Zinc(II) complexes with schiff bases derived from ethylenediamine and salicylaldehyde: The synthesis and photoluminescent properties

Article abstract of DOI:10.1007/s11172-008-0254-x

The effect of the nature of organic ligands and complex formation on the photoluminescent characteristics (relative quantum yield, excited-state lifetime) and thermal stability of tetradentate Schiff bases (H₂L), derivatives of salicylaldehyde

Full text of DOI:10.1007/s11172-008-0254-x

1
880
Russian Chemical Bulletin, International Edition, Vol. 57, No. 9, pp. 1880—1889, September, 2008
Zinc(II) complexes with Schiff bases derived from ethylenediamine
and salicylaldehyde: the synthesis and photoluminescent properties
O. V. Kotova,a S. V. Eliseeva, A. S. Averjushkin, L. S. Lepnev, A. A. Vaschenko, A. Yu. Rogachev,
a
b
b
b
c
b
a
A. G. Vitukhnovskii, and N. P. Kuzmina
aDepartment of Chemistry, M. V. Lomonosov Moscow State University,
Leninskie Gory, 119991 Moscow, Russian Federation.
1
Fax: +7 (495) 939 0998. Eꢀmail: kotova@inorg.chem.msu.ru
P. N. Lebedev Physical Institute, Russian Academy of Sciences,
b
5
3 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 7860. Eꢀmail: alexei@sci.lebedev.ru
Department of Chemistry, University at Albany, State University of New York,
USA, 12222ꢀ0100 New York, Albany.
c
Fax: (518) 442 3462. Eꢀmail: rogachev@albany.edu
The effect of the nature of organic ligands and complex formation on the photoluminescent
characteristics (relative quantum yield, excitedꢀstate lifetime) and thermal stability of tetradenꢀ
1
2
tate Schiff bases (H L), derivatives of salicylaldehyde (H (SAL) , H (SAL) ), oꢀvanillin
(
2
2
2
2
1
H (MO) , H (MO) ) with ethylenediamine and oꢀphenylenediamine, and their zinc(II) comꢀ
2 2
plexes was studied. Zinc(II) complexes were synthesized by the reaction of H L with
2
2
Zn(AcO) •2H O in MeOH at room temperature or under reflux. In the case of H L = H (SAL) ,
2
1
2
2
2
2
H (MO) , H (MO) , complexes of the composition ZnL•H O were isolated irrespective of the
temperature. For H L = H (SAL) , the reaction results in Zn(SAL) •H O at room temperature
and in anhydrous dimeric complex [Zn(SAL) ] under reflux. Density functional calculations of
2
2
2
1
1
2
2
2
1
2
H L and ZnL confirmed that (1) luminescence of these compounds is due to the π—π* transition
2
between orbitals of the organic ligand and (2) enhancement of conjugation of the chain and
introduction of electronꢀdonating substituents lead to a decrease of the energy gap and, thereꢀ
fore, to a bathochromic shift of the emission maximum.
Key words: Schiff bases, zinc(II) complexes, luminescence, organic light emitting diodes.
Transition metal coordination compounds (CC) exꢀ
emission maximum10,11 and a decrease in the quantum
1
2
hibiting luminescent properties are of interest because they
can be used as electroluminescent (EL) materials for orꢀ
ganic light emitting diodes (OLED).1 Determination of
correlations between the composition, structure, and funcꢀ
tional properties (luminescence, thermal stability, volatilꢀ
ity, conductivity, etc.) of starting CC underlies the design
of new EL materials. Investigations of the effect of subsꢀ
tituents in organic ligands on the photophysical properꢀ
yield of photoluminescence, as well as an increase in
1
3
conductivity. However, in each case the study of the
effect of the nature of organic ligands on the composition
and functional properties of the CC based on them is
a subject of separate investigation.
—5
Among different classes of zinc(II) CC, which are of
interest as the EL materials, complexes with organic
ligands containing the azomethine C=N bond and a donor
oxygen or nitrogen atom are of special interest because
these systems exhibit intense luminescence in the blue reꢀ
6
—8
ties of complexes have been documented.
Taking
an aluminum(III) complex with 8ꢀoxyquinoline (AlQ3,
λmax = 525 nm) as an example, it was shown that the
introduction of electronꢀdonating substituents to the benꢀ
zene fragment leads to a bathochromic shift of the emisꢀ
sion maximum and to a hypsochromic shift if the substituꢀ
ent was introduced into the pyridine fragment. This effect
1
,4,14—16
gion of the visible spectrum.
Interest in the search
for new sources of blue luminescence is due to the fact that
efficient EL materials, i.e., red and green luminophores,
are already found among CC and used in OLED, whereas
blue luminophores of comparable efficiency have not been
9
3—5
was explained by the change in the band structure. In addiꢀ
found so far.
Luminescent properties of zinc(II) comꢀ
tion, many researchers pay attention to the effect of introꢀ
duction of conjugated benzene rings into CC. π—π Interꢀ
action between them causes a bathochromic shift of the
plexes are determined only by the organic ligand because
the dꢀshell of the central ion is completely filled. In addiꢀ
tion, the nature of organic ligands in CC considerably inꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1846—1855, September, 2008.
066ꢀ5285/08/5709ꢀ1880 © 2008 Springer Science+Business Media, Inc.
