Dynamic Fluoride Anion Coordinating Si-Containing π-Conjugating Polymers

Article abstract of DOI:10.1021/ma035327+

Three types of poly[dialkylsilylenephenyleneethynylenephenylene]s (dialkyl: 2 for n-dodecylmethyl; 5a for di-n-hexyl; 5b for di-n-hexyl-co-dimethyl) were newly prepared by the AB- and AA-BB-type coupling reactions. These polymers afforded moderate molecul

Full text of DOI:10.1021/ma035327+

2422
Macromolecules 2004, 37, 2422-2426
Dynamic Fluoride Anion Coordinating Si-Containing π-Conjugating
Polymers
Giseop Kw a k ,*,†,§ Mich iya F u jik i,*,†,‡,| a n d Tosh io Ma su d a
Advanced Polymer Science Laboratory, Graduate School of Materials Science, Nara Institute of Science
and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, J apan, CREST-J ST (J apan Science and
Technology Corporation), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, J apan, and Department of
Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, J apan
Received September 8, 2003; Revised Manuscript Received J anuary 23, 2004
ABSTRACT: Three types of poly[dialkylsilylenephenyleneethynylenephenylene]s (dialkyl: 2 for n-do-
decylmethyl; 5a for di-n-hexyl; 5b for di-n-hexyl-co-dimethyl) were newly prepared by the AB- and
AA-BB-type coupling reactions. These polymers afforded moderate molecular weights (Mw) in the range
7.6 × 103-27 × 103. As the amount of tetra-n-butylammonium fluoride (TBAF) increases in a THF solution
of 2, two π-π* absorption bands at 298 and 317 nm due to the phenyleneethynylenephenylene moiety
significantly decreased. Simultaneously, a new intense absorption band at 390 nm characteristic of
intramolecular charge transfer (ICT) appeared in place of the original ICT absorption band at 342 nm.
Also, the intensity of fluorescence band around 393 nm (excited at 342 nm) significantly decreased upon
the addition of TBAF. The changes in UV-vis absorption and fluorescence spectra of the polymers were
readily seen by naked eye. As for 2, a novel three-step titration curve of UV absorbance (or apparent
GPC retention time) as a function of fluoride concentration was found. It was thus assumed that 2
underwent anion charge-driven coil-to-globule conformational transitions in the presence of TBAF. In
addition, the complex forming abilities of the polymers with fluoride ion were greatly dependent on the
nature of alkyl groups attached to the silicon atom of the polymers.
In tr od u ction
to form intramolecular charge transfer (ICT) in both
ground and excited states.4 This led to the idea that the
photophysical properties of the σ-π hetero-junction
polymer may be modulated by changing coordination
number of the silicon atom, if through-space interaction
between the π electron systems bridged by silylene
moiety occurred.
In recent years, silicon-containing π-electron mol-
ecules and silicon-containing polymers have attracted
much attention because of their novel photophysical and
electrical properties due to through-space interaction
between π-electron systems linked by a silylene moiety.1
Indeed, modulation in coordination number of 13 and
14 group main elements such as boron and silicon
causes dramatic changes in photophysical properties of
the π-electron systems.2 Because silicon atom remark-
ably possesses specific affinity toward fluoride ion,
certain tetracoordinate silanes with n-Bu4N+F- (TBAF)
can readily afford the corresponding pentacoordinate
silicates.3 This reaction can often be observed by ordi-
nary colorimetric and fluorometric methods.2a,c,d
From the viewpoint of semiconductor physics, sily-
lene-π-electron alternating copolymers may be re-
garded as a new type of σ-π hetero-junction semicon-
ducting conjugating polymer, in which electronic struc-
ture and photophysical properties could be modulated
by doping fluoride ion as mentioned above. Although
many workers reported synthesis and properties of
various silylene-π-electron copolymers in terms of
potential applications, such as semiconductors, photo-
and electroluminescent materials, and ceramic precur-
sors, little attention has been paid to their ion-sensing
ability so far. Recently, we synthesized well-defined
On the basis of the idea, we tested fluoride ion-sensing
abilities for three types of poly[dialkylsilylenepheny-
leneethynylenephenylene]s (dialkyl: 2 for n-dodecyl-
methyl; 5a for di-n-hexyl; 5b for di-n-hexyl-co-dimethyl)
among many σ-π conjugating polymer candidates by
means of colorimetric and fluorometric methods. Here
we demonstrate that the present polymers significantly
cause changes in UV-vis absorption and fluorescence
in response to fluoride ion. These photophysical changes
were easily observed by naked eye. As for 2, a novel
three-step titration curve of UV absorbance (or retention
time of GPC) as a function of fluoride concentration was
found. Polymer 2 was assumed to undergo coil-to-glob-
ule conformational transitions induced by anion charge
in the presence of TBAF. Additionally, the complex
forming abilities of the polymers with fluoride ion
greatly depended on the nature of alkyl groups attached
to silicon atom of the polymers.
