Synthesis and optical investigations of low molecular weight alkoxy- substituted poly(p-phenylenevinylene)s

Article abstract of DOI:10.1039/a909780e

Low molecular weight poly(2,5-dialkoxy-1,4-phenylenevinylene) polymers with branched, macrocyclic, and cyclic polyethereal alkoxy chains have been prepared by Stille cross-coupling reaction between suitable 2,5-dialkoxy-1,4- diiodobenzenes and (E)-1,2-bis

Full text of DOI:10.1039/a909780e

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J O U R N A L O F
Synthesis and optical investigations of low molecular weight alkoxy-
substituted poly(p-phenylenevinylene)s
a
Francesco Babudri, Stefania R. Cicco, Luca Chiavarone, Gianluca M. Farinola,
b
c
b
b
a
Linda C. Lopez, Francesco Naso* and Gaetano Scamarcio
c
C H E M I S T R Y
a
Centro CNR di Studio sulle Metodologie Innovative di Sintesi Organiche, Dipartimento di
Chimica, Universit aÁ degli Studi di Bari, via Amendola 173, 70126 Bari, Italy. Fax: 0039 080
5442924; E-mail: naso@area.ba.cnr.it
Dipartimento di Chimica, Universit aÁ degli Studi di Bari, via Orabona 4, 70126 Bari, Italy
b
c
INFM ± Dipartimento Interateneo di Fisica, Universit aÁ degli Studi di Bari, via Amendola 173,
0126 Bari, Italy
7
Received 13th December 1999, Accepted 17th April 2000
Published on the Web 8th June 2000
Low molecular weight poly(2,5-dialkoxy-1,4-phenylenevinylene) polymers with branched, macrocyclic, and
cyclic polyethereal alkoxy chains have been prepared by Stille cross-coupling reaction between suitable 2,5-
dialkoxy-1,4-diiodobenzenes and (E)-1,2-bis(tributylstannyl)ethene. Spectral line narrowing in solution and ®lm
under high pulsed excitation intensity is reported. The measurement of the photoluminescence ef®ciency in ®lm
is carried out. The PPV with macrocyclic alkoxy substitution shows higher ef®ciency but lower damage
threshold than the PPV derivative with open alkoxy chains. The potential of these materials in
electroluminescent devices is discussed.
Introduction
Results and discussion
In recent years, new perspectives in the ®eld of semiconductor
devices have been disclosed by the possibility of employing
1. Synthesis of polymers
1
conjugated polymers as active elements. The optical and
The synthesis of polymers 1a±1c (Fig. 1) was performed by
cross-coupling reaction of (E)-1,2-bis(tributylstannyl)ethene 4
with diiododerivatives 3a±3c in re¯uxing benzene in the
presence of tetrakis(triphenylphosphine)palladium(0) as the
catalyst (Scheme 1). Diiodo aromatic derivatives 3a±3c were
electronic properties of semiconductors, associated with the
processing advantages and mechanical properties of polymers,
make these new materials very attractive in the fabrication of
2
light-emitting diodes and other categories that characterise the
®eld of photonic devices. In particular, the observation of
prepared by iodination reaction of the corresponding hydro-
1
1
3
3 2
quinone diethers 2a±2c with HIO ±I (Scheme 1). Diethers
electroluminescence in poly(1,4-phenylenevinylene) (PPV), the
simplest and cheapest poly(arylenevinylene) polymer, has
boosted development of new materials structurally related to
1
2
13
and 2b were prepared according to reported procedures.
2
The cyclic polyether 2c was obtained as shown in Scheme 2,
a
1
4
4
starting from hydroquinone and the dibromo derivative 5,
prepared from the corresponding tetraethylene glycol by
bromination with PBr . Molecular weights, reported in
3
the underivatized PPV, with the aim of modulating the optical
and electrooptical properties. A rapidly emerging area of more
recent interest for applications of these conjugated polymers is
5
Table 1, were determined by gel-permeation chromatography
(
represented by their use as organic semiconductor lasers.
It is also worth noting that oligomers may present similar
electrooptical properties to the corresponding longer chain
GPC) with uniform polystyrene standards. Actually the
6
,7
systems. In particular, ampli®ed spontaneous emission, the
main factor for obtaining laser action, has been reported in
PPV oligomer thin ®lm, with an emission threshold comparable
8
with those noticed in conjugated polymer thin ®lm. Within the
framework of our studies dealing with the application of
organometallic reagents to the synthesis of stereode®ned
9
conjugated polyenes and polymers we have recently reported
the synthesis of poly(2,5-pentyloxy-1,4-phenylenevinylene) by
9
a
the Stille cross-coupling reaction.
procedure affords low molecular weight polymers,
This organometallic
and it
9
a,10
is particularly attractive in view of the large number of
materials which could be obtained and then subjected to
interesting structure±properties relationship studies.
