Self-assembled nanostructures of specially designed Schiff-bases and their zinc complexes: Preparation, characterization and photoluminescence property

Article abstract of DOI:10.1016/j.molstruc.2013.03.051

Four specially designed Schiff bases 2-formyl-4-R-6-(3N-4- hydroxybenzoicacid)-iminomethyl-phenolato (where R = methyl/tert-butyl/chloro for L1, L2, L3 respectively) and 2-(3N-4-hydroxybenzoicacid)-iminomethyl- phenolato (L4) having ability to form hydrogen bonding and their zinc complexes (1-4) have been synthesized and characterized. These complexes gave various types of nano-sized materials via self-assembly in solid state. FE-SEM was employed to investigate their morphology. Using a variety of analytical techniques such as elemental analysis, infrared spectroscopy (FT-IR), ESI-MS and ¹H NMR spectroscopy, a consistent picture of structures of these complexes are obtained. All the Schiff-bases and their zinc complexes exhibit photoluminescence property. Density functional theory calculation has been performed to rationalize the origin of the spectral bands of the ligands as well as the complexes.

Full text of DOI:10.1016/j.molstruc.2013.03.051

Journal of Molecular Structure 1042 (2013) 104–111
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Self-assembled nanostructures of specially designed Schiff-bases and their zinc
complexes: Preparation, characterization and photoluminescence property
Averi Guha a, Ria Sanyal a, Tanmay Chattopadhyay b, YounGyu Han c, Tapan Kumar Mondal d,
Debasis Das a,
⇑
a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
b Department of Chemistry, Panchakot Mahavidyalya, Sarbari, Neturia, Purulia 723 121, India
c Nanotube Research Centre (NTRC), National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
d Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
Four Schiff-bases and their ZnII
complexes has been synthesized with
the variation of para substituent.
Inter/intramolecular hydrogen
bonding generates self-assembled
nanostructures.
They possess plate, sphere, fiber and
granular shape with a size of
ꢁ200 nm depending on the R group.
Powder XRD and FE-SEM was used to
characterize their structure.
ꢀ
ꢀ
DFT calculations were included to
rationalize the electronic spectra.
a r t i c l e i n f o
a b s t r a c t
Article history:
Four specially designed Schiff bases 2-formyl-4-R-6-(3N-4-hydroxybenzoicacid)-iminomethyl-phenolato
(where R = methyl/tert-butyl/chloro for L1, L2, L3 respectively) and 2-(3N-4-hydroxybenzoicacid)-imi-
nomethyl-phenolato (L4) having ability to form hydrogen bonding and their zinc complexes (1–4) have
been synthesized and characterized. These complexes gave various types of nano-sized materials via self-
assembly in solid state. FE-SEM was employed to investigate their morphology. Using a variety of analyt-
ical techniques such as elemental analysis, infrared spectroscopy (FT-IR), ESI-MS and 1H NMR spectros-
copy, a consistent picture of structures of these complexes are obtained. All the Schiff-bases and their zinc
complexes exhibit photoluminescence property. Density functional theory calculation has been per-
formed to rationalize the origin of the spectral bands of the ligands as well as the complexes.
Ó 2013 Published by Elsevier B.V.
Received 9 January 2013
Received in revised form 25 March 2013
Accepted 25 March 2013
Available online 2 April 2013
Keywords:
Zn(II)
Schiff-base
SEM
XRD
Fluoresence properties
1. Introduction
functionally defined precursors encompasses a magnificent
research area due to the fundamental interest of mankind in
materials that have practical applications in chemistry, biology,
physics, and related interdisciplinary fields. Well-structured
deliberate architectures on surface with particular emphasis on
supramolecular self-assembly provide attractive features in this
regard. It is essentially a ‘bottom-up’ approach which allows the
simple and rapid formation of designed functional surface
Nanoscience and nanotechnology are genuinely promising to
the extent that they have significant potential to have a remark-
able impact on our society. Generating nanoparticles from
⇑
Corresponding author. Tel.: +91 33 24837031; fax: +91 33 23519755.
