Novel photoluminescent hemi-disclike liquid crystalline Zn(II) complexes of [N2O2] donor 4-alkoxy substituted salicyldimine Schiff base with aromatic spacer

Article abstract of DOI:10.1016/j.poly.2010.08.017

A series of novel hemi-disclike four coordinated distorted square planar Zn(II) Schiff base complexes containing 4-substituted alkoxy chains on the side aromatic ring [Zn (4-C_nH_2n+1O) ₂ salophen], n = 14, 16, 18 and salophen = N,N′-4-methyl phenylene bis (salicylideneiminato), have been prepared and their mesogenic, photophysical properties were investigated. The phase behavior of these compounds were characterized by differential scanning calorimetry, polarized optical microscopy and variable temperature PXRD study. The ligands are non-mesogenic but the complexes exhibited an unprecedented 2D-hexagonal columnar mesophase (Col_h) in the temperature 175-185 °C range. In the mesophase (Col_h), the molecules self assemble in a columnar stack in antiparallel fashion. All λ_max of the UV-Vis absorption and photoluminescence band occurred at ca. 291-425 and 504-524 nm, respectively. The ligands are non-emissive, but on coordination with Zn(II), the complexes show intense green emission at room temperature in dichloromethane solution (～505 nm, Φ = 20%) as well as in solid (～522 nm, Φ = 9%) at 360 nm excitation. The DFT calculations were performed using Dmol3 program at BLYP/DNP level to obtain the stable electronic structure of the complex. A small LUMO-HOMO band gap (～2.1 eV), presumably suggests a rather strong electronic correlation among the molecules along the column.

Full text of DOI:10.1016/j.poly.2010.08.017

Polyhedron 29 (2010) 3089–3096
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Novel photoluminescent hemi-disclike liquid crystalline Zn(II) complexes
of [N2O2] donor 4-alkoxy substituted salicyldimine Schiff base with aromatic spacer
⇑
Chira R. Bhattacharjee , Gobinda Das, Paritosh Mondal, N.V.S. Rao
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
Article history:
A series of novel hemi-disclike four coordinated distorted square planar Zn(II) Schiff base complexes
containing 4-substituted alkoxy chains on the side aromatic ring [Zn (4ꢀCnH2n+1O)2 salophen], n = 14,
16, 18 and salophen = N,N0-4-methyl phenylene bis (salicylideneiminato), have been prepared and
their mesogenic, photophysical properties were investigated. The phase behavior of these compounds
were characterized by differential scanning calorimetry, polarized optical microscopy and variable
temperature PXRD study. The ligands are non-mesogenic but the complexes exhibited an unprece-
dented 2D-hexagonal columnar mesophase (Colh) in the temperature 175–185 °C range. In the meso-
phase (Colh), the molecules self assemble in a columnar stack in antiparallel fashion. All kmax of the
UV–Vis absorption and photoluminescence band occurred at ca. 291–425 and 504–524 nm, respec-
tively. The ligands are non-emissive, but on coordination with Zn(II), the complexes show intense
Received 14 July 2010
Accepted 12 August 2010
Available online 20 August 2010
Keywords:
Zinc
Schiff base
Metallomesogen
DFT
Fluorescence
green emission at room temperature in dichloromethane solution (ꢁ505 nm,
U = 20%) as well as in
= 9%) at 360 nm excitation. The DFT calculations were performed using DMOL3
solid (ꢁ522 nm,
U
program at BLYP/DNP level to obtain the stable electronic structure of the complex. A small LUMO-
HOMO band gap (ꢁ2.1 eV), presumably suggests a rather strong electronic correlation among the
molecules along the column.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
base ligands are very sparse [17]. Accessibility, ease of derivatiza-
tion has rendered such ligands as versatile synthons for wider
Metallomesogen derived from Schiff-base ligands feature
amongst the earliest and most widely studied class of complexes
[1]. Compounds showing both luminescence and liquid crystallin-
ity are one of the most promising area for the applications in
OLEDs, information storage, sensors, and enhanced contrast dis-
plays [2]. Major research on blue luminescent materials has been
carried out mainly for the metals such as Al, Be, Cu, and Zn [3]. Pho-
toluminescent metallomesogens have been documented with lan-
