Helical and luminescent disubstituted polyacetylenes: Synthesis, helicity, and light emission of poly(diphenylacetylene)s bearing chiral menthyl pendant groups

Article abstract of DOI:10.1021/ma200075y

Diphenylacetylenes containing chiral menthyl groups [C₆H ₅C=C-C₆H₄OCOCH₂OR* and C ₆H₅C=C-C₆H₄CO₂R*, R* = (1R,2S,5R)-(-)-menthyl] are synthesized

Full text of DOI:10.1021/ma200075y

ARTICLE
pubs.acs.org/Macromolecules
Helical and Luminescent Disubstituted Polyacetylenes: Synthesis,
Helicity, and Light Emission of Poly(diphenylacetylene)s Bearing
Chiral Menthyl Pendant Groups
Cathy K. W. Jim,† Jacky W. Y. Lam,*,† Chris W. T. Leung,† Anjun Qin,†,§ Faisal Mahtab,† and
Ben Zhong Tang*,†,§
†Department of Chemistry, The Hong Kong University of Science & Technology (HKUST), Clear Water Bay, Kowloon,
Hong Kong, China
§Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
ABSTRACT: Diphenylacetylenes containing chiral menthyl
groups [C6H5CtCꢀC6H4OCOCH2OR* and C6H5CtCꢀ
C6H4CO2R*, R* = (1R,2S,5R)-(ꢀ)-menthyl] are synthesized
and polymerized by WCl6ꢀPh4Sn catalyst. The structures and
properties of the polymers are characterized and evaluated by
IR, NMR, TGA, UV, CD, and PL analyses. All the polymers are
thermally stable, losing little of their weights when heated to
g250 °C. They also enjoy high photostability, suffering no
change in the molecular weight when exposed to UV irradiation
in air for 24 h. The backbones of the polymers are induced to
helically rotate by the chiral pendants, as verified by their large
specific optical rotations and strong Cotton effects in the
backbone absorption region in the solution and aggregate states.
The polymers emit green light of 493 and 520 nm when their solutions and nanoparticle suspensions in poor solvents are
photoexcited. UV irradiation of the polymer films in air photooxidizes the exposed regions, generating two-dimensional luminescent
photopatterns.
’ INTRODUCTION
polyacetylenes should be thermally stable, thus enabling them to
find useful practical applications such as chiral stationary phase in
the chromatographic drug enantioseparation.12
Optically active polymers with extended π-electron conjuga-
tions are under hot pursuit of scientists because the development
of such polymers may lead to technological innovations in
nonlinear optics, asymmetric electrodes, photonic switching,
and so forth.1,2 One such conjugated polymer is represented
by polyacetylenes, whose conjugated backbones can be induced
to helically rotate when the surrounding environments exert
chiral forces on the polymer chains or when the chiral substit-
uents internally perturb the polymer strands in an asymmetric
fashion.3ꢀ9
Most of the optically active polyacetylenes prepared so far are
monosubstituted, with an overwhelming majority of them being
poly(phenylacetylene) (PPA) and polypropargyl derivatives.7ꢀ9
While the monosubstituted polyacetylenes are more stable than
their unsubstituted polyacetylene parent, their stabilities are still
of concern for many practical applications.10 One way to further
boost the polymer stability is to make a disubstituted polyacety-
lene, which is generally more stable than its monosubstituted
homologue. For example, poly(1-chloro-2-phenylacetylene), a
disubstituted derivative of PPA is so stable that it does not suffer
any decrease in its molecular weight when heated to 200 °C in air
for 20 h. In sharp contrast, PPA decomposes readily when heated
under the same conditions.11 It is envisioned that chiral disubstituted
The synthesis of chiral disubstituted polyacetylenes has,
however, been difficult due to the lack of effective polymerization
systems for disubstituted acetylenes containing functional
groups. So far, only a few helical disubstituted polyacetylenes
have been prepared.13 The chiral groups are incorporated into
the polymer structures through multistep silicon chemistry,
possibly because of the need to avoid the use of polar functional
groups, which are poisoning to TaCl5 and NbCl5, the most
widely used catalysts for the polymerizations of disubstituted
acetylenes.14 If the stereogenic groups can be attached to the
polyacetylene strands via “normal” functional groups such as
ester, it will greatly facilitate the molecular design and polymer
synthesis and significantly enrich the research field of helical
disubstituted polyacetylenes because a vast variety of acetylenes
and chiral building blocks with ester-forming hydroxyl and
carboxyl groups is commercially available.15
Received: January 13, 2011
Revised:
March 7, 2011
Published: March 21, 2011
r
2011 American Chemical Society
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Scheme 1
anticipated to generate materials with both fluorescence and
helicity. With such regard, we have worked on the synthesis of
poly(1-phenyl-1-octyne)s carrying different chiral substituents.18
Disappointedly, the stereogenic polymers exhibit only weak
helicity though they emit intensely in the solution and solid
states. In this paper, we continued our research on helical and
luminescent polyacetylenes. We attached (ꢀ)-menthol, a natu-
rally occurring species, to diphenylacetylene via ester function-
ality. The synthesis is straightforward, involves no complicated
procedures under stringent controlled conditions, and gives
products 1 and 2 in high yields (Scheme 1). In this report, we
tell how the new monomers can be polymerized and report what
the properties of the resultant poly(diphenylacetylene)s exhibit
(P1 and P2; Chart 1).
