Excimer formation in a confined space: Photophysics of ladderphanes with tetraarylethylene linkers

Article abstract of DOI:10.1002/chem.201403806

Communication between chromophores is vital for both natural and non-natural photophysical processes. Spatial confinements offer unique conditions to scrutinize such interactions. Polynorbornene- and polycyclobutenebased ladderphanes are ideal model compo

Full text of DOI:10.1002/chem.201403806

DOI: 10.1002/chem.201403806
Full Paper
&
Ladderphanes
Excimer Formation in a Confined Space: Photophysics of
Ladderphanes with Tetraarylethylene Linkers
Chih-Hsien Chen,[a, b] Kamani Satyanarayana,[a] Yi-Hung Liu,[a] Shou-Ling Huang,[a]
Tsong-Shin Lim,[c] and Tien-Yau Luh*[a]
Dedicated to Professor Armin de Meijere on the occasion of his 75th birthday
Abstract: Communication between chromophores is vital
for both natural and non-natural photophysical processes.
Spatial confinements offer unique conditions to scrutinize
such interactions. Polynorbornene- and polycyclobutene-
based ladderphanes are ideal model compounds in which all
tetraarylethylene (TAE) linkers are aligned coherently. The
spans for each of the monomeric units in these ladder-
phanes are 4.5–5.5 ꢀ. Monomers do not exhibit emission,
because bond rotation in TAE can quench the excited-state
energy. However, polymers emit at 493 nm (F=0.015) with
large Stokes shift under ambient conditions and exhibit dual
emission at 450 and 493 nm at 150 K. When the temperature
is lowered, the emission intensity at 450 nm increases,
whereas that at 493 nm decreases. At 100 K, both monomers
and polymers emit only at 450 nm. This shorter-wavelength
emission arises from the intrinsic emission of TAE chromo-
phore, and the emission at 493 nm could be attributed to
the excimer emission in the confined space of ladderphanes.
The fast kinetics suggest diffusion-controlled formation of
the excimer.
Introduction
so that electron delocalization between chromophores can
readily take place.[10–12] Long-range charge-carrier transport and
electronic coupling have been found in polynorbornene-based
macromolecular arrays.[6] Excimer formation has been
facilitated between two chromophores bridged by oligo-
nucleotides,[13a] cyclodextrins,[13b] cyclohexasilanes,[13c] carbazol-
phanes,[13d] and xanthenes.[13e]
Electronic communication between chromophores is crucial for
artificial optoelectronic applications[1,2] and for natural photo-
synthesis systems.[3] Intrachain exciton and electron migrations
between chromophores appended to a polymeric backbone[4–7]
or associated in a supramolecular scaffold[8] have offered a pow-
erful arsenal to understand various natural light-harvesting
processes. The excitonic effect between chlorophylls has been
proposed to rationalize extremely efficient energy- and elec-
tron-transfer processes in photosynthesis.[3] Relative orientation
and distance among these photophysically active modules
would influence the electronic delocalization and thus bulk
materials properties.[9] The interactions between chromophores
would be more efficient if they could be better aligned along
the polymeric backbone. This consideration has led to substan-
tial efforts in the design and syntheses of chromophore arrays
within supramolecular structures that may provide a confined
spatial environment. The paracyclophane framework has been
demonstrated to bring two chromophores into close proximity
We recently reported a new class of polynorbornene-based
double-stranded ladderphanes.[7,14] This kind of polymer can
also be considered as a linear array of cofacially arranged
arenes in a polymer matrix. Depending on the nature of the
polymeric backbones in 1, the spacing between arene linkers
would be 5.5 ꢀ for double-stranded polynorbornenes[15] and
around 4.5 ꢀ for polycyclobutene-based ladderphanes.[16]
Significant perturbation of photophysical properties has been
observed in these ladderphanes in comparison with those of
corresponding monomers, although these distances are some-
what longer than those in typical p–p interactions between ar-
omatic layers, such as those in DNA molecules[17] and in graph-
ite.[18] To illustrate this, the Soret band splitting with porphyrin
linkers, excimer emission with diethynylene–terphenylene link-
ers, and fluorescence quenching with a range of oligoaryl link-
ers are frequently observed in 1.[14] However, when 16-p-elec-
tron antiaromatic metallacycles are employed as linkers in 1,
the photophysical and electrochemical properties of 1 remain
unchanged in comparison with those of the corresponding
monomer.[19] Since the linkers in 1 are connected to the poly-
meric backbones through a benzylic or an ester group, certain
flexibilities might be expected. These experiments are carried
out at ambient temperature, so that thermally induced mo-
[a] Dr. C.-H. Chen, K. Satyanarayana, Y.-H. Liu, S.-L. Huang, Prof. T.-Y. Luh
Department of Chemistry, National Taiwan University, Taipei, 106 (Taiwan)
E-mail: tyluh@ntu.edu.tw
[b] Dr. C.-H. Chen
Department of Chemical Engineering
Feng Chia University, Taichung, 407 (Taiwan)
[c] Prof. T.-S. Lim
Department of Physics, Feng Chia University, Taichung, 407 (Taiwan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403806.
