Effect of molecular conformation on spectroscopic properties of symmetrical Schiff bases derived from 1,4-phenylenediamine

Article abstract of DOI:10.1016/j.molstruc.2012.12.014

The relationship between the molecular conformation and spectroscopic properties of symmetrical bis-Schiff bases was explored experimentally. Seven samples of compounds p-X-C₆H₄CHNC₆H ₄NCHC₆H₄

Full text of DOI:10.1016/j.molstruc.2012.12.014

Journal of Molecular Structure 1036 (2013) 447–451
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Effect of molecular conformation on spectroscopic properties of symmetrical
Schiff bases derived from 1,4-phenylenediamine
Zhengjun Fang a,c, Chenzhong Cao b,d,e,
⇑
a School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China
b School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
c School of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China
d Hunan Provincial University Key Laboratory of QSAR/QSPR, Xiangtan 411201, China
e Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education, Hunan University of Science and Technology, Xiangtan 411201, China
h i g h l i g h t s
"
"
"
Seven crystals of bis-Schiff bases suitable for X-ray diffraction were obtained.
Term sin is suitable to modify the substituent effects on the mmax
Dihedral angle has a limited effect on the values of dC(C@N).
s
.
s
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 October 2012
Received in revised form 9 December 2012
Accepted 10 December 2012
Available online 19 December 2012
The relationship between the molecular conformation and spectroscopic properties of symmetrical bis-
Schiff bases was explored experimentally. Seven samples of compounds p-X–C6H4CH@NC6H4N@CHC6H4–
p-X (X = OMe, Me, Et, Cl, F, CF3, or CN) were prepared for this study, and their crystal structures were
measured by X-ray diffraction. Their kmax values in ethanol, acetonitrile, chloroform and cyclohexane sol-
vents were measured, and their dC(C@N) values in chloroform-d were determined. The results show that
the mmax is dependent on the substituents at the benzylidene ring and the dihedral angle
molecules, and the term sin( ) is suitable to modify the substituent effects on the max. However, exper-
imental investigations indicate that the dihedral angle has a limited effect on the values of dC(C@N).
This study provides a new understanding for the molecular conformation on spectroscopic properties
of symmetrical Schiff bases.
s of the titled
Keywords:
s
m
Dihedral angle
Absorption spectrum
NMR spectrum
Schiff base
s
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
with a same conjugated chain. Surprisingly, we have recently ob-
served a bathochromic shift of the kmax of benzylideneaniline com-
Schiff bases have recently gained much attention in the design of
catalysts in various chemical processes [1–4], as models for of bio-
logical systems [5–8], and as effective ligands in metal complexes
in the preparation of dyes [9,10]. Most importantly, they have been
shown to have potential applications in optical communications,
and many of them have nonlinear optical behavior [11–13].
Most recent studies have suggested that the UV absorption wave-
length of mesogenic compounds plays an important role in the de-
sign of new materials for nonlinear optical purposes [14,15]. It is
known that the UV maximum absorption wavelength (kmax) of a pla-
nar conjugated molecule is longer than that of the nonplanar one
pared with stilbene, which is in an almost planar conformation
[16]. Nevertheless, the effects of molecularnon-planarityon the kmax
of Schiff bases have rarely been experimentally studied.
Charge distribution of the molecules is central to the optical and
electronic characters of mesogens [17,18]. Several 13C NMR studies
have revealed that the overall electron distribution can be fine-
tuned through the electronic effects of remote substituents
[19,20]. By means of computational study, Neuvonen and co-
workers [21] have proposed that the twist of the aniline ring with
respect of the plane of the C@N unit may affect the 13C NMR chem-
ical shifts of imine carbon in benzylideneanilines. However,
Neuvonen’s conjecture has not been experimentally explored and
still remains complicated.
⇑
Corresponding author at: School of Chemistry and Chemical Engineering,
Hunan University of Science and Technology, Xiangtan 411201, China. Fax: +86 731
58291336.
