Synthesis, photophysical properties, and self-organization of difurobenzosilole derivatives

Article abstract of DOI:10.1002/ejic.201400061

A facile synthetic route was developed to fuse furan rings to a dibenzosilole core for the construction of difurobenzosilole derivatives through an electrophilic double cyclization reaction. Only silafluorene derivatives with electron-donating groups on the periphery participated in this cyclization. Suzuki-Miyaura coupling and deiodination of the diiododifurobenzosiloles resulted in highly emissive silicon-bridged dibenzofuran compounds. These obtained compounds exhibited very high luminescent quantum yields (93 to about 99 %) and good thermal stability. Single crystals of the silicon-bridged dibenzofuran compound with a marginal phenyl group were easily grown and analyzed by single-crystal X-ray diffraction, whereas the deiodinated silicon-bridged dibenzofuran compound showed a high tendency to self-organize into 1D microfibers in various solvents such as hexane, toluene, and THF. Two difurobenzosilole derivatives are synthesized by annulation reactions. The presence of both furan and silole moieties in the π-conjugated molecules results in extremely high fluorescence quantum yields and good thermal stability. Single crystals of one derivative are easily obtained, but the other compound shows a high tendency to self-organize into 1D microfibers in various solvents.

Full text of DOI:10.1002/ejic.201400061

SHORT COMMUNICATION
DOI:10.1002/ejic.201400061
Synthesis, Photophysical Properties, and Self-
Organization of Difurobenzosilole Derivatives
Liangchun Li,[a] Shuhong Li,*[b] Cui-Hua Zhao,[c] and
Caihong Xu*[a]
Keywords: Self-organization / Conjugation / Cyclization / Oxygen heterocycles / Silicon
A facile synthetic route was developed to fuse furan rings to
a dibenzosilole core for the construction of difurobenzosilole
derivatives through an electrophilic double cyclization reac-
tion. Only silafluorene derivatives with electron-donating
groups on the periphery participated in this cyclization. Su-
zuki–Miyaura coupling and deiodination of the diiododifuro-
benzosiloles resulted in highly emissive silicon-bridged di-
benzofuran compounds. These obtained compounds exhib-
ited very high luminescent quantum yields (93 to about 99%)
and good thermal stability. Single crystals of the silicon-
bridged dibenzofuran compound with a marginal phenyl
group were easily grown and analyzed by single-crystal X-
ray diffraction, whereas the deiodinated silicon-bridged di-
benzofuran compound showed a high tendency to self-orga-
nize into 1D microfibers in various solvents such as hexane,
toluene, and THF.
pounds show improved thermal stability.[5] Therefore, the
combination of furan moieties with a silicon-bridged π-con-
jugated molecule is expected to create novel materials with
fine-tuned photophysical properties and thermal stability.
However, there are very few reports on molecules con-
structed from different heteroatom bridges,[6] and no mole-
cule with a Si or O bridge has been reported to date, pre-
sumably because of the very limited synthetic strategies that
are available.
Our persistent interest in the field of π-conjugated ladder
molecules encouraged us to develop ring-fused molecules
possessing both a furan ring and a silole unit. Although
several synthetic methods can lead to benzofuran or di-
benzosilole compounds, it remains a challenge to embed
furan and silole rings in one molecule. In view of their com-
pletely different synthetic methods and intolerance to
strongly acidic or basic conditions, the construction of this
type of molecule should proceed step by step with elaborate
consideration of all the reaction conditions. Herein, we re-
port the synthesis of bridged difurobenzosilole derivatives
on the basis of two annulation reactions. The new ladder-
type π-conjugated molecules containing a furan ring and a
silole ring were constructed, and their fundamental photo-
physical properties and crystal structures were investigated.
Interestingly, subtle changes in the marginal substituents of
the silicon-bridged dibenzofuran compounds led to dif-
ferent solid aggregation behavior; the molecule with mar-
ginal phenyl groups easily formed single crystals, but the
deiodinated silicon-bridged dibenzofuran compound
showed a high tendency to self-organize into 1D microfi-
bers in organic solvents.
