recombination centres for injected carriers20 but the cause of the
spectral shift is not known yet.
The methoxy substituents attached onto the non-conjugated
Experimental
TPD represents N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-
biphenyl-4,4'-diamine. PBD represents 2-(biphenyl-4-yl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole. All the new compounds
were fully characterized with standard spectroscopic techni-
ques. All the physical measurements were performed in CHCl3.
1H NMR spectra were recorded using a JEOL JHM-EX270 FT
NMR spectrometer and are referenced to the residual CHCl3
7.24 ppm. Electronic absorption (UV±vis) and ¯uorescence
spectra were recorded using a Varian Cary 100 Scan Spectro-
photometer and a PTI Luminescence Spectrophotometer
respectively. The corrected energy gaps reported in the
Table 1 were calculated by an addition of 21.9 eV for solvent
interaction.22 The ¯uorescence quantum yields in chloroform
using 9,10-diphenylanthracene as a standard were determined
by dilution method as described by Parker et al.23 The quantum
yields reported here are the average of two independent
measurements. Thermal stabilities were determined by using
Thermal Gravimetric Analyzer (DTA), Shimadzu TGA-40
with a heating rate of 10 ³C min21. The semi-empirical
calculations using the PM3 parameterization were carried
out by MOPAC 6 in the Alchemy 2000 software package. The
experimental details of the LEDs fabrication can be found in
ref. 16 and 21. The general synthetic procedure for the
Wadsworth±Emmons reaction can be found in ref. 15.
ortho-positions of the end-capped phenyl rings have
a
pronounced effect on the solid-state luminescence properties.
In addition to the difference in spectral features, the emission
maxima of both EL and PL are signi®cantly blue-shifted
(y20 nm) in the three-phenyl-ring oligomer, 1, when compared
to that of the corresponding unsubstituted counterparts.21 Our
initial measurement showed that this LED emits blue light at
456 nm with 120 cd m212 at a bias voltage of 7 V which shows
potential for further development of blue LEDs.
There is a bathochromic shift of both PL and EL maxima
with an extension of the chain length as the energy gap is
lowered which is also consistent with the ¯uorescence
behaviour in solution. In addition, all the devices showed
clear rectifying behaviour in their current-voltage character-
istics. The turn-on voltages of these oligomer-based multi-layer
LEDs are all around 4±5 volts.
Based on the initial multilayer device structure mentioned
above, the external quantum ef®ciencies derived from the
measured luminance divided by the current density passing
through the device were calculated and tabulated in Table 1.
The external quantum ef®ciencies are expected to be enhanced
with an increase of chain length; however, there is a dramatic
decrease in the external quantum ef®ciency with a device based
on 3 which is likely due to the severe interchain interaction in
this longer homologue which is known to be detrimental to the
device ef®ciency. One of the remedies to avoid aggregation is by
means of doping an emissive dye into the host material. The
external quantum ef®ciency of an OLED using co-evaporated
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-
diamine and 1 (TPD?1) as an emissive layer improved to
0.82 cd A21 as compared to 0.31 cd A21 using a single 1
emissive layer. The external ef®ciency could be increased to
Improved procedure for the preparation of diethyl 4-[2-(2-
butoxyethoxy)ethoxy]-3,5-dimethoxystyrylbenzylphosphonate
13
To a stirred solution of bisphosphonate 12 (6.92 g, 18.3 mmol)
and 1.2 equivalent of NaH (0.88 g, 22 mmol) in 500 mL
anhydrous DME at 0 ³C was added dropwise a solution of
trialkoxy-substituted benzaldehyde 8 (2 g, 6.1 mmol) over 1 h.
After stirring for 0.5 h at 0 ³C, the reaction mixture was slowly
warmed up to rt. After stirring for 2 h at rt, the solution
mixture was quenched with water. The crude product was
either collected by suction ®ltration or extracted twice with
CH2Cl2, dried over anhydrous MgSO4 and evaporated to
dryness. The crude product was then puri®ed by silica gel
chromatography using the gradient elution technique with
CH2Cl2±ethyl acetate as eluents affording 2.1 g (62%) of 13 and
0.88 g (20%) of 1.
1.3 cd A21 using
a composite emissive layer of 1 and
LiF. Besides the aggregation improvement, this enhancement
may be also due to the improvement of the charge transport
properties in the composite layer. We have also observed
fragmentation of higher homologues i.e. 4 during thermal
evaporation and the LED obtained exhibit signi®cant blue shift
in the EL and PL and higher turn±on voltage which is likely
due to the contribution of its fragments.