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Scheme 1. a) Br2, 71%; b) [Pd(PPh3)4], K2CO3, 7, 15%; c) LAH, 85%; d) 10a:
8b, DMAP, Et3N, 56%; 10b: 9b, DMAP, Et3N, 50%.
crystal X-ray structure of which was determined to confirm the
stereochemistry (Figure S1 in the Supporting Information).[25]
A
Suzuki–Miyaura cross-coupling reaction was utilized to afford
tetraarylethene 5. The benzoate groups in 5 were reduced by
lithium aluminum hydride (LAH) to give 6. Treatment of 6 with
8b afforded the corresponding monomer 10a. Cyclobutene
analogue 10b was prepared from 6 and 9b. Ring-opening
metathesis polymerization (ROMP) of 10a with the first-
generation Grubbs catalyst 12[26] gave the corresponding
polynorbornene-based ladderphanes with E double bonds
2a. On treatment with 11, N-arylpyrrolidine-fused cyclobutene
derivative 10b underwent ROMP stereoselectively to give 2b
with all double bonds in Z-configuration.[27]
tions can take place to enable interactions between adjacent
chromophores. It is envisioned that the photophysical behavior
may be different if the molecular motion can be frozen at low
temperature. In this regard, the intrinsic photophysical proper-
ties of individual chromophores in 1 might be observed.
The photophysics of tetraarylethylenes (TAEs) is rich and
strongly influenced by the environment.[20] In particular, this
class of conjugated systems exhibits aggregation-induced
emission in metal–organic frameworks[21] and modified DNA
strands.[22] It seems likely that some kinds of interactions be-
tween these chromophores might take place on aggregation.
As mentioned above, the spacing between adjacent chromo-
phores in ladderphanes 1 is around 4.5–5.5 ꢀ. We therefore
envisaged that incorporation of TAE moieties as linkers in
1 would offer a useful model to understand the photophysical
behavior of the interactions between these chromophores. We
now wish to report the synthesis and photophysical properties
of ladderphanes 2a and 2b with TAE linkers.
Photophysical properties of ladderphane 2a
The absorption spectrum of monomer 10a is essentially the
sum of the absorptions of tetraarylethylene 6 and aminoben-
zoate 8c (Figure 1).[25] Like other single- and double-stranded
polynorbornenes,[7,14–16,19] the absorption due to the aminoben-
zoate chromophores in ladderphane 2a (lmax =314 nm) exhib-
ited a hypsochromic shift in comparison with that of 10a
(lmax =320 nm, Figure 1), which may be due to release of strain
from the norbornene moiety.[28] On excitation at 350 nm, no
emission was observed from solutions of 10a, 6, and 8c in
CH2Cl2 (1.0ꢂ10À5 to 1.0ꢂ10À3 m). In contrast, 2a showed emis-
sion at lem =493 nm in CH2Cl2 with quantum yield of F=1.5%
(Figure 1, Table 1). This unusually large Stokes shift[25] suggests
that the structure of the excited state could be very different
from that of the ground state (Table 1).[29] In addition, the ab-
Results and Discussion
Synthesis
Diarylethyne 3[23] was prepared by Sonogashira reaction of
methyl 4-bromobenzoate and methyl 4-ethynylbenzoate. With
an excess of bromide ions, trans bromination of 3 took place
to afford tetrasubstituted alkene 4 (Scheme 1),[24] the single-
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cause no other chromophore has lower excited energy.[33] On
the other hand, PET between the aminobenzoate moiety and
an acceptor chromophore linker in ladderphanes might be
possible if their frontier orbitals matched. In this regard, emis-
sion from the corresponding charge-separated state has been
occasionally observed, and the wavelength of this emission
was solvent-dependent.[14f,34] However, the emission profile of
ladderphane 2a remains unchanged in different solvents on
excitation at 350 nm (Figure 2). These observations suggest
that the characteristic emission in 2a does not arise from the
PET process.
