Synthesis and spectroscopic properties of finite Ph 2N-containing oligo(arylenevinylene) derivatives that emit blue to red fluorescence

Article abstract of DOI:10.1021/ol034246v

(Matrix presented) A series of oligo(phenylenevinylene) (OPV) derivatives with finite conjugation units were prepared in short steps from few building blocks. The central and terminal aryl groups of these OPV dyes contain cyano and Ph₂N substituents, respectively, which affect color of fluorescence. The wavelength ranges from 472 nm (blue) to 614 nm (red) depending on the position of the cyano group.

Full text of DOI:10.1021/ol034246v

ORGANIC
LETTERS
2003
Vol. 5, No. 7
1131-1134
Synthesis and Spectroscopic Properties
of Finite Ph2N-Containing
Oligo(arylenevinylene) Derivatives That
Emit Blue to Red Fluorescence
,†
Chien-Le Li,† Shwu-Ju Shieh,‡ Shien-Chang Lin,‡ and Rai-Shung Liu*
Department of Chemistry, National Tsing-Hua UniVersity, Hsinchu,
Taiwan 30043, ROC, and DeVelopment Center, Ritek Display Company,
Hsinchu 30043, ROC
rsliu@mx.nthu.edu.tw
Received February 11, 2003
ABSTRACT
A series of oligo(phenylenevinylene) (OPV) derivatives with finite conjugation units were prepared in short steps from few building blocks.
The central and terminal aryl groups of these OPV dyes contain cyano and Ph2N substituents, respectively, which affect color of fluorescence.
The wavelength ranges from 472 nm (blue) to 614 nm (red) depending on the position of the cyano group.
The synthesis of new oligo(phenylenevinylene) OPV deriva-
tives has attracted considerable attention1 because of their
widespread application in light-emitting diodes,2 chemical
sensors,3 nonlinear optics,4 and organic magnetic materials.5
The energy gaps between the HOMO and LUMO orbitals
of OPV are crucial for the application of these materials.1-5
Although an increase in the number of repeating segments
of phenylenevinylene oligomers may decrease the energy
gap, this approach becomes less effective after a finite length
of oligomers is achieved. A systematic study by Yu6 shows
that no further red shift is observed for the oligomer (eq 2,
Scheme 1) after it reaches 10 aryl groups and nine double
bonds, at which point it shows fluorescent emission at 536
nm. The saturation in the emission wavelength arises from
the limited electron delocalization of a longer oligomer.
The use of OPV dyes for organic light emitting-diodes
(OLED) should enable blue, green, and red fluorescent
emission to achieve a full-color display.7 Common OPV
molecules show fluorescent emission at less than 550 nm
even with extensive conjugation. In this study, we report the
synthesis of functionalized OPV dyes (eq 3) with finite
† National Tsing-Hua University.
‡ Ritek Display Co.
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10.1021/ol034246v CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/14/2003