1

Zinc(II) complexes with Schiff bases
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1881
fluences the packing of molecules in the crystal strucꢀ
ture and the formation of intermolecular contacts,
which also affect the luminescent properties. This enables
targeted variation of the functional properties of zinc(II)
CC by the introducing different substituents into organic
ligands.
The percentage of carbon, hydrogen, and nitrogen was deterꢀ
mined by elemental microanalysis on a C,H,Nꢀanalyzer at the
Center for Pharmaceutical Chemistry of the AllꢀRussian Chemꢀ
ical and Pharmaceutical Research Institute (CPCꢀARCPRI).
IR spectra of the complexes were recorded on a Perkin—
1
7,18
–
1
Elmer FTꢀIRꢀ1600 spectrometer in the region 4000—400 cm
samples were suspended in Nujol and hexachlorobutadiene).
(
In the present work, we study the effect of conjugaꢀ
tion enhancement by replacement of the ethylenediꢀ
amine bridge with the oꢀphenylenediamine one (in the
1
H NMR spectra were recorded on a Bruker Avanceꢀ400
spectrometer (400 MHz).
Mass spectra (EI) of zinc(II) complexes were recorded on a
SSQꢀ710 instrument (Finigan MAT, USA) at CPCꢀARCPRI.
XꢀRay phase analysis of powdered complexes was performed
on a STADI/IP diffractometer (CuꢀKα irradiation). Calculaꢀ
tions of theoretical Xꢀray patterns, as well as indexing were perꢀ
formed using the STOE WinXPOW 1.04 program package.*
Thermal analysis was carried out on a Qꢀ1500 derivatograph
in the temperature range 20—700 °C under nitrogen atmosphere
1
2
1
2
pairs H (SAL) —H (SAL) , H (MO) —H (MO) ) and
2
2
2
2
by introduction of electronꢀdonating substituents (in partiꢀ
cular, methoxy group at position 1,1´ in the pairs
1
1
2
2
H (SAL) —H (MO) , H (SAL) —H (MO) ) on the phoꢀ
2
2
2
2
toluminescent and thermal properties of Schiff bases (H L)
2
and zinc(II) complexes (ZnL•H O).
2
–1
(
sample weight 0.05—0.1 g, heating rate ~10 °C min ).
Photoluminescence spectra of solid samples of the Schiff
bases and zinc(II) complexes were measured on an Ocean Optics
S2000 multichannel spectrometer (an LGIꢀ21 nitrogen laser as
the source of photoexcitation; λexc = 337 nm) at 25 °C and at the
temperature of liquid nitrogen (–196 °C). A correction for the
instrument function was made for all the photoluminescence
spectra. The relative quantum yield of photoluminescence (ϕrel
)
was calculated according to Ref. 22. The accuracy in the deterꢀ
mination of ϕrel was ±10%.
R
—
R´
H
L
CH —CH —
(SAL)1
Excitedꢀstate lifetimes (τobs) were measured on a Model 199s
fluorescent spectrometer (Edinburgh Instruments Ltd.) using
a nitrogen gasꢀdischarge lamp (λexc = 337 nm, τ = 3 ns) and
averaged at least over three independent measurements. The
luminescence decay curves were recorded at a wavelength correꢀ
sponding to the maximum intensity of the sample photoluminesꢀ
cence. To calculate the luminescence decay times for the samꢀ
ples under study, the experimental decay curves were deconvoꢀ
luted with allowance for the decay time of the nitrogen gasꢀ
discharge lamp. All the decay curves were fitted by monoexpoꢀ
nential functions.
2
2
(SAL)2
H
—
CH —CH —
OMe
(MO)1
2
2
OMe
(MO)2
The influence of the ligand nature on the topology of
the frontier orbitals and their position on the energy scale
was evaluated and theoretical modeling of the systems H2L
and ZnL in the framework of the DFT approach was perꢀ
formed.
Theoretical modeling was accomplished in the framework of
the DFT approach using the B3LYP exchangeꢀcorrelation poꢀ
2
3
tential . Zinc(II) ions were described using the Hay—Wadt efꢀ
2
4
fective core potential and the basis set LANL2DZ. For the
atoms of organic ligands, the standard splitꢀvalence basis sets
6ꢀ311G augmented with polarization dꢀfunctions for the nonꢀ
hydrogen atoms and pꢀfunctions for the hydrogen atoms
Experimental
(
6ꢀ311G(d,p)) were used. Full geometry optimization of enol—
Starting compounds Zn(AcO) •2H O (chemically pure
grade), ethylenediamine (chemically pure grade), oꢀphenyleneꢀ
diamine (chemically pure grade), oꢀvanillin (Aldrich), methanol
Aldrich), benzene (chemically pure grade) were used without
additional purification. Salicylaldehyde (pure grade) was puriꢀ
fied by distillation in vacuo (b.p. 93 °C, 25 Torr).
