Spectral and Luminescent Properties of N,N'-Bis(salicylidene)-1,4-Butylenediamine, N,N'-Bis(5-bromosalicylidene)-1,4-Butylenediamine, and Their Complexes with Zinc(II)

Article abstract of DOI:10.1134/S0018143920020034

Abstract: The spectral and luminescent properties of N₂O₂-type aromatic azomethines, N,N'-bis(salicylidene)-1,4-butylenediamine, N,N'-bis(5-bromosalicylidene)-1,4-butylenediamine, and their complexes with Zn(II), have been studied. A

Full text of DOI:10.1134/S0018143920020034

ISSN 0018-1439, High Energy Chemistry, 2020, Vol. 54, No. 2, pp. 87–94. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Khimiya Vysokikh Energii, 2020, Vol. 54, No. 2, pp. 97–104.
PHOTONICS
Spectral and Luminescent Properties of N,N'-Bis(salicylidene)-1,4-
Butylenediamine, N,N'-Bis(5-bromosalicylidene)-1,4-
Butylenediamine, and Their Complexes with Zinc(II)
a,
a
a
a
a
A. N. Borisov *, M. V. Puzyk , E. A. Posadskaya , L. P. Ardasheva , and V. N. Pak
a
Russian State Pedagogical University, St. Petersburg, 191186 Russia
*
e-mail: alexey-borisov@mail.ru
Received October 10, 2019; revised October 22, 2019; accepted October 22, 2019
Abstract—The spectral and luminescent properties of N O -type aromatic azomethines, N,N'-bis(sali-
2
2
cylidene)-1,4-butylenediamine, N,N'-bis(5-bromosalicylidene)-1,4-butylenediamine, and their complexes
with Zn(II), have been studied. All compounds at 293 K fluoresce in solutions and in a polycrystalline state.
An increase in the fluorescence intensity of the azomethines in alcohol solutions occurs upon their conver-
sion to the quinoid form. The main photoluminescent parameters of solutions of the compounds in DMF
and DMSO have been determined. Depending on the presence of bromoaryl substituents in the structure of
the ligands and the solvent properties, the zinc complexes fluoresce with quantum yields up to 50%.
Keywords: luminescence, azomethine bases, Zn(II) complexes
DOI: 10.1134/S0018143920020034
Coordination compounds of transition metals with nated azomethine base: the electronic character of
azomethine bases (Schiff bases) attract the attention of substituents in the aryl fragments, the structure of the
researchers and are considered as precursors for the diamine bridge, and the extension of the π-conjuga-
creation of new functional materials. In particular, tion system [7, 8]. Hence, the structural features of the
complexes of some d elements with aromatic azome- ligand environment, as well as the nature of the inter-
thines of N O -, NO-, and N -types ([M(Schiff)]) molecular interaction, determine the energy of the
2
2
4
possess luminescent [1, 2], catalytic [3], and sensory radiative transition and the luminescence efficiency of
properties [4], as well as the ability to form stable con- the zinc complexes in a polycrystalline form and solu-
ductive polymers [5].
tions.
It is known that some complexes of Zn(II) with
aromatic azomethines (Schiff bases), derivatives of
salicylic aldehyde, intensively fluoresce in the visible
spectral region and can be used to create the emission
layer of electroluminescent devices [6]. The spectral
This work considers the electronic absorption and
emission properties of tetradentate aromatic azome-
thines of N O -type N,N'-bis(salicylidene)-1,4-buty-
2
2
lenediamine (H (salbn-1,4)) and N,N'-bis(5-bromo-
2
and luminescent properties of [Zn(Schiff)] complexes salicylidene)-1,4-butylenediamine
(H (5Br-salbn-
2
are mainly determined by the nature of the coordi- 1,4)), and their complexes with Zn(II) (Scheme 1).
N
N
N
N
Zn
OHHO
O
O
X
X
X
X
(а)
(b)
Scheme 1. Structures of (a) azomethines and (b) their complexes with Zn(II): X = H (H (salbn-1,4) and [Zn(salbn-1,4)]);
2
X = Br (H (5Br-salbn-1,4) and [Zn(5Br-salbn-1,4)]).
2
87

8
8
BORISOV et al.
The choice of azomethine bases for the synthesis of 1,4) (0.27 g, 0.59 mmol, in 15 mL of ethanol) with stir-
the Zn(II) complexes is due to the limiting character ring and heating. A pale yellow precipitate was formed,
of the diamine fragment −(CH ) −, which limits the which was filtered off, washed with hot ethyl alcohol,
2
4
length of the internal conjugation system and favors and dried. The gross formula is C18
H N O Br Zn.
