Photoinduced electron transfer as a probe for the folding behavior of dimethylsilylene-spaced alternating donor-acceptor oligomers and polymers

Article abstract of DOI:10.1021/ma302381y

A series of oligomers and polymers having dimethylsilylene-spaced alternating 4-aminostyrene donor and stilbene acceptor chromophores (two to one) are regioselectively synthesized, and the two donor chromophores are separated by different bridges between two donors. Photophysical tools have been used to examine the folding behavior of these copolymers. Both steady-state and time-resolved fluorescence spectroscopic measurements were examined. The relative intensities (I_CT/I_LE) between emission from charge-separated state (CT emission) and local excited emission of acceptor chromophore (LE emission) increase with increasing number of repeating units, and reach a plateau, when the linkers between the two aminostyrene chromophores are trimethylene bridges. Replacements of these by dimethylene or tetramethylene linker reduce the relative intensities of CT emission of the polymers, owing to the different folding behavior of these polymers. The CT emission intensity of the polymer with rigid piperazine linkers is much lower than that with trimethylene-bridged copolymer of the same degree of polymerization. Slight conformational change of these polymers would lead to slight variation of the distance between donor and acceptor chromophores so that the nonadiabatic interactions in the excited state between donor-acceptor pairs in these oligomers and polymers would be perturbed by such change of conformations.

Full text of DOI:10.1021/ma302381y

Article
pubs.acs.org/Macromolecules
Photoinduced Electron Transfer as a Probe for the Folding Behavior
of Dimethylsilylene-Spaced Alternating Donor−Acceptor Oligomers
and Polymers
Wei-Chih Liao,†,§ Wen-Hao Chen,†,§ Chih-Hsien Chen,*,† Tsong-Shin Lim,‡ and Tien-Yau Luh*,†
†Department of Chemistry, National Taiwan University, Taipei, Taiwan 106
‡Department of Physics, Tunghai University, Taichung, Taiwan 407
S
* Supporting Information
ABSTRACT: A series of oligomers and polymers having
dimethylsilylene-spaced alternating 4-aminostyrene donor and
stilbene acceptor chromophores (two to one) are regiose-
lectively synthesized, and the two donor chromophores are
separated by different bridges between two donors. Photo-
physical tools have been used to examine the folding behavior
of these copolymers. Both steady-state and time-resolved
fluorescence spectroscopic measurements were examined. The
relative intensities (ICT/ILE) between emission from charge-
separated state (CT emission) and local excited emission of
acceptor chromophore (LE emission) increase with increasing
number of repeating units, and reach a plateau, when the linkers between the two aminostyrene chromophores are trimethylene
bridges. Replacements of these by dimethylene or tetramethylene linker reduce the relative intensities of CT emission of the
polymers, owing to the different folding behavior of these polymers. The CT emission intensity of the polymer with rigid
piperazine linkers is much lower than that with trimethylene-bridged copolymer of the same degree of polymerization. Slight
conformational change of these polymers would lead to slight variation of the distance between donor and acceptor
chromophores so that the nonadiabatic interactions in the excited state between donor−acceptor pairs in these oligomers and
polymers would be perturbed by such change of conformations.
between adjacent donor chromophore and acceptor chrom-
phore would be closer in 2 than that in 1 due to, inter alia, the
Thorpe−Ingold effect.14,15 Accordingly, nonadiabatic PET is
observed in 1 (R = Me), whereas adiabatic PET prevails in 2 (R
INTRODUCTION
■
The macroscopic properties of a polymeric material are
determined by, inter alia, an ensemble of different conforma-
tions of folded polymers. The folding behavior may be
rendered from numerous through-space noncovalent inter-
actions along the polymeric backbone.1−10 The differences of
small energetic barriers for each of the conformational states in
a polymer may be added up with the increase of chain length of
a polymer, leading to a more stable conformation.8 Our
knowledge on polymer foldings, however, remains to be
limited. Several probes have been used to examine the folding
of macromolecules by means of circular dichroism, fluorescence
quenching, and excimer formation.11 Photoinduced electron
transfer (PET) has been extensively used for the investigation
of protein conformations.12 The rate of PET is governed by
electronic coupling between reactant and product state, free
energy of reaction, and reorganization energy. These
parameters are particularly sensitive to the distance between
donor and acceptor chromophores. It is envisaged that the use
of PET as a probe might provide information about the nature
of folded polymers. Indeed, we recently found that the size of
the substituents R on silicon affect the mode of PET between
adjacent aminostyrene donor group and stilbene acceptor
moiety separated by dialkylsilylene in 1 and 2.13 The distance
i
= Pr). The photophysical properties of a series of small
molecules 3a−3d having similar repeating units to that of 2
have been examined. Interestingly, the emission profiles of
monomer 3a and dimer 3b are similar to that of 1, whereas
those of trimer 3c and tetramer 3d behave closer to that of 2.13b
It is worth mentioning that the substituents on silicon in these
oligomers are bulky isopropyl groups. Thus, the Thorpe−
Ingold effect might exert similarly in all these small oligomers.
