Controlling conformations in alternating dialkylsilylene-spaced donor-acceptor copolymers by a cooperative thorpe-ingold effect and polymer folding

Article abstract of DOI:10.1002/chem.201102032

A series of dialkylsilylene-spaced copolymers 6 and 7, which contain Me₂Si and iPr₂Si spacer groups, respectively, and have alternating donor and acceptor chromophores, have been designed and regioselectively synthesized by hydrosily

Full text of DOI:10.1002/chem.201102032

DOI: 10.1002/chem.201102032
Controlling Conformations in Alternating Dialkylsilylene-Spaced Donor–
Acceptor Copolymers by a Cooperative Thorpe–Ingold Effect and Polymer
Folding
Chih-Hsien Chen,[a] Yen-Chin Huang,[a] Wei-Chih Liao,[a] Tsong-Shin Lim,[b]
Kuan-Lin Liu,[c] I-Chia Chen,[c] and Tien-Yau Luh*[a]
Dedicated to Professor Leon M. Stock on the occasion of his 80th birthday
Abstract: A series of dialkylsilylene-
spaced copolymers 6 and 7, which con-
tain Me2Si and iPr2Si spacer groups, re-
spectively, and have alternating donor
and acceptor chromophores, have been
designed and regioselectively synthe-
sized by hydrosilylation. The ratio of
the donor and acceptor chromophores
for each repeat unit is 2:1, and the two
donor chromophores are linked by a
trimethylene bridge. A 4-aminostyrene
moiety is used as the donor and a
series of acceptor chromophores with
different reduction potentials are em-
ployed. Both steady-state and kinetic
measurements reveal that the photoin-
duced electron transfer (PET) in 6
obeyed the Marcus theory in which
normal and inverted regions are ob-
served. On the other hand, the iPr2Si-
spaced copolymers 7 exhibit absorption
and emission from the charge-transfer
complexes exclusively due to ground-
state interactions between the donor
and acceptor chromophores. The dis-
crepancy in photophysical behavior
may have arisen from the difference in
distance between the adjacent donor
and acceptor chromophores. The bulki-
ness of the substituents on the silicon
atom (i.e., Me versus iPr) may exert
the Thorpe–Ingold effect on the local
conformation around the silicon atom.
The differences in the small energetic
barriers for each of the conformational
states may be amplified by extending
the distance of the folding structure,
which results in perturbing the confor-
mation of the polymers. These results
suggest that the electronic interactions
between adjacent donor–acceptor pairs
in these copolymers are controlled by
the synchronization of the substitution
effect and corresponding polymeric
structures.
Keywords: Marcus theory · photo-
induced electron transfer · polymer
folding · silicon · substituted sily-
lene · Thorpe–Ingold effect
Introduction
a more stable conformation.[8] Numerous secondary struc-
tures of polymers[3,4] and proteins[5,6] are known to be stabi-
lized by hydrogen bonding. Also, p–p stacking interactions
play an important role in the folding and intermolecular ag-
gregation/assembly of a polymer.[7] Circular dichroism, fluo-
rescence quenching, and excimer formation have frequently
been used as probes for the folding of macromolecules.[9,10]
Nevertheless, knowledge of the precise conformational ar-
rangement of artificial polymers remains a challenging prob-
lem, especially for subnanometric variations in folded struc-
tures. Photoinduced electron transfer (PET) has been
known to be sensitive to subtle changes in distance on the
angstrom scale.[11] In other words, electronic coupling be-
tween the donor and acceptor is strongly perturbed by a
slight variation in the distance between the chromophores.[12]
Indeed, the use of this strategy for investigation of protein
conformation has been carried out.[13] It is envisaged that
the use of PET as a probe might also provide information
about the fine structures of folded polymers.
