Fluorescence improvement of pyridylacrylonitrile by dimethylaminophenyl- substitutions: The effect of packing modes of conjugated compounds

Article abstract of DOI:10.1016/j.molstruc.2012.10.025

Four (dimethylamino)arylacrylonitrile derivatives in the solid state are presented. The compounds were characterized by single crystal X-ray diffraction to investigate the effects of substituents on the resulting crystals lattices and to examine the effects of the factors controlling their solid state on photochemical behavior. The molecules included 2-(phenyl)-3-(4- dimethylaminophenyl)acrylonitrile (I) 2-(2′-pyridyl)-3-(4- dimethylaminophenyl)-acrylonitrile (II), 2-(3′-pyridyl)-3-(4-dimethyl- aminophenyl)acrylonitrile (III) and 2-(3′-pyridyl)-3-(4- dimethylaminophenyl)acrylonitrile (IV). The dimethyl-, CN groups and the position of nitrogen atom in the pyridyl groups affected the nature of the molecular packing and consequently their fluorescence properties. The lattice of each compound is compared with acrylonitriles previously reported and the effect of substitution on the crystal properties with the strongest emission in solid state is examined. Based on analyses of molecular packing in the single crystals, the differences in the fluorescence could be attributed to aggregates that showed non-classical herringbone packing with π-π overlap between neighbor molecules. Also the crystal structure studies showed that the N atom position is located anti to the CN group, giving a more planar geometry. This N-dimethyl substituent for I-IV appears to be general for more planar ground-state geometry about the nitrogen atom and consistent with an amino conjugation effect to obtain sufficient quinoid structures.

Full text of DOI:10.1016/j.molstruc.2012.10.025

Journal of Molecular Structure 1034 (2013) 238–248
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Fluorescence improvement of pyridylacrylonitrile
by dimethylaminophenyl-substitutions: The effect of packing modes of conjugated
compounds
a
a
a
M. Judith Percino a, , Víctor M. Chapela , Margarita Cerón , Guillermo Soriano-Moro ,
⇑
Maria Eugenia Castro a, Francisco J. Melendez b
a Lab. de Polímeros, Centro de Química, Instituto de Ciencias, Universidad Autónoma de Puebla, Complejo de Ciencias, ICUAP, Edif. 103F, 22 Sur y San Claudio, CP 72570 Puebla,
Puebla, Mexico
b Lab. de Química Teórica, Centro de Investigación, Dpto. de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Autónoma de Puebla, Edif. 105-I, 22 Sur y San Claudio, CP 72570
Puebla, Puebla, Mexico
h i g h l i g h t s
"
"
"
X-ray characterization gave evidence about nitrogen position in structure affecting the fluorescence.
interactions determined the packing modes in conjugated compounds.
Molecular structure and CHꢀ ꢀ ꢀ
p
Highly dipole–dipole of A–
p
–D structures exhibit different fluorescence in solution and solid state.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 June 2012
Received in revised form 8 October 2012
Accepted 8 October 2012
Available online 23 October 2012
Four (dimethylamino)arylacrylonitrile derivatives in the solid state are presented. The compounds were
characterized by single crystal X-ray diffraction to investigate the effects of substituents on the resulting
crystals lattices and to examine the effects of the factors controlling their solid state on photochemical
behavior. The molecules included 2-(phenyl)-3-(4-dimethylaminophenyl)acrylonitrile (I) 2-(20-pyri-
dyl)-3-(4-dimethylaminophenyl)-acrylonitrile (II), 2-(30-pyridyl)-3-(4-dimethyl-aminophenyl)acryloni-
trile (III) and 2-(30-pyridyl)-3-(4-dimethylaminophenyl)acrylonitrile (IV). The dimethyl-, ACN groups
and the position of nitrogen atom in the pyridyl groups affected the nature of the molecular packing
and consequently their fluorescence properties. The lattice of each compound is compared with acrylo-
nitriles previously reported and the effect of substitution on the crystal properties with the strongest
emission in solid state is examined. Based on analyses of molecular packing in the single crystals, the dif-
ferences in the fluorescence could be attributed to aggregates that showed non-classical herringbone
Keywords:
Aggregation
Acrylonitrile derivatives
N-dimethylphenyl derivatives
Cyano-substituted
N-dimethylphenyl-acrylonitriles
Crystal structures
packing with
p–p overlap between neighbor molecules. Also the crystal structure studies showed that
the N atom position is located anti to the ACN group, giving a more planar geometry. This N-dimethyl
substituent for I–IV appears to be general for more planar ground-state geometry about the nitrogen
atom and consistent with an ‘‘amino conjugation effect’’ to obtain sufficient quinoid structures.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
(OLEDs), solar cells, organic semiconductor lasers, etc. [1–6]. Typi-
cally, these dyes have a donor–acceptor (D–A) structure that con-
Low molecular weight and oligomeric organic compounds with
optical or electrical properties have been widely used as dyes in or-
ganic electronic devices, including organic light emitting diodes
trols their photophysical properties [2,7,8]. However, most of
these applications require the light emitting materials to be used
in the solid state (e.g. thin films or crystals), while common organic
materials are usually weakly luminescent when concentrated to
the solid states [9] (commonly called a concentration quenching
effect). This is believed to be a result of organic molecules in the
solid state aggregation that can result in emission quenching due
⇑
Corresponding author. Address: Centro de Química, Instituto de Ciencias,
Universidad Autónoma de Puebla, Complejo de Ciencias, ICUAP, Edif. 103H, 22 Sur y
San Claudio, CP 72570 Puebla, Puebla, Mexico. Tel.: +52 222 2295500x7285; fax:
+52 222 2295551.
to strong electronic interactions, hydrogen bonding, or
p–p stack-
ing [10–12]. Chemical, physical and engineering approaches have
E-mail address: judith.percino@correo.buap.mx (M.J. Percino).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.10.025

M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
239
been used by many groups to reduce the effect of aggregation on
fluorescence quenching. In 2001, Luo et al. observed a novel phe-
nomenon that overcomes fluorescence quenching in an aggregate,
which was termed aggregation-induced emission (AIE) [13], and it
has been reported for several molecules [13]. Tang et al. synthe-
sized a series of molecules based on silole and tetraphenylethene
and their derivatives, including 1,1-substituted 2,3,4,5-tetraphe-
nylsiloles [13]. Prasad and co-workers reported aggregation-en-
hanced fluorescence in nano-aggregates due to the hindered
internal rotation of the molecules [14]. Tian et al. designed and
synthesized multibranched triarylamine end-capped triazines that
exhibited enhanced AIE properties [15].
are isomers that contain a cyano group as a strong electron accep-
tor attached to the olefin, a pyridine group as strong acceptor
(in o-, m- and p-positions), and a dimethylamino group which is
an electro-donor. Molecule I is chosen because it lacks an N atom
on the aromatic ring and allow us to evaluate the influence of pyr-
idine. The single crystal structure of each compound is discussed
and correlated to the packing of the structures. We also examine
how their aggregation behavior affects their emission lumines-
cence, even though the differences in the molecular structures of
new compounds are confined to the position of the nitrogen atom.
