Photophysical Properties of Silyl-Substituted Stilbene Derivatives

Article abstract of DOI:10.1002/ejoc.202000397

The effects of silyl groups on the structural, absorption, fluorescence, and photoisomerization properties of stilbenes were investigated. In comparison to that of the parent stilbene (1), fluorescence quantum yields (Φ_f) of Me₃Si-substituted stilbenes 2–4 and 6 in solution were higher. Derivative 5, in which Me₃Si groups are present at all ortho positions of the arene moieties, did not fluoresce at room temperature. The absorption and fluorescence wavelength maxima of Me₃SiMe₂Si-substituted stilbene 7 occurred at longer wavelengths compared to those of the Me₃Si analog 2. The results of theoretical calculations showed that this difference is a consequence of an increase in the HOMO energy of 7 caused by orbital interaction between π-system and the σ(Si–Si) orbital. Stilbene 14, with two Me₃Si-C≡C groups at both para positions, had a high Φ_f (0.95). The calculated transition dipole moment (μ) was well correlated with Φ_f. Derivative 21, which contains Ph₂N and Me₃SiC≡C groups exhibited solvatofluorochromism because it possesses a twisted intramolecular charge transfer (TICT) excited state in which the Ph₂N group and the aromatic ring are orthogonal.

Full text of DOI:10.1002/ejoc.202000397

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Photophysical Properties of Silyl-Substituted Stilbene Derivatives
Authors: Hajime Maeda, Ryo Horikoshi, Minoru Yamaji, Taniyuki
Furuyama, and Masahito Segi
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000397
Link to VoR: https://doi.org/10.1002/ejoc.202000397
01/2020

10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
Photophysical Properties of Silyl-Substituted Stilbene Derivatives
Hajime Maeda*[a], Ryo Horikoshi[a], Minoru Yamaji[b], Taniyuki Furuyama[a][c] and Masahito Segi[a]
[a]
Division of Material Chemistry, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-
1192, Japan
E-mail: maeda-h@se.kanazawa-u.ac.jp
http://kohka.ch.t.kanazawa-u.ac.jp/lab4/lab4.html
[b]
[c]
Division of Molecular Science, Graduate School of Science and Engineering, Gunma University, Ota, Gunma 373-0057, Japan
Japan Science and Technology Agency (JST)-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Supporting information for this article is given via a link at the end of the document.
Abstract: The effects of silyl groups on the structural, absorption,
fluorescence, and photoisomerization properties of stilbenes were
investigated. In comparison to that of the parent stilbene (1),
fluorescence quantum yields (Ff) of Me3Si-substituted stilbenes 2-4
and 6 in solution were higher. The derivative 5, in which Me3Si groups
are present at all ortho positions of the arene moieties, did not
fluoresce at room temperature. The absorption and fluorescence
wavelength maxima of Me3SiMe2Si-substituted stilbene 7 occurred at
longer wavelengths compared to those of the Me3Si analog 2. The
results of theoretical calculations showed that this difference is a
consequence of an increase in the HOMO energy of 7 caused by
orbital interaction between p-system and the s(Si–Si) orbital. Stilbene
14, with two Me3Si-C≡C groups at both para positions, had a high Ff
(0.95). The calculated transition dipole moment (µ) was well
correlated with Ff. Derivative 21, which contains Ph2N and Me3SiC≡C
groups exhibited solvatofluorochromism because it possesses a
twisted intramolecular charge transfer (TICT) excited state in which
the Ph2N group and the aromatic ring are orthogonal.
Scheme 1. Photoreactions of stilbene.
positioned substituents can be employed to design acyclic
stilbenes that have enhanced fluorescence quantum yields.[9]
In an earlier investigation, we explored the intramolecular
photoreactions and fluorescence properties of substances in
which two stilbenes are linked by silicon containing chains.[10] In
the effort, we found that the absorption and fluorescence
wavelength maxima of trans-stilbene derivative 2, in which
trimethylsilyl groups are present at the para-positions of the
phenyl moieties, occur at longer wavelengths than those of the
parent 1. Moreover, we observed that the fluorescence quantum
yield of 2 in solution is 0.08,[10a] a value that is significantly higher
than that of 1. This finding is consistent with the results of earlier
studies showing that the presence of silyl substituents in aromatic
compounds leads to increase fluorescence efficiencies.[11] We
have also found that trimethylsilyl- and trimethylsilylethynyl-
substituted pyrenes,[12] phenanthrenes[13] and naphthalenes[12e, 14]
have increased fluorescence quantum yields. However, no effort
has been carried out thus far to assess the detailed absorption
and fluorescence properties of compounds having silyl groups
introduced at other positions of stilbene.
Based on the earlier observation made in our study with stilbene
derivative 2, we believed that it would be possible to use silyl
groups to design non-cyclic, non-push-pull substituted stilbenes
that emit strong fluorescence. In the study described below aimed
at assessing this proposal, we prepared a selection of stilbene
derivatives that contain silyl and silylethynyl groups at various
positions. The substances were used to determine the effects of
silyl groups on the absorption and fluorescence wavelength and
quantum yields, structures, and photochemical reactivities of
stilbenes.
Introduction
Numerous studies have been conducted in the past exploring
photoreactions of stilbene and its derivatives from both synthetic
and mechanistic viewpoints. When trans- or cis-stilbene in
solution is irradiated with UV light, three types of photoreactions
occur (Scheme 1). One of the most common reaction pathway
taking place involves cis-trans photoisomerization,[1] which occurs
with high quantum yields (Ft-c = 0.55, Fc-t = 0.35)[2]. Another route
is photodimerization[3] to form tetraphenylcyclobutane, which
occurs with an efficiency that depends on stilbene concentration.
The final pathway commonly followed in photoreactions of
stilbenes
is
reversible
cyclization[4]
producing
dihydrophenanthrene intermediates (F ca. 0.1)[5] that undergo
oxidation in the presence of oxidants such as dioxygen or iodine
to form the corresponding phenanthrenes.
Because of the high efficiencies of these three processes,
stilbenes fluoresce with exceptionally low quantum yields (Ff =
0.04[6] for the trans-stilbene and Ff < 0.001[7] for the cis isomer).
In order to create derivatives that strongly emit fluorescence, it is
necessary to incorporate the stilbene moiety into rigid cyclic
frameworks that cause suppression of excited state deactivation
by reversible cis-trans isomerization and photocyclization
processes.[8] It is also known that the push-pull effect of properly
1
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10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
Results and Discussion
Trans-stilbene derivatives 2-6 containing Me3Si groups at ortho,
meta, and para positions (Figure 1) were prepared by using routes
involving McMurry coupling[15] or Horner-Woodward-Emmons
reactions[16] of bromine substituted precursors (Scheme S1). The
formed brominated trans-stilbenes were then lithiated with t-BuLi
and trimethylsilylated with Me3SiCl to form 2-6.
Figure 2. ORTEP plots of X-ray crystallographic data of 5.
× 10–5 M in cyclohexane (Figure 3a, Table 1). Inspection of the
results showed that the absorption maximum of 5 is shifted to
shorter wavelength compared to the parent stilbene 1 whereas
those of 2-4 and 6 are shifted to longer wavelengths. These
findings are consistent with the fact that twisted structure of
stilbenes causes a blue shift in absorption whereas planar
structure causes a red shift.[18]
Fluorescence spectra of 1.0 × 10–5 M cyclohexane solutions of 1-
6 were recorded at room temperature. The results (Figure 3b,
Table 1) showed that the emission maxima of 2-4 and 6 occur at
longer wavelengths than that of 1. In addition, the fluorescence
quantum yields (Ff) of 2-4 and 6 in cyclohexane were in the 0.05-
0.08 range whereas 5 did not fluoresce. Because it is known that
fluorescence intensity of trans-stilbene increases at lower
temperatures,[6, 19] the emission properties of these substances in
a mixture of methylcyclohexane:isopentane (3:1 v/v) at 77 K were
Figure 1. Structures of trans-stilbene (1) and trimethylsilyl-substituted stilbenes
2-6.
Owing to its crystalline nature, the ortho-tetrasubstituted analog 5
was utilized to gain structural information using X-ray
crystallographic analysis.[17] Inspection of the structure (Figure 2)
showed that the arene rings in 5 are planar, and the C(ipso)–
C=C–C(ipso) dihedral angle is 179°. On the other hand, the
C(ortho)–C(ipso)–C=C dihedral angle is 138°. As a result, the two
aromatic rings in 5 are close to orthogonal.
evaluated. The results showed
5 fluoresces under these
conditions at a shorter wavelength than that of 1 (Figure S1). Also,
no phosphorescence was observed from 1-6 at 77 K, a finding
that is consistent with the fact that phosphorescence of stilbene
derivatives occurs only in the presence of heavy atoms.[20]
UV absorption spectra of trans-stilbene 1 and trimethylsilyl-
substituted stilbenes 2-6 were recorded at a concentration of 1.0
Table 1. Absorption and Fluorescence Data, and Calculation Results for 1-7 and 12-24.
Compound
Absorption[a]
Fluorescence
Calculated Results [d]
[a]
[a]
[c]
[b]
labs
(nm)
e
lem
tf
kf
HOMO
(eV)
LUMO
(eV)
Energy gap
(eV)
µ
Ff
(104
mol–
(nm)
(ns)
(107 s–1
)
(a.u.)
