Use of the Pd-promoted Extended One-Pot (EOP) synthetic protocol for the modular construction of poly-(arylene ethynylene) co-polymers [-Ar-C≡C-Ar′-C≡C-]n, opto- and electro-responsive materials for advanced technology

Article abstract of DOI:10.1002/adsc.200404233

We report on the use of our novel multistep/ one-pot/Pd-promoted synthetic strategy, named extended one-pot (EOP), for the preparation of polymeric conjugated systems characterized by a backbone composed of regularly alternating alkyne and arylene moieties of type [-C≡C-A-C≡C-B-]_n. The "A" unit (or module) is in charge of impressing mechanical strength, chemical stability and processability to the polymer, while different "B" units (or modules) have been selected to obtain polymers with different functional properties. With this "modular approach" concept, a series of co-poly(arylene ethynylene)s, of general formula [-Ar-C≡C-Ar′-C≡C-]_n [Ar = 2,5-bis(octyloxy)benzene; Ar′ = 1,10-phenanthroline, hydroquinone, pyridine, tetrafluorobenzene, dithiophene, benzene, and anthracene] potentially useful for the fabrication of sensory, electroactive and light-emitting materials, have been formed. Investigations of the photophysical properties of these materials, both in solution and in the solid state, have demonstrated a large degree of variation of properties depending on the nature of Ar′ and the extension of the conjugation in the polymeric backbone. This EOP synthetic protocol, with its multiple and sequential one-pot Pd-catalyzed processes, is characterized by a very low catalyst charge loading, a consistent cut-down of reaction times, ease of operation and cost with respect to conventional procedures to obtain ethynylated polymers. Moreover, although it is based on the Pd-catalyzed coupling of organostannanes and aromatic halides (Stille reaction), the EOP synthetic route optimizes and reduces the use of tin, because during the multi-step one-pot process, tin intermediates are in situ formed by complete reconversion of tin by-products generated in the course of the transformation. In addition, after formation and isolation of polymeric materials, tin-containing products are recovered and reused to form new reagents for the delivery of the alkyne moiety into the polymer backbone.

Full text of DOI:10.1002/adsc.200404233

FULL PAPERS
Use of the Pd-Promoted Extended One-Pot (EOP) Synthetic
Protocol for the Modular Construction of Poly-(arylene
ꢀ ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
ethynylene) co-Polymers [ Ar C C Ar’ C C ]n, Opto- and
Electro-Responsive Materials for Advanced Technology
Alessandra Micozzia, Monica Ottaviania, Giorgio Giardinaa, Antonella Riccia,
Roberto Pizzoferratob, Tiffany Zillerb, Dario Compagnone,c Claudio Lo Sterzoc,*
a
Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione Dipartimento di Chimica, Box
`
34-Roma 62, Universita “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
b
c
`
`
Facolta di Ingegneria, INFM-Dipartimento di Ingegneria Meccanica, Universita di Tor Vergata, Via del Politecnico 1,
00133 Roma, Italy
Dipartimento di Scienze degli Alimenti, Facolta di Agraria, Universita degli Studi di Teramo, Via Carlo R. Lerici 1,
64023 Mosciano SantꢀAngelo (Teramo), Italy
`
`
Fax: (þ39)-85-807-1509, e-mail: closterzo@unite.it
Received: July 21, 2004; Accepted: October 29, 2004; Published online: January 11, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: We report on the use of our novel multistep/ the conjugation in the polymeric backbone. This
one-pot/Pd-promoted synthetic strategy, named ex- EOP synthetic protocol, with its multiple and sequen-
tended one-pot (EOP), for the preparation of poly- tial one-pot Pd-catalyzed processes, is characterized
meric conjugated systems characterized by a back- by a very low catalyst charge loading, a consistent
bone composed of regularly alternating alkyne and ar- cut-down of reaction times, ease of operation and
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ ꢀ
ylene moieties of type [ C C A C C B ]n. The cost with respect to conventional procedures to obtain
“A” unit (or module) is in charge of impressing me- ethynylated polymers. Moreover, although it is based
chanical strength, chemical stability and processabili- on the Pd-catalyzed coupling of organostannanes and
ty to the polymer, while different “B” units (or mod- aromatic halides (Stille reaction), the EOP synthetic
ules) have been selected to obtain polymers with dif- route optimizes and reduces the use of tin, because
ferent functional properties. With this “modular ap- during the multi-step one-pot process, tin intermedi-
proach” concept, a series of co-poly(arylene ethyny- ates are in situ formed by complete reconversion of
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
lene)s, of general formula [ Ar C C Ar’ C C ]n tin by-products generated in the course of the trans-
[Ar¼2,5-bis(octyloxy)benzene;
Ar’¼1,10-phenan- formation. In addition, after formation and isolation
throline, hydroquinone, pyridine, tetrafluorobenzene, of polymeric materials, tin-containing products are re-
dithiophene, benzene, and anthracene] potentially covered and reused to form new reagents for the de-
useful for the fabrication of sensory, electroactive livery of the alkyne moiety into the polymer back-
and light-emitting materials, have been formed. Inves- bone.
tigations of the photophysical properties of these ma-
terials, both in solution and in the solid state, have
demonstrated a large degree of variation of properties Keywords: C C coupling; conductive polymers; mate-
depending on the nature of Ar’ and the extension of rials science; palladium; Stille reaction
ꢀ
Introduction
um, -silicon, -tin, etc.), both ranging from simple precur-
sors to highly elaborate structures, can be coupled to
In the last 30 years metal-catalyzed cross-coupling reac- form more complex molecules.[1] Among this large fam-
tions have emerged as the most powerful and useful tool ily of reactions we intend to focus on a particular class of
in synthetic organic chemistry, as mainly by means of Cu, transformations, promoted by Pd, in which organic elec-
Ni, Pd catalysts a wide variety of electrophilic partners trophiles are coupled with organotin reagents – namely
(organic triflates, halides and acyl chlorides) and nucle- the Stille reaction.[2] Because of its wide versatility due
ophiles (organo-boron, -aluminium, -copper, -zirconi- to the mildness of the operating conditions, tolerance
Adv. Synth. Catal. 2005, 347, 143–160
DOI: 10.1002/adsc.200404233
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Figure 1. Different uses of the extended one-pot (EOP) synthetic protocol.
of existing functional groups, very large or complete
Unfortunately, the key to the success of the Stille reac-
stereospecificity and the variety of organic residues (al- tion, the use of organotin reagents as nucleophiles,[2,3,8]
kyl, aryl, vinyl, alkynyl, etc.) both allowed as electro- because of the toxicity of tin reagents and the conse-
phile and/or organotin partner, the Stille reaction large- quent related environmental and health concerns, also
ly is the most highly developed and extensively utilized represents the principal drawback toward the conver-
metal-mediated synthetic organic procedure.[2,3]
sion of useful and rationally designed laboratory scale
One of the most efficient synthetic processes per- synthetic procedures into industrial applications, since
formed by the Stille reaction is the coupling between ar- no industrial scale tin-removal procedures are available.
