Structure-directed functional properties of symmetrical and unsymmetrical Br-substituted Schiff-bases

Article abstract of DOI:10.1016/j.molstruc.2013.07.002

This study deals with the investigation of two low molecular weight Schiff base compounds with bromine end groups, differing only by the presence or absence of one methyl group, with the aim of better understanding the solution and the solid state optical

Full text of DOI:10.1016/j.molstruc.2013.07.002

Journal of Molecular Structure 1049 (2013) 377–385
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structure-directed functional properties of symmetrical and
unsymmetrical Br-substituted Schiff-bases
Luminita Marin a, Valeria Harabagiu a, Arie van der Lee b, Adina Arvinte c, Mihail Barboiu a,b,
⇑
a ‘‘Petru Poni’’ Institute of Macromolecular Chemistry of Romanian Academy, 41A Aleea Gr. Ghica Voda, Iasi, Romania
b Institut Européen des Membranes, ENSCM/UM2/CNRS 5635, IEM/UM2, CC 047, Place Eugène Bataillon, F-34095 Montpellier, France
c Centre of Advanced Research in Nanobioconjugates and Biopolymers, ‘‘Petru Poni’’ Institute of Macromolecular Chemistry, 41A Aleea Gr. Ghica Voda, Iasi, Romania
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
Very similar low molecular Schiff bases show different solution and solid state optical, electrochemical and thermic properties.
The presence of a CH3 group on the aromatic increase the energy gap which hinder the electronic transitions.
The introduction of a methyl unit as a bulky group induce large intermolecular distances in the crystal packing.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2013
Received in revised form 27 June 2013
Accepted 2 July 2013
Available online 5 July 2013
This study deals with the investigation of two low molecular weight Schiff base compounds with bromine
end groups, differing only by the presence or absence of one methyl group, with the aim of better under-
standing the solution and the solid state optical, electrochemical and thermic properties generated by the
small structural features. The compounds were structurally characterized by 1H NMR, 13C NMR and FTIR
spectroscopies, single-crystal X-ray diffraction and gas chromatography analysis. Their thermal behavior
was evidenced by polarized light microscopy and differential scanning calorimetry. The single crystal
structure of the compounds reveals the versatility of bromine substituents which can adopt an attractive
or repulsive electron effect, driven by the structural environment. A comparison of the structures shows
Keywords:
Schiff bases
X-ray structures
Crystal packing
Inductive electronic effects
Electrochemistry
that the introduction of a methyl unit as a bulky group hinders completely the p–p stacking interaction
within the structural rows, inducing larger intermolecular distances resulting in quite different functional
physical and chemical properties.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
structural factors, is also endowing dynamic features for this type
of compounds. Moreover, Schiff bases can give rise to extended
Schiff-bases compounds containing one or more CH@N imine
bonds are intensely studied due to their large spectrum of phys-
ico-chemical properties: ability to form various coordination com-
plexes, high thermal stability, semiconducting, liquid crystal,
optical, therapeutic properties. As a consequence of their well-or-
ganized structures, all these features make them useful as thermo
resistant materials, pharmaceutical products, or organic substrates
in electronic and opto-electronic devices [1–5]. The easily procesa-
ble imine bonds [6] provide a direct access to complex systems like
double helices [7], grids [8], borromean rings [9], dynamic poly-
mers [10], capsules [11], polygons [12], etc. The self-organization
of constitutional systems [13] resulting from reversible connectiv-
ity (molecular level) [14] and self-assembly (supramolecular level)
[15] of subcomponents under the pressure of internal and external
conjugated platforms with unique optical, electronic and thermic
properties. Depending on their molecular structure and composi-
tion, they can exist in various crystalline phases and only slight
structural modifications can induce interesting modulation of
functional properties.
On the other hand, recent studies reveal the benefit of the hal-
ogen bonding in the crystal state which lead to impressive elec-
tronic performances that is amplifying triplet generation and
activating triplet emission [16]. The Br atom is almost inert from
an electronic point of view due to two antagonistic electronic ef-
fects: a donor electronic effect due to the lone pair donor 4p orbital
and a withdrawing effect due to a vacant 4d orbital acting as a
weak electron acceptor [17]. Moreover, the Br moieties can be eas-
ily used as anchoring sites for further Suzuki coupling reactions
[18,19].
