Synthesis and photophysical properties of stilbenoid dendrimers via Heck reaction on a tetraphenylethylene core

Article abstract of DOI:10.1246/bcsj.20100064

Two new tetraphenylethylene-cored stilbenoid dendrimers have been efficiently prepared by fourfold Heck reaction with appropriate dendron molecules. The methodology permits highly trans-selective coupling. The optical absorption and emission properties of these systems were studied. The materials display strong emission in the blue region.

Full text of DOI:10.1246/bcsj.20100064

Short Articles
Bull. Chem. Soc. Jpn. Vol. 83, No. 10, 1269–1271 (2010) 1269
Synthesis and Photophysical
Properties of Stilbenoid
c
Dendrimers via Heck Reaction
on a Tetraphenylethylene Core
a
a
b
b
Priyabrata Roy, Debabrata Jana,
and Binay K. Ghorai*
c
Department of Chemistry, Bengal Engineering and Science
University, Shibpur, Howrah 711 103, India
Figure 1. a: Diagonal conjugation (transoid), b: lateral
Received March 5, 2010
conjugation (cisoid), c: cross conjugation,
: con-
E-mail: bkghorai@yahoo.co.in
jugated spacer.
Br
Br
NH2
CO2Me
a
b
CO2Me
Two new tetraphenylethylene-cored stilbenoid dendri-
mers have been efficiently prepared by fourfold Heck reaction
with appropriate dendron molecules. The methodology
permits highly trans-selective coupling. The optical absorp-
tion and emission properties of these systems were studied.
The materials display strong emission in the blue region.
1
2 (65%)
Organic light-emitting devices (OLEDs) have been attracting
considerable attention because of their potential application
within flat-panel displays.1,2 Efficient blue-light OLEDs are
of particular interest because they are desired for use as blue
light sources in full-color display applications;3-8 they can also
serve as hosts for exothermic energy transfer to lower-energy
fluorophores to realize white-light OLEDs.9-13 High-perform-
ance blue-light-emitting materials exhibiting ideal color purity,
good stability, and high fluorescence efficiency remain rela-
tively rare. In particular, dendritic poly(phenylenevinylenes),
also called stilbenoid dendrimers, represent an important group
within this class of material. Several different studies have been
published to date concerning the synthesis and properties of
these materials.14 For example, such compounds have been used
successfully as molecular wires in molecular scale electronics,15
and nanotechnological devices, light emitting diodes,16 organic-
based lasers for fabrication,17 and optoelectronics.17c
d, e
X
5 (66%)
3, X = CO2Me (75%)
4, X = CH2OH (78% )
c
Scheme 1. Reagents and conditions: (a) (i) Br2, HOAc; (ii)
t-BuONO, DMF, 60 °C; (b) 4-t-butylstyrene, Pd(OAc)2, n-
Bu4NBr, KOAc, DMF, 90 °C, 24 h; (c) LiAlH4, THF, rt; (d)
PDC, CH2Cl2, rt; (e) Ph3P+MeI , n-BuLi, THF, ¹20 °C.
¹
of tert-butyl groups to the terminal aryl rings as in the oligo-
meric series the solubility properties can be slightly enhanced.
For synthesis of fully conjugated stilbenoid dendrimer, we
choose tetraphenylethylene two-dimensional scaffolds as a core.
These systems usually have a fully conjugated backbone in
three modes: diagonal conjugation, lateral conjugation, and
cross conjugation. Since the three ³-systems could potentially
interact with each other, the overall photophysical properties of
compound based on Figure 1 were expected to be different from
those of the individual one-dimensional ³-system. All four
corners of this module are attached with identical stilbenoid
peripheral units. Here, phenylenevinylene dendritic arms can
function as light-harvesting antennae.16b This manuscript
focuses on the synthesis and characterization of a tetraphenyl-
ethylene-cored first-generation stilbenoid dendrimers bearing
phenylenevinylene arms as peripheral units. The optical absorp-
tion and emission properties were examined. By incorporation
Results and Discussion
Our studies began with dendron 5, which was prepared from
methyl anthranilate (1) following a slightly modified procedure
as reported by Sengupta and co-workers (Scheme 1).18 Thus,
dibromination of 1 followed by deamination (t-BuONO, DMF)
gave methyl 3,5-dibromobenzoate (2) in 65% yield. Twofold
Heck reaction on 2 with 4-t-butylstyrene under Jeffery’s con-
ditions19 gave 3, which on treatment with lithium aluminum
hydride in THF afforded the benzyl alcohol 4. Conversion of 4
to the corresponding terminal olefin 5 was then accomplished
by pyridinium dichromate (PDC) oxidation followed by Wittig
reaction.
