Effect of conjugation length on intrachain chromophore-chromophore interaction in silylene-spaced divinyloligoarene copolymers

Article abstract of DOI:10.1021/ma047713c

A range of silylene-spaced divinyloligoarene alternating copolymer 3 has been synthesized regioregularly by rhodium-catalyzed hydrosilylation of bis(alkynes) 5 with bis(silanes) 6. Monomeric reference compounds 4 having similar chromophore components were

Full text of DOI:10.1021/ma047713c

1442
Macromolecules 2005, 38, 1442-1446
Effect of Conjugation Length on Intrachain Chromophore-Chromophore
Interaction in Silylene-Spaced Divinyloligoarene Copolymers
Yen-Ju Cheng,† Sourav Basu,† Shr-Jie Luo,†,§ and Tien-Yau Luh*,†,‡
Department of Chemistry and Institute of Polymer Science and Engineering, National Taiwan
University, Taipei, Taiwan 106, and Institute of Chemistry, Academia Sinica, Taipei, Taiwan 106
Received November 8, 2004; Revised Manuscript Received December 1, 2004
ABSTRACT: A range of silylene-spaced divinyloligoarene alternating copolymer 3 has been synthesized
regioregularly by rhodium-catalyzed hydrosilylation of bis(alkynes) 5 with bis(silanes) 6. Monomeric
reference compounds 4 having similar chromophore components were also prepared for comparison. The
silylene moieties serve as insulating spacers between chromophores. Intrachain interaction appears to
be negligible due to the localization of exciton in these conjugated chromophores, resulting in the emission
that closely resembles that of the parent chromophore.
Introduction
investigation on the effect of conjugation length of
chromophore in a series of silylene-spaced regioregular
divinyloligoarene copolymers 3.
Well designed interruption of π-conjugation along the
polymer backbone by insulating spacers allows tuning
of the photophysical properties of the polymers and
furnishes processable materials.1-5 There has been an
increasing study of using silylene moiety as the spacer.2-5
In general, when the silylene spacer contains only one
silicon atom, no conjugative interactions between the π
systems and the silicon moiety is observed. We recently
reported a series of silylene-spaced divinylarene copoly-
mers that exhibit versatile photophysical properties
such as intrachain energy transfer,6 through space
chromophore-chromophore interactions,7 and the trans-
mission of chiroptical properties.8 Theoretical9-11 and
experimental12 studies on inter- or intrachain interac-
tions between chromophores have been extensive. Model
compounds such as paracyclophanes and polystilbene
derivatives provide useful insight for understanding
interchromophore delocalization through space.12 The
extent of such through-space interactions in the excited
state has been suggested to be dependent on the
conjugation length of chromophores. The shorter the
conjugation length, the more is prone to delocalization
over interacting chromophores leading to excimer for-
mation. On the contrary, as the conjugation length
increases, the interactions between chromophores may
be significantly reduced because the geometry relaxation
in the excited state of longer conjugated moieties may
stabilize the localization of exciton in a single chro-
mophore. Indeed, the aforementioned silylene-spaced
divinylbenzene copoymers 1 have been shown to exhibit
molecular-weight dependent aggregate emission profiles
which suggest that significant intrachain chromophore-
chromophore interactions may occur.7,13 In our prelimi-
nary communication, we found that the emission spec-
trum of copolymer 2 is very similar to that of the
corresponding monomer.7 The divinylterthiophene moi-
ety has longer conjugation length than divinylbenzene
chromophore so that exciton may be able to localize in
this chromophore. In this paper, we report a systematic
Results and Discussion
Synthesis. The divinyloligoarene chromophores of
different conjugation lengths and structural variety
were incorporated into the corresponding copolymers
3a-d. Copolymers 3a-d were synthesized by the hydro-
silylation of bis(alkynes) 5 with the corresponding bis-
(silane) 6 in a manner similar to those described
previously (eq 1).5 The corresponding monomeric model
compounds 4a-d were prepared similarly by either
hydrosilylation of bis(silanes) 6 with phenylacetylene
(7) or bis(alkynes) 5 with 2-(phenyl)vinyldimethylsilane
(8) (eqs 2 and 3).7 The preparation of furan-containing
pentaaryls 4d was based on the one-pot furan synthesis
recently developed in our laboratory.14 Thus, treatment
of propargylic dithioacetal 9 with n-BuLi followed by
terephthaldicarboxaldehyde to give the intermediate
† Department of Chemistry, National Taiwan University.
