An iterative approach toward the synthesis of discrete oligomeric p-phenylene vinylene organic dyes employing aqueous Wittig chemistry

Article abstract of DOI:10.1016/j.tetlet.2011.08.040

Photovoltaic research has become increasingly prominent as the search for alternatives to fossil fuels are actively sought. A novel process for the iterative synthesis of discrete donor/acceptor-flanked oligostilbenes, key constituents in dye-sensitized solar cells, is described. The aqueous Wittig process is high yielding, proceeds with high (E)-stereoselectivity and allows facile product purification.

Full text of DOI:10.1016/j.tetlet.2011.08.040

Tetrahedron Letters 52 (2011) 5467–5470
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Tetrahedron Letters
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An iterative approach toward the synthesis of discrete oligomeric p-phenylene
vinylene organic dyes employing aqueous Wittig chemistry
⇑
James McNulty , David McLeod
Department of Chemistry & Chemical Biology, McMaster University, 1280 Main St. West, Hamilton, Ontario, Canada L8S 4M1
a r t i c l e i n f o
a b s t r a c t
Article history:
Photovoltaic research has become increasingly prominent as the search for alternatives to fossil fuels are
actively sought. A novel process for the iterative synthesis of discrete donor/acceptor-flanked oligostilb-
enes, key constituents in dye-sensitized solar cells, is described. The aqueous Wittig process is high
yielding, proceeds with high (E)-stereoselectivity and allows facile product purification.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 4 July 2011
Revised 4 August 2011
Accepted 7 August 2011
Available online 16 August 2011
Keywords:
Dye-sensitized solar cell
Aqeuous Wittig
Triphenylamine
Organic dye
Since the discovery of the photoelectric effect1 by Becquerel the
field of solar capture technology has advanced and accelerated
exponentially over the last decade. In a landmark paper by Grätzel
and co-workers, the first dye sensitized solar cell (DSC) based on a
Ruthenium complex dye2 was reported. A number of Ru(II)
complexes have since been reported, obtaining power conversion
efficiencies above 10% under standard solar illumination.3 Over
the last decade the synthesis of organic dyes as sensitizers in DSCs
has become a dynamic and productive area of research and been
the subject of many recent reviews.4 Organic dyes have shown
impressive photovoltaic performance with efficiencies of 7–
10%.4a,5 Organic chromophores have the advantage of higher molar
extinction coefficients and lower production costs in comparison
to metal-based sensitizers. The synthesis of organic dyes, based
on a wealth of existing synthetic materials (dyes, xerographic
and organic light emitting diode (OLED) materials), has allowed a
diverse array of organic-based chromophores to be investigated
in DSCs. This now includes dyes containing coumarin, indoline, tet-
rahydroquinoline and triarylamine chromophores. Invariably,
these organic dyes possess donor/acceptor moieties linked via an
with heteroatom variations have been advanced by Sun and co-
workers leading to the development of more efficient (up to
7.2%) DSCs.5i–k Oligomeric p-phenylene vinylenes have proven
beneficial bridge structures as they exhibit high luminescent
efficiency, and good stability.6 While the synthesis of oligomeric
p-phenylene vinylene-based organic dyes has been an active area
of research in polymer-based OLEDs,5l the synthesis of discrete
materials with precise control on the length of the conjugated-
p
bridge has become very important in the DSC arena. In recent
work, Ko and co-workers reported a fascinating series of ‘nested’
donor acceptor flanked p-phenylene vinylene sensitizers and dem-
onstrated the efficiency to be a function of the length of the
discrete (E)-stilbene bridge (Fig. 1, 2).5m,n
Although numerous approaches toward the synthesis of
discrete p-phenylene vinylenes exist, in general, these approaches
are lengthy and suffer in terms of overall efficiency. The standard
synthetic routes toward the preparation of p-phenylene vinylenes
employ either transition metal based cross-coupling methods,7 or
Wittig and related olefination methods.8 Both methods provide
moderate yields of (42–86%) with varying (E):(Z) selectivities and
suffer other drawbacks. Transition metal cross couplings require
expensive catalysts, harsh conditions, and large amounts of organic
solvent for purification. The more traditional ylide/carbanion
routes require deprotonation at cryogenic temperatures under
inert atmospheres and are intolerable of acidic protons. Subse-
quent removal of the phosphine oxide side-product is also notori-
ously difficult in Wittig reaction processing. In recent work from
our laboratory, we showed that aqueous Wittig olefination
employing short chain trialkylbenzyl phosphonium salts proceeds
with high yield and high (E)-olefin stereoselectivity9,10a in stilbene
extended p-conjugated ‘bridge’ termed D-p-A systems. The donor
group is normally a nitrogen-containing amine/aniline/triaryl-
amine etc, while the acceptor group is frequently a 2-cyanoacrylic
acid. Triarylamine dyes5 based on this architecture were first
reported by Yanagida and co-workers where DSCs containing a sin-
gle p-phenylene vinylene bridge were shown to exhibit power con-
version efficiencies up to 5.3–6.6% (Fig. 1, 1).5a Other triarylamines
⇑
Corresponding author.
E-mail address: jmcnult@mcmaster.ca (J. McNulty).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.040

