Air-stable, room-temperature emissive disilenes with π-extended aromatic groups

Article abstract of DOI:10.1021/ja108094m

π-Conjugated disilenes with 2-naphthyl or 2-fluorenyl groups on the silicon atoms have been synthesized as air-stable emissive red solids using the bulky 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) groups. The strong π-π* absorptions and distinct

Full text of DOI:10.1021/ja108094m

Published on Web 10/13/2010
Air-Stable, Room-Temperature Emissive Disilenes with π-Extended Aromatic
Groups
Megumi Kobayashi,† Tsukasa Matsuo,*,† Takeo Fukunaga,‡ Daisuke Hashizume,‡ Hiroyuki Fueno,§
Kazuyoshi Tanaka,§ and Kohei Tamao*,†
Functional Elemento-Organic Chemistry Unit and AdVanced Technology Support DiVision, RIKEN AdVanced
Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, and Department of Molecular Engineering,
Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan
Received September 8, 2010; E-mail: matsuo@riken.jp; tamao@riken.jp
Abstract: π-Conjugated disilenes with 2-naphthyl or 2-fluorenyl
groups on the silicon atoms have been synthesized as air-stable
emissive red solids using the bulky 1,1,3,3,5,5,7,7-octaethyl-s-
hydrindacen-4-yl (Eind) groups. The strong π-π* absorptions and
distinct emission at room temperature, both in solution and in the
solid state, have been observed due to the substantial contribution
of the 3pπ*(Si-Si)-2pπ*(carbon π-electron system) conjugation.
The di(2-naphthyl)disilene 3 and di(2-fluorenyl)disilene 4 were
straightforwardly synthesized by the reduction of the corresponding
dibromosilanes with lithium naphthalenide in THF (eq 1). After
removal of any insoluble materials and naphthalene, the disilenes
3 and 4 with an E configuration were isolated as red crystals in
moderate yields, respectively. The disilenes 3 and 4 are quite air-
stable; thus in the solid state, they can survive for more than a half
year with no detectable change as confirmed by the 1H NMR
spectra. In a dilute hexane solution (ca. 10-5 mol/L), they
decompose upon exposure to air, but slowly with a half-lifetime
of ca. 2-4 h, much longer than that of Tip2SidSiTip2 (17 min),10
as estimated by the UV-vis spectroscopy, indicative of the excellent
protection ability of the Eind group.
In 1981, West et al. introduced a concept of kinetic protection
of the otherwise highly reactive SidSi double bond by bulky mesityl
groups, as well as Brook’s and Yoshifuji’s discoveries of the SidC
and PdP double bond compounds, respectively.1 Since then, a
variety of SidSi double bond molecules have been synthesized
employing the appropriately designed bulky groups2,3 or bulky
stable carbenes.4
During the course of our study on the π-conjugated organic
architectures containing the heavy main group elements,5 i.e.,
elemento-organic hybrid materials, we have recently started to
investigate new π-conjugated disilenes and phosphasilenes, com-
prising the SidSi and SidP chromophores.6 Thus, in 2007,
Scheschkewitz’s group7 and we8 independently reported the
synthesis of model systems of disilene analogs of oligo(p-
phenylenevinylene)s (Si-OPVs). In our case, for the synthesis of
diphenyldisilene 1 and 1,4-bis(disilenyl)benzene 2, we developed
the rigid, fused-ring bulky 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-
4-yl (Eind) group as a new protecting group; the Eind groups
encapsulate the reactive SidSi moieties and produce the highly
coplanar π-conjugated frameworks by their perpendicular orienta-
tion to and the interlocking ethyl side chains above and below the
π-frameworks. Accordingly, the 1,4-bis(disilenyl)benzene 2 exhibits
an orange fluorescence even at room temperature due to the efficient
π-conjugation over the skeleton. It is emphasized that such a
coplanar π-framework involving the SidSi chromophore has so
far been achieved only by our Eind group.
Figure 1. Molecualr structures of 3a (top) and 3c (bottom): space filling
models (left), top views (center), and side views (right).
Following the successful achievement of 2 containing two
disilene moieties on the central benzene ring, we have focused on
the photophysical properties of the π-conjugated disilenes by
installation of two π-extended aromatic groups to the central
monodisilene unit.9 We now report the isolation and characterization
of Eind-substituted air-stable disilenes with 2-naphthyl or 2-fluo-
renyl groups, which do contain the highly coplanar π-frameworks
and exhibit extensive emissions at room temperature both in solution
and in the solid state.
The X-ray crystallography of 3 showed that the Si atoms and
the naphthyl groups are disordered over the two positions in the
crystal, corresponding to a mixture of two rotational isomers, s-cis,
s-cis (3a) and s-trans, s-trans (3c), with the occupancy factors of
ca. 0.4/0.6.11 Each molecule has an inversion center at the center
of the SidSi bond. Another s-cis, s-trans isomer (3b) has not been
found in the crystal. Figure 1 shows the molecular structures of 3a
and 3c, together with the space filling models, which demonstrate
that the proximate ethyl groups of the Eind groups effectively
protect the SidSi fragment. Each isomer has an essentially coplanar
† RIKEN, Functional Elemento-Organic Chemistry Unit.
‡ RIKEN, Advanced Technology Support Division.
§ Kyoto University.
9
15162 J. AM. CHEM. SOC. 2010, 132, 15162–15163
10.1021/ja108094m  2010 American Chemical Society

