Induction of photoluminescence and columnar mesomorphism in hemi-disc salphen type Schiff bases via nickel(II) coordination

Article abstract of DOI:10.1016/j.poly.2011.12.001

A series of hemi-disc shaped non-mesomorphic tetradentate salicylaldimine ligands [N,N′-di-(4-hexadecyloxysalicylidene)-l,2-diamino-benzene, N,N′-di-(4-hexadecyloxysalicylidene)-4-Me-l,2-diamino-benzene, and N,N-di-(4-hexadecyloxysalicylidene)-4-NO_2<

Full text of DOI:10.1016/j.poly.2011.12.001

Polyhedron 33 (2012) 417–424
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Induction of photoluminescence and columnar mesomorphism in hemi-disc
salphen type Schiff bases via nickel(II) coordination
⇑
Chira R. Bhattacharjee , Chitraniva Datta, Gobinda Das, Rupam Chakrabarty, Paritosh Mondal
Department of Chemistry, Assam University, Silchar 788011, Assam, India
a r t i c l e i n f o
a b s t r a c t
A series of hemi-disc shaped non-mesomorphic tetradentate salicylaldimine ligands [N,N0-di-(4-hexade-
cyloxysalicylidene)-l,2-diamino-benzene, N,N0-di-(4-hexadecyloxysalicylidene)-4-Me-l,2-diamino-ben-
zene, and N,N-di-(4-hexadecyloxysalicylidene)-4-NO2-l,2-diamino-benzene, (H2L)] were synthesized.
Incorporation of nickel(II) in the tetradentate core via reaction with Ni(OAc)2ꢀ4H2O afforded a series of
four coordinate mesogenic NiL derivatives. The ligands and complexes were characterized by elemental
analyses, FT-IR, UV–Vis, FAB-mass, 1H and 13C NMR (for ligands only). The mesomorphic behavior of the
complexes were probed by polarizing optical microscopy, differential scanning calorimetry and powder
X-ray diffraction technique. The non-mesogenic ligands upon coordination with nickel(II) exhibited
monotropic/enantiotropic phase transition showing rectangular columnar mesophases (Colr) with
c2mm symmetry. A antiparallel dimeric association forming a disc-like arrangement in the mesophase
is proposed on the basis of XRD-study. Solution electrical conductivity measurements are consistent with
the non-electrolytic nature of the complexes. At room temperature with 330 nm excitation, the com-
Article history:
Received 31 October 2011
Accepted 2 December 2011
Available online 13 December 2011
Keywords:
Nickel
Metallomesogen
Columnar mesomorphism
Density functional theory
plexes showed blue emission both in the solid state (ꢁ481 nm,
= 23%) while the ligands are non-emissive. The DFT study carried out at BLYP/DNP level revealed a dis-
torted square planar structure for the nickel(II) complexes.
U
= 7%) and in solution (ꢁ456 nm,
U
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
lanthanides, Zn, Pd, Pt, Au, and Ag have been well documented [6–
10]. We recently developed a series of photoluminescent metallo-
mesogen based on salicylaldimine Schiff-base ligands [11,13–15].