E-mail address: dasdebasis2001@yahoo.com (D. Das).
0022-2860/$ - see front matter Ó 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.molstruc.2013.03.051

A. Guha et al. / Journal of Molecular Structure 1042 (2013) 104–111
105
assemblies [1,2]. These can be brilliantly tuned through the opti-
mal choice of molecular building blocks utilized and stabilized by
hydrogen bonding [3–8], van der Waals interactions [9],
2. Experimental
p–p
2.1. Materials
bonding [10,11] or metal coordination [12,13] between the
blocks. This comprises the most important and handy techniques
for the spontaneous construction of nanostructures such as parti-
cle, fiber, and layer [14,15]. Among them self-assembled entities
generated via hydrogen bonding is the most popular. The choice
of molecular shape and size and especially the arrangement of
hydrogen-bonding donor and acceptor sites are crucial to gener-
ate unique supramolecular nanostructures. Hydrogen bonding is
not random but selective and controlled by the directional
strength of intermolecular interactions [16]. Hydrogen bonds
are formed when a donor (D) with an available acidic hydrogen
atom is brought into direct contact with an acceptor (A). It is
the central theme to regulate the spatial arrangement of function-
alized molecules and ions in the solid state and on surfaces [17–
20]. Nevertheless one of the biggest challenges of supramolecular
chemistry till date is earlier evaluation of molecular systems that
would lead to preprogrammed perfectly controlled self-assembly
[21]. We know that carboxylic acids can form hydrogen-bonding
patterns that contain a center of symmetry, the dimer motif, and
also aggregate in centric one-dimensional chains, catamers, result-
ing from the formation of hydrogen bonds to two or more neigh-
boring building blocks. They are conventional motif-controlling
functional elements in the field of crystal engineering as well as
supramolecular self-assembly [22]. For the development of coor-
dination chemistry, Schiff bases have been widely studied due
to their straightforward synthetic pathways. Moreover their com-
plexes have biological resemblance and excellent catalytic activity
in versatile chemical reactions [23]. They can easily serve as the
N, O or S-donor ligands. The nature, quantity and relative position
of donor atoms and metal centers play an important role for the
construction of controlled supramolecular self-assembled struc-
tures [24]. The motivation to study the specially designed Schiff
bases as guest is due to the fact that they are molecules with a
potential to form intermolecular hydrogen bond and also to
investigate the effect of different para substituent on morphology.
Further, metal complexation allows the formation of more rigid
architectures and because of the different complexation modes,
this kind of interaction can be used for the formation of tem-
plates, which are the basis for extension from the second to the
third dimension. We have investigated the self-assembly, if any,
of newly synthesized Schiff bases (L1–L4) and their zinc com-
plexes (1–4) as depicted in Scheme 1 and attempted to rational-
ize them.
All chemicals were obtained from commercial sources and used
as received. Solvent and reagents were purified and dried accord-
ing to standard procedures [25]. All other reagents were used as
obtained from either Aldrich or Fluka.
2.2. Methods and instrumentation
Reactions were monitored by TLC on silica gel plates (Machery-
Nagel POLYGRAM SIL G/UV254). Visualization of the spots were
carried out by fluorescence quenching with UV light at 254 nm.
The NMR spectra for standard characterization purposes were re-
corded at room temperature with a Bruker AV300 Supercon NMR
spectrometer using the solvent signal as the internal standard in
a 5 mm BBO probe. The 1H NMR spectra were recorded at
300 MHz. The chemical shifts are relative to the signals of the used
solvents: DMSO in DMSO-d6 (d1H = 2.50) [26]. The apparent cou-
pling constants are given in Hertz. The description of the fine struc-
ture means: s = singlet, br.s = broad signal, d = doublet, t = triplet
and sb = singlet with broken heads. Elemental analyses (carbon,
hydrogen, and nitrogen) were performed using a PerkinElmer
24 °C elemental analyzer. Infrared spectra (4000–400 cmÀ1) were
recorded at 27 °C using a Shimadzu FTIR-8400S where KBr was
used as medium. Electronic spectra (800–200 nm) were obtained
at 27 °C using a Shimadzu UV-3101PC where DMSO was used as
a medium as well as a reference. Fluorescence spectra of the com-
plexes in DMSO were recorded in a Perkin–Elmer model LS 55
Luminescence spectrometer. The electrospray mass spectra were
recorded on a MICROMASS Q-TOF mass spectrometer.