In this context, we present here two examples of Schiff-bases
prepared under mild reactional conditions, containing two bro-
mo-phenyl moieties connected via an imine bond, combined with
⇑
Corresponding author at: Institut Européen des Membranes, ENSCM/UM2/CNRS
5635, IEM/UM2, CC 047, Place Eugène Bataillon, F-34095 Montpellier, France.
E-mail address: mihail-dumitru.barboiu@univ-montp2.fr (M. Barboiu).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.07.002

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L. Marin et al. / Journal of Molecular Structure 1049 (2013) 377–385
Table 1
the existence of a methyl substituent in one compound rendering
unsymmetrical and influencing the planarity of the resulted bis-
aromatic conjugated scaffold. It results quite different solid state
optical and thermic properties generated by such small structural
differences.
Crystal data for the compounds 1 and 4.
Compound
1
4
Empirical formula
Space group
a (Å)
b (Å)
c (Å)
C
P21/c
3.97116(11)
5.84810(15)
24.8821(6)
90
13H9Br2N
C14H11Br2N
Pbcn
14.5003(8)
6.1560(3)
28.737(2)
90
2. Experimental section
a
°
b°
92.426(2)
90
577.34(3)
2
90
90
2565.2(3)
8
2.1. Materials and methods
c
°
V (Å3)
Z
4-Bromoaniline (97% purity), 4-bromobenzaldehyde (99% pur-
ity), 4-bromo-3-methylaniline (99% purity) purchased from
Aldrich were used as received. The gas chromatograms were per-
formed using an Agilent Network GC System 6890N instrument.
The infrared (IR) spectra were recorded on a FT-IR Bruker Vertex
70 Spectro-photometer in the transmission mode, by using KBr
pellets.
Size
0.20 Â 0.25 Â 0.30
0.04 Â 0.20 Â 0.35
qcalc (g cmÀ3
)
1.950
1.828
Resolution (Å)
Total data
Unique data
Rint
0.73
24518
1369
0.042
0.0765
71
0.0236
0.0568
0.9163
0.73
11354
2456
0.051
0.1004
154
0.0574
0.1189
1.0443
<r
(I)/I>
Parameters
The proton nuclear magnetic resonance (1H NMR) and carbon
nuclear magnetic resonance (13C NMR) spectra were recorded by
a BRUKER Avance DRX 400 MHz spectrometer using CDCl3 as sol-
vent and tetramethylsilane (TMS) as internal reference. Chemical
shifts are reported in parts per million (ppm).
R
wR2
GOF
The elemental analysis was realized on an elemental analyzer
CHNS 2400 II Perkin Elmer.
The UV–vis absorption and photoluminescence spectra were re-
corded on a Carl Zeiss Jena SPECORD M42 spectro-photometer and
Perkin Elmer LS 55 spectrophotometer respectively, in very diluted
DMF solutions (ffi10À5%) using 10 mm quartz cells fitted with
poly(tetrafluoroethylene) stoppers.
The thermal analysis was performed by using an Olympus BH-2
polarized light microscope equipped with a THMS 600/HSF9I heat-
ing stage. The optical observations were performed by using clean
untreated glass slides. The differential scanning calorimetry (DSC)
measurements were performed with a METTLER Toledo STAR sys-
tem, under nitrogen atmosphere (nitrogen flow 120 ml/min, sam-
ple mass 3–4 mg). The transition temperatures were read at the
top of the endothermic and exothermic peaks.
Wide angle X-ray diffraction measurements (WAXD) were per-
formed on a Bruker D8 Advance diffractometer, using Ni-filtered
Scheme 1. Synthesis of compounds 1 and 4.
Cu Ka radiation (k = 0.1541 nm). An MRI-WRTC – temperature
chamber (with nitrogen inert atmosphere) and an MRI-TCPU1 –
temperature Control and Power Unit were used. The working con-
ditions were 36 kV and 30 mA. The measuring angular range was
1.5–60° in 2h. The samples for X-ray measurements were powders
obtained by grinding single crystals formed by recrystallization
from ethanol. All diffractograms are reported as observed. The sin-
gle crystal diffraction intensities were collected at the joint X-ray
Scattering Service of the Pôle Balard of the University of Montpel-
lier II, France, at 175 K using an Agilent Technologies and a Gemini-
fluoride/acetonitrile, at room temperature under nitrogen atmo-
sphere, using glassy carbon electrodes purchased from BASi – Bio-
analytical Systems, Inc., USA as working electrode, an Ag/AgCl
electrode and a platinum wire as reference and auxiliary elec-
trodes. All reported potentials have been read versus Ag/AgCl.
