Synthesis and properties of through-space conjugated polymers based on π-π Stacked 1,3-biarylpropane tethering units

Article abstract of DOI:10.1002/pola.26738

Novel skipped-π polymers in which the π-components are connected with 2-substituted trimethylene tethering units exhibit bathochromically shifted, broadened ultraviolet absorption with a unique lower-energy absorption band and a largely red-shifted fluorescent emission. These results suggest that through-space π-π interactions owing to a stair-like stacking substructure in these polymers extend the π-conjugation of the components in the ground and excited states. As the photophysical properties of the polymers observed both in a solution and in a dried film are similar to those of the J-aggregates of π-molecules, these polymers may be considered as pseudo J-stacking (or J-like-stacking) polymers.

Full text of DOI:10.1002/pola.26738

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Synthesis and Properties of Through-Space Conjugated Polymers Based
on p–p Stacked 1,3-Biarylpropane Tethering Units
Ryosuke Nomura,1 Ryota Moriai,1 Masaru Kudo,1 Tohru Hoshino,1 Jun-ichi Watanabe,1
Shigeaki Funyu,2 Ken-ich Ishitsuka,2 Sentaro Okamoto1
1Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
2Tsukuba Research Laboratory, Hitachi Chemical Co., Ltd., 48 Wadai, Tsukuba-shi, Ibaraki 300-4247, Japan
Correspondence to: S. Okamoto(E-mail: okamos10@kanagawa-u.ac.jp)
Received 12 January 2013; accepted 18 April 2013; published online
DOI: 10.1002/pola.26738
ABSTRACT:: Novel skipped-p polymers in which the p-compo-
nents are connected with 2-substituted trimethylene tethering
units exhibit bathochromically shifted, broadened ultraviolet
absorption with a unique lower-energy absorption band and a
largely red-shifted fluorescent emission. These results suggest
that through-space p–p interactions owing to a stair-like stacking
substructure in these polymers extend the p-conjugation of the
components in the ground and excited states. As the
photophysical properties of the polymers observed both in a so-
lution and in a dried film are similar to those of the J-aggregates
of p-molecules, these polymers may be considered as pseudo
C
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J-stacking (or J-like-stacking) polymers. 2013 Wiley Periodicals,
Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 00, 000–000
KEYWORDS: conformational analysis; conjugated polymers; flu-
orescence; stacking; UV–vis spectroscopy
p modules, exhibit a folded H-type stacking form in the solu-
tion and in the solid form. Herein, we report the design and
synthesis of new stair-like folded polymers st-P2 and st-P3
(Scheme 1). Polymers st-P2 and st-P3 have triphenyl stack-
ing units 2 and stilbene-type stacking units 3 as tethering
units, respectively, which are expected to have a stair-like
(zigzag) p-stacking structure. It was found that these poly-
mers exhibit unique optical properties that are probably the
result of through-space p–p interactions.
INTRODUCTION The construction of conjugated polymers that
usually consist of an extended p-conjugated system(s) of
newly formed bonds between sp- and/or sp2-hybridized
carbons generated in a polymerization reaction is of great
importance because of their unique properties such as their
electrical conductivity, photoconductivity, electrolumines-
cence, liquid crystallinity, and nonlinear optical properties
and chemical sensing properties.1 Meanwhile, through-space
and through-bond interactions between p systems that are
tethered by one or more saturated carbon atoms in organic
molecules, such as artificial aliphatic–aromatic systems or
systems involving biomolecules such as proteins and DNA,
are significant for understanding the electronic interactions
that affect the spectroscopic properties, reactivities, and
electric conductivities of these systems.2–11 Efforts have
recently been devoted to the development of through-space
p-interacting polymers with nonbonding p–p interactions,
such as those in cyclophane-containing polymers12 or
between pendant aryl moieties.13
EXPERIMENTAL
General Data
NMR spectra using CDCl3 solutions were recorded at 500
and 600 MHz for 1H and at 125 and 150 MHz for 13C,
respectively, on JEOL JMN-ECA500 and 600 spectrometers.
Chemical shifts are reported in parts per million (ppm, d)
relative to Me4Si (d 5 0.00) or residual CHCl3 (d 5 7.26 for
1H and d 5 77.0 for 13C NMR). IR spectra were recorded on
an FT/IR 4100 (JASCO) and are reported in wave numbers
(cm21). Ultraviolet (UV) absorption spectra were recorded
with a SHIMADZU UV-2450 spectrophotometer. Fluorescence
(FL) emission and excitation spectra were recorded with a
SHIMADZU RF-5300PC or JASCO FP-6500 spectrophotometer.