2
2
enol tautomers of the Schiff bases and zinc complexes correꢀ
sponding to the minima on the potential energy surface (PES)
was conducted until a gradient of 10–5 at.u. assuming the point
symmetry group C . In the case of enol—keto forms of organic
molecules, no symmetry restrictions were imposed (point symꢀ
(
2
metry group C ). The spin multiplicities of the Schiff bases and
Synthesis of Schiff bases was carried out according to a stanꢀ
1
1
9
–3
zinc(II) complexes were set equal to 1. The absence of calculated
imaginary harmonic vibrational frequencies suggests that the opꢀ
timized structures correspond to the energy minima on the corꢀ
responding PES. All calculations were carried out using the PC
GAMESS program package.** Despite the fact that theoretical
dard procedure by the reaction of an aldehyde (2•10 mol)
and the corresponding diamine (1•10–3 mol) in benzene. The
yields were ~90—95%.
Synthesis of zinc(II) complexes.20,21 Reactions of solutions of
zinc(II) acetate (1•10–3 mol) and a Schiff base (1•10–3 mol) in
MeOH at room temperature and under reflux led to precipitation
of corresponding zinc(II) complexes. For more complete precipꢀ
itation, the reaction mixtures were cooled to 5 °C. The precipiꢀ
tates were isolated by filtration. The yields were ~70%.
* http://www.stoe.com
** A. A. Granovsky, URL http://lcc.chem.msu.ru/gran/gamess/
index.html

1
882
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Kotova et al.
modeling was performed in the isolated molecule approximaꢀ
tion ignoring the effect of the aggregation state or solvent,
the qualitative estimation of changes in the series of sameꢀ
type compounds is justified, as well as the analysis of the naꢀ
ture of transitions. For visualization (isosurfaces with an inꢀ
crement of 0.035 a.u.) and analysis of the molecular orbitals
Zinc(II) [N,N´ꢀethyleneꢀbis(3ꢀmethoxysalicylidenediamine)]
1
monohydrate Zn(MO) •H O. Found (%): C, 52.5; H, 5.0; N, 6.8.
2
C H N O Zn. Calculated (%): C, 52.8; H, 4.9; N, 6.9. IR,
18
20
2
5
–
1
ν/cm : 3056, 3010, 2948, 2896, 2828 (ν(C—H)); 1660, 1644
(ν(C=N) + δ(O—H)H2O); 1600, 1546, 856, 726, 638, 584,
552 (Ar ring vibrations); 1170, 1102, 1070, 1044 (ν(C—O));
1
+ +
(
MO) of the systems under study, the ChemCraft program*
3400—3000 (ν(O—H)H2O). MS, m/z: 390 [Zn(MO) – H ] .
Zinc(II) [N,N´ꢀ(oꢀphenylene)ꢀbis(3ꢀmethoxysalicylidenediꢀ
was used.
1
2
N,N´ꢀEthyleneꢀbis(salicylidenediamine) H (SAL) . Found (%):
amine)] monohydrate Zn(MO) •H O. Found (%): C, 57.5;
2
2
C, 71.4; H, 5.9; N, 10.4. C H N O . Calculated (%): C, 71.6;
H, 4.6; N, 6.2. C H N O Zn. Calculated (%): C, 57.8; H, 4.4;
1
6
16
2
2
22 20
2
5
H, 5.9; N, 10.5. IR, ν/cm–1: 3052, 3010, 2956, 2930, 2900, 2870
N, 6.1. IR, ν/cm : 3046, 2924, 2810 (ν(C—H)); 1690,
1614 (ν(C=N) + δ(O—H)H2O); 1586, 1540, 862, 788, 738,
584, 560 (Ar ring vibrations); 1192, 1106, 1074, 1048
–1
(
ν(C—H) + (N—H)); 1636 (ν(C=N)); 1577, 858, 775, 742, 648,
5
1
8
62, 473 (Ar ring vibrations); 1372, 1284 (δ(O—H)); 1200, 1150,
1
042, 1021 (ν(C—O)). H NMR (CDCl /Me Si), δ: 13.20 (s, 2 H);
(ν(C—O)); 3600—3000 (ν(O—H)
[Zn(MO) – H ] .
). MS, m/z: 438
H O
2
3
4
2
+ +
.33, 7.29, 7.21, 6.93, 6.84 (all s, 2 H each); 3.90 (s, 4 H).
N,N´ꢀ(oꢀPhenylene)ꢀbis(salicylidenediamine) H (SAL) .
2
2
Found (%): C, 75.7; H, 5.1; N, 9.0. C H N O . Calculated
(
2
0
16
2
2
Results and Discussion
Synthesis and characterization. The Schiff bases
%): C, 75.9; H, 5.1; N, 8.9. IR, ν/cm–1: 3053, 2988, 2927, 2855,
2
6
1
8
712 (ν(C—H) + ν(N—H)); 1612 (ν(C=N)); 1562, 831, 761,
40, 581, 512 (Ar ring vibrations); 1362, 1277 (δ(O—H)); 1192,
1
150, 1044, 1028 (ν(C—O)). H NMR (CDCl /Me Si), δ: 13.05,
1
2
1
2
3
4
H L = H (SAL) , H (SAL) , H (MO) , and H (MO)
2 2 2 2 2
.59, 7.34, 7.33, 7.30, 7.19, 7.03, 6.89 (all s, 2 H each).