Chart 1
’ EXPERIMENTAL SECTION
Materials and Instrumentation. Tetrahydrofuran (THF;
Labscan) and toluene (BDH) were distilled under nitrogen from sodium
benzophenone ketyl immediately prior to use. Dichloromethane
(DCM) was distilled under nitrogen over calcium hydride. All other
solvents were purified using standard procedures. Phenylacetylene (3),
4-iodophenol (4), (ꢀ)-menthoxyacetic acid (6), 4-iodobenzoic acid
(7), (1R,2S,5R)-(ꢀ)-menthol (8), 1,3-dicyclohexylcarbodiimide
(DCC), 4-(dimethylamino)pyridine (DMAP), p-toluenesulfonic acid
monohydrate (TsOH), and other chemicals were purchased from
Aldrich and used as received without further purification.
Number (Mn)- and weight (Mw)-averaged molecular weights and
polydispersity indices (Mw/Mn) of the polymers were estimated by a
Waters Associates gel permeation chromatography (GPC) system
equipped with refractive index and UV detectors. THF was used as
eluent at a flow rate of 1.0 mL/min. A set of linear polystyrenes was used
for the molecular weight calibration. IR spectra were recorded on a
Perkin-Elmer 16 PC FT-IR spectrophotometer. 1H and 13C NMR
spectra were measured on a Bruker ARX 300 NMR spectrometer using
CDCl3 as solvents. Thermogravimetric analysis (TGA) measurements
were carried out under nitrogen on a Perkin-Elmer TGA 7 analyzer at a
heating rate of 10 °C/min.
In our previous work, we succeeded in preparing a group of
chiral disubstituted polyacetylenes bearing ester functionalities, i.
e., poly(phenylpropiolate)s, by inexpensive, “classic” metathesis
catalysts of WCl6ꢀ and MoCl5ꢀPh4Sn.16 Whereas the few
known examples of helical disubstituted polyacetylenes show
Cotton effects in the backbone absorption spectral region with
molar ellipticities ([θ]) smaller than 35 000 deg cm2 dmolꢀ1, our
chiral poly(phenylpropiolate)s, however, exhibit much higher
Cotton effects in the similar spectral region ([θ] up to 115 000
deg cm2 dmolꢀ1).16a,b Unfortunately, the polymers are not
emissive as luminogenic materials with helical conformations
may emit polarized light upon photoexcitation, which may be
utilized for the construction of lightning and orientating layers in
liquid crystal optical display devices, thus obviating the use of
backlight lamps, polyimide films, and polarizing sheets.
Disubstituted polyacetylenes such as poly(1-phenyl-1-al-
kyne)s and poly(diphenylacetylene)s are found to be capable
of emitting strong light upon photoexcitation.17 Thus, attach-
ment of stereogenic groups to the backbones of these polymers is
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UV spectra were measured on a Milton Roy Spectronic 3000 array
spectrophotometer, and the molar absorptivity of the polymers was
calculated on the basis of their monomer repeat units. High-resolution
mass spectra (HRMS) were recorded on a Finnigan TSQ 7000 operat-
ing in a MALDI-TOF mode. Specific optical rotations ([R]D) were
measured on a Perkin-Elmer 241 polarimeter at room temperature
(∼23 °C) with a beam of plane-polarized light of the D line of a sodium
lamp (589.3 nm) as the monochromatic source. Circular dichroism
(CD) measurements were recorded on a Jasco J-720 spectropolarimeter
in 1 mm quartz cuvettes with a step resolution of 0.2 nm, a scan speed of
50 nm/min, a sensitivity of 0.1°, and a response time of 9.5 s. Each
spectrum was the average of 5ꢀ10 scans. Photoluminescence (PL)
spectra were recorded on a Perkin-Elmer LS55 spectrofluorometer with
a xenon discharge lamp excitation.
13C NMR (75 MHz, CDCl3), δ (TMS, ppm): 165.6, 137.6, 131.0, 130.3,
100.4, 75.2, 47.2, 40.9, 34.2, 31.6, 31.4, 26.5, 23.6, 22.0, 20.7, 16.5.
