Photoluminescent tetrahedral d10-metal Schiff base complexes exhibiting highly ordered mesomorphism

Article abstract of DOI:10.1016/j.poly.2015.11.053

A series of four-coordinate d¹⁰-metal complexes of the type [ML₂] {M = Zn, Cd, Hg; L = 4-nitro-2-((octadecylimino)methyl)phenol}, incorporating a new N-alkylated bidentate [N,O]-donor salicylaldimine Schiff base ligand, has been synthesized and characterized by elemental analyses, FT-IR, UV-Vis, ¹H NMR and FAB-mass spectroscopies. The ligand is non-mesomorphic and devoid of any photoluminescence. The zinc(II) and cadmium(II) complexes displayed highly ordered mesophases reminiscent of soft crystals. The phases have been characterized by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD) studies. The complex of mercury(II) decomposed prior to melting. An orthogonal symmetry with a 'herringbone' array for the zinc complex and a primitive triclinic symmetry (p₁) for the cadmium complex, respectively, has been proposed. The complexes exhibited fluorescence at room temperature, both in the solution and in the solid state, with emission maxima in the blue region. Density functional theory (DFT) calculations carried out using the gaussian 09 program at the B3LYP level revealed a distorted tetrahedral geometry around the metal center in all the complexes. Natural bond orbital (NBO) analysis suggested appreciable charge transfer from the ligand to the metal center in the complexes.

Full text of DOI:10.1016/j.poly.2015.11.053

Polyhedron 105 (2016) 150–158
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Polyhedron
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Photoluminescent tetrahedral d10-metal Schiff base complexes
exhibiting highly ordered mesomorphism
Sutapa Chakraborty a, Debraj Dhar Purkayastha a, Gobinda Das a,1, Chira R. Bhattacharjee a,
,
⇑
Paritosh Mondal a, S. Krishna Prasad b, D.S. Shankar Rao b
a Department of Chemistry, Assam University, Silchar 788011, Assam, India
b Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India
a r t i c l e i n f o
a b s t r a c t
A series of four-coordinate d10-metal complexes of the type [ML2] {M = Zn, Cd, Hg; L = 4-nitro-2-((octade-
cylimino)methyl)phenol}, incorporating a new N-alkylated bidentate [N,O]-donor salicylaldimine Schiff
base ligand, has been synthesized and characterized by elemental analyses, FT-IR, UV–Vis, 1H NMR and
FAB-mass spectroscopies. The ligand is non-mesomorphic and devoid of any photoluminescence. The
zinc(II) and cadmium(II) complexes displayed highly ordered mesophases reminiscent of soft crystals.
The phases have been characterized by polarizing optical microscopy (POM), differential scanning
calorimetry (DSC) and powder X-ray diffraction (PXRD) studies. The complex of mercury(II) decomposed
prior to melting. An orthogonal symmetry with a ‘herringbone’ array for the zinc complex and a primitive
triclinic symmetry (p1) for the cadmium complex, respectively, has been proposed. The complexes
exhibited fluorescence at room temperature, both in the solution and in the solid state, with emission
maxima in the blue region. Density functional theory (DFT) calculations carried out using the GAUSSIAN
09 program at the B3LYP level revealed a distorted tetrahedral geometry around the metal center in
all the complexes. Natural bond orbital (NBO) analysis suggested appreciable charge transfer from the
ligand to the metal center in the complexes.
Article history:
Received 8 October 2015
Accepted 25 November 2015
Available online 12 December 2015
Keywords:
Schiff base
Mesophase
Luminescence
X-ray diffraction
Density functional theory
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
to access multifunctional materials [14,15]. Amongst the metals
used, lanthanides have widely been employed in synthesizing
Metal-salicylaldimines have been recognized as an important
class of coordination compounds with applications in diverse fields
ranging from agriculture to optoelectronics [1–3], antimicrobial to
therapeutics [4,5], polymers to dyes [6,7] and drugs to catalysts [8–
10]. Besides, such compounds currently have also earned a place of
interest as liquid-crystalline materials [11]. As for salicylaldimines,
the ease of synthesis as well as the flexibility of the coordination
environment around the imine moiety renders them a versatile
class of ligands in coordination chemistry [9,10].
Materials that are photo-responsive and possess highly ordered
fluid-phases are currently enjoying much interest in applications
related to OLEDs, information storage, sensors, lasers and
enhanced contrast displays, etc. [12,13]. The luminescence proper-
ties of metal complexes coupled with the self-organizing ability of
liquid crystals (LCs) are of abiding interest, offering a viable option
luminescent metallomesogens [16,17]. Luminescence of metal-
lomesogens incorporating d-block metals such as palladium(II)
[18,19], platinum(II) [20,21], iridium(III) [22,23], nickel(II)
[24,25], rhenium(I) [26], gold(I) [27,28], silver(I) [28,29], zinc(II)
[30,31] and copper(I) [32] have also attracted a lot of research
attention. It is pertinent to mention here that complexes of zinc
(II) and its congeners cadmium(II) and mercury(II) display interest-
ing fluorescent properties that originate from a ligand-centered
charge transfer or metal-centered luminescent levels, providing
access to newer functional materials [33,34].
