Improvement of the extended one-pot (EOP) procedure to form poly(aryleneethynylene)s and investigation of their electrical and optical properties

Article abstract of DOI:10.1021/ma0216829

A series of π-conjugated homopolymers of type poly(aryleneethynylene) (PAE), [-Ar-C≡C-]_n, (Ar = 2,5-bis(butoxy)benzene (7a), 2,5-bis(octyloxy)benzene (7b), 2,5-bis(hexadecyloxy)benzene (7c), 3-butylthiophene (7d), and 3-hexadecylthiophene (7e)) have been prepared by further improvement of the palladium-catalyzed Extended One-Pot (EOP) synthetic protocol. With the use of dioxane as solvent and higher reaction temperature (110°C), much higher polymerization degree, improved catalytic efficiency, and increased material purity were obtained. Numerical simulations have been performed in a series of different conjugated polymers in order to evaluate the role of the connection between aromatic rings in the maintaining of an effective electronic conjugation through the polymer chain. Experimentally, the conjugation properties have been investigated by means of photophysical measurements in liquid solution and in solid-state films. The electric transport properties have been characterized in view of applications to electronic devices.

Full text of DOI:10.1021/ma0216829

Macromolecules 2003, 36, 2215-2223
2215
Improvement of the Extended One-Pot (EOP) Procedure To Form
Poly(aryleneethynylene)s and Investigation of Their Electrical and
Optical Properties
†
†
‡
‡
§
|
R. P izzofer r a to, M. Ber liocch i, A. Di Ca r lo, P . Lu gli, M. Ven a n zi, A. Micozzi,
|
,⊥
A. Ricci, a n d C. Lo Ster zo*
INFM-Dipartimento di Ingegneria Meccanica and INFM-Dipartimento di Ingegneria Elettronica,
Facolt a` di Ingegneria, Universit a` di Tor Vergata, Via di Tor Vergata 110, 00133 Roma, Italy,
Dipartimento di Scienze e Tecnologie Chimiche, Universit a` di Tor Vergata, Via della Ricerca
Scientifica, 00133 Roma, Italy, Istituto C.N.R. di Chimica dei Composti Organometallici
(
“
ICCOM-CNR)-Sezione di Roma, Dipartimento di Chimica, Box 34-Roma 62, Universita’
La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy, and Dipartimento di Scienze
degli Alimenti, Facolt a` di Agraria, Universit a` degli Studi di Teramo, Via Spagna 1,
4023-Mosciano S. Angelo, Teramo, Italy
6
Received November 13, 2002; Revised Manuscript Received February 4, 2003
ABSTRACT: A series of π-conjugated homopolymers of type poly(aryleneethynylene) (PAE), [sArsCt
Cs] , (Ar ) 2,5-bis(butoxy)benzene (7a ), 2,5-bis(octyloxy)benzene (7b), 2,5-bis(hexadecyloxy)benzene (7c),
-butylthiophene (7d ), and 3-hexadecylthiophene (7e)) have been prepared by further improvement of
n
3
the palladium-catalyzed Extended One-Pot (EOP) synthetic protocol. With the use of dioxane as solvent
and higher reaction temperature (110 °C), much higher polymerization degree, improved catalytic
efficiency, and increased material purity were obtained. Numerical simulations have been performed in
a series of different conjugated polymers in order to evaluate the role of the connection between aromatic
rings in the maintaining of an effective electronic conjugation through the polymer chain. Experimentally,
the conjugation properties have been investigated by means of photophysical measurements in liquid
solution and in solid-state films. The electric transport properties have been characterized in view of
applications to electronic devices.
In tr od u ction
Organic conductive polymers are fully π-conjugated
polymers that are currently attracting increasing at-
tention due to their unique and unconventional physical
properties that are interesting for the development of
new materials in areas such as microelectronics, optics,
1
,2
sensor technology, and molecular electronics.
In the field of organic conductive π-conjugated poly-
mers, three different classes of materials are mostly
used: (i) polymers made of aromatic rings directly
linked to each other in a linear fashion, the poly-
F igu r e 1. Organic conductive π-conjugated polymers.
moieties is a mandatory requirement in order to obtain
soluble and processable materials, but as a consequence
of the unavoidable steric interactions occurring between
lateral chains on adjacent aromatic rings, the coplanar-
ity of the connected ringssand, thus, conjugation and
related phenomenonasis severely affected.