We have chosen tetraphenylethylene as a core molecule to
prepare stilbenoid dendrimer because this two-dimensional
Published on the web October 5, 2010; doi:10.1246/bcsj.20100064

1270 Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010)
© 2010 The Chemical Society of Japan
350000
300000
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0
2.0
1.5
1.0
0.5
0.0
I
I
I
I
a
6
7
b
300
400
500
600
Wavelength/nm
Figure 2. Absorption (black) and emission spectrum of 7
(blue), emission spectrum of tetraphenylethylene (red) in
CH2Cl2 solutions at C = 1.02 © 10¹6 M.
2.5
800000
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2.0
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8
500000
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0
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Scheme 2. Reagents and conditions: (a) 4-t-butylstyrene,
Pd(OAc)2, n-Bu4NBr, PPh3, KOAc, DMF, 100 °C, 24 h,
45%; (b) 5, Pd(OAc)2, n-Bu4NBr, PPh3, KOAc, DMF,
100 °C, 48 h, 21%.
300
350
400
450
500
550
600
650
scaffold facilitates additional conjugation due to the presence
of four phenyl rings. Moreover, tetraphenylethylene has a
strong emission property that provides opportunities for the
synthesis of tetraphenylethylene-cored stilbenoid dendrimers.
Particularly, in presence of meta-linkages in the phenylene-
vinylene peripheral unit lowers the molecular conjugation.
Both of these effects allow the system to emit blue light, the
primary color that is the hardest to obtain with polymer light-
emitting diodes (PLEDs).
Tetraiodide 6 was prepared from tetraphenylethylene20 by
the fourfold iodination with I2-PhI(OAc)2 in CH2Cl2 after
stirring at room temperature for 60 h in the dark in 76% yield.21
Generally, multifold Heck coupling reaction for stilbenoid
dendrimer preparation afforded low yields due to the insol-
ubility of partially coupled intermediates, which were observed
to precipitate from solution during early reaction times.22 Thus,
fourfold Heck reaction of 6 with 4-tert-butylstyrene derivative
under Jeffery’s phase transfer conditions [10% Pd(OAc)2, n-
Bu4NBr, KOAc, DMF, 100 °C] give rise to the first-generation
dendrimer 7 in 45% yield (Scheme 2). Under the same
conditions, fourfold Heck reaction of 6 with excess of styrene
derivative 5 led to the fluorescent meta-branched X-shaped
tetraphenylethylene-cored dendrimer 8 formed in 21% yield.
UV-Vis and PL Spectroscopy. The optical properties of
the synthesized stilbenoid dendrimers 7 and 8 were investigated
by UV-vis and photoluminescence (PL) spectroscopy on
CH2Cl2 solutions at room temperature. The data obtained are
Wavelength/nm
Figure 3. Absorption (black) and emission spectrum of 8
(blue), emission spectrum of 5 (red) in CH2Cl2 solutions at
C = 1.1 © 10¹6 M.
shown in Figures 2 and 3. These compounds show maximum
absorption wavelengths in the UV or visible region (320 nm).
The peaks at 320 nm were assigned to the particularities of the
stilbene units. The meta-arrangement through which the differ-
ent units are linked prevents efficient delocalization throughout
the conjugated backbone, and as a result, more or less the same
phenomenon observed in absorption spectroscopy. The materi-
als are strongly fluorescent and emit light in the blue region.
Fluorescence quantum yields of 7 and 8 are 0.42 and 0.65.23
In both cases, excitation at 320 nm, that is, at the absorption
maxima of the stilbene units, resulted in fluorescence in the
region 418-420 nm, typical of stilbenoid compounds. Partic-
ularly for compound 8, an additional shoulder peak at 521 nm
was observed (Figure 3). The fluorescence excitation spectrum
of 8 obtained by monitoring the emission at 420 nm and also
520 nm, for both cases produced an excitation band at 320 nm. It
is shown that the compounds 7 and 8 exhibit two emission
bands. The shorter wavelength emission bands of the com-
pounds 7 and 8 coincide with that of the peripheral stilbene
units. The emission bands at the longer wavelengths in both 7
and 8 are different from the emission of a simple tetraphenyl-

P. Roy et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 10 (2010) 1271
ethylene unit. The intriguing phenomenon can be explained by
energy transfer mechanism. The conjugative effect of the
peripheral stilbene unit with the central tetraphenylethylene unit
in both 7 and 8 initiates an efficient energy transfer form the
periphery to the central core for which centrally disposed
tetraphenylethylene unit in each case gives a new emission band
at the longer wavelength (for 7 at 512 nm and for 8 at 521 nm).