‡ Institute of Polymer Science and Engineering, National
Taiwan University.
§ Institute of Chemistry, Academia Sinica.
10.1021/ma047713c CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/20/2005

Macromolecules, Vol. 38, No. 4, 2005
Effect of Conjugation Length 1443
Figure 1. Absorption spectra of 3a (solid line) and 4a (dashed
line), excitation spectrum of 3a (dotted line, λem ) 370 nm) in
CHCl3, and emission spectra of 3a (Mn ) 5700 and 17800, solid
line) and 4a (dashed line, λex ) 310 nm) in CHCl3.
which underwent cyclization under acidic condition to
yield furan-containing pentaaryl 10 in 55% yield. Depro-
tection of the TMS group in 10 afforded 5d in 92% yield
(eq 4). The details are summarized in the Experimental
Section.
Figure 2. Absorption spectra of 3b (solid line) and 4b (dashed
line) and excitation spectrum of 3b (dotted line, λem ) 490 nm)
in CHCl3 and emission spectra of 3b (solid line) and 4b (dashed
line, λex ) 410 nm) in CHCl3.
Photophysical Properties
Figure 3. Absorption spectra of 3c (solid line) and 4c (dashed
line) and excitation spectrum of 3c (dotted line, λem ) 390 nm)
in CHCl3 and emission spectra of 3c (solid line) and 4c (dashed
line, λex ) 324 nm) in CHCl3.
The absorption, emission and excitation spectra of
copolymers 3a-d are shown in Figures 1-4, and those
of the corresponding monomeric compounds 4a-d are
compared in the respective figures. The λmax of the
chromophores were chosen as the corresponding ex-
citation wavelengths in emission spectra for each of
these copolymers. It is noteworthy that the aromatic
moieties in polymers 3b-d have relatively long con-
jugation length in comparison with that in 1. Figures
2-4 show the spectroscopic properties of three repre-
sentative examples, polymer 3b having terphenylene-
tetravinylene chromophore, polymer 3c containing di-
vinyldiphenyloxadiazole chromophore, and polymer 3d
containing divinylpentaaryl chromophore. The excita-

1444 Cheng et al.
Macromolecules, Vol. 38, No. 4, 2005
silylene moieties simply serve as insulating spacer, no
conjugation with the π-system being observed.3f
Experimental Section
Compounds 4c,6a 5c,6a 6b,8 6c,7a and 87a were prepared
according to literature procedures. The absorption spectra were
recorded on a Hitachi U-3310 spectrometer. The photolumi-
nescence and excitation spectra were recorded on a Hitachi
F-4500 fluorescence spectrometer. Gel permeation chroma-
tography (GPC) was performed on a Waters GPC machine
using an isocratic HPLC pump (1515) and a refractive index
detector (2414). THF was used as the eluent (flow rate ) 1
mLmin-1). Waters Styragel HR2, HR3 and HR3 and HR4 (7.8
× 300 mm) were employed for molecular weight determination.
Polystyrene were used as standard (Mn values range from 375
to 3.5 × 106).