2. Experimental section
On the other hand, it is well know that the aromatic amines are
among the most important derivatives for hole transport in de-
vices, therefore triphenylamine have been widely used in opto-
and electroactive materials due to their good electro-donating
and transporting capabilities, as well as their special propeller star-
burst molecular structure [16,17]. As well as cyano-substituted
compounds show good optical and electrical properties due to
their high electron affinities [18]. In particular, some cyano-substi-
tuted compounds have been reported which show unique en-
hanced emission rather than fluorescence quenching in the solid
state [19]. On the other hand, pyridine is an important electron-
acceptor group, due to its high electron affinity. Dailey et al. de-
scribed poly(2,5-pyridinediyl) (PPY) as an efficient electron trans-
port layer in bilayer polymeric LEDs. The OLEDs with a PPY layer
exhibited external quantum efficiency 60 times greater than that
of similar devices without a PPY layer [20]. Epstein et al. described
poly(p-pyridine)- and poly(p-pyridylvinylene)-based polymers as
emissive layers in light emitting devices [21].
The compounds I–IV (Scheme 1) were synthesized and charac-
terized previously [30].
2.1. Crystallization
It is well known that there are two events that lead to crystal-
lization: nucleation and crystal growth. Nucleation is the step
where molecules start moving in the solvent to create clusters
(crystal growth). The crystal growth shows how the crystals can
be formed in different conditions. Changes in the solvent polarity
causes the crystals to grow different along the x, y and z-direction
i.e. crystal might present diverse habits such the development of
faces, the relative dimensions of crystals, the overall shape of crys-
tals, and the form of aggregates of crystals. In Table 2 are shown
the color and habit of the crystals for all compounds.
For I, one crystallization attempt was made in cyclohexane,
There are four main possible packing motifs in organic solid
which is
) = 2.023). But the best results were when I (0.027 g) was dis-
solved in 8 mL of hexane ( = 1.84) and after 24 h the crystals were
obtained. This is an indication that non-polar solvents inhibit the
crystal growth in the y-direction and facilitate its growth in the
x-direction. As results, hexane a non-polar solvent causes that I
crystallize into lath, and also that I growth in the z-direction is
minimal.
For compound II, cyclohexane (non-polar solvent) did not give
good results. II (0.015 g) was dissolved in 9 mL of hexane:1,2-
dichloromethane mixture (3:2) and stored at room temperature
for 2 days to obtain crystals. Hexane is a higher non-polar solvent
compare with cyclohexane, and mixed with a slightly more polar
solvent such CH2Cl2 allowed obtaining adequate crystals for
X-ray determination. The result suggests an effect of the structure
cause that the solvent affects the compound crystallization. Non-
polar solvents inhibit the crystal growth in the three directions.
As results, the mixture of non-polar solvents causes that II crystal-
lize into a crystal irregular block.
On the other hand, out of 4 crystallization attempts for III,
which were as following: it was dissolved into ethanol, a polar sol-
vent (e = 30), mixture of solvents slightly polar with a non-polar
solvent (hexane:ethyl acetate (3:2) as well as in non-polar solvent
cyclohexane). The purpose of these solutions was to obtain the
adequate crystals for X-ray determination. III (0.020 g) dissolved
in 15 mL of hexane:1,2-dichloromethane mixture (5:1) was set
aside at 4 °C for 3 days, after which the crystals were obtained.
The results showed that due to hexane has a dielectric constant
a non-polar solvent (with a dielectric constant
state, named typical herringbone packing without
[22], nonclassical herringbone packing with overlap between
neighbor molecules [23], which is also called slipped -stacking,
lamellar packing or one dimensional (1-D) -stacking [24], and
lamellar packing or two dimensional -stacking [25]. Moreover,
p–p
stacking
(e
p–p
e
p
p
p
introducing polarity, usually as electron-withdrawing groups, into
the molecular structure also could result in lamellar structure. The
design conception is quite concise and explicit. As well as
know, p-type organic semiconductors usually possess a large
p-
conjugated system, and this electronic system would lead to a
herringbone structure for electrostatic repulsion [26].
If polarity is introduced into the chemical structure, the dipole–
dipole attraction would counterbalance the electrostatic repulsion
and this balance might result in lamellar structure. At present, the-
oretical calculations are very useful in the determination of electric
properties and charge distributions of aromatic systems [27]. In
real molecules, electrons charges are distributed by a plot of the
square of the electronic wave function, and the dipole moment re-
sults from the fact that the distributions of positive and negative
charge are not proportional. The dipole moment of a molecule is
determined by integration over the charge distributions. Conse-
quently in order to investigate the position charge in the structure
for different molecules the electric dipole moment values, l, is nor-
mally determined using the B3LYP/6-31+G(d,p) theory level calcu-
lated in Gaussian 09 program [28]. As well as Density Functional
Theory (DFT) methods, as the B3LYP hybrid functional [29] are par-
ticularly attractive because it takes less computational time to
solve a problem that other methods that include electronic
correlation.
(e) of 1.84 and CH2Cl2 of 10.19, the mixture facilitates the crystal
growth in the x, y and z-direction. As results, the mixture causes
In this work, we report a detailed study of the X-ray character-
ization for four molecules 2-(phenyl)-3-(4-dimethylaminophe-
nyl)acrylonitrile (I), 2-(20-pyridyl)-3-(4-dimethyl-aminophenyl)-
acrylonitrile (II), 2-(30-pyridyl)-3-(4-dimethylaminophenyl)acrylo-
nitrile (III), and 2-(30-pyridyl)-3-(4-dimethylaminophenyl)acrylo-
nitrile (IV) (Scheme 1). These molecules are chosen because they
that III crystallize into regular block.
For IV, however hydrogen bonds maintain a large role in the
shape of crystal. The shape of the resulting IV crystals depends
on the ability of a plane to compete for hydrogen bonds. So paired
molecules (hydrogen bonded) are less affected by the addition of a
solvent. The interaction forces of the solvent with the faces of the

240
M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
Scheme 1. Preparation of pyridylacrylonitrile derivatives (I–IV).
crystal affect the stability of that face. A strong binding of the sol-
vent to the crystal impedes the growth of the specific face. Accord-
ingly, the polarity of the solvent in which the IV (0.020 g) was
dissolved in a mixture of methanol:cyclohexane (5:1) and stored
for 4 days at room temperature to obtain the compound in a rect-
angular block form.