3
,4
(
arylene)s (PAs) (I), (ii) polymers made of alternating
5
aryl and ethylene units, the poly(p-arylenevinylene)s
(
PAVs) (II), and (iii) polymers made of alternating aryl
2
a,3
and ethynyl units poly(aryleneethynylene)s
(PAEs)
(III) (Figure 1). In each of these classes, the aromatic
unit may be substituted benzenes (and superior poly-
cyclic aromatic hydrocarbons) and heteroaromatics such
as thiophenes, pyridines, pyrroles, and thiazoles. More-
over, the main chain of these polymeric materials may
be constituted by a sequence of identical moieties or by
a combination of different aromatic moieties, forming
homo- and copolymers, respectively.
Because of the direct ring-to-ring connection, the
disturbing of ring coplanarity by side-chain interactions
is obviously very heavy on PA polymers (I), and in these
systems torsion about the aryl-aryl single bond can
only be overcome by incorporating the PA backbone into
6
a ladder-type structure, with an additional and de-
manding synthetic effort. In polymers of type PAVs (II),
the steric requirements of the R substituents on the Ar
unit directly influence the electronic properties of the
However, for all materials presented in Figure 1, the
presence of side-chain groups (R) on the aromatic
material by affecting the torsion of the conjugated chain
*
Corresponding author. E-mail: losterzo@agr.unite.it.
INFM-Dipartimento di Ingegneria Meccanica, Facolt a` di In-
7
(i.e. the π-π* energy gap).
†
Within this frame we believe that there are a number
gegneria, Universit a` di Tor Vergata.
‡
INFM-Dipartimento di Ingegneria Elettronica, Facolt a` di
of reasons by which PAEs (III) might offer superior
opportunities with respect to the other organic conju-
gated polymers (I-II). (i) In PAEs aromatic rings are
spaced by straightly aligned acetylene moieties; then
the coplanarity of the rings and related conjugation are
Ingegneria, Universit a` di Tor Vergata.
§
Dipartimento di Scienze e Tecnologie Chimiche, Universit a` di
Tor Vergata.
|
Universita’ “La Sapienza”.
Universit a` degli Studi di Teramo.
⊥
1
0.1021/ma0216829 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/15/2003

2
216 Pizzoferrato et al.
Macromolecules, Vol. 36, No. 7, 2003
materials may perform a more effective conjugation with
respect to those of PAV and PA.
Therefore, under real working conditions because of
packing interactions between adjacent chains of polymer
films, and consequent ring torsion, PAE polymers might
display superior performances with respect to PAV and
PA materials.
On the basis of these premises, we concluded that
PAEs are unique materials in the field of organic
conductive polymers.
F igu r e 2. Comparison of the influence of the torsion angle
on the effectiveness of the π-conjugation for the three-ring
models of PAs, PAVs, and PAEs.
We have recently entered this field, reporting a new
and versatile one-pot procedure, named Extended One
Pot (EOP), based on a cascade of Pd-promoted processes
straightforwardly leading from aromatic diiodides to a
wide range of ethynylaromatic homopolymers [sCtCs
Ars]n (IV), ethynylaromatic copolymers [sCtCsArs
CtCsAr′s]n (V), and ethynylaromatic-ethynylmetallo
copolymers [sCtCsArsCtCsM(L)ms]n (VI) (Scheme
less disturbed by the sterical constraint imposed by side-
chain interactions. (ii) Because of the cylindrical sym-
metry of the π-electron distribution on the triple bonds,
an efficient conjugation should be maintained even in
the presence of a certain degree of torsion between
1
). The overall EOP routes leading to IV-VI are
9
rings; in other words, in PAEs conjugation is less
essentially constituted by an interplay of Pd-catalyzed
processes (steps 1 and 3) and an in-situ base-promoted
alkyne/stannane conversion (step 2). The Pd-catalyzed
couplings taking place in the formation of IV and V are
adversely affected by disturbance of the coplanarity of
the rings than in any other conjugated system. Thus,
6
in these systems planarization of aromatic rings is not
a severe requirement in order to maintain conjugation.
17
Stille couplings, while the formation of the organo-
(
iii) Since in organic conjugated materials (opto)elec-
metallic polymers VI in step 3 is achieved by our
original Pd-catalyzed metal-carbon bond formation
tronic properties strongly depend on thin film morphol-
ogy, because electronic overlap between molecules is a
18
procedure.
1
0,11
consequence of relative molecular orientation,
in
In this paper we report on the further improvement
of performances of our EOP synthetic protocol for the
preparation of a range of ethynylated homopolymers of
type poly(aryleneethynylene)s (PAEs), of general for-
mula [sArsCtCs]n (IV), and on the investigation of
their optical and electrical properties, both in solution
and in film, in view of their possible use in device
fabrication.
PAEs by the interplay of the rigid rodlike backbone
structure and the presence of properly chosen side
substituents on the aromatic rings, a control of the
supramolecular assembly may be possible,1 and an
ordered control of thin film morphology might be
achieved.
2
The maintaining of a greater extent of conjugations
and related propertiessin polymeric materials incorpo-
rating an acetylenic linkage between aromatic rings,
with respect to other conjugated polymers, is also
accounted for by quantum-chemistry calculations. Fol-
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of P olym er s. In
Scheme 2 is outlined the three-step palladium-promoted
EOP synthetic protocol we used to form the poly-
(aryleneethynylene) homopolymers [sArsCtCs]n (Ar
) 2,5-bis(butoxy)benzene (7a ), 2,5-bis(octyloxy)benzene
(7b), 2,5-bis(hexadecyloxy)benzene (7c), 3-butylthio-
phene (7d ), and 3-hexadecylthiophene (7e)). The aro-
matic units enter in this synthetic route as 1,4-diodo-
2,5-dialkoxybenzenes (1a -c) and 2,5-diiodo-3-alkyl-
thiophenes (1d ,e), while the acetylenic spacer is con-
veyed into the polymeric structure by the means of its
tributyltin derivative 2.
In the first step the diiodoaromatic compounds (1a -
e) are reacted with tributylethynyltin (Bu3SnsCtCs
H) (2) in refluxing dioxane (110 °C), in the presence of
0.5 mol % of Pd(PPh3)4. In principle, the precise 1:1
stoichiometric ratio between reactants should lead to
the exclusive formation of the products of monocoupling,
the iodo(ethynyl)aromatic compounds (IsArsCtCsH)
(4a -e); however, under these conditions formation of
the biscoupled products (HsCtCsArsCtCsH) (3a -
e) is unavoidable and, as a consequence, an equivalent
amount of unreacted diiodides (IsArsI) (1a -e) is
remaining. In 20 min step 1 is completed and an
equivalent amount of the side product Bu3SnI (per
equivalent of ethynyl group reacted) is formed. Cooling
at 0 °C and addition of LDA caused deprotonation of
the terminal alkyne moieties of 3a -e and 4a -e and the
immediate recombination of the resulting acetylides
with Bu3SnI, to form Bu3SnsCtCsArsCtCsSnBu3
9
lowing the calculation presented by Bunz, in order to
evaluate the influence of the torsion angle on the
π-conjugation of materials of types I-III, we have
calculated and compared the conjugation properties of
a three-ring structure model of compounds of PAs,
PAVs, and PAEs as a function of the torsion angle
between rings (Figure 2). The geometry of the structure
is first relaxed by using standard semiempirical Har-
1
3
tree-Fock methods (PM model ), and then the first
singlet excited state (fundamental optical transition) is
calculated by using a Configuration-Interaction scheme
in an INDO context.14 The energy of the optical transi-
tion is calculated for several values of rotational angle
of the central ring. Figure 2 shows the calculated
transition energy as a function of rotation angle of the
central ring for the three systems.
For the planar structure the PAV system presents a
smaller transition energy with respect to both PAE and
PA. Thus, in the planar case the conjugation property
of the PAV system is larger than those of the other two
cases. This is confirmed by recent experimental and
theoretical results comparing the transport properties
1
5
of three-ring systems of PAV and PAE. By rotating
the central ring, the degree of conjugation reduces for
all systems. However, for a rotation angle larger than
5
0° the PAE system results in a lower transition energy
with respect to those for the PAV and PA systems. This
clearly shows that, for rotation angles larger than 50°,
because of the presence of the acetylenic spacer, PAE

Macromolecules, Vol. 36, No. 7, 2003
One-Pot Procedure To Form Poly(aryleneethynylene)s 2217
Sch em e 1
(
5a -e) and IsArsCtCsSnBu3 (6a -e), respectively,
step can be safely, and more conveniently, carried out
at 0 °C in all cases. The GPC analysis of the polymeric
materials 7a -e obtained by operating under these new
conditions showed that with respect to reactions per-
formed in THF, in all cases a great increase of polymer
molecular weights was obtained, now reaching the 43-
94 DP range. Moreover, in refluxing dioxane, yields of
7a -c significantly improved from the 69-76% range to
the 86-94% range, while yields of 7d -e are maintained
while IsArsI (1a -e) remained unchanged (step 2). This
mixture, brought back to 110 °C, led in an additional
2
-8 h to the formation of the corresponding homopoly-
mers [sArsCtCs]n (7a -e) (step 3). Although both
coupling processes taking place in steps 1 and 3 required
the presence of zerovalent palladium, the catalyst was
only added at once in the first step, with its activity
being maintained throughout the whole synthetic route.
Upon cooling at room temperature, the polymeric
material separated out from the solution as an orange
or orange-brown solid. Precipitation was completed by
addition of methanol; then filtration and repeated
washing with methanol allowed isolation of the purified
material. As experienced before, by fractional distil-
lation of the methanolic rinsing solution, most of the
Bu3SnI formed after completion of step 3 was recovered
and then used to form new Bu3SnsCtCsH, thus
allowing a practically complete recycle of the tin in-
volved in the overall process.
1
9
over the range of 90% (Table 1). Upon obtaining
materials of higher molecular weights, purification of
products became easier. In fact, a crispy solid was
obtained by precipitation (with the exception of 7e), from
which a soluble byproduct and lighter oligomeric frac-
tions could be easily washed out. Finally, it is worthy
of observation that a remarkable catalytic efficiency
1
6b
0
(0.5% of Pd for the overall process of Scheme 2) has
been maintained also by operating under more severe
conditions (110 °C).
2
a,3
With respect to current procedures
used to obtain
It is important to note that when the same transfor-
mations were run in THF solvent, steps 1 and 3 (run at
ethynylaromatic homopolymers [sArsCtCs]n, the EOP
route outlined in Scheme 2 offers some distinct advan-
tages. In Scheme 3 are reported the two most common
strategies used to form (polyaryleneethynylene)s (PAEs):
(route a) the AA/BB approach and (route b) the AB
approach. In both cases the starting material is the
aromatic dihalide ArX2 (8) which is reacted with a
different amount of trimethylsilylacetylene (TMSA) (9)
7
0 °C) required 16 h and 24 h to be accomplished,
1
6
respectively. Moreover, under the former conditions
in order to avoid secondary transformations, treatment
with LDA in step 2 had to be conducted at -20 °C for
alkoxybenzenes 3a -c and 4a -c and at -80 °C for
alkylthiophenes 3d ,e and 4d ,e, while in dioxane this

2
218 Pizzoferrato et al.
Macromolecules, Vol. 36, No. 7, 2003
Sch em e 2
Ta ble 1
a
Determined by GPC (THF, polystyrene standard). b Molecular weight distribution (Mw/Mn). c Degree of polymerization.
2
0
21
in the presence of palladium (Sonogashira -Heck -
Cassar22 coupling). In the first case, the 1:2 ratio
between 8 and 9 lead to the formation of 10 while, in
the second one, despite the accurate 1:1 stoichiometric
ratio between reactants, a mixture of 8, 10, and 12 is
unavoidably obtained, and as a consequence in order

Macromolecules, Vol. 36, No. 7, 2003
One-Pot Procedure To Form Poly(aryleneethynylene)s 2219
Sch em e 3
Sch em e 4
to isolate the only useful precursor 12, an accurate
separation is required. Subsequently, the TMS inter-
mediates 10 and 12 are deprotected, and after isola-
tion,23 the corresponding acetylide derivatives 11 and
4), presenting an alternative route to poly(phenylene-
ethynylene)s, not relying on Pd-catalyzed processes
(Schemes 2 and 3). The ADIMET consists of a very
efficient process, promoted by an in-situ generated/
highly reactive Mo catalyst. Despite its high reactivity,
the active catalyst is conveniently formed in nondried,
nonpurified, off-the-shelf solvents, and the process,
which is very efficient when hydrocarbon monomers are
used, seems to suffer only a few limitations.
1
3 are allowed to react with an equivalent amount of
the dihalide ArX2 (route a) or to self-condense (route b).
In both cases, to achieve a high polymerization degree,
a precise control of the stoichiometric balance is neces-
sary. Since 13 intrinsically offers the precise stoichi-
ometry between the two different functional groups, in
principle, the AB approach (route b) is preferable;
however, because of the cumbersome separation re-
quired to isolate 12 from 10 and 8, the AA/BB approach
is generally preferred.
In vestiga tion of P h otop h ysica l a n d Electr ica l
P r op er ties
Optical and electrical measurements were carried out
in order to characterize the electronic properties of PAEs
The EOP synthetic route (Scheme 2) practically offers
all the advantages of the AB approach (route b of
Scheme 3), without the demanding necessity of dealing
with the isolation of pure hetero-difunctionalized species
7
a -e as opto- and electroactive materials. Toward this
end, photophysical characteristics were investigated
both in solution and in solid-state films, and in the case
of 7b, samples of different degrees of polymerization
6
a -e. In fact, formation of polymers 7a -e efficiently
(7bI, DP ) 40; 7bII, DP ) 90) were examined.
occurs regardless of the relative ratio of the three
coupling partners participating in step 3. The only
determining factor to obtain a polymeric material
constituted by a regularly alternating alkyne/arene
sequence is to have an overall 1:1 iodine/tributyltin
acetylyde moieties ratio, independent of the relative
ratio of the three different coupling partners bearing
these functionalities. Moreover, while intermediates 10
and 11 in route a and 12 and 13 in route b of Scheme 3
need to be isolated and carefully purified prior to the
subsequent uses, in the EOP process the polymerization
process starts with the reaction of diiodides 1a -c with
the tributyltin derivative 2, and besides the halfway
injection of LDA, the transformation straightforwardly
proceeds up to the formation of polymers 7a -e. The
unnecessary separation and purification of intermedi-
ates is a particularly appreciable aspect, especially
considering the delicate nature of aromatic alkynes.
Toward this end, Bunz27 has recently reported the
acyclic diyne metathesis (ADIMET) approach (Scheme
Op tica l Absor p tion a n d P h otolu m in escen ce of
P olym er s in Solu tion . The absorption and emission
spectra obtained for diluted CHCl3 solutions are shown
in Figure 3, and all the relevant spectral parameters
are summarized in Table 2. We note that 7a , 7bII, and
7c show almost identical optical properties in solution.
The absorption spectra of all polymers consist of a
peak at 320 nm and a broad main band centered around
425 nm with a slightly minor blue-shift (417 nm) for
the shorter-chain polymer 7bI. The PL emission spectra,
which are insensitive to the excitation wavelength in
the range from 380 to 460 nm, show a well resolved
splitting into a relatively narrow band, centered around
475 nm, and a broader shoulder, peaked in the range
from 505 to 515 nm and of slightly different relative
intensity depending on the compound. This splitting
and the lack of mirror image similarity between absorp-
tion and emission spectra suggest the influence of
vibronic coupling while the emission bands have been

2
220 Pizzoferrato et al.
Macromolecules, Vol. 36, No. 7, 2003
Ta ble 3. Op tica l P r op er ties of th e P olym er s in
Solid -Sta te F ilm s
absorption λmax emission λmax
emission λmax
in spin-coated in spin-coated in spray-coated
polymer DPa
films (nm)
films (nm)
films (nm)
7
7
7
7
7
7
a
79
40
90
43
94
82
440
424
447
452
405
413
540
585
575
565
630
595
565
580
560
575
655
600
bI
bII
c
d
e
a
Degree of polymerization.
of the solid state (vide infra), suggesting a different
degree of intermolecular interaction or, possibly, the
presence of supramolecular ordering. Moreover, the
little differences observed for the λmax values for com-
pounds 7bI and 7bII, as well as for values in the
literature for samples of different DPs, further support
the evidence that in conjugated polymers optical proper-
ties reach saturation for DP values over 30-40 repeat-
3
3
ing units. The similarity of the optical properties of
benzene-based polymers 7a -7c in solution is particu-
larly meaningful in view of the noticeable differences
observed with respect to the spectra of thiophene-based
polymers. In fact, the absorption spectra of 7d and 7e
are appreciably broader while the emission bands are
not so well resolved in the two structures and are red-
shifted by approximately 50 nm. Moreover, the PL
quantum efficiencies are 1 order of magnitude lower.
Op tica l Absor p tion a n d P h otolu m in escen ce of
Solid -Sta te F ilm s. The solid-state photophysics and
electrical characteristics of the new PAEs were inves-
F igu r e 3. (a) Absorption spectra of the PAEs in CHCl
3
at 295
K. (b) Emission spectra of the PAEs in CHCl at 295 K. The
3
intensities were arbitrarily chosen in order to optimally fit the
graphs.
Ta ble 2. Op tica l P r op er ties of th e P olym er s in Solu tion sa
-4
tigated under ambient conditions and in a vacuum (10
absorption λmax
emission λmax
Torr) on pristine films deposited either by means of
casting, with an average thickness in the range 20-40
µm, or by a spin-coating technique (thickness about 0.1
µm). Excitation for photoluminescence spectra was
provided by the 457-nm line of the Ar-ion laser with a
DPb
(nm)
(nm)
φfl
c
polymer
7
7
7
7
7
7
a
79
40
90
43
94
82
428
417
425
424
414
413
475
475
475
475
550
538
0.14
0.32
0.21
0.21
0.015
0.017
bI
bII
c
d
e
2
power flux in the range 50-200 mW/cm or by the
monochromatized output from a 300-W Xe lamp. The
results regarding spin-coated samples are reported in
Figure 4 and summarized in Table 3. Absorption spectra
of spin-coated samples are strikingly similar to the ones
of liquid solutions apart from a small red-shift of 10-
a
b
All experiments were performed in CHCl3. Degree of polym-
erization. Fluorescence quantum yield.
c
attributed28 to transitions from localized states after a
migration of the excitations along the polymer main
chains.
2
0 nm, for benzene-based polymers, while thiophene
ones are not shifted significantly. A more important
diversification appears in the PL emission spectra
(Figure 4b), and it is even enhanced in casting-deposited
films (data in Table 3). The curves, appreciably broad-
ened but still exhibiting vibronic-like features, show
noticeable differences from each other even within the
class of benzene-based compounds, depending on DP
and the size of decorating chains. This is likely due to
the intermolecular interaction and to a certain degree
of long-range order in the supramolecular structure
induced by the attached chains and the rigid-rod
The spectroscopic features of compounds 7a -c are
consistent with literature reports for the same polymers
prepared by means of conventional methods, even
though quite scattered data are presented by other
groups. Specifically, for 7a , synthesized29 with an aver-
age DP of 138, only the PL emission is reported peaked
at λmax ) 480 nm; for 7b28 with DP ) 23 the absorption
is peaked at λmax ) 449 nm while the main emission is
centered at λmax ) 474 nm; for 7c30 only the absorption
peak was determined at λmax ) 425 nm. The experi-
mental values of PL quantum yields estimated in the
present work are also very similar among the different
2
8
length.
P h ot od egr a d a t ion E ffect s on P h ot olu m in es-
cen ce In ten sity. Solid-state samples also provide a
way to test the stability of polymers under continuos
light excitation, which is a preliminary key point for
applications to light-emitting devices (OLEDs). In this
respect, the high reactivity of singlet oxygen with
double-bond (PAV) polymers has been extensively stud-
ied and well understood, while there are few literature
reports concerning reaction with triple bonds,34 such as
in the case of the present polymers. We observed that
3
1
polymers and lay in the range from 0.14 to 0.32.
Such a striking overall similarity of the optical
properties of these polymers in solution seems to
confirm that these molecules are predominantly gov-
erned by the rigid rod and highly conjugated polymer
backbone and are not influenced by the size of the
attached side chains. On the contrary, the different sizes
of side chains significantly affect the optical properties

Macromolecules, Vol. 36, No. 7, 2003
One-Pot Procedure To Form Poly(aryleneethynylene)s 2221
F igu r e 4. (a) Absorption spectra of solid-state films of the
PAEs deposited by spin-coating. (b) Emission spectra of solid-
state films of the PAEs deposited by spin-coating. The intensi-
ties were arbitrarily chosen in order to optimally fit the graphs.
F igu r e 5. (a) Relative intensity of photoluminescence from a
solid-state film of 7b during irradiation in air with 40 mW/
I
cm2 of 457-nm light from an Ar laser. (b) The same in a
+
films of benzene-based compounds 7a -c showed quite
-4
vacuum (10 Torr).
a stable photoluminescence signal under a CW laser
2
power density up to 5-10 mW/cm in air while a fast
decrease of emission, with a decay time of 600 s
approximately, was monitored as the excitation power
flux was increased above 30 mW/cm2 (Figure 5a),
resulting in a visible permanent bleaching of the irradi-
ated area. Both these effects were much lower when
-
4
irradiation was performed in a vacuum (10
Torr)
(Figure 5b). These results, in fact, confirm the hypoth-
esis of PAEs’ greater stability toward photooxidation
than that of PAV derivatives,34 even though the range
of power density is relatively low. On the other hand,
thiophene-based polymers 7d -e, despite the low values
of quantum efficiency, did not exhibit any appreciable
photooxidation effects.
Electr ica l Con d u ctivity Mea su r em en ts. Electrical
conductivity measurements for each polymer were first
carried out at room temperature in solution-cast films
deposited onto glass substrates with coplanar gap-
separated electrodes (width ) 500 µm, length ) 1 cm).
For measurements at variable temperatures a spray-
casting technique with interdigitated coplanar-electrode
substrates (width ) 60 µm, length ) 0.36 cm) was used,
to increase the detected current. In the latter case, both
aluminum and chromium evaporated electrodes were
used. The I-V curves recorded at room temperature
F igu r e 6. I-V characteristics at 295 K of the PAE films
deposited by casting with aluminum electrodes.
spectra in the solid state (see Figure 4), the conductivity
of films does not exhibit a strong dependence on the
degree of polymerization and on the structure of at-
tached chains. This suggests that the presence of
possible intermolecular interaction or supramolecular
order does not affect the transport behavior, which, on
the other hand, is dominated by injection effects (vide
infra). For higher fields, a trend to a quadratic depen-
dence appears, which suggests the onset of space-charge
limiting effects in the conductive regime. Moreover, Cr
electrodes are generally demonstrated to be better
charge injectors than Al, allowing values of conductivity
up to 3 orders of magnitude higher. Such a large
(
Figure 6) show a general ohmic behavior, that is, a
linear dependence of current on the applied voltage, up
4
to values of the electric field E ) 4 × 10 V/m,
approximately. This allows us to estimate the film
-
9
conductivity σ to values ranging around 4 × 10 S/cm
with Al electrodes, in reasonable agreement with results
reported in the literature29 for 7a prepared with con-
ventional methods. Differently from the fluorescence

2
222 Pizzoferrato et al.
Macromolecules, Vol. 36, No. 7, 2003
2
00K-10M, HT 5 50K-4M, HT 4 5K-600K, HT 3 500K-30K)
and a UV detector. THF (HPLC grade; Aldrich) was the eluent
(flow rate: 1 mL/min).
All manipulations were carried out with Schlenk type
equipment under an atmosphere of argon on a dual manifold/
argon-vacuum system. Dioxane was dried over sodium-
potassium alloy and argon-saturated prior to use. 1,4-Diiodo-
2
(
3
(
,5-bis(butoxy)benzene (1a), 1,4-diiodo-2,5-bis(octyloxy)benzene
1b), 1,4-diiodo-2,5-bis(hexadecyloxy)benzene (1c), 2,5-diiodo-
-butylthiophene (1d ), and 2,5-diiodo-3-hexadecylthiophene
1e) were prepared by using our previously described proce-
1
6
dure. Other chemicals were purchased from Aldrich or Strem
and used as received unless otherwise specified.
P h otoph ysical Ch ar acter ization . Visible absorption spec-
tra were carried out by using a Perkin Helmer Lambda 19
spectrophotometer. Photoluminescence spectra were measured
on a standard setup for photoluminescence using either a
Spectra Physics 2020 argon ion laser or an Oriel xenon lamp
as the excitation source. All absorption and emission studies
3
in solutions were performed in deoxygenated CHCl at con-
centrations at absorption between 0.05 and 0.5 (concentrations
in the range 0.05-0.5 mg/mL) in a quartz cell with a length
of 1 mm. Quantum efficiencies were determined relative to
anthracene in a 10-mm quartz cell by means of a Spex-
Fluoromax instrument at an absorption of 0.04.
F igu r e 7. Activation energy for the electrical conductivity of
a 7a film deposited by spray-casting onto a glass substrate
interdigitated with aluminum electrodes.
For absorption and fluorescence measurements on film, the
polymers were dissolved in CHCl
and 10 mg/mL and were spin-cast on glass substrates. The
3
at concentrations between
5
difference suggests that the transport is mainly domi-
nated by injection mechanisms at electrodes rather than
by bulk processes in the polymer. Within this model,
measurements at variable temperatures indicate that
conduction is thermally activated (thermoionic emission)
with an energy barrier Ea evaluated in the range 0.3-
polymer films were annealed for 10 min at 100 °C and then
slowly cooled to room temperature. The thickness of the films
as measured with a profilometer was typically 100 nm.
