Synthesis, characterization, luminescent properties and biological activities of zinc complexes with bidentate azomethine Schiff-base ligands

Article abstract of DOI:10.1016/j.poly.2018.07.034

A series of zinc complexes with a bidentate azomethine Schiff base ligands derived from 2-tosylaminobenzaldehyde, 2-hydroxy-, 2-hydroxy-5-methoxybenzaldehydes, 2-hydroxynaphthaldehyde and 3,4-dimethoxyphenylethylamine were synthesized by chemical and electrochemical methods. All compounds were characterized on the basis of C, H, N elemental analysis, FT-IR, ¹H NMR, UV-vis and photoluminescence studies. The local atomic structures of complexes were determined from analysis of EXAFS and XANES of Zn K-edges. The molecular structure of bis{2-[(E)-2-(3,4-dimethoxyphenyl)ethyliminomethyl]-4-methoxyphenol}zinc(II) was determined by X-ray single-crystal diffraction. The fluorescence spectra show that these complexes in DMSO solutions at room temperature emit bright blue and green luminescence at 428–497 nm. The assignment and the nature of the bands in experimental UV–Vis spectra of complexes were analyzed using time-dependent (TD) DFT calculations. The azomethines and complexes of zinc have been screened for their antibacterial, protistocidal, and fungistatic activities against Penicillium italicum, Colpoda steinii, Escherichia coli 078 and Staphylococcus aureus P-209, and the results are compared with the activity of sulfadimethoxine, toltrazuril and fundazol.

Full text of DOI:10.1016/j.poly.2018.07.034

Accepted Manuscript
Synthesis, characterization, luminescent properties and biological activities of
zinc complexes with bidentate azomethine Schiff-base ligands
A.S. Burlov, V.G. Vlasenko, Yu.V. Koshchienko, N.I. Makarova, A.A.
Zubenko, Yu.D. Drobin, L.N. Fetisov, A.A. Kolodina, Ya.V. Zubavichus, A.L.
Trigub, S.I. Levchenkov, D.A. Garnovskii
PII:
S0277-5387(18)30430-3
https://doi.org/10.1016/j.poly.2018.07.034
POLY 13299
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
1 June 2018
19 July 2018
21 July 2018
Please cite this article as: A.S. Burlov, V.G. Vlasenko, Yu.V. Koshchienko, N.I. Makarova, A.A. Zubenko, Yu.D.
Drobin, L.N. Fetisov, A.A. Kolodina, Ya.V. Zubavichus, A.L. Trigub, S.I. Levchenkov, D.A. Garnovskii, Synthesis,
characterization, luminescent properties and biological activities of zinc complexes with bidentate azomethine
Schiff-base ligands, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.07.034
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Synthesis, characterization, luminescent properties and biological activities of zinc
complexes with bidentate azomethine Schiff-base ligands
a
b,*
a
a
A. S. Burlov , V. G. Vlasenko , Yu. V. Koshchienko , N. I. Makarova , A. A.
c
c
c
a
d
Zubenko , Yu. D. Drobin , L.N. Fetisov , A.A. Kolodina , Ya. V. Zubavichus , A. L.
d
e
e
Trigub , S. I. Levchenkov , D. A. Garnovskii
a
Institute of Physical and Organic Chemistry of Southern Federal University, Stachki ave. 194/2,
Rostov-on-Don 344090, Russian Federation;
e-mail: anatoly.burlov@yandex.ru
b
Institute of Physics of Southern Federal University, Stachki ave. 194, Rostov-on-Don 344090, Russian
Federation
c
North-Caucasian Zonal Scientific Research Veterinary Institute – Branch of the Federal State Budget
Scientific Institution “Federal Rostov Agricultural Research Centre”, Rostov highway, 0,
Novocherkassk 346421, Russian Federation
d
NRC Kurchatov Institute, 1 Acad. Kurchatov Sq., Moscow 123182, Russian Federation
e
Southern Scientific Centre of Russian Academy of Sciences, Chekhova ave. 41, Rostov-on-Don
3
44006, Russian Federation
Keywords: Azomethines, Zinc complexes, UV-Vis spectra, Photoluminescence,
Biological activity
A B S T R A C T
A series of zinc complexes with a bidentate azomethine Schiff base ligands derived
from 2-tosylaminobenzaldehyde, 2-hydroxy-, 2-hydroxy-5-methoxybenzaldehydes, 2-
hydroxynaphthaldehyde and 3,4-dimethoxyphenylethylamine were synthesized by
chemical and electrochemical methods. All compounds were characterized on the basis
1
of C, H, N elemental analysis, FT-IR, H NMR, UV-vis and photoluminescence studies.
*
Corresponding author
E-mail address: v_vlasenko@rambler.ru
1

The local atomic structures of complexes were determined from analysis of EXAFS and
XANES of Zn K-edges. The molecular structure of bis{2-[(E)-2-(3,4-
dimethoxyphenyl)ethyliminomethyl]-4-methoxyphenol}zinc(II) was determined by X-
ray single-crystal diffraction. The fluorescence spectra show that these complexes in
DMSO solutions at room temperature emit bright blue and green luminescence at 428-
497 nm. The assignment and the nature of the bands in experimental UV-vis spectra of
complexes were analyzed using time-dependent (TD) DFT calculations. The
azomethines and complexes of zinc have been screened for their antibacterial,
protistocidal, and fungistatic activities against Penicillium italicum, Colpoda steinii,
Escherichia coli 078 and Staphylococcus aureus P-209, and the results are compared
with the activity of sulfadimethoxine, toltrazuril and fundazol.
1. Introduction
Zinc(II) complexes of bi-, tri-, tetradentate azomethine ligands receive strong
attention due to their various functional properties. They show antibacterial,
antimicrobial [1, 2], and antitumor [3-9] activity. Many zinc complexes of azomethine
ligands are effective luminophores emitting in the range of 400-450 nm and are in great
demand as photo- and electroluminescent materials [10-20]. Such emitters are the main
constituent of red-green-blue luminescent displays, as well as key luminescent
components in the formation of white radiation by a combination of blue and orange
colors [10].
The photo- and electroluminescent properties of zinc complexes of bidentate
azomethine ligands derived from 2-hydroxybenzaldehyde have been studied by many
authors [21-24]. The maxima of longwave absorption bands of these complexes are
located in the 400-412 nm range, whereas that of photoluminescence at 450-550 nm.
Quantum yields of PL in these zinc complexes are low: 0.007-0.0057 [11, 15, 22]. The
substitution of the hydroxy group in the aldehyde fragment of bidentate azomethine
ligands by the 2-tosylamino group results in formation of chelate zinc complexes with
ZnN coordination site and improvement of their thermal stability [12, 25-30]. Such
4
synthetic available zinc complexes, which luminescent in the blue spectral region λ =
PL
2