The Mn and Mw of polymers were measured with a TOSOH
HLC-8020 GPC unit (eluent: tetrahydrofuran [THF]; calibra-
tion: polystyrene standards) using two TSK-gel columns (23
Multipore HXL-M). All reactions sensitive to oxygen and/or
Recently, we developed a new class of folded stacking poly-
mers that are prepared via the A21B2 type polymerization
of p-units and 1,3-diphenylpropane tethering groups 1 bear-
ing a bulky substituent at the 2-position.14 The tethering
unit 1 promotes a predominant stacking-conformation of p-
units (a closed form) in the polymers as shown in Scheme 1.
As a result, polymers of type P1, derived from 1 and
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diethyl ether solutioꢀn of MeLi (51.8 mL, 1.12 M, 58.0 mmol)
was added at 278 C. After stirring for 30 min at this tem-
perature, a solution of diethyl 3,30-(1,4-phenylene)bis(2-(4-
bromobenzyl)propanoate) (7.14 g, 11.6 mmol) was addedꢀin
THF (24 mL) and the mixture was stirred for 2 h at 278 C.
After addition of 10% aqueous acetic acid, the mixture was
extracted with ethyl acetate. The organic layers were dried
over anhydrous MgSO4, filtered through a pad of Celite, con-
centrated, and chromatographed on silica gel to afford 4,40-
(1,4-phenylene)bis(3-(4-bromobenzyl)-2-methylbutan-2-ol) (4,
6.14 g) in 90% yield. To a mixture of the resulting 4 (2.94 g,
5.00 mmol), imidazole (1.36 g, 20.0 mmol), and N,N-dimethyl-
formamide (DMF, 25 mL), triethylchlorosilane (2.51 mL,
15.0 mmol) was added at 0 ꢀC and the mixture was stirred
for 12 h at room temperature. After addition of saturated
aqueous NaHCO3, the mixture was extracted with ethyl ace-
tate, washed with brine, dried over anhydrous MgSO4, filtered
through a pad of Celite, concentrated, and chromatographed
on silica gel to provide the title compound 2 (3.19 g) as a
mixture of dl- and meso-isomers in 78% yield.
1H NMR (600 MHz, CDCl3) d 5 7.19 (dd, 4H, J 5 2.4, 8.4 Hz,
Ar), 6.80 (d, 4H, J 5 8.4 Hz, Ar), 6.80 (s, 4H, CH2ArCH2)
2.91–2.83 (m, 4H, CH2Ar), 2.38–2.26 (m, 4H, CH2Ar), 2.01–
1.95 (m, 2H, CHCH2Ar), 1.18 (s, 12H, (CH3)2C), 0.97 (t, 18H,
J 5 7.8 Hz, (CH3CH2)3Si), 0.60 (q, 12H, J 5 7.8 Hz,
(CH3CH2)3Si); 13C NMR (150 MHz, CDCl3) d 5 142.0 (2C),
130.9, 130.7, 128.7, 118.9, 76.2, 54.9, 36.4, 35.9, 28.6, 28.2,
28.0, 7.3, 6.9; IR (neat) 2955, 2873, 1488, 1456, 1383, 1237,
1142, 1011, 773.4, 720.3 cm21; HR-MS: m/z 5 calcd For
SCHEME 1 Structure of folded H-stacking polymers (P1) and
stair-like stacking polymers (st-P2 and st-P3).
C
42H64Br2NaO2Si2 [M1Na]1: 837.2709, found 837.2710.
moisture were performed under an argon atmosphere. Dry
solvents were purchased from Kanto Chemicals.
