Thermally stable luminescent zinc-Schiff base complexes: A thiocyanato bridged 1D coordination polymer and a supramolecular 1D polymer

Article abstract of DOI:10.1016/j.poly.2013.08.009

A single thiocyanato bridged 1D polymeric zinc complex with dinuclear asymmetric unit [Zn₂L¹(μ_1,3-SCN)(SCN)] _n (1) and a dimeric zinc complex [Zn₂L²(NCS) (OOCCH₃)(CH₃O

Full text of DOI:10.1016/j.poly.2013.08.009

Polyhedron 65 (2013) 6–15
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Thermally stable luminescent zinc–Schiff base complexes: A thiocyanato
bridged 1D coordination polymer and a supramolecular 1D polymer
Monami Maiti a, Santarupa Thakurta a, Dipali Sadhukhan a, Guillaume Pilet b, Georgina M. Rosair c,
Aline Nonat d, Loïc J. Charbonnière d, Samiran Mitra a,
⇑
a Department of Chemistry, Jadavpur University, Raja S.C. Mullick Road, Kolkata 700 032, India
b Groupe de Crystallographie et Ingénierie Moléculaire, Laboratoire des Multimatériaux et Interfaces, UMR 5615 CNRS–Université Claude Bernard Lyon 1, Bât. Chevreul, 43 bd du 11
Novembre, 1918, 69622 Villeurbanne Cedex, France
c School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh EH14 4AS, UK
d Laboratoire d’Ingénierie Moléculaire Appliquée à l’Analyse, IPHC, UMR 7178 CNRS/UdS, ECPM Bât R1N0, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
A single thiocyanato bridged 1D polymeric zinc complex with dinuclear asymmetric unit [Zn2L1(l1,3
-
Article history:
SCN)(SCN)]n (1) and a dimeric zinc complex [Zn2L2(NCS)(OOCCH3)(CH3OH)2] (2) have been synthesized
using two different Schiff bases N,N/-bis(3-methoxysalicylidenimino)-1,3-diaminopropane (L1H2) and
N,N/-bis(3-ethoxysalicylidenimino)-1,3-diaminopropane (L2H2), respectively. All the ligands and the
complexes have been characterized by microanalytical and spectroscopic techniques. The structures of
the complexes have been conclusively determined by single crystal X-ray diffraction studies. Thermo-
gravimetric analysis has been performed to investigate their thermal stability. Finally, the photolumines-
cence properties of the complexes as well as their respective ligands have been investigated with a
comparative approach.
Received 9 April 2013
Accepted 4 August 2013
Available online 14 August 2013
Keywords:
Zinc(II)–Schiff base polymer
Crystal structure
Thiocyanate bridge
Supramolecular chain
Photophysical studies
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
devices as those three fundamental colors can be mixed to produce
the whole visible spectrum. Interestingly, there is an analogy be-
Zinc is an important transition metal in chemistry as well as in
biology. It plays an important role in cellular phenomena, e.g., en-
zyme catalysis [1], apoptosis [2] and neurotransmission [3]. In
addition, zinc is the second most abundant trace metal after iron
in biological systems and essential to all forms of life [4–7]. Its
d10 configuration permits a wide range of symmetries and various
coordination numbers, i.e., 4, 5 or 6, with suitable geometries (tet-
rahedral, square pyramidal/TBP and octahedral) which is very
important criterion for designing a wide variety of metal–organic
frameworks. Moreover, luminescent compounds are attracting cur-
rent research interest because of their vast applications in emitting
materials for organic light emitting diodes, light harvesting mate-
rials for photocatalysis and fluorescent sensors for organic or inor-
ganic analysts [8]. Hence it is important to understand the
coordination chemistry and luminescence between Group 12 me-
tal ions for developing new fluorescent sensors materials [8]. In
1987 Tang and VanSlyke [9] reported Al(q)3 (q: 8-hydroxyquinoli-
nyl) as the first bilayer organic light-emitting diode (OLED). Since
then, numerous attempts have been made in order to design green,
blue, and red luminescent complexes to develop full color RGB
tween the emission behavior of salicylaldehyde Schiff bases and
8-hydroxyquinoline since both the ligands have similar chromo-
phoric groups i.e., a hydroxy group, a coordinating imine N atom
and a delocalized
p system [10]. Therefore, zinc(II) complexes of
salen type Schiff bases are expected to show good luminescence
properties and the introduction of electron-donating groups into
the salicylidene moiety of Zn(II) complexes enhances the quantum
yield of photoluminescence [11]. Our research group has already
reported some luminescent complexes of different salicylaldimino
Schiff bases with zinc [12–15] as well as lanthanoids [16]. In
continuation of our earlier work, we have synthesized two N2O4
donor compartmental Schiff bases LH2 by the condensation of
one equivalent of 1,3-diamines and two equivalent of
3-alkoxysalicylaldehyde derivatives: N,N/-bis(3-methoxysalicyli-
denimino)-1,3-diaminopropane (L1H2) and N,N/-bis(3-ethoxysali-
cylidenimino)-1,3-diaminopropane (L2H2) (Scheme 1). Zinc
complexes of these ligands have been synthesized in presence of
NaSCN for the both cases to obtain compounds of the following for-
mulation [Zn2L1( 1,3-SCN)(SCN)]n (1) and [Zn2L2(NCS)(OOCCH3)
l
(CH3OH)2] (2).