1dm3cm–1
)
1
295
306
299
298
290
295
325
296
318
346
302
314
298
332
302
360
382
310
338
339
2.55
2.64
2.69
2.00
1.84
2.90
3.74
2.88
2.22
2.91
3.48
1.88
2.18
4.00
1.37
1.83
3.77
1.98
3.50
4.22
352
363
365
355
nd [g]
360
374
379
358
395
372
378
356
390
368
432
419
375
388
387
0.04 [e]
0.08 [f]
0.05
0.06
nd [g]
0.05
0.44
0.05
0.63
0.95
0.40
0.17
0.25
0.42
0.12
0.10
0.50
0.27
0.06
0.18
1.54
0.32
<0.3
<0.3
2.6
–5.765
–5.726
–6.162
–5.738
–5.970
–5.705
–5.630
–5.816
–5.953
–5.698
–6.083
–5.668
–5.778
–5.496
–5.643
–5.063
–5.199
–6.108
–5.523
–6.167
–1.655
–1.760
–1.276
–1.625
–1.478
–1.611
–1.761
–1.841
–1.881
–2.207
–2.050
–1.622
–1.711
–1.957
–1.826
–1.734
–2.014
–2.456
–1.932
–2.654
4.110
3.966
4.886
4.113
4.492
4.094
3.869
3.975
4.072
3.491
4.033
4.046
4.067
3.539
3.817
3.329
3.185
3.652
3.591
3.513
3.15
3.98
2.14
3.26
2.09
3.38
4.29
2.04
3.01
5.07
2.20
2.06
2.93
4.87
2.24
4.15
4.25
4.20
4.24
4.40
2
25.0
>16.7
>20.0
3
4
[h]
[h]
5
–
–
6
0.49
10.2
[h]
[h]
7
–
–
12
13
14
15
16
17
18
19
20
21
22
23
24
0.83
0.99
0.94
6.1
6.0
63.6
101
6.6
0.33
0.75
0.83
4.9
51.5
33.3
50.6
2.4
0.80
1.30
0.39
<0.3
0.35
12.5
38.5
69.2
>20.0
51.4
[a] 1.0×10–5 M in aerated cyclohexane, r.t. [b] Degassed by using freeze-pump-thaw method, 1.0×10–5 M in cyclohexane, r.t. [c] Determined by using kf = Ff/tf. [d]
Calculated by using B3LYP/6-311G**. [e] Data from reference [6]. [f] Data from reference [10a]. [g] Not detected. [h] Not determined.
2
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10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
vessels were irradiated with UV light (l > 280 nm) by using a 450
W high pressure mercury lamp. Analysis of the photolysates gave
the time dependent ratios of the formed cis isomers shown in
Figure 5. The ratio of the cis isomer in the photostationary state
(PSS) was largest for derivative 3, and PSS cis isomer ratio was
found to decrease in the order of 3>4~6>2>1>5. It is known that
PSS isomer ratios in cis-trans photoisomerization reactions of
stilbenes are governed by the potential energy profiles of the
ground S0 and excited S1 and T1 states of the isomers, and the
absorbance of each isomer at the irradiation wavelength.[1] Using
this basis, a possible reason can be given for why the PSS ratio
of the cis isomer was largest in photoreaction of 3. Specifically,
the value might be related to the lower absorbance of the cis-3
due to twisting of the arene rings caused by the ortho-positioned
Me3Si groups. The fact that the proportion of the cis isomer in PSS
decreased with decreasing the steric hindrance except 5 is in
accord with the assumption. Interestingly, the PSS cis isomer ratio
was the lowest for 5, probably because the cis isomer is
essentially impossible to form owing to a large degree of steric
hindrance.
Figure 3. (a) UV absorption and (b) fluorescence spectra of 1-6, 1.0 × 10–5 M in
cyclohexane, lex = 291.5 nm.
Fluorescence lifetimes of 1-6 in cyclohexane (1.0 × 10–5 M) were
determined (Table 1, Figure S2). The fluorescence lifetimes of 2-
6 were found to be shorter than that of 1. The rates of
fluorescence (kf) of 2-6, obtained by using the relationship kf =
Ff/ts, were determined to be in the range of 108 s–1, which is one
order of magnitude larger than that of 1. Transient absorption
measurements, carried out at 200 ns after 308 nm laser pulsing
of cyclohexane solutions of 2-6, showed that no appreciable
transient signals are produced (Figure S3). These findings
indicate that intersystem crossing from excited singlet to excited
triplet states is not efficient, which agrees with the observation of
the absence of phosphorescence.
Next, the absorption spectroscopic properties of the stilbene
derivatives were analyzed by using theoretical methods (Figures
S4-S16). Information about the experimental and calculated
absorption properties of 2 and 5 are given in Figure 4 in terms of
UV absorption spectra (1.0 × 10–5 M in cyclohexane), oscillator
strengths calculated by using TD-SCF/B3LYP/6-311G** without
taking solvent effect into account, and longest wavelength
transitions. The results showed that the longest wavelength
transitions of both substances correspond to a HOMO to LUMO
(p–p*) transition. However, because 5 has a distorted orthogonal
structure, its oscillator strength was smaller than that of 2.
Cis-trans photoisomerization reactions of the stilbene derivatives
were examined. For this purpose, C6D6 solutions of 1-6 in Pyrex
Figure 5. Time dependence of cis ratio in cis-trans photoisomerization reactions
of 1-6, 1.0 × 10–2 M in C6D6, r.t., irradiated by using a 450 W high pressure
mercury lamp.
In order to gain more information about the observed silyl-
substituent effects, disilane-substituted stilbene
7
was
synthesized, and its UV absorption and fluorescence spectra
were recorded (1.0 × 10–5 M in cyclohexane, Figure 6a,b).
Compared to those of 2-6, the molar extinction coefficient (e) of 7
was larger and the absorption maximum was shifted to a longer
wavelength. The fluorescence maximum of 7 was also shifted to
a longer wavelength and its fluorescence quantum yield was 0.44
(Table 1).
Knowledge about the observed effects of Me3SiMe2Si-substitution
is gained by considering reports that show incorporation of
trimethylsilyl groups directly bonded to a thiophene ring causes
an absorption shift to longer wavelength as a consequence of the
cooperative effect of s–p and s*–p* interactions.[11f] Furthermore,
the molar extinction coefficients (e) of thiophenes increase with
increasing transition dipole moments (µ), a value that is defined
by the product of the HOMO and LUMO wave functions. In the
group comprised of 2, 5, and 7, Me3SiMe2Si-substituted stilbene
7 had the largest µ value, and e increased with increasing µ
(Figures 6c, S17). In addition, a good correlation existed between
Figure 4. UV absorption spectra (1.0 × 10–5 M in cyclohexane), oscillator
strengths calculated by TD-SCF/B3LYP/6-311G**, and longest wavelength
transitions of (a) 2 and (b) 5.
3
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10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
Ff and µ. Because the HOMO energy of 7 is higher than that of 2
(Figure 6d), the absorption and fluorescence maxima are shifted
to longer wavelengths.
In order to evaluate why the HOMO energy of 7 is higher than that
of 2, the effects of orbital interactions were examined by using
DFT calculation at the B3LYP/3-21G theoretical level. For
simplification purposes, the stilbenes having a single Me3Si group
or single Me3SiMe2Si group at one para position were selected as
models. The results, portrayed in the orbital diagrams shown in
Figure 7, showed that the energy gap between the HOMO and
LUMO of mono-Me3SiMe2Si-substituted derivative is smaller than
that of the mono-Me3Si-substituted analog, a finding that is
consistent with the actual absorption spectroscopic properties of
7 vs. 2. Also, the s(Si-C) interacts with HOMO of the stilbene core
in the mono-Me3Si derivative. Thus, when considering the orbital
geometry, each of the three s(Si–C) orbitals interacts with the
stilbene HOMO when the Si-Me bond occupies a position
perpendicular to the plane of the aromatic ring. On the other hand,
because the energy of the s(Si–Si) orbital of the mono-
Me3SiMe2Si-stilbene is closer than that of the s(Si–C) of the
mono-Me3Si-analog to the HOMO of the stilbene core, s(Si-Si)
more strongly interacts with the stilbene HOMO than does s(Si–
C).[21] Thus, this interaction causes a greater destabilization of
HOMO in the mono-Me3SiMe2Si-stilbene relative to that in the
Me3Si-substituted analog. Also, only a weak stabilizing interaction
occurs between the unoccupied s*(Si–C) or s*(Si–Si) orbital and
the LUMO of the stilbene core.
Figure 6. (a) UV absorption and (b) fluorescence spectra of 1, 2, 5 and 7, 1.0 ×
10–5 M in cyclohexane, lex = 291.5 nm. (c) Fluorescence quantum yields (Ff)
and transition dipole moments (µ) calculated by using B3LYP/6-311G**. (d)
Energy levels of HOMO and LUMO calculated by using B3LYP/6-311G**.
Figure 7. Orbital diagrams of Me3SiMe2Si-H, Me3SiMe2Si-stilbene, trans-stilbene (1), Me3Si-stilbene, and Me3Si-H (from left to right) calculated by B3LYP/3-21G.
4
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10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
Derivatives 8-11 containing HMe2Si, Et3Si, i-Pr3Si, and t-BuMe2Si
groups at both para positions of stilbene were synthesized in
order to assess their absorption and fluorescence properties
(Figures 8, S18). The results showed that no significant
differences exist between the absorption and fluorescence
wavelength maxima of these substances and those of 2. These
results are related with the fact that the conjugation of the s(Si–
C) orbital with the stilbene core is weak compared to that of s(Si–
Si).
UV absorption spectra of 12-15 (1.0 × 10–5 M in cyclohexane) are
shown in Figure 11a. The absorption bands of 12-15 were
significantly shifted to longer wavelengths compared to those of
1. Especially, the para-substituted derivative 14 displayed the
largest shift among these substances. Fluorescence spectra and
fluorescence quantum yields of 12-15 are displayed in Figure 11b
and Table 1, respectively. Among these stilbenes, 14 has longest
wavelength fluorescence maximum. The fluorescence quantum
yields of these substances were found to be 0.05 (12), 0.63 (13),
0.95 (14), and 0.40 (15), which shows that in the group the para-
substituted analog 14 is the most emissive substance (Figure 11c).
Figure 8. Structures of p,p'-disilylstilbenes 8-11.
In order to examine the effects of extended conjugation, we
prepared silylethynylstilbenes 12-15 by using Sonogashira
coupling reactions[22] of various bromostilbenes with
trimethylsilylacetylene (Figure 9). For comparison purposes,
analogs 16-19 having analogously positioned tert-butylethynyl
groups were also synthesized by using coupling reactions of
bromostilbenes with 3,3-dimethyl-1-butyne.
Figure 11. (a) UV absorption and (b) fluorescence spectra of 1 and 12-15, 1.0
× 10–5 M in cyclohexane, lex = 291.5 nm. (c) Image showing fluorescence of 1
and 14, 1.0 × 10–5 M in cyclohexane, lex = 254 nm.