thanide metals, Pd, Pt, Ag and Au, Cu [4–8]. Zinc analogs are tardy
though many non-mesogenic zinc complexes are known to display
interesting optical properties for LED and sensory device applica-
tions [9–12]. Pyrazole based zinc complexes with non-conven-
tional tetrahedral geometry displaying both supramolecular
mesomorphic order and luminescence has been reported recently
[13]. The Schiff base complexes of first row transition metal with
5-substituted long alkoxy chains forming linear rod like (1D) struc-
ture showing smectic mesophases has been extensively studied
[2,13–16]. In contrast, 4-substituted salen framework based Schiff
use in supramolecular chemistry, nanotechnology and design of
novel (bio-inspired) materials [17]. Quite surprisingly such ligands
with aromatic spacers are not known till date [18]. Recently Dani-
ela Pucci et al. reported some blue light emitting nonlinear
4-substituted Zn(II) complexes with central aliphatic spacer show-
ing SmC mesophase [19]. Choice of metal ion, nature and position
of the substituent on side aromatic ring as well as spacers are
known to greatly influence the mesogenic behavior [20]. Zinc com-
plexes based on salen type framework with high thermal and
redox stability are attractive as low cost accessible building blocks
for newer light-emitting materials [19]. Emission performance of
such systems are dependent on the nature of central bridging dia-
mine core [19]. Though few complexes of VO(IV/V), Ni(II), and
Cu(II) with 4-substituted long alkoxy chains (n = 8–18) of aromatic
rings of salen-type Schiff base ligands with ethylene diamine or
propylene diamine as rigid cores are on record there is no report
on the related zinc(II) complexes with aromatic core as central
linking group [21–23]. Zinc(II) complexes of suitably substituted
2,20,-bipyridyl ligands have been shown to exhibit hexagonal
columnar mesomorphism regardless their rigid central core and
tetrahedral coordination geometry [24,25]. In the present study,
a series of new tetradentate [N2O2] donor Schiff base with a methyl
⇑
Corresponding author.
E-mail address: crbhattacharjee@rediffmail.com (C.R. Bhattacharjee).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.08.017

3090
C.R. Bhattacharjee et al. / Polyhedron 29 (2010) 3089–3096
H2n+1CnO
CHO
n = 14, 16,18
HO
CHO
i
OH
8
OH
H3C
H2N
ii
H3C
H3C
10
11
6
NH2
7
5
12
H
H
4
N
O
N
O
iii
N
N
Zn
1
13
14
9
OH
HO
2
3
H2n+1CnO
OCnH2n+1
15
H2n+1CnO
OCnH2n+1
Scheme 1. (i) CnH2n+1 Br, KHCO3, KI, dry acetone,
D
, 40 h, (ii) glacial AcOH, absolute EtOH
D
, 4 h and (iii) Zn(OAc)2ꢂ2H2O, MeOH, TEA
D, 1 h.
substituted aromatic rigid spacer and 4-substituted long alkoxy
group at the side aromatic rings have been synthesized and their
Zn(II) complexes exhibiting green luminescence were accessed.
Though the ligands are non-mesogenic, the complexes showed
2D-hexagonal columnar mesophases (Colh) hitherto unreported
for such 4-substituted compounds [2,9,11,14–23].
synthetic strategy for the ligands [(L = N,N0-bis-(4-(40-n-alkoxy)-
salicylidene) 4-Me-1,2-phenylenediamine), hereafter abbreviated
as n-mpd, where n indicates the number of carbon atoms in alkyl
chains, n = 14,16,18 and mpd = 4-methyl-1,2-phenylene diamine]
and the mononuclear Zn(II) complexes (Zn-nmpd) are presented
in Scheme 1.
2.2.1. Synthesis of n-alkoxysalicyldehyde (n = 14, 16, 18)
2. Experimental
Alkoxysalicyldehyde derivatives were prepared following the
general method reported in literature [26]. 2,4-Dihydroxybenzal-
dehyde (10 mmol, 1.38 g), KHCO3 (10 mmol, 1 g), KI (catalytic
amount) and 1-bromotetradecane (10 mL, 2.5 g) or 1-bromohexa-
decane (10 mL, 2.8 g) or 1-bromooctaadecane (10 mL, 3 g) were
mixed in 250 mL of dry acetone. The mixture was heated under re-
flux for 24 h, and then filtered, while hot, to remove any insoluble
solids. Dilute HCl was added to neutralize the warm solution,
which was then extracted with chloroform (100 mL). The com-
bined chloroform extract was concentrated to give a purple solid.
The solid was purified by column chromatography using a mixture
of chloroform and hexane (v/v, 1/1) as eluent. Evaporation of the
solvents afforded a white solid product.
2.1. General details
All solvents were purified and dried using standard procedures.