Figure 2. Emission spectra of 2a in Et2O (solid), CHCl3 (dots), and CH2Cl2
(dashes).
As mentioned above, the separation between adjacent link-
ers in polynorbornene-based ladderphanes is about 5.5 ꢀ.[15]
Because of the flexibility of the benzylic moiety and the ester
groups, the adjacent linkers may be brought into closer prox-
imity, and thus bimolecular interactions would take place. Pho-
toinduced bimolecular interactions such as exciton coupling,[35]
charge-transfer complex formation[36,37] and excimer or exciplex
formation[13] are known to take place readily and result in per-
turbation of absorption and/or emission properties.[29,32] As
shown in Figure 1, the absorption band of 2a was only slightly
different (Dl=6 nm) from that of 10a. These results suggest
that neither exciton coupling nor charge-transfer complex
formation between adjacent linkers would take place.
Figure 1. Absorption (black) and emission (gray) spectra of 10a (solid), 2a
(dashes), 8c (dots), and 6 (dash/dots) in CH2Cl2 at ambient temperature.
Table 1. Number-average molecular weight Mn, polydispersity index
(PDI), and photophysical properties (absorption maximum lmax, emission
maximum lem, quantum yield F, Stokes shift for TAE) of monomers and
polymers measured at ambient temperature.
Mn (PDI)[a]
lmax [nm]
lem [nm]
F [%]
[cmÀ1
]
The adjacent TAE chromophores in polymeric ladderphanes
2a are relatively rigid. Unlike monomer 10a, in which rotation
about aryl–aryl bonds can more freely take place, such rotation
in 2a would be more restricted. Since the TAE chromophores
in 2a are in close proximity, a dynamic excimer could readily
be formed, and the excited-state energy would be released as
fluorescence with a large Stokes shift.
[b]
10a
10b
2a
320
318
314
309
–
–
[b]
13700 (1.23)
11100 (1.20)
493
495
1.5
1.4
10770
10850
2b
[a] Determined by GPC with polystyrene references. [b] There is no
emission for 10a and 10b in CH2Cl2.
Variable-temperature emission spectra of ladderphane 2a
sorption and emission spectra do not overlap. Several photo-
physical processes, such as fluorescence resonance energy
transfer (FRET),[30] photoinduced electron transfer (PET),[31] and
other bimolecular interactions,[29,32] might result in large Stokes
shift between absorption and emission spectra. FRET would
not take place with excitation wavelength at 350 nm in 2a, be-
The absorption and emission spectra of 6 and 10a were mea-
sured at 100 K (Figure 3). As mentioned above, neither 6 nor
10a exhibited observable emission at ambient temperature.
However, strong emission at 450 nm in methyltetrahydrofuran
(MTHF) at 100 K was observed for both 6 and 10a (Figure 3). A
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energy. On the other hand, the flexibility of benzylic or ester
groups in 2a might also be increased due to increasing ther-
mal energy. These phenomena would offer the opportunity for
the adjacent TAE linker in 2b to move into closer proximity, so
that direct interaction between these chromophores might
take place during this photophysical process. Therefore, at
higher temperature, the emission at longer wavelength
(493 nm) in 2a might be considered to result from the
formation of an excimer between TAE chromophores.
Kinetic measurements
Figure 3. Absorption (black) and emission (gray) spectra of 6 (dash dot) and
10a (solid) at 100 K in MTHF.
Time-resolved fluorescence spectroscopy with a femtosecond
Ti:sapphire laser equipped with a streak camera was employed
to determine the emission lifetimes of 2a at various tempera-
tures (Table 2). The observed excimer fluorescence decay life-
significant spectral overlap between this low-temperature
emission and the absorption of these small molecules indicates
that this new low-temperature emission at shorter wavelength
may arise from the intrinsic emission of the TAE chromophore.
At ambient temperature, nonradiative torsional deactivation[38]
of aryl moieties in 6 and 10a may quench the excited-state
energy in TAE chromophores and lead to disappearance of this
intrinsic emission. This molecular motion would be significantly
reduced at low temperatures, and the intrinsic emission might
be turned on.
Table 2. Kinetic parameters of 2a: lifetimes (td1, td2, and tr) and apparent
rate constants kr at different temperatures.