Resu lts a n d Discu ssion
poly(silylenephenyleneethynylenephenylene)s by
a
Scheme 1 outlines the synthesis of three types of poly-
[dialkylsilylenephenylene-ethynylenephenylene]s (di-
alkyl: 2 for n-dodecylmethyl; 5a for di-n-hexyl; 5b for
di-n-hexyl-co-dimethyl). Polymer 2 was obtained by the
AB-type coupling reaction. Introduction of a long alkyl
chain such as the n-dodecyl group to the silylene moiety
afforded a good solubility for 2. The molecular weight
(Mw) was evaluated to be 7.6 × 103 (Mw/Mn ) 1.3) by
gel permeation chromatography (GPC). Also, polymers
5a and 5b, containing two n-hexyl groups on silicon,
Pd/Cu-catalyzed AB-type coupling reaction and found
that the combination of silylene and phenyleneethy-
nylenephenylene units in the polymer is effectively able
* Corresponding authors.
† Nara Institute of Science and Technology.
‡ CREST-J ST (J apan Science and Technology Corp.).
§ E-mail: gkwak@ms.aist-nara.ac.jp.
| E-mail: fujikim@ms.aist-nara.ac.jp.
Kyoto University. E-mail: masuda@adv.polym.kyoto-u.ac.jp.
10.1021/ma035327+ CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/13/2004

Macromolecules, Vol. 37, No. 7, 2004
Si-Containing π-Conjugating Polymers 2423
Sch em e 1. Syn th esis of P oly(silylen ep h en ylen eeth yn ylen ep h en ylen e)s (2 a n d 5) fr om th e AB- a n d AA-BB-typ e
Cou p lin g Rea ction s
F igu r e 1. Variation in (a) UV-vis and (b) emission spectra
of 2 upon addition of TBAF (c ) 1.0 × 10-4 M, at 25 °C in
THF).
were synthesized by the AA-BB-type coupling reaction
to compare the affinities toward fluoride ion. Polymers
5a and 5b gave somewhat higher molecular weights
F igu r e 2. Variation of 1H NMR spectra of 2 upon addition of
(5a , Mw ) 27 × 103, Mw/Mn ) 2.4; 5b, Mw ) 8.7 × 103,
TBAF (c ) 4.5 × 10-2 M, at 25 °C in THF-d8).
Mw/Mn ) 2.1) compared to 2. This is because 5a and
5b have better solubility due to the two n-hexyl groups
in terms of the idea that dynamic coordinative interac-
tion between fluoride ion and silicon atom in the
polymer chain leads to a more polar ground state of the
polymer.
1H NMR study on TBAF dependency of 2 may give a
more definitive proof for the existence of the new ICT
complexes, as shown in Figure 2. As the amount of
TBAF increases, the proton signal at 0.545 ppm due to
the methyl group gradually decreases compared to that
than 2.
Figure 1a shows the change in UV-vis absorption of
2 by adding TBAF as a fluoride ion source. As the
amount of TBAF increases, π-π* absorption bands at
298 and 317 nm of phenyleneethynylenephenylene
moiety decreases, while an intense absorption band
characteristic of ICT newly appears at 390 nm in place
of a weak original ICT absorption band at 342 nm. This
significant red shift of ICT absorption band is explicable

2424 Kwak et al.
Macromolecules, Vol. 37, No. 7, 2004
Sch em e 2. P r op osed Hyp er coor d in a te Str u ctu r es In d u ced by a Dyn a m ic In ter a ction betw een th e Silicon Atom
a n d F lu or id e Ion
for the ethyl protons at 0.867 ppm, while new peaks
around +0.12 and -0.14 ppm appear, and all alkyl and
aromatic peaks slightly shift to upfiled by about 0.04
ppm. The new upfield peaks may be attributed to hyper-
(e.g., penta- and hexa-) coordinate structures on the
silylene moiety, because hypercoordinate silicates are
more electron-rich and shielded than the corresponding
tetracoordinate silanes, indicating the higher elec-
tron-density within the same polymer chain after the
addition of TBAF. However, no methyl protons due to
silyl fluoride, which may come from decomposition of
Si-aryl bond, were found.