1-{[(1R,2S,5R)-(ꢀ)-Menthoxycarbonyl]phenyl}-2-phenylacetylene
(2). Into a 500 mL round-bottom flask equipped with a septum and a
stirring bar were added 7.72 g (20 mmol) of 9, 0.2 g (0.29 mmol) of
Pd(PPh3)2Cl2, 55.1 mg (0.29 mmol) of CuI, and 75.4 mg (0.29 mmol)
of PPh3. Dry Et3N (150 mL) and THF (150 mL) and 3.29 mL (3.06 g,
30 mmol) of 3 were then injected by hypodermic syringes. After stirring
at room temperature for 12 h, the mixture was filtered and the
precipitates were washed with diethyl ether. The filtrate was collected,
and after solvent evaporation under reduced pressure, the crude product
was purified by silica gel column chromatography using chloroform/
hexane mixture (1:2 v/v) as eluent. White solid; yield 82.0%. IR (thin
film), v (cmꢀ1): 3060, 2958, 2928, 2870, 2218, 1932, 1806, 1714, 1606,
1560, 1510, 1488, 1456, 1404, 1387, 1368, 1327, 1305, 1290, 1270, 1175,
1140, 1115, 1036, 1017. 1H NMR (300 MHz, CDCl3), δ (TMS, ppm):
8.04 (d, 2H), 7.56 (m, 4H), 7.35 (m, 3H), 4.95 (m, 1H), 2.11 (d, 1H),
1.96 (m, 1H), 1.71 (d, 2H), 1.56 (m, 2H), 1.15 (m, 2H), 0.95 (m, 7H),
0.81 (d, 3H). 13C NMR (75 MHz, CDCl3), δ (TMS, ppm): 165.5,
131.7, 131.4, 130.1, 129.5, 128.7, 128.4, 127.7, 122.7, 92.1
(ꢁCꢀPhCO2), 88.7 (PhCꢁ), 75.0, 47.2, 40.9, 34.3, 31.4, 26.5, 23.6,
22.0, 20.8, 16.5. HRMS (MALDI-TOF): m/z 361.2176 [(M þ 1)þ,
calcd 361.2089].
Monomer Preparation. The (ꢀ)-menthol-containing dipheny-
lacetylenes (1 and 2) were synthesized according to Scheme 1. Typical
procedures for their syntheses are shown below.
1-(4-Hydroxyphenyl)-2-phenylacetylene (5). Into a 500 mL round-
bottom flask equipped with septum and stirring bar were added 4.40 g
(20 mmol) of 4, 0.2 g (0.29 mmol) of Pd(PPh3)2Cl2, 55.1 mg (0.29
mmol) of CuI, and 75.4 mg (0.29 mmol) of PPh3. Dry Et3N (150 mL)
and THF (150 mL) and 2.75 mL (2.56 g, 25 mmol) of 3 were then
injected by hypodermic syringes. After stirring at room temperature for
12 h, the mixture was filtered and the precipitates were washed with
diethyl ether. The filtrate was collected, and after solvent evaporation
under reduced pressure, the crude product was purified by silica gel
column chromatography using chloroform as eluent. A white solid of 5
Polymer Synthesis. All the polymerization reactions were carried
out under dry nitrogen using the standard Schlenk technique, unless
otherwise specified. A typical procedure for the polymerization of 1 is
given below as an example.
1
was obtained in 81% yield. H NMR (300 MHz, CDCl3), δ (TMS,
Into a baked 15 mL Schlenk tube with a stopcock in the side arm was
added 0.31 g of 1. The tube was evacuated under vacuum and then
flushed with dry nitrogen three times through the side arm. Freshly
distilled toluene (2 mL) was injected into the tube to dissolve the
monomer. The catalyst solution was prepared in another tune by
dissolving 16 mg of tungsten(VI) chloride and 17 mg of tetraphenyltin
in 2 mL of toluene. The catalyst solution was aged at room temperature
for 15 min and was transferred to the monomer solution using a
hypodermic syringe. The reaction mixture was stirred at 100 °C for 24
h. The solution was cooled to room temperature, diluted with 15 mL of
chloroform, and added dropwise to 500 mL of methanol through a
cotton filter under stirring. The precipitate was allowed to stand over-
night and then collected by filtration. The polymer was washed with
methanol and dried under vacuum at room temperature to a constant
weight.
Characterization data for P1: Yellow powder; yield 66.3%. Mw
28 800; Mw/Mn 1.3 (GPC, polystyrene calibration). IR (thin film),
v (cmꢀ1): 3084, 3052, 3023, 2952, 2920, 2868, 1780, 1755, 1598, 1503,
1455, 1444, 1400, 1384, 1370, 1342, 1265, 1200, 1165, 1116, 1016. 1H
NMR (300 MHz, CDCl3), δ (TMS, ppm): 6.63, 6.50, 6.23, 6.16, 4.24,
3.20, 2.36, 2.13, 1.66, 1.34, 0.92, 0.78. 13C NMR (75 MHz, CDCl3), δ
(TMS, ppm): 168.5, 148.0, 145.4 (=CPhO), 143.1 (PhC=), 141.3,
131.1, 126.1, 119.4, 80.2, 65.8, 48.1, 40.0, 34.4, 31.5, 25.3, 23.2, 22.3,
21.1, 16.2.
Characterization data for P2: Yellow powder; yield 30.9%. Mw
12 300; Mw/Mn 3.1 (GPC, polystyrene calibration). IR (thin film), v
(cmꢀ1): 3056, 3028, 2953, 2926, 2868, 1716, 1640, 1606, 1490, 1453,
1404, 1387, 1368, 1288, 1272, 1179, 1108, 1017. 1H NMR (300 MHz,
CDCl3), δ (TMS, ppm): 7.26, 7.20, 6.98, 6.79, 6.16, 4.86, 2.26, 1.70,
1.43, 1.25, 0.90. 13C NMR (75 MHz, CDCl3), δ (TMS, ppm): 165.8,
150.4 (=CPhCO2), 146.5 (PhC=), 128.9, 128.4, 126.0, 74.8, 47.2, 41.9,
41.0, 34.3, 31.4, 30.0, 26.4, 23.6, 22.0, 20.8, 18.5, 16.5.
