Characteristic and spectroscopic properties of the Schiff-base model compounds

Article abstract of DOI:10.1016/j.saa.2009.08.045

Two series of conjugated aromatic imines (Schiff-base model compounds) with different central groups and various side-group substitutions have been synthesized and characterized by elemental analysis, differential scanning calorimetry (DSC) technique, hyd

Full text of DOI:10.1016/j.saa.2009.08.045

Spectrochimica Acta Part A 74 (2009) 949–954
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Characteristic and spectroscopic properties of the Schiff-base model compounds
B. Jarza˛bek∗, B. Kaczmarczyk, D. Se˛k
Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34 M. Curie-Skłodowska Str, 41-819 Zabrze, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 May 2009
Accepted 20 August 2009
Two series of conjugated aromatic imines (Schiff-base model compounds) with different central groups
and various side-group substitutions have been synthesized and characterized by elemental analysis,
differential scanning calorimetry (DSC) technique, hydrogen nuclear magnetic resonance (1H NMR),
Fourier transform infrared (FTIR) and ultra-violet and visible light (UV–vis) spectroscopy measurements.
The UV–vis absorption of solutions of these compounds in dimethylacetamid (DMA), chloroform and
methanol was investigated in the optical range from 240 to 450 nm, where two distinct absorption bands:
at 250–280 and 315–360 nm with the different level of absorption have been observed. The influence of
compound molecular structure and polarity of solvent on the absorption spectra and the possible optical
transitions have been discussed. Structure of diamines in the azomethine models fundamentally affected
their spectroscopic properties and conjugation of ␲-electrons.
Keywords:
Schiff-base model compounds
Spectroscopy
Substitution and solvent effects
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
these groups of model compounds. UV–vis spectroscopy seems to
be the simplest and very sensitive method to investigate the opti-
Azomethines (known as Schiff-bases), having imine groups
(CH N) and benzene rings in the main chain alternately, and
being ␲-conjugated, exhibit interest as materials for wide spectrum
applications, particularly as corrosion inhibitors [1], catalyst carri-
ers [2,3], thermo-stable materials [4–6], a metal ion complexing
agents [7] and in biological systems [8,9]. Optical and semicon-
ducting properties of this group of materials have been also widely
investigated from many years because of photo- and electro-
luminescence efficiency with wavelength depended on chemical
structure and interesting non-linear optical (NLO) properties of the
Schiff-bases in their polymeric form [10,11]. Poly(azomethines) are
generally insoluble in common organic solvents, so to evaluate their
properties in solutions, analogous model compounds can bee used.
Synthesis, characterization and spectroscopic properties, particu-
larly substitution effect, tautomerism and influence of polarity and
protic of solvents on the UV–visible spectra of solutions of different
Schiff-base model compounds and polymers have been described
in relatively numerous papers [12–15].
cal transitions and electronic structure for organic and non-organic
compounds, both in form of solid states (thin films, foils) and as liq-
uids (solutions). Therefore the absorption spectra of all investigated
aromatic Schiff-bases are particularly described and discussed in
the present work.
The main aim of this study is to report on the results of typical
investigations, performed to characterize the properties of some
azomethine model compounds with different central group and
various side-group substitutions and to present their absorbance
spectra in the different solvents in detail. The substitution and sol-
vent effect on the possible electronic transitions and conjugation of
␲-electrons and the relationships between the chemical structure
of compounds and their spectroscopic properties are discussed.
2. Experimental
2.1. Synthesis of model compounds
This paper shortly reports synthesis and some typical properties
of two series of the Schiff-base model compounds, obtained by the
condensation process of benzaldehyde and seven various diamines.
The results of elemental analysis, differential scanning calorimetry
(DSC) technique, hydrogen nuclear magnetic resonance (1H NMR),
Fourier transform infrared (FTIR) spectroscopy and ultra-violet and
visible light (UV–vis) absorption have been used to characterize
Syntheses of the investigated model compounds were based on
the condensation reaction of benzaldehyde and aromatic diamine
in the presence of p-toluenesulfonic acid. These processes were car-
ried over by stirring a mixture of 3 mmol of proper diamine, 6 mmol
of bezaldehyde and 0.06 g of p-toluenesulfonic acid in 10 ml of DMA
and heating during 4 h at temperature of 140 ◦C. After cooling the
reaction mixture was poured into 50 ml of ethanol. Resulting pre-
cipitate was filtered, washed with ethanol and crystallized from
ethanol.
∗
The schematic route of condensation reaction, used in this
Corresponding author. Tel.: +48 32 2716077; fax: +48 32 2712969.
E-mail address: bozena.jarzabek@cmpw-pan.edu.pl (B. Jarza˛bek).
work, is shown in Fig. 1. Different diamines: 4,4ꢀ-diamino-biphenyl,
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.08.045

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B. Jarza˛bek et al. / Spectrochimica Acta Part A 74 (2009) 949–954
Fig. 1. Synthetic rout of the under study azomethines.
3,3ꢀ-dimethylbenzidine and 3,3ꢀ,5,5ꢀ-tetramethylbenzidine and
also the analogous diamines only with methylene bridge CH2
between benzene rings, have been used in this reaction, to syn-
thesis Schiff-base model compounds, called as a groups A and B,
respectively. Also 1,4-phenylenediamine has been used to obtain
the simplest of investigated compounds (named in this text as
A0). Chemical structures of all investigated compounds, from both
groups, are presented in Fig. 2.
elemental analysis were performed using a CHNS Perkin Elmer
2400, while the melting points of these compounds were deter-
mined by differential scanning calorimetry (DSC) on a TA-DSC
2010 apparatus. The 1H NMR spectra were recorded on a Var-
ian Inova 300 spectrometer using chloroform (CHCl3) as a solvent
and tetramethylsilan (TMS) as an internal reference. The infrared
spectra were acquired on a DIGITLAB FTS-40A Fourier transform
infrared spectrophotometer in the range of 4000–400 cm−1, at a
resolution of 2 cm−1 and for an accumulated 32 scans and all sam-
ples were analyzed in a form of pellets in potassium bromide.
Our measurements of the optical absorption were preformed
at room temperature on a BECKMAN Acta M-IV (UV–vis–NIR) two
beams spectrophotometer, where a deuterium lamp is a source
2.2. Characterization techniques
Different techniques of the measurements have been used to
characterize the Schiff-bases compounds, synthesized by us. The
Fig. 2. Chemical structures of the azomethine model compounds.

B. Jarza˛bek et al. / Spectrochimica Acta Part A 74 (2009) 949–954
951
3.2. 1H NMR spectroscopy and infrared investigation
of ultraviolet (200–380 nm) and a tungsten lamps are used for
visible (380–800 nm) and near infrared (800–3300 nm) parts of
light. The obtained Schiff-base model compounds are well solu-
ble in the majority of common solvents. Therefore to investigate
optical properties of these compounds we have used three dif-
ferent solvents: the most polarity and aprotic dimethylacetamid
(DMA) CH3CON(CH3)2, the polarity and protic methanol CH3OH and
the non-polar three chloromethan (chloroform) CHCl3. The dipole
moments (Mn) in debyes (D) of these solvents are: 3.92 D for DMA
and 1.60 and 1.01 D for methanol and chloroform, respectively.
Absorption spectra of investigated azomethine solutions were
recorded, using the quartz cells, within the spectral range from
240 to 450 nm, where absorption bands have been observed. The
concentration of these solutions has been on the level 10−5 mol/l,
suitable for the UV–vis optical investigations.
In the 1H NMR spectra of investigated compounds, singlet sig-
nal due to hydrogen atom present in imine group and multiplex
confirming protons in aromatic rings were recognized. The most
interesting fact is a shift of imine group signal position, depending
on the model structure, as it can be seen in Table 1. In the models
from unsubstituted diamines (1A,1B) imine proton signal is shifted
towards lower field in comparison with their substituted analogs.
For the both series, the presence of methyl substitutes causes the
shift of this signal to higher field. Shift of the imine proton sig-
nal towards higher field, indicates the worse conjugation of imine
group ␲ electrons with phenyl ring ␲ electrons. Introduction the
–CH2– bridge between two aromatic rings (group B) influences on
the shift of imine proton signal towards higher fields as compare
to the models with biphenyl type structure (group A).
To analyze the infrared spectra of investigated azomethine com-
pounds the position of C N bond is considered (Table 1). From the
literature data [20,21] follows that free, isolated C N groups absorb
at about 1660 cm−1 and this band is associated with the stretching
vibrations of this group. The frequency of this band strongly falls
with conjugation of the C N group with phenyl ring due to dimin-
ishing at the energy of the C N bonds and delocalization of nitrogen
lone pair into iminic double bond.
3. Results and discussion
Some obtained parameters, of the investigated aromatic Schiff
bases, as the results of elemental analysis, 1H NMR and FT-IR inves-
tigations and the temperature of melting points, are gathered in
Table 1.
3.1. Elemental analysis and thermal properties
As it is seen from Table 1, for the simplest of investigated azome-
thine (A0), with one phenyl ring between nitrogen atoms, the band
characteristic for stretching vibration of the C N group appears at
1617 cm−1. Introduction the additional phenyl ring between the
nitrogen atoms (1A) causes that this band shifts to 1622 cm−1. It
indicates that conjugation of the C N bonds and phenyl ring was
decreased. Separation of these phenyl rings by the CH2 group (1B)
gives the frequency of analyzed band rise to 1625 cm−1, which
proves that the further diminishing at conjugation takes place.
Relatively, the higher influence on the position of this band
is observed after substitution the phenyl ring connected with
nitrogen atom by the methyl groups. Generally, the presence of
substitutes causes the shift of band in the direction to higher wave-
numbers, being more significant for two ortho–methyl groups in
each imine ring. In the case of substituted of four methyl groups
(3A) the shift is equal to 17 cm−1 in comparison to its unsubstituted
model compound (1A). It can be noticed that for azomethine with
two phenyl rings substituted by methyl group in ortho-position
(2A) only 6 cm−1 shift is recorded in relation to unsubstituted model
(1A). Thus it can be proved that the effect of acting of four methyl
groups is more than twice higher than that of two methyl groups
substituting phenyl rings in ortho-positions.
The results of the elemental analyses, of these two groups of
azomethine compounds, are collected in Table 1 and compared
with the calculated values of carbon C, hydrogen H and nitrogen N
contents. This comparison, shows rather good agreement between
calculated and found values and some deficiency of carbon content
may be a result of the difficulties in burning these high-temperature
compounds.
Melting points of the investigated model compounds strongly
depend on their structure. Generally, relation between the struc-
ture and melting points exhibit the same direction for both series
where the presence of subtracts is taken into consideration. Models
1A and 1B, from unsubstituted diamines, melt at higher tempera-
ture, than their analogs with one methyl group at ortho-position
in each aromatic ring (2A, 2B), that is a common rule. However,
the presence of two methyl groups at ortho-position to amine
nitrogen in each ring causes increase melting points of the mod-
els (3A, 3B) in comparison to these with one methyl group (2A,
2B). Similar behavior was observed for polyketaniles [11,16] and
polyimides [17–19]. It seems to indicate that two methyl groups at
ortho-position make the molecule more rigid. Rigidity is also the
reason why presence of two aromatic rings increases the melting
points of the model 1A in comparison with model A0. On the other
hand –CH2– group between aromatic ring in diamine (group B)
lowers the melting point of these models, due to decrease of rigid-
ity, as comparison with the analogs with direct connected rings
(group A).
Analyzing the positions of this band for the compounds from
the group B (with –CH2– bridge connecting the phenyl rings) we
can notice that the shifts of this band are smaller than ascribed
above. In the case of azomethine with two phenyl rings substi-
tuted by two methyl groups in ortho-positions (2B) this shift is
equal to 5 cm−1,while for the azomethine with two phenyl rings
Table 1
Some properties of investigated azomethine compounds.
Elementary analysis–anal. [% 0.1%] (calcul.) [%]
Melting point T [◦C]
1H NMR ı [ppm 0.01]
FTIR ꢀ [cm−1 1 cm−1],
–HC N–(KBr)
C
H
N
A0
84.50 (84.57)
5.37 (5.59)
9.73 (9.84)
138
8.51
1617
1A
2A
3A
86.29 (86.63)
86.52 (86.55)
85.96 (86.48)
5.48 (5.60)
6.13 (6.24)
6.77 (6.79)
7.67 (7.77)
7.30 (7.21)
6.76 (6.73)
239
154
169
8.57
8.42
8.27
1622
1628
1639
1B
2B
3B
83.35 (84.76)
86.04 (86.52)
86.06 (86.45)
8.96 (7.92)
6.60 (6.52)
7.32 (7.04)
7.30 (7.32)
6.86 (6.96)
6.33 (6.51)
151
94
112
8.44
8.37
8.21
1625
1630
1638

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B. Jarza˛bek et al. / Spectrochimica Acta Part A 74 (2009) 949–954
substituted by four methyl groups at 2,6 and 2ꢀ6ꢀ positions (3B)
13 cm−1 shift is observed in relation to azomethine with unsubsti-
tuted rings (1B). It seems to be interesting that almost the same
positions of these bands (1628 and 1630 cm−1) is recorded for
azomethines with two methyl groups (2A and 2B, respectively) and
also in the case of azomethines with four methyl groups the posi-
tion of this band is similar 1639 cm−1 for 3A and 1638 cm−1 for 3B.
It can be concluded that differences in FTIR spectra of the Schiff-
base model compounds, described above, are results of changing
in charge distribution in the compound chains after substitution
between nitrogen atoms.
3.3. UV–vis absorption spectra
Absorbance spectra, within the optical range from 240 (or 260)
to 450 nm, of the solutions in chloroform, methanol and DMA for
groups A and B of the Schiff-base model compounds, obtained by
us, are seen in Figs. 3(a–c) and 4(a–c) suitably. The short-wave’s
border of UV spectrum of solution is limited by a permeability of
solvent, i.e. 240 nm for chloroform and methanol, 260 nm for DMA.
All investigated solutions had the same concentration 10−5 mol/l.
Two absorbance bands at ꢀ1 (250–280) nm and at ꢀ2 (320–380)
nm are distinctly presented, in all these spectra. Both, position and
intensity of absorption bands depend mainly on the chemical struc-
ture of compound but also the polarity of solvent influences these
spectra. The long-wavelength absorption bands, seen at ꢀ2, are due
to interband transitions between delocalized (D→D*) states which
can be derived from interaction of benzene and azomethine dim-
mer ␲-orbitals, while absorption bands at short-wavelength range
(ꢀ1) are attributed to interband localized (L) to delocalized (D*) and
vice versa, i.e. (D to L*) transitions. The intensity of absorption bands
is usually defined by an extinction coefficient (ε) where ε = A/(c · l),
A is the absorbance, c is the concentration of solution, l is the length
of optical way (for instance: the length of quartz cell) but also may
be characterized as the value of absorbance (A) when concentra-
tion of all investigated solutions and conditions of measurements
are the same.
The coordinates of absorption bands maximums, i.e. the wave-
lengths ꢀ1, ꢀ2 and the values of absorbance A1, A2 and also the
ratios (A1/A2) for all investigated azomethine solutions in DMA,
chloroform and methanol are gathered in Table 2, in turn.
3.3.1. Influence of chemical structure
3.3.1.1. Effect of CH3 side-group substitutions. As it is seen in
Figs. 3 and 4, the clearest feature of spectra for both series of
investigated compounds, for all solvents, is the fact that the value
of absorbance A2 decreases and simultaneously A1 increases with
the presence of methyl side-group substitutions in the structure
of molecules. This causes that the value of ratio (A1/A2) distinctly
increases (see Table 2) both for compounds from groups A and
B. The long-wavelength absorption band at ꢀ2 derives from the
interactions of ␲-orbitals of phenyl and imine group and when the
methyl units CH3 (one or two) are incorporated to amine ring (di-
or tetra-methylbenzidin diamine used to obtain azometine com-
pound) these interactions are smaller due to the conformation of
molecule. As it is known, for bezylideneaniline twist of the aniline
ring out of C–N C–C plane is 55◦ [22] and similarly, the molecules of
our investigated compounds are non-planar. Additionally, as effect
of CH3 side-group substitutions the conformation increases and
delocalization of electrons decreases, what leads to the smaller
intensity of absorption band at ꢀ2 (see Figs. 2 and 3). Simultane-
ously, the role of localized states and the localized ⇔ delocalized
transitions increases, what causes the increase of the level of the
short-wavelength absorption band at ꢀ1. However, for A0 com-
pound the intensity of the bands are almost equal, i.e. the values of
ratio A1/A2 are about “1” in all solvents (see Table 2).
Fig. 3. Absorption spectra of investigated compounds (group A) as solutions in the
different solvents: (a) chloroform, (b) methanol, and (c) DMA; wavelength range
260–450 nm, the same for all solvents.
The influence of the presence of CH3 side-groups substitutions
on the positions of absorbance maximum ꢀ1 and ꢀ2 is not so
clearly. For almost all compounds and solvents, the position of the
first maximum ꢀ1 moves insignificantly to the shorter wavelength
(hipsochromic effect) with enlargement the chemical structure.
For compounds from the group A the wavelength of the second
absorbance maximum at ꢀ2 is the longest for one with phenyl
rings substituted by two methyl groups in ortho-position (2A) in
all solvents. In the case of compounds from group B the value
of ꢀ2 moves to the longer wavelength (batochromic effect) with
the increase of amount side-group, except the solutions in chlo-
roform. Simultaneously, the wing of long-wavelength absorption
band at ꢀ2 determines the energy gap of compounds. Insignif-
icant changes of its position indicate on the small influence of
CH3 side-group on the length of conjugated part of compound
molecule.

B. Jarza˛bek et al. / Spectrochimica Acta Part A 74 (2009) 949–954
953
Table 2
Absorption bands parameters of investigated compounds solutions in the different
solvents.
ꢀ1 [nm]
ꢀ2 [nm]
A1
A2
A1/A2
DMA
A0
278.35
350.50
0.198
0.145
1.366
1A
2A
3A
281.87
281.95
278.20
349.87
361.15
347.20
0.232
0.129
0.502
0.328
0.147
0.125
0.709
0.901
4.005
1B
2B
3B
269.60
269.40
270.44
320.90
332.50
340.04
0.362
0.333
0.640
0.244
0.181
0.081
1.485
1.847
7.902
Methanol
A0
265.88
345.75
0.197
0.218
0.904
1A
2A
3A
274.00
274.62
272.25
347.50
348.75
349.13
0.410
0.318
0.528
0.502
0.260
0.096
0.818
1.220
5.485
1B
2B
3B
260.63
257.75
251.00
316.00
327.50
331.87
0.282
0.347
0.538
0.192
0.156
0.057
1.464
2.226
9.376
Chloroform
A0
271.40
350.46
0.233
0.228
1.022
1A
2A
3A
278.43
279.48
275.17
346.99
351.30
348.46
0.203
0.278
0.404
0.226
0.222
0.083
0.901
1.250
5.540
1B
2B
3B
264.67
260.37
253.75
322.01
333.24
332.71
0.299
0.401
0.564
0.181
0.172
0.055
1.650
2.336
10.255
substitutions. As it is seen in Table 2, the ratio (A1/A2) increases
about two times, for group B, compared to results for analogous
compounds from the group A, in all solutions.
3.3.2. Influence the polarity of solvent
For all compounds, from both groups of investigated com-
pounds, the wavelength of the first absorbance maximum ꢀ1 moves
to the longer values (batochromicly) with increase polarity of
solvent, and then the largest value of ꢀ1 is observed for solu-
tions in DMA. The batochromic shift of the wavelength of second
absorbance maximum ꢀ2 is not so evident as in the case of the ꢀ1
value. Only for two compounds from group A and one from group
B the largest value of ꢀ2 is for solutions in DMA. In the case of three
left compounds the differences between values of ꢀ2 in these three
solvents are not so significant. The batochromic displacement of
absorption bands with increase polarity of solvent is usually typi-
cal for ␲ types optical transitions. It seems, that polarity of solvent
more affect some substituted hydroxy Schiff bases, as it is reported
in [13].
Solvents can also influence electronic transitions intensities,
what is considered in [23]. In our investigations the values of
absorbance maximum A1 and A2 seems to be not clearly depended
on polarity of solvents. But the ratio A1/A2 in the most of cases,
both for groups A and B, increases with decrease polarity of solvent
(except A0). In the case of chloroform, almost non-polar solvent, the
values of A1/A2 ratios are the largest, what indicate at the greater
role of localized states and transition between localized and delo-
calized states. Different situation is seen for A0, where the role of
localized states is the largest in the most polar of the used solvents,
i.e. in DMA.
Fig. 4. Absorption spectra of investigated compounds (group B) as solutions in the
different solvents: (a) chloroform, (b) methanol, and (c) DMA; wavelength range
240–400 nm, the same for all figures.
3.3.1.2. Effect of presence of CH2 bridge. To estimate the influence
of the CH2 bridge between phenyl rings (group B) on the results of
absorbance for investigated azomethine compounds one can com-
pare the position and value of absorbance maximum, suitably for
groups A and B (1A with 1B, 2A with 2B, 3A with 3B in turn, in each
solution). Both ꢀ1 and ꢀ2 moves distinctly hipsochromic for com-
pounds from group B in comparison with these for compounds from
group A. The presence of CH2 unit between phenyl rings causes
“break” of the conjugation system and diminishing the length of
conjugated part of molecule what leads to the hipsochromic tran-
sition of the long-wavelength absorption band.
Characteristic feature of these spectra of compounds from series
B, compared to A (except A2 and B2 in methanol) is the increase of
absorbance A1 and simultaneously decrease of A2, what indicate
that due to the presence of CH2 unit between phenyl rings the con-
formation of molecule increase. As it is described above, the same
result, but more clearly, have been observed for CH3 side-group
4. Conclusion
The results of our investigations confirmed that structure of the
diamine in the azomethine model compounds influenced strongly

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B. Jarza˛bek et al. / Spectrochimica Acta Part A 74 (2009) 949–954
on their spectroscopic properties and conjugation of ␲-electrons.
Significant effect is seen when the methyl groups are present
at ortho-positions to nitrogen atom in N-phenylene ring. These
methyl substitutes causes the shift of imine proton signal towards
to higher field at 1H NMR spectra and the shift of the C N bond
position to higher wavelength numbers at infrared spectra, in the
relation to the spectra for analogous unsubstituted model com-
pounds. The same direction of the peaks shift, but not so significant,
have been observed at 1H NMR and FTIR spectra, when –CH2–
bridge connected two aromatic rings, as compare to the spec-
tra of models with biphenyl type structure. These 1H NMR and
FTIR results indicates that conjugation between imine group ␲-
electrons and aromatic ring ␲-electrons and the role of delocalized
states decreased for substituted compounds.
On the basis of the UV–vis absorption of azomethine com-
pounds, it is worth to point out, that the methyl side-group
substitutions cause some different changes of spectra than the pres-
ence of CH2 unit inside the molecule structure. Unexpectedly, the
ortho–methyl groups in imine rings have not cause the signifi-
cant shifts of the absorption bands positions. The main effect of
the substitution CH3 side-groups is the rapid decrease of the long-
wavelength absorption band intensity and simultaneously increase
the level of the short-wavelength absorption band. The same type
of changes of the bands intensity but definitely smaller and addi-
tionally clearly hipsochromic transition of the long-wavelength
band have been observed for compounds with CH2 bridge as com-
pare with the models with biphenyl in their structure. Considering
these changes of spectra we can notice that the methyl side-groups
substituted to imine ring in ortho-position cause the increase of
molecule conformation what leads to the worse conjugation and
decrease of the electrons delocalization. The presence of the –CH2–
unit between phenyl rings also increases conformation but addi-
tionally diminishes the length of conjugated part of molecule.
Acknowledgements
The authors thank Kornelia Górniak MSc., former worked in
the Center for the model compounds preparation and Dr Henryk
Janeczek for DSC measurements. Special thanks for Dr Agnieszka
Iwan for helpful suggestions.
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