Since the ICT absorption band shifted to longer
wavelength by ca. 40 nm after the addition of TBAF,
dramatic change in color from almost colorless to orange
was recognized by naked eye. The schematic chemical
reaction between 2 and TBAF is shown in Scheme 2.
Figure 1b displays variation in fluorescence spectra
F igu r e 3. Plot for fluoride complexation with 2 (c ) 1.0 ×
10-4 M, at 25 °C in THF).
of 2 with TBAF concentration. When 2 is excited at the
original ICT absorption wavelength of 342 nm, the in-
tensity of fluorescence at 393 nm decreased with in-
creasing the TBAF concentration. Furthermore, when
2 is excited at the new ICT absorption wavelength of
390 nm after the addition of 120-fold excess TBAF, a
weak and broad fluorescence band (see, the inset of
Figure 1b) newly appears at 521 nm with a large Stokes
shift. The new ICT excited state is thought to be induced
by dynamic coordinative interaction between fluoride
and silicon atom of the polymer chain. In accordance
with the fluorescence spectra, 2 emitted indigo-blue
light when excited at 342 nm before the addition of
TBAF, while emitting weak yellowish green light when
excited at 390 nm after the addition of 120-fold excess
TBAF.
Figure 3 illustrates the unique three-step relation-
ship between UV absorbance of 2 and fluoride con-
centration: The first step indicates a moderate slope
at less than 25-fold excess TBAF, the second step shows
a steep change in the range from 25 to 100-fold
concentration, and the third step turns to a gentle slope
at more than 100-fold excess TBAF. Because polyelec-
trolyte is known to undergo coil-to-globule change with
an increase in the degree of ionization,5 2 with fluoride
ion may undergo such a conformational transition
accompanied by a change of hydrodynamic volume in
solution.
F igu r e 4. Plot of retention time vs TBAF concentration in 2
solution (c ) 1.73 × 10-3 M, at 40 °C in THF). The correspond-
ing molecular weight was roughly estimated from a calibration
with polystyrene standards.
by the GPC method. Figure 4 shows the unique change
in the apparent retention time in GPC of 2 as a function
of TBAF concentration. As expected, 2 seems to behave
like a polyelectrolyte in the presence of fluoride ion. The
apparent retention time shifted to a shorter retention
time in the initial lower TBAF concentration but shifted
to a longer time at more than 20-fold excess TBAF.
To further support this idea, we tested an apparent
retention time of 2 in the absence and presence of TBAF

Macromolecules, Vol. 37, No. 7, 2004
Si-Containing π-Conjugating Polymers 2425
F igu r e 5. Proposed schematic models of 2 in the three steps: first step: global shape expansion, second step: expanded coil-
to-shrunk coil change, and third step: shrunk coil-to-globule change (the circle signifies hydrodynamic volume in each step). The
pictures show the actual changes in color (a f c) and fluorescence (b f d) (a and b, 2; c and d, 2 + TBAF in THF).
These results led to the idea that the three-step
relationship results from certain critical conformational
changes: first step, global shape expansion due to
charge repulsion within the polymer chain; second step,
subsequent rapid expanded coil-to-shrunk coil change;
and third step, slow shrunk coil-to-globule change. The
proposed anion charge-driven conformational transition
is schematically shown in Figure 5.
In synthetic aromatic silicon chemistry, rate-en-
hanced electrophilic desilylation was reported.6 Actu-
ally, some arylsilane compounds are easily decomposed
by a specific fluoride ion source. The UV-vis spectra of
2, however, clearly exhibited the isosbestic points at 293
and 325 nm, as shown in Figure 1a. Noteworthily, the
UV-vis and fluorescence spectra of 2 after addition of
TBAF were completely recovered to the original spectra
by washing with water. Becasue the TBAF/THF system
is not so effective for desilylation of arylsilanes,7 2 may
afford an air-stable polymer complex with fluoride ion
without any significant decomposition. This may be
uniqueness of the σ-π conjugating polymer, when
compared to silicon-containing π-electron molecular
systems.
Presumably, the methyl group on silicon atom is
responsible for the acceptance of fluoride ion, and long
alkyl groups are responsible for protection of the silicon
atom from the fluoride ion attack. Now we assume that
the fluoride ion attack toward silicon of the polymer
chain might be influenced by the alkyl group size. This
is because 5b, containing the dimethylsilylene moiety,
showed the same tendency as 2, whereas no changes in
both the UV-vis and fluorescence spectra were observed
for 5a with no methyl groups even after addition of more
than a 160-fold excess of TBAF.
