Versatile synthesis of blue luminescent siloles and germoles and hydrogen-bond-assisted color alteration

Article abstract of DOI:10.1002/chem.200903408

(Figure Presented) Deeper in the blue: A versatile procedure has been developed for the preparation of photoluminescent siloles and germoles by palladium-catalyzed cross-coupling of 2,2′-diiodobiaryls with dihydrosilanes and -germanes. The dibenzosiloles and -germoles with ester moieties on the aromatic ring exhibited different emission wavelengths in the solution and crystalline states, attributable to the intermolecular interactions (see figure).

Full text of DOI:10.1002/chem.200903408

COMMUNICATION
DOI: 10.1002/chem.200903408
Versatile Synthesis of Blue Luminescent Siloles and Germoles and Hydrogen-
Bond-Assisted Color Alteration
Yusuke Yabusaki, Norikazu Ohshima, Hitoshi Kondo, Tetsuro Kusamoto,
Yoshinori Yamanoi,* and Hiroshi Nishihara*[a]
Group 14 metalloles are an interesting class of Si-, Ge-, or
Sn-based annulated p-conjugated molecules.[1] The silole
ring system contains a s* orbital on the silicon atom, which
effectively interacts with the p* orbital of the butadiene
unit, producing a low-lying LUMO. Although a variety of
sophisticated silole derivatives have been prepared, the
preparation of these well-designed compounds depends
Recently, a transition-metal-catalyzed process has been
developed to overcome these problems. The groups of Mur-
akami[4] and Shimizu[5] independently reported the pioneer-
ing construction of a silole moiety in the presence of a metal
catalyst. These protocols were limited by high reaction tem-
peratures and multistep procedures for the preparation of
the starting materials. Thus, the development of mild and
general methods for the preparation of silole derivatives has
remained an important goal. During development of orga-
nosilicon compounds containing an aromatic ring, we identi-
fied a new methodology based on transition-metal-catalyzed
arylation of hydrosilanes with aryl halides.[6] Herein, we
report a novel method for constructing the skeletons of
Group 14 metalloles from the corresponding dihydrosilanes
À
heavily on classical Si C bond-formation methods by using
highly reactive organometallic reagents (Scheme 1a).[2,3] This
procedure is not always simple, nor is it safe to implement,
from the perspective of its applications.
À
or dihydrogermanes using Pd-catalyzed E C (E=Si, Ge)
bond formation (Scheme 1b). The effects of the substituents
on the physical properties of these materials will be dis-
cussed in detail.
We examined the cyclic double intramolecular arylation
of a 2,2’-diiodobiaryl with a secondary silane in the presence
of the base (iPr)2EtN, using [Pd(PACHTUNGTRNEUNG
(tBu)3)2] as the catalyst.[7,8]
The results are shown in Scheme 2. The reaction proceeded
smoothly to produce the desired dibenzosiloles or -germoles
as colorless solids in good to high yields (1–16). The reaction
described herein provides a unique method for the prepara-
tion of Group 14 metalloles containing a reactive functional
group on the aromatic rings, some of which (13–16) are diffi-
cult to synthesize when using conventional synthetic meth-
ods. 2,2’-Diiodobithiophene gave an inferior yield for prod-
uct 17; the reduced product was favored. Unfortunately, the
intramolecular cyclization of 1-NpPhSiH2 (Np=naphthyl)
or (tBu)2SiH2 with 2,2’-diiodobiphenyl did not occur at all
under the present conditions due to the steric crowding of
the secondary silanes.
Scheme 1. Strategy for the preparation of Group 14 metalloles (E=Si,
Ge).
[a] Y. Yabusaki, N. Ohshima, H. Kondo, T. Kusamoto, Dr. Y. Yamanoi,
Prof. Dr. H. Nishihara
Department of Chemistry, School of Science
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku
Tokyo 113-0033 (Japan)
Fax : (+81)3-5841-8063
E-mail: yamanoi@chem.s.u-tokyo.ac.jp
nisihara@chem.s.u-tokyo.ac.jp
The photophysical properties of compounds 1–17 were de-
termined by UV-visible and fluorescence spectroscopies, col-
lected in dichloromethane or in the corresponding crystal-
line state. These results are summarized in Table 1. The UV-
visible absorption spectra showed that the position of the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903408.
