Exploring the nitro group reduction in low-solubility oligo-phenylenevinylene systems: Rapid synthesis of amino derivatives

Article abstract of DOI:10.1080/00397911.2020.1731828

A small series of amino oligo-phenylenevinylenes (OPVs) were successfully synthesized from their nitro-analogs in a rapid, simple, and highly efficient fashion employing a sodium sulfide/pyridine system as a reducing agent. In this research, classic and sustainable reduction methodologies including NH₄HCO₂/Zn and a choline chloride/tin (II) chloride deep eutectic solvent (DES) were also evaluated, showing degradation products, incomplete reactivity, and product isolation difficulties in all cases. The straightforward Na₂S/pyridine synthetic protocol proved to maintain the E-E stereochemistry of the OPV backbone that has been previously assembled by the Mizoroki–Heck cross-coupling reaction. Also, the optoelectronic properties were determined and discussed, considering the amino group insertion in these conjugated systems as a contribution for future construction of novel materials with applications in supramolecular electronics, light harvesting, and photocatalysis.

Full text of DOI:10.1080/00397911.2020.1731828

Synthetic Communications
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Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Exploring the nitro group reduction in low-
solubility oligo-phenylenevinylene systems: Rapid
synthesis of amino derivatives
Mauricio Acelas, Andrés Felipe Sierra & César A. Sierra
To cite this article: Mauricio Acelas, Andrés Felipe Sierra & César A. Sierra (2020): Exploring the
nitro group reduction in low-solubility oligo-phenylenevinylene systems: Rapid synthesis of amino
derivatives, Synthetic Communications, DOI: 10.1080/00397911.2020.1731828
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https://doi.org/10.1080/00397911.2020.1731828
Exploring the nitro group reduction in low-solubility
oligo-phenylenevinylene systems: Rapid synthesis of
amino derivatives
ꢀ
ꢀ
Mauricio Acelas, Andres Felipe Sierra, and Cesar A. Sierra
ꢀ
ꢀ
ꢀ
Departamento de Quımica, Grupo de Investigacion en Macromoleculas, Universidad Nacional de
ꢀ
Colombia, Bogota, Colombia
ABSTRACT
ARTICLE HISTORY
Received 11 September 2019
A small series of amino oligo-phenylenevinylenes (OPVs) were suc-
cessfully synthesized from their nitro-analogs in a rapid, simple, and
highly efficient fashion employing a sodium sulfide/pyridine system
as a reducing agent. In this research, classic and sustainable reduc-
tion methodologies including NH4HCO2/Zn and a choline chloride/
tin (II) chloride deep eutectic solvent (DES) were also evaluated,
showing degradation products, incomplete reactivity, and product
isolation difficulties in all cases. The straightforward Na2S/pyridine
synthetic protocol proved to maintain the E-E stereochemistry of the
OPV backbone that has been previously assembled by the
Mizoroki–Heck cross-coupling reaction. Also, the optoelectronic prop-
erties were determined and discussed, considering the amino group
insertion in these conjugated systems as a contribution for future
construction of novel materials with applications in supramolecular
electronics, light harvesting, and photocatalysis.
KEYWORDS
Amino phenylenevinylene;
Mizoroki–Heck reaction;
nitro reduction;
optoelectronic properties
GRAPHICAL ABSTRACT
Introduction
The polyphenylenevinylene (PPV) system is considered a promising organic semicon-
ductor material with remarkable optical and electronic properties. Unnumbered applica-
tions have been described in light-harvesting,[1,2] molecular electronics,[3,4]
photocatalysis,[5] water splitting for hydrogen production,[6,7] chemosensors,[8,9] among
ꢀ
ꢀ
ꢀ
Departamento de Quımica, Grupo de Investigacion en
