Synthesis and properties of disiloxane-bridged cyclophanes bearing heteroaromatics

Article abstract of DOI:10.1246/cl.121271

Disiloxane-bridged cyclophanes 13 bearing heteroaromatics were successfully synthesized. Among them, 3 afforded pronounced excimer fluorescence as well as monomer fluorescence in contrast with 1 and 2. The excimer formation in 3 is accomplished by the fle

Full text of DOI:10.1246/cl.121271

doi:10.1246/cl.121271
Published on the web March 19, 2013
401
Synthesis and Properties of Disiloxane-bridged Cyclophanes Bearing Heteroaromatics
Shin-ichiro Kato, Nanae Yamazaki, Toshihiro Tajima, and Yosuke Nakamura*
Department of Chemistry and Chemical Biology, Graduate School of Engineering,
Gunma University, Kiryu, Gunma 376-8515
(Received December 25, 2012; CL-121271; E-mail: nakamura@gunma-u.ac.jp)
Disiloxane-bridged cyclophanes 1-3 bearing heteroaro-
matics were successfully synthesized. Among them, 3 afforded
pronounced excimer fluorescence as well as monomer fluo-
rescence in contrast with 1 and 2. The excimer formation in 3 is
accomplished by the flexibility of disiloxane linkages and the
relatively long lifetime of the excited singlet state.
All of 1-3 were successfully synthesized by the lithiation of
the corresponding dibromides followed by treatment with 1,3-
dichloro-1,1,3,3-tetramethyldisiloxane (Scheme 1). They were
isolated by column chromatography or recrystallization and
characterized by various spectroscopic methods.17 The low
yields are due to the formation of oligomeric or polymeric
products. It is noteworthy that 3 is much more stable than
cyclophane 3¤ bearing -CH2OCH2- linkages (Chart 2): the latter
gradually decomposed during purification. The increased stabil-
ity in 3 is apparently derived from the introduction of rather
bulky disiloxane-linkages.
The construction of cyclophane frameworks can bring new
properties or functions mainly arising from the transannular
interaction between the constituent aromatics.1 We have so far
synthesized cyclophanes composed of various aromatic nuclei
such as phenanthrene,2-5 carbazole,6-9 and thiophene,10 and
investigated their structures and photophysical properties. Their
photophysical properties, especially fluorescence emission be-
havior, are remarkably affected by the extent of overlap or
distance between the two aromatic nuclei. In these cyclophanes,
the two aromatic nuclei are bridged by carbon-based linkages
such as cyclobutane rings or -CH2OCH2- linkages. The
introduction of silicon atoms as bridging linkages is expected
to alter the structures and the electronic and photophysical
properties of cyclophanes due to the steric and electronic effects
of silicon atoms.11,12 For instance, it is known that the
substitution of silyl groups on some aromatics enhances the
fluorescence intensity.13 From steric aspects, the silicon atoms
may increase the conformational flexibility of cyclophanes.
Herein, we have examined the synthesis and properties of
disiloxane-bridged carbazolophane 1, in order to clarify the
effects of silicon atoms on the electronic and photophysical
properties of carbazolophanes (Chart 1). In addition, 2 and 3
bearing dibenzothiophene and dibenzofuran nuclei, respectively,
were also synthesized. Examples of disiloxane-bridged cyclo-
phanes bearing heteroaromatics have been limited,14,15 although
some aromatic hydrocarbon-based disiloxane-bridged cyclo-
phanes are known.16
1
In the H NMR spectra of 1-3, the aromatic Ha protons for
1-3 are downfield-shifted (¦¤ = 0.23-0.36) relative to those of
the corresponding bis(trimethylsilyl) (bisTMS) derivatives 4-6,
while the Hb and Hc protons are only slightly shifted. These
observations suggest that the overlap between the two aromatic
nuclei is quite small. If the two aromatic nuclei were well
overlapped, the aromatic protons would be generally upfield-
shifted. The downfield shift of Ha results from the deshielding
effect of the opposite aromatic ring. Noticeably, one singlet
signal for the methyl protons on the silicon atoms was observed.
This demonstrates that the rapid conformational changes in 1-3
occur in the NMR time scale at room temperature.18 The
interconversion between the syn- and anti-conformers by the
ring flipping seems to be one of the possible conformational
changes in 1-3. DFT calculations predicted that both the syn-
and anti-conformers in 1-3 are ground-state structures and the
anti-conformer is energetically more favorable than the syn-
conformer (see Figure S8 in the Supporting Information).19
Overall, the anti-conformer is believed to be predominant in
1-3 in a solution phase.
X-ray crystallographic analysis demonstrated that 1-3 adopt
the anti conformation in the solid state as expected (Figure 1).20
Br
Me Me
1) n-BuLi, 2) Cl
Si O Si Cl
Me Me
Me Me
Me Me
Me Me
Hb
Me
Me
Me
Me
Me
Me
1: X = NEt (3%)
2: X = S (8%)
3: X = O (17%)
Si
Si
Si
Si
Si
Si
O
O
O
Hc
Et
X
THF or THF/Et2O, -78 °C to r.t.
(1,2) (3)
Ha
N
N
Et
S
S
O
O
Br
O
O
O
Si
Si
Si
Si
Si
Si
Me
Me
Me
Me
Me
Me
Me Me
Me Me
Me Me
Scheme 1. Synthesis of 1-3.
1
2
3
SiMe3
SiMe3
SiMe3
O
O
O
Et
N
S
O
O
SiMe3
SiMe3
SiMe3
4
5
6
3'
Chart 1.
Chart 2.
Chem. Lett. 2013, 42, 401-403
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