428 – 433 nm with quantum luminescence yields φ = 0.23 – 0.37 and have melting
points above 230 °C, were used as emitting materials for design of OLED devices by
many authors [11, 14, 16, 29, 30].
Since there are much fewer compounds capable of emitting blue light than red
and green, the expansion of the series of luminescence complexes in the 400-500 nm
region is an important and urgent task.
Moreover, the presence of pharmacophore methoxy groups in the amine fragment
and the sulfonamide group in the aldehyde fragment of the complexes can have a
significant effect on their biological properties.
The present work is devoted to the synthesis of zinc complexes of bidentate
azomethine ligands derived from 2-tosylaminobenzaldehyde, 2-hydroxy-, 2-hydroxy-5-
methoxybenzaldehydes,
2-hydroxynaphthaldehyde
and
3,4-
dimethoxyphenylethylamine, and to the study of their structures, photoluminescence
and biological properties.
2. Experimental
2.1. Materials required and general methods
All solvents, zinc acetate dihydrate CAS № 5970-45-6, 2-Hydroxybenzaldehyde
CAS № 90-02-8, 2-Hydroxy-5-methoxybenzaldehyde CAS №: 672-13-9, 2-Hydroxy-1-
naphthaldehyde CAS №708-06-5, 3,4-Dimethoxyphenylethylamine CAS № 120-20-7
were purchased from Alfa Aesar and used as received. 2-(N-Tozylamino)benzaldehyde
has been synthesized using the reported procedure [31].
C, H, N elemental analyses were carried out on a Carlo Erba TCM 480 apparatus
using sulfanilamide as a reference. The metal content was determined gravimetrically in
the analytical laboratory of the Institute of Physical and Organic Chemistry (SFU,
Rostov-on-Don, Russia). Melting points were measured on a Kofler table.
Infrared spectra were recorded on a Varian Excalibur-3100 FT-IR
spectrophotometer for powders of compounds 1b-d and 2a-d.
3

1
H NMR spectra were measured on a Varian Unity-300 (300 MHz) spectrometer
at ambient temperature in CDCl . The chemical shifts (δ) were referenced to residual
3
solvent shifts in the respective deuterated solvents.
UV-Vis spectra were registered on «Varian Cary 100» spectrophotometer.
Photoluminescent spectra were measured on a «Varian Cary Eclipse» fluorescence
spectrophotometer. DMSO of the spectrally pure grade from Sigma-Aldrich was used as
a solvent. All UV-Vis and fluorescence spectra were recorded using standard quartz
-5
cells with an optical path of 1 cm at room temperature at c = 4.0 × 10 M. Fluorescence
quantum yield were determined by Parker-Reese technique [32] using 3-
methoxybenzathrone in toluene (φ = 0.1, λ = 365 nm) as a standard luminophor [33].
ex
2
2
.2. Synthesis of free ligands
.2.1. general procedure for the synthesis of N-[2-[(E)-2-(3,4-
A
dimethoxyphenyl)ethyliminomethyl]phenyl]-4-methylbenzenesulfonamide (1a), 2-[(E)-
2
-(3,4-dimethoxyphenyl)ethyliminomethyl]phenol
(1b),
(1c),
2-[(E)-2-(3,4-
1-[(E)-2-(3,4-
dimethoxyphenyl)ethyliminomethyl]-4-methoxyphenol
and
dimethoxyphenyl)ethyliminomethyl]naphthalen-2-ol (1d).
Solution of 0.36 g (2 mmol) of 3,4-dimethoxyphenylethylamine in 10 ml of
ethanol was added to a solution of 0.55 g (2 mmol) of 2-N-tosylaminobenzaldehyde for
1
a, 0.24 g (2 mmol) of 2-hydroxybenzaldehyde for 1b, 0.30 g (2 mmol) of 2-hydroxy-5-
methoxybenzaldehyde for 1c, 0.34 g (2 mmol) 2-hydroxy-1-naphthaldehyde for 1d in
5 ml of ethyl alcohol. The mixture was refluxed for 2 h. After cooling azomethine
1
precipitates were filtered off, washed with 5 ml of ethanol and recrystallized from
ethanol.
2.2.2.
N-[2-[(E)-2-(3,4-Dimethoxyphenyl)ethyliminomethyl]phenyl]-4-
methylbenzenesulfonamide (1a)
Yellow crystals, m.p. = 110-111 °С. Yield 0.78 g, 89%. Anal. Calc. for
C H N O S: С, 65.73; Н, 5.98; N, 6.38. Found: С, 65.84; Н, 6.01; N, 6.29 %. IR
24
26
2
4
-1
spectrum, selected bands, cm : 3000-2833 ν(CH ), 1633 ν(CH=N), 1328 ν (SO ), 1153
2
as
2
4