The discrepancy on the photophysical properties in these small
molecules may be arisen not only from the Thorpe−Ingold
effect but also from the different folding nature of the oligomers
and thus the polymers. It is, therefore, envisaged that a series of
oligomers and polymers with different degree of polymerization
might offer a platform to realize the folding ensemble of
polymers. We now wish to report a systematic comparison of a
series of oligomers 4 and the corresponding polymers 1 with
Received: November 19, 2012
Revised: January 11, 2013
Published: February 6, 2013
© 2013 American Chemical Society
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different degree of polymerization in the hope of understanding
the change of folding behavior of these oligomers/polymers
with the chain lengths. Dimethylsilylene bridges were chosen in
1 and 4 to eliminate the influence of the Thorpe−Ingold effect
on the photophysical properties of these substrates.
RESULTS AND DISCUSSION
Synthesis of 4. Similar to the synthesis of 3,13b Rh(I)-
■
catalyzed hydrosilylation protocol was used to construct 4
having a dimethylsilylene spacer between donor and acceptor
chromophores. It is interesting to note that the presence of a
stoichiometric amount of NaI (relative to the equivalents of the
amino moiety in the reactants) was essential to facilitate the
hydrosilylation reactions.13,16 The syntheses of 4a−4d are
summarized in Scheme 1. Polymers 114 (n = 14) and 130 (n =
30) were also synthesized by hydrosilylation of 10 with 6. The
details are described in the Supporting Information.
Steady-State Spectroscopies of 114, 130, and 4. The
absorption and emission spectra of 4a−4d, 114, and 130 in
cyclohexane are shown in Figure 1. The absorption profiles of
4a−4d, 114, and 130 were essentially the sum of the absorptions
of the donor and acceptor chromophores. There is no
additional absorption band due to ground-state interactions
between donor and acceptor moieties. The same modes of PET
for 4a−4d, 114, and 130 were observed and could be attributed
to the nonadiabatic electron transfer despite of the difference in
the degree of polymerization. The luminescence spectra of 4a−
4d, 114, and 130 show dual emissions around 360 nm due to the
local excited emission of the acceptor stilbene chromophore
(LE emission) and at 450 nm which may be arisen from the
emission of the charge-separated state of donor−acceptor pair
(CT emission).13,17 The same CT emission wavelengths for 4
a
Scheme 1. Syntheses of 4a−4d
a
n
Reagents and conditions: (a) Me2NC6H4CCH (7), Rh(PPh3)3Cl, NaI, 39%; (b) 7, Rh(PPh3)3Cl, NaI, 56%; (c) BuLi, Me2SiHCl, 99%; (d)
HCCC6H4NMe(CH2)3NMeC6H4CCH (10), Rh(PPh3)3Cl, NaI, 41%; (e) 9, Rh(PPh3)3Cl, NaI, 35%; (f) TBAF, 97%; (g) 6, Rh(PPh3)3Cl,
n
NaI, 35%; (h) 10, Rh(PPh3)3Cl, NaI, 63%; (i) BuLi, Me2SiHCl, 40%; (j) 12, Rh(PPh3)3Cl, NaI, 39%.
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Figure 2. Ratios of intensities of the CT emissions to the LE emissions
of oligomers 4 (solid circle) and polymers 1 (solid circle), 5a (open
triangle), 5b (open diamond), 5c (open square), and 5d (open circle)
in cyclohexane.