2.2. Synthetic procedure for imine-compounds 1, 4
S diffractometer with Mo K
by ab initio (charge-flipping) methods using SUPERFLIP [20] and re-
fined by least-squares methods on F using CRYSTALS [21] against |F|
a
radiation. The structures were solved
The synthesis of the (4-bromobenzylidene)-(4-bromo-phenyl)-
imine (1) has been performed by the acid catalyzed condensation
reaction of the 4-bromobenzaldehyde (2) with 4-bromoaniline
(3) (Scheme 1): 5 mmol (0.88 g) of 3 and 4 ml ethanol were placed
into a round bottom flask fitted with a condenser, a dropwise fun-
nel, a nitrogen inlet and outlet. A solution of 5 mmol (0.93 g) of 2 in
4 ml ethanol was added slowly, under vigorous stirring, as well as
few drops of glacial acetic acid as catalyst. The obtained mixture
was gently refluxed overnight. The reaction mixture was kept for
24 h until the crystals grew sufficiently. The crystals of 1 were fil-
tered off, washed and recrystallized from ethanol. Finally, the ob-
tained single crystals were dried under vacuum for 24 h. The
yield was 87%. The same experimental protocol has been applied
for the synthesis of (4-bromo-3-methyl-benzylidene)-(4-bromo-
phenyl)-imine 4 from 5 mmol (0.94 g) of 4-bromo-3-methylaniline
(5) and 5 mmol (0.93 g) of 4-bromobenzaldehyde (2). The com-
on data having I > 2r(I); R-factors are based on these data. Hydro-
gen atoms were located from difference Fourier synthesis, except
the hydrogen atom in the CH@N linkage of compound 1 which
was placed geometrically. Non-hydrogen atoms were refined
anisotropically. The single crystal parameters of the compounds
1 and 4 are given in Table 1.
Cyclic voltammetry (CV) was employed to study the electro-
chemical behavior of 1 and 4 compounds deposited as thin films
from CHCl3 solution on the glassy carbon (GC) electrode. The cyclic
voltametry (CV) measurements were performed using AUTOLAB
PGSTAT302N system from ECO CHEMIE Utrecht, The Netherlands.
The electrochemical studies were carried out in a single compart-
ment electrochemical cell of 3 ml 0.1 M tetrabutylammonium

L. Marin et al. / Journal of Molecular Structure 1049 (2013) 377–385
379
pound 4 resulted as white single crystals in 83% yield. The gas
2.2.2. (4-Bromo-3-methyl-benzylidene)-(4-bromo-phenyl)-imine (4)
chromatography confirmed the purity of the synthesized com-
pounds to be more than 97% (Table 1S).
2.2.1. (4-Bromobenzylidene)-(4-bromo-phenyl)-imine (1)
1H NMR (CDCl3, 400 MHz): d (ppm) 8.38 (s, 1H: CH@N); 7.77,
7.75 (d, 2H: 12a, 16a); 7.62, 7.60 (d, 2H: 13a, 15a); 7.54, 7.52 (d,
1H: 6a); 7.09 (s, 1H: 3a); 6.92, 6.90 (d, 1H: 5a); 2.43 (s, 3H: CH3).
13C NMR (CDCl3, 100 MHz): d (ppm) 159.07 (C10AH), 150.76
(C4Aipso), 138.78 (C2Aipso), 132.92 (C6AH), 132.09 (C13AH;
C15AH), 130.17 (C12AH, C16AH), 126.19 (C14Aipso), 123.32
(C3AH), 122.15 (C1Aipso), 119.6 (C5AH), 22.9 (C8AH). FTIR
m:
cmÀ1 (KBr): 818, 845, 1297, 1582–1461, 1618), 2852, 2921,
3077–2852. Calcd. for C14H11Br2N (353.06): C 47.63, H 3.14, Br
45.26, N 3.97; found: C 47.75, H 3.21, N 4.09.