omatic electrophiles and alkynyltins,[3a] thus by using bi-
We have been working in this field developing a new
functionalized electrophiles and alkynyltins, oligomeric palladium-based synthetic protocol, named extended
and polymeric conjugated systems characterized by a one-pot (EOP), for the convenient construction of oli-
backbone composed of regularly alternating alkyne gomeric and polymeric alkyne-arylene and alkyne-ary-
and arylene moieties – the polyarylene ethynylenes lene/alkyne-metal skeletons[9] (Figure 1). Although
(PAEs) – might be formed.[4] PAEs are a member of based on the use of tin reagents, the EOP processes
the large family of conjugated polymers, a class of mate- strongly improve the tin economy by optimization and
rials that, by virtue of the extended p-conjugated sys- recycle of tin-containing reagents and by-products.
tems spanning throughout the entire polymer backbone, The EOP essentially consists in a straightforward,
display, to a different degree, a unique combination of three-step, one-pot palladium-promoted cascade of re-
narrow band gaps, rigid-rod character and charge-trans- actions allowing the regular insertion of an alkyne unit
port capability, both in the solid state and in solution.[5] between identical or different aromatic moieties and/
These properties are of great interest for the realization or organometallic fragments, with formation of highly
of optoelectronic devices, like organic light-emitting di- ethynylated organic and organometallic oligomers and
odes (OLED), field-effect transistors (FET), molecular polymers. By varying the stoichiometric ratio between
wires, sensors, photovoltaic cells and information stor- reactants and the different types of aromatic building
age systems, and can be enhanced and/or tuned with blocks and/or organometallic fragments introduced
the incorporation of heteroatoms and/or metal centers in the synthetic route, the EOP synthetic protocol can
into the conjugated backbone.[4–6] Moreover, properly be differently addressed towards the formation of
designed side substitution on aromatic units may ad- i) homo-poly(arylene ethynylene)s[9c,9f] [ Ar C C ]n
dress solubility, processability and solid state morpholo- (Path a), ii) poly(arylene ethynylene-co-metallaethyny-
ꢀ
ꢀ ꢁ ꢀ
gy. Finally, taking advantage of the flexibility of synthet- lene)s[9d,9 g] [ Ar C C M C C ]n (Path b1), and iii)
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
ic organic chemistry, the dimension of these molecular co-poly(arylene ethynylene)s[9a] [ Ar C C Ar' C C ]n
materials can be controlled, allowing both the realiza- (Path b2). Although the different pathways outlined in
tion of nanosized photo- and electro-active objects, the Figure 1 are affording materials with highly different
supramolecularly ordered architectures. For these rea- structural characteristics, the three different processes
sons one of the most intensely explored frontiers in inno- are essentially consisting of identical key transforma-
vative materials science is the replacement of conven- tions that are; iv) a Stille ethynylation reaction
tional inorganic, silicon-based materials with organic (Step 1), v) the in situ formation of ethynyltin deriva-
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
(semi)conductive materials.[7]
tives by reuse of a tin by-product formed in the preced-
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ing step (Step 2), vi) the polymerization process
(Step 3). Since formation of organic homo-polymers
(Path a) and organometallic co-polymers (Path b1) has
already been reported,[9b–g] this paper will focus on key
features of the EOP route leading to co-poly(arylene
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
ethynylene)s [ Ar C C Ar’ C C ]n (Ar = Ar’,
Path b2).
ꢀ
ꢀ
In Step 1 of Path b2 an aromatic dihalide X Ar X
undergoes a Stille reaction with two equivalents of tri-
butyltin acetylide (Bu3Sn C C-H) in the presence of
zerovalent palladium to form the corresponding dieth-
ꢀ ꢁ
ꢀ ꢁ ꢀ ꢀ ꢁ
ynylaromatic species H C C Ar C C-H and two
equivalents of tributyltin halide (Bu3SnX) as side prod-
uct. To this crude mixture LDA is added (Step 2), caus-
ing deprotonation of both terminal acetylene moieties
of the dialkyne and their recombination with Bu3SnX
to form the corresponding bistributyltin species
ꢀ ꢁ ꢀ ꢀ ꢁ
Bu3Sn C C Ar C C-SnBu3. A second aromatic diha-
ꢀ
ꢀ
lide (X Ar’ X) is then added to the same reaction mix-
ture (Step 3), and because of the active presence of the
palladium catalyst remaining from Step 1, a second Stille
reaction takes place leading to the formation of the poly-
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
meric material [ Ar C C Ar’ C C ]n. Although
preliminary investigation of this reaction route has al-
lowed preparation of some representative phenylene-
ethynylene-thienyleneethynylene co-polymers,[9a] better
reaction conditions had to be found for the optimazation
of the procedure and widening the scope of this promis-
ing synthetic tool.
Figure 2. “A” and “B” modules to be combined in the con-
struction of poly(arylene ethynylene) co-polymers according
to the “modular approach concept”.
In the present work we report improvements of the but complementary functions to two different “mod-
EOP synthetic protocol toform co-poly(arylene ethyny- ules”, has been called the A–B modular approach.
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
lene)s [ Ar C C Ar’ C C ]n (Ar = Ar’) (Path b2 of
The 2,5-bis(octyloxy)benzene unit (I) was selected as
Figure 1), in order to achieve materials of increased mo- the “A” module, considering that the two octyloxy side
lecular weight, better purity and wider variety of differ- chains might represent a good compromise for the solu-
ent aromatic units (Ar and Ar’) to be combined in the bility and tractability of the polymeric material with low
poly(arylene ethynylene) sequences. The photophysical effect on functional properties[5d,10,11] (Figure 2).
properties ofthese materials have been also investigated
to ascertain their use in optoelectronic devices.
The “B” units were selected in order to have polymer-
ic materials with potentially useful opto- and/or elec-
tronic properties. Particularly, we have focused our at-
tention on three different potential applications of
poly(arylene ethynylene) materials, as a) sensors, b)
electroactive materials, c) light-emitting polymers. The
Results and Discussion
The synthetic strategy outlined in Path b2 of Figure 1 is 1,10-phenanthroline unit (II) was considered because
potentially a general route to poly(arylene ethynylene) its coordinating ability toward transition metals is ac-
co-polymers with the peculiar possibility of the intro- companied by a severe quenching of polymer fluores-
duction of a broad variety of different aromatic units cence, thus highly sensitive chemosensors might be de-
(Ar = Ar’). Thus, in principle, it allows a rational ap- veloped.[12] Towards the formation of molecular wires
proach for the design of molecular structures useful in and electroactive materials, the hydroquinone unit
advanced technological applications.
(III) was selected to introduce a redox-active unit into
Experimental work was focused on the synthesis the polymer backbone[13] and along with pyridine (IV),
ꢀ
of
ethynylated
A B
type
co-polymers the tetrafluorobenzene (V) and dithiophene (VI) units
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ ꢀ
[ C C A C C B ]n, in which the “A” unit (or “mod- are representing building blocks for the formation of
ule”) impresses mechanical strength, chemical stability polymeric materials displaying different electron-ac-
and processability (solubility, fusibility, filmability) to cepting and/or electron-carrying properties. Finally,
the polymer, while the “B” unit (or “module”) was se- the benzene (VII) and anthracene (VIII) modules
lected according to the desired functionality. This con- were selected as representative cases of photoactive
cept, which is based on the idea to assign two different components in optical materials[14] (Figure 2).
