Phenylenevinylene oligomers by Mizoroki-Heck cross coupling reaction. Structural and optoelectronic characterization

Article abstract of DOI:10.1016/j.molstruc.2016.12.032

In order to study the effect of the molecular structure on the optical properties of totally trans-trans phenylenevinylene oligomers (OPVs), sixteen 1,4-distyrylbenzene derivatives (1a-i and 2a-g) functionalized with different electron-donating (ED) and electron-withdrawing (EW) groups were synthesized by the Mizoroki-Heck cross coupling reaction in moderate to good yields (40–95%). The implemented methodology, with a small modification previously reported by our group, allows obtaining the desired vinyl configuration as well as one novel OPV compound (1h). After structural characterization by several techniques (e.g. FTIR, ¹H, ¹³C and Solid-State NMR), particular emphasis was placed upon the investigation of their optical properties by UV–vis and fluorescence spectroscopies. The results showed that, with only one exception, the ED and EW groups at the ends of OPV systems lead to a bathochromic shift. This effect is intensified with the introduction of methoxy groups on the central ring. Consistent with these, the HOMO-LUMO gaps (ΔE) decreases as the strength of ED and EW substituents increases. The ED and EW substituents also lead to a decrease in the Φ_f values. This contribution in the area of organic electronics can be used as a reference to better select the most appropriate technological application for each OPV and this can be extrapolated to their respective structurally analogous segmented polymer.

Full text of DOI:10.1016/j.molstruc.2016.12.032

Journal of Molecular Structure 1133 (2017) 448e457
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Phenylenevinylene oligomers by Mizoroki-Heck cross coupling
reaction. Structural and optoelectronic characterization
Sandra E. Estrada, Cristian Ochoa-Puentes, Cesar A. Sierra*
ꢀ
Departamento de Química, Universidad Nacional de Colombia, Bogota 111321, Colombia
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to study the effect of the molecular structure on the optical properties of totally trans-trans
phenylenevinylene oligomers (OPVs), sixteen 1,4-distyrylbenzene derivatives (1a-i and 2a-g) function-
alized with different electron-donating (ED) and electron-withdrawing (EW) groups were synthesized by
the Mizoroki-Heck cross coupling reaction in moderate to good yields (40e95%). The implemented
methodology, with a small modification previously reported by our group, allows obtaining the desired
vinyl configuration as well as one novel OPV compound (1h). After structural characterization by several
techniques (e.g. FTIR, 1H, 13C and Solid-State NMR), particular emphasis was placed upon the investi-
gation of their optical properties by UVevis and fluorescence spectroscopies. The results showed that,
with only one exception, the ED and EW groups at the ends of OPV systems lead to a bathochromic shift.
This effect is intensified with the introduction of methoxy groups on the central ring. Consistent with
Received 24 August 2016
Received in revised form
12 December 2016
Accepted 13 December 2016
Available online 18 December 2016
Keywords:
Phenylenevinylene oligomers
Mizoroki-Heck reaction
Optoelectronic properties
these, the HOMO-LUMO gaps (DE) decreases as the strength of ED and EW substituents increases. The ED
and EW substituents also lead to a decrease in the Ff values. This contribution in the area of organic
electronics can be used as a reference to better select the most appropriate technological application for
each OPV and this can be extrapolated to their respective structurally analogous segmented polymer.
© 2016 Published by Elsevier B.V.
1. Introduction
instead of triple bonds in PPAs, allows us to continue the studies on
the influence of the configuration (cis or trans, syn and anti) on the
In the field of polymer electronics, poly(phenylenevinylene)
(PPV) and poly(phenyleneacetylene) (PPA) systems have attracted
considerable attention because of their remarkable optical and
electronic properties. Due to their structural similarities and
specially by using their structurally analogous oligomers, these two
electronically conjugated systems have allowed making very
interesting comparisons between their physicochemical properties,
which has led to comparative studies of chemical stability [1] and
studies on the formation of cyclic systems with applications in
sensing and electronic transport [2e5]. Despite the very good
structural stability shown by these cyclic systems, the tension of the
ring caused by the twists in the vinyl bonds produce a detrimental
in terms of electronic conjugation, especially in PPVs [6]. Although
our research interest includes these two electronic systems, this
work have been focused on phenylene vinylene derivatives since
the presence of double bonds between the aromatic rings in PPVs
optoelectronic properties and its implication over the desired
€
technologic application [7]. In 1970, Horhold et al. [8] reported the
optoelectronic properties of one of the first PPVs. Twenty years
later, Burroughes et al. [9] demonstrated that PPVs can be used as
the active element in organic light-emitting diodes (OLEDs). Since
then, phenylenevinylene derivatives have been exploited in many
applications ranging from materials science to biomedicine, with
notable results in OLEDs [10e13], laser dyes [14], photovoltaic cells
[15e17], chemosensors [18,19], linkers for metal organic frame-
works (MOFs) [20,21], dendrimers [22,23], photoredox catalysts
and photosensitizers [24,25].
Because, PPVs very often are difficult to synthesize and char-
acterize (i.e. poor solubility, heterogeneous structural configura-
tion) [26], oligo phenylenevinylenes (OPVs) have drawn interest as
structural models for PPVs. The synthesis of OPVs is much easier;
allowing to obtain a large number of well-defined chemical struc-
tures with better structural and optoelectronic characterization in a
short time, information that can also be used as experimental data
in theoretical studies to predict structure-property relationships.
Additionally, OPVs act as model compounds to study their
* Corresponding author.
E-mail address: casierraa@unal.edu.co (C.A. Sierra).
http://dx.doi.org/10.1016/j.molstruc.2016.12.032
0022-2860/© 2016 Published by Elsevier B.V.

S.E. Estrada et al. / Journal of Molecular Structure 1133 (2017) 448e457
449
structural analogue PPVs, especially segmented polymers.
Although many routes have been reported for the synthesis of
phenylenevinylene systems, such as Wittig [27e29], Gilch [30],
Wessling [31] and Knoevenagel [32,33], these methodologies pre-
sent some disadvantages like harsh experimental conditions, un-
wanted byproducts and often producing a mixture of cis- and trans-
isomers. On the other hand, the Mizoroki-Heck reaction is one of
the most powerful methods for the formation of C]C bonds under
soft reaction conditions, with minimal structural limitations for
precursors and yielding all-trans vinyl bonds with very few side
reactions [34].
In recent years several studies have been reported on the
structural and optoelectronic properties of numerous substituted
OPVs, with the goal of determining structure-property relation-
ships [34e37]. In order to contribute to the understanding of these
technological attractive materials, here we report the synthesis by
Mizoroki-Heck cross coupling reaction of a series of OPVs with
different ED and EW substituents (Fig. 1) in addition to their
structural and optical characterization. Although several of these
compounds have been previously reported by other synthetic
methodologies, in this contribution they were synthesized for the
first time by the Heck reaction, which also made possible to obtain
OPV 1h, a novel compound difficult to synthesize by most
commonly used methodologies mentioned above, such as the
Wittig reaction. The study of the effects of ED and EW substituent
groups on the properties of OPVs will provide insights about the
structural features that a phenylenevinylene derivative must have
to be valuable in a specific application.
apparatus Stuart SMP10 and are uncorrected. FT-IR spectra were
recorded on a Shimadzu IR Prestige-21 FT-IR spectrometer in the
range of 4000-400 cmꢀ1. Samples were prepared in KBr discs and
ATR technique was used for some of them. 1H NMR (400 MHz) and
13C NMR (100 MHz) were recorded on a Bruker Avance spectrom-
eter using CDCl3 or DMSO-d6 as solvents with tetramethylsilane
(TMS) as internal standard, and KBr was used as filling for solid
NMR analysis. Mass spectra were taken on a Bruker Ultra-
fleXtreme™, MALDI-TOF/TOF mass spectrometer, equipped with a
355 nm solid-state Nd:YAG laser. For LDI measurements, 1 mL of a
2.5 mM stock solution of each compound was applied on the target
surface and evaluated at 25% of laser fluence. Elemental analysis
was performed on a FLASH 2000 elemental analyzer, using the
Eager 300 for EA1112 software. UVevis spectra were measured on a
Thermo Scientific Evolution 300 spectrophotometer in DMF solu-
tions. Fluorescence spectroscopy (FS) was performed on a PTI
QuantaMaster™ 40 spectrofluorometer in DMF solutions. Fluores-
cence quantum yields (Ff) were determined at lexc corresponding
to the absorption maximums. Quinine sulfate in 0.1 M H2SO4
(
Ff ¼ 0.54) was used as standard for determination of fluorescence
quantum yields (Ff) at lexc corresponding to the absorption
maximum of series 1 and to the first absorption maximum of series
2 (l1). Perylene in ethanol (Ff ¼ 0.92) was used as standard for
determination of the Ff at lexc corresponding to the second ab-
sorption maximum of series 2 (l2). The Ff of the nitro compound in
series 1 (OPV 1b) was calculated also using perylene as standard.