1
3
ν (SO ). Н NMR, δ (ppm): 2.35 (s, 3H, CH ), 3.02 (t, 2H, J=7.0 Hz, CH ), 3.80 (s,
s
2
3
2
3
H, OCH ), 3.82-3.87 (m, 5H, OCH , CH ), 6.74-6.82 (m, 3H, C -H), 6.96-7.01 (m,
3
3
2
Ar
3
1
H, C -H), 7.14-7.29 (m, 4H, C -H), 7.60 (d, 1H, J=8.4 Hz, C -H), 7.71 (d, 2H,
Ar
Ar
Ar
3
J=8.4 Hz, C -H), 8.09 (s, 1H, CH=N), 13.13 (s, 1H, NH).
Ar
2.2.3. 2-[(E)-2-(3,4-Dimethoxyphenyl)ethyliminomethyl]phenol (1b)
Yellow crystals, m.p. = 74-75 °С. Yield 0.51 g, 90%. Anal. Calc. for C H NO :
17
19
3
С, 71.56; Н, 6.71; N, 4.91. Found: С, 71.61; Н, 6.68; N, 4.82 %. IR spectrum, selected
-
1
1
bands, cm : 2999-2836 ν(CH ), 1633 ν(CH=N), 1260 ν(Ph-O). Н NMR, δ (ppm): 2.95
2
3
(
t, 2H, J=6.9 Hz, CH ), 3.81 (s, 3H, OCH ), 3.82 (s, 2H, CH ), 3.85 (s, 3H, OCH ),
2
3
2
3
4
3
4
6
.71 (d, 1H, J=1.5 Hz, C -H), 6.75 (dd, 1H, J=8.1 Hz, J=1.8 Hz, C -H), 6.80 (d, 1H,
Ar
Ar
3
3
3
J=8.1 Hz, C -H), 6.85 (t, 1H, J=7.5 Hz, C -H), 6.95 (d, 1H, J=8.4 Hz, C -H), 7.18
Ar
Ar
Ar
3
3
4
(
d, 1H, J=7.8 Hz, C -H), 7.31 (dd, 1H, J=8.5 Hz, J=1.5 Hz, C -H), 8.19 (s, 1H,
Ar
Ar
CH=N), 13.52 (s, 1H, OH).
2.2.4. 2-[(E)-2-(3,4-Dimethoxyphenyl)ethyliminomethyl]-4-methoxyphenol (1c)
Yellow crystals, m.p. = 50-51 °С. Yield 0.54 g, 85%. Anal. Calc. for C H NO :
18
21
4
С, 65.55; Н, 6.71; N, 4.44. Found: С, 65.60; Н, 6.69; N, 4.50 %. IR spectrum, selected
-
1
1
bands, cm : 3000-2842 ν(CH ), 1638 ν(CH=N), 1256 ν(Ph-O). Н NMR, δ (ppm): 2.95
2
3
(
t, 2H, J=6,8 Hz, CH ), 3.76 (s, 3H, OCH ), 3.80 (s, 2H, CH ), 3.81 (s, 3H, OCH ),
2
3
2
3
3
1
.85 (s, 3H, OCH ), 6.69-6.81 (m, 4H, C -H), 6.90 (s, 2H, C -H), 8.15 (s, 1H, CH=N),
3
Ar
Ar
3.00 (s, 1H, OH).
2.2.5. 1-[(E)-2-(3,4-Dimethoxyphenyl)ethyliminomethyl]naphthalen-2-ol (1d)
Yellow crystals, m.p. = 101-102 °С. Yield 0.46 g, 68%. Anal. Calc. for
C H NO : С, 75.20; Н, 6.31; N, 4.18. Found: С, 75.31; Н, 6.28; N, 4.29 %. IR
21
21
3
-1
1
spectrum, selected bands, cm : 3000-2845 ν(CH ), 1630 ν(CH=N), 1286 ν(Ph-O). Н
2
3
NMR, δ (ppm): 2.99 (t, 2H, J=6.8 Hz, CH ), 3.81-3.83 (m, 8H, 2OCH , CH ), 6.73-
2
3
2
3
3
6
.83 (m, 3H, C -H), 6.92 (d, 1H, J=9.3 Hz, C -H), 7.21 (t, 1H, J=7.5 Hz, C -H),
Ar
Ar
Ar
5

3
3
4
7
.38 (t, 1H, J=7,8 Hz, C -H), 7.59 (d, 1H, J=7.5 Hz, C -H), 7.66 (d, 1H, J=2.1 Hz,
Ar
Ar
3
C -H), 7.69 (s, 1H, CH=N), 8.48 (d, 1H, J=8,4 Hz, C -H), 14.51 (s, 1H, OH).
Ar
Ar
2
2
.3. Synthesis of complexes
.3.1. The general procedure for the preparation of complexes 2a-d
Chemical synthesis (CS). Solution of 1 mmol (0.22 g) of zinc acetate dihydrate in
ml of ethanol was added to a solution of 1 mmol of the corresponding azomethine
5
ligand (0.43 g of azomethine 1a, 0.29 g of azomethine 1b, 0.32 g of azomethine 1c, 0.34
g of azomethine 1d) in 20 ml of ethanol. The mixture was refluxed for 2 h. The
precipitates of complexes 2a-d were filtered off, washed with ethanol (5 ml) and
recrystallized from dichloromethane-ethanol (1:2) mixture.
Electrochemical synthesis (ES). A solution of 1 mmol of the corresponding ligand
(0.43 g of azomethine 1a, 0.29 g of azomethine 1b, 0.32 g of azomethine 1c, 0.34 g of
azomethine 1d) in 20 ml of acetonitrile was placed in an electrochemical cell with a
platinum cathode and zinc anode and 0.01 g of [Et N]ClO was added as electrolyte.
4
4
ES were performed during 1 h at 40 mA and 20 V at r. t. The precipitates of
complexes were filtered off and crystallized from a chloroform-ethanol (1:2) mixture.
Electrochemical cell:
-2
Cathode (Pt): 2LH+2е→ 2L +H
2
0
2+
Anode (Zn): Zn – 2e→Zn
-2
2+
Solution: 2L +Zn → ZnL
2
2.3.2. Bis{N-[2-[(E)-2-(3,4-dimethoxyphenyl)ethyliminomethyl]phenyl]-4-
methylbenzenesulfonamide}zinc(II) (2a)
Colorless crystals, m.p. = 247-248 °С. Yield 0.42 g, 90%. Anal. Calc. for
C H N O S Zn: С, 61.42; Н, 5.44; N, 6.01; Zn, 7.12 %. Found: (CS) С, 61.55; Н,
48
50
4
8 2
5.50; N, 6.15; Zn, 7.32 %. (ES) С, 61.38; Н, 5.38; N, 6.14; Zn, 7.20 %. IR spectrum,
-1
selected bands, cm : 3051-2836 ν(CH ), 1630 ν(CH=N), 1258 ν (SO ), 1132 ν (SO ).
2
as
2
s
2
1
Н NMR, δ (ppm): 2.28 (s, 6H, CH ), 2.63-2.73 (m, 2H, CH ), 2.93-3.03 (m, 2H, CH ),
3
2
2
3
.57-3.66 (m, 2H, CH ), 3.69 (s, 6H, OCH ), 3.81 (s, 6H, OCH ), 4.45-4.54 (m, 2H,
2
3
3
6