2.0
1.5
1.0
0.5
0.0
(a)
2.2. Instrumentation
2.2.1. X-ray crystallography and photolysis properties of I–IV
All reflection intensities were measured using a KM4/Xcalibur
(detector: Sapphire3) with enhance graphite-monochromated
Mo Ka radiation (k = 0.71073 Å) under the program CrysAlisPro
(Version 1.171.34.44 Oxford Diffraction Ltd., 2010). The program
CrysAlisPro was used to refine the cell dimensions and data reduc-
tion. The structure was solved with the program SHELXS-97 [31]
and was refined on F2 with SHELXL-97. Analytical numeric absorp-
tion corrections based on a multifaceted crystal model were ap-
plied using CrysAlisPro. The temperature of the data collection
was controlled using the system Cryojet (manufactured by Oxford
Instruments). The H atoms were placed at calculated positions
using the instruction AFIX 43 with isotropic displacement param-
eters having values 1.2 times Ueq of the attached C atoms.
The UV–vis and fluorescence spectra were acquired with a Spec-
trometer SD2000 (Ocean Optics, Dunedin, FL). For compounds in
solution, absorption was measured at room temperature in CHCl3.
In the present work, we determined the quantum efficiency (Uf) for
the four compounds in crystal. For the determination of Uf, the
experimental setup was similar to that described by de Mello
et al. [32], except that compounds I–IV were not prepared in films;
instead, the powdered compounds were placed into Pyrex tubes.
First, the laser emission intensity was measured for the empty
tube, located inside the integrating sphere. Then, the compound
was placed inside the tube and the fluorescence spectrum was
measured. The Uf value of Alq3 was 0.11 and was used as the refer-
ence for our experimental setup. The excitation source for measur-
ing the fluorescence and fluorescence quantum yield (Uf) was a
laser diode at a wavelength of 405 nm.
300
400
500
600
Wavelength/nm
1.0
0.8
0.6
0.4
0.2
0.0
(b)
450
500
550
600
650
700
Wavelength/nm
Fig. 1. Absorbance in CHCl3 (a) and fluorescence emission (in crystal) spectra (b) for
the compounds I phenyl (___), II ortho (- - -), III meta (ꢀ ꢀ ꢀ), and IV para (ꢀ–ꢀ–ꢀ).
Excitation wavelength was 405 nm.
3. Results and discussion
Table 1
3.1. Photophysical properties in solution and solid state
Absorption and emission characteristics of acrylonitrile derivatives.
Compound Abs. (kmax, nm) solid Emis. (kmax, nm) solid Ufa(%) lb
The absorption in solution and fluorescence of crystal of com-
pounds I–IV are given in Fig. 1. The absorption bands observed be-
low 253–262 nm (Fig. 1a) were assigned to n ? pꢁ for electron
transfer from moieties AN(CH3)2 to the structure and the second
state
state
I
II
III
IV
381
395
389
406
507
542
580
616
4.390 6.1959
4.265 4.9477
4.752 6.3947
9.329 8.9342
band at ꢂ379–406 nm was attributed to
p–
pꢁ transitions from
ACNC@CHPh chromophore. A small bathochromic shift was ob-
served for the para position of the pyridine ring. The fluorescence
spectra of I–IV in crystal form (Fig. 1b) showed maximum emission
a
Alq3 11%, as reference.
B3LYP/6-31+G(d,p) theory level calculated in Gaussian 09 program.
b

M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
241
Table 2
Summary of crystallography data for I–IV.
Compounds
I
II
III
IV
Empirical formula
Color, habit
C
17H16N2
C16H15N3
C16H15N3
C16H15N3
Orange, block
Yellow, lath
0.56 ꢃ 0.16 ꢃ 0.05
Orthorhombic
248.32
Yellow, irregular block
0.47 ꢃ 0.35 ꢃ 0.21
Triclinic
Yellow, block
0.28 ꢃ 0.21 ꢃ 0.13
Orthorhombic
249.31
Crystal size (mm3)
Crystal system
Formula weight
Space group
0.31 ꢃ 0.28 ꢃ 0.21
Triclinic
249.31
249.31
Pna21
Pna21
P1
P1
T (K)
a (Å)
b (Å)
c (Å)
110(2)
10.1852(3)
20.905(6)
6.2346(2
90
110(2)
110(2)
40.1893(15)
6.3292(2)
20.2673(7)
90
90
90
5155.3(3)
16
1.285
110(2)
7.25341(16)
12.0241(3)
31.0130(7)
89.971(2)
89.928(7)
72.509(2)
2579.76(19)
8
11.0602(4)
11.3125(4)
12.7314(4)
112.161(3)
103.483(3)
106.265(3)
1309.85(10)
4
a
(°)
b (°)
90
90
c
(°)
V (Å3)
Z
1327.77(7)
4
1.242
528
0.074
Dc (g cmꢄ3)
F (000)
l
1.284
1056
0.078
1.264
528
0.077
2112
0.078
(mmꢄ1
)
k (Å)
2hmax (°)
N
N° (I > 2.0
R1
0.71073
25.13
9000
1191
3.45
0.71073
26.00
32101
7444
4.81
0.71073
25.00
14653
3950
0.71073
27.50
21614
5068
3.79
r(I))
5.95
wR2
8.75
13.57
14.45
10.02
Goodness-of-fit
1.037
0.156 and ꢄ0.186
1.039
0.276 and ꢄ0.4.20
1.074
0.530 and ꢄ0.214
1.078
0.259 and ꢄ0.197
Largest diff peak and hole (e Åꢄ3
)
Fig. 2. Molecular structures. Displacement ellipsoids are drawn at 30% probability level and H atoms are shown as small spheres of arbitrary radii.
wavelengths at 507, 542, 580, and 616 nm, respectively. These
compounds displayed significant differences in electronic spectra
due to the combined effects of the position of the N-atom of the
pyridyl group and the electron-donating functional group attached
on the conjugated structure. The emission spectra were only mea-
sured in solid form because in chloroform the fluorescence of the
four compounds was quenched. As compared to phenyl compound
I, the fluorescence spectra for the pyridine-containing compounds
II–IV exhibited one broader emission band with a red-shifted max-
imum and band width of at least 100 nm. The further red shift of IV

242
M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
Table 3
suggested that the p-conjugation of the main chain was extended
by the para linkage.
Bond lengths (Å) for I, II, III and IV molecules.