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Preparation of Model Compounds
a 1/2 ratio, using Pd(PPh3)4 (1%) as catalyst in refluxing
dioxane (1108C), was studied. Under these conditions
In the formation of co-polymers of type only the 2,5-dibromohydroquinone (2) failed to couple
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
[ Ar C C Ar’ C C ]n by means of the EOP synthet- with 1, while in all the other cases, although different re-
ic protocol (Figure 1), the key point is represented by action times were required (depending on the nature of
Step 3, the second Pd-promoted coupling process, where the dihalide), coupling reactions smoothly proceeded
the in situ formed aromatic bis-tributyltin acetylide spe- with complete conversion of starting materials into the
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
cies (Bu3Sn C C Ar C C SnBu3) must be reacted corresponding coupled products 10–16. The reluctance
ꢀ
ꢀ
with the added bis-bromoaromatic Br Ar Br, in a quite of 2 to undergo coupling with 1 was circumvented by
complex reaction medium containing side products and transforming 2 into the corresponding bis-acetyl (3)
reactants resulting from the two preceding steps. In a and bis-tetrahydropyranyl (THP) (4) derivatives, both
previous work we set optimal conditions to efficiently affording the corresponding coupled products 10 and
perform the EOP route to poly(phenylene ethynylene) 11 under the same general conditions.
and poly(thienylene ethynylene) homo-polymers[9f]
Subsequently, the preparation of model compounds
(Path a of Figure 1). First we decided to investigate the 10–16 was repeated under the real EOP conditions, by
ability of individual dibromides Br-B-Br 2–9 to undergo coupling of dihalides 3–9 with the in situ formed tin ace-
coupling with the tributyltin acetylide moiety of the tylide 1 [Equation (2)]. Now these transformations are
ꢀ ꢁ ꢀ
clean Bu3Sn C C Ph (1), under the optimized condi- taking place in a much more complicated system, be-
tions, to form model compounds Ph C C B C C Ph cause of the presence of by-products and the reactants
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
(10–16) [Equation (1)]. An additional advantage deriv- needed to carry out the in situ formation of 1. Moreover,
ing from this approach is represented by the obtaining of the Pd catalyst only added at the beginning of the first
a series of model compounds whose spectroscopic char- step (formation of 18), has to withstand the addition of
acterization and study of properties might be useful not LDA in the second step (formation of 1), and then
only to characterize the corresponding polymeric mate- must operate again in the third step (formation of 10–
rials (vide infra) but also to predict and rationalize their 16). Under these conditions only the 1,4-dibromo-2,5-
properties.[15]
diacetoxybenzene (3) failed to form the corresponding
coupled product 10,[16] while in all the other cases forma-
tion of model compounds 11–16 took place with un-
changed efficiency.
ð2Þ
Once having assessed the ability of dibromides 4–9 to
efficiently perform, under EOP conditions, coupling
with 1 to form model compounds 11–16, formation of
complementary models 24 and 25, more closely resem-
bling the structure of the corresponding polymeric ma-
terial, was studied [Equation (3)]. In this case, although
ð1Þ
Referring to our previous experience,[9f] the reactivity of the intrinsic reactivity of the B Br and Bu3Sn C C
the aromatic dibromides 2–9 (2¼2,5-dibromohydro- moieties are expected not to be much different than
quinone, 3¼1,4-dibromo-2,5-diacetoxybenzene, 4¼ that of 5 and 8 vs. 1, the presence of the octyloxy side
1,4-dibromo-2,5-ditetrahydropyranylbenzene, 5¼2,6- chains on the central phenyl ring may result in some ster-
ꢀ
ꢀ ꢁ ꢀ
dibromopyridine,
6¼3,8-dibromo-1,10-phenanthro- ic restrictions affecting reaction rate and/or product sta-
line, 7¼5,5’-dibromo-2,2’-dithiophene, 8¼9,10-dibro- bility. It has to be noticed that compounds 24 and 25 re-
moanthracene, 9¼1,4-dibromo-tetrafluorobenzene), quired only slightly longer reaction times with respect to
ꢀ ꢁ
and the commercially available Ph C C-SnBu3 (1) in 12 and 15 to be completely formed and, more important-
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ly, in the case of 25, this product was safely purified by was used to introduce the (masked) hydroquinone moi-
chromatographic separation.[17] This latter result is quite ety into the polymer backbone. 1,4-Dibromobenzene
relevant, because the central unit in models 24 and 25 is (26) was added to the array of dihalides 4–11 to intro-
the very same “A” module that we intend to introduce in duce an unsubstituted benzene moiety as the “B” mod-
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ ꢀ
the A/B type polymer [ C C A C C B ]n and we ule into the polymer backbone. To a certain degree the
have attributed to this unit also the role of imparting sta- molecular weight of polymeric material may be modu-
bility to the polymeric materials.
lated with the duration of the third step (vide infra).
As experienced in the case of the EOP route to
form poly(arylene ethynylene) homo-polymers[9f]
ꢀ
ꢀ ꢁ ꢀ
[ Ar C C ]n, transformations occurring in each step
of the EOP route to co-polymers 27–33, are clearly ac-
counted for by 1H NMR spectroscopy.[18] We describe in
detail in Figure 3 the formation of the poly-{[(2,5-dioctyl-
oxy)-1,4-phenylene acetylene]-co-5,5’-(2,2’-dithiophe-
nylene) acetylene} (31) recorded by 1H NMR spectros-
copy; this being representative of the general transfor-
mations leading to all polymeric materials presented in
this work (Figure 3).
In trace a) of Figure 3 is reported the 1H NMR spec-
trum of a specimen of the reaction mixture[18] sampled
before starting Step 1, thus containing tributyl(ethynyl)-
tin and 1,4-diiodo-2,5-bis(octyloxy)benzene (19) in a
2:1 molar ratio. Although the Pd(PPh3)4 catalyst is pres-
ent, its 1H NMR signals are not detected because of its
low concentration (ffi1%). In trace b) is reported the
spectrum of a sample of the reaction mixture after
20 min of warming at 1108C (end of Step 1). Starting ma-
terials are quantitatively converted into 1,4-diethynyl-
2,5-bis(octyloxy)benzene (20), and tributyltin iodide
side-product. After this step is accomplished, the reac-
tion mixture is first allowed to cool at room temperature,
then brought to 08C, and two equivalents of LDA are
added. Under these conditions the base deprotonates
both terminal alkyne functionalities of 20 forming the
corresponding diethynyl acetylide which reacts with
tributyltin iodide, left over from the first step, forming
1,4-bis(tributyltin)ethynyl-2,5-bis(octyloxy)benzene
(21) (Step 2). The NMR spectrum of this step, which re-
quires additional 20 min, is shown in trace c) of Figure 1,
where only signals of 21 are present. To this reaction
mixture solid 5,5’-dibromo-2,2’-dithiophene (7) is added
(trace d), Figure 3), and the mixture of 21 and 7 is
warmed up to 1108C. The Step 3 is completed in five
ð3Þ
Preparation of Poly(arylene ethynylene) co-Polymers
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
ꢀ
[ Ar C C Ar’ C C ]n
Scheme 1 represents the general EOP route we devised hours at 1108C as reported by the spectrum in trace e)
for the preparation of the poly(arylene ethynylene) co- of Figure 3, where only signals attributable to the crude
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
polymers [ Ar C C Ar’ C C ]n 27–33. While the polymer 31 and the Bu3SnBr side-product are present.