3
3
CH ), 6.51-6.57 (m, 4H, C -H), 6.69 (d, 2H, J=8.1 Hz, C -H), 6.81 (t, 2H, J=7.4 Hz,
2
Ar
Ar
3
3
C -H), 7.08 (d, 4H, J=8.1 Hz, C -H), 7.17-7.22 (m, 4H, C -H), 7.53 (d, 2H, J=9.0
Ar
Ar
Ar
3
Hz, C -H), 8.08 (d, 4H, J=8.1 Hz, C -H), 8.18 (s, 2H, CH=N).
Ar
Ar
2.3.3. Bis{2-[(E)-2-(3,4-dimethoxyphenyl)ethyliminomethyl]phenol} zinc(II) (2b)
Yellow crystals, m.p. = 194-195 °С. Yield 0.28 g, 89%. Anal. Calc. for
C H N O Zn: С, 64.48; Н, 5.63; N, 4.32.; Zn, 10.48 %. Found: (CS) С, 64.62; Н,
34
36
2
6
5.58; N, 5.54; Zn, 10.32 %. (ES) С, 64.58; Н, 5.70; N, 4.28; Zn, 10.54 %. IR spectrum,
-1
1
selected bands, cm : 2985-2835 ν(CH ), 1613 ν(CH=N), 1325 ν(Ph-O). Н NMR, δ
2
3
3
(
ppm): 2.87 (t, 4H, J=7.5 Hz, CH ), 3.68 (s, 6H, OCH ), 3.72 (d, 4H, J=7.5 Hz, CH ),
2
3
2
3
3
2
.81 (s, 6H, OCH ), 6.55-6.56 (m, 2H, C -H), 6.60 (d, 4H, J=8.1 Hz, C -H), 6.71 (d,
3
Ar
Ar
3
3
3
4
H, J=8.1 Hz, C -H), 6.90 (d, 2H, J=8.7 Hz, C -H), 7.03 (dd, 2H, J=7.8 Hz, J=1.5
Ar
Ar
3
Hz, C -H), 7.31 (t, 2H, J=7.8 Hz, C -H), 7.99 (s, 2H, CH=N).
Ar
Ar
2.3.4. Bis{2-[(E)-2-(3,4-dimethoxyphenyl)ethyliminomethyl]-4-methoxyphenol}zinc(II)
(2c)
Yellow crystals, m.p. = 200-201 °С. Yield 0.23 g, 65%. Anal. Calc. for
C H N O Zn: С, 62.30; Н, 5.81; N, 4.04.; Zn, 9.42 %. Found: (CS) С, 62.42; Н, 5.78;
36
40
2
8
N, 4.12; Zn, 9.54 %. (ES) С, 62.15; Н, 5.90; N, 3.98; Zn, 9.37 %. IR spectrum, selected
-1
1
bands, cm : 3000-2842 ν(CH ), 1623 ν(CH=N), 1302 ν(Ph-O). Н NMR, δ (ppm): 2.86
2
3
(
t, 4H, J=7.5 Hz, CH ), 3.68-3.70 (m, 10H, OCH , CH ), 3.74 (s, 6H, OCH ), 3.82 (s,
2
3
2
3
4
4
6
2
H, OCH ), 6.49 (d, 2H, J=3.0 Hz, C -H), 6.55 (d, 2H, J=1.8 Hz, C -H), 6.61 (dd,
3
Ar
Ar
3
4
3
3
H, J=6.9 Hz, J=1.8 Hz, C -H), 6.72 (d, 2H, J=8.1 Hz, C -H), 6.86 (d, 2H, J=9.0
Ar
Ar
3
4
Hz, C -H), 7.01 (dd, 2H, J=9.0 Hz, J=3.2 Hz, C -H), 7.96(s, 2H, CH=N).
Ar
Ar
2.3.5. Bis{1-[(E)-2-(3,4-dimethoxyphenyl)ethyliminomethyl]naphthalen-2-ol}zinc(II)
(2d)
Yellow crystals, m.p. = 204-205 °С. Yield 0.24 g, 65%. Anal. Calc. for
C H N O Zn: С, 68.71; Н, 5.49; N, 3.82.; Zn, 8.91 %. Found: (CS) С, 68.65; Н, 5.52;
42
40
2
6
N, 3.90; Zn, 9.10 %. (ES) С, 68.80; Н, 5.42; N, 3.74; Zn, 9.15 %. IR spectrum, selected
7

-
1
1
bands, cm : 3054-2893 ν(CH ), 1607 ν(CH=N), 1341 ν(Ph-O). Н NMR, δ (ppm): 2.91
2
3
3
(
t, 4H, J=7.2 Hz, CH ), 3.54 (s, 6H, OCH ), 3.75 (s, 6H, OCH ), 3.83 (t, 4H, J=7.0 Hz,
2
3
3
3
CH ), 6.53 (s, 2H, C -H), 6.62-6.68 (m, 4H, C -H), 7.10 (d, 2H, J=9.3 Hz, C -H),
2
Ar
Ar
Ar
3
3
3
7
.22 (d, 2H, J=7.5 Hz, C -H), 7.41 (t, 2H, J=7.5 Hz, C -H), 7.66 (d, 2H, J=7.8 Hz,
Ar
Ar
C -H), 7.74-7.77 (m, 4H, C -H), 8.94(s, 2H, CH=N).
Ar
Ar
2.4. X-ray Crystallography Analysis of zinc complex 2c.
Diffraction data for 2c were collected at 173(2) K on a Bruker APEX2 platform-
CCD X-ray diffractometer system equipped with a graphite monochromatized Mo Kα
X-ray source with a wavelength of λ = 0.71073 Å, 50 kV/40 mA power. Detailed
description of X-ray crystallographic experiment is given in Supplementary Materials.
Crystallographic data for 2c are summarized in Table 1, the fractional atomic
coordinates and isotropic or equivalent isotropic displacement parameters are given in
Tables S1-S4 Supplementary Materials. All calculations were carried out using the
SHELX-97 program [34].
Supplementary data CCDC 1846537 contains the supplementary crystallographic
data
for
2c.
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)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Table 1
Crystal data and structure refinement parameters for complex 2c.
Compound
2c
C H N O Zn
Empirical formula
М
36
40
2
8
694.07
Crystal size, mm
T, K
0.32 × 0.22 × 0.05
173
Crystal system
Space group
a, Å
Monoclinic
C2
18.6407 (9)
8

b, Å
c, Å
8.3884 (4)
12.9283 (6)
125.500 (1)
1645.77 (14)
2

, °
3
V, Å
Z
3
ρ , g/cm
1.401
0.80
728
2.2–31.2
9969
calc.
-1
μ, mm
F(000)
Range (°)

Reflections collected
Unique reflections
Reflections with I > 2(I)
Index ranges
4876
4656
h = -2626
k = -1111
l = -1818
0.026
R (I > 2(I))
1
wR (all reflections)
0.056
2
GOF
0.99
Residual electron density, Δρ / Δρ ,
eÅ
0.61/-0.30
max
min
-3
2
.5. X-ray absorption spectroscopy
The Zn K-edge EXAFS spectra for the complexes 2a-d were recorded at the
Structural Materials Science" beamline of the Kurchatov Synchrotron Radiation
"
Source (Moscow, Russia) [35] with the storage ring operating at electron energy of 2.5
GeV and current of 80−100 mA. A Si(111) channel-cut monochromator was used for
the energy selection. All data were measured in the transmission mode. Sample
thicknesses corresponded to an absorption jump Δμx=0.5–1.0.
EXAFS data (χ (k)) were analyzed using the IFEFFIT data analysis package
exp
[36]. EXAFS data reduction used standard procedures for the pre-edge subtraction and
spline background removal. The radial pair distribution functions around the Zn ions
3
were obtained by the Fourier transformation (FT) of the k -weighted absorption
–1
function χ (k) range of photoelectron wave numbers k = 2.4–13.0 Å . Structural
exp
parameters including interatomic distances (R ), coordination numbers (N ) and distance
i
i
9