Analysis
of
the
spectroscopic
properties
of
the
trimethylsilylethynyl derivative 14 and its tert-butyl analog 18 in
cyclohexane (Figures 12, S19) showed that both the absorption
and fluorescence maxima of 14 appear at longer wavelengths
than those of 18. Also, the fluorescence quantum yields (Ff), µ as
well as the rate constant of fluorescence emission (kf) of 14 were
larger than those of 18 (Table 1).
Figure 9. Structures of trimethylsilylethynyl-substituted stilbenes 12-15 and
their tert-butyl analogs 16-19.
Figure 12. (a) UV absorption and (b) fluorescence spectra of 14 and 18, 1.0 ×
10–5 M in cyclohexane, lex = 291.5 nm.
A
crystal of 14 was generated and subjected to X-ray
crystallographic analysis.[17] Examination of the structure (Figure
10) showed that this substance is highly planar.
Stilbene derivatives 20-24, having a trimethylsilylethynyl group
and an electron donating or withdrawing group at para positions
were synthesized, and their photophysical properties were
determined (Figure 13a). Compared to those of 14, the absorption
and fluorescence maxima of 20 and 21 appeared at longer
wavelengths, however, the maxima of 22-24 occurred at shorter
wavelengths (Figure S20). The opposite directions of the
wavelength maxima changes are in agreement with the HOMO
and LUMO energy gaps of 20-24 determined by using theoretical
methods (Figure S21). The energies of the HOMOs and LUMOs
of the amine-substituted stilbene derivatives 20 and 21 are higher
Figure 10. ORTEP plots of X-ray crystallographic data of 14.
5
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10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
than those of 14, but the difference is larger for the HOMOs.
These changes result in a narrower energy gap and a shift of the
absorption maxima to longer wavelengths. Both the HOMOs and
LUMOs of 22 and 24, which contain electron-withdrawing groups,
are stabilized but the degree is larger in HOMO, resulting in a
larger energy gap and a shift of the absorption maxima to shorter
wavelengths. In the case of derivative 23, which contains the
electron donating methoxy group, the energies of both the HOMO
and LUMO are increased. However, in contrast to the amine
derivatives, the LUMO is more destabilized, resulting in a larger
energy gap and a consequent shift in the absorption maximum to
a shorter wavelength.
It is particularly interesting that the respective dimethylamino- and
diphenylamino-substituted stilbenes 20 and 21 displayed
solvatofluorochromism, where the absorption maximum
wavelengths changed only slightly while the fluorescence maxima
changed greatly with varying solvent polarity (Figure 13b-e, Table
2). In contrast, 22-24 exhibit solvatofluorochromism in much
lesser extent than the amino derivatives. The largest Stokes shifts
in the emissions of 20 and 21 (Dl = 134 nm for 20 and 123 nm for
21) occurred when acetonitrile was the solvent. The fluorescence
quantum yields of 20 and 21 (1.0 × 10–5 M in cyclohexane) were
0.10 and 0.50, respectively. The slopes of Lippert-Mataga plots[23]
of the emission data for 20 and 21 were 7860 and 9509, values
that are comparable with those of related compounds[24] (Table
S1, Figures S22, S23).
Figure 13. (a) Structures of 20-24. (b)(c) UV-vis absorption fluorescence
spectra of 20 and 21, 1.0 × 10–5 M, lex = 360-370 (20) and 380-385 (21) nm.
(d)(e) Images showing fluorescence of 20 and 21 in toluene, THF, CH2Cl2, EtOH,
and CH3CN (from left to right), lex = 254 nm.
Table 2. Solvent Dependence of Photophysical Properties of 20 and 21. [a]
Solvent
20
21
Absorption
Fluorescence
Stokes
shift
Absorption
Fluorescence
Stokes
shift
(cm–1
)
(cm–1
)
labs
(nm)
e
lem
(nm)
labs
(nm)
e
lem
(nm)
(104
mol–
(104
mol–
1dm3cm–1
)
1dm3cm–1
)
cyclohexane
toluene
THF
360
370
368
366
366
365
1.83
1.86
1.41
1.56
1.41
1.76
432
440
469
470
474
499
4630
4300
5852
6046
6225
7357
382
385
382
384
380
380
3.77
3.14
3.82
3.91
3.30
3.93
420
438
464
474
469
503
2368
3143
4626
4945
4994
6435
CH2Cl2
EtOH
CH3CN
[a] 1.0×10–5 M in aerated solvents.
Analysis of the calculated optimized structure of the ground state
of 21 showed that the diphenylamino group is slightly twisted into
a propeller structure like that of Ph3N.[25] Thus, it is expected that
21 could have an intramolecular twisted charge transfer (TICT)
excited state, from which fluorescence is emitted (Figure 14).[26]
In the TICT state, charge would be separated so that a higher
degree of stabilization of the excited state relative to the ground
state will occur in higher polarity solvents. To assess this
possibility, structures of the ground and excited states of 21 were
optimized by using theoretical methods. Analysis of the structures
(Figure 15) showed that nitrogens in both are planar. The angle
of C(ipso)–N–C(ipso) was found to be 37° in the ground state and
75° in the excited state. Comparing the charge on the Ph2N group
with that on the adjacent aromatic ring showed that the charge is
more separated in the excited state than in the ground state.
Figure 14. Schematic representation of twisted intramolecular charge transfer
(TICT) mechanism, adapted from ref[26g] with permission of Prof. Hiroshi
Nakamura and The Japan Society for Analytical Chemistry, 2020.
6
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
Therefore the calculations of 21 suggest that there is some
contribution from TICT excited state in which the amino group is
twisted. Of particular importance is the observation that the
fluorescence quantum yield of 21 is large. Consequently, a
solvatofluorochromic molecule utilizing the highly emissive
property of silylethynyl-substituted stilbene has been successfully
developed.
Experimental Section
Materials and equipment: THF was distilled from CaH2 and then from
Na/Ph2C=O. Spectral grade solvents were used for spectroscopic studies
without further purification. Most other substances were used after
purification by distillation or recrystallization. Melting points were
determined on a Gallenkamp MFB- 595 melting point apparatus. 1H, 13C,
and 29Si NMR spectra were recorded using a JEOL JMN LA-400 (400 MHz
(1H) and 100 MHz (13C), respectively) or a JEOL ECA-500 (500 MHz (1H),
125 MHz (13C), and 100 MHz (29Si), respectively) spectrometer with Me4Si
as an internal standard. IR spectra were determined using a Shimadzu
FTIR-8300 spectrometer. Low- and high-resolution mass spectra were
taken on a JEOL JMS-700 and a JEOL JMS-SM102A instrument,
respectively. UV-vis spectra were recorded using a Hitachi U-2900 or a
JASCO V-570 spectrophotometer. Fluorescence spectra were recorded
using a JASCO FP-8500 spectrophotometer. HPLC separations were
performed on a recycling preparative HPLC instrument (Japan Analytical
Industry Co. Ltd., LC-908 equipped with a JAIGEL-H (GPC) column).
Column chromatography was conducted by using Kanto Chemical Co. Ltd.,
silica gel 60
N
(spherical, neutral, 0.04-0.05 mm). Thin-layer
chromatography was performed with Merck Kiesel gel 60 F254 plates, and
bands were detected by using UV light and a phosphomolybdic acid
ethanol solution with heating. Theoretical calculations were performed by
using a Gaussian 09 software package.
(E)-1,2-Bis[4-(trimethylsilyl)phenyl]ethene (2): To an argon-purged,
stirred mixture of Zn powder (3.90 g, 59.6 mmol) in THF (60 mL) was
added dropwise TiCl4 (3.27 mL, 30 mmol) at 0 °C. To the mixture was
added a THF (20 mL) solution of 4-bromobenzaldehyde (1.85 g, 10 mmol),
and the resulting solution was stirred at 70 °C for 5 h. The mixture was
extracted with CH2Cl2 and sat HCl aq. The organic layer was separated,
dried over MgSO4, filtered, and concentrated in vacuo, giving a residue
that was washed with EtOH to give (E)-1,2-bis(4-bromophenyl)ethene
(0.937 g, 55% yield). White solid; 1H NMR (400 MHz, CDCl3) d 7.02 (s, 2H),
7.37 (d, J = 8.4 Hz, 4H), 7.48 (d, J = 8.4 Hz, 4H) ppm.[10a]
Figure 15. Optimized structures of (a) ground and (b) excited states of 21 and
charge distributions calculated by using B3LYP/6-31+G(**).
