Stepwise synthesis of substituted oligo(phenylenevinylene) via an orthogonal approach

Full text of DOI:10.1021/ja9629928

844
J. Am. Chem. Soc. 1997, 119, 844-845
Scheme 1. Synthesis of OPV Molecules
Stepwise Synthesis of Substituted
Oligo(phenylenevinylene) via an Orthogonal
Approach
Todd Maddux, Wenjie Li, and Luping Yu*
Department of Chemistry and
The James Frank Institute, The UniVersity of Chicago
5725 South Ellis AVenue, Chicago, Illinois 60637
ReceiVed August 29, 1996
In this paper we report the synthesis of a series of all-trans-
substituted oligo(phenylene vinylene) (OPV) compounds. The
motivation of this work stems from our desire to synthesize a
well-defined conjugated block for use as a rigid element in a
diblock copolymer. Organic molecules which possess long
conjugated systems are of interest for their electro-optic proper-
ties.1 In particular, poly(phenylenevinylene) (PPV) has attracted
much attention as an emissive material for use in electrolumis-
cent devices.2 Incorporation of a conjugated block into a diblock
system will provide us with new forms of materials. These
diblock systems are expected to undergo microphase separation,
leading to new forms of self-assembled nanoscale materials
containing a conducting domain which will be interesting for
the study of the quantum confinement effect. OPV molecules
possessing well-defined conjugation lengths and structures may
also serve as model systems for understanding the relationships
between bulk material properties and molecular structures in
conducting polymers.
Scheme 1 shows our strategy in synthesizing these OPVs.
The open-ended OPV chain always has a handle for further
growth or other modification. The key to the overall strategy
is the synthesis of building block molecules, monomers 1 and
2, both of which possess mutually complementary functional
groups at their ends. Monomer 1 possesses an aldehyde at one
end and a vinyl group at the other end. Monomer 2 possesses
an iodo substituent at one end and a phosphonate ester group
at the other end. The iodo terminus of monomer 2 can couple
with the vinyl terminus of monomer 1 under the Heck reaction
conditions. Similarly, the aldehyde terminus of monomer 1 can
couple with the phosphonate terminus of monomer 2 via
Horner-Wadsworth-Emmons chemistry. The strategic place-
ment of the functional groups on monomers 1 and 2 allows for
sequential and alternating addition of these monomers to an OPV
chain as shown in Scheme 1. In order to control the growth of
the OPV chain, it is necessary to effectively block one terminus
of the growing OPV chain which was successfully accomplished
with compound 3. Growth of the OPV chain commences with
compound 3, and thereafter monomers 1 and 2 are added in a
repetitive stepwise fashion (Scheme 1). An alternative approach
to the rapid stepwise synthesis of OPV molecules is to react 2
equiv of monomer 1 with 1 equiv of 1,4-dibromobenzene. The
product would have a conjugation length including five aryl
rings and four double bonds, and in addition it would possess
two aldehyde termini capable of reacting with 2 equiv of
monomer 2. The process could be repeated to quickly build
up longer OPV molecules possessing functionalized termini.
Both monomers and compound 3 are conveniently prepared
from 4-iodo-2,5-dioctoxybenzaldehyde which is itself derived
from 1,4-dihydroxybenzene in three simple steps.
determined from 1H NMR (chemical shifts in parts per million).
The Heck reaction also generates a small amount of regioiso-
mers.5 The minor double bond isomer from the Horner-
Wadsworth-Emmons reaction and the Heck reaction could be
separated from the product by flash column chromatography
using a mixture of hexane and ethyl acetate as the mobile phase.5
Interestingly, in the next Horner-Wadsworth-Emmons reaction
to produce compound 9, no cis product was detected. Reduced
solubility of the longer oligomers poses a problem in the
purification of these compounds, but analytically pure samples
could be obtained through careful flash column chromatography
using mixtures of chloroform and hexane as the mobile phase.
Yields for each of the steps are good to moderate with decreased
solubility of the longer oligomers limiting the yields somewhat.6
Presently, we have succeeded in the synthesis and purification
of compound 10 which has 12 aryl rings, 11 double bonds in
its conjugation pathway, and a molecular weight of 2781.