2.3. Synthesis of starting materials
2. Experimental section
4-Substituted styrenes (4a-d) and 1,4-divinylbenzene (4e) were
prepared by the Wittig reaction (Scheme 1). The general procedure
is as follows: A mixture of methyltriphenylphosphonium bromide
(1.05 equivalents) and potassium carbonate (1.6 equivalents) was
stirred in 50 mL of 1,4-dioxane under a nitrogen atmosphere at
25 ꢁC for 4 h. The corresponding aldehyde (1 equivalent) previously
dissolved in 1,4-dioxane was slowly added to the reaction mixture
and allowed to react at 100 ꢁC for 24 h. After cooling to room
temperature, the mixture was filtered and the solvent evaporated.
The products were purified as colorless oils by silica gel chroma-
tography with hexane or mixtures of heptane:ethyl acetate as
eluent. Once synthesized, the styrenes were used immediately to
avoid polymerization.
2.1. Materials
Aldehydes (3a-e), styrene (4f), 4-acetoxystyrene (4g), 1,4-
dimethoxybenzene (5), dihalobenzenes (7a,b), 4-substituted aryl
halides (8a-d), methyltriphenylphosphonium bromide, potassium
carbonate, bis(dibenzylidene acetone) palladium(0) [Pd(dba)2],
triphenylphosphite [P(OPh)3], triethylamine (TEA) and solvents
were purchased from SigmaeAldrich, Alfa Aesar and Merck and
were used without further purification. Dimethylformamide (DMF)
was dried over anhydrous BaO and stored under nitrogen with
molecular sieve until use. Reactions to get 1a-i and 2a-g com-
pounds were carried out under nitrogen in tightly capped glass
vials with teflon/silicone septum and aluminum caps.
2.3.1. 4-Methoxystyrene (4a)
Obtained as yellow oil with 53% yield from 4-
methoxybenzaldehyde (3a, 1.8 mL, 14.90 mmol), methyl-
triphenylphosphonium bromide (5.590 g, 15.65 mmol) and potas-
sium carbonate (3.296 g, 23.84 mmol). 1H NMR (400 MHz, CDCl3):
2.2. Techniques
Melting points were determined on a digital melting point
Fig. 1. Chemical structure of the synthesized OPV systems (1a-i and 2a-g).

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S.E. Estrada et al. / Journal of Molecular Structure 1133 (2017) 448e457
Scheme 1. Synthetic routes to OPVs 1a-i and 2a-g.
d
3.80 (s, 3H, OCH3), 5.12 (dd, 1H, H-1a, J ¼ 10 Hz), 5.61 (dd, 1H, H-
(6.347,17.77 mmol) and potassium carbonate (3.742 g, 27.08 mmol).
1H NMR (400 MHz, CDCl3):
2.30 (s, 3H, CH3), 5.18 (dd, 1H, H-1a,
J ¼ 10.9 Hz), 5.69 (dd, 1H, H-1b, J ¼ 17.6 Hz), 6.69 (dd, 1H, H-2,
J1 ¼10,9 Hz, J2 ¼ 17.6 Hz), 7.13 (d, 2H, H-5, J ¼ 7,9 Hz), 7.31 (d, 2H, H-
4, J ¼ 8,1 Hz) ppm.
1b, J ¼ 17.6 Hz), 6.65 (dd, 1H, H-2, J1 ¼10,9 Hz, J2 ¼ 17.6 Hz), 6.86 (d,
d
2H, H-5, J ¼ 8,8 Hz), 7.34 (d, 2H, H-4, J ¼ 8,6 Hz) ppm. 13C NMR
(100 MHz, CDCl3):
159.34 ppm.
d 55.27, 111.56, 136.20, 130.40, 127.37, 113.88,
2.3.2. 4-Nitrostyrene (4b)
2.3.4. 4-Cyanostyrene (4d)
Obtained as deep yellow oil with 89% yield from 4-
nitrobenzaldehyde (3b, 2.026 g, 13.41 mmol), methyl-
triphenylphosphonium bromide (5.029 g, 14.08 mmol) and potas-
sium carbonate (2.965 g, 21.45 mmol). 1H NMR (400 MHz, CDCl3):
Obtained as colorless oil with 60% yield from 4-
formylbenzonitrile (3d, 2.030 g, 15.48 mmol), methyl-
triphenylphosphonium bromine (5.808 g, 16.26 mmol) and potas-
sium carbonate (3.424 g, 24.77 mmol). 1H NMR (400 MHz, CDCl3):
d
5.52 (dd, 1H, H-1a), 5.95 (dd, 1H, H-1b), 6.78 (dd, 1H, H-2,
d
5.37 (dd, 1H, H-1a, J ¼ 10.9 Hz), 5.80 (dd, 1H, H-1b, J ¼ 17.6 Hz),
J1 ¼10,9 Hz, J2 ¼ 17.6 Hz), 7.54 (d, 2H, H-4, J ¼ 8,7 Hz), 8.19 (d, 2H, H-
6.65 (dd, 1H, H-2, J ¼ 17.6 Hz), 7.40 (d, 2H, H-4, J ¼ 8,3 Hz), 7,53 (d,
5, J ¼ 8,8 Hz) ppm. 13C NMR (100 MHz, CDCl3):
d
118.63, 134.97,
2H, H-5, J ¼ 8,3 Hz) ppm. 13C NMR (100 MHz, CDCl3):
d 117.74,
143.82, 123.96, 126.82, 147.15 ppm.
135.36, 141.90, 126.75, 132.39, 111.12, 118.91 ppm.
2.3.3. 4-Methylstyrene (4c)
2.3.5. 1,4-Divinylbenzene (4e)
Obtained as colorless oil with 33% yield from 4-tolualdehyde
(3c, 2 mL, 16.92 mmol), methyltriphenylphosphonium bromine
Obtained as colorless oil with 70% yield from tereph-
thalaldehyde (3e, 2.060 g, 15.36 mmol),

S.E. Estrada et al. / Journal of Molecular Structure 1133 (2017) 448e457
451
methyltriphenylphosphonium bromide (11.25 g, 31.49 mmol) and
potassium carbonate (5.52 g, 39.94 mmol). 1H NMR (400 MHz,
trans), 835 cmꢀ1(para-substitution). 13C Solid-State NMR
130.64 (C-1 and C-6), 135.89 (C-2), 117.69 (C-3),
110.27 (C-4), 159.27 (C-5), 127.42 (C-7), 123.76 (C-8), 54.59 (OCH3)
ppm. MALDI-MS: Calc. for C24H22O2: 342.16; found: 342.16. Anal.
Calc. For C24H22O2: C, 84.18; H, 6.48; O, 9.34. Found: C, 83.24; H,
6.49; O, 10.24.
(Solid þ KBr):
d
CDCl3):
d
5.24 (dd, 2H, H-1a, J ¼ 10.8 Hz), 5.75 (dd, 2H, H-1b,
J ¼ 17.6 Hz), 6.70 (dd, 2H, H-2, J ¼ 17.6 Hz), 7.38 (s, 4H, H-4) ppm. 13
C
NMR (100 MHz, CDCl3):
d 113.74, 136.49, 137.14, 126.40 ppm.
