Facile synthetic route to highly luminescent sila[7]helicene

Article abstract of DOI:10.1021/ol4005036

A facile synthetic route to dimethylsila[7]helicene by using a Lewis acid catalyzed double-cyclization reaction for construction of the twisted two phenanthrene moieties is described. Sila[7]helicene exhibited a high fluorescence quantum yield and a realatively large g value (dissymmetric factor) of circularly polarized luminencence (CPL) for small molecules.

Full text of DOI:10.1021/ol4005036

ORGANIC
LETTERS
2013
Vol. 15, No. 9
2104–2107
Facile Synthetic Route to Highly
Luminescent Sila[7]helicene
Hiromi Oyama,† Koji Nakano,‡,§ Takunori Harada, Reiko Kuroda,^
Masanobu Naito,§,# Kazuyuki Nobusawa,r and Kyoko Nozaki*,†
Department of Chemistry and Biotechnology, Graduate School of Engineering,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Department
of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture and
Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan, PRESTO, Japan
Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan,
Department of Chemical Engineering, Fukuoka University, Fukuoka 814-0180, Japan,
Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki,
Noda, Chiba 278-8510, Japan, National Institute of Materials Science, 1-1 Namiki,
Tsukuba, Ibaraki, 305-0044, Japan, and Graduate School of Materials Science, Nara
Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan.
nozaki@chembio.t.u-tokyo.ac.jp
Received February 23, 2013
ABSTRACT
A facile synthetic route to dimethylsila[7]helicene by using a Lewis acid catalyzed double-cyclization reaction for construction of the twisted two
phenanthrene moieties is described. Sila[7]helicene exhibited a high fluorescence quantum yield and a realatively large g value (dissymmetric
factor) of circularly polarized luminencence (CPL) for small molecules.
Helicenes, which consist of ortho-fused aromatic rings
having screw-shaped structures, are known as intriguing
chiral frameworks and are intensively studied in wide
ranges of areas.1 Taking advantage of its chirality, unique
optical properties such as nonlinear optical effects
and circular dichroism (CD) have been well studied.2 On
the other hand, the luminescent nature of helicenes has
been much less studied since the rapid intersystem crossing
from the singlet state to the triplet state lowers the fluo-
rescence quantum yield.3 High quantum yields were only
reported for helicene-like molecules in which the π-con-
jugation is not fully extended to the whole molecules.4
Thus, despite their unique chiral structure, circularly po-
larized luminescence properties of helicenes remain rather
unexplored.4,5 Introduction of heteroatom(s) into the
† The University of Tokyo.
‡ Tokyo University of Agriculture and Technology.
§ Japan Science and Technology Agency.
Fukuoka University.
^ Tokyo University of Science.
# National Institute of Materials Science.
r Nara Institute of Science and Technology.
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r
10.1021/ol4005036
Published on Web 04/15/2013
2013 American Chemical Society

fused polycyclic frameworks adds remarkable changes to
the electronic structures of helicenes, which enables fine-
tuning of various physical properties. However, synthesis
of such heterohelicenes still remains a challenging target
despite the recent progress in photoreactions,6 pericyclic
reactions,7 and transition-metal-catalyzed reactions.4,8
Introduction of silole(s), 1-silacyclopenta-2,4-diene(s),
to a helicene framework would be of particular interest
because of the promising high luminescent and electron-
transporting properties of the silole moiety;9 for exam-
ple, silole-fused π-conjugated compounds are known to
achieve high quantum yields in photoluminescence and
high performance as light-emitting diode devices.10 Thus,
development of silole-fused helicenes is expected to afford
characteristic optoelectronic materials, while their synthe-
ses are currently very limited.11 Herein we report a facile
synthetic strategy toward silole-fused [7]helicene and its
fundamental properties including strong luminescence.
A sila[7]helicene 3 was prepared from an easily accessible
starting material, 2,20,6,60-tetrabromobiphenyl12 (Scheme 1).
The biphenyl moiety was first locked with a silicon bridge,
giving dibenzosilole derivative 1. 2-Ethynylphenyl units
were then introduced to 1 through a double-Negishi cross-
coupling reaction and the subsequent deprotection of
TMS groups by using AgNO3. Owing to the coordination
of Ag to ethynyl moiety, TMS group was selectively
cleaved13 and the silole moiety was left unreacted14 in the
Scheme 1. Preparation of Dimethylsila[7]helicene 3
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deprotection condition to give the desired 1,9-bis-
(2-ethynylphenyl)dibenzosilole 2.15 In order to prepare
the highly twistedtwo phenanthrene moieties, Pt-catalyzed
double cyclization of alkynes was applied.16 We optimized
reaction conditions for the double cyclization to find that
the highest yield of the racemic dimethylsila[7]helicene 3
was obtainedin the presenceofPtCl2 in 1,2-dichloroethane
at 80 ꢀC. Previously, we have reported the synthesis of
oxa-, aza-, and λ5-phospha[7]helicenes from biphenanthryl
derivatives by using Pd-catalyzed carbonꢀheteroatom
bond-formation reactions to construct highly twisted
[7]helicene skeletons.17 The present synthetic route, in
which the phenanthrene moieties are constructed after
the preparation of heterole moiety, gives an alternative
and complementary methodology for the synthesis of
heterole-fused helicenes.
The solid-state structure of (P)-sila[7]helicene 3 was
unambiguously determinedby X-ray crystallographic ana-
lysis (Figure 1). Enantiomerically pure (P)-3 and (M)-3
were obtained via optical resolution of rac-3 by using
preparative HPLC on chiral stationary phase. The enan-
tiopure 3 with the shorter HPLC retention time was
confirmed to be (P)-3 by the refinement of the Flack
parameter with data collected by using Cu KR radiation
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(15) NMR spectra of the obtained compound 2 indicated that it was a
mixture of cis- and trans-conformers. All attempts for separation of
these two conformers were unsuccessful.
Org. Lett., Vol. 15, No. 9, 2013
2105