Table 1 summarizes the absorption and emission properties as
well as the values of the Uf which were measured on compounds
in powder and compared with the Alq3 used as the reference com-
Bond length
I
Bond length
II
III
IV
N(1)AC(3)
N(1)AC(1)
N(1)AC(2)
N(2)AC(11)
C(3)AC(4)
C(3)AC(8)
C(4)AC(5)
C(5)AC(6)
C(6)AC(7)
C(6)AC(9)
C(7)AC(8)
C(9)AC(10)
C(10)AC(11) 1.441(3) C(10A)AC(11A)
C(10)AC(12) 1.496(3) C(10A)AC(12A)
C(12)AC(13) 1.391(3) C(12A)AC(13A)
C(12)AC(17) 1.393(3)
C(13)AC(14) 1.392(3) C(13A)AC(14A)
C(14)AC(15) 1.385(4) C(14A)AC(15A)
C(15)AC(16) 1.378(3) C(15A)AC(16A)
1.371(3) N(1A)AC(3A)
1.447(3) N(1A)AC(1A)
1.454(3) N(1A)AC(2A)
1.151(3) N(2A)AC(11A)
1.407(3) C(3A)AC(4A)
1.414(3) C(3A)AC(8A)
1.380(3) C(4A)AC(5A)
1.409(3) C(5A)AC(6A)
1.406(3) C(6A)AC(7A)
1.454(3) C(6A)AC(9A)
1.381(3) C(7A)AC(8A)
1.356(3) C(9A)AC(10A)
1.378(3) 1.369(6) 1.3614(13)
1.452(3) 1.440(6) 1.4527(14)
1.449(3) 1.454(6) 1.4534(13)
1.155(3) 1.140(6) 1.1480(14)
1.412(3) 1.408(6) 1.4177(14)
1.403(3) 1.423(6) 1.4130(14)
1.370(3) 1.390(6) 1.3738(15)
1.417(3) 1.397(6) 1.4109(14)
1.414(3) 1.402(6) 1.4091(14)
1.435(3) 1.456(6) 1.4418(14)
1.370(3) 1.371(6) 1.3757(14)
1.355(3) 1.367(6) 1.3615(14)
1.434(3) 1.453(6) 1.4388(14)
1.479(3) 1.496(6) 1.4800(14)
1.391(3) 1.381(7) 1.3981(14)
–
pound. The dipole moment values (l) were also determined using
the B3LYP/6-31+G(d,p) theory level calculated in Gaussian 09 pro-
gram [28]. Density Functional Theory (DFT) calculations, are partic-
ularly attractive and versatile because they calculate the properties
of a many-electron system from the functional of the electron den-
sity in minor computational time than other methods that include
electronic correlation based in the calculation of the many-electron
wave function. Specifically, hybrid methods, namely B3LYP, PBE and
PBE0, have been widely used by different authors for calculations of
molecular structures, molecular orbitals energies, excitation ener-
gies, infrared spectrum assignment and 1H chemical shifts, for or-
ganic systems involving interesting optical properties [33–37],
being a good approximation methods for providing accurate results.
1.379(3)
1.387(3) 1.357(8)
1.385(3) 1.352(7) 1.3834(15)
–
1.3862(15)
–
C(16)AC(17) 1.393(3)
–
–
–
–
–
3.2. Crystal structures
C(12A)AC(16A)
N(3A)AC(12A)
N(3A)AC(16A)
N(3A)AC(13A)
N(3A)AC(14A)
N(3A)AC(15A)A
1.386(7) 1.3971(15)
–
–
1.358(6)
1.383(6) 1.3374(15)
1.3415(15)
1.350(3)
1.336(3)
–
–
–
–
To better understand the photophysical behavior of the broad-
ened band and the pronounced red shift of the absorption, the
compounds were characterized by single crystal X-ray. Interac-
tions can lead to the formation of aggregates, which can signifi-
cantly alter the photophysical and photochemical properties of
the molecules resulting in large changes in their spectral character-
istics and luminescence efficiency.
The single crystal data of I–IV and structure refinements are
summarized in Table 2. For compounds I–IV the data were collected
at 110(2) K after the crystals had been flash-cooled from room tem-
perature. X-ray studies of I–IV single crystals were made to demon-
N(1B)AC(3B)
N(1B)AC(1B)
N(1B)AC(2B)
N(2B)AC(11B)
C(3B)AC(4B)
C(3B)AC(8B)
C(4B)AC(5B)
C(5B)AC(6B)
C(6B)AC(7B)
C(6B)AC(9B)
C(7B)AC(8B)
C(9B)AC(10B)
C(10B)AC(11B)
C(10B)AC(12B)
C(12B)AC(13B)
C(13B)AC(14B)
C(14B)AC(15B)
C(15B)AC(16B)
C(12B)AC(16B)
N(3B)AC(12B)
N(3B)AC16B
1.364(3) 1.354(5) 1.3629(13)
1.460(3) 1.457(6) 1.4527(14)
1.452(3) 1.463(6) 1.4526(15)
1.153(3) 1.176(6) 1.1498(15)
1.422(3) 1.424(6) 1.4097(15)
1.412(3) 1.399(6) 1.4146(14)
1.373(3) 1.375(6) 1.3735(14)
1.411(3) 1.409(6) 1.4101(15)
1.415(3) 1.410(5) 1.4069(15)
1.431(3) 1.437(6) 1.4441(14)
1.357(3) 1.385(6) 1.3804(14)
1.365(3) 1.369(6) 1.3617(15)
1.437(3) 1.425(6) 1.4360(15)
1.471(3) 1.486(6) 1.4812(14)
1.402(3) 1.397(6) 1.3952(15)
strate a side-by side (p–p) molecular stacking arrangement. The
crystal structures for the four acrylonitrile derivatives I–IV are
shown in Fig. 2. The X-ray investigations show that the four com-
pounds have the Z-geometry about the ethylene bridge which links
the heterocyclic and aromatic rings. The molecular skeleton is
slightly non-planar: the torsion angles between the acrylonitrile
linkage and the pyridine ring and between the linkage and the p-
dimethylaminophenyl moiety are shown in Tables 3 and 4.