Bis(2-dimethylsilanyl)vinylbiphenyl (6a). A THF solu-
tion of Me2(iPrO)SiCH2MgCl (10 equiv) was evacuated as much
as possible and benzene (30 mL) was introduced. Under N2,
to this mixture was introduced a solution of 4,4′-bis(1,3-
dithiolan-2-yl)biphenyl (1.45 g, 4.0 mmol) and NiCl2(PPh3)2
(0.28 g, 0.4 mmol) in benzene (50 mL). The mixture was
refluxed under N2 for 16 h and then poured into saturated
NH4Cl. The organic layer was separated and the aqueous layer
was extracted twice (2 × 20 mL) with ether. The combined
organic portions were washed twice with 10% NaOH (2 × 20
mL) and brine (20 mL) and dried (MgSO4). The solvent was
removed in vacuo and the residue was triturated with MeOH
to give the solid residue which was mixed with LiAlH4 (304
mg, 8.0 mmol) in benzene (50 mL). The mixture was refluxed
under N2 for 18 h and then cooled to room temperature,
quenched with water, and filtered. The organic layer was
separated, dried (MgSO4) and the solvent was removed in
vacuo to give the residue which was chromatographed on silica
gel (hexane) to afford 6a (0.75 g, 58%); mp 151-153 °C
Figure 4. Absorption spectra of 3d (solid line) and 4d (dashed
line) and excitation spectrum of 3d (dotted line, λem ) 485 nm)
in CHCl3 and emission spectrum of 3d (solid line) and 4d
(dashed line, λex ) 400 nm) in CHCl3.
tion and absorption spectra of polymers 3b-d are very
similar to those of corresponding monomers 4b-d,
respectively. In addition, the emission profiles for
polymers 3b-d are essentially the same as those for
the corresponding monomers 4b-d having the same
divinyloligoarene chromophores. The silicon moiety in
these polymers may serve as an insulating spacer.
These results suggested that the photophysical prop-
erties of 3b-d are very different from those of 1 or
related silylene-spaced mononuclear divinylarene co-
polymers.7a It is thus believed that intrachain through-
space interaction between chromophores in these poly-
mers due to folding of the polymeric chain may not take
place leading to the excimer formation.
As shown in Figure 1, besides the absorption at the
corresponding λmax, there was a tailing absorption
extended to the longer wavelength for copolymers 3a
having divinylbiphenyl chromophores. Similar to those
of divinylbenzene copolymers 1,7 the emission at longer
wavelengths may arise from the through-space intra-
chain interaction between chromophores owing to the
folding aggregate.13 The relative intensity for the longer
wavelength emission in 3a, however, is slightly affected
by increasing the degree of polymerization. Vibronic fine
splittings were vaguely found in these fluorescence
spectra. The intrachain π-π interaction between di-
vinylbiphenyl moieties in 3a might, therefore, be less
favorable than those of divinylbenzene analogues 1
because of its longer conjugation length. These results
are consistent with the models on the interactions of
conjugated systems.12
1
(pentane). H NMR (200 MHz, CDCl3): δ ) 0.24 (d, J ) 3.6
Hz, 12 H), 4.2 (m, 2 H) 6.47 (dd, J1 ) 19.2 Hz, J2 ) 2.5 Hz, 2
H), 6.98 (d, J ) 19.2 Hz, 2 H), 7.48 (d, J ) 8.0 Hz, 4 H), 7.56
(d, J ) 8.0 Hz, 4 H).13C NMR (50 MHz, CDCl3): δ ) -4.0, 126.3,
126.9, 127.0, 137.3, 140.3, 140.8. IR (KBr): ν ) 2966, 2906,
2117, 1660, 1501, 1400, 1249, 1202, 1123, 990, 888, 794, 744,
639. MS: m/z (70 ev) 322. Anal. Calcd for C22H26Si2: C, 74.46;
H, 8.12. Found: C, 74.24; H, 8.27.
4,4′-Bis[2-(dimethylstyrylsilanyl)vinyl]biphenyl (4a).
A mixture of 6a (0.16 g, 0.5 mmol), phenylacetylene 7 (0.102
g, 1 mmol), and RhCl3(PPh3)3 (2.3 mg, 0.5 mol %) in THF (5
mL) was refluxed for 8 h. Removal of the solvent in vacuo and
chromatographic purification on silica gel (pentane) afforded
4a (0.16 g, 61%): mp 185-187 °C (pentane). 1H NMR (200
MHz, CDCl3): δ ) 0.35 (s, 12 H), 6.53 (d, J ) 19 Hz, 2 H),
6.55 (d, J ) 19 Hz, 2 H), 6.95 (d, J ) 19 Hz, 2 H), 6.97 (d, J )