To an argon-purged, stirred THF (50 mL) solution of (E)-1,2-bis(4-
bromophenyl)ethene (0.330 g, 1 mmol) was added dropwise t-BuLi (1.61
M in n-hexane, 1.26 mL, 2.8 mmol) at –78 °C, and the resulting solution
was stirred at –78 °C for 1 h. To the solution was added Me3SiCl (1.26 mL,
8 mmol) at –78 °C, and the solution was stirred at –78 °C for 30 min and
then at room temperature overnight. The mixture was extracted with Et2O
and sat NaHCO3 aq. The organic layer was separated, dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was recrystallized
from EtOH to give (E)-1,2-bis[4-(trimethylsilyl)phenyl]ethene (2, 0.19 g,
59% yield). White solid; 1H NMR (500 MHz, CDCl3) d 0.28 (s, 18H), 7.13
(s, 2H), 7.51-7.54 (m, 8H) ppm; 13C{1H} NMR (125 MHz, CDCl3) d –1.1,
125.8, 128.7, 133.7, 137.7, 140.1 ppm; 29Si NMR (100 MHz, CDCl3) d –4.1
ppm.[10a]
Conclusion
In this study, novel stilbene derivatives containing various types
of silyl substitution patterns were synthesized. The results of
spectroscopic studies showed that the absorption and
fluorescence properties of these compounds change depending
on the position, number and nature of the silyl groups. In particular,
the ortho tetra-substituted stilbene 5 has a highly distorted
structure, and it emits fluorescence only at a low temperature. The
fluorescence quantum yields of these substances increased on
introduction of the silyl groups, and disilane
7
and
(E)-1,2-Bis[2-(trimethylsilyl)phenyl]ethene (3): To an argon-purged,
stirred mixture of Zn powder (7.41 g, 113.24 mmol) in THF (114 mL) was
added dropwise TiCl4 (6.21 mL, 57 mmol) at 0 °C. To the mixture was
added a THF (38 mL) solution of 2-bromobenzaldehyde (2.2 mL, 19 mmol),
and the resulting solution was stirred at 70 °C for 5 h. The mixture was
extracted with CH2Cl2 and sat HCl aq. The organic layer was separated,
dried over MgSO4, filtered, and concentrated in vacuo, giving a residue
that was recrystallized from hexane:toluene = 5:1 to give (E)-1,2-bis(2-
bromophenyl)ethene (2.56 g, 80% yield). White solid; 1H NMR (400 MHz,
CDCl3) d 7.15 (t, J = 7.6 Hz, 2H), 7.34 (t, J = 7.2 Hz, 2H), 7.40 (s, 2H), 7.59
(d, J = 8.0 Hz, 2H), 7.74 (d, J = 7.6 Hz, 2H) ppm.[27]
To an argon-purged, stirred THF (50 mL) solution of (E)-1,2-bis(2-
bromophenyl)ethene (0.495 g, 1.5 mmol) was added dropwise t-BuLi (1.61
M in n-hexane, 3.91 mL, 6.33 mmol) at –78 °C, and the resulting solution
was stirred at –78 °C for 1 h. To the solution was added Me3SiCl (1.9 mL,
15 mmol) at –78 °C, and the solution was stirred at –78 °C for 30 min and
then at room temperature overnight. The mixture was extracted with Et2O
and sat NaHCO3 aq. The organic layer was separated, dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was recrystallized
trimethylsilylethynyl analog 14 displayed strong fluorescence. The
results of theoretical calculations indicated that overlap of the
s(Si–Si) bonds in disilane 7 with the adjacent arene ring orbitals
has a greater effect of frontier orbital energies than does overlap
of s(Si–C) bonds of trimethylsilyl-substituted analogs. The silyl-
substituted stilbenes displayed more efficient fluorescence, and
absorbed and emitted light at longer wavelength than do their tert-
butyl analogs. Finally, stilbene derivatives 20 and 21, having
amine
substituents
at
para-arene
positions,
were
solvatofluorochromic. The results of theoretical calculations
suggested that the structure of 21 changes greatly in proceeding
from the ground to the excited state, and that it possesses a TICT
excited state. The results of this effort are expected to be
informative for workers in the fields of stilbene photochemistry and
highly emissive silicon materials.
7
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
from EtOH to give (E)-1,2-bis[2-(trimethylsilyl)phenyl]ethene (3, 0.175 g,
36% yield). White solid; mp 94-95 °C; 1H NMR (400 MHz, C6D6) d 0.33 (s,
18H), 7.13 (t, J = 6.6 Hz, 2H), 7.25 (t, J = 6.4 Hz, 2H), 7.53 (d, J = 7.3 Hz,
2H), 7.59 (s, 2H), 7.81 (d, J = 7.8 Hz, 2H) ppm; 13C{1H} NMR (100 MHz,
CDCl3) d 0.4, 125.1, 126.8, 129.7, 130.9, 134.7, 139.1, 143.6 ppm; 29Si
NMR (100 MHz, CDCl3) d –4.3 ppm; IR (KBr) 459, 621, 691, 725, 764, 837,
964, 1072, 1123, 1250, 1435, 1470, 2959, 3047 cm–1; MS (EI+) m/z
(relative intensity, %) = 235 (41), 250 (20), 324 (100, M+); HRMS (EI+)
calcd for C20H28Si2: 324.1730, found: 324.1728.
recrystallized
from
EtOH
to
give
(E)-1,2-bis[2,6-
bis(trimethylsilyl)phenyl]ethene (5, 0.106 g, 23% yield). White solid; mp
243-245 °C; 1H NMR (400 MHz, CDCl3) d 0.27 (s, 36H), 6.88 (s, 2H), 7.24
(t, J = 7.6 Hz, 2H), 7.62 (d, J = 7.6 Hz, 4H) ppm; 13C{1H} NMR (125 MHz,
CDCl3) d 1.8, 125.2, 136.2, 137.5, 137.6, 150.0 ppm; 29Si NMR (100 MHz,
CDCl3) d –3.6 ppm; IR (KBr) 621, 687, 760, 852, 891, 1250, 1393, 2959
cm–1; MS (EI+) m/z (relative intensity, %) = 307 (100), 365 (27), 468 (98,
M+); HRMS (EI+) calcd for C26H44Si4: 468.2520, found: 468.2515.
(E)-1,2-Bis[3,5-bis(trimethylsilyl)phenyl]ethene (6): An argon-purged
mixture of 3,5-dibromotoluene (2.375 g, 9.2 mmol), N-bromosuccinimide
(NBS, 1.65 g, 9.2 mmol), and dibenzoyl peroxide (BPO, 66.5 mg, 0.275
mmol) in CCl4 (30 mL) was stirred at 90 °C for 5 h. The formed solid was
removed by filtration. The filtrate was concentrated in vacuo to give a
residue. Silica gel column chromatography (eluent: hexane:AcOEt = 99:1)
followed by recrystallization from EtOH gave 1,3-dibromo-5-
(bromomethyl)benzene (1.294 g, 43% yield). Colorless solid; 1H NMR (400
MHz, CDCl3) d 4.37 (s, 2H), 7.48 (d, J = 1.7 Hz, 2H), 7.60 (t, J = 1.7 Hz,
1H) ppm.[30]
An argon-purged mixture of P(OEt)3 (1.077 mL, 6.0 mmol) and 1,3-
dibromo-5-(bromomethyl)benzene (0.986 g, 3.0 mmol) was stirred at
140 °C for 5 h. The solution was concentrated in vacuo, giving a residue
that was subjected to silica gel column chromatography (eluent:
hexane:AcOEt = 3:7) to give diethyl (3,5-dibromobenzyl)phosphonate
(0.744 g, 64% yield). Yellow oil; 1H NMR (400 MHz, CDCl3) d 1.23-1.30 (m,
6H), 3.07 (d, J = 16.3 Hz, 2H), 4.01-4.14 (m, 4H), 7.39 (t, J = 2.0 Hz, 2H),
7.56 (d, J = 2.0 Hz, 1H) ppm.[30]
To an argon-purged THF (30 mL) solution of 3,5-dibromobenzaldehyde
(1.02 g, 3.86 mmol) and diethyl (3,5-dibromobenzyl)phosphonate (0.744 g,
1.93 mmol) was added t-BuOK (0.325 g, 2.895 mmol), and the resulting
solution was stirred at room temperature for 3 h. To the solution was added
H2O to form white solid. The solid was collected by filtration and washed
with CH2Cl2 to give (E)-1,2-bis(3,5-dibromophenyl)ethene (0.771 g, 81%
yield). White solid; 1H NMR (500 MHz, C6D6) d 5.96 (s, 2H), 7.09 (s, 4H),
7.36 (s, 2H).[30]
(E)-1,2-Bis[3-(trimethylsilyl)phenyl]ethene (4): To an argon-purged,
stirred mixture of Zn powder (14.82 g, 226.5 mmol) in THF (228 mL) was
added dropwise TiCl4 (12.43 mL, 114 mmol) at 0 °C. To the mixture was
added a THF (76 mL) solution of 3-bromobenzaldehyde (7.01 g, 38 mmol),
and the resulting mixture was stirred at 70 °C for 5 h. The mixture was
extracted with CH2Cl2 and sat HCl aq. The organic layer was separated,
dried over MgSO4, filtered, and concentrated in vacuo, giving a residue
that was recrystallized from hexane:toluene = 5:1 to give (E)-1,2-bis(3-
bromophenyl)ethene (4.67 g, 73% yield).1H NMR (CDCl3, 400 MHz) d 7.01
(s, 2H), 7.23 (t, J = 7.6 Hz, 2H), 7.38-7.42 (m, 4H), 7.66 (s, 2H) ppm.[28]
To an argon-purged, stirred THF (50 mL) solution of (E)-1,2-bis(3-
bromophenyl)ethene (0.495 g, 1.5 mmol) was added dropwise t-BuLi (1.61
M in n-hexane, 3.91 mL, 6.33 mmol) at –78 °C, and the resulting solution
was stirred at –78 °C for 1 h. To the solution was added Me3SiCl (1.9 mL,
15 mmol) at –78 °C, and the solution was stirred at –78 °C for 30 min and
then at room temperature overnight. The mixture was extracted with Et2O
and sat NaHCO3 aq. The organic layer was separated, dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was recrystallized
from EtOH to give (E)-1,2-bis[3-(trimethylsilyl)phenyl]ethene (4, 0.173 g,
41% yield). White solid; mp 101-102 °C; 1H NMR (400 MHz, CDCl3) d 0.31
(s, 18H), 7.14 (s, 2H), 7.36 (t, J = 7.4 Hz, 2H), 7.43 (d, J = 7.1 Hz, 2H),
7.54 (d, J = 7.7 Hz, 2H), 7.64 (s, 2H) ppm; 13C{1H} NMR (125 MHz, CDCl3)
d –1.1, 126.6, 128.0, 128.8, 131.7, 132.6, 136.5, 140.9 ppm; 29Si NMR
(100 MHz, CDCl3) d –3.8 ppm; IR (KBr) 459, 621, 694, 725, 752, 799, 837,
895, 964, 1123, 1250, 1404, 2955 cm–1
; MS (EI+) m/z (relative
intensity, %) = 235 (5), 309 (73), 324 (100, M+); HRMS (EI+) calcd for
C20H28Si2: 324.1730, found: 324.1731; Anal. calcd for C20H28Si2: C, 74.00;
H, 8.69, found: C, 73.62; H, 8.82.