first two steps, consisting of the one-pot straightforward Upon cooling to room temperature a large amount of
transformation of the 1,4-diiodo-2,5-bis(octyloxy)ben- red-brown precipitate constituted by the higher mo-
zene (19) into the 1,4-bis(tributyltin)ethynyl-2,5-bis(oc- lecular weights of 31 is formed. This solid material is sep-
tyloxy)benzene (21) are obviously identical in all cases, arated by filtration and washed with dioxane, then redis-
and, were accomplished in 30–40 min, the duration of solved[19] with CHCl3, filtered over Celite, reprecipitated
the third step, the introduction of the “B” module into and washed with methanol. In trace f) of Figure 3 the
the polymeric chain, reflected the same trend observed spectrum of the purified 31 is reported. The absence of
in the formation of the corresponding models 11–16 the high field multiplets in trace f) with respect to trace
and 24–25. Because of the failure of the 1,4-dibromo- e) is due to the elimination of the tributyltin bromide
2,5-diacetoxybenzene (3) to undergo coupling under side-product which has been achieved by washing the
EOP conditions, only the THP-protected derivative 4 precipitate with methanol. An additional crop of poly-
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1
Figure 3. Sections of H NMR spectra for the formation of 31 as outlined in Scheme 1. Spectra were recorded on a sample of
ꢀ ꢁ
the reaction mixture freed from the solvent and redissolved in CDCl3. Trace a) 19 and Bu3Sn C CH before starting Step 1;
trace b) completion of Step 1, mixture of 20 and Bu3SnI; trace c) completion of Step 2, formation of 21; trace d) starting of Step
3, mixture of 21 and 7; trace e) completion of Step 3, mixture of 31 and Bu3SnBr; trace f) isolation of 31.
meric material (consisting of lower molecular weight treatment in Step 2, is an example of the high efficiency,
material – vide infra) is recovered upon addition of and is quite rare even in the case of a single coupling
methanol to the original dioxane solution from which process.[2]
the first crop of precipitate spontaneously formed after
cooling of the reaction mixture.
It is important to remark that the two cross-coupling Molecular Weight Determination and NMR
processes, Step 1 and Step 3, are both sustained by the Characterization
palladium catalyst that is only added once at the begin-
ning of Step 1. The overall use of only 1% of Pd catalyst As mentioned before, in all cases (27–33) the degree of
to perform two coupling processes, despite the base polymerization is directly related to the duration of the
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Scheme 1.
third step. In general, when the polymerization step 13C NMR spectra of the model compound 25 (trace a)
(Step 3) is carried out for more than 2–4 hours, a consis- and low molecular weight fraction of the polymer 32
tent amount of insoluble polymeric material precipi- (trace b) are compared with the solid state CP MAS
tates upon cooling the reaction mixture at room temper- 13C NMR spectrum (trace c) of the insoluble fraction
ature. Polymers 27 and 32 containing benzene and an- of 32. The solution spectra are in fine agreement with
thracene as B moieties in the main chain, respectively, the solid state spectrum indicating the structural homo-
were not soluble in the hot reaction mixture (1108C).
geneity of the material. In the traces b) and c) the inten-
The degree of polymerization (n), calculated for the sity ratio of the 13C NMR signals corresponding to the
polymers 27–33 using the MW values determined by aliphatic and unsaturated carbons are in the expected
GPC (gel permeation chromatography), is reported in 2:1 molar ratio: it is worth noting the presence of two
Table 1. In all cases, a low molecular weight material, signals for the not equivalent alkynyl carbons 5. The
soluble in the reaction mixture, is always obtained at broadness of the 13C NMR signals of trace c) suggests
short reaction times and was precipitated at the end of the amorphous nature of the polymer and likely the
reaction upon addition of methanol (see Experimental presence of rotamers of the aromatic moieties about
Section). Materials of higher molecular weight are the main chain axis. In particular the signals due to car-
formed at longer reaction times and their yield increases bons 1 and 6, diagnostic of the presence of not equiva-
as the reaction time is increased. This fraction spontane- lent aromatic rings along the macromolecular chain, ex-
ously separates upon cooling of the reaction mixture at hibit a line width broader than that observed for the
room temperature and could be analyzed in THF, methyl carbons 8.
chloroform or dichloromethane solution where it
proved to be partially or totally soluble (Table 1). On
the contrary, polymers 27 and 32, that precipitate from Photophysical Properties
the hot reaction mixture, remain mostly insoluble in
all solvents; in this case data obtained by GPC only refer The photophysical properties of co-polymers 27, 29–33
to the soluble fraction whereas the nature of the high were investigated in solution and in the solid state. In the
molecular weight polymer was investigated by solid case of the pyridine- and anthracene-containing poly-
state 13C NMR spectroscopy. In Figure 4 the solution mers 29 and 32 materials of different chain lengths
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Alessandra Micozzi et al.
% insoluble[c]
FULL PAPERS
Table 1. Degree of polymerization of polymers 27–33.
Polymer
Time[a]
n[b]
1 h
2 h
44[d]
45
0
60
6 h
5
0
0
30 min
1 h
2 h
6
7[d]
11
4 h
20[d]
2 h
5
0
5 h
20 h
9
11
60
90
2 h
5 h
14
40
0
50
2 h
3 h
20[d]
28[d]
50
70
22 h
12[d]
0
[a]
Time of the polymerization step (Step 3 of EOP).
Degree of polymerization, calculated on the basis of the Mw value, determined by GPC (THF, polystyrene).
Polymeric material insoluble in common organic solvents.
[b]
[c]
[d]
The n value is relative to the longest part of the soluble polymer (insoluble in dioxane, soluble in THF).
were examined, and, for comparison optical characteris- and Ar’ constituents (A and B modules) and the exten-
tics of the corresponding model compounds 24 and 25 sion of the conjugation of the PAE backbone. Such a be-
were reported. The values of the relevant parameters havior allows a large tunability of the optical properties
are summarized in Table 2. The UV-VIS optical absorp- by introducing different arylenic B modules and, in
tion spectra in dilute CHCl3 solution are reported in Fig- some cases, by varying the conjugation length.
ure 5. The data relative to the homo-polymer poly(ary-
We have noticed that, according to theoretical mod-
lene ethynylene)[9c, f] [ Ar C C ]n [Ar¼2,5-bis(octyl- els[21] for poly(arylene ethynylene) PAE and poly(ary-
oxy)benzene] 34 are also shown for comparison. The lene vinylene) PAV polymers, the low-energy absorp-
curves exhibit a certain degree of variety within the typ- tion band in the range 380–550 nm, depending on the
ical main features of this class of rigid rod copoly- nature of the B module, can be attributed to delocalized
mers.[5d,11,12b,14,20] In this regard, the present data confirm p–p* transitions, while the relatively weaker structures
that the absorption spectra of co-polymers of type between 250 nm and 350 nm are related to a combina-
ꢀ
ꢀ ꢁ ꢀ
ꢀ
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
[ Ar C C Ar’ C C ]n are derived from and can be tion of delocalized and phenyl-localized p–p* transi-
easily interpreted in terms of the properties of the Ar tions.
