Efficient blue emission from siloles

Article abstract of DOI:10.1039/b102221k

2,3,4,5-Tetraphenylsiloles with different 1,1-substituents on the ring silicon atoms, i.e., 1,1-dimethyl-2,3,4,5-tetraphenylsilole (1), 1-methyl-l-(3-chloropropyl)-2,3,4,5-tetraphenylsilole (2), 1-methyl-l,2,3,4,5-pentaphenylsilole (3) and hexaphenylsilol

Full text of DOI:10.1039/b102221k

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Efficient blue emission from siloles
Ben Zhong Tang,*a Xiaowei Zhan,b Gui Yu,b Priscilla Pui Sze Lee,a Yunqi Liub and
Daoben Zhu*b
aDepartment of Chemistry, Hong Kong University of Science and Technology, Clear Water
Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
bCenter for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100080, China. E-mail: zhud@infoc3.icas.ac.cn
Received 8th March 2001, Accepted 23rd July 2001
First published as an Advance Article on the web 18th September 2001
2,3,4,5-Tetraphenylsiloles with different 1,1-substituents on the ring silicon atoms, i.e., 1,1-dimethyl-2,3,4,5-
tetraphenylsilole (1), 1-methyl-1-(3-chloropropyl)-2,3,4,5-tetraphenylsilole (2), 1-methyl-1,2,3,4,5-
pentaphenylsilole (3) and hexaphenylsilole (4), are synthesized and characterized. While all the siloles emit
intense blue light readily observable by naked eyes under normal room illumination conditions, the film of their
acyclic cousin without silicon, namely 1,2,3,4-tetraphenylbutadiene (5), does not fluoresce, revealing the vital
role of the planar and rigid silacyclopentadiene ring in the solid-state photoluminescence process. The electronic
transitions of the siloles can be tuned by varying the 1,1-substituents, and the inductive and conjugating effects
of the aromatic rings confer low LUMO energy levels and high emission efficiencies on the phenyl-substituted
siloles. The electroluminescence device of the 1-phenylsilole 3 shows a high brightness (4538 cd m22 at 18 V)
and an excellent external quantum efficiency (0.65% at 17 V and 94 mA cm22).
offers more possibilities in structural modification and hence
Introduction
property modulation. In this work, we took a molecular
engineering approach and systematically synthesized a series of
siloles with different 1,1-substituents on the ring silicon atoms
(1–4, Scheme 1). All the siloles are stable in air (and hence
easily to handle) and can be readily fabricated into EL devices
by conventional vapor deposition techniques. Similar to the
silole polymer8 discussed above, the EL devices of the siloles
1–4 also emit blue light (lmax y490 nm). The luminance and
external quantum efficiency of the 1-phenylsilole 3 are,
however, 2–3 orders of magnitude higher than those of the
silole polymer.8
Efficient red, green and blue electroluminescent emitters are
indispensable for the design and construction of high quality
full-color display devices.1 Although red and green organic
light-emitting diodes (LEDs) have already achieved high
brightness and efficiency, strong blue emitters are still rare.
Recently, some organometallic complexes containing boron,
aluminium, beryllium and zinc atoms have attracted scientists’
attention as blue-emitting materials due to their high
luminescence efficiency, easy isolation and purification and
high quality of color of the emitted light.2–5
Silicon is a metalloid element and bears both nonmetallic
and metallic characteristics. Organosilicon compounds are in
general much more stable than their cousins of aluminium,
beryllium and zinc complexes. Siloles or silacyclopentadienes
are a group of Si-containing conjugated rings with novel
molecular structures and unique electronic properties.6 Siloles
possess low-lying LUMO energy levels associated with the
s*–p* conjugation arising from the interaction between the s*
orbital of two exocyclic s-bonds on the ring silicon and the p*
orbital of the butadiene moiety.7 As a result, siloles can serve as
efficient electron-transport materials in LED devices.8 Silole
polymers have been found to show interesting photo- (PL) and
electroluminescence (EL) properties;8,9 for example, an EL
device based on a silole-containing polysilane, poly(2’,3’,4’,5’-
tetraphenyl-1’-silacyclopenta-2’,4’-diene-1’,1’-diyl-1,1,2,2,3,3,4,4-
octamethyltetrasilanylene), emitted blue light of 488 nm with a
maximum external quantum efficiency of 0.001% and a
maximum luminance of 5 cd m22 at 22 V.8
Silole is rich in chemistry and a wide variety of silole
derivatives can be readily prepared by ‘‘simple’’ chemical
reactions.6,10 Systematic investigation on the luminescence
properties of the silole derivatives have, however, not been
performed. During our search for highly luminescent materi-
als,11 we were attracted by the structural feature of silole, which
resembles fluorene in terms of ring planarity and rigidity but
Scheme 1 Synthetic route to compounds 1–5.