RMS deviations due to the thermal motion and disorder-induced static deviations of
2
atomic positions, also known as Debye–Waller factors (σ ) were found by non-linear
fitting of the theoretical spectra against the experimental ones.
2
Ri
n

2
N F (k)


k

2i k
2
2
i
2
i

(k)  S0
e
e
sin(2kR  i

k )
, (1)


i
R
k
i1
i
The experimental data were simulated using theoretical photoelectron mean-free-
path (λ), photoelectron backscattering amplitude F (k) and phase functions (formula 1)
i
calculated using the FEFF7 program [37]. The amplitude reduction factor due to
2
extrinsic losses S and the edge energy shift E were calibrated by fitting EXAFS data
0
0
for spectra of reference compounds with known crystal structures. The amplitude
2
0
reductions factors S were found to be equal to 0.9 in all cases.
The accuracy of the fits was estimated by the standard mean-square deviation
criterion (formula 2) (Q-factor),
2
m
w(ki )ki  (k )  k  (k )


exp
i
i
th
i
2
i1
Q 
(2)
m
2
w(ki )ki  (k )


exp
i
i1
where w(ki ) is a weighting function, m is the number of experimental points.
2.6. Quantum-chemical calculations
The GAUSSIAN-03 program package [38] was used for DFT calculations.
Ground-state geometry of isolated molecules of the complexes under study was
optimized using the Becke's three-parameter exchange functional [39], Lee–Yang–Parr
correlation functional (B3LYP) [40] and standard split-valence polarized 6-31G(d) basis
set [41, 42]. The absence of imaginary frequencies in the normal mode analysis ensured
optimized geometry to be the true energy minimum, not a transition state.
The simulation of UV-vis absorption spectra for these complexes was performed
within the Time-Dependent Density Functional Theory (TD-DFT) formalism using the
optimized geometry subject to solvent effects as predicted by the standard polarizable
continuum model (PCM) [43]. The lowest 50 singlet-to-singlet spin-allowed excitation
10

states were taken into account for the calculations of the electronic absorption spectra of
complexes 2a-d.
2.7. Antimicrobial activity assay
Antimicrobial properties of azomethines 1a-d and complexes 2a-d were studied
by means of two-fold serial dilutions in a liquid nutrient medium [44, 45]. Suspensions
5
of bacterial cultures (2 mL) with a concentration of 5.0·10 microbial cells per mL were
mixed with equal amount of test substance solutions at variable concentrations in
special vials (which gave rise to a nominal two-fold decrease in the bacteria
5
concentration to 2.510 per mL). Vials were kept in an incubator for 18 h at 37 °C. In
5
parallel, vials containing nutrient medium and bacteria at a concentration of 2.510 per
mL and only nutrient medium without bacteria were incubated under identical
conditions as positive control and negative control probes, respectively. Standard strains
of bacteria Staphylococcus aureus P-209 and Escherichia coli 078 (field isolates from
the collection of the Rostov regional veterinary laboratory) were used for the
antimicrobial activity tests. Activity of the azomethines and zinc complexes was
compared with that of a commercial antibiotic sulfadimethoxine (chemically pure
grade).
2.8. Antiprotozoal activity assay
Antiprotozoal activity was studied against infusoria Colpoda steinii (field isolate
from the collection of the laboratory of parasitology of the North-Caucasian Zonal
Scientific-Research Veterinary Institute, Russia) using a method described elsewhere
[46]. The tests were carried out in a 96-well microplate typically used for the enzyme-
linked immunosorbent assays (ELISA), 12 wells of the first row were used. A 1:1
mixture of boiled tap and sterile distilled water was used as a medium. Initially the
substance being tested was dissolved in distilled water. Serial dilutions of the test
solutions were prepared as follows:
Solution No.1: 5 mg of the substance under study was dissolved in 50 µL of 70%
aqueous DMSO under stirring, then 5 mL of distilled water was carefully added to give
11

an apparent concentration of the probe 1000 µg/mL, 150 µL of the prepared solution
was placed into the well No.1 of the microplate.
Solutions No.2-12: 150 µL of 1:1 mixture of boiled tap and sterile distilled water
was placed into the wells No.2-12 of the microplate using an automated 8-channel
pipette. Then 150 µL of the solution No.1 was placed into the well No.2 under stirring.
After complete mixing an aliquot of 150 µL of solution No.2 was placed into the well
No.3. A similar procedure was applied to all further wells. In order to provide identical
volumes of solutions in all the wells, 150 µL of the solution was removed from the last
well No.12. Aliquots of the Colpoda steinii protozoa culture suspended in water
preliminary incubated for 3 days (30 µL) were added to all wells in such a way that no
less than 10-15 active infusoria were distinctly visible in the field-of-view of an optical
microscope. The microplate was covered with a lid and left at room temperature (20-22
°C) for 18-20 h.
Test results were controlled as follows: 30 µL of content of each well one-by-one
starting from the well No.12 was transferred onto a clean glass slide and examined
under an optical microscope at a magnification of 10×15 to check for the presence of
living protozoa. The minimum protistocidal concentration of a substance under study
corresponded to the first well encountered with no living infusoria. In a similar way, the
following control solutions were tested:
-
-
blank medium (boiled water + distilled water) – 5 wells;
DMSO (50 µL of 70% DMSO + 5 mL of distilled water serially diluted exactly as in
the case of the tested substances) – 12 wells;
commercial antiprotozoal agent toltrazuril (2.5% solution serially diluted similarly
to the tested substances).
-
2.9. Fungistatic activity assay
Antifungal activity of the compounds was determined by the agar-diffusion
method according to the guideline [47] for fungi culture Penicillium italicum Wehmer
1894) (field isolate, from the collection of micromycetes of the laboratory of
mycotoxicology of the North-Caucasian Zonal Scientific Research Veterinary Institute,
(
12