2.3. Syntheses of complexes
2.3.1. Ligand 1 – L1
The Schiff-base was prepared by condensation reaction be-
tween an aldehyde and a primary amine. An ethanolic solution
(10 mL) of 3-amino-4-hydroxy benzoic acid (10 mmol) was added
dropwise to an ethanolic solution (15 mL) of 2,6-diformyl-4-
methyl-phenol (10 mmol) with constant stirring. The resultant
mixture was refluxed for 6 h and the solvent was reduced by rotary
evaporator. The solid mass thus obtained was recrystallised several
times from dry ethanol to get the pure ligand.
Yield: 69%. Anal. Calcd. for C16H13NO5: C, 64.21; H, 4.37; N, 4.66.
Found: C, 63.28; H, 4.32; N, 4.62%.
Scheme 1. (A) Ligands: L1: X = ACH3, Y = ACHO; L2: X = AC(CH3)3, Y = ACHO; L3: X = ACl, Y = ACHO; L4: X = Y = AH. (B) Complexes: 1: X = ACH3, Y = ACHO; 2: X = AC(CH3)3,
Y = ACHO; 3: X = ACl, Y = ACHO; 4: X = Y = AH.

106
A. Guha et al. / Journal of Molecular Structure 1042 (2013) 104–111
1H NMR (300 MHz, DMSO-d6, 25 °C) d = 15.107 (s, 1H, COOH),
2.3.6. Complex 2
14.728 (br, 1H, OHa), 10.395 (s, 1H, CHO), 10.293 (br, 1H, OHb),
9.142 (s, 1H, N@CH), 7.894 (s, 1H, Hc), 7.803, 7.809 (sb, He), 7.768
(s, 1H, Hd), 7.687, 7.680 (d, 1H, Hf), 7.018, 7.000 (d, 1H, Hg), d
2.221 (s, 3H, CH3).
It was prepared by the same process as above where the Schiff-
base ligand used was L2 instead of L1.
Yield: 68%. Anal. Calcd. for C19H19NO6Zn: C, 53.98; H, 4.53; N,
3.31. Found: C, 53.93; H, 4.51; N, 3.30%.
FT-IR
m
(KBr) [cmÀ1] = 1683(s), 1634(s), 1598(s), 1462(s).
1H NMR (300 MHz, DMSO-d6, 25 °C) d = 14.964 (s, 1H, COOH),
10.279 (s, 1H, CHO), 9.120 (s, 1H, N@CH), 7.972 (s, 1H, Hc), 7.812,
7.813 (sb, 1H, He), 7.770 (s, 1H, Hd), 7.675, 7.654 (d, 1H, Hf),
7.003, 6.971 (d, 1H, Hg), 1.144 (s, 9H, C(CH3)3).
12
ESI-MS m/z = 300.2496 (calculated for
C
H13NO5 + H+:
16
300.2859).
2.3.2. Ligand 2 – L2
FT IR
m
(KBr) [cmÀ1] = 1700(s), 1630(s), 1594(s), 1383(s).
12
It was prepared by adopting similar procedure as for L1 where
the aldehyde used was 2,6-diformyl-4-tert-butyl-phenol instead of
2,6-diformyl-4-methyl-phenol.
ESI-MS m/z = 423.7793 (calculated for
C
H19NO6Zn + H+:
19
423.7449).
Yield: 72%. Anal. Calcd. for C19H19NO5: C, 66.85; H, 5.61; N, 4.10.
Found: C, 66.83; H, 5.60; N, 4.08%.
2.3.7. Complex 3
The same method was repeated where ligand L3 was used in-
stead of L1.