In this work, we report the application of our procedure
to the synthesis of three low molecular weight PPV polymers
with open branched, bridged macrocyclic or cyclic poly-
ethereal alkoxy chains, together with an investigation of their
optical properties. A comparison of light emission under high
laser excitation between solutions and ®lms has been
performed.
Fig. 1 Structures of polymers 1a±1c.
DOI: 10.1039/a909780e
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This journal is # The Royal Society of Chemistry 2000
1573

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properties. In order to ascertain the nature of the chain
terminations we have performed an XPS analysis of 1a±1c for
evaluating the tin and iodine contents. This analysis revealed
low values of atomic concentrations of both the elements
(
under 0.5%), very near to the detection threshold. Therefore,
hydrogen atoms are the most probable end groups of 1a±1c. It
is likely that reductive dehalogenation and destannylation steps
may occur as side reactions during the polymerization process,
and the low molecular weight of the polymers prepared this
way may be ascribed also to the loss of the reactive groups at
the ends of relatively short chains.
2. Optical absorption and low and high excitation intensity
emission spectra of solutions
Scheme 1 Synthesis of PPV derivatives 1a±1c.
Fig. 2 shows the absorption and photoluminescence spectra of
polymers 1a±1c. All the spectra of Fig. 2a show a broad band
ascribed to the p±p* transition. The corresponding peak
wavelengths (lmax) are reported in Table 2, together with the
HOMO±LUMO energy gap E
spectra as the maximum of the ®rst derivative with respect to
g
, evaluated from the absorption
1
energy. The results of Table 2 show the following relation: E
1c)wE (1b)wE (1a). In polymers 1a and 1b, the inductive
g
(
g
g
and mesomeric effects of substituents on the conjugated
backbone can be considered constant, because of the identical
conjugated structure and position of substituents and the
similar nature of the latter. Their degrees of polymerization
(
Table 1) are also similar. Therefore, the observed trend in E
g
values can be ascribed to a reduction of the conjugation length,
associated with the twist of the phenyl rings out of the vinylene
plane. For the polymer 1c, the effect of a lower degree of
polymerization may play a signi®cant role in enhancing the
energy gap value. This is con®rmed by the observed y100 meV
increase of E
g
, associated with a reduction from 7 to 5 of the
1
6
monomeric units in PPV oligomers.
The cw photoluminescence (PL) spectra are shown in
Fig. 2b. The PL spectra are redshifted with respect to the
Scheme 2 Synthesis of compound 2c.
number-averaged molecular weights (M
n
) of rigid rod polymers
are usually overestimated when determined using the randomly
1
5
coiled polystyrene standards.
recorded M values could be used for determining a trend in
Nevertheless, the GPC
n
degrees of polymerization when we consider the structurally
similar polymers 1a±1c. Thus polymer 1a and 1b, although
their M values are not absolute, have very close degrees of
n
polymerization, whereas 1c is a shorter chain oligomer than 1a
and 1b.
When considering these low molecular weight polymers, the
important question about the nature of the terminal groups
arises, which may in principle in¯uence their photophysical
Fig. 2 Absorption (a) and photoluminescence (b) spectra of the 1a, 1b
and 1c polymers in CHCl solution.
3
Table 1 Molecular weights of polymers 1a±1c
a,b
M
n
a,c
/Da
d
Polymer
/Da
M
w
M /M
w
n
N
n
1
1
1
a
a
b
c
2260
2230
1330
3180
3180
1880
1.4
1.4
1.4
8.2
7.4
4.6
b
Determined by gel-permeation chromatography (GPC) with uniform polystyrene standards and THF as a solvent. Number-averaged molecu-
c
lar weight. Weight-averaged molecular weight. Number averaged degree of polymerization.
d
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Table 2 Spectroscopic data of the 1a±1c polymers in solution and ®lm
ABS
max
a
PL
max
b
c
ASE
d
e
Polymer
l
/nm
l
/nm
E
g
/eV
l /nm
max
FWHM/nm
WPL (%)
1
1
1
a
b
c
485
446
425
Ð
553
540
536
Ð
2.33
2.51
2.58
593
570
551
Ð
8
10
20
Ð
5.9¡0.6
15±24¡2
Ð
24
MEH-PPV
a
Ð
10±15¡1
b
Maximum absorption wavelength. Maximum emission wavelength. Value of the p±p* energy gap evaluated from the absorption spectra of
c
d
e
Fig. 1a. Peak wavelength of the narrow emission under high pulsed excitation intensity. Full width half maximum band of the narrow emis-
sion under high pulsed excitation intensity.