Figure 1. (a) Absorption and (b) emission spectra of 4a (black), 4b
(red), 4c (green), 4d (blue), 114 (magenta), and 130 (orange) in
cyclohexane (λex = 320 nm).
and 1 suggest that the mode of interactions between donor and
acceptor chromophores would be the same. Since 4a contains
only one acceptor chromophore and two adjacent donor
chromophores, it seem likely that interaction between adjacent
chromophores in these oligomers 4 and polymers 1 would be
responsible for the CT emission. Interestingly, the relative
intensity of CT emission (ICT) to LE emission (ILE) is increased
when the degree of polymerization increases and reaches a
plateau. A plot of ICT/ILE for these substrates against the degree
of polymerization is shown in Figure 2. It is noteworthy that
the CT emission is more populated when the polymer is more
folded.
Temperature-dependent emission spectra were performed to
investigate the effect of polar solvent such as methyltetrahy-
drofuran (MTHF) on the PET. For example, the spectra of 4b
and 130 at different temperatures are shown in Figure 3. In
comparison with fluorescence spectra measured in cyclohexane,
the emission in MTHF at longer wavelength further red-shifted
and became more prominent because of increasing solvent
polarity. On the other hand, the intensities of the CT emissions
in 4b and 130 decreased significantly together with bath-
ochromic shifts as temperature was lowered. The polar solvents
would be more polar as the temperature decreases,21 and thus,
the charge separation state of donor−acceptor pairs might be
stabilized to lower energy level. The interaction between this
excited state and the higher vibrational level of the ground state
might result in thermal relaxation process to reduce the
opportunity of radiative relaxation.22,23 These results are
consistent with the characteristic property of CT emission in
these oligomers and polymers.24
Figure 3. Temperature-dependent emission spectra of (a) 4b and (b)
130 measured in MTHF: 298 K, black; 270 K, red; 240 K, green; 210
K, blue; 180 K, magenta (λex = 320 nm).
The relative ratios of ICT/ILE for 4a−4d, 114, and 130 are
compared with the degree of polymerization at different
temperatures. As shown in Figure 4a, the ICT/ILE ratios increase
with increasing degree of polymerization and reach a plateau.
Interestingly, the ratios of ICT(T K)/ICT(300 K) for 4a−4d, 114,
and 130 decrease with lowing temperature (Figure 4b). It is
worthy to note that ICT(T K)/ICT(300 K) ratios for 4a were
more sensitive to the change in temperatures, whereas the
variations of ICT(T K)/ICT(300 K) ratios for polymers 114 and
1
30 are less prominent than those for small oligomers. It seems
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Table 1. Fluorescence Lifetimes of 4a−4d, 114, and 130 in
THF
lifetimes
τcs (ps)
τcr (ns)
4a
12
13
11
11
9
7.9
7.1
5.1
4.6
4.5
4.5
4b
4c
4d
114
130
9
results are consistent with those of steady-state measurements
as described in the previous sections.
Replacement of Aliphatic Bridges between Amino-
styrenes. Polymers 1 contain trimethylene bridges to connect
each pair of aminostyrene donor chromophores. It seems likely
that the nature of the tethering chain may also influence the
conformation of such polymers. Accordingly, the trimethylene
linkages in 1 were replaced by four kinds of linkers, and
polymers 5 (5a: ethylene; 5b: 2,2-dimethylpropylene; 5c:
tetramethylene; 5d: piperazine) were thus synthesized (Scheme
2). The properties of 5 are summarized in Table 2. It is
a
Scheme 2. Syntheses of Polymers 5a−5d
Figure 4. (a) Plot of ICT/ILE versus degree of polymerization for 4a−
4d, 114, and 130 in MTHF at different temperatures: 298 K, black; 270
K, red; 240 K, green; 210 K, blue; 180 K, magenta. (b) Ratio of ICT(T
K)/ICT(300 K) at different temperatures for 4a (black), 4b (red), 4c
(green), 4d (blue), 114 (magenta), and 130 (orange) in MTHF.