1,4-Bis(2-(4-ethynylbenzyl)-3-methyl-3-
1,4-Bis(2-(4-bromobenzyl)-3-methyl-3-
((triethylsilyl)oxy)butyl)benzene (5)
((triethylsilyl)oxy)butyl)benzene (2)
To a mixture of 4 (2.94 g, 5.00 mmol), PdCl2(PPh3)2 (280
mg, 0.400 mmol), CuI (76.2 mg, 0.400 mmol), PPh3 (210 mg,
0.800 mmol), and THF (16.7 mL), triethylamine (7.14 mL)
and ethynyltrimethylsilane (2.83 mL, 20.0 mmol) were added
at room temperature. After stirring for 12 h at 80 ꢀC and
confirmation of completion of the reaction by thin-layer
chromatography (TLC) analysis, saturated aqueous NH4Cl
was added to the mixture. The mixture was extracted with
ethyl acetate and the organic layers were washed with brine,
dried over anhydrous MgSO4, filtered through a pad of silica
gel, and concentrated under reduced pressure. A mixture of
the resulting residue, K2CO3 (ꢁ0.1 g), MeOH (10 mL), and
THF (5.0 mL) was stirred at room temperature. After con-
firming the completion of the reaction by TLC analysis (ꢁ3
h), saturated aqueous NH4Cl was added. The mixture was
extracted with ethyl acetate and the organic layers were
washed with brine, dried over anhydrous MgSO4, filtered
through a pad of Celite, concentrated in vacuo, and chroma-
tographed on silica gel to give 4,40-(1,4-phenylene)bis(3-(4-
To a mixture of NaH (55 wt % in oil, 1.92 g, 44.0 mmol) and
dimethyl sulfoxide (DMSO, 50 mL), a solution of diethyl 2-(4-
bromobenzyl)malonate (15.1 g, 46.0 mmol) was added in
ꢀ
DMSO (10 mL) at 0 C. After stirring for 30 min, a solution of
1,4-bis(bromomethyl)benzene (5.28 g, 20.0 mmol) in DMSO
(40 mL) was added and the mixture was stirred for 12 h at
room temperature. After addition of saturated aqueous NH4Cl,
the mixture was extracted with ethyl acetate. The organic
layers were washed with brine, dried over anhydrous MgSO4,
filtered through a pad of Celite, concentrated in vacuo, and
chromatographed on silica gel to yield tetraethyl 2,20-(1,4-phe-
nylenebis(methylene))bis(2-(4-bromobenzyl)malonate) (11.4
g) in 75% yield. A mixture of the resulting tetraethyl 2,20-
(1,4-phenylenebis(methylene))bis(2-(4-bromobenzyl)malonate)
(11.4 g, 15.0 mmol), LiCl (2.55 g, 60.0 mmol), H2O (0.68 mL),
ꢀ
and DMSO (25 mL) was heated to 160 C for 7 h. After cool-
ing to ambient temperature and addition of water, the mixture
was extracted with ethyl acetate. The organic layers were
washed with brine, dried over anhydrous MgSO4, filtered
through a pad of Celite, concentrated, and chromatographed
on silica gel to give diethyl 3,30-(1,4-phenylene)bis(2-(4-bro-
mobenzyl)propanoate) (7.14 g) in 78% yield. To a solution
of anhydrous CeCl3 (14.3 g, 58.0 mmol) in THF (200 mL), a
ethynylbenzyl)-2-methylbutan-2-ol). To
a mixture of the
resulting 4,40-(1,4-phenylene)bis(3-(4-ethynylbenzyl)-2-meth-
ylbutan-2-ol), imidazole (1.36 g, 20.0 mmol), and DMF (25
mL), chlorotriethylsilane (2.51 mL, 15.0 mmol) was added at
ꢀ
0 C. After stirring for 12 h at room temperature, saturated
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temperature, saturated aqueous NH4Cl was added and the
mixture was extracted with ethyl acetate. The organic layers
were washed with brine, dried over anhydrous MgSO4, fil-
tered through a pad of Celite, concentrated in vacuo, and
chromatographed on silica gel to give (E)-tetraethyl 2,20-
((ethene-1,2-diylbis(4,1-phenylene))bis(methylene))bis(2-(4-
bromobenzyl)malonate) (5.00 g) in 58% yield.
1H NMR (600 MHz, CDCl3) d 5 7.41 (d, J 5 8.4 Hz, 1H, Ar),
7.39 (d, J 5 8.4 Hz, 4H, Ar), 7.13 (d, J 5 7.8 Hz, 4H, Ar),
7.05 (d, J 5 7.8 Hz, 4H, Ar), 7.04 (s, 2H, alkene), 4.12 (br q,
J 5 7.2 Hz, 8H, CH2O), 3.22 and 3.16 (2s, each 4H, CH2),
1.17 (t, J 5 7.2 Hz, 12H, CH3).
