Spectroscopic characterization of halogen- and cyano-substituted pyridinevinylenes synthesized without catalyst or solvent

Article abstract of DOI:10.2478/s11696-010-0012-z

An efficient Knoevenagel route using green chemistry conditions was applied for the synthesis of halogen- and cyano- substituted pyridinevinylene compounds. Absorption and fluorescence emission spectra of these conjugated compounds were recorded and compared in order to evaluate the effect of substituents on the electronic properties of pyridinevinylene compounds. The substituents studied were terminal Cl and F, two or three aromatic rings, as well as a cyano group attached to a C=C double bond. The compounds synthesized are: (E)-2-(4-fluorostyryl)pyridine, (E)-2-(4-chlorostyryl)pyridine, (E)-4-(4-chlorostyryl)pyridine, 2,3-diphenylacrylonitrile, 3-phenyl-2-(pyridin-2-yl)acrylonitrile, 3-phenyl-2-(pyridin-3-yl)acrylonitrile, 2-phenyl-3-(pyridin-2-yl)acrylonitrile, 3,3′-(1,4-phenylene)bis(2-phenylacrylonitrile), 3,3′-(1,4-phenylene)bis(2-(pyridin-2-yl)acrylonitrile), and 3,3′-(1,4-phenylene)bis(2-(pyridin-3-yl)acrylonitrile). The solvent-free method used in this work allows obtaining each compound by controlling the reaction temperature. The compounds were characterized by infrared spectroscopy and 1H-NMR spectroscopy.

Full text of DOI:10.2478/s11696-010-0012-z

Chemical Papers 64 (3) 360–367 (2010)
DOI: 10.2478/s11696-010-0012-z
ORIGINAL PAPER
Spectroscopic characterization of halogen- and cyano-substituted
pyridinevinylenes synthesized without catalyst or solvent
a
aM. Judith Percino*, Víctor M. Chapela,aLing-Fa Montiel,aEnrique
Pérez-Gutiérrez,bJosé Luis Maldonado
aCentro de Química, Instituto de Ciencias, Universidad Autónoma de Puebla, Complejo de Ciencias, ICUAP,
Edif. 194, 22 Sur y San Claudio, C.P. 72570 Puebla, Puebla, México
bCentro de Investigaciones en Óptica A.C. (CIO), Lomas del Bosque # 115, Col. Lomas del Campestre,
C.P. 37150 León, Guanajuato, México
Received 23 June 2009; Revised 9 October 2009; Accepted 20 October 2009
An efficient Knoevenagel route using green chemistry conditions was applied for the synthesis of
halogen- and cyano- substituted pyridinevinylene compounds. Absorption and fluorescence emis-
sion spectra of these conjugated compounds were recorded and compared in order to evaluate the
effect of substituents on the electronic properties of pyridinevinylene compounds. The substituents
studied were terminal Cl and F, two or three aromatic rings, as well as a cyano group attached
—
to a C C double bond. The compounds synthesized are: (E)-2-(4-fluorostyryl)pyridine, (E)-2-(4-
—
chlorostyryl)pyridine, (E)-4-(4-chlorostyryl)pyridine, 2,3-diphenylacrylonitrile, 3-phenyl-2-(pyridin-
2-yl)acrylonitrile, 3-phenyl-2-(pyridin-3-yl)acrylonitrile, 2-phenyl-3-(pyridin-2-yl)acrylonitrile, 3,3ꢀ-
(1,4-phenylene)bis(2-phenylacrylonitrile), 3,3ꢀ-(1,4-phenylene)bis(2-(pyridin-2-yl)acrylonitrile), and
3,3ꢀ-(1,4-phenylene)bis(2-(pyridin-3-yl)acrylonitrile). The solvent-free method used in this work al-
lows obtaining each compound by controlling the reaction temperature. The compounds were char-
acterized by infrared spectroscopy and 1H-NMR spectroscopy.
c
ꢀ 2009 Institute of Chemistry, Slovak Academy of Sciences
Keywords: pyridinevinylenes, solvent free synthesis, Knoevenagel condensation
Introduction
reactants, but also on the strength of the employed
bases, nature of the solvents, on their dissociating
powers, as well as side reactions (Bergmann et al.,
1959; Best & Thorpe, 1909).