Introduction
Heteroatom-bridged π-conjugated ladder molecules have
attracted considerable attention owing to their potential ap-
plications in organic electronics and photonics.[1] Hetero-
atom bridges not only stiffen the skeleton but also improve
the electronic structure through orbital interaction, which
results in fascinating properties. Significant progress has
been made in the synthesis of ladder π-conjugated mo-
lecules with a variety of main-group elements.[2] As a repre-
sentative example, π-conjugated molecules with incorpo-
rated furan rings are known for their high fluorescence effi-
ciency and good hole-transporting properties.[3] The re-
cently reported benzodifuran derivatives were demonstrated
to be good hole-transporting materials by Tsuji et al.[3b,3c]
The incorporation of a silicon bridge into π-conjugated
molecules can effectively decrease the LUMO level of the
molecules;[4] this results in good electron-transporting prop-
erties while maintaining fluorescence efficiency comparable
to that of their carbon analogues, and moreover, these com-
[a] Beijing National Laboratory for Molecular Sciences (BNLMS),
Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, P. R. China
E-mail: caihong@iccas.ac.cn
http://chxu.iccas.ac.cn
[b] School of Science, Beijing Technology and Business University,
Beijing 100048, P. R. China
E-mail: lish@th.btbu.edu.cn
http://lxy.btbu.edu.cn
[c] School of Chemistry and Chemical Engineering, Shandong
University,
Jinan 250100, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201400061.
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Scheme 1. Synthesis of difurobenzosilole derivatives.
were essential for the cyclization of our dibenzosilole core
system. Given that the fluorescence of cyclization product
3c was quenched and that 3c showed poor solubility in
common solvents, which prohibited further reaction to turn
on the emission, we used 2d bearing simpler electron-donat-
ing methoxy groups for the annulation. Indeed, under the
same reaction conditions, 2d underwent the double cycliza-
tion reaction in 8 h to afford desired product 3d in 98%
Results and Discussion
The synthetic route is outlined in Scheme 1. The synthesis
started from 1,7-dibromo-2,8-dimethoxy-5,5-diphenyl-5H-
dibenzo[b,d]silole (1), which was obtained in high yield by
silacyclization developed from Huang’s method.[7] Then, ri-
gid rodlike molecules 2a–d containing a dibenzosilole core,
ethynylene linkages, and versatile aryl end groups were syn-
thesized by Sonogashira cross-coupling according to our
previously reported method.[7b]
yield (Scheme 1).
A plausible mechanism of these cyclizations is shown in
With 2a–d in hand, we then took advantage of the in-
stalled methoxy groups for annulation of the furan rings to
the dibenzosilole core to synthesize the difurobenzosilole
compounds. Among a number of recently reported syn-
thetic approaches to benzo[b]furan derivatives,[3,8] the
iodine-induced cyclization reaction systematically devel-
oped by Larock and co-workers is one of the most facile
and efficient approaches.[8a] Following the same procedure,
we first examined the reaction of 2a in anhydrous CH2Cl2
with I2 (2 equiv.). However, no cyclization product was ob-
tained. After many other failed attempts at this cyclization
reaction under various reaction conditions, including Br2/
HCCl3,[8d] I2/NaHCO3/MeCN,[8e] BCl3/N-iodosuccinimide/
CH2Cl2,[8f] and N-bromosuccinimide/DMF,[8g] we finally
realized that the success of the reaction was dependent on
the nature of the electron density of the triple bonds, which
was affected by the periphery substituents.[8a] Therefore, the
annulation reactions of 2b and 2c containing electron-with-
drawing and electron-donating groups, respectively, were
examined under I2/CH2Cl2 conditions. We found that the
reaction of 2c went to completion at room temperature
Figure S1 (Supporting Information). First, addition of the
I2 electrophile to the CϵC bonds produces intermediate A,
B, or C.[8a–8d,8g] The periphery electron-donating methoxy
groups play a crucial role in the electrophilic addition and
the formation of intermediate B, and the reaction then
likely proceeds through intramolecular attack by the oxygen
atoms of the methoxy groups in the ortho positions to the
triple bonds to form five-membered ring intermediate C.
