.9, 68.8, 69.5, 70.0, 71.4,
89.6, 93.7, 93.9, 116.4, 116.9, 129.1, 129.3, 132.2, 134.9,
137.1, 153.4, 153.6. HRMS (ESI) calcd for C52H70FeO2,
782.4720; found 782.4692 [M]þ. Anal. calcd for C52H70FeO2:
C 79.77 H 9.01, found: C 79.49 H 8.95.
Polymer P1: 1H NMR (CDCl3, 400 MHz): d 0.88 (t, J ¼ 6.8
Hz), 1.27 (br), 1.59 (br), 1.97 (m), 2.94 (br), 3.05 (br), 3.35
(br), 3.82 (br), 4.08 (m), 4.14 (br), 4.28 (s), 4.54 (s), 6.54
(m), 6.65 (m), 7.0–7.2 (m) ppm; 13C NMR (CDCl3, 100 MHz):
d 14.1, 22.7, 26.3, 29.5 (m), 31.9, 34.1, 68.9, 69.4, 70.0, 71.5,
89.6, 95.2, 114.0, 116.1, 116.2, 124.9, 133.3, 137.2, 139.6,
142.2, 153.6 ppm.
1H and 13C NMR spectra of P1–P3 and 6 are shown in Sup-
porting Information Figures S1–S8.
1
Polymer P2: H NMR (CDCl3, 400 MHz): d 0.89 (s), 1.26 (br),
RESULTS AND DISCUSSION
1.41 (br), 1.61 (br), 1.97 (br), 2.94 (br), 3.05 (br), 3.35 (br),
3.82 (br), 4.14 (br), 6.54 (m), 6.65 (s), 7.09 (m), 7.7–7.9 (m),
8.02 (m), 8.34 (m) ppm; 13C NMR (CDCl3, 100 MHz): d 14.5,
As shown in Scheme 1, Sonogashira–Hagihara coupling36,37
of
pseudo-p-diethynyl[2.2]paracyclophane
(1)
with
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TABLE 1 Results of Polymerization
phane bridge methylene protons, 2.9–3.9 ppm; cyclophane
and phenylene protons, 6.5–7.2 ppm; ferrocene protons, 4.3
and 4.5 ppm. The Mn value (5500) of P1, calculated from the
1H NMR integral ratio between the ferrocene and cyclophane
bridge methylene protons, indicated that an average of six
p-electron systems (xylyl–ethynyl–phenylene–ethynyl–xylyl
units) were stacked in the primary chain and two ferrocene-
containing p-electron systems (xylyl–ethynyl–phenylene–
ethynyl–ferrocenyl units) were stacked at the chain ends.
Mn
Yield (%) Cacld. Founda Mw/Mn
b
Entry Polym. ArA
1
P1
68
7,800 5,800
2.0
2
P2
62
7,900 7,800
1.9
Optical properties of the through-space conjugated polymers
were investigated to identify the potential applications of
these polymers as single molecular wires. Figure 2(A) shows
the UV–vis absorption spectra of P1 and P3 in dilute CHCl3
solution (1.0 ꢁ 10ꢂ5 M). The absorption spectrum of P3
exhibited typical p–p* absorption bands corresponding to
the arylene-ethynylene moieties at around 315 and 385 nm.
On the other hand, the spectrum of P1 exhibited broad
absorption bands attributable to the metal-to-ligand charge
3
P3
94
7,700 5,400
3.0
a
Calculated by the 1H NMR integral ratio.
b
Estimated by GPC (eluent: CHCl3, polystyrene standards).
1,4-diiodo-2,5-didodecyloxybenzene (2) in the presence of a
catalytic amount of Pd(PPh3)4/CuI was carried out. The
polymerization was monitored by GPC. After reflux for
11 h, monoalkynes were added, and the reaction mixture
was stirred overnight to yield [2.2]paracyclophane-based
through-space conjugated polymers P1–P3. Ferrocene and
anthraquinone were employed as end-capping fluorescence
quenchers for polymers P1 and P2, respectively. Polymer P3
containing xylene at the polymer chain ends was also pre-
pared as the reference polymer. When the molar ratio of
monomers 1 and 2 was 10:11 (Table 1), the number-average
molecular weights (Mn) of polymers P1, P2, and P3 were
found to be 5500, 7800, and 5400, respectively, according to
1
the H NMR integral ratio (vide infra).
Polymers P1–P3 were readily soluble in common organic
solvents such as CH2Cl2, CHCl3, THF, and toluene and hence
could be characterized by 1H and 13C NMR spectroscopy in
CDCl3 solution. The 1H NMR spectrum of P1 is shown in
Figure 1. The signal positions for various groups of protons
were as follows: dodecyl protons, 0.9–2.0 ppm; methylene
unit (AOCH2A) in the dodecyloxy group, 4.2 ppm; cyclo-
FIGURE 2 A UV–vis absorption spectra of P1 and P3 in CHCl3
(1.0 ꢁ 10ꢂ5 M/repeating unit), and (B) fluorescence emission
spectra of P1 and P3 in CHCl3 (1.0 ꢁ 10ꢂ5 M/repeating unit)
excited at 385 nm.