2.3.6. 1,4-Diiodo-2,5-dimethoxybenzene (6) was synthesized as
follows
2.4.2. 1,4-Bis[(E)-2-(4-nitrophenyl)ethenyl]benzene (1b)
1,4-dimethoxybenzene (5, 5.314 g, 38.46 mmol), iodine-
resublimed crystals (11.715 g, 46.16 mmol) and potassium iodate
Obtained as an orange powder with 59% yield from 4-
nitrostyrene (4b, 0.821 g, 5.50 mmol) and 1,4-dibromobenzene
(7a, 0.633 g, 2.68 mmol). m.p.: 292e294 ꢁC; Lit. 288e290 ꢁC [27].
FT-IR (ATR): ῡ 2922 (¼CeH), 1631, 1589 (C]C Ar), 1508, 1335 (NO2),
960 (HC]CH trans), 847 cmꢀ1(para-substitution). 13C Solid-State
(4.116 g, 19.23 mmol) were dissolved in 131 mL of CH3COOH:H2
-
SO4:H2O (120:1:10). The reaction mixture was allowed to stir under
reflux for 24 h to obtain a dark reddish solution. After this, 200 mL
of a saturated solution of sodium sulfite was added inducing pre-
cipitation of the product. The pale yellow solid formed was filtered
and washed with sodium sulfite, ethanol and chloroform to give a
bright white powder (77% yield). m,p.: 173e175 ꢁC; Lit 171e174 ꢁC
NMR (Solid þ KBr):
d 130.33 (C-1 and C-6), 134.90 (C-2), 124.72
(C-3 and C-4), 143.72 (C-5), 122.62 (C-7), 145.83 (C-8) ppm. MALDI-
MS: Calc. for C22H16N2O4: 372.11; found: 372.15. Anal. Calc. For
C22H16N2O4: C, 70.96; H, 4.33; N, 7.52; O, 17.19. Found: C, 69.65; H,
[38]. 1H NMR (400 MHz, CDCl3):
d
3.76 (s, 6H, OCH3), 7.12 (s, 2H, H-
4.61; N, 7.25; O, 17.49.
3) ppm. 13C NMR (100 MHz, CDCl3):
132.41 ppm.
d 67.85, 164.15, 96.21,
2.4.3. 1,4-Bis[(E)-2-phenylethenyl]benzene (1c)
Obtained as a light green solid with 85% yield from styrene (4f,
0.8 mL, 7.23 mmol) and 1,4-dibromobenzene (7a, 0.835 g,
3.54 mmol). m.p.: 254e258 ꢁC; Lit. 254 ꢁC [36]. FT-IR (KBr): ῡ 3024
(¼CeH), 1595, 1561, 1510, 1484, 1446 (C]C), 968 (HC]CH trans),
814 (para-substitution), 690 cmꢀ1(mono-substitution). 1H NMR
2.4. Synthesis of oligo(phenylene vinylene)s 1a-i and 2a-g
The OPV synthesis was done following two different pathways
according to the Mizoroki-Heck reaction (Scheme 1). First; by the
coupling between 4-substituted styrenes and dibromo or diiodo-
benzenes (Method A, Scheme 1), and second; the reaction between
4-substituted bromo or iodobenzenes with 1,4-divinylbenzene
(Method B, Scheme 1).
(400 MHz, CDCl3):
H-6 overlap), 7.37 (t, 4H, H-7), 7.26 (t, H-8) ppm. 13C NMR (100 MHz,
CDCl3): 126.85 (C-1), 136.71 (C-2), 128.59 (C-3), 128.27 (C-4),
d 7.12 (d, 4H, H-3 and H-4), 7.53 (d, 8H, H-1 and
d
137.33 (C-5), 126.52 (C-6), 128.71 (C-7), 127.65 (C-8) ppm. MALDI-
MS: Calc. for C22H18: 282,141; found: 282,148. Anal. Calc. For
The general procedure is as follows: 2.05 equivalents of the
corresponding styrene and 1 equivalent of 1,4-dibromo or 1,4-
diiodobenzene (Method A, to OPVs 1a-d and 2a-f) or 2.05 equiva-
lents of the appropriate aryl halide and 1 equivalent of 1,4-
divinylbenzene (Method B, to OPVs 1e-h), were placed in a 10 mL
headspace crimp vial equipped with a magnetic stir bar. Then, for
both methods, it was added 0.01 equivalents (1 mol%) of Pd(dba)2,
0.1 equivalents (10 mol%) of P(OPh)3 and 5 equivalents of TEA. The
vial was sealed with a PTFE/silicone septum with aluminum cap
and purged-saturated with N2, then 2 mL of solvent was injected
(dry DMF for OPVs-series 1 and 1,4-dioxane for OPVs-series 2,
Scheme 1) and the system was purged once again with N2. The
reaction mixture was vigorously stirred at 110 ꢁC during 48 h for
OPVs-series 1 and 24 h for OPVs-series 2. After finished, highly
fluorescent solid products were obtained.
C22H18: C, 93.57; H, 6.43. Found: C, 92.17; H, 6.28.
2.4.4. 1,4-Bis[(E)-2-(4-acethoxyphenyl)ethenyl]benzene (1d)
Obtained as a light green powder with 71% yield from 4-
acetoxystyrene (4g, 0.79 mL, 5.14 mmol) and 1,4-diiodobenzene
(7b, 0.828 g, 2.51 mmol). m.p.: 300e302 ꢁC; Lit. 298e302 ꢁC [40].
FT-IR (ATR): ῡ 3026 (¼CeH), 2931 (CH3), 1757 (C]O), 1670, 1514,
1502 (C]C Ar), 1209, 1165 (CeOeC), 968 (HC]CH trans),
844 cmꢀ1(para-substitution). 13C Solid-State NMR (Solid þ KBr):
d
134.52 (C-1), 131.42 (C-2), 127.07 (C-3 and C-4), 130.53 (C-5),
123.75 (C-6), 121.91 (C-7), 149.67 (C-8), 18.80 (CH3), 169.86 (C]O)
ppm. MALDI-MS: Calc. for C26H22O4: 398.15; found: 398.22. Anal.
Calc. For C26H22O4: C, 78.37; H, 5.57; O, 16.06. Found: C, 76.27; H,
5.37; O, 15.59.
The hydroxylated OPVs 1i and 2g were obtained by a base-
catalyzed hydrolysis (10 equivalents of KOH in methanol) from 1
equivalent of acetylated derivatives 1d and 2f. As work-up, the
reaction mixture from each hydrolysis was dropwise added over
HCl 2 M to induce precipitation of the products.
The OPVs purification was performed according to their solu-
bility. OPVs-series 1, which had greater difficulties to dissolve in
most common organic solvents, were washed with chloroform and
ethanol. Additionally, compounds 1c, 1f and 1h were recrys-
tallizated from chloroform and OPVs 1a, 1b and 1d from DMF. The
OPVs-series 2 were precipitated from their reaction mixture with
ethanol or water, the solids were filtered and dissolved in chloro-
form and they were passed through celite to finally get fluorescent
solids after concentration by rotary evaporation.
2.4.5. 1,4-Bis[(E)-2-(4-biphenyl)ethenyl]benzene (1e)
Obtained as a green bright powder with 54% yield from 4-
bromobiphenyl (8a, 1.099 g, 4.71 mmol) and 1,4-divinylbenzene
(4e, 0.299 g, 2.30 mmol). m.p.: >300 ꢁC; Lit. > 320 ꢁC [41]. FT-IR
(KBr): ῡ 3021 (¼CeH), 1601, 1579, 1560, 1508 (C]C), 971 (HC]
CH trans), 839 (para-substitution), 718 cmꢀ1(mono-substitution).