Scheme 1
Scheme 3a
skeletons that can emit full range of visible light. Each
terminal phenyl group of these molecules is linked to a
diphenylamine group that can enhance fluorescence decay.8
The central phenyl group is attached with a cyano group to
strengthen the efficiency of the electron delocalization within
conjugation skeletons.9 This design was aimed at producing
an efficient red OPV dye. Molecules with these finite
conjugation skeletons are suitable for sublimation to prepare
OLED thin film.
a Conditions: (1) Pd(OAc)2 (3 mol %), Ph4PBr (20 mol %),
NaOAc (5.0 equiv), DMF, 100 °C, 16 h; (2) Ni(COD)2 (1.1 equiv),
DMF, 40 °C, 12 h; (3) Pd(OAc)2 (10 mol %), Ba(OH)2 (1.8 equiv),
THF, H2O.
Schemes 2 and 3 show the synthetic protocols for the seven
target molecules 12a-c and 13a-d. This approach mini-
mizes the synthetic procedures. Our first goal was to
synthesize central aryl cores 6a-c and 10 via the Horner-
Wadsworth-Emmons reaction, which gave trans double
bonds exclusively (>96%) according to 1H NMR analysis.10
The synthesis of these four trans-stilbene derivatives involves
the repeated use of phosphates 5a and 5b. The benzyl
bromide 3b not only provided the key phosphates 5b but
also the aldehyde 4b. Compounds 3a and 4a were obtained
from commercial sources. The whole synthetic sequence
apparently relies on the availability of compound 3b, which
was obtained in four steps from 2-cyanotoluene 1 (see the
Supporting Information). Another central aryl core 9 was
easily prepared in two steps from the dibromide 7.
Scheme 3 shows the synthesis of target oligomers via
metal-catalyzed coupling reaction. The synthesis of oligomer
12a via a double Heck reaction between commercially
available 11a and 11b is not straightforward. Conventional
conditions (5 mol % Pd(OAc)2, Et3N, toluene, 10 mol %
PPh3, 100 °C) gave the desired product 12a in only 25%
yield after column chromatography. The use of tBu3P(10 mol
%) gave 12a in 51% yield. The best results were obtained
with the phase-transfer reagent Ph4PBr, which gave 12a in
81% yield.11 This coupling reaction gave trans-stilbene in
purities exceeding 96%. This condition worked well for a
series of oligomers including 12b (79%), 13a (84%), 13b
(76%), and 13c (72%). Oligomer 12c was obtained efficiently
by a Ni(COD)-promoted coupling reaction. The availability
Scheme 2
(8) (a) Yang, J.-S.; Chiuo, S.-Y.; Liau, K.-L. J. Am. Chem. Soc. 2002,
124, 2518-2528. (b) Lewis, F. D.; Kalgutkar, R. S.; Yang, J.-S. J. Am.
Chem. Soc. 1999, 121, 12045-12053. (c) Lewis, F. D.; Yang, J.-S. J. Am.
Chem. Soc. 1997, 119, 3834-3835. (d) Lewis, F. D.; Weigel, W. J. Phys.
Chem. A 2000, 104, 8146-8153.
(9) (a) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt,
J. C.; Kannan, R.; Yuan, L.; He, G. S.; Prasad, P. N. Chem. Mater. 1998,
10, 1863-1874. (b) Albota, M.; Beljonne, D.; Bredas, J. L.; Ehrlich, J. E.;
Fu, J. Y.; Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S.
R.; McCordMaughon, D.; Perry, J. W.; Rockel, H.; Rumi, M.; Subramaniam,
G.; Webb, W. W.; Wu, X. L.; Xu, C. Science 1998, 281, 1653-1656.
(10) (a) In a model reaction, the 1,1-regioisomer in the Heck reaction
was separated in less than 4% yield: Peng, Z. H.; Yu, L. Manuscript in
preparation. Also see: (b) Cabri, W.; Candiani, I.; Bedeschi, A.; Santi, R.
J. Org. Chem. 1992, 57, 3558. (c) Cabri, W.; Candiani, I.; Bedeschi, A.;
Santi, R. Synlett 1992, 871.
(11) (a) Manfred, T. R.; Gunther, L.; Renate, S. Angew Chem., Int. Ed.
1998, 37, 481-483. (b) Rajender, S. V.; Kannan, P. N.; Per, J. L.
Tetrahedron Lett. 1999, 40, 2075-2078.
1132
Org. Lett., Vol. 5, No. 7, 2003

