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trum of N-benzylideneaniline from its isoelectronic molecules, (E)-
stilbene [21–25] and azobenzene [26,27], has led to a great number
of theoretical and experimental studies and arguments in the past
three decades [28,29]. However, the effects of molecular non-pla-
nar conformation on the kmax of Schiff bases have rarely been
experimentally studied. Recently, we have verified that the molec-
ular conformation has an important role on the UV spectra of sym-
metrical Schiff bases derived from 1,4-Phenylenediamine (p-
XAAX-p, Fig. 1a) [30]. Nevertheless, the evidence of the molecular
conformation effects on UV spectra of Schiff bases is still scare.
Charge distribution of the molecules is central to the optical and
electronic characters of mesogens [31,32]. Several 13C NMR studies
have revealed that the overall electron distribution can be fine-
tuned through the electronic effects of remote substituents [33–
36]. By means of computational study, Neuvonen and co-workers
[37] have proposed that the twist of the aniline ring respect to
the plane of the C@N unit may affect the 13C NMR chemical shifts
of imine carbon in benzylideneanilines. However, the investigation
of the molecular conformation effects on NMR spectra of p-XAAX-
being cooled to room temperature and purified by recrystallization
from absolute ethanol.
2.2. X-ray crystallography
For X-ray diffraction, suitable crystals of bis-Schiff bases com-
pounds (p-YBBY-p) were obtained by slow evaporation from a bin-
ary solvent mixture of methanol–chloroform (3:1). Colored crystals
were obtained after a few days. For compound Ya–Ye, crystallo-
graphic analyses were performed on a Gemini S Ultra, Oxford plat-
form diffractometer. The crystals Ya, Yb and Yc were measured
with Mo K
tals Yd and Ye were measured with Cu K
a
monochromated radiation (k = 0.71073 Å), while crys-
monochromated radia-
a
tion (k = 1.54184 Å). An empirical absorption correction was
applied. The structures were solved using the direct method and
refined by the full-matrix least-squares method on F2 using the
SHELXL-97 software [41]. All of the non-hydrogen atoms were re-
fined anisotropically, whereas all hydrogen atoms were refined iso-
tropically as a riding mode using the default SHELXL parameters. A
summary of the crystal data and the structure refinements for Ya–
Ye is available in the Supporting Information.
p shows that the dihedral angle
s has a limited effect on the values
of dC(C@N). Our observations of the conformation effects in sym-
metrical Schiff bases p-XAAX-p encouraged us to prepare the other
type of symmetrical Schiff bases derived from 1,4-Phthalaldehyde
(p-YBBY-p, Fig. 1b). When X@Y, p-XAAX-p and p-YBBY-p are iso-
mers. However, the two kinds of symmetrical Schiff bases have dif-
ferent central moiety: in p-XAAX-p, it is central diimine moiety, in
p-YBBY-p, it is central dimethylene moiety. The higher electroneg-
ativity of nitrogen, compared to carbon, and the presence of a lone
pair of electrons in the nitrogen atom, influence the electron distri-
bution. Consequently, it is a worthwhile work to explore the con-
formation effects in p-YBBY-p and provide a further proof for the
effects of molecular conformation on NMR spectrum of symmetri-
cal bis-Schiff bases. These five compounds have been reported by
Iwan et al. [28], Choi et al. [38] and Das et al. [39], but there are
short of their crystal structure.
2.3. Spectral measurement
Absorbance spectra were collected on a LAMBDA-35 UV–vis
spectrometer in a concentration range from 10ꢁ3 to 10ꢁ5 mol/L.
The solvents used in absorption experiments (ethanol, acetonitrile,
chloroform and cyclohexane) were of spectroscopic grade and
were used as purchased. The values of kmax and the maximum
absorption wavenumber tmax for compounds Ya–Ye are listed in
Table 1. The NMR chemical spectra of compounds Ya–Ye were re-
corded in CDCl3 at 293 K. The 13C NMR chemical shifts of the C@N
groups are reported in Table 3, expressed in ppm relative to CDCl3
(77.0 ppm). The detailed analytical data of compounds Ya–Ye are
available in the Supporting Information.
To provide an extending evidence of molecular conformation
effects on the kmax and 13C NMR chemical shifts dC(C@N) of sym-
metrical Schiff bases, we synthesized five samples of symmetrical
bis-Schiff bases p-YBBY-p (Fig. 1b) in this work. In p-YBBY-p, the
3. Results and discussion
substituents
Y
include
H
atom, electron-donating groups
3.1. Description of the crystal structures
(Y = OMe, or Me) and electron-withdrawing ones (Y = Cl, or F).
Their crystal structures and spectroscopic properties were mea-
sured experimentally, and the effects of the molecular conforma-
tion on spectroscopic properties of Ya–Ye (Fig. 1b) were quantified.
A comparison of the X-ray crystal structures revealed a possible
role for the conformation of Ya–Ye (Fig. 2). Compound Ya crystal-
lized in the orthorhombic space group Pbca, while Yb, Yc, Yd, and
Ye all crystallized in the monoclinic space group P21/n.
2. Experimental methods
As shown in Fig. 2, the benzylidene ring of each compound is
nearly co-planar with the N1@C7AC8 or N1A@C7AAC8A, whereas
the aniline ring is twisted significantly from the C4AN1@C7 or
C4AAN1A@C7A. Because of the slight deviation of the benzylidene
ring from the N1@C7AC8 or N1A@C7AAC8A plane, we discuss be-
low the twist of the aniline ring respect to the plane of the C@N
2.1. Sample preparation
Compounds Ya–Ye were all prepared by solid–solid reactions
[40]. The pure p-substituted aniline and 1,4-Phthalaldehyde were
mixed in a 2:1 M ratio, and then the mixture was heated and
melted. The mixture was further stirred for several minutes before
unit only. The dihedral angle
s is defined by atoms C7@N1AC4AC3
or C7A@N1AAC4AAC3A. The values of
s
in Ya–Ye are reported in
N
X
Y
N
N
Y
N
X
Xa:
X = OMe;
Xb: X = Me;
Xc: X = Et;
X = Cl;
Xe: X = F;
Ya: Y = OMe;
Yb:
p-YBBY-p
Y = Me;
Yc: Y = H;
Yd: Y = Cl;
Ye: Y = F;
p-XAAX-p
Xd:
Xf: X = CF3;
Xg:
X = CN
b
a
Fig. 1. General structures of compounds p-XAAX-p(a) and p-YBBY-p(b).
Fang, Zhengjun