FIGURE 1 1H NMR spectrum of P1, 400 MHz, CDCl3.
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CHART 1 Structures of compounds 6 and 7
transfer (MLCT) band in the ferrocene-containing p-electron
systems at around 450 nm, in addition to the p–p* absorp-
tion peaks. The absorption spectrum of model compound 6
(Chart 1), which corresponds to the ferrocene-containing
p-electron system, supported the fact that the broad band
was due to the ferrocene unit (Fig. 3).
Fluorescence emission spectra of P1 and P3 in diluted CHCl3
solution (1.0 ꢁ 10ꢂ5 M) are shown in Figure 2(B). It was
confirmed that this concentration (1.0 ꢁ 10ꢂ5 M) was so
low (Supporting Information Fig. S11) that intermolecular
interactions were negligible. Both polymers P1 and P3 were
excited at 385 nm, and they exhibited almost identical fluo-
rescence spectra with peak maxima at around 415 nm,
which was assigned to emission from the stacked p-electron
systems, on the basis of the emission spectrum of model
compound 7 (Chart 1 and Fig. 3). However, the emission
from P1 was quenched, and the peak intensity (relative fluo-
rescence quantum efficiency: FFL ¼ 0.15) was significantly
lower than that of P3 (FFL ¼ 0.82). Incidentally, emission
from P2 was also quenched by the anthraquinone-containing
p-electron system at the polymer termini (FFL ¼ 0.26), as
shown in Supporting Information Figure S9; UV–vis absorp-
tion and fluorescence emission spectra of P2 are shown in
Supporting Information Figure S12.
FIGURE 3 A UV–vis absorption spectra of 6 and 7 in CHCl3 (1.0
ꢁ 10ꢂ5 M), and (B) fluorescence emission spectra of 6 and 7 in
CHCl3 (1.0 ꢁ 10ꢂ5 M) excited at 365 nm for 6 and 370 nm for 7.
Polymer P1 (Mn ¼ 5800, Mw/Mn ¼ 2.0) was separated into
two fractions, polymer P1a (Mn ¼ 3700, Mw/Mn ¼ 1.2) and
P1b (Mn ¼ 11400, Mw/Mn ¼ 1.5), by size-exclusion column
chromatography. The fluorescence emission spectra of P1,
P1a, and P1b in dilute CHCl3 solution (1.0 ꢁ 10ꢂ5 M) are
shown in Figure 4(A). The shorter the chain length, the
greater was the effectiveness of emission quenching, because
the quencher concentration of P1a was relatively higher
than that of P1 and P1b. The emission spectra of P3 with
compound 6 as an additional quencher are shown in Figure
4(B). To the solution of P3, different concentration of 6—
0.25, 1, 10, and 100 equiv (/repeating unit)—were added.
As expected, the two end-capping ferrocene moieties in P1
effectively quenched the emission from the stacked p-elec-
tron systems via through-space interactions. The Stern–
Volmer plots of P1 and P3 with additional quenchers are
shown in Supporting Information Figure S17. The Stern–
Volmer coefficient (Ksv) of P1 was 7.6 ꢁ 106, which was
approximately 100 times larger than that of P3 containing
On the other hand, no emission peak from compound 6
(CHCl3 solution, excitation wavelength: 315 nm) was detected
(Fig. 3), because of MLCT from the iron moiety [highest
occupied molecular orbital (HOMO) level: approximately ꢂ4.9
eV, estimated by cyclic voltammetry (CV) vs. ferrocene/ferro-
cenium in Supporting Information Fig. S16(A)] to the arylene-
ethynylene moiety [HOMO level: approximately ꢂ5.6 eV, esti-
mated by CV vs. ferrocene/ferrocenium in Supporting Infor-
mation Fig. S16(B)]. In P1, effective intramolecular energy
transfer from the stacked p-electron systems to the terminal
p-electron systems and MLCT occurred via through-space
interactions; the energy transfer could be explained by the
effective overlap between the emission spectrum of 7 (FFL
0.86) and the MLCT band of 6 (Fig. 3).
¼
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hence, this class of polymers can be used as single molecular
wires, in which energy and/or charge transfer would occur
via through-space interactions among the stacked p-electron
systems. Since an electron transfer including MLCT is an
important process for the light-induced reduction of proton
and/or organic molecules, construction of a photocatalyst
system through the electron transfer in the [2.2]paracyclo-
phane-based polymers is the next target.
ACKNOWLEDGMENTS
Financial support ‘‘Grant-in-Aid for Young Scientists (A) (No.
21685012)’’ from the Ministry of Education, Culture, Sports,
Science and Technology, Japan is gratefully acknowledged. This
work was partially supported by The Asahi Glass Foundation
and ‘‘Grant-in-Aid for Young Scientists (A) (No. 24685018)’’
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
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