Russia). A commercial fungicide fundazol was used for comparison. An aqueous
solution of the test compounds or fundazol for comparison were placed on a disc of
filter paper (ND-PMP-1 Paster Central Research Institute of Epidemiology and
Microbiology) in an amount of 15 µg per disc with a diameter of 8 mm.
3. Results and discussion
3.1. Synthesis and spectroscopic properties of azomethines as free ligands and their
derived zinc complexes
The azomethine compounds 1a-d (HL) were obtained by condensation of 2-N-
tosylaminobenzaldehyde (for 1a), 2-hydroxybenzaldehyde (for 1b), 2-hydroxy-5-
methoxybenzaldehyde (for 1c), 2-hydroxy-1-naphthalaldehyde (for 1d) and 3,4-
dimethoxyphenylethylamine in ethanol solutions (Scheme).
13

MeO
MeO
OMe
OMe
XH
N
.
R
Zn(CH COO) 2H O
3 2 2
NH CH CH
2
2
2
XH
O
C H OH
2
5
R
N
X
X
N
R
C H OH
2
5
o
Zn
Zn , CH CN
R
3
OMe
OMe
H
1a-d
2a-d
OMe
OMe
a (R=H, X= NSO2
CH ); b (R=H, X=O); c (R= 5-OCH , X= O);
3
3
d (R= 5,6-cyclo-C H , X=O)
4
4
Scheme. Synthesis of the azomethines 1a-d and zinc complexes 2a-d
14

Zinc complexes 2a-d were obtained by two methods: chemical (refluxing of
azomethine compounds 1a-d with zinc acetate dihydrate at a molar ratio of 2:1 in
ethanol) and electrochemical (by anodic dissolution of zinc in acetonitrile azomethine
solution 1a-d).
The structures of azomethines 1a-d and zinc complexes 2a-d were determined by
1
means of IR, H NMR spectroscopy and elemental analysis.
All azomethine compounds 1a-d are crystalline substances. In the IR spectra of
-1
compounds 1a-d, absorption bands are observed at the 1630-1638 cm ν(CH=N), in the
-1
-1
case of 1a there are also absorption bands at 1328 cm ν (SO ) and 1153 cm ν (SO ),
as
2
s
2
-1
whereas in the cases of 1b-d at 1260, 1256, 1286 cm ν(Ph-O), respectively. Due to the
presence of strong intramolecular hydrogen bond XH...N in the azomethines 1a-d, in
-1
their IR spectra the ν(NH, OH) absorption bands in the region 3000-3500 cm are not
1
observed. In the H NMR spectra of these compounds, the proton signals of CH=N, NH
and OH groups are registered at 7.69-8.19 (1a-d), 13.13 (1a) and 13.00-14.51 ppm (1b-
d), respectively.
According to the elemental analysis data the zinc complexes possess ZnL2
composition. In the IR spectrum of 2a, the absorption bands of ν(CH=N) shift to lower
-1
-1
-1
frequencies by 3 cm , whereas of ν (SO ) and ν (SO ) by 32 cm and 20 cm ,
as
2
s
2
respectively. In the IR spectra of 2b-d, the ν(CH=N) absorption bands also redshift by
-1
-1
1
1
5-23 cm , and the absorption bands ν(Ph-O) blueshift by 45-65 cm . In the H NMR
spectra of 2a-d, the proton signals of the NH and OH groups of azomethines 1a-d
disappear. The proton signals of the CH=N groups downfield by 0.9 (2a) and 1.25 ppm
(2d) or upfield by 0.2 (2b) and 0.19 ppm (2c).
3.2. The X-ray absorption spectroscopy
X-ray absorption spectroscopy (XAS), including extended X-ray absorption fine
structure (EXAFS) and X-ray absorption near-edge structure (XANES) analysis, has
been carried out at the Zn K edge of the zinc coordination compounds 2a-d. From
analysis of the Zn XANES spectra, the local electronic structure of Zn atoms is described.
Fig. 1a displays the background subtracted and normalized Zn K edge XANES spectra and
15

their first derivatives of four complexes 2a-d. The XANES spectra of Zn K-edges,
indicating that the Zn atoms are predominantly present in a divalent oxidation state
occupying the center of tetrahedral configuration. No 1s–3d transitions are expected due to
2+
10
the lack of empty d states in Zn ions (d ).
Analysis of EXAFS of Zn K-edges has been used to provide information about the
quantitative characteristics of the local structure around zinc ions in complexes 2a-d. The
structural data for the complex 2c determined by the X-ray crystallography were used as
the starting model for all Zn(II) complexes in the EXAFS analysis.
3
From the Fourier transforms of the Zn K-edge k -weighted EXAFS spectra (Fig. 1b),
major peak can be observed at 1.52-1.58 Å, corresponding to the Zn–N (2a) and Zn-O/N
(2b-d) distances. Minor shells at > 2 Å are also observed, which give evidence for ligand
coordination. The EXAFS fitting results of the first-neighbor feature show an average
coordination number of 4 (Table 2) for the Zn-N or Zn-N/O shells in the samples,
indicating that the zinc(II) ions have a tetrahedral coordination environment, which is in
accordance to our findings from XANES.
The average distances Zn–N (2a), Zn-O/N (2b-d) and magnitudes of Debye-Waller
factors for all complexes have typical values in similar zinc complexes with tetrahedral
geometry about metal center [12, 48-50].
16

N
8
6
4
2
0
8
1
2
a
0
,2
2a
ts
0
,0
9
640
9650
9660
9670
E, eV
9680
9690
9700
0
1
9
650
9700
9750
9800
0
1
2
3
4
5
6
E, eV
2b
r, Å
O/N
6
4
2
0
0,2
0
,0
9
2b
640
9650
9660
9670
E, eV
9680
9690
9700
0
1
9650
9700
9750
9800
0
1
1
1
2
3
4
5
6
E, eV
8
6
4
2
0
r, Å
2
c
0
,2
2
c
0
,0
9
640
9650
9660
9670
E, eV
9680
9690
9700
0
1
9650
9700
9750
9800
0
2
3
4
5
6
E, eV
8
6
4
2
0
r, Å
0
0
0
0
,3
2
d
,2
,1
2
d
,0
9
640
9650
9660
9670
E, eV
9680
9690
9700
0
9650
9700
9750
9800
0
2
3
4
5
6
r, Å
E, eV
a
b
Fig. 1. Zn K-edge XANES spectra for the zinc complexes 2a-d (a) represented as μ(E)
3
and dμ/dE (insets) (a), MFT EXAFS χ(k)k for zinc complexes 2a-d, experiment - solid
line, best-fit theory - empty circles (b).
Table 2
Parameters of the local structure around zinc ions in complexes 2a-d: coordination
2
numbers (N), interatomic distances (R), and Debye-Waller factors (σ ). Q is the integral
fit quality factor.
2
2
Compound
N
4
R, Ǻ
σ , Ǻ
КС
Q, %
2.1
2a
2.00
0.0033
N
17