(see Figures S1 and S2, Supporting Information). The
degree of twist of 3 was evaluated by the sum of five di-
hedral angles [—C(1)ꢀC(2)ꢀC(3)ꢀC(4), —C(2)ꢀC(3)ꢀ
C(4)ꢀC(5), —C(3)ꢀC(4)ꢀC(5)ꢀC(6), —C(4)ꢀC(5)ꢀ
C(6)ꢀC(7), and —C(5)ꢀC(6)ꢀC(7)ꢀC(8)]. The sum va-
lue was found to be 99.5ꢀ, which is comparable to the
largest value among those of the related hetero[7]-
helicenes (Figure S5a, Supporting Information).17b This
is due to the large angles between the two double bonds of
silole induced by large atomic diameter of silicon. Such a
large angle causes a large overlap between the two terminal
benzene rings of the phenanthrene moieties and there-
fore a strong steric repulsion (Figure S5b, Supporting
Information). The highly twisted structure of 3 was re-
flected in its high tolerence to racemization. In fact,
thermal racemization does not occur even at 220 ꢀC in
o-dichlorobenzene in sealed tube.
of þ2980 (c = 0.10, CHCl3). The values are larger than
those of the related oxa- and aza[7]helicenes, but smaller
than λ5-phospha[7]helicenes, roughly reflecting the degree
of twist. Circular dichroism (CD) of (P)-3 was revealed
to show a large positive signal around 340 nm, followed by
a small negative shoulder (∼300 nm) and a large negative
signal (∼250 nm) (Figure 2b). Such a tendency in the CD
spectrum is similar to those of λ5-phospha[7]helicenes.17
Time-dependent density functional theory (TD-DFT) cal-
culations at the M06/6-31G(d) level of theory were carried
out, and the intense signal around 340 nm was assigned
to a mixed πꢀπ* transition of HOMOꢀ1 to LUMO
(42%) and HOMO to LUMOþ1 (48%) (Figure 2b and
Figure S4, Supporting Information).
Figure 1. X-ray crystallographic structure of (P)-sila[7]helicene
3 (ORTEP drawing with 50% probability).
Optical properties of the obtained sila[7]helicene 3 were
investigated with UVꢀvis absorption and photolumines-
cence (PL) spectroscopy (Figure 2a). The longest absorp-
tion maximum of 3 was 412 nm, which is longer than
phenanthrene (293 nm) and dibenzosilole (286 nm), sug-
gesting the effective delocalization of π-electrons over
the molecule. The absorption edges of 3 was 431 nm, which
is similar to that of λ5-phospha[7]helicene (432 nm)17b and
red-shifted compared to the related aza- and oxa-
[7]helicenes (425 nm for aza[7]helicene and 409 nm for
oxa[7]helicene).14 Upon excitation at 320 nm, compound 3
exhibited a strong blue luminescence with λmax = 450 nm.
The fluorescence quantum yieldsof3 were notably high for
helicene derivatives (23% in CH2Cl2 and 17% in the solid
state), which is a very desirable feature for applications in
circularly polarized luminescent materials.18 Electrochmi-
cal properties were also investigated with differencial pulse
voltammetry to show two oxidation waves at 1.15 and
1.34 V (vs. Fc/Fcþ), which are attributed to the oxidation
of the two phenanthrene moieties (Figure S3, Supporting
Information).
Figure 2. (a) UV (c = 4.2 ꢁ 10ꢀ5 M), PL (c = 4.2 ꢁ 10ꢀ6 M), CD
(c = 4.8 ꢁ 10ꢀ6 M), and CPL (c = 6.8 ꢁ 10ꢀ6 M) spectra of
sila[7]helicene 3 (CH2Cl2 solution). Blue lines in CD and CPL
spectra: (M)-isomer. Red lines: (P)-isomer. The blue and red
bars show the calculated CD spectra. (b) HOMO and LUMO
orbitals of (P)-3.
Chiroptical properties were also investigated. Specific
rotation of enantiopure sila[7]helicene (P)-3 showed [R]D
Finally, we measured CPL properties of the enantiopure
3 in solution. As shown in Figure 2a, enantiopure sila-
[7]helicene (P)-3 and (M)-3 demonstrated CPL activities
(excited at 320 nm), and their CPL spectra are mirror
(18) (a) Montali, A.; Bastiaansen, C.; Smith, P.; Weder, C. Nature 1998,
392, 261–264. (b) Chen, S. H.; Katsis, D.; Schmid, A. W.; Mastrangelo,
J. C.; Tsutsui, T.; Blanton, T. N. Nature 1999, 397, 506–508.
2106
Org. Lett., Vol. 15, No. 9, 2013