likely that the radiative transition from the charge separation
state to ground state in charge recombination process might be
enhanced due to folding of polymer, resulting in more
constrained structure. Presumably, the geometrical reorganiza-
tion of electronic structure in PET process might play an
important role. It is worthy to note that the degree of
bathochromic shift of CT emission for 4a−4d, 114, and 130 are
almost the same at different temperatures. These results
indicate that the energy gap between charge separation state
and ground state would be same in these oligomers and
polymers. Accordingly, the dissimilarity of relative ICT values
between oligomers and polymers may only be influenced by the
folding nature of the oligomers and polymers.
a
Reagents and conditions: (a) NBS, CHCl3; (b) PdCl2(PPh3)2, CuI,
trimethylsilylacetylene (15a, 65%; 15b, 56%; 15c, 59%; 15d, 42%). (c)
K2CO3 (16a, 80%; 16b, 82%; 16c, 85%; 16d, 86%. (d) 6, Rh(PPh3)3Cl
(5 mol %), NaI (2 equiv) (5a, 75%; 5b, 78%; 5c, 75%; 5d, 75%).
Kinetic Measurement of Oligomers. Time-resolved
fluorescence spectroscopy with a femtosecond Ti:sapphire
laser was employed to obtain the emission decay profiles of
oligomers 4a−4d and copolymers 114 and 130. The fluorescence
decay of LE emission was monitored for 4a−4d, 114, and 130 in
THF. Single-exponential fittings were utilized to give the decay
lifetimes, τcs, which were attributed to the charge separation
process (Table 1). The τcs’s of copolymers 1 and oligomers 4
are around 9−13 ps. These results further suggest that the same
mode of charge separation process proceed in 4a−4d, 114, and
130. The lifetimes of charge recombination processes, τcr, were
also measured in THF. As can be seen from Table 1, τcr’s
decrease with increasing degree of oligomerization/polymer-
ization and reached a constant lifetime at 4.5 ns. As the polymer
becomes more folded, τcr turns out to be shorter and τcr of
tetramer 4d was similar to those of 114 and 130. These kinetic
interesting to note that the glass transition temperature (Tg) for
5d having a piperazine bridge is much higher than those for 1
and 5a−5c with simple aliphatic tethers.
Effect of Tether between Two Aminostyrenes on the
Photophysical Properties of 5. The absorption and
emission spectra of 5a−5d in cyclohexane are shown in Figure
5. In a manner similar to those of 4a−4d, 114, and 130, 5a−5d
exhibit LE emissions at 360 nm and CT emission at 450 nm in
cyclohexane. Polymer 5b contains a geminal dimethyl group on
trimethylene bridge. Considering the Mn of 5b, the ICT/ILE ratio
falls onto the curve shown in Figure 2. This may imply that the
conformation of 5b may be similar to that of trimethylene-
bridge polymers 1 with the same degree of polymerization.
Polymer 5c has tetramethylene bridge between two amino-
styrene donor chromophores. The ICT/ILE ratio for 5c is lower
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intramolecular excimer formation is observed only in 1,3-
diphenylpropane, but not in 1,2-diphenylethane and 1,4-
diphenylbutane. DFT calculations on these diphenylalkanes
indicate that both tetramethylene- and ethylene-bridged
substrates would prefer all-anti conformations, whereas the
syn-conformation may contribute significantly in diphenylpro-
pane.26 These results certainly offer a useful clue on the change
of local conformation of two aminostyrene chromophores
linked by these oligomethylene linkage in 5a−5c. Such
variation of local conformation may couple with the polymer
folding behavior to dictate the discrepancy of photophysical
properties of polymers 5a−5c from those of 1.
When piperazine bridge was used, the ICT/ILE ratio of 5d
shows the largest deviation from the curve (Figure 2). The
piperazine ring is rigid, and the energy barrier between boat and
chair conformers of piperazine is around 5−6 kcal/mol.27
Incorporation of piperazine moiety into a polyamide, for
example, would result in a rodlike structure.28 It seems likely
that little contribution of folding would be expected from
piperazine linkers in the polymeric backbone. The small ICT/ILE
ratio of 5d can thus be understood within such framework.