Finally, the methyl groups are removed by SN2 displace-
ment by I– existing in the reaction mixture to give the de-
sired cyclization product.
Intermediate 3d is very useful for further extension of the
π conjugation or for the synthesis of versatile derivatives by
palladium-catalyzed cross-coupling reactions; these reac-
tions can provide derivatives with fine-tuned properties. For
example, the Suzuki–Miyaura reaction of 3d with phenyl-
boronic acid in the presence of Pd(PPh3)4 successfully pro-
duced 5d in 89% yield, whereas treatment of 3d with tBuLi
at –78 °C followed by quenching with methanol afforded
double deiodinated compound 4d in quantitative yield.
The normalized UV/Vis absorption and emission spectra
within 2 h, whereas 2b failed to give the cyclization product, of 2d–5d in CH2Cl2 are shown in Figure 1. Details of their
which indicated that the periphery electron-donating groups photophysical properties are summarized in Table 1. Com-
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pounds 3d, 4d, and 5d exhibit very similar absorption and
emission spectra. Relative to the absorption maximum of
2d, the absorption maxima of the other derivatives are not
significantly redshifted but they are split into two peaks
around 380 nm and there are new absorptions in the 315–
320 nm range, which could be attributed to the newly
formed furan rings.[8k] This indicated that, in solution, the
degree of π conjugation in 3d–5d is similar to that in 2d,
even though furan rings are formed by annulation. The
fluorescence emission maxima of 3d–5d are slightly blue-
shifted relative to that of 2d in CH2Cl2. Notably, 4d and 5d
also exhibit very high quantum yields (ΦF) of 0.93 and 0.99,
respectively, relative to that of 2d, whereas introduction of
iodine groups resulted in a decrease in the fluorescence
quantum yield of 3d down to 0.032. Because of the incorpo-
ration of the Si atom, these compounds displayed high
quantum yields and significant thermal stability, which may
also be ascribed to the formation of the Si-bridged ladder
structure.[5] The decomposition temperatures (Td) corre-
sponding to 5% weight loss of 4d and 5d were 438 and
437 °C, respectively, as evaluated by thermogravimetric
analysis (TGA) under a nitrogen atmosphere, and both
melting points were higher than 300 °C, as analyzed by
differential scanning calorimetry (DSC). The TGA curves
are shown in Figure S2 (Supporting Information).
Single crystals of 3d and 5d suitable for X-ray analysis
were obtained from chloroform and acetone solution,
respectively. The crystallographic parameters are presented
in Table S1 (Supporting Information), and selected geomet-
rical parameters are given in the caption. Both compounds
¯
crystallize in a triclinic crystal system in the P1 space group.
Figure 1. (a) Normalized UV/Vis and (b) fluorescence spectra of
2d–5d in CH2Cl2.
As shown in Figure 2, the molecular structure of 3d in the
crystal exhibits an all-coplanar arrangement, including the
periphery methoxyphenyl groups. The dihedral angles be-
tween the difurobenzosilole core and the methoxyphenyl
rings are only 4.0 and 5.6°, which indicates that the π conju-
gation is extended over the entire molecule. As observed in
our previously reported silicon-bridged p-terphenyl com-
pounds,[9] substituents attached to the sp3-hybridized sili-
con atom could result in inefficient packing. However, com-
pound 3d still organizes in a parallel arrangement with π–
π stacking interactions at a distance of 3.49 Å between the
layers owing to the extended π plane of the molecule (Fig-
ures S3 and S4, Supporting Information). In the solid state,
the framework of 5d twists slightly with dihedral angles of
Table 1. Optical data of 2d–5d.
[a,c]
λabs [nm][a]
logε
λem[nm][a,b]
ΦF
2d
3d
4d
5d
279, 290, 378
320, 373, 389
320, 374, 392
315, 372, 393
4.7
4.8
4.8
4.7
410, 435
406, 428
406, 430
402, 426
0.94
0.03
0.93
0.99
[a] In dichloromethane. [b] Excited at 350 nm. [c] The fluorescence
quantum yield (ΦF) was determined with anthracene as a standard.