To an argon-purged, stirred THF (50 mL) solution of (E)-1,2-bis(3,5-
dibromophenyl)ethene (0.491 g, 1 mmol) was added dropwise t-BuLi (1.77
M in n-hexane, 4.52 mL, 8.02 mmol) at –78 °C, and the resulting solution
was stirred at –78 °C for 1 h. To the solution was added Me3SiCl (1.26 mL,
10 mmol) at –78 °C, and the solution was stirred at –78 °C for 30 min and
then at room temperature overnight. The mixture was extracted with Et2O
and sat NaHCO3 aq. The organic layer was separated, dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was recrystallized
from EtOH to give (E)-1,2-bis[3,5-bis(trimethylsilyl)phenyl]ethene (6, 0.255
g, 54% yield). Light brown solid; mp 146-147 °C; 1H NMR (400 MHz,
CDCl3) d 0.32 (s, 36H), 7.14 (s, 2H), 7.56 (s, 2H), 7.67 (s, 4H) ppm; 13C{1H}
NMR (100 MHz, CDCl3) d 1.0, 128.9, 132.0, 135.6, 137.4, 139.8 ppm; 29Si
NMR (100 MHz, CDCl3) d –3.8 ppm; IR (KBr) 552, 621, 691, 752, 833, 868,
937, 964, 1137, 1246, 1385, 2897, 2955, 3009 cm–1; MS (EI+) m/z (relative
intensity, %) = 219 (18), 381 (19), 396 (27), 453 (43), 468 (100, M+); HRMS
(EI+) calcd for C26H44Si4: 468.2520, found: 468.2524.
(E)-1,2-Bis[2,6-bis(trimethylsilyl)phenyl]ethene (5): To an argon-
purged, stirred THF (100 mL) solution of i-Pr2NH (5.2 mL, 37 mmol) was
added dropwise n-BuLi (1.61 M in n-hexane, 23.1 mL, 37.1 mmol) at –
78 °C, and the resulting solution was stirred at –78 °C for 30 min. To the
solution were added 1,3-dibromobenzene (7.3 g, 30.8 mmol) and then
N,N-dimethylformamide (DMF, 2.99 mL, 37 mmol), and the solution was
stirred at –78 °C for 30 min. To the solution was added dilute H2SO4. The
solution was extracted with AcOEt. The organic layer was separated, dried
over MgSO4, filtered, and concentrated in vacuo, giving a crude residue
containing 2,6-dibromobenzaldehyde (light brown solid, 9.0 g). 1H NMR
(400 MHz, CDCl3) d 7.23 (t, J = 8.0 Hz, 1H), 7.65 (d, J = 8.0 Hz, 2H), 10.27
(s, 1H).[29]
To an argon-purged, stirred mixture of Zn powder (3.90 g, 59.6 mmol) in
THF (60 mL) was added dropwise TiCl4 (3.27 mL, 30 mmol) at 0 °C. To
the mixture was added a THF (20 mL) solution of the crude residue (2.64
g) containing 2,6-dibromobenzaldehyde, and the resulting solution was
stirred at 70 °C for 5 h. The mixture was extracted with CH2Cl2 and sat HCl
aq. The organic layer was separated, dried over MgSO4, filtered, and
concentrated in vacuo, giving a residue that was recrystallized from
hexane:toluene = 10:1 to give (E)-1,2-bis(2,6-dibromophenyl)ethene (0.47
g, 21% yield (2 steps)). White solid; 1H NMR (500 MHz, CDCl3) d 6.94 (s,
2H), 7.00 (t, J = 8.0 Hz, 2H), 7.61 (d, J = 8.0 Hz, 4H) ppm.
To an argon-purged, stirred THF (50 mL) solution of (E)-1,2-bis(2,6-
dibromophenyl)ethene (0.491 g, 1.0 mmol) was added dropwise t-BuLi
(1.77 M in n-hexane, 4.52 mL, 8.02 mmol) at –78 °C, and the resulting
solution was stirred at –78 °C for 1 h. To the solution was added Me3SiCl
(1.21 mL, 10 mmol) at –78 °C, and the solution was stirred at –78 °C for
30 min and then at room temperature overnight. The mixture was extracted
with Et2O and sat NaHCO3 aq. The organic layer was separated, dried
over Na2SO4, filtered, and concentrated in vacuo, giving a residue that was
(E)-1,2-Bis[4-(1,1,2,2,2-pentamethyldisilanyl)phenyl]ethene (7): To an
argon-purged, stirred THF (10 mL) solution of (E)-1,2-bis(4-
bromophenyl)ethene (0.168 g, 0.5 mmol, see preparation of 2) was added
dropwise t-BuLi (1.64 M in n-hexane, 1.8 mL, 3.0 mmol) at –78 °C, and the
resulting solution was stirred at –78 °C for 1 h. To the solution was added
Me3SiMe2SiCl (0.830 g, 10 mmol) at –78 °C, and the solution was stirred
at –78 °C for 30 min and then at room temperature overnight. The mixture
was extracted with Et2O and sat NaCl aq. The organic layer was separated,
dried over Na2SO4, filtered, and concentrated in vacuo, giving a residue
that was subjected to silica gel column chromatography (eluent: hexane)
to give (E)-1,2-bis[4-(1,1,2,2,2-pentamethyldisilanyl)phenyl]ethene (7,
0.145 g, 66% yield). White solid; mp 130-132 °C; 1H NMR (500 MHz,
CDCl3) d 0.44 (s, 18H), 0.71 (s, 12H), 7.50 (s, 2H), 7.82 (d, J = 8.0 Hz, 4H),
7.86 (d, J = 8.6 Hz, 4H) ppm; 13C{1H} NMR (100 MHz, CDCl3) d –4.1, –2.3,
125.8, 128.7, 134.1, 137.3, 139.3 ppm; IR (KBr) 799, 833, 1246, 2951 cm–
1; MS (EI+) m/z (relative intensity, %) = 290 (37), 309 (43), 367 (100), 440
(32, M+); HRMS (EI+) calcd for C24H40Si4: 440.2207, found: 440.2205.
8
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
(E)-1,2-Bis[4-(dimethylsilyl)phenyl]ethene (8): To an argon-purged,
stirred THF (10 mL) solution of (E)-1,2-bis(4-bromophenyl)ethene (0.168
g, 0.5 mmol, see preparation of 2) was added dropwise t-BuLi (1.64 M in
n-hexane, 1.8 mL, 3.0 mmol) at –78 °C, and the resulting solution was
stirred at –78 °C for 1 h. To the solution was added Me2HSiCl (0.946 g, 10
mmol) at –78 °C, and the solution was stirred at –78 °C for 30 min and
then at room temperature overnight. The mixture was extracted with CHCl3
and sat NaCl aq. The organic layer was separated, dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was subjected to
silica gel column chromatography (eluent: hexane:CHCl3 = 40:1) to give
(E)-1,2-bis[4-(dimethylsilyl)phenyl]ethene (8, 0.149 g, 84% yield). White
solid; 1H NMR (500 MHz, CDCl3) d 0.36 (d, J = 4.0 Hz,12H), 4.44 (sept, J
= 3.8 Hz, 2H), 7.14 (s, 2H), 7.50-7.56 (m, 8H) ppm; 13C{1H} NMR (125 MHz,
CDCl3) d –3.8, 125.9, 127.7, 128.9, 137.2, 138.0 ppm; 29Si NMR (100 MHz,
CDCl3) d –17.3 (d, 1J(29Si–1H) = 184.3 Hz) ppm.[31]
(EI+) m/z (relative intensity, %) = 237 (74), 295 (99), 351 (72), 409 (100,
M++1); HRMS (EI+) calcd for C26H40Si2: 408.2669, found: 408.2676.
(E)-1,2-Bis{2-[(trimethylsilyl)ethynyl]phenyl}ethene (12): A mixture of
(E)-1,2-bis(2-bromophenyl)ethene (0.209 g, 0.62 mmol, see preparation of
3), trimethylsilylacetylene (0.5 mL, 3.5 mmol), Pd(PPh3)4 (35 mg, 0.03
mmol), CuI (3 mg, 0.016 mmol), PPh3 (15.7 mg, 0.06 mmol), and Et3N (20
mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
chromatography (eluent: hexane:CH2Cl2 = 50:1) to give (E)-1,2-bis{2-
[(trimethylsilyl)ethynyl]phenyl}ethene (12, 0.220 g, 95% yield). Yellow
solid; 1H NMR (500 MHz, CDCl3) d 0.26 (s, 18H), 7.20 (t, J = 7.5 Hz, 2H),
7.33 (t, J = 7.5 Hz, 2H), 7.48 (d, J = 8.0 Hz, 2H), 7.71-7.73 (m, 4H) ppm;
13C{1H} NMR (125 MHz, CDCl3) d 0.0, 99.7, 103.4, 122.3, 124.3, 127.3,
128.0, 128.7, 132.7, 139.2 ppm.[32]
(E)-1,2-Bis[4-(triethylsilyl)phenyl]ethene (9): To an argon-purged,
stirred THF (10 mL) solution of (E)-1,2-bis(4-bromophenyl)ethene (0.168
g, 0.5 mmol, see preparation of 2) was added dropwise t-BuLi (1.64 M in
n-hexane, 1.8 mL, 3.0 mmol) at –78 °C, and the resulting solution was
stirred at –78 °C for 1 h. To the solution was added Et3SiCl (1.507 g, 10
mmol) at –78 °C, and the solution was stirred at –78 °C for 30 min and
then at room temperature overnight. The mixture was extracted with CHCl3
and sat NaCl aq. The organic layer was separated, dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was subjected to
silica gel column chromatography (eluent: hexane) to give (E)-1,2-bis[4-
(triethylsilyl)phenyl]ethene (9, 0.169 g, 89% yield). White solid; mp 72-
74 °C; 1H NMR (500 MHz, CDCl3) d 0.80 (q, J = 8.0 Hz, 12H), 0.98 (t, J =
8.0 Hz, 18H), 7.14 (s, 2H) ,7.48-7.51 (m, 8H) ppm; 13C{1H} NMR (125 MHz,
CDCl3) d 3.3, 7.4, 125.7, 128.8, 134.6, 137.0, 137.6 ppm; IR (KBr) 710,
(E)-1,2-Bis{3-[(trimethylsilyl)ethynyl]phenyl}ethene (13): A mixture of
(E)-1,2-bis(3-bromophenyl)ethene (0.20 g, 0.62 mmol, see preparation of
4), trimethylsilylacetylene (0.5 mL, 3.5 mmol), Pd(PPh3)4 (35 mg, 0.03
mmol), CuI (3 mg, 0.016 mmol), PPh3 (15.7 mg, 0.06 mmol), and Et3N (20
mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
chromatography (eluent: hexane:CH2Cl2 = 50:1) to give (E)-1,2-bis{3-
[(trimethylsilyl)ethynyl]phenyl}ethene (13, 0.210 g, 95% yield). White solid;
1H NMR (500 MHz, CDCl3) d 0.28 (s, 18H), 7.07 (s, 2H), 7.31 (t, J = 8.0
Hz, 2H), 7.38 (d, J = 7.0 Hz, 2H), 7.44 (d, J = 7.0 Hz, 2H), 7.64 (s, 2H)
ppm; 13C{1H} NMR (125 MHz, CDCl3) d 0.0, 94.4, 104.8, 105.9, 126.8,
128.5, 128.6, 129.9, 131.1, 137.0 ppm.[32]
817, 964, 1007, 1107, 2873, 2954 cm–1
; MS (EI+) m/z (relative
intensity, %) = 323 (22), 351 (21), 379 (76), 408 (100, M+); HRMS (EI+)
(E)-1,2-Bis{4-[(trimethylsilyl)ethynyl]phenyl}ethene (14): A mixture of
(E)-1,2-bis(4-bromophenyl)ethene (0.20 g, 0.62 mmol, see preparation of
2), trimethylsilylacetylene (0.5 mL, 3.5 mmol), Pd(PPh3)4 (35 mg, 0.03
mmol), CuI (3 mg, 0.016 mmol), PPh3 (15.7 mg, 0.06 mmol), and Et3N (20
mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
chromatography (eluent: hexane:CH2Cl2 = 5:1) to give (E)-1,2-bis{4-
[(trimethylsilyl)ethynyl]phenyl}ethene (14, 0.214 g, 95% yield). Yellow
solid; 1H NMR (500 MHz, CDCl3) d 0.26 (s, 18H), 7.08 (s, 2H), 7.42-7.46
(m, 8H) ppm; 13C{1H} NMR (125 MHz, CDCl3) d 0.0, 95.3, 105.1, 122.3,
126.3, 128.8, 132.3, 137.1 ppm.[32]
calcd for C26H40Si2: 408.2669, found: 408.2667.