absorption ones (Stokes shift) owing to the higher degree of
coplanarity of the conjugated backbone in the excited state due
medium, as the mechanism responsible for the spectral line
narrowing. The behaviour of 1b is similar to that of 1a, but the
ampli®cation is less ef®cient. Indeed a narrow spectral line is
obtained with higher excitation levels (Iexcw3.2 mJ cm ) and
the emission peak intensity is an order of magnitude smaller
1
7
to the aromatic to quinoid conversion. This Stokes shift is
bene®cial in reducing self-absorption effects and can be
exploited to investigate optical gain phenomena in organic
2
2
5
materials. This is shown in Fig. 3, in which the emission
spectra of polymers 1a±1c under pulsed high excitation
with the same value of I , in spite of the larger concentration
exc
2
(2.0 mg ml in CHCl with respect to 1.3 mg ml in CHCl3).
1
21
3
intensity are reported. Above a given excitation intensity, the
emission becomes directional and the broad band spectrum
Moreover, the superlinear trend of the peak emission intensity
is not exponential, showing that the ampli®cation of the
spontaneous emission is a less steady phenomenon with respect
ASE
collapses in a narrow spectral line whose peak position (lmax
)
2
1
and full width half maximum (FWHM) are reported in Table 2.
In the insets of Fig. 3 the FWHM and the emission peak
intensity are reported as a function of the excitation intensity
to 1a. This behaviour is enhanced in 1c (2.1 mg ml in CHCl3)
where the FWHM does not change abruptly and a slight
superlinear increase of the peak intensity is evident only for
2
2
2
2
Iexc. In the case of 1a, for Iexcw1.2 mJ cm a narrow spectral
line at 593 nm emerges from the broad band which collapses
into the narrow spectral line with a FWHM of 8 nm for
Iexcw10 mJ cm . For this reason we have restricted the study
of the emission properties in the solid state to 1a and 1b
polymers.
2
2
I
excw1.9 mJ cm . At these values of Iexc, the linear ®t reported
in the inset of Fig. 3a shows that the emission peak intensity
varies exponentially with respect to Iexc. This indicates
3. Low excitation intensity ®lm emission: photoluminescence
ef®ciency
1
ampli®ed spontaneous emission (ASE), i.e., light ampli®ca-
8
tion by stimulated emission after a single pass through the gain
The photoluminescence (PL) ef®ciency in the solid state is a
fundamental parameter to evaluate the possibility of using
these polymers in electroluminescent devices. The results
obtained with the procedure illustrated in the Experimental
section are reported in Table 2. The polymer 1a is approxi-
mately three times less ef®cient than 1b. The greater ef®ciency
of 1b is attributed to the reduction of the interaction between
adjacent chains due to the bridged structure of the alkoxy
1
9
substitution. It is worth noting that the PL ef®ciency is larger
in 1b than in MEH-PPV, which is the prototype polymer for
the fabrication of light emitting diodes based on dialkoxy-
PPV. Furthermore, the PL ef®ciency ratio is maintained in
electroluminescence. Fig. 4 shows the current±voltage (I±V
curve) and the optical power as a function of current (L±I
curve) characteristics for an ITO/1b/Al light emitting diode
(
1
LED). Yellow light becomes visible to the eye above
7.4 V. From the slope of the L±I characteristics we extract
an external electroluminescence ef®ciency g~0.015%, de®ned
as the number of emitted photons per injected carrier.
Signi®cantly, for a similar LED with MEH-PPV, a lower
2
0
value of g~0.008% is reported.
Fig. 3 Normalised luminescence spectra of 1a±1c polymers in CHCl
solution at pulsed excitation intensities below (dashed line) and above
solid line) the threshold for the observation of the spectral line
3
(
narrowing process. In the insets the FWHM and the emission peak
intensity as a function of the excitation intensity are reported.
Fig. 4 Current±voltage and power±current characteristics of an ITO/
1b/Al LED.
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4. High excitation intensity ®lm emission: spectral line
narrowing
Experimental
General
The emission spectra of the 1a and 1b polymer ®lms under high
pulsed excitation intensity are reported in Fig. 5. As in the
solutions, above a given excitation intensity spectral narrowing
of the emission band occurs. The narrow band is distinguished
for intensities above 46 mJ cm in 1a and 21 mJ cm in 1b.