Photopatterning. Photooxidation reactions of the polymer films
were conducted in air at room temperature using 365 nm light obtai-
ned from a Spectroline ENF-280C/F UV lamp at a distance of 1 cm as
light source. The incident light intensity was ∼18.5 mW/cm2. The film -
was prepared by spin-coating the polymer solution (10% w/w in
ppm): 7.50 (m, 2H), 7.42 (d, 2H), 7.32 (m, 3H), 6.82 (d, 2H), 4.85 (s,
1H). 13C NMR (75 MHz, CDCl3), δ (TMS, ppm): 155.6, 133.3, 131.4,
128.4, 127.9, 123.4, 115.7, 115.6, 89.4, 88.1.
1-{[(1R,2S,5R)-(ꢀ)-Menthoxymethylcarbonyloxy]phenyl}-2-phe-
nylacetylene (1). Into a 250 mL round-bottom flask equipped with a
stirring bar were dissolved 3.88 g (20 mmol) of 5, 6.24 g (30 mmol) of
DCC, 0.49 g (4 mmol) of DMAP, and 0.77 g (4 mmol) of TsOH in
100 mL of dry DCM/THF mixture (5:1 v/v). The solution was cooled
to 0 °C with an ice bath, into which 4.29 g (20 mmol) of 6 dissolved in
50 mL of DCM/THF mixture (5:1 v/v) was added under stirring via a
dropping funnel. The reaction mixture was stirred overnight. After
filtration, the solution was concentrated by a rotary evaporator. The
crude product was purified by a silica gel column using chloroform/
hexane mixture (1:2 v/v) as eluent. White solid; yield 79.2%. IR (thin
film), v (cmꢀ1): 2954, 2924, 2830, 1784, 1594, 1572, 1506, 1445, 1407,
1383, 1368, 1343, 1277, 1234, 1203, 1166, 1119, 1041, 1013. 1H NMR
(300 MHz, CDCl3), δ (TMS, ppm): 7.54 (m, 4H), 7.34 (m, 3H), 7.14
(d, 2H), 4.38 (d, 2H), 3.28 (m, 1H), 2.34 (m, 1H), 2.12 (d, 1H), 1.66
(m, 2H), 1.56 (2, 1H), 1.34 (m, 2H), 0.98 (m, 7H), 0.83 (d, 3H). 13C
NMR (75 MHz, CDCl3), δ (TMS, ppm): 169.2, 150.1, 132.8, 131.6,
128.3, 123.0, 121.6, 121.1, 89.5 (ꢁCꢀPhO), 88.5 (PhCꢁ), 80.5, 65.9,
48.2, 40.0, 34.4, 31.5, 25.5, 23.3, 22.3, 21.0, 16.3. HRMS (MALDI-
TOF): m/z 391.2266 [(M þ 1)þ, calcd 391.2195].
(1R,2S,5R)-(ꢀ)-Menthyl-4-iodobenzoate (9). Into a 250 mL round-
bottom flask equipped with a stirring bar were dissolved 3.15 g (20
mmol) of 8, 6.24 g (30 mmol) of DCC, 0.49 g (4 mmol) of DMAP, and
0.77 g (4 mmol) of TsOH in 100 mL of dry DCM/THF mixture (4:1 v/
v). The solution was cooled to 0 °C with an ice bath, into which 5.00 g
(20 mmol) of 7 dissolved in 50 mL of DCM/THF (4:1 v/v) was added
under stirring via a dropping funnel. The reaction mixture was stirred
overnight. After filtration, the solution was concentrated by a rotary
evaporator. The crude product was purified by a silica gel column using
chloroform/hexane mixture (1:2 v/v) as eluent. A white solid of 9 was
obtained in 83.4% yield. 1H NMR (300 MHz, CDCl3), δ (TMS, ppm):
7.78 (d, 2H), 7.76 (d, 2H), 4.93 (t, 1H), 2.09 (d, 1H), 1.92 (m, 1H), 1.71
(m, 2H), 1.49 (m, 2H), 1.11 (m, 2H), 0.94 (m, 6H), 0.79 (d, 3H).
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Table 1. Polymerizations of 1 and 2a
b
b
b
run monomer temp (°C) yield (%)
Mn
Mw
Mw/Mn
1
2
3
1
1
2
80
100
100
17.1
66.3
30.9
5300
7900
1.5
3.0
3.1
9600 28800
4000 12300
a Catalyzed by WCl6ꢀPh4Sn in toluene under nitrogen for 24 h; [M]0 =
0.2 M, [WCl6] = [Ph4Sn] = 10 mM. b Determined by GPC in THF on
the basis of a linear polystyrene calibration.
1,2-dichloroethane) at 2000 rpm for 1 min on a silicon wafer. The
polymer film was dried in a vacuum oven at room temperature overnight.
The photopatterns were generated using a copper photomask and taken
on an optical microscopy (Nikon 80i equipped with Nikon Digital Sight
DS-5Mc-L2 Cooled CCD camera) using UV light source.
’ RESULTS AND DISCUSSION
Figure 1. IR spectra of (A) monomer 1 and (B) its polymer P1
(Table 1, run 2).