The small non-planarity of the molecules permits the visualiza-
tion of the conjugation in the molecule. The charge density is dis-
tributed non-homogenously in the whole structure. The
compounds that contained ACN and pyridine as Z-3-(4-hydroxy-
phenyl-2-(pyridin-4-yl)acrylonitrile [38] showed an overall value
1.384 Å for Car–Car and for the C(7)AC(8) and C(8)AC(9) bond
lengths, 1.348(3) and 1.439(3), respectively, which differed slightly
from the standard values for C@C of 1.330 Å and Csp2–Car of
1.47 Å. On the other hand, in the 2-amino-3-naphthylacrylonitrile
derivatives [39], the C(11)AC(12) double bond length observed
was 1.336(2) Å and for C(12)AN(2), values of 1.415(2) Å for the di-
phenyl-attached to double bonds and 1.307(6) Å and 1.407 Å for
carbazole indicated a significantly larger value. Also the authors
indicated that the bond length of C(1)AC(11) was 1.471(2) in the
diphenyl substituted compound, shorter than the value of
1.560(8) observed for the carbazole substituted compound, indi-
cating that the resonance structure of the naphthalene group
1.381(3)
1.387(3) 1.365(7)
–
1.3876(15)
1.380(3) 1.398(6) 1.3826(15)
1.389(6) 1.3959(15)
1.347(3)
1.335(3)
–
–
N(3B)AC(13B)
N(3B)AC(14B)
N(3B)AC(15B)A
1.337(6)
1.350(6) 1.3348(16)
1.3395(15)
C(7A)AC(8A) = 1.370 Å; C(8A)AC(3A) = 1.403(3); C(3A)AN(1A) =
1.378(3); and C(6A)AC(9A) = 1.435 Å. For III: C(3A)AC(4A) =
1.408(6)Å; C(4A)AC(5A) = 1.390(3) Å; C(5A)AC(6A) = 1.397(6) Å;
C(6A)AC(7A) = 1.402(3) Å; C(7A)AC(8A) = 1.371(6) Å;
C(3A) = 1.423(6A); C(3A)AN(1A) = 1.369(6); and C(6A)AC(9A) =
1.456(6) Å. For IV: C(3A)AC(4A) = 1.417(14)Å; C(4A)AC(5A) =
C(8A)A
1.378(15) Å;
C(5A)AC(6A) = 1.4109(14) Å;
C(6A)AC(7A) =
1.4091(14) Å; C(7A)AC(8A) = 1.3757 (14) Å; C(8A)AC(3A) =
1.4130(14); C(3A)AN(1A) = 1.3614(13); and C(6A)AC(9A) =
1.4418(14) Å. These values indicated that a significant degree of
quinoid structure is present, as shown in the following scheme,
which is due to the high conjugation for the (CH3)2NA in the
structure.
seems to form a better conjugation system with the
p-chromo-
phore of C(11)AC(12) than does the carbazole group. Our four
compounds showed that the dimethylaminophenyl substituent
has the following bond lengths (Table 3).
For
I:
C(3)AC(4) = 1.407(3)Å;
C(4)AC(5) = 1.380(3) Å;
C(7)AC(8) =
+
C(5)AC(6) = 1.409(3) Å;
C(6)AC(7) = 1.406(3) Å;
(H3C)3N
1.381 Å; C(8)AC(3) = 1.414(3); C(3)AN(1) = 1.371(3); and C(6)A
C(9) = 1.454 Å. For II: C(3A)AC(4A) = 1.412(3)Å; C(4A)AC(5A) =
1.370(3) Å; C(5A)AC(6A) = 1.417(3) Å; C(6A)AC(7A) = 1.414(3) Å;

M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
243
Also the olefinic double bond near the three conjugated
substituents has a value slightly higher (C(9)AC(10) 1.356(3) Å;
C(9A)AC(10A) 1.355(3) Å; C(9A)AC(10A) 1.367(6) Å; C(9A)A
C(10A) 1.3615(14) Å) than the corresponding values for other
acrylonitrile derivatives [40,41] and the standard values (C@C
1.330 Å, Csp2–Car 1.470 Å) [42], indicating some delocalization
on the unsaturated bridge units.
Interestingly, the ACN group is clearly in anti position in II and
III structures (Fig. 2), due to the free electron pair from the nitro-
gen atom in the pyridine group, whereas in I it is not present
and in the compound IV, the p-position suppresses the effect.
The structure of I was solved and refined in the noncentrosym-
metric (and polar) space group Pna21 with Z0 = 1. The ORTEP struc-
ture of I is shown in Fig. 3, and the structure of I is ordered. The
absolute configuration (i.e., the direction of the polarity axis) could
not be established by anomalous-dispersion effects in diffraction
measurements on the crystal. The compound I has 4 molecules
per cell. In the crystal packing of compounds I–IV, the compounds
crystallized in a herringbone pattern [42], but other research has
shown that there are four main possible packing motifs in organic
solid states [22–25,43]. With respect to compound I, by introduc-
ing a ACN group attached to the double bond, the packing motifs
suggested that both free variables used to evaluate the occupancy
factors for both disordered formula units were found to be equal
[i.e. 0.884(3) and 0.889(3)]; hence only one free variable was used
for the final refinement, and the occupancy factor for the major
component of the disorder refines to 0.887(2). A closer look at
the reciprocal lattice slices shows some diffuse scattering, which
may indicate the existence of short- or medium-range order in
the crystal structure. The absolute configuration (i.e., the direction
of the polarity axis) could not be established by anomalous-disper-
sion effects in diffraction measurements on the crystal. In the crys-
tal packing of compound III (Fig. 5), from the four molecules
reported, III is a highly non-planar structure, with the cyano group
located in a coplanar conformation with the plane of the double
bond and phenyl group is twisted by a torsion angle between
atoms C(9A)AC(10A)AC(12A)AC(13A) from the pyridine ring at
20.7(6)° and between the atoms that include the ACN
C(11A)AC(10A)AC(12A)AC(16A) at 19.70(6)°. The ACN group is
located in the anti position at the ACN group similar to the struc-
ture for II. The package structure showed that the molecules in this
structure are able to approach each other more closely.
A
AC„Nꢀ ꢀ ꢀACH(
p
)
weak contact seen in sheet of III (of
a
ꢂ2.499 Å) became much shorter than in I. The nitrogen atom in
meta position of III cannot interact intermolecularly with the
CAH of the N(CH3)2 moiety as in compound II. Also there is a con-
tact distance of ꢂ2.876 Å between the different symmetry equiva-
showed a non-classical herringbone packing with a weak
overlap between neighboring molecules (Fig. 3). We found for mol-
ecule I an interaction between CH/
ꢀ ꢀ ꢀAC„N of 2.726 Å. Addition-
ally, we found CH/
ꢀ ꢀ ꢀAC„N interactions of 2.694 Å, the
CH(CH3)ꢀ ꢀ ꢀCH/ of 2.856 Å and 2.872 Å, and C/ ꢀ ꢀ ꢀCH/ interac-
tions of 3.375 Å, whereas the distance between cofacial groups
was 7.1 Å, very large as compared to the expected interaction for
overlap.
For compound II, the structure was solved and refined in the
p–p
p
p
lent molecules, between Hꢀ ꢀ ꢀCH/
p and the H from N(CH3)2.
p
p
p
Therefore, the double bond bearing the dimethylaminophenyl
and the nitrile group introduces sufficient polarity due to the posi-
tion of the pyridine group and almost allows assembly between
one molecule and the other, i.e., the packing showed a slipped
p–p
p–p
p-stacking which could be due to introducing polarity into the
centrosymmetric space group P1 with Z0 = 4 (i.e., there are four
crystallographically independent formula units in the asymmetric
unit). The structure of II is mostly ordered except for one formula
unit that is wholly disordered. The occupancy factor for the major
component of the disordered formula unit refines to 0.558(2). The
crystal is characterized by pseudo-merohedral twinning. The twin
relationship (ꢄ100/0ꢄ10/001) was found via TwinRotMat from
the Platon program [44] and the batch scale factor refines to
0.509(1). The final refinement against F2 was relatively good. The
chemical structure to balance the electrostatic repulsion arising
from the conjugation system. Also the introduction of the ACN
group plays an important role in preventing the interaction
between the two
p faces.