Polymer folding represents, among other things, an ensem-
ble of rotation about the sigma bonds along the polymeric
backbones, through-space electrostatic and steric interac-
tions between non-neighboring sites, and solvophobic ef-
fects.[1–8] The differences in the small energetic barriers for
each of the conformational states may be amplified by ex-
tending the distance of the folding structure, which results in
[a] Dr. C.-H. Chen, Y.-C. Huang, W.-C. Liao, Prof. T.-Y. Luh
Department of Chemistry
National Taiwan University
Taipei, 106 (Taiwan)
Fax : (+886)223644971
E-mail: tyluh@ntu.edu.tw
[b] Prof. T.-S. Lim
Department of Physics
Tunghai University
Taichung, 407 (Taiwan)
Monosilylene-spaced copolymers 1[14] are known to exhib-
it extraordinary photophysical properties;[15–18] furthermore,
different aryl groups (i.e., Ar1 and Ar2) can be regioselec-
tively incorporated in the polymer chain by using a simple
hydrosilylation protocol. The silicon moiety in 1, in general,
[c] K.-L. Liu, Prof. I.-C. Chen
Department of Chemistry
National Tsing Hua University
Hsinchu, 300 (Taiwan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102031.
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Chem. Eur. J. 2012, 18, 334 – 346

FULL PAPER
is considered to be an insulating tetrahedral spacer.[14–19]
Thus, the electronic interactions between chromophores can
be simplified in comparison with conjugated polymers.
Moreover, the steric nature of the substituents on the silicon
atom can have a bearing on the morphological changes.[20]
Such a small variation might fine-tune the features of the
ensembled polymers that originate from the conformational
equilibrium of divinylsilane moiety in 1 due to the Thorpe–
Ingold effect [Eq. (1)]. Accordingly, we chose monosilylene-
spaced copolymer 1 as a skeleton to study the conformation-
al change of the polymeric structure by utilizing the features
of the PET process in polymers.
Results and Discussion
Strategy: To decrease the complexity of conformational
changes in 1, one of the two vinyl moieties in 1 was replaced
by an aryl group so that the equilibrium shown in Equation
(1) could be simplified to Equation (2), in which only syn
and anti equilibrium will be considered.
Figure 1. Absorption (a) and emission (c) spectra of 2 (black) and 3
(gray) in THF (concentration=1ꢁ10À5 m).
on the photophysical properties of the PET processes, 4 and
5 with ethylene and propylene bridges, respectively, were
prepared. There was essentially no difference in their emis-
sion spectra (Figure 2). Based on these preliminary studies,
Aromatic amines are widely used as electron donors in
PET reactions.[21] Because of synthetic convenience, we de-
cided to use two donors symmetrically located at both ends
of the acceptor chromophore. First, model compounds 2 and
3 were synthesized,[22] in which the aminostyrene group was
employed as the donor and the stilbene moiety as the ac-
ceptor. The absorption spectra for 2 and 3 were almost iden-
tical (Figure 1). Both 2 and 3 exhibit dual emission bands at
l=380 nm due to intrinsic local excited (LE) emission of
the stilbene chromophore and at l=500 nm which are likely
to belong to the emission from the charge-separation state
(CT emission; Figure 1). It is interesting to note that the rel-
ative intensity of the LE appeared to be much lower in 3
with two donor chromophores than in 2, which has only one
donor chromophore. In addition, the relative intensity of the
CT emission was much higher in 3 than in 2. Accordingly,
PET would be more efficient in 3 than in 2.
Oligomethylene bridges were designed to link two amino-
styrene chromophores. To test the effect of the chain length
Figure 2. Absorption (a) and emission (c) spectra of 4 (black) and 5
(gray) in THF (concentration=5ꢁ10À6 m).
Chem. Eur. J. 2012, 18, 334 – 346
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335

T.-Y. Luh et al.
polymers 6 and 7 were designed and synthesized for the
conformational investigations.
Photophysical and electrochemical properties of monomers
9 and 13: The photophysical properties, the redox potentials
Eox and Ered, and the relevant properties of monomers 9a–f
are compiled in Table 1. The frontier orbital energies for
each of these chromophores were thus calculated. Accord-
ingly, the PET would be expected from the HOMO of the
aminostyrene moiety to the HOMO of the excited state of
the chromophore in 9.
Synthesis: A rhodium(I)-catalyzed hydrosilylation protocol
was used for the syntheses of 6 and 7 from the correspond-
ing bis-alkyne 8 and bis-silyl hydride 9 or 10 shown in Equa-
tion (3).[14,22] The presence of one equivalent of NaI was es-
sential to facilitate the rhodium-catalyzed hydrosilylation re-
actions.[18] Bis-alkyne 8 was obtained from the Sonogashira
reaction of 11 and trimethylsilylacetylene followed by desi-
lylation. Bis-silyl hydrides 9 and 10 containing acceptor
chromophores were prepared from the metal/halogen ex-
change between bis-bromide 12 and nBuLi followed by re-
action with R2SiHCl, where R was a methyl or isopropyl
group. The acceptor chromorphores in 9 and 10 were chosen
so that the relative free-energy differences in the PET pro-
cess for 6 could be tuned. In addition, the lengths of these
acceptor chromophores were kept as close as possible to
maintain similar center-to-center distances between the
donor and acceptor chromophores in 6 and 7.