Transition metal complexes with salen-type ligands have been
extensively studied mainly due to their ability to catalyze an extre-
mely broad range of chemical transformations, including the asym-
metric ring-opening of epoxides, aziridination, cyclopropanation,
epoxidation of olefins and formation of cyclic and linear polycarbon-
ates [16–19]. Moreover, nonlinear optical (NLO) properties of such
materialshavealsobeenexploredinrecentyears [20–23]. Neverthe-
less, application studies on metal–salen derivatives remained sparse
intheliterature,eventhoughthesecompoundsareknowntobepho-
toluminescent for a long time [20–25]. Choice of metal ion, nature
andposition of thesubstituentsonside aromaticringas wellasspac-
ersare knownto greatlytunethemesogenicas wellas photophysical
behavior. For some compounds even a minor changes within the
spacer can lead to major differences in molecular organization and
in turn liquid-crystalline behavior [26–31]. Metal–salen complexes
with 5-substituted alkoxy or alkyl chains exhibiting smectic meso-
morphism are well documented [32–35]. Metallomesogen based
on 4-substituted salen-type framework Schiff base ligands has been
sparsely investigated [26–30]. Recently we have reported a series of
structurally analogous 4-substituted zinc(II), oxovanadium(IV),
Ni(II) as well as some copper(II) complexes, using cyclohexane/phe-
nylene diamine spacer exhibited different type of columnar phase
The technologies based on physical properties such as light emis-
sion or charge transport ability and related materials are currently
receiving significant attention owing to their potential applications
such as displays, solar cells, active components for image and data
treatment storage etc. [1–5]. Of specific interest are luminescent li-
quid-crystalline materials that are considered very attractive for po-
tential applications in optoelectronic devices because of their
excellent charge-transport properties [1,3–10]. Combining lumines-
cence properties in soft materials to generate new electronic devices
is a fast growing field of research [3–10]. Design and synthesis of
such luminescent photoresponsive liquid crystals is significant par-
ticularly in the context of their potential applications in organic light
emitting diodes (OLEDs), information storage, sensors, and en-
hanced contrast displays [11,12]. Metallomesogens (metal contain-
ing liquid crystals) are ideal candidates for tuning smart
multifunctional propertiesowingtothecombinationofoptical, elec-
tronic and magnetic characteristics. Studies on light-emitting meso-
gens were mainly focused on organic compounds, while
luminescent metallomesogens caught the fancy of researchers
rather recently. Emissive metallomesogen with metals such as
⇑
Corresponding author. Tel.: +91 03842 270848; fax: +91 03842 270342.
E-mail address: crbhattacharjee@rediffmail.com (C.R. Bhattacharjee).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.12.001

418
C.R. Bhattacharjee et al. / Polyhedron 33 (2012) 417–424
!
ꢀ
ꢁ
[11,13,15,36–38]. Complexes with shorter alkoxy substituent,
[VO(4-CnH2n+1O)2salen]ClO4 (n = 3, 8, 10) with similar spacer lacked
liquid crystalline behavior [26]. When complexed with non-discoid
ligands, a molecular shape with a reduced length-to-breadth ratio is
formed favouring a disc-like metallomesogens [39]. Such structures
tend to form columnar mesophases [39]. Compounds exhibiting
columnar phases with axially linked discs are of particular interest
as potential one-dimensional photoconductors, semiconductors, or-
ganic light emitting diodes (OLED) and photovoltaic cells [40–50]. A
series of analogous nickel(II) complexes with cyclohexane spacer
showing columnar rectangular mesophase have been reported re-
cently by us [15]. However, there appears to be no record of nickel(II)
complexes with rigid aromatic spacer. As a part of our continued and
systematic investigation directed towards soft materials, in this arti-
cle we describe synthesis of newer tetradentate Schiff base ligands
bearingaromaticspacer with differentelectron withdrawing/donat-
ing substituent, and induction of luminescence and columnar meso-
morphism via formation of hemi-disc shaped nickel(II) complexes.
The ligands are non-mesogenic devoid of any luminescence.
1
2
d2E
dN2
dl
dN
g
¼
¼
~
v
ðrÞ
mðrÞ
Global softness is the inverse of global hardness with a factor of
half
!
ꢀ
ꢁ
1
g
d2N
dE2
dN
d
S ¼
¼
¼
~
2
l
~
ðrÞ
v
v
ðrÞ
By applying finite difference approximation the global hardness
and softness are expressed as:
IE ꢂ EA
1
g
¼
;
S ¼
2
IE ꢂ EA
where IE and EA are the first vertical ionization energy and the elec-
tron affinity of the molecule, respectively. Using Koopmans’ theo-
rem IE and EA can be approximated as negative of EHOMO and
ELUMO, respectively and thus chemical hardness and chemical soft-
ness can be written as [56].
ELUMO ꢂ EHOMO
2
2. Experimental
g
¼
and S ¼
;
respectively:
2
ELUMO ꢂ EHOMO
2.1. Physical measurements
2.3. Materials
The C, H and N analyses were carried out using PE2400 elemen-
tal analyzer. The 1H NMR spectra were recorded on Bruker DPX-
400 MHz spectrometer in CDCl3 (chemical shift in d) solution with
TMS as internal standard. 13C NMR spectra were recorded on a JEOL
AL300 FT NMR spectrometer. Molar conductance of the com-
pounds was determined in CH2Cl2 (ca. 10ꢂ3 mol Lꢂ1) at room tem-
perature using MAC-554 conductometer. UV–Vis absorption
spectra of the compounds in CH2Cl2 were recorded on a Shimadzu
UV-160PC spectrophotometer. Photoluminescence spectra were
recorded on a Shimadzu RF-5301PC spectrophotometer. The fluo-
rescence quantum yield in dichloromethane was determined by
dilution method using 9,10-diphenyl anthracene as standard.