The structure IV was solved and refined in the centrosymmetric
space group P1 with Z0 = 2. The structure is highly ordered. The
molecules are almost planar because the torsion angle between
atoms C(5A)AC(6A)AC(9A)AC(10A) is ꢄ9.98(19)°, between
C(11A)AC(10A)AC(12A)AC(13A) is 7.11(15)°, whereas between
of C(9A)AC(10A)AC(12A)AC(16A) is 5.92(17)°. The ACN group is
located on one side, similar to the structure for I. The packing
structure showed closer interactions than those in compounds
I–III due to the nitrogen atom being located in the para position
in the pyridine group, making the molecule exhibit a dipolar
moment higher than for I–III, Table 1. The molecules are able to
approach each other more closely. The H(CH3)ꢀ ꢀ ꢀAC„N intermo-
lecular interaction between molecules A and B is 2.634 Å, the
R factor [F2 > 2 (F2)] is about 0.048. The final difference Fourier
r
map was flat and the residual peaks were no larger than
0.28 e Åꢄ3. Neighboring molecules in close contact that played an
important role in crystal packing were AC„Nꢀ ꢀ ꢀAC„N with a dis-
tance 3.323 Å, the AC„Nꢀ ꢀ ꢀH of the CN(CH3) moiety that were sep-
arated by 2.692 Å, and the 2.676 Å contact between Nꢀ ꢀ ꢀH of the
pyridine and the AN(CH3) moiety (Fig. 4). This network of contacts
leads to each molecules interacting with the others molecules in
the lattice. Apparently the presence of the nitrogen atom in ortho
position is a reason for such differences in the crystal packing com-
pared with the compound I, because the ACN group is located anti
at the nitrogen atom due to the steric hindrance arising from the
free pair of electrons from the pyridine group. Even though the
H(CH3)ꢀ ꢀ ꢀC/
p
interaction
is
3.573 Å,
while
that
for
AN(Py)ꢀ ꢀ ꢀAH(CH3) is ꢂ2.510 Å for molecules B and ꢂ2.596 Å for
molecules A. The Cꢀ ꢀ ꢀAH(CH3)2 interaction is ꢂ2.712–2.769 Å in
molecules A and B, respectively. Also present are interactions of
the C/
p
ꢀ ꢀ ꢀCsp2 atoms of the double bond of 3.361 Å in molecule
structure possesses a large
p-conjugated system, and thus is elec-
A. The packing also indicates that the molecules showed a head-
to-tail orientation along the b axes with an interplanar distance
of about 3.363 Å for A and 3.505 Å for B; these distances are consis-
tron rich, the system would lead to a herringbone structure for
electrostatic repulsion. However, it is clear that polarity is intro-
duced due to the position of the nitrogen atom and therefore the
dipole–dipole attraction would counterbalance the electrostatic
repulsion carried out to adopt the II structure with a weak interac-
tent with p–p interactions, since p-stacking distances are typically
3.4–3.6 Å in organic materials [45–47]. On the other hand, for
organic molecules, aggregation is a common phenomenon that oc-
curs in solutions of high concentrations and it has the deleterious
side effect of emission quenching due to strong electronic interac-
tion between
p
–p
stacking which is also called slipped
p-stacking
(Fig. 4) with a weak
p–p
overlap between neighboring molecules.
The structure III was solved and refined in the noncentrosym-
metric space group Pna21 with Z0 = 4 (i.e. there are four crystallo-
graphically independent molecules in the asymmetric unit). The
structure is partly ordered, and the formula units A and D are found
to be wholly disordered over two orientations. A first refinement
tions, hydrogen bonding, or p–p stacking. Since molecule IV exhib-
ited emission stronger than I–III, this behavior could be attributed
to its aggregated state (Fig. 6). According to the four main possible
packing motifs in organic solid states [22–25,48], molecule IV
showed a lamellar packing, two-dimensional (2-D)
p–p-stacking

244
M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
Table 4
Torsion angles (°) in crystal structures of molecules (I–IV).
Bond angle
I (°)
174.6(2)
Bond angle
II (°)
1.0(3)
1.3(3)
ꢄ177.9(2)
179.9(2)
ꢄ178.1(2)
2.7(3)
ꢄ1.1(4)
ꢄ1.0(3)
ꢄ179.2(2)
1.4(3)
179.8(2)
0.2(4)
178.6(2)
ꢄ2.3(3)
172.1(2)
ꢄ9.7(4)
1.2(4)
ꢄ177.9(2)
ꢄ176.5(2)
4.3(3)
III (°)
176.2(4)
IV (°)
C(1)AN(1)AC(3)AC(4)
C(2)AN(1)AC(3)AC(8)
C(2)AN(1)AC(3)AC(4)
C(1)AN(1)AC(3)AC(8)
N(1)AC(3)AC(4)AC(5)
C(8)AC(3)AC(4)AC(5)
C(3)AC(4)AC(5)AC(6)
C(4)AC(5)AC(6)AC(7)
C(4)AC(5)AC(6)AC(9)
C(5)AC(6)AC(7)AC(8)
C(1A)AN(1A)AC(3A)AC(4A)
C(2A)AN(1A)AC(3A)AC(8A)
C(2A)AN(1A)AC(3A)AC(4A)
C(1A)AN(1A)AC(3A)AC(8A)
N(1A)AC(3A)AC(4A)AC(5A)
C(8A)AC(3A)AC(4A)AC(5A)
C(3A)AC(4A)AC(5A)AC(6A)
C(4A)AC(5A)AC(6A)AC(7A)
C(4A)AC(5A)AC(6A)AC(9A)
C(5A)AC(6A)AC(7A)AC(8A)
C(9A)AC(6A)AC(7A)AC(8A)
C(6A)AC(7A)AC(8A)AC(3A)
N(1A)AC(3A)AC(8A)AC(7A)
C(4A)AC(3A)AC(8A)AC(7A)
C(7A)AC(6A)AC(9A)AC(10A)
C(5A)AC(6A)AC(9A)AC(10A)
C(6A)AC(9A)AC(10A)AC(11A)
C(6A)AC(9A)AC(10A)AC(12A)
C(9A)AC(10A)AC(12A)AC(13A)
C(11A)AC(10A)AC(12A)AC(13A)
N(3A)AC(12A)AC(13A)AC(14A)