19 Hz, 2 H), 7.23-7.60 (m, 18 H). 13C NMR (75 MHz, CDCl3):
δ ) -2.5, 126.5, 126.9, 127.0, 127.4, 127.6, 128.1, 128.5, 137.4,
138.3, 140.3, 144.3, 145.0. IR (KBr): ν ) 3030, 2989, 2985,
2899, 1912, 1604, 1574, 1448, 1401, 1335, 1250, 1198, 989, 837,
793, 733. MS m/z 526. Anal. Calcd for C36H38Si2: C, 82.07; H,
7.27. Found: C, 82.35; H, 7.40.
2,5-Bis{2,4-(2-dimethyl(2-styryl)silanylvinyl)phenyl-
vinyl}-1,4-dibutoxybenzene (4b). Under N2, a mixture of
phenylacetylene (7) (0.102 g, 1.0 mmol), 6b (0.30 g, 0.5 mmol),
and RhCl(PPh3)3 (2.3 mg, 0.5 mol %) in THF (5 mL) was
refluxed for 4 h. Removal of the solvent and chromatographic
separation on silica gel (hexane) afforded 4b (0.34 g, 85%); mp
Conclusions
In summary, we have systematically investigated the
influence of conjugation length of chromophores on the
photophysical behavior of silylene-spaced divinylarene
copolymers 3. When the chromophores in silylene-
spaced copolymer are mononuclear divinylarenes,
through-space delocalized interchromophoric contact
prevails, leading to emission at longer wavelengths due
to intrachain aggregation.7a When the conjugation
length of chromophore increases, the intrachain interac-
tion appears to be negligible due to the localization of
exciton in these longer conjugated chromophores, re-
sulting in the emission that closely resembles that of
the parent chromophore. Our results suggest that the
1
167-168 °C. H NMR (300 MHz, CDCl3): δ ) 0.31 (s, 12 H),
1.01 (t, J ) 7.4 Hz, 6 H), 1.53-1.61 (m, 4 H), 1.80-1.90 (m, 4
H), 4.05 (t, J ) 6.3 Hz, 4 H), 6.52 (d, J ) 19.1 Hz, 4 H), 6.94
(d, J ) 19.1 Hz, 4 H), 7.00-7.60 (m, 24 H). 13C NMR (75 MHz,
CDCl3): δ ) -2.5, 13.9, 19.5, 31.6, 69.2, 110.5, 114.2, 123.5,
126.5, 126.7, 126.8, 127.1, 127.4, 128.1, 128.4, 128.5, 137.4,
137.8, 138.3, 144.5, 144.9, 151.1. HRMS: calcd for C54H62Si2O2,
798.4288; found, 798.4285. Anal. Calcd for C54H62Si2O2: C,
81.15; H, 7.82. Found: C, 80.96; H, 7.87.
1,4-Bis{[3-phenyl-5-(4-trimethylsilylethynylphenyl)-
furyl]-2-yl}benzene (10). Under argon atmosphere, a 2.5 M

Macromolecules, Vol. 38, No. 4, 2005
Effect of Conjugation Length 1445
n-BuLi in hexane solution (4.1 mL, 10.2 mmol) was introduced
dropwisely to a THF (20 mL) solution of propargylic dithio-
acetal 9 (3.22 g, 8.5 mmol) at -78 °C, and the mixture was
stirred for 50 min. To this mixture was added slowly a THF
(10 mL) solution of terephthaldicarboxaldehyde (0.52 g, 3.9
mmol) at -78 °C. The reaction mixture was stirred for 1 h at
-78 °C, and gradually warmed to room temperature. After
further stirring for 1 h at room temperature, TFA (1.59 mL,
14.8 mmol) was added and the mixture was stirred at room
temperature overnight. The reaction mixture was quenched
with saturated NH4Cl (30 mL), and the organic layer was
separated. The aqueous layer was extracted with Et2O (40 mL
× 2). The combined organic layer was washed with saturated
NaHCO3 (40 mL × 2), dried (MgSO4), filtered, and evaporated
in vacuo. The resulting residue was purified by flash column
chromatography (silica gel, CH2Cl2/hexane 1:4) to afford the
desired 10 as a pale yellow solid (1.52 g, 55%): mp 184-185
(2.3 mg, 0.5 mol %) under N2. The mixture was refluxed for 4
h. Using the same purification procedure as described above,
3b was obtained (0.29 g, 81%); Mn ) 6600, PDI ) 2.32. IR
(KBr): ν ) 3043, 2955, 2869, 1597, 1507, 1418, 1247, 1197,
1063, 1025, 839, 732 cm-1. 1H NMR (300 MHz, CDCl3): δ ) δ
0.31 (s, 12 H), 0.98-1.03 (m, 6 H), 1.54 (m, 4 H), 1.85 (m, 4
H), 4.04 (m, 4 H), 6.51 (d, J ) 19.2 Hz, 2 H), 6.93 (d, J ) 19.4
Hz, 2 H), 7.07-7.13 (m, 4 H), 7.41-7.46 (m, 18 H). Anal. Calcd
for C48H56O2Si2: C, 79.95; H, 7.83. Found: C, 79.38; H, 7.52.
