One-pot synthesis and study of spectroscopic properties of oligo(phenylenevinylene)s

Article abstract of DOI:10.1016/j.tetlet.2015.10.033

Two series of OPVs (oligo(phenylenevinylene)), that is, ((1E,1′E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dibenzene derivatives and 4-((E)-4-((E)-styryl)styryl)pyridine derivatives with different functional groups of varying electronic properties have been synthesized by one-pot Wittig-Heck methodology. The synthesized derivatives have been studied for their optical properties. Amongst them the ((1E,1′E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dibenzene derivatives with appropriate changes in the end group showed a significant impact on the UV absorption and emission spectra. Particularly NO₂-OPV showed distinct solvatochromism in the wavelength range of 218 nm in different solvents. Whereas 4-((E)-4-((E)-styryl)styryl)pyridine derivatives showed clear acidochromism which can be detected visually as well as spectroscopically.

Full text of DOI:10.1016/j.tetlet.2015.10.033

Tetrahedron Letters 56 (2015) 6617–6621
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
One-pot synthesis and study of spectroscopic properties of oligo
(phenylenevinylene)s
⇑
Krupa N. Patel, Ashutosh V. Bedekar
Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara 390 002, India
a r t i c l e i n f o
a b s t r a c t
Two series of OPVs (oligo(phenylenevinylene)), that is, ((1E,10E)-(2,5-dimethoxy-1,4-phenylene)bis
(ethene-2,1-diyl))dibenzene derivatives and 4-((E)-4-((E)-styryl)styryl)pyridine derivatives with differ-
ent functional groups of varying electronic properties have been synthesized by one-pot Wittig–Heck
methodology. The synthesized derivatives have been studied for their optical properties. Amongst them
the ((1E,10E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dibenzene derivatives with appropriate
changes in the end group showed a significant impact on the UV absorption and emission spectra.
Particularly NO2–OPV showed distinct solvatochromism in the wavelength range of 218 nm in different
solvents. Whereas 4-((E)-4-((E)-styryl)styryl)pyridine derivatives showed clear acidochromism which
can be detected visually as well as spectroscopically.
Article history:
Received 11 September 2015
Revised 6 October 2015
Accepted 8 October 2015
Available online 9 October 2015
Keywords:
One-pot synthesis
Oligo(phenylenevinylene)s
Wittig–Heck reaction
Solvatochromism
Acidochromism
Ó 2015 Elsevier Ltd. All rights reserved.
A variety of conjugated molecules have been studied for their
wide range of applications in different fields. Molecules with
Owing to the importance of this class of compounds, many
methods are known for the synthesis of oligo(phenylene-viny-
lene)s. Generally the conjugation in the molecule is built by
condensation reactions namely Wittig, Knoevenagel, Perkin etc.,
or by metal mediated coupling reactions such as Mizoroki–Heck
or Sonogashira reactions.
p-conjugation are capable of allowing the mobility of electrons
through continuous delocalization due to their structural and
orbital arrangements.1 Amongst the conjugated molecules, oligo
(phenylenevinylene) (OPVs), representing the model class of com-
pounds of poly(phenylenevinylene) (PPV), have been extensively
studied due to their high stability and good luminescent efficiency.
One-pot synthesis is referred to a protocol, comprising of sev-
eral simultaneous synthetic steps. More than often the combina-
tion of these operations results in lesser consumption of reagents
required for reactions and work-up. This technique can effectively
reduce reaction time, may avoid purification of unstable, toxic, or
volatile intermediates, and generally support concept of green syn-
thesis. Recently several procedures have been developed for mak-
ing useful molecules and intermediates by adopting one-pot or
domino or tandem synthetic schemes.21 As part of our interest in
the synthesis of conjugated molecules from readily available mate-
rials we have developed one-pot dehydrohalogenation–Heck
or Wittig–Heck,22 Wittig–Suzuki,23 and oxidation-Wittig–Heck24
reaction protocols. In this Letter we further present one-pot Wit-
tig–Heck methodology to synthesize ((1E,10E)-(2,5-dimethoxy-
1,4-phenylene)bis(ethene-2,1-diyl))dibenzene derivatives and
4-((E)-4-((E)-styryl)styryl)pyridine derivatives (OPVs) where dif-
ferent functional group of varying electronic properties can be
easily introduced. The OPVs which mainly differ in the end group
can exhibit considerably different physical and optical properties.