In addition, when other halide ion sources such as
Cl-, Br-, and I- n-Bu4N+ salts were added to 2 instead
of TBAF, no changes in both UV-vis and fluorescence
spectra were observed. This indicates that the polymer
2 possesses a high selectivity toward fluoride ion rather
than the competing anions.
addition of TBAF. A unique three-step linear relation-
ship between UV absorbance (or retention time in GPC)
of 2 and fluoride concentration was obtained. This
relationship indicated anion charge-driven conforma-
tional transition (coil-to-globule change) of 2 in the
presence of TBAF. The fluoride complex forming abili-
ties of the present polymers were greatly dependent on
the nature of the alkyl group attached to the silicon
atom. The present work may be the first demonstration
of fluoride ion sensing ability based on σ-π conjugated
organosilicon polymers, and our results should provide
a new design concept for new silicon-containing conju-
gating polymers directed toward highly selective and
highly sensitive anion recognition materials.
Exp er im en ta l Section
Syn th esis of Mon om er s. 4-[(4-Br om op h en yl)d od ecyl-
m eth ylsilyl]p h en yla cetylen e (1). Compound 1 was synthe-
sized by modifying the literature method,4 and dichlorodode-
cylmethylsilane was used as a starting material. The data were
as follows. Yield: 50%, colorless liquid. IR (KBr): 3303, 2955,
1
2923, 2853, 1253, 1100, 823 cm-1. H NMR (CDCl3, δ): 7.58-
7.20 (m, 8H, aromatic), 3.10 (s, 1H, ethynyl), 1.58-0.88 (m,
25H, dodecyl), 0.51 (s, 3H, methyl) ppm. 13C NMR (CDCl3, δ):
138.2, 136.0, 134.2, 131.3, 131.0, 124.1, 122.8, 83.6, 77.8, 33.5,
31.9, 29.5, 29.3, 29.1, 23.6, 22.7, 14.1, 13.8 ppm. Anal. Calcd
for BrC27H37Si: C, 69.06; H, 7.94. Found: C, 69.16; H, 7.98.
Bis[4-br om op h en yl]d ih exylsila n e (3). The compound 3
was synthesized by modifying the literature method,4 and
dichlorodi-n-hexylsilane was used as a starting material. The
data were as follows. Yield: 64%, colorless liquid. 1H NMR
(CDCl3, δ): 7.54-7.26 (m, 8H, aromatic), 1.45-0.75 (m, 26H,
hexyl) ppm. 13C NMR (CDCl3, δ): 136.3, 135.0, 131.0, 124.1,
33.3, 31.4, 23.5, 22.6, 14.1, 12.3 ppm.
Bis[4-eth yn ylph en yl]dih exylsilan e (4a). A 100 mL round-
bottomed flask was equipped with a reflux condenser, a three-
way stopcock, and a magnetic stirring bar and flushed with
dry nitrogen gas. Triethylamine (500 mL), (Ph3P)2PdCl2 (40
mg, 40 µmol), CuI (100 mg, 0.53 mmol), PPh3 (100 mg, 0.35
mmol), trimethylsilylacetylene (1.85 g, 18.8 mmol), and 3 (4.0
g, 7.84 mmol) were placed in the flask, and the mixture was
stirred for 4 h at 80 °C. After the completion of reaction had
been confirmed by TLC, the resulting solution was filtered,
and then the solution was evaporated. Methanol (ca. 100 mL),
THF (ca. 100 mL), and NaOH (ca. 0.5 g) were added to the
crude product, and stirred for 1 h. After the volatiles had been
evaporated, the product was extracted with diethyl ether,
washed with water, and dried over anhydrous sodium sulfate.
After diethyl ether was evaporated, the crude product was
Con clu sion
The photophysical properties of 2, which are related
to intramolecular charge transfer, significantly changed
upon addition of fluoride ion. Indeed, dramatic changes
in UV-vis absorption and fluorescence occurred upon

2426 Kwak et al.
Macromolecules, Vol. 37, No. 7, 2004
purified by flash column chromatography (eluent, hexane) to
give the desired product (yield 0.6 g, 22%) as colorless liquid.
1H NMR (CDCl3, δ): 7.44 (s, 8H, aromatic), 3.10 (s, 1H,
ethynyl), and 1.48-0.76 (m, 26H, hexyl) ppm. 13C NMR (CDCl3,
δ): 137.5, 134.6, 131.2, 122.8, 83.7, 77.8, 33.3, 31.4, 23.5, 22.6,
14.1, 12.2 ppm.
