Highly coplanar (E)-1,2-Di(1-naphthyl)disilene involving a distinct CH-π interaction with the perpendicularly oriented protecting eind group

Article abstract of DOI:10.1246/cl.131043

An air-stable emissive di(1-naphthyl)disilene protected by the bulky Eind groups (Eind: 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) has been obtained by reducing the corresponding dibromosilane (Eind)(1-Naph)SiBr2 with lithium naphthalenide. The X-ray crystallography shows a highly coplanar (E)-1,2-di-(1-naphthyl)disilene skeleton, favorable for the efficient π-conjugation involving the Si=Si unit, together with a distinct CHπ interaction between the peri-H atom of 1-naphthyl groups and the aromatic ring of the perpendicularly oriented Eind groups.

Full text of DOI:10.1246/cl.131043

CL-131043
Received: November 7, 2013 | Accepted: December 4, 2013 | Web Released: December 10, 2013
Highly Coplanar (E)-1,2-Di(1-naphthyl)disilene Involving a Distinct CH-π Interaction
with the Perpendicularly Oriented Protecting Eind Group#
Megumi Kobayashi,1 Naoki Hayakawa,2 Koichi Nakabayashi,2 Tsukasa Matsuo,*1,2,3
Daisuke Hashizume,4 Hiroyuki Fueno,5 Kazuyoshi Tanaka,5 and Kohei Tamao*1
1Functional Elemento-Organic Chemistry Unit, RIKEN Advanced Science Institute,
2-1 Hirosawa, Wako, Saitama 351-0198
2Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University,
3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502
3JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
4Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science,
2-1 Hirosawa, Wako, Saitama 351-0198
5Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,
Nishikyo-ku, Kyoto 615-8510
(E-mail: t-matsuo@apch.kindai.ac.jp)
An air-stable emissive di(1-naphthyl)disilene protected by
the bulky Eind groups (Eind: 1,1,3,3,5,5,7,7-octaethyl-s-hydrin-
dacen-4-yl) has been obtained by reducing the corresponding
dibromosilane (Eind)(1-Naph)SiBr2 with lithium naphthalenide.
The X-ray crystallography shows a highly coplanar (E)-1,2-di-
(1-naphthyl)disilene skeleton, favorable for the efficient π-
conjugation involving the Si=Si unit, together with a distinct
CH-π interaction between the peri-H atom of 1-naphthyl groups
and the aromatic ring of the perpendicularly oriented Eind
groups.
A large variety of organic π-electron architectures are
currently known; their properties primarily depend on the
number and arrangement of the double bonds. In recent years,
the incorporation of double bonds of the heavier main group
elements into carbon π-conjugated systems have attracted much
attention because of their potentially useful properties as
functional materials.1 For example, in organosilicon chemistry,
various types of π-conjugated molecules containing a Si=Si
double bond have been developed2-8 by taking advantage of
steric protection with the appropriately designed bulky sub-
stituents. We have introduced a series of fused-ring bulky
“Rind” (1,1,3,3,5,5,7,7-octa-R-substituted-s-hydrindacen-4-yl)
groups in this field.9 This paper is concerned with the synthesis
of di(1-naphthyl)disilene 1 stabilized by Eind (R = Et) groups
(Figure 1).
Figure 1. 1,2-Di(1-naphthyl)disilene
thyl)disilene 2.
1 and 1,2-di(2-naph-
twisting of the naphthyl group from the Si=Si double bond.10
However, we are still interested in examining how much the
peri-H atom causes the geometrical changes by introducing the
1-naphthyl group in the disilene unit.
In 2010, we reported 2-naphthyl counterpart 2 as the first
air-stable and room-temperature emissive disilene,2b existing as
a mixture of two conformers in the crystals, 2(s-trans, s-trans)
and 2(s-cis, s-cis), in the ratio of 6:4. In both isomers, the highly
coplanar skeleton including the Si=Si unit is effectively
encapsulated by the perpendicularly oriented Eind groups, in
which the proximate ethyl side chains interlock with one another
above and below the π-framework. In 2012, we demonstrated for
the first time that disilene 2 can emit light in an organic light-
emitting diode (OLED), which has opened a new platform for
the development of functional elemento-organic materials and
devices.2e
The synthetic route to disilene 1 starting from (Eind)Br (3)
is outlined in Scheme 1.11 Disilene 1, isolated as a red powder, is
air-stable in the solid state for more than 2 years, similar to 2,
while a solution of 1 gradually decomposes upon exposure to
1
air, as monitored by the H NMR spectroscopy.
Disilene 1 was found by X-ray analysis to form two pseudo-
polymorphs (forms I and II) of red crystals, depending on
the crystallization conditions. Although fine microcrystals of
form I were obtained from a dilute benzene solution of 1 with
no crystal solvent, form II with 3 equiv of crystal THF was
crystallized from a suspension of 1 in THF. The geometry of 1
in both polymorphs is nearly identical, having a coplanar (E)-
1,2-di(1-naphthyl)disilene skeleton with an inversion center at
the center of the Si=Si bond. Thus, only the molecular structure
of form II is presented in Figure 2. Rather surprisingly, the peri-
In these previous studies, the 2-naphthyl group was
employed rather than the 1-naphthyl group, because the presence
of a hydrogen atom at the peri-position in the latter might cause
432 | Chem. Lett. 2014, 43, 432–434 | doi:10.1246/cl.131043
© 2014 The Chemical Society of Japan