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absorption maximum (ca. 290–340 nm), ascribed to the p–p*
transition of the silole ring, depended on the identity of sub-
stituent of the aromatic ring. The siloles exhibited fluores-
cence maxima at 350–410 nm under the irradiation at
250 nm.
The fluorescence quantum yields depended significantly
on the aromatic substituent groups. Dibenzosiloles with
electron-neutral or -releasing groups on their aromatic rings
were weakly fluorescent (FF =<0.01–0.08). This was in a
sharp contrast to the high fluorescence efficiencies observed
with an electron-accepting group (13 and 14, FF =0.32). The
Stokes shift was also found to depend on the identity of the
substituent. Dibenzosiloles and -germoles 13–16 exhibited
smaller Stokes shifts than did other Group 14 metalloles.
These results suggested smaller structural differences be-
tween the ground and excited states.
The dibenzosilole molecule 14 yielded an emission maxi-
mum at 351 nm in solution and this emission peak was shift-
ed to 392 nm in the crystalline bulk solid with a concomitant
increase in the emission quantum yield (Figure 1).[9] The
Scheme 2. Palladium-catalyzed one-pot synthesis of dibenzosiloles and di-
benzogermoles.
Figure 1. Fluorescence spectra and photographs (excited at 250 nm) of 14
at room temperature a) in dichloromethane and b) in the solid state.
Table 1. Photophysical properties of Group 14 metalloles 1–17.[a]
[e]
[f]
Compound UV/Vis
labs [nm][b]
loge Fluorescence FF
lem [nm]
lem [nm][d] FF
redshift in the emission maximum and enhancement of lu-
minescence efficiency can be explained as follows: The
solid-state structure of 14, derived from single-crystal X-ray
analysis, revealed that variations in the molecular packing
played a major role in the crystalline-state fluorescence be-
havior (Figure 2).[10] In the packed model of 14, hydrogen
bonding between the carbonyl and the aromatic hydrogen
1
2
3
4
5
6
7
8
289
4.06 352
3.80 355
0.03 352
0.04 355
0.14
0.16
0.02
0.03
0.23
292
285
288
299
302
290
297
331
333
330
332
325
327
324
323
340
3.97 345
4.16 346
3.99 362
4.12 369
4.25 354
4.18 360
4.14 391
4.08 404
4.09 357
3.99 389
4.38 353
4.34 351
4.43 354
4.35 356
4.07 420
<0.01 349
0.01 350
0.01 367
0.02 369
<0.01 362
<0.01 363
0.07 382
0.08 400
0.01 363
<0.01 392
0.32 363
0.32 392
0.02 363
0.05 394
0.13
<0.01
<0.01
0.17
0.11
0.01
<0.01
0.33
0.59
À
(C=O···H C: 2.40 ꢁ), along with the C=O···C=O interaction
(which forms a motif parallel to the weak carbon–oxygen in-
teraction, 3.20 ꢁ) (Figure 2c), produce a compact aggrega-
tion structure in the crystalline state with a J-type staggered
intermolecular p–p overlapping (3.40 ꢁ) geometry through-
out the crystal.[11] A similar redshift was observed for diben-
zogermole 16, and the X-ray structure was very similar to
that of 14 and contained dimer units p stacked with the aid
9
10
11
12
13
14
15
16
17
0.07
0.18
[g]
[g]
0.34
À
of C=O···H C (2.38 ꢁ) and C=O···C=O (3.22 ꢁ) interac-
tions.[12] These interactions produced a redshift in the photo-
luminescent emission in the crystalline state relative to the
emission spectrum in solution. The influence of the ester
substituent can also be illustrated in a comparison of the X-
ray structures of 10.[13] No strong interactions were present-
[a] All measurements were carried out in dichloromethane at room tem-
perature. [b] Absorption maximum of the longest wavelength. [c] Excited
at 250 nm. [d] Crystalline solid-state fluorescence. [e] Determined with
respect to anthracene in EtOH (FF =0.27). [f] Absolute quantum yield
measurement. [g] Fluorescence in the solid state could not be measured
because the compound was an oil.