(Mw) and number-average molecular weight (Mn) of the
polymers were evaluated using gel permeation chromatogra-
phy (Shimadzu A10 instruments, Polymer Laboratories, PLgel
Mixed-B (300 mm in length) as a column, and HPLC-grade
tetrahydrofuran as eluent at 40 °C), based on a calibration
with polystyrene standards. The retention time of the poly-
mer was measured on the same chromatography instrument.
UV-vis, emission, and IR spectra were measured on J ASCO
UV-550, J ASCO FP-6500, and HORIBA FT-730 spectrometers,
respectively.
Bis[4-et h yn ylp h en yl]d im et h ylsila n e (4b ). The com-
pound 4b was prepared using bis[4-bromophenyl]dimethylsi-
lane, similar to 4a . The data were as follows. Yield: 25%, white
1
solid. H NMR (CDCl3, δ): 7.46 (s, 8H, aromatic), 3.09 (s, 1H,
ethynyl), and 0.55 (s, 6H, methyl) ppm. 13C NMR (CDCl3, δ):
138.9, 134.0, 131.3, 122.9, 83.6, 77.8, 29.7 ppm.
Ack n ow led gm en t. The authors are grateful to Mr.
S.-Y. Kim, N. Hotta, and M. Ishikawa for technical
assistance.
Syn th esis of P olym er s. P oly[(n -d od ecylm eth yl)sily-
len ep h en ylen eeth yn ylen ep h en ylen e] (2). Triethylamine
(10 mL), (Ph3P)2PdCl2 (21.0 mg, 30 µmol), CuI (5.7 mg, 30
µmol), and 1 (469.6 mg, 1.0 mmol) were placed in the flask,
and the mixture was stirred under reflux for 2 days. After the
resulting solution was filtered, the solution was concentrated
and poured into a large amount of acetone under stirring to
precipitate the polymer formed. The data were as follows.
Yield: 89%, brown gummy solid. IR (KBr): 2955, 2923, 2851,
1252, 1105, 823 cm-1. 1H NMR (CDCl3, δ): 7.62-7.29 (br, 8H,
aromatic), 1.48-0.79 (br, 25H, dodecyl), 0.53 (br, 3H, methyl)
ppm. 13C NMR (CDCl3, δ): 136.0, 134.3, 130.7, 124.1, 90.3,
33.6, 31.9, 29.6, 29.3, 23.7, 22.7, 14.1, 13.9 ppm. Anal. Calcd
for C27H37Si: C, 83.44; H, 9.34. Found: C, 76.13; H, 8.53. The
significant difference between the calculated and found values
may be ascribed to the fact that organosilicon polymers
containing unsaturated functional groups such as ethynylene
and vinylene groups tend to cross-link at a relatively low
temperature and be thermally converted to silicon carbides
in high char yield.4,8
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P o ly [d i a lk y ls i ly le n e p h e n y le n e e t h y n y le n e p h e n -
ylen e]s (Dia lk yl: 5a for Di-n -h exyl; 5b for Di-n -h exyl-
co-d im eth yl). Triethylamine (10 mL), (Ph3P)2PdCl2 (21.0 mg,
30 µmol), CuI (5.7 mg, 30 µmol), 3 (255 mg, 0.5 mmol), and 4
(4a , 130 mg, 0.5 mmol; 4b, 200 mg, 0.5 mmol) were placed in
the flask, and the mixture was stirred in reflux for 2 days.
After the resulting solution was filtered, the solution was
concentrated and poured into a large amount of methanol
under stirring to precipitate the formed polymer. The data
were as follows. 5a . Yield: 95%, brown solid. 1H NMR (CDCl3,
δ): 7.77-7.13 (br, 8H, aromatic), 1.81-0.56 (br, 26H, hexyl)
ppm. 13C NMR (CDCl3, δ): 137.0, 134.7, 130.8, 123.9, 90.2,
33.3, 31.4, 23.6, 22.6, 14.1, 12.3 ppm. 5b. Yield: 90%, light
brown solid. 1H NMR (CDCl3, δ): 7.79-7.03 (br, 8H, aromatic),
1.92-0.10 (br, 32H, alkyl) ppm. 13C NMR (CDCl3, δ): 137.0,
136.4, 134.7, 134.0, 131.6, 130.8, 129.0, 128.4, 90.1, 33.2, 23.4,
22.4, 14.2, 12.3 ppm.
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Mea su r em en ts. 1H (400 MHz) NMR spectra were mea-
sured in THF-d8 solution at 25 °C on
a J EOL EX-400
spectrometer. Chemical shift was referred to the solvent (3.57
and 1.72 ppm for THF). The weight-average molecular weight
MA035327+