Highly fluorescent blue-emitting materials from the Heck reaction of triphenylvinylsilane with conjugated dibromoaromatics

Article abstract of DOI:10.1055/s-2006-951506

A new family of conjugated, highly photoluminescent blue-emitting organosilicon materials prepared from the Heck reaction of triphenylvinylsilane with various dibromoaromatics is described. The products are prepared in one high-yielding step (60-79%) from readily available starting materials. Their high photoluminescent quantum yields (67-98%) coupled with moderate glass transition temperatures (Tg, 69-138 °C) make them excellent candidates for active materials in organic light-emitting diodes (OLEDs). Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-951506

LETTER
3009
Highly Fluorescent Blue-Emitting Materials from the Heck Reaction of
Triphenylvinylsilane with Conjugated Dibromoaromatics
H
ighly Fluorescent Blu
e
e-Emitting
e
Materials Yoon Lo, Alan Sellinger*
Institute of Materials Research and Engineering (IMRE), 3 Research Link, 117602, Singapore
Fax +6568727528; E-mail: alan-sellinger@imre.a-star.edu.sg
Received 13 April 2006
Abstract: A new family of conjugated, highly photoluminescent
blue-emitting organosilicon materials prepared from the Heck reac-
tion of triphenylvinylsilane with various dibromoaromatics is de-
scribed. The products are prepared in one high-yielding step (60–
79%) from readily available starting materials. Their high photolu-
minescent quantum yields (67–98%) coupled with moderate glass
transition temperatures (Tg, 69–138 °C) make them excellent can-
didates for active materials in organic light-emitting diodes (OLE-
Ds).
Key words: Heck reaction, organosilicon, photoluminescent,
OLEDs, vinyl silanes
Figure
1
Space-filling model of 1,4-bis[(E)-2-(triphenylsi-
lyl)vinyl]-benzene (3a) showing the ‘dumbbell-like’ structure that
suppresses crystallization.19
Of the vast number of reports related to Heck chemistry,1–4
relatively few describe the cross coupling of aromatic
halides with vinyl silanes.5–8 Of these reports, the formed
Si-vinyl-aromatic containing molecules are often used as
vinylating agents to replace the more toxic vinyltin com-
pounds used in Stille coupling.9–12
photoluminescence.18 Lastly, triphenylvinylsilane is a
commercially available, air- and moisture-stable solid.
As shown in Scheme 1, we employ bis(tri-tert-butylphos-
phine)palladium(0) {Pd[P(t-Bu)3]2} as catalyst, dicyclo-
hexylmethylamine as base/HBr scavenger and toluene as
solvent.20 The reactions are run at 80 °C and are generally
complete within 4–16 h – much more milder conditions
than previously reported for vinylsilane-based Heck cou-
pling.6–8 The reaction progress was monitored by TLC and
MALDI-TOF. Product purification is very straightfor-
ward using precipitation and column chromatography
techniques to produce target molecules in 60–79% yield.
Product intermediate 2d was prepared using known pro-
cedures.21
Our interest is to prepare new Si-vinyl-aromatic contain-
ing materials using Heck chemistry, as opposed to other
more arduous protocols,13,14 and study their properties to-
wards application in organic light-emitting diodes (OLE-
Ds). We have previously reported the Heck coupling of
octavinylsilsesquioxane with various monohalo function-
alized aromatics and their resultant thermal, optical and
device properties.15–17 As an extension of this work, we re-
port here the Heck coupling of triphenylvinylsilane with
various dibromoaromatics to form blue-emitting materials
with excellent thermal and optical properties.
MALDI-TOF spectra show a very narrow distribution of
peaks for all the compounds suggesting formation of sin-
gle molecules respectively for all cases. A representative
spectrum is shown in Figure 2 for compound 3e. These re-
sults also confirm the complete suppression of double
Heck additions across individual vinyl groups as no mol-
ecules with three or four additions of triphenylvinylsilane
could be found. As a control experiment we reacted tri-
phenylvinylsilane with a 2.2-fold excess of 1-bromo-4-
fluorobenzene and <1% of the double Heck product was
observed by GC-MS.