N,N´ꢀEthyleneꢀbis(3ꢀmethoxysalicylidenediamine) H (MO) .
studied in the present work were obtained following stanꢀ
1
2
19
dard procedures in high yields (~90%). The composition
Found (%): C, 65.6; H, 6.3; N, 8.6. C H N O . Calculatꢀ
1
8
20
2
4
–
1
H2L was confirmed by elemental analysis, IR spectroscoꢀ
ed (%): C, 65.9; H, 6.1; N, 8.5. IR, ν/cm : 3087, 3006, 2932,
1
py, and H NMR spectroscopy. The IR spectra exhibit
2
7
1
915, 2841, 2585 (ν(C—H) + ν(N—H)); 1633 (ν(C=N)); 837,
92, 731, 621 (Ar ring vibrations); 1324, 1250, 1081, 1048,
010 (δ(O—H)); 1190, 1170, 1132 (ν(C—O)). 1H NMR
the characteristic bands of the ν(C=N) (in the region
–1
–1
1640—1610 cm ), ν(O—H) (1400—1000 cm ), aromatꢀ
–
1
(
CDCl /Me Si), δ: 13.59, 8.33, 6.91, 6.85, 6.79 (all s, 2 H each);
ic ring, ν(C—H) and ν(N—H) (3100—2500 cm ) vibraꢀ
tions. The appearance of the ν(N—H) absorption bands in
the IR spectra and the presence of singlet signals at
3
4
3
.96 (s, 4 H); 3.89 (s, 6 H).
N,N´ꢀ(oꢀPhenylene)ꢀbis(3ꢀmethoxysalicylidenediamine)
2
H (MO) . Found (%): C, 69.7; H, 5.8; N, 8.2. C H N O .
1
2
22 20
2
4
δ ~8.33—8.62 in the H NMR spectra confirm the formaꢀ
–
1
Calculated (%): C, 70.2; H, 5.3; N, 7.5. IR, ν/cm : 3464, 3369,
058, 3014, 2955, 2925, 2837 (ν(C—H) + ν(N—H)); 1613
ν(C=N)); 736, 648, 584, 539 (Ar ring vibrations); 1400, 1378,
tion of hydrogen bonds O—H...N=C between the hydroxꢀ
3
(
2
5,26
yl hydrogen and the nitrogen atom.
Zinc(II) complexes were obtained by reactions of
zinc(II) acetate solution with corresponding Schiff base in
methanol at room temperature or on heating:
1
366, 1324, 1246 (δ(O—H)); 1205, 1094, 1040 (ν(C—O)).
1
H NMR (CDCl /Me Si), δ: 13.20, 8.62, 7.34, 7.23, 7.11, 7.02,
3
4
6
.88 (all s, 2 H each); 3.91 (d, 6 H).
Zinc(II) [N,N´ꢀethyleneꢀbis(salicylidenediamine)] monohyꢀ
1
drate Zn(SAL) •H O. Found (%): C, 54.8; H, 4.7; N, 8.1.
Zn(AcO) •2H O + H L → ZnL•nH O.
(1)
2
2
2
2
2
C H N O Zn. Calculated (%): C, 54.9; H, 4.6; N, 8.0. IR,
1
6
16
2
3
–
1
ν/cm : 3046, 3018, 2950, 2922, 2930, 2900, 2870 (ν(C—H));
2
1
In the case of H L = H (SAL) , H (MO) , and
2
2
2
1
8
1
654, 1634 (ν(C=N) + δ(O—H)H2O); 1596, 1550, 1530, 1514,
60, 850, 744, 740, 634, 608, 572 (Ar ring vibrations); 1184,
142, 1124, 1090 (ν(C—O)); 3300—2900 (ν(O—H)H2O). MS,
2
H (MO) , the composition of the products of reaction (1)
2
is independent of temperature and the ZnL•H O comꢀ
2
1
plexes are isolated. For H L = H (SAL) , reaction (1) at
1
+ +
2
2
m/z: 330 [Zn(SAL) – H ] .
1
room temperature results in Zn(SAL) •H O, whereas an
anhydrous complex [Zn(SAL) ] is formed under reflux.
Zinc(II)
[bisꢀ(N,N´ꢀethyleneꢀbis(salicylidenediamine)]
2
1
1
[
Zn(SAL) ] . Found (%): C, 58.5; H, 4.7; N, 8.5. C H N O Zn .
2
2
32 28
4
4
2
–
1
Calculated (%): C, 57.9; H, 4.2; N, 8.5. IR, ν/cm : 2898, 2846
ν(C—H)); 1530, 1540 (ν(C=N)); 1436, 946, 860, 734 (Ar ring
vibrations); 1184, 1130, 1125, 1090 (ν(C—O)). MS, m/z: 330
The IR spectra of the zinc complexes exhibit the chaꢀ
racteristic absorption bands of the ν(C—H), ν(C=N),
ν(C—O) stretching vibrations, and the aromatic ring viꢀ
brations. The presence of water molecules in the comꢀ
(
1
+ +
[
Zn(SAL) – H ] .
Zinc(II) [N,N´ꢀ(oꢀphenylene)ꢀbis(salicylidenediamine)] monoꢀ
pounds ZnL•H O is indicated by a broad O—H stretching
2
2
hydrate Zn(SAL) •H O. Found (%): C, 60.2; H, 4.2; N, 7.1.