(E)-1,2-Bis[4-(triisopropylsilyl)phenyl]ethene (10): To an argon-purged,
stirred THF (10 mL) solution of (E)-1,2-bis(4-bromophenyl)ethene (0.168
g, 0.5 mmol, see preparation of 2) was added dropwise t-BuLi (1.64 M in
n-hexane, 1.8 mL, 3.0 mmol) at –78 °C, and the resulting solution was
stirred at –78 °C for 1 h. To the solution was added i-Pr3SiCl (1.928 g, 10
mmol) at –78 °C, and the solution was stirred at –78 °C for 30 min and
then at room temperature overnight. The mixture was extracted with CHCl3
and sat NaCl aq. The organic layer was separated, dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was subjected to
silica gel column chromatography (eluent: hexane) to give (E)-1,2-bis[4-
(triisopropylsilyl)phenyl]ethene (10, 0.199 g, 81% yield). White solid; mp
155-157 °C; 1H NMR (500 MHz, CDCl3) d 1.09 (d, J = 7.5 Hz, 36H), 1.41
(sept, J = 7.5 Hz, 6H), 7.14 (s, 2H) ,7.48-7.51 (m, 8H) ppm;13C{1H} NMR
(125 MHz, CDCl3) d 10.8, 18.6, 125.6, 128.9, 134.5, 135.6, 137.4 ppm; IR
(E)-1,2-Bis{3,5-bis[(trimethylsilyl)ethynyl]phenyl}ethene
(15):
A
mixture of (E)-1,2-bis(3,5-dibromophenyl)ethene (0.30 g, 0.59 mmol, see
preparation of 6), trimethylsilylacetylene (1.5 mL, 10.8 mmol), Pd(PPh3)4
(70 mg, 0.06 mmol), CuI (6 mg, 0.03 mmol), PPh3 (31.4 mg, 0.12 mmol),
and Et3N (20 mL) in THF (30 mL) was stirred at 50 °C under an argon
atmosphere overnight. The resulting mixture was filtered. The filtrate was
concentrated in vacuo, giving a residue that was subjected to silica gel
column chromatography (eluent: hexane:CH2Cl2 = 10:1) to give (E)-1,2-
bis{3,5-bis[(trimethylsilyl)ethynyl]phenyl}ethene (15, 0.325 g, 96% yield).
Yellow solid; mp 275-276 °C; 1H NMR (500 MHz, CDCl3) d 0.25 (s, 36H),
6.99 (s, 2H), 7.48 (s, 2H), 7.51 (s, 4H) ppm; 13C{1H} NMR (125 MHz,
CDCl3) d 2.0, 97.3, 105.9, 125.9, 130.5, 132.0, 136.6, 139.1 ppm; IR(KBr)
845, 984, 1161, 1250, 1585, 2156, 2958 cm–1; MS (EI+) m/z (relative
intensity, %) = 549 (29), 564 (100, M+); HRMS (EI+) calcd for C34H44Si4:
564.2520, found: 564.2518.
(KBr) 817, 964, 1010, 1103, 1365, 1461, 1596, 2866, 2943, 3016 cm–1
MS (EI+) m/z (relative intensity, %) = 379 (33), 407 (69), 449 (100), 492
(80, M+); HRMS (EI+) calcd for C32H52Si2: 492.3608, found: 492.3606.
;
(E)-1,2-Bis[4-(tert-butyldimethylsilyl)phenyl]ethene (11): To an argon-
purged, stirred THF (10 mL) solution of (E)-1,2-bis(4-bromophenyl)ethene
(0.168 g, 0.5 mmol, see preparation of 2) was added dropwise t-BuLi (1.64
M in n-hexane, 1.8 mL, 3.0 mmol) at –78 °C, and the resulting solution was
stirred at –78 °C for 1 h. To the solution was added t-BuMe2SiCl (1.507 g,
10 mmol) at –78 °C, and the solution was stirred at –78 °C for 30 min and
then at room temperature overnight. The mixture was extracted with CHCl3
and sat NaCl aq. The organic layer was separated, dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was subjected to
silica gel column chromatography (eluent: hexane) to give (E)-1,2-bis[4-
(tert-butyldimethylsilyl)phenyl]ethene (11, 0.172 g, 84% yield). White solid;
mp 139-142 °C; 1H NMR (500 MHz, CDCl3) d 0.26 (s, 12H), 0.88 (s, 18H),
7.14 (s, 2H), 7.49-7.51 (m, 8H) ppm; 13C{1H} NMR (125 MHz, CDCl3) d –
6.2, 16.9, 26.5, 125.5, 128.9, 134.8, 137.5, 137.6 ppm; IR (KBr) 540, 625,
679, 775, 822, 968, 1010, 1107, 1250, 1466, 1596, 2855, 2928 cm–1; MS
(E)-1,2-Bis[2-(3,3-dimethylbut-1-yn-1-yl)phenyl]ethene (16): A mixture
of (E)-1,2-bis(2-bromophenyl)ethene (0.198 g, 0.59 mmol, see preparation
of 3), 3,3-dimethyl-1-butyne (0.3 mL, 2.4 mmol), Pd(PPh3)4 (35 mg, 0.03
mmol), CuI (3 mg, 0.016 mmol), PPh3 (15.7 mg, 0.06 mmol), and Et3N (20
mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
chromatography (eluent: CHCl3) followed by HPLC (GPC, eluent: CHCl3)
9
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
to give (E)-1,2-bis[2-(3,3-dimethylbut-1-yn-1-yl)phenyl]ethene (16, 0.144 g,
72% yield). Yellow solid; mp 217-218 °C; 1H NMR (500 MHz, CDCl3) d 1.38
(s, 18H), 7.18 (t, J = 7.4 Hz, 2H), 7.26-7.29 (m, 2H), 7.41 (d, J = 7.4 Hz,
2H), 7.68-7.71 (m, 4H) ppm; 13C{1H} NMR (125 MHz, CDCl3) d 28.0, 31.0,
79.0, 99.7, 121.4, 123.4, 126.3, 127.6, 128.0, 131.8, 131.9, 135.9, 136.1
dimethylaniline (0.459 g, 76% yield). Yellow-green solid; 1H NMR (400
MHz, CDCl3) d 2.99 (s, 6H), 6.71 (d, J = 8.8 Hz, 2H), 6.83 (d, J = 16.0 Hz,
1H), 7.03 (d, J = 16.0 Hz, 1H), 7.33 (d, J = 8.4 Hz, 2H), 7.39-7.44 (m, 4H)
ppm.[34]
A mixture of (E)-4-(4-bromostyryl)-N,N-dimethylaniline (0.100 g, 0.30
mmol), trimethylsilylacetylene (1 mL, 7.0 mmol), Pd(PPh3)4 (38 mg, 0.033
mmol), CuI (10 mg, 0.053 mmol), PPh3 (20 mg, 0.076 mmol), and Et3N (10
mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
chromatography (eluent: hexane:CHCl3 = 1:1) to give (E)-N,N-dimethyl-4-
{4-[(trimethylsilyl)ethynyl]styryl}aniline (20, 0.079 g, 75% yield). Green
solid; 1H NMR (400 MHz, CDCl3) d 0.25 (s, 9H), 2.99 (s, 6H), 6.71 (d, J =
9.0 Hz, 2H), 6.87 (d, J = 16.0 Hz, 1H), 7.06 (d, J = 16.0 Hz, 1H), 7.40-7.42
(m, 6H) ppm; 13C{1H} NMR (125 MHz, CDCl3) d 0.0, 40.4, 94.4, 105.5,
112.3, 120.9, 123.5, 125.3, 125.7, 127.7, 129.8, 132.2, 138.4, 150.2
ppm.[35]
ppm; IR (KBr) 760, 972, 1199, 1361, 1450, 1474, 2341, 2862, 2966 cm–1
;
MS (EI+) m/z (relative intensity, %) = 295 (33), 325 (40), 340 (100, M+);
HRMS (EI+) calcd for C26H38: 340.2191, found: 340.2204.
