Folding of alternating dialkylsilylene-spaced donor-acceptor copolymers: The oligomer approach

Article abstract of DOI:10.1002/chem.201102031

A series of oligmers with donor-acceptor pairs separated by diisopropylsilylene (iPr₂Si) spacers, composed of monomer 4b, dimer 5, trimer 6, and tetramer 7, were synthesized to scrutinize the folding behavior. Monomer 4a with a dimethylsilylene (Me₂Si) spacer was also prepared for comparison. The 4-aminostyrene moiety was used as the donor and the stilbene moiety as the acceptor. Both steady-state and time-resolved fluorescence spectroscopic measurement were made. Regardless of the substituents on the silicon atom, the emission spectra of 4a and 4b exhibit both local excited (LE) emission of the acceptor chromophore and emission from the charge-separated state (CT emission), which are similar to that of the corresponding Me₂Si-spaced copolymer 2a with the same donor and acceptor chromophores, but different from that of the copolymer with the iPr₂Si spacer 2b. Dimer 5 behaves like 4 and 2a. As the chain length of the oligomers increases, the emission properties of the higher homologues become prone to that of 2b. Thus, tetramer 7 exhibits emission from the charge-transfer complex, which is essentially same as that of 2b. Moreover, charge-transfer absorptions emerge in 6 and 7. These results suggest that the folding nature of oligomers approaches that of the corresponding polymer, as the degree of oligomerization increases, and the electronic interactions between adjacent donor-acceptor pairs are controlled by the steric effect of the substituents on the silicon atoms and concomitant amplification of the stabilizing energy by extending the distance of the folding structure. Copyright

Full text of DOI:10.1002/chem.201102031

FULL PAPER
DOI: 10.1002/chem.201102031
Folding of Alternating Dialkylsilylene-Spaced Donor–Acceptor Copolymers:
The Oligomer Approach
Chih-Hsien Chen,[a] Wen-Hao Chen,[a] Yi-Hung Liu,[a] Tsong-Shin Lim,[b] and
Tien-Yau Luh*[a]
Dedicated to Prof. Tak-Hang Chan for his outstanding contributions to organosilicon chemistry
Abstract: A series of oligmers with
donor–acceptor pairs separated by dii-
sopropylsilylene (iPr2Si) spacers, com-
posed of monomer 4b, dimer 5, trimer
6, and tetramer 7, were synthesized to
scrutinize the folding behavior. Mono-
mer 4a with a dimethylsilylene (Me2Si)
spacer was also prepared for compari-
son. The 4-aminostyrene moiety was
used as the donor and the stilbene
moiety as the acceptor. Both steady-
state and time-resolved fluorescence
spectroscopic measurement were made.
Regardless of the substituents on the
silicon atom, the emission spectra of 4a
and 4b exhibit both local excited (LE)
emission of the acceptor chromophore
and emission from the charge-separat-
ed state (CT emission), which are simi-
lar to that of the corresponding Me2Si-
spaced copolymer 2a with the same
donor and acceptor chromophores, but
different from that of the copolymer
with the iPr2Si spacer 2b. Dimer 5 be-
haves like 4 and 2a. As the chain
length of the oligomers increases, the
emission properties of the higher ho-
mologues become prone to that of 2b.
Thus, tetramer 7 exhibits emission
from the charge-transfer complex,
which is essentially same as that of 2b.
Moreover, charge-transfer absorptions
emerge in 6 and 7. These results sug-
gest that the folding nature of oligo-
mers approaches that of the corre-
sponding polymer, as the degree of oli-
gomerization increases, and the elec-
tronic interactions between adjacent
donor–acceptor pairs are controlled by
the steric effect of the substituents on
the silicon atoms and concomitant am-
plification of the stabilizing energy by
extending the distance of the folding
structure.