131.9, 130.9, 130.4, 128.9, 127.5, 126.4, 126.3, 125.7, 125.6,
34.6, 31.2; MALDI-TOF MS: m/e: 2006.6907 (MH+).
This work is supported by UGC [No. 37-93/2009(SR)],
UGC-SAP and CSIR (research fellowship to D.J.), New Delhi.
We thank MHRD, New Delhi for providing our Department
LC-MS and Spectrofluorimeter instrument.
Conclusion
References
In conclusion, we have successfully synthesized and
characterized a monodisperse stilbenoid first generation dendri-
mer in a straightforward manner using fourfold Heck coupling
reaction between tetraphenylethylene derivatives and an ap-
propriate stilbenoid unit. The protocol permits a highly trans-
selective synthetic route to stilbenoid compounds. The meta-
substitution pattern causes all chromophores to be independent,
and all of the compounds present absorption wavelengths in the
UV or visible region, and the materials exhibit strong emission
in the blue region.
1
2
3
T. Fuhrmann, J. Salbeck, MRS Bull. 2003, 28, 354.
C. W. Tang, S. A. Van Slyke, Appl. Phys. Lett. 1987, 51, 913.
C. Hosokawa, M. Eida, M. Matsuura, K. Fukuoka, H.
Nakamura, T. Kusumoto, Synth. Met. 1997, 91, 3.
Y. Duan, Y. Zhao, P. Chen, J. Li, S. Y. Liu, F. He, Y. G. Ma,
Appl. Phys. Lett. 2006, 88, 263503.
H.-T. Shih, C.-H. Lin, H.-H. Shih, C.-H. Cheng, Adv.
Mater. 2002, 14, 1409.
4
5
6
J. Y. Shen, C. Y. Lee, T.-H. Huang, J. T. Lin, Y.-T. Tao,
C.-H. Chien, C. T. Tsai, J. Mater. Chem. 2005, 15, 2455.
7
C.-C. Wu, Y.-T. Lin, K.-T. Wong, R.-T. Chen, Y.-Y. Chien,
Experimental
Adv. Mater. 2004, 16, 61.
8
R. C. Chiechi, R. J. Tseng, F. Marchioni, Y. Yang, F. Wudl,
General. Solvents and reagents were dried and purified by
distillation before use as follows: tetrahydrofuran from sodium
benzophenone ketyl, dichloromethane and chloroform from
P2O5, DMF from CaH2, and pyridine from solid KOH. Unless
otherwise noted, all reactions were carried out under an inert
atmosphere in flame dried flasks. After drying, organic extracts
were evaporated under reduced pressure and the residue was
column chromatographed on silica gel (Spectrochem, particle
size 100-200 mesh), using an ethyl acetate-petroleum ether
(60-80 °C) mixture as eluent unless specified otherwise.
Typical Experimental Procedure for Preparation of
Stilbenoid Dendrimers 7 and 8. A mixture of dried KOAc
(70 mg, 0.714 mmol), n-Bu4NBr (153 mg, 0.476 mmol) in DMF
(6 mL) was stirred for 15 min in a double necked round-
bottomed flask in argon atmosphere at room temperature. To
the reaction mixture were added 6 (100 mg, 0.119 mmol), PPh3
(12 mg, 0.047 mmol), and 4-tert-butylstyrene (114 mg, 0.717
mmol) simultaneously. The mixture was stirred for another
15 min. A catalytic amount of Pd(OAc)2 was added and heated
for 24 h at 100 °C. The mixture was cooled to room temperature
and extracted with hexane and ether mixture (1:1) (15 mL). The
organic layer was washed with water (3 © 5 mL) and brine
(5 mL), and dried over anhydrous Na2SO4. Solvent was
removed in a rotary evaporator and the crude product was
purified using column chromatography (silica gel/ethyl acetate:
petroleum ether 1:99) to yield the compound 7 (51 mg, 45%) as
white solids.