402
Figure 1. X-ray crystal structures of (a) 1, (b) 2, and (c) 3.
The structures of 1-3 resemble one another: the two aromatic
nuclei take an offset conformation with slight overlap, although
the interplanar distance between them is ca. 3.0 ¡. The arrange-
ment of two carbazole nuclei in 1 is also similar to that of the
carbazolophane bearing -CH2OCH2- linkages by Tani et al.21
The absorption spectra of 1-3 are illustrated in Figure 2
along with those of the corresponding bis(TMS) derivatives 4-6.
The spectral shapes of 1-3 are almost the same as those of 4-6,
respectively, with almost twofold absorbance. This behavior
indicates that the intramolecular interaction between the aro-
matics is small in the ground state, as suggested by the 1H NMR
spectra and X-ray structures. However, the longest absorption
maximum of 1 is only slightly red-shifted as compared to 4
accompanying the small hyperchromic effect, probably resulting
from the head-to-tail arrangement of the transition dipole
moments of the carbazole chromophores in the anti conforma-
tion of 1 as pointed by Tani et al.21
The fluorescence spectra of 1-3 are also shown in Figure 2.
Similar to 4 and 5, both 1 and 2 exhibited the monomer-like
emission with vibrational structures characteristic of carbazole
or dibenzothiophene, reflecting the small overlap of aromatics
in the predominant anti-conformers.22 Interestingly, 3 afforded
broad, red-shifted excimer-like emission23 with marked intensity
along with monomer-like emission, in contrast with 1 and 2,
suggesting the conversion from the anti- to syn-conformer in the
excited state of 3, Thus, 3 partially adopts the syn conformation
necessary for the excimer formation in the excited state.
Such dynamic behavior in 3 was confirmed by fluorescence
lifetime measurements. When monitoring the emission at
420 nm, predominantly corresponding to the excimer emission,
Figure 2. Absorption and fluorescence spectra of 1-6 in
CH2Cl2 at room temperature. The concentrations of compounds
¹1
were 1 © 10¹5-1 © 10¹6 mol L
.
the rise component of ¸ = 0.78 ns was observed along with the
decay component of ¸ = 7.6 ns (Figure S6).24 On monitoring the
emission at 350 nm, mainly in the region of monomer emission,
the decay component of ¸ = 0.80 ns was detected (Figure S5).24
Such rise and decay components definitely demonstrate the
excimer formation from monomer. The emission spectra at 77 K
also support the dynamic behavior in 3. Noticeably, excimer-like
emission was missing in 2-methyltetrahydrofuran (MTHF) at
77 K (Figure S4).24 This observation apparently indicates that
the excimer forms via the conformational change in the excited
state, since the conformation was frozen in a rigid matrix at
77 K.
The observation of remarkable excimer fluorescence in 3 is
probably ascribed to the longer lifetime of the singlet excited
state of dibenzofuran than dibenzothiophene: ¸f for 6 is 4.5 ns,
while that for 5 is 0.7 ns. Within the lifetime of the excited state,
3 can undergo the conformational change. This conformational
change is facilitated by the flexible disiloxane-linkages.
Chem. Lett. 2013, 42, 401-403
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