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J. McNulty, D. McLeod / Tetrahedron Letters 52 (2011) 5467–5470
O
O
HO
HO
CN
CN
Ph
N
Ar
n
15a
25n
Ph
N
Ar
Figure 1. Core architecture of oligoene 1 and p-phehylene vinylene 2 D-p-A organic sensitizers for DSCs.
phine hydrobromide. These salts were dissolved in water in the
presence of LiOH and aldehyde 5 and upon warming, underwent
smooth olefination to yield the extended stilbenes 4a–e of exclu-
sive (E)-configuration. The alcohols 4a and 4b were subsequently
converted to the corresponding phosphonium salts and, again,
underwent olefination with aromatic aldehydes to yield the
dimeric p-phenylene vinylenes without incident (see Supplemen-
tary data). These results proved aldehyde 5 to be an ideal extender
unit suitable for the elaboration of oligostilbenes under our
aqueous Wittig conditions. Removal of the water soluble
phosphine oxide was not problematic in any of the reactions inves-
tigated. The stilbenes precipitate upon cooling of the reaction
mixture and are separated cleanly by suction filtration.
1) Et3P-HBr
OH
OH
n
4
2)
3
CHO
5
HO
water,LiOH
Scheme 1. Proposed iterative route toward discrete p-phenyl vinylene oligomers.
synthesis, in contrast to the reaction with triphenylphosphine
derived reagents. Additionally, triethylphosphine oxide (and other
short-chain phosphine oxides) is completely water soluble,
enabling rapid product purification. We also showed that trialkylb-
enzylphosphonium salts were available directly from the reaction
of benzyl alcohols with the air stable salt triethylphosphine hydro-
bromide. This ‘off-the-shelf’ type process circumvents the direct
use of both triethylphosphane and benzylic halides.10 On the basis
of these results, we considered the development of a discrete, iter-
ative route toward the synthesis of oligostilbenes using the readily
available11 bifunctional aldehyde/alcohol 5 as a strategic synthon,
the overall chemistry is outlined in Scheme 1. Conversion of a suit-
ably functionalized benzylic alcohol 3 directly to the trialkylbenzyl
phosphonium salt (step 1) would occur as described,10 aqueous
Wittig olefination using the bifunctional aldehyde would yield
the (E)-stilbene 4 (step 2), functionalized with the terminal ben-
zylic alcohol. Such a process was therefore expected to be iterative
allowing for the synthesis of oligomeric p-phenyl vinylenes. On the
basis of our prior aqueous Wittig studies, we were also confident
that the process would not require the use of protecting groups.
In this Letter we report the development of this general process
and a direct application demonstrating the synthesis of a triaryl-
We next desired to extend these feasibility studies to prepare an
oligomeric D-p-A type system using this methodology. Due to the
reactivity of the acrylic acid functionality, the most logical route
appeared to be to start at the triarylamine terminus and build
the oligostilbene toward the cyanoacrylate end, with Knoevenagel
capping as the last step in the process. The process developed is
outlined in Scheme 3. Thus, diphenylamine 6 was arylated with
iodobenzene using the Ullman-type coupling developed by Wolf
and Xu,12 forming triphenylamine 8 in good yield. Vilsmaier–
Haack formylation of 8 gave the mono-aldehyde 9 selectively,
which was reduced yielding the triarylamine-functionalized
benzylic alcohol ‘starter unit’ 10. Compound 10 was now subject
to our iterative olefination process described above. Conversion
to the triethyl phosphonium salt 11 proceeded without incident.
Compound 11 underwent aqueous Wittig reaction with the bifunc-
tional ‘extender’ aldehyde 5 to give the functionalized stilbene 12,
completing the first iterative step. The iterative process was now
continued through conversion of 12 to its corresponding triethyl-
phosphonium salt and aqueous Wittig olefination with another
equivalent of aldehyde 5 to yield the oligomeric stilbene 13,
successfully completing the second iterative cycle. Both olefins
were introduced with complete (E)-configurational control.
amine/cyanoacrylate flanked stilbene (D-p-A system).
We initiated study on the feasibility of the aqueous Wittig
reaction using unprotected aldehyde 5 with a panel of functional-
ized benzyl alcohols 3a–e, Scheme 2. The substituents were chosen
to gauge the compatibility of the process in the presence of various
halogens and an alkyl group. All benzylic alcohols underwent
conversion to the necessary phosphonium salts using triethylphos-
Having demonstrated the successful double iterative aqueous
Wittig process with 5, we now investigated the terminal capping
step. The benzylic alcohol in 13 underwent Swern oxidation
yielding the aromatic aldehyde 14 which was subjected to a
1)
P
X
H
R1
CH2OH
R1
No Solvent
OH
°
100 C, 8 h
Ar
2) H2O, Base, t, T
CH2OH
R2
3a
R1
R2
4a
R1=H, R2=H
R1=H, R2=H
4b R1=F, R2=H
4c R1=Br, R2=H
3b R1=F, R2=H
3c R1=Br, R2=H
5
OHC
R2
4d
R1=H, R2=Cl
3d
R1=H, R2=Cl
4e R1=H, R2=CH3
3e R1=H, R2=CH3
Scheme 2. Use of bifunctional aldehyde 5 as an extender unit in p-phenylene vinylene synthesis.