Salicylaldimine based metallomesogens have been accessed
mostly with palladium(II) [35], copper(II) [36,37], oxovanadium
(IV) [37,38], nickel(II) [38], iron(III) [39] or manganese(III) [40].
While examples of zinc are scarce [30,31,41], those that incorporate
cadmium and mercury ions appear to be virtually non-existent.
Apart from a very early account of mesomorphic cadmium alka-
noates [42] or with porphyrins as ligands [43], only sporadic
mention of cadmium based mesogens has been made in the litera-
ture. An unpublished reference to uncharacterized cadmium and
⇑
Corresponding author. Tel.: +91 03842 270848; fax: +91 03842 270342.
E-mail address: crbhattacharjee@rediffmail.com (C.R. Bhattacharjee).
Present address: Polymers and Advanced Materials Laboratory, National Chemical
1
Laboratory, Pune 411008, India.
http://dx.doi.org/10.1016/j.poly.2015.11.053
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

S. Chakraborty et al. / Polyhedron 105 (2016) 150–158
151
mercury complexes of di-thiobenzoate with ill-defined phases was
made in 1993 [44]. Aquated cadmium ions, [Cd(H2O)4]2+, upon
complexation with non-ionic oligo(ethylene oxide) surfactants,
have been reported to form liquid crystalline phases [45,46].
Further, another interesting variety of complexes, tetrachloromet-
allates [MCl4]2À (M = Zn and Cd), with n-alkylpyridinium as a coun-
ter-cation were reported to exhibit smectic mesomorphism [47].
Mesomorphic zinc complexes were previously reported to be either
square planar or trigonal-bipyramidal [30,48]. The zinc metal ion
usually prefers a tetrahedral geometry, which often results in the
loss of the mesomorphism [49,50]. Tetrahedral zinc complexes
exhibiting mesomorphism have appeared only in the current dec-
ade and are limited to only a few examples [40,50–54], and the ones
that are both luminescent and liquid crystalline are rare. In fact
there appears to be only a couple of instances in the literature of
the latter type [41,51–53]. As against tetradentate ‘salen’ type
ligands, which enforce a planar geometry, bidentate ligands with
long alkyl arms, giving greater flexibility, allow the metal ion to
acquire the thermodynamically stable tetrahedral geometry,
ensuring enhanced order in the molecular self-assembly.
Supplies Ltd, UK). The XRD apparatus (X’Pert PRO MP, PANalytical),
employing Cu (l = 0.15418 nm) radiation, consisted of
K
a
a
focusing elliptical mirror for beam preparation optics providing a
well-focused line beam, a fast high resolution multi-channel solid
state detector (PIXCEL) and operated at 45 kV and 30 mA rating.
Collimation with 20 mrad Soller slits on the input as well as the
diffracted beam side provides very good vertical resolution.
Quantum chemical calculations on the zinc(II), cadmium(II) and
mercury(II) complexes were carried out using the GAUSSIAN 09
program at the B3LYP level.
2.3. Synthesis and characterization
2.3.1. Synthesis of 4-nitro-2-((octadecylimino)methyl)phenol, HL
A
methanolic solution of 5-nitro salicylaldehyde (0.84 g,
5 mmol) was added to a methanolic solution of octadecylamine
(1.35 g, 5 mmol). The solution mixture was heated under reflux
with a few drop of acetic acid as a catalyst for 3 h to yield the
yellow Schiff base. The compound was collected by filtration and
re-crystallized from methanol.
Accordingly, we report here the synthesis of a series of lumines-
cent tetrahedral zinc(II), cadmium(II) and mercury(II) complexes of
a new flexible one ring N-alkylated [N,O]-donor rod shaped salicy-
laldimine Schiff base ligand. The ligand is non-mesomorphic and
lacks any fluorescence. Except for the mercury(II) complex, which
decomposes before melting, the zinc(II) and cadmium(II) com-
plexes exhibit highly ordered mesophases.
Yield: ꢀ1.81 g (82.49%). Anal. Calc. for C25H42N2O3 (418.63): C,
71.66; H, 10.04; N, 6.68. Found: C, 71.66; H, 10.06; N, 6.65%. FAB
Mass, m/z: 418.3 [M]+. 1H NMR (300 MHz, CDCl3; Me4Si at 25 °C,
4
ppm) d: 15 (s, 1H; H5), 8.31 (s, 1H; H1), 8.23 (d, J(H,H) = 3.0 Hz,
3
4
1H; H2), 8.19 (dd, J(H,H) = 6.0 Hz, J(H,H) = 3.0 Hz, 1H; H3), 6.90 (d,
3J(H,H) = 6.0 Hz, 1H; H4), 3.66 (t, J(H,H) = 6.0 Hz, 2H; @NACH2),
0.88 (t, J(H,H) = 6.0 Hz, 3H; CH3), 1.36 (m, ACH2 of the methylene
side chain proton). IR
3
3
(mmax
,
cmÀ1
,
KBr): 3428 OH), 2917
(
m
(m
as(CAH), CH3), 2848 ( s(CAH), CH3), 1672 (
m
m
C@N), 1280 (mCAO).