Amongst ((1E,10E)-(2,5-dimethoxy-1,4-phenylene)bis-(ethene-
2,1-diyl))dibenzene derivatives we have synthesized seven com-
pounds, OPV 1–7, by the given one-pot procedure. The synthesis
These
p-conjugated molecules are investigated at length for their
absorption and fluorescence behaviour, and can be tuned by chem-
ical functionalization and external control (e.g., solvent, tempera-
ture, pH). Further modulation of the optoelectronic properties of
OPVs is possible by introducing different substituents which are
capable of other interactions such as hydrogen bonding and
p–p
stacking leading to applications in various fields. The optical and
electronic properties of these molecules strongly depend on
structural features and hence can be modulated by variations in
conjugation length2–4 and donor–acceptor strengths.2a,4b,5 These
molecular systems have gained great interest owing to their
potential application in fields, such as organic light emitting
diodes (OLED),6,7 field effect transistors (FETs),8,9 nonlinear optical
devices,10,11 solar cells12,13 and chemical sensors.14–18 The oligo
(phenylenevinylene) (OPV)-based materials have received a great
deal of attention, due to their specific photophysical properties
and high quantum yields.19,20
⇑
Corresponding author. Tel.: +91 0265 2795552.
E-mail address: avbedekar@yahoo.co.in (A.V. Bedekar).
http://dx.doi.org/10.1016/j.tetlet.2015.10.033
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

6618
K. N. Patel, A. V. Bedekar / Tetrahedron Letters 56 (2015) 6617–6621
R
OMe
OMe
OMe
K2CO3
Pd(OAc)2
OHC
I
CH3PPh3I
dppp
DMA
R
I
2
1a-1g
OMe
3
R
140oC, 24h
OPV1, R = H, 82%
OPV2, R = CH3, 68%
OPV3, R = Cl, 85%
OPV4, R = Br, 28%
OPV5, R = NO2, 75%
OPV6, R = N(CH3)2, 45%
OPV7, R = dioxole, 47%
One-pot Wittig-Heck reaction
Scheme 1. One-pot Wittig–Heck synthetic methodology for the preparation of OPV 1–7.
N
BrPh3P
2
OMe
OMe
OMe
K2CO3
Br
CHO
4
Pd(OAc)2
dppp,DMA
140oC, 24h
OHC
OPV 8, 79%
OMe
2
N
N
5
6
Scheme 2. One-pot Wittig–Heck synthetic methodology for the preparation of OPV 8.
in situ Wittig reaction with 2,5-dimethoxyterephthalaldehyde 5
and Mizoroki–Heck reaction with 4-vinyl pyridine 6 giving the
final product (OPV 8) (Scheme 2).
On the other hand OPVs 9, 10 and 11 were synthesized from 4-
bromobenzyltriphenylphosphonium bromide 4. This undergoes
in situ Wittig reaction with substituted aldehydes (1a, 1e, 7) fol-
lowed by one-pot Mizoroki–Heck reaction with vinyl pyridine 6
furnishing pyridine terminated OPVs (Scheme 3).
PPh3Br
OHC
R
K2CO3
Br
Pd(OAc)2
dppp,DMA
140oC, 24h
4
N
N
R
1a, R=H
1e, R=NO2
7, R=OMe
OPV9, R=H, Y = 85%
OPV10, R=OCH3, Y = 64%
OPV11, R=NO2, Y = 96%
6
Photophysical properties of OPV 1–7
Scheme 3. One-pot Wittig–Heck methodology for the synthesis of OPV 9–11.