T [K]
td1[a] [ns]
td2[c] [ns]
tr[a] [ps]
kr[e] [109 sÀ1
]
[d]
[d]
298
200
150
140
130
120
100
0.5
1.0
1.8
1.9
2.0–10.0[b]
2.0–10.0[b]
2.0–10.0[b]
–
–
–
–
[d]
[d]
0.4
0.6
1.0
1.7
104
115
172
215
9.62
8.70
5.81
4.65
As shown in Figure 4, the emission intensity of 2a at
493 nm gradually increased from 298 to 150 K. The enhance-
ment of the emission intensity may be due to decreased ther-
mal quenching. Below 150 K, a new emission band at shorter
wavelength gradually emerged. The emission maximum of 2a
was shifted to 450 nm at 100 K, and the profile is similar to
those of 6 and 10a under the same conditions. Apparently,
dual emission occurs from 2a. The emission at 450 nm could
be observed only at low temperature, whereas the emission at
493 nm became dominant at higher temperature. At higher
temperature, the decrease in emission intensity at 450 nm for
2a followed the same trend as observed for monomers 6 and
10a. The thermal energy would enhance molecular motions,
and thus bond rotation in TAE might quench the excited-state
2.0–10.0[b]
–
[d]
[a] Detected at 520–540 nm. [b] The exact decay lifetimes could not be
obtained due to instrumental limitations. [c] Detected at 440–460 nm.
[d] No signal was detected. [e] The rate was estimated from tr.
times td1 of 2a, which were wavelength-independent, were
500 ps at 298 K and 1.0 ns at 200 K.[25] The increase of td1 may
be due to decreased thermal relaxation. When the temperature
reached 150 K, on excitation, the features of the time-resolved
fluorescence varied significantly with emission wavelength.
Representative decay profiles are shown in Figure 5, and the
details are described in the Supporting Information.[25] The
fluorescence at 440–460 nm displays an instrument-limited rise
followed by slow decay. However, the fluorescence at 520–
540 nm shows a slow rise (tr) followed by a slow decay from
the excimer (td1). These results suggested that at least two ex-
cited species were involved in 2a at 150 K, and this observa-
tion is consistent with the temperature-dependent emission
spectra described above (Figure 4). The fluorescence decay td2
at shorter wavelength (440–460 nm) may arise from the intrin-
sic emission of the TAE chromophore in 2a. The lifetime td1,
for the longer-wavelength (520–540 nm) emission of 2a at
150 K would be attributed to the excimer emission, in
a
manner similar to those described above for higher
temperatures.
Figure 4. Temperature-dependent emission spectra of 2a at different tem-
peratures in MTHF (298 K: black solid; 250 K: gray solid; 200 K: black dashes;
150 K: gray dashes; 140 K: black dots; 130 K: gray dots; 120 K: black dash/
dots; 110 K: gray dash/dots; 100 K: black dash/dot/dots). Excitation wave-
length: 350 nm.
The tr values gradually increase from 150 to 120 K (Table 2).
These results suggest that tr may be responsible for the forma-
tion of a dynamic excimer.[39] The estimated apparent rate con-
stants kr (=1/tr), including formation, dissociation, and decay
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Figure 6. Rate constant versus temperature for 2a (squares) and 2b (circles).
a difference might arise from the shorter span of the poly-
cyclobutene skeleton.
TAE excimer formation in ladderphanes
Interactions between two chromophores can be reflected by
photophysical processes.[29] In general, if the chromophores are
in close proximity, such interactions may lead to ground-state
interactions between chromophores provoking the formation
of a charge-transfer complex or exciton delocalization, which
may result in the appearance of a new absorption and/or emis-
sion at longer wavelength, if allowed. Occasionally, vibronic
structure may be observed in the emission profiles.[39,40] Alter-
natively, fluorescence quenching and shortening of lifetimes
are frequently found in various systems. Indeed, a number of
ladderphanes with different aromatic linkers are known to
show such behavior.[14d,e] The third possibility is the formation
of an excited dimer, which may exhibit a new structureless
emission at longer wavelength.[29] In particular, if the
chromophores are in a confined space, such as anthracene
appended in polynorbornene,[41] terphenylene–diethynylene-
linked ladderphane,[14d] and surface attachment of pyrene
on porous silica,[42] excimer emissions are observed.
