Effects of substituents on silicon atoms upon absorption and fluorescence properties of 1,3,6,8-tetrakis(silylethynyl)pyrenes

Article abstract of DOI:10.1016/j.tetlet.2017.10.008

Synthesis, UV–vis absorption, and fluorescence spectroscopic properties of 1,3,6,8-tetrakis(silylethynyl)pyrenes 2–10 were studied. Absorption maxima of CH₂Cl₂ solutions of these compounds appeared at 437–445 nm, and molar absorption coefficients (ε) of most of these compounds exceeded 10⁵ L mol^?1 cm^?1. Fluorescence emissions measured in dilute CH₂Cl₂ solutions are observed in visible region, and their intensities remarkably increased compared with that of pyrene. Fluorescence spectra obtained from concentrated CH₂Cl₂ solutions exhibited broad excimer emissions when steric bulk of substituents on silicon atoms is sufficiently low. Molecular orbital calculations indicated that HOMO-LUMO energy gap decreased with increasing the number of phenyl groups on silicon atoms, and that the silyl groups act as electron-donating groups to tetraethynylpyrene core. Resonances in ²⁹Si NMR spectra shifts to upfield with increasing the number of phenyl groups on silicon atoms due to the shielding effect of phenyl groups.

Full text of DOI:10.1016/j.tetlet.2017.10.008

Accepted Manuscript
Effects of substituents on silicon atoms upon absorption and fluorescence prop-
erties of 1,3,6,8-tetrakis(silylethynyl)pyrenes
Hajime Maeda, Tomokazu Shoji, Masahito Segi
PII:
S0040-4039(17)31276-5
https://doi.org/10.1016/j.tetlet.2017.10.008
TETL 49370
DOI:
Reference:
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Tetrahedron Letters
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Revised Date:
Accepted Date:
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4 October 2017
Please cite this article as: Maeda, H., Shoji, T., Segi, M., Effects of substituents on silicon atoms upon absorption
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Tetrahedron Letters
jo u r n a l h o m e p a g e : w w w . e ls e v i e r . c o m
Effects of substituents on silicon atoms upon absorption and fluorescence properties
of 1,3,6,8-tetrakis(silylethynyl)pyrenes
Hajime Maeda, Tomokazu Shoji and Masahito Segi
Division of Material Chemistry, Graduate School of Natural Science and Technology Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192,
Japan.
ARTICLE INFO
ABSTRACT
Article history:
Received
Synthesis, UV-vis absorption, and fluorescence spectroscopic properties of 1,3,6,8-
tetrakis(silylethynyl)pyrenes 2-10 were studied. Absorption maxima of CH
2 2
Cl solutions of
Received in revised form
Accepted
Available online
these compounds appeared at 437-445 nm, and molar absorption coefficients () of most of these
5
–1
–1
compounds exceeded 10 Lmol cm . Fluorescence emissions measured in dilute CH
solutions are observed in visible region, and their intensities remarkably increased compared
with that of pyrene. Fluorescence spectra obtained from concentrated CH Cl solutions exhibited
2 2
Cl
2
2
broad excimer emissions when steric bulk of substituents on silicon atoms is sufficiently low.
Molecular orbital calculations indicated that HOMO-LUMO energy gap decreased with
increasing the number of phenyl groups on silicon atoms, and that the silyl groups act as
electron-donating groups to tetraethynylpyrene core. Resonances in Si NMR spectra shifts to
upfield with increasing the number of phenyl groups on silicon atoms due to the shielding effect
of phenyl groups.
Keywords:
Tetrakis(silylethynyl)pyrene
Absorption
2
9
Fluorescence
Excimer
2
9
Si NMR
2
009 Elsevier Ltd. All rights reserved.
Introduction
In previous studies on fluorescence of pyrene derivatives, we
have treated only trimethylsilyl group as a silyl component. In
this study, several 1,3,6,8-tetrakis(silylethynyl)pyrenes having
In recent years, fluorescence properties of silyl- and
silylethynyl-substituted aromatic hydrocarbons get a great deal of
attention. In most cases, fluorescence intensity increases when
silyl and silylethynyl groups are attached on aromatic rings such
1-4
1,5,6
1,7
8
as naphthalene, anthracene,
phenanthrene, and perylene
fluorophores. Our previous studies also indicate that when silyl
and silylethynyl groups are attached on pyrene, fluorescence
1
,9-14
intensity dramatically increases.
quantum yield ( ) of pyrene is 0.32, however, those of 1,3,6,8-
tetrakis(trimethylsilyl)pyrene and 1,3,6,8-
tetrakis(trimethylsilylethynyl)pyrene increase up to 0.56 and
For example, fluorescence
15
f
1
11
0
.99, respectively.
The increasing phenomenon in fluorescence quantum yield is
not fully understood but interpreted as follows. Silyl groups act
to increase transition moments by - and *-* conjugation and
16
extend -systems of arenes. In addition, silicon atom does not
hardly suffer heavy atom effect unlike germanium and tin atoms,
that causes intersystem crossing to enhance decay path to excited
1,10,17
triplet state.
Effect of acetylene linkage is attributed to
extension of -conjugation and relaxation of steric hindrance
11
between arenes and silyl groups, however, when more than two
acetylene groups are attached, decay path to excited triplet via
3,8,14
intersystem crossing often increases.
Thus, interpretation of
effect of silicon and acetylene linkage is progressing, however,
study of effect of substituents on silicon atoms is not enough.
Scheme 1. Synthesis of 1,3,6,8-tetrakis(silylethynyl)pyrenes 2-
10.
—
——