images of each other. The dissymmetric factor g (normalized
difference in emission of right-handed and left-handed
circularly polarized light)19 was determined to be 3.5 ꢁ
10ꢀ3 for 470 nm. Recently, carbon-bridged 1,10-bitriphe-
nylenes, carbo[7]helicenes, have been reported to exihibit
the highest g value (g = 3.0ꢀ3.2 ꢁ 10ꢀ3) among small
organic molecules in nonaggregated state and/or without
host matrix ever reported.4 Accordingly, sila[7]helicene is
also classified into the chiroptical organic molecules with
a high g value and a high fluorescence quantum yield. The
g value of 3 (3.5ꢁ 10ꢀ3) is in the same order as those of
other hetero[7]helicenes (∼1.5 ꢁ 10ꢀ3) such asλ5-phospha-
[7]helicene, oxa[7]helicene, and aza[7]helicene. It is sug-
gestedthat the dissymmetric factor derives mainly from the
helical biphenanthryl moiety while the heterole moiety
plays essential roles in the luminescent properties. How-
ever, these hetero[7]helicenes showed much lower quan-
tum yield in photoluminescence. Thus, the strong emission
and large absolute g value were endowed to sila[7]helicene
owing to the combination of silole moiety and the distorted
helical structure.
acid catalyzed double cyclization reaction. The incorpora-
tion of silole to the helicene framework resulted in a high
luminescence quamtum yield, and characteristic chiroptics
properties. The quantum yield of 3 is the second highest
value for helicenes,4 demonstrating the importance of this
molecular design. Especially, solid state luminescent is
notably high in helicene derivatives (Φ = 17%).20 Further-
more, considering the possible extension of the synthetic
approach (incorporation of other elements and easy
derivatization), it can be concluded that our approach is
promising to open up new aspects in chiroptics materials.
Acknowledgment. This work was supported by a
Grant-in-Aid for Scientific Research on Innovative Areas
“Emergence of highly elaborated π-space and its function”
(21108508) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We thank Prof. Takuzo
Aida (The University of Tokyo) for the CD spectra and DPV
measurement and Prof. Tsuyoshi Kawai (Nara Institute of
Science and Technology) for CPL measurement.
In conclusion, we developed an easily accessible syn-
thetic route to provide silole-fused [7]helicene via a Lewis
Supporting Information Available. Experimental pro-
cedures, crystallographic data, differential pulse voltam-
mogram, and molecular orbitals of 3, the degree of twist
of the related hetero[7]helicenes, 1H and 13C NMR spec-
tra, and X-ray data for 3 (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(19) (a) Riehl, J. P. Chem. Rev. 1986, 86, 1–16. (b) Furumi, S.; Sakka,
Y. Adv. Mater. 2006, 18, 775–780. (c) Jeong, S. M.; Ohtsuka, Y.; Ha,
N. Y.; Takanishi, Y.; Ishikawa, K.; Takazoe, H. Appl. Phys. Lett. 2007,
90, 211106. (d) Circular Dichroism: Principles and Applications, 2nd ed.;
Berova, N., Nakanishi, K., Woodly, R. W., Eds.; Wiley-VCH: New York,
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(20) No Φ values in solid states are reported from Tanaka et al.
The authors declare no competing financial interest.
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