Table 1. Physical Properties of Oligo(arylenevinylene)
Derivatives
c
d
λuv
λPL
oxid E1/2
redn E1/2
compd (max)a (max) Φfb
(mV)
(mV)
12a
12b
12c
13a
13b
13c
13d
398
386
428
386
433
447
429
480
472
541
510
560
614
577
0.86 839, 1039
0.96 938
0.37 965
-1677, -1743
0.84 830, 1220
0.61 862, 1001 -1565, -1790
0.41 911, 1018 -1399, -1611, -1799
0.32 875 -1502, -1643, -1823
a Measured in CH2Cl2; fluorescence was recorded by irradiation at the
absorption maximum. b Measured in CH2Cl2 with fluorescein in 0.1 M
NaOH (Φf ) 0.95) as a standard. c In CH2Cl2 (0.1 M nBu4NPF6 as a
supporting electrolyte). d In THF (0.1 M nBu4NClO4 as a supporting
electrolyte).
emission wavelengths. The observed trend for λUV (max) and
λPL (max) for this series of compounds is consistent with
our expectation: 13c > 13b = 13d > 13a. For 12b, the
dihedral angle between the two central phenyl groups is ca.
56.2°, whereas the corresponding angle in 12a is 39.7°.14
For compound 12b, the preference for a high nonplanarity
accounts for a poor electron delocalization across these two
central groups. The data in Table 1 indicate that a wide range
of emission wavelengths (475-615 nm) can be achieved
from these finite OPV backbones with the introduction of a
cyano position at a suitable position.
Table 1 shows electrochemical data from cyclic voltam-
metry with Bu4NPF6 (0.10 M in CH2Cl2) and Bu4NClO4
(0.10 M in THF) as supporting electrolytes in anodic
oxidation and cathodic reduction respectively.15 Compounds
12a-c and 13a-d show quasi-reversible or two quasi-
reversible anodic redox couples which are closely overlapped.
This process corresponds to removal of electrons from
diphenylamino group. The occurrence of a cathodic redox
process depends on the presence of a cyano group which
can decrease the LUMO energy level. No reduction behavior
was observed for unfunctionalized OPV species 12a and 13a,
consistent with our expectations. Two or three reversible
cathodic redox couples were observed for 12c, 13b-13c,
which corresponds to sequential reductions at the cyanophe-
nyl groups. Figure 2 shows the CV curves for compounds
12a and 13c. One exception case is the dicyano OPV 12b
which does not show a cathodic redox couple. This may be
attributed to the nonplanarity of the two central phenyl groups
(vide infra), which results in a high energy level for LUMO
orbital.
Figure 1. Comparison of 12a-c (a) and 13a-d (b) UV and PL
spectra in solution (∼5 × 10-5 M in CH2Cl2).
of 1,2-vinyl diborate 1412 effected a double-Suzuki coupling
of aryl bromide 10, giving 13d in 72% yield. These seven
oligomers were characterized by NMR, Mass and elemental
analysis. HLPC analysis of these samples showed that the
purity exceeded 95%.
Parts a and b of Figure 1 show UV and PL spectra of
OPV dyes 12a-c and 13a-d with their spectral data and
quantum efficiency13 given in Table 1. The cyano groups of
12b, 13b, and 13c lie in conjugation with the remote Ph2N
group whereas those of 12c and 13d will conjugate with the
proximate Ph2N group. Accordingly, the former is expected
to show a greater bathochromic effect than the latter relative
to their parent OPVs 12a and 13a. Notably, the UV and PL
spectra of 12b are changed very little where the correspond-
ing spectra of 12c show a significant red-shift. An additional
double bond in compound 13a resulted in a red-shift of PL-
emission by 30 nm compared to that of OPV 12a. The
introduction of a cyano group to the central phenyl in 13a
produced a more pronounced change in the absorption and
The OLED device was prepared from OPV 12b because
of its high quantum yields. The device structure is ITO/CuPc-
(15 nm)/NPB(40 nm)/12b(2%) in DNA (30 nm)/Alq3(30 nm)
(14) Calculation of dihedral angles between two phenyl planes is
performed on the ground-state structure using MOPAC-AM1 computations.
(15) Carbon electrode was used as a working electrode and a platinum
wire as a counter electrode, all potentials were recorded versus Ag/AgCl
(sat’d) as a reference electrode. Ferrocenium/ferrocene redox couple in CH2-
Cl2/nBu4NPF6 occurs at Eo′) + 0.57 V vs Ag/AgCl (satd), in THF/nBu4-
NClO4 occurs at Eo′) + 0.55 V vs Ag/AgCl (satd).
(12) Gu, Y.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2001, 373-
380.
(13) Brannon, J. H.; Magde, D. Absolute quantum yield determination
by thermal blooming. Fluorescein, J. Phys. Chem. 1978, 82, 705-709.
Org. Lett., Vol. 5, No. 7, 2003
1133

of 12b (472 nm). The turn on voltage at 3.5 V and low
working voltages (540 cd/m2 at 6.0 V and 3120 cd/m2 at
7.4 V with CIE coordinates of x ) 0.15; y ) 0.18) and the
maximum brightness 11190 cd/m2 at 10.5 V indicated the
suitability of these functionalized OPV for OLED devices.
In summary, we prepared a series of Ph2N-containing OPV
dyes based on a concise synthetic approach involving few
building blocks. These OPV molecules were functionalized
with cyano groups to show a wide range of PL emission
wavelengths (472-615 nm). The relation of the cyano
positions to the photochemical properties of these OPV has
been studied by UV, PL and electrochemistry. The suitability
of these oligomers for OLED device is reflected by a EL
device of OPV 12b which shows a blue emission with good
EL efficiency.
Acknowledgment. We thank the National Science Coun-
cil and Ministry of Education for financial support of this
work.
Figure 2. Cyclic voltammogram of oligo(arylenevinylene) 12a and
13c.
/Mg-Ag (10:1)(50 nm)/Ag(100 nm) which CuPc (copper
phthalocyanine) and NPB (4,4′-bis(N-naphthyl)-N-phenyl-
amino)biphenyl) were used as hole injector and hole transport
layers, respectively, and DNA (9,10-di(2-naphthyl)anthra-
cene) functioned as a host material layer. Such a device gave
a blue EL emission at 460 nm very close to the PL spectra
Supporting Information Available: Synthetic proce-
dures, spectral data, and electrochemical data of oligomers;
EL-devices of oligomer 12b. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL034246V
1134
Org. Lett., Vol. 5, No. 7, 2003