C H N O Zn. Calculated (%): C, 60.5; H, 4.0; N, 7.1. IR,
ν/cm : 2920, 2820, 2898 (ν(C—H)); 1614 (ν(C=N) +
δ(O—H) ); 1586, 1530, 854, 800, 748, 602, 552 (Ar ring viꢀ
brations); 1170, 1152, 1126, 1034 (ν(C—O)); 3400—2800
2
–1
absorption band in the region 3500—3200 cm , which is
absent in the IR spectrum of [Zn(SAL) ] . The Xꢀray data
for Zn(SAL) •H O and [Zn(SAL) ] were reported earliꢀ
For the products formed in the reaction of zinc(II)
acetate with H(SAL) , a comparison was made of the theꢀ
oretical diffraction patterns calculated from the data of
20
16
2
3
1
–
1
2
1
1
2
2
H O
2
27,28
er.
2
+ +
1
(
ν(O—H)
). MS, m/z: 378 [Zn(SAL) – H ] .
H O
2
1
1
*
http://www.chemcraftprog.com
Xꢀray analysis for Zn(SAL) •H O and [Zn(SAL) ] with
2 2

Zinc(II) complexes with Schiff bases
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1883
I
a
b
I
1
2
1
2
3
13
23
33
43
53
63
73 2θ/deg
3
13
23
33
43
53
63
73 2θ/deg
1
1
Fig. 1. Experimental (1) and theoretically calculated (2) Xꢀray diffraction patterns of Zn(SAL) •H O (a) and [Zn(SAL) ] (b).
2
2
the experimental diffraction patterns of the complexes isoꢀ
lated (Fig. 1). For instance, the diffraction pattern of the
product of the roomꢀtemperature reaction (1) matches
For ZnL•H O (see Fig. 2) in the temperature range
2
20—150 °C, is characterized by slow weight loss correꢀ
sponding to elimination of water molecules. The tempeꢀ
rature, at which the second stage of weight loss beꢀ
1
the theoretically calculated pattern for Zn(SAL) •H O,
2
1
whereas the diffraction pattern of the compound obtained
under reflux matches that calculated for the dimer comꢀ
gins, decreases in the order Zn(SAL) •H O (320 °C) >
2
2
2
Zn(SAL) •H O (290 °C) > Zn(MO) •H O (230 °C) ≅
2
2
1
1
1
plex [Zn(SAL) ] .
Zn(MO) •H O (225 °C). For [Zn(SAL) ] , the stage
2
2
2
Thermal analysis. Thermal stability is an important
of slow weight loss is absent and transformations begin
at ~320 °C.
characteristic of the compounds to be used as the EL maꢀ
terials.2
,3,29
Usually, it is experimentally determined from
Photoluminescent properties. All the Schiff bases and
zinc(II) complexes based on them studied in the present
work emit in the visible region on the UV excitation
(Fig. 3). The position of emission maximum, the quantum
yield of luminescence, and the excitedꢀstate lifetime deꢀ
pend on the structure of CC and the nature of the organic
the weight loss curves. The thermal stability is often charꢀ
acterized by the temperature of the onset of weight loss.
Thermal analysis was carried out for the Schiff bases and
zinc(II) complexes (Fig. 2). The weight loss curves of H2L
have two portions, the total weight loss is 100%, and
the temperatures of the onset of weight loss decrease in the
ligand. The photoluminescence spectra of H L and zinc(II)
2
2
1
2
order H (MO) (250 °C) > H (MO) (210 °C) > H (SAL)
complexes recorded at the temperature of liquid nitrogen
exhibit a hypsochromic shift and a splitting of the emission
2
2
2
1
(
160 °C) > H (SAL) (100 °C).
2
Δm/m (%)
Δm/m (%)
a
b
0
0
5
3
2
4
6
8
0
0
0
0
2
3
20
4
4
2
40
60
80
1
1
1
00 200
300
400
500
600
Т/°C
100
200
300
400
500
600
Т/°C
1
2
Fig. 2. Weight loss curves of the Schiff bases (a) and zinc(II) complexes (b) under nitrogen atmosphere: a: H (SAL) (1), H (SAL) (2),
2
2
1
2
1
1
2
1
2
H (MO) (3), and H (MO) (4); b: [Zn(SAL) ] (1), Zn(SAL) •H O (2), Zn(SAL) •H O (3), Zn(MO) (4), and Zn(MO) (5).
2
2
2
2
2

1
884
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
a
Kotova et al.
b
2
2
20
18
16
ν~•10–3/cm–1
16 ν~•10–3/cm–1
2
6
24 22
20
18
2
Zn(MO) •H O
2
H (MO)2
2
1
Zn(MO) •H O
2
1
H (MO)
2
2
Zn(SAL) •H O
2
H (SAL)2
[Zn(SAL)1]2
2
1
1
H (SAL)
2
Zn(SAL) •H O
2
4
20
470
520
570
620
670
λ/nm
370
420
470
520
570
620
670
λ/nm
Fig. 3. Photoluminescence spectra of the Schiff bases (a) and the zinc(II) complexes (b) at –196 (solid lines) and 25 °C (dashed lines);
solid samples; λexc = 337 nm.
band into several components compared to the spectra
recorded at 25 °C (see Fig. 3).
orbitals with close energies, which are localized on differꢀ
ent fragments (enol and ketone) takes place; this leads
to narrowing of the energy gap between the HOMO
and LUMO (Fig. 6). Photoluminescence of the zinc(II)
complexes is also due to energy transfer between
the HOMO and LUMO of the deprotonated ligands with
the HOMO mainly being πꢀbonding in character and
the LUMO mainly being π*ꢀantibonding in character.