Keywords: oligomers
·
photoin-
duced electron transfer · polymer
folding · substituted silylene · sili-
con · Thorpe–Ingold effect
Introduction
stable conformation.[11] It is, therefore, envisaged that an in-
crease of repetitive units in oligomers can achieve a structur-
al feature with similar folding behavior of a polymer. The
use of oligomers to mimic the optoelectronic properties of
conjugated polymers has been extensive.[12] Moreover, fol-
damers provide another useful model for the folding of bio-
macromolecules.[2]
We recently found that the size of the substituents on the
silicon atom in dialkylsilylene-spaced divinyl arene copoly-
mers (e.g., 1) may exert the Thorpe–Ingold effect, thus re-
sulting in a discrepancy in the photophysical and morpho-
logical behavior of the polymers.[13–15] The silylene moiety in
1 is, in general, considered to be an insulating tetrahedral
spacer.[13–19] Fluorescence resonance energy transfer
(FRET),[17] chiroptical property transfer,[18] and photoin-
duced electron transfer (PET)[15,19] can readily take place be-
tween chromophores separated by the silylene moieties. To
illustrate this approach, both the local emission (LE) of ac-
ceptor chromophores and emission from the charge-separat-
ed state (CT emission) are observed for 2a, whereas emis-
sion from the charge-transfer complex is found for 2b.[15]
The difference in the photophysical properties might be at-
tributed to the change in the distance between the donor
and acceptor chromophores in 2a and 2b. Such a morpho-
logical discrepancy may have arisen from the change in the
The macroscopic functional properties of polymeric materi-
als should be closely related to the intrachain folding and in-
termolecular interactions of polymeric molecules. The fold-
ing nature in general is governed by numerous through-
space noncovalent interactions along the polymeric back-
bone.[1–9] A polymeric molecule is folded so that the loss of
entropy due to conformational change can be compensated
by the enthalpic gain through such intrachain interactions.[10]
The differences in small energetic barriers for each of the
conformational states may be amplified by extending the
distance of the folding structure, thus resulting in a more
[a] Dr. C.-H. Chen, W.-H. Chen, Y.-H. Liu, 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)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102031.
Chem. Eur. J. 2012, 18, 347 – 354
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347

local conformation that surrounds the silicon atom due to
the steric effect of the substituents. Interestingly, both the
LE and CT emission are observed in the model monomeric
compounds 3a and 3b, which contain the same pair of
donor and acceptor chromophores; albeit, the relative inten-
sities of these two emissions are somewhat different. Unlike
2b, no charge-transfer absorption and the corresponding
emission were observed for 3b with isopropyl substituents
on the silicon atom. It is envisaged that, in addition to the
Thorpe–Ingold effect, other factors such as polymer folding
may also play a pivotal role on the photophysical discrepan-
cies in 2. To mimic the folding nature of 2, we have designed
and synthesized monomers 4 and oligomers 5–7 of different
chain lengths with the same structural features of the repeti-
tive units in 2.
Scheme 1. a) nBuLi, iPr2SiHCl (73%); b) MnO2 (90%); c) 4-BrC6H4PO-
(OEt)2, KOtBu (66%); d) 10, [Rh(PPh3)3Cl], NaI (76%); e) nBuLi,
A
ACHTUNGTRENNUNG
ꢀ
ꢀ
iPr2SiHCl (79%); f) HC CC6H4NMe
(PPh3)3Cl], NaI (36%).
ACHTUNGNERT(UNNG CH2)3NMeC6H4C CH (16), [Rh-
AHCTUNGTRENNUNG
Results
Synthesis: Similar to the synthesis of 2,[15] the rhodium(I)-
catalyzed hydrosilylation protocol was used to construct 4–7,
with silylene spacers between the donor and acceptor chro-
mophores. It is interesting to note that the presence of a sto-
ichiometric amount of NaI (relative to the equivalents of
the amino moiety in the reactants) was essential to facilitate
the hydrosilylation reactions.[15,19] Thus, monomers 4a and
4b were synthesized from the hydrosilylation of 10 with 8
and 9, respectively [Eq. (1)]. The synthesis of dimer 5 is
summarized in Scheme 1. Scheme 2 outlines the synthesis of
key intermediate 22 for trimer 6 and tetramer 7, which are
shown in Scheme 3.[20]
Scheme 2. a) N-methylaniline (33%); b) NBS (99%); c) AlMe3 (66%);
ꢀ
d) LAH (97%); e) I2 (70%); f) TMSC CH, [Pd
N
ꢀ
(72%); g) TIPSC CH, [Pd
(99%); i) 15, [Rh
aluminum hydride, NBS=N-bromosuccinimide, TBAF=tetrabutyl-
ammonium fluoride, TEA=N,N,N-triethylamine, TIPS=triisopropylsilyl,
TMS=trimethylsilyl.