Figure 5. Time-resolved fluorescence profiles of 2a at 150 K with different
detection wavelengths. a) 440–460 nm and b) 520–540 nm.
of the excimer in such processes, are listed in Table 2. When
the temperature reached 100 K, the feature of the time-re-
solved fluorescence became independent of wavelength. This
phenomenon indicates that excimer formation in 2a would be
significantly restricted, and these results are consistent with
the observation in steady-state measurements, in which
intrinsic emission becomes predominant.
As mentioned above, neither TAE core 6 nor monomers 10a
and 10b exhibit any emission at ambient temperature. How-
ever, polymers 2a and 2b show fluorescence at 493 nm with
quantum yields of 0.015, and 0.014, respectively. It is notewor-
thy that there are no spectral overlaps between the absorption
and emission profiles of these polymers. In other words, the
Stokes shifts appear to be fairly large. Apparently, the induc-
tion of fluorescence from the TAE chromophore should be at-
tributed to the confined space in polymeric ladderphanes. As
described above, the span between two adjacent chromo-
phores in 2a and 2b would be 5.5 and 4.5 ꢀ, respectively, and
the benzylic ester linking groups would be quite flexible. Con-
sequently, electronic interaction between adjacent chromo-
phores would be feasible. It seems likely that excimers would
be readily formed in 2a and 2b.
Polynorbornene- versus polycyclobutene-based ladder-
phanes
At ambient temperature, the photophysical properties of 2b,
including absorption and emission profiles as well as quantum
yield, were almost same as those of 2a (Table 1, Figure S3 of
the Supporting Information).[25] The temperature-dependent
emission spectra of 2b also showed similar behavior to those
of 2a (Supporting Information, Figure S4),[25] whereby the
emission maxima gradually shifted from 495 to 450 nm on
lowering the temperature. The kinetic properties of 2b were
also measured at different temperatures. Basically, the kinetic
parameters of 2b were similar to those of 2a except that the
kr values of 2b were slightly larger than those of 2b at 120–
150 K (Figure 6). It seems likely that the formation of excimer
could more readily occur for 2b than that for 2a, and such
Conclusion
We have demonstrated the design and synthesis of polynor-
bornene- and polycyclobutene-based ladderphanes with TAE
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1
solid (3.22 g, 71%). M.p. 211–2128C; H NMR (CDCl3, 400 MHz): d=
3.95 (s, 6H), 7.60 (d, J=8.4 Hz, 4H), 8.11 (d, J=8.4 Hz, 4H);
13C NMR (CDCl3, 100 MHz): d=52.5, 117.4, 128.8, 129.4, 130.2,
linkers. Structurally, the TAE chromophores are aligned cofacial-
ly with interchromophoric distances of about 4.5–5.5 ꢀ. The
photophysical behaviors of these polymers were examined in
detail. In contrast to monomeric species containing the same
chromophore, excimer emission is observed at ambient tem-
perature in these polymers. At low temperatures (<150 K),
a new intrinsic emission gradually emerged and became pre-
dominant at 100 K. Note that, even in concentrated solution at
ambient temperature, the TAE excimer was not observed for
monomers 6, 10a, and 10b. The confined space in ladder-
phanes offers a useful platform to facilitate these interchromo-
phore interactions resulting in intriguing photophysical proper-
ties that would otherwise not be feasible. Our results may offer
a possible pathway for aggregation-induced emission involving
TAE chromophores.
~
144.1, 165.7ppm; IR (KBr): n=3026, 2948, 2848, 1724, 1603, 1507,
1434, 1403, 1276, 1109, 1019, 857, 823 cmÀ1
.
(E)-1,2-Bis(4-hexyloxyphenyl)-bis(4’-methoxycarbonyl)stilbene
(5): Under N2, a mixture of 2-(4-hexyloxyphenyl)-4,4,5,5-tetrameth-
yl-1,3,2-dioxaborolane (2.28 g, 7.5 mmol),
4 (0.88 g, 3 mmol),
Pd(PPh3)4 (30.0 mg, 2.6ꢂ10À2 mmol), and TBAB (0.10 g, 0.3 mmol)
in toluene (30 mL) was mixed with K2CO3 solution (2m, 8 mL) and
the mixture was heated to 908C for 24 h. The mixture was then
poured into water (50 mL) and extracted with EtOAc (50 mLꢂ2).