Polymer 3c. To a solution of 6c (123 mg, 0.5 mmol) and 5c
(135 mg, 0.5 mmol) in THF (2.5 mL) was added Rh(PPh3)3Cl
(2.3 mg, 0.5 mol %) under N2. Using the same purification
procedure as described above, 3c was obtained (210 mg, 82%);
Mn ) 15600, PDI ) 2.97. IR (KBr): ν ) 2949, 1608, 1541, 1489,
1247, 1180, 1014, 984, 836, 727, 668. 1H NMR (400 MHz,
CDCl3): δ ) 0.36 (br s, 12 H), 6.55 (d, 4 H), 6.70 (d, 4 H), 6.8-
7.1 (m, 4 H), 7.45 (s, 4 H), 7.61 (s, 4 H), 8.11 (s, 4 H). Anal.
Calcd: C, 74.37; H, 6.24. Found: C, 75.45; H, 7.22.
1
°C. H NMR (500 MHz, CDCl3): δ ) 0.27 (s, 18 H), 6.83 (s. 2
H), 7.3-7.5 (m, 10 H), 7.53 (d, J ) 8.3 Hz, 4 H), 7.54, (s, 4 H),
7.68 (d, J ) 8.3 Hz, 4 H);13C NMR (100 MHz, CDCl3): δ )
0.0, 95.3, 105.1, 110.8, 122.0, 123.4, 125.3, 125.9, 127.5, 128.8,
129.8, 130.2, 132.4, 134.0, 148.0, 151.9. IR (KBr): ν ) 2966,
2157, 1673, 1606, 1493, 1251, 929, 865, 842, 764, 698. HRMS:
calcd for C48H42O2Si2, 706.2723; found, 707.2736. Anal. Calcd:
C, 81.54; H, 5.99. Found: C, 81.18; H, 5.74.
1,4-Bis{[3-phenyl-5-(4-ethynylphenyl)furyl]-2-yl}ben-
zene (5d). To a solution of 10 (0.42 g, 0.6 mmol) in methanol
(100 mL) and THF (125 mL) was added potassium hydroxide
(0.38 g, 6 mmol). The mixture was stirred at room temperature
for 1 h. After removal of the solvent, the product was extracted
with chloroform then purified by flash chromatography to give
5d (0.31 g, 92%): mp 342 °C (dec). 1H NMR (500 MHz,
CDCl3): δ ) 3.14 (s, 2 H), 6.84 (s. 2 H), 7.3-7.5 (m, 10 H),
7.53 (d, J ) 8.3 Hz, 4 H), 7.55, (s, 4 H), 7.70 (d, J ) 8.3 Hz, 4
H). 13C NMR (100 MHz, CDCl3): δ ) 78.1, 83.7, 110.9, 120.9,
123.5, 125.3, 125.9, 127.6, 128.7, 128.8, 129.8, 130.6, 132.6,
134.0, 148.0, 151.8. IR (KBr): ν ) 3293, 3059, 2926, 2110, 1625,
1493, 1392, 1246, 1018, 840, 765, 697. HRMS: calcd for
C42H26O2, 562.1933; found, 562.1932. Anal. Calcd: C, 89.66;
H, 4.66. Found: C, 89.39; H, 4.48.