Monomer Synthesis. With a view to synthesizing polymers
with both fluorescence and helicity, we designed two dipheny-
lacetylene derivatives containing chiral menthyl units according
to the synthetic routes shown in Scheme 1. The Pd-catalyzed
cross coupling of 3 and 4 gave 5, whose esterification reaction
with 6 in the presence of DCC, DMAP, and TsOH furnished the
desirable product 1. Monomer 2 was obtained by esterification
reaction of 7 with 8, followed by Sonogashira coupling of the
resultant compound (9) with 3. All the reactions could be carried
out at ambient conditions, and all the intermediates and mono-
mers were obtained in high yields. We characterized their
molecular structures by standard spectroscopic methods and
obtained satisfactory analysis results (see Experimental Section
for details).
Polymerization. In our previous study, we succeeded in
polymerizing diphenylacetylenes containing ester unit by
WCl6ꢀPh4Sn.19 We thus tested whether the same catalyst can
initiate the polymerizations of 1 and 2. Stirring a toluene solution
of 1 at 80 °C in the presence of WCl6ꢀPh4Sn under nitrogen for
24 h gives a polymeric product with Mw of 7900 in a low yield
(∼17%) (Table 1, run 1). GPC analysis of the residue by
evaporation of the filtrate shows that it is oligomeric species
with molecular weights of several hundreds. Raising the tem-
perature to 100 °C has improved the polymerization result
significantly: the yield and molecular weight of the polymer are
more than 3.5-fold higher than those obtained at 80 °C. A higher
temperature may make the catalyst more active and thus greatly
accelerates the polymerization reaction.
Under the same experimental conditions, 2 was converted into
a polymer with a lower molecular weight in a lower yield
(Table 1, no. 3). Why the polymerization behavior of 2 is
different from that of 1 is unclear at present but may be related
to the difference in the steric crowdedness around the triple
bond. Attachment of (ꢀ)-menthyl pendant to the diphenylace-
tylene structure directly through the ester functionality generates
sterically more bulky monomer 2, making it more difficult to be
polymerized into high molecular weight polymer in high yield by
the tungsten catalyst. On the other hand, the polar ester group in
1 may be well shielded by the methylene group. This reduces its
toxic interaction with the transition-metal catalyst and hence
enhances the polymerizability of the monomer.
Figure 2. 1H NMR spectra of 1 and its polymer P1 (Table 1, run 2) in
CDCl3. The solvent peaks are marked with an asterisk.
structures (see Experimental Section for details). An example of
the IR spectrum of P1 is shown in Figure 1, with that of its
monomer (1) given in the same figure for comparison. The
spectrum of P1 resembles that of 1, exhibiting a strong band at
1740 cmꢀ1 associated with CdO stretching vibration. This result
indicates that the ester functionality remains intact after the
polymerization. Since no peaks related to CtC stretching
vibration at ∼2222 cmꢀ1 are observed in the spectra of the
monomers, no further information on the structures of the
polymers can be obtained. We thus characterized the polymers
Structural Characterization. The molecular structures of the
polymers are characterized by spectroscopic methods, and all
give satisfactory data corresponding to their expected molecular
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Figure 4. TGA thermograms of P1 (Table 1, run 2) and P2 recorded
under nitrogen at a heating rate of 10 °C/min.
Figure 3. 13C spectra of (A) 1 and (B) its polymer P1 (Table 1, run 2)
in CDCl3. The solvent peaks are marked with an asterisk.
by NMR analyses, which prove that the acetylenic triple bonds of
the monomers have been consumed and transformed to the
polyene double bonds of the polymers by the polymerization
reactions.
Figure 2 shows the 1H NMR spectra of 1 and its polymer P1 in
chloroform-d. The phenyl protons linked to the acetylene triple
bond of 1 resonate at δ 7.54, 7.34, and 7.14. These absorption
peaks, however, disappear after polymerization. The transforma-
tion of the acetylene triple bond of 1 to the double bond of P1 by
the polymerization reaction downfield shifts the phenyl proton
resonances, which are now observed at δ 6.63, 6.50, and 6.16.
The peaks are broad because the aromatic rings are directly
attached to a rigid polymer backbone. No other unexpected
signals are observed, and all the peaks can be readily assigned,
verifying the high purity of its molecular structure. Similar results
are obtained when compared the spectrum of P2 with that of 2,
proving that the molecular structure of the polymeric product is
indeed P2, as shown in Chart 1.
Figure 5. GPC traces of P1 before and after UV irradiation in air for
different time intervals.
bulky groups are unstable.11 The first examples of optically active
polyacetylenes are a group of poly(1-alkyne)s with small chiral
groups, which are very sensitive to oxygen, light, and heat and
must be stored in a refrigerator under nitrogen in the dark.3
Thanks to the stable aromatic rings along the polymer backbone,
our polymers can be stored under ambient temperatures in vials
in air, with no observable color and morphology changes.