2. Experimental section
2.1. Materials
2.3.2. Synthesis of the zinc(II), cadmium(II) and mercury(II) complexes,
[ML2] (M = Zn, Cd, Hg)
All chemicals used were of the reagent grade quality and used
without further purification. The solvents were dried before use
following standard procedures.
General procedure: To a methanolic solution of the ligand, HL
(0.084 g, 0.2 mmol),
a
methanolic solution of Zn(OAc)2Á2H2O
(0.02 g, 0.1 mmol), Cd(OAc)2Á2H2O (0.026 g, 0.1 mmol) or Hg
(OAc)2 (0.032 g, 0.1 mmol) was added separately. The mixture
was stirred for 2 h at room temperature. A creamy white solid
formed in each case which was immediately filtered, washed with
diethyl ether and re-crystallized from dichloromethane/methanol
(1:1).
2.2. Instrumentation and methods
The C, H and N analyses were carried out using a Carlo Erba
1108 elemental analyzer (USA). The 1H NMR spectra of the com-
pounds were recorded on a Bruker Avance II, 300 MHz (for the
ligand)/400 MHz (for the complexes) spectrometer (Bruker, India)
in CDCl3 (chemical shift in d) solution with TMS as the internal
standard. Mass spectra were recorded on a Jeol SX-102 spectrom-
eter (Jeol, Japan) with fast atom bombardment. UV–Vis absorption
spectra of the compounds were recorded in CH2Cl2 on a Shimadzu
UV-1601PC spectrophotometer (Shimadzu, Asia Pacific, Pte. Ltd.,
Singapore). Photoluminescence spectra were recorded on a Shi-
madzu RF-5301PC spectrophotometer (Shimadzu, Asia Pacific,
Pte. Ltd., Singapore). The fluorescence emission quantum yields
(EQY) in degassed dichloromethane solution were determined by
the standard optical dilution method using 9,10-diphenylan-
thracene (EQY = 0.96, in cyclohexane) as the standard. Infrared
spectra were recorded on a Perkin Elmer BX series spectropho-
tometer (Perkin Elmer, USA) on KBr disks in the 400–4000 cmÀ1
range. The optical textures of the different phase of the compounds
were studied using a Nikon optiphot-2-pol polarizing microscope
(Nikon Corporation, Tokyo, Japan) attached with Instec hot and
cold stage HCS302, with an STC200 temperature controller of
0.1 °C accuracy. The thermal behavior of the compounds were
studied using a Pyris-1 system linked to a Perkin Elmer differential
scanning calorimeter (DSC; Perkin Elmer, Switzerland) with a heat-
ing or cooling rate of 5 °C/min. Variable temperature powder X-ray
diffraction (PXRD) experiments were performed in the transmis-
sion geometry with the samples in a glass capillary (Capillary Tube
[ZnL2]: Yield: ꢀ0.07 g (70%). Anal. Calc. for C50H82N4O6Zn
(900.64): C, 66.62; H, 9.10; N, 6.22. Found: C, 66.63; H, 9.12; N,
6.20%. FAB Mass, m/z: 898.6 [MH]+. 1H NMR (400 MHz, CDCl3;
4
Me4Si at 25 °C, ppm) d: 8.30 (s, 1H; H1), 8.25 (d, J(H,H) = 4.0 Hz,
3
4
1H; H2), 8.18 (dd, J(H,H) = 8.0 Hz, J(H,H) = 4.0 Hz, 1H; H3), 6.84 (d,
3J(H,H) = 8.0 Hz, 1H; H4), 3.62 (t, J(H,H) = 8.0 Hz, 2H; @NACH2),
0.88 (t, J(H,H) = 8.0 Hz, 3H; CH3), 1.55 (m, ACH2 of the methylene
side chain proton). IR (
2849 ( s(CAH), CH3), 1656 (mC@N), 1277 (mCAO).
3
3
m mas(CAH), CH3),
max, cmÀ1, KBr): 2920 (
m
[CdL2]: Yield ꢀ0.08 g (74%). Anal. Calc. for C50H82N4O6Cd
(947.67): C, 63.31; H, 8.65; N, 5.91. Found: C, 63.33; H, 8.67; N,
5.87%. FAB Mass, m/z: 948.5 [MH]+. 1H NMR (400 MHz, CDCl3;
4
Me4Si at 25 °C, ppm) d: 8.30 (s, 1H; H1), 8.24 (d, J(H,H) = 4.0 Hz,
3
4
1H; H2), 8.19 (dd, J(H,H) = 8.0 Hz, J(H,H) = 4.0 Hz, 1H; H3), 6.93 (d,
3J(H,H) = 8.0 Hz, 1H; H4), 3.66 (t, J(H,H) = 8.0 Hz, 2H; @NACH2),
0.88 (t, J(H,H) = 8.0 Hz, 3H; CH3), 1.71 (m, ACH2 of the methylene
side chain proton). IR (
3
3
m mas(CAH), CH3),
max, cmÀ1, KBr): 2919 (
2850 ( s(CAH), CH3), 1639 (mC@N), 1275 (mCAO).