The pseudohalide ion, SCNꢀ has coordinated the metal ions in
both terminal and bridging modes. While bridging the adjacent
metal centers thiocyanate furnishes various coordination modes
⇑
Corresponding author. Fax: +91 033 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.08.009

M. Maiti et al. / Polyhedron 65 (2013) 6–15
7
N
N
OHC
OH
reflux
+
H2N
NH2
OHHO
OR
OR
2 mmol
RO
1 mmol
LH2
-2 H+
R = CH3 for L1H2 and C2H5 for L2H2
N
O
N
O
O
O
R
R
L2-
Scheme 1. The synthetic routes and the mode of coordination of the Schiff base ligands.
as depicted in Scheme 2(A–E). In different bi and polynuclear com-
2-hydroxy-3-ethoxybenzaldehyde, 1,3-propylenediamine (i.e. 1,3-
diaminopropane) and sodium thiocyanate were purchased from
Aldrich Chemical Company. Zinc acetate dihydrate was purchased
from E. Merck, India.
plexes these bridging modes have been identified as
[17], 1,1-SCN (B) [18], 1,3-NCS (C) [19], 1,1,3-SCN (D) [20] and
1,1,1,3-SCN (E) [21]. The first three modes of bridging usually form
dinuclear complexes [17,22,23]. 1,1,3-SCN, 1,1,1,3-SCN and also
l1,1-NCS (A)
l
l
l
l
l
l
l
1,3-NCS modes are prone to form polynuclear 2D networks with
2.2. Physical measurements
double SCNꢀ bridging in presence of bidentate ligands [24–26]
and 1D chain structures with single or double SCNꢀ bridging in
presence of tri or tetradentate ligands [27,28]. The complexes re-
The Fourier Transform Infrared spectra were recorded in the
range 4000–400 cmꢀ1 on a Perkin-Elmer RX I FT-IR spectropho-
tometer with solid KBr pellets. C, H, and N microanalyses were car-
ried out with a Perkin-Elmer 2400 II elemental analyzer. 1H NMR
spectra of the Schiff base ligands and that of complex 2 were re-
corded on a Bruker 300 MHz FT-NMR spectrometer using trimeth-
ylsilane as internal standard in CDCl3 solvent. UV–Vis absorption
spectra were recorded on a Specord 205 (Analytik Jena) spectrom-
eter. Steady state emission and excitation spectra were recorded
on a Horiba Jobin Yvon Fluorolog 3 spectrometer working with a
continuous 450W Xe lamp. For the measurement in the solid state,
ported herein contain pendant SCNꢀ ligands. Moreover
l1,3-bridg-
ing SCNꢀ has lead to a single chain 1D polymer.
In this paper, we have reported the synthetic details, spectral
characterizations, thermal investigations, along with X-ray crystal
structures and photophysical studies of 1 and 2 where thiocyanate
acts as terminal ligand as well as end-to-end bridging in presence
of a potentially hexadentate Schiff base.
2. Experimental
the spectrometer was fitted with a Quanta-
U integrating sphere
from Horiba. Detection was performed with a Hamamatsu R928
photomultiplier. All spectra were corrected for the instrumental
functions. When necessary, a 370 nm cutoff filter was used to elim-
inate the second order artifacts. The ligand emission lifetime in
EtOH solution was measured in time-resolved mode, by monitor-
ing the decay at the maximum of the emission spectrum using a
Jobin Yvon FluoroHub single photon counting controller, fitted
with a 303 nm Jobin Yvon NanoLED. The decays were analyzed
with DataStation v2.4. The same set up was used for solid state life-
time measurements using 2 mm inner diameter quartz tube and
the front face configuration of the instrument. Luminescence quan-
tum yields in solution were measured according to conventional
procedures, with diluted solutions (optical density <0.05), using
2.1. Materials
All chemicals and solvents employed for the syntheses were
of analytical grade and used as received without further
purification. O-vanillin (i.e. 2-hydroxy-3-methoxybendaldehyde),
M
N
C
S
S
C
N
N
C
S
M
M
N
C
C
S
N
C
S
Rhodamine 6G in methanol (
are 15%.
U = 0.86) as Ref. [29]. Estimated errors
M
M
M
M
M
M
M
M
M
M
Solid state studies and quantum yields were determined using a
Hamamatsu Quantaurus-QY integrating sphere according to the
procedure described in Ref. [30a,b]. The luminescence quantum
D
E
A
B
Scheme 2. Different coordination modes of thiocyanate ligand.
yield U is given by:

8
M. Maiti et al. / Polyhedron 65 (2013) 6–15
H9 H9
H8 H8
H8 H8
H7 H7
H5
H6
H4
H5
H4
H3
H3
H2
N
N
N
N
OH7
HO
OH6
HO
O
OCH2CH3
2
O
OCH3
CH2
CH3
1
CH3
1
L2H2
L1H2
Scheme 3. Proton numbering scheme of Schiff base ligands.
Table 1
Crystal structure parameters of the complexes 1 and 2.