Both P1 and P2 are completely soluble in common organic
solvents, such as THF, DCM, chloroform, and dioxane and can
be readily fabricated into tough solid films by spin-coating or
solution-casting process. They enjoy high thermal stability. As
shown in Figure 4, P1 loses merely 5% of its weight when heated
to ∼300 °C (Td). The polymer strand is well wrapped by the
bulky, stable phenyl rings, which imparts P1 with high resistance
to thermolysis. The Td (∼250 °C) of P2 is lower than P1,
presumably due to its lower molecular weight.
The 13C NMR spectroscopy offers more informative char-
acterization data. For example, the spectrum of P1 shows no
acetylene carbon resonances of 1 at δ 89.5 and 88.5 (Figure 3).
On the other hand, two new peaks appear at δ 145.4 to 143.1.
Since the resonance peaks of the backbone carbons of poly-
(diphenylacetylene)s carrying naphthalene pendants have been
reported to locate at δ 145.2 and 143.7.19a It thus seems
reasonable to assign the peaks at δ 145.4 and 143.1 to the
backbone olefin carbon resonances of P1. Similarly, P2 displays
no peaks associated with the acetylene carbon absorptions of 2 at
δ 92.1 and 88.7 but exhibits backbone olefin carbon resonances
at δ 150.4 and 146.5. This also once again proves that the
diphenylacetylene polymerization is realized through the con-
version of the triple bonds to the double bonds.
We also investigated the photostability of the polymers. We
irradiated the solid powders of P1 in air under stirring by UV light
from a Spectroline ENF-280C/F UV lamp placed ∼5 cm away
from the center of the Schlenk tube (incident light intensity = 13
Solubility and Thermal Stability. It is known that mono-
substituted polyacetylenes without functional bridges and/or
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mW/cm2). The change in the molecular weight of the polymer at
different time intervals was followed by GPC analysis. As shown
in Figure 5, the polymer suffers no degradation after exposure to
UV irradiation for 0.5 h. Even prolonging the time to 24 h,
practically no change was observed in its GPC curve, suggesting
that it possesses a high photostability. P1 contains many aromatic
rings, which may act as radical sponges to trap, deactivate, and/or
annihilate the destructive radical species formed in the UV
irradiation process.
Chiroptical Properties. Acetylene polymerizations initiated
by Mo- and W-based catalysts are known to yield polymers with
irregular or random stereostructures, containing chain segments
with cisꢀtransoid and transꢀcisoid conformations.16 Many
research groups have formed that stereoregular cis geometrical
structure is indispensable for helix induction.5,6,8 Will our chiral
poly(diphenylacetylene)s (P1 and P2) prepared from tungsten
catalyst take helical conformation? To check this, we measured
the [R]D values of the polymers at room temperature (Table 2).
The [R]D value of P1 in CHCl3 is very large (ꢀ1728.6°) and is
more than 90-fold higher than its monomer 1 (ꢀ19.3°) in the
same solvent, suggestive of chirality contribution from the
polyene backbone and the formation of helical conformation
with an excess in one handedness. The [R]D value of the polymer
changes with solvent, but the direction remains unchanged, in
contrast to our previous observation that not only the magni-
tudes of [R]D’s of the solutions of chiral monosubstituted
poly(phenylacetylene)s change but also their signs reverse even
when the polarities of the solvents are similar.9f,g A disubstituted
polyacetylene chain is generally more rigid than a monosubsti-
tuted one,20 and it is envisioned that the high chain stiffness of P1
has helped keep its helical-sense preference unaltered by the
solvent perturbation.
the peaks at 370 and 425 nm should be associated with the
absorptions of the polyacetylene backbone. The ground-state
electronic transitions vary little with the solvent: the UV spectra
measured in DCM, toluene, and CHCl3 are basically the same as
that performed in THF. Similarly, the backbone of P2 absorbs at
wavelengths longer than 330 nm. No spectral change was
observed when the measurement was carried out in other
solvents.
The CD spectra of P1 and P2 measured in different solvents
are shown in the lower parts of Figures 6 and 7. The THF
solution of P1 displays a strong CD band at 380 nm with a molar
ellipticity as high as ꢀ74 500 deg cm2 dmolꢀ1, whereas its
monomer (1) is CD-inactive, giving a flat line parallel to the
abscissa. Thus, the strong Cotton effect observed at 380 nm
should be related to the absorption of the polyene backbone,
unambiguously confirming that the main chain indeed takes a
helical conformation with a large excess of one-handedness. To
check how the chain conformation of P1 varies with response to
Similarly, the [R]D value of P2 is much higher than its
monomer. Its magnitude varies with solvent but its sign remains
unchanged. There is also no correlation between the [R]D value
and the solvent polarity. Interestingly, however, although both
P1 and P2 possess the same kind of chiral moiety [i.e.,
(1R,2S,5R)-(ꢀ)-menthyl], the [R]D of the former is up to 3-fold
larger than that of the latter in magnitude and opposite to that of
the latter in sign, demonstrating that the chiroptical properties of
polyacetylenes can be readily tuned by changing their molecular
structures.