Dissimilarity between SiMe2- and SiiPr2-spaced copolymers
based on steady-state spectroscopic analysis: As shown in
Equation (3), a series of polymers 6 was designed so that
the differences in the free energy DGo for the charge separa-
tion processes were systematically tuned. By using the
Weller equation,[23] the DGo values in the electron-transfer
process for 6 were estimated based on the data for the cor-
responding monomers 9 and 13 (Table 2).[22]
The absorption and emission spectra of 6, 7, and the cor-
responding acceptor monomers 9 in cyclohexane and THF
are compared in Figures 3–9. Interestingly, the absorption
spectra for 6 were essentially solvent independent. However,
the absorption spectra for 7 exhibited noticeable additional
absorption bands at longer wavelengths in cyclohexane and
THF, which are attributed to the absorption for the charge-
transfer complex.[24] Indeed, these lower-energy absorption
bands shifted to further longer wavelengths with relatively
higher intensities in the more-polar THF solvent than in cy-
clohexane. These results clearly indicate that interactions
between the donor and acceptor chromophores at the
ground state might be more significant in 7, and thus the ex-
citation pathways for 6 and 7 might be different.
The emission spectra of 6 and 7 exhibited completely dif-
ferent trends. The emission profile of 6a in cyclohexane was
almost identical to that of the corresponding monomer 9a
and the full-width half maxima (FWHM) of the emission
band was only slightly broader in THF (Figure 3b). The
emission intensities of 6a in 2-methyltetrahydrofuran
(MTHF) remained similar at diffferent temperatures in the
range 298–180 K (Figure 3d). In the emission spectrum of
7a, besides the LE emission of the acceptor chromophore at
l=424 and 446 nm, a longer wavelength emission at approx-
imately l=450–500 nm started to emerge (Figure 3c). Inter-
estingly, the emission intensities of 7a were enhanced with
Table 1. The photophysical and electrochemical properties of monomers 9 and 13.
[a]
[a]
[c]
Compound
lmax
lem
E0À0
[eV]
F[b]
to
Eox
[V]
Ered
[V]
HOMO[d]
[eV]
LUMO[e]
[eV]
[nm]
[nm]
[ns]
9a
9b
9c
9d
9e
9 f
13
365
424, 446
396
350, 366
345, 360
344
3.02
3.41
3.62
3.78
4.05
4.11
3.54
0.12
0.80
0.26
0.79
0.28
0.10
–
0.20
1.00
0.18
0.95
1.95
0.74
–
0.69
0.89
1.24
–
–
–
À2.48
À2.71
À2.48
À2.37
À2.39
À2.18
–
À5.30
À5.50
À5.85
–
–
–
À2.67
À2.90
À2.67
À2.56
À2.58
À2.37
À1.40
324, 273
320, 308
292
265
243
343
–
305
0.33
À4.94
[a] The lmax and lem values were measured in THF. [b] The F value was determined using coumarin-1 (F=0.99 in ethyl acetate) and 2,5-diphenyl-1,3,4-
oxodiazole (F=1.0 in ethanol) as a standard. [c] Fluorescence lifetime to was estimated by single-exponential fitting of the fluorescence decay profile.
[d] HOMOs were estimated by oxidation potentials Eox of 9a–c and 13. The Eox values of 9d–f were not measured because they were beyond the instru-
ment limitation (1.3 V versus Ag/Ag+). [e] LUMOs were estimated by reduction potential Ered of 9 and that of 13 was obtained by HOMO+E0À0
.