As for the solution, in 1b ®lms the narrow band is centred at
Macherey-Nagel silica gel 60 (particle size 0.040±0.063 mm) for
ash chromatography and Macherey-Nagel aluminium sheets
¯
with silica gel 60 F254 for TLC were used. GC/mass spectro-
metry analyses were performed on a Hewlett-Packard 5890 gas
chromatograph equipped with a HP-1 capillary column and
2
2
22
1
13
HP MSD 5970B mass selective detector. H NMR and
C
5
70 nm and the observation of the spectral line narrowing is
NMR spectra were recorded on a Bruker AM 500 spectrometer
at 500 MHz and at 125.7 MHz respectively, or on a Varian XL
more critical than in 1a ®lms. In fact, the 1b ®lm is damaged
after few excitation pulses, although measurements were
performed under vacuum. The lower threshold can be ascribed
both to a better uniformity of 1b ®lms, which guarantees the
waveguide effect of the light, and to the much higher PL
2
00 at 200 MHz and at 50.7 MHz respectively using the
1
3
residual CHCl signal at d 7.24 ppm as the standard for the H
1
3
data and the triplet centred at d 77.00 for the C data. FT-IR
spectra were registered on a Perkin-Elmer 1710 spectrometer
on KBr pellets. Molecular weights were determined with a
Hewlett-Packard HP 1050 liquid chromatograph instrument
using THF as a solvent and a Plgel 5m Mixed-D 30067.5 mm
1
9
ef®ciency. In solution, PL ef®ciencies are comparable,
explaining the higher threshold observed for 1b.
1
2
13
14
column. Diethers 2a, 2b, dibromide 5, and the monomer
2
1
Conclusion
4 were prepared as reported. Tetrakis(triphenylphosphine)-
palladium( ) was a commercial product. Benzene was distilled
0
In conclusion, we have employed the Stille methodology for the
synthesis of low molecular weight alkoxy PPV derivatives with
branched 1a and bridged 1b, 1c substituents. The investigation
of their optical properties represents further evidence that
oligomers may be used beside high molecular weight materials
in optical and electrooptical applications.
immediately prior to use from sodium±benzophenone in a
nitrogen atmosphere. Reactions sensitive to oxygen and
moisture were conducted under a nitrogen atmosphere. Films
with thickness larger than 200 nm were formed by spin coating
on quartz substrates.
We have correlated the optical properties with the sub-
stituent structure, demonstrating that bridged macrocyclic
functionalization is bene®cial for increasing the emission
ef®ciency of dialkoxy-PPVs and may have useful applications
for LEDs. On the other hand, open chain dialkoxy substitution
appears more suitable for laser action.
Summing up, the validity of our approach appears clearly
demonstrated. Indeed, we have applied one of the most
versatile carbon±carbon bond forming reactions to the
production of stereode®ned PPV systems. After our work,
the possibility of using such a process appears well established
and this opens a highway to such systems. Furthermore, we
have found that the materials obtained in a straightforward
and operationally simple manner, in spite of their low
molecular weight, present photophysical properties of special
interest whose ®ne tuning, in principle, is made possible by the
versatility of the synthetic procedure set up.
Synthesis
1,4-Diiodo-2,5-bis(2-methylbutoxy)benzene (3a). 1,4-Bis(2-
methylbutoxy)benzene 2a (5.45 g, 21.6 mmol), I (4.88 g,
19.2 mmol), HIO3 (4.60 g, 26.2 mmol), and H SO4 (30%
aqueous solution, 6 mL) in CCl
2
2
4
(13 mL) and acetic acid
(
28 mL) were heated at 75 ³C overnight under stirring. The
resulting mixture was cooled, diluted with water (50 mL) and
extracted with ethyl acetate (3630 mL). The organic phase was
washed with an aqueous solution of sodium thiosulfate (10%,
50 mL) and water (50 mL), dried over anhydrous sodium
sulfate, and the solvent evaporated at reduced pressure. The
crude product was crystallised twice from ethanol, and 5.69 g
1
(
NMR (500 MHz, CDCl ): d 0.94 (t, J~7.5 Hz, 6H), 1.04 (d,
52% yield) of white crystals were obtained (mp 74±76.5 ³C). H
3
J~6.8 Hz, 6H), 1.25±1.35 (m, 2H), 1.54±1.64 (m, 2H), 1.83±
.91 (m, 2H), 3.69 (dd, J~8.7, 6.4 Hz, 2H), 3.77 (dd, J~8.7,
1
5
.7 Hz, 2H), 7.11 (s, 2H) ppm. GC/MS (70 eV) m/z (%) 502
z
M , 4), 432 (2), 362 (100), 304 (4), 236 (2), 207 (4), 110 (12), 43
(
(
47). C16
.65%.