To confirm whether the macromolecular chains are really
spiraling in a helical sense, we measured the circular dichroism
(CD) spectra of the polymer solutions. Before the CD analysis,
we performed UV measurements to check where the backbone
absorbs. The upper panels of Figures 6 and 7 show the UV
spectra of P1 and P2 in different solvents. In THF, P1 exhibits
absorption peaks centered at 270, 370, and 425 nm. Since its
monomer does not absorb at wavelengths longer than 310 nm,
Figure 6. UV (upper panel) and CD (lower panel) spectra of monomer
1 in CHCl3 and P1 in different solvents at room temperature. Con-
centration (mM): ∼0.38 (CD) and 0.12 (UV). The spectral data in
toluene below 290 nm were not taken because of the interference by the
solvent absorption.
Table 2. Specific Optical Rotations of Monomers and Polymers in Different Solvents
[R]D, deg (c, g/dL)
solventa
1
P1
2
P2
CHCl3
DCM
THF
ꢀ19.3 (0.015)
ꢀ1728.6 (0.014)
ꢀ1450.0 (0.018)
ꢀ1718.8 (0.016)
ꢀ1212.5 (0.016)
ꢀ17.0 (0.125)
þ765.7 (0.042)
þ694.7 (0.036)
þ690.6 (0.036)
þ573.0 (0.050)
toluene
a Debye solvent polarizability functions of the solvents: 0.71 (CHCl3), 0.73 (DCM), 0.68 (THF), and 0.31 (toluene).
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Figure 8. UV (upper panel) and CD (lower panel) spectra of P1 in
THF and THF/hexane mixtures with different hexane fractions at room
temperature. Concentration (mM): ∼0.13 (CD and UV).
Figure 7. UV (upper panel) and CD (lower panel) spectra of monomer
2 in CHCl3 and P2 in different solvents at room temperature. Con-
centration (mM): ∼1 (CD), 0.1 (UV). The spectral data in toluene
below 290 nm were not taken because of the interference by the solvent
absorption.
The CD spectrum suffers little change when up to 40% of hexane
is added to the THF solution, but the peak ellipticity increases
slightly (in absolute term) afterward.
the change in its surrounding environment, we carried out the
CD measurement in different solvents. The spectral pattern of
P1 remains the same when the solvent is changed from THF to
DCM, toluene, and CHCl3, indicating the same screw sense
dominates in these solvents. The Cotton effects measured in
DCM and toluene, are, however, slightly lowered, suggesting that
some of the helical chains have reversed their screw sense.
CD analysis verifies that P2, like its cousin P1, possesses a
helical conformation. In THF, it shows a [θ] value of þ17 900
deg cm2 dmolꢀ1 at ∼400 nm associated with the helicity of the
polymer chain. Thanks to the stiff backbone, the polymer
possesses a stable helical conformation and is thus less suscep-
tible to the solvent perturbation, as revealed by the large
similarity in the CD spectra obtained in DCM, toluene, and
CHCl3.
Some chiral molecules and polymers are found to exhibit lower
or amplified CD signals when aggregated in poor solvents or
fabricated as thin films in the solid state.21 Which case can be best
described for our system? To address this question, we studied
the chiroptical properties of the polymers in THF/hexane and/
or THF/H2O mixtures. Since hexane and water are nonsolvents
for P1 and P2, their polymer chains must have been aggregated in
the solvent mixtures with high fractions of these solvents. Dilute
polymer solutions are used for the investigation to prevent the
precipitation of the polymers. As shown in Figure 8, addition of
small amount of hexane (<50%) into the THF solution of P1
causes no significant change in its UV spectrum. Afterward, the
peak intensity increases slightly, but the spectral pattern remains
unchanged. Similar results are also obtained in the CD analyses.
The effect of water on the optical and chiroptical properties of P1
is more pronounced. The UV spectra measured in THF/H2O
mixtures are resembled to that in THF, irrespective of the water
content (Figure 9). A level-off tail is observed in THF/H2O
mixtures with water fractions larger than 10%, due to the light
scattering or Mie effect of the polymer aggregates.22 The mixtures
are, however, visually transparent and macroscopically homoge-
neous, suggesting that the polymer aggregates are nanometer
sized.23 Addition of 10% of water in the THF solution of P1 alters
little its helicity. When the water content is increased to 20%, the
Cotton effect at 388 nm is intensified by more than 2-fold and a new
peak is emerged at 430 nm. Further increment of the amount of
water in the solvent mixture weakens the backbone CD absorptions,
but their ellipticities are still higher than those in pure THF.
Why the polymer shows different responses to the presence of
hexane and water? Being hydrophobic in nature, P1 should
aggregate more readily in THF/H2O mixtures even at low water
contents. In the aggregates, the polymer chains are located in the
close vicinity, which promotes exciton coupling24 and hence
results in the emergence of a new CD peak at the longer
wavelength region. The steric effect and/or hydrophobic inter-
action imposed by the neighboring strand, on the other hand,
have stabilized the helical conformation and thus intensified the
CD signals. In a mixture with a “lower” water fraction, the
polymer stands of P1 may slowly assemble to form more regular
or ordered aggregates, while in a mixture with a “higher” water
content, the fast agglomeration of the polymer strands may
generate aggregates with more random structures, which weak-
ens the interstrand interaction and the associated Cotton effects.
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Figure 11. CD spectra of thin films of P1 and P2 at room temperature.