Parameters
1
2
Empirical formula
Formula weight
T (K)
C
21H20N4O4S2Zn2
C26H35N3O8SZn2
680.37
100(2)
587.31
150
Crystal system
Space group
a (Å)
orthorhombic
Pbca
orthorhombic
P2(1)2(1)2(1)
7.8386(9)
16.2279(17)
23.188(2)
2949.7(6)
4
1.532
1.748
1408
11.5131(4)
18.4518(7)
22.0191(7)
4677.7(3)
8
1.668
2.265
2384
b (Å)
c (Å)
V (Å3)
Z
Dcalc (Mg mꢀ3
)
l
(mmꢀ1
F (000)
)
h Range (°)
Total data
1.850–27.860
11002
2.51–27.59
71919
Unique data
5561
6782
Observation data [I > 2
R1[I > 2 (I)]
(I)]
r
(I)]
3395
6355
0.0225
0.0523
1.013
0.064
0.30, ꢀ0.41
r
0.0394
0.0418
1.1101
0.039
wR2[I > 2
r
Goodness of fit (GOF) on F2
Rint
Residuals (e Åꢀ3
)
0.54, ꢀ0.46
Fig. 1. 1H NMR spectra of L1H2, L2H2 and Complex 2 in CDCl3 solvent.
Eik ꢀ ð1 ꢀ AÞE0k
LeðkÞA
U
¼
ð1Þ
ð2Þ
Luminescence quantum yields in solutions were determined on
optically diluted samples (optical density <0.05) according to Eq.
(3) [30c]:
Where
L0ðkÞ ꢀ LiðkÞ
!
R
ꢀ
ꢁ
ꢀ
ꢁ
nx2
n2ref
IemxðkÞdðkÞ
o:d:ref
o:d:x
A ¼
L0ðkÞ
R
Ux
¼
Uref
ꢁ
ꢁ
ꢁ
ð3Þ
Iemref ðkÞdðkÞ
in which Ei(k) is the integrated luminescence of the film caused by
direct excitation, E0(k) is the integrated luminescence of the film
caused by illumination from the sphere, A is the absorbance of
the film, Le(k) is the integrated excitation profile from the empty
integration sphere (without the sample), L0(k) is the integrated exci-
tation profile when the sample is diffusively illuminated by the
integration sphere and Li(k) is the integrated excitation profile upon
direct excitation of the sample by the incident beam. The integrated
spectra were corrected for the wavelength sensitivity of the photo-
multiplier and the spectral response of the sphere using correction
functions furnished by the supplier.
in which the subscripts x and ref refer respectively to the sample
and the reference, n is the refractive index of the solution, Iem(k)
is the emitted intensity at wavelength k, and o.d. is the optical den-
sity of the sample.
2.3. Syntheses
2.3.1. Synthesis of the Schiff base ligands
Both the ligands were synthesized in similar ways as
previously reported [31]. L1H2 is the 1:2 condensation product of
Table 2
IR spectral data (cmꢀ1) of ligands L1H2 and L2H2 and complexes 1 and 2.
Compound
m(C@N)
m(C–OPhenolic
)
m
(O–H/H2O)
m
(Zn–N)
m
(SCNꢀ)
m
(COOꢀ)
L1H2
L2H2
1
1641
1640
1622
1627
1254
1260
1226
1224
3449
3448
–
–
–
454
439
–
–
–
–
–
2127–2109
2086
2
3432
1564–1413

M. Maiti et al. / Polyhedron 65 (2013) 6–15
9
1,3-diaminopropane with 3-methoxy-2-hydroxybenzaldehyde, and
L2H2 is the 1:2 condensation product of 1,3-diaminopropane with
3-ethoxy-2-hydroxybenzaldehyde. 1H NMR for L1H2 (d, 300 MHz):
2.11(t, J = 6.45, 2H8), 3.74 (t, J = 8.1, 2H7), 3.91 (s, 3H1), 6.79–6.95
(m, 1H2, 1H3, 1H4), 8.37 (s, 1H5), 13.94 (s, 1H6), ppm; 1H NMR for
L2H2 (d, 300 MHz): 1.27 (t, J = 6.98, 3H1), 2.09 (t, J = 6.7, 2H9), 3.73
(t, J = 6.5, 2H8), 4.12 (q, 2H2), 6.76–6.94 (m, 1H3, 1H4, 1H5), 8.37 (s,
1H6), 13.94 (s, 1H7), ppm (Scheme 3 and Fig. 1).
and 2, respectively, have been shifted considerably towards lower
frequencies compared to that of the free ligands indicating the coor-
dination of the imine nitrogen atom to the metal center [35]. Well de-
fined bands observed at 3449 and 3448 cmꢀ1 in the spectra of L1H2
and L2H2, respectively, are due to O–H stretching, which disappeared
in the spectra of 1 upon deprotonation of the O–H group during com-
plexation; while a band at 3432 cmꢀ1 in the spectrum of 2 may be
attributed to the presence of coordinated methanol molecules. The
phenolic C–O stretching bands at 1254 and 1260 cmꢀ1 in the spectra
of L1H2 and L2H2, respectively were shifted to 1226 and 1224 cmꢀ1 in
case of 1 and 2, respectively, supporting the deprotonation and coor-
dination of the phenolic oxygen donors to the metal centre. The li-
gand coordination to the metal centre were indicated by a band
appearing at 454 cmꢀ1 for 1 and at 439 cmꢀ1 for 2, which were
2.3.2. Synthesis of the complexes
2.3.2.1. [Zn2L1(
l1,3-SCN)SCN]n (1). Zinc acetate dihydrate (0.439 g,
2 mmol) was dissolved in 10 mL methanol. A methanolic solution
of the Schiff base (L1H2) (0.342 g, 1 mmol) was added to it followed
by drop wise addition of aqueous solution of NaSCN (0.162 g,
2 mmol). The mixture was allowed to stir for 40 min with gentle
heating. The bright yellow solution was filtered and kept in
refrigerator for crystallization by slow evaporation. After 2 days
colorless plate shaped single crystals suitable for X-ray
crystallography were obtained, yield 0.440 g (75%). Anal. Calc. for
mainly assigned to
teristic sharp
SCN bands at 2127–2109 cmꢀ1 in 1 and at 2086 cmꢀ1 in
2, respectively. In 1 the SCN band is bifurcated indicating two differ-
m(Zn–N) in each case. Both complexes show charac-
m
m
ent coordination modes of the SCNꢀ ligand. In 2, acetate ligands dis-
play asymmetric and symmetric stretching vibrations at 1564 and
1413 cmꢀ1, respectively. Thus the characteristic vibrational bands
of the above mentioned groups in complexes 1 and 2 give a superfi-
cial idea about their structures which are very much consistent to
their crystal structures.