CONTACT Cesar A. Sierra
casierraa@unal.edu.co
ꢀ
Macromoleculas, Universidad Nacional de Colombia, Kra 30 No 45-03, Bogota, Colombia.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
M. ACELAS ET AL.
others.[10] However, on the basis of their length and structural defects concerning het-
erogeneous geometrical configuration around double bonds, the overall polymer elec-
tronic conjugation is compromised. Hence, the oligo-phenylenevinylene (OPV) moiety
emerges as a rational model approach offering the possibility to explore by extrapolation
the unique optical, electronic and thermal properties of their high molecular weight
analogs, overcoming the problems mentioned above and other important solubil-
ity issues.[11]
Different synthetic approaches, including Gilch,[12] Knoevenagel,[13] and Wittig[14]
reactions have been widely used to access the OPV skeleton. More recently, the
Wittig–Horner reaction has shown high E-selectivity, facilitating the synthesis of
more complex structures such as OPV-hexasubstituted benzenes and OPV-functional-
ized azafullerenes.[15,16] Nevertheless, the Mizoroki–Heck reaction, considered the
method of choice for arylation of alkenes, also offers a powerful and convenient
strategy to provide pure anti all-trans materials under mild conditions as demon-
strated by the efforts made by our research group in terms of efficiency and stereo-
selectivity improvement.[17,18] Alternative and sustainable routes for obtaining anti
trans-trans OPVs in high yields have been subjected to recent studies, leading to the
development of efficient Pd supported heterogeneous catalytic systems.[19–21] It
should be disclosed that Pd-coupling reactions employing sterically hindered deacti-
vated bromoarenes (e.g., 1-Bromo-2-nitrobenzene) are considered a synthetic chal-
lenge which has been addressed with bulky phosphine ligands, the use of ionic
liquids, longer reaction times and high temperatures to support the oxidative add-
ition of the C-X bond.[22–24]
On the other hand, supramolecular hydrogen-bonded architectures employing
OPVs have also recently attained special recognition for applications in electronic
devices. It has been demonstrated that secondary interactions between these
p-conjugated systems result in materials with macromolecular improved optical and
electronic properties associated with the self-aggregation of their units.[25] To achieve
larger degrees of supramolecular polymerization, several nitrogen-containing groups
(e.g., urea, melamine) have been employed to functionalize the OPV moiety.[26,27]
Thus, amino OPV derivatives could serve as simple molecular models for conjugated
superstructures. Although the preparation of isolated examples of amino OPV deriva-
tives has been carried out by acid hydrolysis[14] of amide OPV precursors and
[28,29]
reduction from NO2-OPVs employing different reducing agents such as SnCl2
and Na2S,[30,31] a rational evaluation of the most relevant reaction conditions leading
to NH2-OPV has not yet been performed from poorly soluble starting materials.
Therefore, herein we describe a concise chemical exploration of synthetic features
conducted to obtain amino OPV derivatives employing substituted styrenes and aryl
bromide-iodide moieties through the Mizoroki–Heck reaction. This, as the basis for
the stereoselective (E-E) p-conjugated backbone construction (Scheme 1), yielding a
small series of amino-OPVs expeditiously prepared taking advantage of the best
reduction reaction conditions found in this study. Additionally, the optoelectronic
characterization, regarding the nature of the nitro and amino substituents and their
position around the OPV structure, are examined providing insights for future
applications in the field of photoluminescent materials.