Infrared spectra were recorded on a Perkin–Elmer L 120-000A
spectrometer on KBr disc. Mass spectra were recorded on a Jeol
SX-102 spectrometer with fast atom bombardment. The optical
textures of the different phase of the compounds were studied
using a polarizing microscope (Nikon optiphot-2-pol) attached
with Instec hot and cold stage HCS302, with STC200 temperature
controller of 0.1 °C accuracy. The thermal behavior of the com-
pounds were studied using a Perkin–Elmer differential scanning
calorimeter (DSC) Pyris-1 spectrometer with a heating or cooling
rate of 5 °C/min. Variable temperature powder X-ray diffraction
(PXRD) of the samples were recorded on a Bruker D8 Discover
The materials were procured from Tokyo Kasei and Lancaster
Chemicals. All solvents were purified and dried using standard pro-
cedures. Silica (60–120 mesh) from Spectrochem was used for
chromatographic separation. Silica gel G (E-Merck, India) was used
for TLC.
2.4. Synthesis of hexadecyloxysalicyldehyde
Alkoxysalicyldehyde derivatives were prepared following re-
ported method [11,13,15,36–38]. 2,4-Dihydroxybenzaldehyde
(10 cm3, 1.38 g), KHCO3 (10 cm3, 1.00 g), KI (catalytic amount)
and 1-bromohexadecane (10 cm3, 2.8 g) were mixed in 250 mL of
dry acetone. The mixture was heated under reflux for 24 h, and
then filtered, while hot, to remove any insoluble solids. Dilute
HCl was added to neutralize the warm solution followed by extrac-
tion with chloroform (100 cm3). The combined chloroform extract
was concentrated to give a purple solid. The solid was purified by
column chromatography using a mixture of chloroform and hex-
ane (v/v, 1/1) as eluent. Evaporation of the solvents afforded a
white solid product.
Synthesis of 16-opd and 16-mpd are reported in our earlier
observations [11,13].
instrument using CuKa radiation.
2.4.1. Synthesis of N,N0-bis(4-(40-hexadecyloxy)-salicylidene)-4-NO2-
1,2-phenylenediamine (16-npd)
2.2. Computational analysis
An ethanolic solution of 2-hydroxy-(4-hexadecyloxy)-salicylal-
dehyde (0.39 g, 1 mmol) was added to an ethanolic solution of
4-NO2-1,2-phenylenediamine (0.07 g, 0.5 mmol). The solution
mixture was refluxed with a few drops of acetic acid as catalyst
for 3 h to yield the yellow Schiff base N,N0-bis(4-(40-n-alkoxy)-sal-
icylidene)-4-NO2-1,2-phenylenediamine. The compound was col-
lected by filtration and recrystallized from absolute ethanol to
obtain a pure compound.
Yield: 0.33 g, 75%. FAB Mass (m/e, fragment): m/z: calc. 841.6;
found: 842.6 [M+H+]; Anal. Calc. for C52H79N3O6: C, 74.1; H, 9.4;
N, 4.9. Found: C, 74.2; H, 9.5; N, 4.8%. 1H NMR (400 MHz, CDCl3):
d = 13.02 (s, 1H, H5), 8.78 (s, 1H, H4), 7.79 (d, J = 8.4 Hz, H9), 7.25
(d, 2H, H6), 7.18 (t, J = 8.2 Hz, 2H, H1), 6.69 (d, J = 2.5 Hz, 2H, H3),
6.97 (dd, J = 2.4 Hz, J = 8.3 Hz, 2H, H2), 3.98(t, J = 6.6 Hz, 2H, –
OCH2), 0.96 (t, J = 6.7 Hz, 6H, CH3), 0.98 (m, –CH2 of methylene pro-
All the structures were completely optimized using the hybrid
HF-DFT method, labeled as BLYP. The BLYP functional is comprised
of a hybrid exchange functional as defined by Becke and the non-
local Lee–Yang–Parr correlation functional [51]. We used DFT
semicore pseudopotential with double numerical basis set plus
polarization functions (DNP), which is comparable with the Gauss-
ian 6-31G(d,p) basis set in size and quality [52]. All structures were
relaxed without any symmetry constraints. Convergence in energy,
force, and displacement was set as 10ꢂ5 Hartree (Ha), 0.001 Ha/Å,
and 0.005 Å, respectively. All calculations were performed with
the DMol3 program package [52–54].