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Properties of Silyene-Spaced Copolymers
FULL PAPER
Table 2. Number-average molecular weight Mw, polydispersity index (PDI), and kinetic parameters of 6 and 7.
was lowered (Figure 5d). Pre-
sumably, the solvent polarity
may be enhanced as the tem-
perature decreases,[26] and thus
the charge-separation state
might be stabilized to a lower
energy level. It seems likely
that the interaction between
this excited state and the higher
vibrational level of the ground
state might result in a thermal-
relaxation process to decrease
the opportunity of radiative re-
laxation.[27,28] In contrast, the
temperature-dependent behav-
ior of emission profiles of 7c
was similar to that observed for
7b, as described above (Fig-
ure 5e).
6
7
[a]
[b]
Mw
A
ÀDGo
tCS
kCS ꢁ10À10
[sÀ1
Mw
A
td
kd ꢁ10À9
[sÀ1
[eV]
[ps]
45
N
]
[ps]
E
]
a
b
c
d
e
f
17000
(1.6)
13000
(2.3)
20000
(2.4)
17800
(1.8)
10600
(1.8)
11800
(1.8)
0.40
0.56
1.01
1.27
1.52
1.79
1.7
2.9
6500
(2.4)
7000
(1.6)
6200
(2.0)
38
12.0
5.6
6.4
G
N
176
155
34
9
E
ACHTUNGTRENNUNG
10.6
7.0
150
6.7
6.2
N
ACHTUNGTRENNUNG
14
19
35
ACHTUNGTRENNUNG
5.2
5200
(1.7)
160
T
ACHTUNGTRENNUNG
2.7
ACHTUNGTRENNUNG
[a] Weller equation: ÀDGo =E0À0Àe
[Eox(D)ÀEred(A)]+C/RDA
,
where C=e2/4peoes =3.04ꢁ10À29 JmÀ1 and
R
DFT calculations of model compounds Ma–f.[22] [b] Two-exponential fitting was utilized to give two lifetimes
(see Figures S4–S6 in the Supporting Information),[22] in which the shorter component tCS was the major route
for the fluorescence decay (~99%).
When the ÀDGo value for
decreasing temperature (Figure 3e). The formation of the
the donor–acceptor couple was
charge-transfer complex in 7a might be plausible.
raised to 1.52 eV for 6e, the emission profiles in cyclohex-
ane and THF again became a single LE emission at l=
350 nm (Figure 6b) and were essentially temperature inde-
pendent (Figure 6d). These properties were similar to those
of 6a, as described above. In other words, the emission spec-
tra of 6a and 6e may have arisen mainly from the LE emis-
sion of the acceptor moiety and no emission at longer wave-
lengths arising from the CT emission being observed. On
the other hand, the emission properties of 7e in THF at am-
bient temperature and in MTHF at variable temperatures
appeared to be similar to those of 7b and 7c (Figure 6c,e).
The presence of the geminal diisopropyl groups on the sil-
icon atoms in 7 would exert the Thorpe–Ingold effect,[20]
which may lead to modification of the average spacing and
orientation between the donor and acceptor chromophores
in 7 relative to those in 6. Accordingly, the mode of the in-
teractions between these donor and acceptor chromophores
might lead to the discrepancy in photophysical behavior be-
tween these two series of copolymer. The absorption spectra
of 6 were essentially the sum of all the contributing chromo-
phores (in Figures 3–6), whereas those spectra of 7 consisted
of an additional solvent-dependent absorption band at the
longer wavelength because of the interactions between the
donor and acceptor chromophores at the ground state; fur-
thermore, the fluororescence observed for 7 might be attrib-
uted to the emission that arises from this charge-transfer
The dissimilarities in the emission properties between 6
and 7 were more prominent when ÀDGo became larger. The
emission spectrum of 6b in cyclohexane at ambient temper-
ature was the same as that of the corresponding monomeric
acceptor chromophore 9b (Figure 4b). It is worth noting
that a shoulder appeared at approximately l=480 nm in
THF. These dual fluorescence bands were assigned as the
LE of the acceptor moiety at a shorter wavelength, and the
CT emission at a longer wavelength.[25] When the two
methyl substituents on the silicon atom in 6b were replaced
by two isopropyl groups, the emission spectrum of 7b dis-
played a new emission profile at l=420–450 nm with a vi-
bronic structure (Figure 4c). This emission behavior is con-
sistent with the observation of a charge-transfer absorption
band in 7b, thus suggesting a strong intrachain interaction
between the chromophores at the ground state. In the tem-
perature-dependent emission spectra (Figure 4d,e), the
emission intensities of 6b were similar at different tempera-
tures, whereas those of 7b became stronger when the tem-
perature was lowered. It is noteworthy that the variation in
the emission intensities with temperature for 7b appeared
to be more prominent than those for 7a.