1H NMR (300 MHz, DMSO-d6, 25 °C) d = 15.109 (s, 1H, COOH),
14.734 (br, 1H, OHa), 10.389 (s, 1H, CHO), 10.287 (br, 1H, OHb),
9.026 (s, 1H, NH), 7.968 (s, 1H, Hc), 7.812, 7.809 (sb, 1H, He),
7.768 (s, 1H, Hd), 7.674, 7.650 (d, 1H, Hf), 6.987, 6.965 (d, 1H, Hg),
1.152 (s, 9H, C(CH3)3).
Yield: 68%. Anal. Calcd. for C15H10NO6ClZn: C, 44.92; H, 2.51; N,
3.49. Found: C, 44.90; H, 2.48; N, 3.47%.
1H NMR (300 MHz, DMSO-d6, 25 °C) d = 15.233 (s, 1H, COOH),
10.108 (s, 1H, CHO), 9.341 (s, 1H, N@CH), 7.910 (s, 1H, Hc), 7.806,
7.804 (sb, He), 7.767 (s, 1H, Hd), 7.561, 7.558 (d, Hf), 7.154, 7.069
(d, Hg).
FT IR
m
(KBr) [cmÀ1] = 1685(s), 1623(s), 1516(s), 1460(s).
12
ESI-MS m/z = 342.3321 (calculated for
C
H19NO5 + H+:
19
342.3659).
FT IR
m
(KBr) [cmÀ1] = 1717(s), 1634(s), 1534(s), 1365(s).
12
ESI-MS m/z = 424.0319 (calculated for
C
H10NO6ClZn + Na+:
15
2.3.3. Ligand 3 – L3
424.0658).
The same procedure as for L1 was repeated by using 2,6-difor-
myl-4-chloro-phenol as the aldehyde.
2.3.8. Complex 4
Yield: 70%. Anal. Calcd. for C15H10NO5Cl: C, 56.35; H, 3.15; N,
4.36. Found: C, 56.31; H, 3.12; N, 4.34%.
A similar synthetic strategy was employed as for other com-
plexes where ligand L4 was used instead of L1.
1H NMR (300 MHz, DMSO-d6, 25 °C) d = 15.105 (s, 1H, COOH),
14.730 (br, 1H, OHa), 10.392 (s, 1H, CHO), 10.285 (br, 1H, OHb),
9.049 (s, 1H, NH), 7.995 (s, 1H, Hc), 7.806, 7.804 (sb, He), 7.767 (s,
1H, Hd), 7.561, 7.558 (d, Hf), 7.018, 7.001 (d, Hg).
Yield: 68%. Anal. Calcd. for C14H11NO5Zn: C, 49.66; H, 3.27; N,
4.41. Found: C, 49.63; H, 3.25; N, 4.38%.
1H NMR (300 MHz, DMSO-d6, 25 °C) d = 13.804 (s, 1H, COOH),
9.052(s, 1H, N@CH) and the peaks between d 7.959 to d 6.718 cor-
respond to the aromatic hydrogens of the two benzene rings.
FT IR
m
(KBr) [cmÀ1] = 1680(s), 1629(s), 1528(s), 1384(s).
12
ESI-MS m/z = 320.7762 (calculated for
320.7049).
C
15
H10NO5Cl + H+:
FT IR
m
(KBr) [cmÀ1] = 1695(s), 1556(s), 1338(s).
12
ESI-MS m/z = 361.6230 (calculated for
C
H11NO5Zn + Na+:
14
361.6108).
2.3.4. Ligand 4 – L4
The same synthetic strategy was used as for L1 where salicyal-
dehyde was used instead of 2,6-diformyl-4-methyl-phenol.
Yield: 65%. Anal. Calcd. for C14H11NO4: C, 65.37; H, 4.31; N, 5.44.
Found: C, 65.35; H, 4.30; N, 5.42%.