Global hardness (g) of an electronic system is defined [55] as
the second derivative of total energy (E) with respect to the num-
~
mðrÞ
ber of electrons (N) at constant external potential,

C.R. Bhattacharjee et al. / Polyhedron 33 (2012) 417–424
419
ton in side chain); IR (
m
max, cmꢂ1, KBr): 3513 (
C@N), 1294 (m
m
OH), 2922 (
m
as(C–H),
spectra of the complexes assigned to Ni–O and Ni–N stretching
vibrations that are not observed in the spectra of the ligands fur-
nished evidence for [N,O] binding mode of the ligand. The mC@N
stretching frequency is rather independent of the length of alkoxy
side chain in both ligands and their complexes. The FAB-mass spec-
tra of the compounds matched well with their formula weights.
Solution electrical conductivity of complexes recorded in CH2Cl2
CH3), 2873 ( s(C–H), CH3), 1625 (
m
m
(C–O)).
2.4.2. Synthesis of nickel(II) complexes
General procedure. The ligand 16-opd (0.79 g, 1 mmol) or 16-
mpd (0.81 g, 1 mmol) or 16-npd (0.84 g, 1 mmol) was dissolved
in minimum volume of absolute ethanol. To this, an equimolar
amount of nickel acetate Ni(OAc)2ꢀ4H2O (0.02 g, 0.1 mmol) in
methanol was then added slowly and stirred for 2 h at room tem-
perature. A red solid formed immediately was filtered, washed
with diethyl ether and recrystallized from chloroform–ethanol
(1:1).
Ni-16opd. Yield = 0.60 g (75%). FAB Mass (m/e, fragment): m/z:
calc. 852.5; found: 853.5 [M+H+]; Anal. Calc. for C52H78N2O4Ni: C,
73.1; H, 9.2; N, 3.2. Found: C, 73.1; H, 9.2; N, 3.1%. 1H NMR
(400 MHz, CDCl3): d = 8.22 (s, 1H, H4), 7.69 (d, J = 8.5 Hz, H9), 7.17
(d, 2H, H6), 7.10 (t, J = 8.4 Hz, 2H, H1), 7.16 (dd, J = 2.3 Hz, J = 9.1,
2H, H8), 6.61 (d, J = 2.4 Hz, 2H, H3), 6.49 (dd, J = 2.4 Hz, J = 8.2 Hz,
2H, H2), 3.95 (t, J = 6.8 Hz, 2H, –OCH2), 0.91 (t, J = 6.8 Hz, 6H,
(10ꢂ3 M) was found to be <10
X
ꢂ1 cmꢂ1 molꢂ1, much lower than is
expected for a 1:1 electrolyte, thus confirming the non-electrolytic
nature of the complex. 1H NMR spectra of the ligands showed two
characteristic signals at d = 13.4–13.8 ppm corresponding to the
OH proton and at d = 8.5 ppm due to the imine proton. Moreover
upon complexation, lack of proton signal corresponding to the OH
group of the free ligands and the upfield shift of the imine proton fur-
ther attested the coordination of azomethine-N.
3.2. Photophysical properties
The electronic spectra of the free ligands exhibited three bands
(Fig. 1) in the region of ꢁ288–366 nm assigned to
p–
p⁄ transitions,
CH3), 0.87 (m, –CH2 of methylene proton in side chain); IR (mmax
cmꢂ1, KBr): 2922 (
as(C–H), CH3), 2870 ( s(C–H), CH3), 1611
C@N), 534 ( Ni–N), 459 ( Ni–O).