Compound 7: 1H NMR (400 MHz, CDCl3): ¤ 7.40 (d, 8H,
J = 8.0 Hz), 7.34 (d, 8H, J = 8.0 Hz), 7.03 (d, 4H, J = 16.0 Hz),
7.01 (d, 4H, J = 16.0 Hz), 1.31 (s, 36H); 13C NMR (75 MHz,
CDCl3): ¤ 150.7, 135.8, 134.7, 131.8, 129.7, 128.3, 127.7,
126.2, 126.0, 125.6, 34.6, 31.3; MALDI-TOF MS: m/e:
964.2755 (M+).
Compound 8: 1H NMR (500 MHz, CDCl3): ¤ 7.55 (br s,
8H), 7.51 (d, 8H, J = 8.0 Hz), 7.49 (br s, 4H), 7.48 (d, 16H,
J = 8.0 Hz), 7.42 (d, 8H, J = 8.0 Hz), 7.40 (d, 16H, J = 8.0 Hz),
7.17 (d, 12H, J = 16 Hz), 7.10 (d, 12H, J = 16 Hz), 1.33 (s,
72H); 13C NMR (150 MHz, CDCl3): ¤ 150.8, 138.1, 134.4,
Adv. Mater. 2006, 18, 325.
9
J.-H. Jou, Y.-S. Chiu, R.-Y. Wang, H.-C. Hu, C.-P. Wang,
H.-W. Lin, Org. Electron. 2006, 7, 8.
10 C. H. Chuen, Y. T. Tao, F. I. Wu, C. F. Shu, Appl. Phys.
Lett. 2004, 85, 4609.
11 J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett. 1995, 67,
2281.
12 T.-H. Liu, Y.-S. Wu, M.-T. Lee, H.-H. Chen, C.-H. Liao,
C. H. Chen, Appl. Phys. Lett. 2004, 85, 4304.
13 Y.-S. Wu, S.-W. Hwang, H.-H. Chen, M.-T. Lee, W.-J.
Shen, C. H. Chen, Thin Solid Films 2005, 488, 265.
14 C. J. García-Martínez, E. Díez-Barra, J. Rodríguez-López,
Curr. Org. Synth. 2008, 5, 267.
15 D. Bloor, in Introduction to Molecular Electronics, ed. by
M. C. Petty, M. R. Bryce, D. Bloor, Edward Arnold, London,
1995, pp. 1-28.
16 a) J. N. G. Pillow, M. Halim, J. M. Lupton, P. L. Burn,
I. D. W. Samuel, Macromolecules 1999, 32, 5985. b) M. Halim,
J. N. G. Pillow, I. D. W. Samuel, P. L. Burn, Adv. Mater. 1999, 11,
371. c) J. Tolosa, C. Romero-Nieto, E. Díez-Barra, P. Sánchez-
Verdú, J. Rodríguez-López, J. Org. Chem. 2007, 72, 3847.
17 a) A. Dodabalapur, M. Berggren, R. E. Slusher, Z. Bao, A.
Timko, P. Schiortino, E. Laskowski, H. E. Katz, O. Nalamasu,
IEEE J. Sel. Top. Quantum Electron. 1998, 4, 67. b) M. Berggren,
A. Dodabalapur, Z. Bao, R. E. Slusher, Adv. Mater. 1997, 9, 968.
c) W. J. Oldham, Jr., R. J. Lachicotte, G. C. Bazan, J. Am. Chem.
Soc. 1998, 120, 2987. d) T. Maddux, W. Li, L. Yu, J. Am. Chem.
Soc. 1997, 119, 844.
18 S. Sengupta, S. K. Sadhukhan, R. S. Singh, N. Pal,
Tetrahedron Lett. 2002, 43, 1117.
19 T. Jeffery, in Advances in Metal-Organic Chemistry, ed. by
S. L. Liebeskind, JAI Press, Greenwich, CT, 1996, Vol. 5, p. 153.
20 R. E. Buckles, G. M. Matlock, Org. Synth., Coll. Vol. 1963,
IV, 914.
21 S. Sengupta, Synlett 2004, 7, 1191.
22 S. Wang, W. J. Oldham, Jr., R. A. Hudack, Jr., G. C. Bazan,
J. Am. Chem. Soc. 2000, 122, 5695.
23 Quantum yields were measured in CH2Cl2 solutions using
pyrene (¯ = 0.32 in cyclohexane) as standard (errors within 15%).