The materials were procured from Tokyo Kasei and Lancaster
Chemicals. Silica (60–120 mesh) from Spectrochem was used for
chromatographic separation. Silica gel G (E-Merck, India) was used
for TLC. The C, H and N analyses were carried out using PE2400 ele-
mental analyzer. The 1H NMR spectra were recorded on a Bruker
DPX-400 MHz spectrometer in CDCl3 (chemical shift in d) solution
with TMS as internal standard. 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 fluorescence quan-
tum yield in dichloromethane were determined by dilution meth-
od using 9,10-diphenyl anthracene as standard. Infrared spectra
were recorded on a Perkin–Elmer L 120-000A spectrometer on
KBr disc. The optical textures of the different phase of the com-
pounds were studied using a polarizing microscope (Nikon optip-
hot-2-pol) attached with Instec hot and cold stage HCS302, with
STC200 temperature controller of 0.1 °C accuracy. The thermal
behavior of the compounds were studied using a Perkin–Elmer dif-
ferential 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
0
2.2.2. Synthesis of N,N-bis-(4-(40-n-octadecyloxy)-salicylidene)-4-Me-
1,2-phenylenediamine (18-mpd)
An ethanolic solution of 2-hydroxy-(4-octadecyloxy)-salicylal-
dehyde (0.33 g, 1 mmol) was added to an ethanolic solution of 4-
Me-1,2-phenylenediamine (0.06 g, 1 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)-salicylid-
ene)-4-Me-1,2-phenylenediamine. The compound was collected
by filtration and recrystallised from absolute ethanol to obtain a
pure compound. Yield: 0.279 g (75%); m.p., 126 °C. 1H NMR
(400 MHz, CDCl3): d = 13.07 (s, 1H, H9), 8.77 (s, 1H, H4), 7.74 (d,
J = 8.54 Hz, H5), 7.24 (d, 2H, H7), 7.14 (t, J = 8.43 Hz, 2H, H1), 7.13
(dd, J = 2.41 Hz, J = 9.0, 2H, H6), 6.61 (d, J = 2.44 Hz, 2H, H3), 6.49
(dd, J = 2.44 Hz, J = 8.29 Hz, 2H, H2), 3.97 (t, J = 6.8 Hz, 2H, –
OCH2), 2.1 (s, 1H, H8), 0.89 (t, J = 6.8 Hz, 6H, CH3), 0.86 (m, –CH2
of methylene proton in side chain). 13C NMR (75.45 MHz; CDCl3;
Me4Si at 25 °C, ppm) d = 134.03 (–C1), 118.14 (–C5), 141.7 (–C11),
135.82 (–C10), 128.03 (–C6), 120.93 (–C7), 140.34 (–C12), 162.52
(–NCH), 113.00 (–C13), 164.12 (–C14), 108.13 (–C3), 162.79 (–C15),
D8 Discover instrument using Cu Ka radiation. Quantum chemical
calculation on Zn-18mpd was carried out using density functional
theory (DFT) as implemented in DMol3 package.
2.2. Synthesis and analysis
The Schiff base ligands, were synthesized by condensation of 4-
alkoxysubstituted aldehyde with the 4-methyl-1,2-phenylene dia-
mine with slight modification of the literature procedure [26]. The
115.64 (–C2). IR (mmax;cmꢀ1 , KBr): 3500 (mOH), 2925 (mas(C–H), CH3),

C.R. Bhattacharjee et al. / Polyhedron 29 (2010) 3089–3096
3091
2921 (mas(C–H), CH2), 2873 (
ms(C–H), CH3), 2850 (mas(C–H), CH2), 1629
3. Results and discussion
(m
C@N), 1298 (mC–O).
3.1. Spectral investigation
2.2.3. Synthesis of N,N0-bis-(4-(40-n-hexadecyloxy)-salicylidene)-4-
Me-1,2-phenylenediamine (16-mpd)
The characterizations of the compounds were made by elemen-
tal analyses, FT-IR, 1H NMR, and mass spectrometry. The analytical
data of the compounds Table 1, are in good agreement with the
proposed formulae of the compounds. The shift of mCN vibrational
Yield: 0.21 g (73%); m.p., 128 °C. 1H NMR (400 MHz, CDCl3):
d = 13.01 (s, 1H, H9), 8.72 (s, 1H, H4), 7.77 (d, J = 8.54 Hz, H5),
7.24 (d, 2H, H7), 7.15 (dd, J = 2.41 Hz, J = 9.0, 2H, H6), 7.14 (t,
J = 8.43, 2H, H1), 6.49 (dd, J = 2.14 Hz, J = 8.29 Hz, 2H, H2), 6.45 (d,
J = 2.13 Hz, 2H, H3), 3.91 (t, J = 6.8 Hz, 2H, –OCH2), 2.5 (s, 1H, H8),
0.98 (t, J = 6.8 Hz, 6H, CH3), 0.82 (m, –CH2 of methylene proton in
stretching frequency at ca. 1625 cmꢀ1 to lower wave number (
Dm
ꢁ30 cmꢀ1) and absence of mOH mode upon chelation, clearly sug-
gested the coordination of azomethine N and phenolate oxygen
to the metal. The mC@N stretching frequency is rather independent
of the length of alkoxy side chain in both ligands and their com-
plexes. A comparison of the 1H NMR spectral data of the ligands
with those of the Zn(II) complexes showed the absence of the phe-
nolic –OH and a significant shift (8.324–8.197 ppm) in the peak
positions of the –N@CH in the spectrum of the metal complex, sug-
gesting binding through the phenolate anion and the azomethine
nitrogen atom of the ligand to the metal ion [27]. The FAB-mass
spectra Table 1, of the Zn(II) complexes are concordant with their
formula weights. These features compare well with a related struc-
turally analogous fluorescent but non-mesogenic 4-methoxy
substituted-salophen-zinc complex (Fig. 1) [9], and a dodecyloxy-
bis-(salicylaldiminato)zinc(II) complexes stacked in 3D aggrega-
tion with no reported mesogenicity [11].