2.4. Electron microscopic observation
Field-emission scanning electron microscope (FE-SEM) images
were taken using a Hitachi S-4800 field emission electron micro-
scope at 15 kV. Specimens were prepared by placing a 3 lL drop
of the dispersion on an amorphous carbon supporting film
mounted on a standard grid. The drop was then blotted off with
a filter paper, followed by evaporation to dryness in vacuo. These
dried samples on the grids were coated with 2–3 nm thickness of
osmium using a Meiwaforsis Neoc AN.
1H NMR (300 MHz, DMSO-d6, 25 °C) d = 13.598 (s, 1H, COOH),
10.762 (br, 1H, OHa), 10.659 (br, 1H, OHb), 9.088 (s, 1H, NH) and
the peaks between d 7.959 and d 6.718 correspond to the aromatic
hydrogens of the two benzene rings.
FT IR
m
(KBr) [cmÀ1] = 1690(s), 1513(s), 1380(s).
12
ESI-MS m/z = 280.2679 (calculated for
280.2308).
C
H11NO4 + Na+:
14
2.5. Spectroscopic analyses
2.3.5. Complex 1
methanolic solution (10 mL) of zinc(II) acetate hydrate
A
FT-IR spectra were obtained on a Shimadzu FTIR-8400S spec-
trometer using KBr method. X-ray diffraction (XRD) was performed
(5 mmol) was added dropwise to a methanolic solution (10 mL)
of pure Schiff-base ligand L1 (5 mmol) with constant stirring. The
complex began to precipitate out immediately after the completion
of addition of ligand. The stirring was continued for another 2 h, fil-
tered and kept in a CaCl2 desiccator.
on a Rigaku R-AXIS IV X-ray diffractometer monochromated Cu K
radiation (40.0 kV, 30.0 mA) at room temperature.
a
2.6. Computational methods
Yield: 68%. Anal. Calcd. for C16H13NO6Zn: C, 50.48; H, 3.44; N,
3.68. Found: C, 50.43; H, 3.40; N, 3.65%.
Full geometry optimizations were carried out using the density
functional theory method at the B3LYP level for ligands L1–L4 and
the complex 1–4 [27]. All elements except Zn were assigned the 6-
31+G(d) basis set. The LANL2DZ basis set with effective core poten-
tial was employed for the Zn atom [28]. The vibrational frequency
calculation was performed to ensure that the optimized geome-
tries represent the local minima and there are only positive eigen
values. All calculations were performed with Gaussian03 program
1H NMR (300 MHz, DMSO-d6, 25 °C) d = 15.15 (s, 1H, COOH),
10.500 (s, 1H, CHO), 9.091 (s, 1H, N@CH), 7.891 (s, 1H, Hc), 7.811,
7.817 (sb, He), 7.768 (s, 1H, Hd), 7.687, 7.680 (d, 1H, Hf), 6.898,
6.801 (d, 1H, Hg), d 2.355 (s, 3H, CH3).
FT IR
m
(KBr) [cmÀ1] = 1717(s), 1634(s), 1534(s), 1354(s).
12
ESI-MS m/z = 403.6750 (calculated for
C
16
H13NO6Zn + Na+:
403.6468).

A. Guha et al. / Journal of Molecular Structure 1042 (2013) 104–111
107
package [29] with the aid of the GaussView visualization program.
Vertical electronic excitations based on B3LYP optimized geome-
tries were computed using the time-dependent density functional
theory (TD-DFT) formalism [30] in DMSO using conductor-like
polarizable continuum model (CPCM) [31].
1700–1720 cmÀ1 which is assigned for ACOOH group. Thus it is
evident that that the (SI Figs. 1–8 in Supporting information)
ACOOH group do not participate in complex formation, but they
form lateral-type hydrogen-bond which assists in the formation
of nanostructures via self assembly (Scheme 2). In addition, there
are bands in the range 3000–3400 cmÀ1, 2200–2800 cmÀ1 and at
ꢁ1630 cmÀ1 for phenolic AOH groups, ACAH skeletal vibration
and –CHO groups respectively.