Corresponding author. Tel.: +81-76-264-6290; fax: +81-76-234-4800; e-mail: maeda-h@se.kanazawa-u.ac.jp

2
Tetrahedron Letters
–
5
sevral kinds of substituents on silicon atoms are prepared, and
effects of substituents on silicon atoms upon absorption and
fluorescence properties are investigated.
UV-vis absorption spectra of 1-10 are measured in 1 x 10
CH Cl solutions (Figure 2, Table 1). The absorption maxima of
2-10 shift by 100-108 nm to longer wavelengths compared with
that of 1, and reach to visible region. MeO and CF groups on
M
2
2
3
Results and discussion
phenyl ring do not appreciably influence on the absorption
maxima. Most of molar absorption coefficients () of 2-10
showed more than 1 x 10 Lmol cm , indicating that these are
molecules that absorb light very efficiently.
Synthesis of 1,3,6,8-tetrakis(silylethynyl)pyrenes is shown in
5
–1
–1
Scheme 1. Bromination of pyrene (1) with Br
2
(4 equiv) gave
,3,6,8-tetrabromopyrene (11). Sonogashira coupling reaction of
1 with trimethylsilylacetylene produced 1,3,6,8-
1
1
tetrakis(trimethylsilylethynyl)pyrene (3). Desilylation of 3 by
KOH/MeOH gave 1,3,6,8-tetraethynylpyrene (12), which was
lithiated and then silylated to give various 1,3,6,8-
tetrakis(silylethynyl)pyrenes (2, 4-10). For measurement of
absorption and fluorescence, extremely pure samples purified by
silica gel column chromatography, recycling preparative HPLC
(GPC), and recrystallization were used.
–
5
The colors of selected compounds in solid state and 1 x 10
2 2
M CH Cl solutions under UV irradiation conditions are shown
in Figure 1. Visual color of the solids turns from orange to
yellow in the order of increasing bulkiness of substituents on
silicon atoms. The hypsochromic shift might be associated with
the aggregation pattern that is determined by bulkiness of
1
8
substituents. Strong emission of fluorescence can be seen from
CH Cl solutions of 1,3,6,8-tetrakis(silylethynyl)pyrenes under
–5
Figure 2. UV-vis absorption spectra of 1-10, 1.0 x 10 M in
aerated CH Cl .
2 2
2
2
UV irradiation with the naked eye, although fluorescence from
unsubstituted pyrene 1 is weakly visible under the same
–
5
Figure 3. Fluorescence spectra of 1-10, 1.0 x 10 M in aerated
CH Cl
2
2
, ex = abs in Table 1.
2 2
Figure 1. Colors of (a) solids and (b) fluorescence in CH Cl
–
5
solutions (1.0 x 10 M, ex = 254 nm) of selected compounds.
conditions.
29
Table 1. Photophysical properties, calculated data, and Si NMR chemical shifts of compounds 1-10.
a
d
absorption
fluorescence
calculation
29
e
Si
f
comp
ound
a,b
b,c
energy
gap
charge density