13C Solid-State NMR (Solid þ KBr):
d 135.05 (C-1, C-2, C-5, C-8, Ci,
Cm), 125.67 (C-3, C-4, C-6, C-7, Co, Cp) ppm. MALDI-MS: Calc. for
C34H26: 434.20; found: 434.18. Anal. Calc. For C34H26: C, 93.97; H,
6.03. Found: C, 91.66; H, 6.38.
2.4.6. 1,4-Bis[(E)-2-(4-cyanophenyl)ethenyl]benzene (1f)
Obtained as
a yellow powder with 90% yield from 4-
bromobenzonitrile (8b, 1.123 g, 6.16 mmol) and 1,4-
divinylbenzene (4e, 0.392 g, 3.01 mmol). m.p.: 274e276 ꢁC; Lit.
279e281 ꢁC [42]. FT-IR (KBr): ῡ 3026 (¼CeH), 2216 (C^N), 1630,
1596, 1499 (C]C), 958 (HC]CH trans), 830 cmꢀ1(para-substitu-
2.4.1. 1,4-Bis[(E)-2-(4-methoxyphenyl)ethenyl]benzene (1a)
Obtained as a light green powder with 65% yield from 4-
methoxystyrene (4a, 0.8 mL, 5.98 mmol) and 1,4-
dibromobenzene (7a, 0.688 g, 2.92 mmol). m.p.: 296e300 ꢁC; Lit.
310e311 ꢁC [39]. FT-IR (KBr): ῡ 3019 (¼CeH), 2955, 2837 (CH3),
1603, 1573, 1515, 1465 (C]C), 1252, 1029 (CeOeC), 969 (HC]CH
tion). 1H NMR (400 MHz, CDCl3):
d 7.56 (s, 4H, H-1), 7.13 (d,
Jtrans ¼ 16.3 Hz, 2H, H-3), 7.22 (d, Jtrans ¼ 16.3 Hz, 2H, H-4), 7.60 (d,
4H, H-6), 7.65 (d, 4H, H-7) ppm. 13C NMR (100 MHz, CDCl3):

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d
126.92 (C-1), 110.78 (C-2), 127.19 (C-3), 131.67 (C-4), 136.60 (C-5),
2.4.11. 2,5-Dimethoxy-1,4-bis[(E)-2-(4-nitrophenyl)ethenyl]
127.41 (C-6), 132.55 (C-7), 141.65 (C-8), 118.99 (CN) ppm. MALDI-
MS: Calc. for C24H16N2: 332.131; found: 332.145. Anal. Calc. For
benzene (2b)
Obtained as a red powder with 95% yield from 4-nitrostyrene
(4b, 0.707 g, 4.74 mmol) and 1,4-diiodo-2,5-dimethoxybenzene
(6, 0.902 g, 2.31 mmol). m.p. 272e274 ꢁC; Lit. 274e278 ꢁC [45].
FT-IR (ATR): ῡ 3067 (¼CeH Ar), 2924, 2839 (CH3), 1587, 1466 (C]C
Ar), 1512, 1333 (NO2), 1211 (CeOeC), 964 (HC]CH trans),
C
24H16N2: C, 86.72; H, 4.85; N, 8.43. Found: C, 85.30; H, 4.87; N,
8.42.
827 cmꢀ1(para-substitution). 1H NMR (400 MHz, DMSO-d6):
d 3.50
2.4.7. 1,4-Bis[(E)-2-(4-methoxycarbonylphenyl)ethenyl]benzene
(1g)
(s, 6H, CH3eO), 7.00 (s, 2H, H-2), 7.11 (d, Jtrans ¼ 16.5 Hz, 2H, H-4),
7.23 (d, Jtrans ¼ 16.5 Hz, 2H, H-5), 7.41 (d, Jortho ¼ 8.9 Hz, 4H, H-7),
7.80 (d, Jortho ¼ 8.9 Hz, 4H, H-8) ppm. 13C Solid-State NMR
Obtained as a light green powder with 71% yield from methyl 4-
iodobenzoate (8c, 1.348 g, 5.14 mmol) and 1,4-divinylbenzene (4e,
0.327 g, 2.51 mmol). m.p.: >300 ꢁC; Lit. > 321 ꢁC [43]. FT-IR (KBr): ῡ
3019 (¼CeH), 2954 (CH3), 1721 (C]O), 1679, 1646, 1607, 1566 (C]C
Ar), 1282, 1108 (CeOeC), 962 (HC]CH trans), 813 cmꢀ1(para-sub-
(Solid þ KBr):
d 143.06 (C-1), 104.80 (C-2), 123.71 (C-3 and C-7),
120.72 (C-4), 129.25 (C-5), 145.78 (C-6), 126.18 (C-8), 151.24 (C-9),
52.93 (CH3O) ppm. MALDI-MS: Calc. for C24H20N2O6: 432.132;
found: 432.157. Anal. Calc. For C24H20N2O6: C, 66.66; H, 4.66; O,
22.20; N, 6.48. Found: C, 64.10; H, 4.87; O, 22.87; N, 6.16.
stitution). 13C Solid-State NMR (Solid þ KBr):
d 130.06 (C-1 and C-6),
134.90 (C-2), 122.52 (C-3 and C-4),137.63 (C-5),128.20 (C-7),128.68
(C-8), 53.37 (OCH3), 164.77 (C]O) ppm. MALDI-MS: Calc. for
C
26H22O4: 398.152; found: 398.149. Anal. Calc. For C26H22O4: C,
2.4.12. 2,5-Dimethoxy-1,4-bis[(E)-2-(4-methylphenyl)ethenyl]
benzene (2c)
78.37; H, 5.57; O, 16.06. Found: C, 76.97; H, 5.53; O, 16.27.
Obtained as
a yellow powder with 53% yield from 4-
methylstyrene (4c, 0.654 g, 5.53 mmol) and 1,4-diiodo-2,5-
dimethoxybenzene (6, 1.052 g, 2.70 mmol). m.p. 183e185 ꢁC; Lit.
176e178 ꢁC [45]. FT-IR (ATR): ῡ 3049 (¼CeH Ar), 2912, 2829 (CH3),
1514, 1493, 1460 (C]C Ar), 1259, 1047 (CeOeC), 962 (HC]CH
trans), 846 cmꢀ1 (para-substitution). 1H NMR (400 MHz, CDCl3):
2.4.8. 1,4-Bis[(E)-2-(4-acetylphenyl)ethenyl]benzene (1h)
Obtained as a pale yellow powder with 58% yield from 4-
bromoacetophenone (8d, 1.113 g, 5.59 mmol) and 1,4-
divinylbenzene (4e, 0.355 g, 2.73 mmol). m.p.: 296e298 ꢁC. FT-IR
(KBr): ῡ 3022 (¼CeH), 1672 (C]O), 1609, 1551, 1514 (C]C), 964
(HC]CH trans), 837 cmꢀ1(para-substitution). 13C Solid-State NMR
d
2.29 (s, 6H, CH3), 3.85 (s, 6H, CH3eO), 7.05 (s, 2H, H-2), 7.37 (m,
6H, H-4, H-7), 7.02 (d, Jtrans ¼ 16.5 Hz, 2H, H-5), 7.09 (d,
(Solid þ KBr):
d 127.44 (C-1 and C-6), 134.77 (C-2), 123.93 (C-3 and
Jortho ¼ 7.9 Hz, 4H, H-8) ppm. 13C NMR (100 MHz, CDCl3):
d 21.26
C-4),136.33 (C-5),129.60 (C-7),137.17 (C-8), 25.18 (CH3),197.38 (C]
O) ppm. MALDI-MS: Calc. for C26H22O2: 366.162; found: 366.211.
Anal. Calc. For C26H22O2: C, 85.22; H, 6.05; O, 8.73. Found: C, 84.34;
H, 6.02; O, 8.67.
(CH3), 56.41 (CH3eO), 137.33 (C-1), 109.12 (C-2), 126.62 (C-3),
122.24 (C-4), 128.81 (C-5), 135.10 (C-6), 129.35 (C-7), 126.51 (C-8),
151.49 (C-9) ppm. MALDI-MS: Calc. for C26H26O2: 370.193; found:
370.196. Anal. Calc. For C26H26O2: C, 84.29; H, 7.07; O, 8.64. Found:
C, 83.42; H, 7.13; O, 8.45.