A
AHCTUNTGRENN(GNU PPh3)3Cl], NaI (56%); j) TBAF, 50%. LAH=lithium
AHCTUNGTRENNUNG
348
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Oligomerization in Donor–Acceptor Copolymers
FULL PAPER
in these disordered structures of 4b (Figure 1c) is different
from that in the anti conformation in both 4a and 4b (Fig-
ure 1a,b). The X-ray data for 4a and 4b offer direct evi-
dence to show that the size of the substituents on the silicon
atom should exert the Thorpe–Ingold effect on the orienta-
tion of the remaining groups attached to this silicon atom,
and thus the conformation of the molecules.
Photophysical properties of 4: As mentioned previously,[15]
the simple donor–acceptor monomeric pairs 3a and 3b ex-
hibit similar photophysical properties. This observation indi-
cates that the substituents on the silicon atom might have
little effect on the adjacent chromophore–chromophore in-
teraction separated by the silylene spacers in these mono-
mers. It is noteworthy that polymers 2a and 2b consist of
two donors and one acceptor species in each of the repeti-
tive units. Monomers 4a[15] and 4b and oligomers 5–7 with
the same ratio of donor and acceptor chromophores as
those in 2 were thus synthesized in the hope that they could
serve as a better model for the photophysical properties of
polymers 2. The absorption and emission spectra of 4a and
4b are shown in Figure 2. Interestingly, the absorption pro-
files of both 4a and 4b were the sum of the absorptions of
the donor and acceptor chromophores with no characteristic
band at relatively longer wavelengths for the charge-transfer
complex,[21] which was observed in 2b.[15] Moreover, the ab-
sorption spectra of 4a and 4b were independent of the sol-
vent polarity. The emission profiles exhibited dual emissions
at l=360 and 450 nm in cyclohexane, and the latter emis-
sion shifted to l=510 nm in THF. The emission band at the
shorter wavelength was assigned as the LE of the acceptor
chromophore and the emission band at the longer wave-
length could have arisen from the CT emission. In nonpolar
cyclohexane, the relative intensity of the LE was stronger
than that of the CT emission, whereas the relative intensities
were reversed in THF. The behavior of monomers 4a and
4b, in general, are similar to copolymer 2a, and the size of
the substituents (Me versus iPr) on the silicon atom ap-
peared to be irrelevant.
Scheme 3. a) [Rh
(56%); c) nBuLi, iPr2SiHCl (79%); d) 22, [RhACHTUNGTRENNUNG
G
ACHTUNGTREN(UNNG PPh3)3Cl], NaI
Crystal structures of 4:[20] The ORTEP structures of 4a and
4b are shown in Figure 1. Both crystals belong to the P21/n
space group with a center of symmetry. It is noteworthy that
4a with dimethylsilylene spacers exclusively shows an anti
conformation for both the vinylsilane and stilbene moieties.
The center-to-center distance between the aniline donor and
stilbene acceptor chromophores for the single crystal 4a is
10.47 ꢁ. On the other hand, 4b with diisopropyl linkers ex-
hibits 50% of the crystal structure similar to that of 4a and
the center-to-center distances between the aniline and stil-
bene chromophores of this structure is 11.07 ꢁ. In addition,
another 50% constitutes disorder structures of equal
amounts of syn and anti conformations for the vinylsilane
groups, whereas the center-to-center distances between the
aniline and stilbene chromophores of this structure is
9.79 ꢁ. Interestingly, the orientation of the stilbene moiety
Figure 2. Absorption spectra (solid) of 4a (black) and 4b (gray) in THF
and emission spectra of 4a (black) and 4b (gray) in cyclohexane (a)
and THF (d). Concentration=5ꢂ10À6 m; excitation wavelength=
330 nm.