3. Results and discussion
The ligands were further characterized by 1H NMR spectroscopy
(see SI Figs. 9–12 in Supporting information), where for L1, L2 and
L3 (Scheme 1) a singlet signal at d ꢂ 15.1 corresponds to the hydro-
gen of carboxylic acid group, the broad peaks at d ꢂ 14.7 and 10.3
stands for phenolic OHa and OHb respectively, the aldehyde hydro-
gen arises at d ꢂ 10.4 and that for the imine hydrogen at 9.1. The
peaks in between d 7.0 to d 8.0 correspond to the aromatic hydro-
gens of the two benzene rings present in the ligand system where
Hc and Hd appear as singlet at d ꢂ 7.9 and 7.8 respectively but
metacoupled He give singlet with broken heads at d ꢂ 7.803,
7.809. The peaks at d ꢂ 7.687, 7.680 are for Hf which is orthocou-
pled with Hg and with broken heads for metacoupled with He, dou-
blet at d ꢂ 7.018, 7.000 are for Hg which doublet orthocoupled with
Hf. The three hydrogens of the methyl group for L1 appear as a sin-
glet at d 2.3 nine tert-butyl hydrogens at d 1.3 for L2. For the L4 the
bands show a slight shifting with respect to the others at d ꢂ 13.6
for carboxylic acid group, 10.8, 10.7 (each for the two hydrogens of
OHa and OHb), 9.1 for imine hydrogen.
3.1. Synthesis and characterization
All the Schiff-base ligands (L1–L4) were prepared by following
the same condensation procedure (see SI file). The compounds
with carboxylate functional group can be conveniently character-
ized by IR spectroscopy. Generally, the peak at wave numbers
around 1680–1700 cmÀ1, which is assignable to the C@O stretch-
ing vibration, indicate a hydrogen bonding mode between the
COOH groups. The peak in the range of 1680–1690 cmÀ1 indicates
the existence of an acid–acid dimer, whereas the peak at
1700 cmÀ1 and above indicates a lateral-type hydrogen-bond
[32–36]. We have carried out FT-IR measurements of all Schiff
bases (L1–L4) and their zinc complexes (1–4). The characteristic
IR band frequencies are summarized in Table 1.
All the ligands have a peak in the range 1680–1690 cmÀ1 for the
stretching frequency of the carbonyl group of ACOOH group. On
the other hand, the zinc complexes exhibit a peak in the range
Table 1
Infrared spectral data in cmÀ1
.
Functional groups
L1
L2
L3
L4
1
2
3
4
COOH group
CHO group
Imine group
Skeletal vib.
1683
1634
1598
1462
1685
1623
1516
1460
1680
1629
1528
1384
1690
–
1513
1380
1717
1634
1534
1354
1700
1630
1594
1383
1717
1634
1534
1365
1695
–
1556
1338
(A)
(B)
Scheme 2. Two different types of hydrogen-bonding modes between the COOH groups displaying (A) acid–acid dimer (B) lateral type hydrogen bonding.

108
A. Guha et al. / Journal of Molecular Structure 1042 (2013) 104–111
The complexes (1–4) were also characterized by proton NMR
(L1, L2, L3 and L4) were observed to be different depending on
the ligand group at para-position, i.e., ACH3, AC(CH3)3, ACl and
(see SI Figs. 31–34 in Supporting information) where each spec-
trum resembles that of the corresponding ligand except the fact
that the phenolic OH groups which gets deprotonated during com-
plexation have vanished.
AH. The ligand L1 showed sheet shape of ꢁ2
lM nm length and
ꢁ200 nm diameter (Fig. 1A). The ligand L2 illustrated spherical
particles of size ꢁ100 nm (1C). The ligand L3 exhibited amorphous
agglomerates of size ꢁ200 nm (1E). The ligand L4 showed granular
shape with the size of ꢁ250 nm (1G). We observed significant
change in the ligands morphology when zinc ion was added to
them (i.e. in the complex). The complexes, 1 and 4, both showed
fiber like shape (Fig. 1B and H), but their size was quite different.