Synthesis of longer oligomers is still in progress. All of the
spectroscopic studies and elemental analysis results are consis-
tent with the proposed molecular structures.7
The absorption and emission spectra for the series of OPVs
that possess an aldehyde end group are shown in Figure 1. All
of the oligomers show strong and broad absorption in the visible
region. Tripling the conjugation length from oligomer 4 to
oligomer 8 results in a red shift of 32 nm. However, little or
no red shift is noticed after the conjugation length reaches eight
aryl rings and seven double bonds. The saturation in λmax has
been observed previously and arises because of the limitations
to electron delocalization in the longer oligomers.8 Thus, the
effective conjugation length in this series of oligomers is reached
at compound 6. The absorption wavelength maximum in the
oligomers converges to that of PPV over relatively short
The Horner-Wadsworth-Emmons reaction did give a de-
tectable amount of the cis product (approximately 5%) as
(5) (a) In a model reaction, the 1,1-regioisomer in the Heck reaction
was separated in less than 4% yield: Peng, Z. H.; Yu, L. P., Paper in
preparation. Also see: (b) Cabri, W.; Candiani, I; Bedeschi,A.; Santi, R. J.
Org. Chem. 1992, 57, 3558. (c) Cabri, W.; Candiani, I; Bedeschi,A.; Santi,
R. Synlett 1992, 871.
(6) The yields for each coupling step: 6, 77%; 8, 84%; 10, 59%; 7,
84%; 9, 73%.
(7) (10) Anal. Calcd for C192H266O13: C, 82.89; H, 9.58. Found: C, 82.63;
H, 9.58. (9) Anal. Calcd for C159H224O10I1: C, 78.93; H, 9.20; I, 5.25.
Found: C, 79.06; H, 9.20; I, 5.42.
* Address correspondence to this author at the Department of Chemistry.
(1) Skotheim, T. A., Ed. Handbook of ConductiVe Polymers; Marcel
Dekker; Basel, 1989.
(2) (a) Burroughs, J. H.; Bradley, D. D.; Brown, A. R.; Marks, R. N.;
Mackey, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347,
539. (b) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.;
Holmes, A. B. Nature 1993, 365, 628.
(3) Heck, R. F. Org. React. 1982, 27, 345.
(4) Boutagy, J.; Thomas, R. Chem. ReV. 1974, 74, 87.
S0002-7863(96)02992-7 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 4, 1997 845
Figure 1. Absorption and emission spectra for compounds 6, 8, and
10 taken in dilute solutions of chloroform.
Table 1. Thermal Transition Temperatures and Florescent
Quantum Yield of OPVs
Figure 2. Photograph of liquid crystal texture of compound 8 taken
between crossed polarizers (magnification 200× (reproduced at 70%
of original size), temperature 138 °C).
6
7
8
9
10
Tm (°C)
Tc (°C)a
Φfl (%)b
97
67
87
87
90
158
81
113
176
89
105
185
82
The phase texture shows strong patterns characteristic of a
nematic phase with a disclineation of 1.
75
431
λ
max (nm)
441
457
460
463
It should be pointed out that various syntheses of different
OPVs have been reported in the past.11-13 Schenk et al. have
investigated the charging capacity of OPV molecules and the
charge distribution within the OPV chain.11 Katz et al. have
studied the fluorescence dynamics of OPVs both in solution
and the solid state and observed little difference between them.12
The work presented in this paper is distinct from others in that
our approach is open ended and very versatile, and it enables
us to synthesize longer oligomers that more closely resemble
organic polymeric conducting materials. Due to the fact that
the Heck and Horner-Wadsworth-Emmons reactions tolerate
a wide variety of functional groups, it should be possible to
synthesize many different OPV molecules of various lengths
and structures. This is useful for fine-tuning the band gap in
emissive organic materials. Also noteworthy is the use of two
reaction types to construct the same functionality; this eliminates
the need of protecting groups. The use of a stilbene analog as
the building blocks allows for efficient and fast construction of
the OPV chain. After each step a functional group (either an
aldehyde or iodo group) is left for further chemical manipulation
which may include either continued elongation or reaction with
an end-functionalized polymer to form novel diblock copoly-
mers. Controlled bidirectional growth is also possible, enabling
a very rapid construction of OPV compounds possessing
functional groups at both ends.
In summary, a general strategy for the stepwise synthesis of
long conjugated molecules has been demonstrated. The strategy
employs the use of two reaction types to grow the conjugated
chain stepwise, eliminating the need for protecting group
chemistry. A similar strategy has recently been reported for
the synthesis of dendritic molecules.14 The molecules reported
here showed high fluorescence quantum yields and a liquid
crystalline phase. We intend to use these molecules in the
production of novel diblock copolymers.
Acknowledgment. This work was supported by the National
Science Foundation and Air Force Office of Scientific Research.
Support from the National Science Foundation Young Investigator
program is gratefully acknowledged. This work also benefited from
the support of the NSF MRSEC program at the University of Chicago.
a Clearing temperature from the liquid crystalline phase to the
isotropic phase. b Excited at λmax
.
conjugation lengths which suggests the validity of using OPV
to better understand the electronic properties of electroactive
PPV materials.
Solutions of these oligomers give a brilliant green fluores-
cence (Figure 1). With the exception of oligomer 6, each of
the oligomers exhibits three main features in their fluorescence
spectrum in accord with theory.9 There is a major band with a
shoulder to the red of moderate intensity which is itself
overlapped with a barely discernible weak band to the red. The
emission spectrum of compound 6 is very broad and shows none
of the fine structure present in the other spectra. The two main
bands, discernible in other compounds, merged together in the
fluorescence spectrum of compound 6 into a broad band with
relatively higher intensity than the bands in each of the other
oligomers. At present, it is not clear why this should be the
case, but it does appear to follow the general trend that in
oligomers possessing aldehyde end groups the intensity of the
second band grows as the conjugation length decreases. The
fluorescence quantum yields of each of the oligomers are shown
in Table 1. No clear trend exists in the quantum yields, but it
is interesting to note that the yields are substantially higher than
the yields normally reported in PPV, suggesting that defect sites
and impurities act to reduce fluorescence quantum yields in
conjugated polymers. No change in any of the emission spectra
was observed when fluorescence measurements were made over
a wide range of concentrations, suggesting that excimer forma-
tion does not occur.
This series of all-trans-OPVs are rigid rod molecules, and
thus it is not too surprising that each of the oligomers, except
compound 6, manifests a reversible thermotropic liquid crystal-
line phase.10 The melting temperature (Tm) and clearing
temperature (Tc) for each oligomer are shown in Table 1. These
observations were made using polarized light microscopy to
detect the mesoscopic state. The melting temperature and
clearing temperature show a rough correlation with molecular
weight. The temperature range over which the liquid crystalline
phase persists becomes broader with increasing conjugation
length and the melting temperature becomes higher as the length
increases in a series with the same end group. Figure 2 shows
a micrograph of the liquid crystalline texture of compound 6.
Supporting Information Available: Experimental details and NMR
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JA9629928
(8) Graham, S. C.; Bradley, D. D. C.; Friend, R. H.; Spangler, C. Synth.
Met. 1991, 41.
(11) Schenk, R.; Gregorious, H.; Meerholz, K.; Heinze, J.; Mullen, K.
J. Am. Chem. Soc. 1991, 113, 2634.
(9) Gartstein, Y. N.; Rice, M. J.; Conwell, E. M. Phys. ReV. B 1995, 52,
(12) Katz, H. E.; Bent, S. F.; Wilson, W. Z.; Schilling, M. L.; Ungashe,
S. B. J. Am. Chem. Soc. 1994, 116, 6631.
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(10) (a) Bao, Z. N.; Cai, R. B.; Yu, L. P. Macromolecules 1993 26, 5281.
(b) Memeger, W. J. Macromolecules 1989, 22, 1577.
(13) Tour, J. M. Chem. ReV. 1996, 96, 537.
(14) Zeng, F.; Zimmerman, S. C. J. Am. Chem. Soc. 1996, 118, 5326.