Figure 1. ORTEP (30% probability level) drawing of a) 4a, b) 4b with-
out disorder, and c) 4b with disorder of the vinylsilane groups.
Chem. Eur. J. 2012, 18, 347 – 354
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T.-Y. Luh et al.
Photophysical properties of oligomers 5–7: The absorption
and emission spectra of dimer 5, trimer 6, and tetramer 7
with the diisopropylsilylene spacer in THF are shown in
Figure 3. The photophysical properties of 5 are almost iden-
the folding nature of copolymer 2b should be similar to that
of oligomer 7.
As mentioned above (Figure 3a), the absorption of the
acceptor stilbene chromophore in these oligomers 5–7 is at
approximately l=300–350 nm and that of the charge-trans-
fer complex is at approximately l=350–400 nm. Because 5
does not exhibit any absorption beyond l=350 nm, no
change in emission spectra was observed at different excita-
tion wavelengths. On the other hand, the emission profiles
for 6 and 7 are excitation-wavelength dependent (Figure 4).
Figure 3. a) Absorption and b) emission spectra of 5 (c, lex =330 nm),
6 (a, lex =380 nm), 7 (g, lex =380 nm), and 2b (d, lex =330 nm)
in THF. Concentration=1ꢂ10À6 m.
tical to those of 4a and 4b. Interestingly, the absorbance at
approximately l=350–400 nm increased with increasing oli-
gomeric chain length. This absorption band could be attrib-
uted to the charge-transfer absorption for the formation of
the charge-transfer complex as observed in 2b. The emission
spectrum of dimer 5 was similar to those spectra of copoly-
mer 2a and the corresponding monomers 4a and 4b (Fig-
ure 3b), in which both LE and CT emission at l=375 and
510 nm were observed. Interestingly, trimer 6 exhibited an
emission band at l=430 nm with a vibronic structure in
THF. The emission band further shifted to l=450 nm in
tetramer 7, which was similar to the emission of copolymer
2b with the diisopropylsilylene spacer. The discrepancy in
the emission profiles among these diisopropylsilylene-
spaced oligomers 5–7, monomers 4b, and copolymer 2b
may arise from the difference in the number of repetitive
units in the oligomeric and polymeric chains. In other words,
when the degree of oligomerization was four (as in 7) or
above, the ground-state interaction between adjacent
donor–acceptor pairs should become similar, thus resulting
in the formation of charge-transfer complexes. Presumably,
Figure 4. Emission spectra of a) 6 and b) 7 in THF at different excitation
wavelengths (c: lex =330 nm, a: lex =350 nm, g: lex =360 nm,
d: lex =380 nm).
When the shorter excitation wavelength was employed, the
emission maxima at approximately l=500 nm were ob-
served for both 6 and 7. Presumably, local excitation of the
acceptor stilbene chromophore might take place followed
by charge transfer, thus leading to CT emission at this wave-
length. The relative intensities of the emission band at ap-
proximately l=500 nm for both 6 and 7, however, de-
creased when the excitation wavelength increased from l=
330 to 380 nm. In addition, the emission of the charge-trans-
fer complex at l=430–470 nm was also observed. Alterna-
tively, excitation at the longer wavelength (l>360 nm) due
to the absorption of the charge-transfer complexes in 6 and
7 gave an emission band only at approximately l=430–
470 nm, which is attributed to the emission of the charge-
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Oligomerization in Donor–Acceptor Copolymers
FULL PAPER
transfer complexes between the donor and acceptor chro-
mophores of these oligomers.
The normalized excitation spectra of 6 and 7 are shown in
Figure 5. The relative intensities at approximately l=360–
400 nm increased when the monitoring wavelengths gradual-
the CT emission in polymer 2a at approximately l=510 nm
gradually shifted to a long wavelength with lowering intensi-
ties as the temperature decreased.[15] Similar behavior was
observed for 4 and 5 in methyltetrahydrofuran (MTHF; Fig-
ure 6a–c). This observation appears to be the characteristic
property of CT emissions in these systems. When the tem-
perature is lowered, the polarity of the solvent should be en-
hanced.[22] The energy level of CT state should thus be low-
ered, thereby resulting in a decrease in the energy gap be-
tween the CT and ground states.[23] Moreover, the LE for 4
and 5 at approximately l=370 nm is somewhat enhanced at
lower temperatures (Figure 6a–c). The thermal relaxation of
Figure 5. Excitation spectra of a) 6 and b) 7 in THF with different moni-
tored wavelengths (c: lex =570 nm, a: lex =550 nm, g: lex
525 nm, d: lex =500 nm, l: lex =480 nm).