Ni-16mpd. Yield = 0.22 g, 76%. FAB Mass (m/e, fragment): m/z:
,
which involves molecular orbitals essentially localized on the C@N
group and the benzene ring. The absorption maxima are red shifted
in 16-npd ligand owing to the presence of the NO2 chromophore in
the ligand. The complexes exhibited two intense red shifted bands
(Fig. 2) at ꢁ312–325 nm and ꢁ383–401 nm resulting from the me-
tal-perturbed ligand-centered transitions, which have the same
origin as the two principal bands of the ligand spectrum. In addi-
tion, the complexes also displayed a broad unstructured low inten-
sity band centered at ꢁ447–462 nm assigned to ligand to metal
charge transfer transitions (LMCT) charge transfer transition
(N ? Ni2+) which might have obscured the d–d bands.
m
m
(m
m
m
calc. 866.5; found: 867.5 [M+H+]; Anal. Calc. for C53H80N2O4Ni: C,
73.3; H, 9.2; N, 3.2. Found: C, 73.4; H, 9.3; N, 3.1%. 1H NMR
(400 MHz, CDCl3): d = 8.19 (s, 1H, H4), 7.72 (d, J = 8.5 Hz, H9), 7.21
(d, 2H, H6), 7.11 (t, J = 8.4 Hz, 2H, H1), 7.16 (dd, J = 2.33 Hz, J = 9.1,
2H, H8), 6.61 (d, J = 2.4 Hz, 2H, H3), 6.49 (dd, J = 2.44 Hz,
J = 8.28 Hz, 2H, H2), 3.92 (t, J = 6.8 Hz, 2H, –OCH2), 0.91 (t,
J = 6.8 Hz, 6H, CH3), 0.87 (m, –CH2 of methylene proton in side
chain); IR (
CH3), 1613 (
m
m
max, cmꢂ1, KBr): 2922 (
C@N), 532 ( Ni–N), 461 (m
mas(C–H), CH3), 2870 (
ms(C–H),
The photoluminescence properties of the compounds were
investigated in dichloromethane solution and in thin films, at room
temperature. The ligands are non-emissive, but their correspond-
ing Ni(II) complexes displayed strong blue emission both in
solution and solid state (Fig. 3), usually observed in similar
Schiff-based metal complexes, with the emission maximum
m
Ni–O).
Ni-16npd. Yield = 0.64 g, 75%. FAB Mass (m/e, fragment): m/z:
calc. 897.5; found: 898.5 [M+H+]; Anal. Calc. for C52H77N3O6Ni: C,
69.4; H, 8.6; N, 4.6. Found: C, 69.3; H, 8.5; N, 4.5%. 1H NMR
(400 MHz, CDCl3): d = 8.23 (s, 1H, H4), 7.77 (d, J = 8.5 Hz, H9), 7.25
(d, 2H, H6), 7.17 (t, J = 8.4 Hz, 2H, H1), 7.18 (dd, J = 2.33 Hz, J = 9.1,
2H, H8), 6.64 (d, J = 2.45 Hz, 2H, H3), 6.42 (dd, J = 2.4 Hz,
J = 8.2 Hz, 2H, H2), 3.93 (t, J = 6.5 Hz, 2H, –OCH2), 0.90 (t,
J = 6.8 Hz, 6H, CH3), 0.88 (m, –CH2 of methylene proton in side
centered at ꢁ454–488 nm originating from
centered excited state [57–60,32]. The emission maxima for typical
compound Ni-18opd in solid state (ꢁ481 nm, = 7%) is consider-
ably red shifted with respect to that recorded in solution
= 23%). In solid state after complexation, the free
p–
p⁄ singlet ligand-
U
chain); IR (
CH3), 1616 (
m
m
max, cmꢂ1, KBr): 2921 (
C@N), 531 ( Ni–N), 457 (m
mas(C–H), CH3), 2870 (
ms(C–H),
(ꢁ456 nm,
U
m
Ni–O).
rotation of the flexible bonds of the ligand is reduced and hence en-
ergy dissipation through non-radiative channels decreases leading
to shift of emission wavelength to lower energy. Moreover in the
solid state, a larger electronic delocalization and intermolecular
aromatic interaction leads to a lowering of energy of the electronic
states [57–60,32]. The spectral data are summarized in Table 1.