m
[HgL2]: Yield: ꢀ0.07 g (67%). Anal. Calc. for C50H82N4O6Hg
(1035.85): C, 57.92; H, 7.91; N, 5.40. Found: C, 57.95; H, 7.93; N,
5.54%. FAB Mass, m/z: 1036.6 [MH]+. 1H NMR (400 MHz, CDCl3;
4
Me4Si at 25 °C, ppm) d: 8.30 (s, 1H; H1), 8.24 (d, J(H,H) = 4.0 Hz,
3
4
1H; H2), 8.18 (dd, J(H,H) = 8.0 Hz, J(H,H) = 4.0 Hz, 1H; H3), 6.89 (d,
3J(H,H) = 8.0 Hz, 1H; H4), 3.66 (t, J(H,H) = 8.0 Hz, 2H; @NACH2),
0.88 (t, J(H,H) = 8.0 Hz, 3H; CH3), 1.46 (m, ACH2 of the methylene
3
3

152
S. Chakraborty et al. / Polyhedron 105 (2016) 150–158
side chain proton). IR (
m
max, cmÀ1, KBr): 2916 (
C@N), 1273 ( CAO).
m
as(CAH), CH3), 2848
(003) and (004) reflections of a lamellar lattice (Table 2) [56,57].
(m
s(CAH), CH3), 1642 (
m
m
The (001) reflection is attributed to the smectic layer-spacing, d.
Additionally, in the wide-angle region multiple harmonics corre-
sponding to spacings of 5.0 Å (111) and 4.9 Å (200) were
observed. Also, a broad-scattering halo at ꢀ4.6 Å for the molten
alkyl chains was observed. Presence of three strong reflections in
the wide angle region indexed to (111), (200) and (112) is a sig-
nature of the SmE phase [56,57]. The phase was also validated by
the characteristic ‘platelet texture’ observed in the POM study.
The calamitic molecules thus are presumed to be arranged in lay-
ers with orthorhombic symmetry and a herringbone array, typical
3. Results and discussion
3.1. Synthesis and characterization
The ligand was synthesized by the condensation of 5-nitrosali-
cylaldehyde and octadecylamine. The complexes were prepared by
slow addition of a methanolic solution of 1 equivalent of M(OAc)2Á
nH2O (M = Zn, Cd; n = 2 or Hg; n = 0) to a methanolic solution of the
ligand (2 equivalents) (Scheme 1). Structures were ascertained by
elemental analysis, 1H NMR, FT-IR, UV–Vis and FAB-mass spec-
troscopy. The FAB-mass spectral data were in compliance with
the calculated formula weights of the compounds. The IR spectrum
of the ligand exhibited a broad band at ꢀ3432 cmÀ1 due to the
presence of a phenolic-OH group. The C@N stretching vibration
of the ligand was located at ꢀ1672 cmÀ1. In the complexes, the
mCN vibrational stretching frequency was shifted to a lower wave
for an SmE phase (Fig. 6) [57]. The aromatic core p–p interactions
and hydrophobic interactions of the alkyl chains renders them to
act as segregated parts. The lattice parameters of the SmE
phase are deduced as a = 9.89 Å, b = 6.05 Å, c = 24.65 Å and
Vcell = 1461.24 Å3 (Table 2). The calculated interlayer distance of
24.65 Å is about half the length of the fully extended molecule as
computed (vide infra) by DFT (42.1 Å). The molecular area for the
proposed arrangement, AM (65.38 Å2) is deduced from the relation
AM = VM/d; (molecular volume, VM = 1608.28 Å3, density,
q = 1 g/
number (
D
m
ꢀ30 cmÀ1) and
mOH mode was absent. This clearly sug-
cm3). Assuming a parallel arrangement of the molecules perpen-
gests the coordination of the azomethine nitrogen and phenolate
oxygen atoms to the metal in the complexes. The 1H NMR spec-
trum of the ligand consisted of a singlet at d 15 ppm due to the
phenolic-OH proton and another singlet at d 8.31 ppm due to the
imine proton. A relatively small upfield shift (ꢀ0.01 ppm) of the
AN@CH proton and the absence of a signal for the phenolate pro-
ton in the 1H NMR spectra of the metal complexes further suggests
coordination through the phenolate oxygen and the azomethine
nitrogen atoms of the ligand. DFT studies (vide infra) revealed the
geometry of the complexes to be distorted tetrahedral. The com-
puted metal-O/N bond lengths and the bond angles (vide infra)
complied well with related crystallographically characterized
non-mesomorphic tetrahedral zinc-Schiff base complexes [55].
dicular to layer direction, the calculated AM value would corre-
spond to
a
cross-sectional area per aliphatic arm,
Ach = AM/2 = 32.69 Å2, which is considerably larger than the cross-
sectional area of a stretched molten alkyl arm (Ach = 20.915
+ 0.01593, T = 22.85 Å2 at 122 °C). The alkyl arms are therefore
believed to be considerably interdigitated within the layers [16].
The Vcell/VM ratio (= 0.9) also suggests approximately one molecule
per unit cell.