18 2 3 2
the tetrahedral structure of the coordination site The results of elemental analysis, calculated/found,
ZnN O ]. The luminescent properties of azome- %: С 40.34/40.26, Н 3.36/3.49, N 5.23/5.32. IR spec-
[
2
2
−
1
thines H (salbn-1,4) and H (5Br-salbn-1,4) and the trum, ν/cm : ν(С=N) 1623, ν(Ar−O) 1280, ν(H O)
2
2
2
corresponding zinc complexes have not been studied 3650−3350. The yield was 68%.
previously.
The IR transmission spectra of azomethines and
Zn(II) complexes were measured in the range of
–
1
4
000–400 cm (tablets with KBr) on a Shimadzu IR-
EXPERIMENTAL
Prestige 21 IR Fourier spectrometer.
The synthesis of azomethines H (salbn-1,4) and
2
The electronic absorption spectra (EAS) of solu-
tions of azomethine and Zn(II) complex were mea-
sured at 293 K in the range of 200–700 nm on a Shi-
madzu UV 2550 PC spectrophotometer.
H (5Br-salbn-1,4) was carried out under the condi-
tions of general acid catalysis with glacial acetic acid.
2
Synthesis of H (salbn-1,4). N,N'-bis(salicylidene)-
,4-butylenediamine was obtained by the condensa-
2
1
The luminescence spectra of polycrystalline sam-
ples, as well as solutions of Zn(II) complex in ethanol,
DMF, and DMSO at 293 K, were obtained using a
Flyuorat-02-Panorama spectrofluorimeter. The rela-
tion reaction of butylenediamine-1,4 (Merck, 99%,
0
3
.149 g, 1.69 mmol) and salicylic aldehyde (0.412 g,
.38 mmol, in 10 mL of ethanol). After heating and
stirring the resulting solution for 15 min, a lemon yel-
low precipitate was formed. The compound was fil-
tered, washed with cold ethyl alcohol and ether, and
dried. The results of elemental analysis (gross formula
C H N O ), calculated/found, %: C 72.97/72.84,
tive luminescence quantum yields (Ф ) were calcu-
rel
lated using the Parker formula [9]:
−
Aст
2
x
(
1 − 10 )S nx
A
Φ
rel = Φst
,
18
20
2
2
−
2
st st
x
−
1
(
1 − 10 )S n
H 6.76/6.85, N 9.46/9.54. IR spectrum, ν/cm :
ν(С = N) 1632, ν(Ar–O) 1285, ν(O–H) 2864. The
yield was 86%.
where Ф is the luminescence quantum yield of the
st
standard; S and S are the areas under the curves of
x
st
Synthesis of H (5Br-salbn-1,4). N,N'-bis(5-bro-
2
the true luminescence spectra of the sample and the
mosalicylidene)-1,4-butylenediamine was obtained
by the condensation reaction of butylenediamine-1,4
standard, respectively; A and A are the absorbances
x
st
of solutions at an excitation wavelength; and n and n
x
st
(
Merck, 99%, 0.039 g, 0.44 mmol) and 5-bromosali-
are the refractive indices of solutions. An aqueous
solution of fluorescein in 0.1 M NaOH (λ
cylic aldehyde (Aldrich, 98%, 0.18 g, 0.88 mmol, in
5 mL of ethanol). Upon mixing the reactants, a yel-
low solution was formed, which was then heated for
0 min. The yellow precipitate formed was filtered off,
washed with ethanol and ether, and dried. The gross
formula is C H N O Br . The results of
=
max
1
5
20 nm, Ф = 0.85 at 293 K [10]) was used as a stan-
st
dard. The accuracy of Ф determination was 10%.
rel
1
18
18
2
2
2
RESULTS AND DISCUSSION
Electronic absorption spectra of solutions of
H (salbn-1,4) and H (5Br-salbn-1,4) in ethanol,
dichloromethane, DMF, and DMSO are due to spin-
allowed transitions of different orbital nature. Table 1
presents the main parameters of the spectra.
elemental analysis,
calculated/found,
%:
С
4
7.58/47.46, Н 3.96/4.02, N 6.17/6.15. IR spectrum,
−1
2
2
ν/cm : ν(С=N) 1633, ν(Ar−O) 1281, ν(O−H) 2864.
The yield was 80%.