C
21H20N4O4Zn2S2 (587.31): C, 42.91; H, 3.41; N, 9.53. Found: C,
42.95; H, 3.53; N, 9.43%. FT-IR (KBr, cmꢀ1):
(C@N) 1622,
(C–OPhenolic) 1226, (Zn–N) 454.
m
m
m
2.3.2.2. [Zn2L2(NCS)(OOCCH3)(CH3OH)2] (2). Compound 2 was pre-
pared in a similar fashion as in case of 1, using 1 mmol of L2H2
(0.370 g) in methanol as Schiff base ligand. Yellow needle shaped
single crystals suitable for X-ray diffraction were obtained after
three days. Crystals were isolated by filtration and were air dried,
yield 0.537 g (79%). Anal. Calc. for C26H35N3O8SZn2 (680.37): C,
45.86; H, 5.14; N, 6.17. Found: C, 45.77; H, 5.21; N, 6.15%. FT-IR
3.2. 1H NMR spectroscopy
1H NMR spectroscopy has been used to extract information
regarding the formation of the Schiff base ligands (L1H2 and L2H2)
and mode of coordination of the ligand L2H2 with the ZnII centers
in complex 2. We could not obtain NMR spectrum of complex 1
due to its insolubility and possibility of rupture of polymeric struc-
ture in conventional solvents. The NMR proton numbering scheme
of the ligands are represented in Scheme 3 and the corresponding
spectra of the ligands and complex 2 are represented in Fig. 1. In
the NMR spectra of the free ligands no broad peak was observed
in the region d 5.0 to d 8.0 ppm, indicating the absence of free –
NH2 functionality, supporting the formation of the ligands [36].
The broad peak at d ꢂ 13.94 ppm stands for the phenolic OH6 and
OH7 of the ligands L1H2 and L2H2, respectively. The peaks in be-
tween d = 6.76 and 6.95 ppm correspond to the aromatic protons
of the ligands. The protons H5 and H6 attached to the imino carbon
of L1H2 and L2H2, respectively are strongly downfield shifted
(d ꢂ 8.37 ppm) due to the influence of both phenolic –OH and imino
N groups in its close vicinity. The methylene protons (H7 for L1H2
and H8 for L2H2) being very close to the imino nitrogen, are
deshielded and appear as a 2H triplet at d ꢂ 3.7 ppm. The three
methyl hydrogens (H1) attached to the aromatic oxygen of L1H2 ap-
pear as a 3H singlet at d ꢂ 3.91 ppm but for L2H2 these hydrogens
are intervened by a –CH2 group and appear at upper field as a triplet
at d ꢂ 1.27 ppm. Additionally, spectrum of L2H2 contains a quartet
at d ꢂ 4.12 ppm due to the intervening –CH2 group protons (H2)
which are in the paramagnetic influence of the aromatic oxygen.
The complex 2 was also characterized by proton NMR (Fig. 1)
where it resembles that of the corresponding ligand except the fact
that signal of the phenolic OH proton which gets deprotonated
during complextation is missing. Moreover, it is clearly observed
that the signals of H3–H6 are broadened compared to the free li-
gand (L2H2) possibly due to spin–lattice relaxation caused by para-
magnetic impurities in the complex [37]. 1H NMR spectrum of 2
contains three additional singlet at d ꢂ 2.01, 3.48 and 5.29 ppm,
respectively. d ꢂ 2.01 ppm can be assigned to the three protons
of the coordinated CH3COOꢀ group, d ꢂ 3.48 and 5.29 ppm are
due to the three methyl proton and hydroxyl proton respectively,
of the coordinated CH3OH. The results obtained from the NMR
spectra are in agreement with the structure obtained by the X-
ray analysis.
(KBr, cmꢀ1):
asym(COOꢀ) 1564,
m
(C@N) 1627,
m
(C–OPhenolic) 1224,
m(O–H) 3432,
m
m
sym(COOꢀ) 1413,
m
(Zn–N) 439. 1H NMR (d,
300 MHz): 1.33 (t, J = 6.98, 3H1), 2.01 (s, 3H of CH3COOꢀ), 2.22 (t,
J = 6.7, 2H9), 3.48 (s, 3H of CH3OH), 3.90 (t, J = 6.5, 2H8), 4.22 (q,
2H2), 5.29 (s, OH of CH3OH), 6.65–6.91 (m, 1H3, 1H4, 1H5), 8.13
(s, 1H6), ppm (Scheme 3 and Fig. 1).