When the ÀDGo value was 1.01 eV, the emission spectrum
of 6c showed two emission bands at l=360 and 450 nm in
cyclohexane due to LE and CT emissions, respectively (Fig-
ure 5b). When THF was employed, the relative LE intensity
(l=360 nm) was significantly decreased. In addition, the rel-
ative intensity of the CT emission at approximately l=
530 nm became very prominent with a bathochromic shift in
comparison with that observed in cyclohexane. On the other
hand, the lem values for 7c at l=450 and 480 nm in cyclo-
hexane and THF, respectively (Figure 5c), were different
from those observed for the LE and CT emissions for 6c.
The intensities of the CT emission in 6c decreased signifi-
cantly together with bathochromic shifts as the temperature
complex.[25]
.
Steady-state observation of the Marcus-inverted region in 6:
The ÀDGo values for the charge-separation processes in the
copolymers 6d and 6 f were estimated to be 1.27 and
1.79 eV, respectively. The emission behaviors of 6d (Fig-
ure 7a) and 6c were comparable. However, the relative in-
tensity of the LE versus CT emission for 6d appeared to be
more prominent than those for 6c in both cyclohexane and
THF solvents. It is worth mentioning that the DGo value was
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337

T.-Y. Luh et al.
Figure 3. a) Absorption spectra of 6a (black) and 7a (gray) in cyclohexane (a) and THF (c); concentration=1.0ꢁ10À5 m. b) Emission spectra of 6a
in cyclohexane (a) and THF (d) and 9a (c) in THF. c) Emission spectra of 7a in cyclohexane (a) and THF (d) and 10a (c) in THF. Var-
iable-temperature emission spectra of d) 6a and e) 7a in MTHF (c: 298 K, a: 270 K, g: 240 K, d: 210 K, l: 180 K; concentration=1.0ꢁ
10À5 m; excitation wavelength=370 nm).
more negative for 6d than for 6c. As expected, the emission
properties of 6 f (Figure 8a) were similar to those of 6e be-
cause they represented relatively large ÀDGo values. The
temperature-dependent emission spectra of 6d displayed a
classic behavior of CT emission, as observed for 6c (Fig-
ure 7b). Furthermore, 6 f behaved in a similar manner to 6e
when the temperature was changed (Figure 8b).
A plot of the integrated intensity ratios of CT (FCT) and
LE (FLE) emissions for 6 against the ÀDGo value is shown in
Figure 9. It is interesting to note that the FCT/FLE ratios in-
creased with increasing ÀDGo value, reached a maximum,
and then decreased again. These observations, based on the
steady-state spectra, suggest that there was no CT emission
when the ÀDGo value was smaller than 0.5 eV or larger than
1.5 eV. The charge-transfer process became predominant
when the ÀDGo value was around 1 eV. These steady-state
results matched nicely with the Marcus theory.[29] On the
contrary, no similar behavior was found with copolymer 7.
Time-resolved fluorescence spectroscopy: Time-resolved
fluorescence spectroscopy with a femtosecond Ti/sapphire
laser and a streak camera was employed to obtain the emis-
sion decays of 6 and 7 (see Figures S1–S3 in the Supporting
Information). The LE emission wavelengths of the corre-
sponding acceptor chromophores in 6 were monitored, and
the decay lifetimes attributed to charge-separation process
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Properties of Silyene-Spaced Copolymers
FULL PAPER
Figure 4. a) Absorption spectra of 6b (black) and 7b (gray) in cyclohexane (a) and THF (c); concentration=1.0ꢁ10À5 m. b) Emission spectra of 6b
in cyclohexane (a) and THF (d) and 9b (c) in THF. c) Emission spectra of 7b in cyclohexane (a) and THF (d) and 10b (c) in THF. Var-
iable-temperature emission spectra of d) 6b and e) 7b in MTHF (c: 298 K, a: 270 K, g: 240 K, d: 210 K, l: 180 K; concentration=1.0ꢁ
10À5 m; excitation wavelength=370 nm).
were obtained (Table 2). The rate constants of charge sepa-
ration processes kCS were thus calculated.[30] When the
DGo values became more negative, the kCS values increased
initially, reached a maximum, and then decreased. A plot of
kCS against ÀDGo is shown in Figure 10. Notably, the
Marcus-inverted region was observed for the first time
based on forward PET processes in a polymeric system.