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This journal is # The Royal Society of Chemistry 2001
DOI: 10.1039/b102221k

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1-Methyl-1,2,3,4,5-pentaphenylsilole (3). This compound
was prepared by a procedure similar to that of 1, using
dichloro(methyl)phenylsilane instead of dichloro(dimethyl)-
silane; the product was isolated as a faintly greenish–yellow
crystalline solid in 78% yield (4.6 g). Mp~170–171 uC. 1H
NMR (300 MHz, CDCl3): d 7.68 (m, 2H), 7.39 (m, 3H), 7.04
(m, 12H), 6.88 (m, 8H), 0.81 (s, 3H). FT-IR (KBr): 3055, 3023,
1596, 1488, 1441, 1429, 1298, 1267, 1251, 1110, 1088, 1074,
1027, 937, 913, 798, 762, 738, 726, 710, 697, 498, 474 cm21. MS
(EI): m/z 476 (calcd for C35H28Si 476.69). Anal. Calcd for
C35H28Si: C, 88.19; H, 5.92. Found: C, 88.19; H, 6.06%.
Experimental
Materials
High-purity lithium (Acros), diphenylacetylene, dichloro(di-
methyl)silane, dichloro(3-chloropropyl)methylsilane, dichloro-
(diphenyl)silane and dichloro(methyl)phenylsilane (all from
Aldrich) were used as received without further purification.
Tetrahydrofuran (THF) was freshly distilled from sodium
benzophenone ketyl under nitrogen prior to use.
Instrumentation
Hexaphenylsilole (4). This compound was prepared by a
procedure similar to that of 1, using dichloro(diphenyl)silane
instead of dichloro(dimethyl)silane; the product was isolated as
a faintly greenish–yellow crystalline solid in 68% yield (4.6 g).
Mp~186–187 uC. 1H NMR (300 MHz, CDCl3): d 7.68 (m,
4H), 7.43 (m, 6H), 7.20–6.85 (m, 20H). FT-IR (KBr): 3055,
3024, 1597, 1485, 1441, 1429, 1298, 1111, 1074, 1027, 790, 764,
741, 713, 697, 509 cm21. MS (EI): m/z 538 (calcd for C40H30Si
538.76). Anal. Calcd for C40H30Si: C, 89.18; H, 5.61. Found: C,
89.46; H, 5.56%.
Melting points were determined on a Yanaco MP-500 melting
point apparatus. FT-IR spectra were recorded on a Perkin–
Elmer System 2000 spectrophotometer using KBr pellets.
Electronic absorption spectra were taken in dilute chloroform
solutions (y0.05 mM) on a General TU-1201 UV-vis spectro-
1
photometer. H NMR spectra were obtained in chloroform-d
on a Bruker ARX300 NMR spectrometer and the spectral data
are expressed in d scale relative to the internal standard of
tetramethylsilane. Mass spectra were measured on AEI-MS50
using electron impact (EI) mode. Elemental analyses were
carried out on a Carlo Erba Model 1106 elemental analyzer.
1,2,3,4-Tetraphenylbutadiene (5). To a solution of diphenyl-
acetylene (1.8 g, 10 mmol) in dry THF (5 mL) was added clean
lithium shavings (140 mg, 20 mmol). The reaction mixture was
stirred at room temperature for 2 h under nitrogen. The
unreacted lithium was filtered off and the filtrate was washed
with water. The organic layer was extracted with diethyl ether
and dried over magnesium sulfate. The solvent was removed
and the residue was purified by flash chromatography over
silica gel using n-hexane as eluent. Recrystallization from
ethanol gave a pale yellow crystal in 75% yield (1.35 g).
Mp~183–184 uC. 1H NMR (300 MHz, CDCl3): d 7.41–7.30
(m, 10H), 7.02 (m, 6H), 6.74 (m, 4H), 6.30 (s, 2H). FT-IR
(KBr): 3081, 3060, 3026, 1634, 1596, 1489, 1443, 1357, 1073,
1027, 917, 870, 786, 762, 752, 704, 692, 574, 549, 477 cm21. MS
(EI): m/z 358 (calcd for C28H22 358.48). Anal. Calcd for
C28H22: C, 93.81; H, 6.19. Found: C, 93.96; H, 6.11%.
