Organosilicon compounds with blue photoluminescence properties

Article abstract of DOI:10.1071/CH04059

A series of π-conjugated compounds and related Si compounds with blue photoluminescent properties have been prepared. Structures of products were confirmed by NMR spectra and mass spectrometry. Photoluminescent properties are discussed. Based on luminesce

Full text of DOI:10.1071/CH04059

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2004, 57, 811–814
Organosilicon Compounds with Blue Photoluminescence Properties
Guorong Zheng,A Wei Li,A Zixing Wang,A and Ping LuA,B
A
Department of Chemistry, Zhejiang University, Zhejiang 310027, P.R. China.
B Author to whom correspondence should be addressed (e-mail: pinglu@zju.edu.cn).
A series of π-conjugated compounds and related Si compounds with blue photoluminescent properties have been
prepared. Structures of products were confirmed by NMR spectra and mass spectrometry. Photoluminescent prop-
erties are discussed. Based on luminescent properties, these Si compounds might find utility as emitters in organic
light-emitting diode displays.
Manuscript received: 5 March 2004.
Final version: 18 May 2004.
Introduction
(Table 1), 2a, 2b, and 2c are more flexible than 1a, 1b,
and 1c.
Electroluminescence (EL) devices have been studied because
of their potential application in full-colour flat-panel dis-
plays.[1] Compared to other display technologies, organic
light-emitting diodes (OLEDs) show unique advantages, for
example, low driving voltage, wide viewing angle, easy pro-
cessing, high brightness, and ultra-thin layers.[2–5] However,
the lifetimes of OLEDs limit their application in large-area
displays. The synthesis of new organic materials with bet-
ter properties is an effective way to solve this restrictive
problem. Organic compounds and polymers based on an
sp2-hybridized carbon skeleton[6,7] have been developed for
this purpose. Compared to polymers, organic compounds are
easier to purify by sublimation, have exact molecular weight,
and give rise to high colour purity.[6]
Results and Discussion
Synthesis and Thermal Properties
The synthetic routes are described in Scheme 1. A Grig-
nard reaction between p-tolylmagnesium bromide and
dichlorodimethylsilane yields dimethylditolylsilane 3. Treat-
ment of 3 with N-bromosuccinimide and dibenzoyl per-
oxide in carbon tetrachloride yields the corresponding
bis(bromomethyl)phenyldimethylsilane 4. Bis[4-(diethoxy
phosphoryl)methyl]phenyldimethylsilane 5 was prepared by
an Arbuzov reaction between 4 and triethylphosphite in
toluene.
Conjugated compounds 1a–1c were synthesized by a
Wittig–Horner reaction between the appropriate carbonyl
compound and bis[4-(diethoxyphosphoryl)methyl]biphenyl
With sp2-conjugated systems, there are at least three
practical considerations. The rigidity of the molecule will
definitely influence device fabrication. The solubility of the
molecule will affect its preparation and purification. Finally,
a red shift of the absorption and emission is observed as
the molecular weight increases. To investigate these, several
organosilicon compounds have been synthesized:[7] silicon
has empty d orbitals that can conjugate with carbon p orbi-
tals to form d–p conjugation. Silicon partially interrupts the
conjugated systems. The net result is that the absorption
and emission could be blue-shifted and the colour purity
improved. On the other hand, these molecules are more
flexible and easier to fabricate.
A new type of organic light-emitting material with sili-
con inserted into an sp2-hybridized carbon skeleton has been
synthesized by a Wittig–Horner reaction in moderate yields
(Scheme 1). Both the absorption and emission λmax of 2a
and 2b are blue-shifted. In the case of 2c the absorption is
blue-shifted, while the emission λmax is split into two peaks
(see Fig. 3). Based on their relative lower melting points
in good yield. Products were characterized by 1H NMR, 13
C
NMR, and mass spectrometry (MS). Compounds 1a, 1b, and
1c show poor solubility and high thermal stability.Analogous
silicon-containing compounds (2a, 2b, and 2c) were highly
soluble in common organic solvents.
Photoluminescence Properties
Table 1 lists the λmax for the UV absorption and photolumi-
nescence (PL) spectra of six compounds in tetrahydrofuran
(THF).As shown, the absorption λmax arising from the π–π*
transition of the diphenyl-based conjugated compounds (1a,
1b, and 1c) ranged from 364 to 369 nm, while the absorp-
tion λmax of the silicon-containing compounds (2a, 2b, and
2c) ranged from 327 to 336 nm. These were blue-shifted an
average of 35 nm compared to those of the diphenyl-based
compounds (1a, 1b, and 1c). In the PL spectra, the emission
λmax of 1a, 1b, and 1crangedfrom416to439 nm, whilethose
© CSIRO 2004
10.1071/CH04059
0004-9425/04/080811