The ΦF is the average value of repeated measurements within Ϯ5%
error.
about 16.4 and 21.0° between the methoxyphenyl groups bridged difurobenzosilole framework; the HOMO and
and the central scaffold because of the introduced phenyl LUMO energy levels of 5d are –4.81 and –1.32 eV, respec-
groups. Compound 5d exhibits parallel stacking in a similar tively. The phenyl groups in 5d have almost no contribution
fashion to that found in 3d with a relatively weak π–π inter- to the HOMO and LUMO. With a marginally different
action (distance ca. 3.60 Å; see Figures S5 and S6, Support- structure, compound 4d shows comparable values of the
ing Information). Therefore, it might be suggested that HOMO and LUMO (Figure S7, Supporting Information).
there exists a similar phenomenon in the crystal packing of Interestingly, 4d failed to give single crystals for X-ray
4d bearing two hydrogen atoms instead of the iodine atoms analysis but showed a strong tendency to self-organize into
of 3d. Single-point calculations [B3LYP/6-31G(d)level] by 1D microfibers in various solvents such as hexane, toluene,
using the geometry derived from the crystal structure dem- and THF (Figure 2; Figures S8–S12, Supporting Infor-
onstrated that the highest occupied molecular orbital mation). The microfibers self-organized by the slow-evapo-
(HOMO) and the lowest unoccupied molecular orbital ration method have a diameter in the range from 0.5 to
(LUMO) are significantly delocalized over the entire 10 μm and a length up to 5.0 mm, and they were charac-
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Figure 2. ORTEP drawing of 3d viewed from (a) the top and (b) the side with thermal ellipsoids shown at the 50% probability level;
hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Si1–C1 1.873(2), Si1–C12 1.870(2), C1–C2 1.397(3), C2–
C3 1.395(3), C3–C4 1.399(3), C4–C5 1.376(3), C5–C6 1.387(3), C6–C7 1.480(3), C7–C8 1.393(3), C8–C9 1.379(3), C9–C10 1.396(3), C10–
C11 1.397(3), C11–C12 1.393(3), C1–C6 1.429(3), C7–C12 1.425(3), C3–C13 1.444(3), C13–C14 1.361(3), C14–O1 1.398(3), O1–C4
1.370(3), C9–O2 1.371(3), O2–C15 1.397(3), C15–C16 1.362(3), C16–C10 1.444(3), C13–I1 2.077(2), C16–I2 2.066(2); Si–C1–C6
108.48(16), C1–C6–C7 115.3(2), C6–C7–C12 114.8(2), C7–C12–Si1 109.00(17), C12–Si1–C1 92.08(10), C3–C13–I1 122.52(17), C14–C13–
I1 129.33(18), C10–C16–I2 121.88(17), C15–C16–I2 130.38(18). SEM images of microfibers: (c) 4d self-assembled microfibers in toluene
on a large scale and (d) an individual microfiber; (e) 4d self-assembled microfibers in THF on a large scale and (f) an individual microfiber.
13C NMR spectra were measured with a Bruker 400 MHz or
terized by scanning electron microscopy (SEM). As illus-
600 MHz spectrometer. UV/Vis absorption spectra and fluores-
trated by the SEM images in Figure 2, the microfibers ob-
cence spectra measurements were performed at room temperature
tained from toluene and THF solution show fairly smooth
with a Shimadzu UV–1601PC spectrometer and a Hitachi F–4500
surface and a more consistent microstructure. These micro-
spectrometer, respectively. Melting points were determined by
fibers of the novel organic compound are rather intriguing
differential scanning calorimetry (DSC) experiments by using an
EXSTAR6000 DSC6220 instrument under a nitrogen atmosphere.