The combined organic layer was washed with brine (100 mL) and
water (100 mL) and dried (MgSO4). The solvent was removed in
vacuo to give a residue, which was purified by chromatography on
silica gel (hexane/CH2Cl2, 2/1) to give 5 as a yellow solid (0.29 g,
15%). M.p. 102–1038C; 1H NMR (CDCl3, 400 MHz): d=0.92 (t, J=
7.0 Hz, 6H), 1.30–1.50 (m, 16H), 1.70–1.80 (m, 4H), 3.87 (s, 6H),
3.90 (t, J=7.0 Hz, 4H), 6.66 (d, J=8.8 Hz, 4H), 6.90 (d, J=8.8 Hz,
4H), 7.07 (d, J=8.4 Hz, 4H), 7.74 ppm (d, J=8.4 Hz, 4H); 13C NMR
(CDCl3, 100 MHz): d=14.2, 22.7, 25.8, 29.3, 31.7, 52.0, 67.9, 113.7,
127.9, 128.9, 131.2, 132.3, 134.9, 139.9, 148.6, 157.8, 166.7ppm; IR
Experimental Section
General
1H and 13C NMR spectra were recorded on a Varian 400 Unity Plus
(400 MHz) at ambient temperature. Samples for 1H and 13C NMR
measurements were dissolved in CDCl3 and CHCl3. Residual proton
signals of the deuterated solvents were used as internal standard.
The chemical shift of CHCl3 was calibrated at d=7.26 ppm in
1H NMR spectra and d=77.0 ppm in 13C NMR spectra. All 13C NMR
spectra were recorded with complete proton decoupling. High-res-
olution mass spectra were obtained with a Jeol JMS-700 mass
spectrometer by using the FAB method in 3-nitrobenzyl alcohol
matrix. Gel permeation chromatography (GPC) was performed on
a Waters GPC instrument by using an isocratic HPLC pump (1515)
and a refractive-index detector (2414). THF was used as eluent
(flow rate 1 mLminÀ1). Waters Styragel HR2, HR3, HR3, and HR4
(7.8ꢂ300 mm) columns were employed for molecular weight de-
termination, and polystyrenes were used as the standard (Mn
values ranging from 375 to 3.5ꢂ106). Absorption spectra were re-
corded on a Hitachi U-3310 spectrophotometer, and emission
spectra on a Hitachi F-4500 fluorescence spectrophotometer. The
quantum yield was obtained by using a spectrophotometer with
an integrating sphere (Hamamatsu C9920-02).
~
(KBr): n=3035, 2928, 2856, 1723, 1603, 1506, 1434, 1276, 1244,
1176, 1103, 937, 833, 759 cmÀ1; HRMS (FAB): m/z calcd for C42H48O6:
648.3451; found: 648.3475.
(E)-1,2-Bis(4-hexyloxyphenyl)-bis(4’-hydroxymethyl)stilbene (6):
Compound 5 (0.32 g, 0.5 mmol) was added to LAH (40 mg,
1.0 mmol) in Et2O (10 mL) at 08C under N2. The reaction mixture
was stirred at RT for 4 h and quenched with water (1 mL). Et2O
(20 mL) was then added and the mixture was washed with brine
(50 mL) and water (50 mL) and dried (NaSO4). The solvent was re-
moved in vacuo to give a residue, which was purified by chroma-
tography on silica gel (CH2Cl2/EtOAc, 10/1) to give 6 as a white
1
solid (0.27 g,90%). M.p. 178–1798C; H NMR (CDCl3, 400 MHz): d=
0.92 (t, J=7.0 Hz, 6H), 1.30–1.50 (m, 16H), 1.70–1.80 (m, 4H), 3.89
(t, J=6.6 Hz, 4H), 4.59 (s, 4H), 6.63 (d, J=8.8 Hz, 4H), 6.91 (d, J=
8.8 Hz, 4H), 6.97 (d, J=8.4 Hz, 4H), 7.08 ppm (d, J=8.4 Hz, 4H);
13C NMR (CDCl3, 100 MHz): d=14.2, 22.7, 25.9, 29.4, 31.7, 65.2, 67.8,
113.5, 126.2, 131.5, 132.3, 136.0, 138.4, 139.2, 143.6, 157.4ppm; IR
~
(KBr): n=3229, 3041, 2926, 2862, 1604, 1507, 1466, 1439, 1410,
1239, 1172, 1110, 1014, 965, 825 cmÀ1; HRMS (FAB): m/z calcd for
C40H48O4: 592.3552; found: 592.3567.