Electrical characterizations were performed with a 4155B
Agilent semiconductor parameter analyzer. Variable-temper-
ature measurements were carried out in an Oxford cryogenic
apparatus.
0
.5 eV (Figure 7 for 7a ). Moreover, with respect to the
HOMO and LUMO levels reported in the literature for
similar polymers,35 and the Fermi levels of the two
metals, these features suggest a hole-transport mech-
anism for conductivity.
P r epar ation of th e P oly(ar ylen eeth yn ylen e) Hom opoly-
m er s 7a -e. The procedure for the preparation of poly{ethynyl-
[2,5-di(butoxy)]benzene} (7a ) is representative. To a solution
3 4
of 0.012 g (0.011 mmol) of Pd(PPh ) and 1.000 g (2.10 mmol)
of 1,4-diiodo-2,5-bis(butoxy)benzene in 40 mL of dioxane was
added 0.662 g (2.10 mmol) of tributyl(ethynyl)tin. After 20 min
Con clu sion s
1
16
of stirring at 110 °C, H NMR analysis indicated complete
consumption of the tin reagent. After cooling to 0 °C, 1.16 mL
The Pd-promoted Extended One-Pot (EOP) synthetic
protocol represents a convenient way to access poly-
(2.31 mmol) of a solution of LDA (2.0 M solution in THF/
heptane/ethylbenzene) was added, and the mixture was al-
lowed to warm to room temperature. A sample of the reaction
(
aryleneethynylene)s (PAEs), a special class of π-
conjugated polymers offering unique opportunities as
advanced materials for innovative optoelectronic ap-
plications. Thanks to the improved performances here
presented, by this novel synthetic route, good quality
polymeric materials are obtained, whose optoelectronic
characteristics are in line with those of similar materials
obtained with more demanding conventional proce-
dures. Simplicity and straightforwardness, high yield,
safety, and economy are key characteristic of this
1
16b
mixture examined by H NMR analysis indicated complete
conversion of the alkyne moieties into the corresponding
tributyltinalkynyl functionalities. Subsequently, the mixture
was refluxed for 2 h and, after cooling, concentrated to a small
volume, and treated with methanol. The precipitate thus
formed was collected by filtration, washed repeatedly with
methanol, and dried under vacuum. 0.568 g (86%) of product
was obtained as an orange solid. Evaporation under reduced
pressure of the washings afforded a dark oil which was
-
2
submitted to vacuum distillation (Kugelrohr, 120 °C/10
mmHg), giving 0.802 g (92%) of tributyltin iodide.
procedure, whose further exploitation, in parallel with
other innovative approaches,2
7,36
might lead to a wider
The spectroscopic and analytical features of polymers 7a -e
use of a very promising class of advanced materials.
were in agreement with reported data.1
6b
Exp er im en ta l Section
Ack n ow led gm en t . This work was supported by
the Consiglio Nazionale delle Ricerche (C.N.R. Roma),
with the “Progetto Finalizzato C.N.R.-Materiali
Speciali per Tecnologie Avanzate II”, contract n.
97.00946.PF34, and the “Progetto Finalizzato C.N.R.-
Materiali e Dispositivi per l’Elettronica a Stato Solido”,
contract n. 97.01350.PF48.
Gen er a l P r oced u r es. Elemental analyses were performed
by the Servizio Microanalisi of the Dipartimento di Chimica
of Universit a` di Roma “La Sapienza”. IR spectra were recorded
on a Nicolet FT 510 instrument in the solvent subtraction
1
13
mode, using a 0.1 mm CsI cell. H NMR and C NMR spectra
were recorded on a Bruker AC300P spectrometer at 300 and
1
7
5 MHz, respectively. The H NMR chemical shifts (ppm) are
1
relative to Me
4
Si, assigning the residual H impurity signal
1
3
in the solvent at 7.24 ppm (CDCl ). The C NMR chemical
shifts are calibrated to the C triplet of CDCl at 77.00 ppm.
3
Refer en ces a n d Notes
1
3
3
Molecular weights were determined (relative to polystyrene
standard) on a Perkin-Elmer gel permeation chromatograph
(
1) Handbook of Conducting Polymers; Skotheim, T. J ., Elsen-
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(GPC) equipped with a set of Waters Styragel columns (HT 6

Macromolecules, Vol. 36, No. 7, 2003
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