A mixture of the resulting dibromide (4.31 g, 5.00 mmol),
LiCl (1.70 g, 40.0 mmol), H2O (0.36 mL), and DMSO (16.7
ꢀ
mL) was stirred for 12 h at 160 C. After cooling to ambient
temperature and addition of water, the mixture was
extracted with ethyl acetate. The organic layers were washed
with brine, dried over anhydrous MgSO4, filtered through a
pad of Celite, concentrated in vacuo, and chromatographed
on silica gel to give (E)-diethyl 3,30-(ethene-1,2-diylbis
(4,1-phenylene))bis(2-(4-bromobenzyl)propanoate) (2.44 g)
in 68% yield. To a solution of the resulting diester (3.59 g,
5.00 mmol) in THF (16.7 mL), MeMgI (1.46 M in diethyl
ꢀ
ether, 41.1 mL, 60.0 mmol) was added at 0 C and the mix-
ture was stirred for 12 h at room temperature. After addi-
tion of aqueous 1 M of HCl, the mixture was extracted with
ethyl acetate. The organic layers were washed with brine,
dried over anhydrous MgSO4, filtered through a pad of Celite,
concentrated in vacuo, and chromatographed on silica gel to
give (E)-4,40-(ethene-1,2-diylbis(4,1-phenylene))bis(3-(4-bro-
mobenzyl)-2-methylbutan-2-ol) (2.25 g) in 73% yield. To a
mixture of the resulting diol (3.45 g, 5.00 mmol), imidazole
(1.36 g, 20.0 mmol), and DMF (25 mL), chlorotrimethylsilane
(2.51 mL, 15.0 mmol) was added at 0 ꢀC and the mixture
SCHEME 2 Synthesis of tethering units 2 and 3.
aqueous NaHCO3 was added. The mixture was extracted
with diethyl ether and the organic layers were washed with
brine, dried over anhydrous MgSO4, filtered through a pad of
Celite, concentrated in vacuo, and chromatographed on silica
gel to produce the title compound 5 (2.16 g) as a mixture of
dl- and meso-isomers in 61% yield through three steps.
1H NMR (600 MHz, CDCl3) d 5 7.24 and 7.23 (2d, J 5 7.8
Hz, 4H, Ar), 6.91 (br d, J 5 7.8 Hz, 4H, Ar), 6.81 and 6.80
(2s, 4H, Ar), 2.99 (s, 2H, acetylenic H), 2.94–2.84 (m, 4H,
CH2), 2.43–2.37 (m, 2H, CH2), 2.34–2.28 (m, 2H, CH2), 2.04–
1.99 (m, 2H, CH), 1.17 (s, 12H, CCH3), 0.96 (t, J 5 7.8 Hz,
18H, SiCH2CH3), 0.59 (q, J 5 7.8 Hz, 12H, SiCH2); 13C NMR
(150 MHz, CDCl3) d 5 144.2, 139.5, 131.8, 128.9, 128.8,
118.8, 84.0, 76.2, 54.8, 36.6, 36.5, 36.3, 28.6, 28.5, 28.2, 7.2,
6.9; IR (neat) 3080, 2956, 2874, 1507, 1456, 1415, 1237,
1142, 1017, 719.3, 670.1 cm21; HR-MS: m/z 5 calcd For
C
46H66KO2Si2 [M1K]1: 745.4238, found 745.4231.
(E)-1,2-Bis(4-(2-(4-bromobenzyl)-3-methyl-3-
((triethylsilyl)oxy)butyl)phenyl)ethane (3)
To a mixture of diethyl 2-(4-bromobenzyl)-2-(4-iodobenzyl)-
malonate (6.57 g, 11.5 mmol), (E)-diethyl 2-(4-bromobenzyl)-
2-(4-(2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)vinyl)
benzyl)ma6lonate (5.45 g, 10.0 mmol), Cs2CO3 (6.52 g, 20.0
mmol), and DMF (44 mL), a solution of Pt(PPh3)4 (622 mg,
0.50 mmol) was added in DMF (22 mL) at room tempera-
SCHEME 3 Synthesis of polymers st-P2 and st-P3: Reagents:
(a) Me3SiCCH, Cl2Pd(PPh3)2, CuI, Et3N, THF; (b) K2CO3, MeOH,
THF; (c) Et3SiCl, imidazole, DMF; (d) Pd(PPh3)4, CuI, Et3N, THF;
and (e) Pd(PPh3)4, K2CO3, (n-Bu)4NBr, THF.
ꢀ
ture. After stirring for 12 h at 120 C and cooling to ambient
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ꢀ
was stirred for 12 h at room temperature. After addition of
saturate aqueous NaHCO3, the mixture was extracted with
diethyl ether. The organic layers were washed with brine,
dried over anhydrous MgSO4, filtered through a pad of Celite,
concentrated in vacuo, and chromatographed on silica gel to
give the title compound 3 (2.94 g) as a mixture of dl- and
meso-isomers in 64% yield.
stirred for 3 days at 80 C. After cooling to ambient tempera-
ture and addition of saturated aqueous NaHCO3, the mixture
was extracted with CH2Cl2. The combined organic layers
were washed with H2O, dried over Na2SO4, and the aqueous
layer was extracted with CH2Cl2. The combined organic
layers were washed with H2O and then dried over Na2SO4.