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Scheme 1. Approaches employed in this work to assemble the amino-OPV backbone
Table 1. Preliminary formation of amino and nitro OPV derivatives via direct Mizoroki-Heck cou-
pling reaction.
Entry
1
Styrene
Product
Yield, %
80
1a
3a
2
3
NR
5a
1b
1c
4a
5a
aMultiple products formation.
NR: no reaction
Results
Synthesis of nitro-OPVs
Mizoroki–Heck reaction conditions for the preparation of OPVs have been previ-
ously optimized by our research group and were employed to successfully provide
the desired nitro-OPV model 3a (Table 1, entry 1).[32] Alternatively, preliminary
efforts to access amino OPV derivatives also involving direct Mizoroki–Heck cou-
pling reaction of 4-aminostyrene 1b and 2,5-diiodo-1,4-dimethoxybenzene 2a (Table
1, entry 2) were unpromising; the use of aminostyrene precursor gave multiple prod-
ucts (as observed by TLC) that were not possible to isolate by column
chromatography.

4
M. ACELAS ET AL.
The observed chemical behavior can be attributed to the low stability exhibited by
the 4-aminostyrene which probably involves a spontaneous polymerization caused by
the nucleophilic attack of the amino group directly on the activated b-position of the
double bond generating a zwitterionic propagating species.[33] To prevent this, amine
protection was carried out by treatment with acetyl chloride. The easily formed N-(4-
vinylphenyl) acetamide 1c was immediately brought under the same Pd(0)-catalyzed
coupling reaction conditions. However, only a 5% of the desired N-acetyl OPV deriva-
tive 5a (purified by column chromatography) was obtained among multiple products
(Table 1, entry 3). These exploratory outcomes confirmed that aminostyrene derivatives
are not suitable substrates to obtain the amino OPV compounds under these reaction
conditions. Nevertheless, the successful preparation of the nitro OPV model compound
3a suggested that a reduction could be a feasible synthetic alternative to amino OPVs.
Having established a convenient route to obtain nitro OPVs, a small library of com-
pounds was prepared employing a combination of styrene and 1,4-dihalide precursors
to afford different nitro-substituted OPV on both central and flanking aryl rings (Table
2). All nitro OPV products were obtained as stable solids. Noteworthy, the trans stereo-
selective preparation of compounds 3a and 3b was successfully achieved employing the
Mizoroki–Heck reaction as an alternative route to classic Wittig conditions, whereas a
mixture of cis and trans isomers was obtained as reported by Saikachi and Muto.[34]
No significant yield variation was observed regardless of the electronic and steric
nature of the 1,4-dihalide arene precursors. Due to poor solubility of the nitro OPV
compounds in the most common deuterated solvents (CDCl3, DMSO-d6), pyridine-
d5 was alternatively employed to obtain the 1H-NMR spectra in some cases.
Compounds 3d and 3g were not soluble in any of the available deuterated solvents
(pyridine-d5 or DMSO-d6), and thus 13C CP/MAS-NMR was employed as a charac-
terization technique. However, the acquisition of solid-state NMR data was only pos-
sible for 3g.
It should be highlighted that NMR spectroscopy confirms characteristic[3] J coupling
constants (ꢀ16.0 Hz) for the protons on both E double bonds of the nitro OPV prod-
ucts, indicating that the symmetry of the substituents in the 1,4-dihalide precursor has
no effect on the reaction stereochemical preference. Thus, the unique configuration is a
consequence of the employed conditions, being the phosphite ligand responsible for the
steric hindrance that promotes control in the orientation of the substrates during the
arene-alkene coupling as suggested on previous research results.[17]
Reduction of nitro-OPVs
Initial reduction experiments were carried out employing a solid mixture of ammonium
formate and zinc grounded in a mortar, followed by the addition of the nitro OPV 3a
(Table 3, entry 1).[35] After 15 min, TLC revealed that no reaction was taking place
under these conditions indicating that the use of a grinding auxiliary might be required
to enhance the catalytic hydrogen transfer.[36] As an attempt to improve the reactivity
through a classical approach, methanol, one of the most commonly used solvents for
this transformation, was used, followed by the addition of few drops of acetic acid
(Table 3, entry 2). The reaction mixture was then heated to reflux; however, no product

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5
Table 2. Pd-catalyzed coupling reaction for direct synthesis of nitro OPV precursors.
Entry
1
Styrene, 1
Aryl halide, 2
Product, 3
Yield, %
80
1a
3a
3b
2
3
69
55
1d
1f
2a
3c
3d
3e
4
5
62
87
1g
1h
1d
6
7
8
56
70
40
3f
2b
2c
1d
3g
1a
1e
1f
3h
3i
9
37
50
2d
10
3j