Figure 3. Structural parameters for CH-π interaction defined
in ref 12. O: Center of the ring. I: Foot of a perpendicular line
from H atom. Dpln: Distance of H atom from the π-plane. Dpx1
Distance between O and I. ¡: C-H£π access angle.
:
Table 1. Photophysical data of disilenes 1 and 2
Emission
Scheme 1. Synthesis of 1,2-di(1-naphthyl)disilene 1.
Absorption
¾
Stokes shift
Compd Statea
-
max/nm
(ΦF)
¹1
¹1
-
max/nm /cm¹1 M
/cm
1
2
THF
Solid
THF
Solid
521
504
9.5 © 103 614 (< 0.01) 2910
635 (0.05)
2.5 © 104 586 (< 0.01) 2780
619 (0.23)
aIn a THF solution or in the solid state.
Figure 2. Molecular structures of 1 (form II): top view (left)
and side view (right). Hydrogen atoms of Eind groups and THF
molecules are omitted for clarity. Selected atomic distances (¡),
bond angles (deg), and torsion angle (deg): Si1-Si1* =
2.1688(7), Si1-C1 = 1.8955(13), Si1-C29 = 1.8831(14), C37£
C1 = 3.284(2), H37£C1 = 2.457, C1-Si1-Si1* = 124.77(5),
C29-Si1-Si1* = 117.82(5), C29-Si1-C1 = 117.39(6), Si1*-
Si1-C29-C30 = 4.93(12).
Figure 4. Solid-state color and emission of 1 at room temper-
ature in ambient air.
H atoms do not cause any steric repulsion with the Eind-
containing disilene moiety, but seems to participate in the CH-π
interaction with the benzene ring of the perpendicularly oriented
Eind groups (vide infra).
follows: (1) The UV-vis spectrum of 1 in THF shows an
absorption peak at 521 nm (¾ = 9.5 © 103), which is 17 nm red-
shifted from that of 2 (504 nm),2b indicating the efficient π-
conjugation over the di(1-naphthyl)disilene skeleton. (2) Di-
silene 1 exhibits a rather weak emission at room temperature,
both in solution and in the solid state (Figure 4). The emission
maximum of 1 appears at 614 nm in THF. The Stokes shift of 1
is estimated to be 2910 cm¹1, similar to that of 2 (2780 cm¹1),2b
indicating the rigid framework of di(1-naphthyl)disilene.
To elucidate the nature of bonding in di(1-naphthyl)disilene,
DFT computations were carried out for 1(s-trans, s-trans) at the
B3LYP/6-31G** level by using the Gaussian 09 program
package.13 The optimized structure well reproduces the X-ray
molecular structure found in the crystals. The missing 1(s-cis,
s-cis) conformer is also found at an 8.85 kcal mol¹1 higher level
with a distorted di(1-naphthyl)disilene skeleton.11 The frontier
molecular orbitals of 1(s-trans, s-trans) are depicted in Figure 5.
Although the HOMO is mainly represented by the 3pπ(Si-Si)
orbital, the LUMO involves the substantial contribution of the
3pπ*(Si-Si)-2pπ*(1-naphthyl) conjugation, which is essentially
the same as that of 2.2b The HOMO and LUMO levels of 1
(¹4.209 and ¹1.572 eV) are, respectively, slightly higher and
lower than those of 2(s-trans, s-trans) (¹4.304 and ¹1.478
eV).2b Thus, the total HOMO-LUMO gap in 1 (2.637 eV) is
slightly smaller than that in 2(s-trans, s-trans) (2.826 eV), being