side chain). IR (mmax;cmꢀ1 , KBr): 3434 (
2918 ( as(C–H), CH2), 2844 ( s(C–H), CH3), 2845 (
C@N), 1289 ( C–O).
m
OH), 2916 (
mas(C–H), CH3),
m
m
m
as(C–H), CH2), 1622
(m
m
2.2.4. Synthesis of N,N0-bis-(4-(40-n-tetradecyloxy)-salicylidene)-4-
Me-1,2-phenylenediamine (14-mpd)
Yield: 0.19 g (74%) m.p., 130 °C. 1H NMR (400 MHz, CDCl3): d
13.12 (s, 1H, H9), 8.77 (s, 1H, H4), 7.74 (d, J = 8.54 Hz, H5), 7.24
(d, 2H, H7), 7.23 (dd, J = 2.81 Hz, J = 9.0, 2H, H6), 7.15 (t, J = 8.43,
2H, H1), 6.66 (d, J = 2.44 Hz, 2H, H3), 6.43 (dd, J = 2.44 Hz,
J = 8.29 Hz, 2H, H2), 3.97 (t, J = 6.8 Hz, 2H, –OCH2), 2.5 (s, 1H, H8).
0.99 (t, J = 6.3 Hz, 6H, CH3), 0.88 (m, –CH2 of methylene proton in
side chain). IR (mmax;cmꢀ1 , KBr): 3433 (
2919 ( as(C–H), CH2), 2849 ( s(C–H), CH3), 2850 (
1626( C@N), 1287( C–O).
m
OH), 2918 (
m
as(C–H), CH3),
m
m
mas(C–H), CH2),
m
m
3.2. Photophysical properties
2.2.5. Synthesis of mononuclear zinc(II) complexes (Zn-nmpd
n = 14,16,18)
The electronic absorption spectra (Fig. 2), of n-mpd (n = 14, 16,
18) recorded in dichloromethane (10ꢀ4 M), consisted of two peaks
at 291 and 333 nm and a very-low intensity band at ꢁ364 nm. The
2.2.5.1. General procedure. The ligand 18-mpd (0.075 g,0.1 mmol)
or 16-mpd (0.062 g,0.1 mmol) or 14-mpd (0.060 g.0.01 mmol)
was dissolved in minimum volume of absolute ethanol. To this,
an equimolar amount of zinc acetate Zn(OAc)2ꢂ2H2O (0.02 g,
0.1 mmol) in methanol was then added slowly and stirred for 2 h
at room temperature. A yellow solid formed immediately was fil-
tered, washed with diethyl ether and recrystallised from chloro-
form–ethanol (1:1).