After filtration and evaporation, the residue was dissolved in
CH2Cl2 and poured into a large amount of MeOH. The sol-
vents were decanted and the precipitated polymer was
washed several times with diethyl ether and dried under
reduced pressure. Polymer st-P3 (0.275 g) was obtained in
70 % yield as a yellow solid.
1H NMR (600 MHz, CDCl3) d 5 7.28 (d, 4H, J 5 7.8 Hz, Ar),
7.23 (d, 4H, J 5 7.2 Hz, Ar), 6.97 (s, 2H, alkene), 6.94 (d, 4H,
J 5 8.4 Hz, Ar), 6.83 (d, 4H, J 5 7.2 Hz, Ar), 2.99–2.89 (m,
4H, CH2Ar), 2.40–2.33 (m, 4H, CH2Ar), 2.08–2.03 (m, 2H,
CHCH2Ar), 1.22 (s, 12H, CCH3), 0.98 (t, 18H, J 5 8.1 Hz,
SiCH2CH3), 0.61 (q, 12H, J 5 7.4 Hz, SiCH2CH3); 13C NMR
(150 MHz, CDCl3) d 5 141.79, 141.75, 134.9, 131.0, 130.7,
129.2, 127.7, 126.2, 119.0, 76.2, 54.9, 36.6, 28.2, 7.2, 6.8; IR
(neat) 2953, 2873, 1488, 1456, 1237, 1178, 1143, 1011,
1H NMR (600 MHz, CDCl3) d 5 7.45–7.17 (br m, overlap
with CDCl3 peak, 10H, Ar), 7.09–6.78 (br, 12H, Ar and
CH@CH), 2.95–2.81 (br, 4H, CH2Ar), 2.48–2.30 (br, 4H,
CH2Ar), 2.07–2.01 (br, 2H, CHCH2Ar), 1.23–1.05 (br, 12H,
(CH3)2C), 1.03–0.86 (br m, 18H, (CH3CH2)3Si), 0.65–0.50 (br,
12H, (CH3CH2)3Si); 13C NMR (150 MHz, CDCl3) d 5 136.7,
131.8, 129.3, 128.9, 128.4, 127.1, 126.6, 126.2, 76.3, 55.0,
36.3, 28.3, 7.2, 6.9; IR (neat) 3022, 2953, 2874, 1514, 1458,
1383, 1365, 1143, 1040, 1016, 818.6, 741.5, 723.2 cm21; Mn
5 1.61 3 104, Mw 5 3.50 3 104, PDI 5 2.16 based on GPC
analysis (polystyrene standard).
963.3, 720.3 cm21
50H70Br2NaO2Si2 [M1Na]1: 939.3179, found 939.3150.
;
HR-MS: m/z
5
calcd For
C
Synthesis of Polymer st-P2 from 2 and 5
To a mixture of 2 (245 mg, 0.300 mmol), Pd(PPh3)4
(34.7mg,0.0300 mmol), and CuI (5.71 mg, 0.0300 mmol), a
solution of 5 (212 mg, 0.300 mmol) was added in THF (3.00
mL) and then Et3N (1.50 mL) at room temperature and the
ꢀ
mixture was stirred for 3 days at 80 C. After cooling to am-
RESULTS AND DISCUSSION
bient temperature and addition of saturated aqueous
NaHCO3, the mixture was extracted with CH2Cl2. The com-
bined organic layers were washed with H2O and then dried
over Na2SO4. After filtration and evaporation, the residues
were dissolved in CH2Cl2 and poured into a large amount of
MeOH. The solvents were decanted and the precipitated
polymer was washed several times with diethyl ether and
dried under reduced pressure. Polymer st-P2 (401 mg) was
obtained in 98 % yield as a light yellow viscosity solid.
Synthesis
The requisite tethering units 2 and 3 were readily synthe-
sized starting from diethyl a-(4-bromophenylmethyl)-malo-
nate14 via the a-alkylation of the malonate and selective
Sonogashira and Suzuki–Miyaura coupling reactions (Scheme
2). As a model compound, 2-H2 (X 5 H) was also prepared
similarly.