1H NMR (CDCl3, 400 MHz): d (ppm) 8.36 (s, 1H: CH@N); 7.76,
7.74 (d, 2H: 11a, 15a); 7.61, 7.59 (d, 2H: 12a, 14a); 7.51, 7.49 (d,
2H: 2a, 6a); 7.09, 7.06 (d, 2H: 3a, 5a). 13C NMR (CDCl3,
100 MHz):
d
(ppm) 159.23(C9AH), 150.54(C4Aipso), 134.88
3. Results and discussion
(C13Aipso), 132.24(C2AH; C6AH), 132.09(C12AH, C14AH),
130.19 (C11AH, C15AH), 126.19(C10Aipso), 122.54(C3AH,
3.1. Synthesis and structural characterization
C5AH), 119.6(C1Aipso). FTIR
1478, 1620, 1723, 1895, 3062–2880. Calcd. for
m
: cmÀ1 (KBr): 814, 829, 1587–
C
13H9Br2N
The compounds 1 [2] and 4 have been synthesized by classical
acid condensation reaction (Scheme 1) [5,22]. Single crystals have
been obtained by recrystallization from ethanol. The 1H, 13C NMR
(339.03): C 46.06, H 2.68, Br 47.14, N 4.13; found: C 45.93, H
2.74, N 4.21.
Fig. 1. Crystal structure of compound 1: (a) side view in stick representation; (b) space-filling representation of interdigitated parallel rows which are alternatively stratified
in compact stacked blocks; and (c) side view of stacking offset-face-to-face interactions between two molecules of 1 in the crystal.

380
L. Marin et al. / Journal of Molecular Structure 1049 (2013) 377–385
Fig. 2. Crystal structure of compound 4 in stick representation along: (a) a, (b) b, and (c) c axes.
Fig. 3. Packing interaction areas (translucent red surfaces) in the single crystal structures of compounds (a) 1 and (b) 4. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
and FTIR spectra are in agreement with the proposed formula
(Fig. 1, 2S). No significant differences have been observed, in the
chemical shifts of either the N@CH proton or the aromatic protons,
between the two synthetized azomethines, which indicates the
weak influence of the inductive repulsive effect of the methylene
unit. Despite the traces of reagents indicated by the GC measure-
ments, no signals of initial compounds have been detected by
NMR.

L. Marin et al. / Journal of Molecular Structure 1049 (2013) 377–385
381
Table 2
ing of the CAH bond in the ACH@NA group, respectively (Fig. 3S)
[23]. No significant absorption bands were observed at 1685 cmÀ1
(due to the aldehyde group) or at 3345 cmÀ1 (due to the amine
linkage). The weak absorption peaks at 3077–2991 and 1916–
Summary of photophysical properties. The energy band gap (Eg) was estimated from
the following equation: Eg = h  c/kmax, where h is the Planck constant, c is the light
velocity, and k is the wavelength of the absorption, in the optical absorption spectra.
Emg ax/Eegdge/Egint (eV)
1723 cmÀ1 were assigned to the CAH stretching (
mCAH) and the
Compound
kabs (nm)
kem (nm)
Stokes shift
CAH out-of-plane (
cCAH) deformations of the phenylene rings,
1 (solution)
4 (solution)
1 (film)
278, 3
278, 3
286, 3
289, 3
414
425
–
3.81/3.02/3.3
3.85/3.09/3.17
3.61
89
103
–
respectively, while the strong absorption peaks within the 1587–
1461 cmÀ1 and 845–708 cmÀ1 domains were attributed to the mC@C
and cCAH of the 1,4-phenylene rings. Upon comparing the two
spectra, as expected, the asymmetric and symmetric stretching
modes of CH3 in the 4 are assigned to the absorption bands at
2921 and 2852 cmÀ1 respectively, while the band at 1297 cmÀ1
is assigned to the CACH3 stretching mode. These findings are in
very good agreement with the proposed structure and literature
values [24,25]. The FTIR spectra registered in the melted state re-
veals much more intense absorption bands than in the solid state,
reflecting the disruption of intermolecular forces and thus increas-
ing mobility of atoms. In the case of compound 1, the wave number
of the absorption bands slightly move to blue, indicating that ther-
mal energy favors more the conjugation of the aromatic system,
the linkages being weaker and their vibrations and deformations
stronger. Contrary, the absorption bands of the compound 4
slightly moves to red, probably due to the hyperconjugation effect
of the CH3 unit which opposites to the withdrawing effect of imine
nitrogen and increase the stability of the system (Fig. 4S). The FTIR
spectra, registered at room temperature, after cooling, are similar
to the FTIR spectra before heating.