Kinetic Measurements of 5a−5d. Two-exponential
fittings were utilized to give the decay lifetimes, τcs and τ0, in
which τ0 was attributed to the intrinsic radiative transition. The
kinetic parameters of 5 are shown in Table 2. The τcs’s and their
relative weight percent for 5b are similar to those for 1 and 4,
but τcs’s of 5a, 5c, and 5d are somewhat longer than that for 5b
with smaller weight percent. Longer lifetime of charge
separation with relatively smaller weight percent might indicate
less efficient of PET in the system. It seems apparent that
different aliphatic tethers might construct different folding
behavior to alter PET processes in these systems. On the other
hand, the τcr’s of 5 are also affected by the tethers. The τcr of 5b
with 2,2-dimethylpropylene bridge is 4.7 ns, and that of 5d with
a piperazine bridge is 8.5 ns. These kinetic results of 5 are
consistent with those of 4a−4d, 114, and 130.
Table 2. Number-Average Molecular Weight (Mn), Repeated
Unit (n), Polydispersity Index (PDI), Glass Transition
Temperature (Tg), and Lifetimes (τcs, τ0, and τcr) of 114, 130,
and 5a−5d
lifetimes
d
Tg
(°C)
τcr
a
a
b
c
c
Mn
PDI
n
τcs (ps)
τ0 (ps)
(ns)
114
130
5a
8200
17800
4600
6800
5500
3400
1.4
1.8
1.7
2.1
1.8
1.4
14
30
8
103
101
82
9 (100%)
9 (100%)
28 (77%)
11 (98%)
15 (95%)
17 (61%)
4.5
4.5
8.3
4.7
7.8
8.5
180 (23%)
180 (2%)
180 (5%)
5b
5c
11
9
99
87
5d
6
173
180 (39%)
a
b
Measured in THF by gel permeation chromatography. Degree of
c
polymerization. The emission decay profiles were measured in THF
from LE emissions, and two-exponential fitting was utilized to give two
d
lifetimes with relative weight percent. The emission decay profiles
were measured in THF from CT emissions, and single-exponential
fitting was used to give lifetimes.
CONCLUSIONS
■
In summary, we have demonstrated that the photophysical
properties of polymers 1 and 5 and the related oligomers 4a−
4d having alternating donor and acceptor chromophores
separated by dimethylsilylene moieties. CT emissions were
observed for all of these substrates, but the relative intensities
were significantly affected by the degree of polymerization and
by the nature of tethering groups between two aminostyrene
donors. A slight conformational change of these polymers
would lead to slight variation of the distance between donor
and acceptor chromophores Since the PET process is very
sensitive to distance between chromophores, the nonadiabatic
interactions in the excited state between donor−acceptor pairs
in these oligomers and polymers would be significantly
perturbed with the variation of conformations. Our results
have offered a useful platform to elucidate the folding nature of
these silicon-containing polymers. Extension to other polymeric
systems is in progress in our laboratory.
Figure 5. (a) Absorption and (b) emission spectra of 5a (black), 5b
(red), 5c (green), and 5d (blue) in cyclohexane.
than that expected for trimethylene-bridge polymers 1 with
same number of repeating units (Figure 2). These results
suggest that the CT emission would be less efficient in 5c
containing tetramethylene linkers than that in 1 having
trimethylene connectors. It is noteworthy that the ethylene-
bridged copolymer 5a exhibits even much smaller ICT/ILE ratio
in comparison with that for 1. Apparently, the chain lengths of
these oligomethylene linkages determine the folding nature of
these polymers and thus the process of PET in the overall
photophysical processes in these copolymers.
EXPERIMENTAL SECTION
■
General. Gel permeation chromatography (GPC) was performed
on a Waters GPC machine with an isocratic HPLC pump (1515) and
a refractive index detector (2414). THF was used as the eluent (flow
rate = 1.0 mL/min). Waters Styragel HR2, HR3, and HR4 columns
(7.8 × 300 mm) were employed for determination of relative
molecular weight using polystyrene as standard (Mn values ranged
The photophysical properties of 1,ω-diphenylalkanes have
been investigated in detail.25 It is interesting to note that
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
Synthesis details and H and 13C NMR spectra of all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tyluh@ntu.edu.tw (T.-Y.L.); chhschen@ntu.edu.tw
(C.-H.C.).
Author Contributions
§These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Council and the National
Taiwan University 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.
Jager, M. Chem. Rev. 2006, 106, 1785−1813. (d) Berezin, M. Y.;
Achilefu, S. Chem. Rev. 2010, 110, 2641−2684.
̈
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