J. McNulty, D. McLeod / Tetrahedron Letters 52 (2011) 5467–5470
5469
CHO
I
CH2OH
Ph
a
b
c
NH
Ph
N
+
N
N
8
9
10
6
7
d
OH
Cl
OH
PEt3
e
f
N
N
N
11
12
13
g
CN
COOH
O
h
N
N
14
15
Scheme 3. Reagents and conditions: (a) NaOtBu, Cu2O, NMP, 90 °C, 24 h, 81%; (b) POCl3, DMF, 35 °C, 24 h, 88%; (c) NaBH4, DCM, EtOH, rt, 2 h, 95%; (d) PEt3ÁHCl, 100 °C, 8 h,
95%; (e) LiOH, 5, H2O, 80 °C, 4 h, 96%; (f) (i) PEt3ÁHCl, 100 °C, 8 h (ii) LiOH, 5, H2O, 80 °C, 4 h, 93%; (g) oxalyl chloride, DMSO, TEA, DCM, À78 °C, 2 h, 81%; (h) cyanoacetic acid,
piperidine, CH3CN, reflux, 2.5 h, 89%.
Knoevenagel condenstaion reaction with cyanoacetic acid to yield
the desired D- -A type oligostilbene (or p-phenylene vinylene) 15.
Supplementary data
p
The UV–vis spectrum of compound 13 showed broad absorption
with two intense kmax at 303 and 378 nM. The UV–vis spectrum
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.08.040.
of the D-p-A flanked stilbene 15 similarly exhibited broad absorp-
tion, however the kmax values were red-shifted to 308 and 384 nM,
respectively, and was similar to literature data for analogous
compounds.13 In addition, the extinction coefficients of 15 were
significantly increased, relative to 13, in extending to the full
donor/acceptor flanked oligomer, as expected.
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We thank the NSERC, Cytec Canada Inc. and McMaster Univer-
sity for financial support of this work.

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