2
2
2
2
2
2
1.92
2.02
1.91
2.02
1.91
2.01
0.0040
0.0040
0.0041
0.0041
0.0039
0.0039
O
N
O
N
O
N
2
b
1.0
2.6
1.7
2
c
2d
EXAFS fitting: Δr= 1.0-2.0 Å
3.3. The X-ray diffraction
The molecular structure and atom numbering scheme of the complex 2c are
shown in Fig. 2. The molecule of 2c has C2 symmetry. The Zn(II) center displays a
distorted tetrahedral geometry with a N O ligand environment. The azomethine ligands
2
2
act as the bidentate chelating species and are coordinated to the central ion in trans-
positions through deprotonated phenolic oxygen atoms O(1) and O(1a) and azomethinic
nitrogen atoms N(1) and N(1a). The dihedral angle between the planes N(1)O(1)Zn(1)
and N(1a)O(1a)Zn(1) is equal to 78.93º.
The six-membered chelate rings adopt a half-boat conformation, with the Zn
atom deviating 0.274 Å from the plane formed by N(1)C(8)C(6)C(1)O(1) (the
maximum mean-plane deviation for C(8) is 0.028 Å). As can be seen from data of
selected bondlengths for 2c (Fig.2), the agreement between EXAFS and crystallography
was excellent for the major shells (distances underestimated by only 0.02 Å).
18

Fig. 2. Structure of the complex 2c with labeling and thermal ellipsoids drawn at
the 50% probability level. Selected bond lengths (Å): Zn(1)–N(1) 2.0005(12),
Zn(1)–O(1) 1.9188(10). Selected bond angles (º): O(1)–Zn(1)–N(1) 96.69(4),
O(1)–Zn(1)–O(1a) 128.53(7), O(1)–Zn(1)–N(1a) 109.13(5).
Fig. 3. The crystal packing of 2c. Dashed lines indicate hydrogen bonds.
19

The crystal packing of the compound is shown in Fig. 3. In the crystal lattice of
2c (Fig. 3), all oxygen atoms of each ligand are involved in hydrogen bond interactions.
There exists intermolecular hydrogen bonds between the two O(3), O(4) of methoxy
o
o
groups with H-C(3) of methoxyphenol unit 2.585 Å, 173.03 and 2.747 Å, 118.47 ,
respectively), intermolecular contacts between O(2) of methoxy group with H-C(9)
o
(
2.772 Å, 138.45 ) of (CH ) fragments and hydrogen bonding of oxygen atom of
2
2
phenol unit O(1)H-C(7) with methoxy group of neighbouring molecules (2.681 Å,
o
1
72.39 ).
3.4. Electronic absorption spectra and photoluminescence of the zinc complexes
The optical properties of the zinc complexes 2a-d were investigated by UV-Vis
and photoluminescence (PL) spectroscopy in DMSO solutions at room temperature. The
results of these studies are summarized in Table 3. The electronic absorption spectra of
2a-d are shown in Fig. 4.
UV-Vis spectra of the complexes 2a-d in the spectral range from 300 to 470 nm
reveal broad longwave absorption bands with maxima at 354-399 nm (ε = 15260-20890
-1
-1
M cm ). The complex 2d with the 2-hydroxynaphthaldehyde based ligand additionally
-1
-1
has a second higher energy band with λmax = 318 nm (ε = 21530 M cm ) in the same
spectral range (Table 3, Fig. 4).
Structural modification of the aldehyde fragment of bidentate azomethine ligands
of zinc complexes, associated with the substitution of the 2-tosylamino group (1a, λmax
=
354 nm) by 2-hydroxy group (1b, λmax = 367 nm) leads to an absorption band’s
bathochromic shift by 13 nm. Additional bathochromic shift (Δλ = 32 nm) in the
absorption spectrum of 1c (λmax = 399 nm) in comparison with 1b is achieved due to the
introduction of the donor methoxy group into the 2-hydroxybenzylidene fragment.
Table 3
UV-Vis and photoluminescence (PL) data of 2a-d in DMSO at 293 K: ε – molar
extinction coefficient; φ – quantum yield of the fluorescence.
20

Photoluminescence
Absorption
Compound
Excitation
Emission
-1
-1
λ , nm (ε, M сm )
φ
max
λ , nm
λ , nm
max
max
3
2
3
2
3
54 (15260)
77 (31500)
67 (15960)
73 (26100)
99 (18200)
352
278
366
275
398
275
397
318
273
2
a
428
445
497
0.15
0.45
0.11
2b
2
c
2
73sh (27880)
98 (20890)
318 (21530)
71sh (37550)
3
2d
443
0.04
2
sh - Shoulder
Fig. 4. UV-Vis spectra of compounds 2a-d in DMSO.
21

All complexes 2a-d demonstrate bright blue and green photoluminescence (Table
, Fig. 5). The fluorescence maxima are located in the wide spectral range of 428 – 497
3
nm.
(
b)
(
a)
(
c)
Fig. 5. Normalized emission spectra (λ = 365 nm) of compounds 2a-d in DMSO (a).
ex
Solutions photographs of compounds 2a-d in DMSO: before radiation (no emission)
(
b); during radiation (photoluminescence, λ = 365 nm) at room temperature (c).
ex
In 2a-d the effect of substituents in the aldehyde fragment of ligands on spectral-
fluorescent characteristics is similar to their effect on EAS. The maxima of the emission
bands of 2b (λmax = 445 nm) and 2c (λmax = 497 nm) are bathochromically shifted by 15-
69 nm in comparison with 2a (λmax = 428 nm).
The fluorescence excitation spectra of 2a–d fit well their absorption spectra,
indicating that their fluorescence was correctly assigned to zinc complexes ZnL (Table
2
3, Fig. 6).
22