24 2 2
H O I : Calcd C, 38.27; H, 4.82; Found C, 38.57; H,
4
1
7,19-Diiodo-2,15-dioxabicyclo[14.2.2]icosa-1(19),16(20),17-
triene (3b). The synthesis of this compound was performed
following the same procedure reported for compound 3a,
starting from 2,15-dioxabicyclo[14.2.2]icosa-1(19),16(20),17-
triene 2b (1.64 g, 5.9 mmol), I
1.26 g, 7.2 mmol), and SO
(3 mL) and acetic acid (11 mL) at 50 ³C for
h. The crude product was puri®ed by crystallisation from
2
(1.36 g, 5.3 mmol), HIO
4
(30% aqueous solution,
3
(
H
2
1
5
.8 mL) in CCl
4
petroleum ether. 0.97 g (31% yield) of a white solid (mp 99±
1
1
8
1
4
2
2
3
02 ³C) were obtained. H NMR (500 MHz, CDCl ): d 0.94 (s,
H), 1.02±1.12 (m, 2H), 1.14±1.23 (m, 2H), 1.24±1.41 (m, 4H),
.54±1.63 (m, 2H), 1.68±1.78 (m, 2H), 4.19 (ddd, J~11.8, 8.3,
.1 Hz, 2H), 4.27 (ddd, J~11.8, 6.1, 4.2 Hz, 2H), 7.23 (s,
H) ppm. C NMR (125 MHz, CDCl
7.94, 28.51, 69.54, 87.25, 124.52, 151.78 ppm. GC/MS (70 eV)
1
3
3
): d 23.95, 26.77, 27.44,
z
m/z (%) 528 (M , 18), 362 (100), 149 (2), 135 (2), 69 (6), 55 (18),
41 (20). Anal. Calcd for C H O I : C, 40.93; H, 4.96. Found:
18 26 2 2
C, 40.81; H, 4.84%.
Fig. 5 Emission spectra of 1a and 1b polymer ®lms at pulsed excitation
intensities below (dashed line) and above (solid line) the threshold for
the observation of the spectral line narrowing process.
4-(2-{2-[2-(2-Bromoethoxy)ethoxy]ethoxy}ethoxy)phenol
(6). In a light stream of nitrogen, an aqueous solution of
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NMe OH (1.1 M, 32 mL, 35.2 mmol) was added dropwise to a
4
was evaporated under reduced pressure, and the residue was
stirred solution of hydroquinone (3.89 g, 35.3 mmol) and
dibromide 5 (11.30 g, 35.3 mmol) in DMF (80 mL). The
resulting mixture was warmed for 6 h at 80 ³C, then diluted
with water (250 mL) and extracted with ethyl acetate
2 2
diluted with water and extracted with CH Cl (3650 mL). The
crude polymer obtained from evaporation of the solvent was
washed thrice with hexane. The solid was recovered after each
washing by centrifugation. Low molecular weight fractions
were eliminated by extraction in a Soxhlet apparatus with
ethanol for 24 hours, followed by a second extraction with
(
(
3650 mL). The organic phase was washed with water
56100 mL), dried over anhydrous sodium sulfate, and the
solvent removed under reduced pressure. A thick light yellow
1
oil was obtained (5.03 g, 41% yield). H NMR (200 MHz,
CHCl
obtained ( 0.33 g, 60% yield) was dried at 90 ³C and 10 mbar.
The polymer is soluble in CH Cl and CHCl3, insoluble in
ethanol. IR (KBr) n 3062, 2960, 2922, 1508, 1459, 1421, 1257,
3
. After evaporation of the solvent, the dark red powder
22
CDCl
2
3
): d 3.43±3.57 (m, 2H), 3.63±3.97 (m, 12H), 4.01±4.17 (m,
H), 6.70 (s, 1H, OH), 6.72±6.88 (m, 4H) ppm. GC/MS (70 eV)
2
2
z
21 1
m/z (%) 350 (Mz2, 4), 348 (M , 4), 241 (11), 239 (11), 153 (17),
51 (17), 110 (38), 109 (100), 107 (88), 81 (17), 65 (12). Anal.