(E)-1,2-Bis[3-(3,3-dimethylbut-1-yn-1-yl)phenyl]ethene (17): A mixture
of (E)-1,2-bis(3-bromophenyl)ethene (0.20 g, 0.59 mmol, see preparation
of 4), 3,3-dimethyl-1-butyne (0.3 mL, 2.4 mmol), Pd(PPh3)4 (35 mg, 0.03
mmol), CuI (3 mg, 0.016 mmol), PPh3 (15.3 mg, 0.06 mmol), and Et3N (20
mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
chromatography (eluent: CHCl3) followed by HPLC (GPC, eluent: CHCl3)
to give (E)-1,2-bis[3-(3,3-dimethylbut-1-yn-1-yl)phenyl]ethene (17, 0.188 g,
94% yield). Yellow solid; mp 168-171 °C; 1H NMR (500 MHz, CDCl3) d 1.33
(s, 18H), 7.05 (s, 2H), 7.26-7.30 (m, 4H), 7.38 (d, J = 6.9 Hz, 2H), 7.55 (s,
2H) ppm; 13C{1H} NMR (125 MHz, CDCl3) d 28.0, 31.0, 78.8, 98.7, 124.4,
125.7, 128.4, 128.5, 129.6, 130.7, 137.1 ppm; IR (KBr) 694, 802, 868, 895,
964, 1292, 1361, 1474, 1597, 2221, 2866, 2966 cm–1; MS (EI+) m/z
(relative intensity, %) = 155 (18), 325 (45), 340 (100, M+); HRMS (EI+)
calcd for C26H38: 340.2191, found: 340.2190.
(E)-N,N-Diphenyl-4-{4-[(trimethylsilyl)ethynyl]styryl}aniline (21): To
an argon-purged, stirred THF (15 mL) solution of diethyl (4-
bromobenzyl)phosphonate (0.921 g, 3.00 mmol)[33] and 4-
(diphenylamino)benzaldehyde (0.840 g, 3.07 mmol) was added t-BuOK
(1.008 g, 8.98 mmol) at 0 °C, and the resulting solution was stirred at room
temperature for 3 h. The solution was extracted with CH2Cl2 and sat NaCl
aq. The organic layer was dried over Na2SO4, filtered, and concentrated in
vacuo, giving
a residue that was subjected to silica gel column
chromatography (eluent: CH2Cl2) to give (E)-4-(4-bromostyryl)-N,N-
diphenylaniline (1.260 g, 98% yield). Yellow solid; 1H NMR (400 MHz,
CDCl3) d 6.91 (d, J = 16.4 Hz, 1H), 7.02-7.06 (m, 5H), 7.08-7.12 (m, 4H),
7.23-7,28 (m, 4H), 7.35 (t, J = 8.4 Hz, 4H), 7.44 (d, J = 8.5 Hz, 2H) ppm.[34]
A mixture of (E)-4-(4-bromostyryl)-N,N-diphenylaniline (0.426 g, 1.0 mmol),
trimethylsilylacetylene (1 mL, 7.0 mmol), Pd(PPh3)4 (114 mg, 0.099 mmol),
CuI (10 mg, 0.053 mmol), and Et3N (10 mL) in THF (30 mL) was stirred at
50 °C under an argon atmosphere overnight. The resulting mixture was
filtered. The filtrate was concentrated in vacuo, giving a residue that was
(E)-1,2-Bis[4-(3,3-dimethylbut-1-yn-1-yl)phenyl]ethene (18): A mixture
of (E)-1,2-bis(4-bromophenyl)ethene (0.20 g, 0.59 mmol, see preparation
of 2), 3,3-dimethyl-1-butyne (0.3 mL, 2.4 mmol), Pd(PPh3)4 (35 mg, 0.03
mmol), CuI (3 mg, 0.016 mmol), PPh3 (15.3 mg, 0.06 mmol), and Et3N (20
mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
chromatography (eluent: CHCl3) followed by HPLC (GPC, eluent: CHCl3)
to give (E)-1,2-bis[4-(3,3-dimethylbut-1-yn-1-yl)phenyl]ethene (18, 0.190 g,
96% yield). White solid; mp 240-241 °C; 1H NMR (500 MHz, CDCl3) d 1.32
(s, 18H), 7.03 (s, 2H), 7.36 (d, J = 8.6 Hz, 4H), 7.40 (d, J = 8.6 Hz, 4H)
ppm; 13C{1H} NMR (125 MHz, CDCl3) d 28.3, 31.1, 77.3, 103.9, 123.2,
124.1, 127.1, 127.7, 127.8, 132.3, 138.8 ppm; IR(KBr) 556, 837, 949, 968,
1120, 1292, 1361, 1458, 1512, 2341, 2866, 2966 cm–1; MS (EI+) m/z
(relative intensity, %) = 155 (10), 325 (38), 340 (100, M+); HRMS (EI+)
calcd for C26H38: 340.2191, found: 340.2198.
subjected to silica gel column chromatography (eluent: hexane:CH2Cl2
=
10:1) to give (E)-N,N-diphenyl-4-{4-[(trimethylsilyl)ethynyl]styryl}aniline
(21, 0.399 g, 90% yield). Yellow solid; mp 121-123 °C. 1H NMR (500 MHz,
CDCl3) d 0.25 (s, 9H), 6.94 (d, J = 16.0 Hz, 1H), 7.02-7.12 (m, 9H), 7.25-
7.28 (m, 4H), 7.37 (d, J = 8.0 Hz, 2H), 7.40-7.44 (m, 4H) ppm; 13C{1H}
NMR (125 MHz, CDCl3) d 0.0, 97.8, 105.2, 121.6, 123.1, 123.3, 124.6,
126.0, 126.1, 127.4, 129.1, 129.3, 131.0, 132.3, 137.8, 147.4, 147.6 ppm;
IR (KBr) 694, 756, 837, 860, 964, 1281, 1493, 1589, 2152, 2341 cm–1; MS
(EI+) m/z (relative intensity, %) = 214 (18), 443 (100, M+); HRMS (EI+)
calcd for C31H29NSi: 443.2069, found: 443.2079.[36]
(E)-1,2-Bis[3,5-bis(3,3-dimethylbut-1-yn-1-yl)phenyl]ethene (19):
A
mixture of (E)-1,2-bis(3,5-dibromophenyl)ethene (0.29 g, 0.59 mmol, see
preparation of 6), 3,3-dimethyl-1-butyne (0.7 mL, 4.8 mmol), Pd(PPh3)4 (70
mg, 0.06 mmol), CuI (6 mg, 0.032 mmol), PPh3 (32 mg, 0.12 mmol), and
Et3N (20 mL) in THF (30 mL) was stirred at 50 °C under an argon
atmosphere overnight. The resulting mixture was filtered. The filtrate was
concentrated in vacuo, giving a residue that was subjected to silica gel
column chromatography (eluent: CHCl3) followed by HPLC (GPC, eluent:
CHCl3) to give (E)-1,2-bis[3,5-bis(3,3-dimethylbut-1-yn-1-yl)phenyl]ethene
(19, 0.281 g, 95% yield). White solid; mp 205-206 °C; 1H NMR (500 MHz,
CDCl3) d 1.32 (s, 36H), 6.97 (s, 2H), 7.33 (s, 2H), 7.40 (s, 4H) ppm; 13C{1H}
NMR (125 MHz, CDCl3) d 27.9, 31.0, 78.2, 98.9, 124.4, 128.3, 128.6, 133.7,
136.9 ppm; IR (KBr) 687, 879, 953, 1269, 1361, 1508, 1585, 1685, 2322,
2866, 2970 cm–1; MS (EI+) m/z (relative intensity, %) = 340 (8), 485 (15),
500 (100, M+); HRMS (EI+) calcd for C38H44: 500.3443, found: 500.3450.
(E)-Trimethyl({4-[4-(trifluoromethyl)styryl]phenyl}ethynyl)silane (22):
To an argon-purged, stirred THF (15 mL) solution of diethyl (4-
bromobenzyl)phosphonate (0.299 g, 0.97 mmol)[33] and 4-
(trifluoromethyl)benzaldehyde (0.270 mL, 2.0 mmol) was added t-BuOK
(0.327 g, 2.91 mmol) at 0 °C, and the resulting solution was stirred at room
temperature for 3 h. The solution was extracted with CH2Cl2 and sat NaCl
aq. The organic layer was dried over Na2SO4, filtered, and concentrated in
vacuo, giving
a
residue that was subjected to silica gel column
chromatography
(eluent: CHCl3) to give (E)-1-bromo-4-[4-
(trifluoromethyl)styryl]benzene (0.293 g, 90% yield). White solid; 1H NMR
(400 MHz, CDCl3) d 7.11 (s, 2H), 7.40 (d, J = 8.4 Hz, 2H), 7.51 (d, J = 8.8
Hz, 2H), 7.57-7.63 (m, 4H) ppm.[37]
A mixture of (E)-1-bromo-4-[4-(trifluoromethyl)styryl]benzene (0.097 g,
0.30 mmol), trimethylsilylacetylene (1 mL, 7.0 mmol), Pd(PPh3)4 (38 mg,
0.033 mmol), CuI (6 mg, 0.032 mmol), PPh3 (20 mg, 0.076 mmol) and Et3N
(10 mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
chromatography (eluent: hexane:CHCl3 = 10:1) to give (E)-trimethyl({4-[4-
(trifluoromethyl)styryl]phenyl}ethynyl)silane (22, 0.040 g, 39% yield). White
solid; mp 109-111 °C; 1H NMR (500 MHz, CDCl3) d 0.26 (s, 9H), 7.11 (d,
J = 16.0 Hz, 1H), 7.15 (d, J = 16.0 Hz, 1H), 7.45-7.49 (m, 4H), 7.58-7.62
(m, 4H) ppm; 13C{1H} NMR (125 MHz, CDCl3) d 0.0, 95.6, 104.9, 122.8,
(E)-N,N-Dimethyl-4-{4-[(trimethylsilyl)ethynyl]styryl}aniline (20): To
an argon-purged, stirred THF (15 mL) solution of diethyl (4-
bromobenzyl)phosphonate (0.304 g, 0.99 mmol)[33] and 4-
(dimethylamino)benzaldehyde (0.298 g, 2.00 mmol) was added t-BuOK
(0.336 g, 2.99 mmol) at 0 °C, and the resulting solution was stirred at room
temperature for 3 h. The solution was extracted with CH2Cl2 and sat NaCl
aq. The organic layer was dried over Na2SO4, filtered, and concentrated in
vacuo, giving
a residue that was subjected to silica gel column
chromatography (eluent: CHCl3) to give (E)-4-(4-bromostyryl)-N,N-
10
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202000397
European Journal of Organic Chemistry
FULL PAPER
124.1 (q, 1J(13C–19F) = 270.6 Hz), 125.8 (q, 3J(13C–19F) = 4.8 Hz), 126.5,
126.6, 128.0, 129.5 (q, 2J(13C–19F) = 29.8 Hz), 130.4, 132.4, 136.7, 140.5
ppm; IR (KBr) 841, 864, 972, 1069, 1126, 1169, 1254, 1323, 1415, 2156
cm–1; MS (EI+) m/z (relative intensity, %) = 157 (6), 329 (100), 344 (86,
M+); HRMS (EI+) calcd for C20H19F3Si: 344.1208, found: 344.1213.