C O M M U N I C A T I O N S
di(2-naphthyl)disilene framework; the Si-Si-C-C torsion angles
are 2.25(14)° for 3a and 9.57(11)° for 3c, respectively. The SidSi
bond distances of 2.1623(18) Å for 3a and 2.1667(12) Å for 3c
are in the range of those for typical disilenes.2 Similar rotational
isomers have also been found in the crystal of 4.11
appreciable contribution of the 3pπ*(Si-Si)-2pπ*(naphthyl) conjuga-
tion. Thus, while the HOMO levels of 3a-c (-4.249, -4.286, and
-4.304 eV) are comparable to that of 1 (-4.253 eV), the LUMO levels
of 3a-c (-1.554, -1.488, and -1.478 eV) are lower than that of 1
(-1.330 eV), which are in good agreement with the electrochemical
experimental results already mentioned.
Table 1. Photophysical Data of Disilenes 1-4
The outstanding stability15 of 3 and 4 would provide opportuni-
ties for future electronic and optoelectronic applications. Further
investigations are now in progress.
λ
max(abs)/
nm
Stokes shift/
cm-1
compounda
ꢀ/cm-1 M-1
λ
max(em)/nm (ΦF)
1
2
3
hexane
hexane
THF
solid
THF
solid
461
543
504
2.4 × 104
3.0 × 104
2.5 × 104
3.0 × 104
n.d.b
Acknowledgment. We thank the Ministry of Education, Culture,
Sports, Science, and Technology of Japan for the Grant-in-Aid for
Specially Promoted Research (No. 19002008). We also thank Dr.
Yayoi Hongo, Mr. Takashi Nakamura, Dr. Masayoshi Nishiura,
and Dr. Zhaomin Hou (RIKEN) for their kind help with the mass
spectrometry, solid-state NMR spectroscopy, and thermal analysis.
The numerical calculations were partly performed at the Super-
computer Laboratory, Institute for Chemical Research, Kyoto
University.
612 (0.10)
586 (0.01)
619 (0.23)
575 (0.01)
600 (0.20)
2080
2780
4
510
2200
a Solution or solid. b Not detected.
Some photophysical and electrochemical features are found as
follows: (1) The UV-vis spectrum of 3 in THF shows an absorption
maximum at 504 nm (ꢀ ) 2.5 × 104), which is 43 nm red-shifted
from that of 1 (λmax ) 461 nm),8 indicating the π-conjugation
extension over the di(2-naphthyl)disilene skeleton (Table 1). The
absorption peak of 4 appears at 510 nm with a higher molar
extinction (ꢀ ) 3.0 × 104). (2) The disilenes 3 and 4 exhibit a
weak but distinct fluorescence at room temperature in THF, thus
in contrast to the fact that the diphenyldisilene 1 does not show
any fluorescence in solution. The emission maxima are found at
586 nm for 3 and 575 nm for 4. The Stokes shifts are calculated to
be 2780 cm-1 for 3 and 2220 cm-1 for 4, which are comparable to
that of 2 (2080 cm-1)8 but much lower than those observed for
Supporting Information Available: Experimental details, CIF for
3 and 4, DFT calculations, and full listing for ref 14. This material is
available free of charge via Internet at http://pubs.acs.org.
References
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(3) The disilenes exhibit strong emissions in the solid state, as shown
in Figure 2. The quantum yields (ΦF) are 0.23 for 3 and 0.20 for
4, respectively, 20 times stronger than those in solution. The weaker
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waves are observed at -2.50 V for 3 and -2.59 V for 4, whose
values are lower than that of 1 (-2.66 V).
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Figure 2. Photographs of 3 and 4 in air.
DFT calculations were performed for 3a-c at the B3LYP/6-
31G(d,p) level using the Gaussian 03 suite of programs.14 The
optimized structures of 3a and 3c reproduced the X-ray molecular
structures. The missing isomer 3b is also found to be a local minimum
with a coplanar di(2-naphthyl)disilene skeleton.11 These isomers have
nearly the same energies; the relative energies are 0.00 (3c), 0.72 (3b),
and 1.70 (3a) kcal mol-1. Whereas the HOMOs are essentially
represented by the 3pπ(Si-Si) orbital, the LUMOs involve the
(13) (a) Shepherd, B. D.; West, R. Chem. Lett. 1988, 17, 183. (b) Kira, M.;
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(15) These disilenes have a high thermal stability with the decomposition points
of >280 °C under an argon atmosphere in a sealed tube. The temperatures
at 5% weight loss (Td5) are 356 °C for 3 and 308 °C for 4, respectively.
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