Polymers 6c and 6d exhibited dual emissions, from which
the CT emissions at longer wavelengths were prominent (in
Figure 5b and 7a). The fluorescence decay at these wave-
lengths as a result of the CT emission were monitored, and
these lifetimes were 4.5 and 2.6 ns for 6c and 6d, respective-
ly. The corresponding rate constant of the charge-recombi-
nation process was calculated to be kCR =2.2 and 3.8ꢁ
108 sÀ1. The free-energy changes of these relaxation process-
es of 6c and 6d (determined by ÀDGCR =E0À0 +DGo) were
ÀDGCR =2.63 and 2.52 eV, respectively. It is worth mention-
ing that these kCR values fitted nicely into the inverted
region. The emission lifetimes td for 7 and the rate of the
deactivation processes kd are also listed in Table 2. It is in-
teresting to note that the emission-decay profile of 7a was
treated by a two-exponential fitting to give two lifetimes
td =38 and 176 ps. The magnitude of the longer lifetime was
similar to other td values of 7b, 7c, and 7e, and the magni-
tude of the shorter lifetime was close to tCS values of 6a.
These results were consistent with the steady-emission spec-
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339

T.-Y. Luh et al.
Figure 5. a) Absorption spectra of 6c (black) and 7c (gray) in cyclohexane (a) and THF (c); concentration=1.0ꢁ10À5 m. b) Emission spectra of 6c
in cyclohexane (a) and THF (d) and 9c (c) in THF. c) Emission spectra of 7c in cyclohexane (a) and THF (d) and 10c (c) in THF. Vari-
able-temperature emission spectra of d) 6c and e) 7c in MTHF (c: 298 K, a: 270 K, g: 240 K, d: 210 K, l: 180 K; concentration=1.0ꢁ
10À5 m; excitation wavelength=330 nm).
tra of 7a, in which the emission spectra showed a broad
band that possibly involved two different radiative-relaxa-
tion processes from the LE emission of the acceptor and the
emission of the charge-transfer complex. The kd values for 7
were similar, whereas the kCS and kCR values for 6 followed
a bell-shape distribution (Table 2).
It seems likely that the electronic coupling between the
LE and CT states in 6 and 7 would be different, thus leading
to different radiative-relaxation pathways. These results reit-
erate the discrepancy of the alkyl substituents on the silicon
atom on the conformations of 6 and 7 due to the Thorpe–
Ingold effect, which would dictate the photophysical behav-
ior of the donor–acceptor pair in these copolymers. Indeed,
a broad transient maximum at l=530 nm was characterized
in 6c, but there was no transient feature at l=530 nm in 7c
(Figure 11). These results further confirmed that the transi-
ent species of the excited state were different for 6c and 7c.
Thorpe–Ingold effect and polymer folding: It is known that
silylene-spaced divinylarene and related copolymers are
highly folded.[13,14] Through-space interactions between non-
adjacent divinylbenzene chromophores readily take place to
result in the appearance of emission bands at longer wave-
lengths with vibronic fine structures.[15] To rule out such a
possibility, copolymers 14a and 14b were synthesized by re-
placing the N-methyl amino groups in 6 and 7 with ethyli-
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Properties of Silyene-Spaced Copolymers
FULL PAPER
Figure 6. a) Absorption spectra of 6e (black) and 7e (gray) in cyclohexane (a) and THF (c); concentration=1.0ꢁ10À5 m. b) Emission spectra of 6e
in cyclohexane (a) and THF (d), and 9e (c) in THF. c) Emission spectra of 7e in cyclohexane (a) and THF (d), and 10e (c) in THF.
