Highly branched polyphenylenes with 1,3,5-triphenylbenzene fragments via cyclocondensation of acetylaromatic compounds and Ni0-catalyzed dehalogenation: Synthesis and light emission

Article abstract of DOI:10.1021/ma0347114

Highly branched photoluminescent polyphenylenes (PP) containing 1,3,5-triphenylbenzene (TPB) fragments were prepared via combination of cyclocondensation of acetylaromatic compounds and Ni⁰-catalyzed dehalogenation. To develop optimal condition

Full text of DOI:10.1021/ma0347114

Macromolecules 2003, 36, 8353-8360
8353
Highly Branched Polyphenylenes with 1,3,5-Triphenylbenzene
Fragments via Cyclocondensation of Acetylaromatic Compounds and
Ni0-Catalyzed Dehalogenation: Synthesis and Light Emission
Ir in a A. Kh otin a ,*,† Olga E. Sh m a k ova ,‡ Dia n a Yu . Ba r a n ova ,†
Na ta lia S. Bu r en k ova ,† An a sta sia A. Gu r sk a ja ,† P eter M. Va letsk y,† a n d
Lyu d m ila M. Br on stein §
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
Vavilova str. 28, 117813, Moscow, Russia; Department of Chemistry, University of Houston,
Houston, Texas 77204; and Department of Chemistry, Indiana University,
Bloomington, Indiana 47405
Received May 27, 2003; Revised Manuscript Received August 21, 2003
ABSTRACT: Highly branched photoluminescent polyphenylenes (PP) containing 1,3,5-triphenylbenzene
(TPB) fragments were prepared via combination of cyclocondensation of acetylaromatic compounds and
Ni0-catalyzed dehalogenation. To develop optimal conditions for cyclocondensation of acetylaromatic
compounds, we studied a model reaction of 1,3,5-triphenylbenzene formation using different solvents
and catalysts. The maximum 85% yield of TPB was achieved using H2SO4 as a catalyst in ethanol-
toluene medium at 50 °C for 5 h. Cyclocondensation polymerization of 4,4′-diacetyl diphenyl ether or
4,4′-diacetylbenzene in the presence of equimolar amounts of acetophenone (in the optimal conditions
with respect to the soluble polymer yield) did not produce photoluminescent PP due to various defects in
the polymer chain. Defect-free PP with starlike fragments were synthesized using Ni0-catalyzed
polymerization of aromatic bromides obtained by modification of 1,3,5-tri(p-bromophenyl)benzene. The
molecular weights of the polymers were 6700 and 8600 Da. The maximum photoluminescence in solution
(quantum yield of 96%) was obtained for the highly branched polymer with starlike TPB fragments,
bearing no Br or acetyl groups. The PP of this kind also show very bright fluorescence in the solid state
under UV irradiation at 360 nm so they can be considered as promising materials for OLED applications.
In tr od u ction
groups) can be either soluble or insoluble depending on
their structure.7,20,27-30 At the same time, often the
length (or structure) of the substituents is not sufficient
to separate the macromolecules and to prevent a self-
quenching of luminescence due to migration of excita-
tion energy. The solution of this problem allowing
synthesis of effective blue emitters can be found in
synthesis of polymers with very branched fragments
containing no other substituents than benzene rings.
The example was shown in our recent work,31 where
the presence of branched aromatic fragments (in this
case, in poly(phenylenevinylenes)) provided high lumi-
nescence intensity and blue luminescence maximum
shift (at about 350 nm). The aromatic starlike blocks
are luminophores themselves so they determined emis-
sion of the polymer molecules. This shows that the
incorporation of branched phenylene blocks in side
chains of the macromolecules impart the luminescence
to the polymers, if the branched phenylene structure is
defect-free.
Polyphenylenes (PP) are π-conjugated polymers draw-
ing a considerable interest from the mid-1960s as
thermally stable materials.1 Nowadays, polyphenylenes
are again in the focus of attention because of their
unusual electrical and optical properties such as electri-
cal conductivity, optical nonlinearity, luminescence,2,3
and therefore the possibility of using in organic light
emitting diodes (OLEDs).4,5 Wide band gaps allowing
emission of blue light (400-450 nm) are typical for
polyphenylenes. The development of effective long-lived
blue emitters remains a significant challenge in the field
of OLEDs that makes polyphenylenes the polymers of
interest.6,7 PP can be synthesized using different
avenues.8-15 The majority of recent works were focused
on linear PP.7,16,17 One of the successful methods to
synthesize linear PP is Ni0-catalyzed homopolymeriza-
tion of aromatic dibromides or dichlorides.14,18-26 This
method leads to defect-free structures and seems to be
promising for synthesis of polyphenylenes designed for
optoelectronic devices.