To clarify the effects of molecular conformation on the kmax and
13C NMR chemical shifts dC(C@N) of Schiff bases, we synthesized
seven samples of symmetrical bis-Schiff bases p-XBAX-p (Fig. 1)
E-mail address: czcao@hnust.edu.cn (C. Cao).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.12.014

448
Z. Fang, C. Cao / Journal of Molecular Structure 1036 (2013) 447–451
after a few days. For compound a, crystallographic analyses were
performed on a Rigaku Saturn 724 CCD diffractometer; for b,
X-ray diffraction was performed with a Bruker Smart CCD Apex II
platform diffractometer; for c, d, e, f, and g, X-ray diffraction were
performed with a Gemini S Ultra, Oxford platform diffractometer.
The crystals a, b, c, e, f, and g were measured with Mo K
chromated radiation (k = 0.71073 Å), and crystal d was measured
with Cu K monochromated radiation (k = 1.54184 Å). An empiri-
a mono-
a
cal 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 [23]. All of
the non-hydrogen atoms were refined anisotropically, whereas
all hydrogen atoms were refined isotropically as a riding mode
using the default SHELXL parameters. A summary of the crystal
data and the structure refinements for a–g is available in the
Supporting Information.
CCDC 885615, 885616, 885617, 892132, 892515, 892516 and
892517 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via http://www.
ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033).
Fig. 1. General structures of compounds a–g.
in this work. In p-XBAX-p, the substituents X include electron-
donating groups (X = OMe, Me, or Et) and electron-withdrawing
ones (X = Cl, F, CF3, or CN). Their crystal structures and spectro-
scopic properties were measured experimentally, and the effects
of the molecular conformation on spectroscopic properties of a–g
were quantified.
2. Experimental methods
2.1. Sample preparation
2.3. Spectral measurement
Compounds a–g were all prepared by solid–solid reactions [22].
The pure p-substituted benzaldehyde and p-phenylenediamine
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 being cooled to room temperature and purified by recrystal-
lization from absolute ethanol.
Absorbance spectra were collected on a LAMBDA-35 UV–vis
spectrometer in a concentration range from 10ꢀ3 to 10ꢀ5 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 a–g are listed in Ta-
ble 1. The NMR chemical spectra of compounds a–g were recorded
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 a–g are
available in the Supporting Information.
2.2. X-ray crystallography
Crystals of bis-Schiff bases p-XBAX-p suitable for X-ray diffrac-
tion were obtained by slow evaporation from a binary solvent mix-
ture of methanol–chloroform (3:1). Colored crystals were obtained
3. Results and discussion
Table 1
Values of kmax (cm) and tmax (cmꢀ1) for compounds a–g.
3.1. Description of the crystal structures
ex
No
Compounds
rP
r
kmax
tmax
Solvent
cc
A comparison of the X-ray crystal structures revealed a possible
role for the conformation of a–g (Fig. 2). Compounds a, b, e, and g
crystallized in the monoclinic space groups P2(1), P21/n, P21/n and
P21/n, respectively, while c, d, and f all crystallized in the triclinic
space group Pꢀ1. The CF3 groups in compound f were observed
to be disordered over many sites. The results are in good agree-
ment with the previous results [24,25]. The high symmetry of
the CF3 group, coupled with the relatively low energy barrier for
rotation about the 3-fold symmetry axis, was suggested to be the
reason for the high level of disorder observed over many positions
[24].
In all seven cases, the conformation observed for the molecules
is the anti-form with respect to the C@N bond. As shown in Fig. 2,
the benzylidene ring of each compound is nearly co-planar with
the C4AC7@N1 or C15AC14@N2, whereas the aniline ring is
twisted significantly from the C7@N1AC8 or C11AN2@C14. Be-
cause of the slight deviation of the benzylidene ring from the
C4AC7@N1 or C15AC14@N2 plane, we discuss below the twist
of the aniline ring with respect of the plane of the C@N unit only.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
a
b
c
d
d
f
g
a
b
c
d
d
f
g
a
b
c
d
d
f
g
a
b
c
d
d
f
ꢀ0.27
–0.17
ꢀ0.15
0.23
0.06
0.54
ꢀ0.50
ꢀ0.17
ꢀ0.13
ꢀ0.22
0.06
ꢀ0.12
ꢀ0.70
ꢀ0.50
ꢀ0.17
ꢀ0.13
ꢀ0.22
0.06
ꢀ0.12
ꢀ0.70
ꢀ0.50
ꢀ0.17
ꢀ0.13
ꢀ0.22
0.06
ꢀ0.12
ꢀ0.70
ꢀ0.50
ꢀ0.17
ꢀ0.13
ꢀ0.22
0.06
354.23
350.90
350.75
354.38
348.14
359.14
371.54
354.52
350.83
350.70
354.09
348.45
358.35
370.38
357.73
354.34
354.69
357.81
351.02
361.93
375.96
357.88
355.52
356.05
360.10
354.70
364.90
374.94
28230
28498
28510
28218
28724
27844
26915
28207
28504
28514
28241
28699
27906
26999
27954
28221
28194
27948
28488
27630
26599
27942
28128
28086
27770
28193
27405
26671
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
0.66
ꢀ0.27
ꢀ0.17
ꢀ0.15
0.23
0.06
0.54
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Chloroform
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
0.66
ꢀ0.27
ꢀ0.17
ꢀ0.15
0.23
0.06
0.54
0.66
ꢀ0.27
ꢀ0.17
ꢀ0.15
0.23
0.06
0.54
The dihedral angle
C14@N2AC11AC10. The values of
The value of = 180° corresponds to the co-planarity of the aniline
ring and the C7@N1AC8 or C11AN2@C14 plane. It should be noted
that is clearly affected by the substitution of the aromatic ring.
s
is defined by atoms C7@N1AC8AC9 or
s
in a–g are reported in Fig. 2.
s
ꢀ0.12
g
0.66
ꢀ0.70
s

Z. Fang, C. Cao / Journal of Molecular Structure 1036 (2013) 447–451
449
Table 2
Correlation results for a–g, in acetonitrile, chloroform and cyclohexane.
ex
cc
ex
cc
Solvent
mmax
mmax
¼
¼
q
q
r
r
þ
qprp þ constant
sin( ) + qprp sin( ) + constant
Constant
ex ex
s
s
cc cc
qp
R
R2
s
F
n
Eqs.
ex
q
cc
Ethanol
1451.91
3476.75
1415.12
3343.64
1552.13
3776.59
1067.63
2582.79
ꢀ1030.3
ꢀ1816.19
ꢀ936.17
ꢀ1655.12
ꢀ993.24
ꢀ1689.74
ꢀ1078.39
ꢀ1952.50
28636.01
28627.64
28633.11
28621.65
28384.33
28380.29
28152.23
28145.88
0.9904
0.9976
0.9919
0.9967
0.9879
0.9970
0.9934
0.9965
0.9810
0.9952
0.9838
0.9935
0.9759
0.9941
0.9868
0.9929
102.50
51.56
89.00
56.52
117.63
58.37
76.55
55.97
103.10
413.41
121.36
303.88
81.03
335.15
149.20
280.86
7
7
7
7
7
7
7
7
Acetonitrile
Chloroform
Cyclohexane
(4)
(5)
(6)
(7)
(8)
(9)
ex
cc
follows (Eq. (1)) [28], where
coefficients.
q
and qp are the corresponding
ð1Þ
Table 3
13C NMR shifts (ppm) of the C@N carbons in compounds a–g.
ex ex
mmax
¼
q
r
þ qprp þ constant
cc cc
No
X
rF
rR
dC(C@N) No
X
F
rF
rR
dC(C@N)
Therefore, in this work, we tried to correlate the mmax of a–g in
ethanol with Eq. (1), and obtained Eq. (2). The results are good, and
the correlation coefficient was as high as 0.9904.
1
2
3
4
OMe 0.29 ꢀ0.56 158.93
5
6
7
0.45 ꢀ0.39 158.22
Me
Et
Cl
0.01 ꢀ0.18 159.63
0.00 ꢀ0.15 159.65
0.42 ꢀ0.19 158.19
CF3 0.38
0.16 158.08
0.15 157.79
CN
0.51
ex
cc
mmax ¼ 28636:01 þ 1451:91
r
ꢀ 1030:3rp
R ¼ 0:9904; R2 ¼ 0:9810; s ¼ 102:50; n ¼ 7; F ¼ 103:10
ð2Þ
Table 4
It is interesting to note that the dihedral angle
s of compounds
Correlation results of dC(C@N) for a–g.
changes with the substituent X. Does the dihedral angle have an
impact on the UV absorption energy of molecules? Recently, we
observed that the correlation results of benzylideneanilines with
Eq. (1) were somewhat worse, presumably because of the twist
of the benzylidene ring [16]. Thus, in this study, we want to con-
Regression equation
R
R2
s
F
n
No
dC(C@N) = 159.53ꢀ3.63
rFꢀ0.83 rR
0.9989 0.9978 4.74 ꢁ 10ꢀ2 906.00
7
(10)
dC(C@N) = 159.61ꢀ3.93
0.9903 0.9807 0.14
0.8352 0.6976 0.56
0.1508 0.0228 1.00
101.67
4.61
7
7
7
(11)
(12)
(13)
rF + 96.01 rR sin(s)
sider the effects of
the aniline ring are co-planar (
gation between them. However, when they are in the orthogonal
orientation ( = 90°) the conjugation is the weakest. Therefore,
we can take a sine function of
ing the term sin(
s
on the tmax of a–g. When the C@N unit and
dC(C@N) = 158.05 + 948.98
rF sin( )ꢀ7.34 rR
dC(C@N) = 159.53ꢀ89.07
rF sin( )ꢀ24.01 rR
sin(
s
= 0° or 180°) there prevails conju-
s
0.05
s
s
s)
s
to describe the interaction. Apply-
ex
s
) to modify the parameters
r
and
r
p, we carried
cc
out a correlation analysis once again (Eq. (3)). The correlation of Eq.
(3) is much better than that of Eq. (2), and its standard error is only
51.56 cmꢀ1, which is approximately half of Eq. (2). This confirms
that the effect of the twist of the aniline ring with respect of the
plane of the C@N unit is also an important factor influencing the
substituent effects on the kmax, though it is not as important to
the kmax as the effects of conjugation unity or the substituents on
the compound.
Among the seven compounds, the maximum value of
s
occurs in a
(
(
s
s
= 163.25°) and d (s = 163.49°), and the minimum is found in c
= 136.08°).
3.2. Absorption spectra
ex
Table 1 summarizes the kmax (column 5) and the corresponding
mmax (column 6) of a–g in protic solvent (ethanol), electron pair
donating solvent (acetonitrile), and those with no specific sol-
vent–solute interactions (chloroform, cyclohexane). The absorp-
tion spectra of a–g in ethanol are shown in Fig. 3. For
comparison, the maximum absorbance is normalized to 1 for all
measurements.
As can be seen from Fig. 3, all spectra show four bands. These
values are similar to those observed in related Schiff base com-
pounds [26,27]. The maximum absorption bands at about 2.6–
2.9 ꢁ 104 cmꢀ1 are assigned to the intramolecular charge transfer
band of the azomethine C@N group. It is noted that a bathochromic
shift is observed with the increasing electron-withdrawing or elec-
tron-donating ability of X. Compound g had the highest kmax, fol-
lowed by compound f, d, a, b, c, and e in order of increasing
kmax. This observation is in accordance with our previous study
[16]. In our previous research, it was verified that substituent ef-
mmax ¼ 28627:64 þ 3476:75
r
sinð
s
Þ ꢀ 1816:19
r
p sinð Þ
s
cc
R ¼ 0:9976; R2 ¼ 0:9952; s ¼ 51:56; n ¼ 7; F ¼ 413:41
ð3Þ
To further verify this result, the absorbance spectra of a–g were
experimentally measured in acetonitrile, chloroform and cyclohex-
ane. We correlated the tmax of a–g in acetonitrile, chloroform and
cyclohexane, respectively, and obtained Eqs. (4)–(9) (Table 2).
The results of Eqs. (5), (7) and (9) are excellent. This is further proof
that the substituent effects upon the kmax are affected by the twist
of the aniline ring with respect to the rest of the molecules, and the
term sin(s) is available to scale the effects.
3.3. NMR spectra
By computational study, Neuvonen et al. proposed that the
s affected the dC(C@N) of benzylideneanilines [21].
To experimentally validate this claim, we examined the effects of
dihedral angle
fects had obvious impacts on the tmax of molecules, and the tmax
s
on the dC(C@N) of a–g.
Table 3 summarizes the dC(C@N) values of a–g. As shown in
ex
was correlated with the excited-state parameter (
r
) and Ham-
cc
mett parameter (
r
P) for substituted benzenes. The equation is as
Table 3, the dC(C@N) of compounds range from 157.79 to

450
Z. Fang, C. Cao / Journal of Molecular Structure 1036 (2013) 447–451
Fig. 2. Representative solid state molecular structures of a–g. Displacement ellipsoids are drawn at the 50% probability level, and H atoms are shown as small spheres of
arbitrary radii.
a
sults are not in accordance with the previous research [19,29,30],
perhaps because compounds a–g are symmetrical in structure.
To give insight into the substituent effects on dC(C@N) in sym-
metrical bis-Schiff bases, the dC(C@N) values in Table 3 were first
b
c
d
1.2
e
f
g
correlated with rF and rR parameters (rF and rR are the inductive
0.8
0.4
0.0
parameter and resonance parameter, respectively), and Eq. (10)
was obtained. The results are shown in Table 4. The correlation re-
sults are excellent, and the deviation is only 4.74 ꢁ 10ꢀ2
.
Is the twist of the aniline ring a factor influencing the dC(C@N)
in symmetrical bis-Schiff bases? We modified the parameters rF
and rR with the parameter sin(s). However, good to poor correla-
tions were observed when the term sin(
s) was used to modify rR
(Eq. (11)), rF (Eq. (12)), or both rF and rR (Eq. (13)) (Table 4). This
suggests the effects of the twist of aniline ring on the dC(C@N) val-
ues of symmetrical bis-Schiff bases are not obvious.
2.4
3.0
3.6
4.2
4.8
Wavenumber/cm-1 (×104)
The behaviors described above can be understood by the prop-
erties of kmax and dC(C@N). kmax depends on the HOMO–LUMO gap,
and a larger conjugation generally leads to a lesser gap. The smaller
the torsion of the C@N unit and the aniline ring is the greater con-
jugation is. Thus, the absorbance energy is affected by the twist of
an aniline ring. However, the dC(C@N) value depends on the charge
properties of imine carbon. The charge of imine carbon depends on
the whole molecular orbitals of molecules, not just the frontier
molecular orbitals. Therefore, the effects of aniline ring torsion
on electronic density of imine carbon are limited. This rationaliza-
tion about the dC(C@N) is supported by several studies concerning
Fig. 3. Normalized absorbance spectra for a–g in ethanol.
159.65 ppm. When substituent X is electron-withdrawing, the dC
(C@N) decreases with increasing electron-withdrawing capability.
This indicates that electron-withdrawing substituents X cause
shielding of the imine carbon. The electron-donating ones also be-
have in the same way: with the increasing capability of electron
donating, there is also a decreasing dC(C@N). The experimental re-

Z. Fang, C. Cao / Journal of Molecular Structure 1036 (2013) 447–451
451
the investigation of substituent effects on the dC(C@N) [31–33], in
which the dC(C@N) values are well correlated with the parameters
rF and rR
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4. Conclusion
A comparison of the crystal structures of compounds a–g shows
that the dihedral angle
the aromatic ring. The dihedral angles
effects of substituents on the max, and the term sin(
s
is clearly affected by the substituents on
influences the electronic
) is suitable
s
m
s
to modify the effect. Surprisingly, the twist of the aniline ring with
respect to the plane of the C@N unit has a limited role on the
dC(C@N) in a–g, and a good correlation between the dC(C@N) and
the substituent parameters is obtained. The results of this investi-
gation indicate the importance of the molecular conformation
effects upon the absorbance spectra. The spectral behavior result-
ing from molecular conformation must be considered to fully
understand the spectroscopic properties of these potential optical
materials.
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The project was supported by the National Natural Science
Foundation of China (Nos. 21272063, 21072053), the Scientific Re-
search Fund of Hunan Provincial Education Department (Nos.
10K025, 12C0625) and the Scientific Research Fund of Hunan Insti-
tute of Engineering (No. Xj0924).
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Appendix A. Supplementary data
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Chem. 68 (2003) 2151.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.
12.014. These data include MOL files and InChiKeys of the most
important compounds described in this article.
[33] H. Neuvonen, K. Neuvonen, P. Pasanen, J. Org. Chem. 69 (2004) 3794.