3.2. Scanning electron microscopy studies and powder X-ray
diffraction
Field-emission scanning electron microscopy (FE-SEM) was
used to analyze the self-assembled nanostructure of the resulting
compounds (Fig. 1). The morphologies of the Schiff-base ligands
The complex 1 have ꢁ1
l
M length and ꢁ100 nm diameter whereas
complex 4 have ꢁ1 M length and ꢁ50 nm diameter. In case of the
l
Fig. 1. FE-SEM images of the designed Schiff-bases (A: L1, C: L2, E: L3 and G: L4) and their zinc complexes (B: 1, D: 2, F: 3, H: 4).

A. Guha et al. / Journal of Molecular Structure 1042 (2013) 104–111
109
Complex 1
Complex 2
Complex 3
Complex 4
20
40
60
80
2
θ
(deg)
Fig. 2. XRD pattern of the zinc complexes: a = 1; b = 2; c = 3; d = 4.
complexes 2 and 3, both have same rod shape (Fig. 1D and F) with a
size of ꢁ1 M length and ꢁ200 nm diameter.
l
This change in morphology of the various ligands indicates that
the morphology of supramolecular aggregation can be controlled
by introducing groups of various steric properties. The methanolic
mixture of Zinc(II) acetate and Schiff-bases forms a precipitate
each time we had attempted for crystallization. Just for the aim
to investigate the inherent physical nature of the complexes, pow-
der XRD of the complexes were carried out. Fig. 2 shows the XRD
for the precipitated samples. It reveals that all the complexes are
crystalline solids to some degree. Complex 2 and 4 appears to be
the most crystalline in the set followed by complex 1 and then 3
which can be considered to be almost amorphous. It is probably
again the substitution at para position which governs the crystal-
linity of the samples and hence their XRD patterns.
Thus it can be suggested that the bulky ligands have a tendency
to form a crystalline structure although it is difficult to describe the
detailed molecular packing structures from the patterns. 2 and 3
have similar crystal peaks at 9.1°, 16.4°, and 19.4°. It gives support
to the fact that they form similar inter and intramolecular hydro-
gen bonds in the complexes and their rod like shape in Fig. 1B
and C is supposed to result from these interactions. The diffraction
of 4 shows a sharp peak at 6°, which is mainly attributed to the for-
mation of imine groups and the cleavage of intra-molecular hydro-
gen bond [37]. Notably, the change in crystalline nature is mainly
dependant on some factors induced by a ligand, such as steric hin-
drance and hydrophobic force [38]. Thus, it is likely that the crys-
talline nature can be regulated by the modification of substituent
groups in Schiff-bases.
Fig. 3. The absorption (inset) and emission spectra of the (A) Schiff bases (B) their
zinc complexes in DMSO at a concentration of 10À4 (M) at room temperature.
so that an unequivocal assignment for the spectral properties of
Schiff-bases is often laid aside [30–32]. In our case too we have faced
similar difficulties. Moreover, solubility of the compounds in other
solvents were quite poor making spectral characterization further
difficult. However, with a view to the fact that L1 contains a substi-
tuted benzaldehyde type chromophore the absorption bands at
ꢂ267 nm and ꢂ367 nm seems to be originating from the open form
and intramolecularly hydrogen bonded closed form of L1, respec-
tively. It is probably the greater degree of stability due to the pres-
ence of the intramolecular hydrogen bonding (IMHB) interaction
that the closed form absorbs at lower energy region (kabs ꢂ 367 nm).
Though the assignment of the lowest energy absorption band
(kabs ꢂ 467 nm) remains a matter of debate, a direct comparison
with spectroscopic properties of structurally related compounds
3.3. Absorption and fluorescence properties
All the designed Schiff-bases exhibit three absorption bands
while their zinc complexes show two peaks which are summarized
in Table 2. The absorption spectra of L1 have been recorded in
DMSO solvent and presented in Fig. 3a. L1 is found to exhibit three
absorption bands peaking at kabs ꢂ 267 nm, 316 nm and 467 nm.
Usually it is difficult to perturb the spectral properties of Schiff-
bases by external agents (like change of solvent polarity, pH, etc.)