Variable-temperature emission spectra of d) 6a and e) 7e in MTHF (c: 298 K, a: 270 K, g: 240 K, d: 210 K, l: 180 K; concentration=1.0ꢁ
10À5 m; excitation wavelength=290 nm).
dene moieties. No PET process between the stilbene chro-
mophore and para-alkyl-substituted styrene moieties would
be expected as a result of the mismatch of the frontier orbi-
tal energies. The absorption and emission spectra of 14a and
14b are shown in Figure 12. Similar fluorescence spectra of
14a and 14b as a result of the intrinsic emission of the stil-
bene chromophore suggest that no intrachain through-space
interactions that lead to the longer-wavelength emissions in
these polymers would take place, no matter whether the
substituent on the silicon atom is a methyl or isopropyl
group. These results indicate the absorption and emission
features observed in 7 would arise from the charge-transfer
complex.
The experimental evidence discussed above suggests the
importance of the Thorpe–Ingold effect on the conformation
of the dialkylsilylene-spaced copolymers 6 and 7, which
would bring the adjacent donor–acceptor chromophores in
proximity. As such, the mode of interaction between these
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Figure 7. a) Emission spectra of 9d (c) in THF and 6d in cyclohexane
(a) and THF (d). b) Variable-temperature emission spectra of 6d in
MTHF (c: 298 K, a: 270 K, g: 240 K, d: 210 K, l: 180 K;
concentration=1.0ꢁ10À5 m; excitation wavelength=290 nm).
Figure 8. a) Emission spectra of 9 f (c) in THF and 6 f in cyclohexane
(a) and THF (d). b) Variable-temperature emission spectra of 6 f in
MTHF (c: 298 K, a: 270 K, g: 240 K, d: 210 K, l: 180 K;
concentration=1.0ꢁ10À5 m; excitation wavelength=280 nm).
chromophores would be different depending on the steric
bulkiness of the substituents on the silicon atom. For the
purpose of establishing the importance of the interactions
between adjacent donor–acceptor chromophores modulated
by silylene moieties in 6 and 7, the monomeric model
donor–acceptor pairs 2 and 15 were synthesized. The ab-
Figure 9. The relative ratios of CT/LE emission intensity of 6a–f in THF.
sorption spectra were essentially independent of the solvent
polarity (see Figure S7 in the Supporting Information), and
there is no characteristic charge-transfer absorption for 2
and 15. The emission spectra of 2 and 15 are compared with
those of 6c and 7c in Figure 13a,b, respectively. The emis-
sion spectrum of 2 in cyclohexane showed LE emission of
the acceptor moiety at l=370 nm and a tailing signal
toward l=450 nm. When THF was used as the solvent, dual
emissions were observed at l=370 and 510 nm, and these
two emissions were assigned to LE and CT emissions, re-
spectively, which is similar to those of 6c. The emission
spectrum of 15 also displayed the LE and CT emissions at
l=370 and 450 nm, respectively, in cyclohexane; further-
more, the CT emission shifted to l=510 nm in THF. Inter-
estingly, the emission profiles of 15 were similar to those of
2 or 6c, but completely different from those of 7c under the
same conditions. Unlike the emissions of 7c, no emission as
a result of the charge-transfer complex was observed at all
for 15, although both 7c and 15 contain the iPr2Si linker be-
tween the donor and acceptor chromophores. As discussed
above, the bulky isopropyl substituents would exert the
Thorpe–Ingold effect to bring the adjacent chromophores
into proximity. Compound 15 is a small molecule. Rotation
about the sigma bonds might occur relatively freely, thus
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Properties of Silyene-Spaced Copolymers
FULL PAPER
Figure 10. Logarithmic plots of rate constants versus free-energy change
of the forward (*) and backward (*) electron-transfer reactions.
Figure 12. The normalized absorption (c) and emission (a) spectra
of 14a (black) and 14b (gray) measured in THF.
Figure 13. The emission spectra of a) 2 (a) and 6c (c) and b) 15
(a) and 7c (c) in cyclohexane (black) and THF (gray). Excitation
wavelength=330 nm.
Figure 11. Transient absorption spectra of a) 6c and b) 7c (*: 0 ps, ~:
1 ps, ~: 3 ps, &: 10 ps).
leading to an ensemble of different conformers. The distance
between the donor and acceptor chromophores might not
be fixed in 15. Accordingly, the mode of interaction between
the donor and acceptor chromophores for 2 and 15 would
be similar. It is worth noting that the relative intensity of
the CT emission of 15 was much stronger than that of 2. Pre-
sumably, the Thorpe–Ingold effect remained to play an im-
portant role. On the other hand, silylene-spaced copolymers
would be highly folded and the Thorpe–Ingold effect may
generate a variation in the local conformation. Such a
change in the local conformation may lead to a different
folding nature of a polymer. It seems likely that the folding
of polymer 7c might further perturb the relative distance
between the adjacent donor and acceptor chromophores,
thus resulting in different photophysical behavior.