C(14A)AC(15A)AC(16A)AN(3A)
ꢄ0.80(16)
ꢄ170.1(2)
ꢄ174.1(4)
ꢄ2.11(17)
177.45(11)
179.63(11)
ꢄ177.38(10)
2.21(16)
10.5(3)
8.6(7)
ꢄ6.0(3)
178.6(2)
ꢄ0.8(3)
0.0(3)
ꢄ6.5(7)
179.3(4)
1.9(6)
ꢄ1.3(7)
ꢄ0.40(17)
ꢄ2.01(16)
178.77(11)
2.65(16)
0.9(3)
0.3(6)
ꢄ176.7(2)
ꢄ1.0(3)
176.7(2)
0.3(3)
ꢄ178.3(4)
0.0(6)
178.4(4)
0.7(7)
ꢄ179.0(4)
ꢄ1.6(6)
ꢄ1.7(8)
176.7(4)
ꢄ0.9(8)
ꢄ178.4(4)
20.7(6)
ꢄ157.0(4)
0.2(7)
C(9)AC(6)AC(7)AC(8)
C(6)AC(7)AC(8)AC(3)
N(1)AC(3)AC(8)AC(7)
C(4)AC(3)AC(8)AC(7)
ꢄ178.06(10)
ꢄ0.87(17)
178.00(10)
ꢄ1.58(16)
170.81(11)
ꢄ9.98(19)
ꢄ2.40(19)
ꢄ179.63(11)
ꢄ175.50(11)
7.11(15)
ꢄ178.77(19)
0.6(3)
C(7)AC(6)AC(9)AC(10)
C(5)AC(6)AC(9)AC(10)
C(6)AC(9)AC(10)AC(11)
C(6)AC(9)AC(10)AC(12)
C(9)AC(10)AC(12)AC(13)
C(11)AC(10)AC(12)AC(13)
C(9)AC(10)AC(12)AC(17)
C(11)AC(10)AC(12)AC(17)
C(17)AC(12)AC(13)AC(14)
C(10)AC(12)AC(13)AC(14)
C(12)AC(13)AC(14)AC(15)
C(13)AC(14)AC(15)AC(16)
C(14)AC(15)AC(16)AC(17)
C(13)AC(12)AC(17)AC(16)
C(10)AC(12)AC(17)AC(16)
C(15)AC(16)AC(17)AC(12)
ꢄ172.8(2)
4.7(4)
ꢄ1.4(3)
177.9(2)
176.0(2)
ꢄ4.7(3)
ꢄ4.7(3)
174.62(19)
ꢄ0.1(3)
179.3(2)
ꢄ1.1(4)
1.5(4)
1.1(3)
1.1(4)
ꢄ177.90(10)
2.3(8)
0.94(18)
C(10A)AC(12A)AC(13A)AC(14A)
ꢄ177.99(19)
ꢄ177.90(10)
ꢄ0.6(3)
1.0(3)
ꢄ178.4(2)
ꢄ0.7(3)
C(16A)AN(3A)AC(12A)AC(13A)
C(16A)AN(3A)AC(12A)AC(10A)
C(9A)AC(10A)AC(12A)AN(3A)
C(11A)AC(10A)AC(12A)AN(3A)
C(12A)C(13A)AC(14A)AC(15A)
C(12A)AN(3A)AC(16A)AC(15A)
C(13A)AC(14A)AC(15A)AC(16A)
C(16A)AC(12A)AC(13A)AN(3A)
C(14A)AC(15A)AC(16A)AC(12A)
C(9A)AC(10A)AC(12A)AC(16A)
C(11A)AC(10A)AC(12A)AC(16A)
C(10A)AC(12A)AC(13A)AN(3A)
C(13A)AN(3A)AC(14A)AC(15A)
C(13A)AC(12A)AC(16A)AC(15A)
C(10A)AC(12A)AC(16A)AC(15A)
C(16A)AC(12A)AC(13A)AC(14A)
N(3A)AC(15A)AC(16A)AC(12A)
C(1B)AN(1B)AC(3B)AC(4B)
C(2B)AN(1B)AC(3B)AC(8B)
C(2B)AN(1B)AC(3B)AC(4B)
C(1B)AN(1B)AC(3B)AC(8B)
N(1B)AC(3B)AC(4B)AC(5B)
C(8B)AC(3B)AC(4B)AC(5B)
ꢄ0.5(3)
178.60(18)
4.3(3)
ꢄ174.84(18)
ꢄ0.6(3)
ꢄ0.6(3)
ꢄ0.5(3)
3.6(7)
1.7(7)
ꢄ162.6(4)
5.92(17)
ꢄ171.47(10)
19.7(6)
ꢄ179.4(4)
ꢄ3.2(7)
ꢄ1.24(17)
ꢄ1.05(16)
177.60(11)
0.78(16)
ꢄ4.5(7)
178.8(4)
0.20(19)
ꢄ2.7(3)
ꢄ4.1(3)
176.0(2)
177.3(2)
176.4(2)
ꢄ3.6(3)
1.6(3)
ꢄ5.6(6)
173.0(4)
ꢄ174.5(4)
4.2(6)
ꢄ1.25(15)
ꢄ0.39(15)
178.74(10)
179.62(9)
ꢄ178.18(9)
0.99(15)
ꢄ0.18(16)
ꢄ0.66(15)
ꢄ179.99(9)
0.70(15)
ꢄ179.6(4)
1.6(6)
C(3B)AC(4B)AC(5B)AC(6B)
C(4B)AC(5B)AC(6B)AC(7B)
C(4B)AC(5B)AC(6B)AC(9B)
C(5B)AC(6B)AC(7B)AC(8B)
C(9B)AC(6B)AC(7B)AC(8B)
C(6B)AC(7B)AC(8B)AC(3B)
ꢄ0.2(6)
ꢄ1.1(6)
178.9(4)
1.1(6)
1.9(3)
179.9(2)
ꢄ3.6(3)
178.2(2)
1.7(4)
ꢄ179.0(4)
ꢄ179.92(9)
0.3(6)
0.11(16)
N(1B)AC(3B)AC(8B)AC(7B)
C(4B)AC(3B)AC(8B)AC(7B)
ꢄ178.0(2)
179.6(4)
ꢄ1.7(6)
ꢄ175.9(4)
4.1(7)
ꢄ1.9(7)
176.4(4)
ꢄ20.5(7)
157.9(4)
0.1(7)
178.21(9)
ꢄ0.95(15)
179.03(10)
ꢄ1.65(18)
ꢄ0.51(17)
ꢄ179.80(10)
176.98(10)
ꢄ2.35(14)
0.28(18)
1.9(3)
C(7B)AC(6B)AC(9B)AC(10B)
C(5B)AC(6B)AC(9B)AC(10B)
C(6B)AC(9B)AC(10B)AC(11B)
C(6B)AC(9B)AC(10B)AC(12B)
C(9B)AC(10B)AC(12B)AC(13B)
C(11B)AC(10B)AC(12B)AC(13B)
N(3B)AC(12B)AC(13B)AC(14B)
C(14B)AC(15B)AC(16B)AN(3B)
C(10B)AC(12B)AC(13B)AC(14B)
C(16B)AN(3B)AC(12B)AC(13B)
C(16B)AN(3B)AC(12B)AC(10B)
C(9B)AC(10B)AC(12B)AN(3B)
C(11B)AC(10B)AC(12B)AN(3B)
9.9(4)
ꢄ172.1(2)
0.0(4)
179.4(2)
176.9(2)
ꢄ3.7(3)
0.4(3)
0.1(4)
ꢄ0.3(8)
ꢄ0.11(17)
179.75(19)
ꢄ0.2(3)
ꢄ179.61(19)
ꢄ3.7(3)
175.68(18)
ꢄ178.80(9)

M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
245
Table 4 (continued)
Bond angle
I (°)
Bond angle
II (°)
III (°)
IV (°)
C(12B)AC(13B)AC(14B)AC(15B)
C(13B)AC(14B)AC(15B)AC(16B)
C(12B)AN(3B)AC(16B)AC(15B)
C(16B)AC(12B)AC(13B)AN(3B)
C(14B)AC(15B)AC(16B)AC(12B)
C(9B)AC(10B)AC(12B)AC(16B)
C(11B)AC(10B)AC(12B)AC(16B)
C(10B)AC(12B)AC(13B)AN(3B)
C(13B)AN(3B)AC(14B)AC(15B)
C(13B)AC(12B)AC(16B)AC(15B)
C(10B)AC(12B)AC(16B)AC(15B)
C(16B)AC(12B)AC(13B)AC(14B)
N(3B)AC(15B)AC(13B)AC(12B)
ꢄ0.3(3)
0.1(4)
0.0(4)
ꢄ0.1(7)
0.3(7)
161.1(4)
ꢄ20.5(6)
ꢄ178.6(4)
0.1(7)
ꢄ2.33(16)
178.34(9)
ꢄ0.51(17)
ꢄ1.12(15)
178.23(9)
0.56(15)
ꢄ0.1(6)
178.3(4)
0.95(17)
Fig. 3. Herringbone packing motif with a weak p–p stacking of compound I from the view of the b axes, with schematic intermolecular interactions or close contacts.
Fig. 4. Herringbone packing motif with a slipped p-stacking indicated a weak p–p overlap of II compound.
which is apparently important for the emission properties of
compound. It is generally believed that the stacking motif act-
ing as active layers [26] and the polarity conferred to the molecule
structure by the electron-withdrawing action from the pyridine
group in the para position results in the lamellar structure. Another
factor to consider is that although IV contains a large electron-rich
p–p

246
M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
Fig. 5. Herringbone packing motif with a slight slipped
p
-stacking of compound III, and showing the interaction (dashed lines).
Fig. 6. Crystal packing motif with
p–
p
stacking of compound IV along the c axes and showing close interactions (dashed lines).