Considerable attention has been given to the de-
velopment of reactions under solvent-free conditions
in recent years (Tanaka & Toda, 2000). The Knoeve-
nagel condensation reaction was originally reported
by Emil Knoevenagel (1896). Since then, systematic
studies have been reported in literature (Cope, 1937).
For example, a series of alkylidene cyanoacetic esters
were prepared by condensing ten simple ketones with
methyl cyanoacetate using secondary amines such as
piperidine and diethylamine as catalysts (Krauch &
Kunz, 1969). Other authors (Patai & Israeli, 1960a,
1960b, 1960c; Patai & Zabicky, 1960) have shown that
the Knoevenagel condensation is a complex reaction
that depends not only on the relative reactivity of the
Meanwhile, conjugated organic oligomers and poly-
mers have become a major class of technologically im-
portant materials. Conjugated organic polymers have
the advantages of low cost, easy modification of prop-
erties by appropriate substitution and solvent pro-
cessability. In particular, poly(p-phenylenevinylene)
PPV and its derivatives have received considerable
attention during the last two decades (Burroughes
et al., 1990; Friend et al., 1999). In order to in-
—
—
crease the electron affinity of PPV, –C N sub-
—
stituents have been attached either to the double
bonds (Friend et al., 1999; Jin et al., 2003; Gillis-
*Corresponding author, e-mail: jpercino@siu.buap.mx

M. J. Percino et al./Chemical Papers 64 (3) 360–367 (2010)
361
z
z
W
W
O
+
Y
X
Y
U
X
U
U
I
II
III
I
U
X
Y
W
II
Z
III
X
Y
W
Z
a
b
c
d
e
C
C
C
C
N
N
C
C
N
C
C
H
H
CN
CN
CN
a
b
c
Cl
F
H
a
b
c
d
e
f
C
C
C
C
C
N
N
N
C
C
N
C
C
H
H
H
CN
CN
CN
F
Cl
Cl
H
H
H
N
C
C
C
C
N
C
C
C
Fig. 1. Synthesis of (E)-2-(4-fluorostyryl)pyridine IIIa, (E)-2-(4-chlorostyryl)pyridine IIIb, (E)-4-(4-chlorostyryl)pyridine IIIc, 2,3-
diphenylacrylonitrile IIId, 3-phenyl-2-(pyridin-2-yl)acrylonitrile IIIe, and 3-phenyl-2-(pyridin-3-yl)acrylonitrile IIIf .
sen et al., 2001; Mitschke & Bäuerle, 2000; Detert
& Sugiono, 2000) or to the aromatic rings (Pinto
et al., 2000) of PPV. The introduction of cyano
and other electron withdrawing groups into PPV
reduces the barrier for electron excitation in the
molecule and improves the electron transport proper-
ties (Pinto et al., 2000). Several conjugated polymers
containing cyano-distyrylbenzene units as emitting
chromophores, such as poly(phenylenecyanovinylene)s
(Liu et al., 2002), poly(N-alkyl-3,6-carbazole), and
poly[bicarbazolyl-ene-alt-phenylenebis(cyanovinylene)
(Roncalli, 1997) have been synthesized (Liu et al.,
2002; Roncalli, 1997; Greenham et al., 1993; Boucard
et al., 1997; Boucard, 2001; Bellete˛te et al., 2005) via
the Knoevenagel or Wittig reactions (Detert & Sug-
iono, 2000; Pinto et al., 2000). However, these routes
generally gave low molecular mass, insoluble and in-
fusible materials.
“Green chemistry” syntheses without catalyst or
solvent have previously been applied to obtain several
oligophenylenevinylenes (Dubey et al., 2007; Wang &
Cheng, 2004; Bigi et al., 2000) or styrylpyridines and
interesting intermediates (Percino et al., 1997, 2006,
2007, 2008; Percino & Chapela, 2000; Chapela et al.,
2003). In this work, the same solvent-free, catalyst-
free conditions were used to synthesize ten conjugated
molecules. Pyridinevinylene compounds with terminal
Cl or F as well as molecules with a –CN group on the
and were distilled before use. IR spectra of the prod-
ucts were recorded on a Vertex model 70 Bruker 750
FT-IR spectrophotometer by ATR. 1H-NMR spec-
tra were obtained on a Varian 300-MHz NMR spec-
trometer in CDCl3. UV-VIS absorption spectra were
recorded at room temperature in CHCl3 on a Spec-
trometer SD2000 Ocean Optics with the light source
UV-VIS DT 1000 CE from Analytical Instrument
System, using quartz cells. Photoluminescence spec-
tra were recorded using the spectrometer mentioned
above as the detector and a Mineralight UVGL-58 UV
lamp as the excitation source. Melting points were
measured using an SEV (0–300◦C) apparatus and they
are uncorrected.