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3 Figure 4. Trace a) solution 13C NMR spectrum of the model
compound 25; trace b) solution 13C NMR spectrum of low
molecular weight fraction polymer 32; trace c) solid state
CP MAS 13C NMR spectrum of the insoluble fraction of 32.
This attribution is supported by the dependence of the
absorption spectra as a function of the degree of poly-
merization n which has been investigated for two sys-
tems and is reported in Figure 6. For the case of anthra-
cene-based co-polymer 32 (Figure 6a) the effectiveness
of p-conjugation gives rise to the progressive red-shift of
the low-energy band as the chain length increases from
the model compound 25 to the polymer with n¼28.
On the other hand, the position of the high-energy
band varies much less with n as expected for transitions
due to localized states. For pyridine-based 29 (Fig-
ure 6b) the same value of the peak wavelength is ob-
served for different values of n and this can be explained
by the possible interruption of p-conjugation due to the
distortion of the planarity of the aromatic rings in the
polymer backbone,[22] which localizes the excitation
within a few monomeric units.[5d,12b] As matter of facts,
the spectrum of 29 exhibits the lowest value of lmax
among all the other systems (see Figure 5). Other specif-
ic features of PAE co-polymers can be verified by com-
paring the spectra relative to the different B modules.
For instance, it has been reported[5d] that donor lateral
chains can induce a bathochromic shift of the absorption
spectra. In the present materials, this effect can be ob-
served by increasing the number of alkoxy-substituted
benzene rings passing from the co-polymer 27 (with ab-
sorption lmax ¼405 nm) to homo-polymer 34 (lmax
¼
418 nm). A similar value of bathochromic shift is ob-
served in the cases of polymers 30 and 33 containing
1,10-phenanthroline or tetrafluorobenzene units, re-
spectively, while the larger value measured in 31
Figure 5. Normalized absorption spectra of the PAEs in
CHCl3. For the values of the degree of polymerization n
not reported in the figure see Table 2.
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Table 2. Optical properties of the PAE polymers in solution and solid state.
n[a]
In CHCl3 solution
In spin-coated films
Absorption lmax [nm] Emission lmax [nm] Quantum yield Absorption lmax [nm] Emission lmax [nm]
34 40
27 45
24 model* 374
29 11
17
418
405
470
445
420
428
430
426
460
500
490
545
565
465
0.40
0.47
0.78
0.65
0.55
0.51
0.57
0.18
0.60
0.27
0.20
0.44
420
407
–
–
–
390
415
438
–
488
–
585
555
–
–
–
545
575
650
–
622
–
390
390
391
410
444
20
30
5
31 14
25 model* 470
32
8*
28
482
530
417
33 10
420
580
[a]
Degree of polymerization.
* lmax refers to the low-energy band.
Figure 7. Normalized emission spectra of the PAEs in CHCl3.
For the values of the degree of polarization n not reported in
the figure see Table 2.
(lmax ¼444 nm) confirms the effect of the introduction of
thiophene units into conjugated chains.[5d] Accordingly,
the very large bathochromic shift observed in 32
(lmax ¼482 nm) verifies the tendency of the anthracene
unit to an extended conjugation[20a] with a complete
loss of its typical absorption structures in the range
300–400 nm. Finally, it should be noted that the absorp-
tion spectra measured in concentrated solution (10 mg/
mL, not reported here) do not show appreciable differ-
ences with respect to dilute ones, which rules out the for-
mation of molecular aggregates.
A similar variability and tunability of the optical prop-
erties as a function of the nature of the alternating B
module can be observed in the photoluminescence
(PL) spectra reported in Figure 7. However, as for ab-
sorption, all the curves present some characteristic fea-
tures of PAE polymers[5d,12b,20], i.e., a high-energy peak
Figure 6. a) Normalized absorption spectra of the model
compound 25 and the co-polymer 32 in CHCl3 for different
values of the degree of polymerization n; b) normalized ab-
sorption spectra of the model compound 24 and the co-poly-
mer 29 in CHCl3 for different values of the degree of poly-
merization n.
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with a shoulder on the low-energy side, that in some cas-
es become two well-resolved bands, separated by 25–
40 nm (15–19 meV). Such structures are attributed to
vibronic transitions while the lack of mirror image sim-
ilarity between absorption and emission spectra sug-
gests a certain degree of vibronic coupling. Moreover,
the emission bands are generally slightly narrower
than the absorption bands, which is consistent with emis-
sion from localized states after migration of the excita-
tions along the polymer main chains.[11] The PL quantum
yields of emission in diluted solutions reach reasonably
high values in the range 0.2–0.8 (see Table 1), that are
comparable with what is reported for similar sys-
tems[11,20c] and make these materials quite interesting
for electrooptical applications.
As expected, in the two systems investigated at differ-
ent degrees of polymerization, i.e., 29 and 32, the values
of quantum yield increase with the decrease of n. For the
case of anthracene-based 32 the reduction of quantum
yield parallels a change of the emission spectral shape
so that the low-energy vibronic peak becomes the pre-
dominant one (see Figure 8a). These effects can be ex-
plained with the migration of the excitations over longer
distances along the main chain, which increases the vi-
bronic coupling and the chance of non-radiative de-exci-
tation due to traps and defects. Interestingly, the de-
crease of quantum yield with n is also found in pyri-
dine-based 29, even though to a less extent than in 32,
whereas absorption spectra are quite independent of n
(see Figure 6b). We can speculate that this suggests a
certain degree of delocalization of the relaxed excited Figure 8. a) Normalized emission spectra of the model com-
pound 25 and the co-polymer 32 in CHCl3 for different values
of the degree of polarization n; b) normalized emission spec-
tra of the model compound 24 and the co-polymer 29 in
CHCl3 for different values of the degree of polarization n.
state.
The solid-state photophysics of PAEs are quite inter-
esting since interchain interactions can occur and influ-
ence the emissive properties of thin films deeply. The ab-
sorption spectra of spin-coated thin films (not reported
here, see values of lmax in Table 1) do not show any ap-
preciable difference with respect to solution. This simi-
larity rules out the formation of aggregates as well as
the forced planarization of the p-system in the solid
state. On the other hand, PL emissions of pristine films
exhibit very large values of red shift, as high as 150 nm
in 31, with respect to solution. Moreover, the bands
are broadened and present no vibronic structures (see
Figure 9), with the exception of 27 that exhibits also a
high-energy peak resembling the solution one. Such a
behavior can be interpreted as indicating the formation
of excimers, that are complexes between a polymer
chain in the excited state and an adjacent one in the
ground state. Excimers are very common in other conju-
gated polymers and have been extensively studied in
systems such as poly(p-phenylene)s, poly(p-phenylene
vinylene)s and fluorene ladder-type polymer
(LPPP)[23] but many questions are still open about their
formation. It is a necessary requirement that two chains
Figure 9. Normalized emission spectra of solid-state films of
be parallel over a length of at least several monomers, the PAEs deposited by spin-coating. For the values of the de-
gree of polarization n not reported in the figure see Table 2.
with a distance between the chains of 0.3 to 0.4 nm.
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Alessandra Micozzi et al.