abs
monomer
excimer

HOMO LUMO
1
2
3
NMR
 (ppm)
log 
1
2
3
R R R
Si
(
nm)

em (nm)
em (nm)
(eV)
(eV)
Si
R R R
(
eV)
1
2
3
4
5
6
7
8
9
337
437
437
439
439
440
440
440
442
445
4.69
4.89
5.00
5.02
5.13
5.05
5.06
5.16
5.03
5.12
370
445
442
444
448
446
448
449
448
452
474
587
559
559
542
554
558
555
557
–7.217
–7.131
–7.043
–7.040
–6.999
–7.169
–7.170
–7.432
–7.087
–7.033
1.896
0.657
0.745
0.748
0.780
0.623
0.621
0.351
0.700
0.735
9.113
7.788
7.788
7.788
7.779
7.792
7.791
7.783
7.787
7.768
–
–
–
–
–
g
1.678
2.012
2.139
2.226
2.097
2.093
2.098
2.217
2.354
–1.042
–1.342
–1.442
–1.505
–1.420
–1.416
–1.428
–1.534
–1.662
0.636
0.670
0.697
0.721
0.677
0.677
0.670
0.683
0.692
–17.4
–7.4
–
–21.1
–21.5
–20.6
–25.1
–28.5
g
h
1
0
[
nd
a
–5
1-10] = 1 x 10 M in CH
2
Cl
2
.
b
c
d
e
f
Excitation wavelength (ex) is abs in this Table.
[
2 2
1-10] = saturated concentration in CH Cl .
Calculated by HF/3-21G.
Mülliken charge.
1
00 MHz, in CDCl
Not measured.
Not detected.
3
.
g
h

–
5
Fluorescence spectra of 1-10 are also measured in 1 x 10
CH Cl solutions (Figure 3, Table 1). Fluorescence intensities of
,3,6,8-tetrakis(silylethynyl)pyrenes 2-10 are remarkably strong
compared with that of 1, and fluorescence quantum yields are
almost quantitative ( =0.9~1). Stokes shifts are only 5-9 nm,
M
silicon atoms increases, the energy gap between HOMO and
LUMO decreases (6 > 9 > 10), and bathocromic shifts of
absorption maxima are observed. In addition, when the number
of aromatic groups on silicon atoms increases, positive charge
2
2
1
1
2
3
f
density on both silicon (Si) and silyl group (R R R Si) increases.
It means that electron donating ability of silyl groups increases
with increasing the number of aromatic groups on silicon atoms.
indicating that structural change upon excitation is very small.
The difference of fluorescence intensity depending on
substituents was not so particularly observed.
29
Si NMR chemical shifts of these compounds are all negative,
and the resonances are shifted upfield with increasing the number
of aromatic groups on silicon atoms. For example, resonances of
Si on 3, 6, 9 and 10 appear at –17.4, –21.1, –25.1, and –28.5
It is known that when fluorescence of pyrene is measured
under concentrated conditions, only excimer (excited dimer)
19,20
emission is observed.
Figure 4 shows results of fluorescence
ppm, respectively (Figure 6). This is in good agreement with the
measurements of compounds 1-10 at saturated concentration in
29
–
2
order of Si NMR resonances of Me
4 3
Si (0 ppm) > PhMe Si (–4.5
CH
2 2
Cl (ca. 1-2 x 10 M). Compounds 3, 4, and 6-9 shows
ppm) > Ph
interpreted to shielding effect by electronegative substituents.
2 2 3
Me Si (–6.5 ppm) > Ph MeSi (–11.5 ppm), which is
typical broad excimer emissions whose maxima appears at 554-
22,23
5
1
59 nm. These are 80-85 nm longer than excimer emission from
. On the other hand, excimer emission from 5 (i-Pr Si) is weak,
Si) is not observed. These results can be
3
and that from 10 (Ph
3
explained by disturbance of formation of excimers because of
bulkiness of substituents on silicon atoms. Excimer fluorescence
from 2 (Me HSi) is observed at the longest wavelength among
2
the investigated derivatives, whose maxima appears at 587 nm.
This result is associated with the formation of more stable
excimer, due to easy approach of two molecules by small steric
hindrance. There are some suggested structures of pyrene
excimers such as parallel and orthogonal, sandwich type and
20,21
partial overlap structures.
Although we do not have any
evidence for the structures of excimers, we would like to suggest
possible structures of excimers formed from 1,3,6,8-
tetrakis(silylethynyl)pyrenes as shown in Figure 5. Silylethynyl
groups might prevent close approach of the two molecules, with
keeping parallel - interaction of two tetraethynylpyrene cores.
29
Figure 6. Si NMR spectra (100 MHz) of (a) 9 and (b) 3 in
CDCl
3
.
Figure 4. Fluorescence spectra of 1-10, saturated concentration
in aerated CH Cl , ex = abs in Table 1.
2 2
Figure 5. Possible structure of excimers.
Figure 7. Molecular orbitals and energy levels of HOMO and
LUMO calculated by HF/3-21G.
HOMO and LUMO levels, energy gaps of their levels, charge
densities on silicon (Si), those of substituents on silicon
1
2
3
1
2
3
(R R R ), and their sum (R R R Si) obtained from molecular
Molecular orbitals and energy levels of HOMO and LUMO of
orbital calculations are listed in Table 1. Energy gap between
HOMO and LUMO is influenced by the number of aromatic
groups on silicon atoms. As the number of aromatic groups on
1
, 3, 10 and 12 calculated by HF/3-21G are shown in Figure 7.
Energy gap between HOMO and LUMO decreases in the order