6
M. ACELAS ET AL.
Table 3. Reduction conditions evaluated for the synthesis of amino OPVs.
Entry
Reducing agent
Solvent
Temperature, ꢁC
Time, h
Yield, %
1
2
3
4
5
6
7
8
9
NH4HCO2/Zn
NH4HCO2/Zn
SnCl2·H2O
SnCl2·H2O
Na2S
Na2S
Na2S
Na2S
SnCl2·2H2O
Nonea
r.t.
118
78
110
90
115
100
100
120
0.25
12
8
1
2
0.5
0.5
1
NR
NR
MeOH/AcOH
EtOH/AcOEt (1:1)
HCl (37%)
Pyridine/HCl (37%)
Pyridine
<5b
<5b
<5b
82
1,4-dioxane
22
18
20
PEG 600
Choline chloride (DES)
4
aMechanochemical reaction.
bMultiple product formation.
NR: no reaction
formation was observed. These results can be attributed to the poor solubility of the
OPV in methanol, preventing the appropriate interaction between reactants in solution.
Then, to improve the OPV solubility, SnCl2 in a mixture of EtOH/AcOEt was
explored (Table 3, entry 3). Although complete solubility of the nitro OPV was reached
under reflux conditions, and successful reduction of similar nitro-substituted conjugated
systems have been reported,[28] conversion of 3a into the corresponding amino deriva-
tive was not observed, and multiple products were formed instead. The same chemical
behavior was observed when concentrated HCl (37%) was used as solvent (Table 3,
entry 4).
The use of HCl in combination with Na2S has been previously reported for selective
nitro group reductions.[30] However, under these conditions, the model compound 3a
was again converted into multiple products (Table 3, entry 5). An analog methodology
involving the use of sodium sulfide/pyridine has already been reported, but its applica-
tion was limited to a single OPV example.[31] Our exploratory synthetic efforts con-
firmed that this reduction system serves as a rapid and efficient approach toward amino
OPV derivatives (Table 3, entry 6). A greener approach exploring the use of water-
miscible, less harmful solvents was unsuccessful. 1,4-dioxane, PEG-600, and a choline
chloride/tin (II) chloride deep eutectic solvent (DES)[37] gave the desired product but in
low yields (Table 3, entries 7, 8 and 9). It should be noted that the purification proced-
ure for these last three systems became tedious, considering that an emulsion was
formed after pouring the reaction mixture in water, therefore preventing the formation
of the solid amino OPV product although TLC indicated reaction completion.
Remarkably, after reaction is complete with the best reaction conditions (Table 3,
entry 6), the obtained solid product was recovered by filtration followed by washing
cycles with water and a 1:1 water/ethanol mixture. No further purification procedures
were required. By evaluating the characteristic coupling constant values observed for
1
vinylic protons, H-NMR spectra confirmed that the E-E stereochemistry was main-
tained (Figure 1).

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SYNTHETIC COMMUNICATIONSV
7
Figure 1. 1H-NMR spectrum for compound 4a, aromatic region expansion.
To demonstrate the effectiveness of this synthetic methodology, the best reaction con-
ditions were used to reduce all other nitro substituted OPV systems previously prepared
by Mizoroki–Heck reaction (Table 4). Complete reduction of the nitro group was
achieved in all cases in moderate to high yields. Reduction of nitro-OPVs 3d and 3g
required a minor procedure modification to enhance solubility, and consisted in heating
the reacting mixture (OPV 3d/3g, pyridine, and Na2S) at 100 ꢁC on a capped vial for
12 h, after which the reaction crude was also poured into water and the target product
recovered by filtration. It should be noted that the insertion of other substituents
around the OPV scaffold (compounds 4c–e) led to a significant decrease in the reduc-
tion reaction yields, although no side-products were observed by TLC. Interestingly, the
reduction reaction with OPVs containing the nitro group in the central ring (3i and 3j,
Scheme 2) gave a mixture of products as confirmed by MALDI-MS and NMR experi-
ments. Reports indicate that the use of sulfide as a reducing agent for p-substituted
nitrobenzenes promotes the formation of azo and azoxy compounds as by-products.[38]
Upon thorough examination of the MALDI spectrum, a self-radical unimolecular
induced fragmentation pattern is observed for all signals due to the presence of the
amino group. However, signals at 592 and 502 m/z confirm the presence of the azo
compound and its subsequent double bond cleavage product along with the desired
amino OPV with the corresponding [M þ H]þ ion signal at 298 m/z (see
Supplementary Data).
Despite the efforts that were made to isolate the major amino OPV products 4i and
4j via recrystallization and purification by column chromatography, HPLC, and NMR
results indicate the side-product remains in a 0.28:1 ratio (see Supplementary Data).
The formation of the azo compound can be ascribed to the reaction between the nitro-
syl reduction intermediate[39] and a preformed amine molecule in the medium, as
depicted on Scheme 3. The azo dimerization starts with the addition of the amine on
the nitrosyl group and then the elimination of water affords the N¼N bond.

8
M. ACELAS ET AL.
Table 4. Sodium sulfide/pyridine reduction of other nitro OPV systems.
Entry
Nitro OPV
Product
Yield, (%)
82
1
3a
3b
4a
4b
2
3
76
36
3c
4c
4
5
43
39
4d
4e
3d
3e
6
7
8
96
93
75
3f
4f
3g
4g
3h
4h
Scheme 2. Products observed under the evaluated reaction conditions for nitro-OPVs 3i and 3j

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9
Scheme 3. Proposed mechanism of OPV azo-dimerization
Monitoring the reduction of compound 3j by HPLC confirms the coelution of
two substances with different UV–Vis absorption spectra (see supplementary data).
MALDI-MS showed the [M þ H]þ ion of the expected amino-OPV at 348 m/z as
well as other peaks at 590 and 522 that can be assigned to the azo by-product frag-
mentation ions (see Supplementary Data). It should be highlighted that under the
employed reduction conditions, the nitrile group was unaffected, and the formation
of possible thioamide-type side-products was not observed in the MS spectrum.[40]
The overall experimental data show that placing the nitro group in the central ring
of the OPV moiety seems to stabilize the reduction nitrosyl intermediate due to the
electron-donating character of the styrene substituents, thus favoring the formation
of the azo by-product.
Optoelectronic properties of OPVs
The photophysical properties of compounds 3a–j and 4a–h were studied by UV–Vis
and fluorescence spectroscopy to evaluate the effect of the NO2 and NH2 groups pos-
ition around the OPV conjugated scaffold (Figures 2 and 3; Table 5).
Initial spectral examination for all compounds shows a double band absorption
ꢂ
ꢂ
caused by the expected p!p electronic transition and the additional n!p electronic
transition. This ascribed to the OMe group in the central ring, and the presence of an
additional group containing a heteroatom (OMe and NMe2) in the flanking aryl rings,
that can be observed for both NO2 and NH2 OPVs 3a–e and 4a–e. An analog absorp-
tion behavior is observed for all other NH2-OPVs 4f–h, where a shoulder associated
ꢂ
with the n!p transition of the amine group is also noticed. The change in the elec-
tronic nature of the substituents over the flanking aryl rings indicates that the electron-
donating character of the amino group causes a hypsochromic effect for both absorption
and emission maxima in all cases, except for compounds 3e and 4e, where a redshift of
8 nm is observed in the absorption. This phenomenon can be attributed to the simultan-
eous presence of a strong electron-donating group and the different position of the
nitro group in the aromatic ring. The blueshift is also exhibited when the NO2 and
NH2 groups are changed from position 4- to position 3-, and from position 3- to pos-
ition ꢃ2, producing a modification on the electronic density of the OPV structure.
For the nitro series, a redshift of 5 and 37 nm in the absorption spectra was observed
when Me– (compound 3f) and OMe– (compound 3a) groups are bonded to the central
ring compared to the reference structure 3h. Furthermore, the same electronic behavior

10
M. ACELAS ET AL.
483
435
476
396
325
351
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
499
404
476
406
497
330
344
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
517
392
563
404
538
328
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
354
475
408
334
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
345
550
521
398
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
Figure 2. Normalized absorption and emission spectra of OPVs 3a–j and 4a–e in MeCN.

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11
430
454
392
468
402
497
322
326
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
479
395
586
404
503
326
326
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
400
507
486
474
379
320
324
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
459
474
376
354
322
279
300
400
500
600
300
400
500
600
Wavelenght (nm)
Wavelenght (nm)
Figure 3. Normalized absorption and emission spectra of OPVs 4a–h in MeCN.
is observed when additional groups are bonded to the flanking aryl rings, and the NO2
is located in position ꢃ3, where a redshift of 6 and 8 nm was found for compounds 3c
and 3d, respectively, in comparison with the reference structure 3h. Noteworthy, when
the nitro group is moved to position 2-, an opposite effect is exhibited with a blueshift
of 6 nm for compound 3e, also employing OPV 3h as reference.
This trend can be seen for the amino series as well, with a redshift of 3 and 53 nm
corresponding to Me (OPV 4f) and –OMe (OPV 4a) substituents, respectively, when

12
M. ACELAS ET AL.
Table 5. Photophysical properties for OPVs 3a–j and 4a–h in MeCN.
OPV
kmax (nm)
kem (nm)
e (L molꢃ1cmꢃ1
)
DE (eV)
Uf
3a
435
351sh
396
325sh
404
330sh
406
344sh
483
92,800
2.45
0.01
a
3b
3c
3d
3e
476
499
15,900
3332
2.73
2.69
2.63
2.77
–
a
–
a
476
497sh
517
3100
–
a
392
19,100
–
328sh
403
353
398
334
345
402
326sh
392
322sh
395
326sh
404
326sh
400
324sh
379
320sh
354
538sh
563
475
550
408
521
468
497sh
430
454sh
479
3f
61,750
9200
22,300
69,150
83,800
37,350
2.64
2.97
2.99
3.05
2.92
2.68
0.05
–
a
3g
3h
3i
3j
4a
0.09
a
–
–
a
0.26
0.50
0.42
0.06
0.37
0.20
0.31
0.25
4b
4c
4d
4e
4f
32,700
8300
3.19
2.79
2.71
2.96
2.99
3.17
2.99
503sh
586
48,200
32,100
40,600
28,150
45,800
486
507sh
474
4g
4h
459
474
279sh
376
322sh
aNo appreciable emission.
OPV 4h is used as reference. This bathochromic effect is also observed when the -OMe
group is kept in the central ring, and other substituents are included in the flanking
aryl rings, displaying redshifts of 19, 28, and 24 nm for compounds 4c, 4d, and 4e,
respectively, compared to OPV 4h. This absorption pattern is attributed to the increas-
ing electron-donating character of the central ring substituents (H < Me < OMe), induc-
ing a generalized bathochromic effect. Alternatively, when the nitro group is placed on
the central ring, compounds 3i and 3j exhibit a blue shift of 64 and 53 nm in the
absorption spectra compared to reference compound 3h, respectively.
Regarding the emission spectra, an additional band is observed for all compounds
bearing two or more electron-donating groups (compounds 3d, 3e, 4a–c, and 4e) except
for compound 4d, where a broad emission band is observed due to overlapping. The
insertion of additional electron-donating groups (OMe, NMe2, and NH2) around the
OPV scaffold seems to modify the vibronic states of the molecule, inducing a double
band emission. It should be noted that a significant Stokes shift of 182 nm is displayed
for compound 4d, ascribed to the contribution of the electron-donating effect of the
two substituents in the flanking aryl ring, lowering self-absorption phenomena.
However, for compound 4e, a significant decrease in the Stokes shift of 86 nm, suggests
that the position of the amino group plays a major role in the OPV energetic levels.
Also, a strong bathochromic effect is obtained for the emission spectra when a

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13
combination of electron-withdrawing groups (NO2 and CN) are placed around the
OPV skeleton leading to a Stokes shift of 170 nm in compound 3j.
The attenuation coefficients e of 4-nitro (3a, 3f) and 4-amino compounds (4a, 4f) are
significantly higher compared to their 3-substituted analogs as a result of a modification
on the electronic density on the OPV conjugated backbone. Additionally, the position
of the nitro group in OPV 3e, significantly increases the attenuation coefficient com-
pared to the other 4-substituted OPVs 3c and 3d. Interestingly, compound 4c exhibits
the lowest e value among all amino OPV derivatives, possibly due to the presence of
the Cl atom and its moderate electron-withdrawing effect. No additional trends regard-
ing this property were observed. It should be disclosed that the attenuation coefficient
for compound 3a had been previously reported in DMF (e ¼ 38,600 Lꢄmolꢃ1ꢄcmꢃ1).[32]
However, the difference in the obtained value relies in the strong solvatochromism
exhibited by this particular compound considering that, this time, MeCN was used as
the solvent.[41]
The optical gap DE showed an increase for the 3-substituted OPVs compared to their
4-substituted analogs. Also, a stronger inductive effect exerted by the central ring sub-
stituents is related to a decrease in the DE value. Compounds 3d and 4d exhibit a
higher DE value compared to the other flanking aryl ring substituted OPVs 3c, 3d, 4c,
and 4d. This property can also be attributed to the modification in the position of the
nitro and amino group around the OPV scaffold allowing a molecular energy level
modulation. Additionally, it can be observed that nitro OPVs have a very low quantum
fluorescence yield (Uf) associated with the lack of appreciable emission at low absorb-
ance values in most cases. It was only possible to determine the Uf values for com-
pounds 3a, 3f, and 3h. Alternatively, an increase in the quantum yield is observed for
3-amino OPVs compared to 4-amino compounds. The overall obtained Uf values indi-
cate that the efficiency of the conversion of absorbed light into emitted light is slightly
improved by the reduction of the nitro group in the OPV backbone. Moreover, it was
found that compound 4d displayed the lowest Uf among all other amino OPV deriva-
tives. This observation was attributed to the intramolecular hydrogen bonding between
the –NMe2 and –NH2 vicinal groups, which generates additional vibronic internal con-
version processes, leading to the dissipation of the excitation energy and to the decrease
in the radiative relaxation pathway.[42] Although the Uf values for the prepared amino
OPVs are not high enough for emissive materials applications, alternative photocatalytic
properties can be explored. However, it is expected that their supramolecular self-
assembly leads to a significant improvement in their photophysical properties.
Conclusions
In summary, we have demonstrated that the preparation of the amino OPV backbone
can be achieved via Mizoroki–Heck coupling reaction and subsequent reduction of the
aromatic nitro group upon treatment with a Na2S/pyridine system. This methodology
proved to be a convenient strategy for low solubility NO2–OPVs when the nitro groups
are located on the flanking rings, and serves as an expedite synthetic pathway to design
a variety of amino OPV-type building blocks and diversify future development of
hydrogen-bonded supramolecular polymers with promising technological applications.

14
M. ACELAS ET AL.
Also, the photophysical properties are reported, indicating a high dependence on the
type of substituent (EWG or EDG) and its position around the OPV, as result of the
modification in the electron density of the molecule.
Experimental part
Materials
All reagents were purchased from Merck (Kenilworth, NJ), Alfa Aesar (Haverhill, MA),
and Sigma-Aldrich Chemical Co. (St. Louis, MO), and used without further purification.
The reaction progress was monitored using thin-layer chromatography on PF254 TLC
aluminum sheets from Merck. Column chromatography was performed using silica gel
(60–120 mesh). The melting points (uncorrected) were determined using a Stuart
SMP10 digital melting point apparatus. IR spectra were recorded on an FTIR Thermo
Scientific Nicolet iS10 spectrophotometer coupled to a diamond ATR Smart iTR cell.
Mass spectra were collected from m/z 0 to 1000 in reflectron positive ion mode using
an UltraFlextreme MALDI-TOF-MS (Bruker, Billerica, MA) with a laser solid source of
Nd:YAG and 1000 Hz acquisition frequency employing the laser desorption ionization
(LDI) method at a 25% laser fluency. Elemental analyses were performed on a Thermo
Scientific analyzer (Flash 2000) and were within 0.4 of theoretical values. NMR spectra
1
(1H and 13C) were measured on a Bruker Ultrashield-400 spectrometer (400 MHz H
NMR and 100 MHz 13C NMR), using CDCl3, DMSO-d6, methanol-d4, pyridine-d5 or
mixtures as solvents and references. J values are reported in Hz; chemical shifts are
reported in ppm (d) relative to the solvent peak (residual CHCl3 in CDCl3 at 7.26 ppm
for protons, residual DMSO at 2.50 ppm for protons and residual pyridine at 8.74 ppm
for protons). Signals were designated as follows: s, singlet; d, doublet; dd, doublet of
doublets; ddd, doublet of doublets of doublets; t, triplet; td, triplet of doublets; q, quar-
tet; m, multiplet; br., broad. 13C CP/MAS-NMR spectra were acquired using a Bruker
AVANCE III instrument operating at 9.4T using KBr as filler. The absorption and emis-
sion spectra were recorded on 1 ꢅ 10ꢃ5 M solutions in MeCN using a Thermo
Scientific Evolution 300 spectrophotometer and a PTI QuantaMasterTM 40 spectrofluor-
ometer. The maxima of absorption kmax and emission kem were directly obtained from
the corresponding spectra. The attenuation coefficient e was obtained from the analysis
of OPV solutions in the 2.0ꢃ18.0 mM range. The optical HOMO-LUMO gap was calcu-
lated using the onset value of the absorption spectrum in the direction of longer wave-
lengths.[43] Finally, the fluorescence quantum yields Uf were acquired according to the
reference method described in the literature using quinine sulfate (in 0.1 M H2SO4)
as standard.[44]
Synthesis
General procedure for the synthesis of nitro oligo-phenylenevinylenes OPVs 3a–3g. To a
10 mL crimp vial equipped with a magnetic stirrer, the corresponding styrene
(2.0 mmol), aryl 1,4-dihalide precursor (1.0 mmol), bis(benzylidene)acetone palladium
(0) (2.0 mol%) and triphenylphosphite (9.0 mol%) were added. The vial was then sealed
with a PTFE septum-aluminum cap followed by purge-saturation with Ar. Dry DMF
(3 mL) and triethylamine (2.0 mmol) were subsequently added via syringe. The reaction

R
SYNTHETIC COMMUNICATIONSV
15
mixture was stirred at 110 ꢁC for 24 h. After the reaction is complete, the crude was
filtered over celite, and precipitation of the product was induced by the addition of
cold water (75 mL). Recrystallization from a 2:1 DMF:1,4-dioxane afforded the
desired OPVs.
4,40-((1E,1’E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl)) bis(nitrobenzene) 3a. Red
solid, mp 275–278 ꢁC; yield 0.259 g (80%) from 1-nitro-4-vinylbenzene (0.240 g,
1.61 mmol), 1,4-diiodo-2,5-dimethoxybenzene (0.314 g, 0.81 mmol), Pd(dba)2 (18.4 mg,
0.03 mmol), P(PhO)3 (45.0 mg, 0.145 mmol), and Et3N (0.163 g, 1.61 mmol). IR (ATR,
1
cmꢃ1): 3068, 2226, 1604, 1506, 1219, 921, 844. H-NMR (400 MHz, CDCl3) d(ppm):
3.94 (s, 6H), 7.59 (s, 2H), 7.63 (d, J ¼ 16.5 Hz, 2H), 7.73 (d, J ¼ 8.8 Hz, 4H), 7.99 (d, J
¼ 16.5 Hz, 2H), 8.25 (d, J ¼ 8.8 Hz, 4H). 13C-NMR (100 MHz, CDCl3) d(ppm): 56.6,
110.7, 124.8, 127.8, 128.3, 128.6, 128.7, 130.6, 145.3, 147.4, 152.9. MALDI-MS Calc. for
C24H20N2O6 432.132; found [M]þ 432.292. Anal. Calcd for C24H20N2O6: C, 66.66; H,
4.66; N, 6.48%. Found: C, 66.81; H, 4.57; N, 6.55%.
General procedure for the synthesis of amino oligo-phenylene vinylene derivatives 4a–e.
On a 50 mL round bottom flask equipped with a condenser and a magnetic stirrer, the
corresponding OPV (1 mmol) was dissolved in 3.0 mL of pyridine and taken to ebulli-
tion. Then, a solution of 6 mmol sodium sulfide hydrate (approx. 60% Na2S) in 4 mL of
water (approx. 1.5 M) was added while maintaining a gentle reflux. At this point, the
color of the reaction mixture changes to dark brown. After the addition is complete, the
reaction was stirred and heated at 115 ꢁC for 30 additional minutes. After reaction com-
pletion, the crude was poured over 50 mL of cold water. The solid product was filtered
before being washed with water (20 mL) and a water/ethanol (1:1) mixture (20 mL). No
further purification procedures were required.
4,40-((1E,1’E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dianiline 4a. Yellow solid.
Yield 0.256 g (82%), mp 189–191 ꢁC. From OPV 3a (0.327 g, 0.76 mmol) and Na2S
(0.589 g (60%), 4.53 mmol). IR (ATR, cmꢃ1): 3446, 3369, 3213, 3034, 3001, 2954, 1608,
1
1514, 1207, 1039. H-NMR (400 MHz, Pyridine-d5) d(ppm): 3.73 (bs, NH), 3.90 (s, 6 H),
6.67 (d, J ¼ 8.5 Hz, 4H), 7.02 (d, J ¼ 16.4 Hz, 2H), 7.10 (s, 2H), 7.29 (d, J ¼ 16.4 Hz,
2H), 7.37 (d, 8.5 Hz, 4H). 13 C-NMR (100 MHz, Pyridine-d5) d(ppm): 56.4, 108.9, 115.2,
119.7, 126.6, 127.8, 128.7, 146.0, 151.3. MALDI-MS Calc. for C24H24N2O2 372.183;
found [M]þ 372.316. Anal. Calcd for C24H24N2O2: C, 77.39; H, 6.49; N, 7.52%. Found:
C, 77.04; H, 6.42; N, 7.28%.
Acknowledgments
We want to thank Laboratorio de Espectrometria de Masas – PTG – Universidad Industrial de
Santander for data acquisition.
Disclosure statement
No potential conflict of interest was reported by the author(s).

16
M. ACELAS ET AL.
Funding
This work was financially supported by Colciencias [project Nꢁ FP44842-025-2017], DIB-UNAL
[project with code HERMES 36990] and the Teaching Assistant Scholarship Program of
Universidad Nacional de Colombia.
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