The diffractogram at 80 °C (Fig. 5b) consisted of a number of
sharp reflections in both the low as well as wide angles, consistent
with the existence of a crystalline phase. Moreover, the diffrac-
togram was devoid of any diffuse band at wide angles, thus point-
ing to the crystalline nature of the material.
The PXRD profile of the cadmium(II) complex, recorded at
210 °C (Fig. 7), exhibited one very sharp and a number of relatively
less intense reflections in the small angle region. The first and fifth
peak are indexed to (001) and (002) reflections, respectively, cor-
responding to a layered structure (Table 2). Other small and mid-
angle reflections, however, indicated a three dimensional order of
the mesophase. A poorly resolved diffuse halo at 4.6 Å in the wide
angle region indicated lateral short-range order of the molten alkyl
chains. On the basis of these features, a highly ordered mesophase
reminiscent of soft crystals has been suggested, possessing both a
layer arrangement and a 3D structure [58–60]. The observed pat-
tern could be indexed to a primitive triclinic lattice (p1) with lattice
3.2. Mesomorphic properties
The phase behavior of the compounds was studied by polarizing
optical microscopy (POM), differential scanning calorimetry (DSC)
and variable temperature powder-XRD techniques. The phase
sequence, transition temperatures and associated enthalpies are
summarized in Table 1. The ligand is non-mesomorphic, the
greater conformational flexibility of the uncoordinated ligand
molecule is believed to be one plausible reason for its non-
mesomorphic character. On complexation, this flexibility is
reduced, which might be responsible for induction of mesomor-
phism in the zinc(II) and cadmium(II) complexes. The mercury(II)
complex, however, decomposed at 270 °C precluding any meso-
morphic study. Upon cooling the sample from isotropic melt, the
zinc(II) complex showed a ‘platelet texture’ (Fig. 1) at 127 °C, typ-
ical of the smectic E (or crystal E) phase [56]. The DSC profile
(Fig. 2) showed four transitions on heating and two in the cooling
cycle. The peaks below the melting point are believed to have
arisen from a crystal-to-crystal transition which could not be
detected in the POM study. In the case of the cadmium(II) complex,
a fibrous texture was observed (Fig. 3) upon cooling at 228 °C. The
DSC trace (Fig. 4) revealed two transitions each in the heating and
cooling scans. In contrast to the zinc complex, interestingly the
mesophase for the cadmium(II) complex was quite stable over a
wide temperature range (231–88 °C). The reversibility of the
thermal behavior of the zinc(II) and cadmium(II) complexes was
established by DSC through repeated heating and cooling runs.
The powder XRD profile of the zinc(II) complex, recorded at
122 °C (Fig. 5a), consisted of one very sharp and three relatively
weak reflections in the low angle region with a reciprocal spacing
ratio of 1:2:3:4. The spacing could be assigned to (001), (002),
parameters a = 9.9 Å, b = 21.7 Å, c = 37.1 Å;
c
a = 47°, b = 68.6°,
= 57° and Vcell = 4870 Å3. The mesophase with a triclinic lattice
may thus be designated as MT [59]. The molecular volume,
VM = 1462.45 Å3 was calculated assuming a density close to 1.2 g/
cm3. Customarily, a density of 1 g/cm3 is presumed for liquid crys-
talline mesophases, resulting from organic molecules or metal-
lomesogens incorporating lighter metals. In some instances,
higher density values (
q
ꢁ 1.1–1.2 g/cm3) were reported for some
smectic gold enriched dendrimers [61], discotic gold pyrazolate
compounds [62], discotic cyclopalladated metallomesogens [63],
some potassium salts of crown ethers [64] and columnar hexaalky-
loxytriphenylenes [65]. A higher density has thus been considered
for the newly synthesized cadmium(II) complex, considering a
heavier metal core. Thus approximately three molecules per unit
cell in the triclinic lattice in the soft crystalline phase has been
worked out.
Variation in the type of mesophase formed on changing the
metal from zinc(II) to cadmium(II) and the lack of mesomorphism
or thermal instability of the complex of the other congener, mer-
cury(II), is not discernible at this moment. A noticeable aspect is

S. Chakraborty et al. / Polyhedron 105 (2016) 150–158
153
Scheme 1. (i) Glacial acetic acid, methanol, reflux 3 h, (ii) M(OAc)2ÁnH2O (M = Zn, Cd; n = 2 or Hg; n = 0), Methanol, stir, 2 h.
Table 1
Phase transition data of [ZnL2] and [CdL2].
Compounds
[ZnL2]
Ta (°C)
Transitionb
D
H (kJ molÀ1
)
trsS (J KÀ1 molÀ1
)
D
85.7
104.5
121.1
133.6
127.1
114.2
Cr–Cr1
16.9
17.9
36.9
22.8
25.2
37.2
47.1
47.4
93.6
56.1
62.9
96.0
Cr1–Cr2
Cr2–SmE
SmE–I
I–SmE
SmE–Cr2
[CdL2]
116.0
235.8
231.2
88.6
Cr–MT
MT–I
I–MT
62.6
15.4
14.9
47.4
160.9
30.2
29.5
MT–Cr
131.0
a
DSC peak temperature.
Cr: crystal, SmE: smectic E or soft crystal E, MT = mesophase triclinic.
b
Fig. 3. Fibrous texture of [CdL2] upon cooling at 228 °C.
Fig. 1. Platelet texture of [ZnL2] upon cooling at 127 °C.
Fig. 4. DSC thermogram of [CdL2].
the significant variation in the ionic radius of the metal centers
(Zn2+ = 0.74 Å; Cd2+ = 0.95 Å; Hg2+ = 1.02 Å) [54,66].
3.3. Photophysical properties
The absorption spectra (Fig. 8) of the compounds were recorded
in dichloromethane solution (10À5 M) at room temperature. The
ligand showed three bands centered at 260, 323 and 403 nm,
respectively, attributed to the
matic rings and the C@N fragment (Table 3). Upon complexation, all
the
p⁄ bands were virtually unaltered in the cadmium(II) and
p–
p⁄ transition, localized on the aro-
p–
mercury(II) complexes. The absorption spectrum of the zinc(II)
complex, however, exhibited only two bands at 257 and 343 nm.
The low-intensity ligand centered
ꢀ400 nm, observed in the spectra of the ligand and the cadmium
p–
p⁄ transition peak at
Fig. 2. DSC thermogram of [ZnL2].

154
S. Chakraborty et al. / Polyhedron 105 (2016) 150–158
Fig. 7. PXRD profile of [CdL2] at 210 °C.
Table 2
PXRD data of [ZnL2] and [CdL2].
Compound
Mesophasea
Lattice constants (Å)
dobsd (Å) (dcalcd (Å))b
Miller indices
hklc
[ZnL2]
SmE (at 122 °C)
d = 24.65 Å
24.64(24.65)
12.33(12.32)
8.22(8.22)
6.17(6.16)
5.05(5.05)
4.95(4.95)
4.56diffuse
(001)
(002)
(003)
(004)
(111)
(200)
VM = 1608.28 Å3
AM = 65.38 Å2
a = 9.89 Å
b = 6.05 Å
c = 24.65 Å
Vcell = 1461.24 Å3
[CdL2]
MT (at 210 °C)
a = 9.9 Å
b = 21.7 Å
c = 37.1 Å
27.33(27.14)
18.37(18.05)
15.82(15.91)
14.64(14.28)
13.65(13.57)
11.62(11.51)
10.15(10.29)
9.75(9.80)
9.08(9.08)
(001)
(011)
(012)
(010)
(002)
(013)
Fig. 5. PXRD pattern of [ZnL2] recorded at 122 °C (a) and (b) at 80 °C.
a
= 47°
b = 68.6°
c
= 57°
(01À1)
Vcell = 4870 Å3
(111)
VM = 1462.45 Å3
(110)
8.14(8.14)
6.80(6.80)
(113)
(025)
6.38(6.33)
6.22(6.24)
3.95(3.95)
3.80(3.80)
(1À1À1)
(1À1À2)
(1À3À3)
(23À1)
(2À2À3)
3.16(3.16)
4.56diffuse
a
Molecular volume, VM is calculated using the formula: VM = M/k
qNA, where M is
the molecular weight of the compound, NA is the Avogadro number,
q
is the
density (ꢀ1 g cmÀ3 for the zinc(II) and 1.2 g cmÀ3 for the cadmium(II) com-
plexes), k(T) is a temperature correction coefficient at the temperature of the
experiment (T). k(T) = VCH2(T°)/VCH2(T); where VCH2(T) = 26.5616 + 0.02023T is
the volume of a methylene group (in Å3) at a given temperature (in °C), and T
° = 25 °C [75].
Vcell; unit cell volume: in the orthorhombic lattice, Vcell = abc; in triclinic lattice,
1/2
Vcell = abc (1 À cos2
a
À cos2b À cos2
c
+ 2 cos
a
cosb cos
c
)
.
b
c
dobsd and dcalcd are the experimentally observed and calculated diffraction
spacings, respectively. The distances are given in Å.
hkl are the Miller indices of the reflections in SmE and MT phases.
Fig. 6. The ‘herringbone’ model for the self-assembled rod-like complex molecules
in the smectic E phase.
performed both in dichloromethane solution and in the solid state.
The ligand is non-emissive, the probable reason being the greater
conformational flexibility. The complexes, however, exhibited fluo-
rescence at room temperature with emission maxima at ꢀ479 nm
(Fig. 9 and Table 3). The emission observed at room temperature
(II) and mercury(II) complexes, appeared as a weak shoulder in the
spectrum of the zinc(II) complex, while the second absorption peak
was considerably bathochromically shifted (ꢀ33 nm) compared to
the free ligand. Photoluminescence studies of the compounds were

S. Chakraborty et al. / Polyhedron 105 (2016) 150–158
155
in dichloromethane solution at room temperature and are found
to be 0.20, 0.16 and 0.09, respectively for the zinc, cadmium and
mercury complexes. The lowest value of the quantum yield for
the mercury complex might be due to the ‘heavy atom’ effect
[67]. The solid state emission spectra of the complexes were
recorded on a uniform thin film. The emission band of the com-
plexes was quite broad and exhibited an unusual blue shift
(ꢀ467 nm) compared to the solution spectra, while the emission
intensities were largely quenched. In the solid state somewhat lar-
ger aggregation causes enhanced intermolecular interactions,
which explains the reduced luminescence intensity [68]. Further
it has been argued that a tetrahedral geometry or formation of a
non-planar molecular shape, as in the present case, prevent inter-
chromophoric contacts leading to a blue shift of the emission max-
ima with respect to those observed in solution [50].
3.4. DFT study
Fig. 8. UV–Vis spectra of the ligand and the complexes (CH2Cl2; 10À5 M).
Density functional theory (DFT) calculations at the B3LYP level
of theory on the ligand and its zinc(II), cadmium(II) and mercury
(II) complexes were carried out using the GAUSSIAN 09 program
[69]. The geometries of all the species were fully optimized at
the gradient corrected DFT level using a three parameters fit of
Becke’s hybrid functional combined with the Lee–Yang–Parr corre-
lation functional, termed as B3LYP [70,71]. For the zinc, cadmium
and mercury atoms the Los Alamos effective core potential plus
double zeta (LanL2DZ) basis set were employed [72] and the 6-
311G(d,p) basis set was employed for all non-metal atoms. The
gas phase ground state geometries of the ligand and its zinc, cad-
mium and mercury complexes were fully optimized using the
restricted B3LYP methods without imposing any symmetry con-
straints. Calculations for vibrational frequencies were performed
alongside each geometry optimization to ensure the stability of
the ground state, as denoted by the absence of imaginary frequen-
cies. Natural bond orbital (NBO) calculations were performed with
the NBO code included in the GAUSSIAN 09 program at the same level
of theory [73].
Table 3
UV–Vis and photoluminescence data of the ligand and metal complexes.
Compounds
p
?
p⁄
aPL
aPL
(k, nm;
e
, l molÀ1 cmÀ1
)
(k, nm; solution)
(k, nm; solid)
HL
260 (17464)
323 (12689)
403 (5090)
–
–
ZnL2
CdL2
HgL2
257 (37547)
343 (28107)
400sh (7894)
475
479
470
470
467
466
259 (22961)
324 (16478)
400 (6557)
259 (30310)
322 (20880)
399 (8397)
a
PL: photoluminescence; sh: shoulder.
3.5. Geometry optimization
DFT calculations were performed in order to get better insight
into the electronic structure of the ligand as well as its complexes
with zinc, cadmium and mercury. Optimized geometries of the
ligand, zinc, cadmium and mercury complexes (Figs. 10a–d) and
computed significant geometric parameters of the complexes were
evaluated at B3LYP level (Table 4). All the metal (M = Zn, Cd, Hg)
centers are tetra coordinated with two [N,O] donor bidentate
ligands through two nitrogen atoms of two C@N groups and two
oxygen atoms of two phenolic groups. The average MAO and
MAN bond lengths in the complexes are 1.971 and 2.087, 2.159
and 2.290, and 2.266 and 2.357 Å, respectively (Table 4). The aver-
age O1AMAO2 bond angles are 128.4°, 135.8° and 139.3° and the
average N1AMAN2 bond angles are found to be 125.3°, 129.0° and
137.6°, respectively, around the zinc, cadmium and mercury
atoms, which deviate substantially from those expected for a
square planar motif, indicating a distorted tetrahedral geometry.
The molecular lengths of the zinc(II), cadmium(II) and mercury
(II) complexes computed from DFT are found to be 42.1, 42.4 and
42.7 Å, respectively.
Fig. 9. Photoluminescence spectrum of [CdL2] (10À5 M; kex. = 325 nm).
3.6. Natural bond orbital (NBO) analysis
in the complexes stems from the intra-ligand p–
p⁄ transition. Che-
lation provides rigidity to the ligand and reduces energy loss via
non-radiative de-activation. The luminescence intensities of the
complexes were found to decrease in the order: Zn > Cd > Hg. The
emission quantum yields of the complexes have been determined
DFT with natural bond orbital (NBO) analysis allows deeper
insight into the nature of metal–ligand bonding. The natural
charges and electronic configuration of the atoms of the complexes
and free ligand evaluated by natural bond orbital (NBO) analysis

156
S. Chakraborty et al. / Polyhedron 105 (2016) 150–158
Fig. 10a. Optimized structure of the ligand, HL.
Fig. 10b. Optimized structure of [ZnL2].
Fig. 10c. Optimized structure of [CdL2].
Fig. 10d. Optimized structure of [HgL2].
Table 4
are summarized in Table 5. The calculated natural charges on the
metal ions zinc, cadmium and mercury are considerably lower
than the formal charge of +2. The electronic populations on the
px, py and pz orbitals of the zinc, cadmium and mercury atoms in
their respective complexes are found to be (0.0990, 0.1133 and
0.1026), (0.0730, 0.0800 and 0.0700) and (0.0529, 0.0695 and
0.0640), respectively. The dxy, dzx, dyz, dx2Ày2 and dz2 orbitals of zinc,
cadmium and mercury atoms in their complexes, however, are
occupied by more than 1.99 eÀ. Comparing the atomic charges in
the free ligand and its complexes with zinc, cadmium and mercury,
it can be suggested that atomic charge re-distribution occurs on all
the atoms. The calculated natural atomic charges on zinc, cadmium
and mercury are found to be +1.405, +1.487 and +1.394, respec-
Selected bond lengths (Å) and bond angles (°) for the zinc(II), cadmium(II) and
mercury(II) complexes evaluated at the B3LYP level.
Structural parameters
Zn
Cd
Hg
MAO1
MAO2
MAN1
MAN2
O1AMAO2
N1AMAN2
O1AMAN1
O2AMAN2
O1AMAN2
O2AMAN1
1.971
1.971
2.087
2.088
128.4
125.3
93.0
92.8
108.6
112.1
2.159
2.158
2.289
2.291
135.8
129.0
86.2
86.0
111.8
114.4
2.269
2.263
2.354
2.361
139.3
137.6
83.7
83.5
108.7
113.8

S. Chakraborty et al. / Polyhedron 105 (2016) 150–158
157
Table 5
Natural atomic charges and natural electron configuration of selected atoms of the ligand and zinc(II), cadmium(II) and mercury(II) complexes evaluated at the B3LYP level.
Atoms
Ligand
Charge
À0.614
Zn2+
Cd2+
Hg2+
Configuration
Charge
Configuration
Charge
Configuration
Charge
Configuration
O1
O2
N1
N2
M
[core]2S1.722p4.89
À0.795
À0.800
À0.634
À0.629
1.405
[core]2S1.672p5.123p0.01
[core]2S1.672p5.123p0.01
[core]2S1.352p4.263p0.01
[core]2S1.352p4.263p0.01
[core]4S0.303d9.974p0.31
À0.807
À0.812
À0.637
À0.633
1.487
[core]2S1.682p5.123p0.01
[core]2S1.682p5.123p0.01
[core]2S1.362p4.263p0.01
[core]2S1.362p4.253p0.01
[core]5S0.304d9.985p0.22
À0.785
À0.779
À0.613
À0.68
[core]2S1.692p5.093p0.01
[core]2S1.692p5.083p0.01
[core]2S1.362p4.243p0.01
[core]2S1.362p4.243p0.01
[core]5S0.445d9.976p0.196d0.01
À0.433
[core]2S1.392p4.033p0.01
1.394
tively, which reflects ligand-to-metal charge transfer in all the
complexes. According to the NBO, the electronic configuration of
Zn is: [core] 4s0.303d9.974p0.31, which corresponds to 18 core elec-
trons, 10.27 valence electrons (on 4s and 3d atomic orbitals) and
0.31 Rydberg electrons (mainly on 4p orbitals), giving 28.58 elec-
trons. This is consistent with the calculated natural atomic charge
on the zinc atom (+1.405) in its complex. The electronic configura-
tion of Cd is: [core] 5s0.304d9.985p0.22, with 36 core electrons, 10.28
valence electrons (on 5s and 4d atomic orbitals) and 0.22 Rydberg
electrons (mainly on 5p orbitals) leading to 46.50 electrons, which
matched well with the calculated natural charge on the cadmium
atom (+1.487) in the complex. The electronic configuration of the
mercury atom in its complex is calculated to be [core]
6s0.445d9.976p0.196d0.01 that matches well with the calculated natu-
ral charge on the mercury atom (+1.394) in the complex. The nat-
ural atomic charges on the oxygen atoms of the ligand bound to
metal ions change from À0.614 to À0.795 and À0.800 in the zinc
complex, to À0.807 and À0.812 in the cadmium complex and to
À0.785 and À0.779 in the mercury complex, while the atomic
charges on the nitrogen atoms of the ligand change from À0.433
to À0.634 and À0.629, to À0.637 and 0.633, and to À0.618 and
À0.613 on complexation with zinc, cadmium and mercury, respec-
tively (Table 5). The optimized geometry also revealed that the dis-
luminescent, decomposed prior to melting. DFT calculations carried
out using the GAUSSIAN 09 program at the B3LYP level revealed a dis-
torted tetrahedral geometry for all the complexes. Examples of
tetrahedral coordination geometry exhibiting liquid crystallinity
are rather uncommon. The application potential of the newly
synthesized complexes as multi-functional materials are avenues
to be explored in the near future.
Acknowledgements
S.C. gratefully acknowledges the receipt of INSPIRE Research
Fellowship (Code: IF110692) from the Department of Science and
Technology, India. D.D.P. thanks the University Grants Commission
(UGC) for financial support under the Research Fellowship
Scheme for Meritorious Students (RFSMS). P.M. thanks the Depart-
ment of Science and Technology (Ministry of Science & Technol-
ogy), New Delhi, India for financial support. Sophisticated
Analytical Instrumentation Facility, North Eastern Hill University,
Shillong and Central Drug Research Institute, Lucknow are
acknowledged for some spectral results. The authors also thank
DBT e-Library Consortium (DeLCON) of Bio-Informatics Centre,
Assam University, India.
tance between the oxygen and aromatic carbon atoms (OACar.
)
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