With the tethering units 2 and 3 in hand, polymers st-P2
and st-P3 (Scheme 3) were synthesized. The dibromotri-
phenyl unit 2 was polymerized with diethynyl-triphenyl unit
5, which was derived from 4, via a Sonogashira coupling
reaction to furnish st-P2 (Mn 5 3.17 3 103, Mw 5 4.57 3
103: determined by GPC, eluent: THF, calibration: polystyrene
standards) in 98% yield after reprecipitation from THF using
methanol. Polymer st-P3 (Mn 5 1.17 3 104, Mw 5 2.77 3
104: determined by GPC, THF/polystyrene standards) was
obtained in 57% yield via the Suzuki coupling polymeriza-
tion of unit 3 and bis-boryl compound 6.
1H NMR (600 MHz, CDCl3) d 5 7.28–7.16 (br m, overlap
with CHCl3 peak, Ar), 7.00–6.88 (br m, Ar), 6.88–6.76 (br m,
Ar), 3.00–2.87 (br m, 4H, CH2Ar), 2.50–2.26 (br m, 4H,
CH2Ar), 2.08–1.96 (br, 2H, CHCH2Ar), 1.17 (br s, 12H, CCH3),
0.97–0.91 (br m, 18H, (CH3CH2)3Si), 0.59–0.52 (br m, 12H,
(CH3CH2)3Si); 13C NMR (150 MHz, CDCl3) d 5 143.7, 143.34,
143.3, 142.0, 139.6, 139.5, 139.4, 139.3, 132.1, 131.2, 130.9,
130.7, 129.0, 128.8, 120.3, 118.9, 88.9, 76.3, 76.2, 54.9, 36.6,
36.4, 36.3, 36.0, 28.6, 28.5, 28.4, 28.3, 28.2, 28.1, 7.2, 6.9; IR
(neat) 3046, 2958, 2875, 1606, 1513, 1458, 1414, 1383,
1365, 1260, 1143, 1107, 1017, 801.3, 742.5, 723.2 cm21; Mn
5 3.17 3 103, Mw 5 4.57 3 103, polydispersity index (PDI)
5 1.44 based on Gel permeation chromatography (GPC)
analysis (polystyrene standard).
Conformational Analysis of Triphenyl Compound 2
Our previous study on conformational analyses of 2-substi-
tuetd 1,3-diphenylpropane 1 using FL, 1H NMR measure-
ment, and MM2 calculation clarified that 1 superiorly exists
in a closed (stacked) conformation in solution.14 We carried
out a similar investigation for elucidating the conformation of
compound 2-H2, a model compound for a tethering unit of
polymer st-P2. Figure 1(a) compares the FL properties of
toluene, diphenyl stacking compound 1 (X 5 H), and triphenyl
unit 2-H2. Both 1 and 2-H2 exhibited only a benzene-excimer
Synthesis of Polymer st-P3 from 3 and 1,4-Bis((E)-2-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)vinyl)
benzene (6)
To a mixture of 3 (408 mg, 0.500 mmol), tetra-n-buthylam-
monium bromide (161 mg, 0.500 mmol), 6 (191 mg, 0.500
mmol), THF (1.67 mL), and aqueous 2 M K2CO3 (1.0 mL), a
solution of Pd(PPh3)4 (23.1 mg, 0.0200 mmol) was added in
THF (1.67 mL) at room temperature. The mixture was
emission (kem
without FL from a single benzene ring (kem
5332 nm for 1 and 331 nm for 2-H2)
max
5285 nm
max
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absorbance linearly increased as the concentration increased
in the range of 1026–1024 M. The FL quantum yields of DT
and st-P2 were determined by using 9-anthracenecarboxylic
acid in CH2Cl2 as a standard (UF 5 0.442).15 The results are
summarized in Table 1.
UV spectra of st-P2 consisted of absorptions based on ben-
zene ring and dialkyltolan units, and the absorbance was
enhanced compared with those of 2-H2 and DT. Absorption
spectra by tolan units of st-P2 (kmax 5 293 nm (log E 5
4.44), 311 nm (log E 5 4.40)) had a similar shape but red-
shifted about 5 nm compared with that of DT. In addition, it
is noteworthy that an additional broadened absorption band
appeared at lower energy (320–400 nm) in the spectrum of
st-P2. As mentioned above, as intermolecular interaction
was ruled out at the concentration used for the analysis, this
result suggests that extended conjugation via through-space
interactions with the triphenyl stacking units might exist.
FIGURE 1 (a) Normalized FL spectra of 1, 2-H2, and toluene.