2.4. Crystallographic data collection and structure refinements
Intensity data were collected using Mo
Ka radiation
(k = 0.71069 Å) with a Nonius Kappa CCD diffractometer at 150 K
for 1 and on a Bruker X8 Apex2 diffractometer at 100 K for 2. Struc-
tures were solved by direct methods using the SIR97 [32] program
for 1 and by SHELXS 97 program [33] for 2. Structures were refined
by full-matrix least-squares methods with the program CRYSTALS
[34] for 1 and SHELXL 97 [33] for 2. In complex 2, one of the CH3 moi-
eties of the two ethoxy substituents is disordered over two posi-
tions with 60.0(5)% occupancy for the major component (C25A).
Selected crystallographic data, experimental conditions and rele-
vant features of the structural refinements for all the complexes
are summarized in Table 1.
3. Results and discussion
3.1. Fourier transform infrared spectra
Fourier transform infrared spectra of both complexes were ana-
lyzed and compared with those of the corresponding ligands L1H2
and L2H2 and the stretching vibrations are listed in Table 2. Strong
sharp absorption bands at 1641 and 1640 cmꢀ1 in the spectra of
L1H2 and L2H2, respectively, due to the C@N stretching indicate the
formation of the desired Schiff base ligands. Condensation of all the
primary amine groups have been confirmed by the absence of the
N–H stretching bands in the region 3150–3450 cmꢀ1. In the com-
plexes, the mC@N stretching vibrations at 1622 and 1627 cmꢀ1 in 1

10
M. Maiti et al. / Polyhedron 65 (2013) 6–15
Table 3
3.3. Crystal structure descriptions
Selected bond distances (Å) and bond angles (°) in 1 and 2.
3.3.1. [Zn2L1(
l1,3-SCN)(SCN)]n (1)
1
2
The X-ray structural analysis reveals that complex 1 has a zig-
zag polymer chain structure. A perspective view of the dimeric
asymmetric unit of 1 is presented in Fig. 2 and the bond distances
and angles around the metal centers are listed in Table 3. The
asymmetric unit of 1 contains a penta coordinated and a hexacoor-
dinated Zn ions (Zn1 and Zn2, respectively) connected by doubly
bridging phenoxo oxygens (O9 and O23) of a hexadentate Schiff
base ligand. There are three thiocyanate ligands, one pendant from
Zn2, and the other two bridge two adjacent molecular units form-
ing an undulating infinite chain. The Zn1. . .Zn2 separation (3.203 Å)
is within the range of previously reported complexes [38,39].
Zn1 is penta coordinated with a distorted square pyramidal
Bond distance (Å)
Zn1–N11
Zn1–N15
Zn1–O9
Zn1–O23
Zn1–S31
Zn2–N33
Zn2–N43
Zn2–O2
2.044(3)
2.037(3)
2.044(3)
2.040(3)
2.465(12)
1.983(4)
1.953(4)
2.610(3)
2.017(3)
2.027(3)
2.481(3)
Zn1–N10
Zn1–N14
Zn1–O8
Zn1–O22
Zn1–O33
Zn1–O35
Zn2–O1
2.0328(16)
2.0616(17)
2.0576(13)
2.0317(14)
2.1512(15)
2.3198(15)
2.6500(14)
2.0324(13)
2.0200(13)
2.5153(16)
1.9456(14)
1.9523(18)
Zn2–O8
Zn2–O9
Zn2–O23
Zn2–O24
Zn2–O22
Zn2–O23
Zn2–O29
Zn2–N1
Bond angles (°)
N11–Zn1–N15
N11–Zn1–O9
N11–Zn1–O23
N11–Zn1–S31
N15–Zn1–O9
N15–Zn1–O23
N15–Zn1–S31
O9–Zn1–O23
O9–Zn1–S31
O23–Zn1–S31
N33–Zn2–N43
N33–Zn2–O2
O9–Zn2–N33
N33–Zn2–O23
N33–Zn2–O24
N43–Zn2–O2
N43–Zn2–O9
N43–Zn2–O23
N43–Zn2–O24
O2–Zn2–O9
97.46(14)
89.67(12)
148.34(13)
101.87(10)
159.46(12)
88.97(12)
105.04(9)
75.28(11)
92.19(8)
106.31(8)
124.06(15)
77.33(13)
113.18(13)
111.69(13)
80.41(13)
86.85(13)
108.41(13)
113.32(13)
85.74(13)
66.69(11)
141.95(11)
146.98(11)
76.16(11)
145.72(11)
69.57(11)
N10–Zn1–N14
O35–Zn1–N10
O33–Zn1–N10
O22–Zn1–N10
O8–Zn1–N10
O8–Zn1–N14
O22–Zn1–N14
O33–Zn1–N14
O35–Zn1–N14
O8–Zn1–O22
O8–Zn1–O33
O8–Zn1–O35
O22–Zn1–O33
O22–Zn1–O35
O33–Zn1–O35
O1–Zn2–O8
O1–Zn2–O22
O1–Zn2–O23
O1–Zn2–O29
O1–Zn2–N1
O8–Zn2–O22
O8–Zn2–O23
O8–Zn2–O29
O8–Zn2–N1
100.74(6)
90.43(6)
90.26(6)
167.07(6)
90.35(6)
168.33(6)
90.43(6)
91.25(6)
88.24(6)
78.16(5)
92.36(6)
88.03(5)
96.07(6)
83.34(5)
179.21(5)
66.53(5)
145.50(5)
145.61(5)
74.89(5)
83.98(6)
79.01(5)
147.32(5)
109.95(6)
111.64(6)
68.81(5)
120.86(6)
108.33(7)
83.04(6)
84.57(6)
119.78(7)
geometry with Addison parameter,
s
= 0.022 (
s
= |bꢀ
a
|/60° where
b and are the two largest angles around the central atom.
a
s
= 0
for a perfect square pyramidal and 1 for a perfect trigonal bipyra-
midal geometry) [40]. The N2O2 chromophore of (L1)2ꢀ forms the
basal plane and the apical position is occupied by S31 end of a
bridging thiocyanate ligand. Zn1 is displaced by 0.410 Å towards
the apical S31 atom from the mean square plane passing through
N11, N15, O9 and O23. The second metal centre, Zn2 is tetrahedral
with
s
4 = 0.947 (s4 is the geometry index for four-coordinated
+ b)}/141°, where
complexes and is defined as
s
4 = {360° ꢀ (
a
a
and b – the two largest h angles in the four-coordinate species)
[41]. Zn2 is connected through the phenoxo O atoms of (L1)2ꢀ
(O9 and O23; average Zn–O 2.022 Å) and two thiocyanate N atoms,
N43 of a pendant SCNꢀ and N33 of a
l
-bridging SCNꢀ ligands.
There are two additional much longer (average 2.545 Å) connec-
tions to the methoxo O atoms (O2 and O24) of (L1)2ꢀ
.
The Zn1 atom of each asymmetric unit is connected to the Zn2
atom of an adjacent molecule by the basal–apical coordination
O2–Zn2–O23
O2–Zn2–O24
O9–Zn2–O23
O9–Zn2–O24
O23–Zn2–O24
mode of l
1,3-SCNꢀ ligand. Thus a single thiocyanato bridged undu-
lating molecular chain develops along the crystallographic a axis
(Fig. 3).
O22–Zn2–O23
O22–Zn2–O29
O22–Zn2–N1
O23–Zn2–O29
O23–Zn2–N1
O29–Zn2–N1
3.3.2. [Zn2L2(NCS)(OOCCH3)(CH3OH)2] (2)
Fig. 4 represents a perspective view of the asymmetric unit of
complex 2 with the atom labeling scheme. Selected bond lengths
and angles are summarized in Table 3. Complex 2 is a dinuclear
cluster with a very similar Zn–ligand chelate arrangement to com-
plex 1. However the geometry of Zn1 differs in both complexes. In
1 it is square pyramidal but it is octahedral in complex 2. The four
equatorial sites are occupied by two imine nitrogen (N10, N14) and
two phenoxo oxygen (O8, O22) atoms of (L2)2ꢀ. Two methanol
molecules are coordinated to Zn1 at trans-axial positions. Zn1 is
displaced by 0.082 Å from the equatorial plane towards the apical
oxygen atom O33. The twelve cis-angles [78.16(5)°–100.74(6)°]
show significant deviations from ideal octahedral bond angles
and the three trans-angles [167.07(6)°–179.21(5)°] less so. The
average equatorial bond distances (2.0459 Å) are smaller than
the average axial bond distances (2.2355 Å) (Table 3); these fea-
tures indicate a z-out type Jahn–Teller distortion from the ideal
octahedral geometry. The axial Zn–O lengths are quite different
with the longer Zn–O distance 2.3198(15) cf. 2.1512(15) Å belong-
ing to the methanol ligand which is involved with intra and inter-
molecular hydrogen bonding.
The second metal center (Zn2) has distorted tetrahedral geom-
etry (s4 = 0.846) [41] very similar to that in complex 1. The tetrahe-
dron around Zn2 is formed by the bridging phenoxo oxygens (O8,
O22) of the Schiff base ligand, the N1 end of a terminal thiocyanate
ligand and a monodentate (
two longer contacts to the ethoxo oxygens (O1, O23) of (L2)2ꢀ
g
1) CH3COOꢀ anion. Again there are
Fig. 2. Perspective view of the asymmetric unit of complex 1, H-atoms have been
removed for clarity.

M. Maiti et al. / Polyhedron 65 (2013) 6–15
11
Fig. 3. View of the polymeric arrangement in 1 running along axis a, H-atoms have been removed for clarity.
3.4. Photoluminescence studies
3.4.1. Absorption and emission of the ligands in EtOH
The UV–Vis absorption spectra of the Schiff-base ligands L1H2
and L2H2 were recorded at room temperature in EtOH solutions
(Fig. 6). The two spectra exhibit three distinct absorption bands
centered at 264 nm (
cmꢀ1 for L2H2), 330 nm
2660 Mꢀ1 cmꢀ1 for L2H2) and 421 nm (
L1H2 and 1050 Mꢀ1cmꢀ1 for L2H2). They can be assigned to
e -
= 13300 Mꢀ1 cmꢀ1 for L1H2 and 12050 Mꢀ1
(
e
= 2780 Mꢀ1 cmꢀ1 for L1H2 and
= 1360 Mꢀ1 cmꢀ1 for
e
p
?
p⁄ and n ? p⁄ transitions of the Schiff-base ligands. A similar
absorption spectrum had been previously recorded for the N,N/-
bis(salicylidene)-1,3-diaminopentane L0H2 [13], but the substitu-
tion of the phenol rings by methoxy groups in L1H2, or ethoxy
groups in L2H2, resulted in a 20 nm bathochromic shift of all the
absorption bands. Finally, a shoulder is also observed at ca.
295 nm with
e
= 4290 Mꢀ1 cmꢀ1 for L1H2 and 3570 Mꢀ1 cmꢀ1 for
L2H2.
Upon excitation into the UV domain at 274 nm, the two ligands
gave rise to broad fluorescence spectra with maxima pointing both
at 474 nm. The corresponding excitation spectra are dominated by
two intense bands at 274 and 350 nm (Fig. 7). Interestingly, the
strong excitation bands centred at ca. 350 nm do not correspond
to any of the transitions recorded in the absorption spectra. This
behaviour is typical of salen-based compound and has been corre-
lated to the presence of an equilibrium between the ketoamine and
the enolimine tautomers due to the formation of strong Nꢃ ꢃ ꢃH
bonds between the imino-phenolate moiety [43].
Fig. 4. Perspective view of the asymmetric unit of complex 2, H-atoms have been
removed for clarity.
(Table 3). A similar feature of longer methoxy contacts compared
to phenoxy is also seen in an alike trizinc methoxy phenoxy Schiff
base complex [42].
The emission spectra of L1H2 and L2H2 are presented in Fig. 8.
Interestingly, the emission maxima are bathochromically shifted
by 30 nm, in comparison with the ligand L0H2, as a result of the
incorporation of the electron donating methoxy or ethoxy group
on the salen skeleton. Similar absorption and emission maxima
have been previously reported for a tridentate Schiff base ligand
with a methoxy group in ortho position of the phenol moiety
[44]. The corresponding quantum yields have been determined in
EtOH with the high dilution method relative to rhodamine 6G
The dinuclear units of 2 are connected by bifurcated OHꢃ ꢃ ꢃO
hydrogen bonds which form a chain along the crystallographic a
axis. The hydrogen atom (H33) belonging to the methanol mole-
cule coordinated to a zinc metal ion (Zn1) of one dimer forms inter-
molecular hydrogen bond with the non-coordinated oxygen atom
(O32) of the carboxylate group coordinated to a zinc metal ion
(Zn2) of a neighboring dimer. This oxygen atom (O32) is also in-
volved in an intra-hydrogen bond with the hydrogen atom (H35)
belonging to a second methanol molecule, positioned on the same
side and coordinated to the second zinc ion (Zn1) (Fig. 5, Table 4).
(
U = 0.86) [29]. The measured quantum yields amount to
8.9 ꢁ 10ꢀ3 and 4.6 ꢁ 10ꢀ3 for L1H2 and L2H2, respectively and they

12
M. Maiti et al. / Polyhedron 65 (2013) 6–15
Fig. 5. Hydrogen bonded chain propagated along the crystallographic a axis.
Table 4
Hydrogen bond dimensions for complex 2.
D–Hꢃ ꢃ ꢃA
d(D–H) Å
d(Hꢃ ꢃ ꢃA) Å
d(Dꢃ ꢃ ꢃA) Å
<(DHA)°
O33–H33ꢃ ꢃ ꢃO32#1
O35–H35ꢃ ꢃ ꢃO32
0.860(16)
0.873(18)
1.826(18)
1.82(2)
2.659(2)
2.667(2)
163(3)
163(3)
Symmetry transformations used to generate equivalent atoms: #1: x ꢀ 1,y,z.
Fig. 7. Excitation spectra of the ligands L1H2 (red, kem = 474 nm) and L2H2 (blue,
kem = 478 nm) in EtOH, normalized on the maximum of the low energy excitation
band. (Colour online.)
3.4.2. Luminescence properties of the complexes
The photophysical properties of the complexes were examined
in the solid state. Selected data are summarized in Table 6. Upon
excitation of the ligand at ca. 350 nm, the two complexes display
broad emission spectra with maxima at 497 and 512 nm for 1
and 2, respectively (Fig. 9). The overall emission spectra are batho-
chromically shifted in comparison to the free ligands. Although
very weak, these luminescence emission spectra are typical for
salen-type Schiff–base Zn(II) complexes and can be assigned to
Fig. 6. UV–Vis absorption spectra of L1H2 (red) and L2H2 (blue) (Cligand = 2 ꢁ 10ꢀ4
M
in EtOH). (Colour online.)
are similar to the value previously obtained for the ligand N,N/-
bis(salicylidene)-1,3-diaminopentane L0H2 (9.3 ꢁ 10ꢀ3 in MeOH)
[13]. From these results, one can notice that L1H2 is more emissive
than L2H2, which is probably due to the replacement of the methyl
groups by ethyl groups, the later bringing additional vibrational
quenching from the C–H bonds of the alkyl chains. The lumines-
cent decays have been measured by Time Correlated Single Photon
Counting (TCSPC) Lifetime Spectroscopy with a NanoLED emitting
at 303 nm. The decay curves can only be conveniently fitted with
bi-exponential models, with lifetimes in the nanosecond range (Ta-
ble 5). These bi-exponential fluorescence decay profiles may be re-
lated to the presence of two species in solution, as for instance the
ketoamine and the enolimine isomers.
intraligand
p ?
p⁄ transitions [45,46]. A similar emission pattern
had been previously recorded for the polymeric Zn(II) complex
with ligand L0H2. However, the introduction of methoxy or ethoxy
substituents in the ortho positions of the phenol rings is responsi-
ble for a 60 nm red shift of the emission maxima, as recorded for
the free ligands. The corresponding excitation spectra are pre-
sented in Fig. 10 and show a strong excitation from the UV region
to 420 nm, as observed in the case of the free ligands. However,
saturation phenomena in the solid state prevented an accurate
determination of the maxima of excitation.