band at ꢁ291 and ꢁ333 nm are attributed to a
p–p* transition
localized on the aromatic rings, whereas the band at ꢁ364 nm is
due to the n– * excitation of the lone pair on the imine nitrogen
atom to the * orbital on the C@N fragment [19]. The Zn(II) com-
p
p
plexes showed two intense bands at ꢁ313 and ꢁ375 nm resulting
from the metal-perturbed ligand-centered transitions, which have
the same origin as the two principal bands of the ligand spectrum
[19]. Further, on complexation the bands at ꢁ364 nm of the ligands
are red shifted to ꢁ425 nm, which appeared as a shoulder in the
spectra. This has originated from the donation of lone pair
2.2.5.2. Zn-18mpd. Yield = 0.0.07 g (75%) 1H NMR (400 MHz,
CDCl3): d, 8.21 (s, 1H, H4), 7.70 (d, J = 8.51 Hz, H5), 7.20 (d, 2H,
H7), 7.12 (t, J = 8.34, 2H, H1), 7.01 (dd, J = 2.35 Hz, J = 8.59, 2H,
H6), 6.56 (d, J = 2.44 Hz, 2H, H3), 6.39 (dd, J = 2.34 Hz, J = 8.19 Hz,
2H, H2), 3.97 (t, J = 6.8 Hz, 2H, –OCH2), 2.1 (s, 1H, H8), 0.89 (t,
J = 6.8 Hz, 6H, CH3), 0.86 (m, –CH2 of methylene proton in side
Table 1
Mass spectrometric and elemental analysis data of Zn-nmpd.a
Complex
FAB+
%C
%H
%N
chain). IR (mmax;cmꢀ1 , KBr): 2920 (mas(C–H), CH3), 2850 (mas(C–H), CH2),
1612 ( C@N).
m
Zn-18mpd
Zn-16mpd
Zn-14mpd
928 (929)
872 (873)
815 (816)
73.2 (73.5)
72.1 (72.7)
70.4 (70.9)
9.4 (9.6)
9.5 (9.3)
8.5 (8.8)
3.1 (3.0)
3.3 (3.2)
3.2 (3.4)
2.2.5.3. Zn-16mpd. Yield = 0.03 g (70%) 1H NMR (400 MHz, CDCl3):
d, 8.04 (s, 1H, H4), 7.70 (d, J = 8.51 Hz, H5), 7.25 (d, 2H, H7), 7.10
(t, J = 8.34 Hz, 2H, H1), 7.09 (dd, J = 2.35 Hz, J = 8.59, 2H, H6), 6.54
(d, J = 2.44 Hz, 2H, H3), 6.32 (dd, J = 2.34 Hz, J = 8.19 Hz, 2H, H2),
3.97 (t, J = 6.8 Hz, 2H, –OCH2), 2.1 (s, 1H, H8) 0.89 (t, J = 6.8 Hz,
6H, CH3), 0.84 (m, –CH2 of methylene proton in side chain). IR
a
Calculated values are in brackets.
(
mmax;cmꢀ1 , KBr): 2919 (
C@N).
mas(C–H), CH3), 2849 (mas(C–H), CH2), 1583
(m
N
N
2.2.5.4. Zn-14mpd. Yield = 0.06 g (75%) 1H NMR (400 MHz, CDCl3):
d, 8.21 (s, 1H, H4), 7.70 (d, J = 8.51 Hz, H5), 7.21 (d, 2H, H7), 7.16
(t, J = 8.34 Hz, 2H, H1),7.01 (dd, J = 2.35 Hz, J = 8.54, 2H, H6), 6.56
(d, J = 2.44 Hz, 2H, H3), 6.39 (dd, J = 2.34 Hz, J = 8.19 Hz, 2H, H2),
3.97 (t, J = 6.8 Hz, 2H, –OCH2), 2.1 (s, 1H, H8), 0.99 (t,
J = 6.8 Hz, 6H, CH3), 0.89 (m, –CH2 of methylene proton in side
Zn
O
O
H3CO
OCH3
chain). IR (mmax;cmꢀ1 , KBr): 2919 (mas(C–H), CH3), 2851 (mas(C–H), CH2),
1589( C@N).
m
Fig. 1. Zinc salophen complex with 4-methoxy substituent.

3092
C.R. Bhattacharjee et al. / Polyhedron 29 (2010) 3089–3096
Table 2
UV-Vis and photoluminescence data of ligands (n-mpd)and Zn-nmpd complexes.
Compounds
p
?
p
*
n?
p*
PLa
(Solution)
PLa
(Solid)
(e
, l molꢀ1 cmꢀ1
)
(
e
, l molꢀ1 cmꢀ1
)
18-mpd
292 (11300)
331 (14200)
366 (9900)
425 (8100)
364 (9100)
426 (8200)
366 (9300)
425 (8400)
–
Zn-18mpd
16-mpd
314 (11200)
377 (14100)
504
–
520
522
524
291 (11400)
333 (14400)
Zn-16mpd
14-mpd
316 (11400)
379 (14400)
507
–
293 (11200)
332 (14600)
Zn-14mpd
314 (11300)
376 (11500)
509
Fig. 2. UV–Vis absorption spectra of 18-mpd and Zn-18mpd in CH2Cl2.
a
Photoluminescence data of Zn-nmpd.
electrons to the metal further attesting the coordination of azome-
thine [28,29].