Synthesis of compounds IIIa–IIIf
The condensation reaction scheme is presented in
Fig. 1. Compounds IIIa to IIIf were obtained from
the corresponding methylpyridine or pyridylacetoni-
trile I with the corresponding aldehyde II at the
molar ratio of 1 : 1. The mixtures were reacted at
a temperature between 120◦C and 140◦C with reac-
tions times between 20–40 h. All reactions were car-
ried out at reflux in the absence of any condens-
ing agent or solvent. At all times during the reac-
tion, the mixtures appeared oily with a brown to red
color. The mixtures were treated with a solution of
2 M NaOH to precipitate the products as powders.
These products were purified by recrystallization from
—
—
C
C linkage were synthesized in order to evaluate
their absorption and emission properties. Only tem-
perature was used to control the reactions; the prod-
ucts were precipitated using a 2 M NaOH solution.
Each compound was characterized by IR, 1H-NMR,
UV-VIS, and fluorescence spectroscopy.
1
hexane or cyclohexane and characterized by IR, H-
NMR spectroscopy, UV-VIS, and fluorescence spec-
troscopy.
Synthesis of compounds VIa–VId
Experimental
The condensation reaction proceeded according to
Fig. 2. Compounds VIa to VId were obtained from
the corresponding acetonitrile derivative IV with 2-
pyridinecarbaldehyde or terephthalaldehyde V. Molar
ratio of the acetonitrile derivative to the other reac-
tant was 1 : 1 for compound VIa and 2 : 1 for VIb–VId.
2-Methylpyridine, 4-methylpyridine, benzyl cyan-
ide, 2-pyridylacetonitrile, 3-pyridylacetonitrile, ben-
zaldehyde, 4-fluorobenzaldehyde, 4-chorobenzaldehy-
de, 2-pyridinecarbaldehyde, and terephthalaldehyde
were acquired from Aldrich Chemical Co. (México)

362
M. J. Percino et al./Chemical Papers 64 (3) 360–367 (2010)
z
z
CN
CN
O
Y
Y
+
X
X
U
U
IV
V
VI
IV
U
X
V
Y
Z
VI
a
U
C
X
C
Y
N
Z
a
b
c
C
C
N
C
N
C
a
b
c
N
C
C
H
CHO
CHO
H
b
c
d
C
C
N
C
N
C
C
C
C
CN
N
CN
CN
N
Fig. 2. Synthesis of 2-phenyl-3-(pyridin-2-yl)acrylonitrile VIa, 3,3ꢀ-(1,4-phenylene)bis(2-phenylacrylonitrile) VIb, 3,3ꢀ-(1,4-phenylene)bis(2-
(pyridin-2-yl)acrylonitrile) VIc, and 3,3ꢀ-(1,4-phenylene)bis(2-(pyridin-3-yl)acrylonitrile) VId.
Table 1. Conditions, yields, and properties of compounds shown in Figs. 1 and 2
Temperature Time Yield
M.p.
◦C
Compound
Appearance
Reference
Solubility
◦C
140
h
30
%
16
19
11
40
75
IIIa
IIIb
IIIc
IIId
IIIe
Brown powder
Brown powder
Brown powder
78
Percino et al. (2007,
2008)
CHCl3, THF, acetone,
EtOH, MeOH, DMSO
120
21.5
20.75
51
81–82
88–89
85–86
63
Smith (1947)
CHCl3, THF, acetone,
MeOH, DMSO
135
THF, acetone, EtOH,
MeOH, DMSO
120–140
120
Yellow–transparent
crystal
Gierschner et al.