Calcd for C14 Br: C, 48.15; H, 6.06. Found: C, 47.85; H,
3
1202, 1103, 1046, 801 cm . H NMR (500 MHz, CDCl ): d
1
0.94±1.02 (m, 6H), 1.03±1.17 (m, 6H), 1.28±1.43 (m, 2H), 1.57±
1.72 (m, 2H), 1.89±2.04 (m, 2H), 3.75±3.95 (m, 4H), 7.16 (br s,
21 5
H O
1
2H), 7.51 (br s, 2H) ppm. C NMR (125 MHz, CDCl
3
5
.94%.
3
): d
11.50, 16.85, 26.42, 35.09, 74.28, 110.05, 122.70, 127.48,
151.15 ppm. Anal. Calcd for (C18
Found: C, 80.73; H, 9.03%.
2
,5,8,11,14-Pentaoxabicyclo[13.2.2]nonadeca-
26 2 n
H O ) : C, 78.78; H, 9.55.
1(18),15(19),16-triene (2c). A solution of 6 (2.51 g, 7.2 mmol)
in DMSO (36 mL) and NMe OH in water±DMSO (6.5 mL of
4
an aqueous solution 1.1 M, 7.2 mmol, diluted with DMSO±
water 80% up to 36 mL) were added dropwise at the same time
to DMSO (300 mL, previously degassed by bubbling a nitrogen
stream for 1 h), warmed at 80 ³C under a vigorous stirring. The
slow addition required about 4 h. The resulting mixture was
cooled to room temperature and stirred overnight. Most of the
solvent was then removed by distillation under reduced
pressure (80 Torr, 85±90 ³C). When the volume of the residue
of the distillation was reduced to about 30 mL, the distillation
was stopped, and water (200 mL) was added. The resulting
mixture was extracted with dichloromethane (3650 mL), the
organic phase washed with water (36100 mL), and dried over
anhydrous sodium sulfate. The solvent was then removed
under reduced pressure. The crude product was puri®ed by
Poly[2,15-dioxabicyclo[14.2.2]icosa-1(19),16(20),17-trien-
7,19-ylenevinylene] (1b). This polymer was prepared follow-
1
ing the procedure above described for the polymer 1a, starting
from 17,19-diiodo-2,15-dioxabicyclo[14.2.2]icosa-1(19),16(20),
1
7-triene 3b (0.25 g, 0.47 mmol), (E)-1,2-bis(n-tributylstanny-
l)ethene 4 (0.29 g, 0.48 mmol) and tetrakis(triphenylphos-
phine)palladium( (0.017 g, 0.015 mmol), in anhydrous
0
)
benzene (20 mL) for 7 days. After the workup above described
for the polymer 1a, a dark red solid was obtained (0.082 g, 58%
yield). IR (KBr) n 3054, 2923, 2851, 1592, 1494, 1460, 1417,
2
1 1
1
245, 1186, 1049, 999, 970, 874, 691 cm . H NMR (500 MHz,
CDCl ): d 0.70±2.10 (m, 20H), 3.90±4.10 (m, 4H), 7.24 (br s,
3
1
H), 7.5 (br s, 2H) ppm. C NMR (125 MHz, CDCl ): d 24.16,
3
2
3
2
7.43, 27.93, 28.41, 28.54, 68.90, 112.66, 123.23, 128.62,
50.26 ppm. Anal. Calcd for (C H O ) : C, 79.95; H, 9.39.
¯
ash chromatography (silica gel, ethyl acetate±petroleum ether
1
Found: C, 81.66; H, 9.83%.
2
0
28 2 n
1
0 : 30). A yellow oil was obtained (0.51 g, 26% yield). H NMR
7
(
3
200 MHz, CDCl ): d 3.20±3.28 (m, 4H), 3.32±3.40 (m, 4H),
.66±3.74 (m, 4H), 4.22±4.30 (m, 4H), 6.92 (s, 4H) ppm.
3
1
3
C
Poly[2,5,8,11,14-pentaoxabicyclo[13.2.2]nonadeca-
(18),15(19),16-trien-16,18-ylenevinylene] (1c). This polymer
was prepared following the procedure above described for
the polymer 1a, starting from 16,18-diiodo-2,5,8,11,14-pen-
taoxabicyclo[13.2.2]nonadeca-1(18),15(19),16-triene 3c (0.19 g,
3
NMR (50 MHz, CDCl ): d 68.66, 70.25, 70.43, 72.32, 118.06,
1
z
1
1
53.71 ppm. GC/MS (70 eV) m/z (%) 268 (M , 100), 180 (2),
36 (10), 121 (5), 110 (24), 109 (5), 108 (11), 82 (13), 80 (9), 73
: C, 62.67; H, 7.51.