in agreement with the experimental data, a slightly longer
Some structural features (given below) are mentioned only
for form II. (1) An s-trans, s-trans conformer of 1, theoretically
¹1
8.85 kcal mol more stable than the s-cis, s-cis isomer,11 has
been found only in the crystals in contrast to the 2-naphthyl
counterpart 2,2b probably because of the presence of the peri-H
atom of the 1-naphthyl groups and/or the crystal packing
situation. (2) The 1-naphthyl groups are highly coplanar with the
Si=Si bond having a Si-Si-C-C torsion angle of 4.93(12)°,
similar to that of 9.57(11)° in 2(s-trans, s-trans).2b The Si=Si
bond length of 2.1688(7) ¡ is in the standard range of those for
disilenes.1c (3) It is noteworthy that the peri-H atom of the 1-
naphthyl group is in close contact with the benzene ring of the
Eind group in the crystals, judging from the three pertinent
parameters for the CH-π interaction (Figure 3), defined by
Takahashi, Nishio, and their co-workers.12 Thus, form II has
Dpln = H37£π = 2.46 ¡, Dpx1 = 1.14 ¡, and ¡ = angle C37-
H37£π = 153°. These data are all consistent with the intrinsic
nature of the CH-π interaction.12 It is further noted that the
observed Dpln of 2.46 ¡ is the shortest edge for a large number of
¹1
known C(sp2)-H£π distances and in an about 1 kcal mol
stabilized region.12
Table 1 summarizes the photophysical data of 1 together
with those of 2 for comparison. Several features are found as
Chem. Lett. 2014, 43, 432–434 | doi:10.1246/cl.131043
© 2014 The Chemical Society of Japan | 433

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3
4
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We thank the Ministry of Education, Culture, Sports,
Science and Technology of Japan for the Grant-in-Aid for
Specially Promoted Research (No. 19002008) and Scientific
Research on Innovative Areas “Stimuli-responsive Chemical
Species” (Nos. 24109003 and 25109543). We thank Dr. Y.
Hongo (RIKEN) for her kind help with the mass spectrometry.
We are grateful to Materials Characterization Support Unit,
RIKEN Center for Emergent Matter Science, for the elemental
analyses of samples synthesized in this study. The synchrotron
radiation experiments were performed at BL26B1 in SPring-8
with the approval of RIKEN (Proposal No. 20100007). The
numerical calculations were partly performed at the Super-
computer Laboratory, Institute for Chemical Research, Kyoto
University. We thank Professor A. Sudo and Mr. S. Sugita for
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thank Professor N. Tokitoh and Dr. T. Sasamori for their
valuable discussions.
6
7
8
9
References and Notes
#
Dedicated to Professor Renji Okazaki for celebration of his 77th
birthday in recognition of his great contribution to the main group
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434 | Chem. Lett. 2014, 43, 432–434 | doi:10.1246/cl.131043
© 2014 The Chemical Society of Japan