Synthesis of branched polyphenylenes containing
1,3,5-triphenyl-substituted benzene rings was described
in early 1990s.32-34 The synthesis was based on a AB2-
type monomer which, in turn, was synthesized from
1,3,5-tribromobenzene by substitution of one bromine
group with MgX group by Grignard reaction33 or with
B(OH)2 group by Suzuki reaction.32-34 The polymers
prepared by these methods had low molecular weight,
and the majority of them were soluble in common
solvents. However, neither their structure nor optical
properties were reported in refs 32-34. One might
speculate that these methods should result in the
formation of a number of defects in polymers; the
Normally linear poly-p-phenylenes (PPP) are in-
soluble in organic solvents that strongly limits their
potential application for OLED. A reasonable way to
increase solubility is to insert side groups in linear
macromolecules. PPP containing certain substituents in
phenyl rings (for example, alkyl, aryl, alkoxy, or acyl
† Russian Academy of Sciences.
‡ University of Houston.
§ Indiana University.
* Corresponding author: e-mail khotina@ineos.ac.ru; Fax (+7
095)135-5085.
10.1021/ma0347114 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/02/2003

8354 Khotina et al.
Macromolecules, Vol. 36, No. 22, 2003
polymers might also contain a significant amount of
terminal bromo groups, but these issues were not
discussed.
contains many defect fragments due to side reactions.
The major side reactions are an incomplete cyclization,
i.e., dimerization instead of cyclization with formation
of â-methylchalcone groups
To synthesize branched polymers with solely phenyl-
ene rings in their structure, one can suggest the
methods allowing generation of a new phenyl ring
during polymer chain formation. The Diels-Alder reac-
tion11,13 is a method generating the hexaphenyl- and
pentaphenylbenzene rings. A large family of well-
defined macromolecules, consisting of several hexa-
phenylbenzenes, were described recently in refs 35-37.
However, hexaphenylbenzene fragments are not lumi-
nescent.31,38 Improvement comes from planarization of
the phenyl rings, but the system becomes insoluble. To
impart solubility to such a system, extra substituents
are required, but this adds to the complexity of the
synthesis.
Another polymer reaction resulting in the formation
of 1,3,5- and 1,2,4-trisubstituted benzene rings is cyclo-
trimerization of aromatic ethynyl compounds reported
in the 1970s and discussed in a review article.39 Recent
works40-42 reported the development of this synthetic
method and first showed the luminescent properties of
the branched polyphenylenes prepared in this way. The
emission efficiency was found to be from 15% to 46% in
solution.
and formation of linear and cyclic vinylene-containing
products and also pyrylium salts.10 According to refs 44
and 47, this method leads to oligomers containing on
average 7-8 TPB units and about two â-methylchalcone
fragments. These oligomers are soluble in chloroform,
benzene, dioxane, DMF, and other conventional solvents
due to highly branched structure; however, they bear
defect fragments and are characterized by high poly-
dispersity (Mw/Mn ) 6.8-9.9).
Thus, trimerization cyclocondensation developed in
the 1970s leads to soluble branched polyphenylenes
containing 1,3,5-triphenyl-substituted benzene rings
and a number of defect fragments. The defect fragments
were of no concern when thermal stability of a polymer
was a target property, but this method could not be
considered as a viable route for synthesis of polymers
with light emitting properties.
The modern demands for OLEDs require a very
robust synthetic procedure to obtain highly emissive PP
with high yields. Since these PP should be both soluble
in organic solvents and defect-free to provide high
emission, we suggest here a combination of two tech-
niques: (i) cyclocondensation of monoacetyl aromatic
compounds with trimer formation (a central part of a
starlike polymer)31 and (ii) Ni0-catalyzed dehalogenation
for synthesis of branched defect-free polymers. In this
paper we also report the modification of the cyclo-
condenation procedure, allowing us to obtain cyclo-
trimers with a high yield, and also structure and
properties of branched PP with starlike fragments.
Some works, summarized in a review article,10 de-
scribed polyphenylene synthesis based on trimerization
cyclocondensation of diacetylaromatic compounds and
acetophenone in the presence of acidic catalysts to give
exclusively 1,3,5-substituted triarylbenzenes.43,44
Since 1,3,5-triphenylbenzene (1, TPB) is an important
“building block” of branched polyphenylenes, there were
attempts to optimize the synthesis of 1 starting from
the early work by Wirth et al.,45 describing synthesis of
1,3,5-triphenylbenzene with no more than 50% yield by
cyclocondensation of acetophenone in the presence of
various acids. Further optimization of this reaction
carried out with triethylformate as a ketalizing agent
in benzene using gaseous HCl as a catalyst is described
in refs 10 and 44. It allowed synthesis of 1 with about
75% yield. Use of tetrachlorosilane in dry ethanol,
resulting in HCl and tetraethoxysilane (the latter is a
ketalizing agent), led to formation of 1,3,5-triphenyl-
benzene with a comparable yield.46
Exp er im en ta l Section
1
Gen er a l Da ta . H and 13C NMR spectra were recorded on
a Bruker AM-400. FTIR spectra were recorded on a Bruker
IFS 48 instrument. Mass spectrometry was performed using
AEIMS-30 and Kratos MS-890. Molecular weights of the
polymers were determined by gel permeation chromatography
Cyclotrimerization polycondensation of the equimolar
amounts of diacetyl- and monoacetyl-containing ben-
zenes results in the polymer where chain growth occurs
due to interaction of three acetyl groups:43
(GPC) at
a flow rate of 1 mL/min CHCl3 with injector
Rheodyne, Milton-Roy UV detector, and PL-gel columns with
100, 500, and 5 × 103 Å pore sizes and with polystyrene stand-
ard. In some cases Mh n was determined by the ebullioscopy
method using the EP-68. Molecular weights were also deter-
mined from sedimentation data using ultracentrifuge MOM
3180 (Hungary) at T ) 25 ( 0.1 °C in THF or DMF.
Yields of 1 in model reactions have been determined by
quantitative thin-layer chromatography.48 Liquid chromatog-
raphy was performed with a LC-31 Bruker, with UV detector
(λ ) 254 nm) at room temperature with the glass column
J esser, silica gel Separon SGX, and methanol as an eluent at
5 mL/min. UV/vis spectra were taken on a Hitachi 150-20
spectrometer. The emission spectra were obtained from solu-
tions in CHCl3 with a J obin-Yvon 3CS spectrofluorimeter.
Quantum yields were calculated according to ref 49 using
quinine sulfate in 0.1 N sulfuric acid as a reference (φf ) 0.55).
The optical path length was 1 cm. The absorbance of the
samples and the references was kept below 0.2 (in CHCl3) at
the excitation wavelength.
Flash chromatography was performed with a silica gel 60,
230-400 mesh ASTM (Merck or Macherey-Nagel) column.
Ma ter ia ls. All reagents and chemicals were purchased from
Aldrich Chemical Co. Acetophenone was distilled before use.
Ethyl orthoformate was distilled over K2CO3. Benzene was
This reaction has two drawbacks: (i) it may lead to the
gel formation and should be stopped before gelation
occurs (by precipitation in ethanol), and (ii) a polymer

Macromolecules, Vol. 36, No. 22, 2003
Highly Branched Polyphenylenes 8355
Ta ble 1. Th e 1 Yield in th e Acetop h en on e Cyclocon d en sa tion in Differ en t Solven ts a t 20 °C
yield of TPB in different solvents, %
ethanol:toluene
catalyst amount,
run
catalyst
wt %
time, h
benzene
toluene
ethanol
) 1:1 (vol)
1
2
3
4
5
6
7
HCl (gas)
HCl (gas)
H2SO4, conc
CH3C6H4SO3H
BF3‚O(C2H5)2
CH3SO3H
120a
120a
20
15
30
1
2
62
76
13
25
27
71
72
40
76
28
15
57
70
69
60
70
5
5
6
65
72
12
10
49
70
71
96
96
96
96
96
20
20
63
70
CF3SO3H
a
Here and in Table 2 concentration of HCl (gas) is in mL/min.
dried with P2O5 and then distilled over Na. Gaseous hydrogen
chloride was prepared by dropwise addition of concentrated
H2SO4 to NaCl. DMF was distilled over CaH2 in dry argon.
All other reagents and starting materials were used as
received. 4,4′-Diacetyldiphenyl oxide was prepared by standard
Friedel-Crafts acylation of diphenyl ether with acetyl chloride
in the presence of anhydrous AlCl3 in dichloroethane.50 After
recrystallization from ethanol, the product had mp of 101-
102 °C.50
Syn t h et ic P r oced u r es. Synthesis of 1,3,5-Tri(4-bromo-
phenyl)benzene (2). 1,3,5-Tri(4-bromophenyl)benzene was syn-
thesized by cyclocondensation of p-bromoacetophenone (60 g,
0.3 mol) in the presence of 1.2 mol excess of ethyl orthoformate
(65.1 mL, 0.36 mol) in dry benzene (100 mL), bubbling gaseous
hydrogen chloride through the solution for 1 h at room
temperature. When H2SO4 was used as a catalyst, 100 mL of
ethanol-toluene mixture (1:1 vol) contained 20 g (11 mL) of
concentrated H2SO4 and the same amount of ethyl ortho-
formate; the reaction was carried out at 50 °C for 5 h. The
precipitate obtained after pouring reaction solution in ethanol
was isolated, rinsed with acetone, and recrystallized from
chloroform. In HCl/benzene medium the yield was 50%, and
in H2SO4/ethanol-toluene, the yield reached 62%; mp ) 267-
268 °C (262 °C51). MS: m/z ) 543 [M+]. Elemental analysis
data for C24H15Br3: Calculated, %: C, 53.07; H, 2.78; Br, 44.14.
Found, %: C, 53.48; H, 3.08; Br, 43.48.
Synthesis of p-Ethynylacetophenone. A Schlenk tube was
charged with p-bromoacetophenone (4.529 g, 0.0228 mol) and
purged with argon. The temperature was raised to 70 °C, and
triethyamine (3 mL), (Ph3P)2PdCl2 (0.05 g), Ph3P (1.0 g),
trimethylsilylacetylene (3.9 mL, 0.0273 mol), and CuI (0.01
g) were added in argon counterflow. The mixture was stirred
at 70 °C for 4 h. The resulting solution was mixed with
chloroform and washed with water. The organic solution was
dried over CaCl2 overnight, and then the solvent was evapo-
rated. The resulting solid was charged with 20 mL of 50 wt %
NaOH, 10 mL of ethanol, 2 mL of THF, and 0.01 g of dibenzo-
18-crown-6. The mixture was stirred for 3 h at 50 °C. After
that the reaction solution was mixed with water. The product
was extracted with diethyl ether and purified with column
chromatography (silica gel as adsorbent, a mixture hexane:
chloroform ) 2:1 as an eluent). Yield is 27%; mp ) 66-68 °C
(67-68.5 °C52). MS: m/z ) 132. Elemental analysis data for
%: C, 66.84; H, 3.55; Br, 27.57. FTIR: 1680 cm-1 for CdO
and 2110 cm-1 for CtC.
Polyphenylenes with Acetyl End Groups. In a typical experi-
ment (synthesis of polyphenylene I from 4,4′-diacetyldiphenyl
ether and acetophenone as monomers), equimolar amounts of
4,4′-diacetyldiphenyl ether (4.6 mL, 0.04 mol) and aceto-
phenone (1.0 g, 0.04 mol) were mixed with 48.7 mL of ethyl
orthoformate (1.2 mol per every acetyl group) and dry solvent
(70%). The reaction mixture was stirred about 30 min (until
the monomer dissolved) and charged with a calculated amount
of an acidic catalyst at a reaction temperature (see Table 2).
After reaction completion, the reaction mixture was poured
into ethanol. The yellow precipitate was isolated, washed with
ethanol, and dried in a vacuum desiccator at room tempera-
ture.
The polymer II based on p-diacetylbenzene, and acetophe-
none was synthesized in the same way.
Synthesis of the Polyphenylenes by Ni0-Catalyzed Reaction.
Polymer IV was prepared by the method described elswhere.53
Anhydrous NiCl2 (2.6 mg, 0.02 mmol), Ph3P (11.2 mg, 0.043
mmol), bipy (3.2 mg, 0.02 mmol), 4-bromobenzene (0.067 g,
0.425 mol), and 1,3,5-tri(4-bromophenyl)benzene (0.23 g, 0.425
mmol) were placed in a Schlenk tube purged with argon. The
tube was evacuated and filled with argon five times. Then dry
DMF (1 mL) was added by syringe purged with argon. The
reaction mixture was quickly heated to 90 °C and then stirred
at this temperature (for 6 h). In the first 10 min the reaction
mixture became red-brown, which indicated the catalytic
complex formation.53 The resulting solution was poured into
methanol; the precipitate was collected, washed with diluted
HCl, water, and methanol, and dried in a vacuum desiccator
at room temperature for 10 h. Yield was 52%. Elemental
analysis data for C30H24: Calculated, %: C, 93.75; H, 6.25.
Found, %: C, 92.52; H, 5.97; Br not found. Polymer III was
prepared using the same procedure.
Resu lts a n d Discu ssion
The first approach to synthesis of defect-free poly-
phenylenes with highly branched structure was to
optimize formation of TPB fragments using cyclo-
condensation trimerization. The optimum conditions for
synthesis of polyphenylenes using cyclocondensation of
acetylaromartic compounds were thought to be benzene
as a solvent and gaseous HCl as a catalyst.10 However,
when we carried out an optoelectronic study of the
polyphenylenes obtained in these conditions, they showed
no luminescence both in solutions and in a solid state.
This was especially puzzling as model cyclotrimers with
a 1,3,5-triphenyl-substituted benzene core, which are
the main structural blocks of the polyphenylenes,
showed luminescence with quantum yields in solutions
in the range 29-72%.31
C
10H8O: Calculated, %: C, 83.30; H, 5.56. Found, %: C, 83.19;
H, 5.60. FTIR: 1680 cm-1 for CdO, 2110 cm-1 for CtC, and
3100 cm-1 for tC-H.
Synthesis of [1,3-Di(4-bromophenyl)-5-(4-phenylethynyl-4-
phenylacetyl)]benzene (3). A Schlenk tube purged with argon
was charged with 1,3,5-tri(p-bromophenyl)benzene (2) (5 g,
0.0092 mol), 4-ethynylacetophenone (1.15 g, 0.090 mol), tri-
ethylamine (2 mL), and DMF (3 mL) and then heated to 80
°C under stirring. After that a catalytic system consisting of
(Ph3P)2PdCl2 (0.05 g), Ph3P (1.0 g), and CuI (0.001 g) was
added. The reaction mixture was stirred at 70-80 °C for 8 h.
After cooling, the resulting solution was poured into methanol
and the precipitate was collected. The final compound was
purified with column chromatography (silica gel as adsorbent,
chloroform as eluent). The yield was about 20%; mp ) 248-
250 °C. MS: m/z ) 606 [M+]. Elemental analysis data for
We suggested that these polyphenylenes show no
luminescence because of its quenching by defect groups.
We believe that the higher yield of 1,3,5-triphenyl-
benzene (1) in a model reaction of acetophenone cyclo-
condensation could determine the enhanced probability
C
34H22Br2: Calculated, %: C, 67.35; H, 3.66; Br, 26.36. Found,

8356 Khotina et al.
Macromolecules, Vol. 36, No. 22, 2003
Ta ble 2. Ch a r a cter istics of P P Syn th esized in Differ en t Rea ction Con d ition s
catalyst
run
polymer
catalyst
HCl, gas
amount, wt %
solvent
benzene
T,°C
time, h
yield, %
70
73
54
mp, °C
η
inh, dL/g
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
180
19
20
21
22
23
24
25
26
27
28
I
II
I
120
20
0.3
0.2
140-150
155-170
95-115
0.11
0.13
0.01
0.04
0.05
0.08
0.06
0.02
0.14
0.10
0.05
0.06
0.05
0.05
0.07
0.05
0.04
0.05
0.14
0.07
0.08
0.05
0.02
0.03
0.06
0.07
0.06
0.02
H2SO4, conc
5
10
15
20
30
50
20
20
20
15
20
30
15
15
20
30
30
40
50
30
5
ethanol
63
71
79
100-115
105-125
110-140
110-145
130-180
165-175
135-150
105-115
125-135
120-140
120-155
125-135
105-110
105-115
110-135
110-145
115-140
115-145
110-130
90-120
20
96
7 ins, 56 sa
25 ins, 38 s
73
78
35
70
68
51
72
27
52
66
76
66
65
64
20
II
I
I
ethanol/toluene
50
20
2
CH3-C6H4- SO3H
BF3‚O(C2H5)2
ethanol
96
ethanol/toluene
50
20
2
96
I
I
ethanol/toluene
50
30
2
96
ethanol
2
CH3SO3H
10
15
20
20
30
20
96
40
62
76
60
100-130
110-130
135-150
93-135
ethanol/toluene
50
20
2
96
40 ins, 20 s
135-160
a
“s” stands for soluble fraction; “ins” stands for insoluble fraction; no notation means no insoluble polymer.
of the formation of the targeted fragments in the
cyclocondensation polymerization of acetylaromatic com-
pounds. On the basis of this, we studied cycloconden-
sation of acetophenone searching for reaction conditions
leading to the higher 1 yield.
Mod el Cyclocon d en sa tion of Acetop h en on e. The
cyclocondensation of acetophenone was carried out in
polar and nonpolar solvents in the presence of different
acidic catalysts. As acetophenone cyclocondensation
proceeds through a carbocationic mechanism,10,54 one
might assume that this reaction should be inefficient
in polar solvents since polar solvent should suppress the
carbocation formation. Moreover, if the reaction medium
is ethanol, additional complication is that ethanol is a
reaction product so its abundance in the reaction
solution should further suppress the reaction.
F igu r e 1. Temperature dependences of the 1 yield for
different catalysts: HCl (1), H2SO4 (2), p-toluenesulfonic acid
(3), BF3‚O(C2H5)2 (4), and CH3SO3H (5) in the toluene-ethanol
mixture. Reaction time: 30 min.
Nevertheless, a similar yield of 1 was obtained using
not only benzene or toluene with HCl (76% in both
cases) but also an ethanol/toluene mixture or pure
ethanol as a solvent (70-72%). When sulfuric acid was
used as a catalyst instead of HCl, the yields were close
to about 70% and equal in benzene, toluene, and
toluene/ethanol but slightly lower in ethanol (Table 1).
Use of other acids listed in Table 1 leads to lower yields.
To figure out the composition of the reaction products,
the mixture obtained in ethanol/toluene as a solvent
(Table 1, run 3) with low yield of 1 was studied by liquid
chromatography. This composition allowed easier detec-
tion of byproducts because of their higher content. The
product of acetophenone cyclocondensation was found
to contain 10.6% 1, 60.6% acetophenone, 26.7% mixture
of â-methylchalcone and pyrylium salt, and 2.1% of
other products of acetophenone condensation, probably
analogous to those described in ref 10. This examination
shows that comparatively low yields in the 1 syntheses
can be mainly explained by (i) formation of byproducts
of the condensation of acetophenone and triethyl ortho-
formate (pyrylium salts) and (ii) incomplete aceto-
phenone condensation (â-methylchalcone formation).
The temperature dependence of the yield of 1 in
toluene solution shows different behavior for different
catalysts (Figure 1). For gaseous HCl, increase of
reaction temperature resulted in decrease of the yield
of 1. This may occur because of a side reaction of HCl
addition to intermediate vinyl ether and also because

Macromolecules, Vol. 36, No. 22, 2003
Highly Branched Polyphenylenes 8357
F igu r e 2. Dependences of the 1 yield on synthesis duration
for H2SO4/ethanol (1) and H2SO4/ethanol-toluene (2). Reaction
temperature: 50 °C.
Sch em e 1
F igu r e 3. 13C NMR spectra of polyphenylenes I synthesized
in H2SO4/ethanol (1) and HCl/benzene (2) media: (*) fragments
of 1 (1,3,5-triphenylbenzene); (+) â-methylchalcone fragments;
notation ∨ is used to denote the diacetyldiphenyl ether end
groups.
(DAB) (see Scheme 1) in the presence of equimolecular
amount of acetophenone with triethylformate10,44 in the
mixture ethanol:toluene ) 1:1 (vol) in the presence of
different acidic catalysts (Table 2).
of HCl concentration decrease at higher temperatures.
For p-toluenesulfonic acid and methanesulfonic acid, the
yield increases with increase of the reaction tempera-
ture. Temperature dependence of the yield goes through
a maximum value when the reaction is carried out with
boron trifluoride diethyl etherate and sulfuric acid.
Apparently, it is caused by faster formation of byprod-
ucts at temperatures higher than 50 °C for these two
catalysts.
The yield of 1 depends also on reaction time. The yield
strongly increased during the first 2 h and reached 85%
for 5 h as shown in Figure 2 for two reaction solutions:
H2SO4 in ethanol and ethanol/toluene at 50 °C. This
temperature was chosen as it allows the highest yield
of 1 for H2SO4 as a catalyst (see Figure 1). This
dependence can be explained by increase of aceto-
phenone conversion becoming quantitative (100%) for
5-6 h. Thus, 85% yield of 1 can be obtained in rather
mild conditions (without aggressive gaseous HCl), but
this value is still far from quantitative. At the same
time, increase of the yield of 1 inspired us to try
cyclocondensation for polymer synthesis considering
that the synthetic procedure might be further optimized
during polymerization. Since, in the case of polymers,
all “byproducts” are fixed in the polymer structure as
defect fragments, it should strongly influence the PP
properties.
Cyclocon d en sa tion P olym er iza tion of 4,4′-Di-
a cetyl Dip h en yl Eth er a n d Acetop h en on e. Al-
though for cyclocondensation polymerization with di-
functional monomers we used the general conditions
found for the synthesis of 1, we had also to take into
account that in polymer synthesis the reaction duration
and temperature are restricted by possible and undesir-
able gel formation. Thus, conditions should be modified
accordingly to give the highest yield of the soluble
polymer in mild conditions and for the shortest time.
We studied cyclocondensation polymerization of 4,4′-
diacetyl diphenyl ether (DADPE) or 4,4′-diacetylbenzene
The conditions presented in Table 2 show that yields
higher than 70% for soluble polymers were obtained in
the conditions similar to ones found for 1 (Table 1). For
BF3‚O(C2H5)2, the reaction temperature was increased
to 50 °C, which led to shorter duration of the synthesis
before gelation. As follows from the data presented in
Table 2, the highest yield of soluble polymer (79%) was
obtained with sulfuric acid at 20 °C for 96 h. It should
be noted that the best yield of 1 (85%, Figure 2) was
obtained in the same conditions, but at higher temper-
ature (50 °C). For polymer synthesis, this temperature
resulted in fast gelation and was not used.
The structure of the polymers I based on DADPE was
studied using
13C NMR spectroscopy. A signal assign-
ment was performed in accordance with ref 44. The 13C
NMR spectra (Figure 3) of two polymer I samples
obtained in runs 1 and 6 (Table 2) show a similar set of
signals assigned to aromatic carbons. In addition, the
13C NMR spectrum of the polymer I obtained in run 6
contains the signals at 18.58 ppm and at 190.20 ppm
characteristic of methyl and carbonyl groups of â-
methylchalcone defect fragments,44
while in the 13C
NMR spectrum of the product from run 1, these signals
are absent.
At the same time, the FTIR spectra of the polymer I
samples from runs 1 and 6 show a weak band at 1660
cm-1, which is characteristic of a carbonyl group of a
â-methylchalcone fragment. Moreover, the spectrum of
the sample from run 6 contains an additional band at
1720 cm-1 characteristic of ester group appearing as a
result of â-methylchalcone group hydrolysis. Thus,
judging by FTIR and 13C NMR spectra, both polymer I
samples contain defect fragments, but the polymer from
run 6 shows an enhanced amount of these fragments
compared to the polymer from run 1.

8358 Khotina et al.
Macromolecules, Vol. 36, No. 22, 2003
Sch em e 2
Ta ble 3. GP C Da ta of P olyp h en ylen es I fr om Ru n s 1, 6,
a n d 21 fr om Ta ble 2
run from
Table 2
catalyst
HCl
H2SO4
BF3‚O(C2H5)2
Mn
Mw
Mw/Mn
1
6
21
2560
2020
1430
5850
6480
2620
2.3
3.2
1.8
Three polymer I samples prepared with different
catalysts (runs 1, 6, and 21, Table 2) were examined by
GPC (Table 3). Comparison of the GPC data with the
relatively low values of inherent viscosity (especially for
polymers from runs 6 and 21) shows high branching in
the polymers I.55
The polyphenylenes II based on DAB and aceto-
phenone, i.e., the polyphenylenes consisting of solely
phenylene groups, were synthesized in two systems: in
benzene with HCl and in ethanol/toluene mixture with
H2SO4 (Table 2, runs 2 and 10). The polymer from run
2 prepared with HCl has the higher molecular weight
than the one from run 10 as follows from values of
inherent viscosity and melting temperatures. Besides,
the latter polymer (from run 10) bears the higher
amount of defect fragments judging by the FTIR spectra
(presence of the band at 1660 cm-1). Unlike the poly-
mers based on DADPE (I), where NMR signals are well
resolved, for the polymers II, 13C NMR spectra are not
useful for their characterization as the NMR signals
mainly overlap.
Thus, analysis of soluble branched PP obtained by
cyclocondensation polymerization showed that they
always contain some defect fragments and do not show
photoluminescence. Synthesis of branched defect-free
oligophenylene with 13 phenyl ring (tridecaphenyl or
1,3,5-tri[1,3-diphenyl(phenyl-5-yl)phenyl-4′-yl]ben-
zene) was described by us earlier31 using cycloconden-
sation of [1-(4′-acetylphenyl)-3,5-diphenyl]benzene (see
Scheme 2). This molecule presents a second generation
of PP dendrimer with 1 as a dendrimer center.
Here all byproducts were eliminated by purification
after synthesis, and the final product showed intense
photoluminescence in solution (quantum yield ) 72%)
at wavelength of about 370 nm (Figure 4). We believe
that the higher generations of such dendrimers might
result in enhancement of photoluminescence properties.
However, as always with dendrimers of a higher gen-
eration, the synthesis is usually laborious and time-
consuming. Another possibility is to lengthen the arms
of the dendrimer center (obtained by cyclocondensation
of acetylaromatic compounds) using some polymeriza-
tion technique (for example, Ni0-catalyzed dehalogena-
tion) providing defect-free structure. This approach
(described in the next section) should result in the
branched polymers containing starlike fragments.
F igu r e 4. Photoluminescence spectra of tridecaphenyl (1),
polyphenylene III (2), polyphenylene IV (3), and 1,3,5-tri(4-
bromophenyl)benzene (4).
Ni0-Ca ta lyzed Syn th esis of High ly Br a n ch ed P P
w ith Sta r lik e F r a gm en ts. As discussed above and
reported in refs 18, 26, and 53, dehalogenation of
dibromo- or dichloroaromatic compounds catalyzed by
Ni0 complexes leads to defect-free PP structures. For
synthesis of defect-free branched PP with TPB starlike
fragments, we first synthesized the corresponding bro-
mides and then used them as monomers in Ni0-
catalyzed polymerization.