Synthesis of [Zn(salbn-1,4)] · H O. To obtain the
2
complex,
a
saturated aqueous solution of
The high-intensity absorption bands with maxi-
mums in the range of 215–220 and 250–250 nm cor-
respond to ππ* transitions in the aryl groups of azome-
thines [11]; their position is practically not affected by
a change in the spectroscopic solvent polarity param-
eter Z (Table 1, Fig. 2). The band with a maximum in
Zn(CH COO) ⋅ 2H O (0.17 g, 0.77 mmol) was added
3
2
2
to a solution of H (salbn-1,4) in ethanol (0.23 g,
2
0
.77 mmol, in 10 mL of alcohol). The resulting mix-
ture was stirred and heated. The pale yellow precipi-
tate formed was filtered off, washed with hot ethanol,
and dried. The gross formula is C H N O Zn. The
1
8
20
2
3
the region of 315 nm (326 nm for H (5Br-salbn-1,4))
is due to ππ* transitions in the azomethine groups
2
results of elemental analysis, calculated/found, %:
С 57.19/57.06, Н 5.30/5.42, N 7.41/7.50. IR spec-
[
12].
−
1
trum, ν/cm : ν(С=N) 1628, ν(Ar−O) 1278, ν(H O)
2
In solvents with high Z values (alcohols) and high
3
550−3350. The yield was 73%.
nucleophilicity (DMF, DMSO), an additional broad
band of medium intensity with a maximum in the
region of 400 nm appears in EAS of azomethines,
Synthesis of [Zn(5Br-salbn-1,4)] · H O. A satu-
2
rated aqueous solution of Zn(CH COO) · 2H O (0.13 g,
3
2
2
0
.59 mmol) was added to a solution of H (5Br-salbn- which corresponds to nπ* transitions in bipolar keto-
2
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SPECTRAL AND LUMINESCENT PROPERTIES
89
Table 1. Characteristics of EAS of azomethines and Zn(II) complexes in different solvents
C H OH,
DMSO,
Z = 71.1 kcal mol
DMF,
Z = 68.5 kcal mol
CH Cl ,
2 2
Z = 64.2 kcal mol −1
2
5
Z = 79.6 kcal mol−1
−1
−1
Compound
λmax, nm (logε)
λmax, nm (logε)
λmax, nm (logε)
λmax, nm (logε)
H (salbn-1,4)
215 (4.60); 255 (4.28);
315 (3.87);
[375] (3.63);
415 (3.33)
2
3
15 (3.78); [375] (2.90);
255 (4.36);
315 (3.90)
[
278] (3.79); 313 (3.79);
[403] (3.42)
402 (3.40)
H (5Br-salbn-1,4)
220 (4.74); 250 (4.26);
277] (3.5); 326 (3.79);
224 (4.83);
253 (4.31);
326 (3.92)
2
3
4
26 (3.89);
15 (3.04)
326 (3.58);
415 (2.36)
[
409 (3.34)
[
Zn(salbn-1,4)]
[259] (4.38); [325]
[272] (4.00); [321]
(3.45); 367 (3.80)
−
−
−
(3.25); 366 (3.72)
[
Zn(5Br-salbn-1,4)]
[ ]” Shoulder.
[269] (4.10);
[268] (4.10);
373 (3.91)
−
374 (3.90)
“
“
−” Solubility less than 0.05 mg/mL.
imine structures—quinoid forms [13, 14]. This band is azomethines (Schiff bases) and the corresponding
absent in acetonitrile and dichloromethane.
complexes [Zn(Schiff)] is due to radiative degradation
of short-lived (τ < 1 μs) singlet electronically excited
In contrast to azomethines, the zinc complexes are
soluble only in DMF and DMSO and have very lim-
ited solubility in ethanol (<0.1 mg/mL). The low sol-
1
states of (π*−π) type.
Apparently, the necessary condition for fluores-
ubility of such complexes is explained by the formation cence of azomethine bases is the presence of aryl-type
of cluster structures at the expense of intermolecular hydroxy groups in their structure. We showed in our
donor–acceptor bonds Zn–O–Zn [15]. The solving experiments that azomethines—derivatives of benzal-
action of DMF and DMSO is obviously due to coor- dehyde, 3-methoxybenzaldehyde, and 2-aminobenz-
dination of solvent molecules, coupled with destruc- aldehyde, not containing aryl hydroxy groups, do not
tion of intermolecular bonds.
luminesce in the solid form and in solutions.
Upon complexation, the position of the bands cor-
Fluorescence is characteristic of aromatic azome-
responding to ππ* transitions in the aryl groups thines of the N O type in the quinoid form, which is
2
2
remains practically unchanged. At the same time, the
formation of the coordination site leads to the destruc-
tion of keto-imine structures and, hence, disappear-
ance of the “quinoid” band, as well as an insignificant
bathochromic shift of the “azomethine” band.