4
Tetrahedron
of 1 > 12 > 3 > 10, which indicates that conjugation is extended
References and notes
in the order of 1 < 12 < 3 < 10. Energy levels of HOMO and
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electronegativity. HOMO and LUMO levels of 3 and 10 are
higher than those of 12, indicating that silyl groups behave as
electron-donating groups. This is also supported by calculated
positive charge density on silyl groups, for example, 0.670 (3)
and 0.692 (10).
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visible dyes with high fluorescence quantum yields, without
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1
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1
x 10 M CH
2
Cl
2
showed significantly larger fluorescence
Cl
intensity than that of 1. Excimer emissions obtained in CH
2
2
2
2
under saturated concentration depended on the bulkiness of silyl
groups. From moleccular orbital calculations, it is concluded that
silyl groups act as electron-donating substituents and influence to
extension of -conjugation. Si NMR chemical shifts of selected
compounds indicate that the shielding effect by phenyl groups on
silicon atoms is involved. Further studies for the utilities of these
visible dyes with high fluorescence quantum yields are now
under investigation.
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Acknowledgments
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This study was financially supported by Grant-in-Aids for
Scientific Research (C) (20550049, 23550047, 26410040,
Supplementary Material
1
7K05777), from the Ministry of Education, Culture, Sports,
Supplementary data associated with this article (synthetic
procedures and spectral data of compounds 2-10) can be found in
the online version, at http..
Science and Technology (MEXT) of Japan. This work was also
supported by the Collaborative Research Program of Institute for
Chemical Research, Kyoto University (grant # 2013-5, # 2014-
20). H.M. is also grateful for financial support from The Mazda
Foundation, A-STEP (Adaptable and Seamless Technology
Transfer Program through target-driven R&D, JST), and
Kanazawa University SAKIGAKE Project.

3
Highlights
Fluorescence intensities of 1,3,6,8-tetrakis(silylethynyl)pyrenes are very large.
Excimer emissions are observed when steric bulk of silyl groups is low.
Electron-donating characters of silyl groups are evaluated by calculations.
Graphical Abstract
Effects of substituents on silicon atoms upon
absorption and fluorescence properties of
1
,3,6,8-tetrakis(silylethynyl)pyrenes
Hajime Maeda*, Tomokazu Shoji and Masahito Segi