812
G. Zheng et al.
S
CHO
CHO
S
S
1a
O
O
O
ButOK
O
(EtO)2PH2C
CH2P(OEt)2
toluene
O
1b
O
1c
NBS/CCl4
BrH2C
CH2Br
Si
Si
S
CHO
CHO
S
Si
3
4
5
S
2a
(a) Mg/Et2O
P(OEt)3
(b) (CH3)2SiCl2
O
O
O
ButOK
Br
CH3
Si
O
(EtO)2PH2C
CH2P(OEt)2
Si
toluene
O
2b
Si
O
2c
Scheme 1. Synthesis of organic emitters.
Table 1. Properties of organic emitters with or without silicon
insertion
1a-UV
1a-Em
2a-UV
2a-Em
350
300
250
200
150
100
50
Compound
no.
mp [◦C]
Absorption
λmax [nm]
Emission
λmax [nm]
Quantum
yield
1a
1b
1c
2a
2b
2c
280 (dec.)
280 (dec.)
260–262
185–186
138–140
110–112
369
365
364
335
327
336
416, 439
416, 438
428
391
378
0.55
0.21
0.0075
0.061
0.016
0.0052
418, 438
of 2a, 2b, and 2c are between 378 and 438 nm, respectively
(Figs 1 and 2).
0
250
300
350
400
450
500
550
600
Luminescence was observed for both 1c and 2c (Fig. 3)
although the quantum yields were low, being 0.0075 and
0.0052, respectively. As previously reported,[8] no lumine-
scence was found when the linkage was a 1,4-phenylene
ring.
In the case of the silicon compounds, the backbone is
more flexible. This may be because of the longer Si C
bond (1.91 Å) compared to the C C bond (1.53 Å). This will
Wavelength (nm)
Fig. 1. UV-vis and emission spectra of 1a and 2a.
disturb the coplanarity of the π-conjugated compounds and
reduce their crystallinity. In other words, the compounds with
silicon insertion should be good candidates for blue emitters
in OLED displays.