Thermogravimetric analysis (TGA) was performed with an
for potential application in electronic devices such as or-
ganic field-effect transistors.[10]
EXSTAR6000 TG/DTA6300 instrument under a nitrogen atmo-
sphere at a heating rate of 10 °Cmin–1. Elemental analyses were
obtained with a FLASH EA1112 Microanalytical facility. MS spec-
tra were determined by using a Shimadzu GC–MS QP2010 spec-
Conclusions
We demonstrated an efficient route to synthesize the first
examples of difurobenzosilole compounds by a convenient
electrophilic double iodocyclization reaction. Iodide hetero-
cyclic derivative 3d is a useful intermediate that can be used
to construct molecules containing the difurobenzosilole
skeleton. Further reactions of 3d afforded difurobenzosilole
derivatives 4d and 5d. As our initial design, the presence of
both furan and silole moieties in the π-conjugated molecule
resulted in extremely high fluorescence quantum yields and
good thermal stability, which are quite appealing for the
application of these derivatives in electronic devices. More-
over, single crystals of 5d were easily obtained, but in con-
trast, compound 4d showed a high tendency to self-orga-
nize into 1D microfibers in various solvents such as hexane,
toluene, and THF. Further investigation of their electronic
properties and expanding this synthetic approach to other
heteroatom-bridged π-conjugated molecules are currently
underway in our laboratory.
trometer, and MALDI–TOF spectra were collected with a Bruker
Reflex III spectrometer.
Synthesis of Compounds 2d: A 25 mL flask was charged with dibro-
modibenzosilole 1 (0.54 mmol), Pd(PPh3)4 (62 mg, 0.054 mmol),
and CuI (21 mg, 0.11 mmol). Then, piperidine (6 mL) and aryl-
acetylene (1.62 mmol) were added. The mixture was stirred at 80 °C
for 8 h and then concentrated. Water (10 mL) was added, and the
mixture was extracted with dichloromethane. The combined or-
ganic layer was washed with brine (20 mL), dried with MgSO4,
filtered, and concentrated under reduced pressure. The residue was
purified by silica gel column chromatography to afford 2d in 69%
1
yield, m.p. 285 °C. H NMR (400 MHz, [D6]acetone): δ = 7.94 (s,
2 H), 7.84 (s, 2 H), 7.72 (d, J = 7.1 Hz, 4 H), 7.40–7.49 (m, 10 H),
6.96 (d, J = 8.6 Hz, 4 H), 4.04 (s, 6 H), 3.84 (s, 6 H) ppm. 13C
NMR (150 MHz, [D6]acetone): δ = 163.8, 160.8, 151.0, 139.0,
136.1, 133.7, 133.6, 131.1, 129.1, 128.6, 116.5, 115.0, 114.0, 105.9,
95.1, 85.8, 56.4, 55.7 ppm. HRMS (EI): calcd. for C44H34O4Si
654.2226; found 654.2230.
Synthesis of Compound 3d: A 250 mL flask was charged with 2d
(0.75 g, 1.15 mmol), CH2Cl2 (120 mL), and solid I2 (1.75 g,
6.87 mmol). The mixture was stirred at room temperature for 8 h.
Saturated Na2S2O3 solution (80 mL) was added, and the mixture
was extracted with chloroform. The combined organic layer was
Experimental Section
General: All experiments were performed under a nitrogen atmo-
sphere unless otherwise noted. 3,7-Dibromo-2,8-dimethoxy-5,5-di- washed with brine (20 mL), dried with MgSO4, filtered, and con-
phenyl-5H-dibenzo[b,d]silole (1) was prepared by a previously re-
centrated under reduced pressure. The residue was recrystallized
from chloroform/acetone to give 3d (0.983 g, 1.12 mmol) in 98%
yield as yellow-green crystals, m.p. 322 °C. 1H NMR (400 MHz,
ported procedure.[7b] All solvents were dried by general procedures
1
and distilled under an inert atmosphere before use. H NMR and
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CDCl3): δ = 8.15 (d, J = 8.6 Hz, 4 H), 7.95 (s, 2 H), 7.75 (d, J =
9.1 Hz, 6 H), 7.46 (t, J = 7.1 Hz, 2 H), 7.41 (t, J = 7.0 Hz, 4 H),
7.04 (d, J = 8.6 Hz, 4 H), 3.89 (s, 6 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 160.6, 156.8, 154.0, 146.7, 135.8, 133.1, 132.9, 131.9,
130.4, 129.1, 128.4, 127.2, 122.7, 114.2, 104.3, 59.8, 55.6 ppm.