Synthesis
The synthesis of compounds 1 and 3–5 was reported in the
literature,12 but all these compounds were not fully character-
ized, and only UV and/or IR data were given. Here, we report
the simple one-pot synthesis of these compounds and present
the full characterization data including elemental analysis, IR,
1H NMR and MS. Compounds 1–5 were prepared by treating
diphenylacetylene with lithium followed by hydrolysis or
reaction with dichlorosilicons as shown in Scheme 1.
1,1-Dimethyl-2,3,4,5-tetraphenylsilole (1). To a solution of
diphenylacetylene (4.5 g, 25 mmol) in dry THF (20 mL) was
added clean lithium shavings (350 mg, 50 mmol). The reaction
mixture was stirred at room temperature for 2 h in a dry
nitrogen atmosphere. The mixture was then diluted with
120 mL of THF, followed by the addition of 1.6 g (12.5 mmol)
dichloro(dimethyl)silane. After refluxing for 5 h, the reaction
mixture was cooled and filtered and the filtrate was washed
with water. The organic layer was extracted with diethyl ether
and dried over magnesium sulfate. The solvent was removed
and the residue was purified by flash chromatography over
silica gel using 10% diethyl ether in petroleum ether as eluent.
Recrystallization from ethanol gave a faintly greenish–yellow
crystal in 70% yield (3.6 g). Mp~177–178 uC. 1H NMR
(300 MHz, CDCl3): d 7.12 (m, 6H), 7.01 (m, 6H), 6.96 (m,
4H), 6.84 (m, 4H), 0.49 (s, 6H). FT-IR (KBr): 3077, 3058, 3023,
2956, 1595, 1488, 1441, 1297, 1245, 1089, 1075, 1029, 938, 913,
836, 795, 777, 743, 709, 697, 579 cm21. MS (EI): m/z 414 (calcd
for C30H26Si 414.62). Anal. Calcd for C30H26Si: C, 86.91; H,
6.32. Found: C, 86.50; H, 6.42%.
LED fabrication and luminescence measurements
Thin films of 1–5 were deposited on quartz substrates by vapor
vacuum deposition technique, whose PL spectra were recorded
on a Hitachi F-4500 spectrofluorometer using 360 nm as the
excitation wavelength. Substrates used were indium–tin oxide
(ITO) glass with a sheet resistance of 100 V square21. The ITO-
coated glass substrates were etched, patterned, and washed
with water, acetone and isopropyl alcohol sequentially. The EL
devices using the siloles as the emitting layers were fabricated
ontheITOsubstrates. N,N’-Diphenyl-N,N’-bis(3-methylphenyl)-
1,1’-biphenyl-4,4’-diamine (TPD) and tris(8-hydroxyquinoline)-
aluminium (Alq3) were used as the hole- and electron-transport
layers, respectively. The TPD (y50 nm), silole (y50 nm) and
Alq3 (y30 nm) layers and aluminium cathode were deposited
onto the ITO substrate by vacuum evaporation at pressure
close to 1026 Torr. The deposition rate for the organic layers
was about 2 A s21. Luminance was measured with an LS-1
˚
1-Methyl-1-(3-chloropropyl)-2,3,4,5-tetraphenylsilole (2).
This compound was prepared by a procedure similar to that of
1, using dichloro(3-chloropropyl)methylsilane instead of
dichloro(dimethyl)silane; the product was isolated as a faintly
greenish–yellow crystalline solid in 74% yield (4.4 g). Mp~94–
portable luminometer. Current–voltage characteristics for
the LEDs with a structure of ITO/TPD(50 nm)/silole(50 nm)/
Alq3(30 nm)/Al were measured with a PA meter/DC voltage
source (HP4140B). All the measurements were performed in
ambient atmosphere at room temperature.
1
95 uC. H NMR (300 MHz, CDCl3): d 7.13 (m, 6H), 7.01 (m,
6H), 6.93 (m, 4H), 6.81 (m, 4H), 3.48 (t, 2H, J 6.7), 1.84 (m,
2H), 1.14 (m, 2H), 0.51 (s, 3H). FT-IR (KBr): 3056, 3024, 2954,
1596, 1573, 1489, 1441, 1298, 1252, 1089, 1075, 1027, 1003, 937,
915, 806, 790, 762, 711, 697, 578, 483 cm21. MS (EI): m/z 477
(calcd for C32H29SiCl 477.12). Anal. Calcd for C32H29SiCl: C,
80.56; H, 6.13. Found: C, 80.62; H, 6.12%.