The luminescence quantum yields and lifetimes of the Zn(II)
complexes have been checked in the solid state (Table 6). 1 and

M. Maiti et al. / Polyhedron 65 (2013) 6–15
13
Fig. 8. Emission spectra of L1H2 (red, kexc = 342 nm) and L2H2 (blue, kexc = 358 nm)
in EtOH normalized according to their luminescence quantum yields. (Colour
online.)
Fig. 9. Normalized emission spectra of
kexc = 358 nm) in the solid state. (Colour online.)
1
(red, kexc = 342 nm) and
2
(blue,
Table 5
Photophysical properties of L1H2 and L2H2 in absolute EtOH solution at room
temperature.
Ligand Absorption kab/nm
(log[
/Mꢀ1 cmꢀ1])
Excitation
kmax/nm
Emission
kmax/nm (U) (Ai /%) [i = 1,2]
Lifetime si/ns
e
L1H2
264 (4.12), 330
(3.44), 421 (3.13)
264 (4.08), 330
(3.42), 421 (3.02)
342
359
474
s1 = 3.0 ns (71)
s2 = 5.6 ns (29)
s1 = 1.7 ns (39)
s2 = 5.4 ns (61)
(8.9 ꢁ 10ꢀ3
478
)
L2H2
(4.6 ꢁ 10ꢀ3
)
Table 6
Photophysical properties of the polymeric complexes 1 and 2 in the solid state.
Complex
Excitation
Emission
Lifetime
kex/nm
kem/nm
s
i/ns (Ai /%) [i = 1,2]
1
2
342
359
497
512
s
s
s
s
1 = 2.3 ns (58)
2 = 4.1 ns (42)
1 = 2.8 ns (43)
2 = 6.4 ns (57)
Fig. 10. Normalized excitation spectra of
kexc = 512 nm) in the solid state. (Colour online.)
1
(red, kem = 497 nm) and
2
(blue,
494 nm, close to the maximum observed in the solid state, which
indicated that the complex is stable in solution and also that com-
plexation drives the equilibrium between the ketoamine and enoli-
mine tautomers observed for the free ligand, towards the
enolimine isomer as single species. The corresponding excitation
spectrum is characterized by an intense band centered at
370 nm, which is in good agreement with the absorption spectrum
(Fig. 11).
However, the complex is not much stronger emissive in solu-
tion than in the solid state and a luminescence quantum yield be-
low 1% has been measured. Time decay profiles have also been
recorded and satisfactorily fitted to bi-exponential decays with
lifetimes of the same order of magnitude than the one measured
2 give rise to very weak emission bands and their quantum yields
are too low to be determined accurately (<1%). In contrast to other
highly luminescent Zn(salen) complexes [14], the weak lumines-
cent of 1 and 2 may be attributed to additional non-radiative deac-
tivation pathways, possibly through electron transfer phenomenon
resulting from the presence of isothiocyanate anions, as previously
observed for other coordination compounds [47,48]. Bi-exponen-
tial luminescence decays have been recorded and the correspond-
ing lifetimes are presented in Table 6. For each polymer, a short
component with a lifetime of ca. 2 ns has been measured, in agree-
ment with the lifetime reported for monometallic Schiff base Zn(II)
complexes (1.41–2.52 ns) [45,46]. In that case, a longer component
with a lifetime around 5–6 ns is also observed, in agreement with
the values measured for the free ligands.
for the free ligand in EtOH solution
2 = 9.48 ns, 33%).
(s1 = 0.58 ns, 67%;
s
The photophysical properties of complex 2 were also examined
in DMSO solutions (Fig. 11). The UV–Vis absorption spectra is
bathochromicaly shifted in comparison to the one of the free li-
gands and exhibits two main bands centered at 279 nm
3.5. Thermogravimetric analysis
(e
= 22460 Mꢀ1 cmꢀ1) and 370 nm
(
e
= 12050 Mꢀ1 cmꢀ1). Upon
The study of the thermal stability of the complexes is very
important in order to use them as emissive materials. The thermal
stability of the complexes 1 and 2 have been investigated using
excitation into the transitions of the Schiff-base ligand at
370 nm, a broad fluorescence is observed with a maximum at

14
M. Maiti et al. / Polyhedron 65 (2013) 6–15
Fig. 11. UV–Vis absorption spectrum of 2 (purple) in DMSO and corresponding excitation (red, kem = 494 nm) and emission spectra (blue, kex = 370 nm). (Colour online.)
TGA over a temperature range 25–600 °C in a single run in a dy-
namic nitrogen atmosphere.
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