Random copolymers 16 with different ratios of Me2Si and
iPr2Si linkers (Me2Si/iPr2Si=9:1, 4:1, and 1:1) were synthe-
sized. The absorbance of the charge-transfer absorption at
approximately l=400 nm increases with increasing molar
fractions of the iPr2Si moieties (Figure 14a). The relative in-
tensity of the emission at l=510 nm (Figure 14b), as a
result of the CT emission from the donor–acceptor pair
linked by the Me2Si spacer, decreases with decreasing molar
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343

T.-Y. Luh et al.
fraction of the Me2Si linkers in 16. In the meantime, the
emission at l=450 nm, which arises from the emission from
the iPr2Si-spaced donor–acceptor pair, became stronger
when the molar fraction of the iPr2Si spacer increases. As
expected, the intensity of the emission band at approximate-
ly l=450 nm increases with decreasing temperature, where-
as that of the emission band at around l=500–550 nm de-
creases with concomitant bathchromic shift at the same
time. These behaviors are same as the temperature-depen-
dent emissions of 6c and 7c discussed previously. In other
words, when the donor and acceptor chromophores were
spaced by the iPr2Si group, a charge-transfer complex would
be formed between these chromophores in the polymers
and the emission of the Me2Si-spaced donor–acceptor pair
in 16 exhibits typical CT emissions as those observed for 6c.
These results suggest that concomitance of the Thorpe–
Ingold effect and polymer folding is indispensable to control
the electronic interactions between adjacent donor–acceptor
chromophores.
Conclusion
In summary, we have demonstrated the fundamental differ-
ences between the photophysical properties of 6 and 7
owing to a difference in the alkyl substituents on the silylene
spacer. The variation of the steric environment around the
silicon atom may generate different conformations and fold-
ing so that the spatial arrangement and orientation of the
donor and acceptor chromophores could be adjusted. The
free-energy differences between the LE and CT states for 6
could readily be tuned and dual emissions were observed in
those polymers with appropriate the DGo values for the
charge-separation processes. In particular, the Marcus-in-
verted region was observed in both steady-state and fast ki-
netic measurements for 6. In contrast, the photophysical be-
havior is completely different when the substituents on the
silicon atom are changed from dimethyl to diisopropyl
groups. Because of the synchronized Thorpe–Ingold effect
exerted by the bulky isopropyl substituents and the pertur-
bation of the conformation of the polymer, the adjacent
donor and acceptor chromophores in 7 may be brought
closer. A charge-transfer complex may thus be formed from
a strong interaction between these neighboring chromo-
phores at the ground state. Evidence to support the impor-
tance of folding on the photophysical behavior of 6 and 7
can be illustrated by a systematic investigation on the oligo-
mers with the same structural features of 6 and 7.[31]
Experimental Section
Figure 14. a) Absorption and b) emission spectra of 6c, 7c, and 16 with
different ratios of Me2Si and iPr2Si spacers (c: 6c, a: 16 Me2Si/
iPr2Si=9:1, g: 16 Me2Si/iPr2Si=4:1, d: 16 Me2Si/iPr2Si=1:1, l:
7c; concentration=4.5ꢁ10À6 m). c) Variable-temperature emission spec-
tra of 16 (SiMe2/SiiPr2 =9:1) in MTHF (c: 298 K, a: 270 K, g:
240 K, d: 210 K, l: 180 K; concentration=1.0ꢁ10À5 m; excitation
wavelength=330 nm).
General: Gel-permeation chromatography (GPC) was performed on a
Waters GPC machine with an isocratic HPLC pump (1515) and a refrac-
tive-index detector (2414). THF was used as the eluent (flow rate=
1.0 mLminÀ1). Waters Styragel HR2, HR3, and HR4 columns (7.8ꢁ
300 mm) were employed for determination of the relative molecular
weight with polystyrene as a standard (Mn values ranged from 375 to
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Properties of Silyene-Spaced Copolymers
FULL PAPER
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Acknowledgements
This work was supported by the National Science Council and the Na-
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