FULL PAPERS
The distance must be small enough for their p orbitals to ried out (Route b). Although this procedure presents
overlap but not so strongly that the ground state can distinct advantages (no isolation of the alkyne, no buta-
form a new stable molecule (aggregate). This is consis- diyne defects) with respect to the one outlined in Path
tent with the rigid-rod nature of the present polymers a), to the best of our knowledge it has only received little
and the trend to form a long-range ordered supramolec- attention.
ular structure in the solid state.[11] Moreover, this
The use of the “classical” Stille protocol to form ethy-
hypothesis is supported by the fact that PL spectra of nylated co-polymers is outlined in Route c. Formation of
PAEs in solid-state solution with inert polymers,[15] the bis-alkyne 37 can be achieved either by the same
such as polystyrene and poly(methyl methacrylate), TMSA-dihalide coupling used in the first part of Route
show a partial recovery of the characteristic emission a or by reaction of 34 with two equivalents of
ꢁ
bands of the liquid solution; as expected considering R3SnC CH in the presence of zerovalent Pd. In both
the supposed greater separation between PAE chains. cases the bis-alkyne 37 must be isolated. In addition
ꢁ
Further investigations, however, are needed to clarify when 37 is formed by the use of R3SnC CH, two
this important point.
equivalents of R3SnX are formed (and generally dis-
carded) as by-product. Transformation of 37 into
ꢀ ꢁ ꢀ ꢀ ꢁ ꢀ
R3Sn C C Ar’ C C SnR3 (40) then requires the use
Comparison of EOP Synthetic Protocol and Current
Procedures towards Formation of Models and
Polymers
of additional tin reagents.[29]
Bunz[30] has recently adapted the acyclic diyne meta-
thesis (ADIMET) approach (Route e), originally devel-
oped for preparation of homo-poly(arylene ethyny-
Formation of either model compounds 10–16 and 24– lene)s, to the formation of co-poly-(arylene ethyny-
25 as well as polymers 27–33, essentially consist in the lene)s. In this synthetic route, two distinct Pd-catalyzed
coupling of sp- and sp2-hybridized carbon atoms. In processes are necessary to form the two partners (41 and
this respect, the milestone for the accomplishment of 42) that then need to be isolated and purified before be-
this transformation is represented by the palladium-cat- ing mixed and reacted under ADIMET conditions to
alyzed cross-coupling reaction between vinyl/aryl hal- form the corresponding co-polymer 43. The ADIMET
ides
and
terminal
acetylenes
(Sonogashira– itself consists on a very efficient process, promoted by
[24]
ꢀ
Heck Cassar coupling). In fact, although over the an in situ generated/highly reactive Mo catalyst. Howev-
years this reaction has received a number of significant er high temperatures are required, regiorandom distri-
improvement and developments,[25] it still is mostly bution of the two monomers is obtained, and hetero-
used according to its original protocol. It is necessary atom-containing monomers display relatively sluggish
to compare the effectiveness of the EOP synthetic pro- reactivity.[30,31]
tocol with respect to this and other current procedures
For polymers 30 and 33 it is possible to make a direct
towards the formation of PAE co-polymers (Figure 10). comparison between the classical and the EOP proce-
Route a represents the traditional and currently most dures outlined in Route a and Route d of Figure 10.
used route[5] composed of three consecutive separate Starting in both case from 1,4-diiodo-2,5-bis(octyloxy)-
¼
steps consisting in i) a Pd-catalyzed TMSA-dihalide benzene [34, Ar
2,5-bis(octyloxy)benzene],
coupling to form the TMS-protected bisethynylaromat- Grummt[32] and Weder[20e] first prepared the 1,4-bis(tri-
ic intermediate (36), ii) TMS deprotection to form the methylsilyl)ethynyl-2,5-bis(octyloxy)benzene [36, Ar
¼
free alkyne (37) iii) Pd-catalyzed coupling of the termi-
nal alkyne (37) with a second aromatic dihalide (38) to the TMS protecting groups formed the 1,4-bis(ethyn-
2,5-bis(octyloxy)benzene] and then by removal of
¼
form the polymer (39). The major disadvantage in this yl)-2,5-bis(octyloxy)benzene [37, Ar
2,5-bis(octyl-
procedure is that both the TMS intermediate (36) and oxy)benzene]. This was subsequently coupled with 6
the terminal alkyne intermediate (37) need to be isolat- and 9 to form 30[32] and 33,[20e] respectively. In both cases
ed and purified before the following step. Nevertheless, overall yields are acceptable (60–70%), however these
it remains the most used because of its great reliability procedures require isolation and purification of both
and flexibility, in fact each step can be optimized accord- the TMS (36) and the terminal alkyne (37) intermedi-
ing to the particular nature of the substrate under trans- ates. Moreover, in both cases the overall reaction times
formation.
to carry out the three steps are longer than 24 h.
By the use of the EOP synthetic protocol (Route d),
A one-pot reaction has been carried out by Heitz[27]
and other authors,[28] who report an optimization of an the 1,4-diiodo-2,5-bis(octyloxy)benzene [34, Ar 2,5-
efficient in situ sequential TMS deprotection-acety- bis(octyloxy)benzene], is straightforwardly converted
lene/aryl halide coupling procedure devised by Haley[26] in 1,4-bis(tributyltin)ethynyl-2,5-bis(octyloxy)benzene
¼
¼
to form cyclo-oligoacetylenes. After formation and iso- [40, Ar 2,5-bis(octyloxy)benzene], and, by direct addi-
lation of the trimethylsilyl intermediate (36), the remov- tion of 6 or 9 into the reaction mixture 30 and 33 are re-
al of the trimethylsilyl group and the polycondensation, spectively formed. While the characteristics of the ob-
under phase-transfer conditions, are simultaneously car- tained products are comparable, using the EOP route
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Figure 10. Comparison of the EOP synthetic protocol and current procedures towards the formation of PAE co-polymers.
no isolation of intermediates is needed, final products The recovered Bu3SnBr was subsequently utilized in the
are isolated in 60–94% yields, and, in the case of 30 reaction with the lithium acetylide ethylenediamine
[33]
ꢁ
the reaction time is much shorter. Finally, the EOP route complex, to form new Bu3SnC CH [Equation (4)],
only requires the use of 1% of Pd catalyst to accomplish which is the coupling partner of 1,4-diiodo-2,5-bis(octyl-
the overall formation of 30 and 33, while Route a re- oxy)benzene (19) at the beginning ofthe EOP routeout-
quires two distinct aliquots, each of 3–5%, of catalyst lined in Scheme 1.
to form the same materials.
ð4Þ
Recovery and Recycle of the Bu3SnBr – The Tin
Economy
The recovery of the Bu3SnBr at the end of the polymer-
ꢁ
Upon completion of the third step of the EOP synthetic ization process, and its re-conversion into Bu3SnC CH
protocol outlined in Scheme 1, higher molecular weight allows us to draw a tin cycle (Scheme 2) which further
polymeric materials 27–33 spontaneously precipitated, improves the convenience of the EOP synthetic proto-
while lower fractions were separated by precipitation col to form ethynylated polymers.
with methanol. The crude polymeric material was then
To our knowledge, there is only one report in the liter-
purified by redissolution with appropriate solvents and ature in which alkynylstannanes are formed without the
reprecipitation and washing with methanol. The metha- need of forming and isolating the terminal alkyne pre-
nolic rinsing solution containing lower oligomers, resid- cursors. Buchwald reported on the use of bis(tributyltin)
ual phosphine ligand, and the Bu3SnBr side product, oxide and tetrabutylammonium fluoride (TBAF) to di-
upon evaporation of the volatiles leaves a dark-viscous rectly convert alkynylsilanes into alkynylstannanes[34]
residue, from which in all cases it was possible to recover (Scheme 3). Nevertheless, we believe that our EOP pro-
more than 90% of pure Bu3SnBr by vacuum distillation. cedure (Figure1) is more convenient since i) in the for-
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Alessandra Micozzi et al.