The study of luminescent Zn(II) complexes is not only of
fundamental interest, but also significant for different applications
(optical amplifiers, optical waveguides, OLEDs, etc.) [19]. Photolu-
minescence study of the Zn(II) complexes are carried out at room
temperature in dichloromethane solution and also in the solid
state (Fig. 3). The ligands are non-emissive while the correspond-
ing Zn(II) complexes emit intense green light (Fig. 4) at ꢁ505–
522 nm with high emission quantum yield of ꢁ20% (solution),
and 9% (solid) when excited at 360 nm, Table 2. For Zn(II) com-
plexes, no emission originating from metal-centered MLCT/LMCT
excited states is expected due to redox insensitive stable
Zn2+(d10) configuration [19,30–32]. The intense emission observed
at room temperature in the complex stems from the intraligand
Fig. 5. Overlap of UV–Vis and fluoroscence spectra of Zn-18mpd.
(p–p
*) fluorescence, and the role of the central Zn2+ ion is to pro-
vide stability to the ligand [30–32]. The solid state emission spec-
tra of the complexes were recorded by placing a uniform powder
sheet between two quartz plates. The emission band at ꢁ504 nm
in solution is red shifted by ꢁ20 nm in all the complexes. This shift
is due to the intermolecular aromatic interaction which is weaker
in solution than that in the solid [3,19,13]. Moreover in the solid
state, a larger electronic delocalization leads to a lowering of
energy of the electronic states. On the basis of overlap of the fluo-
rescence emission (508 nm) with the lowest energy UV–Vis
absorption band (425 nm), (Fig. 5) the lowest excited singlet state
energy is experimentally calculated to be ꢁ260 kJ molꢀ1
.
Fig. 3. Photoluminescence spectrum of Zn-18mpd in solid (1) and in solution (2).
3.3. Thermal microscopy and differential scanning calorimetric
studies: mesomorphic behavior
The ligands N ,N0-bis-(4-(40-n-alkoxy)-salicylidene)-4-Me-1,2-
phenylenediamine are non-mesogenic. Polarized optical micros-
copy studies of the complexes with alkoxy chain lengths, n = 14,
16, 18 exhibited monotropic mesomorphic behavior in the temper-
ature range 175–185 °C. Upon cooling the sample from isotropic
phase, a sphereulitic growth appeared, which coalesce to a focal
conic fan texture (Fig. 6) with large homeotropic regions, suggest-
ing a hexagonal columnar mesophase (Colh). A similar observation
has been reported earlier for platinum metal complexes [33]. Large
homeotropic regions are caused by the bulky alkoxy groups in the
molecules. Pertinent here is to mention that all other previously
reported metallomesogens with 4-substituted alkoxy-salen type
Fig. 4. Fluorescent image of Zn-18mpd in solid state.

C.R. Bhattacharjee et al. / Polyhedron 29 (2010) 3089–3096
3093
phase transition. The clearing temperatures of the complexes
which usually correspond to the breakdown of the packing and
columnar stacking showed a marginal increase (5–10 °C incre-
ment) with decrease in alkoxy chain length. The isotropic to
columnar (I–Colh) phase transition temperatures also exhibited a
similar dependence on alkoxy chain length. Self-assembly into
columnar phases and strong electronic interaction within the col-
umns, render them as potential charge carrier system [22]. The
p–p orbital overlap of the rigid aromatic spacer further augment
such charge transport along the columns holding promises for
application in the area of molecular electronics, optoelectronics,
photoconductivity etc. [1,34].
Fig. 6. Optical micrograph of the fan like textures of Zn-16mpd at 169 °C (cooling
cycle).
3.4. X-ray diffraction studies
Powder X-ray diffraction (PXRD) pattern (Fig. 8) of Zn-18mpd at
Schiff base showed either smectic, lamellar (Coll) or rectangular
(Colr) columnar mesophase [21–23]. DSC thermograms of the com-
plexes Zn-18mpd and Zn-16mpd (Fig. 7), exhibited solid–solid, so-
lid–liquid transitions in heating and isotropic liquid to mesophase
and then to solid in the cooling cycle, however, the Zn-14mpd
complex exhibited an additional solid–solid (Cr1–Cr2) phase tran-
sitions in the heating cycle Table 3. This solid–solid transition
could not be detected in thermal microscopy. All the complexes
are highly viscous imposing severe restriction in the molecular
mobility. However, on pressing the sample with a needle on glass
cover slips, fluidity could be observed. The DSC trace for the com-
175 °C was recorded on a Bruker D8 Discover instrument using Cu
Ka
radiation. In the low-angle region, four sharp peaks Table 4, one
very strong and three weak reflections are seen whose d-spacings
p
p p
are in the ratio of 1: 3: 4: 7. Identifying the first peak with the
Miller index [10], the ratios confirm a two-dimensional hexagonal
lattice [35]. In the wide angle region three diffuse reflections are
plex Zn-16mpd at 182.9 °C (D
H = 6.7 kJ molꢀ1) is due to I–Colh
Fig. 8. X-ray diffraction pattern of Zn-18mpd at 175 °C.