403
Furthermore, the ratio of monomer and excimer emission
intensity in 3 was dependent on solvents (Figure S3).24 The
relative intensity of excimer emission in less polar cyclohexane
is lower than that in dichloromethane. The nature of solvents
is probably associated with the feasibility of conformational
change and/or the stability of the overlapped conformation.
In summary, we have successfully synthesized disiloxane-
bridged cyclophanes 1-3 bearing heteroaromatics. Noticeably, 3
afforded pronounced excimer fluorescence as well as monomer
fluorescence in contrast with 1 and 2. The excimer formation in
3 is accomplished by the flexibility of disiloxane linkages and
the relatively long lifetime of the excited singlet state. The use of
disiloxane linkages can endow cyclophanes with fascinating
properties.
Mézailles, L. Ricard, F. Mathey, P. Le Floch, J. Am. Chem.
Soc. 2001, 123, 6654. b) A. Moores, C. Defieber, N.
Mézailles, N. Maigrot, L. Ricard, P. Le Floch, New J. Chem.
2003, 27, 994.
15 Quite recently, disilane-bridged carbazolophanes were re-
ported. W. Nakanishi, S. Kamata, S. Hitosugi, H. Isobe,
Chem. Lett. 2012, 41, 1652.
16 a) H. Maeda, H. Yagi, K. Mizuno, Chem. Lett. 2004, 33, 388.
b) K. Imai, S. Hatano, A. Kimoto, J. Abe, Y. Tamai, N.
Nemoto, Tetrahedron 2010, 66, 8012.
17 Selected spectral data of 1-3 are as follows: 1: 1H NMR
(600 MHz, CDCl3): ¤ 8.66 (s, 4H), 7.64 (d, J = 8.0 Hz, 4H),
7.44 (d, J = 8.0 Hz, 4H), 4.39 (q, J = 7.2 Hz, 4H), 1.45 (t,
J = 7.2 Hz, 6H), 0.37 (s, 24H); 13C NMR (CDCl3, 125 Hz): ¤
140.88, 130.21, 128.85, 125.57, 122.89, 107.91, 37.50,
13.79, 0.99; UV-vis (CH2Cl2): -max (¾) 269 (57800), 299
(12700), 333 (4700), 348 (5300) nm; HR-FAB-MS (NBA,
positive): m/z calcd for C36H46O2N2Si4 650.2600, found
650.2600 ([M + Na]+). 2: 1H NMR (CDCl3, 300 MHz): ¤
8.61 (s, 4H), 7.79 (d, J = 7.9 Hz, 4H), 7.53 (d, J = 7.9 Hz,
4H), 0.47 (s, 24H); 13C NMR (CDCl3, 125 Hz): ¤ 140.55,
134.88, 134.78, 130.83, 126.06, 122.16, 0.74; UV-vis
(CH2Cl2): -max (¾) 260 (36600), 281 (9200), 292 (11300),
315 (3800), 328 (4300) nm; MALDI-TOF-MS (Dith, posi-
tive): m/z 629.46 [(M + H)+]; Anal. Calcd (%) for
C32H36O2S2Si4¢0.07CHCl3: C, 60.43; H, 5.70%. Found C,
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan (No. 24550040). The authors
thank Dr. Toshitada Yoshihara and Prof. Dr. Seiji Tobita (Gunma
Univ.) for the measurements of emission spectra at low
temperatures.
References and Notes
1
a) Cyclophanes I in Topics in Current Chemistry, ed. by F.
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1
60.45; H, 6.41%. 3: H NMR (600 MHz, CDCl3): ¤ 8.45 (s,
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2
3
4
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(¾) 255 (30300), 286 (32100), 290 (32900), 301
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18 Similar fast conformational changes were observed in the
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19 The energy differences between the syn- and anti-conformers
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20 CCDC-905888 (1), CCDC-905890 (2), and CCDC-905889
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Chem. Lett. 2013, 42, 401-403
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/