To sum up, the luminescence is caused by the π→π*ꢀtranꢀ
sition between the ligand orbitals (ligandꢀtoꢀligand charge
transfer).
The results of theoretical modeling and analysis of the
30,31
molecular orbitals (Figs 4 and 5) and the published
data
showed that the broad band in the photoluminescence
spectra of the Schiff bases is due to the energy transfer
between the highest occupied (HOMO) and the lowest
unoccupied (LUMO) MO, the HOMO and LUMO mainꢀ
ly being πꢀbonding and π*ꢀantibonding in character, reꢀ
spectively. Therefore, this band can be assigned to the
π→π*ꢀtransition. In addition, photoinduced proton transꢀ
3
2—38
Analysis of published data8,30,31,39—41 showed that both
conjugation enhancement and the introduction of elecꢀ
tronꢀdonating substituents should lead to a considerable
decrease in the energy difference between the HOMO and
LUMO. This is confirmed by the results of theoretical
modeling with evaluation of the frontier orbital enerꢀ
gies (see Fig. 6), viz., a replacement of the ethylenediꢀ
amine bridge by the oꢀphenylenediamine one in the pairs
fer is characteristic of excited Schiff base molecules.
The absorption of a light quantum results in transfer of
a proton in the enol—enol structure of the starting Schiff
base (Scheme 1, A) to form the enol—keto tautomer
(
see Scheme 1, B). The HOMO of the Schiff base in the
enol—keto form (B) is mainly localized on the fragment
containing the NH group (see Fig. 4). On going from the
form A to B (see Scheme 1), the HOMO is split into two
1
2
1
2
H (SAL) —H (SAL) , and H (MO) —H (MO) , as
2
2
2
2
well as introduction of the electronꢀdonating methꢀ
Scheme 1
1
1
oxy groups in the pairs H (SAL) —H (MO) , and
2
2
2
2
H (SAL) —H (MO) . A similar decrease in the energy
2
2
difference between the frontier orbitals under the action of
these two factors can be also observed for the zinc(II) comꢀ
plexes (Fig. 7) becausee their energy band structures are
mainly determined by the organic ligands. The theoretiꢀ
cally calculated changes in positions of the HOMO and
LUMO on conjugation enhancement and introduction of
electronꢀdonating groups agrees with the experimental
data, namely, the photoluminescence spectra of H L and
2
ZnL•H O exhibit a bathochromic shift of the emission
2
band maximum (see Fig. 3, Table 1).
It is known that deprotonation of organic ligands and
1
0
complexation with d ꢀmetals also considerably decrease

Zinc(II) complexes with Schiff bases
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1885
a
b
c
i
j
k
d
l
e
f
m
n
g
o
h
p
1
2
Fig. 4. The highest occupied (a—h) and lowest unoccupied (i—p) molecular orbitals of the Schiff bases H (SAL) (a, b, i, j), H (SAL)
2
2
1
2
(
c, d, k, l), H (MO) (e, f, m, n), H (MO) (g, h, o, p) in the enol—enol tautomeric forms (a, c, e, g, i, k, m, o) and in the enol—keto
2
2
forms (b, d, f, h, j, l, n, p) (obtained from B3LYP/6ꢀ311G(d,p) calculations).

1
886
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Kotova et al.
a
e
b
f
c
g
d
h
Fig. 5. The highest occupied (a—d) and lowest unoccupied (e—h) molecular orbitals of the zinc(II) complexes with Schiff bases
1
2
1
2
Zn(SAL) (a, e), Zn(SAL) (b, f), Zn(MO) (c, g), and Zn(MO) (d, h) (obtained from B3LYP/LANL2DZ/6ꢀ311G(d,p) calculations).
Table 1. Photoluminescence characteristics of the Schiff
bases and zinc(II) complexes
the energy gap between the HOMO and LUMO. The reꢀ
sults of theoretical modeling and comparison of the fronꢀ
tier orbital energies of the Schiff bases in the initial
enol—enol form (A), and the zinc(II) complexes confirm
this regularity (see Fig. 6 and 7). Therefore, the photoluꢀ
minescence spectra of the zinc(II) complexes should exꢀ
hibit a bathochromic shift of the emission maximum relaꢀ
tive to the spectra of the starting ligands. However, analyꢀ
sis of the experimental data revealed the opposite effect
Compound
λmax/nm ϕrel/rel. units
τobs/ns
H (SAL)1
501
564
571
597
436
540
538
457
570
0.04
1.00
0.01
0.06
0.69
0.11
0.10
0.21
0.03
0.9
2.2
0.4
0.7
2.0
1.5
0.1
1.2
0.2
2
2
H (SAL)
2
H (MO)1
2
2
H (MO)
2
1
Zn(SAL) •H O
2
(
see Table 1, Fig. 3). Analogous violations of regularities
Zn(SAL)1]2
[
42,43
have been documented
and explained by the intramoꢀ
2
Zn(SAL) •H O
2
lecular proton transfer in the excited molecule of the startꢀ
ing Schiff base (Fig. 8). This leads to the bathochromic
1
Zn(MO) •H O
2
2
Zn(MO) •H O
2
shift of the H L photoluminescence maximum. Complexꢀ
2

Zinc(II) complexes with Schiff bases
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1887
–
1.38
–1.59
Sn (n > 1)
E/eV
–
1.54
LUMO
–
1.78
–1.86
–
1.98
S1
–
2.02
–
2.17
i
3
.70
TS*
4
.52
3
.29
3
.96
3
.50
3.92
4
.84
A
4
.09
A
Fl
–
5.27
–
5.29
–
5.67
–
5.74
–6.11
TS
–
5.78
–
5.90
HOMO
–
6.38
S0
A
B
A
B
A
B
A
B
H L
2
H (SAL)1 H (SAL)2 H (MO)1 H (MO)2
2
2
2
2
Sn (n > 1)
Fig. 6. Theoretical frontier orbital diagrams of tautomeric forms
A and B of the Schiff bases.