Table 4
PXRD data of Zn-18mpd.
dobs (Å)a
I
hkb
dcalc (Å)c
Parametersd
Fig. 7. DSC thermogram of Zn-16mpd.
Zn-18mpd
22.14
12.69
10.98
8.19
7.4
4.9
3.7
vs
w
w
w
br
br
s
10
11
20
21
h0
22.12
12.67
10.84
8.21
a = 25.5 Å
S = 566 Å2
Vm = 1722 Å3
h = 3.04
Table 3
DSC data of the Zn-nmpd complexes.
N = 1
Compounds
Zn-18mpd
T ( °C)a
Transitionb
D )
H (kJ molꢀ1
hCh
ho
101.6
182.9
175.8
161.1
Cr–Cr1
Cr1–I
I–Colh
14.9
16.4
3.2
a
b
c
dobs is experimentally and theoretically measured diffraction spacings at 175 °C.
[hk] are indexation of the reflections.
dcalc is experimentally and theoretically measured diffraction spacings at
Colh–Cr1
2.1
Zn-16mpd
Zn-14mpd
123.3
192.2
182.9
175.7
Cr–Cr1
Cr1–I
I–Colh
17.5
19.2
6.7
175 °C.
d
The mesophase parameters are deduced from the following mathematical
expressions for the columnar phase <d10> = 1/Nhk
Nhk is the number of hk reflections and a = 2<d10>/ 3 (for Colh). Molecular volume
Vm is calculated using the formula: Vm = M/k
the compound, NA is the Avogadro number,
P
[
hkdhk(h2 + k2 + hk)1/2] where
p
Colh–Cr1
5.4
q
q
NA where M is the molecular weight of
101.6
120.0
197.3
184.4
182.3
Cr–Cr1
Cr1–Cr2
Cr2–I
I–Colh
Colh–Cr2
7.7
17.2
18.0
2.2
is the volume mass (ꢃ1 g cmꢀ3), and k
(T) is a temperature correction coefficient at the temperature of the experiment (T).
k = VCH (T0)/VCH (T), T0 = 25 °C. VCH (T) = 26.5616 + 0.02023. The intracolumnar
2
2
2
repeating distance h is calculated directly from the estimated molecular volume Vm
according to the equation: h = NVm/S, in which N is the number of molecules (here N
3.8
a
Transition temperature obtained from onset peak.
Cr: crystal; Colh: hexagonal columnar.
is chosen equal to 1), and S is the lattice area (columnar cross-section). For a hex-
p
b
agonal lattice: S = a2 3/2.

3094
C.R. Bhattacharjee et al. / Polyhedron 29 (2010) 3089–3096
H3C
N
N
O
Zn
O
H2n+1CnO
OCnH2n+1
Fig. 9. Organization of single molecule in a unit cell.
seen. The broad one centered at 4.9 Å corresponds to the liquid like
order of the aliphatic chains. The relatively sharper one at about
3.7 Å, well separated from the broad one is due to the stacking of
the molecular cores on top of one another [35]. The presence of an-
other weak and somewhat broad halo at 7.4 Å (ꢁh0 = 2h0) suggest a
staggered dimeric interactions of the molecules along the column
[36]. The intracolumnar repeating periodicity (h) along the colum-
nar axis, the columnar cross section (S) and the molecular volume
(Vm) are related analytically through the relation hS = NVm where N
is the number of molecules within the fraction of the column.
Assuming one molecule per unit cell and the density of the com-
pound in the Colh mesophase to be close to 1 g cmꢀ3, it turns out
that the columnar cross-sections (S = 566 Å2) are made of a single
molecule (Fig. 9). A comparison of the radii (22.1 Å) of half disc
molecule in its extended form as obtained from DFT calculation
and the lattice constant (25.5 Å) clearly reflect substantial interdig-
itation of the antiparallel molten chains in the columnar stack
(Fig. 10), as postulated by Swager et al. and others for related mate-
rials [37–42]. This back to back arrangement leads to a discoid-like
shell around the rigid aromatic core of the complex molecule.
Moreover the observed stacking distance h0 (3.7 Å) being larger
than the intracolumnar repeating distance, h (3.04 Å), suggest
strong undulations of the columnar cores about the columnar axis
[43–45]. These results indicate a lateral expansion of the randomly
Fig. 10. Face-to-face arrangement of two adjacent molecules of Zn-18mpd in a
column.