Results and discussion
Electronic absorption
UV absorption spectra of the chloroform solutions of 1–5 are
shown in Fig. 1A. All the siloles 1–4 exhibit two peaks at y250
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tuned by molecular engineering of the inductive effects of the
1,1-substituents.
Tamao and coworkers have investigated the substituent
effects in an analoguous system, i.e., 3,4-dimethyl-2,5-bis(tri-
methylsilyl)siloles, and found that the inductive effect plays a
major role in modifying the electronic structures of the siloles.13
Our observation in the 2,3,4,5-tetraphenylsilole system is thus
consistent with Tamao’s early finding. Theoretical calculations
reveal that the phenyl group does not only lower the LUMO
energy level but also slightly raises the HOMO energy level of a
silole,13 suggesting that the extended s*–p* conjugation over
the phenyl ring on the silicon atom may also play a role in red-
shifting the absorption spectrum of a silole. Fig. 1B shows the
absorption spectra of the thin solid films of the siloles, which
are similar to those measured in the solutions. Thus the
electronic transitions of the siloles are primarily determined by
their molecular structures, which are insignificantly affected by
their aggregation states.
Photoluminescence
The great similarity between PL and EL spectra of many light-
emitting materials suggests that similar mechanisms are at play
or that the same emitting species (a singlet exciton) is involved
in each case.14 Because of the simplicity of the PL measure-
ments, we first checked the PL behaviors of the siloles 1–4 and
their structural cousin 5. Fig. 2 shows the PL spectra of the thin
solid films of the siloles. All the siloles emit strong blue light
upon photoexcitation. Compared to those of the siloles with
1,1-dialkyl substituents (1 and 2), the PL spectra of the siloles
with phenyl substituents on the ring silicon atoms (3 and 4) are
located in the longer wavelength region. Thus the phenyl rings
induce red-shifts in the PL spectra of the siloles, probably again
due to the inductive and s*–p* conjugating effects of the
aromatic rings.
The thin film of the acyclic carbon analog of the siloles (5),
however, emits hardly observable PL signals. The rotation of
the unfixed cisoid structure of 5 in three-dimensional space may
unfavorably perturb the electronic structure, giving a non-
planar structure with inefficient p-conjugation. The fixation of
the cis-cisoid butadiene moiety in the siloles by the silicon atom
creates a polar and rigid ring structure. It is well known that
increasing planarity and rigidity helps to increase fluorescence
efficiency as it enhances the free mobility of the p electrons and
charge transportation.14 Increased rigidity can also help by
reducing vibrational–rotational interactions in the excited state
that can lead to intersystem crossing. Fluorene ring and acyclic
biphenyl are the ‘‘classic’’ examples in this regard, with the
former possessing a much higher quantum efficiency than the
Fig. 1 Absorption spectra of (A) chloroform solutions of 1–5 and (B)
thin films of 1–4.
and y360 nm; the former is attributed to the p–p* transition of
the phenyl groups and the latter is due to the p–p* transition of
the silacyclopentadiene ring. The band gaps (Eg) of the siloles
are estimated from the onset positions of the absorption
bands11 to be 2.97, 2.97, 2.93 and 2.90 eV, respectively, while
the Eg of 5 is 3.35 eV. The large difference between the band
gaps of the siloles and their acyclic cousin
5 clearly
demonstrates the significant structural and electronic effects
of the ring silicon atom on the electronic transition of the
p-conjugated system. The fixation of the labile cis-cisoid
butadiene structure by the silicon atom gives a fluorene-like
planar and rigid ring structure9 and the unique orbital
interactions such as s*–p* conjugation lower the LUMO
energy levels;9 the synergistic interplay of the two effects leads
to the observed low Eg of the siloles.
The 1,1-substituents on the silicon atom affect the absorp-
tion spectra of the siloles in a unique way. Replacing one of the
two 1,1-dimethyl groups in 1 with a 3-chloropropyl group
brings about little change to the absorption peak in the long
wavelength region; the lmax of 2 being practically identical to
that of 1. This may be understood by the fact that methyl and
propyl both belong to the same category of electron-donating
alkyl groups. Substitution of the methyl group by a phenyl
group (3), however, red-shifts the absorption spectrum.