Initially we studied OPVs 1–7 by UV–visible and florescence
spectroscopy as they are structurally similar to two methoxy sub-
stituents at 2 and 5 positions of the central ring. The only difference
is the terminal functional group which shows distinct variation in
its optical properties. It is established that OPVs having Z–Z, Z–E
and E–E stereochemistry have distinctly different optical and phys-
ical properties.25 We have determined that all the OPVs in our study
are of E–E isomers as established by 1H NMR analysis. We have
studied the UV–Visible absorbance and florescence emission of
begins with the Wittig reaction of 4-substituted benzaldehydes
1a–1g with methyltriphenylphosphonium iodide
2 resulting
in situ generation of 4-substituted styrene which will eventually
undergo Mizoroki–Heck reaction with 1,4-diiodo-2,5-dimethoxy-
benzene 3 in the same pot to give the final products (OPV 1–7)
(Scheme 1).
In the similar manner OPV 8 was synthesized using 4-bromo
benzyltriphenylphosphonium bromide
4 which can undergo
Figure 1. (a) UV–Visible spectra for OPV 1–7. (b) PL spectra for OPV 1–7.

K. N. Patel, A. V. Bedekar / Tetrahedron Letters 56 (2015) 6617–6621
6619
Figure 2. (a) UV–Visible and (b) PL spectra of NO2–OPV in different solvents.
Figure 3. Emission spectra for OPVs (a) 8, (b) 9, (c) 10 and (d) 11.
solution of OPV 1–7 in toluene and the observed two characteristic
broad absorption peaks for all the compounds attribute to different
electronic transitions (Fig. 1). All the compounds showed higher
energy band around 330 nm, whereas it shifted to 350 nm in OPV
5 with electronegative nitro end group. Another absorption band,
that is, lower energy band varied from around 390 nm for most
OPVs. It was seen at 420 nm for OPV 6 with the presence of -N
(CH3)2 end group and 431 nm for the OPV 5 with –NO2 moiety.
Hence it can be concluded that with only the end group modifica-
tion of OPVs showed the kmax over the range of 41 nm. The emission
spectra for all these compounds in toluene also showed distinct
variation with the change in the end group functionality. These
changes result due to dipole–dipole interactions between solvent
and OPVs as well as due to aggregation.26 It was observed that all

6620
K. N. Patel, A. V. Bedekar / Tetrahedron Letters 56 (2015) 6617–6621
Table 1
Absorption and emission values for pyridine terminated OPVs
OPV
kmax (nm)
kmax after protonation (nm)
kem (nm)
kem after protonation (nm)
Shift in emission peak (nm)
8
9
10
11
352, 408
318, 332
364
427
368
417
376
480
421
455
508
618
496
604
517
138
75
149
9
316, 366
the OPVs except the one with –NO2 end group showed vibronic
features (due to distinct transitions associated with a typical C–C
stretching motion strongly coupled to the electronic system) sug-
gesting the dominance of single molecule species. Also the absence
of such vibronic features in OPVs containing –NO2 indicate to the
presence of highly aggregated species in solution.26
new absorption band appears as a broad peak at 427 nm. Further
in order to study the substituent effect we have systematically
studied OPV 9–11 with electron releasing and electron withdrawing
substituents. OPV 9 (R = H) showed absorption at 318 and 332 nm
which after protonation shifted to 368 nm. Also the OPV 10
with –OMe functional group absorbs at 364 nm whereas OPV 11
with chromophoric –NO2 group shows absorption peaks at 316
and 366 nm. Upon protonation the absorption peak of OPV 10
shifts from 364 to 417 nm, such high bathochromic shift can be
attributed to the presence of electron releasing –OMe group which
can stabilize the pyridyl nitrogen after protonation. In case of 11
(R = NO2) protonation leads to a smaller bathochromic shift,
that is, from 366 to 376 nm due to the presence of electron
withdrawing –NO2 group.
The emission spectra of all the pyridine terminated OPVs
showed emission peak in the visible region of electromagnetic
spectrum and showed a bathochromic shift after protonation.