HRMS (EI): calcd. for C42H28O4SiI2 877.9846; found 877.9854.
Synthesis of Compound 4d: tBuLi (1.5 m in pentane, 0.27 mL,
0.4 mmol) was added over 45 min to a solution of 3d (88 mg,
0.1 mmol) in dry THF (10 mL) dropwise at –78 °C. The reaction
mixture was stirred for 1 h at –78 °C. Then, methanol (2 mL) was
added, and the mixture was allowed to reach room temperature.
The reaction mixture was quenched with a saturated NH4Cl aque-
ous solution and extracted with chloroform. The organic layer was
washed with brine, dried with MgSO4, filtered, and concentrated
under reduced pressure. The mixture was purified by recrystalli-
zation from dichloromethane/methanol to give 4d (62 mg,
0.099 mmol) in 99% yield as yellow-green crystals, m.p. Ͼ350 °C.
Owing to the poor solubility of this compound, a 13C NMR spec-
[2]
[3]
1
trum with satisfactory resolution could not be obtained. H NMR
(400 MHz, CDCl3): δ = 8.02 (s, 2 H), 7.90 (s, 2 H), 7.82 (d, J =
8.4 Hz, 4 H), 7.72 (d, J = 7.5 Hz, 4 H), 7.43–7.34 (m, 6 H), 7.00
(d, J = 8.4 Hz, 4 H), 6.86 (s, 2 H), 3.88 (s, 6 H) ppm. HRMS (EI):
calcd. for C42H30O4Si 626.1913; found 626.1920.
Synthesis of Compound 5d: A 50 mL flask was charged with 3d
(88 mg, 0.10 mmol), Pd(PPh3)4 (12 mg, 0.01 mmol), and K2CO3
(41 mg, 0.30 mmol). Then, phenylboronic acid (37 mg, 0.30 mmol)
and a mixture of toluene (20 mL) and ethanol (5 mL) were added.
The mixture was stirred at 80 °C for 24 h and then concentrated.
Water (30 mL) was added, and the mixture was extracted with chlo-
roform. The combined organic layer was washed with brine, dried
with MgSO4, filtered, and concentrated under reduced pressure.
The residue was purified by a silica gel column chromatography to
afford 5d (69 mg, 0.089 mmol) in 89% yield, m.p. Ͼ350 °C. 1H
NMR (400 MHz, CDCl3): δ = 8.09 (s, 2 H), 7.77 (s, 2 H), 7.65 (d,
J = 7.5 Hz, 4 H), 7.61 (d, J = 8.2 Hz, 4 H), 7.51 (d, J = 7.6 Hz, 4
H), 7.46 (t, J = 7.5 Hz, 4 H), 7.39 (t, J = 7.0 Hz, 4 H), 7.33 (t, J =
7.5 Hz, 4 H), 6.87 (d, J = 8.3 Hz, 4 H), 3.83 (s, 6 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 159.9, 156.9, 151.5, 146.1, 135.8,
133.4, 133.1, 131.2, 130.8, 130.1, 130.0, 129.2, 128.6, 128.2, 127.7,
125.3, 123.4, 116.4, 114.1, 104.1, 55.5 ppm. HRMS (EI): calcd. for
C54H38O4Si 778.2539; found 778.2546.
[4]
[5]
[6]
CCDC-935131 (for 3d) and -935132 (for 5d) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[7]
[8]
Supporting Information (see footnote on the first page of this arti-
cle): Scheme of the plausible mechanism, TGA curves, crystallo-
graphic parameters and crystal packing of 3d and 5d, and SEM
images of the self-assembled microfibers of 4d.
Acknowledgments
The authors gratefully acknowledge the National Natural Science
Foundation of China (NSFC) (grant numbers 50673094 and
20774102) for financial support.
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Received: January 16, 2014
Published Online: March 6, 2014
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim