Oligo(phenylene vinylene)-poly(methylstyrene) hybrids: Controlled step-wise molecular wiring of oligo(phenylene vinylene)

Article abstract of DOI:10.1016/j.tetlet.2006.02.088

Well defined oligo(phenylene vinylene) grafted polymers known as oligo(phenylene vinylene)-poly(methylstyrene) hybrids have been developed using a step-wise synthetic protocol, where the length of the OPV can be controlled systematically to achieve specif

Full text of DOI:10.1016/j.tetlet.2006.02.088

Tetrahedron Letters 47 (2006) 2731–2734
Oligo(phenylene vinylene)–poly(methylstyrene) hybrids:
controlled step-wise molecular wiring of oligo(phenylene vinylene)
Rethi Madathil,* Raman Parkesh and Sylvia M. Draper
Science Foundation Ireland, Physics Department and Chemistry Department, Trinity College, Ireland
Received 15 September 2005; revised 9 February 2006; accepted 15 February 2006
Available online 3 March 2006
Abstract—Well defined oligo(phenylene vinylene) grafted polymers known as oligo(phenylene vinylene)–poly(methylstyrene)
hybrids have been developed using a step-wise synthetic protocol, where the length of the OPV can be controlled systematically
to achieve specific optoelectronic properties. The process allows the structural modification of attached OPV at a molecular level
either by varying the chain length or by changing functionalities. The step-wise generation of OPV chains on the backbone of a
highly soluble polymer ensures solubilization in a variety of solvents and also the exhibition of interesting optical properties.
Ó 2006 Elsevier Ltd. All rights reserved.
The organic semiconducting materials, poly(phenylene
vinylene) and oligo(phenylene vinylene) (OPV), have
attracted intense research interest because of their
attractive electroluminescent properties.1 The realization
of optoelectronic devices with OPV as the active layer,
requires the design of new structures with specific opto-
electronic features. Great success has been achieved in
developing OPV based supramolecular structures such
as dendrimers2 tetrahedral OPVs3 and self-assembled
structures.4
ited to a maximum of three.6–11 In this context a strategy
that integrates long chain OPV oligomers on flexible
polymers is interesting.
The present work explores the step-wise wiring of OPV
into higher oligomers on mechanically flexible
poly(methylstyrene). The development of OPV chains
via step-wise addition of a stilbenoid unit on a highly
soluble polymer ensures solubilization in a variety of
solvents. The proposed method gives a choice of grow-
ing unmodified OPV or other substituted forms. The
high solubility of the polymer hybrids at each step per-
mits the further attachment of stilbenoid units.
From an applications point of view, mechanical flexibil-
ity is highly desired to obtain deposition as a thin film
over a large area. Successful physical blending depends
on the compatibilty of the components, where the
intractable nature of OPVs is disadvantageous. Only
soluble alkoxy substituted OPVs are considered as suit-
able candidates for blending.5 New strategies that result
in high quality blends of OPVs with flexible inert poly-
mers are therefore technologically important. Recently
developed copolymers of OPV with non-conjugated
polymers are promising due to their higher processabil-
ity.6–11 However, OPVs with greater chain length are
insoluble and hence difficult to incorporate directly into
processable polymers. Consequently, OPV-copolymers
so far designed have chain lengths of the OPV unit lim-
We selected poly(methylstyrene) as the inert polymer
backbone to undergo OPV wiring. The precursor poly-
mer
poly(4-methylstyrene-co-4-chloromethylstyrene),
P1 consisting of 10 equiv of 4-methylstyrene and 1 equiv
of 4-chloromethylstyrene was synthesized using benzoyl
peroxide as the polymerization initiator12 (Scheme 1).
The high content of methylstyrene in the precursor poly-
mer induces solubility. The chloromethyl functionalities
on P1 were then converted to phosphonates13 by treat-
ment with a large excess of triethylphosphite at 140 °C
to give P2 (Scheme 1). The P2 polymers were subse-
quently subjected to a Wittig–Horner reaction14 with
4-(diethoxymethyl)benzaldehyde followed by treatment
with 1 M HCl to obtain the first generation polymer,
10PMS-OPVC2. Monomer m3, necessary for further
attachment of OPV was prepared as illustrated in
Scheme 2. We have established a simple protocol for
the synthesis of m3 in bulk quantities.15
Keywords: Hybrid polymer; Step-wise wiring; Optoelectronic
properties.
*
Corresponding author. E-mail: madathir@tcd.ie
Present address: Pharmacology Department, University of Oxford,
Oxford, UK.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.088

2732
R. Madathil et al. / Tetrahedron Letters 47 (2006) 2731–2734
]
]
]
x
]
y
O
O
]
y
[
]
]
y
[
[
]
y
[
[
[
[
[
(a)
x
x
x
(b)
(a)
+
+
O
O
CH3
CH3
m1
CH3
Cl
m2
CH3
P
O
P
CH3
Cl
O
O
2
O
P1
P2
m1 : m2 = 10 : 1
]
x
]
y
]
x
]
y
m3
[
O
[
[
O
O
[
O
10 PMS -OPVC2
(d)
(c)
10 PMS -OPVC3
+
P2
]
]
y
CH3
[
H3C
[
O
x
step-wise
synthesis
10 PMS -OPVC3
m3 / (a)
CH3
O
O
O
z
10 PMS -OPVC2
Scheme 1. Synthesis of 10PMS-OPVC2. Reagents and conditions: (a)
benzoyl peroxide, 130 °C, 6 h, (b) triethylphosphite, 140 °C, 24 h, (c)
potassium-tert-butoxide, THF, rt, 24 h, (d) 1 M HCl, 3–5 min.
O
(n = 4 - 9)
10 PMS -OPVCn
n = z+1
Scheme 3. Step-wise synthesis of 10PMS-OPVCn polymer hybrids
with chain length n = 4–9. Reagents and conditions: (a) potassium-
tert-butoxide, THF, rt, 24 h and then 1 M HCl, 3–5 min.
O
Cl
O
O
Cl
(a)
(b)
(c)
O
O
O
P
P
P
]
Cl
O
O
]
y
]
x
O
[
]
y
[
[
[
O
O
O
x
(a)
1
2
m3
+
O
CH3
CH3
Scheme 2. Synthesis of monomer m3. Reagents and conditions: (a)
triethylphosphite, 120 °C, 3 h, (b) hexamethylene tetraamine, acetic
acid, 100 °C, 3 h and then concd HCl, 100 °C, 15 min, (c) triethyl
orthoformate, N-bromosuccinimide, ethanol, rt, 12 h.
P
z
Z + 1
O
O
O
10 PMS -OPVn
10 PMS -OPVCn
Aldehyde functionalized 10PMS-OPVC2 was systemat-
ically subjected to Wittig reaction with monomer m3, in
a step-wise manner to attach stilbenoid units as shown
in Scheme 3. To ensure the complete transformation
of OPV at each stage to the next higher oligomer, a large
excess of monomer m3 was used. The polymers obtained
were semi-crystalline, bright yellow powders.
]
x
]
y
[
[
]
]
[
[
(a)
y
x
+
CH3
O
O
CH3
P
O
O
2
These aldehyde functionalized 10PMS-OPVCn poly-
mers were then reacted with 4-diethylbenzylphospho-
nate to obtain phenylene end-functionalised 10PMS-
OPVn (Scheme 4). Hybrid polymer 10PMS-OPV2 was
synthesized by treating P2 with benzaldehyde. All the
polymers were highly soluble in a variety of solvents
such as THF, ethyl acetate, dichloromethane and tolu-
ene. The absorption and fluorescence spectra of poly-
mers 10PMS-OPVCn are depicted in Figure 1.
10 PMS -OPV2
Scheme 4. Synthesis of 10PMS-OPVn polymer hybrids. Reagents and
conditions: (a) potassium-tert-butoxide, THF, rt, 24 h.
Polymers 10PMS-OPVC3–9 showed only a slight change
in fluorescent emission characteristics as depicted in Fig-
ure 1. (10PMS-OPVC2 at 462 nm, 10PMS-OPVC3 at
467 and 10PMS-OPVC4–9 at 472–478 nm) with intense
emission in the blue-green region. The electron with-
drawing nature of the aldehyde group in these polymers
enhances delocalization of electrons in a similar fashion
in all these polymers, resulting in broad absorption
bands.
The absorbance maxima are red-shifted with increased
chain length. The polymers, where n is 2 and 3–4 exhib-
ited absorption maxima at 337 nm and ꢀ360 nm,
respectively, indicating a decrease in the energy of
absorption with increase in chain length. Hybrid poly-
mers 10PMS-OPVC5–9 exhibit absorption features com-
parable to each other with maxima between 370 and
380 nm. 10PMS-OPVC2 showed blue coloured emission
at 440 nm.
Absorption spectra of phenylene end-functionalised
10PMS-OPVn polymers show significant changes in

R. Madathil et al. / Tetrahedron Letters 47 (2006) 2731–2734
2733
Acknowledgements
10PMS-OPVCn
n = 2
n = 3
n = 4
n = 5
n = 7
n = 9
Dr. R. Madathil acknowledges the Science Foundation
Ireland (CINSE project) for financial support.
References and notes
1. Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem.,
Int. Ed. Engl. 1992, 37, 402.
2. Meier, H.; Lehmann, M. Angew. Chem., Int. Ed. 1998, 37,
643.
300
360
420
400 450 500 550 600 650
Wavelength (nm)
3. Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F. J. Am.
Chem. Soc. 2000, 122, 7467.
4. El-ghayoury, A. P.; Schenning, H. J.; Van Hal, P. A.; Van
Duren, J. K. J.; Janssen, R. A. J.; Meijer, E. W. Angew.
Chem., Int. Ed. 2001, 40, 3660.
Figure 1. UV–vis absorption and fluorescence spectra (kexc 337 nm for
n = 2, 360 nm for n = 3 and 373–380 nm for n = 4–9 for 10PMS-
COPVn polymers in THF solution).
5. Pron, A.; Rannou, P. Prog. Polym. Sci. 2002, 27, 135.
6. Zheng, M.; Sarker, A. M.; Gurel, E. E.; Lahti, P. M.;
¨
the spectral characteristics with chain length as illus-
trated in Figure 2. As the chain length increases, a signif-
icant shift in absorption towards low energy is observed
(absorption maxima for 10PMS-OPV2 at 300 nm, for
n = 3–4, 330–335 nm and for n = 5–9 between 370 and
380 nm) indicating the increase in effective conjugation
length. Absorption spectra were similar to the corre-
sponding OPV oligomers.14
Karasz, F. E. Macromolecules 2000, 33, 7426.
7. Zyung, T.; Hwang, D.; Kang, I.; Shim, H.; Hwang, W.;
Kim, J. Chem. Mater. 1995, 7, 1499.
8. Aguiar, M.; Karasz, F. E.; Akcelrud, L. Macromolecules
1995, 28, 4598.
9. Hay, M.; Klavetter, F. L. J. Am. Chem. Soc. 1995, 117,
7112.
10. Akcelrud, L. Prog. Polym. Sci. 2003, 28, 875.
11. Hochfilzer, C.; Rasch, S.; Winkler, B.; Huslage, J.;
Leising, G. Synth. Met. 1997, 85, 1271.
The fluorescent spectra of 10PMS-OPVn were highly
structured with defined fluorescent bands, compared to
the broad spectra of 10PMS-OPVCn polymer hybrids.
The spectral bands were shifted to the lower energy with
increase in conjugation length (10PMS-OPV2 at
358 nm, 397–420 nm for n = 3, 460 nm for n = 4, for
n = 5–9 between 460 and 480 nm). As the chain length
increases the emission colour changes from violet to
green (Fig. 2).
12. Park, E. S.; Kim, H. S.; Kim, M. N.; Yoon, J. S. Eur.
Polym. J. 2004, 40, 2819.
13. Kauffman, J. M.; Moyna, G. J. Org. Chem. 2003, 68, 839.
14. Meier, H.; Gerold, J.; Kolshorn, H.; Baumann, W.; Bletz,
M. Angew. Chem., Int. Ed. 2002, 41, 292.
15. General synthetic procedures: All the reagents and chem-
icals used for the syntheses were purchased from Aldrich.
Solvents were of analytical, HPLC and anhydrous grade.
The monomers, 4-methylstyrene and 4-chloromethylstyr-
ene were passed through neutral alumina columns
(Aldrich) to remove the inhibitor before use.
In summary, this work explores a step-wise synthetic
strategy to obtain oligo(phenylene vinylene)–poly(meth-
ylstyrene) hybrids where the length of the OPV can be
controlled systematically to achieve specific optoelec-
tronic properties. The protocol allows the structural
modification of attached OPV at a molecular level
either by varying the chain length or by changing the
functionalities.
Synthesis of poly(4-methylstyrene-co-4-chloromethylstyr-
ene), P1: Methylstyrene (11.82 g, 0.1 mol), 1.53 g
(0.01 mol) of chloromethylstyrene and benzoyl peroxide
(240 mg) were dissolved in 20 mL of toluene in a reaction
flask and purged with pure N2 gas for 20 min. The flask
was then placed on a preheated hotplate at 80 °C with
stirring. Then the temperature was slowly raised to 130 °C.
The reaction was allowed to proceed for 6 h under an N2
atmosphere. The copolymer was precipitated by pouring
into excess methanol, then filtered, purified by repeated
reprecipitation from a solution of the polymer in toluene
using methanol and dried at room temperature for 24 h.
Elemental analysis conformed that copolymer P1 contains
3–3.5% of chlorine (calculated ꢀ2.66%). 1H NMR
(400 MHz, CDCl3) d (ppm): 7.1–6.5 (aromatic region),
4.52 (CH2–Cl), 2.2–2.3 (CH3–Ar).
10PMS-OPVn
n = 2
n = 3
n = 4
n = 5
n = 10
Conversion of chloromethyl groups to phosphonate
groups (copolymer P2): In a round bottom flask, 6 g of
P1 and 10 mL of triethylphosphite (excess, ꢀ10 times)
were added and refluxed for 24 h at 140 °C with stirring.7
The phosphonate polymer P2 was precipitated using
a large excess of methanol and then filtered. Further
purification of the product was carried out by dissolving
in THF and reprecipitating using methanol. The prod-
uct was then filtered and dried to give a white amor-
360
420
Wavelength (nm)
420
480
540
600
300
360
1
phous powder (yield 97%). H NMR (400 MHz, CDCl3)
Figure 2. UV–vis absorption and fluorescence spectra (kexc 300 nm for
n = 2, 330 nm for n = 3–4 and 365 nm for n = 5–9 for 10PMS-OPVn
polymers in THF solution).
d (ppm): 7.1–6.5 (aromatic region), 3.8–4.1 (Ar–PO–
O2–C2H4–C2H6), 2.95–3.12 (Ar–CH2–PO–), 1.08–1.15

2734
R. Madathil et al. / Tetrahedron Letters 47 (2006) 2731–2734
(Ar–PO–O2–C2H4–C2H6), 2.2–2.3 (CH3–Ar). Synthesis of
(m, 4H, PO–O2C2H4–), 3.14–3.22 (d, 2H, J = 21.9 Hz, Ar–
CH2–PO), 1.22–1.27 (t, 6H, J = 7.2 Hz, PO–O2C2H4–
C2H6).
hybrid polymer 10PMS-OPVC2: In a round bottom flask,
2 g of P2, and 2 g of 4-(diethoxymethyl)benzaldehyde
(ꢀ6 equiv) were dissolved in 50 mL of anhydrous THF.
Potassium-tert-butoxide (1.5 g) was added and the flask
was closed tightly and the reaction stirred at room
temperature for 24 h under a nitrogen atmosphere.8 The
reaction mixture was precipitated using methanol and
filtered. The product was dissolved in THF and reprecipi-
tated using methanol to ensure complete removal of
reagents. The product was then dissolved in 100 mL of
THF containing 5 mL of 1 M HCl and stirred for 2–3 min
to remove the aldehyde protection, precipitated using
methanol, filtered and dried to give a pale yellow semi-
crystalline powder (yield 95%). 1H NMR (400 MHz,
CDCl3) d (ppm): 9.7–9.94 (OHC–Ar), 6.2–7.3, 7.6–7.9,
(aromatic region), 2.2–2.3 (CH3–Ar).
Synthesis of (4-diethoxymethyl-benzyl)-phosphonic acid
diethyl ester (m3): In a round bottom flask, 20 g of 2,
23.1 g of triethyl orthoformate, 30–50 mL of ethanol and
400 mL of DCM were taken and stirred overnight at room
temperature.11 The reaction mixture was washed with cold
5% sodium hydroxide solution followed by water and the
organic layer was extracted into DCM, dried using MgSO4
and evaporated. Purification was conducted by running
column chromatography using hexane–ethyl acetate
(20 mL:80 mL) to obtain m3 as a viscous colourless liquid
(yield 95%). Elemental analysis: found—C = 57.89%,
H = 8.37% (calcd—C = 58.1%, H = 8.24%); Mass
found—331.1 (M+H+) (calcd—330.16); 1H NMR
(400 MHz, CDCl3) d (ppm): 7.42 (d, J = 7.6 Hz, 2H,
Ar), 7.31 (d, J = 7.6 Hz, 2H, Ar), 5.51 (s, 2H, Ar–CH–
O–), 4.00–4.11 (m, 4H, PO–O2C2H4–), 3.14 (d, 2H, J =
21.9 Hz, Ar–CH2–PO), 1.22–1.26 (m, 12H, PO–O2C2H4–
C2H6).
Synthesis of 4-(diethoxyphosphorylmethyl)benzylchloride
(1): a,a0-Dichloro-p-xylene (52.53 g, 0.3 mol) was taken in
a round bottom flask containing a magnetic stirring bar.
The flask was sealed using a rubber septum and immersed
in an oil bath and the temperature was gradually raised to
120 °C to melt the compound. To the melt, 25 g (0.15 mol)
of triethylphosphite was added slowly using a syringe over
a period of 1 h with stirring. Needles were attached to the
rubber septum before starting the addition of tri-
ethylphosphite, to avoid pressure building inside the flask.
After the addition, the reaction was continued for 30 min.
The progress of the reaction was monitored using thin
layer chromatography in ethyl acetate. The reaction
mixture was then cooled to room temperature to give a
solid product. Purification was conducted by column
chromatography using hexane–ethyl acetate (20
mL:80 mL) mixture to afford a viscous colourless liquid9
(50% yield). Elemental analysis: C = 51.9%, H = 6.4%,
Cl = 12.9% (calcd—C = 52.09%, H = 6.56%, Cl =
General synthetic procedure for 10PMS-OPVCn polymer
hybrids: In a round bottom flask, 1.0 g of 10PMS-OPVC2
and 2.0 g of m3 (ꢀ10 equiv) were dissolved in 50 mL of
anhydrous THF. Potassium-tert-butoxide (1.25 g) was
added and the reaction flask was closed tightly and
reaction mixture stirred at room temperature for 24 h
under a nitrogen atmosphere.8 The product was precipi-
tated using methanol and filtered. The product was
dissolved in THF and reprecipitated using methanol to
ensure complete removal of the reagents. The product was
then dissolved in 100 mL of THF containing 5 mL of
0.1 M HCl and stirred for 2–3 min to remove the aldehyde
protection, then precipitated using methanol, filtered and
dried to obtain 10PMS-OPVC3 as a yellow semi-crystal-
1
line product (yield 96%). H NMR (400 MHz, CDCl3) d
1
12.81%); Mass found—257 (M+H+) (calcd—256.09); H
(ppm): 6.0–7.9 (aromatic region), 9.7–9.94 (OHC–Ar).
The same experimental procedure was repeated step-wise
to generate all the 10PMS-OPVCn (n = 4–9) polymer
hybrids. The product obtained in one step was used as the
starting material for the next step to obtain step by step
OPV wiring.
NMR (400 MHz, CDCl3) d (ppm): 7.27–7.33 (m, 4H, Ar),
4.56 (s, 2H, Ar. CH2–Cl), 3.99–4.03 (m, 4H, PO–O2C2H4–),
3.14 (d, 2H, J = 21.9 Hz, Ar–CH2–PO), 1.24 (t, 6H,
J = 7.2 Hz, PO–O2C2H4–C2H6).
Synthesis of (4-formyl-benzyl)-phosphonic acid diethyl
ester (2): In a round bottom flask, 25 g (0.09 mol) of 1,
25.33 g (0.181 mol) of hexamethylene tetraamine and
50 mL of 50% acetic acid were taken and the flask fitted
with a reflux condenser. The mixture was refluxed under
stirring for 3 h at 100 °C and then 15 mL of HCl was
added and reflux continued for 15 min.10 The mixture was
then cooled to room temperature. The organic layer
was extracted into DCM, dried using MgSO4 and evap-
orated to give a liquid. Purification was conducted by
column chromatography using hexane–ethyl acetate
(20 mL:80 mL). Compound 2 (21 g, yield 90%) was
obtained as a viscous liquid. Elemental analysis: C =
55.5%, H = 6.6.l% (calcd—C = 56.25%, H = 6.69%);
Synthesis of 10PMS-OPVn polymer hybrids: In a general
procedure, 50 mg of 10PMS-OPVC2 and 200 mg of
diethyl-4-benzylphosphonate (high excess) were dissolved
in 10 mL of anhydrous THF. Potassium-tert-butoxide
(100 mg) was added and the flask was closed tightly and
the reaction stirred at room temperature for 24 h under an
N2 atmosphere. The product was then precipitated using
methanol and filtered.8 The product was dissolved in THF
and reprecipitated using methanol to ensure complete
removal of the reagents. The product was precipitated
using methanol, filtered and dried to give 10PMS-OPVC3
as a yellow semi-crystalline product (yield 96%). 1H NMR
(400 MHz, CDCl3) d (ppm): 6.0–7.9 (aromatic region),
9.7–9.94 (OHC–Ar). A similar procedure was used for
converting all 10PMS-OPVCn (n = 4–9) polymers to
generate 10PMS-OPVn (n = 4–9) polymer hybrids.
1
Mass found—277.14 (M+H+) (calcd—276.07); H NMR
(400 MHz, CDCl3) d (ppm): 9.97 (s, 1H, Ar–CHO), 7.79
(d, 2H, J = 7.6 Hz, Ar), 7.45–7.48 (m, 2H, Ar), 3.99–4.1