–
1.48
E/eV
S1
–
1.66
–
2.03
LUMO
–
2.20
B
A
Fl
3
.55
3
.13
3
.81
3
.35
S0
ZnL
Fig. 8. Scheme of electronic transitions for photoluminescence
–
5.03
–
5.16
4
2,43
of the Schiff bases and zinc(II) complexes
: A is absorption,
–
5.55
HOMO
–
5.47
Fl is fluorescence, TS is transition state, and i is intramolecular
proton transfer in the excited molecule.
1
Zn(SAL)2
Zn(MO)1
Zn(MO)2
Zn(SAL)
Fig. 7. Theoretical frontier orbital diagrams of the zinc(II) comꢀ
plexes.
zinc(II) ion is a distorted square pyramid; however, this is
due to coordination of the water molecules in the first case
and to coordination of the oxygen atom of the neighboring
molecule and dimer formation in the second case. The
decrease in the gap width Δ(HOMO—LUMO) and the
bathochromic shift of the emission maximum in the phoꢀ
toluminescence spectra are apparently caused by the forꢀ
ation is accompanied by transformations of these tautoꢀ
meric forms and, as a consequence, by a hypsochromic
shift of the emission maximum in the photoluminescence
spectra of the zinc(II) complexes compared to the starting
Schiff bases.
1
1
Taking the zinc(II) complexes with H (SAL) as an
mation of binuclear clusters in the case of [Zn(SAL) ]
2
2
example, one can determine how their composition and
structure affect the photoluminescent properties. For inꢀ
and, as a consequence, by the change in the MO structures
and the nature of transitions. In particular, from the reꢀ
sults of theoretical modeling of cluster compounds of
1
stance, the formation of the dimeric complex [Zn(SAL) ]
2
1
0
causes a bathochromic shift of the emission maximum
d ꢀmetals with hydroxy/oxy bridges it follows that the
HOMO is still the πꢀbonding orbital of the ligand, whereas
the LUMO is the σ*ꢀantibonding orbital, which is mainly
due to the formation of the metal—oxygen bond and is
1
1
compared to Zn(SAL) •H O and the starting H (SAL)
2
2
(
see Table 1, Fig. 3). The change in positions of the emisꢀ
1
1
sion maxima of Zn(SAL) •H O and [Zn(SAL) ] is exꢀ
2
2
5
,44
plained by different electronic structures of these comꢀ
localized on the metal center.
Thus, in the case of
2
7,28
1
pounds due to the differences in the crystal structures.
In both complexes, the coordination polyhedron of the
[Zn(SAL) ] one likely deals with the change in the nature
2
of the energy transition, inducing the photoluminescence.

1
888
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Kotova et al.
Here, the emission is due to the charge transfer from the
ligand πꢀorbital to the 5sꢀorbital of the zinc(II) ion.
4. A. V. Metelitsa, A. S. Burlov, S. O. Bezuglyi, I. G. Borodkiꢀ
na, V. A. Bren´, A. D. Garnovskii, V. I. Minkin, Coordinats.
Khim., 2006, 32, 894 [Russ. J. Coord. Chem., 2006, 32, 858
Analysis of the relative quantum yields and photolumiꢀ
nescence decay times (see Table 1) revealed no regular
effect of the modification of organic ligands and complexꢀ
ation; only certain trends can be mentioned. For instance,
the introduction of a methoxy group leads to a decrease
in the relative quantum yields and the excitedꢀstate lifeꢀ
times of both H L and ZnL•H O, whereas replacement of
(
Engl. Transl.)].
5
6
. S.ꢀL. Zheng, X.ꢀM. Chen, Aust. J. Chem., 2004, 57, 703.
. S. Lamansky, P. Djurovich, D. Murphy, F. AbdelꢀRazzaq,
H.ꢀE. Lee, C. Adachi, P. E. Burrows, S. R. Forrest, M. E.
Thompson, J. Am. Chem. Soc., 2001, 123, 4304.
7. Y. W. Shi, M.ꢀM. Shi, J.ꢀC. Huang, H.ꢀZ. Chen, M. Wang,
X.ꢀD. Liu, Y.ꢀG. Ma, H. Xu, B. Yang, Chem. Commun.,
2006, 1941.