(2000)
CHCl3, THF, acetone,
EtOH, MeOH, DMSO
29.5
Yellow crystal
Castle and Seese
(1955), Tetsuzo et al.
(1969)
CCl4, CHCl3, THF,
acetone, EtOH,
MeOH, DMSO
IIIf
120
120
22.5
3:30
34
50
Brown–transparent
crystal
101
CHCl3, THF, acetone,
EtOH, MeOH, DMSO
VIa
Red powder
97–98
Castle and Seese
(1955)
CHCl3, THF, acetone,
EtOH, MeOH, DMSO
VIb
VIc
140
120
2:30
1:30
55
73
Orange powder
Yellow–brown crystal
223–224
215–216
DMSO
CCla4, CHCla3, THFa,
acetonea, DMSOa
VId
120
3
78
Yellow powder
114–115
DMSOa
a) Soluble only by heating.
The mixtures were reacted at temperatures between
120◦C and 140◦C using reaction times of 2–5 h, shorter
than those for compounds III (Fig. 1). All reactions
were carried out at reflux in the absence of any con-
densing agent or solvent. At all times during the reac-
tion, the mixtures appeared oily with a brown to red
color. All mixtures were treated with a solution of 2
M NaOH to precipitate the products as powders. The
products were purified by recrystallization from hex-
ane, chloroform, or cyclohexane, followed by charac-
Results and discussion
Figs. 1 and 2 show the reactions used to produce
compounds with and without cyano groups in their
structure. To obtain the target compounds, the re-
action temperature, time, and the presence of sub-
stituents –F, –Cl, and –CN play important roles in
the product formation. Table 1 shows the yields and
measured properties of each compound. The yields ob-
tained here were better than those obtained by other
reactions reported previously under the same condi-
tions (Percino et al., 2006).
1
terization by IR, H-NMR, UV-VIS, and fluorescence
spectroscopy.

M. J. Percino et al./Chemical Papers 64 (3) 360–367 (2010)
Table 2. Spectral characterization of compounds shown in Figs. 1 and 2
363
Compound
Spectral characterization
IR (KBr), ν˜/cm−1: 3067 (m), 3006 (m), 1638 (m, C C), 1585 (s), 1505 (s), 1467 (s), 1432 (s), 1227
—
IIIa
—
F
(s, C—F), 977 (s, out-of-plane δ-CH)
a´
3
1H NMR (300 MHz, CDCl3), δ: 8.583 (d, 1H, H-d), 7.604 (d, 1H, J(a,a )
(m, 2H, H-b), 7.344 (m, 1H, H-c), 7.142 (m, 4H Ph), 7.078 (d, 1H, J(a,a )
ꢀ
ꢀ
= 16.2 Hz, H-a’), 7.543
= 16.2 Hz, H-a)
N
c
d
3
a
b
b´
IR (KBr), ν˜/cm−1: 3076 (m), 3046 (s), 3004 (m), 1636 (m, C C), 1583 (s), 1491 (s), 1469 (s), 1431
—
IIIb
—
Cl
(s), 975 (s, out-of-plane δ-CH), 818 (s, C—Cl)
a´
3
1H NMR (300 MHz, CDCl3), δ: 8.583 (d, 1H, H-d), 7.651 (t, 2H, H-b), 7.589 (d, 1H, J(a,a )
Hz, H-a’), 7.482 (d, 2H, Ph), 7.319 (d, 2H, Ph), 7.143 (d, 1H H-c), 7.123 (d, 1H, J(a,a )
ꢀ
= 16.2
N
c
d
3
ꢀ
= 16.2 Hz,
a
b
b´
H-a)
IR (KBr), ν˜/cm−1: 3070 (m), 3029 (m), 1635 (m, C C), 1593 (s), 1548 (m), 1490 (s), 1412 (s), 972
—
IIIc
—
Cl