Compared to azomethines, an additional band of
medium intensity with a maximum at 367 nm appears
in EAS of the complexes. In the literature [16], infor-
mation is available about its assignment to ligand-to-
formed upon excitation as a result of photoinduced
proton transfer to the nitrogen atom of the imino
group [19]. However, when considering the photo-
chemical mechanism of the formation of the lumines-
cent tautomer, it should be kept in mind that crystal-
line azomethines initially contain an insignificant
amount of the quinoid form not associated with pho-
toinduced proton transfer. In particular, in the X-ray
photoelectron spectra of azomethines of the N O
2
2
metal charge transfer (LMCT), which is consistent type, low-intensity components are always present
with the tetrahedral structure of the coordination site. corresponding to quinoid oxygen and nitrogen with
The LMCT bands, as a rule, shift bathochromically localized and delocalized positive charge [20], which
with increasing the solvent parameter Z [17]; however, disappear upon complexation.
due to the low solubility of the complexes, this feature
could not be verified.
At 293 K, the luminescence spectra of polycrystal-
line samples of H (salbn-1,4) and H (5Br-salbn-1,4)
2
2
At 293 K, the studied compounds luminesce in represent broad unstructured bands with maximums
polycrystalline form and in solutions. According to the at 493 and 510 nm, respectively (Fig. 2, curves 1, 2).
literature data [18], photoluminescence of aromatic The observed bathochromic shift in the fluorescence
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9
0
BORISOV et al.
8
5
1
60
40
20
6
4
2
0
2
4
3
250
300
350
400
450
λ, nm
200
300
400
λ, nm
Fig. 1. Electronic absorption spectra of H (salbn-1,4) in (1) CH CN, (2) C H OH, (3) CH Cl , (4) DMF, and (5) DMSO. С =
2
3
2
5
2
2
0.1 mg/mL, 293 K.
maximum of H (5Br-salbn-1,4) is due to the partici- tion of a luminescent tautomer as a result of photoin-
2
duced proton transfer.
pation of bromoaryl substituents in the conjugation of
p orbitals, which reduces the energy of the radiative
transition. Unlike azomethines, the fluorescence
bands of solid samples of the complexes are multicom-
ponent: 422, 475, and 518 nm for [Zn(salbn-1,4)] and
As an example, Fig. 3 shows the dynamics of
changes in the EAS of an ethanol solution of azome-
thine H (salbn-1,4) upon the gradual addition of
2
alkali. The concentration of NaOH in alcohol was
chosen so as to minimize the effect of dilution, and the
total amount of NaOH added was equivalent to depro-
tonation of both hydroxy groups. The gradual increase
in the absorption in the region of 400 nm correspond-
4
43, 495, and 515 nm for [Zn(5Br-salbn-1,4)] (Fig. 2,
curves 3, 4). Presumably, the first two components of
the emission bands of the complexes have a vibrational
nature and correspond to stretching vibrations of C=C
bonds in aryl groups, and the additional splitting of the ing to nπ* transition in the quinoid form unambigu-
radiative level is due to specificity of intermolecular ously indicates an increase in its concentration. The
interaction in the crystalline state.
decrease in the intensity of the bands with maximums
at 215 and 255 nm indicates a decrease in the aroma-
ticity of the system, and the decrease in the intensity of
Since luminescence of azomethines is due to the
presence of the quinoid form, it should be expected
that an increase in its content will increase the inten-
sity of emission bands. To convert azomethines to the
the ππ* transition band in the imino group (λ
=
max
3
13 nm) indicates the participation of this group in
stabilization of the anion formed. The changes in EAS
quinoid (salt) form, we used deprotonation of aryl of azomethine that occur upon its deprotonation are in
hydroxy groups with alkali in an alcohol solution. good agreement with Scheme 2 of delocalization of
Obviously, this excludes the possibility of the forma- the negative charge in the anions formed:
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SPECTRAL AND LUMINESCENT PROPERTIES
91
1
2.0
×0.5
1
.5
2
4
1.0
3
0.5
0
400
500
λ, nm
600
Fig. 2. Luminescence spectra of polycrystalline samples at 293 K of (1) H (salbn-1,4), (2) H (5Br-salbn-1,4), (3) [Zn(salbn-
2
2
1
,4)], and (4) [Zn(5Br-salbn-1,4)]. λ = (1, 2) 420 and (3, 4) 337 nm.
ex
(
CH ) ...¼
(CH ) ...¼
(CH ) ...¼
2 n
2
n
2 n
N
N
O
N
O
H
+
OH−
X
X
O
X
X
X
X
(I)
(II)
(CH ) ...¼
(CH ) ...¼
(CH ) ...¼
2 n
2
n
2 n
N
O
N
O
N
O
(V)
(IV)
(III)
Scheme 2. Stabilization of the anion of azomethine base in an alcohol solution upon an addition of alkali (X = H or Br).