=
ly changed from l=570 to 480 nm. It is noteworthy that the
relative enhancement at approximately l=360–400 nm was
more prominent in 7 that in 6. The band at approximately
l=360–400 nm in these excitation spectra would be contrib-
uted to by the charge-transfer absorption, and the emission
band at l=330 nm in the excitation spectra should be as-
signed as the absorption of the stilbene chromophore. As
mentioned above, the characteristic CT emission of 6 and 7
appeared at a longer wavelength (at approximately l=
500 nm) and that of the radiative relaxation from the
charge-transfer complex occurred at a shorter wavelength
(l=430–470 nm). Consequently, when the shorter monitor-
ing wavelengths were chosen, the band at approximately l=
360–400 nm in the excitation spectra was enhanced.
Temperature-dependent emission spectra of 4–7: Com-
pounds 4a, 4b, and 5 exhibit dual emission (Figures 2 and
3), which should consist of the LE of the acceptor stilbene
chromophore at l=370 nm and the CT emission at approxi-
mately l=510 nm. We have previously demonstrated that
Figure 6. Temperature-dependent emission spectra of a) 4a, b) 4b, and
c) 5 in THF at different temperatures (c: 295 K, a: 270 K, g:
240 K, d: 210 K, l: 180 K). Excitation wavelength=330 nm.
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T.-Y. Luh et al.
Table 1. The fluorescence decay lifetimes t of 2–7.
the corresponding excited state might be restricted at lower
temperatures.[24]
The temperature-dependent emission spectra of 6 and 7
represented completely different behavior from 4 and 5
(Figure 7a,b), that is, the intensity increased with decreasing
t [ps]
Measured at
l=360–380 nm
Measured at
l=440–460 nm
4a
4b
5
12
10
10
–
–
–
–
6
175
160
7
2a
2b
–
9[a]
150[a]
[a] Ref. [15].
5 are close to the fluorescence decay lifetime of the LE
emission for 2a (i.e., t=9 ps). On the other hand, the t val-
ues of the charge-transfer complexes observed in 6 and 7
are also similar to that observed in 2b. It is worth mention-
ing that the kinetic results are consistent with the observed
photophysical properties of the steady state in these oligo-
mers and polymers.
Discussion
We examined the detailed photophysical properties of mon-
omers 4a and 4b, dimer 5, trimer 6, and tetramer 7, which
have similar repetitive units as that of polymers 2a and 2b
(described in the accompanying report).[15] Two distinct cate-
gories were found in the photophysical behavior of these
molecules. Small molecules 4 and 5 and polymer 2a exhibit
similar emission profiles with typical LE and CT emission,
no matter whether the substituents on the silicon atom are
methyl or isopropyl. On the other hand, both the CT emis-
sion and emission from the charge-transfer complexes for
oligomers 6 and 7 were observed upon excitation at l=
330 nm (Figure 4). This CT emission disappeared when the
excitation wavelength was l=370 nm. In this regard, oligo-
mers 6 and 7 gave similar photophysical properties from the
charge-transfer complex between the adjacent aminostyrene
donor and stilbene acceptor chromophores as that of poly-
mer 2b.