Excited at 254 nm (2-H2, 1024 M), 262 nm (1, 1024 M), and 262
nm (toluene, 2 3 1025 M) in CH2Cl2. (b) Comparison of the
chemical shifts of the aromatic protons of 1, 2-H2, toluene, and
p-xylene (CDCl3, 500 MHz).
for toluene). This result indicates that compounds 1 and 2-
H2 preferentially exist in a p–p stacked conformation in the
1
excited state. As evident from the results of H-NMR analyses
of 1 and 2-H2 [Fig. 1(b)], the aromatic protons are shifted
up-field compared to those of toluene and p-xylene owing to
the shielding effect in the stacked form in the ground state.14
Therefore, these data suggest that triphenyl compound 2-H2
exists predominantly in a folded (stacked) conformation in
the ground and excited states.
Properties of Polymer st-P2
Next, we investigated the optical properties of polymer
st-P2. Figure 2 shows the UV absorption and FL spectra of
st-P2 along with those of 2-H2 and di(4-tolyl)acetylene (DT)
in CH2Cl2. 2-H2 and DT are model compounds for the struc-
ture components of st-P2. The UV and FL spectra of DT and
st-P2 were each measured using a 1025 M solution, whereas
those of 2-H2 were obtained using a 1024 M solution. The
concentration (M) of st-P2 was adjusted such that the con-
centration of the repeated unit was 1025 M. To compare
their properties, the resulting absorbance and FL intensity
values were adjusted to be 1025 M of tolan or triphenyl unit
concentration. In these solutions, intermolecular interaction
(an aggregation effect) can be ruled out, because the UV
FIGURE 2 UV absorption and FL spectra of 2-H2, DT, and st-P2.
UV absorption: 2-H2 (1024 M), DT (1025 M), and st-P2 (1025 M)
in CH2Cl2. FL: excited at 254 nm (2-H2, 1024 M), 287 nm (DT,
1025 M), and 305 nm (st-P2, 1025 M) in CH2Cl2.
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TABLE 1 Optical Properties of 2-H2, DT, and Polymer st-P2
a
a
UV kmax (nm)
FL kmax (nm)
c
Compound
In Solution (log E)
Film
In Solution (UF
)
Film
2-H2
230 (3.85)
–
331
–
DT
229 (4.10), 287 (4.33), 306 (4.27)
229 (4.54), 293 (4.44), 311 (4.40)
–
311 (0.001)d
386 (0.016)e
–
Polymer st-P2
293, 312
450
a
c
Absorption and emission spectra were recorded in dilute CH2Cl2 solu-
tions at room temperature.
The quantum yield (UF) was calculated based on the measurements of
1025
CH2Cl2 solutions by using 9-anthracenecarboxylic acid as
standard.
M
a
b
The sample solutions were excited at 254, 287, and 305 nm for 2-H2,
d
DT, and st-P2, respectively. Film of st-P2 was excited at 313 nm.
Excited at 287 nm.
Excited at 293 nm.
e
Figure 2(b) shows the FL spectra of these compounds. 2-H2
exhibited benzene-excimer emission at approximately 330
nm. Polymer st-P2 exhibited a smaller benzene-excimer
emission and a smaller emission from a single tolan moiety,
and the spectra was largely red-shifted (kmax 5 386 nm) and
its intensity (UF 5 0.016) remarkably increased compared
with that of DT (UF 5 0.001). These results can be attrib-
uted to the overlapping of the benzene-excimer emission
region (nearly, 330 nm) with that of the lower-energy
absorption band of st-P2. The bathochromic shift of the
emission of st-P2 may originate from the extended conjuga-
tion via through-space interaction owing to a stacking struc-
ture in the stair-like folded polymer.
substrate and removing the solvent, was carried out for the
samples of st-P2 [Fig. 4(b)]. It showed broad spectra without
any sharp peak, and therefore a solid of st-P2 had no crys-
talline form.
In summary, skipped-p polymer st-P2, comprising phenylene
and phenylene–ethynylene–phenylene units connected by tri-
methylene tethers, exhibited largely red-shifted, broadened
UV absorption and red-shifted FL emission compared with
those of dimethyltolan (DT). It can be concluded that these
properties are attributed to the extended conjugation via
through-space interactions owing to a stair-like stacking
structure.
Properties of Polymer st-P3
The space-filling model shown in Figure 3 is the result of
molecular mechanics calculation (MM2) (nonsolvent condi-
tions) for a part of st-P2. It indicates the folded stair-like
structure as a superior conformation.