Table 2
Electronic and photoluminescence spectral data in nm.
L1
1
L2
2
L3
3
L4
4
Absorption
Emission
267, 316, 467
404, 401,529
527
292, 466
402, 558
266, 316, 472
404, 401,521
520
296, 461
403, 533
267, 319, 475
406, 399,522
523
298, 457
407, 538
285, 349, 468
405 404,525
524
300, 436
408
530

110
A. Guha et al. / Journal of Molecular Structure 1042 (2013) 104–111
[39–41] tends to attribute the band to the zwitterionic species. Also
this observation seems to be self-authenticated by the fact that the
zwitterionic species is capable of undergoing substantial stabiliza-
tion through delocalization of its formal charges whence it absorbs
in the lower energy region.
in high yield by a simple synthetic approach. The Schiff-bases have
been designed in such a way that they may form inter and/or intra
molecular hydrogen bonding, a property which has been proved to
be instrumental in making self-assembled materials. The morphol-
ogy of the Schiff-base ligands are observed to be markedly influ-
enced by the substituent groups present in the backbone of the
ligand systems. Also, for a particular ligand the morphology gets
changed on complexation with Zn(II) ion. The crystalline nature
of the complexes differs on varying the substituent on the ligand
backbone. Thus, the present study demonstrates the tuning possi-
bilities of supramolecular arrays giving an insight into the new
strategies for fabricating nanostuctural architectures. The synthe-
sized compounds also have pronounced fluorescent property. So,
they may have potential for applications in heterogeneous cataly-
sis and as contrast agents for bio-imaging and that work is under-
way in this laboratory. In order to get an in-depth idea of the origin
of the UV–vis spectral properties of the ligands as well as of the
complexes, DFT calculations have been performed whose results
are significantly well matched with the experimental values.
To understand the spectral bands specially the low energy
transitions of the ligands, TD-DFT calculations on the optimized
geometries of L1–L4 are performed in DMSO solvent. The low en-
ergy bands are only found in the zwitterionic forms of the li-
gands. Other calculated spectral transitions also well reproduced
the experimental bands with this form of the ligands (Fig. 4
and see SI Figs. 21–28 in Supporting information). To study the
effect of Zn2+ ion on the spectral properties of the ligands, the
respective Zn(II) complexes in penta-coordinated geometries
(two coordination site have been occupied by MeOH as is as-
signed by ESI-MS study from the spectra given in SI Figs. 13–20
in Supporting information) have been optimized and TD-DFT cal-
culations were performed. The optimized structures of the ligands
and complexes are given in SI Figs. 29 and 30 in Supporting infor-
mation. There are no significant change in the spectral bands of
the complexes compared to free ligand values except the blue-
shifting of the low energy bands (see SI Tables 1–8 in Supporting
information).
The emission spectra of L1 recorded at kex ꢂ 316 nm and
467 nm are presented in Fig. 3a. It is usual to observe that excita-
tion at the respective species produces the corresponding local
emission maxima at kem ꢂ 400 nm and 528 nm. However, for
kex ꢂ 316 nm a little hump at kem ꢂ 528 nm is observed corre-
sponding to the local emission from the zwitterionic species.
It is not unlikely that electronic excitation at the higher energy
excitation wavelength of 316 nm may lead to formation of the
zwitterionic species on the excited state surface to some extent.
The emission spectra of the four ZnII complexes are depicted in
Fig. 3b and inset show the absorption spectra of those species.
The complexation with Zn2+ ion is not found to perturb the spectral
properties of L1 significantly except for a red-shift on the emission
profile to kem ꢂ 558 nm with accompanied lowering of intensity.
We have observed similar results for the other ligands (L2–L4)
and their zinc complexes (2–4).
Acknowledgements
The authors wish to thank the CSIR, New Delhi (01/(2464)/11/
EMR-II dt 16-05-2011 to DD) for financial support. The generous
help of Dr. Bijan Pal, Research Scholar, Department of Chemistry,
University of Calcutta, India, for fluorescence study is also duly
acknowledged.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.03.
051.
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