FULL PAPERS
Scheme 2.
loring of polymers for certain applications has come
within reach.
We have also demonstrated that the physical charac-
teristics of these materials prepared by the EOP syn-
thetic protocol are identical to those of materials pre-
pared by more synthetically demanding, conventional
procedures. Moreover, the recovery of the Bu3SnBr at
the end of the polymerization process, and its re-conver-
ꢁ
Scheme 3.
mer method it is necessary to first prepare and isolate sion into Bu3SnC CH (which, in turn, is the reagent we
the alkynylsilanes, and then to transform them into the use to introduce the ethynyl moiety into the polymer
alkynylstannanes. We obtain instead the alkynylstan- backbone) let us consider the Bu3Sn- moiety as a recy-
nanes directly from the aromatic halide. ii) in our proce- clable carrier of the ethynyl group into the polymer
dure, alkynylstannanes are formed by addition of a base structure.
and recycling of the tributyltin group of the tributyltin
In comparison with current procedures to prepare or-
acetylide reagent. In order to achieve the same result, ganic and organometallic ethynylated polymers,[5] the
Buchwald needs to introduce bis(tributyltin) oxide and key aspects of the process depicted in Figure 1 are: 1)
TBAF in the reaction media.
The polymers are accessed without the otherwise man-
datory need of purification and handling of aromatic di-
functionalized terminal alkynes, materials of a delicate
nature whose handling might require special precau-
tions.[35] 2) The multifold and sequential palladium-cat-
Conclusions
The Pd-driven EOP synthetic protocol has the potential alyzed cross-coupling processes takes place with high ef-
of being addressed toward the formation of three differ- ficiency, needing only a very low catalyst charge loading.
ent main classes of highly ethynylated materials: i) or- 3) Since tin acetylides are formed by reuse of the Bu3SnI
ganic homo-polymers, ii) organic co-polymers, and iii) generated as by-product in Step 1, no extra instable and
organometallic co-polymers. Taking advantage of the toxic tin reagents are needed to form these key inter-
enormous capability of the palladium catalyst to effi- mediate. 4) The recovery and the recycle of the tin re-
ciently perform the coupling of an extremely large vari- agent allow us to overcome the major drawback of the
ety of organic and organometallic substructures, an al- Stille reaction, that is the environmental concern related
ternative and convenient access to innovative materials with the use of toxic and costly organostannanes.[29] In
for advanced technology has been disclosed. In this re- this respect, we believe that our EOP synthetic protocol
spect, is worthy of note to underline the concept of a is in agreement not only with the desirability for Stille
“modular approach”, which consists in the formation protocols catalytic in tin,[36] but also with the “atom
of highly ethynylated material in which one “module” economy” concept recently developed by Trost,[37] and
is in charge of impressing mechanical stability and proc- the design of environmentally benign chemical process-
essability to the material and a second “module” is in es.[38]
charge of imparting functional properties to the materi-
al. Thus, by combination of appropriate “modules” Ar
and Ar’ the opportunity for a systematic and rational tai-
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film, the polymers were dissolved in CHCl3 at concentrations
of 10 mg/mL and were spin-cast on glass substrates. No anneal-
ing procedures were generally carried out on pristine films. The
thickness of the films as measured with a profilometer was typ-
ically 100 nm.
Experimental Section
General Procedures
Elemental analyses were performed by the Servizio Microana-
`
lisi of the Dipartimento di Chimica of Universita di Roma “La
Sapienza”. IR spectra were recorded on a Nicolet FT 510.
1H NMR and 13C NMR spectra were recorded on a Bruker
AC300P spectrometer at 300 and 75 MHz, respectively. Chem-
ical shifts (ppm) are reported in d values relative to Me4Si; for
1H NMR, CHCl3 (d¼7.24) and for 13C NMR, CDCl3 (d¼77.0)
were used as internal standard. Solid state cross polarization
magic angle spinning (CP MAS) 13C NMR spectra were re-
corded on a Bruker Avance 300 operating at a 13C Larmor fre-
quency of 75.47 MHz. Samples were charged in a zirconia rotor
(4 mm o.d.) and spun at the magic angle, calibrated with KBr,
at 5000 Hz. Spectra were acquired using high power decou-
pling and a pulse length of 1.93 ms (908 pulse), contact time of
2 ms and a delay of 3 s between the scans. The chemical shifts
are referred to tetramethylsilane used as external standard.
Molecular weights were determined (relative to polystyrene
standard) on a Perkin-Elmer gel permeation chromatography
(GPC) equipped with a set of Waters Styragel columns (HT 6
200K-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). Mass spectra were performed on an
electrospray ionization time of flight (ESI-TOF) Micromass
spectrometer.
Solvents, including those used for NMR and chromatogra-
phy, were thoroughly degassed before use. Dioxane was dried
over sodium and argon-saturated prior to use. Chromato-
graphic separations were performed with 70–230 mesh silica
gel (Merck).
All manipulations were carried out with Schlenk-type
equipment under an atmosphere of argon on a dual mani-
fold/argon-vacuum system. Liquids transferred by syringe or
cannula.
Preparation of Model Compounds 10 – 16 by Coupling
of the Dibromides 3–9 with
Tributyl(phenylethynyl)tin (1)
The procedure for the preparation of 5,5’-bis(phenylethynyl)-
2,2’-dithiophene (14) is representative. A Schlenk tube was
loaded with 5,5’-dibromo-2,2’-dithiophene (7) (0.147 g,
0.45 mmol) and Pd(PPh3)4 (0.006 g, 0.0051 mmol). After three
ꢁ
cycles of vacuum/argon, were added by syringe Bu3SnC CPh
(1) (0.371 g, 0.90 mmol) and dioxane (15 mL). The reaction
mixture was heated at 1108C and stirred for 3 h. 1H NMR anal-
ysis of a sample showed the complete conversion of starting
materials. After cooling to room temperature, ether (20 mL)
and an aqueous saturated solution of KF (40 mL) were added,
and the resulting solution was stirred for 30 min. After filtra-
tion over cotton wadding, the solution was transferred into a
separatory funnel. The organic layer was washed with water
(3Â25 mL), dried over sodium sulfate and filtered. Celite
(5 g) was added to the filtrate, and the mixture was evaporated
to dryness. The residue was chromatographed on a silica gel
column (40Â3 cm) and eluted with hexane-CH2Cl2 (6:4).
Pure product 14 was obtained as a yellow crystalline solid;
yield: 0.160 g (93%).