4 (film)
–
3.65
–
Emg ax – energy band gap calculated for wavelength of the absorption maxima; Eegdge
–
energy band gap calculated for wavelength of the absorption edge; and Eignt – energy
band gap calculated for wavelength of the intercept of absorption and emission
curves.
3.2. Solid state structures of imino compounds 1, 4
Fig. 4. Absorption and emission behavior of 1 (continuous line) and 4 (dashed line).
The crystal structures of 1 and 4 were determined on crystals
obtained from ethanol solutions at room temperature. They reveal
the expected azomethine compounds and the unit cells contain ten
molecules of 1 and eight molecules of 4, respectively. The molecule
of 1 present a planar structure indicating a very good conjugation
The FTIR spectra show typical bands arising from imine groups
presenting
a
strong absorption band at 1620 cmÀ1 and at
2880 cmÀ1 specific to the the AC@NA vibration and axial stretch-
Fig. 5. Polarized light microscopy photographs (200Â) of compounds 1 and 4 (a) before and (b) after heating/cooling cycle.

382
L. Marin et al. / Journal of Molecular Structure 1049 (2013) 377–385
Fig. 6. XPRD diffraction patterns of compounds 1 (left) and 4 (right) at room temperature. The top figures represent the diffractograms before thermal treatment along with
the Le Bail fits using the cell parameters of the single crystal structure determinations. Blue lines are the observed diffraction patterns and red lines the calculated patterns.
The arrows indicate some of the diffraction peaks belonging to the second phase. The bottom figures represent the superimposed XPRD patterns before and after thermal
treatment; the blue lines are the patterns before treatment and the red lines those after thermal treatment.
of imine linkage with phenyl moieties (the twisting angles of aro-
matic rings are both of 2.02°). In the crystal each molecule of 1 pre-
sents a tight contact with the two neighboring ones by stacking
with opposite orientations of the inner imine group side-arms as
previously described (Fig. 1) [2]. The occupancy of both imine
atoms is corresponding to 50% N and 50% C while the CAH – atom
is 50% present in the two positions (Fig. 1c). Each duplex of 1 asso-
ciates in the crystal lattice by phenyl–phenyl offset-face-to-face
(off) interactions (the average distance of 3.97 Å and the offset an-
gle of about 20.8°) [26]. They are restricted to one direction form-
ing interdigitated parallel rows which are alternatively stratified in
compact stacked blocks (Fig. 1b).
3.7 Å, forming molecular layers with the inter-layers distance
about 12.4 Å, resulting in a fish backbone supramolecular architec-
ture (Fig. 1). The layers are weakly connected by type-II BrABr
interactions (3.801 Å; H1 = 92.80°; H2 = 166.00°), to be compared
with the cis-type
H1 = 71.36°; H2 = 108.64° [27]. The intermolecular and inter-rib-
bons distances are close to the interplanar distance of 3.5 Å,
I BrABr intralayer interaction (3.971 Å;
p–p
pointing for presumed electrical properties. The molecules of 4
presents a non-planar distorted geometry with the torsion angles
CACAC@N and C@NACAC of À15.07° and 20.6°, respectively, indi-
cating a very low conjugation of imine linkage with phenyl moie-
ties (Fig. 2). The molecules of 4 are in close contact filling the
Within a row the distance between two Br–Br atoms of two dif-
ferent stacked ligands is 3.97 Å, indicating a slight cohesive inter-
action between the halogen atoms, the van der Waals radius of
Br being 1.85 Å, which enhances slightly the stabilization of the
structure. This fact sustains the hypothesis of a push–pull system
in which the two bromine atoms play antagonic role – withdraw-
ing and repulsive, respectively – the sense being established by the
nitrogen atom electronegativity. This is possible due to the two
antagonistic electronic effects of bromine atom – the donor elec-
tronic effect due to the lone pair donor 4p orbital and the with-
drawing effect due to a vacant 4d orbital – which become in a
different way predominant, forced by the nitrogen electronegativ-
ity. In the crystal two different blocks are arranged into almost two
orthogonal planes (Fig. 1a) with the inter-ribbons distance around
space of the crystal, but in contrast to compound 1 no p–p stacking
interactions are present within the crystal. The integrity of the
crystal packing is assured by weak hydrophobic CH3ACH3 and,
again, by the interlayer halogen BrABr (d = 3.881 Å) contacts. The
latter contacts have a trans type I configuration with H1 = 112.5°
and H2 = 152.9°. In addition to these interlayer contacts, there is
an attractive CHÁ Á ÁBr contact of 2.95 Å with a CHÁ Á ÁBr angle of
141.7°. The molecules are forming ribbons along b axis with an
intermolecular distance of 6.13 Å, the packing being driven by self
assembling of aromatic rings and methyl groups. The ribbons are
packed together two by two, with an inter-ribbons distance of
3.79 Å, while the d-spacing between the twice packs is 4.10 Å.
The packs form molecular layers, with the interlayer distance
about 14.7 Å.

L. Marin et al. / Journal of Molecular Structure 1049 (2013) 377–385
383
Fig. 3 shows the packing interactions in the structures of 1 and
3.4. Thermal analysis
4, as calculated from the gradient of the electron densities derived
from the atomic coordinates [28,29]. It is clearly shown that the
packing of the molecules within the layers in 1 is mostly driven
The polarized light microscopy (PLM) measurements showed
that compounds 1 and 4 present well-defined crystalline states
at room temperature (Fig. 5) which rapidly melt during the first
heating scan with a short appearance of marbled textures not reli-
able with stable mesophases observed for other systems [31,32].
The melting point of the less compact structure of the 4 (81 °C)
is substantially lower than the melting point of dense structure
of 1 (147 °C) in accordance with observed crystal structure. When
the isotropic liquid is cooled, the crystalline state appears in the
form of terraces and spherules, respectively at 130 °C (1) and
43 °C (4).
by
p–p stacking interactions between the molecules within the
rows which are in close contact via van der Waals interactions so
that all available void space between the molecules is filled. Oppo-
sitely, only weak van der Waals interactions may be observed in
the structure of the compound 4, presenting a less dense structure
than compound 1.
The X-ray crystallographic results allow the following conclu-
sions to be made:
ꢀ In term of specific self-assembly the compounds 1 and 4
self-organize in the solid state in interdigitated parallel
rows which are alternatively stratified in compact stacked
blocks.
ꢀ The larger intermolecular distances observed between rows and
blocks in the crystal matrix, indicate that compound 4 presents
a lower dipole moment than compound 1 because of the Me
electron-donor bulky group on the aniline ring which favors
an increase of the dihedral angle and strongly disturb the planar
interactions between molecules.
The differential scanning calorimetry (DSC) measurements
(Fig. 5S) are consistent with PLM observations: melting with the
short occurrence of the marbled texture (endothermic left tailed
peak at 143 °C (1) and 81 °C (4)) and crystallization processes (exo-
thermic peak at 135 °C (1) and 49 °C (4)). The melting and crystal-
lization temperature values determined by DSC are close by those
measured by PLM, under the same conditions (10 °C/min). The DSC
and PLM measurements registered with various heating/cooling
rates (10, 5, 1 °C) show only a melting and, respectively, cooling
process, indicating that, in the absence of a flexible spacer, the
mesophase could not be stabilized.
The X-ray diffraction patterns of the compounds 1 and 4 have
been recorded for the samples obtained by recrystallization from
ethanol and then for the samples thermally treated (heating up
to the melted state and cooling at room temperature) (Fig. 6). It ap-
pears that already before the thermal treatment the samples are
not single-phased, because not all peaks can be indexed using
the cell parameters of the single-crystal structure determinations.
Both 1 and 4 display a pronounced impurity peak at a slightly low-
er angle than the main (002) diffraction peak of the single-crystal
ꢀ The bromine substituents play an important role in the selfas-
sembling of the molecules due to the attractive forces between
they which indicate a dynamic electronic effect drawn by the
structural environment.
ꢀ In term of specific self-organization the compounds 1 and 4
reveal an attractive example of solid-state high (1) or low (4)
density structures resulted by synergetic intermolecular
p–p
stacking, halogen–halogen, halogen–H, and hydrophobic CH3–
CH3 interactions between different molecules.
3.3. UV–vis and fluorescence spectra
The ultraviolet (UV–vis) absorption and fluorescence spectro-
scopic data in diluted dimethylformamide (DMF) solution
(5 Â 10À4 M) for the azomethine compounds 1 and 4 are summa-
rized in Table 2. The absorption spectra are characterized by two
absorbtion bands: the first one at 278 nm (4.46 eV) assigned to
the benzenoid transitions and the second one at 325 and 322 nm,
respectively (3.81, 3.85 eV) characteristic to the spin-allowed
p–
pà transitions involving the azomethine-phenyl-bromine frame-
work (Fig. 4).
The marked broadening of the absorption peaks can be related
to non-planar conformations adopted in the solution as previously
described [2]. The incorporation of the CH3 group do not result in
notable effects on the position of kmax band for the compounds 1
and 4. The energy gap has been calculated for wavelength of the
absorption maxima, edge absorption and for the intercept of
absorption and emission spectra (Table 2). The values of the optical
gaps (Eogpt) estimated from their absorption maxima are higher
than those usually observed for azomethine compounds incorpo-
rating in their structure electron-withdrawing units which pro-
mote an enhanced conjugation [3,22].
The emission spectra of the compounds 1 and 4 have been mea-
sured by excitation at 278 nm. They are luminescent materials
which emit weak violet (414 nm) and bluish light (425 nm),
respectively (Fig. 4). Interestingly enough, the compounds present
large Stokes shift values (89 and 103 nm, respectively), resulting in
an overlap missing between the absorption and emission spectra of
the compound 4 and a negligible overlap in the case of 1. This pre-
vents the reabsorption of the emitted light and so undesired loses,
aspect very important for applications based on detection of emit-
ted light or light emitting devices [5,30].
Fig. 7. Comparison of voltammetric profiles in (a) the reduction and (b) the
oxidation range for 1 (solid line) and 4 (dotted line).

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L. Marin et al. / Journal of Molecular Structure 1049 (2013) 377–385
Table 3
Cyclic voltammetry data of the compounds 1 and 4.
ox
onset
red
onset
Code
Ered (V)
Eox1 (V)
Eox2 (V)
IP (eV)
AE/AE (eV)
Eg (eV)
E
(V)
E
(V)
1
4
À0.6
À1.08
À1.34
1.06
0.98
1.15
1.07
1.39
1.29
5.46
5.38
3.8
3.43
1.66
1.95
À0.97
crystalline phases. The (002) peak corresponds to the packing
direction of the layers formed by the interdigitated parallel rows
and which are hold together by very weak BrABr interactions
and in the case of compound 4, by a slightly stronger CHÁ Á ÁBr inter-
action. These interactions are so weak that other interactions may
be possible to self-assemble the 3D structure, which explains qual-
itatively the formation of a second polymorphic crystal form for
both 1 and 4. Based on the relative intensities of the strongest
peaks of both forms, it is estimated that about 80% of the crystals
are of the form determined by single-crystal diffractions, and that
the remaining 20% belongs to an unknown polymorph.
Upon thermal treatment the behavior of 1 and 4 is different. The
diffraction intensities in both cases diminish, but for 1 a new poly-
morph appears to develop at the expense of both room-tempera-
ture polymorphs, whereas no signs of a new polymorph appear
for 4. Peak broadening is insignificant upon the thermal treatment,
meaning that the coherently diffracting domains do not become
smaller. The drop in relative intensity of 4 is therefore a bit surpris-
ing, since the appearance of an important signal originating from
an amorphous part cannot be detected as well.
To establish the electronic structure of 1 and 4 compounds, the
relative energy levels of the highest occupied molecular orbital
(HOMO or
p level) and the lowest unoccupied molecular orbital
(LUMO or pà level) have been estimated from cyclic voltammetric
measurements on the basis that the oxidation process corresponds
to electrons removal from HOMO energy level, while reduction
corresponds to electron addition to the LUMO energy level of
ox
material [34] The oxidation onset potentials (E
) and reduction
onset
red
onset potentials (E
) of a material can be correlated to the ioni-
onset
zation potential and electron affinity according to the relationship
proposed by Bredas et al. [35]. The onset potentials are obtained at
the intersection of the baseline charging current with the tangent
drawn at the rising currents. A summary of the peak potentials
(Eox, Ered), oxidation and reduction onset potentials of the two com-
pound is given in Table 3. The ionization potentials (IP) and elec-
tron affinity (AE) have been calculated, according to equations:
ox
IP ¼ E
þ 4:4 ðeVÞ
þ 4:4 ðeVÞ
onset
red
AE ¼ E
onset
Eg ¼ IP À AE ðeVÞ
3.5. Electrochemical experiments
where Eox and Ered are the onset potentials of oxidation and reduc-
tion measured in the system, while Eg is the energy gap.
As can be seen from Table 3, the HOMO level of the compound 1
is higher and its electrochemical gap is smaller compared to the
compound 4, due to the better conjugation and thus better planar-
ity of the molecules.
The electrochemical behavior of the compound 1 deposited as a
film on the glassy carbon electrode was evaluated by cyclic voltam-
metry at a scan rate of 100 mV/s using 0.1 M tetrabutylammonium
fluoride in acetonitrile as supporting electrolyte. As shown in Fig. 7,
when scanning the potential on the negative range, only one
reduction peak appears around À1.08 V attributed to the reduction
of imino group. According to literature, the reduction of imines
with at least one phenyl substituent on the C@N double bond is
a two-electron process involving saturation of the carbon–nitrogen
double bond [33]. The first electron uptake corresponds to the for-
mation of radical-anion while the second electron uptake is due to
the further reduction of the radical-anion:
4. Conclusions
In conclusion the simple symmetrical and unsymmetrical Schiff
bases discussed here present complete different constitutional,
optical and thermic properties. Interesting enough, the bromine
end group prove a versatile electronic effect which accounts for
selfassembling of the molecules and significant influence of prop-
erties. The presence of a CH3 bulky group on the aromatic back-
bone influences its electronic properties by decreasing the HOMO
energy level and the increasing of energy gap which hinder the
electronic transitions. Moreover, the shifting of emission from vio-
let to bluish leads to an overlap missing between absorption and
emission spectra which prevent the reabsorption of the emitted
light. Comparing the crystal packing of the two compounds, the
introduction of a Me unit as a bulky group in the chemical struc-
PhCH@NPh þ eÀ ! PhCH@NPhÅÀ
PhCH@NPhÅÀ þ Hþ ! PhCÅHANHPh
PhCÅHANHPh þ eÀ þ Hþ ! PhCH2ANHPh
Because the radical obtained from protonation of the radical-
anion is more easily reduced than the starting imine, the potentials
of the first and second electron uptake by the imine practically
overlaps. Applying the potential in the positive range, two oxida-
tion peaks at +1.15 V and +1.39 V were registered due to the elec-
tron transfer processes taking place at the terminal bromine.
Compared to compound 1, the reduction peak corresponding to
compound 4 occurs at more negative potential value (À1.34 V)
(Fig. 7a) and with a lower current intensity, indicating a more dif-
ficult reduction process. The explanation for this negative shift
might be the hyperconjugation effect due to the presence of the
methyl group which diminishes the electron density of the mole-
cules. Similar to reduction process, the oxidation peaks of the com-
pound 4 is left shifted to lower potential values (+1.07 V and
+1.29 V) indicating an easier oxidation, facilitated by the elec-
tron-donor methyl group (Fig. 7b).
ture of 4, hinders completely the weak
p–p stacking interaction
within the rows, inducing larger intermolecular distances which
significantly differentiates the properties of compounds 1 and 4.
The crystalline structure obtained by crystallization from the
melted state shows closer intermolecular distances when com-
pared to the supramolecular architecture of the crystalline struc-
ture obtained by recrystallization from ethanol. More precisely,
about 80% of the crystals are of the form determined by single-
crystal diffraction, and the remaining 20% belongs to an unknown
polymorph. Considering the simplicity of this molecules, these ba-
sic molecular synthons can further find possible applications on
the synthesis of more complex heteroarchitectures of applicative
interest such as carbonic anydrase inhibitors [36], solid state

L. Marin et al. / Journal of Molecular Structure 1049 (2013) 377–385
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The research leading to these results has received funding from
the Romanian National Authority for Scientific Research, CNCS –
UEFISCDI Grant, Project Number PN-II-ID-PCCE-2011-2-0028 and
European Union’s Seventh Framework Programme (FP7/2007-
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