3. Results and discussion
3.1. Synthesis and structural assessment
The compounds (16-opd/16-mpd/16-npd and Ni-16opd/Ni-
16mpd/Ni-16npd) could be achieved through a facile and straight-
forward procedure [11,13,15,36–38]. The synthetic strategy for the
3.3. Thermal microscopy and differential scanning calorimetry study
0
ligands [(L = N,N -bis(4-(40-n-hexadecyloxy)-salicylidene)1,2-phen-
The thermal behavior of the compounds (Table 2) was investi-
gated by polarizing optical microscopy (POM) and differential
scanning calorimetry (DSC) study. The ligands are found to be
non-mesomorphic. However, on co-ordination to nickel(II) ion
mesomorphism was induced, reflecting conformational rigidifica-
tion of the ligand on complexation. Monotropic mesomorphism
was encountered for Ni-16npd and Ni-16mpd complexes. The Ni-
16opd showed enantiotropic mesomorphic behavior. Polarized
optical microscopy of a representative complex (Ni-16opd) re-
vealed that, upon cooling the sample from isotropic phase, a spher-
ulitic growth appeared which coalesce to a fan-like texture (Fig. 4)
at ꢁ130 °C with large homeotropic regions, suggesting a columnar
mesophase (Col). The mesophase is stable down to room tempera-
ture. The DSC thermogram (Fig. 5) for the complex (Ni-16opd)
exhibited two transitions in heating and two in cooling cycle.
ylenediamine/4-Me-1,2-phenylenediamine/4-NO2-1,2-phenylene-
diamine), hereafter abbreviated as 16-opd/16-mpd/16-npd] and the
Ni(II) complexes (Ni-16opd/Ni-16mpd/Ni-16npd) are presented in
Scheme 1. The complexes (Ni-16opd/Ni-16mpd/Ni-16npd), were
prepared by the reaction of appropriate ligand with nickel acetate
(1:1 molar ratio) in ethanol/methanol and recrystallized from meth-
anol/CH2Cl2; the complexes were isolated as red colored solids in
good yields. The compounds were characterized by 1H and 13C
NMR, FT-IR, UV–Vis spectroscopy and elemental analysis. From
the IR study, it was found that the shift of
frequency at ca.1625 cmꢂ1 to lower wave number (
and absence of
mCN vibrational stretching
D
m
ꢁ 30 cmꢂ1
)
m
OH mode upon chelation, clearly suggested the coor-
dination of azomethine-N and phenolate-O to the metal. Appear-
ance of additional bands at ꢁ450–480 and ꢁ527–549 cmꢂ1 in the

420
C.R. Bhattacharjee et al. / Polyhedron 33 (2012) 417–424
C16H33
O
CHO
i
HO
CHO
X=H, CH3, NO2
OH
X
OH
ii
X
H2N
NH2
7
8
X
9
6
7
8
4
H
H
9
6
4
N
N
O
H
H
1
Ni
N
N
iii
1
2
O
5
OH
2
HO
3
Oc16H33
C16H33
O
3
C16H33
O
OC16H33
Scheme 1. (i) C16H33Br, KHCO3, KI, dry acetone,
D
, 40 h, (ii) glacial AcOH, absolute EtOH,
D
, 4 h and (iii) Ni(OAc)2ꢀ4H2O, MeOH, Stirr, 2 h.
0.5
1
1. Ni-opd
2. Ni-mpd
2.5
2.0
1.5
1.0
0.5
0.0
1. 16-opd.
2. 16-mpd.
3. 16-npd.
2
3. Ni-npd
3
0.4
0.3
0.2
0.1
0.0
3
1
2
300
400
500
600
280
320
360
400
440
480
Wavelength(nm)
Wavelength(nm)
Fig. 2. Absorption spectra of the complexes.
Fig. 1. Absorption spectra of the ligands.
The melting temperatures of the complexes showed a decreasing
trend with X = H, CH3, and NO2 in that order (Table 2). The isotrop-
isation temperatures, however, reflected a different trend with the
nitro substituted complex exhibiting highest value (209.2 °C) and
the methyl substituted one showing the least (125.2 °C). Another
interesting aspect is the quite low enthalpy values for the phase
transitions in the unsubstituted complex (Ni-16opd) compared to
those observed for the substituted ones (Ni-16mpd and Ni-
16npd). In general, complexes with higher molecular weights tend
to exhibit larger enthalpy changes during mesophase to isotropic
liquid transitions [27]. Relatively high enthalpies for mesophase
transition to isotropic state in the substituted complexes, Ni-
16mpd and Ni-16npd could be indicative of the formation of a
higher order columnar phase in this system [61].