Polymer 3d. To a solution of 6c (0.1 g, 0.4 mmol) and 5d
(0.26 g, 0.4 mmol) in THF (2.5 mL) was added Rh(PPh3)3Cl
(1.9 mg, 0.5 mol %) under N2. The mixture was refluxed for 4
h. Using the same purification procedure as described above,
1
3d was obtained (0.25 g, 71%); Mn ) 13200, PDI ) 2.75. H
NMR (400 MHz, CDCl3): δ ) 0.33 (br s, 12 H), 6.4-6.6 (m, 4
H), 6.8 (s, 2 H), 6.9-7.1 (m, 4 H), 7.3-7.6 (m, 22 H), 7.6-7.8
(m, 4 H). IR (KBr): ν ) 2961, 1673, 1604, 1447, 1408, 1255,
1048, 990, 843. Anal. Calcd: C, 83.12; H, 5.98. Found: C,
84.21; H, 6.67.
Acknowledgment. We thank the National Science
Council and the Ministry of Education of the Republic
of China for support. Y.J.C. thanks the Yen-Chuang
Foundation for a best thesis award (2004) and the
Chinese Chemical Society, Taipei, Republic of China,
for outstanding graduate research (2004).
References and Notes
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1,4-Bis{[3-phenyl-5-(4-[2-(dimethylstyrylsilanyl)-vinyl]-
phenyl)furyl]-2-yl}benzene (4d). Under N2, a mixture of 8
(0.107 g, 0.66 mmol), 5d (0.187 g, 0.33 mmol), and RhCl(PPh3)3
(1.5 mg, 0.5 mol %) in CHCl3 (3 mL) was refluxed for 6 h.
Removal of the solvent and chromatographic separation on
silica gel (CH2Cl2) to afford a yellow solid, 4d (0.17 g, 58%):
1
mp 201-203 °C. H NMR (400 MHz, CDCl3): δ ) 0.34 (s, 12
H), 6.54 (d, J ) 19.1 Hz, 2 H), 6.57 (d, J ) 19.1 Hz, 2 H), 6.82,
(s, 2 H), 6.98 (d, J ) 19.1 Hz, 4 H), 7.3-7.55 (m, 24 H), 7.56
(s, 4 H), 7.73 (d, J ) 8.4 Hz, 4 H). 13C NMR (100 MHz,
CDCl3): δ ) -2.5, 109.3, 110.1, 123.9, 125.2, 125.8, 126.5,
126.9, 127.3, 127.5, 127.6, 128.1, 128.6, 128.8, 130.0, 134.2,
137.4, 138.2, 138.3, 144.3, 145.0, 147.7, 152.5. IR (KBr): ν )
2960, 1680, 1604, 1507, 1408, 1254, 1051, 989, 843, 798, 765,
699. HRMS: calcd for C62H54O2Si2, 886.3662; found, 886.3661.
Anal. Calcd: C, 83.93; H, 6.13. Found: C, 83.55; H, 6.44.
Polymer 3a. A mixture of 6a (48 mg, 0.15 mmol), 5a (30
mg, 0.15 mmol), and RhCl(PPh3)3 (4.6 mg, 0.005 mmol) in THF
(2.5 mL) was refluxed under N2 for 4 h. Methanol was added.
The precipitate was collected and redissolved in THF and then
precipitated again with methanol. The product was collected
by filtration and washed with methanol to give 3a (67 mg,
1
86%). H NMR (200 MHz, CDCl3): δ ) 0.33 (br s, 6 H), 6.56
(d, J ) 19 Hz, 2 H), 6.98 (d, J ) 19 Hz, 2 H), 7.3-7.6 (m, 8 H).
IR (KBr): ν ) 3032, 2959, 1646, 1604, 1496, 1402, 1252, 1050,
988, 837, 796, 645. Anal. Calcd for C18H18Si: C, 82.38. H, 6.91.
Found: C, 81.51; H, 6.03.
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RhCl(PPh3)3
time
(h)
temp
(°C)
%
yield
(M x 10-3
)
Mn
DPI
2.0
2.0
6
4
40
refluxing THF
75
86
5700
17800
2.8
3.2
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