(s, out-of-plane δ-CH), 827 (s, C—Cl)
1H NMR (300 MHz, CDCl3), δ: 8.613 (dd, 2H, H-c), 7.506 (m, 6H, Py, Ph), 7.294 (d, 1H, J(a,a )
= 16.2 Hz, H-a’), 7.033 (d, 1H, J(a,a ) = 16.2 Hz, H-a)
ꢀ
a´
3
b
c
3
ꢀ
a
N
b
c
IIId
c
IR (KBr), ν˜/cm−1: 3099 (m), 3056 (m), 3032 (m), 2218 (s, CN), 1678 (m, C C), 1492 (m), 1447
—
—
—
(m), 760 (s, CH, CR2—CH)
CN
b
1H NMR (300 MHz, CDCl3), δ: 7.896 (m, 2H, H-b), 7.687 (m, 2H, H-d), 7.529 (s,1 H, H-a), 7.494
a
c
(m, 6H, Ph)
b
IIIe
IR (KBr), ν˜/cm−1: 3081 (w), 3054 (m), 3006 (m), 2215 (s, CN), 1676 (w, C C), 1600 (m), 1579
—
—
e
—
(m), 1567 (m), 779 (m, CH, CR2 CH)
—
CN
1H NMR (300 MHz, CDCl3), δ: 8.643 (d, 1H, H-d), 8.468 (s, 1H, H-a), 7.997 (m, 2H, H-b), 7.811
N
d
e
(m, 2H, H-e), 7.493 (m, 3H, Ph), 7.295 (m, 1H, H-c)
a
b
b´
c
IIIf
IR (KBr), ν˜/cm−1: 3097 (w), 3046 (m), 2212 (s, CN), 1660 (w, C C), 1603 (m), 1572 (m), 754 (m,
—
—
d
—
CH, CR2—CH)
CN
c
1H NMR (300 MHz, CDCl3), δ: 8.911 (d, 1H, H-c), 8.620 (dd, 1H, H-b), 7.961 (m, 2H, H-dd’), 7.55
N
(s, 1H, H-a), 7.486 (m, 5H, Py, Ph)
d´
a
b
VIa
IR (KBr), ν˜/cm−1: 3058 (w), 3008 (m), 2247 (m, CN), 1679 (m, C C), 1586 (s), 1552 (s), 749 (m,
—
—
c
—
CH, CR2—CH)
b
b
´
1H NMR (300 MHz, CDCl3), δ: 8.751 (d, 1H, Hd), 8.216 (s, 1H, H-a), 7.907 (m, 2H, Hb), 7.466 (m,
CN
5H, Ph), 7.007 (m, 1H, H-c)
d
N
a
VIb
IR (KBr), ν˜/cm−1: 3059 (m), 3030 (m), 2218 (s, CN), 1669 (s, C C), 1597 (m), 1540 (m), 1497
—
—
—
(m), 759 (m, CH, CR2—CH)
a´
1H NMR (300 MHz, DMSO-d6), δ: 8.138 (s, 2H, H-a), 8.098 (s, 4H, Ph-b), 7.844 (m, 10H, Py)
CN
CN
b
a
VIc
IR (KBr), ν˜/cm−1: 3075 (m), 3054 (m), 3012 (m), 2219 (s, CN), 1679 (m, C C), 1606 (m), 1583
—
—
—
(s), 755 (m, CH, CR2—CH)
e
1H NMR (300 MHz, DMSO-d6), δ: 8.722 (m, 2H, H-d), 8.546 (s, 2H, H-a), 8.180 (s, 4H, Ph-e), 8.030
N
CN
CN
N
d
(m, 4H, H-b), 7.514 (m, 2H, Hc)
a
b´
b
c
VId
IR (KBr), ν˜/cm−1: 3057 (m), 3011 (m), 2215 (s, CN), 1647 (m, C C), 1586 (m), 1568 (m), 759 (m,
—
—
—
CH, CR2—CH)
c
N
1H NMR (300 MHz, DMSO-d6), δ: 8.655 (s, 2H, H-b), 8.290 (s, 2H, H-a), 8.251 (s, 4H, Ph-c), 8.196
CN
b
CN
N
(m, 6H, Py)
a
FTIR and NMR characterization
lar characteristic bands at 1638 cm−1, 1636 cm−1, and
1635 cm−1, respectively, assigned to the double bond
—
ν(–C C–) of alkenes conjugated with an aromatic
—
FTIR spectra of compounds IIIa–IIIc showed simi-

364
M. J. Percino et al./Chemical Papers 64 (3) 360–367 (2010)
ring (Bellamy, 1975; Nakanishi & Solomon, 1977), as
well as bands at 977 cm−1, 975 cm−1, and 972 cm−1
a
1.0
0.8
0.6
0.4
0.2
0.0
,
respectively, characteristic of a –C—H bond out of
plane bending vibration for a double bond in the trans
configuration. Also, bands at 1227 cm−1 and 822 cm−1
were assigned to ν(C—F) and ν(C—Cl), respectively.