We chose triphenylvinylsilane as a precursor in this study
for a variety of reasons. First, in order for a material to be
considered useful for application in OLEDs, it generally
must form amorphous thin films on the order of 25–75
nm. In the case of triphenylvinylsilane, the bulky triphen-
ylsilyl groups provide a ‘dumbbell-like’ molecular shape
that tends to suppress crystallization, as shown in Figure 1
for compound 3a. Second, the bulkiness of the triphenyl-
silyl groups prevents double Heck addition across a single
vinyl group, which has been found in other vinylsilane
systems.12,16 Third, the formed linkage provides extended
electron delocalization from the aromatic moiety to sili-
con through s*–p* interactions, that potentially provides
materials with semiconducting properties and enhanced
Thermal properties of the compounds were investigated
using differential scanning calorimetry (DSC), and ther-
mal gravimetric analysis (TGA), see Table 1. DSC indi-
cates moderate to high glass transition temperatures (Tg)
values ranging from 69–138 °C. The compounds are com-
pletely amorphous as indicated by no melting or crystalli-
zation events via DSC. TGA demonstrates the high
thermal stability of these compounds with 5% mass loss in
N2 ranging from 350–451 °C.
SYNLETT 2006, No. 18, pp 3009–3012
0
3
.1
1
.2
0
0
6
Advanced online publication: 25.10.2006
DOI: 10.1055/s-2006-951506; Art ID: S07406ST
© Georg Thieme Verlag Stuttgart · New York

3010
M. Y. Lo, A. Sellinger
LETTER
Pd[P(t-Bu)3]2
Si
Br Ar Br
+
Si
Ar
NCy2Me
Si
toluene, 80 °C
2a–g
3a–g
1
a
b
d
N
N
c
e
S
N
N
f
Scheme 1 Heck coupling of triphenylvinylsilane with various dibromoaromatics.
Figure 3 Photoluminscent spectra for compounds 3a–e.
Figure 2 Representative MALDI-TOF spectrum of compound 3e,
9,10-bis[(E)-2-(triphenylsilyl)vinyl]anthracene.
In summary, a new family of highly fluorescent blue-
emitting, amorphous, and thermally stable organosilicon
materials have been synthesized from a straightforward
Heck-chemistry protocol using readily available starting
materials. Further work in our laboratory will be directed
towards evaluating these materials as emitters and charge
transport materials in OLEDs.
Photoluminescence (PL) spectra were measured in tolu-
ene and peak emission values are reported in Figure 3 and
Table 1. The materials in this study emit deep blue (3a–c),
blue (3d), and sky blue (3e,f). PL quantum yields (FFL) in
toluene were calculated relative to 9,10-diphenylan-
thracene as standard. The calculated quantum efficiencies
were relatively high, ranging from 67–98%, as shown in
Table 1.
Triphenylvinylsilane, 9,10-dibromoanthracene, 4-4¢-dibromo-
biphenyl, 2,7-dibromo-9,9-dimethyl-9H-fluorene, 1,4-dibromo-
benzene, 4,7-dibromobenzo[c][1,2,5]thiadiazole, bis(tri-t-butyl-
phosphine)-palladium(0) and dicyclohexylmethylamine were pur-
chased from Sensient Imaging Technologies, Pacific Chem Source,
Synlett 2006, No. 18, 3009–3012 © Thieme Stuttgart · New York

LETTER
Highly Fluorescent Blue-Emitting Materials
3011
Table 1 Thermal and Optical Properties of Compounds 3a–g
c
Molecule
Mol. Wt.
647.0
Tg (°C)a
75
Td (°C)a
366
labsmax (nm)b
308
lemmax, (nm)b
395
FFL
3a
3b
3c
3d
3e
3f
0.81
0.98
0.88
0.85
0.70
0.67
723.1
89
395
323
361
763.1
106
138
69
392
342
372
1169.7
747.1
451
374
421
350
394
486
705.0
85
367
399
481
a Measured in nitrogen.
b Measured in toluene. PL spectra obtained by irradiating at respective UV/Vis absorbance maximum wavelengths.
c 9,10-Diphenylanthracene used as standard.
Aldrich, Strem and TCI and used as received. All reactions were
performed using Schlenk techniques in an argon atmosphere with
anhydrous solvents.
Compound 3a
1H NMR (CDCl3): d = 6.96–7.09 (m, 4 H), 7.26–7.52 (m, 20 H),
7.59–7.66 (m, 14 H). 13C NMR (CDCl3): d = 123.47, 126.99,
127.92, 129.61, 133.54, 134.42, 136.03, 148.26. 29Si NMR
(CDCl3): d = –16.66. MS (MALDI; Ag+ adduct, 100%): m/z calcd:
755.16; found: 753.47. Isolated yield: 60%.