Fig. 11. Undulations of molecules within a column.
Fig. 12. DFT optimized structures of (a) 18mpd and (b) Zn-18mpd.

C.R. Bhattacharjee et al. / Polyhedron 29 (2010) 3089–3096
3095
Table 5
piled tilted disc within the column (Fig. 11). The tetrahedral
Selected bond lengths and bond angles of Zn-
18mpd.
methyl substituent in the phenylene spacer is presumed to force
the neighboring molecules above and below the plane by an angle
in order to be well stacked in the column.
Structure parameter
Zn-18mpda
Zn–O(1)
Zn–O(2)
Zn–N(1)
Zn–N(2)
O(1)–Zn–O(2)
N(1)–Zn–N(2)
N(1)–Zn–O(2)
O(1)–Zn–N(1)
O(1)–Zn–N(2)
N(2)–Zn–O(2)
2.084
2.081
1.966
1.966
98.6
80.9
92.3
162.6
162.5
92.4
3.5. Theoretical study of 18-mpd and Zn-18mpd
As efforts to obtain single crystal of the complexes failed, quan-
tum chemical calculation is performed to investigate the electronic
structure of the ligand, 18-mpd and its Zn(II) complex (Fig. 12),
using density functional theory (DFT). Full geometry optimization
of the complex without symmetry constrain has been carried out
using DMOL3 program package at BLYP/DNP level [46]. BLYP is the
most widely used exchange-correlation functional suggested ex-
change potential by Becke [47] with gradient corrected correlation
provided by Lee et al. [48]. DNP is the double numerical with polar-
ization basis set (all core electrons included), whose size though
comparable to 6-31G** basis set of Hehre et al. [49] is believed to
be much more accurate. Highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) diagram
of the complex is presented in (Fig. 13) .The electron density of the
HOMO is localized mainly on the aromatic rings. The HOMO and
LUMO energies of the complex calculated to be ꢀ4.61 and
a
Bond lengths are reported in Å and bond
angles in °.
ence of regular
r and dative bonding, respectively. The bond an-
gles 96.6° and 80.9° for O1–Zn–O2 and N1–Zn–N2, respectively,
around the zinc atom deviate substantially from the tetrahedral
values indicating a distorted planar four coordinate geometry. A
short central bridge, methyl substituted aromatic core in the pres-
ent complexes presumably prevented the formation of a tetrahe-
dral environment around zinc(II), leading to a distorted planar
geometry [19] Flexible spacers such as alkyl chain, (CH2)n between
two imine nitrogen donor atoms in such Schiff base complexes,
however, are known to facilitate a distorted tetrahedral geometry
usually preferred by zinc [19].
ꢀ2.51 eV, respectively,
DE = 2.1 eV matched well with those re-
ported for similar Schiff base complexes of zinc [3]. The HOMO
(ꢀ4.55 eV)-LUMO (2.63 eV) gap for the ligand was found to be
1.92 eV. The length and bent angle of the molecule in its extended
form are found to be ꢁ44 Å and 98°, respectively, from DFT calcu-
lation. Some of the selective geometric parameters of optimized
Zn(II) complex, evaluated by DFT calculation at BLYP/DNP level
are shown in Table 5. The average Zn–O1 and Zn–N1 bond lengths
of the complex are 1.96 and 2.08 Å, respectively, implying the pres-
4. Conclusion
A series of new hemi-disclike mononuclear zinc(II) complexes
of tetradentate [N2O2] donor Schiff base bearing long alkoxy sub-
stituent of variable chain length in 4-position of the side aromatic
ring and a central methyl substituted aromatic spacer group have
been successfully synthesized. All the ligands are non-mesogenic,
however, the complexes exhibited unprecedented monotropic
hexagonal columnar mesophase in the temperature range 175–
185 °C. The columnar mesophases (Colh) are presumed to have
been built from the stacking of flat and rigid phenylenediamine
spacer. The complexes exhibited intense green fluorescence both
in solid and solution state. Based on spectral and DFT study, four
coordinated distorted square planar geometry have been conjec-
tured. The strategy adopted herein can be effectively employed
to access newer green emitting metallomesogens with tunable
molecular construction motifs leading to smart multifunctional
materials.
Acknowledgments
The authors are thankful to SAIF, NEHU and CDRI, Lucknow for
analytical and spectral data. GD acknowledges financial support
from DST and UGC, Govt. of India. Thanks are also due to Dr. R.C.
Deka, Tezpur University for computational facility.
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