Monomer 10a: Compound 8b [freshly prepared from the corre-
sponding carboxylic acid 8a (0.15 g, 0.60 mmol) and (COCl)2
(0.1 mL, d=1.5 gmLÀ1, 1.20 mmol)] in CH2Cl2 (5 mL) was added to
a stirred solution containing 6 (0.15 g, 0.25 mmol), 4-dimethylami-
nopyridine (one crystal), and Et3N (1 mL) in CH2Cl2 (15 mL) at 08C .
The mixture was stirred at RT overnight. CH2Cl2 (30 mL) was then
added and the mixture was washed with saturated NaHCO3
(30 mL) and water. The organic layer was dried (MgSO4). After filtra-
tion, the solvent was evaporated in vacuo to give a residue, which
was purified by chromatography on Et3N-treated silica gel (hexane/
CH2Cl2, 1/1) to afford 10a as white solid (0.15 g, 56%). M.p. 119–
Time-resolved fluorescence experiments
Light from
a mode-locked Ti:sapphire laser (repetition rate:
76 MHz; pulse width: <200 fs) was passed through an optical
parametric amplifier to produce the desired wavelength. The fluo-
rescence of the sample was reflected by a grating (150 grooves/
mm; 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 with 1.0ꢂ10À5 m concentration in cuvet.
The signal was collected ten times to decrease signal-to-noise
ratio.
1
1208C; H NMR (CDCl3, 400 MHz): d=0.92 (t, J=6.6 Hz, 6H), 1.30–
1.50 (m, 16H), 1.53 (d, J=7.8 Hz, 2H), 1.63 (d, J=7.8 Hz, 2H), 1.70–
1.80 (m, 4H), 2.92–3.05 (m, 8H), 3.05–3.12 (m, 4H), 3.25–3.35 (m,
4H), 3.89 (t, J=6.4 Hz, 4H), 5.22 (s, 4H), 6.12 (s, 4H), 6.37 (d, J=
8.4 Hz, 4H), 6.64 (d, J=8.4 Hz, 4H), 6.93 (d, J=8.4 Hz, 4H), 7.01 (d,
J=8.0 Hz, 4H), 7.14 (d, J=8.0 Hz, 4H), 7.88 ppm (d, J=8.4 Hz, 4H);
13C NMR (CDCl3, 100 MHz): d=14.2, 22.7, 25.8, 29.4, 31.7, 45.4, 46.7,
50.5, 52.1, 65.6, 67.8, 110.8, 113.5, 116.1, 126.9, 131.2, 131.3 132.3,
134.5, 135.6, 136.0, 139.2, 143.6, 150.2, 157.3, 166.6ppm; IR (KBr):
Synthesis
(E)-1,2-Dibromo-bis(4’-methoxycarbonyl)stilbene (4): A solution
of bromine in CCl4 (1m, 12 mL, 12 mmol) was added dropwise to
a slurry of 3 (2.98 g, 10.0 mmol) in CCl4 (50 mL) at 08C. The reac-
tion mixture was stirred for 1 h and then added to 10% aqueous
sodium sulfite (50 mL). After extraction with Et2O, the organic layer
was collected, dried (MgSO4), and the solvent removed in vacuo.
The residue was recrystallized from ethanol to give 4 as a white
~
n=3055, 2928, 2850, 1756, 1700, 1603, 1508, 1472, 1377, 1271,
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1242, 1176, 1098, 1014, 972, 826 cmÀ1; HRMS (FAB): m/z calcd for
C72H79N2O6 [M+H]: 1067.5938; found: 1067.5978.
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Monomer 10b: Compound 9b [freshly prepared from the corre-
sponding carboxylic acid 9a (0.14 g, 0.60 mmol) and (COCl)2
(0.1 mL, d=1.5 gmLÀ1, 1.20 mmol)] in CH2Cl2 (5 mL) was added to
a stirred solution consisting of 6 (0.15 g, 0.25 mmol), DMAP (one
crystal), Et3N (1 mL), and CH2Cl2 (15 mL) at 08C. The mixture was
stirred at RT overnight. CH2Cl2 (30 mL) was then added and the
mixture was washed with saturated NaHCO3 (30 mL) and water.