All materials were characterized with 1H NMR, 13C NMR and 29Si
NMR. All NMR analyses were carried out in CDCl3 solvent on a
Bruker DPX 400 MHz NMR. Tetramethylsilane (TMS) was used as
an internal standard. Matrix-Assisted Laser Desorption/Ionization
Time-of-Flight mass spectrometry (MALDI-TOF) was performed
on a Bruker Autoflex MALDI Tandem TOF/TOF mass spectrome-
ter. Dithranol was used as the matrix and silver trifluoromethane-
sulfonate (AgTFA) as the ion source when necessary.
Photoluminescence (PL) spectra were recorded on a Perkin-Elmer
LS50B luminescence spectrometer. Solution PL were measured in
toluene. All thermal gravimetric calorimetry (TGA) analyses were
carried out on a Thermal Analysis TGA 2050 module under N2 with
a flow rate of 60 mL/min and a heating rate of 10 °C/min to 900 °C.
Differential scanning calorimetry (DSC) analyses were performed
under N2 on a Thermal Analysis DSC 2920 module with a scanning
rate of 10 °C/min. Photoluminescent quantum yields (PLQE, FFL)
were calculated based on a previously described method. 9,10-
Diphenylanthracene was selected as reference for the PLQE mea-
surement. The relative quantum efficiency was calculated accord-
ing to the equation:
Compound 3b
1H NMR (CDCl3): d = 6.73–7.01 (m, 4 H), 7.30–7.48 (m, 14 H),
7.51–7.59 (m, 24 H). 13C NMR (CDCl3): d = 122.38, 123.27,
127.08, 127.26, 127.95, 129.63, 134.48, 136.05, 139.33, 148.28.
29Si NMR (CDCl3): d = –16.62. MS (MALDI; Ag+ adduct, 100%):
m/z calcd: 831.19; found: 829.59. Isolated yield: 72%.
Compound 3c
1H NMR (CDCl3): d = 1.54 (s, 6 H), 6.96–7.09 (m, 4 H), 7.30–7.52
(m, 20 H), 7.59–7.66 (m, 16 H). 13C NMR (CDCl3): d = 27.25,
46.83, 120.45, 120.83, 122.35, 126.00, 126.33, 127.95, 129.62,
134.60, 136.10, 137.43, 149.14, 154.55. 29Si NMR (CDCl3): d =
–16.46. MS (MALDI; Ag+ adduct, 100%): m/z calcd: 871.22;
found: 871.49. Isolated yield: 77%.
Compound 3d
1H NMR (CDCl3): d = 1.54 (s, 9 H), 6.90–7.06 (m, 4 H), 7.35–7.44
(m, 50 H), 7.51–7.68 (m, 13 H). 13C NMR (CDCl3): d = 31.43,
42.53, 119.41, 120.11, 120.25, 123.06, 124.26, 124.49, 126.20,
127.10, 127.30, 127.66, 127.88, 129.52, 132.99, 134.76, 136.04,
144.51, 146.49, 148.33. 29Si NMR (CDCl3): d = –16.54. MS (MAL-
DI; 100%): m/z calcd: 1168.55; found: 1169.11. Isolated yield:
72%.
F
FL(sample) = [Astd/Asample][Fsample/Fstd][nsample/n std]FFL(std)
where std is the 9,10-diphenylanthracene reference, A the absorp-
tion at the excitation wavelength, F the total integrated emission, n
the refractive index of the solvent, and FFL the quantum yield.
General Procedure for Heck Reaction 3a–g
Procedure for the Preparation of 3a
Compound 3e
Into a flame-dried 25-mL round-bottom flask were added triphen-
ylvinylsilane 1 (0.3 g, 1.05 mmol, 1 equiv), 1,4-dibromobenzene 2a
(0.53 mmol, 0.5 equiv) and bis(tri-t-butylphosphine)palladium(0)
{Pd[P(t-Bu)3]2} (5.34 mg, 0.01 mmol, 2 mol%). The flask was
evacuated and refilled with argon three times. Anhyd toluene (15
mL) was added followed by nitrogen degassed dicyclohexylmethyl-
amine (Cy2NCH3) (0.24 mL, 1.16 mmol, 2.2 equiv). The reaction
was stirred for 24 h at 80 °C and monitored by TLC and MALDI-
TOF. Upon completion, the reaction mixture was cooled to r.t. and
filtered through a short Celite®/MgSO4 column to remove Pd
particles and the amine hydrochloride. The filtrate was concentrated
and precipitated into stirring acidified MeOH. The precipitate was
filtered and purified via column chromatography using CH2Cl2–
hexane (3:1) as eluent. After chromatography, the solution was
concentrated and reprecipitated into acidified MeOH and collected
by filtration. Compound 3a was obtained as a yellow powder
(60% yield).
1H NMR (CDCl3): d = 6.85–6.99 (m, 4 H), 7.42–7.51 (m, 18 H),
7.71–7.80 (m, 12 H), 8.25–8.27 (m, 8 H). 13C NMR (CDCl3):
d = 125.27, 126.13, 128.10, 128.57, 129.79, 134.41, 134.49,
135.14, 136.10, 147.01. 29Si NMR (CDCl3): d = –16.91. MS (MAL-
DI; Ag+ adduct, 100%): m/z calcd: 855.19; found: 855.53. Isolated
yield: 60%.
Compound 3f
1H NMR (CDCl3): d = 6.90–7.04 (m, 4 H), 7.35–7.51 (m, 20 H),
7.56–7.68 (m, 12 H). 13C NMR (CDCl3): d = 127.75, 127.79,
127.89, 129.70, 130.18, 134.32, 136.13, 136.57, 144.05. 29Si NMR
(CDCl3): d = –16.55. MS (MALDI; Ag+ adduct, 100%): m/z calcd:
813.12; found: 813.46. Isolated yield: 79%.
Synlett 2006, No. 18, 3009–3012 © Thieme Stuttgart · New York

3012
M. Y. Lo, A. Sellinger
LETTER
(15) Sellinger, A.; Laine, R. M. US Patent 6517958, 2003.
(16) Sellinger, A.; Tamaki, R.; Laine, R. M.; Ueno, K.; Tanabe,
H.; Williams, E.; Jabbour, G. E. Chem. Commun. 2005,
3700.
(17) Lo, M. Y.; Sellinger, A. Chem. Rec. 2006, 6, 157.
(18) Yamaguchi, S.; Tamao, K. Chem. Lett. 2005, 34, 2.
(19) The space-filling model was generated using Materials
Studio software package from Accelrys Inc. using a
molecular mechanics (MM) method.
Acknowledgment
Financial support provided by the Institute of Materials Research
and Engineering (IMRE) and the Agency for Science, Technology
and Research (A*STAR), Republic of Singapore. The authors thank
Dr. Sundarraj Sudhakar of IMRE for the preparation of compound
2d and Ms. Ting Ting Lin for generating space-filling model in
Figure 1. Correspondence and requests should be addressed to A.S.
(20) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.
(21) A flame-dried flask was charged with t-BuONa (5.8 g, 60
mmol) and dried at 110 °C under vacuum for 1 h. After
cooling to r.t., 4,4¢-dibromobiphenyl (3.12 g, 10 mmol),
aniline (1.83 mL, 20 mmol) and Pd[P(t-Bu)3]2 (25.5 mg,
5·10–2 mmol) were added followed by 40 mL of toluene. The
reaction mixture was purged using argon for a few minutes
and then heated to 70 °C under an atmosphere of argon until
all the amine was consumed as monitored by TLC. The
reaction mixture was then cooled to r.t. followed by the
addition of 4-bromo-tert-butylbenzene (3.47 mL, 20 mmol),
Pd[P(t-Bu)3]2 (25 mg, 5·10–2 mmol) and toluene (20 mL).
The reaction mixture was then stirred at 90 °C for 15 h.
Finally the reaction mixture was cooled, extracted with H2O
and the organic layer dried with MgSO4 and concentrated to
get an oily residue which was then suspended in MeOH to
get a grey solid which was filtered and dried. The crude
product was used as such for the next step. To a solution of
the compound above (4.9 g, 8.33 mmol) in 300 mL DMF
was added dropwise at r.t. a solution of N-bromosuccinimide
in 100 mL DMF. After addition the reaction mixture was
stirred at r.t. overnight. The reaction mixture was
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precipitated by pouring into acidified H2O and the solids
obtained were filtered, washed thoroughly with H2O and
dried. Yield: 98%. The product(2d) obtained was used in the
next step without any further purification.
(14) Cheng, Y.-J.; Luh, T.-Y. Macromolecules 2005, 38, 4563.
Synlett 2006, No. 18, 3009–3012 © Thieme Stuttgart · New York