Fig. 6. UV-Vis (1), fluorescence emission (λ = 400 nm) (2) and fluorescence excitation
ex
(λobs = 450 nm) spectra of 2d in DMSO.
The fluorescence efficiency of the studied compounds also varies over a wide
range (φ = 0.04-0.45), depending on their structure (Table 3). Complexes 2a-c (φ =
.11-0.45) demonstrate higher quantum yields of fluorescence than their analogue 2d (φ
0.04). The introduction of an electron donating 5-methoxy group into the aldehyde
0
=
fragment leads to a significant quenching of the fluorescence of 2c (φ = 0.11) in
comparison with 2b (φ = 0.45).
Such spectral characteristics make them promising for use as emitters in the
design of light-emitting organic diodes. Earlier, based on zinc complexes with similar
ligands, OLED devices were made that demonstrated significant changes in
luminescence characteristics depending on the structure of the complex [11, 23, 24, 30].
These devices emitted in the green region of the spectrum with a brightness of 480 to
2
2
1
470 cd/m (at 12-14 V) and a current density of up to 30 mA/cm .
Thus, the above data allows us to conclude that the structural modification of the
bidentate azomethine ligands described in this paper (substitution of the N-tosylamino
23

group by the hydroxy group, introduction of the 5-methoxy group as well as
benzannelation) has a significant effect on the optical properties of 2a-d.
3.5. Interpretation of UV-vis spectra based on TD-DFT calculations
The ground state fully optimized geometry was used to evaluate the excitation
energies and oscillator strengths of the electronic excitations of compounds 2a-d in
DMSO solution using TD DFT method. The solvent effect was simulated using the
polarizable continuum model (PCM). In Fig. 7, the calculated electronic transitions are
characterized by vertical lines with the height proportional to the oscillator strength, the
theoretical absorption spectra have been simulated by considering a constant FWHM of
30 nm and compared to the experimental absorption spectra of 2a-d. The calculated
vertical excitation energies, wavelengths, oscillator strengths, and percentage
contributions of dominant electronic transitions for complexes 2a-d are collected in
Table 4, along with the experimental transition wavelengths for comparison.
24

0,6
0,3
0,0
2a
200
250
300
350
400
450

, nm
0,9
0,6
0,3
2b
0
,0
,9
200
250
300
350
400
450
0

, nm
0,6
0,3
0,0
2c
200
250
300
350
400
450

, nm
1,2
0,9
0,6
0,3
0,0
2d
200
250
300
350
400
450

, nm
Fig. 7. Comparison of experimental (blue) and TD-DFT calculated (dash red) UV-vis
spectra for complexes 2a-d. Vertical bars represent individual calculated electronic
transitions with the height proportional to the oscillator strength.
Table 4
Calculated wavelengths (λ ) and related experimental values (λ ), energies (E),
max
exp
oscillator strengths (f), involved molecular orbitals and their contributions for different
electronic transitions in 2a-d according to TD-DFT calculations.
complex λ , nm λ , nm E, eV
orbital transition (contribution, %)
f
exp
max
25

HOMO-1  LUMO (54 %)
HOMO-2  LUMO (26 %)
HOMO  LUMO (82 %)
HOMO-1  LUMO (16 %)
HOMO-1  LUMO (27 %)
HOMO-2  LUMO (61 %)
HOMO-1  LUMO+1 (48 %)
HOMO-2  LUMO+1 (25 %)
HOMO-1  LUMO (80 %)
HOMO  LUMO (85 %)
HOMO  LUMO+1 (80 %)
HOMO-1  LUMO+1 (81 %)
HOMO  LUMO (91 %)
HOMO-1  LUMO(87 %)
HOMO  LUMO+1 (83 %)
HOMO-1  LUMO+1 (83 %)
HOMO  LUMO (89 %)
HOMO-1  LUMO(82 %)
HOMO  LUMO+1 (75 %)
HOMO-2  LUMO (50 %)
HOMO-1  LUMO+1 (40 %)
HOMO-3  LUMO (84 %)
HOMO-2  LUMO (39 %)
HOMO-1  LUMO+1 (43 %)
HOMO-5  LUMO (59 %)
HOMO-4  LUMO+1 (24 %)
3
3
74.18
67.14
3.314
3.377
3.433
0.057
0.029
0.021
0.122
2
a
354
3
3
61.15
55.12
3
.491
3
3
3
3
3
3
3
3
3
3
3
46.12
43.23
38.47
38.17
3.582
3.612
3.663
3.666
0.064
0.099
0.044
0.120
0.034
0.049
0.132
0.047
0.028
0.077
0.261
2b
367
399
93.13 3.1537
86.25 3.2099
76.93 3.2893
72.78 3.3259
2
c
73.33
68.96
60.42
3.321
3.360
3.440
3
3
3
98
82
18
3
3
3
59.92
58.39
56.95
3.445
3.460
3.473
0.024
0.017
0.023
2d
3
11.26
3.983
0.260
26

As can be seen from Fig. 7 the calculated electronic absorption spectra are in
good agreement with experiment. This makes it possible to reliably relate the
experimental absorption bands to certain electronic transitions forming them.
Analysis of energy characteristics, isosurfaces (Figs. 8-11), and populations (S5-
S8 Supplementary Materials) of frontier MOs for 2a-d demonstrates specific
regularities due to the effect of benzaldehyde fragment structural modification of
bidentate azomethine ligands. As can be seen from Figs. 8-11 and the data in Table 4,
the longwave band in the absorption spectra of zinc complexes 2b-d is determined,
*
mainly, by  electronic transitions between quasi-degenerate HOMO and HOMO-1
and LUMO and LUMO+1. The calculation correctly reproduces the experimental
bathochromic shift for the 2b-d complexes, due to the modification of the benzaldehyde
fragment of the ligand, which correlates with the energy gap for these complexes.
Because of the small difference in the energy values between quasi-degenerate vacant
frontier MOs (~ 0.02 eV) for complexes 2a, b, higher electronic transitions with
maximal λmax are realized between HOMO-1  LUMO, whereas for complexes 2c, d
such are HOMO  LUMO. For all 2b-d complexes the electron density for LUMO and
LUMO+1 is localized on both benzaldehyde fragments (Figs. 8-11). The electron
density for HOMO and HOMO-1 is localized on both benzaldehyde fragments only for
2c (Fig. 10), whereas for 2b,d it is delocalized throughout the ligand (excluding zinc
*
ions) (Fig. 9, 11). Thus, in case 2c, the electronic transitions have the (L ) (L )
1
1
*
ILCT and n(O') (L ) nature, whereas in 2b,d the transitions have a mixed
1
*
*

(L ) (L ) and (L ) (L ) LLCT/ILCT type, where O' are atoms of 4-methoxy L
2
1
1
1
1
benzaldehyde fragments and L is (3,4-dimethoxyphenyl)ethyliminomethyl moiety.
2
27

Fig. 8. Molecular orbital energy level diagram for 2a and contour plots of frontier MOs.
The arrows show principal one-electron excitations contributing to the specified low-
lying electronic transitions.
28

Fig. 9. Molecular orbital energy level diagram for 2b and contour plots of frontier MOs.
The arrows show principal one-electron excitations contributing to the specified low-
lying electronic transitions.
Fig. 10. Molecular orbital energy level diagram for 2c and contour plots of frontier
MOs. The arrows show principal one-electron excitations contributing to the specified
low-lying electronic transitions.
29