New bromide was prepared starting from 1,3,5-tri(p-
bromophenyl)benzene51 (2) synthesized by cycloconden-
sation of p-bromoacetophenone. The synthetic procedure
was based on the conditions developed for the synthesis
of 1.To prevent 3D cross-linking in Ni0-catalyzed reac-
tion with tribromide, we needed either to use some
monobromide (to decrease the overall functionality of
the system) or to convert tribromide to dibromide (to
substitute one bromo group (out of three) in 2 for the
other group (see Scheme III). However, reaction with
equimolar amounts of phenylboronic acid and 2 resulted
in nonselective substitution: by TLC the reaction
product contained a mixture of mono- and disubstituted
products along with remaining 2. Our attempts to
separate these products from each other using flash
chromatography were unsuccessful.
To obtain a monomer with two bromo groups, 2 was
treated with 4-ethynylacetophenone, prepared by Scheme
IV. In this case, the reaction also resulted in a mixture
of products with different functionality, but because of
the presence of polar COCH3 groups, all products were
easily separated from each other using column chroma-
tography with chloroform:hexane ) 1:2 (vol) mixture as
an eluent. Compound 4 was obtained with 20% yield.
Its purity was confirmed by mass spectrometry, elemen-
tal analysis, and FTIR.
Polymer III was prepared by Ni0-catalyzed polymer-
ization from 3 in DMF in the presence of anhydrous

Macromolecules, Vol. 36, No. 22, 2003
Highly Branched Polyphenylenes 8359
Sch em e 3
Sch em e 4
Ta ble 4. Ch a r a cter istics of P olym er s P r ep a r ed by
Ni0-Ca ta lyzed P olym er iza tion
polymer
mp, °C
Mh w, Da
PL QY, %
III
IV
210-216
101-110
6700
8600
17
96
ards.56 As polymer III cannot form additional branches
(monomer 3 is strictly difunctional), the value of mo-
lecular weight allows calculating the number of repeat-
ing units. Since molecular weight of a repeating unit is
446, polymer III contains 15 repeating units. For
polymer IV, this value cannot be calculated as the
branching is terminated by addition of bromobenzene.
However, if one assumes that a repeating unit in
polymer IV is a disubstituted phenyl ring, this polymer
contains 113 phenyl rings. Compared to tridecaphenyl,
polymer IV contains more phenyl rings by a factor of 9;
yet, the branched structure with prevailing TPB frag-
ments is preserved.
F igu r e 5. 1H NMR spectrum of polyphenylene IV showing
the signals area of aromatic protons.
NiCl2, Zn, Ph3P, and bipy in high-purity argon at 80
°C. Compound 2 containing three terminal bromine
atoms was also used for direct polymer synthesis
(polymer IV), but in this case, the equimolar amount of
bromobenzene was added to prevent gelation.
The photoluminescence (PL) data for the polymers III
and IV are shown in Table 4 and Figure 4. Unlike 1,
its brominated analogue 2 shows nearly no photolumi-
nescence. The photoluminescence spectra of polymers
III and IV and tridecaphenyl in chloroform solutions
show blue emission, which is typical for completely
aromatic structures.6,15 Tridecaphenyl containing 13
benzene rings and 1,3,5-triphenyl-substituted benzene
exhibits an intense photoluminescence peak at 360 nm
with a PL quantum yield (QY) of 72%.31 The spectra of
the polymers differ depending on the polymer structure.
The spectrum of polymer IV shows peaks at 360 and
380 nm (Figure 4); the position of the former fully
coincides with the position of the tridecaphenyl peak.
This polymer is characterized by a remarkably high PL
QY (96%, Table 4). Since polymer IV is highly branched
and resembles the tridecaphenyl structure, the similari-
The structure of polymers III and IV was character-
ized by 1H NMR and 13C NMR spectroscopy. The 1H
NMR spectrum of the polymer IV (Figure 5) contains
the signals at 7.37-7.56 ppm, which are characteristic
for the protons of monosubstituted aromatic rings, and
the signals at 7.70-7.95 ppm, which can be assigned to
the remaining aromatic protons. In the 13C NMR
spectrum of the polymer III, the signal at 29.75 ppm is
characteristic of methyl protons of the acetyl groups.
Molecular weights of polymers III and IV were
examined by sedimentation in an ultracentrifuge. This
method allows obtaining accurate values of weight-
average molecular weight without using relative stand-

8360 Khotina et al.
Macromolecules, Vol. 36, No. 22, 2003
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spectrum of polymer III peaked at about 400 nm. We
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to the presence of acetyl end groups enhancing interac-
tion between neighboring chains, while the red shift
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branching and lack of starlike fragments. The important
property of these PP is very bright fluorescence in the
solid state under UV irradiation at 360 nm, so these
polymers can be considered as promising materials for
OLED applications.
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Ack n ow led gm en t. Financial support for this work
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