The organic layer was dried (MgSO4). After filtration, the solvent
was evaporated in vacuo to give a residue, which was purified by
chromatography on Et3N-treated silica gel (hexane/CH2Cl2, 1/1) to
1
afford 10b as white solid (0.12 g, 50%). M.p. 123–1248C; H NMR
(CDCl3, 400 MHz): d=0.92 (t, J=7.0 Hz, 6H), 1.30–1.50 (m, 16H),
1.70–1.80 (m, 4H), 2.93 (dd, J=10.6, 6.4 Hz, 4H), 3.55 (d, J=6.4 Hz,
4H), 3.65 (d, J=10.6 Hz, 4H), 3.89 (t, J=6.4 Hz, 4H), 5.23 (s, 4H),
6.14 (s, 4H), 6.62 (d, J=8.8 Hz, 4H), 6.64 (d, J=8.4 Hz, 4H), 6.93 (d,
J=8.8 Hz, 4H), 7.01 (d, J=8.4 Hz, 4H), 7.15 (d, J=8.4 Hz, 4H),
7.91 ppm (d, J=8.4 Hz, 4H); 13C NMR (CDCl3, 100 MHz): d=14.2,
22.7, 25.9, 29.4, 31.7, 46.5, 48.9, 65.7, 67.8, 112.0, 113.5, 117.4, 127.0,
131.2, 131.3, 132.4, 134.3, 136.0, 139.2, 143.7, 153.1, 157.3,
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~
166.5ppm; IR (KBr): n=3056, 2928, 2851, 1758, 1701, 1603, 1523,
1509, 1473, 1378, 1272, 1242, 1177, 1098, 1014, 972, 826 cmÀ1
;
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HRMS (FAB): m/z calcd for C66H71N2O6 [M+H]: 987.5312; found:
987.5339.
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Polymer 2a: A solution of 12 (6.5 mg, 8.0ꢂ10À3 mmol) in CH2Cl2
(0.5 mL) was added to a solution of 10a (43 mg, 4ꢂ10À2 mmol) in
CH2Cl2 (5 mL). The reaction mixture was stirred for 2 h at RT, after
which ethyl vinyl ether (0.1 mL) was added and the mixture was
stirred for 10 min. The mixture was concentrated and the residual
solution was added to MeOH (15 mL). The precipitate was collect-
ed, rinsed with MeOH, and dried in vacuo to give 2a as a pale
1
gray powder (40 mg, 93%). H NMR (CDCl3, 400 MHz): d=0.85–0.95
(br, 6H), 1.20–2.50 (br, 24H), 2.60–3.50 (br, 16H), 3.80–4.00 (br, 4H),
5.00–5.60 (br, 8H), 6.40–6.70 (br, 8H), 6.80–7.20 (br, 12H), 7.80–
8.05 ppm (br, 4H); 13C NMR (CDCl3, 100 MHz): d=15.7, 22.6, 25.7,
29.3, 31.6, 39.2, 45.1, 46.9, 49.2, 49.9, 65.4, 67.8, 111.4, 113.6, 117.0,
126.1, 126.5, 128.6, 131.4, 132.5, 134.5, 136.4, 139.3, 143.7, 150.9,
157.5, 166.7 ppm; GPC: Mn =13700 (PDI=1.23).
Polymer 2b. In a manner similar to that described above for 2a,
reaction of 10b (39 mg, 4ꢂ10À2 mmol) and 11 (6.8 mg, 8ꢂ
10À3 mmol) yielded 2b as a pale gray powder (35 mg, 91%).
1H NMR (CDCl3, 400 MHz): d=0.80–1.00 (br, 6H), 1.20–1.55 (br,
12H), 1.65–1.80 (br, 4H), 3.00–3.70 (br, 12H), 3.80–3.95 (br, 4H),
5.10–5.30 (br, 4H), 5.50–5.85 (br, 4H), 6.30–6.70 (br, 8H), 6.80–7.20
(br, 12H), 7.80–8.05 ppm (br, 4H); 13C NMR (CDCl3, 100 MHz): d=
14.2, 22.7, 25.9, 29.4, 31.7, 40.7, 53.8, 65.8, 67.8, 110.5, 113.5, 117.4,
125.8, 126.8, 128.1, 131.4, 132.3, 133.9, 136.4, 139.1, 143.7, 150.0,
157.3, 166.3 ppm; GPC: Mn =11100 (PDI=1.20).
Acknowledgements
This work was supported by the Ministry of Science and Tech-
nology of the Republic of China and the National Taiwan Uni-
versity. We thank Professor Huan-Cheng Chang for allowing us
to use their femtosecond laser facilities for kinetic measure-
ments.
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Keywords: confinement
·
excimers
·
ladderphanes
·
photophysics · ring-opening polymerization
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Received: June 4, 2014
Published online on October 24, 2014
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ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim