Synthesis, spectroscopic and physicochemical investigations of environmentally benign heterocyclic Schiff base derivatives as antibacterial agents on the bases of in vitro and density functional theory

Article abstract of DOI:10.1016/j.jphotobiol.2013.01.007

A series of Schiff bases were synthesized by the reaction of 4-dimethylamino-benzaldehyde and corresponding active amines under microwave irradiation. Results obtained from spectroscopic (FT-IR, ¹H NMR, ¹³C NMR, GC-MS) and elemental

Full text of DOI:10.1016/j.jphotobiol.2013.01.007

Journal of Photochemistry and Photobiology B: Biology 120 (2013) 82–89
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Journal of Photochemistry and Photobiology B: Biology
journal homepage: www.elsevier.com/locate/jphotobiol
Synthesis, spectroscopic and physicochemical investigations of environmentally
benign heterocyclic Schiff base derivatives as antibacterial agents on the bases
of in vitro and density functional theory
Abdullah M. Asiri a,b, Salman A. Khan a, , Hadi M. Marwani a,b, Kamlesh Sharma c
⇑
a Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
b Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
c Molecular Science Research Institute, School of Chemistry, University of the Witwatersrand, P.O. Wits 2050, Johannesburg, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Schiff bases were synthesized by the reaction of 4-dimethylamino-benzaldehyde and corre-
sponding active amines under microwave irradiation. Results obtained from spectroscopic (FT-IR, 1H
NMR, 13C NMR, GC–MS) and elemental analyses of synthesized compounds were in a very good agree-
ment with their chemical structures. UV–Vis and fluorescence spectroscopy measurements provided that
all compounds are good absorbent and fluorescent. Fluorescence polarity study demonstrated that these
compounds were sensitive to the polarity of the microenvironment provided by different solvents. In
addition, spectroscopic and physicochemical parameters, including singlet absorption, extinction coeffi-
cient, Stokes shift, oscillator strength and dipole moment, were investigated in order to explore the ana-
lytical potential of synthesized compounds. The antibacterial activity of these compounds was first
studied in vitro by the disk diffusion assay against two Gram-positive and two Gram-negative bacteria.
The minimum inhibitory concentration was then determined with the reference of standard drug tetra-
cycline. The results displayed that compound 1 was better inhibitors of both types of the bacteria (Gram-
positive and Gram-negative) than tetracycline. Based on the density functional theory calculations, LUMO
and density of compounds (1–5) were calculated to support the antibacterial activities.
Received 29 November 2012
Received in revised form 5 January 2013
Accepted 9 January 2013
Available online 09 February 2013
Keywords:
Schiff bases
Physicochemical studies
Tetracycline
Antibacterial activity
Density functional theoretical calculations
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
antiamoebic [11], antinociceptive [12], anticancer [13], antidepres-
sant and anti-inflammatory [14].
Azomethines are compounds having a formula RAN@CAR0,
where R is aryl group, and R0 is either an alkyl or aryl group [1].
They are known as Schiff bases by the name of German chemist
Hugo Schiff. Schiff bases are very versatile as compounds can have
a variety of different substituents [2]. In general, Schiff bases have a
wide range of potential applications in various biological fields [3].
Thanassi and his coworkers reported the Schiff base activity
against tuberculosis [4]. Heterocyclic Schiff base derivatives have
been also synthesized and their anticancer, antiamoebic, antibacte-
rial, antifungal, antiviral and anti-HIV activity have been reported
[5,6]. It has been established by various researchers that Schiff
bases exhibit anti-inflammatory, anti-proliferative and antipyretic
properties [7–9]. They are used as intermediates for the formation
of various heterocyclic compounds by a nucleophilic addition reac-
tion. Schiff base derivatives containing heterocyclic rings, for
example, pyrazole, isoxazole and thiozole, have been found to pos-
sess considerable biological activities, such as, antimicrobial [10],
On the other hand, Schiff base derivatives are widely used in
materials science fields, such as, third order non-linear optics
(NLOs) [15], optical switching [16], electrochemical sensing [17],
Langmuir films and photoinitiated polymerization [18]. Physico-
chemical characteristics, such as, solvatochromic, piezochromic,
oscillator strength, dipole moment, florescent quantum yield and
photostability, are also the most important studies for determining
the behavior of compounds [19,20]. In accordance, the present
study was aimed to report the synthesis of heterocyclic Schiff bases
by the microwave radiation and to investigate their antibacterial
activity on the bases of in vitro and density functional theory calcu-
lations. In addition, the analytical efficiency of heterocyclic Schiff
base derivatives was investigated by studying spectroscopic and
physicochemical parameters.
2. Experimental
2.1. Chemicals and reagents
⇑
4-Dimethylamino-benzaldehyde and heterocyclic amines were
purchased from Acros Organic. Other reagents and solvents (A.R.)
Corresponding author. Tel.: +966 2 6952293; fax: +966 2 6952292.
E-mail address: sahmad_phd@yahoo.co.in (S.A. Khan).
1011-1344/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotobiol.2013.01.007

A.M. Asiri et al. / Journal of Photochemistry and Photobiology B: Biology 120 (2013) 82–89
83
were obtained commercially and used without further purification,
except dimethylformamide (DMF), ethanol and methanol.
chloroform and ethanol (9:1), giving crystals. Physicochemical data
of the synthesized compounds are given in Table 1.
2.2. Apparatus
2.3.1. N-[4-(dimethylamino)phenyl methylidene]-1,3-benzothiazol-2-
amine (1)
Melting points were recorded on a Thomas Hoover capillary
melting apparatus without correction. FT-IR spectra were recorded
on a Nicolet Magna 520 FT-IR spectrometer. 1H NMR and 13C NMR
experiments were performed in CDCl3 on a Brucker DPX 600 MHz
spectrometer using tetramethyl silane (TMS) as internal standard
at room temperature. UV–Vis electronic absorption spectra were
acquired on a Shimadzu UV-1650 PC spectrophotometer. Absorp-
tion spectra were collected using a 1 cm quartz cell. Steady state
fluorescence spectra were measured using Shimadzu RF 5301 PC
spectrofluorphotometer with a rectangular quartz cell. Emission
spectra were monitored at right angle. All fluorescence spectra
were blank subtracted before proceeding in data analyses.
FT-IR (KBr) vmax cmꢀ1: 2904 (CAH), 1575 (C@C), 1526 (HC@N),
1151 (CAN); 1H NMR (CDCl3) d: 9.74 (s, 1H, CHolefinic), 7.94 (d,
CHaromatic, J = 7.2 Hz), 7.80 (d, CHaromatic, J = 7.8 Hz), 7.75 (d,
CHaromatic, J = 9 Hz), 6.75 (d, CHaromatic, J = 9 Hz), 7.56–7.26 (m, 4H,
CHaromatic), 3.12 (s, NACH3), 3.09 (s, NACH3); 13C NMR (CDCl3) d:
190.42, 165.19, 154.35, 151.66, 133.95, 132.96, 127.57, 126.23,
125.15, 124.50, 122.12, 121.69, 119.13, 111.00, 43.17, 40.21; GCMS
(m/z, %): 283 (M + 1, 68); Anal. Calc. for C16H15N3S: C, 68.30, H;
5.37; N, 24.93, Found: C, 68.27; H, 5.27; N, 24.85.
2.3.2. 2-[4-(Dimethylamino)phenylmethylidene)amino]-4,5,6,7-
tetrahydro-1-benzothiophene-3-carbonitrile (2)
2.3. General procedure for the synthesis of Schiff bases
FT-IR (KBr) vmax cmꢀ1: 2914 (CAH), 1625 (C@C), 1560 (C@N),
A mixture of 4-dimethylamino-benzaldehyde (5.8 mmol) and
corresponding active amine (5.8 mmol) in anhydrous methanol
(15 mL) were mixed together in the presence of few drops of acetic
acid. The reaction mixture was heated inside a microwave oven for
2–5 min (at 210 Watts, i.e. 30% microwave power). The progress of
reaction was monitored by TLC with a solvent system of chloro-
form and methanol (8:2). A good yield was obtained after comple-
tion of the reaction, and the product was recrystallized from
1167 (CAN); 1H NMR (CDCl3) d: 9.75 (s, 1H, CHolefinic), 8.29 (d,
CHaromatic, J = 7.6 Hz), 7.86 (d, CHaromatic, J = 7.8 Hz), 7.75 (d,
CHaromatic, J = 9.0 Hz), 6.37 (d. CHaromatic, J = 9 Hz), 2.70–1.82 (m,
8H, ACH2A), 3.10 (s, NACH3), 3.07 (s, NACH3); 13C NMR (CDCl3)
d: 190.39, 158.56, 134.65, 133.86, 131.89, 130.71, 115.10, 112.58,
111.12, 104.34, 43.39, 40.78, 25.25, 25.47, 23.11, 22.89, 22.11,
5.16; GCMS (m/z, %): 311 (M + 1, 68); Anal. Calc. for C18H19N3S:
C, 69.87; H, 6.19; N, 13.58, Found: C, 69.82; H, 6.11; N, 13.53.
Table 1
Physicochemical data of the synthesized compounds (1–5).
Compound
Molecular formula
C16H15N3S
mp (°C)/Crystallization
Yield (%)
84.8
Reaction time (min)
4 min
1
177/CHCl3
N
S
2
3
C18H19N3S
195/CH3OH
226/CHCl3
90.4
85.4
3 min
2 min
NC
S
C20H22N4O
CH3
N
CH3
O
N
4
5
C14H17N3O
152/CH3OH
184/CH3OH
86.7
83.8
3.5 min
CH3
H3C
NC
N
O
CH3
C18H19N3SO
5 min
O
S
C2H5

84
A.M. Asiri et al. / Journal of Photochemistry and Photobiology B: Biology 120 (2013) 82–89
ACH3); 13C NMR (CDCl3) d: 190.34, 161.94, 161.74, 160.39, 146.24,
133.00, 132.58, 130.00, 114.00, 62.39, 61.63, 60.92, 43.74, 15.18,
15.11, 15.05, 14.40, 14.35, 14.32, 14.17; GCMS (m/z, %): 327
(M + 1, 76); Anal. Calc. for C18H19N3OS: C, 66.43; H, 5.88; N,
12.91, Found: C, 66.38; H, 5.78; N, 12.86.
Table 2
Antibacterial activity of Schiff base derivatives (1–5), positive control: Tetracycline
and negative control (DMSO) measured by the Halo Zone Test (Unit, mm).
Compounds Corresponding effect on microorganisms
A. hydrophila Y. enterocolitica L. monocytogenes P. aeruginosa
1
2
3
4
5
14.2 0.5
10.2 0.3
10.5 0.3
9.8 0.3
22.5 0.4
15.2 0.3
13.4 0.4
12.5 0.5
14.8 0.3
20.0 0.5
–
15.2 0.5
11.0 0.2
9.6 0.4
8.6 0.4
10.8 0.4
12.0 0.5
–
18.2 0.4
10.4 0.2
10.2 0.4
10.8 0.1
12.4 0.4
14.0 0.5
–
2.4. Organism culture and in vitro screening
Antibacterial activity was studied by the disk diffusion method
with minor modifications [21]. Aeromonas hydrophila, Yersinia
enterocolitica, Listeria Monocytogenes and Pseudomonas aeruginosa
were subcultured in BHI medium and incubated for 18 h at 37 °C.
The bacterial cells were then suspended, according to the McFar-
land protocol, in saline solution to produce a suspension of about
12.2 0.2
Tetracycline 13.0 0.5
DMSO
–
10ꢀ5 CFU mLꢀ1. An amount of 10
lL of this suspension was mixed
with 10 mL of sterile antibiotic agar at 40 °C and poured onto an
agar plate in a laminar flow cabinet. Five paper disks (6.0 mm
diameter) were fixed onto the nutrient agar plate. 1 mg of each
Table 3
Minimum inhibition concentration (MIC) of Schiff base derivatives (1–5) products,
positive control: Tetracycline.
Bacterial strain
MIC (
l
g mLꢀ1) compound
Positive control
tested compound was dissolved in 100
dard solutions of 10, 20, 25, 50 and 100
individually poured onto each disk plate. Tetracycline (30
l
l l
L DMSO to prepare stan-
g/ L. All solutions were
g/disk)
1
2
3
4
5
l
A. hydrophila
16
32
16
16
64
256
128
256
256
256
256
256
128
64
64
128
32
32
32
32
32
was used as standard drug (positive control). A DMSO poured disk
was used as negative control. The susceptibility of the bacteria to
tested compounds was determined by the formation of an inhibi-
tory zone after 18 h of incubation at 36 °C. Table 2 reports the inhi-
bition zones (mm) of each compound and the control.
Y. enterocolitica
L. monocytogenes
P. aeruginosa
128
256
128
2.3.3. 4-[(4-Dimethylamino-benzylidene)-amino]-1,5-dimethyl-2-
phenyl-1,2-dihydro-pyrazol-3-one (3)
The minimum inhibitory concentration (MIC) was evaluated by
the macro dilution test using standard inoculums of 10ꢀ5
-
FT-IR (KBr) vmax cmꢀ1: 2893 (CAH), 1644 (C@C), 1656 (C@O),
1578 (C@N), 1133 (NAN); 1H NMR (CDCl3) d: 9.65 (s, 1H, CHolefinic),
7.78 (d, CHaromatic, J = 2.4 Hz), 6.72 (d, CHaromatic, J = 3.00 Hz), 7.26–
7.48 (m, 5Haromatic), 3.20, (s, NACH3), 2.98 (s, NACH3), 2.56 (s,
NACH3), 1.25 (s, CH3); 13C NMR (CDCl3) d: 190.38, 161.31,
157.93, 151.87, 138.10, 135.05, 129.30, 129.06, 126.48, 125.87,
123.99, 122.80, 119.94, 111.81, 110.95, 40.24, 37.84, 10.24; GCMS
(m/z, %): 336 (M + 1, 45); Anal. Calc. for C20H22N4O: C, 71.83; H,
6.63; N, 16.75, Found: C, 71.76; H, 6.56; N, 16.69.
CFL mLꢀ1. Serial dilutions of tested compounds, previously dis-
solved in dimethyl sulfoxide (DMSO), were prepared to final
concentrations of 512, 256, 128, 64, 32, 16, 8, 4, 2 and 1 lg/mL. A
2.3.4. (N,N-Dimethyl amino-benzylidene)-(3,4-dimethyl-isoxazol-5-
yl)-amine (4)
FT-IR (KBr) vmax cmꢀ1: 2912 (CAH), 1609 (C@C), 1574 (C@N),
1121 (CAN); 1H NMR (CDCl3) d: 8.69 ((s, 1H, CHolefinic), 7.80(d,
CHaromatic, J = 8.4 Hz), 6.71 (d, CHaromatic, J = 9.00 Hz), 3.07, (s,
NACH3), 2.22 (s, NACH3), 2.04 (s, CH3); 13C NMR (CDCl3) d:
166.06, 161.78, 160.38, 153.09, 131.21, 123.58, 111.42, 104.54,
40.11, 10.34, 6.72; GCMS (m/z, %): 245 (M + 1, 56); Anal. Calc. for
C14H17N3O: C, 69.11; H, 7.04; N, 17.27, Found: C, 69.08; H, 6.98;
N, 17.23.
2.3.5. 2-[4-(Dimethylamino)phenylmethylidene amino]-4-methyl-5-
propanoylthiophene-3-carbonitrile (5)
FT-IR (KBr) vmax cmꢀ1: 2907 (CAH), 1613 (C@C), 1535 (C@N),
1169 (CAN); 1H NMR (CDCl3) d: 9.79 (s, 1H, CHolefinic), 8.46 (d,
CHaromatic, J = 7.2 Hz), 7.94 (d, CHaromatic, J = 8.4 Hz), 7.79 (d,
CHaromatic, J = 6.6 Hz), 6.73 (d, CHaromatic, J = 9 Hz); 4.38 (t, ACH2-
ACH3), 3.13 (s, NACH3), 3.11 (s, NACH3), 2.52 (s, CH3), 1.40 (q, CH2-
H
R
N
CHO
Microwave radiation
C2H5OH (2 - 5) min
+ H2N
R
N
N
Fig. 1. (a) Electronic absorption spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 1 in
different solvents. (b) Emission spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 1 in
different solvents.
Scheme 1. Synthetic route of Schiff bases.

A.M. Asiri et al. / Journal of Photochemistry and Photobiology B: Biology 120 (2013) 82–89
85
Fig. 2. (a) Electronic absorption spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 2 in
different solvents. (b) Emission spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 2 in
different solvents.
Fig. 3. (a) Electronic absorption spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 3 in
different solvents. (b) Emission spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 3 in
different solvents.
volume of 100 lL was added to each tube of a 24 h old inoculum.
and at 1526–1578 cmꢀ1 for azomethine group (ACH@NA). The ab-
sence of absorption band at 1700–1750 cmꢀ1 further confirmed
the conversion of ACHO group to ACH@NA group.
The MIC, defined as the lowest concentration of the tested com-
pound which inhibits the visible growth after 18 h, was deter-
mined visually after incubation for 18 h at 37 °C, the results are
presented in Table 3. Tests using DMSO and tetracycline as nega-
tive and positive controls were also performed.
In addition, evidence for the formation of Schiff base derivative
was obtained from the 1H NMR spectra, which provide diagnostic
tools for the positional elucidation of the protons. The structural
assignments of the NMR spectra are given in Section 2. Assign-
ments of the signals are based on the chemical shifts and intensity
patterns. 1H NMR spectra of Schiff bases showed peaks of aromatic,
methyl and olefinic (AN@CHA) proton. These were all singlets and
each peak indicated to the intensity of one proton in 600 MHz 1H
NMR. The 1H NMR spectra of Schiff bases (1–5) showed sharp sin-
glet at d 8.69–9.79 ppm, indicating to the presence of azomethine
(ACH@NA) proton. The two sharp singlet peaks at d 2.22–
3.13 ppm indicated to the ACH3 group which attached to the nitro-
gen. The appearance of multiplets at d 6.37–8.46 ppm was due to
aromatic protons. 13C NMR spectra also displayed signals in the
range of d 111.00–119.94 ppm and at d 125.15–131.21 ppm due
to aryl carbon and azomethine carbon, respectively. In the mass
spectrum, compound 1 showed a peak at m/z 283, matching its
molecular formula C16H15N3S. A peak at m/z 311 was observed
for compound 2, which is in conformity with the molecular for-
mula of C18H19N3S.
2.5. Computational method
The computational calculation of compounds (1–5) was per-
formed using Spartan’04 Windows graphical software, with den-
sity functional theory, DFT/6-31G⁄ basis set. This method has
been previously used successfully for small molecule’s calculations
[22]. Fundamental frequencies of optimized structure were also
calculated and assigned as minima (all real frequencies, no nega-
tive frequencies).
3. Results and discussion
3.1. Characterization of heterocyclic Schiff base derivatives
The Schiff base derivatives were synthesized in good yield by
the reaction of 4-dimethylamino-benzaldehyde and the corre-
sponding active amines according to previously reported proce-
dure (Scheme 1) [23]. Resultant compounds were stable in the
solid state as well as in the solution state. Data obtained from spec-
tral studies (FT-IR, 1H NMR, 13C NMR and GCMS) and elemental
analyses of synthesized compounds were consistent with their
chemical structures. Assignments of selected characteristic of FT-
IR band positions provided significant indication for the formation
of the Schiff base derivatives. The FT-IR spectra of Schiff bases (1–
5) showed absorption bands at 2893–2914 cmꢀ1 for aliphatic CAH
3.2. Spectral behavior of heterocyclic Schiff base derivatives in
different media
Absorption and emission spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3
compounds (1–5) in various non-polar, polar aprotic and protic
solvents were studied (Figs. 1a–5b). Calculated physicochemi-
cal parameters obtained from steady state absorption and

86
A.M. Asiri et al. / Journal of Photochemistry and Photobiology B: Biology 120 (2013) 82–89
Fig. 5. (a) Electronic absorption spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 5 in
different solvents. (b) Emission spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 5 in
different solvents.
Fig. 4. (a) Electronic absorption spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 4 in
different solvents. (b) Emission spectra of 1 ꢁ 10ꢀ5 mol dmꢀ3 of compound 4 in
different solvents.
4.2 Å because this value is comparable to the radius of a typical aro-
fluorescence spectra are tabulated in Tables 4–8. A close examina-
tion of Figs. 1a–5a displays that the polarity of solvent has minimal
to no effect on absorption maxima, indicating the weak polar char-
acter of compounds (1–5) in the ground state. However, the emis-
sion spectra of these compounds are broad and red shifted as the
solvent polarity increases, as shown in Figs. 1b–5b. The red-shift
in chloroform to DMSO indicates that photoinduced intramolecular
charge transfer (ICT) occurring in the singlet excited state [24]. As a
result, the polarity of compounds (1–5) increases on excitation.
ꢀ
m
matic fluorophore [27]. Stokes shifts (
D
ss) of compounds (1–5) in
different solvents were calculated, as shown in Tables 4–8, using
the following the equation [24]:
ꢀ
mss
ꢀ
mex
ꢀ
mem
D
¼
ꢀ
ð3Þ
ꢀ
ꢀ
where mex and mem denote the wavenumbers of excitation and emis-
sion maxima (cmꢀ1), respectively.
In addition, the change in dipole moment (Dl) between the ex-
cited singlet and ground state can be calculated using solvatochro-
mic shift method [28,29]. In this method, the dimensionless
microscopic solvent polarity parameters, ENT , can be obtained from
Eqs. (4) and (5):
3.3. Determination of dipole moment
The solvatochromic behavior in compounds (1–5) allows one to
ET ðsolventÞ ꢀ 30:7
determine the difference in the dipole moment between the ex-
ETN
¼
ð4Þ
ð5Þ
32:4
cited singlet and the ground state (
can be obtained using the simplified Lippert–Mataga equation as
D
l
=
le
ꢀ
lg). This difference
28; 591
kmaxðnmÞ
ET ðkcal molꢀ1Þ ¼
follows [24,25]:
2
2ðle
ꢀ
lg
Þ
ꢀ
D
D
mss
¼
D
f þ Const:
ð1Þ
where kmax is the wavelength of peak in the red region of the intra-
molecular charge transfer absorption of compounds (1–5). The
effective number of electrons transition from the ground to excited
state is usually described by the oscillator strength, which provides
the absorption area in the electronic spectrum. The oscillator
strength, f, can be calculated using the following equation [30]:
Z
hca3
D ꢀ 1
n2 ꢀ 1
f ¼
ꢀ
ð2Þ
2D þ 1 2n2 þ 1
ꢀ
where
D
mss is the Stokes-shift [26], which increases with increasing
the solvent polarity pointing to stronger stabilization of the excited
state in polar solvents, h denotes Planck’s constant, c refers to the
speed of light in vacuum and a is the Onsager cavity radius. Param-
f ¼ 4:32 ꢁ 10ꢀ9
eð
m m
Þd
ð6Þ
ꢀ
ꢀ
is the extinction coefficient (L molꢀ1 cmꢀ1), and
repre-
ꢀ
m
eters D and n of the Onsager polarity function (
D
f), in Eq. (2), cor-
where
e
respond to the dielectric constant and refractive index of the
solvent, respectively. The Onsager cavity radius was chosen to be
sents the numerical value of wavenumber (cmꢀ1). Oscillator
strength values of compounds (1–5) in different solvents are

A.M. Asiri et al. / Journal of Photochemistry and Photobiology B: Biology 120 (2013) 82–89
87
Table 4
Spectral data of compound 1 in different solvents.
ET (kcal molꢀ1
)
kab (nm)
kem (nm)
e
(Mꢀ1 cmꢀ1
)
f
l
Debye
D
mss (cmꢀ1
)
ꢀ
Solvent
D
f
DMSO
EtOH
CHCl3
CH3CN
Dioxan
0.266
0.305
0.217
0.274
0.148
68.07
69.22
69.56
69.73
70.59
420
413
411
410
405
509
502
493
503
479
56,350
55,910
55,700
54,220
53,320
0.93
0.96
0.90
0.97
0.81
9.13
9.16
8.85
9.18
8.33
4163
4292
4047
4509
3814
Table 5
Spectral data of compound 2 in different solvents.
ET (kcal molꢀ1
)
kab (nm)
kem (nm)
e
(Mꢀ1 cmꢀ1
)
f
l
Debye
D
mss (cmꢀ1
)
ꢀ
Solvent
D
f
DMSO
EtOH
CHCl3
CH3CN
Dioxan
0.266
0.305
0.217
0.274
0.148
65.87
68.23
67.59
68.23
68.23
434
419
423
419
419
512
509
495
510
509
36,800
34,400
32,300
32,150
35,800
0.51
0.58
0.44
0.54
0.60
6.84
7.17
6.27
6.92
7.29
3510
4220
3438
4258
4220
Table 6
Spectral data of compound 3 in different solvents.
ET (kcal molꢀ1
)
kab (nm)
kem (nm)
e
(Mꢀ1 cmꢀ1
)
f
l
Debye
D
mss (cmꢀ1
)
ꢀ
Solvent
D
f
DMSO
EtOH
CHCl3
CH3CN
Dioxan
0.266
0.305
0.217
0.274
0.148
78.54
81.92
79.86
80.31
80.08
364
349
358
356
357
459
418
456
433
449
42,100
34,300
41,000
44,800
42,200
0.95
0.64
0.98
0.89
0.96
8.55
6.87
8.62
8.19
8.52
5686
4730
6003
4995
5739
Table 7
Spectral data of compound 4 in different solvents.
ET (kcal molꢀ1
)
kab (nm)
kem (nm)
e
(Mꢀ1 cmꢀ1
)
f
l
Debye
D
mss (cmꢀ1
)
ꢀ
Solvent
D
f
DMSO
EtOH
CHCl3
CH3CN
Dioxan
0.266
0.305
0.217
0.274
0.148
72.38
74.26
73.68
73.31
73.49
395
385
388
390
389
460
455
456
455
455
40,060
41,200
41,720
44,300
46,000
0.57
0.65
0.64
0.65
0.68
6.92
7.32
7.25
7.32
7.48
3577
3996
3843
3663
3728
Table 8
Spectral data of compound 5 in different solvents.
ET (kcal molꢀ1
)
kab (nm)
kem (nm)
e
(Mꢀ1 cmꢀ1
)
f
l
Debye
D
mss (cmꢀ1
)
ꢀ
Solvent
D
f
DMSO
EtOH
CHCl3
CH3CN
Dioxan
0.266
0.305
0.217
0.274
0.148
62.28
64.83
64.39
65.27
64.97
459
441
444
438
440
558
545
530
547
525
29,300
28,500
29,380
28,850
33,000
0.45
0.49
0.42
0.52
0.48
6.61
6.76
6.28
6.94
6.68
3865
4327
3654
4549
3679
Table 9
Some selected descriptors calculated computationally using DFT/RB3LYP method.
Compound
Total energya
LUMOb
Molecular weight
Volumec
Densityd
1
2
3
4
5
ꢀ1181.15113
ꢀ1259.75681
ꢀ1069.52813
ꢀ783.137730
ꢀ1334.96584
ꢀ1.65
ꢀ1.59
ꢀ0.88
ꢀ1.58
ꢀ1.60
281.383
309.437
334.423
243.310
325.436
309.89
339.67
364.29
265.62
357.62
0.908
0.911
0.918
0.916
0.910
a
b
c
Total energy is in atomic unit (au).
LUMO are given in electron Volt (eV).
Volume is given in Å3.
d
Density is the molecular weight/molecular volume.

88
A.M. Asiri et al. / Journal of Photochemistry and Photobiology B: Biology 120 (2013) 82–89
reported in Tables 4–8. In addition, the transition dipole moment
) for compounds (1–5) from ground to excited state in Debye
was estimated in different solvents (Tables 4–8) using the following
energy, lowest unoccupied molecular orbital (LUMO), molecular
weight, volume and density, are summarized in Table 9 and
Fig. 6. The antibacterial activity of the Schiff bases (1–5) is a func-
tion of descriptor LUMO and density. LUMO is an electronic param-
eter that measures the electrophilicity of the molecules. When a
molecule acts as a lewis acid, incoming electrons are received in
its LUMO. Molecules with low-lying LUMO are more capable to ac-
cept electrons than those having high energy LUMO, and thus they
show higher activity. Density reflects the types of atoms and how
tightly they are packed in a molecule. It is defined as the ratio of
molecular weight/molecular volume. Density is negatively corre-
lated with activity. This means that if the molecule is compact, it
will reduce the density and thereby increase the activity.
The compounds with sulfur atoms i.e. 1, 2 and 5, have shown
lower LUMO values. These compounds have also lower density val-
ues (Table 9). Molecule 1 has the lowest LUMO energy and the low-
est density as compared to all the other molecules. Thus, one can
conclude the highest antibacterial activity of compound 1. This
finding strongly supported to the experimental results.
(l
relation [31]:
f
l2
¼
ð7Þ
4:72 ꢁ 10ꢀ7
ꢁ
ꢀ
m
3.4. Antimicrobial activity: disk-diffusion and microdilution assay
Compounds (1–5) were tested for their antibacterial activities
by disk-diffusion method using nutrient broth medium [contained
(g/L): beef extract 3 g; peptone 5 g; pH 7.0] [32]. The Gram-positive
bacteria and Gram-negative bacteria utilized in this study con-
sisted of A. hydrophila, Y. enterocolitica, L. monocytogenes and P.
aeruginosa. In the disk-diffusion method, sterile paper disks
(0.5 mm) impregnated with compound, dissolved in dimethylsulf-
oxide (DMSO) at concentration 100 lg/mL, were used. The paper
disks impregnated with the solution of the tested compound were
then placed on the surface of the media inoculated with the micro-
organism. The plates were incubated at 35 °C for 24 h, the growth
inhibition zones are shown in Table 2. Schiff base derivatives were
further checked by MIC method, as illustrated in Table 3.
4. Conclusions
Some Schiff bases were synthesized by the reaction of
4-dimethylamino-benzaldehyde with the corresponding active
heterocyclic amines under microwave irradiation. In addition,
studying spectroscopic and physicochemical properties of hetero-
cyclic Schiff base derivatives may show considerable promise
toward their potential applications. The antibacterial activity of
3.5. Computation
Compounds (1–5) were calculated using density functional the-
ory, DFT/RB3LYP method. Some selected descriptors such as total
Fig. 6. Electron density map of structures (1–5), calculated with DFT/RB3YPL method.

A.M. Asiri et al. / Journal of Photochemistry and Photobiology B: Biology 120 (2013) 82–89
89
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Acknowledgments
This Project was funded by SABIC and Deanship of Scientific Re-
search (DSR), King Abdulaziz University, Jeddah, under Grant No.
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