The observed IR peaks are consistent with the ex-
pected structures (Table 2).
From 1H-NMR, principal evidence (Silverstein &
Webster, 1997) for the formation of compounds IIIa–
IIIc were the doublets due to protons in the trans po-
sition of a double bond at δ: 7.604–7.550 and 7.079–
7.025 each with J = 16.2 Hz and J = 16.2 Hz for IIIa,
δ: 7.294–7.240 and 7.033–6.979 with J = 16.2 Hz and J
= 16.2 Hz for IIIb, and δ: 7.591–7.537 and 7.126–7.072
with J = 16.2 Hz and J = 16.2 Hz for IIIc. IR char-
acterization of compounds IIId–IIIf showed bands as-
200
250
300
350
400
450
Wavelength/nm
b
1.0
0.8
0.6
0.4
0.2
0.0
—
signed to stretching ν(C C) of a double bond shifted
—
to a higher wavelength than typical thus suggesting
that the –CN group is attached to the double bond.
The signals at 1678 cm−1, 1679 cm−1, 1676 cm−1
,
and 1660 cm−1 were assigned as ν(C C) of an alkene
—
—
conjugated with an aromatic ring, and the bands for
IIId–IIIf at 743 cm−1, 739 cm−1, and 754 cm−1 were
assigned to a δ(C—H) vibration on a double bond.
Bands at 2218 cm−1, 2215 cm−1, and 2212 cm−1 were
1
300 400 500 600 700 800 900 1000
Wavelength/nm
assigned to the –CN group. From H-NMR, the prin-
cipal evidence for compounds IIId–IIIf was the singlet
appearing with the chemical shifts of 7.59, 8.46, and
7.55, respectively. This signal was caused by a unique
proton attached to a double bond.
Fig. 3. Normalized absorption (a) and emission (b) spectra of
compounds IIIa (solid line), IIIb (dotted line), and IIIc
(dashed line) in chloroform.
The reaction conditions for VIa–VId allowed ob-
taining these compounds with good yields in shorter
reaction times than the other compounds, and with-
out the separation problem after the reaction. These
attributes may be explained by high reactivity of -
CH2CN through the nitrogen atom in the pyridine
ring and –CN. Characterization of VIa–VId by IR
spectroscopy indicated the presence of the following
der at around 278 nm. The shape of the spectra is also
typical for the trans configuration of styrylpyridines
(Giglio et al., 2000). However, no shift of the absorp-
tion wavelength expected from the presence of –F or
–Cl was observed.
Emission spectra of compounds IIIa and IIIc af-
ter a 368 nm excitation are shown in Fig. 3b. Photo-
luminescence spectra of the molecules in CHCl3 are
characterized by a strong emission with a maximum
at 491 nm and a shoulder of weaker intensity at 418
nm. The difference of isomers IIIb and IIIc emission
wavelengths (Fig. 3b) is probably caused by the π-
delocalization length regulated by ortho and para link-
ages to the pyridine ring and the substituent effects
considering that IIIb shows a shoulder with higher in-
tensity, 418 nm, than the other two compounds, 491
nm.
characteristic bands: signals at 1679 cm−1, 1669 cm−1
,
1679 cm−1, and 1647 cm−1 were assigned to ν(C C)
—
—
of an alkene conjugated with an aromatic ring where
–CN is attached to the alkene double bond; bands at
749 cm−1, 759 cm−1, 736 cm−1, and 759 cm−1 for
each compound were assigned to a δ(C—H) vibra-
tion of a double bond; and bands at 2247 cm−1, 2218
cm−1, 2219 cm−1, and 2215 cm−1 were assigned to
1
the –CN group. From H-NMR, principal evidence of
compounds VIa–VId was the singlet at δ: 8.212, 8.138,
8.546, and 8.290, respectively, caused by a unique pro-
ton attached to a double bond.