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Synthesis of Blue Luminescent Siloles and Germoles
COMMUNICATION
Figure 2. Packing arrangement in the crystal structure of 14. a) Packing structure, b) hydrogen bond (A–B), and c) carbonyl interactions (A–C).
ed between the dibenzosilole molecules in the molecular
packing of 10. Accordingly, the small redshift observed for
other dibenzosiloles may have been the result of a lack of
aggregation in the crystalline state. This is the first example
of siloles that show a hydrogen-bond-assisted luminescent
color change.[14]
Finally, we investigated systematically the effects of sub-
stituents on phosphorescence of dibenzosiloles for the first
time. All dibenzosiloles 1, 2, 5, 6, 9, 10, 13, and 14 exhibited
long-lived phosphorescence (tp =1–4 s) in the region 450–
650 nm (Table 2 and Figure 3) at 77 K in glassy 2-methylte-
trahydrofuran. The phosphorescence quantum yields were
low (FP =0.01–0.11), but were somewhat higher than that of
tetraphenylsilane (FP <0.01) due to a more extensively con-
jugated electronic structure. The substituents on the diben-
zosilole ring influenced the phosphorescent quantum
yield.[15] The electron-neutral and -donating substituted di-
benzosilole derivatives showed higher quantum efficiencies
relative to the electron-withdrawing substituted derivatives.
In summary, the synthesis of dibenzosiloles and -germoles
obtained through an intramolecular Pd-catalyzed cyclization
pathway is described. Compared with the conventional syn-
thetic strategies for achieving siloles and germoles, the pres-
ent cyclization has several advantages in terms of functional
group tolerances. The luminescence properties could be
tuned through the molecular structure, particularly the elec-
tronic effects of the substituents.[16–18] The remarkably high
efficiency of the solid-state fluorescence, with a longer emis-
sion shift observed for 14, was achieved through the forma-
tion of a J-type aggregation structure. The structure–proper-
ty relationships described herein will be valuable for the
future design and synthesis of new organic electrolumines-
cent materials containing Group 14 metalloles as a key
building unit.
Table 2. Phosphorescence of dibenzosiloles.[a]
[c]
Compound
Phosphorescence
tp [s]
Fp
lem [nm][b]
1
2
5
6
490
491
505
505
545
547
526
526
2.87
2.91
3.05
3.69
1.26
0.98
1.71
1.56
0.11
0.11
0.06
0.09
0.04
0.08
0.01
0.02
9
10
13
14
Experimental Section
[a] Measured in 2-methyltetrahydrofuran at 77 K. [b] Excited at labs
.
General: All experiments were carried out under an argon atmosphere in
oven-dried glassware. See the Supporting Information for details of char-
acterization of individual compounds.
[c] Absolute quantum yield measurement.
Typical experimental procedure for palladium-catalyzed intramolecular
cyclization of 2,2’-diiodobiaryl with a secondary silane: The procedure
for palladium-catalyzed cyclization is illustrated by the synthesis of 9,9-
diethylsilafluorene (1). N,N-diisopropylethylamine (270 mL, 1.51 mmol)
and diethylsilane (130 mL, 1.00 mmol) were added to a solution of 2,2’-
diiodobiphenyl (204 mg, 0.503 mmol) and bis(tri-tert-butylphosphine)pal-
ladium(0) (12.4 mg, 0.024 mmol) in THF (1.0 mL) under a nitrogen at-
mosphere. After stirring for 3 d at room temperature, the reaction mix-
ture was quenched with water, extracted with CH2Cl2 three times, and
dried with Na2SO4. The solvent was evaporated under reduced pressure,
and the crude product was purified by column chromatography on silica
Figure 3. Phosphorescence of 10 in 0.1 mm 2-methyltetrahydrofuran at
77 K (UV lamp, 254 nm).
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Y. Yamanoi, H. Nishihara et al.