Further red-shift is induced when the two methyl groups in 1
are replaced by two phenyl rings in 4. Clearly the electron-
withdrawing phenyl groups exert a bathochromic effect on the
electronic transitions of the siloles. In other words, the
electronic structures and properties of the siloles can be readily
Fig. 2 Photoluminescence spectra of thin solid films of siloles 1–4.
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Table 1 Performance of the EL devices of the silolesa
Turn-on Emission
Silole voltage/V maximum/nm mW
Brightnessb/ External quantum
efficiencyc (%)
1
2
3
4
9
7
11
12
494
494
494
488
6.8
0.7
138.7
36.3
0.026
0.0045
0.65
0.31
aConfiguration of the devices: ITO/TPD/silole/Alq3/Al. bAt 14 (1), 12
(2), 18 (3) and 21 V (4). cAt 141 (1), 7.14 (2), 94 (3) and
1.9 mA cm22 (4).
applied voltage is increased. The device has a turn-on voltage of
11 V, a high brightness of 139 mW (corresponding to a
luminance of 4538 cd m22) at 18 V, and an excellent external
quantum efficiency of 0.65% at 17 V and 94 mA cm22. This is
one of the best results for blue EL devices using aluminium as
the cathode.14,16,17
The EL data of the silole devices are summarized in Table 1.
All the devices emit blue light of y490 nm. The substituents on
the ring silicon atoms largely affect the EL properties of the
siloles. The siloles with electron-withdrawing and s*–p*
conjugating 1,1-(di)phenyl group(s) (3 and 4) show much
higher brightness and external quantum efficiencies than their
counterparts with alkyl groups (1 and 2). Moreover, the high
melting points of 3 and 4 should be emphasized as a very
important attribute to prevent crystallization that leads to
device failure. While the device performance of 2 is only slightly
better than that of Adachi’s silole polymer,8 the 1-phenylsilole
3 greatly outperforms the 1,1-dialkylsilole 2—it is truly
remarkable that the EL properties of the siloles can be
manipulated to such a great extent (2 orders of magnitude) by
simply varying the type of the 1-substituent.
Fig. 3 Electroluminescence spectra of the ITO/TPD/silole/Alq3/Al
devices.
latter.15 The difference in the PL properties of the silole rings
and their acyclic cousin 5 is even more dramatic and striking,
presenting an excellent example demonstrating the importance
of molecular planarity and rigidity in the photophysical
process.
Electroluminescence
The efficient PL emission from the siloles prompted us to check
their EL behaviors, whereas 5 was not further studied because
of its PL inactivity. The EL devices were fabricated using the
silole, TPD and Alq3 as the light-emitting, hole-transport and
electron-transport layers, respectively. Fig. 3 shows the EL
spectra of the siloles, which are similar to their PL spectra with
the emission maximums located at y490 nm. This suggests
that the blue emission comes mainly from the radiative decay of
the singlet excitons of the siloles formed by the recombination
of holes and electrons injected from the TPD and Alq3 layers,
respectively.
The current–voltage–brightness curves of the EL device of a
silole, that is 3, are given in Fig. 4. The device shows an
excellent diode behavior: under the forward bias (a positive
voltage applied to the ITO electrode), the current increases
exponentially with an increase in the applied voltage after
exceeding the turn-on voltage. In contrast, under reverse bias,
no obvious increase in the current density is observed when the
Conclusions
Four siloles are synthesized by a simple chemical reaction in a
one-pot procedure. The structural planarity and rigidity endow
the siloles with excellent luminescencent properties, which can
be further tuned by varying the 1,1-substituents on the ring
silicon atoms. The EL device of the 1-phenylsilole 3 emits blue
light with a high brightness and an excellent external quantum
efficiency. Noting that a figure of 100 cd m22 is considered
bright enough for a flat panel display,16,17 the extremely high
luminance of 3 (4538 cd m22) makes the silole a promising
candidate for luminescent materials to be used in LED devices
and systems.
Acknowledgements
We thank the financial support by the National Natural
Science Foundation of China, the Postdoctoral Foundation of
China, a Knowledge Innovation Program of the Chinese
Academy of Sciences, the K. C. Wong Education Foundation
of Hong Kong, the Research Grants Council of the Hong
Kong Special Administrative Region, China, and the Joint
Laboratory for Nanostructured Materials and Technology
between the Chinese Academy of Sciences and the Hong Kong
University of Science & Technology.
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