Notably, only monomer 4a and polymer 2a have the di-
methylsilylene spacer, whereas the rest have the bulky diiso-
propylsilylene spacers. The crystal structures of 4a and 4b
clearly demonstrated that the bulkiness of the substituents
on the silicon atom may exert the Thorpe–Ingold effect,
thus leading to different conformations around the silicon
atom. The change from the CT emission to the charge-trans-
fer complex emission in 4–7 indicates that there is a discrep-
ancy in the electronic coupling between the donor and ac-
ceptor chromophores. It seems likely that such electronic
coupling might be relatively weak in 4 and 5 and strong in 6
and 7. In other words, the electron-transfer processes might
be altered from non-adiabatic to adiabatic regimes.[21] As
mentioned in the accompanying report,[15] the electronic
coupling for PET in 2a was estimated to be around 25 cmÀ1
and those for 6 and 7 were calculated on the basis of the
Figure 7. Temperature-dependent emission spectra of a) 6 and b) 7 in
THF at different temperatures (c: 295 K, a: 270 K, g: 240 K,
d: 210 K, l: 180 K). Excitation wavelength=330 nm.
temperature no matter whether the excitation wavelengths
were l=350 or 370 nm. The excitation wavelength at l=
350 nm for 6 and 7 might trigger the occurrence of the CT
emission (Figure 4). However, the increase in the emission
intensity during lowing of the temperature should dominate
so that the variation of the CT emission, if any, could not be
detected. The similar emission behavior between 6, 7, and
polymer 2b further suggests that the folding nature of 6 and
7 should take place in the same manner of that of 2b.
Time-resolved fluorescence spectroscopy: Time-resolved
fluorescence spectroscopy with a femto-second Ti/sapphire
laser equipped with a streak camera was employed to obtain
the fluorescence decay lifetimes t of 4–7 in THF. The results
are compared with the lifetimes of 2a and 2b in Table 1.
The decay of the LE emissions for 4 and 5 were monitored
at l=360–380 nm, and the fluorescence decays of the
charge-transfer complexes for 6 and 7 were monitored at
l=440–460 nm. The t values of monomers 4a,b and dimer
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Oligomerization in Donor–Acceptor Copolymers
FULL PAPER
model developed by Mulliken and Hush[21,25] to be 600 and
1000 cmÀ1 per repeating unit, respectively.[20]
pulse laser. The fluorescence of the sample was reflected by a grating
(150 gmmÀ1; BLZ: 500 nm) and detected by an optically triggered streak
camera (Hamamatsu C5680) with a time resolution of about 0.3 ps. The
sample was prepared in a concentration of 1.0ꢂ10À5 m and an ultramicro
cuvette with a path length of 1 mm was used to maintain the excitation at
the same time. The signal was collected ten times to decrease the signal-
to-noise ratio.
Intriguingly, the substituents on the silicon atom for 4b
and 5–7 were isopropyl groups, which should exert a similar
Thorpe–Ingold effect on the local conformation around the
silicon atom. The discrepancy in the photophysical behavior
could only have arisen from the number of the repetitive
units in these oligomers. The similarity in photophysical
properties between polymer 2b and larger oligomers 6 and
7 suggests that the folding nature might be similar. In other
words, as the chain length of the oligomer becomes longer,
they should be more folded. Through-space intrachain van
der Waal interactions might restrict the degree of freedom
of the oligomeric chain to bring the adjacent donor–accept-
or chromophores closer. As such, this geometrical perturba-
tion may change the nature of the electronic coupling from
weak to strong by increasing the degree of polymerization.
It is believed that synchronization of the Thorpe–Ingold
effect and the conformational changes of the oligomers may
occur in these larger oligomers 6 and 7 and polymer 2b. A
similar enhancement was not observed either in small mole-
cules 4b and 5 with the diisopropylsilylene linkers or in
polymer 2a with the dimethylsilylene spacers. Our results
suggest that the folding nature of trimer 6 and tetramer 7
should emerge to be similar to that of the corresponding
polymer 2b in these silylene-spaced copolymers.
Acknowledgements
We thank the National Science Council and the National Taiwan Univer-
sity of the Republic of China for support. We are grateful to Professor
Huan-Cheng Chang for allowing us to use the femtosecond laser facilities
for the kinetic measurements. Thanks are also due to Professor Yu Wang
for helpful discussion.
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Conclusion
In summary, we have demonstrated that an increase in the
oligomer chain length in 4–7 should gradually shift the pho-
tophysical properties due to a different folding nature. The
presence of bulky isopropyl substituents together with con-
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Experimental Section
General: The absorption spectra were measured on a Hitachi U-3310
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Received: July 1, 2011
Revised: August 25, 2011
Published online: December 8, 2011
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