Absorption spectra and the corresponding fluores-
cence of compounds IIId–IIIf with one –CN group at-
tached to the double bond are shown in Fig. 4a. In
contrast to IIIa–IIIc (Fig. 3a), the broad absorption
peak for IIId–IIIf consists of only one maximum at 309
nm (IIId), 324 nm (IIIe), and 314 nm (IIIf ). This ab-
sorbance was assigned to a π–π* transition of the con-
UV-VIS and fluorescence spectra
UV absorption spectra of compounds IIIa–IIIc are
shown in Fig. 3a. Two bands are observed: a strong
absorption one at 311–318 nm which is typical for the
π–π* transition of conjugated segments, and a shoul-

M. J. Percino et al./Chemical Papers 64 (3) 360–367 (2010)
365
a
a
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
200
250
300
350
400
450
250
300
350
400
450
500
550
Wavelength/nm
Wavelenght/nm
1.0
(b)
b
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
300 400 500 600 700 800 900 1000
Wavelenght/nm
400
500
600
700
800
Wavelength/nm
Fig. 5. Normalized absorption (a) and emission (b) spectra
of compounds VIa (solid line), VIb (dashed line), VIc
(dotted line), and VId (dot–dashed line) in chloroform.
Fig. 4. Normalized absorption (a) and emission (b) spectra of
compounds IIId (solid line), IIIe (dashed line), and IIIf
(dot–dashed line) in chloroform.
jugated segments. No appreciable bathochromic shift
caused by the presence of the –CN group in the struc-
ture was observed.
523 nm (VIb) suggest that introduction of the cyano
group into the structure of VIa, VId, and VIb led to
a weak bathochromic effect. The VId and VIb isomers
have a cyano group on the vinylene moiety in the alpha
position. However, a large bathochromic effect can be
seen in Fig. 5b for VIc and VIb compared to VIa,
which could be caused by the addition of an aromatic
ring into the structure.
The fluorescence spectra of IIId–IIIf are shown in
Fig. 4b. The maximum emission wavelengths for com-
pounds IIId and IIIf were almost identical at 500 nm
and 499 nm, respectively, while IIIe emitted at 488
nm. These three compounds did not display any sig-
nificant changes of their electronic spectra due to the
electron-withdrawing functional group attached.
Fig. 5 shows the absorption and fluorescence spec-
tra of compounds VIa–VId. Each compound showed
one broad absorption peak with the maxima at 321
nm (VIa), 318 nm (VIa), 380 nm (VIa), and 359 nm
(VId). Even though VIb is a compound with three aro-
matic rings, compared to VIa, both absorbed light at
around 318 nm. The absorption maxima for VIc and
VId were red shifted suggesting that π-conjugation of
the compound’s main chain was interrupted by meta
linkages reducing the π-conjugation length (Bellamy,
1975; Nakanishi & Solomon, 1977; Williams & Flem-
ing, 1980; Kang et al., 1997).
Conclusions
A series of highly soluble pyridinevinylene deriva-
tives containing either two or three rings (pyridine
or benzene rings) were prepared by the reaction
of various benzaldehydes with methylpyridine, ben-
zyl cyanide or different pyridylacetonitriles as well
as by reacting 2-pyridinecarbaldehyde or terephtha-
laldehyde with benzyl cyanide or different pyridy-
lacetonitriles applying the Knoevenagel method under
green chemistry conditions (without solvent or cata-
lyst). Electron withdrawing groups in the para posi-
tions of the rings enabled tuning the electronegativ-
ity of the chromophores without significant changes
of the electronic spectra; whereas CN substituents on
the vinylene moiety of the compounds led to weak
The emission spectra of compounds VIa–VId are
shown in Fig. 5b. Their photoluminescence maxima
at 387 nm (VIa), 433 nm (VIc), 475 nm (VId), and

366
M. J. Percino et al./Chemical Papers 64 (3) 360–367 (2010)
bathochromic shifts. For compounds with an aromatic
ring, a strong effect of the compound structure on the
absorbance and fluorescence was observed.
10.1002/(SICI)1521-4095(200005)12:10<757::AID-ADMA
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