The OPV 8 shows a clear emission band at 480 nm which quickly
disappears on protonation with methanolic HCl and a new band
appears at 618 nm. Such a large bathochromic shift can be
Further spectral examination indicates that the kem for OPV 1–7
varied in the range of 64 nm, that is, from 442 nm to 506 nm. The
Stoke shifts for most OPVs were around 52 nm whereas the NO2–
OPV showed distinctly high kem at 506 nm with the Stoke shift of
75 nm. Another important observation about the NO2–OPV was
seen in its solvatochromism. The sample of NO2–OPV dissolved
in different solvents such as toluene, chloroform, tetrahydrofuran,
dichlorobenzene, acetonitrile, dimethylsulfoxide were evaluated to
measure the absorption as well as emission of these solutions. The
absorption peak varied over a range of 42 nm, between 390 and
432 nm for different solvents. But in case of emission the role of
solvent was prominent as the emission ranged spanned over
218 nm, that is, from 432 nm in methanol to 650 nm in acetonitrile
and DMSO, see Figure 2. Such strong solvatochromism can be
attributed to combination of electrostatic OPV–solvent interac-
tions, intramolecular charge transfer and aggregate formation in
solution.26 Solvent stabilization of intramolecular charge transfer
in the excited state also leads to such high solvatochromism.
attributed to the long conjugation of
p-bond across the pyridine
nitrogen and the electron donating –OMe group. The OPV 9, 10
and 11 show an emission band at 421, 455 and 508 nm,
respectively, and after protonation new band appears at 496, 604
and 517 nm, respectively.
It is observed that there was 75 nm bathochromic shift in OPV 9
whereas 149 nm bathochromic shift was observed in the case of
OPV 10 with –OMe substitution where the positive charge of pro-
tonated pyridine will be stabilized by the push–pull mechanism.
On the other hand the OPV 11 showed only 9 nm shift due to the
presence of electron withdrawing –NO2 group. Florescence spectra
for OPV 9, 10 and 11 are shown in Figure 3. The absorption and
emission wavelengths are summarized in Table 1.
Hence, in this Letter we have presented the synthesis of oligo
(phenylenevinylene)s by one-pot Wittig–Heck methodology in
good yields.27 The OPVs with different substituents showed vary-
ing optical properties depending on the end group substituents.
The OPV–NO2 showed solvatochromism with the wavelength
range of 218 nm in different solvents. The pyridine terminated
OPVs showed acidochromism which was studied spectroscopically.
Photophysical properties of OPV 8–11
The presence of terminal pyridine group in OPV 8–11 was
explored using its pyridyl ring that can prove to be an active site
for the protonation and hence can drastically alter the absorption
and emission of the compound. The pyridine terminated OPVs 8–
11 were spectroscopically studied for their protonation behaviour.
All the pyridine terminated compounds were readily soluble in
organic solvents. They were highly fluorescent in diluted solutions
and display a spectral sensitivity to pH in absorption spectra as
well as in fluorescence spectra at the low concentration range of
0.01 mM. All the compounds exhibit highly pH-dependent absorp-
tion and emission. The absorption spectrum of pyridine terminated
OPVs showed a strong absorption in the range of 300–400 nm. The
absorption near 300 nm can be assigned to p–
p⁄ electronic transi-
tion while the absorption band near 400 nm can be due to n–p⁄
transition. All the pyridine terminated OPVs showed a clear bath-
ochromic shift on addition of HCl. This acid-induced change is
clearly detected visually (blue to orange) as well as studied by
measuring its absorbance and emission in methanol by addition
of methanolic HCl solution. The absorption and emission change
is reversible, as the blue emission of free sample can be fully
restored on the addition of ammonia. The pH-dependent change
can be readily attributed to the protonation–deprotonation process
of the terminal pyridine groups, which allows the reversible
inter-conversion between the cationic and neutral forms of OPVs.
The quinonoid form formed due to the protonation of pyridine
nitrogen, absorbs at higher wavelength as compared to the ben-
zenoid form. The changes of these compounds occur in the visible
range, making it suitable for applications as visually sensing mate-
rial. Solution of OPV 8 shows absorption at 352 and 408 nm which
is red shifted on acidification with dilute methanolic HCl and the
Acknowledgment
We thank Council of Scientific and Industrial Research (CSIR),
New Delhi, India for a research fellowship to K.N.P.
Supplementary data
Supplementary data (characterization data and copies of the
spectra) associated with this article can be found, in the online ver-
sion, at http://dx.doi.org/10.1016/j.tetlet.2015.10.033.