gel to give 1 (105 mg, 88%).[2a] 1H NMR (500 MHz, CDCl3): d=7.83 (d,
2H, J=7.8 Hz), 7.62 (d, 2H, J=7.1 Hz), 7.43 (dt, 2H, J=1.3, 7.6 Hz),
7.26 (t, 2H, J=7.3 Hz), 0.91–1.01 ppm (m, 10H); 13C NMR (125 MHz,
CDCl3): d=148.5 (Cq), 137.3 (Cq), 133.2 (CH), 130.1 (CH), 127.1 (CH),
120.8 (CH), 7.6 (CH3), 3.8 ppm (CH2); EIMS: m/z: 238 [M+].
A. Wakamiya, S. Yamaguchi, Org. Lett. 2004, 6, 3707; r) S. Yamagu-
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690, 5365.
[3] Recently, the development of an intramolecular sila-Friedel–Crafts
reaction as a novel synthetic method for dibenzosilole derivatives
was reported, see: S. Furukawa, J. Kobayashi, T. Kawashima, J. Am.
Chem. Soc. 2009, 131, 14192.
[4] The synthesis of silole skeletons through the iridium-catalyzed
[2+2+2] cycloaddition of silicon-bridged 1,6-diynes with alkynes,
ruthenium-catalyzed double trans-hydrosilylation of 1,4-diarylbuta-
1,3-diynes, and ring-closing metathesis of silicon-tethered dienes or
trienes has been reported, see: a) T. Matsuda, S. Kadowaki, T.
Goya, M. Murakami, Org. Lett. 2007, 9, 133; b) T. Matsuda, S. Ka-
dowaki, M. Murakami, Chem. Commun. 2007, 2627; c) T. Matsuda,
Y. Yamaguchi, M. Murakami, Synlett 2008, 561.
Photochemical measurements: The solutions were degassed by three
freeze–pump–thaw cycles prior to measurements. Photoluminescent
quantum yields were measured by using the optically dilute method (A=
ꢀ0.1). Fluorescence quantum yields were determined in dichloromethane
relative to anthracene (FF =0.27 in EtOH) at RT.[19] Fluorescent quan-
tum yields of 1–16 in the solid state and phosphorescent quantum yields
of 1, 2, 5, 6, 9, 10, 13, and 14 at 77 K were recorded with a Hamamatsu
Photonics C9920-02G Absolute PL Quantum Yield Measurement
System.
[5] The palladium-catalyzed intermolecular coupling of 2-(arylsilyl)aryl
triflates has been reported, see: a) K. Mochida, M. Shimizu, T.
Hiyama, J. Am. Chem. Soc. 2009, 131, 8350; b) M. Shimizu, K. Mo-
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[7] The intramolecular cyclization in the presence of (iPr)2EtN was
slightly superior to that in the presence of Et3N.
[8] The coupling reaction of 2,2’-dibromobipheyl with diethylsilane
gave the corresponding dibenzosilole 1 in 26% yield. The reduced
product, biphenyl, was obtained as a major product by the observa-
tion of GC–MS.
Acknowledgements
The authors thank Kimiyo Saeki and Dr. Aiko Sakamoto of the Elemen-
tal Analysis Center of The University of Tokyo for assistance with the el-
emental analysis measurements. We also thank Hideki Waragai for the
measurement of photoluminescent quantum yields of 1–16 in the crystal-
line bulk solid. This work was financially supported by Grant-in-Aids for
Young Scientists (B) (No. 21750036), Scientific Research for Priority
Area “Coordination Programming” (area 2107, No. 21108002), and the
Global COE Program for “Chemistry Innovation through the Coopera-
tion of Science and Engineering” from the Ministry of Education, Cul-
ture, Sports, Science, and Technology, Japan.
Keywords: aggregation
luminescence · siloles
·
cross-coupling
·
germoles
·
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[11] Generally, crystal-packing structures with intermolecular electronic
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see the Supporting Information.
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Synthesis of Blue Luminescent Siloles and Germoles
COMMUNICATION
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Received: December 11, 2009
Published online: April 1, 2010
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5585