Next, the optical properties of stilbene-type polymer st-P3
were investigated. Figure 5 compares the UV absorption and
FL spectra of st-P3 with those of model compounds 4,
40-dimethylstilbene (DMeSt) and 1,4-bis((E)-4-methylstyryl)-
benzene (BSt) in CH2Cl2. The UV spectra were each meas-
ured using a 1026 M solution, whereas FL spectra were
measured using a 1027 M solution. The concentration (M) of
st-P3 was adjusted such that the concentration of the
repeated unit was 1026 or 1027 M. In these solutions, inter-
molecular interaction (an aggregation effect) can be ruled
out, because the UV absorbance linearly increased as the
concentration increased in the range of 1027–1025 M. The
results are summarized in Table 2.
The FL of st-P2 was further red-shifted in the film form [Fig.
4(a)], and the absorption and excitation spectra of the film
showed an additional broad absorption band nearly 400 nm
that is not present in the UV spectrum of the polymer in the
solution. This increase in the absorption at lower energy
may be attributed to intramolecular through-space interac-
tions as well as intermolecular interactions via aggregation.
X-ray diffraction (XRD) analysis of polymer powder peeled
from the dried film, prepared by spin-coating on a glass
FIGURE 3 The space-filling model based on the result of molecular mechanics calculation (MM2) (nonsolvent conditions) for a
part of st-P2.
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TABLE 2 Optical Properties of DMeSt, BSt, and Polymer st-P3
a
a
UV kmax (nm)
FL kmax (nm)
c
Run Compound
In Solution (log E)
In Solution (UF )
1
2
3
DMeSt
BSt
303 (4.49), 317 (4.47)
360 (4.45)
360 (0.104)d
414 (0.884)e
Polymer st-P3 324 (4.75), 364 (4.60), 447 (0.207)f
a
Absorption and emission spectra were recorded in dilute CH2Cl2 solu-
tions at room temperature.
b
The sample solutions were excited at 324, 364, and 317 nm for
DMeSt, BSt, and st-P3, respectively.
c
The quantum yield (UF) was calculated based on the measurements of
1027
standard.
M CH2Cl2 solutions by using 9-anthracenecarboxylic acid as a
d
Excited at 303 nm.
Excited at 360 nm.
Excited at 324 nm.
e
f
Similar to st-P2, polymer st-P3 exhibited a unique absorp-
tion band in the lower energy region (>400 nm) which can-
not be explained by superimposing the spectra of DMeSt
and BSt [Fig. 5(a)]. A largely red-shifted emission of st-P2
was also observed compared with those of the structural
p-components (DMeSt and BSt) [Fig. 5(b)]. Therefore, it can
be concluded again that these properties may be attributed
to a through-space p–p interaction that arises from the
folded conformation of the polymer.
FIGURE 4 (a) Normalized UV absorption, FL, and excitation
spectra of a CH2Cl2 solution (1025 M) and a film of st-P2. (b)
XRD analysis of polymer powder peeled from the film of st-P2.
CONCLUSIONS
We synthesized novel skipped-p polymers in which p-compo-
nents are connected with 2-substituted trimethylene tether-
ing units and demonstrated that their stair-like stacking
structure resulted in unique photophysical properties. The
stair-like stacking polymers st-P2 and st-P3 exhibited batho-
chromically shifted, broadened UV absorption behavior
involving unique lower-energy absorption bands and largely
red-shifted FL emission. These results suggest that through-
space p–p interactions owing to the stacking substructure in
these polymers extend the p-conjugation of the components
in the ground and excited states. As the photophysical prop-
erties of the polymers observed both in the solution and in
the dried films are similar to those of J-aggregates of
p-molecules, these polymers may be considered as pseudo
J-stacking (or J-like-stacking) polymers. UV-absorbing and
fluorescent polymers are important as functional macromole-
cules with various prospective applications.16–20 Further
investigations to determine a more detailed mechanism of
the optical properties and the potential applications of this
type of polymer, such as hole transporting or nonlinear opti-
cal materials, are underway in our laboratories.
REFERENCES AND NOTES
FIGURE 5 UV absorption and normalized FL spectra of BSt, DMeSt,
and st-P3. (a) UV absorption: BSt (1026 M), DMeS (1026 M), and st-
P3 (1026 M) in CH2Cl2. (b) FL: excited at 364 nm (BSt, 1027 M), 324
nm (DMeS, 1027 M), and 317 nm (st-P3, 1027 M) in CH2Cl2.
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