(
15), 64 (10). Anal. Calcd for C14
H
20
O
5
Found: C, 62.58; H, 7.32%.
0
0
(
.36 mmol), (E)-1,2-bis(n-tributylstannyl)ethene
.36 mmol) and tetrakis(triphenylphosphine)palladium(
4 (0.22 g,
)
0
16,18-Diiodo-2,5,8,11,14-pentaoxabicyclo[13.2.2]nonadeca-
0.013 g, 0.012 mmol), in anhydrous benzene (20 mL) for
days. After the workup above described for the polymer
a, consisting of Soxhlet extraction with hexane for removing
1(18),15(19),16-triene (3c). The synthesis of this compound
was performed following the same procedure reported for
compound 3a, starting from 2c (0.32 g, 1,2 mmol), I (0.28 g,
2
9
1
low molecular weight fractions and a second extraction with
methanol, a dark red solid was obtained (0.088 g, 83% yield,
mp 130±140 ³C). IR (KBr) n 2956, 2925, 2854, 1598, 1489, 1459,
1
solution, 0.4 mL) in CCl (0.5 mL) and acetic acid (2 mL) at
.1 mmol), HIO
3 2 4
(0.13 g, 0.7 mmol), and H SO (30% aqueous
4
50 ³C for 4 h. The crude product was puri®ed by ¯ash
chromatography (silica gel, ethyl acetate±petroleum ether
21
1
1
417, 1260, 1186, 1106, 1050, 948, 802 cm
500 MHz, CDCl ): d 2.58±3.40 (m, 8H), 3.45±3.85 (m, 4H),
.00±4.35 (m, 2H), 4.45±4.70 (m, 2H), 6.85±7.75 (m, 4H) ppm.
.
H NMR
(
4
3
7
0 : 30).
A white solid was obtained (mp 128±131 ³C, 0.29 g, 46%
13
C NMR (125 MHz, CDCl
23.80, 128.59, 152.14 ppm. Anal. Calcd for (C H O ) : C,
3
): d 70.53, 70.84, 72.22, 114.73,
1
yield). H NMR (500 MHz, CDCl
3
): d 3.26±3.32 (m, 2H), 3.34±
1
6
1
6
20 5 n
3
(
.43 (m, 6H), 3.68±3.73 (m, 2H), 3.78±3.84 (m, 2H), 4.20±4.26
5.74; H, 6.90. Found: C, 62.51; H, 7.65%.
1
m, 2H), 4.39±4.46 (m, 2H), 7.33 (s, 2H) ppm. C NMR
3
(
1
125 MHz, CDCl ): d 70.60, 71.31, 71.34, 72.28, 88.68, 126.67,
3
z
54.39 ppm. GC/MS (70 eV) m/z (%) 520 (M , 100), 360 (17),
Optical measurements
2
33 (9), 205 (15), 134 (12), 106 (10), 73 (28), 53 (25), 45 (63), 43
Absorption and emission spectra. Absorption measurements
were performed with a Varian DMS 100 double beam
spectrometer. For cw PL spectra, the solutions were excited
with the 441.6 nm line of a He±Cd ion laser. Absorption and cw
(
18 5 2
71). Anal. Calcd for C14H O I : C, 32.33; H, 3.49. Found: C,
3
1.78; H, 3.43%.
2
1
Poly[2,5-bis(2-methylbutoxy)-1,4-phenylenevinylene] (1a). A
mixture of 1,4-diiodo-2,5-bis(2-methylbutoxy)benzene 3a
PL spectra of solutions with a concentration of y20 mg l
were recorded in order to make negligible inter-chain
interactions. PL measurements under high pulsed excitation
energy were performed exciting the solutions with the output of
a dye laser pumped by a 10 Hz Q-switched Nd : YAG laser
providing y10 ns pulses. The output wavelength of the dye
laser was settled at the maximum absorption of the solutions
and ®lms. The pump laser beam was ®rst expanded and then
(
(
1.00 g, 2.0 mmol), (E)-1,2-bis(n-tributylstannyl)ethene
1.03 g, 2.0 mmol) and tetrakis(triphenylphosphine)palla-
) (0.069 g, 0.06 mmol), in anhydrous benzene (30 mL)
4
dium(
0
was re¯uxed in a nitrogen atmosphere. After 3 days a dark red
precipitate slowly separated from the reaction mixture. After
5
days the mixture was cooled to room temperature, the solvent
J. Mater. Chem., 2000, 10, 1573±1579
1577

View Article Online
focused onto the samples, by a cylindrical lens for obtaining a
narrow rectangular excitation area (0.2468 mm). The inci-
dence angle was nearly 90 degrees and the emission was
collected from the side of the sample. Films were kept under a
2
5
vacuum of 10 mbar to prevent photooxidation and the
solutions were contained in a 160.563 cm cuvette. The
emission was spectrally analysed by a system comprising a
monochromator and a CCD detector, having a spectral
resolution of 1.5 nm. All measurements were performed at
room temperature.