2
2
the ethylenediamine bridge by the oꢀphenylenediamine one
causes ϕrel and τobs to increase for the Schiff bases, but to
8
. C. PérezꢀBolivar, V. A. Montes, P. Anzenbacher, Inorg.
Chem., 2006, 45, 9610.
decrease for the zinc(II) complexes. Among the H L comꢀ
2
9
. R. Pohl, P. Anzenbacher, Org. Lett., 2003, 5, 2769.
pounds studied, the highest relative quantum yield was
1
1
0. S. Mizukami, H. Houjou, K. Sugaya, E. Koyama, H. Tokuꢀ
hisa, T. Sasaki, M. Kanesato, Chem. Mater., 2005, 17, 50.
1. M. E. Germain, T. R. Vargo, P. G. Khalifah, M. J. Knapp,
Inorg. Chem., 2007, 46, 4422.
2
observed for H (SAL) derived from oꢀphenylenediamine
2
and salicylaldehyde. A strong photoluminescence quenchꢀ
ing and decrease in the photoluminescence decay time
observed for the ethylenediamine and oꢀvanillin derivative
12. W.ꢀK. Lo, W.ꢀK. Wong, W.ꢀY. Wong, J. Guo, K.ꢀT. Yeung,
Y.ꢀK. Cheng, X. Yang, R. A. Jones, Inorg. Chem., 2006,
45, 9315.
1
(
H (MO) ) is probably due to the π—πꢀstacking in the
2
4
5,46
structure of this compound.
The complexation with
1
3. V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier,
R. Silbey, J.ꢀL. Brédas, Chem. Rev., 2007, 107, 926; Y. Shiꢀ
rota, H. Kageyama, Chem. Rev., 2007, 107, 953.
zinc(II) leads to the increase in ϕ and τobs for the ethylꢀ
rel
1
1
enediamine derivatives H (SAL) and H (MO) , whereas
2
2
a decrease in these values is observed for the oꢀphenyleneꢀ
1
1
4. P. G. Gozzi, L. S. Dolci, A. Garelli, M. Montalti, L. Prodi,
N. Zaccheroni, New. J. Chem., 2003, 27, 692.
5. T. Sano, Y. Nishio, Y. Hamada, H. Takahash, T. Usuki,
K. Shibata, J. Mater. Chem., 2000, 10, 157.
2
2
diamine derivatives H (SAL) and H (MO) .
2
2
In conclusion, we have studied the effect of conjugaꢀ
tion enhancement, introduction of electronꢀdonating subꢀ
stituent, and complexation on the photoluminescent and
16. S. H. Hwang, P. Wang, C. N. Moorefield, J.ꢀChe. Jung,
J.ꢀY. Kim, S.ꢀW. Lee, G. R. Newkome, Macromol. Rapid
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17. N. Yoshida, K. Ichikawa, M. Shiro, J. Chem. Soc., Perkin
Trans., 2000, 2, 17.
thermal properties of tetradentate Schiff bases (H L), saꢀ
2
1
licylaldehyde and oꢀvanillin derivatives H (SAL) ,
H (SAL) , H (MO) , and H (MO) , and the correspondꢀ
2
2
1
2
2
2
2
ing zinc(II) complexes. Peculiarities of the synthesis of the
zinc(II) complexes with Schiff bases have been considered.
Explanation of changes in the relative quantum yields and
the luminescence decay times of the modified Schiff bases
and complexes requires more detailed investigations inꢀ
volving recording of time resolved spectra, theoretical
modeling of excited states of molecules, determination of
the crystal structure of each compound, etc. Nevertheless,
the results obtained in this study allow the zinc(II) comꢀ
plexes obtained to be recommended as EL materials for
the design of OLED with allowance for their thermal staꢀ
bility and photoluminescent properties.
1
8. W. Lu, M. C. W. Chen, K.ꢀK. Cheung, Ch.ꢀM. Che, Orgaꢀ
nometallics, 2001, 20, 2477.
9. H. Schiff, Ann. Suppl., 1864, 3, 43.
0. M. J. O´Conner, B. O. West, Aust. J. Chem., 1967, 20, 2077.
1. G. E. Bately, D. P. Graddon, Aust. J. Chem., 1967, 20, 885.
1
2
2
22. C. de Mello Donegá, S. A. Junior, G. F. de Sa, Chem. Comꢀ
mun., 1996, 1199.
2
2
2
3. C. Lee, W. Yang, R. G. Parr, Phys. Rev. B, 1988, 37, 785.
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1
991, 113, 4917.
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000, 122, 1045.
2
2
2
2
7. D. Hall, F. H. Moore, J. Chem. Soc. (A), 1966, 1822.
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The authors are grateful to A. V. Mironov (M. V.
Lomonosov Moscow State University) for the measureꢀ
ment of the Xꢀray patterns of powdered zinc(II) complexes.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 07ꢀ02ꢀ00495a,
2
3
0
9ꢀ02ꢀ00546a, and 09ꢀ03ꢀ00850a).
3
1. S.ꢀL. Zheng, J.ꢀP. Zhang, X.ꢀM. Chen, Z.ꢀL. Huang, Z.ꢀY.
Lin, W.ꢀT. Wong, Chem. Eur. J., 2003, 9, 3888.
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Received September 11, 2007;
in revised form April 28, 2008
4