In our earlier reports, we have found that, the VO-16opd
showed lamellar columnar mesomorphism (Coll) [36]. However,
Zn-16opd/Zn-16mpd complexes exhibited columnar rectangular
(Colr/Colh) mesophase [11,13].
3.4. XRD-study
The mesophase structure was confirmed by temperature-
dependent X-ray diffraction analysis (Table 3). The spectrum
was recorded for a typical compound, Ni-16opd at 130 °C. In
the low angle region two fundamental sharp reflections were ob-
served characteristic of
a rectangular columnar mesophase
(Fig. 6). Further, there are two diffuse reflections in the wide an-
gle region. One corresponds to 4.9 Å, for molten alkyl chains, the

C.R. Bhattacharjee et al. / Polyhedron 33 (2012) 417–424
421
1. In solution.
2. In solid.
400
200
0
1
2
400
500
600
Fig. 4. POM texture of Ni-16opd complex.
Wavelength(nm)
Fig. 3. Emission spectra of the Ni-16opd complex.
50
45
40
Table 1
UV–Vis and photoluminescence data of the compounds.
Compounds
p
?
p⁄
LMCT
, l molꢂ1 cmꢂ1
PLa
(solution)
PLa
(solid)
(e
, l molꢂ1 cmꢂ1
)
(
e
)
16opd
292 nm (22000)
329 nm (26800)
363 nm (18981)
315 nm (2770)
385 nm (3610)
289 nm (19300)
330 nm (23100)
365 nm (18110)
314 nm (2200)
385 nm (2670)
295 nm (13400)
335 nm (16851)
367 nm (21850)
326 nm (3450)
402 nm (3800)
Ni-16opd
16mpd
447 nm (1800)
448 nm (1310)
462 nm (2820)
456 nm
454 nm
468 nm
481 nm
478 nm
488 nm
Ni-16mpd
16npd
60
80
100
120
140
160
Temperature(0C)
Ni-16npd
Fig. 5. DSC thermogram of Ni-16opd complex.
a
Photoluminescence data.
Table 2
POM and DSC data of the complexes.
relatively sharper one at about 3.6 Å, well separated from the
broad peak is due to the regular stacking of the molecules within
the columnar mesophase. In the wide angle region the presence
of another less broad peak at about 7.6 Å may indicate the forma-
tion of dimers (Fig. 7) along the axis of the column. Moreover,
due to the absence of (21) peak in the XRD spectrum, the symme-
try of the lattice can be further assigned to a c2mm plane group
[39,62,63]. The lattice constants of this rectangular phase are
a = 38.2 Å and b = 15.4 Å. The lattice parameter a = 38.2 Å is larger
than the radius of the half-disc shaped molecule (ꢁ20.6 Å). There-
fore the hemi-disc molecules in the mesophase are believed to
organize themselves in an anti-parallel fashion (Fig. 7). A similar
type of molecular self assembly was reported earlier
[15,37,39,64].
Compounds
Ni-16opd
T (°C)
Transition
D )
H (kJ molꢂ1
108.2 (heating)
135.0 (heating)
133.9 (cooling)
87.1 (cooling)
84.4 (heating)
125.2 (heating)
78.3 (heating)
209.2 (heating)
Cr–Colr
Colr–I
I–Colr
Colr–Cr
Cr–Colr
Colr–I
Cr–Colr
Colr–I
2.9
2.6
2.5
0.93
Ni-16mpd
Ni-16npd
12.8
12.9
14.8
15.4
highest occupied molecular orbital (HOMO) for the nickel com-
plex (Ni-16opd) is shown in (Figs. 9 and 10). In all the com-
plexes, electron density of the HOMO is mainly localized in
between Ni–O and Ni–N bonds, while the electron density of
the LUMO is localized on nickel atoms. The Ni–O (apical) inter-
action in the dimer (Figs. 9 and 10) expectedly occur between
molecular orbitals of sigma (O) and pi⁄(Ni). The HOMO, LUMO
energy difference of the complexes depends on the electronic
nature of the substituents [65–67]. The energy differences
decrease in the order of Ni-16mpd > Ni-16opd > Ni-16npd. The
HOMO–LUMO energy separation can be used as a measure of
3.5. DFT-study
The optimized structure of the complex is shown in (Fig. 8).