The addition of alkali causes not only changes in decrease in the degree of ππ stacking interaction, and
EAS of alcohol solutions of azomethines, but also a coordination of solvent molecules.
predictable increase in the intensity of the excitation
For solutions of the complexes, the positions of the
and luminescence spectra (Fig. 3, inset).
maximums and the shape of the emitting bands are
The studied zinc complexes intensively luminesce independent of the energy of the excitation light, and
in dilute solutions in DMF and DMSO. Compared to the fluorescence excitation spectra of the complexes
solid samples, the fluorescence spectra of solutions are in good agreement with the spin-allowed optical
lose their multicomponent character, which is proba- transitions in EAS (Fig. 4, curves 1, 2). This confirms
bly due to destruction of the cluster structures, a the assignment of luminescence of the studied com-
HIGH ENERGY CHEMISTRY Vol. 54 No. 2 2020

9
2
BORISOV et al.
A
1.6
I, rel. units
0
.2
1
.2
.8
.4
0
0
.1
0
0
0
300
400
500
600
λ, nm
300
400
λ, nm
Fig. 3. Dynamics of changes in EAS, excitation, and luminescence spectra (inset) of H (salbn-1,4) in ethanol solution (C =
2
–3
0
.05 mg/mL) upon the addition in 0.2-mL portions of 1.0 × 10 M NaOH solution in C H OH.
2 5
plexes to radiative transitions from the lowest-energy responsible for nonradiative deactivation of electroni-
1
electronically excited state of the (π*–π) type. The cally excited states. The decrease in the values of Фrel
probability of internal conversion from the higher to in the case of bromo-substituted azomethine and the
lowest electronically excited state is close to unity.
The values of the luminescence quantum yields
complex is due to the fact that the presence of heavy
bromine atoms increases the rate of the processes of
nonradiative energy degradation.
Ф
for dilute solutions of the azomethines and the
rel
complexes are given in Table 2. It follows from the data
that passing to Zn(II) complexes leads to a significant
CONCLUSIONS
increase in the average value of Ф : in the case of a
rel
solution of [Zn(salbn-1,4)] in DMSO, up to 52%. The
decrease in the efficiency of nonradiative processes on
passing to [Zn(Schiff)] complexes is due to the more
In this work, the spectral and luminescent proper-
ties of the azomethine bases H (salbn-1,4) and
2
2
H (5Br-salbn-1,4) and their complexes with zinc have
been studied. It has been found that the conversion of
the azomethines to the salt (quinoid) form leads to a
“
rigid” structure of azomethines in the coordinated
state.
On the other hand, the conformational “flexibil- significant increase in the fluorescence quantum
ity” of the aliphatic diamine bridge favors the tetrahe- yield. The high luminescence quantum yields in the
dral structure of the coordination site [ZnN O ]. This, blue-violet spectral region of solutions of the Zn(II)
2
2
in turn, hinders the stacking interaction of complex complexes (~50%) are determined by the nature of the
molecules and the appearance of additional ππ states ligand environment and the solvent.
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SPECTRAL AND LUMINESCENT PROPERTIES
93
A
I, rel. units
3
2
0
.4
.3
.2
3
0
0
2
1
1
4
0.1
5
0
300
400
500
600
λ, nm
Fig. 4. (1) Luminescence excitation and (2) absorption spectra of a solution of [Zn(salbn-1,4)] in DMSO (293 K). Luminescence
spectra (λ = 370 nm, 293 K) of [Zn(salbn-1,4)] in (4) DMSO (C = 2.5 × 10– M) and (5) ethanol (saturated solution), and
5
ex
(
3) fluorescein in 0.1 M NaOH/H O.
2
Table 2. Some parameters of photoluminescence of azomethines and Zn(II) complexes
λmax, nm (Фrel)
Solvent/Compound
H (salbn-1,4)
H (5Br-salbn-1,4)
[Zn(salbn-1,4)]
[Zn(5Br-salbn-1,4)]
2
2
Polycrystalline sample
DMF
DMSO
493 (−)
515 (−)
422, 475, 518 (−)
450 (0.01)
445 (0.52)
443, 495, 515 (−)
452 (0.05)
453 (0.05)
452 (<0.01)
453 (0.03)
455 (0.02)
455 (<0.01)
0.32
0.18
−
C H OH
445 (0.21)
2
5
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Translated by A. Tatikolov
HIGH ENERGY CHEMISTRY
Vol. 54
No. 2
2020