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eluent to give pure product as pale yellow solid (0.144 g, 82.3%). Melting point:
176 °C (Lit. 28 177–178 °C).
1H NMR (CDCl3, 400 MHz)
d 7.58 (d, J = 7.2 Hz, 2H, aromatic protons of
terminal benzene ring), 7.52 (d, J = 16.4 Hz, 1H, E olefinic protons), 7.37–7.41
(m, aromatic protons of terminal benzene rings), 7.26–7.30 (m, aromatic
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Synthesis of 4,40-((1E,10E)-(((1E,10E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-
2,1-diyl))bis(4,1-phenylene))bis(ethene-2,1-diyl))dipyridine OPV 8
A two neck round bottom flask was charged with 2,5-dimethoxyterephtha-
laldehyde (0.150 g, 0.773 mmol), 4-bromo benzyl triphenylphosphine bromide
(0.792 g, 1.546 mmol), 4-vinyl pyridine (0.178 g, 1.546 mmol), K2CO3 (0.854 g,
6.180 mmol), palladium acetate (0.0034 g, 0.0154 mmol), dppp (0.0095 g,
0.023 mmol), TBAB (0.049 g, 0.154 mmol) in N,N-dimethyl acetamide (10 mL)
and was stirred at 140oC under N2 atmosphere for 24 h. The cooled mixture
was then poured in water (100 mL) and extracted with ethyl acetate
(3 ꢀ 50 mL). The organic layer was washed with water (2 ꢀ 20 mL) and dried
over anhydrous sodium sulfate. The solution was concentrated under reduced
pressure to obtain a viscous liquid, which was purified by column chromatog-
raphy on silica gel and petroleum ether–ethyl acetate mixture as eluent to give
pure product as orange solid (0.336 g, 79.40%).
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27. Typical procedure for the synthesis of OPVs
Melting point: >250 °C (d) (Lit. 29)
1H NMR (400 MHz, CDCl3) d 8.60 (d, J = 6 Hz, 4H), 7.64–7.30 (m, 14H), 7.18–
7.14 (m, 4H), 7.10–7.018 (m, 4H), 3.96 (s, 6H).
General procedure for the synthesis of OPV 9–11:
Synthesis of 4-((E)-4-((E)-styryl)styryl)pyridine OPV 9
A two neck round bottom flask was charged with, 4-bromo benzyl triph-
enylphosphine bromide (0.500 g, 0.976 mmol), benzaldehyde (0.124 g,
1.171 mmol), 4-vinyl pyridine (0.122 g, 1.171 mmol), K2CO3 (0.539 g,
3.90 mmol), palladium acetate (0.0021 g, 0.00976 mmol), dppp (0.0060 g,
0.014 mmol), TBAB (0.031 g, 0.097 mmol) in N,N-dimethyl acetamide (10 mL)
and was stirred at 140 °C under N2 atmosphere for 24 h. The cooled mixture
was then poured in water (100 mL) and extracted with ethyl acetate
(3 ꢀ 50 mL). The organic layer was washed with water (2 ꢀ 20 mL) and dried
over anhydrous sodium sulfate. The solution was concentrated under reduced
pressure to obtain a viscous liquid, which was purified by column chromatog-
raphy on silica gel and petroleum ether–ethyl acetate mixture as eluent to give
pure product as yellow solid (0.229 g, 85%). Melting point: 280oC (Lit. 30 283–
284 °C).
1H NMR (400 MHz, CDCl3) d 8.60 (d, J = 6.0 Hz, 2H), 7.61–7.51 (m, 6H), 7.41–
7.30 (m, 6H), 7.19 (d, J = 16.4 Hz, 1H), 7.13 (d, J = 16.4 Hz, 1H), 7.05 (d,
J = 16.4 Hz, 1H).
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Yang, J.-X.; Yu, X.-Q.; Wu, Y.-Z. New J. Chem. 2005, 29, 479.
General procedure for the synthesis of OPV 1–7:
Synthesis of 4,40-((2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl)dibenzene
OPV 1
30. Siegrist, A. Helv. Chim. Acta 1980, 63, 1311.
A two neck round bottom flask was charged with 1, 4-diiodo dimethoxy