Figure 10 depicts the UV and CD spectra of P2 in THF and
THF/water mixtures with different water fractions. The UV
spectrum of P2 shifts slightly upward even when a large amount
of water is added to its THF solution. Unlike P1, the CD
spectrum of P2 changes little upon aggregate formation, pre-
sumably due to its relatively lower molecular weight, which forms
fewer aggregates with smaller sizes in the aqueous mixtures.
Similar to those in THF and THF/H2O mixtures, the spectra
of thin films of the polymers depict strong CD bands associated
with the helicity of the polyacetylene chains (Figure 11). New
Cotton effects are observed at the longer wavelengths due to the
stronger interstrand interaction in the solid state.
Figure 9. UV (upper panel) and CD (lower panel) spectra of P1 in
THF and THF/water mixtures with different water fractions at room
temperature. Concentration (mM): ∼0.13 (CD and UV).
Light Emission. It now becomes clear that the present
polymers are optically active in both solution and aggregate
state. The next question is: will they emit strong light upon UV
exposure? Figure 12A depicts the PL spectra of P1 in THF and
THF/hexane mixtures. In pure THF, P1 emits a green light at
493 nm when photoexcited at 424 nm. The fluorescence
quantum yield (ΦF) determined by using fluorescein (ΦF =
79% in 0.1 M NaOH solution) is 25.4%. Addition of hexane into
its THF solution causes little spectral change but increases
slightly the light emission. The PL intensity remains unchanged
when up to 80% of hexane is added but drops afterward.
Generally, aggregation quenches the PL of fluorophores due to
the increase in the short-range interactions.25,26 On the other
hand, aggregation can positively restrict the intramolecular
motions of the fluorophores, which block the channels for the
excitons to decay nonradiatively. Whether aggregation quenches
or boosts the PL of fluorophores depends on their molecular
structures. In P1, the first effect seems to be prevailed, and its PL
thus becomes lower in THF/hexane mixtures with high hexane
contents.
The destructive effect of aggregation on the PL of P1 is more
obvious in THF/H2O mixture. Since water is such a nonsolvent
for P1, addition of 20% of water into its THF solution has already
aggregated its chains and weakened its PL (Figure 12B). The
higher the water content, the weaker is the light emission.
Although aggregation reaches its maximum in the solid state,
the thin film of P1 shows reasonable strong green light upon UV
irradiation.
Figure 10. UV (upper panel) and CD (lower panel) spectra of P2 in
THF and THF/water mixtures with different water fractions at room
temperature. Concentration (mM): ∼0.10 (CD and UV).
The phenomenon observed in P1 is not an isolated case but is
also found in P2. In THF, P2 emits at 520 nm with an ΦF value of
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Figure 12. PL spectra of P1 in (A) THF/hexane and (B) THF/water mixtures with different hexane and water fractions. Concentration: 10 μM;
excitation wavelength: 424 nm.
more sensitive tool than the CD spectroscopy for detecting
aggregate formation in the polymer.
Photopatterning. Since P1 and P2 are emissive in the solid
state, we explored their potential use as PL imaging materials.27
UV irradiation of a thin film of P1 through a mask bleaches the
exposed regions, while the unexposed regions remain emissive. A
photopattern is thus generated without performing the develop-
ment process (Figure 14).
’ CONCLUSION
In this work, diphenylacetylenes bearing stereogenic (ꢀ)-
menthyl groups are synthesized and polymerized by
WCl6ꢀPh4Sn, furnishing poly(diphenylacetylene)s with high
molecular weights in moderate yields. All the polymers are
soluble in common organic solvents and are thermally stable
(Td g 250 °C). They show large specific optical rotations and
strong CD signals in the absorption region of the polyene
backbone in the solution and aggregate states, suggesting that
their main chains are helically rotating in a preferred screw sense
by the chiral pendants. Strong green lights are observed when
their solutions and nanoaggregates in poor solvents are photo-
excited. UV irradiation of their films photooxidizes the exposed
parts, generating two-dimensional fluorescent photopatterns.
Figure 13. PL spectra of P2 in THF and THF/water mixtures with
different water fractions. Concentration: 10 μM; excitation wavelength:
400 nm.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail chjacky@ust.hk (J.W.Y.L.), tangbenz@ust.hk (B.Z.T.);
Ph þ852-2358-7375 (8801); Fax þ852-2358-1594.
’ ACKNOWLEDGMENT
Figure 14. Two-dimensional fluorescent photopattern generated by
photooxidation of P1. The photograph was taken under a fluorescence
optical microscope.
This project was partially supported by the grant from Nissan
Chemical Industries, Ltd., the Research Grants Council of Hong
Kong (604509 601608, and HKUST2/CRF/10), the Innovation
and Technology Commission (ITP/008/09NP and ITS/168/
09), the University Grants Committee of Hong Kong (AoE/
P-03/08), and the National Science Foundation of China
(20974028). B.Z.T. thanks the support from the Cao Guangbiao
Foundation of Zhejiang University.
9.79% (Figure 13). Addition of small amount of water (<40%)
into its THF solution has enhanced its light emission. Further
increment of the water content has, however, decreased the PL,
with the intensity at 90% water content being one-third of that in
pure THF. Such result demonstrates that the PL technique is a
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dx.doi.org/10.1021/ma200075y |Macromolecules 2011, 44, 2427–2437