2.4.9. 1,4-Bis[(E)-2-(4-hydroxyphenyl)ethenyl]benzene (1i)
2.4.13. 1,4-Bis[(E)-2-(4-cyanophenyl)ethenyl]-2,5-
dimethoxybenzene (2d)
Obtained as a green powder with 85% yield from 1d (0.3 g,
0.75 mmol) and KOH (0.422 g, 7.53 mmol) in MeOH (3 mL). m.p.:
>300 ꢁC; Lit. 300 ꢁC [44]. FT-IR (ATR): ῡ 3308 (OeH), 3020 (¼CeH),
1603, 1591, 1502, 1452 (C]C Ar), 1385 (CeO), 1250 (CeO), 964
(HC]CH trans), 833 cmꢀ1(para-substitution). 1H NMR (400 MHz,
Obtained as
a yellow powder with 75% yield from 4-
cyanostyrene (4d, 0.67 mL, 5.22 mmol) and 1,4-diiodo-2,5-
dimethoxybenzene (6, 0.993 g, 2.55 mmol). m.p. 284e286 ꢁC; Lit.
297 ꢁC [46]. FT-IR (ATR): ῡ 3055 (¼CeH Ar), 2993, 2835 (CH3), 2220
(C^N), 1597, 1493, 1465 (C]C Ar), 1209 (CeOeC), 956 (HC]CH
trans), 815 cmꢀ1 (para-substitution). 1H NMR (400 MHz, CDCl3):
DMSO-d6):
d
7.52 (s, 4H, H-1), 7.15 (d, Jtrans ¼ 16.4 Hz, 2H, H-3), 7.01
(d, Jtrans ¼ 16.4 Hz, 2H, H-4), 7.43 (d, Jortho ¼ 8.59 Hz, 4H, H-6), 6.78
(d, Jortho ¼ 8.5 Hz, 4H, H-7) ppm. 13C NMR (100 MHz, DMSO-d6):
d
3.95 (s, 6H, CH3eO), 7.13 (s, 2H, H-2 overlap), 7.13 (d, 2H, H-4
overlap), 7.59 (d, Jtrans ¼ 16.4 Hz, 2H, H-5), 7.63 (d, 8H, H-7, H-8)
ppm. 13C NMR (100 MHz, CDCl3):
56.26 (CH3eO), 110.49 (C-1),
d
126.82 (C-1), 136.81 (C-2), 128.57 (C-3), 125.38 (C-4), 157.77 (C-5),
128.28 (C-6), 116.07 (C-7), 128.73 (C-8) ppm. MALDI-MS: Calc. for
22H18O2: 314.131; found: 314.133. Anal. Calc. For C22H18O2: C,
d
C
109.29 (C-2), 126.49 (C-3), 126.81 (C-4), 127.47 (C-5), 142.22 (C-6),
126.96 (C-7), 132.48 (C-8), 151.81 (C-9), 119.10 (CN) ppm. MALDI-
MS: Calc. for C26H20N2O2: 392.152; found: 392.169. Anal. Calc. For
84.05; H, 5.77; O, 10.18. Found: C, 80.18; H, 5.80; O, 12.02.
2.4.10. 1,4-Bis[(E)-2-(4-dimethoxyphenyl)ethenyl]-2,5-
C26H20N2O2: C, 79.57; H, 5.14; O, 8.15; N, 7.14. Found: C, 78.27; H,
dimethoxybenzene (2a)
5.23; O, 8.29; N, 7.21.
Obtained as
a yellow powder with 40% yield from 4-
methoxystyrene (4a, 0.67 mL, 5.09 mmol) and 1,4-diiodo-2,5-
dimethoxybenzene (6, 0.969 g, 2.48 mmol). m.p. 212e214 ꢁC; Lit.
209e210 ꢁC [45]. FT-IR (ATR): ῡ 3059 (¼CeH Ar), 2908, 2833 (CH3),
1600, 1573, 1510, 1464 (C]C Ar), 1249, 1043, 1030 (CeOeC), 966
(HC]CH trans), 821 cmꢀ1(para-substitution). 1H NMR (400 MHz,
2.4.14. 2,5-Dimethoxy-1,4-bis[(E)-2-phenylethenyl]benzene (2e)
Obtained as a yellow powder with 45% yield from styrene (4f,
0.69 mL, 5.98 mmol) and 1,4-diiodo-2,5-dimethoxybenzene (6,
1.139 g, 2.92 mmol). m.p.: 178e180 ꢁC; Lit. 177e178 ꢁC [45]. FT-IR
(ATR): ῡ 3057 (¼CeH), 2933, 2827 (CH3), 1595, 1498, 1460 (C]C
Ar), 1259, 1047 (CeOeC), 960 (HC]CH trans), 750 cmꢀ1 (mono-
CDCl3):
d 3.92 (s, 6H, CH3eO, terminal rings), 3.83 (s, 6H, CH3eO,
central ring), 7.11 (s, 2H, H-2), 7.35 (d, Jtrans ¼ 16.4 Hz, 2H, H-4), 7.06
substitution). 1H NMR (400 MHz, CDCl3):
d 3.86 (s, 6H, CH3eO), 7.07
(d, Jtrans ¼ 16.4 Hz, 2H, H-5), 7.49 (d, Jortho ¼ 8.7 Hz, 4H, H-7), 6.90 (d,
(s, 2H, H-2 overlap), 7.42 (d, Jtrans ¼ 16.5 Hz, 2H, H-4), 7.05 (d,
Jtrans ¼ 16.3 Hz, 2H, H-5 overlap), 7.49 (d, Jortho ¼ 7.3 Hz, 4H, H-7),
7.29 (t, 4H, H-8), 7.18 (t, 2H, H-9) ppm. 13C NMR (100 MHz, CDCl3):
Jortho ¼ 8.7 Hz, 4H, H-8) ppm. 13C NMR (100 MHz, CDCl3):
d 56.40
(CH3eO, terminal rings), 55.33 (CH3eO, central ring), 151.38 (C-1),
108.97 (C-2), 126.55 (C-3), 121.18 (C-4), 128.33 (C-5), 130.75 (C-6),
127.78 (C-7), 114.10 (C-8), 159.21 (C-9) ppm. MALDI-MS: Calc. for
d
56.39 (CH3eO), 151.54 (C-1), 109.18 (C-2), 123.24 (C-3), 127.49 (C-
4, C-5), 137.84 (C-6), 128.64 (C-7), 126.60 (C-8), 128.97 (C-9) ppm.
MALDI-MS: Calc. for C24H22O2: 342.162; found: 342.211. Anal. Calc.
For C24H22O2: C, 84.18; H, 6.48; O, 9.34. Found: C, 82.46; H, 6.53; O,
C
26H26O4: 402.183; found: 403.23. Anal. Calc. For C26H26O4: C,
77.59; H, 6.51; O, 15.90. Found: C, 75.62; H, 6.38; O, 16.01.

S.E. Estrada et al. / Journal of Molecular Structure 1133 (2017) 448e457
453
9.51.
been reported that compounds with trans-configuration adopts a
more-planar conformation, which is beneficial for the electronic
conjugation and promotes the improvement of the photo-
luminescence intensity [49]. On the other hand, cis-conformation
leads to structural torsions which decrease the electron cloud
density on the olefinic segments leading to small blue shifts in the
absorption spectrum, affecting the molar absorption coefficients,
and fluorescence properties compared with the trans-conformation
[50].
As it was mentioned above, most of the compounds synthesized
here have already been reported in the literature, mainly by Wittig
and Horner-Wadsworth-Emmons (HWE) reactions, which produce
mixtures of cis-cis and cis-trans OPV isomers. However, the
implementation of additional procedures could be necessary in
order to obtain the trans product, if this configuration is required.
Some examples are the modification of the original Wittig reaction
[28], the isomerization reaction using hot iodine solution [39],
photoisomerization [51] or the use of additional steps to separate
the isomers [36]. However, the extra steps reduce the yield of the
final product, increase the probability of impurities and increase
costs of the process.
2.4.15. 1,4-Bis[(E)-2-(4-acetoxyphenyl)ethenyl]-2,5-
dimethoxybenzene (2f)
Obtained as
a yellow powder with 58% yield from 4-
acetoxystyrene (4g, 0.68 mL, 4.47 mmol) and 1,4-diiodo-2,5-
dimethoxybenzene (6, 0.850 g, 2.18 mmol). m.p. 220e222 ꢁC; Lit.
231 ꢁC [47]. FT-IR (ATR): ῡ 3004 (¼CeH Ar), 2932 (CH3), 1754 (C]
O), 1617, 1504 (C]C Ar), 1214 (CeOeC methoxy), 1197 (CeOeC
ester), 969 (HC]CH trans), 828 (para-substitution) cmꢀ1. 1H NMR
(400 MHz, CDCl3):
d 2.31 (s, 6H, CH3eC]O), 3.92 (s, 6H, CH3eO),
7.10 (m, 8H, H-2, H-5, H-8), 7.43 (d, Jtrans ¼ 16.5 Hz, 2H, H-4), 7.55 (d,
Jortho ¼ 8.5 Hz, 4H, H-7) ppm. 13C NMR (100 MHz, CDCl3):
d 21.18
(CH3C]O), 56.37 (CH3eO), 169.50 (C]O), 149.97 (C-1), 109.15 (C-
2), 126.52 (C-3), 127.95 (C-4), 123.40 (C-5), 135.67 (C-6), 127.49 (C-
7), 121.74 (C-8), 151.53 (C-9) ppm. MALDI-MS: Calc. for C28H26O6:
458.173; found: 458.226. Anal. Calc. For C28H26O6: C, 73.35; H, 5.72;
O, 20.94. Found: C, 72.20; H, 5.67; O, 20.32.
2.4.16. 1,4-Bis[(E)-2-(4-hydroxyphenyl)ethenyl]-2,5-
dimethoxybenzene (2g)
Obtained as a yellow powder with 80% yield from 2f (0.5 g,
1.09 mmol), KOH (0.617 g, 10.99 mmol) and MeOH (5 mL). m.p.:
294e296 ꢁC; Lit. 302 ꢁC [47]. FT-IR (ATR): ῡ 3363 (OeH), 3000
(¼CeH), 1604, 1585, 1514, 1494 (C]C Ar), 1354 (CeOH), 1259
(CeOH), 1024 (CeOeC), 958 (HC]CH trans), 821 cmꢀ1(para-sub-
Besides the advantage of yielding products with pure trans-
configuration, the methodology used allows to synthesize the novel
OPV 1h (R ¼ eCOCH3) from the corresponding ketone derivative (4-
bromoacetophenone) and 1,4-divinylbenzene. Which by other
methodologies, like the Wittig reaction (using the proper pre-
cursors) may generate several products by a competing reactions or
the need of extra synthetic steps like protection-deprotection of
carbonyls. As drawback, it was found that series 1 compounds have
very limited solubility in most common solvents, making very
difficult the purification and characterization processes and also
the optoelectronics measurements that require the preparation of
solutions with well-established concentrations.
stitution). 1H NMR (400 MHz, DMSO-d6):
d 3.88 (s, 6H, CH3eO), 7.20
(s, 2H, H-2), 7.22 (d, 4H, H-4 and H-5), 7.40 (d, Jortho ¼ 8.5 Hz, 4H, H-
7), 6.78 (d, Jortho ¼ 8.5 Hz, 4H, H-8), 9.59 (s, 2H, OH) ppm. 13C NMR
(100 MHz, DMSO-d6):
d 56.60 (CH3eO), 151.18 (C-1), 109.18 (C-2),
66.82 (C-3), 129.12 (C-4), 119.78 (C-5), 128.19 (C-6), 129.09 (C-7),
116.09 (C-8), 157.70 (C-9) ppm. MALDI-MS: Calc. for C24H22O4:
374.152; found: 374.183. Anal. Calc. For C24H22O4: C, 76.99; H, 5.92;
O, 17.09. Found: C, 72.41; H, 5.81; O, 18.78.
3.2. Optoelectronic properties of oligomers
3. Results and discussion
In order to characterize the effect of electron-donating (ED) and
electron-withdrawing (EW) groups on the photophysical proper-
ties, the synthesized OPVs were studied by UVevis and FS tech-
niques. Fig. 2 shows the normalized absorption and emission
spectra of OPVs 1a-i (Fig. 2, left) and 2a-g (Fig. 2, right) in DMF
(1ꢂ10ꢀ5 M), ordered by their increase in the bathochromic shift.
The UVevis absorption (labs), molar attenuation coefficient (ε),
3.1. Synthesis and characterization of oligomer structure
Because 4-substituited styrenes are not readily available due to
their tendency to polymerize during purification, transport or
storage, they were synthesized by the Wittig reaction and once
obtained they were used immediately in the Mizoroki-Heck reac-
tion. As shown in Scheme 1, two different paths can be used to
accomplish the reaction. In the first case (method A) an excess of 4-
substituted styrene was used, while in the second case (method B)
the excess was for the 4-halobenzene. In both cases, the excess of
terminal rings precursors was given to try to ensure that di-
coupling products were preferentially obtained and not those of a
single coupling.
In this work, we used a modification of the classic Mizoroki-
Heck methodology using triphenylphosphite [P(OPh)3] instead of
triphenylphosphine (PPh3), which has been proved previously to be
more efficient [48]. In general, the sixteen oligomers were obtained
with high to moderate yields with a stereoselectivity towards
compounds with totally trans-trans configuration. The high ster-
eoselectivity during the formation of the vinyl bonds in the prod-
ucts was confirmed by FT-IR (presence of bands between 980 and
958 cmꢀ1, corresponding to trans-HC]CH vibrations and absence
of bands from 730 to 665 cmꢀ1, corresponding to cis-HC]CH vi-
brations) and by 1H NMR; the proton-proton coupling constants “J”
in vinyl bonds was found between 12 and 18 Hz, which is typical for
bonds in trans configuration.
HOMO-LUMO gap (DE), photoluminescence emission (lem) and
fluorescence quantum yields (Ff) are summarized in Table 1.
Despite the poor solubility of series 1 compounds, efforts were
made in order to obtain the required concentration for optoelec-
tronic analysis, and it was made applying ultrasonic and heating
techniques. The HOMO-LUMO gaps (
analysis of the absorption edge according to the equation
,h is the absorption coefficient, h is the
¼ A,(h
E)1/2, where
Planck constant, is the frequency of light and A is the absorbance.
By drawing the (h curves and extending the linearity edge
A)2 vs h
of absorbance to the intersect with the energy axis, the E value is
DE) were determined from the
s
n
n
-
D
s
n
n
n
D
obtained [52,53]. The fluorescence quantum yields (Ff) were
calculated according to a method described in the literature [54],
using the following equation:
!
2
h
f
F
ðOPVÞ
ðSTÞ
ðOPVÞ
FfðOPVÞ
¼ FfðSTÞ
f
F
ðSTÞ
h
ðOPVÞ
ðSTÞ
where Ff(OPV) and Ff(ST) are the fluorescence quantum yield of the
sample and of standard, respectively (the subscript f describe
fluorescence). f(OPV) and f(ST) are the absorption factors, the fraction
To emphasize the importance of the pure trans-configuration in
the vinyl bonds of OPV systems for some specific applications, it has

454
S.E. Estrada et al. / Journal of Molecular Structure 1133 (2017) 448e457
Fig. 2. Normalized absorption and emission spectra of the OPVs 1a-i (series 1) and 2a-g (series 2) in DMF, ordered by their bathochromic effect.
of the light impinging on the sample that is absorbed:
f(OPV) ¼ 1e10eA(OPV) and f(ST) ¼ 1e10eA(ST). F(OPV) y F(ST) are the in-
tegrated intensities (areas under the emission curve) of sample and
eCOOCH3 < eCN < eCOCH3 < eNO2 for series 1, and eCN < eNO2
for series 2.
In series 1, the OPV 1d, substituted with acetoxy groups
(R ¼ eOCOCH3) is a constitutional isomer of 1g (R ¼ eCOOCH3). The
direct interaction between the oxygen and the benzene ring over
1d produce a small red shift effect (3 nm in UVevis and 5 nm in FS
with respect to unsubstituted OPV 1c), which become much greater
by changing the oxygen by carbonyl group on OPV 1g (19 nm in
UVevis and 24 nm in FS with respect to unsubstituted OPV 1c),
suggesting that EW substituents have a greater bathochromic effect
than ED substituents. This also is corroborated for series 2; the
presence of the acetoxy group on OPV 2f produce a slight bath-
ochromic shift compared to the reference compound 2e.
The ED groups over OPVs of series 1 have similar effects. The
OPVs 1a (R ¼ eOCH3) and 1i (R ¼ eOH), have both the same shift of
13 nm in UVevis and shifts of 16 and 18 nm in FS, respectively. The
bathochromic effect produce by EW groups in OPV systems is very
similar for the compounds 1g (R ¼ eCOOCH3) and 1f (R ¼ eCN),
both with a shift of 19 nm in UVevis and of 24 and 27 nm in FS,
respectively.
For OPV 1h (R ¼ eCOCH3) it was found that the ketone sub-
stituent presents one of the largest bathochromic shifts (25 nm in
UVevis and 34 nm in FS), due possibly to the formation of a larger
electronic conjugation favored by the inductive effect over the
benzene by the carbonyl group directly attached to it. This also
could explain the largest bathochromic effect in the series 1 given
by the nitro group in OPV 1b (R ¼ eNO2) with a red shift of 51 nm in
standard spectra, respectively. h(OPV)
y h(ST) are the refractive
indices of the solvent used in the sample and reference solutions,
respectively.
The absorption spectra in series 1 and 2 (Fig. 2) showed that
both ED and EW groups at the two ends of the OPV structure leads
to a bathochromic shift compared with the OPV with hydrogen
atoms in that positions (OPVs 1c and 2e). Similar effect is observed
in the emission spectra. This trend is observed for most of the
substituents, except for the compound 1e (R ¼ eC6H5), where the
phenyl group caused a hypsochromic effect, might be due to the
extended electronic conjugation of the biphenyl system.
Disregarding the exception presented by compound 1e, the ED
and EW groups generally have a red shift that is more intensified
with EW groups. The same behavior has been reported in similar
studies of OPVs with ED and/or EW substituents on terminal rings
[35e37]. It has been shown that the trend depends on the ED or EW
strength of the substituents. In this work, it was observed that the
ED groups studied have a moderate effect on the bathochromic
shift and this shift increases with the increase of electron donating
strength of the substituent in the order eOCOCH3 < eOCH3 < eOH
for series 1, and eCH3 < eOCOCH3 < eOCH3 < eOH for series 2. On
the other hand, the bathochromic effect is intensified by the pres-
ence of the EW groups, and the trend also is related with the EW
strength
of
the
substituents
with
the
order

S.E. Estrada et al. / Journal of Molecular Structure 1133 (2017) 448e457
455
Table 1
Photophysical data for OPVs 1a-i and 2a-g in DMF (1ꢂ10ꢀ5 M).
OPV
labs (nm)
ε (Lmolꢀ1cmꢀ1
)
DE (eV)
lem (nm)
Ffa
1a
355 (sh)
e
3.08
404 (sh)
e
R ¼ OCH3
370 (max)
387 (sh)
408 (max)
51,800
e
422 (max)
0.66
e
1b
58,800
2.75
3.20
540 (max)
0.06b
R ¼ NO2)
1c
343 (sh)
357 (max)
375 (sh)
346 (sh)
360 (max)
378 (sh)
346 (max)
363 (sh)
360 (sh)
376 (max)
395 (sh)
360 (sh)
376 (max)
396 (sh)
367 (sh)
382 (max)
400 (sh)
357 (sh)
370 (max)
390 (sh)
l1 ¼ 338
l2 ¼ 398
l1 ¼ 361
l2 ¼ 448
l1 ¼ 328
l2 ¼ 395
l1 ¼ 342
l2 ¼ 418
l1 ¼ 327
l2 ¼ 393
l1 ¼ 328
l2 ¼ 395
l1 ¼ 338
l2 ¼ 398
e
388 (sh)
e
R ¼ H
62,000
e
406 (max)
0.86
e
1d
e
3.17
392 (sh)
411 (max)
e
R ¼ OCOCH3
60,400
e
0.80
e
1e
49,500
e
3.28
3.03
380 (sh)
398 (max)
418 (sh)
0.72
e
R ¼ C6H5
1f
e
e
R ¼ CN
55,400
e
433 (max)
0.60
e
1g
e
3.01
2.97
3.06
430 (max)
440 (max)
424 (max)
e
R ¼ COOCH3
72,600
e
0.63
e
1h
e
e
R ¼ COCH3
73,300
e
0.64
e
1i
R ¼ OH
e
e
49,700
e
0.70
e
2a
26,600
54,000
17,300
38,600
32,700
56,400
13,400
21,900
18,400
27,300
26,600
43,400
13,400
30,200
2.85
2.50
2.88
2.71
2.89
2.88
2.85
437 (max)
461 (sh)
N/Ac
0.48
0.85b
N/Ac
R ¼ OCH3
2b
R ¼ NO2
2c
434 (max)
460 (sh)
480 (max)
0.50
R ¼ CH3
0.95b
0.45
2d
R ¼ CN
2e
0.53b
0.86
436 (max)
458 (sh)
437 (max)
461 (sh)
437 (max)
462 (sh)
R ¼ H
0.94b
0.56
2f
R ¼ OCOCH3
2g
R ¼ OH
a
0.92b
0.55
0.88b
Quinine sulfate was used as standard for determination of fluorescence quantum yields (Ff) at lexc corresponding to the absorption maximum of series 1 and to the first
absorption maximum of series 2 (l1).
b
Perylene was used as standard for determination of Ff at lexc corresponding to the absorption maximum of OPV 1b and to the second absorption maximum of series 2 (l2).
No appreciable emission in DMF. sh: shoulder; max: wavelength of maximum absorption.
c
UVevis and 134 nm in FS, such effect changes the absorption and
emission properties of the OPV from the ultraviolet region to the
visible region of the electromagnetic spectrum.
530 nm. Recently it was reported that the emission of 2b changes
from 432 to 650 nm with solvents such as methanol, toluene,
chloroform, THF, dichlorobenzene, acetonitrile and DMSO [34].
Although the shift of ED and EW groups seems to be lower in
OPVs of series 2 that in the series 1 with respect to unsubstituted
OPVs for each series, it is important to note that the absorption and
emission wavelengths of OPVs in series 2 are more shifted toward
the visible region, indicating that the presence of two ED methoxy
groups over the central ring strongly increases the bathochromic
shift of OPVs. Comparing the reference compounds in each series, it
is clear that the methoxy groups in the central ring of the OPV 2e
produce a large bathochromic shift of 36 nm in the UVevis and
30 nm in the FS spectra with respect to unsubstituted OPV 1c. A
similar study reported by Chaieb, also showed that the introduction
of electron donating groups on the central ring leads to a red shift
[36].
The shifts in the wavelength generated by ED groups in com-
pounds of series 2 are smaller with respect to the reference com-
pound in this series (OPV 2e). The shifts obtained for OPVs 2c
(R ¼ eCH3) and 2f (R ¼ eOCOCH3) are very small and similar, with
shifts of 1 and 2 nm for absorption wavelengths l1 and l2 respec-
tively, and shifts of ꢀ2 and 1 nm in the emission spectrum,
respectively. The red shift increases slightly when increasing the ED
ability in the OPVs 2a (R ¼ eOCH3) and 2g (R ¼ eOH) with shifts in
the absorption spectrum of 11 nm (l1) and 5 nm (l2) and only of
1 nm in the emission spectrum.
The shift tends to increase with strongly EW groups in series 2.
For OPV 2d (R ¼ eCN) shifts of 15 nm (l1) and 25 nm (l2) in the
UVevis and 44 nm in the FS spectrum were obtained.
As in series 1, the OPV with nitro groups of series 2 (compound
2b), possess the highest bathochromic effect. Although this OPV did
not show appreciable emission in solution (DMF), the absorption
spectrum showed the greatest shift with values of 34 nm (l1) and
55 nm (l2). Interestingly, this compound also presents a particular
behavior, in solvents like acetonitrile or DMF shows a negligible
emission; in chloroform a weak orange emission above 600 nm,
while in diethyl ether and toluene present moderate emission at
The bathochromic effect of the methoxy groups at the central
ring also shows that by comparing compounds substituted with
acetoxy groups in series 1 and 2, compounds 1d and 2f, the
methoxy groups generate a shift of 35 nm in the absorption spec-
trum and 26 nm in the emission spectrum, values that are com-
parable but lower than those generated for methoxy groups in the
reference compounds 2e and 1c.
When the terminal groups are ED, the bathochromic effect of

456
S.E. Estrada et al. / Journal of Molecular Structure 1133 (2017) 448e457
the methoxy groups in the central ring tends to be lower; com-
pound 2a (R ¼ eOCH3) with shifts of 28 nm in the UVevis and
15 nm in the FS spectra compared to 1a (R ¼ eOCH3). Another
example is OPV 2g (R ¼ eOH) with shifts of 28 nm in the UVevis
and 13 nm in the FS spectrum compared to 1i (R ¼ eOH). When the
terminal groups are EW, the bathochromic effect due to the
methoxy groups in the central ring tends to increase; OPVs 2d
(R ¼ eCN), shifts 42 nm in the absorption spectrum and 47 nm in
the emission spectrum compared to 1f (R ¼ eCN) and 2b
(R ¼ eNO2), with shifts of 40 nm in the UVevis with respect to 1b
(R ¼ eNO2). These results show that the bathochromic shift caused
by the methoxy groups at the central ring is influenced by the
presence of other ED or EW substituents at the end rings of the
molecule, suggesting a change on the electronic environment of the
PV system.
When the Ff values are measured at the lexc of the second ab-
sorption maximum l2 similar trend was observed:
(
)
eCH3 > H > OCOCH3 > OH > OCH3 > CN. In this case, the quantum
yields obtained for OPVs with methyl and hydrogen atoms at the
end rings were quite similar and the same trend in decrease of Ff
with the presence of substituents was found. The lower Ff value
obtained for the cyano compound regards to the absence of emis-
sion of the nitro-compound (OPV 2b) also corroborate that the EW
groups have the biggest effect over the decrease of Ff values related
with ED substituents.
From this study, it can be inferred that the phenylenevinylene
type systems possess valuable optoelectronic properties, which can
be adjusted with the inclusion of substituents with strongly ED
and/or EW nature. The inclusion of these groups in the structure of
PV derivatives produces a bathochromic shift while leads to a
The effect of the position of methoxy groups over the red shift
was also evaluated by comparing the displacements caused by
these groups in isomers 1a (OCH3 groups at the end rings) and 2e
(OCH3 groups at the central ring). The shift generated by methoxy
groups in the OPV 1a is 13 nm in UVevis and 16 nm in FS with
respect to unsubstituted compound 1c (R ¼ H), while the shift
caused for methoxy groups in the compound 2e is much higher,
36 nm in UVevis and 30 nm in FS versus 1c, which might indicate
that the presence of two methoxy groups over the same ring
strongly intensifies the red shift.
decrease in the DE values. This could be quite beneficial when the
goal is to take advantage of renewable sources such as solar energy
to excite molecules in order to make them useful in various ap-
plications (i.e. as organic photo-redox catalysts, components in the
active layer of photovoltaic cells, conductive materials, etc.). It was
also noticed that unsubstituted OPVs exhibit excellent fluorescence
quantum yields. The inclusion of ED groups such as acetoxy, hy-
droxyl, phenyl, methyl and methoxy, improve the absorption and
emission properties but with a decrease in quantum yields.
Nevertheless, these remain high, making these systems potential
candidates for applications in OLEDs and laser dyes.
With respect to the molar attenuation coefficient (ε), no clear
trends were observed (Table 1). OPVs of series 2 showed values of ε
lower than those obtained for OPVs of series 1. The
tained (see Table 1) shows that the energy gap decreases according
DE values ob-
4. Conclusions
to the following trend: C6H5 > eH > eOCOCH3 > eOCH3 > eOH
Sixteen model OPVs were synthesized by Mizoroki-Heck cross
coupling reaction. The methodology reported could be used as
synthetic route for phenylenevinylene systems with highly desired
functional groups in their molecular structure, with high stereo-
selectivity to trans-trans configuration of the vinyl bonds and
allowing the synthesis of compounds that present difficulties to be
obtained by other synthetic methodologies. The optoelectronic
study in DMF solutions showed that ED and EW groups at the end
rings lead a red shift in absorption and emission spectrums, with a
magnitude that depends of the electronic strength of the sub-
stituents. In addition, inclusion of methoxy groups at the central
rings increases the red shift into the visible region of the electro-
>
eCN
eH > eOCOCH3, eCH3 > eOCH3, eOH > eCN > eNO2 (series 2).
The EW groups in the OPV system lead to smaller E values,
being the lowest the eNO2 (2.75 eV for OPV 1b; 2.50 eV for OPV 2b).
The ED groups also contribute to decrease the E with respect to
> eCOOCH3 > eCOCH3 > eNO2 (series 1) and
D
D
unsubstituted OPVs, once again with the exception of OPV
substituted with phenyl rings (OPV 1e), which in this case presents
the greatest ED(3.28 eV). The trend shows that DE decrease with
the increase of ED and EW strength of the substituents, in agree-
ment with the increase in the bathochromic shift. This is reason-
able, considering that the increase in wavelength implies a decrease
in energy required for excitation. The same trend was found by Wu
and coworkers for OPVs substituted with other ED groups [35]. The
magnetic spectrum. However,
DE values decrease with increasing
of ED and EW strength of the substituents. Properties like molar
attenuation coefficient and fluorescence quantum yield were
calculated without a clear trend. Nevertheless it was found that ED
and EW substituents decrease the Ff values with respect to
unsubstituted OPVs.
DE values obtained for OPVs of series 2 are much lower than ob-
tained for OPVs of series 1, which indicate that the introduction of
methoxy groups in central ring favors the reduction of the energy
gap.
Furthermore, regarding the fluorescence quantum yields (Ff), it
is noted that for series 1 both ED and EW groups leads to a decrease
in the Ff values compared to the unsubstituted compound 1c, ac-
Acknowledgments
cording to the order:
H > eOCOCH3 > eC6H5 > eOH >
This work was supported by the Administrative Department of
Science, Technology and Innovation through the projects No. 608-
2011 and 534-2011. Sandra Estrada is grateful to the National
Doctoral Program of COLCIENCIAS (Grant No. 528-2011) for provi-
sion of a doctoral fellowship.
eOCH3 > eCOCH3 > eCOOCH3 > eCN. Although there is no clear
trend, it was observed that ED groups such as eOCOCH3, eC6H5,
eOH and eOCH3 reduce the quantum yield relative to the reference
OPV 1c from 0.86 to 0.80, 0.72, 0.70 and 0.66 respectively. This
decrease is intensified with EW groups such as eCOCH3, eCOOCH3
and eCN up to values of 0.64, 0.63 and 0.60 respectively. Chaieb and
coworkers also found a similar trend; high values of Ff for OPVs
with ED groups and low values for OPVs with EW substituents [36].
In the case of OPVs of series 2, similar to that found for series 1,
the presence of ED and EW groups at the end rings leads to a
decrease in the Ff value relative to the reference OPV 2e. The Ff
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2016.12.032.
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The ED and EW substituent reduce the Ff value from 0.86 to 0.45.
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