Photoluminescence ef®ciency. Comparison of the emission
intensity with that of a standard material gives approximate
values, because the emission angular distribution is not
isotropic as for the solutions, but it is strongly related to
refractive index of the material and to the morphology of the
Fig. 7 Measured spectra for a ®lm of 1a polymer in the three
con®gurations illustrated in Fig. 5. The PL spectra have been enlarged
®lm. For this reason the PL ef®ciencies were assessed by means
of an integrating sphere which redistributes isotropically the
emission of the ®lm regardless of its angular dependence over
the interior surface.
3
by a factor of 10 for clarity. The absorption band and the PL spectrum
(short long dashed lines) taken with the sample outside the sphere are
reported to display the re-absorption effect.
We followed the experimental procedure described by de
2
2
Mello et al. The experimental set-up for the measurement is
shown in Fig. 6. We used a modi®ed Ocean Optics integrating
sphere with an internal diameter of 50 mm and a sample port
with a diameter of 8 mm. The samples were ®xed on a Te¯on
support and put inside the sphere through the sample support.
The ®lms were excited with the 441.6 nm line of a He±Cd laser.
The beam laser was guided into a 600 mm optical ®bre and
focused onto the sample with a lens, having the optical axis
forming an angle of 8 degrees with respect to the normal. Laser
power was kept below 0.1 mW, with a beam diameter at the
sample of 4 mm. The signal was collected with an identical ®bre
at 90 degrees coupled with the spectrometer. This con®guration
of sample excitation and collection of the signal does not
require the use of a diffusive baf¯e to prevent direct
illumination of the optical ®bre. The assessment of the PL
ef®ciency involves three steps outlined in Fig. 6. In the ®rst
con®guration, the sphere is empty (Fig. 6a); this measurement
allows to determine the laser light incident onto the sample.
The second measurement is made with the sample placed inside
the sphere and the laser light directed on the sphere wall
shown in Fig. 7. The correction was made comparing the
photoluminescence spectra taken with the sample inside and
outside the sphere.
Electroluminescence ef®ciency. Light emitting diodes have
been prepared by spin-coating a 58 nm thick 1b PPV layer onto
indium-tin-oxide (ITO) coated glass from a tetrachloroethane
solution. ITO glass was previously treated with an oxygen
plasma in a reactive ion etching (RIE) reactor. This treatment,
lowering the ratio Sn : In and increasing the oxygen concentra-
23
tion on the material surface, leads to a lower turn-on voltage.
Al top contacts of 3 mm63 mm area were thermally
evaporated. I±V, and L±I characteristics were measured in
air at room temperature with a Hewlett-Packard HP4145B
semiconductor analyser and a calibrated photodiode.
Acknowledgements
This work was ®nancially supported by Ministero dell'
Universit aÁ e della Ricerca Scienti®ca e Tecnologica, Rome,
by the University of Bari (National Project ``Stereoselezione in
Sintesi Organica. Metodologie ed Applicazioni''), and by
Consiglio Nazionale delle Ricerche, Rome (National Project
(
Fig. 6b). This con®guration provides the way to evaluate the
secondary emission, i.e. emission due to illumination of the
sample by diffuse laser light. The third con®guration is similar
to the second one, but with the collimated laser beam directed
on the sample (Fig. 6c). The corresponding measured spectra
for 1a are shown in Fig. 7. The line at 441.6 nm corresponds to
the detection of the laser light and the band at longer
wavelengths to the detection of the emission sample. The
area under the laser pro®le is proportional to the unabsorbed
laser photons, while the area under the band is proportional to
the emitted photons. The evaluation of these areas in the three
``Progetto Finalizzato Materiali Speciali per Tecnologie
Avanzate II''). We are very grateful to Drs Fabio Palumbo
and Ernesto Mesto of CNR Centro di Chimica dei Plasmi- Bari
for the XPS analyses.
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1579