In summary, a novel homologous series of highly soluble,
multiple alkoxy end-substituted oligophenylenevinylenes con-
taining up to six phenyl rings were synthesized. We have shown
that the multiple solubilizing groups substituted at the end of
oligophenylenevinylenes can enhance solubility of higher
homologues of oligomers but do not disrupt the co-planarity
of the p-conjugation backbone. In addition, we have
theoretically and experimentally shown that the electron
donating groups substituted at the end of OPVs can lead to
a faster approach to a limit of convergence and hence promote
a shorter effective conjugation length relative to those of the
unsubstituted and lateral substituted OPVs. The emission
spectra also exhibit a similar tendency of saturation as the
conjugation length increases. End-substituted OPVs containing
more than three phenyl rings exhibit very high ¯uorescence
quantum yields. Multi-layer LEDs using these end-substituted
OPVs as emissive layers were fabricated by vacuum evapora-
tion technique and were investigated. We have found that there
is a bathochromic shift of the PL and EL maxima with an
increase of chain length in the series. All these oligomer-based
multi-layer LEDs show very low turn-on-voltage. Further-
more, there is an increase in the external quantum ef®ciency
with an increase in chain length; however, the higher
homologues of these oligomers suffered from severe interchain
interaction leading to a dramatic decrease in the external
quantum ef®ciency. Nevertheless, the interchain interaction
can be alleviated by doping the oligomer into the host matrix.
1H NMR (270 MHz, CDCl3) d 7.43 (d, J~7.56 Hz, 2H),
7.26 (dd, J~8.19 Hz, J~2.16 Hz, 2H), 6.98 (s, 2H), 6.70 (s,
2H), 4.12 (t, J~5.22 Hz, 2H), 3.99 (m, 4H), 3.82 (s, 12H), 3.84
(t, J ~ 4.98 Hz, 2H), 3.70 (m, 2H), 3.58 (m, 4H), 3.49 (t,
J~6.72 Hz, 2H), 3.14 (d, J~21.87 Hz, 1H), 1.55 (m, 2H), 1.32
(m, 2H), 1.23 (t, J~6.89 Hz, 6H), 0.89 (t, J~7.29 Hz, 3H).
1,4-Bis-4-{[2-(2-butoxyethoxy)ethoxy]-3,5,-dimethoxy}styryl-
benzene 1:. 1H NMR (270 MHz, CDCl3) d 7.42 (s, 4H), 6.99 (d,
J~16.2 Hz, 2H), 6.91 (d, J~16.2 Hz, 2H), 6.66 (s, 4H), 4.10 (t,
J ~ 4.86 Hz, 4H), 3.82 (s, 12H), 3.77 (t, J~5.00 Hz, 4H), 3.64
(m, 4H), 3.54 (m, 4H), 3.40 (t, J~6.75 Hz, 4H), 1.50 (m, 4H),
1.28 (m, 4H), 0.84 (t, J~7.16 Hz, 6H). MS (FAB) m/z 722.6
(Mz). HRMS(FAB) C42H58O10Na: calc. 745.3928 found
745.3918. Found: C, 69.78; H, 8.09. C42H58O10 requires C,
69.72; H, 8.17%. Mp~66 ³C.
1,2-Bis(4-{4-[2-(2-butoxyethoxy)ethoxy]-3,5-dimethoxystyr-
yl}phenyl)ethene 2. 1H NMR (270 MHz, CDCl3) d 7.43 (s, 8H),
7.05 (s, 2H), 6.99 (d, J~16.2 Hz, 2H), 6.91 (d, J~16.2 Hz, 2H),
6.66 (s, 4H), 4.10 (t, J~5.25 Hz, 4H), 3.83 (s, 12H), 3.75 (t,
J~5.13 Hz, 4H), 3.64 (m, 4H), 3.53 (m, 4H), 3.40 (t,
J~6.675 Hz, 4H), 1.51 (m, 4H), 1.29 (m, 4H), 0.84 (t,
J~7.43 Hz, 6H). MS (FAB) m/z 824.6 (Mz). HRMS(FAB)
C50H64O10: calc. 824.4499 found 824.4513. Mp~110 ³C.
J. Mater. Chem., 2000, 10, 1805±1810
1809