The complexes are neutral with Ni2+ in d8-system. We optimized
singlet and triplet state of the complexes and found singlet
square planar geometry is more stable. The 3D isosurface plots
of the lowest unoccupied molecular orbital (LUMO) and the

422
C.R. Bhattacharjee et al. / Polyhedron 33 (2012) 417–424
Table 3
XRD-data of the Ni-16opd.
Compound
Ni-16opd
Dobs. (Å)a
Dcalc. (Å)b
hkc
Parametersd
19.3
14.1
10.3
7.6
4.9
3.5
19.2
14.3
10.4
20
11
31
Colr – c2mm
a = 38.2 Å
N
O
N
O
b = 15.4 Å
Ni
RO
S = 588.3 Å2
Vm = 1551 Å3
h = 2.1 Å
RO
OR
O
N
OR
O
N
Ni
Scol = 294.1
a
Dobs. is experimentally and theoretically measured diffraction spacings at
130 °C.
b
Dcalc. is experimentally and theoretically measured diffraction spacings at
130 °C.
c
hk are indexation of the reflections.
Mesophase parameters, molecular volume Vm is calculated using the formula:
Fig. 7. Dimeric interaction of molecules.
d
Vm = M/k
gadro number,
qNA, where M is the molecular weight of the compound, NA is the Avo-
q
is the volume mass (ꢃ1 g cmꢂ3), and k(T) is a temperature cor-
rection coefficient at the temperature of the experiment (T). k = VCH2 (T0)/VCH2 (T),
T0 = 25 °C. VCH2 (T) = 26.5616 + 0.02023 (T). h is the intermolecular repeating dis-
tance deduced directly from the measured molecular volume and the lattice area
according to h = Vm/S. For the Colr phase, the lattice parameters a and b are deduced
p
h2/a2 + k2/b2, where a, b
from the mathematical expression: a = 2d20 and 1/dhk
=
are the parameters of the Colr phase, S is the lattice area, Scol is the columnar cross-
section (S = ab, Scol = S/2).
6000
4000
2000
0
Fig. 8. Optimized structure of Ni-16opd.
3.8
7.6
11.4
15.2
19.0
2θ (0)
22.8
26.6
30.4
34.2
Fig. 6. XRD-pattern of Ni-16opd.
kinetic stability of the molecule and could indicate the reactivity
pattern of the molecule. A small HOMO–LUMO gap implies a low
kinetic stability and high chemical reactivity, because it is
energetically favorable to add electrons to LUMO or to extract
electrons from a HOMO. We further calculated the chemical
softness values for both model complexes from their HOMO
and LUMO energies. The chemical softness values of hydrogen,
methyl and nitro-substituted complexes are 2.058, 2.051 and
3.643 eVꢂ1, respectively. The higher softness value obtained for
the later complex indicated the lower stability of the complex
compared to the hydrogen and methyl containing one. Some of
the selective geometric parameters of optimized hydrogen,
methyl and nitro-substituted nickel complex, evaluated by DFT
calculation at BLYP/DNP level are reported in Table 4. From
DFT data, it is noticed that the complexes have an average
Ni–O and Ni–N bond lengths are in the range of 1.93–1.92 and
2.00–1.99 Å, respectively. The average bond angles are in the
range of 94.70–92.8 and 83.20–83.6 for O1–Ni–O2 and N1–
Fig. 9. LUMO energy diagram of Ni-16opd.
Ni–N2, respectively, around the nickel atom deviate substantially
from the tetrahedral values indicating a distorted planar four
coordinate geometry. The dihedral angles O(1)O(2)N(1)N(2) and
N(1)O(1)O(2)N(2) as computed from DFT are found to lie in
the range 15–20° (Table 4) reflecting the deviation from planar-
ity. A strained conformation of the [N2O2]-donor tetradentate li-
gand with long pendant alkyl side chains is believed to have
caused the deviation from planar symmetry. Moreover, a short
rigid central spacer group in the present complexes presumably
prevented the formation of a tetrahedral environment around
nickel(II), leading to a distorted square planar geometry. The
length of the complexes based on the fully extended structure
is found to be ꢁ40.7 Å (measured from the two terminal end
of the side alkyl chain).

C.R. Bhattacharjee et al. / Polyhedron 33 (2012) 417–424
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Ni-opd
Ni-mpd
Ni-npd
Ni–O(1)
Ni–O(2)
Ni–N(1)
Ni–N(2)
O(1)–Ni–O(2)
N(1)–Ni–N(2)
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