M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
247
Fig. 7. The absence of emission of luminescence of I in THF solution (a) and visible yellow–green emission in solid form in cyclohexane under UV illumination (b) and image
of a single crystal of I under UV source (c).
p
-conjugated system that would contribute to a herringbone struc-
substituted I–IV described in the present study exhibit significant
differences in their fluorescence behavior in solid state compared
to that in solution. The absorption spectra of these derivatives were
fairly insensitive to the nitrogen atom position as well as to the sol-
vent polarity. The insensitivity of the absorption spectra of these
molecules to solvent polarity is indicative of the dipole moments
of their ground and excited states being nearly the same. Their
fluorescence spectra in bulk, however, only for the molecule IV
exhibited a marked bathochromic shift with the para-position,
indicating that the dipole moments of their emissive states are sig-
nificantly higher compared to that of their ground states. These re-
sults suggest that direct excitation of these molecules in solvent
leads to a nonemissive locally excitation state possessing nearly
the same dipole moment as the ground state, which subsequently
transforms into a more polar emissive state via intramolecular
charge transfer process. On the other hand, the intermolecular
ture by electrostatic repulsion, the strong dipole–dipole attraction
of IV, stronger even than those in I–III, counterbalances the
electrostatic repulsion, possibly resulting in the lamellar structure.
Furthermore, the double bonds bearing the dimethylaminophenyl
and the nitrile group introducing the para position of the pyridine
group occur in pairs, providing a structure for the molecular
arrangement that gives a strong
p–p overlap, i.e., a more cofacial
arrangement that affects the photophysical properties (Fig. 6).
The differences between the crystal structures of I, II, III and IV
suggest the important role of the position of the nitrogen atom in
the molecular arrangements, which would be helpful to obtain p–p
stacking to increase luminescence. The molecular stacking modes
of molecule IV might be a promising feature since they exhibit par-
allel molecular stacking with face-to-face p–p interactions that are
favorable for carrier transport, while the weak molecular transition
dipole interaction is beneficial for luminescence. The stacking
modes of molecular dipoles rely on both the molecular stacking
modes and the orientation of the lowest allowed transition dipole
direction in the molecules. These intermolecular interactions play
an important role that results in quenching of luminescence in
solution (Fig. 7). The compound I in solution with THF did not ex-
hibit luminescence, but in crystal form in the growing media is
clearly luminescent.
interactions ACHꢀ ꢀ ꢀ
p
and ACHꢀ ꢀ ꢀN could be an important role in
the fluorescence in solid state due to these interactions are not oc-
cur in solution form, contributing to the bulk characteristics. The
crystal habit of I–IV could be different when the crystallization
conditions were changed, however the selection of best solvent is
not always easy. The results on the crystal morphology have indi-
cation that the substrate-solvent interaction and clustering were
tentatively supposed to be the reason for a variety of habits under
different crystallization conditions, i.e., the solvent can attach to
the crystal surface to affect the growth direction of the crystals.
The differences for these molecules strongly depend on the nature
of substituents that caused predominantly the formation of highly
luminescent aggregates in the solid state. Also the differences in
behavior might to be attributed to the dipole–dipole interactions
4. Conclusion
The molecular basis of the luminescence behavior of the bulk
materials has significant importance from the point of view of
designing highly luminescent materials. The dimethylamino-CN
and ACHꢀ ꢀ ꢀ and ACHꢀ ꢀ ꢀN which offset the electrostatic repulsions
p

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M.J. Percino et al. / Journal of Molecular Structure 1034 (2013) 238–248
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Supplementary material
X-ray crystallographic data. CCDC 905648, CCDC 905649, CCDC
905650 and CCDC 905651 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or
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Acknowledgements
We want to thank VIEP-BUAP (Project PEZM-NAT11-G) and
PROMEP-SEP (Thematic network of collaboration) for the financial
support and Dr. Maxime A. Siegler (Johns Hopkins University) for
the assistance with X-ray crystallography.
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