Preparation of Model Compounds 11 – 16 by EOP
Synthetic Protocols
The typical procedure was as follows: to a solution of iodoben-
zene (17) (1 mmol) and Pd(PPh3)4 (0.5 mol % equivalent) in
2,5-Dibromo-hydroquinone (2),[39] 1,4-dibromo-2,5-diacet-
oxybenzene (3),[13] 1,4-dibromo-2,5-tetrahydropyranylben-
zene (4),[40] 3,8-dibromo-1,10-phenanthroline (6)[41] were pre-
pared by known methods; 2,5-di(octyloxy)benzene (19) was
prepared by using our previously described procedure.[9a, c]
Other chemicals were purchased from Aldrich and used as
received unless otherwise specified.
ꢁ
dioxane was added tributyl(ethynyl)tin (Bu3SnC CH)
(1 mmol). After 20 minutes of stirring at 1108C, 1H NMR anal-
ysis showed the complete conversion of starting materials. Af-
ter cooling to 08C, LDA (2.0 M solution in THF/heptane/ethyl-
benzene, 1.1 mmol) was added and the mixture was allowed to
warm up to room temperature. 1H NMR analysis of a sample of
the reaction mixture indicated complete conversion of phenyl-
acetylene into phenylethynyltin. Subsequently, the dibromide
(4–9; 0.5 mmol) was added to the flask and the mixture was re-
1
fluxed for 1–3 h (12 h for dibromide 9). H NMR analysis
Photophysical Characterization
showed the complete conversion of the two coupling partners.
After cooling at room temperature, the products 11–16 were
isolated by the same procedure used for the previously describ-
ed method.
Visible absorption spectra were carried out by using a Perkin-
Elmer Lambda 18 spectrophotometer. Photoluminescence
spectra were measured on a standard set-up for photolumines-
cence using a Spectra Physics 2020 Argon Ion Laser, a 411-nm
InGaN diode laser or an Oriel Xenon lamp as the excitation
sources. The different excitation sources were used so as to ex-
cite each co-polymer at the wavelength of the main absorption
band. Absorption and emission studies in solutions were gener-
ally performed in deoxygenated CHCl3 at concentrations in the
range 0.01–0.05 mg/mL in a quartz cell with a length either of
1 mm or 10 mm. Quantum efficiencies were determined rela-
tive to anthracene and Rhodamine 101 in a 10-mm quartz
cell at a concentration of 5Â10ꢀ4 mg/L (absorption less than
0.01). For absorption and fluorescence measurements on
Preparation of 1,4-Bis(2-pyridylethynyl)-2,5-
dioctyloxybenzene (24)
To a solution of 1,4-diiodo-2,5-dioctyloxybenzene (19; 0.296 g,
0.505 mmol) and Pd(PPh3)4 (0.006 g, 0.0051 mmol) in dioxane
ꢁ
was added tributyl(ethynyl)tin (Bu3SnC CH) (0.331 g,
1.01 mmol). After 20 minutes of stirring at 1108C, H NMR
1
analysis showed the complete conversion of starting materials.
After cooling to 08C, LDA (2.0 M solution in THF/heptane/
Adv. Synth. Catal. 2005, 347, 143–160
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157

Alessandra Micozzi et al.
FULL PAPERS
ethylbenzene; 0.52 mL, 1.03 mmol) was added and the mixture
was allowed to warm up to room temperature. 1H NMR anal-
ysis of a sample of the reaction mixture indicated the complete
formation of 1,4-bis[(tributyltin)ethynyl]-2,5-dioctyloxyben-
zene (21).
2-Bromopyridine (22) (0.181 g, 1.11 mmol) was then added
to the flask and the mixture was refluxed for 3 h. 1H NMR anal-
ysis showed the completion of the reaction. After cooling, the
mixture was filtered over a celite pad, concentrated to a small
volume and treated with pentane. The precipitate thus formed
was collected by filtration and washed repeatedly with pentane
to remove Bu3SnBr. The residue was chromatographed on a
short silica gel column (10Â2.5 cm). Elution with CH2Cl2-ace-
tone (9:1) produced a yellow band, which, after removal of the
solvent, gave the pure product 24; yield: 0.162 g (60%). An an-
alytical sample was obtained by sublimation (1308C/10ꢀ6 mm
Hg).
obtain formation of the corresponding co-polymers. Materials
of different molecular weights are formed depending from the
duration of this step (Table 1). Formation of polymer 33 re-
quired a longer reaction time (22 h). In general, after cooling
to room temperature, insoluble material separated from the di-
oxane solution. After removal of the solid by filtration, the di-
oxane solution was filtered over a celite pad, concentrated to a
small volume and treated with methanol, until a solid precipi-
tate was obtained, which was washed repeatedly with methanol
and dried under vacuum. The original crop of material which
spontaneously precipitated upon cooling the reaction mixture
was treated with chloroform. In the case of polymers 27, 30, 31
and 32, this material showed to be only partially soluble in
THF, chloroform or dichloromethane. This solid was collected
by filtration and separately analyzed. From the chloroform sol-
ution, polymeric material was obtained by the same procedure
adopted for the dioxane solution.
Supporting Information
Preparation of 1,4-Bis(9-anthracenylethynyl)-2,5-
dioctyloxybenzene (25)
Characterization data of compounds 10–16, 24, 25, and 27–33.
To a solution of 1,4-diiodo-2,5-dioctyloxybenzene (19; 0.298 g,
0.508 mmol) and Pd(PPh3)4 (0.006 g, 0.0051 mmol) in dioxane
Acknowledgements
ꢁ
was added tributyl(ethynyl)tin (Bu3SnC CH) (0.337 g,
1.02 mmol). After 20 minutes of stirring at 1108C, H NMR
1
This work was partially supported by the Ministero Italiano del-
analysis showed the complete conversion of starting materials.
After cooling to 08C, LDA (2.0 M solution in THF/heptane/
ethylbenzene; 0.52 mL, 1.04 mmol) was added and the mixture
was allowed to warm up to room temperature. 1H NMR anal-
ysis of a sample of the reaction mixture indicated the complete
formation of 1,4-bis[(tributyltin)ethynyl]-2,5-dioctyloxyben-
zene (21).
9-Bromoanthracene (23; 0.292 g, 1.07 mmol) was then add-
ed to the flask and the mixture was refluxed for 3 h. 1H NMR
analysis showed the completion of the reaction. After cooling,
the mixture was filtered over a celite pad, concentrated to a
small volume and treated with pentane. The precipitate thus
formed was collected by filtration and washed repeatedly
with pentane to remove Bu3SnBr. The residue was chromato-
graphed on a silica gel column (35Â2.5 cm). Elution with hex-
ane-CH2Cl2 (8:2) produced a first orange band, which, after re-
moval of the solvent, gave the pure product 25; yield: 0.254 g
(68%). An analytical sample was obtained by sublimation
(1508C/10ꢀ6 mm Hg).
`
lꢀUniversita e della Ricerca Scientifica e Tecnologica (M.I.U.R.
Roma), with the contract COFIN 2003 “Studio del meccanismo
di reazione e applicazione nella sintesi della catalisi da palladio
per la formazione di polimeri poli(etinilaromatici) e poli(metal-
loetinilaromatici), materiali funzionali per applicazioni innova-
tive in optoelettronica”. The authors gratefully acknowledge
`
Dott. Patrizia Oliva and Prof. Alfonso Grassi (Universita di
Salerno) for CP MAS 13C NMR measurements and stimulating
discussions.
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