Fluorescence quantum yields: Fluorescence quantum yields in solutions
were obtained by using relative methods.[40] Cyclohexane solutions of 1-4,
6, 7, 12-24 (1.0 × 10−5 M) were degassed by using freeze-pump-thaw
cycles to exclude the influence of dioxygen. Fluorescence spectra were
recorded at 298 K using a 1 cm path length cell. Fluorescence quantum
yields (Ffs) were determined by using 1 as a standard with a known Ff of
0.04 [6]. The Ffs were calculated employing the equation,
(E)-{[4-(4-Methoxystyryl)phenyl]ethynyl}trimethylsilane (23): To an
argon-purged, stirred THF (15 mL) solution of diethyl (4-
bromobenzyl)phosphonate (0.310 g, 1.01 mmol)[33] and p-anisaldehyde
(0.243 mL, 2.00 mmol) was added t-BuOK (0.327 g, 2.91 mmol) at 0 °C,
and the resulting solution was stirred at room temperature for 3 h. The
solution was extracted with CH2Cl2 and sat NaCl aq. The organic layer was
dried over Na2SO4, filtered, and concentrated in vacuo, giving a residue
that was subjected to silica gel column chromatography (eluent: CHCl3) to
give (E)-1-bromo-4-(4-methoxystyryl)benzene (0.185 g, 60% yield). White
solid; 1H NMR (500 MHz, CDCl3) d 3.84 (s, 3H), 6.89-6.92 (m, 3H), 7.07
(d, J = 16.0 Hz, 1H), 7.35 (d, J = 8.0 Hz, 2H), 7.41-7.50 (m, 4H) ppm.[38]
A mixture of (E)-1-bromo-4-(4-methoxystyryl)benzene (0.05 g, 0.173
mmol), trimethylsilylacetylene (1 mL, 7.0 mmol), Pd(PPh3)4 (40 mg, 0.035
mmol), CuI (3 mg, 0.016 mmol), PPh3 (18 mg, 0.069 mmol) and Et3N (10
mL) in THF (30 mL) was stirred at 50 °C under an argon atmosphere
overnight. The resulting mixture was filtered. The filtrate was concentrated
in vacuo, giving a residue that was subjected to silica gel column
Ff(spl) = Ff(std) × [A(std)/A(spl)] × [I(spl)/I(std)] × [n(spl)/n(std)]2
where Ff(spl) and Ff(std) are the fluorescence quantum yields of the
sample and standard, respectively, A(spl), I(spl), and n(spl) are the
absorbance at excitation wavelength (lex), integrated fluorescence
intensities obtained from spectra in which the horizontal axis is converted
from wavelength to wavenumber, and refractive index of solvents. A(std),
I(std), and n(std) and are those of the standard. Excitation wavelengths
(lex = 291.5 nm) and concentrations of solutions of sample and standard
(1.0 × 10−5 M) were identical in each determination.
Fluorescence lifetimes: Fluorescence lifetimes (τf) were determined with
a time-correlated single-photon counting fluorimeter system (C11367-25)
from HAMAMATSU PHOTONICS K.K.
chromatography (eluent: hexane:CHCl3
= 10:1) to give (E)-{[4-(4-
Transient absorption: A XeCl excimer laser (308 nm, Lambda Physik,
Lextra 50) was used as the excitation laser light source. The details of the
detection system for the time profiles of the transient absorption have been
reported elsewhere.[41] Transient absorption spectra were obtained using
a Unisoku USP-T1000-MLT system, which provides a transient absorption
spectrum with one laser pulse.
methoxystyryl)phenyl]ethynyl}trimethylsilane (23, 0.017 g, 32% yield).
White solid; mp 220-222 °C; 1H NMR (500 MHz, CDCl3) d 0.27 (s, 9H),
3.83 (s, 3H), 6.90 (d, J = 8.0 Hz, 2H), 6.92 (d, J = 16.0 Hz, 1H), 7.07 (d, J
= 16.0 Hz, 1H), 7.40-7.46 (m, 6H) ppm; 13C{1H} NMR (125 MHz, CDCl3) d
0.0, 55.3, 94.8, 105.3, 114.2, 121.5, 125.8, 126.0, 127.8, 129.2, 129.8,
132.2, 137.8, 159.4 ppm; IR(KBr) 760, 837, 868, 972, 1034, 1177, 1250,
1512, 1597, 2156, 2341, 2959 cm–1; MS (EI+) m/z (relative intensity, %) =
146 (16), 277 (88), 306 (100, M+); HRMS (EI+) calcd for C20H22OSi:
306.1440, found: 306.1443.
Acknowledgements
(E)-4-{4-[(Trimethylsilyl)ethynyl]styryl}benzonitrile (24): To an argon-
This study was financially supported by a Grant-in-Aid for
Scientific Research (C) (JP20550049, JP23550047, JP26410040,
JP17K05777 to HM) and (B) (JP26288032 and JP18H02043 to
MY) from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of Japan. This study was also supported by
the Collaborative Research Program of the Institute for Chemical
Research, Kyoto University (grant #2013-5, #2014-20). H.M. is
also grateful for financial support from The Murata Science
Foundation, Izumi Science and Technology Foundation, Shibuya
Science Culture and Sports Foundation, A-STEP (Adaptable and
Seamless Technology Transfer Program through target-driven
R&D, JST, Japan), and Kanazawa University SAKIGAKE Project.
We thank Professor Soichiro Kyushin at Gunma University for
helpful discussion about transition dipole moment. Professor
Hiroshi Ikeda and Assistant Professor Yasunori Matsui at Osaka
Prefecture University are acknowledged for helpful discussion
about theoretical calculations. We are also indebted to Professor
Tsuyoshi Asakawa and Associate Professor Akio Ohta at
Kanazawa University for permission to use a UV-vis absorption
spectrophotometer (Hitachi U-2900) and thank Taisuke Shiina
and Rikako Nakada at Kanazawa University for the verification of
13C NMR spectrum of 24.
purged,
stirred
THF
(15
mL)
solution
of
diethyl
(4-
bromobenzyl)phosphonate (0.304 g, 0.99 mmol)[33] and 4-
cyanobenzaldehyde (0.258 g, 1.97 mmol) was added t-BuOK (0.327 g,
2.91 mmol) at 0 °C, and the resulting solution was stirred at room
temperature for 3 h. The solution was extracted with CH2Cl2 and sat NaCl
aq. The organic layer was dried over Na2SO4, filtered, and concentrated in
vacuo, giving
a residue that was subjected to silica gel column
chromatography (eluent: CHCl3) to give (E)-4-(4-bromostyryl)benzonitrile
(0.270 g, 96% yield). White solid; 1H NMR (500 MHz, CDCl3) d 7.08 (d, J
= 16.5 Hz, 1H), 7.15 (d, J = 16.5 Hz, 1H), 7.38-7.42 (m, 2H), 7.50-7.54 (m,
2H), 7.56-7.60 (m, 2H), 7.63-7.67 (m, 2H) ppm.[39]
A
mixture of 4-(4-bromostyryl)benzonitrile (0.097 g, 0.34 mmol),
trimethylsilylacetylene (1 mL, 7.0 mmol), Pd(PPh3)4 (40 mg, 0.035 mmol),
CuI (5 mg, 0.026 mmol), PPh3 (20 mg, 0.076 mmol) and Et3N (10 mL) in
THF (30 mL) was stirred at 50 °C under an argon atmosphere overnight.
The resulting mixture was filtered. The filtrate was concentrated in vacuo,
giving a residue that was subjected to silica gel column chromatography
(eluent:
hexane:AcOEt
=
1:1)
to
give
(E)-4-{4-
[(trimethylsilyl)ethynyl]styryl}benzonitrile (24, 0.042 g, 39% yield). White
solid; mp 184-185 °C; 1H NMR (500 MHz, CDCl3) d 0.26 (s, 9H), 7.09 (d,
J = 16.0 Hz, 1H), 7.17 (d, J = 16.0 Hz, 1H) ,7.45-7.49 (m, 4H), 7.57 (d, J =
8.0 Hz, 2H), 7.64 (d, J = 8.0 Hz, 2H) ppm; 13C{1H} NMR (125 MHz, CDCl3)
d –0.1, 95.9, 104.8, 110.8, 118.9, 123.1, 126.7, 126.9, 127.5, 131.5, 132.4,
132.5, 136.2, 141.5 ppm; IR (KBr) 760, 837, 1250, 2156, 2341 cm–1; MS
(EI+) m/z (relative intensity, %) = 83 (100), 301 (9, M+); HRMS (EI+) calcd
for C20H19NSi: 301.1287, found: 301.1283.
Keywords: Stilbene • Silicon • Absorption • Fluorescence •
Solvatofluorochromism
Photoisomerization of 1-6: C6D6 (0.5 mL) solutions of stilbene derivatives
1-6 (0.005 mmol) in NMR test tubes (5 mmf) were sealed. Then the
solutions were irradiated by using a 450 W high pressure mercury lamp
(Ushio, UM-452) for 6 h at room temperature, maintained by using
circulated cooling water. The ratio of cis/(cis+trans) was determined by the
integral ratio of 1H NMR (400 MHz) of photolysates for each irradiation time.
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Entry for the Table of Contents
Absorption and fluorescence properties of silyl-substituted stilbenes were investigated. The derivative in which four Me3Si groups were
introduced at the ortho positions did not fluoresce at room temperature. The para bis-Me3SiC≡C-substituted analog has a high
fluorescence quantum yield (0.95). The stilbene containing Ph2N and Me3SiC≡C groups exhibited solvatofluorochromism while
maintaining a good emission quantum yield (0.50).
Key Topic
Fluorescence
14
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