Organosilicon Compounds with Blue Photoluminescence Properties
813
spectrophotometer. Quantum yields are calculated based on POPOP
(1,4-bis(5-phenyloxazol-2-yl)benzene) as standard.[9]
1b-UV
300
1b-Em
2b-UV
2b-Em
250
200
150
100
50
Typical Procedure for the Synthesis of Symmetrical Compounds
(1a, 1b, and 1c)
4,4ꢀ-Bis[(diethoxyphosphoryl)methyl]biphenyl (4 mmol) and a car-
bonyl compound (8 mmol, see Scheme 1) were dissolved in 30 mL
of toluene. The mixture was heated under stirring to 110◦C. To this
solution, solid potassium tert-butoxide (12 mmol) was added in one
portion. After refluxing for 3 h, solvent was removed by rotary evap-
oration. The residue was dissolved in chloroform, precipitated from
methanol, and filtered. The product was purified by recrystallization
from chloroform/methanol.
Compound 1a, yield 85%, decomposition above 280◦C (Found: C
78.0, H 4.8. Calc. for C24H18S2: C 77.8, H 4.9%). δH ((D6)DMSO)
7.00 (2H, d, J 16.3), 7.08 (2H, dd, J1 4.6, J2 3.7), 7.24 (2H, d, J 3.7),
7.47 (2H, d, J 4.6), 7.50 (2H, d, J 16.3), 7.66 (4H, d, J 8.3), 7.72 (4H, d,
J 8.3); m/z (EI) 370 (100%, M+).
0
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 2. UV-vis and emission spectra of 1b and 2b.
Compound 1b, yield 84%, decomposition above 280◦C (Found: C
84.9, H 5.4. Calc. for C24H18O2: C 85.2, H 5.4%). δH ((D6)DMSO)
6.56–6.58 (4H, m), 7.04 (2H, d, J 16.4), 7.19 (2H, d, J 16.4), 7.66 (4H,
d, J 8.4), 7.72 (6H, d, J 8.4). δC (CDCl3) 110.1, 112.8, 117.5, 126.6,
127.3, 127.6, 136.6, 139.2, 143.7, 153.5. m/z (EI) 338 (100%, M+).
Compound 1c, yield 80%, mp 260–262◦C (Found: C 95.0, H 5.8.
Calc. for C40H26: C 94.8, H 5.2%). δH (CDCl3) 7.11 (2H, dd, J1 7.9,
7.2), 7.34 (4H, dd, J 7.3, 7.2), 7.40 (2H, t, J 7.2), 7.72–7.76 (12H, m),
7.82 (6H, dd, J 6.4, 7.9). δC 119.9, 120.1, 120.5, 124.7, 126.9, 127.1,
127.3, 128.5, 128.9, 130.2, 130.3, 136.5, 136.8, 137.0, 139.5, 139.9,
140.4, 141.7. m/z (EI) 506 (23.15%, M+).
14
12
10
8
1c-UV
1c-Em
2c-UV
2c-Em
6
Synthesis of 2a and 2b
4
Bis[4-(diethoxyphosphoryl)methyl]phenyldimethylsilane (5 mmol) and
a carbonyl compound (10 mmol, Scheme 1) were dissolved in 30 mL of
toluene. Potassium tert-butoxide (12 mmol) was added. The mixture
was stirred at room temperature for 4 h and the solvent was removed.
The residue was purified by column chromatography (silica gel) using
n-hexane/dichloromethane (20 : 1); 2a and 2b were obtained.
Compound 2a, yield 72%, mp 185–186◦C (Found: C 72.4, H 5.8.
Calc. for C26H24S2Si: C 72.9, H 5.6%). δH (CDCl3) 0.56 (6H, s), 6.92
(2H, d, J 16.1), 6.99–7.00 (2H, m), 7.06 (2H, d, J 3.4), 7.18 (2H, d,
J 5.0), 7.26 (2H, d, J 16.1), 7.44 (4H, d, J 7.9), 7.50 (4H, d, J 7.9). δC
(CDCl3) −0.007, 124.5, 126.8, 128.0, 128.6, 129.9, 130.6, 136.9, 139.9,
140.1, 145.2. m/z (EI) 428 (73.62%, M+).
2
0
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. UV-vis and emission spectra of 1c and 2c.
Conclusion
Compound 2b, yield 76%, mp 137–140◦C (Found: C 78.5, H 5.9.
Calc. for C26H24O2Si: C 78.8, H 6.1%). δH (CDCl3, TMS) 0.55 (6H,
s), 6.34–6.41 (4H, m), 6.92 (2H, d, J 16.3), 7.02 (2H, d, J 16.3), 7.39
(2H, s), 7.43 (4H, d, J 7.9), 7.49 (4H, d, J 7.9). δC (CDCl3, TMS) 0.002,
111.2, 114.1, 119.3, 128.1, 129.4, 136.9, 139.9, 140.1, 144.6, 155.6. m/z
(EI) 396 (77.84%, M+).
In conclusion, a series of blue emitters were synthesized
in moderate yields by a Wittig–Horner reaction, and fully
characterized by spectroscopy. Their thermal and optical
properties were investigated. The results indicated that these
new silicon-inserted compounds are highly promising mate-
rials for the development of stable, blue, electroluminescent
emitters. Further investigation of the thermal behaviour and
photoluminescence properties of these new molecules is in
progress.
Synthesis of 2c
Compound 2c was prepared as above except that the mixture was stirred
at 110◦C for 10 min.
Compound 2c, yield 70%, mp 110–112◦C (Found: C 89.3, H 5.9.
Calc. for C42H32Si: C 89.3, H 5.7%). δH (CDCl3) 0.67 (6H, s), 7.07
(2H, dd, J1 3.05, J2 3.07), 7.29–7.36 (8H, m), 7.59–7.62 (4H, m), 7.65–
7.67 (6H, m), 7.70 (4H, d, J 7.55), 7.77 (2H, d, J 7.45). δC (CDCl3) 0.9,
122.9, 123.0, 123.5, 127.7, 129.9, 130.3, 130.4, 131.5, 131.9, 132.0,
137.6, 139.8, 139.9, 141.0, 141.5, 142.5, 142.8, 144.6. m/z (EI) 564
(8.02%, M+).
Experimental
Instruments
1H and 13C NMR spectra were obtained on a BrukerAVANCE DMX500
spectrometer operating in the FT mode. Five percent w/v solutions in
CDCl3 or (CD3)2SO were used to obtain NMR spectra. Tetramethyl-
silane (TMS) was used as an internal standard. A Hewlett Packard
5989B electron-impact mass spectrometer was used to obtain mass
spectra. Fluorescence measurements were made with an RF-5301
spectrofluorometer (Shimadzu, Kyoto, Japan) equipped with a xenon
lamp. UV-vis absorption spectra were recorded on a Shimadzu UV-265
Acknowledgments
P.L. thanks the National Science Foundation (20074032,
20374045) and the Ministry of Education of China for
financial support.

814
G. Zheng et al.
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